Operando resonant soft X-ray emission spectroscopy of the LiMn 2 O 4 cathode using an aqueous electrolyte solution.

The Mn 3d electronic-structure change of the LiMn 2 O 4 cathode during Li-ion extraction/insertion in an aqueous electrolyte solution was studied by operando resonant soft X-ray emission spectroscopy (RXES). The Mn L 3 RXES spectra for the charged state revealed the Mn 4+ 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 Mn 3+ and Mn 4+ states. The degree of charge transfer is significantly different between the Mn 3+ and Mn 4+ states, indicating that the redox reaction takes place on the strongly-hybridized Mn 3d-O 2p orbital rather than the localized Mn 3d orbital.