First-Principles Prediction of Electrochemical Electron-Anion Exchange: Ion Insertion without Redox

It is widely assumed that the gain or loss of electrons in a material must be accompanied by its reduction or oxidation. Here, we report a system in which the insertion/deinsertion of an electron occurs without any reduction or oxidation. Using first-principles methods, we demonstrate this effect in the Y₂CF₂-[Y₂C]²⁺(e^-)₂ material system, where (e^-) indicates a lattice site containing a bare electron. We present a model in which Y₂CF₂ is in contact with a fluoride-containing electrolyte and the application of a positive voltage drives fluorination while a negative voltage reverses the process. We show that this chemistry does not change the oxidation states of the host lattice, causes no significant volume expansion, and occurs rapidly at room temperature. Finally, we demonstrate that this mechanism of ion insertion may enable a broad class of anion shuttle battery electrodes, some with gravimetric capacities nearly double those employed in intercalation-type Li-ion batteries.