Work function-tunable Li–Ag alloy anodes for modulating interface barriers in sulfide-based solid-state batteries

Li-alloy anodes have emerged as promising alternatives to the Li metal for all-solid-state batteries (ASSBs) due to their ability to mitigate interfacial side reactions and suppress dendrite growth. However, Li-ion diffusion limitation at the alloy/electrolyte interface remains a challenge. Herein, we propose a work function-tunable Li–Ag alloy design to reduce the electrostatic barrier at the Li–Ag/β-Li3PS4 interface based on a first-principles-informed thermodynamic model. Our calculations show that among Li–Ag alloys, Li-rich phases Li3Ag and Li11Ag2 exhibit ultralow Li diffusion barriers (∼0.1 eV) and favorable Li vacancy formation energies, enabling rapid Li diffusion. Our analysis of work functions of Li–Ag alloys as functions of orientation and termination reveals that surface termination is the primary determinant, with orientation playing a secondary role. A Li-terminated surface substantially lowers the work function, thereby reducing the interfacial potential drop and promoting Li-ion transport. While the influence of orientation is comparatively minor, its optimization can further reduce the barrier. Notably, the Li3Ag phase with a Li-terminated (111) surface achieves an interfacial potential drop as low as 0.20 V, much lower than that of the Ag-terminated (111) surface. Our findings highlight surface termination as a critical design principle and suggest that synergistic optimization of alloy composition, termination, and orientation is essential to minimize interfacial barriers in ASSBs.