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.
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P.G.B. is indebted to the Engineering and Physical Sciences Research Council (EPSRC), Henry Royce Institute for Advanced Materials (under grant IDs EP/R00661X/1, EP/S019367/1, EP/R010145/1) and the Faraday Institution Next Generation Li-ion Cathodes project CATMAT (under grant ID FIRG016) for financial support.
The authors declare no competing interests.
Peer review information Nature Energy thanks Yong Yang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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House, R.A., Marie, JJ., Pérez-Osorio, M.A. et al. The role of O2 in O-redox cathodes for Li-ion batteries. Nat Energy (2021). https://doi.org/10.1038/s41560-021-00780-2