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 FeF2.67O0.33·0.33H2O 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- ↔ O2-) 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.
Keywords: DFT; Li+-ion battery; cationic and anionic electrochemistry; high-capacity electrode material; proton-coupled charge transfer.