Lithium-Rich Layered Cathode Materials Containing 3d and 4d Transition Metals for Lithium Ion Battery
Authors: Watanabe Aruto , Orikasa Yuki , Nakanishi Koji , Mori Takuya , Yamamoto Kentaro , Nishijima Motoaki , Hashimoto Kenji , Kinoshita Hajime , Uchimoto Yoshiharu 
Published: February 2020

Layered rock salt type structure such as LiCoO2 and Li(Mn,Co,Ni)O2 have been used as the cathode materials for lithium ion battery. However, the practical capacity is not enough to require the future demand for the large-scaled energy devices such as electric vehicles or electricity storage systems. The high capacity cathode materials should be developed for the next generation battery system. Layered lithium-rich oxides (xLi2MnO3-(1-x)Li(Mn,Co,Ni)O2) are one of promising candidates and the many researchers have studied them [1-4]. For the practical applications, the irreversible reaction in the initial charge-discharge, the capacity fade and the voltage decrease should be addressed. Recently, the lithium-rich compounds containing 4d transition metals were reported as the interesting cathode materials [5]. In this study, layered lithium-rich compounds containing 3d and 4d transition metals were synthesized and the reaction mechanism was investigated. These knowledges will be useful to develop the next-generation cathode materials.

The cathode materials were synthesized by the polymerized complex method. Metal sources were dissolved into nitric acid and citric acid and ethylene glycol were added into this solution with stirring and heated at 200 °C. The obtained polymer was calcined at 400°C for 3 hours. The precursor was pressed into pellets (10 mm diameter) and heated at 850°C for 48 hours. The obtained powders were characterized by X-ray diffraction and scanning electron microscopy. For the charge-discharge measurements, the prepared powder, acetylene black and polyvinylidene difluoride binder were mixed at a weight ratio of 8:1:1 with 1-methyl-2-pyrrolidone solvent. The slurry was coated on the Al foil and dried in a vacuum oven at 80°C. The electrodes were roll-pressed and used as the working electrode. Li foil was used as the counter electrode material and 1 mol/L LiPF6 in a 3:7 volume ratio of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was used as the electrolyte. The two-electrode cells were assembled in an argon-filled glove box.

X-ray diffraction study showed the prepared samples exhibit the layered rock salt structure without distinct impurities. Scanning electron microscopy showed the particle size of the powder was sub-micrometer. The charge-discharge capacity of this material was about 200 mAh/g. The charge-discharge profile represented a gradual slope and low polarization compared with the case of xLi2MnO3-(1-x)Li(Mn,Co,Ni)O2. Moreover, the irreversible reaction at the high potential caused by oxygen evolution did not occur in the first cycle and showed good cycle performance. This result suggested that lithium-rich layered oxides containing 3d and 4d transition metals have one of promising cathode materials. We will discuss the reaction mechanism of this system in the presentation.

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