Stabilizing Low-Coordinated O Ions To Operate Cationic and Anionic Redox Chemistry of Li-Ion Battery Materials

Conventional electrochemical processes are mainly operated by cationic redox chemistry. Developing cumulative cationic and anionic redox chemistry offers a transformative approach to increase the energy storage capacity of Li-ion batteries and active sites of catalysts. However, realizing the reversible anionic redox reaction to increase the specific capacity in Li-ion battery materials is a large challenge because uncontrollable anion-anion combination and gas evolutions cause poor cyclic performance. Here, we use open-framework metal-fluorides (FeF₃·0.33H₂O) to demonstrate cumulative cationic and anionic redox reactions to be realized through O substitution. Experimental studies verified that O substitution could form reductive O ions, and stabilizing this reductive low-coordinated O by p-d orbital hybridization and hydrogen-transfer-mediated O-H bond formation plays an important role in operating anionic electrochemistry. O substitution also exhibits an improved cyclic performance beyond the insertion-reaction capacity (150 mA h/g) of FeF₃·0.33H₂O (225 and 300 mA h/g). Theoretical calculations show that FeF_2.67O_0.33·0.33H₂O exhibits a 50% higher insertion-reaction capacity (225 mA h/g) than FeF₃·0.33H₂O (150 mA h/g) before structural collapse, which is attributed to cumulative cationic (Fe³⁺ ↔ Fe²⁺) and anionic (O^- ↔ O^2-) redox reactions based on our electronic structure analysis. The present study opens a new avenue to develop cationic and anionic electrochemistry to improve the storage capacity and cyclic performance through stabilizing low-coordinated O ions.