Phonon RIXS signatures reveal polaron formation in oxygen redox Li- and Na-ion battery cathodes

Abstract

Oxygen redox (OR) in Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ (LRNMC) and Na_0.67Mg_0.28Mn_0.72O₂ (NMMO) has been associated with the formation of embedded molecular O₂ due to the appearance of their distinctive features in RIXS, while it is unclear whether OR also affects the oxygen left in the lattice. Here we use high-resolution oxygen K-edge resonant inelastic X-ray scattering (RIXS) at threshold excitation (527.5 – 529.5 eV) revealing lattice responses due to OR. We find that both cathodes show pronounced multiphonon progressions, which are either altered or activated upon charging. The first progression, with a fundamental energy loss ∼67-74 meV, matches the A_1g lattice-oxygen mode observed by Raman spectroscopy, confirming its bulk origin. The second progression that exhibits a fundamental loss of ∼98 meV appears only at the highest state of charge and is resonant with the new pre-edge states at 527.5 eV. The latter mode emerges concurrently with the characteristic trapped-O₂ RIXS signal and is strongly coupled to an occupied electronic band near the Fermi level, indicating that OR may not only contribute to the formation of molecular O₂, but also perturbs lattice oxygen states, likely via polaron-forming oxidized lattice oxygen.