Electronic-Structure Analyses of Li-Ion-Battery Electrodes By Soft X-Ray Absorption/Emission Spectroscopy Including Operando experiments

Abstract

Improving the energy density and power density of electrode materials for Li-ion batteries (LIBs) is highly important to further develop electric and hybrid-electric vehicles. For the improvements, understanding the charge-discharge mechanisms from a viewpoint of the electronic structure is indispensable. Recently, electronic-structure analyses of the electrode materials using soft x-ray spectroscopy have been of particular importance.In this study we demonstrate operando Mn L 3 soft x-ray emission spectroscopy (XES) for LiMn2O4 [1] which is a typical cathode material of LIB. We developed an operando cell consisting of the LiMn2O4 cathode, a counter electrode and an electrolyte solution [2] by modifying the in situ cell for fuel cell catalysts [3]. The charge-discharge experiments were performed by cyclic voltammetry. The operando XES experiments were carried out using ultrahigh-resolution x-ray emission spectrometer [4] at BL07LSU of SPring-8. The XES spectra were analyzed by theoretical analyses based on the configuration-interaction full-multiplet (CIFM) calculation [5-7].In the operando Mn L 3 XES study for LiMn2O4, we revealed that the open-circuit voltage (OCV) state is almost the same as the initial state consisting of the Mn3+ and Mn4+ states. For the charged state, the Mn L 3 XES spectrum largely changed corresponding to the oxidation of Mn3+ to Mn4+ state at the Mn3+ site for the OCV. The spectrum for the discharged state almost returned to the spectrum for the OCV, while the small difference indicates that the Mn3+ state is slightly enhanced for the discharged state. Thus, the Mn 3d electronic states were reversibly changed for the charge-discharge process [2]. Moreover, charge-transfer (CT) effects between the Mn 3d and O 2p orbitals for each valence state were clarified. For the Mn4+ state, a negative CT energy was determined by the CIFM calculation, suggesting a very strong CT effect from the O 2p to Mn 3d orbitals. In the presentation, XES and soft X-ray absorption spectroscopy studies for other electrode materials will also be reported, and the relationship between the electronic structure and electrochemical performance will be discussed.