Chemical Origins of Electrochemical Overpotential in Surface-Conversion Nanocomposite Cathodes

A new branch of promising nanocomposite cathode materials for rechargeable batteries based on non-intercalation materials has been recently discovered. However, all the nanocomposite cathodes reported thus far suffer from a large overpotential in the first charge, which hinders the activation and lowers the energy efficiency. Here, a series of model nanocomposites consisting of MnO and various metal fluorides (LiF, NaF, KF, RbF, CsF, MgF2, CaF2, and AlF3) to identify the key parameters affecting the activation and overpotential in the first charge are evaluated. It is demonstrated that the F 1s binding energy of the metal fluorides is a plausible indicator of the overpotential in the first charge as well as the subsequent reversible discharge capacity. The stability of the cation in the electrolyte and its solvation nature are also shown to affect the overall activation process. Finally, it is proposed that appropriate tuning of the binding energy of metal fluorides (e.g., by forming solid solutions such as LiCsF2) is a feasible approach to reduce the overpotential and increase the reversible capacity. The findings broaden the current understanding of surface-conversion nanocomposite chemistries, thus providing guidelines for the design of nanomixture cathode materials for rechargeable batteries.