(Invited) O-Redox Cathodes the Role of Trapped O2

Abstract

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. Understanding the O-redox process and the nature of oxidised oxygen has proved very challenging.Using a range of techniques including XAS, RIXS, STEM, NMR, diffraction and DFT, applied across a wide range of alkali metal rich transition metal oxides, reveals that O2- is oxidised to O2.1-4 The O2 is either evolved from the surface or trapped in voids formed in the bulk by reorganisation of vacancies within the structure. Although O2 can be reduced back to O2-, the process is not energetically reversible, explaining the much lower voltage on discharge compare with the first charge (approx. 1eV less), the phenomenon of so-called voltage hysteresis. Furthermore, it is possible to suppress O2 formation, trapping hole states on O2- and obtaining energetic (voltage) and structural reversibility.4 Such behaviour points the way towards high energy density cathodes for Li-ion batteries. House, R. A. et al. First cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O2 trapped in the bulk. Nature Energy 8, 777–785 (2020).House, R. A. et al. The role of O2 in O-redox cathodes for Li-ion batteries. Nature Energy 1–9 (2021).Boivin, E. et al. The Role of Ni and Co in Suppressing O‐Loss in Li‐Rich Layered Cathodes. Advanced Functional Materials 2003660 (2020).House, R. A. et al. Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes. Nature 577, 502–508 (2020).