Enhanced Li Stripping/Plating Performance Enabled by Tin-Indium Double Interlayers in Sulfide-Based All-Solid-State Li Metal Batteries

In recent years, the development of high-energy-density and highly safe energy devices has accelerated for a sustainable society. In terms of safety, all-solid-state batteries have attracted significant attention because they employ nonflammable solid electrolytes (SEs) instead of conventional liquid electrolytes. Among SE candidates, sulfide SEs are particularly promising due to their high ionic conductivity and excellent formability. For achieving higher energy density, the use of Li metal as a negative electrode is ideal because it has a high theoretical capacity (3860 mA h g −1 ) and the lowest redox potential (−3.04 V vs. standard hydrogen electrode). However, Li filaments (dendrites) can lead to short-circuiting, thereby hindering the practical application of sulfide-based all-solid-state Li metal batteries. One of the causes of short-circuiting is a reductive decomposition of SEs at the interface with Li metal. In the case of Li 3 PS 4 SE, the reductive reaction causes the volume expansion and the formation of microcracks at the interface. Then, Li metal deposits along the cracks during cycling, resulting in short-circuiting [1] . To enhance the reduction tolerance of SEs, a promising strategy is the addition of lithium halides, which are thermodynamically stable with Li metal. In fact, Li 3 PS 4 –LiF and Li 3 PS 4 –LiI SEs exhibit high stability against Li metal [2,3] . Another cause is void formation at the Li/SE interface during Li stripping. [4] These voids decrease the effective contact area and lead to current concentration during subsequent Li plating, which promotes the growth of Li filaments. Therefore, maintaining interfacial contact at the Li/SE interface is essential. To address this issue, inserting alloy interlayers at the interface could preserve interfacial contact even after Li stripping and provide favorable sites for Li plating. In this study, we aimed to suppress short-circuiting in sulfide-based all-solid-state Li metal batteries by inserting Li alloys at the interface. As the interlayer metals, Sn and In were selected because they do not form solid solutions with Li, allowing the alloy layers to remain at the interface even after cycling. Sn, In, or a Sn–In double interlayer was sputtered on one side of the 54Li 3 PS 4 ·46LiI glass (LPSI) pellet, and Li foil was attached. Before cycling, XRD patterns at the interfaces were obtained. The most Li-rich alloys (Li 17 Sn 4 and Li 13 In 3 ) were observed at the interfaces. We also confirmed that these alloy interlayers remained at the interfaces after cycling. To examine the effects of these interlayers on Li plating, the voltage profiles during Li plating were compared. The cells with the In or Sn-In double interlayer demonstrated significantly decreased initial overpotentials compared with those without an interlayer. SEM observation after Li plating revealed that Li was plated non-uniformly in the cells without an interlayer. In contrast, Li metal was uniformly plated in the cells with the Sn–In interlayer. These results suggest that it is suggested that Li 13 In 3 acts as a favorable Li plating site and contributes to uniform Li plating. To evaluate the effect of the interlayers on Li stripping, Li was stripped at a constant current density, and the stripped capacity was used as an indicator of their ability to maintain interfacial contact. The insertion of metal interlayers increased the stripped capacities, and the cell with the Sn interlayer exhibited the highest stripped capacity. The co-insertion of both Sn and In also resulted in a higher stripped capacity than In alone. These results indicate that Li 17 Sn 4 is effective for maintaining interfacial contact at the Li/SE interface. In cycling tests using Li symmetric cells, the cell with the Sn–In double interlayer exhibited improved Li stripping and plating performance compared to the other cells. In summary, the insertion of Sn contributed to maintaining interfacial contact during Li stripping, and the insertion of In decreased overpotential during Li plating and promoted dense and uniform Li plating. By inserting both Sn and In, their beneficial effects were combined, enabling more stable operation of all-solid-state Li metal batteries. Reference s : [1] M. Otoyama et al., ACS Appl. Mater. Interfaces , 13 (2021) 5000. [2] T. Asakura et al., J. Mater. Chem. A , 13 (2025) 1942. [3] M. Suyama et al., Electrochimica Acta , 286 (2018) 158. [4] J. A. Lewis et al. , Nat. Mater. , 20 (2021) 503.