Understanding the Role of Borohydride Doping in Electrochemical Stability of Argyrodite Li6PS5Cl Solid-State Electrolyte.

This work elucidates the mechanism by which lithium borohydride (LiBH4) doping into argyrodite-type Li6PS5Cl (LBH-LPSCl) solid-state electrolyte (SSE) enhances electrochemical stability. State-of-the-art electrochemical performance is achieved with 5 wt% borohydride. Symmetric cells achieve critical current density (CCD) of 7.3 mA cm-2, versus 2.6 mA cm-2 for baseline-LPSCl. All solid-state batteries (ASSBs) employing lithium metal and NMC811 cathode are stable over 400 cycles at 0.5C, with capacity retention of 83%. An anode-free ASSB (AF-ASSB) is stable over 600 cycles, with capacity loss of 0.04% per cycle. 5LBH-LPSCl allows for enhanced low temperature operation, down to -14 °C. Yet the difference in electrolytes' bulk microstructures and hardnesses are minimal, while ionic conductivity is incrementally improved (≈50%). Theoretical modeling indicates limited effect of substitution on thermodynamic stability of PS4 3- units, which decompose when contacting Li. Instead, enhanced electrochemical stability is site-specific kinetic effect: In situ electrodeposition experiments using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) reveal tri-layer SEI based predominately on Li3P/LiBH4/Li2S that blocks electrons while facilitating ion transport. This SEI manifests reduced interface resistance and accelerated nucleation and growth of metallic Li. With baseline-LPSCl the SEI based on Li3P/Li2S is substantially thicker, generating localized stresses that promote interfacial cracking while cycling.