Insights into the Transition from Ionic Insulator to Conductor for Li-Mn-Cl-Based Halospinel

Abstract Spinel-based halide solid electrolytes (HSEs) have recently gained attention as they incorporate cost-effective and earth-abundant central metals, but their low ionic conductivity remains a bottleneck. To address this issue, we introduce aliovalent substitution of Zr4+ into Li2MnCl4 spinel using mechanochemical synthesis, which creates both vacancies and disorder. Zr4+ introduction creates new octahedral and tetrahedral Li sites, along with vacancies, which facilitates long-range Li+ transport through additional face-sharing polyhedrons, leading to a conductivity enhancement by more than four orders of magnitude (∼0.5 mS·cm−1) for Li1.2Mn0.6Zr0.4Cl4. This conductivity further increases with higher Zr substitution; however, Li2ZrCl6 precipitates as an impurity phase. Further improved oxidative stability and compatibility of Li1.2Mn0.6Zr0.4Cl4 with NMC 811 cathodes is verified by in-situ X-ray photoelectron spectroscopy, FIB-SEM, and an ASSB assembled with this solid electrolyte as the catholyte. Overall, this work provides valuable structural insights for the design of cost-effective Li+ ion conductive spinel halides for future solid-state battery applications.