Probing Interfacial Electrolyte Chemistry and SEI Failure Mechanisms on Lithium-Metal Anodes via Establishing a Multichannel Analysis Paradigm

Abstract

The formation and failure of the solid electrolyte interphase (SEI) govern the reversibility, stability, and safety of Li metal anodes. However, the dynamic construction process of the SEI and the associated multiphase product evolution originating from interfacial electrolyte decomposition remain insufficiently understood. Herein, we establish a multichannel interfacial product analysis paradigm that enables the tracking of distinct products during electrolyte decomposition and SEI evolution across gaseous, volatile, solid, dead Li, and thermal-response pathways. Using traditional and fluorinated carbonate electrolytes as model systems, we demonstrate that the traditional electrolyte undergoes continuous carbonate fragmentation, forming an SEI enriched in organic and carbonate species with limited passivation capability, thereby inducing interfacial reconstruction, dead Li accumulation, and pronounced gas evolution under thermal stress. In contrast, fluorinated electrolytes follow a more convergent decomposition pathway to form a LiF-rich composite SEI, which reduces dead Li formation, stabilizes Li plating/stripping, and markedly suppresses the release of interfacial gases during thermal decomposition. More broadly, this integrated multitechnique characterization system provides a powerful analytical tool to precisely resolve SEI formation and failure processes, thereby offering scientific insights and practical engineering strategies for developing safer and highly reversible Li-metal batteries.

Introduction

Lithium-metal anodes are widely regarded as one of the most promising anode candidates for the next generation of energy-storage systems with high energy density, owing to their ultrahigh theoretical specific capacity of 3860 mAh g^–1 and extremely low standard electrode potential of −3.04 V versus the standard hydrogen electrode (SHE). , However, their practical application is severely hindered by the intrinsically unstable Li metal/electrolyte interface, which is manifested by nonuniform Li deposition, continuous morphological reconstruction, and unavoidable interfacial parasitic reactions. , During repeated Li plating/stripping, persistent interfacial side reactions between Li metal and the electrolyte continuously consume active Li and electrolyte components, leading to decreased Coulombic efficiency and increased interfacial impedance. , Meanwhile, the interfacial morphological reconstruction and local volume changes associated with Li plating/stripping induce repeated rupture and regeneration of the solid electrolyte interphase (SEI), further aggravating interfacial instability. , The SEI is mainly derived from the reductive decomposition of electrolyte components on the low-potential Li metal surface, and its composition, structure, and spatial distribution critically govern Li⁺ transport and electronic blocking behavior at the Li metal interface. − Although SEI formation inevitably consumes active Li inventory and electrolyte components, a stable and functional SEI is essential for regulating Li⁺ transport, suppressing electron leakage across the interphase, mitigating continuous interfacial side reactions, and enabling reversible Li plating/stripping. Therefore, elucidating the formation, evolution, and failure mechanisms of the SEI is crucial for developing highly reversible and long-life Li metal batteries.

Constructing a stable SEI requires regulation of the reductive decomposition pathways of the electrolyte at the Li metal interface and the resulting distribution of decomposition products. , However, the dynamic decomposition behavior of electrolytes on Li-metal surfaces, the evolution of intermediate products, and the coupled evolution among different product channels remain insufficiently understood. Conventional ex situ techniques, such as X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), , and scanning electron microscopy (SEM), provide valuable information about the local composition, depth distribution, and morphology of the formed SEI. Nevertheless, these techniques mainly capture postreaction states and are limited in revealing the dynamic evolution and reaction pathways involved in SEI formation. , Therefore, integrated characterization approaches capable of tracking SEI formation in situ or quasi in situ and quantitatively or semiquantitatively analyzing key interfacial products, including gaseous species, volatile organic products, solid SEI components, and inactive Li, are essential for overcoming current mechanistic limitations and enabling rational SEI design.

In this study, as illustrated in Scheme , we propose and validate a multichannel interfacial product analysis framework to elucidate electrolyte reductive decomposition pathways on Li metal surfaces and the resulting SEI evolution. The methodological significance of this work does not lie simply in the parallel use of multiple characterization techniques, but in establishing a product-resolved correlation workflow that connects different interfacial product reservoirs and failure events. Specifically, online electrochemical mass spectrometry (OEMS) was employed to monitor gaseous products generated during Li plating/stripping, while online electrochemical gas chromatography–mass spectrometry (OE-GCMS) was used to identify volatile organic decomposition products and resolve organic fragmentation pathways. In situ surface-enhanced infrared absorption spectroscopy (in situ SEIRAS), titration mass spectrometry (TMS), and ToF-SIMS were further used to correlate these gaseous/volatile decomposition pathways with retained solid SEI chemistry and spatial chemical distribution. Ethanol-titration TMS, SEM, and EIS-DRT were then used to link SEI chemistry with inactive metallic Li accumulation, deposition morphology, and SEI-related interfacial kinetics. Finally, online pyrolysis mass spectrometry (OP-MS) was introduced to evaluate the thermal-response behavior of cycled interfacial residues.

1. Multichannel Interfacial Product Analysis Framework for Resolving Electrolyte Decomposition and SEI Evolution on Lithium-Metal Anodes.

^a Interfacial electrolyte species near the Li metal surface undergo electron-transfer-induced reductive decomposition during Li plating/stripping. This interfacial decomposition process generates multiple classes of products, including gaseous species such as H₂, C₂H₄, and CO₂; volatile organic products such as alcohols, ethers, and esters; solid or quasi-solid SEI components such as LiF, Li₂CO₃, and ROCO₂Li; and electrochemically inactive metallic Li. By integrating OEMS, OE-GCMS, in situ SEIRAS, TMS, and OP-MS, this analytical framework enables product-resolved tracking of electrolyte decomposition, SEI construction, inactive Li formation, and interfacial thermal stability. This multichannel analysis paradigm provides mechanistic insights into the relationship between interfacial reductive chemistry and SEI failure, offering guidance for the rational design of electrolytes and SEIs in lithium metal batteries.

With conventional (EC/EMC) and fluorinated carbonate electrolytes (FEC/FEMC) as model systems, this integrated approach enables a systematic comparison of interfacial product generation, SEI compositional evolution, inactive Li formation, Li plating/stripping reversibility, and interfacial thermal stability. By correlating electrolyte decomposition pathways with SEI construction and failure behavior, this work provides a product-resolved mechanistic framework and an effective characterization strategy for guiding the development of safer, highly reversible, and stable Li-metal batteries.

Results and Discussion

Online Identification of Interfacial Decomposition Products and Pathways

To elucidate the initial reductive behavior of different electrolytes at the Li-metal interface, Li/Cu half-cells were employed as a model system, with Li serving as both the Li source and counter electrode and Cu as the substrate for Li deposition. During the test, the cell was first galvanostatically discharged at a current density of 1 mA cm^–2 for 1 h to drive Li⁺ reduction and deposition on Cu, concomitantly inducing initial SEI formation. The deposited Li was then stripped by galvanostatic charging at 1 mA cm^–2 to 1.0 V vs Li/Li⁺. This protocol provides a model platform to probe interfacial electrolyte decomposition as well as the evolution of gaseous and volatile byproducts during Li plating/stripping and initial SEI formation.

As shown in Figure a, OEMS reveals pronounced gas evolution in both electrolytes during the initial Li plating and stripping. In the EC/EMC electrolyte, H₂, C₂H₄, and CO₂ signals emerge as the potential decreases. C₂H₄ is generally considered a characteristic product of EC ring-opening reduction, indicating EC decomposition at the low-potential Li-metal interface. CO₂ formation may be associated with decarboxylation or further decomposition of carbonate solvents and their reduced intermediates. H₂ evolution is typically attributed to the reduction of trace water, protic impurities, or hydrogen-containing interfacial species on highly reactive Li surfaces. , In contrast, the FEC/FEMC electrolyte shows markedly suppressed C₂H₄ evolution but more pronounced H₂ and CO₂ signals, suggesting a reductive decomposition pathway distinct from that of the EC/EMC. In particular, the higher H₂ evolution in the FEC/FEMC electrolyte may be related to the reduction of HF, generated from fluorinated solvents or lithium-salt decomposition, on freshly exposed Li surfaces.

1.

Online identification of gaseous and volatile organic decomposition products during Li plating/stripping in EC/EMC and FEC/FEMC electrolytes. (a) Voltage profiles and corresponding OEMS traces of H₂, C₂H₄, and CO₂ during the first Li plating/stripping cycle in Li/Cu half-cells. The EC/EMC electrolyte shows pronounced C₂H₄ evolution associated with EC reduction, whereas the FEC/FEMC electrolyte exhibits suppressed C₂H₄ evolution and stronger H₂/CO₂ signals. (b) Integrated amounts of H₂, C₂H₄, and CO₂ over repeated cycles, revealing distinct gas-evolution trends for the two electrolyte systems. (c) Electrochemical online GC-MS total ion chromatograms (TICs) of volatile organic decomposition products generated during Li deposition. The EC/EMC electrolyte produces a broader distribution of volatile products, while FEC/FEMC shows a more convergent product profile. (d) Standard mass spectra and molecular structures of the assigned volatile products corresponding to the numbered peaks in (c).

The gas evolution in each cycle was further quantified by integrating the corresponding OEMS signals. As shown in Figure b, H₂ evolution gradually decreases in both electrolytes with cycling, indicating that the initially formed SEI partially mitigates direct contact between fresh Li and water, protic impurities, or hydrogen-containing byproducts. For the EC/EMC system, the amount of C₂H₄ increases progressively with cycling, suggesting that EC ring-opening reduction is not effectively suppressed by the initial SEI and persists on newly exposed Li surfaces during repeated plating/stripping. In contrast, CO₂ evolution in the FEC/FEMC system gradually decreases, indicating that fluorinated carbonate-related decarboxylation/reductive decomposition is progressively suppressed after the initial cycles. This trend suggests that the interfacial layer derived from fluorinated solvent decomposition provides a degree of interfacial passivation.

To identify volatile organic decomposition products beyond gaseous species, an OE-GCMS was employed to separate and analyze volatile components generated during Li deposition. As shown in Figure c, in addition to the small-molecule gases detected by OEMS, OE-GCMS identified various volatile organic components, including alcohols, ethers, esters, and carbonyl-containing compounds. Figure d presents the database-matched mass spectra and proposed molecular structures of the assigned volatile products recorded with the MS detector. These products indicate that carbonate electrolytes undergo multistep reductive decomposition on the Li metal interface. Notably, the EC/EMC system produces a broader range of volatile organic products, suggesting more complex organic fragmentation reactions at the Li metal interface. By comparison, the FEC/FEMC system yields markedly fewer volatile organic products with a more convergent product distribution, indicating that organic fragmentation side reactions are partially suppressed in the fluorinated electrolyte. In addition, the silicon- and fluorine-containing signals observed by OE-GCMS may originate from reactions between HF and the chromatographic column or other silicon-based components in the system. This observation further supports the involvement of HF-related side reactions in the FEC/FEMC system and is consistent with the higher H₂ evolution observed by OEMS.

Based on these online analysis results, Figure summarizes the possible decomposition pathways of the two electrolytes at the Li metal interface. In the EC/EMC system, EC can undergo ring-opening reduction to form reduced products such as lithium ethylene dicarbonate (LEDC), accompanied by C₂H₄ evolution. EMC may further undergo hydrolysis, transesterification, C–O bond cleavage, and alkoxy fragment recombination in the presence of trace amounts of water or under a reductive interfacial environment, producing volatile organic products such as methanol, ethanol, ethyl formate, and dimethyl carbonate (DMC). In the case of FEC/FEMC, FEC reduction may involve C–F bond cleavage, LiF formation, CO₂ release, and the generation of carbonyl-containing volatile fragments. The distinct product-evolution trends indicate that the SEI formed in the conventional EC/EMC system is insufficient to fully passivate the Li metal interface, whereas FEC/FEMC effectively enhances SEI passivation and suppresses continuous electrolyte decomposition through a more convergent decomposition pathway and a more inorganic-rich interfacial product distribution. It should be noted that, because EC/EMC and FEC/FEMC differ in both the cyclic and linear carbonate components, the observed interfacial differences are interpreted at the electrolyte-formulation level rather than being assigned to the individual contribution of FEC or FEMC alone.

2.

Proposed interfacial decomposition pathways of EC/EMC and FEC/FEMC electrolytes on Li metal. EC undergoes reductive ring opening to form lithium ethylene dicarbonate-type products, accompanied by C₂H₄ evolution. EMC participates in hydrolysis, transesterification, C–O bond cleavage, and alkoxy-fragment recombination, generating volatile organic products such as methanol, ethanol, ethyl formate, and DMC. In contrast, FEC reduction involves C–F bond cleavage, LiF formation, CO₂ release, and the formation of VC-derived or carbonyl-containing intermediates.

Chemical Composition and Spatial Distribution of Solid SEIs

After establishing the differences in the evolution of gaseous and volatile organic decomposition products between the two electrolyte systems, it is essential to elucidate how these initial decomposition pathways translate into differences in the solid SEI composition. Given that the solid SEI deposited on the Li metal surface directly determines interfacial passivation and the subsequent stability of Li plating/stripping, acid-sensitive carbonate/organic lithium carbonate species, interfacial functional group evolution, and the depth distribution of solid SEI products in the postcycling SEI were further analyzed.

As shown in Figure a, an H₂SO₄-titration-based TMS method was first employed to quantitatively analyze acid-sensitive inorganic/organic lithium carbonate species within the SEI. Components such as Li₂CO₃ and ROCO₂Li can react with H₂SO₄ and release CO₂, as represented by the following reactions:

$${\mathrm{Li}}_2{\mathrm{CO}}_3+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{Li}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2\uparrow$$

$$2{\mathrm{ROCO}}_2\mathrm{Li}+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{Li}}_2{\mathrm{SO}}_4+2\mathrm{ROH}+2{\mathrm{CO}}_2\uparrow$$

3.

Quantitative and in situ SEIRAS analyses of carbonate/organic lithium carbonate species in SEIs derived from EC/EMC and FEC/FEMC electrolytes. (a) Schematic illustration of H₂SO₄-titration mass spectrometry (TMS) for quantifying acid-sensitive carbonate/organic lithium carbonate species in the SEI. Li₂CO₃, ROCO₂Li-type species, and other acid-reactive carbonate-containing components can react with H₂SO₄ to release CO₂, enabling stoichiometric evaluation of carbonate inventory within the SEI. (b) CO₂ evolution amounts measured by H₂SO₄-titration MS from cycled electrodes in EC/EMC and FEC/FEMC electrolytes over repeated cycles. The EC/EMC-derived SEI shows continuous accumulation of acid-sensitive carbonate/organic lithium carbonate species, whereas the FEC/FEMC-derived SEI exhibits much lower CO₂ evolution. (c, d) In situ SEIRAS spectra collected during SEI formation in EC/EMC and FEC/FEMC electrolytes, respectively. EC/EMC promotes the continuous formation of ROCO₂Li- and Li₂CO₃-related SEI species, whereas FEC/FEMC suppresses alkyl carbonate accumulation and exhibits an additional feature near 1860 cm^–1, possibly associated with FEC-derived polymeric or carbonyl-containing interfacial species, potentially including VC-derived polymeric components.

Thus, the amount of evolved CO₂ is used here to evaluate the total acid-sensitive carbonate inventory retained in the SEI. As shown in Figure b, CO₂ evolution in the EC/EMC system increases steadily with cycling, indicating the continuous accumulation of acid-sensitive carbonate-containing SEI components during repeated Li deposition and stripping. By contrast, the FEC/FEMC system exhibits an overall lower level of CO₂ evolution, which stabilizes after an initial increase, suggesting that this fluorinated electrolyte suppresses the continuous accumulation of acid-sensitive inorganic/organic lithium carbonates.

To monitor the dynamic SEI formation process, in situ SEIRAS was employed to track interfacial functional-group evolution under polarization. As shown in Figure c, in the EC/EMC system, multiple characteristic peaks associated with inorganic and organic lithium carbonate SEI species gradually intensify as the potential decreases from 1.6 to 0.1 V vs Li/Li⁺. The peak near 1680 cm^–1 is attributable to the CO stretching vibration of alkyl carbonate or organic lithium carbonate species (ROCO₂Li), while the peaks in the 1490–1450 cm^–1 region are mainly associated with CO₃ ^2– vibrations in Li₂CO₃. The intensification of these features indicates the sustained formation of ROCO₂Li and Li₂CO₃ at the low-potential interface, consistent with the higher CO₂ release observed by TMS. In contrast, the FEC/FEMC system shows a markedly different SEIRAS response (Figure d), with only weak growth of the ROCO₂Li-associated CO peak, indicating suppressed formation of alkyl carbonate SEI species. Notably, a new feature near 1860 cm^–1 emerges as the potential decreases. Considering the complexity of FEC reduction, this band is tentatively assigned to FEC-derived polymeric or carbonyl-containing interfacial species, potentially including VC-derived polymeric components. ,

To further resolve the spatial composition and depth distribution of the postcycling SEI, ToF-SIMS depth profiling was conducted on the cycled electrodes (Figure ). Representative fragments, including C₂HO^–, LiCO₃ ^–, and LiF₂ ^–, were selected to probe organic oxygen-containing SEI species, carbonate SEI species, and LiF-related fluorinated inorganic components, respectively. , Figure a and b present the raw depth profiles and normalized intensity variations over sputtering time for both electrolyte systems. It should be noted that the ToF-SIMS depth profiles are used here for relative comparison under identical sputtering conditions. Because organic/carbonate-rich and LiF-rich inorganic SEIs may exhibit different sputtering yields and matrix-dependent secondary-ion responses, sputtering time should not be directly converted into absolute physical depth or SEI thickness without independent crater-depth calibration. In the EC/EMC system, C₂HO^– exhibits a high initial intensity in the shallow region and gradually decays with sputtering time, indicating a substantial presence of organic oxygen-containing SEI species derived from solvent reduction. In contrast, LiF₂ ^– signals in the EC/EMC system are relatively weak in both intensity and depth distribution, suggesting that fluorinated inorganic components are not the dominant SEI species in this system. In the FEC/FEMC system, the LiF₂ ^– intensity is significantly enhanced and remains relatively high over longer sputtering times, indicating that low-potential reductive decomposition favors the formation of LiF-related fluorinated inorganic components, which are distributed more continuously throughout the SEI. Meanwhile, C₂HO^– and LiCO₃ ^– signals are comparatively weak, suggesting a lower proportion of organic oxygen-containing and carbonate species. Figure c–e show the normalized fragment intensities for direct comparison, illustrating that the EC/EMC-derived SEI is enriched in C₂HO^–, whereas the FEC/FEMC-derived SEI exhibits stronger and more continuously distributed LiF₂ ^– signals.

4.

Depth-resolved ToF-SIMS analysis of SEIs derived from EC/EMC and FEC/FEMC electrolytes. (a, b) ToF-SIMS depth profiles and normalized intensity evolution of representative fragments, including C₂HO^–, LiCO₃ ^–, and LiF₂ ^–, for cycled electrodes in EC/EMC and FEC/FEMC electrolytes, respectively. C₂HO^–, LiCO₃ ^–, and LiF₂ ^– are selected as characteristic fragments of organic oxygen-containing species, carbonate species, and LiF^–-related fluorinated inorganic components, respectively. (c–e) Direct comparison of normalized LiF₂ ^–, C₂HO^–, and LiCO₃ ^– signals between the two electrolyte systems, with inset 3D ToF-SIMS reconstructions. The EC/EMC-derived SEI is enriched in organic oxygen-containing species, whereas the FEC/FEMC-derived SEI exhibits a stronger and more continuous LiF₂ ^– distribution, indicating the formation of a LiF-rich fluorinated inorganic SEI.

Collectively, these analyses indicate that the SEI formed in the EC/EMC system is primarily characterized by the accumulation of carbonate/organic lithium carbonate and organic oxygen-containing species, whereas the FEC/FEMC system produces a composite SEI enriched in LiF-related fluorinated inorganic components and potentially accompanied by VC-derived polymeric species or polycarbonate-like CO species. These compositional differences provide a mechanistic basis for subsequent analyses of the interfacial passivation capability and degradation behavior in the two electrolyte systems.

Dead-Li Accumulation and Electrochemical Reversibility

The above TMS, SEIRAS, and ToF-SIMS results reveal pronounced differences in the chemical composition and spatial distribution of the SEI derived from the two electrolytes. To further establish the correlation between SEI compositional differences and interfacial failure behavior, ethanol titration mass spectrometry, SEM, and electrochemical impedance spectroscopy coupled with EIS-DRT were employed to systematically compare metallic dead Li accumulation, Li deposition morphology, and interfacial kinetic evolution in the two electrolyte systems.

As shown in Figure a, ethanol-titration-based TMS was used to quantify metallic “dead Li” in the cycled electrodes. Metallic Li reacts with ethanol to release H₂ according to the reaction 2Li + 2C₂HOH → 2C₂H₂OLi + H₂↑; therefore, the amount of H₂ evolved can be used to evaluate the content of residual metallic dead Li based on stoichiometry. As shown in Figure b, the H₂ evolution in the EC/EMC system increases markedly with cycling, indicating continuous accumulation of metallic dead Li during repeated Li plating/stripping. In contrast, the FEC/FEMC system shows significantly lower H₂ evolution with only a slight increase upon cycling, suggesting that this electrolyte effectively mitigates metallic dead Li formation. The H₂ evolution was further converted into dead-Li-related capacity loss and compared with the total irreversible capacity loss (Figure c). The total capacity loss of the EC/EMC system continuously increases with cycling, with dead-Li-related loss accounting for a substantial fraction, indicating that electronically isolated metallic Li is a major contributor to irreversible capacity loss. By contrast, the FEC/FEMC system exhibits much lower total capacity loss and dead-Li-related capacity loss, suggesting that its interfacial structure helps suppress metallic dead Li formation and reduce the associated irreversible capacity loss. It should be noted that dead-Li-related capacity loss represents only part of the total irreversible loss, while the remaining loss may arise from SEI formation, electrolyte consumption, and other parasitic reactions.

5.

Quantification of metallic dead Li, deposition morphology, and interfacial kinetics in EC/EMC and FEC/FEMC electrolytes. (a) Schematic illustration of ethanol-titration MS for quantifying metallic dead Li through H₂ evolution. (b) H₂ evolution amounts measured by ethanol-titration MS from cycled electrodes over repeated cycles, reflecting the accumulation of metallic dead Li in different electrolytes. (c) Comparison between total capacity loss and dead-Li-related capacity loss calculated from ethanol-titration MS. (d, e) SEM images of cycled Li deposits formed in EC/EMC and FEC/FEMC electrolytes, respectively, showing the transition from rough, porous deposits to a more compact morphology. (f, g) DRT maps of Li||Li symmetric cells using EC/EMC and FEC/FEMC electrolytes, together with schematic illustrations of the corresponding SEI evolution. The EC/EMC electrolyte promotes metallic dead-Li accumulation, rough Li deposition, and stronger SEI-related interfacial responses, whereas FEC/FEMC suppresses dead-Li formation and stabilizes the Li metal interface.

SEM images of the cycled electrodes further reveal distinct Li-deposition morphologies in the two systems. As shown in Figure d, the Li electrode surface in the EC/EMC system exhibits a rough, porous, and mossy-like morphology, indicating nonuniform Li deposition and continuous interfacial reconstruction. Such a loose deposit structure can readily cause partial metallic Li to lose electronic contact with the conductive network, thereby forming metallic dead Li. In contrast, the FEC/FEMC system shows a more uniform and compact electrode surface (Figure e), indicating that the composite SEI formed in this electrolyte helps regulate Li deposition and suppress dead Li generation. The differences in deposition morphology and dead Li accumulation are also reflected in the interfacial kinetic behavior. EIS-DRT analysis shows that the major difference between the two systems lies in the SEI-related relaxation region (Figure f and g). The EC/EMC system exhibits a stronger SEI-related relaxation response that continuously evolves during cycling, suggesting more active interfacial processes, likely associated with repeated SEI rupture and reconstruction and rough Li deposition. In comparison, the FEC/FEMC system shows a weaker and more stable SEI-related response, indicating that the composite SEI mitigates the continuous evolution of SEI-related interfacial processes and maintains more stable interfacial kinetics.

To further verify the impact of these interfacial differences on electrochemical reversibility, the cycling behaviors of the two electrolyte systems were compared in Li/Cu half-cells and Li||Li symmetric cells. As shown in Figure a, the EC/EMC system exhibits rapidly decaying and highly fluctuating Coulombic efficiency in Li/Cu half-cells, accompanied by more pronounced polarization growth, indicating poor Li plating/stripping reversibility. In contrast, the FEC/FEMC system maintains high and stable Coulombic efficiency with lower voltage polarization over extended cycling, suggesting a more stable interface during repeated Li plating/stripping. Consistent trends are observed in Li||Li symmetric cells (Figure b): the EC/EMC system shows progressively increasing overpotential and eventual instability, whereas the FEC/FEMC system maintains a low and stable overpotential over prolonged cycling. Combined with the lower dead Li content, more uniform deposition morphology, and more stable SEI-related relaxation response shown in Figure , these results indicate that the composite SEI formed in the FEC/FEMC system helps reduce active Li deactivation and continuous interfacial reconstruction, which are closely associated with its enhanced Li plating/stripping reversibility and cycling stability.

6.

Electrochemical reversibility and cycling stability of Li-metal electrodes in EC/EMC and FEC/FEMC electrolytes. (a) Coulombic efficiency of Li/Cu half-cells cycled at 1 mA cm^–2 with a capacity of 1 mAh cm^–2; the inset shows representative voltage profiles during the 55th–57th cycles. (b) Galvanostatic cycling voltage profiles of Li||Li symmetric cells at 1 mA cm^–2 and 1 mAh cm^–2; the inset shows enlarged voltage profiles from 150 to 160 h. Compared with EC/EMC, the FEC/FEMC electrolyte enables higher Coulombic efficiency, lower polarization, and more stable long-term Li plating/stripping behavior.

Thermal-Response Behavior and Interfacial Thermal Stability

The above results indicate significant differences in the composition, spatial distribution, and interfacial failure behavior of the SEIs derived from the two electrolytes. In addition to its impact on the reversibility of Li plating/stripping at room temperature, the thermal response behavior of the cycled SEI and interfacial residues under elevated temperatures is a crucial metric for evaluating the safety of Li metal anodes. Therefore, in this study, online pyrolysis mass spectrometry (OP-MS) was employed to analyze the thermal outgassing behavior of cycled Cu electrodes retaining SEI components and residual inactive Li-related species. This experiment is a model interfacial-residue test under a defined Ar atmosphere and temperature-programmed condition, rather than a direct evaluation of full-cell thermal safety or thermal runaway behavior. As shown in Figure b, the gas evolution characteristics of the two systems are markedly distinct. First, the H₂ evolution exhibits obvious system dependence: the FEC/FEMC system displays a concentrated H₂ release peak in the lower-temperature region, whereas the EC/EMC system exhibits a broad release in the midtemperature region and a continuously increasing trend during the high-temperature stage. Since H₂ generation may involve reactions between residual metallic Li and hydrogen-containing interfacial components or trace proton sources, thermal cracking/dehydrogenation of organic components in the SEI, and enhanced contact reactions triggered by Li melting and redistribution at high temperatures, the H₂ evolution during the temperature-programmed process cannot be simply equated to the dead Li content. Instead, it should be regarded as the combined result of residual Li, hydrogen-containing interfacial components, and SEI thermal decomposition. Compared to H₂, the evolution of C₂H₄ and CO₂ more directly reflects the thermal stability of the organic carbonates, lithium alkyl carbonates, and inorganic carbonate components within the SEI. Notably, the onset temperature of gas evolution for the FEC/FEMC-derived SEI is lower than that of the EC/EMC system. This is likely related to the presence of a thermally sensitive polymeric film (such as VC-derived polymeric components (poly-VC)) on its surface, which can locally decompose at the early low-temperature stage and generate early trace gas signals. However, the total evolved amounts of C₂H₄ and CO₂ in the FEC/FEMC system remain significantly lower than those in the EC/EMC system (Figure b inset), indicating its restricted overall thermal decomposition. By contrast, the EC/EMC-derived SEI has a higher onset temperature for gas evolution, but is subsequently followed by intense and large-scale C₂H₄ and CO₂ release peaks. This “delayed but high-intensity” outgassing behavior indicates that its SEI bulk phase is rich in thermally unstable organic and carbonate components. Combined with the SEIRAS and ToF-SIMS analyses, it can be concluded that the FEC/FEMC-derived SEI is more inclined to form a LiF-rich inorganic framework accompanied by limited FEC-derived polymeric/organic components, thereby effectively suppressing the thermal decomposition of the dominant organic/carbonate components. Consequently, the organic/carbonate-rich SEI derived from EC/EMC is more prone to bulk thermal decomposition or reactions with residual active Li during the heating process, whereas the FEC/FEMC-derived interfacial residues exhibit much lower total C₂H₄/CO₂ evolution under the OP-MS testing condition, consistent with the formation of a LiF-rich/inorganic-enriched SEI with limited accumulation of thermally labile organic and carbonate species.

7.

Thermal-response behavior and interfacial thermal stability of cycled electrodes revealed by online pyrolysis mass spectrometry. (a) Schematic illustration of the online pyrolysis mass spectrometry (OP-MS) setup. The cycled Cu electrode retaining the SEI and interfacial residues was placed in the pyrolysis unit and heated under Ar carrier gas flow, and the evolved gases were delivered to the mass spectrometer through a cold trap. (b) Temperature profile and corresponding OP-MS gas-evolution rates of H₂, C₂H₄, and CO₂ from cycled electrodes in EC/EMC and FEC/FEMC electrolytes during heating to 300 °C at 2 °C min^–1; the inset shows the integrated gas amounts. The FEC/FEMC-derived interface exhibits an earlier but weaker gas-evolution onset, possibly associated with thermally responsive polymeric SEI components, whereas the EC/EMC-derived interface shows stronger C₂H₄ and CO₂ release at higher temperatures, indicating pronounced thermal decomposition of organic/carbonate-rich SEI components. The significantly suppressed C₂H₄/CO₂ evolution in the FEC/FEMC system suggests reduced thermal outgassing of retained interfacial residues under the defined OP-MS testing condition.

Conclusion

In this work, we established a multichannel interfacial product analysis framework to elucidate the formation, evolution, and failure mechanisms of the SEI on Li metal anodes. By integrating OEMS, OE-GCMS, TMS, in situ SEIRAS, ToF-SIMS, EIS-DRT, and OP-MS, this framework enables product-resolved correlation of interfacial electrolyte decomposition with SEI chemistry, inactive Li formation, Li plating/stripping reversibility, interfacial kinetics, and the thermal response behavior of cycled interfacial residues.

Using EC/EMC and FEC/FEMC electrolytes as model systems, we demonstrate that distinct reductive decomposition pathways lead to markedly different SEI structures and failure behavior. The conventional EC/EMC electrolyte undergoes continuous carbonate decomposition at the Li metal interface, producing abundant gaseous products and volatile organic fragments and forming an organic/carbonate-rich SEI with limited passivation capability. This interphase promotes continuous SEI reconstruction, metallic dead Li accumulation, nonuniform Li deposition, unstable interfacial kinetics, and pronounced thermally induced gas evolution. In contrast, the fluorinated FEC/FEMC electrolyte follows a more convergent decomposition pathway, suppressing complex organic fragmentation and promoting the formation of a LiF-rich composite SEI. This inorganic-enriched interphase mitigates dead Li formation, regulates the Li deposition morphology, stabilizes SEI-related interfacial processes, and suppresses thermal decomposition gas release.

Although Li||Cu and Li||Li cells were used here as model systems to isolate Li metal interfacial processes, practical lithium metal batteries involve additional coupled factors. Therefore, this work should be regarded as a mechanistic model study rather than a complete practical cell demonstration. Nevertheless, the product-resolved multichannel framework established here is transferable to more realistic cell configurations, including full cells and pouch cells, where it can be used to correlate anode-side interfacial products with practical degradation- and safety-relevant behaviors.

Overall, this study reveals the intrinsic correlation between interfacial electrolyte decomposition pathways, SEI structural evolution, and interfacial failure behavior, providing mechanistic insight into the structure–function relationship of SEIs on Li metal anodes. The product-resolved analytical paradigm proposed here offers a transferable framework for evaluating Li metal interfacial stability and guiding the rational construction of highly reversible and thermally stable SEIs.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.6c00892.

• Experimental details; measurement methods; related characterizations (OEMS, OE-GCMS, and in situ SEIRAS, TOF-SIMS, SEM, OP-MS) (PDF)

Y.T., W.T., and Y.Q. contributed to the design of the research and performed the experimental data analysis. Y.Y. conducted the measurements and analysis of in situ SEIRAS data. D.N., Y.Z., and Y.Q. supervised the work. All authors discussed the results, cowrote, and commented on the manuscript.

The authors declare no competing financial interest.

Published as part of JACS Au special issue “Future Perspectives on Battery Chemistries”.