Directing Ion Transport and Interfacial Chemistry in Pnictogen-Substituted Thio-LISICONs

Aliovalent substitution is a ubiquitous strategy to increase ionic conductivity in solid-state electrolytes, often by many orders of magnitude. However, the structural and compositional changes that occur upon aliovalent substitution are highly interrelated, and a deep understanding of how substitutions simultaneously impact ion transport and the chemical evolution of interfaces during electrochemical cycling remain as prevailing challenges. Here, we interrogate aliovalent pnictogen substitution of Li₄GeS₄ in the series Li_3.7Ge_0.7Pn_0.3S₄ (Pn = P, As, Sb) and unravel the impact on ion transport processes and degradation during electrochemical cycling. High-resolution powder X-ray diffraction and pair distribution function analysis reveal that all substituted compounds exhibit an anisometric distortion of the Li₄GeS₄ structure. Temperature-dependent potentiostatic electrochemical impedance spectroscopy reveals that aliovalent substitution increases the room-temperature lithium ionic conductivity by 2 orders of magnitude. Curiously, aliovalent substitution results in a simultaneous increase in the Arrhenius prefactor and decrease in the activation barrier, which contribute to the significant increase in lithium-ion conductivity. We attribute this apparent violation of the "Meyer-Neldel" entropy-enthalpy compensation to the introduction of Li⁺ vacancies that elicit a redistribution of the lithium substructure. Electrochemical stability and cycling performance were interrogated by critical current density tests on symmetric cells with Li electrodes coupled with virtual electrode X-ray photoelectron spectroscopy measurements. In all substituted compounds, we observe the growth of electronically conductive phases that result in continual growth of the solid electrolyte interphase and increase in interfacial impedance during electrochemical cycling. We find that electrochemical instability against Li⁰ is predominantly driven by reduced Ge species. Taken together, our study presents holistic insights into the structural and compositional factors that drive ionic conductivity and electrochemical degradation in lithium metal sulfide solid-state electrolytes.