Progress in in-situ electrochemical nuclear magnetic resonance for battery research

Abstract

A thorough understanding of the fundamental electrochemical and chemical processes in batteries is crucial to advancing energy density and power density. However, the characterizations of such processes are complex. In-situ electrochemical nuclear magnetic resonance (EC-NMR) offers the capability to collect real-time data during battery operation, furnishing insights into the local structures and ionic dynamics of materials by monitoring changes in the chemical environment around the nuclei. EC-NMR also has the advantages of being both quantitative and non-destructive. This paper systematically reviews the design of EC-NMR approach, and delves into the applications and progress of EC-NMR concerning battery reaction mechanisms, failure mechanisms, and overall battery systems. The review culminates in a comprehensive summary of the perspective and challenges associated with EC-NMR.

Graphical abstract

1. Why do we use in-situ NMR?

The advancement of high-energy-density rechargeable batteries (e.g., lithium/sodium-ion batteries) relies fundamentally on a comprehensive understanding of intrinsic electrochemical reactions, ionic transfer mechanisms, elementary chemical and electrochemical processes, material structures, and their intercorrelations. Insights into the theories provide significant guidance for innovating electrochemical energy storage systems and enhancing their performance, where in-situ characterizations have played pivotal roles. Due to the possible generation of short-lived intermediates, structural relaxation, and dynamic bonding evolution, particularly at the interface, during the operation of batteries, it becomes highly challenging to achieve an accurate comprehension through post-mortem detection, which underscores the necessity to develop and utilize in-situ/operando characterization techniques. Notably, synchrotron radiation X-ray diffraction allows for well-resolved observations of crystalline phase changes, while X-ray absorption spectroscopy and resonant inelastic X-ray scattering can detect local structures around heavy atoms (e.g., Mn, Fe) and even some light atoms (e.g., O, F). Additionally, in-situ surface-enhanced Raman scattering and infrared spectroscopy have been routinely employed to discern the local bonding evolution at the electrochemical interface.

This review emphasizes recent advancements in in-situ electrochemical nuclear magnetic resonance (EC-NMR). NMR proves to be a robust tool for elucidating variations in the chemical environment surrounding nuclei, providing valuable insights into both crystalline and amorphous phases, along with nonstoichiometric species, and local structures. Its distinctive advantages lie in its quantitative capability of tracing phase changes and structural transformations during electrochemical reactions and its non-destructive nature, allowing in-situ detection while avoiding information loss due to material instability. The real-time data acquisition in in-situ NMR captures relatively more comprehensive and quantitative analysis results, enabling a nuanced understanding of battery operation and failure mechanisms. Moreover, NMR serves as a unique technique for characterizing ionic dynamics, which is a significant concern related to charge transfer and polarization in high-performance batteries. Additionally, magnetic resonance imaging (MRI) leverages NMR's sensitivity to visualize spatial distribution, yielding crucial details on concentration gradients, diffusion rates, dendrite growth, and facilitating non-destructive, in-situ studies on a spatial scale.

This paper undertakes a comprehensive exploration, commencing with the evolving landscape of EC-NMR probes and EC-NMR cells. Delving into the challenges and developmental history, it illuminates the trajectory followed by researchers in refining the tools that facilitate probing the core of electrochemical processes. Afterward, the applications of in-situ NMR technique are introduced from the perspective of the fundamental mechanisms that determine the performance of battery materials and systems. The narrative expands to examine the diverse applications and progress made by EC-NMR in deciphering the complex reaction and ionic transfer mechanisms of battery materials, unraveling the failure mechanisms of materials, and contributing to the holistic understanding of battery systems. The review ultimately distills the prospects and challenges lying ahead of EC-NMR. By encapsulating the advances and setbacks, this paper provides a roadmap for enthusiasts and scholars at the intricate intersection of electrochemistry and NMR to glimpse the promising future and obstacles that await exploration.

2. The development of EC-NMR for battery research

There are several obstacles to implementing and applying EC-NMR to battery research: noise from potentiostat would affect the simultaneous achievement of high signal-to-noise ratio (SNR) NMR signals and electrochemical tests of batteries, incompatibility between magic angle spinning (MAS) and conductive components and applied current may block the acquirement of high-resolution spectra, and magnetic field inhomogeneity arising from diverse battery components would cause line-broadening. To surmount these hurdles, researchers are actively engaged in the rational design of EC-NMR cells and probes, aiming to optimize SNR, sensitivity, and resolution in experimental tests. Efforts are focused on seeking innovative solutions to unlock the full potential of EC-NMR for comprehensive insights into electrochemistry and solid-state ionics in batteries.

EC-NMR for battery was first realized by integrating the current collector of the cell with the probe's coil by Gerald II et al., in 2000 (Fig. 1a) [1]. They used coin cells as in-situ cells, but the metal shell was replaced with Teflon. Utilizing a copper disk on the Teflon shell as the current collector and NMR detector circuit conductor, they positioned the carbon sample near the copper disk for optimal NMR signal sensitivity. However, practical NMR acquisition omitted the separator and lithium counter electrode to minimize signals from lithium-ion in the electrolyte. The method also suffered from low sensitivity and could not achieve simultaneous detection of NMR signals and electrochemical tests, rendering the information ‘quasi-in-situ’. Subsequently, Letellier et al. advanced effective EC-NMR testing enabled by commercial static probes equipped with Bellcore-type plastic bag cells [[2], [3], [4], [5]]. The EC-NMR cell's structure involved pressing together the carbon anode, separator, and lithium counter electrode, infiltrating the separator with electrolyte, and sealing the assembly in a plastic bag with a copper collector (Fig. 1b). However, challenges surfaced compared to traditional coin cells, including insufficient pressure for the optimal component contact, inadequate airtightness, and a propensity for electrolyte leakage. While this EC-NMR provided in-situ information, it fell short of fulfilling all EC-NMR test requirements and possible RF noise from potentiostat. A novel cylindrical cell design was then introduced (Fig. 1c), featuring a polypropylene casing that requires only unscrewing a cap for assembly. The cathode, separator, and anode are sequentially placed into the battery, sealed with the cap, and connected to the copper mesh current collector through punched holes. This design significantly reduces the sealed area, minimizing the risk of electrolyte leakage and simplifying assembly by reducing metal components. Consequently, cylindrical cells exhibited improved cycling performance with no electrolyte leakage during 60 h of cycling tests and 158 h of EC-NMR tests [6]. Subsequent enhancements by Kayser et al. [7] and the integration of 3D printing technology resulted in well-sealed cylindrical cells capable of extended operation, tested up to 2400 h. More recently, pouch cells have been explored as in-situ cells directly (Fig. 1d) [8], laying the foundation for the application of in-situ NMR technique to real commercial cells.

Fig. 1.

Progress in the development of EC-NMR cells and probes for battery research. (a) Coin cell. Reproduced with permission [1]. Copyright 2000, Elsevier. (b) Bag cell. Reproduced with permission [2]. Copyright 2003, American Institute of Physics. (c) Cylindrical cell [6]. Reproduced with permission [6]. Copyright 2011, Elsevier. (d) Pouch cell. Reproduced with permission [8]. Copyright 2018, Springer Nature. (e) STRAFI probe. Reproduced with permission [9]. Copyright 2012, Elsevier. (f) ATMC probe. Reproduced with permission [10]. Copyright 2016, Elsevier. (g) Parallel-plate probe. Reproduced with permission [12]. Copyright 2020, Institute of Physics. (h) Inside-out probe. Reproduced with permission [8]. Copyright 2018, Springer Nature.

Further development contributes to the design of specific probes that integrate the electromagnetic shielding scheme with the potentiostat to enhance the SNR and operability. Fu et al. pioneered the development of a stray-field-imaging (STRAFI) NMR probe for battery research with integrated design in 2012 (Fig. 1e) [9]. Afterward, Pecher et al. developed a specifically designed automatic tuning matching cycler (ATMC) EC-NMR probe (Fig. 1f) [10], which allows for ‘on-the-fly’ recalibration of resonant circuits, and facilitates a straightforward and highly-shielded connection of electrochemical cells inside the RF coil. Recent progress contributes to the design of NMR and MRI for analogous commercial cells and large cells. Parallel-plate RF probes (Fig. 1g) were developed [11,12], which provided enhanced sensitivity and an ideal orientation of the electromagnetic field for EC-NMR detection of large cells. Furthermore, with the availability of MRI probes, Andrew et al. successfully observed the state of charge and defects of a pouch cell placed in a cylindrical holder with water as the detection medium by using an “inside-out” MRI probe (Fig. 1h) [8].

In solid-state NMR, interactions cause spectral line broadening, and MAS is a common technique to average these interactions. However, in the case of batteries, rotating at the magic angle is not feasible due to interference caused by the cell current collector and conductive electrodes, which generate eddy currents by cutting the static magnetic field of the spectrometer. As a result, most current EC-NMR tests operate at static conditions, posing significant challenges in analyzing spectra, particularly for paramagnetic cathode materials. To address the challenge, in-situ MAS NMR was proposed and realized by using a jelly roll cell placed in a 4 mm rotor [13]. Most metallic components were eliminated and the active material was cast onto a cellulosic substrate in the jelly cell. Despite challenges such as solvent loss and the need for electrolyte filling during rotation, which may impact sample sealing and result in nonuniform material distribution, in-situ MAS NMR using the approach showed significantly narrower linewidths compared to that under static conditions (Fig. 2). This attempt opens an avenue for further exploration, such as using solid-state electrolytes (SSEs) to mitigate electrolyte volatilization and incorporating a photoresistor for current rate control, could enhance the applicability of in-situ MAS NMR in studying battery systems.

Fig. 2.

(a) The photo of a commercial 4 mm rotor (left) and component diagram of in-situ MAS cell (right). (b) The deconvolution of ⁷Li NMR spectra of the LiCoO₂/graphite cell (fully charged to 4.1 V) at MAS of 10 kHz (left) and static condition (right), respectively. Reproduced with permission [13]. Copyright 2019, American Chemical Society.

3. Reaction and ionic transfer mechanisms in electrodes and electrolytes by EC-NMR

In battery studies, EC-NMR has proven invaluable for delving into the underlying mechanisms of electrodes, SSEs, and interfaces, shedding light on the correlation between their structures and performance. The technique enables the extraction of critical information regarding the chemical composition, local structure, and ionic dynamics of battery materials and interfaces.

Anodes. When it comes to anode materials, the application of EC-NMR focuses on revealing the phase transition and structural evolution processes occurring during the charge/discharge cycles. This application proves especially adept at detecting nonstoichiometric and amorphous that emerge throughout the electrochemical process of anodes. The NMR shift due to the Knight shift induced by conductive anodes, coupled with the elusive nature of possibly generated short-lived intermediates and structural relaxation, make the ex-situ tests inadequate to capture these compounds. EC-NMR studies of anode including silicon, lithium, etc. yield significant insights into the reaction mechanisms.

Si-based anodes that can deliver a high theoretical capacity of up to 4200 mAh g⁻¹ is a typical instance. Various techniques including XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), and TEM (transmission electron microscope) have been extensively applied to understand the reaction of Si-based anodes. The formation of amorphous Li–Si alloys and transformation between crystalline and amorphous phases hamper the understanding, whereas the application of EC-NMR achieved very important results. Key et al. conducted groundbreaking studies on Si anode structural changes upon discharge by applying in-situ ⁷Li NMR [14], unveiling the formation of a series of Li–Si alloys (Li₁₂Si₇, Li₇Si₃, Li₁₃Si₄, Li₁₅Si₄, Li_15+δSi₄) during Li insertion. Moreover, excess lithium-induced self-discharge reactions were monitored using in-situ NMR, disclosing the gradual disappearance of the nonstoichiometric Li_15+δSi₄ phase with chemical shift at −10 ppm (Fig. 3a). The kinetics of lithium-silicide phase transitions in Si nanowire electrodes was also revealed by Ogata et al. using EC-NMR [15]. They found that the slow kinetics of the transition from amorphous to crystalline phase in Li-rich Li_xSi phases prevented the observation of another phase transformation associated with the lithiation of the crystalline phase c-Li_3.75Si to form c-Li_{3.75+(0.2–0.3)}Si. Freytag et al. explored the lithiation and de-lithiation of a-SiO and Si by EC-NMR, highlighting differences in reaction behavior [16], with Si exhibiting the Li₁₅Si₄ phase near the end of the lithiation, leading to capacity loss. In contrast, this crystalline phase was not observed in SiO anode, and as a result, demonstrated a more symmetric lithiation/de-lithiation behavior, as evidenced by Kitada et al.’s work [17].

Fig. 3.

(a) The in-situ⁷Li NMR spectra obtained during the first discharge process. Reproduced with permission [14]. Copyright 2009, American Chemical Society. (b) MRI time series showing the distribution of ⁷Li electrolyte concentration (top) and the ⁷Li chemical shift image of metals (bottom) during battery charging process. Reproduced with permission [19]. Copyright 2015, American Chemical Society. (c) The full set of the in-situ¹⁷O NMR spectra of the same Li₂MnO₃/Li half-cell battery as a function of specific capacity. Reproduced with permission [28]. Copyright 2022, John Wiley and Sons. (d) Schematic illustration of stress caused by Li–In alloying reaction and local discrepant diffusion coefficients (left) and the varied ion transport pathways and possible phase transition in Li₁₀GeP₂S₁₂ solid electrolyte at low and high current densities (right). Reproduced with permission [29]. Copyright 2023, John Wiley and Sons.

Beyond Si-based anodes, lithium metal as the ideal anode, confronts safety and low columbic efficiency challenges arising from the formation of lithium dendrites and mossy metal. These microstructures pose significant safety hazards and have hindered the commercialization of lithium metal batteries. Thus, understanding the lithium microstructure is crucial, and EC-NMR blazes a unique trail for this exploration. By leveraging the bulk susceptibility (BMS) effect that yields distinct chemical shifts in different morphologies, EC-NMR enables the quantitative study of lithium microstructures during cycling. Bhattacharyya et al. conducted pioneering work in observing and quantifying lithium metal microstructures [18], attributing changes in signal strengths to the formation or stripping of mossy/dendritic lithium. By calculations based on the skin depth of the metal under RF excitation, they provided a method to quantify the amount of mossy/dendritic lithium. Although their work didn't distinguish between moss and dendrites. Chang et al. later advanced the field by employing MRI techniques to decouple different lithium microstructures using chemical shift imaging (CSI) [19]. They decerned distinct chemical shifts for lithium dendrites (270 ppm), mossy lithium (260 ppm), and lithium metal (250 ppm) (Fig. 3b), offering insights into the transition process of lithium metal from bulk-moss-dendrite. Subsequent research investigated the effects of various factors such as temperature, stack pressure, salt type, salt type, solvent and salt concentration on the lithium deposition process, aiming to enhance the safety performance and cycling stability of lithium batteries [[20], [21], [22]].

Cathodes. Compared to anode materials, the exploration of paramagnetic cathode materials through EC-NMR faces considerable challenges, primarily stemming from three challenges in this domain: (1) Hyperfine interactions induced by paramagnetism in paramagnetic cathode materials result in severe broadening of linewidth of even MHz, complicating the interpretation of spectra. (2) The chemical shifts of paramagnetic materials are influenced by factors such as orientation, shape, and packing density due to BMS effect, leading to more complex spectral analysis. (3) BMS effects arising from paramagnetic and metallic components exert a potent influence, causing magnetic field inhomogeneity thus line broadening and possible additional shifts [[23], [24], [25]]. Despite that, nuclei in some cathode materials are influenced by a relatively weak paramagnetic effect, displayed in-situ NMR spectra with promising resolution. For instance, the evolution of the intensities in different Na⁺ sites in V-based Na₃V₂(PO₄)₂F₃ cathode during cycling can be well-elucidated by using in-situ ²³Na NMR [26], revealing the structural evolution. For cathode with strong paramagnetic effect, efforts have been made to investigate spinel Li_1.08Mn_1.92O₄ cathode by Grey's group. The necessary calibration of spin-spin relaxation time to conduct quantitative analyses was clearly illustrated for such material using in-situ NMR technique, due to the possible varied paramagnetic effect derived from the valence change of Mn-ions [27]. The quadrupolar Carr-Purcell-Meiboom-Gill pulse sequence has also been applied to acquire in-situ ¹⁷O NMR of Li₂MnO₃ to monitor the O local structural changes in real time during the electrochemical cycling (Fig. 3c) [28]. Furthermore, recent development of automatic tuning matching cycler in-situ NMR technique along with the application of automated frequency sweep NMR experiments makes it possible to detect ultra broad signals of paramagnetic cathodes, e.g. ³¹P NMR of LiFePO₄ [10].

Electrolytes. EC-NMR techniques also can offer valuable insights into ionic transport properties at both micro- and macro-scale. EC-NMR coupled with ⁶Li isotope tracer technique enabled the investigation of the lithium-ion transport pathways and phase transitions of an LGPS (Li₁₀GeP₂S₁₂) solid-state electrolyte. By discharging samples to a real capacity at diverse current densities, the researchers examined chemical compositions and decomposition processes within the samples, identifying the key transport phases and potential phase transitions at diverse current densities (Fig. 3d). This study suggested a hypothesis involving differences in the diffusion coefficient of localized lithium-ion, emphasizing the impact of space charge layer on lithium-ion transport at interfaces [29]. Additionally, Gao et al. employed EC-NMR to explore the evolution of solid electrolyte interphase (SEI) during the charge/discharge process of sodium metal batteries [30]. The results indicated that the prior reduction of the difluoro(oxalato)borate contributes to SEI formation, effectively preventing the decomposition of the carbonate solvents and offering robust protection for sodium metal anode.

Beyond the above application, a scanning image-selected in-situ spectroscopy approach has been utilized to study the reaction mechanisms of cathode and anode, simultaneously [31]. Taking LiCoO₂/Li₄Ti₅O₁₂ cells as examples, they reveal the solid solution reaction mechanism of LiCoO₂ and the two-phase reaction mechanism of Li₄Ti₅O₁₂ in one experiment. Recently, Afonso et al. developed a new operando NMR device for low-temperature experiments [32]. By investigating LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622)/graphite full-cells, they indicate that operando NMR could detect the onset of metallic lithium deposition on graphite at low temperatures and fast charging.

4. Evolution and failure mechanism of battery materials and systems during cycling by EC-NMR

Understanding the failure mechanism is the key to enhancing the battery life. Recent progress in the realization of in-situ cells for long-term cycling makes it possible to utilize EC-NMR to track the battery composition and structure changes during cycling, which offers invaluable insights into the root causes of the failure of battery materials and systems.

Evolution of metal battery systems using model cells. The work by Kayser et al. revealing the formation and evolution of mossy/dendritic lithium over an extended period (Fig. 4a), showcases the power of NMR measurements in characterizing Li metal microstructures [7]. This capability provides critical insights into the failure mechanism of batteries, aiding in the development of strategies to mitigate dendrite growth and prolong battery life. While dendrite growth may be uniform during long-term testing, shorter NMR tests can still offer valuable information for inferring battery health and lifetime. Xiang et al. and Grey et al. independently conducted a quantitative study on the failure process of lithium metal batteries [33,34], identifying a two-stage process for capacity loss dominated initially by SEI and later by dead lithium (Fig. 4b). Their quantitative approach, validated against titration gas chromatography and mass spectrometry titration, highlights the feasibility and reliability of NMR in quantifying inactive lithium. In a related context, Liang et al. studied sulfide-based all-solid-state lithium batteries (SSLMBs), comprehensively examining the formation and sources of inactive lithium [35]. Their finding regarding the impact of solid-state electrolyte (SSE) chemistry on lithium activity, identification of ‘dead lithium’ modes, and the faster corrosion rate of mossy/dendritic lithium contribute valuable knowledge to the understanding of failure mechanisms in SSLMBs. These studies collectively demonstrate the versatility of EC-NMR in unraveling the intricacies of battery failure, facilitating advancements in battery design and longevity.

Fig. 4.

(a) The pseudocolor plot and integral evolution of Li microstructure signals in Li metal/graphite cell. Reproduced with permission [7]. Copyright 2018, Royal Society of Chemistry. (b) Operando⁷Li NMR results of Cu||LiFePO₄ cell during cycling [33]. Copyright American Association for the Advancement of Science. (c) In-situ²³Na MRI images of Na||Cu cells. Reproduced with permission [36]. Copyright 2020, Springer Nature.

For sodium metal batteries, the study by Xiang et al. on the failure mechanism of sodium metal batteries using MRI provides crucial insights into the formation and evolution of sodium metal microstructures (SMSs) during the deposition/stripping process (Fig. 4c) [36]. The observed linear increase in overpotential associated with growing SMSs and the subsequent transition to violent electrolyte decomposition, mossy SMS formation, and rapid battery failure underscores the importance of controlling overvoltage below the transition voltage. This study suggests that adjustments to solvents and electrolytes, aimed at either reducing overvoltage or increasing the transition voltage, are essential strategies for achieving long-term stable cycling of sodium metal batteries. Such detailed examinations of failure mechanisms contribute significantly to the development of effective strategies for enhancing the performance and safety of sodium-based energy storage systems.

Pouch cells. The expanding application of EC-NMR from laboratory half-cells to pouch cells systems marks a significant advancement in EC-NMR research for batteries. This broader scope allows researchers to inquire into the electrochemical processes, reaction mechanisms, and failure mechanisms in more realistic and practical settings, providing valuable information about the performance and behavior of batteries under conditions closer to real-world applications. This transition to studying larger and more complex cell configurations reflects the growing importance of understanding the intricacies of battery systems as a whole, contributing to the development of safer, more efficient, and longer-lasting energy storage solutions.

The innovative use of inside-out MRI for detecting the state of charge and defects in commercial pouch cells by Andrew et al., in 2018 represents a notable breakthrough [8]. This method, relying on induced or permanent magnetic field changes generated by the battery itself, overcomes the challenges posed by the conductive shell of pouch batteries and eliminates the need for an RF-penetrating magnetic field. This inside-out MRI method combined with single-point ramped imaging with T₁ enhancement (SPRITE) enhances sensitivity and safety, allowing for precise measurements of current-induced magnetic field close to the cell surface. This approach provides a unique tool for assessing the manufacturing quality and health status of commercial pouch cells. However, it's important to recognize the restrictions pertaining to MRI probes and magnet bore sizes, which limits its applicability to particular battery systems.

5. Perspective

The evolution of in-situ electrochemical nuclear magnetic resonance (EC-NMR) techniques has significantly advanced the understanding of underlying mechanisms in battery systems and materials. By overcoming challenges in the design of EC-NMR probes and cells, researchers have achieved real-time monitoring of battery reaction mechanisms, failure processes, and overall system dynamics. The development of sophisticated EC-NMR probes, including STRAFI probes, Automatic Tuning Matching Cycler (ATMC) probes, and parallel-plate RF probes, has enabled enhanced sensitivity and improved accuracy in studying various battery materials. The study of anode materials, particularly Si-based anodes, has benefited from EC-NMR, shedding light on phase transitions, and the impact of lithiation on different materials. Additionally, EC-NMR has played a pivotal role in examining the formation of lithium microstructures, such as dendrites and moss, providing essential insights into the safety hazards associated with lithium batteries. Furthermore, the application of EC-NMR in studying cathode materials, solid-state electrolytes, and SEI has expanded its utility, offering a comprehensive view of battery performance. Long-term studies using EC-NMR have revealed the formation and evolution of microstructures, aiding in understanding failure mechanisms and strategies for extending battery life. The extension of EC-NMR techniques to pouch cells, as demonstrated by inside-out MRI, showcases the versatility of this method in practical battery configurations. Overall, EC-NMR has become an invaluable tool for unraveling the intricacies of battery systems and guiding advancements in energy storage research.

The last two decades have witnessed significant progress in the development of in-situ cells and probes, yet their limitation to static states introduces challenges related to spectral broadening and reduced resolution due to persistent interactions. Despite the advances, in-situ MAS NMR, designed to achieve high-resolution spectra, faces hurdles in maneuverability that need further attention for future refinement. The evolution of in-situ MAS NMR, coupled with the exploration of novel methodologies like dynamic nuclear polarization (DNP), and the advancement of high spatial-resolution MRI techniques, presents avenues for achieving higher resolution and more comprehensive insights in both temporal and spatial dimensions. The ongoing development of these techniques holds the promise of overcoming existing challenges and unlocking new possibilities for precise characterization in electrochemical studies.

Indeed, EC-NMR emerges as a powerful tool, unraveling the intricate interplay within battery systems over time and space. Its ability to delve into crucial aspects like material phase transitions, evolution, ion transport dynamics, and kinetics offers unparalleled insights, enriching our comprehension of battery functionality and failure mechanisms. Despite lingering challenges, the optimism lies in the potential solutions: innovative pulse designs, hardware enhancements, and the crafting of representative model systems. This trajectory of refinement positions EC-NMR as a robust characterization instrument poised to unlock the mysteries of advanced electrochemical energy materials and devices, paving the way for transformative advancements in the realm of energy storage.

CRediT authorship contribution statement

Yong Jiang: Writing – original draft. Mengmeng Zhao: Writing – original draft. Zhangquan Peng: Writing – review & editing, Supervision, Conceptualization. Guiming Zhong: Writing – review & editing, Supervision, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Biographies

Yong Jiang is currently pursuing his Ph.D. at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences. He received his B.E. degree from Beijing University of Chemical Technology in 2021. His research interests include using characterization methods such as NMR to study interface electrochemistry.

Zhangquan Peng obtained his Bachelor degree in Wuhan University in 1997, and received his MSc and PhD degrees from Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences (CAS) in 2000 and 2003, respectively. He is currently a group leader at Dalian Institute of Chemical Physics, Chinese Academy of Sciences. His current research focus on the spectroelectrochemistry and interfacial electrochemistry battery and electrocatalytic systems.

Guiming Zhong obtained a Bachelor degree in Wuhan University in 2006, and received his PhD degree from Xiamen University in 2013. Currently, he is an associate scientist fellow at Dalian Institute of Chemical Physics, Chinese Academy of Sciences. His current research focus on the interfacial electrochemistry and solid-state ionic in batteries by NMR spectroscopy.