The effect of negative electrode and cyclable lithium on the bulk redox mechanisms of lithium-and manganese-rich positive electrodes

Abstract

Lithium- and manganese-rich (LMR) oxides are of commercial interest offering high energy density at a competitive cost, but their redox behavior on extended cycling remains poorly understood. Most mechanistic understanding have been derived from lithium-metal half-cell studies, which do not faithfully reflect practical full-cell electrochemistry as the cathode potential and cyclable lithium evolves dynamically. Here, we study an exemplar cobalt-free LMR (0.3Li2MnO3●0.7LiMn0.5Ni0.5O2) material using multiple full-cell architectures that span a wide range of lithium inventory retention and definitively show that, under the tested full-cell operating and electrode-harvesting conditions, LMR materials do not undergo a conversion from oxygen to manganese redox upon extended cycling. Across all full-cell configurations – including those that preserve lithium inventory most effectively and maintain access to the low-voltage lithium insertion/removal sites for hundreds of cycles – we find no evidence for the emergence of bulk high-spin trivalent manganese. Instead, manganese reduction is observed only in relithiated half-cell environments, indicating that manganese reduction originates from system level factors, and not from a genuine shift in the cathode redox mechanisms. These results demonstrate that half-cell interpretations do not automatically transfer to full-cells and refute prevailing notions that manganese reduction drives voltage fade and that oxygen redox is fundamentally irreversible.