Operando resonant soft X-ray emission spectroscopy of the LiMn₂O₄ cathode using an aqueous electrolyte solution

The Mn 3d electronic-structure change of the LiMn₂O₄ cathode during Li-ion extraction/insertion in an aqueous electrolyte solution was studied by operando resonant soft X-ray emission spectroscopy (RXES). The Mn L₃ RXES spectra for the charged state revealed the Mn⁴⁺ state with strong charge-transfer from the O 2p to Mn 3d orbitals dominates, while for the open-circuit-voltage and discharged states it is ascribed to the mixture of sites with Mn³⁺ and Mn⁴⁺ states. The degree of charge transfer is significantly different between the Mn³⁺ and Mn⁴⁺ states, indicating that the redox reaction takes place on the strongly-hybridized Mn 3d-O 2p orbital rather than the localized Mn 3d orbital.