What’s in a redox couple? The case of Li-rich oxides in the context of battery cathodes

Li-ion battery technology is central to achieve a sustainable society. However, the energy density of current Li-ion batteries is limited by the relatively low capacity of conventional cathodes, in comparison with anodes. Specifically, current battery electrode materials are limited by their inability to reversibly accommodate redox changes of more than one electron per total transition metal content. The formal activity of transition metal centers is traditionally employed to account for chemical changes during these redox reactions. In oxides, recent studies suggest that bands with a large oxygen character can supply additional charge beyond the amount compensated at transition metal centers. However, this novel reactivity has strained our understanding of redox chemistry in solids, to the point of questioning how formal redox couples are defined.

X-ray absorption spectroscopy (XAS) is the premier tool to define the electronic state of solid compounds. By combining probes of both transition metal and oxygen species, partitioning of the charge compensation can be achieved. In this talk, we revisit fundamental concepts of redox chemistry in a series of oxides with layered rock-salt structure, using 4d and 5d metals as models of O-centered activity. Spectra will be presented upon Li deintercalation in oxides that should theoretically tap into the same redox couples. Contrary to this assumption, we reveal the significant diversity of states that can be achieved. The results challenge our tendency to box oxides into simple classifications of redox transitions and highlight the versatility of the covalent bond to compensate charge.