Distinguishing intrinsic and X-ray beam-induced O2 in battery cathodes

Lithium-rich cathodes offer one of the few routes to increase the energy density of Li-ion batteries. They store charge through oxidation and reduction of the oxide ions in addition to the transition metal ions. The mechanism by which oxide ions store charge (i.e. oxygen redox) remains an important and hotly debated topic in the battery field. Resonant Inelastic X-ray Scattering (RIXS) at the O K-edge has provided evidence that oxidised O2- takes the form of O2 in discrete molecular form trapped within pores inside the transition metal oxide. A recent high-profile study has argued that this O2 is an artefact of the X-ray beam, rather than a direct product of the electrochemical charging process. This is based primarily on observations of the same spectroscopic features from O2 arising in conventional cathodes which do not nominally undergo oxygen redox, i.e. LiNi0.8Co0.1Mn0.1O2 (NMC811).

We are proposing to perform neutron PDF on charged Li-rich NMC and NMC811 to identify the presence or absence of trapped molecular O2. Neutron PDF is bulk average, independent of X-ray beam damage complications and, importantly, has been employed in the past using Polaris, ISIS, to successfully demonstrate the presence of O2 in Li-rich NMC. We aim to provide further independent evidence to reinforce our conclusion that O2 is beam-induced in NMC811 and electrochemically induced in Li-rich NMC. This data addresses a reviewers comment in a recent paper submission of ours to Nature Materials.