Induced Structural Disorder Enables High Ionic Conductivity and Capacity in Li2Fe1− x Mn x Cl4 Cathodes

The development of high-energy-density solid-state batteries is aided by the discovery of ionically conductive cathode active materials which can enable the production of cathode composites with high active material loadings. Here, we show that the ionic conductivity of Li2FeCl4 can be more than doubled by increasing the structural disorder via a facile mechanochemical and low-temperature annealing procedure. We find that the metastable Fd3̅m Li2FeCl4 phase produced by mechanochemical synthesis undergoes a room temperature structural re-ordering on the scale of months, which lowers the ionic conductivity and raises concerns for the shelf-life of metastable materials. We demonstrate that the structural disorder can be “locked-in” by introducing Mn2+ into the system without significantly impacting the ionic conductivity. We further elucidate the origin of the phase transitions and the role of enthalpic stabilization on ionic conductivity through density functional theory and machine-learning force field molecular dynamics calculations. These reveal a 3-dimensional Li+ conduction pathway in the disordered structure, which is not present when the material is ordered. By engineering high-conductivity disordered, Fd3̅m, phases of Li2FeCl4 and Li2Fe0.75Mn0.25Cl4, solid-state batteries containing zero catholyte are demonstrated with high voltage plateaus of 3.7 V and capacities of 115 mAh/gCAM and 90 mAh/gCAM, respectively. Considering the high cathode loadings, we demonstrate that the materials Li2FeCl4 and Li2Fe0.75Mn0.25Cl4 can be used to produce solid-state battery cathodes with higher energy densities than that of LiFePO4, with similar cost effectiveness.