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 (O2- → 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→Li1.67Mn2.73+O2.1F0.2) activated a two-electron redox of Mn4+/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.