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 O2 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 O2, most of which is trapped in the bulk and the remainder of which evolves from the surface. The model extends the O2 formation and evolution at the surface, which is well known and well characterized, into the electrode particle bulk as caged O2 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.