The Origin of the Oxygen Redox Activity in Layered and Cation-Disordered Li-Excess Cathode Materials

Abstract

With increasing complexity of technology comes a demand for higher-energy density Li-ion batteries, which requires high-energy density cathode materials. Reversible oxygen redox in Li-excess materials can substantially expand the search space of high capacity and energy density cathodes, because it can deliver excess capacity beyond the theoretical transition metal (TM) redox capacity, reducing the necessity of heavy and expensive TM ions.[1-4] Nevertheless, the structural and electronic origin of the oxygen redox process is not understood, preventing the rational design of better cathode materials with oxygen redox. In this talk, we explain how specific chemical and structural features in layered and cation-disordered Li-excess cathode materials introduce labile oxygen electrons that can be easily extracted and participate in the practical capacity of these materials. Our ab initio calculations demonstrate that Li excess and cation disorder create unhybridized oxygen 2p states in lithium-metal-oxide cathodes that promote oxygen redox, which can create extra capacity beyond the TM redox capacity and lead to peroxo-like species when local distortion in the chemical bond is allowed (Figure 1).[5] Furthermore, we explain how this specific oxygen redox process competes with the TM redox, providing clear guidelines for the design of high-capacity cathodes with optimized TM or O redox.