Amorphous iron fluorosulfate as a high-capacity cathode utilizing combined intercalation and conversion reactions with unexpectedly high reversibility

To achieve the desirable dual characteristics of high-capacity performance and low-cost production for the batteries of tomorrow, leveraging of multi-redox reactions of Earth-abundant transition metals in electrodes is fundamentally important. Here we identify an amorphous iron fluorosulfate electrode, a-LiFeSO4F, that can exploit both the intercalation and conversion reactions with a stable reversibility. The a-LiFeSO4F electrode delivers a capacity of 360 mAh g−1 with ~98.6% capacity retention after 200 cycles even at an elevated temperature (60 °C). In contrast to the conventional intercalation/conversion-type electrodes, the reversible cycle stability is attributed to the inherent amorphous structure of a-LiFeSO4F, whose structural integrity is not severely disturbed even after the conversion reaction, allowing its continuation as an intercalation host. We believe that this cycle stability of the intercalation/conversion reaction can be generally extended to various amorphous intercalation materials, offering new insights into the design of high-capacity electrodes through the exploitation of multi-mechanistic lithiation processes.