Mechanism of Exact Transition between Cationic and Anionic Redox Activities in Cathode Material Li2FeSiO4

The discovery of anion redox activity is promising for boosting the capacity of lithium ion battery (LIB) cathodes. However, fundamental understanding of the mechanisms that trigger the anionic redox is still lacking. Here, using hybrid density functional study combined with experimental soft X-ray absorption spectroscopy (sXAS) measurements, we unambiguously proved that Li (2– x ) FeSiO 4 performs sequent cationic and anionic redox activity through delithiation. Specifically, Fe 2+ is oxidized to Fe 3+ during the first Li ion extraction per formula unit (f.u.), while the second Li ion extraction triggered the oxygen redox exclusively. Cationic and anionic redox result in electron and hole polaron states, respectively, explaining the poor conductivity of Li (2– x ) FeSiO 4 noted by previous experiments. In contrast, other cathode materials in this family exhibit diversity of the redox process. Li 2 MnSiO 4 shows double cationic redox (Mn 2+ –Mn 4+ ) during the whole delithiation, while Li 2 CoSiO 4 shows simultaneous cationic and anionic redox. The present finding not only provides new insights into the oxygen redox activity in polyanionic compounds for rechargeable batteries but also sheds light on the future design of high-capacity rechargeable batteries.