Operando electrochemical fluorination to achieve Mn 4+ /Mn 2+ double redox in a Li 2 MnO 3 -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- → O 2 ) in a Li 2 MnO 3 -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 (Li 2 MnO 3 →Li 1.67 Mn 2.73+ O 2.1 F 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.