Operando soft x-ray emission spectroscopy of LiMn₂O₄ thin film involving Li-ion extraction/insertion reaction

We developed an electrochemical in situ cell for soft x-ray emission spectroscopy (XES) to accurately investigate the redox reaction and electronic structure of transition metals in the cathode materials for Li-ion battery. The in situ cell consists of a Li-metal counter electrode, an organic electrolyte solution, and a cathode on a membrane window which separates the liquid electrolyte from high vacuum and can pass the incoming and emitted photons. In this study, the Mn 3d electronic structure of LiMn₂O₄ thin-film electrodewas clarified by the operando XES. At the charged state, the XES spectrum changed significantly from the open-circuit-voltage (OCV) state, suggesting oxidation of the Mn³⁺ component through Li-ion extraction. Upon discharge up to 3.0 V vs. Li/Li⁺, the XES spectrum almost returned to its profile at the OCV state with small difference, indicating the valence change of Mn: Mn^3.6+ → Mn⁴⁺ → Mn^3.3+ corresponding to the OCV, charged, and discharged states.