Charge-discharge mechanism of electrode materials for Li-ion batteries as seen via soft x-ray absorption/emission spectroscopy

Daisuke Asakura, Eiji Hosono, Yusuke Nanba, Hideharu Niwa, Hisao Kiuchi, Jun Miyawaki, Haoshen Zhou, Masaharu Oshima, Yoshihisa Harada
To improve the performance of electrode materials for Li-ion batteries, understanding the charge-discharge properties from a view point of electronic structure, i.e., revealing redox reaction of the transition-metal (TM) in the electrode materials during Li-ion insertion/extraction is highly important. Core-level photoemission spectroscopy and x-ray absorption spectroscopy at the TM K edge have widely been used to study the valence change of the electrode materials during the charge/discharge
more » ... ocess. However, these methods are not so suitable to extract the information of TM 3d orbitals such as crystal-field splitting and charge-transfer (CT) effect. In order to clarify the TM 3d electronic structure in detail, we utilized high-energy-resolution soft x-ray emission spectroscopy (XES) which can element-selectively reveal the TM 3d orbital below the Fermi level. In this study, the Mn 3d electronic structure of LiMn 2 O 4 was investigated. Spinel-type LiMn 2 O 4 is a typical cathode material for Li-ion batteries [1]. The average valence of Mn is expected to be Mn 3.5+ at the initial state. At the charged state (Li-extracted Mn 2 O 4), the Mn should be oxidized from Mn 3.5+ to Mn 4+. LiMn 2 O 4 powder was fabricated by a sol-gel method. The cubic crystal structure was confirmed by x-ray diffraction. The electrochemical experiments were carried out according to Ref. 2. Soft x-ray absorption spectroscopy (XAS) and XES measurements at the Mn L 2,3 edges were performed at BL07LSU in SPring-8. The total electron-yield mode was employed for the XAS. The XES measurements were carried out using an ultra-high-resolution XES spectrometer, HORNET [3]. The energy resolution was set to E/E = 3200. All the XAS and XES measurements were performed at room temperature. Figure 1 shows the Mn L 2,3-edge XAS of LiMn 2 O 4 for the initial, fully-charged, and discharged states. According to previous XAS studies for several Mn compounds and multiplet calculations [4], the multiplet structure of the initial-state spectrum suggests that Mn 3+ and Mn 4+ states coexist. In the L 3 region, the peaks at 640 and 642 eV were ascribed to the Mn 3+ state and those at 640.8 and 643 eV were attributed to the Mn 4+ state. At the charged state, the Mn 3+ component decreased and the Mn 4+ character was enhanced, suggesting the oxidation of Mn by charging. Fig. 1: Mn L 2,3-edge XAS for LiMn 2 O 4 during charge-discharge.