Understanding the Role of Borohydride Doping in Electrochemical Stability of Argyrodite Li 6 PS 5 Cl Solid‐State Electrolyte

Abstract This work elucidates the mechanism by which lithium borohydride (LiBH 4 ) doping into argyrodite‐type Li 6 PS 5 Cl (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 PS 4 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 Li 3 P/LiBH 4 /Li 2 S 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 Li 3 P/Li 2 S is substantially thicker, generating localized stresses that promote interfacial cracking while cycling.