The role of O2 in O-redox cathodes for Li-ion batteries

The energy density of Li-ion batteries can be improved by storing charge at high voltages through the oxidation of oxide ions in the cathode material. However, oxidation of O2- triggers irreversible structural rearrangements in the bulk and an associated loss of the high voltage plateau, which is replaced by a lower discharge voltage, and a loss of O-2 accompanied by densification at the surface. Here we consider various models for oxygen redox that are proposed in the literature and then describe a single unified model involving O2- oxidation to form O-2, most of which is trapped in the bulk and the remainder of which evolves from the surface. The model extends the O-2 formation and evolution at the surface, which is well known and well characterized, into the electrode particle bulk as caged O-2 that can be reversibly reduced and oxidized. This converged understanding enables us to propose practical strategies to avoid oxygen-redox-induced instability and provide potential routes towards more reversible, high energy density Li-ion cathodes. Oxygen redox in Li-rich oxide cathodes is of both fundamental and practical interest in Li-ion battery development. Bruce and team examine the current understanding of oxygen-redox processes, especially those concerning O-2 formation, and discuss strategies that can harness oxygen redox with suppressed side effects.