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