In Situ Characterization Techniques for Investigating the Reaction Mechanisms of Lithium–Sulfur Batteries: Progress, Application, and Future

ABSTRACT

Lithium–sulfur (Li–S) batteries remain constrained by polysulfide shuttling, sluggish redox kinetics, and Li dendrite growth under practical conditions. Introducing catalysts at the three‐phase interface of cathode/electrolyte/sulfur can markedly accelerate polysulfide redox reactions, thereby suppressing the shuttle of soluble intermediates and improving the utilization of sulfur active species. Modifying the Li anode can concomitantly alleviate Li dendrites to enhance cycling stability. Despite these advances, the interaction between catalytic centers and reactive sulfur species and the mechanistic evolution of polysulfide species during real‐time charge/discharge remains far from fully resolved. Therefore, in situ characterization techniques are crucial for the real‐time study of phase transitions and interface reactions. However, a systematic review that establishes a reaction‐interface/research‐question‐driven framework for technique selection remains lacking, particularly one that integrates diverse operando characterizations with the multi‐electron and multistep electrochemical mechanisms of Li–S batteries. Against this backdrop, this review focuses on three key mechanisms in Li–S batteries: cathode catalysis, polysulfide shuttle behavior, and anode SEI evolution. It establishes a framework that matches research objectives with in situ characterization techniques. Our aim is to help researchers quickly and rationally select and combine appropriate in situ characterization techniques in studies of different mechanisms, thereby more accurately revealing the nature of the reactions and guiding the material and system design of high‐performance Li–S batteries.

This review begins by revisiting the electrochemical principles of Li–S batteries and the kinetic challenges. Then, we summarize the latest advances regarding in situ characterization techniques, focusing on their design, operating principle, and practical applications. Drawing on their applications in resolving different mechanisms, we classify in situ characterization techniques into three categories. Finally, we discuss the existing limitations of these techniques and propose constructive strategies to promote their development and application in Li–S battery research.

1. Introduction

Lithium‐sulfur batteries are widely regarded as important candidates for next‐generation energy storage devices owing to their high theoretical energy density (2600 Wh kg⁻¹), abundant sulfur resources, low cost, and relatively environmentally friendly characteristics [1, 2, 3]. However, several critical barriers hinder the commercialization of Li–S batteries, among which sluggish reaction kinetics are one of the central impediments that undermine reversible capacity utilization and durable high‐rate performance [4, 5, 6].

In recent years, the rapid development of electrocatalysis and interface chemistry has provided a clear path to overcome these kinetic challenges [7, 8]. By constructing catalytically active functional components at the cathode, the adsorption and conversion behavior of S/LiPSs can be controlled at the reaction interface, reducing the energy barrier of key steps and optimizing electron/ion transport, thereby significantly accelerating the overall reaction kinetics [9, 10, 11]. However, electrocatalysis in Li–S batteries does not proceed at a single, stationary boundary. Instead, it is governed by two evolving three‐phase interfaces, namely cathode‐S₈‐electrolyte and cathode‐Li₂S₂/Li₂S‐electrolyte [12, 13, 14]. Because the discharge/charge of the sulfur cathode involves complex multi‐electron, multi‐step reaction pathways, the three‐phase interface continuously generates, evolves, and reconstructs during cycling, often resulting in a significant difference between the actual working and the initial state. Consequently, dynamic, real‐time, and direct tracking of the catalyst's structural evolution at the three‐phase interface and its interactions with polysulfides is essential for understanding the electrocatalytic reaction mechanism of Li–S systems, thereby guiding the synthesis and preparation of high‐performance electrocatalysts [15, 16, 17].

Furthermore, performance improvements in Li–S batteries are not limited to catalytic regulation on the cathode side. In recent years, multi‐dimensional engineering strategies on the anode, including electrolyte solvation structure adjustment, Li metal surface/interface modification, and artificial SEI construction, have been demonstrated to mitigate parasitic reactions induced by polysulfide crossover and stabilize Li deposition, thereby promoting higher reversibility and extended lifespan at the full‐cell level [18, 19, 20]. It is worth emphasizing that the growth of Li dendrites on the Li metal anode and the formation of the SEI film constitute a highly non‐equilibrium evolution process [21]. This process not only directly determines the battery's safety and cycle life but also leads to the accumulation of side reactions [22, 23, 24]. This indirectly affects the reaction environment of the sulfur cathode and the migration behavior of polysulfides [25]. Therefore, a deep understanding of the SEI evolution at the Li metal/electrolyte interface is a necessary prerequisite for achieving synergistic optimization of “cathode catalysis‐anode/electrolyte modification” at the material level, and thus constructing high‐energy‐density and high‐reliability Li–S batteries.

Over recent decades, numerous studies have reported in situ characterization techniques for elucidating the reaction mechanisms of Li–S batteries [35, 36, 37, 38]. As shown in Figure 1a, researchers have shown increasing interest in various in situ characterization techniques in recent years. These techniques help elucidate multi‐phase reaction kinetics in cathode and interface evolution behavior at the anode. Notably, as shown in Figure 1b, since 2011, researchers have gradually combined various in situ characterization techniques with Li–S electrochemical testing, laying a solid foundation for addressing key challenges in Li–S batteries [26, 27, 28, 29, 30, 31, 32, 33, 34]. For example, in situ XRD enables real‐time monitoring of the dynamic evolution of polysulfide crystallinity and phase structures at the cathode of Li–S batteries [39]. In situ TEM can provide nanoscale insights into Li dendrite growth and SEI formation [40]. Furthermore, other advanced in situ characterization techniques, such as in situ XAS, NMR, AFM, Raman, UV–vis, FTIR, and XPS, have provided supplementary information for understanding the structural changes, morphological evolution, and behavior of sulfur and polysulfide compounds in Li–S batteries [41, 42, 43, 44]. Understanding these fundamental mechanisms will guide the further optimization and design of electrode/electrolyte materials and battery structures, thereby contributing to improved electrochemical performance of Li–S batteries.

FIGURE 1.

(a) Publications utilizing in situ characterization techniques for Li–S batteries from January 1, 2011, to December 31, 2025, were quantified through a search on the Web of Science database. The search was conducted using the query “In situ XRD/XAS/XPS/FTIR/TEM/AFM/UV–vis/Raman/NMR Li–S batteries” within the “Topic” field. (b) Chronology of developments in in situ characterization techniques for Li–S batteries since 2011. Reprinted with permission [26]. Copyright 2011, American Chemical Society. Reprinted with permission [27]. Copyright 2013, Elsevier. Reprinted with permission [28]. Copyright 2013, Wiley‐VCH. Reprinted with permission [29]. Copyright 2014, American Chemical Society. Reprinted with permission [30]. Copyright 2015, Wiley‐VCH. Reprinted with permission [31]. Copyright 2015, Wiley‐VCH. Reprinted with permission [32]. Copyright 2016, Wiley‐VCH. Reprinted with permission [33]. Copyright 2017, American Chemical Society. Reprinted with permission [34]. Copyright 2017, Sage Publications.

Existing reviews typically compare characterization techniques primarily on the basis of their spectral signatures, yet often fail to establish explicit links between in situ techniques and the distinct reaction interfaces in Li–S batteries. Building on this, we develop a framework that integrates in situ/operando characterization with three key mechanisms of Li–S batteries, including the cathode and catalyst reaction mechanisms, tracking polysulfide evolution and shuttle behavior, and electrode‐electrolyte interphase evolution. First, in Section 2, we introduce the basic electrochemical reaction mechanism of Li–S batteries and highlight the key challenges. Section 3 introduces the basic principles of commonly used in situ characterization techniques and the design of in situ batteries. Building upon this foundation, we systematically review the significant progress made in recent years in in situ research on elucidating redox mechanisms, polysulfide dissolution/migration behavior, and the dynamic evolution of Li metal dendrites and SEI, emphasizing the crucial role of advanced characterization techniques in revealing the essence of electrochemical reactions and driving performance improvements in Li–S batteries. Furthermore, based on specific mechanisms, we categorize various in situ characterization techniques into three main types: (3.1) cathode and catalyst reaction mechanisms; (3.2) tracking polysulfide evolution and shuttle behavior; and (3.3) electrode‐electrolyte interphase evolution. Section 4 builds on the classification of in situ/operando characterization techniques discussed in Section 3 and further examines their synergistic application strategies for resolving key scientific issues in Li–S batteries. Focusing on three different research targets: the working mechanism of cathodes/catalysts, the tracking of polysulfide intermediates, and interfacial evolution processes. This section highlights how in situ characterization techniques with complementary information dimensions can be rationally integrated to enable multiscale correlative analysis. Finally, in Section 5, we summarize the current challenges facing in situ characterization techniques and explore potential future development directions in this field. We hope this review will deepen the systematic understanding of researchers about in situ characterization of Li–S electrochemistry, promote its rational application in Li–S battery research, and ultimately drive the development of high‐performance Li–S batteries that better meet actual operating conditions.

2. Electrochemical Reaction Mechanisms and Challenges in Li–S Batteries

2.1. Overall Electrochemical Reaction Mechanism

As shown in Figure 2a, a typical Li–S battery consists of a sulfur‐based cathode, a Li metal anode, and an electrolyte composed of ether‐based organic solvents, typically a mixture of 1,3‐dioxolane (DOL) and dimethoxyethane (DME), with lithium bis (trifluoromethane sulfonyl) imide (LiTFSI) as the supporting electrolyte [45, 46]. During the discharge process, Li undergoes oxidation at the anode, releasing Li⁺ and electrons. The electrons pass through an external circuit, generating an external current. Meanwhile, sulfur species at the cathode accept these electrons. This process triggers a multi‐step reduction reaction, ultimately forming Li₂S [47]. Throughout this electrochemical reaction, current is generated via the redox reactions between Li and sulfur. In an ideal scenario, where the S₈ molecule is fully reduced to Li₂S, a total of 16 electrons are consumed. The overall reaction is represented by Equation (1) [48]:

$$16\mathrm{L}{\mathrm{i}}^{+}+{\mathrm{S}}_8+16{\mathrm{e}}^{-}\to 8\mathrm{L}{\mathrm{i}}_2\mathrm{S}$$ (1)

FIGURE 2.

(a) Schematic diagram and (b) a typical charge and discharge voltage curve of Li–S batteries.

However, the actual reaction process in Li–S batteries is highly complex. During discharge, S₈ is not directly reduced to Li₂S in a single step; instead, it undergoes a series of intricate structural and compositional transformations, involving various reaction intermediates [16, 49]. This multi‐stage process encompasses five primary reaction stages. The specific sequence of reactions is as follows:

$${\mathrm{S}}_8+2\mathrm{L}{\mathrm{i}}^{+}+2{\mathrm{e}}^{-}\leftrightharpoons \mathrm{L}{\mathrm{i}}_2{\mathrm{S}}_8$$ (2)

$$\mathrm{L}{\mathrm{i}}_2{\mathrm{S}}_8+\frac{2}{3}\mathrm{L}{\mathrm{i}}^{+}+\frac{2}{3}{\mathrm{e}}^{-}\leftrightharpoons \frac{4}{3}\mathrm{L}{\mathrm{i}}_2{\mathrm{S}}_6$$ (3)

$$\mathrm{L}{\mathrm{i}}_2{\mathrm{S}}_6+\mathrm{L}{\mathrm{i}}^{+}+{\mathrm{e}}^{-}\leftrightharpoons \frac{3}{2}\mathrm{L}{\mathrm{i}}_2{\mathrm{S}}_4$$ (4)

$$\mathrm{L}{\mathrm{i}}_2{\mathrm{S}}_4+2\mathrm{L}{\mathrm{i}}^{+}+2{\mathrm{e}}^{-}\leftrightharpoons 2\mathrm{L}{\mathrm{i}}_2{\mathrm{S}}_2$$ (5)

$$\mathrm{L}{\mathrm{i}}_2{\mathrm{S}}_2+2\mathrm{L}{\mathrm{i}}^{+}+2{\mathrm{e}}^{-}\leftrightharpoons 2\mathrm{L}{\mathrm{i}}_2\mathrm{S}$$ (6)

Figure 2b illustrates the typical charge and discharge voltage profile of a Li–S battery. As depicted in the figure and Equations (2) and (3), during the initial stage of the discharge phase, the elemental sulfur (S₈) ring accepts Li⁺ and electrons from the conductive framework, resulting in the formation of soluble long‐chain polysulfides (Li₂S₈, Li₂S₆) [50, 51, 52]. This transformation corresponds to the first plateau observed in the discharge curve (approximately 2.4 V) and is accompanied by a distinct color change in the electrolyte. This stage contributes roughly 25% of the total discharge capacity [53, 54]. As shown in Equations (4) and (5), as the discharge progresses, the long‐chain polysulfides undergo successive reduction, forming short‐chain intermediates such as Li₂S₄ and Li₂S₂ [55, 56, 57]. These intermediates maintain some solubility, which results in increased electrolyte viscosity and electrode passivation, causing the discharge voltage to enter the second plateau (around 2.1 V) [58, 59, 60]. Ultimately, the polysulfides are completely reduced to insoluble Li₂S, which precipitates onto the surface of the cathode, forming a dense insulating layer, as described in Equation (6). This final stage contributes approximately 75% of the discharge capacity and is pivotal in determining the battery's total discharge performance [61, 62].

2.2. Major Challenges

As outlined in the previous section, the complex electrochemical reactions and distinctive charge and discharge characteristics of Li–S batteries give rise to several critical challenges in their practical application. This section summarizes the four primary challenges encountered by Li–S batteries: the polysulfide shuttle effect, the low electrical conductivity and volume expansion of the sulfur cathode, the instability of the Li anode, and sluggish reaction kinetics of polysulfides. These challenges will be explored in detail in the subsequent discussion.

1. Shuttling Effect

The polysulfide shuttle effect is a major contributor to capacity degradation in Li–S batteries. This phenomenon arises from the uncontrolled migration of polysulfides between the anode and cathode through the electrolyte, resulting in active material loss and capacity fading [63, 64]. Specifically, during discharge, sulfur reacts with Li⁺ to generate soluble Li₂S_x. Driven by a concentration gradient, these polysulfides migrate from the sulfur cathode toward the Li anode, where they react with metallic Li to form insoluble Li₂S [25, 65]. In addition to depleting active material from the cathode, this process causes the accumulation of insoluble Li₂S on the anode surface, leading to passivation of the Li metal and further deterioration of battery performance [59, 66]. During charging, short‐chain polysulfides dissolve into the electrolyte and migrate back toward the cathode [67]. Over successive cycles, the repeated shuttling of polysulfides not only disrupts the electrochemical balance between the electrodes but also accelerates the loss of active materials, further diminishing battery performance [68, 69].

The polysulfide shuttle effect in Li–S batteries manifests most directly as accelerated capacity fading and depressed coulombic efficiency, phenomena that are consistently observed even in early cycles across ether‐based Li–S chemistries [70, 71]. Reported studies under representative test conditions show that shuttle‐involved cycling can lead to measurable sulfur loss and a concomitant drop in coulombic efficiency, while the long‐term decay is nonlinear because it is coupled to the evolving anode surface state, SEI growth, and polysulfide concentration gradients rather than a fixed per‐cycle loss ratio [72, 73, 74]. This naturally explains why the shuttle also induces self‐discharge: even at open circuit, continued parasitic conversion lowers OCP and gradually drains stored energy [75]. Consequently, beyond cycling metrics, the shuttle effect directly undermines calendar‐life‐relevant retention behavior, making it a primary obstacle for Li–S batteries to satisfy long‐duration energy storage requirements in practical use cases [76, 77].

• 2.Sulfur Cathode

At room temperature, sulfur is a characteristic insulator, with an electronic conductivity of only 5 × 10⁻³⁰ S/cm. Similarly, the ion/electronic mixed conductivity of Li₂S and Li₂S₂, which are formed upon full discharge, remains extremely low [78, 79]. Due to the poor electron transport capabilities within the sulfur particles, electrochemical reactions are primarily confined to the particle surface, impeding efficient electron transport into the particle interiors and thereby limiting the overall reaction extent. Although the theoretical specific capacity is 1675 mAh g⁻¹, its actual first‐cycle capacity typically falls below 1200 mAh g⁻¹ under practical and conventional conditions, primarily due to its low conductivity and surface‐limited reactions [80, 81, 82]. As cycling progresses, the surface of the cathode becomes progressively covered by an insoluble Li₂S passivation layer, isolating the internal active materials and preventing their participation in surface reactions, which accelerates capacity degradation. Furthermore, when the discharge rate exceeds 0.5 C, the sulfur cathode's limited charge transfer efficiency cannot sustain high current loads, resulting in a rapid decline in the capacity of the Li–S battery [83, 84].

The primary cause of volume expansion in the sulfur cathode is the structural expansion and contraction induced by phase transitions and the density difference between sulfur and Li₂S during charge and discharge cycles. Specifically, the density of sulfur is 2.07 g/cm³, while that of Li₂S is 1.66 g/cm³, leading to a significant volumetric expansion of approximately 80%. With repeated expansion and contraction, the sulfur particles tend to detach from the conductive additives and the current collector, leading to the loss of active substances [85, 86]. Simultaneously, the expansion of the sulfur particles compresses the electrode pores, thereby obstructing electrolyte infiltration and polysulfide diffusion. This phenomenon increases interfacial mass transfer resistance, further diminishing the overall conductivity of the sulfur cathode. The coupling of these two phenomena accelerates the degradation of battery performance over time [87, 88].

• 3.Li Metal Anode

Upon initial contact with the electrolyte, the Li anode undergoes a reduction reaction that leads to the formation of a solid SEI, which critically influences the kinetics of subsequent Li deposition [89, 90]. The chemical composition of this initial protective layer primarily consists of a mixture of organic Li carbonate and inorganic Li fluoride [91]. However, its limited mechanical strength and insufficient ion transport capacity pose a significant risk of failure. During cycling, spatial and temporal inhomogeneities in Li⁺ flux drive preferential Li deposition at surface defects, giving rise to high‐curvature protrusions [92, 93, 94]. The tip‐enhanced electric field effect accelerates the growth of these protrusions in subsequent cycles, eventually evolving into dendritic or moss‐like structures. Additionally, the substantial volume fluctuation during Li deposition and stripping causes continuous rupture and reorganization of the brittle SEI film [1, 95, 96]. This repeated deformation exposes fresh Li surfaces, which further consume the electrolyte for interface repair. Consequently, this process accelerates the irreversible consumption of active Li and gradually undermines the uniformity of the electrode‐electrolyte interface. Over time, the proportion of organic components in the dynamically reconstructed SEI film can increase interfacial impedance [14, 47, 97].

• 4.Sluggish Reaction Kinetics

Sluggish conversion kinetics and high reaction energy barriers are fundamental challenges in Li–S batteries, directly leading to various performance‐related issues. The discharge process, particularly its final stage, is crucial, as the lower‐voltage conversion of soluble polysulfides to insoluble Li₂S₂/Li₂S contributes most of the capacity [54 ^, 61]. Theoretical calculations indicate that this liquid‐solid transformation (Li₂S₄ to Li₂S) is an entropy‐reducing reaction, requiring the overcoming of a significant energy barrier [98, 99]. As a result, it serves as the rate‐determining step in the reduction of sulfur species, making its kinetics critical to the overall discharge performance of Li–S batteries. The insulating nature of Li₂S promotes passivation layer formation on its surface, hindering Li‐ion diffusion. Additionally, the decomposition potential of Li₂S exceeds its thermodynamic equilibrium value, leading to severe electrochemical polarization and reduced coulombic efficiency [100]. Furthermore, Li₂S nucleation is inherently non‐uniform and requires overcoming a high nucleation energy barrier. Overall, the sluggish liquid‐solid transformation kinetics of Li₂S₄ to Li₂S result from multiple material‐related constraints, necessitating a comprehensive optimization approach incorporating electron/ion transport enhancement, interface engineering, and material design [101, 102]. Improving the reaction kinetics of this step can enhance discharge capacity and efficiency while mitigating the shuttle effect, reducing active sulfur loss, and improving coulombic efficiency and rate performance [103]. Moreover, speeding up this process can reduce side reactions between liquid polysulfides and the Li anode, preventing the formation of an unstable SEI layer [104].

To overcome these challenges, it is crucial to gain a thorough understanding of the changes occurring in the various components of Li–S batteries during charge and discharge cycles. However, the electrochemical processes during the charging and discharging of Li–S batteries are highly complex, and the exact impact of the chemical and physical states of the electrode materials on redox reactions remains poorly understood. As such, the application of advanced characterization techniques is critical for capturing the dynamic changes within different parts of the Li–S battery throughout cycling. The subsequent section will provide an overview of the fundamental principles behind various in situ characterizations employed in Li–S batteries, along with a discussion of their specific applications. Additionally, we will identify the in situ characterization methods best suited for monitoring the structural and electrochemical transformations occurring during the charge and discharge processes of Li–S batteries.

3. Principles and Applications of In Situ/Operando Characterization in Li–S Batteries

In situ spectroscopy enables time‐resolved monitoring of reaction systems under real‐time conditions, and is therefore widely used to capture the formation and decay of key intermediates in catalysis, as well as the dynamic evolution of the valence state/coordination environment of the catalytic center [105]. Its core advantage lies in directly reading observable measurements such as bonding vibrations, surface adsorption configurations, and electronic structure changes through the electromagnetic radiation, magnetic resonance, or probe‐sample interactions, thereby providing a verifiable chain of evidence for reaction pathways and rate‐controlling steps [106]. It should be noted that in situ characterization and operando characterization are not identical. Specifically, in situ characterization refers to measurements performed under a defined environment, electrochemical state, or simulated working condition, but does not necessarily require the device to operate in a complete and realistic electrochemical configuration. By contrast, operando characterization imposes more stringent requirements: the measurement should be conducted under conditions that approximate real device operation as closely as possible, while the corresponding electrochemical data are collected simultaneously and directly correlated with the characterization signals. In general, in situ characterization requires only the simulation of working conditions, whereas operando characterization requires real working conditions [107, 108, 109].

The operation of a Li–S battery can be conceptualized as a completely enclosed “black box,” where elucidating microscopic state changes during cycling remains challenging [110]. Traditional electrochemical measurement techniques provide limited insight into these dynamic transformations, while conventional ex situ characterization methods fail to capture the real‐time evolution of the battery's internal state during charge and discharge [43, 111]. In situ characterization techniques, however, allow real‐time tracking of electrode state changes throughout cycling, yielding crucial insights into dynamic processes [112, 113, 114]. This capability is essential for unraveling the intricate reaction mechanisms governing Li–S battery operation and advancing the development of more efficient sulfur cathodes, electrolytes, and functional additives. This section explores the principles and applications of commonly used in situ characterization techniques in Li–S batteries, and discusses them according to their practical application scenarios. Table 1 summarizes representative in situ/operando characterization techniques used in Li–S battery research and their main application scenarios.

TABLE 1.

Working principles of various in situ characterization techniques and their applications in Li–S batteries.

Application scenario	In situ/Operando characterization technique	Detection principle	Representative applications in Li–S batteries	Advantages	Limitations
Cathode and catalyst reaction mechanisms	In situ/operando XAS	X‐ray absorption analysis for elemental electronic and structural insights	Tracking the dynamic evolution of catalytically active centers	High elemental selectivity	Limited temporal resolution
	In situ/operando XRD	X‐rays interacting with a periodic atomic lattice produce a characteristic diffraction pattern	S8/Li2S phase‐transition process	Real‐time tracking of crystalline phase changes	Limited sensitivity to soluble, amorphous, or poorly crystalline LiPSs
Tracking polysulfide evolution and shuttle behavior	In situ/operando NMR	Revealing molecular structure via nuclear spin resonance in a magnetic field	Detection of polysulfide intermediates by 1H and 7Li NMR.	Capability to probe chemical environments at the molecular and ionic scales	Complex cell configuration and difficult discrimination of polysulfide species
	In situ/operando Raman	Molecular vibration scattering for chemical bond and structure analysis	Detection of polysulfide intermediates	Sensitive to many sulfur‐containing species with suitable calibration and spectral deconvolution	Limited penetration depth and restricted spatial representativeness
	In situ/operando UV–vis	Detect and identify compounds by their absorption of UV and visible light	Evolution of soluble polysulfides	Real‐time and intuitive observation of soluble polysulfide evolution	Inability to observe solid‐phase products
Electrode‐electrolyte interphase evolution	In situ/operando AFM	Imaging based on the interaction between the probe tip and the sample	Direct observation of interphase‐film transformation	High temporal resolution	Complex in situ cell configuration and limited spatial resolution
	In situ/operando XPS	Electron energy spectrum for elemental and chemical state identification	Surface elemental composition, chemical states, and interfacial bonding	Sensitivity to surface/interfacial chemical states	Complex electrochemical in situ cell configuration and surface‐sensitive probing only
	In situ/operando FTIR	Infrared spectrum generation via light interference and the Fourier transform	Particularly suitable for probing interfacial chemical processes	Minimal disturbance to the cell configuration	Low spatial resolution and limited detection sensitivity
	In situ/operando TEM	High‐energy electron imaging of ultra‐thin samples	Suitable for probing elusive transient intermediates	High temporal resolution	Complex cell configuration and potential beam‐induced damage
Complementary in situ/operando characterization techniques	In situ/operando EPR	Detection of paramagnetic species with unpaired electrons	Monitoring of sulfur radical intermediates during cycling	Sensitivity to radicals and paramagnetic intermediates	Detects only paramagnetic species; susceptible to electrolyte interference
	In situ/operando SANS	Probing nanoscale structural changes through neutron–nucleus interactions	Li2S deposition and distribution changes in porous carbon cathodes	Nanoscale‐averaged structural information; suitable for probing internal changes in porous electrodes	Low spatial resolution; demanding in situ cell requirements

3.1. Cathode and Catalyst Reaction Mechanisms

3.1.1. In Situ/Operando XAS

X‐ray absorption spectroscopy (XAS) can distinguish the electronic structure of different oxidation states and unoccupied states by the edge shift, white line intensity, and pre‐edge characteristics of the XANES region, and further reflect the influence of factors such as local symmetry and crystal field effect [113]. Figure 3a shows the basic principle of x‐ray absorption testing. Under the conditions of transmission geometry, approximately uniform sample, and negligible scattering, when x‐rays pass through the sample, their intensity attenuation conforms to the Beer‐Lambert law:

$$I_t=I_0{e}^{-\mu \left(E\right)t}$$ (7)

FIGURE 3.

(a) Schematic diagram of XAS. (b) Graphical representation delineating the characteristic spectral regions in x‐ray absorption spectroscopy: pre‐edge features, XANES, and EXAFS components. (c) Schematic diagram of the in situ button cell. (d) Evolution of S K‐edge XANES spectra during a full galvanostatic cycle. (e) Standard normalized S K‐edge XANES spectra of S, Li₂S, and polysulfide solutions (Li₂S₄, Li₂S₆, and Li₂S₈), together with XANES spectra acquired at selected voltages during cycling. (f) Galvanostatic charge/discharge profiles obtained at 0.1C rate, with concomitant variation in peak 2470.9 intensity that serves as a spectroscopic marker for polysulfide intermediates throughout cycling. Reprinted with permission [119]. Copyright 2023, American Association for the Advancement of Science. (g) Comparison of EXAFS Curves of Fe K‐Edge for 1Fe‐N/C, 2Fe‐N/C, and 3Fe‐N/C with Experimental and Calculated Fe K‐Edge XANES Spectra. (h) Operando Fe K‐Edge FT‐EXAFS Spectra Recorded During Discharge‐Charge Cycles and the Evolution of Local Coordination and Oxidation States of Fe Atoms. Reprinted with permission [120]. Copyright 2024, American Chemical Society.

I ₀ denotes the incident x‐ray intensity, while I _t represents the transmitted x‐ray intensity after the beam passes through the sample. The parameter μ(E) is the energy‐dependent absorption coefficient, and t is the sample thickness [115, 116]. The x‐ray absorption spectrum typically comprises two distinct components: XANES and EXAFS [42]. Figure 3b demonstrates that as x‐ray energy approaches an element's absorption threshold, a sharp rise in absorption coefficient occurs, creating a distinct edge. This edge position reflects the element's oxidation state, whereas pre‐edge characteristics reveal local symmetry details. Notably, XANES spectra exhibit high sensitivity to coordination environments. The EXAFS originates from the interference effects of outgoing photoelectrons scattered by neighboring atoms. By Fourier transforming the oscillatory EXAFS signal into a radial distribution function, one can extract structural parameters, such as metal ligand bond lengths and coordination numbers in a specific element manner for a wide range of materials, including both crystalline and amorphous samples [117]. In summary, the XANES spectral features yield essential data regarding oxidation states and local electronic symmetry, whereas EXAFS oscillations enable precise determination of structural parameters, including interatomic distances, coordination spheres, and adjacent atomic species [118].

Compared with conventional ex situ XAS, in situ XAS enables characterization under conditions closer to actual material operation, thereby allowing the real‐time tracking of changes in the local electronic structure and coordination environment of the target element. For Li–S batteries, LiPSs and their discharge products are highly sensitive to air and moisture and can be readily oxidized during cell disassembly, making it difficult to accurately capture the true evolution of sulfur species during battery operation. Therefore, ex situ XAS is generally limited to probing the intrinsic electronic structure and local coordination environment of catalysts before reaction. By contrast, in situ XAS allows spectra to be collected in real time in sealed cells under practical charge‐discharge conditions, enabling more reliable tracking of sulfur species conversion, valence state changes of catalytically active sites, and the dynamic evolution of local coordination structures. This provides direct evidence for elucidating the sulfur redox mechanisms and the nature of catalysis in Li–S batteries.

In Li–S battery research, sulfur K‐edge x‐ray absorption spectroscopy (S K‐edge XAS) serves as an element‐specific local probe, which can maintain atomic‐scale sensitivity to changes in the chemical speciation and valence state/local electronic structure of sulfur species, thus providing direct evidence for the redox evolution in the sulfur cathode reaction process [62] In a notable study, Xu et al. modified a CR2032 button cell by drilling a 4.5 mm diameter window in its shell (Figure 3c) to facilitate in situ XAS measurements [119]. To eliminate the potential interference from sulfur species originating from LiTFSI, 1.0 M LiClO₄ dissolved in a 1:1 (v/v) mixture of DOL and DME was employed as the electrolyte in all sulfur K‐edge XAS measurements. The methodology enabled in situ tracking of the sulfur cathode's chemical state transformations throughout electrochemical cycling, capturing dynamic redox processes with temporal resolution. As illustrated in Figure 3d‐f, by comparing the XANES spectra of sulfur species standards with those obtained during cycling, and by correlating the spectroscopic features with the electrochemical data, the transformation processes of polysulfides in the presence of Co‐SAs/NC catalysts were elucidated. This work established a systematic methodological framework for deciphering the reaction mechanisms of polysulfides in Li–S batteries.

For the XAS analysis of coordination changes in the local environment of metal catalysts during electrochemical operation, Cai et al. changed the number of iron atoms in the iron assembly, and ex situ XAS was initially employed to characterize the structure and local coordination environment of 1Fe‐N/C, 2Fe‐N/C, and 3Fe‐N/C catalysts (Figure 3g). Subsequently, in situ XAS measurements revealed how the adsorption‐desorption and electrochemical conversion kinetics of polysulfides evolved with the increasing incorporation of single iron atoms at the catalyst's metal center. As shown in Figure 3h, during the electrochemical cycle, the 2Fe‐N/C catalyst exhibited the emergence of an Fe─S bond, with its peak intensity reaching a maximum at approximately 25% discharge depth and gradually diminishing during the charging process. This reversible interaction between Fe and S plays a crucial role in maintaining catalytic stability, highlighting the dependency of catalytic activity on the dynamic interaction between the iron metal center and sulfur species within the catalyst [120].

Although XAS offers distinct advantages, including strong element selectivity and the ability to probe local electronic structure and coordination environment, it still faces several limitations in Li–S battery research. For example, in situ cells typically require special windows, simplified cell configurations, or relatively low sulfur loadings, which may deviate from practical Li–S batteries [121]. In addition, the reaction process in Li–S batteries involves the rapid formation, diffusion, and deposition of polysulfides. Because XAS scans require a finite acquisition time, conventional in situ XAS may yield time‐averaged information when reaction intermediates are short‐lived, making it difficult to capture transient species [122].

In conclusion, XAS has emerged as a crucial technique for analyzing the real‐time evolution of battery reaction mechanisms and material transformations at the atomic and electronic levels. By leveraging in situ XAS, researchers can dynamically track changes in the chemical state, coordination environment, and charge transfer processes of sulfur species, thereby elucidating the stepwise transformation pathway of S₈→Li₂S_n→Li₂S within the sulfur cathode. Furthermore, in situ XAS enables the assessment of the adsorption and catalytic capabilities of sulfur host materials toward LiPSs. By capturing the dynamic evolution of catalytically active centers, it offers critical guidance for the rational design of highly efficient sulfur hosts. In summary, in situ XAS overcomes a fundamental limitation in Li–S battery research by providing element‐specific local structural and electronic information that evades conventional characterization methods. This capability is instrumental in optimizing reaction kinetics, suppressing side reactions, and enhancing the cycling stability of Li–S batteries. The advancement of in situ XAS has significantly propelled progress in Li–S batteries, establishing it as a vital characterization technique for overcoming the fundamental technical challenges of this energy storage technology.

3.1.2. In Situ/Operando XRD

X‐ray diffractometers constitute a powerful, non‐destructive analytical tool for analyzing crystalline materials and determining their atomic spacing. As shown in Figure 4a, these instruments operate on the principle of constructive interference of monochromatic x‐rays as they interact with a crystalline sample [43]. The principle of XRD lies in how x‐rays scatter when interacting with atoms in a regularly ordered crystal lattice. The resulting diffraction patterns, generated by this scattering, reveal key information about the material's structural periodicity and crystal orientation. This technique has been extensively employed in battery research to elucidate phase composition and assess the crystallinity of active materials [36]. The interplanar spacing information extracted from these diffraction patterns adheres to Bragg's law, expressed as follows:

nλ  =  2dsinθ

(8)

d denotes the interplanar lattice spacing responsible for producing the diffraction peak, λ represents the wavelength of the incident x‐ray, θ is the diffraction angle, and n is an integer indicating the order of reflection [43, 123]. For a known x‐ray wavelength, the interplanar spacing d can be determined from θ. A schematic of the XRD setup is shown in Figure 4b. By systematically scanning the sample over a range of 2θ angles, the resulting diffraction peaks are transformed into a series of d spacings. Because each crystalline compound exhibits a unique set of d spacings, unknown substances can be accurately identified through comparison with these established values [124].

FIGURE 4.

(a) Schematic Representation of the XRD Principle. (b) Schematic diagram of XRD apparatus. (c) In situ XRD was employed to evaluate the redox kinetics of LiPSs in Li–S batteries based on Mn‐CoP(11)@S, CoP@S, and Mn‐CoP(19)@S. Reprinted with permission [126]. Copyright 2022, Wiley‐VCH. (d) Schematic representation of the modified coin cell designed for in situ XRD measurements. (e) In situ XRD data plotted as a function of time. Reprinted with permission [127]. Copyright 2020, American Chemical Society.

For Li–S batteries, in situ XRD offers clear advantages over ex situ XRD. Ex situ XRD requires cell disassembly at selected charge/discharge states, followed by electrode retrieval for measurement; during exposure, the sample may undergo phase changes, making it difficult to fully reflect the actual electrochemical reaction process. Therefore, ex situ XRD is generally used mainly to characterize the crystalline phases of electrode materials themselves, whereas it is less suitable for probing Li₂S₂/Li₂S generated during electrochemical reactions. By contrast, in situ XRD enables real‐time monitoring of structural evolution in the sulfur cathode during battery operation, thereby continuously providing dynamic information on crystalline S₈ consumption and Li₂S formation/decomposition. This is of great significance for elucidating the reaction mechanisms of Li–S batteries and evaluating how electrode materials regulate Li₂S nucleation/decomposition and sulfur‐conversion kinetics.

In situ XRD was first introduced to the battery field in 1978, when Chianelli et al. developed an in situ XRD cell to investigate the behavior of lithium/titanium sulfide batteries [125]. The foundation for battery material studies using in situ XRD was established by this groundbreaking 1978 work, which served as the initial demonstration of this technique's potential. In a recent investigation, Zhou et al. employed in situ XRD to monitor the phase evolution of sulfur species and the formation of Li₂S in CoP@S, Mn‐CoP(11)@S, and Mn‐CoP(19)@S‐based batteries during charge and discharge cycling [126]. As illustrated in Figure 4c, the α‐S₈ peaks at 23.065°, 25.832°, 26.706°, and 27.699° in the Mn‐CoP(19) sample disappeared more rapidly than those in CoP@S and Mn‐CoP(11)@S. At the end of discharge, the Li₂S peak at 26.997° gradually intensified and subsequently diminished during charging. These observations indicate that Mn‐CoP(19) exhibits significantly enhanced conductivity and catalytic activity compared to Mn‐CoP(11) and CoP, thereby improving the redox kinetics of sulfur. Furthermore, Chien et al. refined the design of a button cell by combining in situ XRD with an intermittent current interruption technique [127]. As shown in Figure 4d, a 7 mm diameter aperture was introduced into the stainless‐steel casing of a CR2025 coin cell to facilitate in situ measurements. Both sides of the aperture were sealed with Kapton tape to maintain cell integrity. The cathode, Celgard separator, and Li metal anode were assembled between two beryllium (Be) windows, ensuring uniform stacking pressure and optimal structural alignment. It enables the simultaneous monitoring of potential (E), internal resistance (R), and diffusion resistance coefficient (k) in Li–S batteries (Figure 4e). Their findings showed that sulfur species underwent distinct phase changes. These transformations were found to correspond closely with the fluctuation of internal resistance during battery operation.

In recent years, in situ XRD has emerged as a critical tool in modern materials science due to its real‐time monitoring capabilities and adaptability to various environmental conditions. It provides valuable information on crystalline phase evolution, particularly S₈ consumption and Li₂S formation/decomposition. Meanwhile, the structural transformations in battery materials can also be precisely characterized through in situ XRD analysis. Both electrode reaction mechanisms and phase transition dynamics are clearly revealed by this approach. Consequently, this technology not only enhances the accuracy and reliability of analytical results but also provides a robust platform for the discovery of novel materials and the optimization of existing ones. However, in situ XRD also has certain limitations. Because polysulfide intermediates are typically soluble and poorly crystalline, their diffraction peaks are difficult to directly detect by XRD. Therefore, in situ XRD mainly reveals the transformation of sulfur species indirectly through the phase‐transition behavior of S₈ and Li₂S, but is generally less sensitive to soluble, amorphous, or poorly crystalline polysulfide intermediates.

3.2. Tracking Polysulfide Evolution and Shuttle Behavior

3.2.1. In Situ/Operando NMR

Nuclear magnetic resonance (NMR) spectroscopy is based on the magnetic resonance properties of nuclear isotopes [62]. It exploits the specific nuclear spin states of atomic nuclei in an external magnetic field, where the spin orientation relative to the magnetic field results in distinct energy levels [36]. The core principle of NMR is that when a radiofrequency pulse of a specific frequency is applied, atomic nuclei in the spin state absorb energy and transition to higher energy levels. This technique provides critical information about atomic environments, including the local chemical environments, molecular dynamics, and speciation [128, 129].

In situ NMR spectroscopy is a powerful tool for studying electrochemical devices, such as redox flow batteries and electrocatalytic reactors, as it enables the monitoring of structural and electronic changes in materials during electrochemical processes [130, 131, 132]. Letellier et al. pioneered the use of in situ ⁷Li NMR spectroscopy in battery systems to investigate the reversible electrochemical reactions of Li⁺ in graphitic carbon electrodes. Specifically, they used in situ NMR spectroscopy on ⁷Li to indicate the precise state of vacancies and Li, whether embedded or inserted, and the energy barrier that causes the hysteresis between battery charging and discharging during the extraction process [133].

Dedicated probe design is central to in situ NMR spectroscopy. In Li–S batteries, ⁷Li NMR has proven to be a sensitive technique for detecting both dissolved and solid Li⁺ species, enabling the identification of discharge and ex situ charge products. Zhang et al. designed two Li₂S@graphene cathode materials to investigate the electrocatalytic and domain effects of graphene on Li₂S. Figure 5a displays the in situ NMR signals of the Li₂S@graphene/Li battery during charge and discharge processes. The narrow linewidth ⁷Li NMR peak near 2.3 ppm indicates high Li⁺ mobility in the Li₂S@graphene cathode, while the peak at 246 ppm represents the overall Li characteristics of the Li anode. In Figure 5b, the two ⁷Li NMR signals of the Li₂S@graphene cathode correspond to two distinct Li⁺ environments in the solution. The ⁷Li NMR signals remain stable across different states of charge (SOC) and depths of discharge (DOD), demonstrating that Li₂S@graphene acts as a solid interception barrier. This barrier effectively confines active materials within the nanocapsules during cycling and significantly suppresses the outflow of polysulfides [134].

FIGURE 5.

(a, b) In situ NMR stacking spectra of the Li₂S@graphene cathode at different SOC and depths of DOD. Reprinted with permission [134]. Copyright 2024, Wiley‐VCH. (c) Three‐dimensional ¹H NMR characterization coupled with DFT calculations reveals the binding energetics between aromatic disulfides and polysulfides (Li₂S₂). Depth‐dependent NMR spectra demonstrate invariant solvent signatures (DOL: 3.84/4.90 ppm; DME: 3.47/3.30 ppm) in conventional electrolytes during progressive discharge states. Reprinted with permission [135]. Copyright 2021, Elsevier. (d) ¹H NMR spectral evolution of DMTS‐enhanced electrolyte system containing sulfur‐saturated solutions at progressive discharge states. Characteristic solvent resonances appear at (A) 4.80 ppm and (B) 3.78 ppm (DOL), (C) 3.48 ppm and (D) 3.29 ppm (DME), with the distinctive DMTS signature at (E) 2.48 ppm. Discharge profile of S/C cell in 30 vol% DMTS modified electrolyte cycled at 0.3 C and the operando ¹H NMR spectra at different discharge states. Reprinted with permission [136]. Copyright 2020, Elsevier.

During the charge and discharge process, ether‐based solvents (e.g., DOL/DME) in the electrolyte form specific solvation structures with Li⁺. In situ ¹H NMR tracks changes in chemical shifts and monitors the real‐time concentration variations of electrolyte components, enabling the analysis of solvation structures and interfacial reaction mechanisms. Zhou et al. conducted in situ ¹H NMR studies on electrolytes at different depths of discharge. Figure 5c reveals a characteristic 2.25 ppm signal in the initial discharge phase, indicative of ·SR radical‐derived organic sulfide generation. Subsequent spectral evolution, manifested through peak displacement and intensity modulation, elucidates the mechanistic progression of S─S bond scission in the PDATTS cathode, ultimately yielding sulfonate intermediates (RSLi) and precipitated Li sulfides [135]. Liu et al. introduced a DMTS‐modified electrolyte and compared ¹H NMR peak intensities under various conditions. They observed that during discharge, the DMT peak intensity weakened, while several polysulfide methyl group peaks appeared. As discharge continued, all peaks except DMT intensified, reaching a maximum at 2.10 V, after which their intensities decreased until the end of discharge. This indicated a gradual reduction in dimethyl sulfide concentration. The ¹H NMR spectra confirmed that DMTS radicals exchanged with S radicals, forming dimethyl polysulfides and inhibiting the generation of soluble polysulfides (Figure 5d) [136]. In situ ¹H NMR validated the reduction pathway and desulfurization mechanism in this novel system, providing a theoretical foundation for the improved capacity retention observed.

Overall, the dynamic application of in situ NMR technology in Li–S batteries provides a unique perspective with local chemical resolution to reveal the polysulfide shuttle effect, chemical evolution pathways, and electrode‐electrolyte interaction mechanisms. By in situ monitoring of chemical shifts and intensity changes in ¹H, ⁷Li, and other nuclei, this technique can help reveal the dynamic equilibrium of sulfur species transformation and the accumulation patterns of the shuttle effect during charge and discharge cycles. Additionally, in situ NMR can analyze the coordination structures of polysulfides with solvent molecules or additives, clarifying the regulatory mechanisms of solvation structures on the solubility and reaction activity of LiPSs. These insights guide the design of low‐solubility electrolyte systems, advancing the development of high‐performance Li–S batteries.

Nevertheless, in situ NMR still has several intrinsic limitations. For example, the ⁷Li NMR signal may originate simultaneously from LiTFSI, the Li anode, and LiPSs, which increases the uncertainty of spectral assignment and interpretation. Moreover, in situ NMR requires specially designed cells that are compatible with the magnet and radio‐frequency coil, and these configurations differ to some extent from practical coin‐cell or pouch‐cell systems [137]. Therefore, although in situ NMR can provide dynamic information, it remains limited in terms of spatial resolution, quantitative analysis, and the simulation of realistic operating conditions.

3.2.2. In Situ/Operando Raman

When photons undergo inelastic scattering with quantized molecular vibrations, the resulting energy shifts (first characterized by C.V. Raman in 1928) form the basis of Raman spectroscopic analysis [138]. When incident light interacts with a material and is scattered by its molecules, the majority of the scattered light retains the same wavelength as the incident light, a phenomenon known as Rayleigh scattering [139]. However, a small fraction of the scattered light exhibits a wavelength shift, which is determined by the chemical structure of the material. This shifted component is referred to as Raman scattering [140, 141]. As shown in Figure 6a, the fundamental principle of Raman spectroscopy is based on the Raman effect, where the Raman shift is directly related to the vibrational energy levels of the molecules, providing fingerprint information about the material. Through quantitative examination of vibrational peak attributes, Raman analysis provides simultaneous determination of chemical constituents, molecular architectures, and crystalline forms.

FIGURE 6.

(a) Schematic illustration of the Raman scattering principle. (b) Schematic diagram of the electrochemical in situ Raman spectroscopy setup. (c) Time‐resolved Raman spectra and discharge curves of Li–S batteries with S@CoS/CNT and S@Ni_0.5Co_2.5S₄/CNT cathodes. Reprinted with permission [144]. Copyright 2024, Wiley‐VCH. (d) Battery device used for in situ Raman testing. (e) Corresponding cyclic voltammetry (CV) curves and in situ Raman spectra during discharge, along with a comparison between the voltage‐dependent experimental concentration from in situ Raman spectra and the simulated concentration derived from density functional theory (DFT) calculations. Reprinted with permission [145]. Copyright 2024, Springer Nature. In situ time‐resolved Raman spectroscopy during the discharge process of batteries equipped with (f) PP separators and (g) VN/Mo₂C@CNS separators. (h) Schematic diagram illustrating the mechanism of Li–S batteries with VN/Mo₂C@CNS separators. Reprinted with permission [146]. Copyright 2024, Elsevier.

In situ Raman spectroscopy integrates the capabilities of Raman spectroscopy with electrochemical testing systems, enabling real‐time monitoring of electrochemical reactions under operando conditions. This technique provides a powerful tool for understanding electrochemical reaction mechanisms at the molecular level [142]. Real‐time monitoring and data acquisition are central to in situ Raman spectroscopy. Advanced data analysis software can process spectral data in real time, extracting parameters such as characteristic peak positions and intensities, and generating reaction kinetic curves. This real‐time feedback is crucial for process optimization and control. Ex situ Raman measurements are typically performed after the cell has been charged or discharged to a selected voltage, followed by electrode or separator retrieval under an inert atmosphere, washing, sealing, and subsequent Raman analysis. However, this approach can only provide static information at a specific time point. Moreover, sulfur species may change during cell disassembly and sample handling, meaning that ex situ results may not fully represent the true internal state of the working cell. Therefore, the ability of in situ Raman spectroscopy to monitor the dynamic evolution of sulfur species in real time during charge‐discharge processes is particularly important for elucidating polysulfide conversion and Li₂S deposition/decomposition behavior in Li–S batteries.

Since LiPSs exhibit high Raman activity, Raman spectroscopy has emerged as a powerful tool for studying Li–S batteries [143]. Wang et al. pioneered the integration of Li–S battery devices with Raman spectroscopy and electrochemical workstations, constructing a custom in situ electrochemical testing platform (Figure 6b) to investigate the electrocatalytic conversion process of sulfur species in the S@Ni_0.5Co_2.5S₄/CNT cathode under real electrochemical conditions. In contrast to the S@CoS cathode, the Raman peaks corresponding to soluble polysulfide intermediate on the S@Ni_0.5Co_2.5S₄/CNT cathode side nearly disappeared, demonstrating that Ni_0.5Co_2.5S₄ effectively catalyzes the conversion of long‐chain LiPSs into solid Li₂S (Figure 6c) [144]. By employing time‐resolved Raman spectroscopy, the concentration changes of polysulfides during the charge and discharge process can be semi‐quantitatively or quantitatively with appropriate calibration, enabling the establishment of an accurate reaction kinetic model. This approach is essential for elucidating the conversion kinetics of polysulfide. Liu et al. utilized in situ Raman spectroscopy to detect specific reaction intermediates during the discharge CV scan (Figure 6d,e). The study achieved molecular‐level resolution of the potential‐dependent phase evolution, offering definitive experimental validation of the multi‐step sulfur reduction process in Li–S batteries [145]. Their findings established Li₂S₄ as the critical intermediate dictating SRR reaction rates, while confirming the dual role of Li₂S₆ as the dominant discharge‐phase polysulfides and principal shuttle‐effect contributor.

The evaluation of the shuttle effect inhibition is another critical application of in situ Raman spectroscopy. Since the dissolution and diffusion of polysulfides are the root causes of the shuttle effect, the effectiveness of electrolyte additives or separator modifications in mitigating the shuttle effect can be quantitatively assessed by comparing the distribution changes of polysulfides before and after their introduction. For instance, Sun et al. investigated the inhibitory effect of VN/Mo₂C@CNS separator modifiers on polysulfides using in situ Raman spectroscopy. As shown in Figure 6f–h, the S₄ ²⁻ and S₆ ²⁻ signals were detected on the second voltage plateau near the unmodified separator side, indicating a severe shuttle effect. In contrast, the LiPSs peaks in the VN/Mo₂C@CNS battery remained weak throughout the discharge process, demonstrating that VN/Mo₂C@CNS significantly suppresses the shuttle effect as a separator modification layer [146].

In situ Raman spectroscopy enables the dynamic tracking of the generation, transformation, and dissolution behavior of LiPSs during the charge and discharge process. It provides detailed insights into the concentration changes of LiPSs, directly correlating these with the shuttle effect and capacity decay mechanisms. Through time‐resolved analysis, this technique can monitor spatially resolved species distributions when coupled with confocal configurations, offering a theoretical foundation for suppressing side reactions. Compared with other in situ characterization techniques, in situ Raman spectroscopy exhibits high sensitivity toward key reaction intermediates in Li–S batteries. These polysulfide species possess distinct S‐S vibrational modes and S‐Li‐related vibrational features, enabling Raman spectroscopy to effectively distinguish different sulfur species. However, because Raman mainly probes information from the region near the laser‐irradiated area, it is difficult to obtain bulk or spatially averaged information comparable to that provided by in situ XRD, XAS, or NMR. In addition, laser irradiation may induce local heating of the material, which can compromise the accuracy of the measurements. Overall, by providing a powerful dynamic perspective, in situ Raman spectroscopy plays an indispensable role in deciphering the complex reaction network of Li–S systems, accelerating their transition from fundamental research to practical applications.

3.2.3. In Situ/Operando UV–Vis

Ultraviolet–Visible spectroscopy (UV–vis spectroscopy) is a spectroscopic analytical technique that uses the absorption of ultraviolet and visible light by a substance to determine its composition, structure, or concentration. Its fundamental principle involves analyzing the structure, composition, and concentration of a substance by measuring its absorption of specific wavelengths of light, which arises from electronic transitions within the molecules of the material. UV–vis spectroscopy is frequently applied to study the optical properties and reaction kinetics of materials, particularly homogeneous and molecular and heterogeneous materials [147].

UV–vis spectroscopy is a widely used analytical tool for observing polysulfides in Li–S batteries. It has proven to be a powerful technique for studying sulfur species in solution, enabling both qualitative and quantitative analysis of intermediates in Li–S batteries. Its key advantages include high sensitivity to low‐concentration samples and the ability to provide rapid qualitative analysis. Since the in situ UV–vis technique was first introduced in 2013 to investigate the evolution of intermediates, it has become one of the most important and effective methods for elucidating the actual reaction pathways in Li–S systems [28, 62]. Different polysulfides exhibit distinct absorption peaks in UV–vis spectra, allowing in situ UV–vis to monitor changes in sulfur species within the electrolyte during charge and discharge processes. Notably, long‐chain polysulfides absorb at longer wavelengths compared to short‐chain polysulfides [56]. It should be noted that both in situ UV–vis and ex situ UV–vis spectroscopy play important roles in Li–S battery research, but they differ in their characterization focus. Specifically, ex situ UV–vis spectroscopy is most commonly used to evaluate the polysulfide adsorption capability of materials and their ability to suppress polysulfide diffusion, for example, by examining the adsorption of a single polysulfide species, such as Li₂S₆, on a catalyst. By contrast, in situ UV–vis spectroscopy enables real‐time spectral acquisition during battery charge and discharge, allowing continuous tracking of the conversion of soluble polysulfide species. It is therefore more suitable for elucidating sulfur cathode reaction mechanisms and polysulfide conversion kinetics. These two approaches are complementary, which ex situ UV–vis verifies the polysulfide‐trapping capability of materials; in situ UV–vis further reveals how they regulate the sulfur redox reaction process.

In a recent study, Liu et al. designed an in situ UV–vis absorption cell (Figure 7a) and observed significant differences in the overall trends of the in situ UV spectra for S@HCS and S@ZCO‐QDs@HCS materials [148]. Compared to the S@HCS electrode, the S@ZCO‐QDs@HCS electrode exhibited higher concentrations of S₆ ²⁻ and S₈ ²⁻, attributed to the redox reaction between ZCO quantum dots and LiPSs. This result demonstrates the efficient catalytic effect of the ZCO‐QDs catalyst on the dissociation reaction of Li₂S₆ (S₆ ²⁻ → 2S₃·⁻) (Figure 7b,c). Zeng et al. utilized in situ UV–vis spectroscopy to detect changes in polysulfide concentrations during discharge, confirming that S₃ ⁻ radicals play a key intermediate role in polysulfide conversion reactions [149]. Their findings revealed that DHCP effectively promotes the generation of S₃·⁻ radicals during discharge (Figure 7d,e). Similarly, Wang et al. identified a novel discharge mechanism in the SRVCR cathode using in situ UV–vis spectroscopy [150]. As shown in Figure 7f–i, two prominent peaks at ∼470 and 530 nm were observed after discharge, corresponding to the formation of short‐chain polysulfides (Li₂S₄) and long‐chain polysulfides (Li₂S₆). During the charging process, the opposite movement of these peaks indicated the reverse transformation of Li₂S₃ to Li₂S₆, suggesting that Li₂S₆ and Li₂S₃ are the primary discharge intermediates. This mechanism differs significantly from that observed in traditional Li–S batteries.

FIGURE 7.

(a) Schematic illustration of fabricating an in situ UV–vis Li–S test cell. Contour plots of the in situ UV–vis spectra (left) and the corresponding discharge profiles (middle) for (b) the S@HCS electrode and (c) the S@ZCO‐QDs@HCS electrode (right: color scale). Reprinted with permission [148]. Copyright 2021, Wiley‐VCH. In situ UV–vis spectroscopy of Li–S cell with (d) HCP/S and (e) DHCP/S cathodes and relevant galvanostatic charge‐discharge curves. Reprinted with permission [149]. Copyright 2022, Wiley‐VCH. In situ UV–vis spectra of (f) the RUB/S cathode and (h) the SRVCR cathode during discharge. First‐order derivative plots of the in situ UV–vis spectra for (g) the RUB/S cathode and (i) the SRVCR cathode during discharge. Reprinted with permission [150]. Copyright 2022, Elsevier.

In situ UV–vis spectroscopy provides a critical experimental foundation for elucidating the shuttle effect and reaction kinetics in Li–S batteries by dynamically monitoring changes in the light absorption characteristics of polysulfides in the liquid phase. Furthermore, by analyzing the dynamic changes in polysulfide concentrations within the electrolyte, the adsorption capacity of sulfur host materials or the inhibitory effects of electrolyte additives on the shuttle effect can be quantitatively evaluated. This approach offers essential experimental support for optimizing electrolyte composition, designing efficient sulfur host materials, and validating reaction mechanisms. However, in situ UV–vis spectroscopy is intrinsically more suited to analyzing concentration changes of polysulfides rather than directly probing crystal structures, chemical bonds, or valence states. Therefore, it is generally difficult for this technique to directly elucidate the specific interaction mechanisms between polysulfides and electrodes. In addition, its response to solid‐phase products formed at the end of discharge, such as Li₂S₂/Li₂S, is relatively weak, and these species usually do not produce well‐defined absorption peaks in the electrolyte as soluble polysulfides do [151]. Thus, in situ UV–vis spectroscopy is more suitable for analyzing the overall evolution trends and conversion kinetics of soluble polysulfides, whereas issues related to solid‐product formation, interfacial chemistry, and changes in catalytically active sites still require further validation using complementary in situ characterization techniques.

3.3. Electrode–Electrolyte Interphase Evolution

3.3.1. In Situ/Operando AFM

In 1983, the scanning tunneling microscope (STM) was invented, enabling the observation of material surface morphology, measurement of local density of states on sample surfaces, and even the precise positioning and manipulation of atoms. This breakthrough made it possible to study surfaces at the atomic scale [152]. However, STM is limited to conductive samples, prompting scientists to explore more versatile nanoscale characterization methods. In 1986, Binnig et al. developed the atomic force microscope (AFM). Unlike STM, AFM employs a micro‐cantilever with a fine tip to scan the sample surface, forming images by detecting the interaction forces between the tip and the sample. This innovation addressed the limitations of STM, allowing AFM to characterize the surfaces of most materials, regardless of their conductivity [153, 154].

For Li–S batteries, the main distinction between in situ AFM and ex situ AFM lies in the testing environment and the type of information obtained. In situ AFM is more focused on revealing reaction mechanisms and dynamic evolution processes, whereas ex situ AFM is primarily used to characterize morphological outcomes at specific states. Specifically, in ex situ AFM, the cell is disassembled after being charged or discharged to a selected state, and the electrode is removed, sealed, transferred under an inert atmosphere, and then examined. This approach is mainly used to compare changes in electrode surface roughness, deposit distribution, and interfacial morphology before and after cycling or at different electrochemical states. It is relatively straightforward to implement and is suitable for comparative morphological analysis. However, the sample may undergo changes during disassembly, especially for air‐sensitive species such as LiPSs and SEI/CEI layers. By contrast, in situ AFM is typically performed in an electrolyte or electrochemical AFM cell, allowing real‐time observation of morphological evolution at the electrode/electrolyte interface during battery operation. It is therefore better suited for studying dynamic processes such as LiPSs conversion, Li₂S/Li₂S₂ nucleation, interphase formation, and Li deposition/stripping at the anode.

In Li–S batteries, a stable and dense CEI layer is critical for interface protection. Monitoring the interface structure and dynamics of insoluble Li₂S₂/Li₂S at the cathode side in situ AFM and describing its dynamic evolution within operando batteries are essential for understanding the underlying mechanisms and optimizing battery performance. Lang et al. employed operando AFM to visualize, in real time, the dynamic nucleation, growth, deposition, and dissolution of insoluble Li₂S₂ and Li₂S at the cathode/electrolyte interface in a Li/LiPSs system on highly oriented pyrolytic graphite (HOPG), revealing at the nanoscale that irreversible accumulation of Li₂S₂ is a key contributor to capacity decay (Figure 8a,b) [32]. In another study, using the same material platform under LiFSI electrolyte and 60°C conditions, they found that a functional interphase formed in situ at the cathode interface, where polysulfides were trapped by a LiF network. This interface improved the reversibility of interfacial reactions, suppressed the shuttle effect, and benefited the high‐temperature performance of Li–S batteries. (Figure 8c) [155]. Mahankali et al. were the first to introduce AFM‐SECM into the Li–S system, performing nanoscale analysis of single/multi‐particle discharge deposits during the charging oxidation process, directly distinguishing the coexistence of conductive and insulating regions within the particles. As shown in Figure 8d, they proposed a capacity decay pathway: the accumulation of insulating products generated by the reaction of intermediate polysulfides with Li₂S at high potentials. First, at the charging potential, the conductive phase oxidizes and generates intermediate polysulfides, while the insulating phase becomes electrochemically irreversible and undergoes a chemical/electrochemical coupling reaction with the intermediate LiPS. When the potential rises to >2.6 V, the consumption of generated Li₂S₆ by Li₂S leads to a decrease in probe current, accompanied by a rapid recovery in particle height/volume and an expansion of the insulating region in the phase map, indicating the generation and deposition of inactive insulating products. Ultimately, the accumulation of these inactive insulating products on the Li₂S surface simultaneously hinders further oxidation and limits subsequent reduction, resulting in increased internal resistance and decreased utilization of active materials, thus forming a closed and verifiable capacity decay pathway [156].

FIGURE 8.

(a) Schematic illustration of nanoscale processes at the highly oriented pyrolytic graphite (HOPG) cathode/polysulfides‐electrolyte interface, directly observed using in situ AFM. (b) Schematic illustration of the nucleation, growth, and dissolution processes of Li₂S₂/Li₂S during discharge. Reprinted with permission [32]. Copyright 2016, Wiley‐VCH. (c) Schematic illustration of the formation process of the cathode‐side interphase during charge and discharge at room temperature and 60°C. Reprinted with permission [155]. Copyright 2017, Wiley‐VCH. (d) In situ AFM mapping of the Li–S cathode surface, the line profiles of particles identified as I and II show height changes at corresponding potentials. Reprinted with permission [156]. Copyright 2019, American Chemical Society.

From the morphological evolution of polysulfides‐related depositions to the mechanical instability of the electrode/electrolyte interface, in situ AFM, with its nanometer‐scale spatial resolution and in situ dynamic observation capabilities, provides an important interfacial evidence chain for the complex electrochemical behavior of Li–S batteries. In situ AFM can not only track the nucleation, growth, and fracture processes of interfacial films, such as CEI/deposited layers, but also monitor the dynamic evolution of the electrode/electrolyte interface in real time. Nevertheless, it still has several limitations in Li–S battery research. First, the AFM testing configuration is relatively complex, requiring stringent control over the sealing of the electrochemical cell, probe stability, and electrolyte compatibility. The measurements are also susceptible to disturbances from electrolyte flow, bubble formation, and probe‐sample interactions. In addition, the scanning area of AFM is limited, and local observations may not fully represent the macroscopic reaction behavior of the entire electrode. Therefore, AFM results should generally be interpreted in combination with complementary characterization techniques and electrochemical measurements [157].

3.3.2. In Situ/Operando XPS

X‐ray photoelectron spectroscopy (XPS) is a widely utilized surface analysis technique based on the photoelectric effect [158, 159]. This method enables qualitative and quantitative analysis of elements within the top 1–20 nm of a material's surface. The fundamental principle of photoelectron excitation in XPS is illustrated in Figure 9a,b. Typically, an Al Kα x‐ray source is employed to generate x‐rays of specific energy, which irradiate the material surface, inducing electron transitions from inner atomic energy levels. By measuring the kinetic energy (E_k) and binding energy (E_b) of the emitted photoelectrons, detailed information regarding the elemental composition and chemical states within the sample can be obtained [116, 160].

FIGURE 9.

(a, b) Schematic illustration of the photoelectron excitation principle. (c) Schematic representation of the custom‐designed XPS sample holder for battery cycling and in situ XPS characterization. XPS spectra of core‐level (d) S 2p and (e) F 1s at the Li‐electrolyte interface region during charge/discharge cycles. (f) Evolution of the atomic concentration of various sulfur‐based species during charge/discharge cycles, derived from the S 2p peak area. (g) Ratios of terminal sulfur atoms to bridging sulfur atoms (S_B ⁰/S_T ¹⁻) and disulfide atoms to sulfur atoms (S²⁻/S¹⁻), calculated from the S 2p peak area. Reprinted with permission [33]. Copyright 2017, American Chemical Society. (h) In situ high‐resolution XPS spectra of C 1s and F 1s of FG protection layer before, after 1st and 5th cycles. Reprinted with permission [161]. Copyright 2021, Elsevier.

In situ XPS is a surface‐sensitive technique that can be implemented in in situ/quasi‐in situ configurations. Additionally, due to the significant spectral separation between adjacent elements at the same energy level, this technique exhibits strong capabilities in qualitative elemental identification and defect chemistry studies [112]. In most Li–S battery studies, ex situ XPS is mainly used to analyze the surface elemental composition, valence‐state distribution, and chemical bonding environment of catalyst materials, thereby providing a basis for identifying their structural features and potential active sites. By contrast, in situ XPS focuses more on real‐time tracking of the dynamic evolution of interfacial species during battery operation, making it particularly useful for observing polysulfide conversion and the formation and evolution of SEI/CEI layers in Li–S batteries. For example, S 2p spectra can be used to monitor the transformation among sulfur species such as S₈, lithium polysulfides, and Li₂S/Li₂S₂, whereas Li 1s and F 1s spectra can be used to analyze the generation and evolution of interfacial components such as Li₂S, LiF, and lithium‐salt decomposition products.

Nandasiri et al. were the first to employ in situ XPS to obtain a comprehensive, molecular‐level understanding of parasitic reactions and the subsequent evolution of the SEI layer [33]. The specialized in situ XPS stage depicted in Figure 9c features a Teflon‐mounted configuration with Li anode and graphite cathode, utilizing gold wire interconnects for external potentiostatic control during surface analysis. To address the high vapor pressure issue of the DOL/DME electrolyte, they introduced [bmpyr]⁺[TFSI]⁻ as a cosolvent, which exhibits excellent chemical stability. Through in situ XPS analysis of the S 2p and F 1s spectra before and after charge and discharge cycles, it was observed that the sulfur dianion (S²⁻) concentration increased progressively during discharge, indicating the formation of insoluble and electronically insulating Li₂S through polysulfides reduction. Similarly, the F 1s spectra revealed a significant increase in Li–F and C–F species, while the CF₃ signal (associated with the original TFSI⁻) decreased, suggesting its decomposition near the Li anode (Figure 9d,e). Furthermore, as cycling progressed, sulfur species (S_B ⁰ and S_T ¹⁻) exhibited exponential growth, accompanied by a high S_B ⁰/S_T ¹⁻ ratio (>3), indicating continuous contamination of the Li anode during charge and discharge cycles (Figure 9f,g). In a recent study, Lu et al. designed a novel functional, selective, and rigid‐flexible coupled LiF‐intercalated graphene (LiF‐GN) SEI as an anode protective layer for Li–S batteries. To further elucidate the in situ reaction process and mechanism of the LiF‐GN SEI, XPS analysis of the C 1s spectra at the 0th, 1st, and 5th cycles demonstrated the gradual reduction of the Sp³ C‐F bond to form the Sp² C═C bond, leading to graphene formation [161]. Concurrently, in situ XPS of the F 1s spectra revealed the progressive formation of LiF during cycling (Figure 9h).

Compared with other in situ characterization techniques, the major advantage of in situ XPS lies in its ability to directly provide information on surface elemental composition, valence states, and chemical‐bonding changes. However, its limitations are also evident, especially in liquid‐electrolyte systems such as Li–S batteries. First, XPS generally imposes stringent environmental requirements; therefore, in situ XPS often requires specially designed cell configurations [162]. Second, the probing depth of XPS is typically limited to only a few nanometers. While this makes it highly suitable for surface analysis, it also means that the detected signals can be strongly affected by surface overlayers. During charge and discharge in Li–S batteries, relatively thick SEI/CEI layers and Li₂S deposits can form on the electrode surface, making it difficult for XPS to probe the underlying catalytically active sites or deeper sulfur species. Overall, in situ XPS is a powerful analytical technique capable of acquiring high‐resolution core‐level spectra of key elements while providing spatially resolved chemical imaging and insights into the formation of chemical species. This technique is particularly valuable for elucidating the chemical evolution of electrode surfaces and the mechanisms underlying interfacial reactions. For Li anodes, in situ XPS enables the analysis of the dynamic formation and decomposition processes of SEI components, such as LiF, Li₂O, and organic compounds. These capabilities significantly enhance our understanding of the roles of transient species in parasitic reactions at the molecular level and the subsequent evolution of the SEI/CEI layers during the cycling of Li–S batteries.

3.3.3. In Situ/Operando FTIR

Infrared spectroscopy involves the absorption of specific frequencies of infrared light by molecular bonds that vibrate at corresponding frequencies. The resulting transmittance curve, recorded as a function of wavenumber, constitutes the infrared spectrum. Infrared spectroscopy, classified as a molecular vibrational spectroscopy, is selective to vibrations that induce a change in the molecular dipole moment. The Michelson interferometer serves as the core component of Fourier transform infrared spectroscopy (FTIR). The optical configuration in Figure 10a,b employs a beam splitter to divide incoming radiation, directing one portion to a translating mirror and the other to a fixed reflector, creating the interferometric path difference. Upon recombination, these beams generate an interference pattern, which is subsequently subjected to a Fourier transformation to yield the infrared spectrum [163, 164]. In situ FTIR offers the capability to monitor molecular vibrational dynamics in real time, coupled with the advantage of rapid spectral acquisition. These attributes have established FTIR as a pivotal analytical tool for elucidating reaction mechanisms in Li–S batteries. Kalra et al. developed a specialized in situ cell for FTIR measurements, illustrated in Figure 10c,d [165]. The electrochemical cell integrates a sulfur‐impregnated carbon nanofiber cathode, liquid electrolyte, porous separator, and Li metal anode within a hermetically sealed CR2032 configuration, creating an optimized system for real‐time spectroscopic characterization.

FIGURE 10.

(a) Schematic illustration of the Michelson interferometer principle. (b) Schematic diagram of the Fourier Transform Infrared device. (c) Schematic diagram of in situ FTIR electrochemical experiments using a Li–S cell on an ATR crystal in an FTIR spectrometer. (d) Enlarged schematic of the ATR crystal/cathode interface, highlighting the penetration depth of the reflected IR beam into the working electrode, which is approximately 1–2 µm. Reprinted with permission [165]. Copyright 2017, American Chemical Society. (e, f) In situ ATR‐FTIR spectra of Li electrodes at different potentials during deposition and stripping in various electrolytes. Reprinted with permission [166]. Copyright 2023, Elsevier. (g) A schematic representation of an infrared spectrometer operating during the cyclic voltammetry process of LSB. The Fourier transform analysis highlights spectral variations associated with electrolyte decomposition and SEI formation. The top view of the spectral evolution over time focuses on the C‐O‐C vibrational band, with data points corresponding to the integrated intensity within the 987–1288 cm⁻¹ spectral region. This visualization provides insights into real‐time chemical transformations occurring at the electrolyte/electrode interface during cycling. Reprinted with permission [167]. Copyright 2023, Elsevier. (h) In situ FTIR spectra of the AC‐S‐CMA₁₆₀₀ electrode material during cyclic voltammetry (1.7 → 2.7 → 1.7 V) at a scan rate of 0.1 mV s⁻¹. Symmetric and asymmetric stretching modes of O─H and C═O are monitored for H₂O and Li₂CO₃, respectively. Reprinted with permission [168]. Copyright 2024, Elsevier.

In Li–S battery systems, FTIR can be used to characterize changes in sulfur species, electrolyte components, and functional groups on the electrode surface during electrochemical reactions. In situ FTIR and ex situ FTIR provide distinctly different types of information in Li–S batteries. Ex situ FTIR requires cell disassembly and sample sealing under an inert atmosphere, during which soluble polysulfides may be lost, redistributed, or oxidized. By contrast, in situ FTIR can directly capture changes in characteristic bands as a function of voltage or time under operating conditions, thereby revealing interfacial transformation processes in Li–S batteries more faithfully. Therefore, compared with ex situ FTIR, in situ FTIR provides stronger evidence for the regulatory role of materials in polysulfide conversion.

In situ ATR‐FTIR measurements have established themselves as a useful methodology for characterizing transient species generated during electrolyte breakdown in Li–S electrochemical systems. This vibrational spectroscopy technique uniquely combines electrolyte bond tracking with direct observation of SEI compositional evolution and anode‐electrolyte interfacial phenomena during electrochemical cycling, providing essential molecular‐level insights. In a recent study, Wu et al. employed in situ FTIR spectroscopy to elucidate the SEI formation dynamics and interfacial evolution mechanisms in 1 M LiTFSI‐DOL/DME electrolytes, with particular focus on the functional role of LiNO₃ additives in modifying SEI characteristics [166]. Figure 10e,f demonstrates distinct FTIR spectral variations between LiTFSI‐DOL/DME electrolytes with and without 2 wt% LiNO₃ additive during Li electrodeposition/dissolution cycles, revealing the additive's substantial influence on interfacial chemistry. In the presence of LiNO₃, the decomposition rate of TFSI⁻ is markedly slower, and a new peak emerges at 1210 cm⁻¹, attributed to the reaction of TFSI⁻ with DOL to form ROSO₂Li‐like components. These observations confirm that LiNO₃ additives promote the formation of protective interfacial species that significantly enhance electrolyte stability against reductive degradation during repeated deposition/dissolution processes. This results in improved coulombic efficiency and long‐term cycling stability of the battery.

Santos et al. investigated various vibrational bands associated with organic functional groups in the electrolyte solvent (DOL/DME) using in situ FTIR spectroscopy [167]. As shown in Figure 10h, the study identified the following characteristic vibrations: C–C group vibrations of DOL (800–1000 cm⁻¹), C–O–C group vibrations of both DOL and DME (1000–1250 cm⁻¹), C–H group vibrations of DOL and DME (2800–3050 cm⁻¹), and C–H vibrations associated with the HCO₂Li⁺ group (1300–1600 cm⁻¹). The researchers observed a gradual decrease in the absorbance intensity of the C–O–C band during charge and discharge cycling, suggesting partial electrolyte decomposition, primarily during the charging phase. This finding indicates that, in addition to electrode degradation, an additional chemical process contributes to battery failure, becoming increasingly significant over multiple charge and discharge cycles. In another study by Santos et al., in situ FTIR was employed to investigate electrolyte degradation and SEI formation when water was introduced as an additive in a 1.0 M LiTFSI/1.0% LiNO₃ electrolyte with a 1:1 mixture of DOL and DME [168]. As illustrated in Figure 10g, in situ FTIR provides real‐time insights into electrolyte degradation, chemical changes, SEI formation, and related phenomena. This technique is highly valuable for monitoring electrolyte evolution and understanding the interactions between additives and electrolytes.

In situ FTIR can accurately analyze the types of chemical species and transformation pathways at the electrode/electrolyte interface by monitoring the position and intensity changes of specific infrared vibration peaks in real‐time. Compared to transmission FTIR, ATR‐FTIR provides enhanced surface sensitivity and in situ compatibility, making it particularly suitable for interfacial studies. Through in situ analysis of characteristic functional groups (such as the C–O vibration peaks of ether solvents and the S–S/S–Li vibration peaks of polysulfides), this technique provides direct evidence for understanding the regulatory effects of electrolyte solvation structures on polysulfide solubility. It also supports the design of low‐solvation‐energy electrolytes, offering critical insights for optimizing Li–S battery performance. However, researchers should also note that in situ FTIR mainly provides spectrally averaged information, and its spatial resolution is relatively limited compared with other in situ characterization techniques. In addition, FTIR has limited detection sensitivity for certain sulfur species; for example, some S–S or Li–S vibrational bands may be weak and can be easily obscured by background signals from the electrolyte. Therefore, in situ FTIR is more suitable for monitoring electrolyte evolution and SEI/CEI transformation processes.

3.3.4. In Situ/Operando TEM

Transmission electron microscopy (TEM) utilizes a high‐energy electron beam (typically 80–300 kV) to penetrate ultrathin samples (<100 nm), forming images through interactions such as elastic and inelastic scattering between electrons and the sample. TEM enables direct visualization of materials at the atomic or near‐atomic resolution under favorable conditions, providing critical insights into crystallographic structure and chemical composition when coupled with EDS. The integration of advanced electron detection systems and aberration correctors has significantly enhanced the temporal and spatial resolution of TEM [169]. In recent years, TEM has emerged as a powerful and indispensable tool for atomic‐resolution characterization of nanomaterials and microscopic materials. Initially applied to battery research, TEM was primarily used for ex situ imaging of pristine electrode materials or electrodes cycled in liquid electrolytes. However, traditional TEM cannot capture real‐time changes within samples, such as electrochemical phenomena occurring in operando rechargeable batteries. By incorporating external stimuli (such as electrical, optical, mechanical, thermal, or cooling inputs) and utilizing specialized sample holders (e.g., electrically biased holders or heated chips), complex processes under device‐operating conditions can be studied [170, 171].

Compared with ex situ TEM, which mainly observes the intrinsic structure of the material itself, in situ TEM offers a unique perspective for elucidating the structure‐performance evolution mechanisms of electrode materials in electrochemical systems, owing to its exceptional spatial resolution and real‐time observation capabilities. This technology can deeply analyze the intrinsic mechanism of capacity decay, electrode degradation, and safety issues during electrochemical cycling by dynamically tracking the morphological evolution, volume change, composition distribution, and interface evolution under working conditions. Since the first in situ TEM study of LIB electrode materials was reported in 2010, the technique has made significant strides in advancing research on rechargeable battery systems, establishing itself as a key characterization method that bridges microscopic dynamic processes with macroscopic electrochemical performance [172].

In situ TEM enables real‐time observation of the reaction processes in Li–S batteries, providing critical insights into the detailed mechanisms and pathways of electrochemical reactions between Li and sulfur at the nanoscale. However, the application of in situ TEM to study sulfur cathodes in Li–S batteries remains limited due to the inherent sensitivity of sulfur to electron beams, which can induce radiation‐induced decomposition or structural damage. To address this challenge, Kim et al. proposed an electrochemical reaction model involving Li⁺ and sulfur nanoparticles confined within carbon nanotubes [31]. For the first time, they utilized a specialized holder in a transmission electron microscope to observe the real‐time lithiation of sulfur confined in cylindrical carbon pores. Given that the high‐vacuum environment of TEM is incompatible with the liquid electrolytes used in Li–S batteries, solid electrolytes or micro liquid cells are essential for such studies. For instance, Yang et al. designed a solid‐state Li–S nanobattery within a transmission electron microscope and successfully observed the electrochemical lithiation of the sulfur cathode using in situ TEM (Figure 11a) [173].

FIGURE 11.

(a) Schematic illustration of the electrochemical setup for in situ TEM observation of solid‐state Li–S nanobatteries. Reprinted with permission [173]. Copyright 2016, Wiley‐VCH. (b) Cryo‐TEM images depicting the structural characteristics of Li metal using polypropylene (PP) and phosphotungstic acid‐functionalized polypropylene (PTA‐PP) separators, along with corresponding energy‐dispersive x‐ray spectroscopy (EDS) results. Reprinted with permission [175]. Copyright 2024, Wiley‐VCH. (c) Schematic representation of the sample preparation process for Cryo‐TEM observation. (d) Cryo‐TEM characterization of the Li/PEO and Li/S‐CPE interfaces, including the morphology of Li deposited on copper grids, alongside corresponding elemental mapping and SAED patterns in PEO‐LiTFSI and S‐CPE systems. Reprinted with permission [176]. Copyright 2022, Wiley‐VCH.

A rapidly emerging trend in TEM is the use of Cryo‐TEM, which expands the range of sample types that can be analyzed by stabilizing atomic structures and mitigating electron beam‐induced heating effects. Cryo‐TEM enables direct observation of Li and its SEI layer at atomic resolution under carefully prepared cryogenic and low‐dose conditions [174]. Wang et al. achieved in situ polymerization of polylipoic acid (PTA) on a functionalized industrial polypropylene (PP) separator and utilized Cryo‐TEM to obtain atomic‐scale structural and chemical information, revealing the evolution of the sensitive Li deposition surface [175]. As illustrated in Figure 11b, Li deposition in the PP separator system exhibits a dendritic morphology, whereas it forms uniformly spherical structures under the PTA‐PP separator. Elemental mapping indicates that the copper electrode surface corresponding to the PTA‐PP separator is enriched with F, suggesting a significant increase in the LiF component. Cryo‐TEM analysis further demonstrates that the PTA coating induces the formation of a LiF‐rich SEI layer, which enhances Li⁺ diffusion and effectively suppresses the growth of Li dendrites. Su et al. designed an all‐solid‐state Li metal battery with a Li|S‐CPE|FCC configuration. To compare the performance differences between Li|S‐CPE|Li and Li|PEO‐LiTFSI|Li batteries, they analyzed the Li/PEO‐LiTFSI and Li/S‐CPE interfaces using Cryo‐TEM [176]. Figure 11c illustrates the sample preparation process for Cryo‐TEM observation. Briefly, a copper grid was placed between the SPE and the Li foil. After cycling, the copper grid was subjected to Cryo‐TEM imaging. As shown in Figure 11d, the interface of the sulfur‐composite polymer electrolyte (S‐CPE) exhibits a smooth morphology without the formation of excessive interfacial layers. This observation confirms that the S‐CPE effectively inhibits Li dendrite formation and mitigates excessive reactions between reduced Li dendrites and the ether groups in polyethylene oxide (PEO).

Although in situ TEM enables real‐time observation of dynamic processes, such as electrode interfacial evolution in Li–S batteries at the nanoscale, and provides important evidence for elucidating their reaction mechanisms, this technique still faces several practical limitations. First, electron‐beam irradiation may induce structural rearrangement of sulfur‐containing species, electrolytes, or interfacial components. This issue is particularly significant for beam‐sensitive systems such as polysulfides, Li₂S, and organic electrolytes, where beam effects may substantially affect the reliability of experimental results [177, 178]. By employing rapid freezing techniques, Cryo‐TEM enables near‐atomic‐resolution static structural analysis of sensitive materials, addressing the limitations of conventional characterization methods where intermediates and electrode/electrolyte interfaces are prone to decomposition or crystallization during sample preparation or testing.

3.4. Complementary In Situ/Operando Characterization Techniques

In addition to the aforementioned in situ characterization techniques, in situ electron paramagnetic resonance (EPR) and in situ small‐angle neutron scattering (SANS) also provide important complementary insights into the reaction mechanisms of Li–S batteries. Unlike conventional in situ characterization methods, which are mainly used to track polysulfide species, catalyst structures, and electrode/electrolyte interfacial evolution, in situ EPR can sensitively capture the formation and transformation of transient polysulfide radical intermediates during electrochemical reactions [179, 180]. In contrast, in situ SANS enables nanoscale analysis of sulfur‐species deposition behavior, pore‐structure changes, and their evolution pathways. The combination of these two techniques helps reveal the complex reaction processes in Li–S batteries from the complementary perspectives of radical chemistry and nanopore structure [181, 182].

Electron paramagnetic resonance (EPR) is a spectroscopic technique highly sensitive to species containing unpaired electrons, enabling the selective detection of radicals, paramagnetic centers, and transition‐metal active sites generated during electrochemical reactions. In Li–S battery systems, in situ EPR can directly capture signals from sulfur radicals such as S₃·⁻ and S₂·⁻, while tracking their dynamic evolution as a function of charge‐discharge potential in real time. As shown in Figure 12a, Li et al. systematically investigated the formation and transformation of sulfur radicals on an Fe₃O₄ substrate using in situ EPR [183]. The results showed that, within the discharge region of 2.3–2.0 V, distinct zero‐field splitting signals appeared in the EPR spectra, confirming the formation of triplet radical pairs, including [S₄·⁻‐S₄·⁻], [S₃·⁻‐S₃·⁻], and [S₂·⁻‐S₂·⁻]. By further analyzing the g factor, axial zero‐field splitting parameter D, and rhombic parameter E, the researchers distinguished radical pairs with different chain lengths and quantitatively evaluated their spin interactions. Compared with the substrate‐free system, Fe₃O₄ significantly enhanced the EPR signal intensity, indicating that it helps stabilize triplet sulfur radical pairs and thereby promotes sulfur‐species conversion kinetics. In addition, combined with synchrotron‐based near‐edge x‐ray absorption fine structure (NEXAFS) analysis, this study further confirmed the chemical states of radical intermediates, providing direct evidence for elucidating the role of magnetic substrates in regulating sulfur redox processes.

FIGURE 12.

(a) Identification of voltage‐dependent sulfur radical pairs in Li–S batteries by EPR and sulfur K‐edge NEXAFS, including [S₄·⁻‐S₄·⁻], [S₃·⁻‐S₃·⁻], and [S₂·⁻‐S₂·⁻] species and their electronic/valence characteristics. Reprinted with permission [183]. Copyright 2025, American Chemical Society. (b) Operando SANS combined with PGRF modeling reveals the nanoscale phase evolution, morphology, and size changes of Li₂S/Li₂S₂ deposits during Li–S battery discharge/charge. Reprinted with permission [187]. Copyright 2022, Springer Nature. (c) Operando SANS reveals the location‐ and size‐dependent precipitation behavior of Li₂S/S₈ within microporous carbon electrodes in Li–S batteries. (d) Operando neutron scattering coupled with electrochemical impedance analysis tracks the cycling‐dependent evolution of sulfur species from Li₂S₈ solution, correlating voltage profiles, SANS intensity changes, solution resistance, Warburg behavior, and DRT‐derived charge‐transfer processes of S₈ and Li₂S. Reprinted with permission [188]. Copyright 2019, American Chemical Society.

It should be noted that EPR only responds to paramagnetic species and is therefore difficult to use for directly detecting most diamagnetic species, such as S₈, Li₂S_n, and Li₂S. Accordingly, in situ EPR is more suitable for identifying and tracking radical intermediates, rather than serving as an independent quantitative tool for sulfur‐species analysis [184]. Combining it with complementary techniques, such as in situ Raman and UV–vis spectroscopy, can help establish correlations among radical‐intermediate formation, soluble polysulfide evolution, and final lithium sulfide deposition, thereby providing a more comprehensive understanding of the multistep sulfur‐conversion mechanism in Li–S batteries.

Small‐angle neutron scattering (SANS) is a structural characterization technique based on differences in scattering length density at the nanoscale. It is well‐suited for resolving phase transitions, pore‐structure evolution, particle deposition, and liquid‐phase infiltration behavior over a length scale of approximately 1–100 nm. In Li–S battery systems, in situ/operando SANS is mainly used to track the nanoscale nucleation, growth, deposition, and dissolution of solid discharge products, such as Li₂S₂ and Li₂S. As discharge proceeds, changes in the SANS scattering intensity can reflect the formation of solid sulfide phases and their morphological evolution. For porous carbon/sulfur composite cathodes, in situ SANS can further determine whether sulfur species or Li₂S enter the internal pore channels or predominantly deposit on the external surface of the carbon framework, thereby providing important guidance for pore‐structure regulation, sulfur‐loading optimization, and improved active‐material utilization [185, 186].

Prehal et al. combined SANS with a plurigaussian random‐field model to quantitatively reveal the structural evolution of Li₂S₂/Li₂S nanocomposite discharge products in Li–S batteries [187]. As shown in Figure 12b, compared with small‐angle x‐ray scattering (SAXS), SANS allows the scattering length density contrast to be tuned by using deuterated solvents, thereby providing higher material discrimination and enabling effective differentiation between amorphous Li₂S₂ and nanocrystalline Li₂S. Real‐time SANS results showed that Li₂S₂ nanoparticles with a size of approximately 2.8 nm first precipitated during discharge, followed by their further solid‐state conversion into Li₂S crystallites of approximately 10 nm. This study provides quantitative nanoscale evidence for the formation of the Li₂S₂ intermediate phase and its conversion into Li₂S, offering important support for understanding the conversion pathway of solid sulfides in Li–S batteries. Risse et al. further employed operando SANS to monitor Li–S batteries in real time over ten consecutive cycles, and combined this analysis with simultaneous electrochemical impedance spectroscopy to systematically reveal the precipitation/dissolution kinetics of S₈ and Li₂S. By using deuterated electrolytes, such as deuterated THF/DME, the researchers effectively matched the scattering length density of the electrolyte with that of the carbon substrate, reducing the background scattering from the microporous structure by approximately 95% and thereby markedly enhancing the resolvability of the precipitated phases [188]. The experimental results showed that neither S₈ nor Li₂S precipitated inside the micropores of the carbon electrode during cycling; instead, they were primarily deposited on the external surface of the carbon fibers. By further fitting the SANS data with a spherical‐particle model, the authors quantitatively obtained the average size and size‐distribution characteristics of Li₂S particles. Notably, the dissolution of Li₂S exhibited a pronounced size‐dependent behavior: smaller particles dissolved preferentially, whereas larger particles dissolved progressively at later stages (Figure 12c,d). This heterogeneous dissolution process can explain the appearance of local maxima in the scattering intensity during charge‐discharge cycling.

Overall, operando SANS offers unique advantages for Li–S battery research. It can not only distinguish whether solid sulfides precipitate inside pore channels or on the external surface of the carbon framework, but also quantitatively track the evolution of the size, morphology, and spatial distribution of nanoscale Li₂S₂/Li₂S discharge products. When combined with techniques such as electrochemical impedance spectroscopy, SAXS, Raman spectroscopy, or XAFS spectroscopy, SANS can complement information on the chemical conversion of sulfur species from the perspective of nanostructural reconstruction, thereby providing direct structural evidence for elucidating key issues such as limited sulfur utilization, deposition‐induced pore blockage, and capacity decay.

4. Correlative In Situ/Operando Characterization for Li–S Batteries

In situ characterization techniques hold revolutionary significance in Li–S batteries. By enabling real‐time monitoring of material evolution, interfacial behavior, and reaction pathways under operating conditions, these techniques provide critical experimental support for elucidating the complex reaction mechanisms and failure origins of Li–S batteries. However, the sulfur redox process involves multiple coupled reactions, including solid‐liquid‐solid conversion, polysulfide migration, Li₂S deposition, and interfacial transformation, spanning diverse chemical, structural, spatial, and temporal scales. Therefore, a single technique can usually capture only one dimension of the reaction process and may suffer from limited sensitivity, restricted field of view, or insufficient representativeness. Accordingly, multimodal in situ/operando characterization is necessary to establish a comprehensive and reliable mechanistic picture. This is particularly important for probing the charge‐discharge reaction mechanisms of Li–S batteries and also provides a more robust basis for rational materials design and device optimization under practical conditions. In the following section, we classify the different applications of various in situ characterization techniques in Li–S batteries according to their respective advantages, as illustrated in Figure 13.

FIGURE 13.

Schematic illustration of the application of in situ/operando characterization techniques in Li–S batteries.

First, at the cathode/catalyst interface, in situ XRD, XAS, and SANS can synergistically reveal sulfur‐species conversion, catalytic‐site evolution, and discharge‐product deposition behavior across different length scales. Specifically, in situ XRD can track, in real time, the transformation of crystalline sulfur into lithium sulfide during discharge and its reversible decomposition during charge by monitoring the attenuation of α‐S₈ diffraction peaks and the emergence/disappearance of characteristic Li₂S peaks. By contrast, in situ XAS offers the advantage of elemental selectivity. Therefore, in situ XAS can not only track the evolution of the chemical states of sulfur species during cycling, but also reveal the dynamic interactions, coordination reconstruction, and electron‐transfer processes between polysulfides and catalytically active centers. Meanwhile, in situ SANS can serve as an important complementary technique for resolving the nanoscale nucleation, growth, and dissolution of solid discharge products such as Li₂S/Li₂S₂, as well as their mass‐transport behavior associated with pore structures. In particular, for porous carbon/catalyst composite cathodes, SANS can distinguish whether discharge products form inside the pore channels or predominantly deposit on the external surfaces of the carbon framework or catalyst, thereby providing structural evidence for understanding pore blockage, limited active‐material utilization, and interfacial reaction heterogeneity. Therefore, in the study of the cathode/catalyst interface, in situ XRD focuses more on the phase transition process of crystalline sulfur species, in situ XAS is more suitable for analyzing the electronic/coordination structure evolution of catalytic centers, and in situ SANS can further supplement nanoscale deposition and pore mass transfer information. The combination of the three can construct a systematic understanding of sulfur conversion kinetics and catalytic mechanisms from three levels: crystal phase, local coordination, and nano morphology.

Second, for short‐lived reaction intermediates that undergo rapid conversion between solution and solid phases and are difficult to capture, in situ NMR, in situ Raman spectroscopy, in situ UV–vis spectroscopy, and in situ EPR can be integrated as a complementary characterization suite to enable multidimensional analysis of soluble polysulfides, radical intermediates, and their conversion pathways. In situ NMR can distinguish the concentration distribution of polysulfides with different chain lengths in the electrolyte in real time by monitoring changes in the chemical shifts, peak shapes, and peak intensities of nuclei such as ¹H and ⁷Li. In addition, NMR can further provide information on the coordination environment between polysulfides and solvents, lithium salts, or functional additives, offering a basis for understanding how solvation structures regulate sulfur‐conversion behavior.

In situ Raman spectroscopy, by leveraging the molecular vibrational fingerprints of polysulfide anions and its time‐resolved acquisition capability, can identify key sulfur intermediates during discharge and charge. This technique is particularly suitable for evaluating the ability of interphase layers, separator modification layers, or electrolyte additives to suppress polysulfide migration and the shuttle effect. Compared with Raman spectroscopy, in situ UV–vis spectroscopy focuses more on the quantitative analysis of liquid‐phase polysulfides. Polysulfides with different chain lengths exhibit characteristic absorption bands in the ultraviolet‐visible region, and the synchronous or opposite evolution of their peak positions and intensities can reflect changes in polysulfide chain‐length distribution, further indicating differences in liquid‐phase reaction pathways during charge and discharge.

On this basis, in situ EPR can further capture paramagnetic sulfur radical intermediates, such as S₃·⁻ and S₂·⁻. By analyzing changes in radical signal intensity as a function of potential or time, in situ EPR can reveal the generation, stabilization, and consumption of radical species, and correlate these processes with polysulfide disproportionation, chain scission, and the formation of solid Li₂S/Li₂S₂. It should be emphasized that NMR, Raman, and UV–vis spectroscopy mainly focus on soluble sulfur species or species with vibrational/optical activity, whereas EPR responds only to paramagnetic radicals. Therefore, the combined use of these in situ techniques can establish correlations among liquid‐phase concentration distribution, molecular structural fingerprints, spectral quantification, and radical chemistry, enabling a more comprehensive understanding of the rapid, multistep, and highly coupled sulfur‐conversion processes in Li–S batteries.

Although sulfur K‐edge in situ/operando XAFS can monitor the valence‐state changes and conversion pathways of cathodic sulfur species during battery operation, including the evolution of S₈, soluble polysulfides, and Li₂S₂/Li₂S, recent Li–S battery studies have increasingly applied in situ/operando XAS to probe the catalyst side. In particular, XAS is well suited for revealing the local coordination environment, electronic‐structure modulation, and dynamic reconstruction of transition‐metal catalytic active sites during charge‐discharge processes [180, 189].

Third, at the electrode‐electrolyte interface, the formation, rupture, and reconstruction of SEI/CEI layers are often coupled with the nucleation, growth, and stripping of lithium dendrites, constituting a typical nonequilibrium interfacial evolution process. Therefore, relying on a single morphological or chemical characterization technique is insufficient to fully elucidate the underlying failure mechanisms. Instead, it is necessary to integrate multiple in situ techniques capable of providing complementary information on interfacial chemical states and structural morphology.

At the level of morphological evolution, in situ AFM can perform real‐time scanning of lithium‐metal surfaces in liquid‐electrolyte environments, thereby directly visualizing dynamic processes such as surface roughening, particle growth, crack formation, and SEI/CEI film coverage and rupture during electrodeposition/stripping. With relatively high spatial resolution, this technique can reveal the correlation between lithium deposition morphology and interphase stability, providing direct evidence for understanding dendrite initiation and local current‐density distribution. Furthermore, in situ TEM enables observation of lithium dendrite growth behavior, SEI nanostructures, and mechanical deformation of interfacial layers at higher spatial resolution, helping to elucidate the structural instability mechanisms of lithium‐metal anodes during cycling at the nanoscale.

At the level of interfacial chemistry, in situ XPS, owing to its surface sensitivity and elemental/chemical‐state resolution, can resolve in real time the formation, transformation, and decomposition of key SEI/CEI components during cycling. For example, the dynamic evolution of LiF, organic carbonates, inorganic sulfur/oxygen species, and lithium‐salt decomposition products can be used to track electrolyte reduction, side‐reaction accumulation, and SEI reconstruction, thereby elucidating the mechanism of interfacial chemical evolution at the molecular level. Meanwhile, in situ FTIR can monitor the reaction pathways of electrolyte solvents, anions, and organic SEI components during charge and discharge through real‐time changes in vibrational‐band position, intensity, and lineshape, further revealing the continuous accumulation of electrolyte decomposition and interphase formation.

Overall, in situ AFM and TEM mainly provide information on lithium‐deposition morphology, dendrite growth, and SEI/CEI nanostructural evolution, whereas in situ XPS and FTIR focus on resolving SEI/CEI components, electrolyte decomposition products, and interfacial side‐reaction pathways. The synergistic application of these techniques can effectively establish the intrinsic correlation between electrode‐electrolyte interfacial evolution and battery cycling stability.

5. Summary and Prospects

In summary, this review provides a comprehensive review of the application of advanced characterization techniques in investigating the mechanisms underlying Li–S batteries, highlighting key advancements in the field. Particular attention is given to the fundamental principles, distinct advantages, and specific application contexts of in situ characterization methods. These techniques play a critical role in advancing the understanding of the electrochemical processes within Li–S batteries, particularly with regard to the multi‐step transformation of sulfur electrode materials, the polysulfide shuttle effect, and the dynamics of electrolyte interfaces. In situ characterization techniques have greatly advanced the understanding of Li–S battery failure mechanisms. They enable real‐time monitoring of material evolution, interfacial chemistry, and reaction kinetics. Despite these advancements, several critical challenges remain. These challenges primarily concern the resolution, sensitivity, and overall applicability of current methods. Consequently, there is an urgent need for technological innovations and methodological advancements to address these limitations. The article concludes with a discussion on the existing constraints and outlines potential future directions for the development of in situ characterization techniques, as shown in Figure 14.

FIGURE 14.

Current challenges and future prospects of in situ characterization technology.

The reaction kinetics of Li–S batteries are characterized by the rapid dissolution and diffusion of polysulfides, the continuous phase transitions of sulfur species (S₈→Li₂S_x→Li₂S), and the dynamic reconstruction of the electrode‐electrolyte interface. These processes typically span broad timescales from microseconds to hours and exhibit spatial heterogeneity across a range of nanometers to micrometers. However, existing characterization techniques face significant challenges in meeting the simultaneous demands for high temporal resolution and high spatial resolution. For instance, in situ XRD typically requires integration times ranging from several seconds to minutes, which may fail to capture transient intermediates such as short‐chain polysulfides. In situ TEM can achieve atomic‐scale resolution; however, when conducted in a liquid electrolyte environment, the electron beam often induces sample degradation, leading to a reduction in resolution. Similarly, in situ XPS typically sacrifices spatial resolution to maintain a vacuum environment, thereby failing to provide an accurate representation of the chemical state at the solid‐liquid interface [169, 190, 191].

Second, the operating conditions of Li–S batteries impose stringent requirements on the design of in situ characterization devices. Current in situ testing configurations are often misaligned with real‐world application scenarios. Most laboratory‐scale in situ tests are conducted using button cells and low‐load electrodes, which do not accurately represent the performance of high‐load or soft‐pack batteries used in commercial applications. For instance, specific techniques, such as in situ UV–vis, are highly susceptible to interference from the absorption peaks of the electrolyte solvent, which complicates the accurate quantification of polysulfide concentration. Similarly, NMR struggles with signal shielding from the Li metal anode when using liquid electrolytes. Additionally, the probe contact pressure during in situ AFM testing can induce local stress distortion on the electrode surface, which may lead to discrepancies between the observed and actual behavior of the SEI formation mechanism [157, 192, 193].

Third, Li–S batteries represent a highly complex system, wherein the spectral signals of intermediate products often exhibit significant overlap. For instance, in situ Raman spectroscopy struggles to distinguish individual polysulfides. To solve this problem, Yan et al. have demonstrated that the combination of surface‐enhanced Raman scattering (SERS) and chemometric techniques can improve the separation of these overlapping characteristic peaks. However, the introduction of the probe interface may disrupt the actual reaction environment, potentially affecting the accuracy of the measurements [194].

It is important to note that, compared to Li‐ion batteries, the application of in situ characterization techniques for Li–S batteries remains underdeveloped. The solid‐liquid‐solid multi‐phase reaction in Li–S batteries involves the complex coupling of multiple physical fields, including electron and ion transport, interfacial electrochemistry, and mechanical stress, making it challenging for a single technique to fully analyze these processes [195]. To fully understand the electrochemical transfer pathways within the LSB system, researchers must integrate multiple in situ characterization techniques that complement each other, enabling a more comprehensive exploration of the electrochemical transformation processes [196, 197]. For instance, by integrating advanced techniques such as XAS, Raman spectroscopy, and AFM, it becomes possible to simultaneously monitor changes in elemental valence states, molecular vibrations, and phase mechanics, providing a holistic view of the system's behavior. Furthermore, the advancement of light source and detector technologies plays a critical role in the future development of in situ characterization techniques. In order to transition from laboratory‐scale models to real‐world applications, it is crucial to develop in situ characterization methods tailored to practical batteries. This includes the development of in situ test cells compatible with high‐load sulfur electrodes and solid electrolytes, the integration of pressure and temperature control modules to better reflect actual application conditions, and the creation of in situ pressure‐electrochemical joint measurement technologies for soft‐pack batteries to investigate the effects of electrode expansion on battery performance during cycling.

The evolution of in situ characterization technology necessitates a deep integration of materials science, chemistry, physics, and computational science. In the future, by overcoming the limitations of spatiotemporal resolution, enabling comprehensive analysis, and developing characterization platforms that closely mimic real‐world systems, in situ techniques are expected to unravel the “black box” reactions within Li–S batteries. This will facilitate a deeper understanding of their behavior and accelerate the commercialization of high‐performance battery systems.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

Research data are not shared.