Abstract
We developed operando soft X-ray emission spectroscopy (XES) techniques to accurately investigate the redox reactions and electronic structures of cathode materials of lithium-ion batteries. The operando cell consists of a special electrode tip with Si3N4 membrane window, Li-metal counter electrode and electrolyte solution. Operando Mn 2p XES for LiMn2O4 thin film with electrochemical charge/discharge reactions was performed using ultrahigh-resolution XES spectrometer at BL07LSU of SPring-8. The XES spectra showed a reversible Mn 3d electronic-structure change during the charge-discharge process. A redox reaction of Mn3+ ⇔ Mn4+ by charge/discharge was revealed. We also found a strong charge-transfer effect between the Mn 3d and O 2p orbitals for the Mn4+ state. The change of the charge-transfer effect between the Mn3+ and Mn4+ states suggests the Mn-O bond strength should change during charge-discharge process, which would be related to degradation of the cycle performance.
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