Alkaline-earth metal substitution stabilizes the anionic redox of Li-rich oxides

Abstract

Anionic oxygen redox chemistry in Li-excess transition metal oxides has emerged as a new paradigm to increase the energy density of rechargeable batteries. However, the anionic redox of oxygen mostly forms the O–O bond, leading to partly irreversible discharge. To address this issue, we demonstrate a new strategy to achieve dual functions of activating cation activity and enhancing oxygen anion redox activity in Li₂MnO₃ through a light metal (Mg, Ca and Al) substitution of Li. Our first-principle thermodynamic calculations show that Mg²⁺ substitution in Li₂MnO₃ prefers locating at the Li⁺ site, which induces the formation of reductive Mn-ions with electrochemical activity. The substituted local structure of (Li–O–Mg)/2(Li–O–Mn) becomes more electrochemically stable than the specific Li–O–Li local structure in Li₂MnO₃, which effectively retards O–O combination. Therefore, the reversible capacity (245 mA h g⁻¹) of the optimal substituent compound Li_1.5Mg_0.5MnO₃ is significantly improved compared with that (115 mA h g⁻¹) of Li₂MnO₃. More importantly, this substitution strategy can be effectively extended to other Li-rich electrodes such as Li₃VO₄ and Li₃NbO₄ with Mg-substitution activating cationic redox. The study opens a new avenue in improving the reversible capacity of Li-rich cathode materials through low-cost element substitution.