Graphical abstract

In this study, lithium metal was treated with SnF₂ to form an artificial solid electrolyte interphase (SEI) layer. The formed composite layer consisting of LiF and Li-Sn alloy effectively suppressed lithium dendrite growth, improving the electrochemical performance of lithium metal batteries.

1. Introduction

In response to increasing global efforts toward carbon neutrality and environmental sustainability, numerous policies and initiatives targeting CO₂ emission reductions have been implemented worldwide. Secondary batteries have emerged as a pivotal enabling technology in this transition. Since the commercialization of lithium-ion batteries (LIBs) by Sony in 1991, LIBs have dominated the secondary battery market, gradually replacing lead–acid, nickel–cadmium (Ni–Cd), and nickel–metal hydride (Ni–MH) batteries. Today, LIBs are widely utilized in smartphones, drones, electric vehicles, and energy storage systems (ESSs). However, for applications such as electric vehicles and ESSs, an energy density of at least 300 Wh/kg is required, which is challenging to achieve with conventional LIB systems employing metal oxide cathodes and graphite anodes. Consequently, research into next-generation batteries offering higher energy and power densities has rapidly accelerated [1-5].

Lithium (Li) metal is considered a highly promising anode material for next-generation batteries owing to its high theoretical specific capacity (3680 mAh/g), low redox potential (–3.04 V vs. SHE), and low density (0.534 g/cm³). Nevertheless, the practical implementation of Li metal anodes is severely hindered by the growth of Li dendrites during repeated cycling. These dendrites originate from non-uniform Li⁺ deposition and lead to continuous side reactions with the electrolyte, resulting in the formation of electrically isolated “dead Li.” Over time, uncontrolled dendrite growth may penetrate the separator, ultimately causing internal short circuits and catastrophic cell failure [6-12]. To address these challenges, significant efforts have been devoted to suppressing dendrite formation and stabilizing the Li metal surface. For example, He et al. [13] fabricated a LiF-rich solid electrolyte interphase (SEI) by reacting NF₃ gas with Li metal, thereby enhancing mechanical robustness and ionic conductivity. Wang et al. [14] constructed a three-dimensional Li^⁺ host structure on a Cu current collector to facilitate homogeneous Li-ion flux. Cheng et al. [15] introduced nanoscale additives to promote uniform Li deposition and suppress dendritic growth. Despite extensive research into various strategies for suppressing Li dendrite growth, formidable technical challenges remain that hinder the reliable and practical application of Li metal anodes.

In this study, SnF₂ was spray-coated onto Li metal to form an artificial SEI layer comprising LiF and Li-Sn phases (LiF@Li-Sn). This composite layer significantly enhanced the air stability of Li metal and reduced interfacial resistance, as confirmed by electrochemical impedance spectroscopy (EIS). Furthermore, in situ optical microscopy analysis demonstrated that the composite layer effectively suppressed Li dendrite growth, ensuring uniform Li deposition during cycling. Full cells incorporating NMC622 cathodes and LiF@Li-Sn anodes exhibited excellent cycling stability, retaining over 79.4% of their initial capacity after 300 cycles at 1 C, thereby validating the efficacy of the proposed surface modification strategy. These results clearly highlight the potential of SnF₂-based surface modification in achieving stable and durable Li metal anodes, providing a facile and scalable approach for next-generation Li metal batteries (LMBs).

2. Experimental Section

2.1 Preparation of composite coating layer on lithium metal

To prepare LiF@Li-Sn composite layer, a coating solution was first prepared. SnF₂ and KPF₆ were dissolved at a weight ratio of 1:5 in 40 mL of dimethyl ether (DME). The solution was sonicated for 1 hour to ionize the SnF₂. Subsequently, the solution was centrifuged at 4000 rpm for 10 minutes. The resulting coating solution was then spray-coated onto the Li metal surface at a dry room. After spray coating, the Li metal was washed with dimethyl carbonate (DMC) to remove any unreacted residues and dried in a convection oven at 60°C for 30 minutes.

2.2 Electrochemical measurements

For electrochemical performance test of LMBs, NMC622 electrodes were firstly prepared. The cathode slurry was composed of NMC622, Super P (carbon black, Timcal), and PVDF binder (polyvinylidene fluoride, KF1300, Kureha) in a weight ratio of 90:5:5, and mixed using a Thinky mixer (ARE-310, Thinky) at 2000 rpm for 12 minutes. The prepared slurry was cast onto Al foil (Welcos) with a thickness of 120 µm and dried in a convection oven at 110°C for 1 hour and in 110°C vacuum oven for 24 hours. Two types of coin cells (CR2032, Welcos) were assembled in a glove box filled with argon gas to evaluate the electrochemical performance. For the Li//Li symmetric cells, Li metals with a thickness of 200 µm (Honjo metal) were used as both anode and cathode with 1 M LiTFSI in DOL/DME (1:1, v/v) as the electrolyte. In the NMC622 full cells, as- prepared NMC622 electrode, Li metal, and 1 M LiPF₆ in EC/EMC (1:2, v/v) were employed as cathode, anode, and electrolyte, respectively. Chronoamperometry (CA), Tafel analysis, and EIS were conducted using potentiostat (VSP, Biologics) in a two-electrode configuration. Galvanostatic charge and discharge tests were performed using a battery cycler (SERIES 4000, MACCOR).

2.3 Materials characterization

Field-emission scanning electron microscopy (FE-SEM) was used to observe the surface morphology of the composite coating layer on Li metal. Measurements were carried out with an electron beam at 15 kV and 10 mA. The Li dendrite growth behavior during plating was monitored in real time using a digital optical microscope under a current density of 3 mA/cm². Elemental composition of the composite coating layer was analyzed using X-ray photoelectron spectroscopy (XPS), with all data calibrated to the C 1s peak at 284.5 eV.

3. Results and Discussion

Fig. 1 shows the surface and cross-sectional morphology of bare Li metal and LiF@Li-Sn metal. The bare Li metal surface (Fig. 1a) is smooth with low roughness, as confirmed by the 3D topography image. This smooth surface is susceptible to localized Li deposition, which can result in dendrite growth. In contrast, the LiF@Li-Sn metal surface (Fig. 1b) is rough and covered with granular structures. The 3D topography image shows that the surface roughness is much higher than that of bare Li metal. This roughness is due to the formation of the LiF@Li-Sn layer created by the reaction between SnF₂ and Li. The cross-sectional images further illustrate the difference. The bare Li metal (Fig. 1c) has no protective layer, leaving it exposed to the electrolyte, which increases the risk of dendrite formation. On the other hand, the LiF@Li-Sn metal (Fig. 1d) has a uniform coating layer with a thickness of about 7.85 μm. This layer is composed of two main components: LiF, which is an ion-conductive but electronically insulating material, and Li-Sn, which provides good electrical conductivity and mechanical strength [16]. The LiF layer acts as a barrier against dendrite growth, while the Li-Sn layer maintains electrical contact. This dual-layer structure improves the stability of the Li metal surface by preventing direct contact with the electrolyte and distributing the current density more evenly. This structure is expected to enhance the cycling performance of Li metal anodes.

Fig. 2 provides elemental analysis of the LiF@Li-Sn composite layer through EDS, confirming the chemical composition and the spatial distribution of key elements. The EDS spectrum shows prominent peaks for tin (Sn) and fluorine (F), indicating that Sn and LiF are present in the composite layer. A peak for phosphorus (P) is also detected, which is likely due to the use of KPF₆ in the SnF₂ solution preparation. Despite thorough washing, P remains detectable, which suggests that PF₆- ions are either strongly adsorbed onto the LiF surface or chemically transformed during the SnF₂ treatment process. PF₆- can decompose on the Li surface, forming phosphate or other phosphorus-containing compounds [17]. The EDS mapping images show that Sn (green) and F (red) are uniformly distributed across the composite layer, indicating a homogeneous structure. However, the exact chemical state of P cannot be determined through EDS alone. Further analysis using XPS is necessary to confirm whether P exists as PF₆-, phosphate, or other species.

Fig. 3 presents the XPS analysis of the LiF@Li-Sn composite layer, confirming the chemical composition and the transformation of the SnF₂ precursor into a stable structure. In Fig. 3a, the P 2p XPS spectrum exhibits a major peak at 136.3 eV, which is attributed to P-F bonding in Li_xPF_y species or undecomposed PF₆- residues. A minor peak at 133.8 eV is ascribed to lithium phosphate compounds (Li_xPF_yO_z) formed on the lithium surface [18, 19]. The Sn 3d XPS spectrum (Fig. 3b) reveals two distinct peaks at 485.0 eV (Sn 3d_5/2) and 493.4 eV (Sn 3d_3/2), consistent with metallic Sn (Sn⁰) or a Li-Sn alloy (Li_xSn_y) [20]. These energies are significantly lower than that of SnF₂ (Sn 3d_5/2 at ~487.0 eV), confirming that the SnF₂ precursor has been fully converted according to [20]:

SnF2+2Li→LixSny+2LiF

This transformation produces a conductive Li-Sn alloy while generating LiF as an ion-conductive, electronically insulating protective layer. The formation of LiF and Li–Sn alloy is also confirmed by the Li 1s and F 1s XPS spectra in Fig. 3c and 3d, where peaks at 55.5 eV (Li 1s) and 684.3 eV (F 1s) correspond to LiF, and a peak at 55.9 eV (Li 1s) indicates the presence of Li-Sn alloy [21]. These findings demonstrate that the LiF@Li-Sn composite layer is composed of an electron-conductive Li-Sn alloy and an ion-conductive LiF layer, collectively providing enhanced interfacial stability for Li metal anodes.

Fig. 4 presents the comparative air stability of bare Li metal and LiF@Li-Sn composite over a period of 30 minutes of air exposure. The bare Li metal shows rapid surface degradation upon exposure to air. Initially, the clean metallic surface quickly begins to discolor within 1 minute, indicating the onset of oxidation. This discoloration intensifies over time, becoming progressively darker at 3 to 5 minutes. After 10 minutes, the surface exhibits a darkened and uneven appearance, and by 30 minutes, it is almost completely black. This severe discoloration is attributed to the formation of lithium oxide (Li₂O), lithium hydroxide (LiOH), and potentially lithium carbonate (Li₂CO₃), caused by the reaction of lithium with atmospheric oxygen, moisture, and CO₂ [22]. This result clearly demonstrates the poor air stability of bare Li metal. In contrast, the LiF@Li-Sn composite (bottom row) shows significantly enhanced air stability. The surface maintains a relatively uniform appearance for the first 5 minutes, with only minimal discoloration appearing after 10 minutes. Even after 30 minutes of exposure, the surface remains largely intact, with only slight darkening in some regions. This enhanced stability is attributed to the protective function of the LiF@Li-Sn layer. The LiF component provides an ion-conductive but electronically insulating barrier, effectively blocking direct contact between the reactive lithium and atmospheric species.

Fig. 5a and 5b shows the CA curves of bare Li and LiF@Li-Sn electrodes, which were used to determine the Li⁺ diffusion coefficients. Using Cottrell equation [23], the diffusion coefficients of Li⁺ at bare Li and LiF@Li-Sn were calculated as 1.08 × 10^-7 cm²/s and 1.28 × 10^-6 cm²/s, respectively. The enhanced diffusion in the LiF@Li-Sn electrode is attributed to its porous surface structure and the lithiophilic properties of LiF. The Tafel plots (Fig. 5c) further highlight the superior electrochemical activity of LiF@Li-Sn. The LiF@Li-Sn composite exhibits an exchange current density (i₀) of 0.242 A/cm², which is significantly higher than that of bare Li metal (0.042 A/cm²). This indicates faster charge transfer kinetics at the LiF@Li-Sn interface, attributed to the conductive Li-Sn alloy and the stable, ion-conductive LiF layer. In contrast, bare Li exhibits a lower i₀ due to unstable surface reactions and limited active area for charge transfer. Nyquist plots in Fig. 5d strongly support these Tafel analysis findings. After the cycle, the charge transfer resistance (R_ct) of LiF@Li-Sn is 25.2 Ω, significantly lower than that of bare Li (164.3 Ω). This low R_ct directly aligns with the high exchange current density observed in the Tafel plots, confirming that LiF@Li-Sn provides a more efficient charge transfer pathway.

Fig. 6 presents the cycling performance of Li symmetric cells with bare Li metal and LiF@Li-Sn at two current densities: 2 mA/cm² (Fig. 6a) and 5 mA/cm² (Fig. 6b). In Fig. 6a, the LiF@Li-Sn cell starts with an initial overpotential of 40 mV, which decreases to 27 mV after 250 cycles, indicating stable and efficient Li plating/stripping. In contrast, the bare Li cell shows a high and unstable overpotential, which increases rapidly, reflecting poor interfacial stability due to continuous SEI formation and dendrite growth. In Fig. 6b, the differences are more pronounced. The bare Li cell starts with a high overpotential of 200 mV, which further rises during cycling, indicating severe interfacial degradation. In contrast, the LiF@Li-Sn cell maintains a low initial overpotential of 50 mV, which only increases to 150 mV after 200 cycles, demonstrating superior stability. These results confirm that the LiF@Li-Sn composite effectively suppresses dendrite formation and maintains low interfacial resistance, providing consistent Li plating/stripping even under high current conditions.

Fig. 7 provides direct visual evidence of the Li deposition behavior on bare Li metal (Fig. 7a) and LiF@Li-Sn (Fig. 7b). In Fig. 7a, bare Li metal rapidly develops dendrites within 1 minute, which grow significantly over time, forming dense, thick structures by 10 minutes. This rapid dendrite formation indicates poor interfacial stability, where uncontrolled Li deposition leads to continuous SEI formation and severe surface roughening. In contrast, Fig. 7b shows that LiF@Li-Sn maintains a smooth and stable surface without any visible dendrite formation, even after 20 minutes. The absence of dendrites demonstrates that the LiF@Li-Sn composite effectively suppresses uncontrolled Li growth. The LiF layer acts as an ion-conductive but electronically insulating barrier, preventing direct Li-electrolyte contact, while the Li-Sn alloy promotes uniform Li deposition. These observations confirm that the LiF@Li-Sn composite not only enhances surface stability but also ensures safe and uniform Li deposition, making it an effective protective layer for Li metal anodes.

Fig. 8 shows the electrochemical performance of NMC622 full cells with bare Li metal and LiF@Li-Sn metal. In Fig. 8a and 8b, both cells show similar initial capacities around 170 mAh/g, but the Li//NMC622 cell rapidly decays to 9.8 mAh/g after 300 cycles, while the LiF@Li-Sn//NMC622 cell maintains 130.8 mAh/g (79.4%), indicating superior cycling stability. This improvement is attributed to the LiF@Li-Sn layer, which prevents direct Li-electrolyte contact and minimizes side reactions. Fig. 8b further supports this result, where the Li//NMC622 cell shows erratic coulombic efficiency (CE) and a sharp capacity drop after 215 cycles, caused by dendrite formation and electrolyte depletion. In contrast, the LiF@Li-Sn//NMC622 cell maintains a stable CE around 98.8%, reflecting consistent Li plating/stripping. The rate performance in Fig. 8c reveals that the LiF@Li-Sn//NMC622 cell maintains high capacities of 184.6, 181.2, 174.5, 186.7, 157.8, and 150.2 mAh/g at 0.1 C to 5 C, outperforming the Li//NMC622 cell, which suffers greater capacity loss at high rates. These results confirm that the LiF@Li-Sn layer significantly enhances cycling stability, CE, and rate performance.

4. Conclusion

In this study, a scalable SnF₂-based spray coating method was used to form a LiF@Li-Sn as artificial SEI layer on Li metal, significantly enhancing the electrochemical performance of LMBs. The artificial SEI layer, composed of ion-conductive LiF and conductive Li-Sn alloy, effectively suppressed Li dendrite growth and maintained a stable interface. This dual protective structure not only provided efficient ion transport but also ensured uniform Li plating and stripping, preventing continuous SEI formation and interfacial degradation. Electrochemical analysis confirmed the superior performance of the LiF@Li-Sn electrode. In symmetric cells, the LiF@Li-Sn electrode maintained a stable overpotential without visible dendrite formation, demonstrating excellent interfacial stability. In NMC622 full cells, the LiF@Li-Sn//NMC622 cell achieved a high capacity retention of 79.4% (130.8 mAh/g) after 300 cycles, significantly outperforming the Li//NMC622 cell, which rapidly decayed to 9.8 mAh/g. The LiF@Li-Sn//NMC622 cell also exhibited high coulombic efficiency (~98.8%) and superior rate performance, delivering high capacities across various current densities, including 150.2 mAh/g at 5 C. These results highlight the potential of the SnF₂ based composite coating as a practical strategy for stabilizing Li metal anodes for high energy density LMBs.

Article information

Funding

This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2022R1C1C1006536 and No. RS-2023-00221237).

Conflict of interest

Minho Yang serves as an editor of the Journal of Powder Materials editing but has no role in publishing this article. Except for that, the authors declare no competing financial interests or personal relationships.

Data Availability Statement

All dataset files used in this study are already provided in the manuscript.

Author Information and Contribution

Yeong Hoon Jeon: MS candidate; writing-original draft, visualization. Seul Ki Choi: Ph.D candidate; writing-original draft, visualization. Yun Seung Nah: conceptualization, investigation. Wonil Shin: Ph.D candidate; writing-review & editing. Yong-Ho Choa: Professor, supervision, writing-review & editing. Minho Yang: Professor; conceptualization, writing-review & editing, writing-original draft, supervision, funding acquisition.

Acknowledgments

None.

Fig. 1.

Field-emission scanning electron microscopy images of (a, c) bare Li metal and (b, d) LiF@Li-Sn metal, with 3D surface topography maps shown as insets.

Fig. 2.

Energy-dispersive X-ray spectroscopy (EDS) analysis of the LiF@Li-Sn metal surface: (a) Scanning electron microscopy image and (b) corresponding EDS spectrum. Elemental mapping images of (c) Sn and (d) F.

Fig. 3.

X-ray photoelectron spectra of LiF@Li-Sn: (a) P 2p, (b) Sn 3d, (c) Li 1s, and (d) F 1s.

Fig. 4.

Optical images showing the air stability of (top) bare Li metal and (bottom) LiF@Li-Sn metal.

Fig. 5.

Chronoamperometry analysis results of (a) bare Li metal and (b) LiF@Li-Sn metal. (c) Tafel plot of bare Li metal and LiF@Li-Sn metal. (d) Nyquist plot of lithium symmetric cells after the first cycle with bare Li metal and LiF@Li-Sn metal.

Fig. 6.

Lithium symmetric cell data of bare Li and LiF@Li-Sn at (a) a current density of 2 mA/cm² with a capacity of 1 mAh/cm² and (b) a current density of 5 mA/cm² with a capacity of 1 mAh/cm².

Fig. 7.

In situ optical microscopy images of lithium deposition behavior on (a) bare Li and (b) LiF@Li-Sn over time.

Fig. 8.

Electrochemical performance of NMC622 full cells with bare Li and LiF@Li-Sn: (a and b) Galvanostatic charge and discharge profiles after the first and 300th cycle. (b) Cycling performance at 1 C. (c) Rate performance at various rates.

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