When Electrolytes Are Semiconductors: A Feature, Not a Bug for Solid-State Batteries

The development of stable and efficient solid electrolytes is essential for advancing solid-state battery technologies. In this study, we present a comparative study of three sulfide-based electrolytes, Li6PS5Cl (LPSCl), Li6PS5Br (LPSBr), and Li10GeP2S12 (LGPS), combining Density Functional Theory (DFT) and hybrid (HSE06) simulations for electrochemical, charge carrier transport, and structural characterization. DFT and HSE06 simulations revealed semiconductor-like direct band gaps for LPSCl, with a 2.45 eV (DFT) −3.30 eV (HSE06) and 2.32 eV (DFT) −3.34 eV (HSE06) for LPSBr, and indirect band gap with 2.13 eV (DFT) −3.22 eV (HSE06) for LGPS, along with work functions of 3.40 eV for the argyrodites and 3.67 eV for LGPS. Scanning Kelvin Probe (SKP) analyses, performed at both micrometric and nanometric resolution, showed consistently negative surface potentials and interfacial polarons associated with electron tunneling through the surface of the electrolyte. Potentiostatic electrochemical impedance spectroscopy (PEIS) and cyclic voltammetry (CV) confirmed enhanced ionic conductivity with increasing temperature. While LPSCl and LGPS exhibited stable behavior at almost all temperatures, from −20 to 60 °C, LPSBr displayed noise-like activity at 0 °C with Au symmetric electrodes. This integrated experimental/theoretical approach highlights differences in electronic structure, interfacial charge distribution, and electrochemical stability, all showing affinity to react with lithium, providing key insights for the design and optimization of solid electrolytes for next-generation batteries.