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 Li_xRuO₃ 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