Toward Optimization of the Chemical/Electrochemical Compatibility of Halide Solid Electrolytes in All-Solid-State Batteries

All-solid-state batteries (ASSBs) that rely on the use of solid electrolytes (SEs) with high ionic conductivity are the holy grail for future battery technology, since it could enable both greater energy density and safety. However, practical application of ASSBs is still being plagued by difficulties in mastering the SE–electrode interphases. This calls for a wide exploration of electrolyte candidates, among which halide-based Li + conductors show promise despite being not stable against Li or Li x In y negative electrodes, hence the need to assemble cells with a dual SE design. In the work described herein, we studied the electrochemical/chemical compatibility of Li 3 InCl 6 against layered oxide positive electrode (LiNi 0.6 Mn 0.2 Co 0.2 O 2, NMC622), carbon additive, and Li 6 PS 5 Cl under both cycling and aging conditions. Combining electroanalytical and spectroscopic techniques, we provide evidence for the onset of electrochemically driven parasitic decomposition reactions between Li 3 InCl 6 and NMC622/carbon at lower potentials (3.3 V vs LiIn/In) than theoretically predicted in the literature. Moreover, to combat chemical incompatibility between dual SEs, we propose a new strategy that consists of depositing a nanometer-thick (1 or 2 nm) surface protective layer of Li 3 PO 4 made by atomic layer deposition between Li 3 InCl 6 and Li 6 PS 5 Cl. Through this surface engineering process with highly conformal and pinhole-free thin films, halide-based solid-state cells showing spectacular capacity retention over 400 cycles were successfully assembled. Altogether, these findings position halide SEs as serious contenders for the development of ASSBs.