Control of Two Solid Electrolyte Interphases at the Negative Electrode of an Anode-Free All Solid-State Battery based on Argyrodite Electrolyte

Anode-free all solid-state batteries (AF-ASSBs) employ "empty" current collector with three active interfaces that determine electrochemical stability; lithium metal - Solid electrolyte (SE) interphase (SEI-1), lithium - current collector interface, and collector - SE interphase (SEI-2). Argyrodite Li₆PS₅Cl (LPSCl) solid electrolyte (SE) displays SEI-2 containing copper sulfides, formed even at open circuit. Bilayer of 140 nm magnesium/30 nm tungsten (Mg/W-Cu) controls the three interfaces and allows for state-of-the-art electrochemical performance in half-cells and fullcells. AF-ASSB with NMC811 cathode achieves 150 cycles with Coulombic efficiency (CE) above 99.8%. With high mass-loading cathode (8.6 mAh cm^-2), AF-ASSB retains 86.5% capacity after 45 cycles at 0.2C. During electrodeposition of Li, gradient Li-Mg solid solution is formed, which reverses upon electrodissolution. This promotes conformal wetting/dewetting by Li and stabilizes SEI-1 by lowering thermodynamic driving force for SE reduction. Inert refractory W underlayer is required to prevent ongoing formation of SEI-2 that also drives electrochemical degradation. Inert Mo and Nb layers likewise protect Cu from corroding, while Li-alloying layers (Mg, Sn) are less effective due to ongoing volume changes and associated pulverization. Mechanistic explanation for observed Li segregation within alloying Li_xMg layer is provided through mesoscale modelling, considering opposing roles of diffusivity differences and interfacial stresses.