Synergistic Al3+ and F− Co-doping boosts ionic conductivity of Li2MnCl4 for all-solid-state batteries

Manganese-based halides like Li 2 MnCl 4 are attractive for their cost-effectiveness and inverse spinel structure with ion channels. Their use as solid-state electrolytes is limited by low ionic conductivity at room-temperature and poor stability at the lithium interface. Current strategies focus on single-cation doping, which often offers limited conductivity improvements or causes side effects. This study proposes a dual-site cation-anion co-doping strategy to modify lattice defects and Li + migration pathways, thereby enhancing interfacial stability with metallic lithium. Lattice distortion is greatly induced, and Li + vacancy concentration is optimized in Li 1.7 Al 0.1 MnCl 4- x F x ( x  = 0–0.7) by substituting Al 3+ at octahedral sites and partially replacing Cl − with F − . Structural and electrochemical tests show that moderate co-doping maintains the cubic structure and greatly improves Li + transport. The best Li 1.7 Al 0.1 MnCl 3.9 F 0.1 exhibits a room-temperature ionic conductivity of 5.2 × 10 −5  S cm −1 , nearly ten times higher than pristine Li 2 MnCl 4 , with a reduced activation energy of 0.45 eV. The distribution of relaxation time analysis further reveals that interface-related relaxation processes remain relatively stable over long time scales. Stable lithium stripping/deposition was demonstrated for hundreds of hours at 0.1 mA cm −2 in lithium-symmetric batteries. However, polarization during long cycling indicates challenges with interfacial stability. This work underscores the impact of dual-site co-doping in developing low-cost, high-performance manganese-based halide solid-state electrolytes.