Operando electrochemical fluorination to achieve Mn⁴⁺/Mn²⁺ double redox in a Li₂MnO₃-like cathode

While the Li-Mn-O cathode is highly attractive owing to the low cost, earth-abundance and low environmental impact of its component elements, its large-scale deployment is severely impeded by its deficient energy density. Herein, we used the electrochemical voltage itself to induce oxygen release (O^2- → O₂) in a Li₂MnO₃-like cathode, electrolyte decomposition, and in operando doping of F into the lattice in the first cycle. The drastic changes in the cathode composition and structure (Li₂MnO₃→Li_1.67Mn^2.73+O_2.1F_0.2) activated a two-electron redox of Mn^4+/2+ with an ultrahigh capacity of 326 mA h g^-1. Both the design strategy of the two-electron redox and the electrochemical fluorination may be valuable for treating other electrodes as well.