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

Electrochemical stability of anode-free all solid-state battery (AF-ASSB) based on argyrodite Li₆PS₅Cl (LPSCl) requires control of two distinct solid electrolyte interphases, SEI-1 and SEI-2. With an "empty" current collector used in AF-ASSB there are three active interfaces; lithium metal - SE interphase (SEI-1), lithium - current collector interface, and collector - SE interphase (SEI-2) where copper sulfides are formed even without external current. Bilayer of 140nm magnesium / 30nm tungsten (Mg/W-Cu) controls these and allows for state-of-the-art electrochemical performance in half-cells and full cells. 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, which also drives electrochemical degradation. Inert Mo and Nb layers likewise protect Cu, while Li-active layers (Mg, Sn) do not due to pulverization. Mechanistic explanation for observed Li segregation within alloying Li_xMg layer is provided through mesoscale modelling, considering diffusivity and stress.