An operando soft x-ray emission study of LiMn 2 O 4 with an aqueous electrolyte solution

Daisuke Asakura, Eiji Hosono, Hideharu Niwa, Hisao Kiuchi, Jun Miyawaki, Yusuke Nanba, Haoshen Zhou, Masaharu Oshima, Yoshihisa Harada
Improvement of the energy density and power density of electrode materials for Li-ion batteries (LIBs) are highly important to further develop electric and hybrid-electric vehicles. In order to improve the performances, understanding the charge-discharge mechanisms of the electrode materials from a viewpoint of the electronic structure is indispensable. Soft x-ray spectroscopy, which directly reveals the 3d orbital of transition metals, has recently attracted much attention for the
more » ... or the electronic-structure analyses of the electrode materials. In this study we demonstrate operando soft x-ray emission spectroscopy (XES) for LiMn 2 O 4 with an aqueous electrolyte solution. LiMn 2 O 4 is a typical cathode material for LIB 1 and works as a cathode even for an aqueous electrolyte solution 2 as well as organic electrolyte solutions used for general LIB. The high-rate charge-discharge property for LiMn 2 O 4 with an aqueous electrolyte solution is promising for large-scale energy storage, while a high voltage cannot be obtained for the aqueous LIBs because of the narrow voltage window for H 2 O. We developed an in situ cell consisting of the LiMn 2 O 4 cathode, a Pt-wire counter electrode, Ag/AgCl reference electrode, and a 1 M LiNO 3 /H 2 O electrolyte solution by modifying the in situ cell for fuel cell catalysts 3. The operando XES experiments were carried out using ultrahigh-resolution XES spectrometer 4 at BL07LSU of SPring-8. We selected 642 eV as the excitation energy in which the Mn 3+ character should be enhanced for the Mn L 3-edge absorption spectrum. The charge-discharge experiments were performed by cyclic voltammetry. The operando XES measurements were performed for the second charge-discharge cycle. The Mn 2p-3d-2p resonant XES spectra revealed a clear difference between the pristine LiMn 2 O 4 powder and open-circuit-voltage (OCV) state (i.e. before the second cycle) (Fig. 1). For the powder sample, the charge-transfer (CT) excitation below 637 eV is large and dd-excitation peaks appear from 637 to 641 eV. The configuration-interaction full-multiplet (CIFM) calculation 5,6 (not shown) suggests Fig. 1: Operando Mn 2p-3d-2p XES spectra for LiMn 2 O 4 .