Enhanced Li-ion conductivity of Li2FeCl4 cathode material by partial Br substitution

Chloride-based Li2FeCl4 has emerged as a promising candidate high-voltage, highly deformable cathode material for all-solid-state lithium-ion batteries. However, further enhancement of Li-ion conductivity is required for practical application. In this study, we synthesized Br-substituted Li2FeCl3.8Br0.2 and examined how partial anion substitution influences both the crystal structure and Li-ion conductivity. X-ray diffraction confirmed that a single-phase cubic framework was retained and that the lattice constant remained unchanged despite Br incorporation. As a result of AC impedance measurements, the ionic conductivity was confirmed to increase approximately twofold, from 2.0 × 10−5 S/cm for the pristine material to 4.0 × 10−5 S/cm for the Br-substituted sample. In an effort to uncover the underlying mechanism of this enhancement, first-principles calculations (Density Functional Theory) combined with genetic algorithm – driven structural optimization were performed. The calculations indicated that Br substitution promoted a more disordered occupancy of Li ions across the sites along the conduction pathways. These results demonstrate that targeted anion substitution effectively tunes the Li-site energy landscape and controls Li-ion conductivity in chloride cathode materials.