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)}FeSiO₄ performs sequent cationic and anionic redox activity through delithiation. Specifically, Fe²⁺ is oxidized to Fe³⁺ 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)}FeSiO₄ noted by previous experiments. In contrast, other cathode materials in this family exhibit diversity of the redox process. Li₂MnSiO₄ shows double cationic redox (Mn²⁺-Mn⁴⁺) during the whole delithiation, while Li₂CoSiO₄ 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.