Iodine-driven artificial SEI layer for high performance lithium metal batteries in an SO₂-based electrolyte

Abstract

Lithium metal anodes offer high theoretical capacity with a low redox potential; however, an unstable solid electrolyte interphase (SEI), lithium dendrite formation, and limited cycling performance limit their practical applications. While SO₂-based inorganic electrolytes provide high ionic conductivity and inherent nonflammability, their high reactivity with lithium forms thick and unstable SEI layers that are primarily composed of LiCl and lithium–sulfur–oxy compounds. In this study, we demonstrate an SEI engineering strategy using a molecular iodine (I₂)-containing LiAlCl₄–3SO₂ electrolyte to control the composition of the SEI formed at the interface of the lithium metal electrode. The I₂ acts as a redox mediator that promotes the formation of Li₂S and Li₂O-enriched artificial SEI layers in lithium metal, replacing the typical LiCl-dominant interfacial film. This Li₂S/Li₂O-rich artificial SEI exhibited enhanced Li⁺ transport properties and mechanical stability compared to the native SEI, thereby suppressing lithium dendrite formation and mitigating parasitic reactions during cycling. The electrochemical evaluation demonstrated improved cycling stability in both Li//Li symmetric cells and Li//LiFePO₄ full cells across various current densities.

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1 Introduction

The growing demand for energy storage systems with high energy density for electric vehicles and grid-scale applications has driven the development of advanced lithium-ion battery systems.^1–4 The lithium metal anode is considered the most promising next-generation lithium storage material because of its high theoretical capacity (∼3860 mA h g⁻¹) and low redox potential (−3.04 V vs. SHE).^5–7 However, the practical implementation of lithium metal anodes remains hindered by several challenges, including interfacial instability, low coulombic efficiency, uncontrolled lithium dendrite growth, and continuous side reactions during cycling.^5,8–10 Extensive efforts have been made to improve interfacial stability by modifying electrolyte formulations to control the interfacial reactions of lithium metal with the electrolyte,^11–13 modifying the surface of the lithium metal anode,^14–16 and engineering robust artificial solid electrolyte interphase (SEI) layers^17–20 to regulate the lithium deposition behavior. Recently, additive-assisted artificial SEI engineering has also attracted growing attention as an effective strategy to regulate Li⁺ flux, homogenize interfacial reactions, and suppress dendrite formation. For instance, Wang et al. demonstrated that additive-derived inorganic species incorporated into SEI layers can significantly enhance ionic conductivity and mechanical stability, leading to improved lithium deposition uniformity.²¹ Moreover, the controlled interfacial chemistry combined with three-dimensional lithiophilic host structures enables confined and homogeneous lithium deposition even at high areal capacities.²² In addition, the construction of artificial inorganic/organic composite SEI layers using functional additives has been reported to simultaneously enhance the ionic conductivity, interfacial robustness, and electrochemical reversibility of lithium metal anodes, enabling stable cycling even under high-voltage conditions.²³ These recent studies highlight the critical role of additive-driven artificial SEI design in controlling lithium deposition behavior and stabilizing lithium metal anodes. Although most of these efforts have focused on organic electrolyte systems, they still suffer from volatility, flammability, and limited long-term stability. Recent attention has shifted toward non-organic electrolytes with intrinsic nonflammability.^2,24–28 Among alternative electrolyte systems, nonflammable SO₂-based liquid electrolytes, such as LiAlCl₄–3SO₂ (Li–SO₂), offer significant benefits including nonflammability and high ionic conductivity (∼100 mS cm⁻¹).²⁹ However, Li–SO₂ liquid electrolytes are highly reactive to lithium metal, leading to the formation of thick and unstable SEI layers mainly composed of LiCl and lithium–sulfur–oxide compounds.^30–37 Li₂S and Li₂O are beneficial for lithium metal anodes because of their contribution to the electrochemical and mechanical stability of the SEI layer.^38–40 Li₂S is an ion-conductive sulfide compound that facilitates lithium-ion transport through the SEI,^41–43 while Li₂O is a mechanically rigid compound that helps maintain the structural integrity of the interphase during cycling.^44–46 However, despite these benefits, the lithium metal electrode in Li–SO₂ electrolyte suffers from unstable cycling behavior and continuous side reactions with the electrolyte. This suggests that the amount of Li₂S and Li₂O formed in the SEI of the lithium electrode might be insufficient to effectively suppress dendritic growth or maintain SEI integrity over prolonged cycling. Therefore, augmenting the formation of Li₂S and Li₂O in the SEI of the lithium electrode may be essential for improving the interfacial stability of lithium metal anodes using Li–SO₂ electrolytes. In this study, we demonstrate an interfacial engineering strategy that utilizes lithium iodide as an additive for lithium metal anodes with Li–SO₂ electrolyte, enabling the in situ generation of molecular iodine (I₂) via a spontaneous redox disproportionation reaction.^47–51 When lithium metal electrodes are pretreated by controlled immersion in this I₂-containing Li–SO₂ electrolyte (I₂–Li–SO₂), I₂ acts as a catalytic mediator to facilitate the preferential formation of the Li₂S and Li₂O-enriched SEI via I₂/I⁻ redox transformation.^48,49 While a highly rough native SEI was formed in pristine Li–SO₂ electrolyte, mainly composed of LiCl with limited ion-conductive components, the engineered interphase was relatively uniform and compact, thus showing significantly enhanced electrochemical stability. The resulting protective layer effectively mitigates lithium dendrite formation and suppresses parasitic side reactions to enable uniform lithium deposition and improve long-term cycling performance. This approach provides a practical strategy for stabilizing lithium metal electrodes in the Li–SO₂ electrolyte system and advances high-performance lithium metal battery technologies.

2 Experimental section

2.1 Electrolyte synthesis

The pristine LiAlCl₄–3SO₂ electrolyte was prepared by mixing the same molar amounts of LiCl and AlCl₃ powder in a sealed bottle, followed by blowing SO₂ gas with stirring to promote the chemical reaction between the SO₂ gas and the powder mixture. After the completion of the reaction, the resulting electrolyte was transferred to an Ar-filled glovebox to prevent moisture exposure. For the preparation of iodine-containing electrolytes, LiI, LiCl, and AlCl₃ powders were mixed at a molar ratio of 0.1 : 0.9 : 1, respectively, during the initial powder-mixing stage, enabling in situ I₂ formation within the electrolyte system. The subsequent synthesis procedure was identical to that of the pristine electrolyte, including SO₂ gas introduction, reaction completion, and collection under inert atmospheric conditions. For the concentration-dependent study, the LiI fraction was further varied while keeping the total salt content constant, using LiI : LiCl : AlCl₃ molar ratios of 0.05 : 0.95 : 1 and 0.20 : 0.80 : 1, and the same SO₂ introduction and handling procedures were applied.

2.2 Electrochemical measurements

CR2032 coin-type symmetric Li//Li cells were assembled using 200 µm-thick and 18 µm-thick lithium metal (Honjo metal, Japan), glass fiber filter separators (GC50, Advantec), and an SO₂-based electrolyte in an Ar-filled glovebox. LiFePO₄ (LFP) cathodes were prepared by hand-mixing using an agate mortar, combining 80 wt% active material (LFP; China), 10 wt% conductive agent (Super-P, TIMCAL, Switzerland), and 10 wt% polyvinyl fluoride binder (KF1100, Kureha Chemical Industry Co., Ltd, Japan) dissolved in N-methyl-2-pyrrolidinone (99.9 wt%, Sigma Aldrich, USA). The resulting slurry was cast onto aluminium foil using a Baker film applicator, vacuum-dried overnight at 200 °C, and compressed at 200 kg cm⁻². Both Li//Li symmetric cells and Li//LFP full cells were assembled in an Ar-filled glovebox and left for 12 h at 25 °C before electrochemical evaluation using a TOSCAT battery testing system. For Li//LFP full-cell testing, two types of LiFePO₄ cathodes were employed: areal loadings of 12 mg cm⁻² for 0.5C and 9 mg cm⁻² for 2C current density evaluation, corresponding to areal capacities of 2 mA h cm⁻² and 1.5 mA h cm⁻², respectively. After calendering, the electrode densities were ∼2.3 g cc⁻¹ (12 mg cm⁻²) and ∼2.2 g cc⁻¹ (9 mg cm⁻²). The cells were subjected to galvanostatic charge–discharge cycling to evaluate the electrochemical performance of pristine and I₂-modified lithium anodes paired with LFP cathodes. CV and LSV tests of the electrolytes were performed using a potentiostat (VSP-300, BioLogic) in a three-electrode beaker-type cell (with glassy carbon as the working electrode and Li-metal foil as the counter and reference electrodes) at a scan rate of 0.1 mV s⁻¹ with a voltage window of 0.005–4.5 V for CV tests and 0.005–5 V for LSV tests.

2.3 Electrochemical characterization

Raman spectra of the electrolytes were acquired using a Raman spectrometer (DXR3xi, Thermo Fisher Scientific) to analyze the molecular structure and composition of SO₂-based electrolytes. EIS was performed using an impedance-analyzing potentiostat (VSP-300, BioLogic) with an AC amplitude of 10 mV over a frequency range of 1 mHz to 1 MHz to evaluate interfacial resistance characteristics. The surface morphology and cross-sectional structures of lithium metal electrodes before and after cycling in LiAlCl₄–3SO₂ electrolytes were examined using field-emission SEM (FE-SEM; JEOL JSM-7000F). For detailed microstructural analysis, focused ion beam FE-SEM (Helios) was employed. Cross-sectional TEM analysis was performed using a transmission electron microscope (JEM-2010, JEOL). XPS (Thermo Fisher Scientific) was used to analyze the surface chemical composition and oxidation states of lithium metal electrodes after electrochemical cycling. All XPS profiles were calibrated by aligning the C 1s peak to 284.8 eV to ensure consistent binding energy referencing.

3 Results and discussion

3.1 Formation and characterization of the Li₂S/Li₂O-enriched artificial SEI layer

Fig. 1 presents a comprehensive overview of the interfacial engineering strategy for lithium metal electrodes using I₂-containing LiAlCl₄–3SO₂ systems (I₂–Li–SO₂). The electrolyte was prepared by dissolving LiCl, AlCl₃, and LiI with SO₂ gas, followed by the spontaneous formation of liquid electrolyte with I₂, showing a color change to deep purple (Fig. S1). This color change originates from the disproportionation reaction of iodide ions and the successful generation of I₂ species in the electrolyte. Raman spectroscopy (Fig. S2) revealed characteristic peaks at 209, 417, and 627 cm⁻¹ corresponding to I₂ vibrations in the I₂–Li–SO₂ system.⁵² We further examined the LiI concentration dependency (in the 0.05–0.20 M range, compared to that of LiCl in the first synthesis process) on the performance of the iodine-containing LiAlCl₄–3SO₂ electrolyte. Fig. S3 shows that the 0.05 M LiI electrolyte exhibits a lighter color and weaker normalized I₂-related Raman intensity as well, whereas the 0.10 and 0.20 M electrolytes show similar dark coloration with nearly identical Raman features. Based on these results, the 0.10 M condition was selected as the representative I₂-modified electrolyte for electrochemical characterization, and its redox behavior and electrochemical stability were further examined by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) (Fig. S4). The I₂-containing electrolyte shows a narrower electrochemical stability window than the pristine LiAlCl₄–3SO₂ electrolyte.^48,53,54 The decrease in the electrochemical window of I₂–Li–SO₂ electrolyte owing to electrochemical reduction of I₂ at approximately 3.7 V (vs. Li/Li⁺) suggests that the I₂-containing Li–SO₂ electrolyte is insufficient for lithium rechargeable batteries. Despite this electrochemical limitation, the I₂–Li–SO₂ system can be used for the surface modification of lithium metal anodes. Fig. 1a shows a schematic of the surface modification process by immersing lithium metal into unmodified Li–SO₂ and I₂–Li–SO₂ electrolytes (Fig. S5). Fig. 1b and c show the top-view and cross-sectional scanning electron microscopy (SEM) images of the Li metal after immersion in the pristine and I₂-containing electrolytes for 24 h, respectively. Fig. 1b shows the formation of a native SEI layer with an irregular, flake-type morphology of lithium metal after immersion in Li–SO₂ electrolyte for 24 h. Fig. 1c shows a uniform SEI layer with a round morphology after dipping in the I₂–Li–SO₂ electrolyte, which is mainly composed of spherical particles forming a conformal layer with thickness of approximately 2 µm. SEM observation shows that at the electrolyte with 0.10 M LiI, Li metal has the most uniform rounded/conformal surface layer, whereas dipping of Li metal in the 0.05 and 0.20 M electrolytes led to denser film-like layers with different thicknesses (Fig. S6). X-ray photoelectron spectroscopy (XPS) was employed to obtain insights into the chemical composition and interfacial chemistry of the lithium metal surfaces after the immersion process. Fig. 1d shows the XPS profiles of lithium metal after immersion in the I₂–Li–SO₂ electrolyte (I₂-modified lithium). The Li 1s spectrum shows distinct peaks at 54.5 eV (ref. 55) and 53.5 eV,⁵⁶ corresponding to Li₂S and Li₂O, respectively. We further compared the surface SEI chemistry after iodine treatment at different LiI levels (0.05 and 0.20 M) by XPS analysis of the Li 1s, O 1s, S 2p, Cl 2p, and I 3d regions (Fig. S7). XPS spectra after dipping (Fig. S7) show that the chemical compositions of the SEIs formed by dipping Li metal in the electrolytes are almost the same. All of this indicates significantly higher surface concentrations of these inorganic compounds compared to lithium metal immersed in pristine Li–SO₂ electrolyte (Fig. S8). The O 1s and S 2p spectra of lithium metal after immersion in the I₂–Li–SO₂ electrolyte further confirm this observation, showing distinct Li₂O and Li₂S peaks at 528.8 and 162.5 eV, respectively. In contrast, the Li 1s spectrum of lithium metal immersed in pristine Li–SO₂ electrolyte shows a peak at 56.2 eV for LiCl.⁵⁷ The O 1s and S 2p spectra are dominated by peaks corresponding to the Li–S–O compound, with only minor contributions from Li₂O and Li₂S. This comparison reveals that the introduction of I₂ into the Li–SO₂ electrolyte alters the reaction chemistry of lithium metal with the SO₂-based electrolyte, leading to changes in the main SEI species on the surface of the lithium metal through the following reactions:⁴⁹

2I⁻ + SO₂ + 6Li → Li₂S + 2Li₂O + I₂ (1)

Li₂SO₄ + I₂ + 8Li → Li₂S + 4Li₂O + 2I⁻ (2)

Fig. 1 Artificial Li₂S/Li₂O-rich SEI formation via I₂-modified LiAlCl₄–3SO₂ electrolyte. (a) Schematic of lithium metal surface modification using LiAlCl₄–3SO₂ and I₂-containing LiAlCl₄–3SO₂. Surface and cross-sectional SEM images of modified lithium electrodes immersed in (b) LiAlCl₄–3SO₂ and (c) I₂-containing LiAlCl₄–3SO₂. (d) XPS profiles showing the Li 1s, O 1s, S 2p, Cl 2p, and I 3d regions of the lithium metal surface after treatment with I₂-containing LiAlCl₄–3SO₂.

I₂ in the electrolyte acts as a chemical mediator, promoting the formation of SEI components such as Li₂O and Li₂S, which are expected to enhance the interfacial stability of the lithium metal electrode. To verify this I₂-mediated chemistry, ex situ Raman spectra were collected for the I₂–Li–SO₂ electrolyte before and after lithium dipping (Fig. S9), together with XPS analysis of lithium subjected to sequential treatment (pristine Li–SO₂ dipping followed by I₂–Li–SO₂ dipping) as shown in Fig. S10. After dipping, the Raman bands for AlCl₄⁻ decrease in intensity, indicating the interfacial reaction between lithium and the electrolyte, whereas the characteristic I₂ band remains essentially unchanged (Fig. S9).⁵⁸ Notably, the Raman band near ∼170 cm⁻¹ decreases after lithium dipping, suggesting a change in the iodine species associated with weak iodine–iodide interactions.^52,59 This observation is consistent with reversible I₂/I⁻ interfacial mediation during the dipping process, rather than irreversible consumption of iodine. Consistently, the absence of a detectable I 3d signal was observed in the XPS spectra (Fig. 1d), confirming that iodine was not incorporated as a SEI component. It should be noted that the XPS spectra of Li metal after the sequential treatment show that I₂ can transform a pre-formed SEI to a Li₂S/Li₂O-rich composition without iodine incorporation, supporting that I₂ acts as a catalytic mediator rather than a consumable additive (Fig. S10).

3.2 Lithium deposition behavior on I₂-modified electrodes

Fig. 2 shows top-view SEM images of lithium deposits formed on the lithium metal electrode, obtained after three cycles using pristine lithium and the I₂-modified lithium with Li–SO₂ electrolyte at various current densities and areal capacities. These images offer critical insights into how surface pretreatment affects the morphology of lithium deposits under various electrochemical conditions (0.5–3 mA cm⁻² for current density and 1–5 mA h cm⁻² for areal capacity). In the pristine lithium electrode (Fig. 2a), lithium deposits appear relatively flat and uniform under mild conditions, such as 0.5 mA cm⁻² and 1 mA h cm⁻². However, as the current density and areal capacity increased, the lithium metal surface became rough, with lithium filament growth. This dendritic morphology results from the uneven Li⁺ flux and high interfacial reactivity of lithium metal with the Li–SO₂ electrolyte. In contrast, the I₂-modified lithium electrode (Fig. 2b) shows rounded and compact deposits under all conditions. Even at high current densities, the I₂-modified lithium maintained a relatively smooth surface without needle-shaped deposits, while a floc-like overlayer was formed. SEM-EDS mapping (Fig. S11) indicates that this overlayer shows stronger O, S, Cl, and Al signals compared to the surface features formed on pristine lithium. This suggests that the pre-formed artificial SEI was partially preserved during cycling and continued to function as a protective scaffold that guides more uniform Li deposition and suppresses dendritic growth under harsh conditions. This comparison suggests that the Li₂S and Li₂O-enriched SEI layer effectively controlled lithium nucleation and growth in the Li–SO₂ electrolyte system, thereby maintaining uniform lithium-ion flux and preventing high-surface-area dendrite formation. This uniform and rounded lithium deposition behavior indicates that ion transport in the I₂-modified SEI is dominated by fast lateral Li⁺ diffusion along the SEI/Li interface rather than vertical Li penetration.^60,61 The Li₂S/Li₂O-rich inorganic interphase with abundant grain boundaries provides rapid interfacial diffusion pathways, which effectively homogenize the surface Li-ion flux and suppress the localized current amplification responsible for dendritic growth.⁶²

Fig. 2 Effect of current density and areal capacity on Li deposition morphology. FIB-SEM images of the Li deposit for (a) the pristine lithium metal electrode and (b) I₂-modified Li-metal electrode in a LiAlCl₄–3SO₂ inorganic electrolyte after three cycles under various areal capacity and current density conditions.

3.3 Electrochemical performance of I₂-modified lithium electrodes

Fig. 3 shows that the lithium symmetric cells with an artificial SEI layer on the lithium metal anode exhibited stable operation for over 1000 cycles at current densities of 1 and 3 mA cm⁻², with a fixed areal capacity of 3 mA h cm⁻² with the Li–SO₂ electrolyte. At a current density of 1 mA cm⁻² (Fig. 3a), the pristine lithium electrode exhibited a slightly lower average overpotential than the modified electrode, but showed potential fluctuations between 2400 and 3600 h (corresponding to 400–600 cycles), which originated from non-uniform lithium deposition and an unstable SEI layer on the surface of the pristine lithium electrode (Fig. S12). In contrast, the I₂-modified electrode maintains stable electrochemical deposition and stripping of lithium metal without abrupt changes in the overpotential for over 6000 h (2000 cycles). In particular, voltage profiles enlarged near 5000 h (Fig. S13) clearly show that the I₂-modified Li//Li cell retains periodic plating/stripping oscillations without a short-circuit-like flat response, indicating that the slight long-term potential evolution arises from gradual interfacial reorganization and mass-transport polarization rather than catastrophic failure.^63–66 Although the modified electrode exhibited a slightly higher overpotential compared with pristine lithium, the Li₂S and Li₂O-enriched SEI effectively suppressed parasitic reactions and enabled uniform lithium deposition. At the higher current density of 3 mA cm⁻² and areal capacity of 3 mA h cm⁻² (Fig. 3b), the performance difference becomes more pronounced. The pristine lithium electrode showed a sharp increase in overpotential after 200 h (corresponding to 100 cycles) with voltage fluctuations, followed by a short-circuit failure at 562 h (281 cycles) (see the enlarged profile in Fig. S14). Such degradation of the pristine lithium electrode is attributed to the accelerated SEI breakdown and aggressive dendrite growth under high current conditions. In contrast, the I₂-modified lithium electrode demonstrates stable charge–discharge behavior for over 2000 h (1000 cycles), indicating that the modified SEI layer effectively withstood high current conditions and suppressed dendrite formation during extended cycling. Note that the electrolyte with 0.10 M LiI showed the most favorable overall performance among the investigated formulations (Fig. S15). To further evaluate interfacial stability, Li//Li symmetric cell tests were conducted at both low (1 mA h cm⁻²) and high (5 mA h cm⁻²) areal capacities as shown in Fig. S16. The I₂-modified lithium maintained stable cycling behavior even at high areal capacity, whereas pristine lithium suffered from early short-circuit failure. Moreover, under 18 µm thin-lithium conditions, the I₂-modified lithium electrode exhibited a uniform and dense interfacial structure as revealed by cross-sectional SEM analysis (Fig. S17), and demonstrated stable and reversible plating/stripping behavior in the corresponding electrochemical test (Fig. S18), confirming the strong protective capability of the artificial SEI. In addition, to identify the current density limitation of the I₂-modified lithium electrode, Li symmetric cell rate tests were performed from 0.1 to 10 mA cm⁻² at a fixed areal capacity of 1 mA h cm⁻². The I₂-modified lithium maintained stable operation across the entire current range, indicating that the artificial SEI effectively accommodates fast Li⁺ transport over a wide kinetic regime (Fig. S19).⁶⁷

Fig. 3 Electrochemical performance and interfacial resistance of pristine and I₂-modified lithium electrodes, and electrochemical performance of Li//Li symmetric cells with pristine and I₂-modified lithium electrodes. Cycling behavior at current densities of (a) 1 mA cm⁻² and (b) 3 mA cm⁻² with a fixed areal capacity of 3 mA h cm⁻². (c) EIS analysis after 12 h at open-circuit voltage. Nyquist plots after 2, 10, and 50 cycles for (d) pristine lithium and (e) I₂-modified lithium electrodes.

This remarkable improvement in stability also highlights the role of the artificial SEI layer in enabling the reliable operation of lithium metal electrodes. Because the cycling stability of lithium electrodes is closely related to the interfacial resistance, electrochemical impedance spectroscopy (EIS) (Fig. 3c–e) at different cycles was employed to analyze the resistance of the naturally formed SEI and preformed Li₂S/Li₂O artificial SEI on the lithium metal electrode. As shown in Fig. 3c, the SEI resistance (R_sei) of the I₂-modified lithium before testing was higher than that of pristine lithium, indicating that the artificial SEI layer already formed on the electrode surface prior to cycling (Table S1). This preformed layer, while increasing the initial impedance, may provide a more compact and denser interface with the electrolyte compared to that of the pristine lithium electrode. Fig. 3d shows the EIS profiles of the pristine lithium metal electrodes after 2, 10, and 50 cycles. After 10 cycles, the pristine lithium electrode showed an increase in R_sei, possibly due to the thickening of the SEI caused by side reactions between the lithium and electrolyte during cycling. However, after 50 cycles, a sharp decrease in R_sei was observed, which can be attributed to SEI breakdown and surface structural changes during repeated cycling (Table S2).⁶⁸ In contrast, the EIS spectra of the I₂-modified lithium metal electrode exhibited a different behavior compared to those of the pristine lithium electrode, as shown in Fig. 3e. Although the initial R_sei was higher than that of pristine lithium, the resistance of the SEI layer for the I₂-modified lithium electrode gradually decreased with cycling. This suggests that the Li₂S/Li₂O-rich artificial SEI enabled highly stable lithium deposition and stripping over extended cycles.⁶⁹ Overall, the I₂-modified lithium shows a higher initial SEI resistance and charge-transfer resistance due to the presence of the preformed inorganic layer, and both resistances gradually decrease with cycling and become comparable to those of pristine lithium. The exchange current density of the Li electrode is well known to reflect the electrochemical kinetics for the Li deposition/dissolution reaction according to the complete overpotential–current relationship.⁷⁰ Since R_ct is inversely related to effective charge-transfer kinetics under an identical symmetric cell, the reduction of R_ct for the I₂-modified electrode indicates that the artificial SEI does not hinder, and may even facilitate the effective interfacial exchange kinetics during cycling.

Fig. 4 shows cross-sectional SEM images of lithium metal electrodes obtained from cycled Li//Li symmetric cells with the Li–SO₂ electrolyte after 2, 20, and 50 cycles at a current density of 3 mA cm⁻² and an areal capacity of 3 mA h cm⁻². The pristine lithium electrodes showed severe morphological degradation over extended cycling (Fig. 4a–c and S20a). The thickness of the porous surface layer dramatically increased from ∼7 µm after 2 cycles to ∼30 µm after 20 cycles, ultimately reaching 100–180 µm after 50 cycles. This progressive thickening indicates continuous parasitic reactions between the lithium metal and electrolyte, leading to the formation of highly porous lithium deposits. In contrast, I₂-modified lithium electrodes maintained a relatively stable morphology even after 50 cycles, with the interfacial layer thickness remaining below ∼20 µm (Fig. 4d–f and S20b). The surface layer, consisting of both SEI components and lithium deposits, remained thin and dense. This compact interfacial layer maintained on the I₂-modified lithium electrode indicates that Li transport is dominated by fast lateral interfacial diffusion rather than vertical Li penetration, which is mechanically suppressed by the rigid inorganic Li₂S/Li₂O framework.^71,72 Such morphological and dimensional stability of the electrode is well maintained, effectively suppressing continuous parasitic reactions with the electrolyte and leading to enhanced long-term cycling stability.

Fig. 4 Cross-sectional SEM analysis of SEI evolution during cycling, and cross-sectional SEM analysis of lithium metal electrodes after symmetric cell cycling. (a–c) Pristine lithium electrode after cycling at a current density of 3 mA cm⁻² with areal capacity of 3 mA h cm⁻² for 2, 20, and 50 cycles, respectively. (d–f) I₂-modified lithium electrode after cycling under identical conditions, demonstrating the formation of dense, uniform SEI layers.

Fig. 5 and 6 show the XPS profiles of lithium metal electrodes from the Li//Li symmetric cells after various cycles. For the pristine lithium electrodes (Fig. 5), the Li 1s spectra were dominated by the peak at 56.2 eV, corresponding to LiCl, which progressively accumulates and remains as the main SEI component throughout cycling. Very small peaks for Li₂S (54.5 eV) and Li₂O (53.4 eV) were observed, indicating the limited formation of beneficial inorganic species in the SEI of the pristine lithium electrode. The O 1s spectra of the pristine lithium electrode showed that S–O and C–O compounds were the major components of the SEI layer, while the Li₂O observed at 528.8 eV remained weakly visible throughout the cycling. The S 2p spectra showed the progressive increase of the peaks for the Li–S–O compound with cycling, while the Li₂S signal at 162.5 eV remained consistently weak. These results indicate that the SEI of pristine lithium mainly consisted of LiCl, with Li₂S and Li₂O as minor constituents. In contrast, the XPS profiles of the I₂-modified lithium electrode (Fig. 6) showed that the composition of the SEI was modified by introducing iodine. The Li 1s spectra revealed strong peaks for Li₂S (54.5 eV) and Li₂O (53.5 eV), which remained prominent even after 50 cycles together with LiCl (56.2 eV). However, with continued cycling, the intensity of the LiCl peak progressively increased, suggesting the gradual development of LiCl-containing outer layers on the initially formed Li₂S/Li₂O-based layer. This configuration was further supported by the O 1s and S 2p spectra, which revealed the enrichment of Li₂S and Li₂O in the modified SEI layer of the I₂-modified lithium metal electrode. Notably, the O 1s spectra showed that the intensity of the peaks for the S–O and Li–S–O species became more pronounced during cycling, while the Li₂O peak remained consistently detectable. The S 2p spectra indicated an increase in Li–S–O species with the presence of Li₂S. The Cl 2p spectra showed only LiCl peaks, with no additional byproducts. These findings indicate that there were two layers in the SEI layer of the I₂-modified lithium electrode. The Li₂S/Li₂O-rich inner layer remained structurally intact throughout cycling, while the outer layer, mainly composed of LiCl and Li–S–O compounds, showed a gradual increase in thickness during cycling. The cross-sectional TEM (Fig. S21)^73,74 and depth-profiling XPS collected at different cycle numbers (Fig. S22) further support the proposed bilayer structure, revealing a dense and continuous SEI containing Li₂O/Li₂S domains. The depth-resolved XPS confirms that Li₂S/Li₂O components remain detectable up to 100 cycles, indicating a persistent inorganic-rich inner framework. This structural feature of the SEI was consistent with the EIS fitting results (Table S2). Although the pre-formed inorganic SEI initially increases interfacial resistances, both R_sei and R_ct progressively decrease and then stabilize upon cycling, indicating that the interphase becomes progressively more ion-conductive and kinetically stable while retaining its robust inorganic framework rather than undergoing SEI breakdown. Such a preformed layer can accommodate repeated interfacial cycling, thereby sustaining stable electrochemical performance. In contrast, the pristine lithium electrode develops a thick and mechanically fragile SEI.

Fig. 5 XPS spectra evolution of pristine lithium metal during cycling. XPS analysis of the surface layer of the pristine lithium metal for (a) Li 1s, (b) O 1s, (c) S 2p, and (d) Cl 2p after dipping, 10 cycles, 20 cycles, 50 cycles, and 100 cycles using LiAlCl₄–3SO₂.

Fig. 6 XPS spectra evolution of I₂-modified lithium metal during cycling. XPS analysis of the surface layer of the I₂-modified lithium metal electrode for (a) Li 1s, (b) O 1s, (c) S 2p, and (d) Cl 2p after dipping, 10 cycles, 20 cycles, 50 cycles, and 100 cycles using LiAlCl₄–3SO₂.

To validate the practical applicability of the I₂-modified lithium metal anodes, the full-cell performance was evaluated using LiFePO₄ (LFP) cathodes in Li–SO₂ electrolyte systems. Fig. 7 presents a comparison of the electrochemical performance of the pristine and I₂-modified lithium anodes paired with LFP cathodes under different current density conditions. At a current density of 1 mA cm⁻², both electrodes exhibited initial coulombic efficiencies exceeding 99.3% (Fig. 7a). However, the long-term cycling performance (Fig. 7b) reveals distinct differences in stability. The I₂-modified lithium anode maintains a superior capacity retention of 63.9% after 600 cycles, while the pristine lithium anode shows only 39.7% of its initial capacity after the same number of cycles, which can be attributed to the physicochemical properties of the SEI layer of each lithium electrode. At a higher current density of 3 mA cm⁻², both systems demonstrated comparable initial coulombic efficiencies of over 96.9% (Fig. 7c). However, the I₂-modified anode demonstrated superior high-rate performance, maintaining better capacity retention throughout the extended cycling (Fig. 7d). These results confirm that the artificial SEI not only stabilizes the interface but also facilitates lithium-ion transport, supporting high-rate battery operation.

Fig. 7 Electrochemical performance of pristine and I₂-modified lithium metal anodes with LiFePO₄ cathodes, and electrochemical performance of LiFePO₄ cathodes with pristine and I₂-modified lithium metal anodes in LiAlCl₄–3SO₂ electrolyte. (a) The initial galvanostatic charge–discharge voltage profiles and (b) cycling performance at 0.5C (1 mA cm⁻²) with an areal capacity of 2 mA h cm⁻². (c) Initial voltage profiles and (d) cycling performance at 2C (3 mA cm⁻²) with an areal capacity of 1.5 mA h cm⁻².

4 Conclusions

We demonstrated that a Li₂S and Li₂O-enriched artificial SEI layer, pre-formed by dipping a metal electrode in an I₂-containing LiAlCl₄–3SO₂ electrolyte, effectively suppressed dendritic lithium growth and mitigated side reactions between the lithium metal and electrolyte. This inorganic interphase exhibited high chemical and electrochemical stability during repeated cycling without continuous side reactions. Symmetric lithium cells with the modified electrode achieved long-term cycling stability over 1000 cycles even under high current densities. Li//LFP full cells demonstrated significantly improved capacity retention of 63.9% after 600 cycles. These results show that forming a stable Li₂S/Li₂O-enriched artificial SEI through an I₂-containing LiAlCl₄–3SO₂ pretreatment is an effective route to improve the durability of lithium metal anodes in SO₂-based electrolyte systems. While the present study validates the effectiveness of iodine-mediated interfacial stabilization in coin-type cells, translating SO₂-based inorganic electrolytes to pouch-cell formats remains challenging due to SO₂ gas evolution during the sealing process.

Author contributions

Jiwhan Lee: conceptualization, data curation, formal analysis, investigation, visualization, methodology, writing – original draft, writing – review & editing. Seung Do Mun: conceptualization, formal analysis, investigation, methodology. Seong Hoon Choi: conceptualization, investigation. Hansu Kim: writing – original draft, writing – review & editing, project administration, supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors state that all data supporting the results of this study are provided in the article and its supplementary information (SI). Additional raw data are available from the corresponding author upon reasonable request. Supplementary information is available. See DOI: https://doi.org/10.1039/d5ta08305b.

Acknowledgements

This research was supported by the Hyundai Motor Company.

Notes and references

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