Solid State NMR Studies of Li2MnO3and Li-Rich Cathode Materials: Proton Insertion, Local Structure, and Voltage Fade

Abstract

High energy densities of lithium-rich transition metal oxides cannot be sufficiently maintained on cycling due to high-voltage first-cycle activation and the subsequent structural changes. These changes can be seen as a continuous decrease of the average voltage with cycling, known as voltage fade. Electrochemical and chemical insertion of protons has been reported suggesting that protons generated in the electrolyte could be involved in electrochemical cycling which could play a similar role in the “Li₂MnO₃ component” of lithium rich transition metal oxides. Here, electrochemical insertion of structural protons, changes in lithium occupancy at various states of charge, and changes in local structure have been investigated via a combination of local probes including solid state NMR, X-ray absorption spectroscopy and first principle calculations. While significant evidence is found for the deposition of non-structural proton-bearing species on electrodes, which accumulate with extensive cycling, structural proton insertion is not found to be a significant process directly effecting voltage fade. The electrochemical activity of disordered Li₂MnO₃, synthesized at low temperature, is also investigated and its Li removal/insertion properties measured quantitatively with NMR. Major reordering of Li sites and subsequent local structural transitions are observed by NMR and are found to be synchronous with voltage fade.