Operando Soft X-Ray Absorption Study on Electronic Structure of Lithium-Rich Cathode Materials; Li2MnO3 and Li2RuO3

High capacity cathode materials of lithium ion secondary battery are desirable to apply to the large-scaled energy devices such as electric vehicles or electricity storage systems. Recently, lithium-rich layered materials such as Li2MnO3 and Li2RuO3 are attended as interesting cathode materials because of their a few times larger theoretical capacity than that of LiCoO2 [1].The reaction mechanism of the lithium-rich cathodes has not been fully understood, especially at the high potential. In this study, Li2MnO3 and Li2RuO3 electrodes were prepared, and the electronic structure change of Li2MnO3 and Li2RuO3 were investigated by using operandosoft X-ray absorption spectroscopy under charge-discharge reaction of these electrodes. The cathode materials were synthesized by solid-state reaction method. Li2MnO3 was prepared from a stoichiometric amount of LiOH-H2O and MnO2. They were dispersed in acetone and ground by a ball milling machine for 3 hours at a speed of 400 rpm using 2 mm ZrO2 beads as grinding media. After drying, the mixture powder was pressed into pellets (10 mm diameter.) and heated at 450 °C for 24 hours in air and then calcined at 400 °C for 48 hours. Li2RuO3 was prepared from LiOH-H2O and RuO2in the same way, however, heated at 1000 °C for 15 hours in air and then ground for 3 hours before calcined at 900 °C for an hour. The cathode for operando X-ray absorption measurements was prepared from a paste by mixing 80 wt% of as-prepared cathode active materials, 10 wt% of acetylene black and 10 wt% of polyvinylidene difluoride binder in 1-methyl-2-pyrrolidone solvent. This paste was coated on the platinum-sputtered silicon nitride thin film. Li4Ti5O12 was used as the counter electrode material and 1 mol/L LiPF6 in an acetonitrile solvent was used as an electrolyte. The operando soft X-ray absorption spectroscopy measurement was carried out at BL27SU of SPring-8. The oxygen K-edge XANES spectra for Li2MnO3 and Li2RuO3 during first charge reaction were measured. For the charge reaction of Li2MnO3, the peaks located at 529.5 eV and 532.0 eV were progressively weaken and broadened (Figure 1a). It implies that the main reaction during the charge process is the oxygen evolution because of the small contribution of Mn 3d - O 2p orbital. For Li2RuO3 (Figure 1b), on the other hand, the peaks at 529.5 eV and 532.0 eV were shifted to the lower energy during the delithiated process from x=2.00 to x=1.00 in LixRuO3 and these peaks were sharpen from x=1.00 to x=0.48. In the Ru L3-egde XANES spectra, the peak was shifted to the higher energy from x=2.00 to x=1.00, however not changed from x=1.00 to x=0.48. These results suggest that the Li2RuO3 charge reaction includes two process, (1) the oxidized reaction of Ru4+→Ru5+, (2) O2 charge compensation owing to the hybridization state between Ru 4d and O 2porbital. This work revealed the contribution of the Ru-O hybrid orbital during the charge process directly and was a guide to design the high-capacity cathode materials in the future. Reference [1] M. Sathiya, K. Ramesha, G. Rousse, D. Foix, D. Gonbeau, A. S. Prakash, M. L. Doublet, K. Hemalatha, and J.-M. Tarascon, Chem. Mater. 25(2013), 1121−1131