Optimization of Calcination Temperature in Preparation of a High Capacity Li-Rich Solid-Solution Li[Li0.2Ni0.18Co0.03Mn0.58]O2 Material and Its Cathode Performance in Lithium Ion Battery
Authors: Nomura Fumihiro , Tanabe Toyokazu , Ohsaka Takeo , Gunji Takao , Matsumoto Futoshi 
Published: February 2020

In recent years, Li-rich solid-solution layered oxide materials (LLOs) comprising layered LiMO2 (M: transition metals) and Li2MnO3 have attracted much interest as a cathode material for lithium ion battery (LIB) because some materials exhibit capacities as high as 250 mAh g-1 in the voltage range of 2.0 and 4.8 V. Because the Li2MnO3 structure can be reformulated with Li[Li1/3Mn2/3]O2, both of Li2MnO3 and LiMO2 can be considered to be of layered α-NaFeO2-type rock salt structure. The structural compatibility between Li2MnO3, which is electrochemically inactive and has a large theoretical capacity, and LiMO2, which is electrochemically active but offers lower capacity, allows for the structural integration of these components at an atomic level. As a result, the electrochemically inactive Li2MnO3 can participate in the charge/discharge process after activation with oxygen release from the lattice during the charging process and the capacity of LiMO2 can be improved because the Li2MnO3 component acts to stabilize the layered structure of LiMO2 when more than 50% of the Li+ ions are deintercalated. These materials are charged to above 4.5 V (vs. Li/Li+) to fully activate the Li2MnO3 component, and after activation, the cathodes are charged to 4.5 V to reach discharge capacities over 250 mAh g-1. In our previous paper, in order to find the optimal composition of the LLOs exhibiting higher cathode performance, which are composed of Li2MnO3, LiCo1/3Ni1/3Mn1/3O2 and LiNi0.5Mn0.5O2, the selected 75 samples having different composition were synthesized under identical preparation conditions except for the composition and the dependence of the performance of the cathode material on the percentages of Li2MnO3, LiCo1/3Ni1/3Mn1/3O2 and LiNi0.5Mn0.5O2 in the LLO samples was examined in the viewpoint of discharge capacity, retention of discharge capacity, average discharge voltage, energy density and rate capability. In each viewpoint, using ternary phase diagrams of the percentages of Li2MnO3, LiCo1/3Ni1/3Mn1/3O2 and LiNi0.5Mn0.5O2 in the LLO samples, the dependence of each performance on the percentages of Li2MnO3, LiCo1/3Ni1/3Mn1/3O2 and LiNi0.5Mn0.5O2 in the LLO samples was evaluated. The results concluded that among the LLOs examined Li[Ni0.208Li0.183Co0.033Mn0.575]O2 (Li2MnO3 (55%) - LiNi1/2Mn1/2O2 (35%) - LiNi1/3Co1/3Mn1/3O2 (10%)) possesses the best composition as cathode material for LIBs. In this case, Li[Ni0.208Li0.183Co0.033Mn0.575]O2 was synthesized only under the calcination condition of 900 ℃ for 12 h in air. Changing the calcination temperature can be expected to lead to a structural change and consequently a change in cathode performance. Therefore, in order to clarify the relationship between calcination temperature and cathode performance, in other words, between the degree of crystallinity of LLOs and cathode performance, the LLO samples were synthesized at different calcination temperatures in the rage of 800-1100℃ and their structural and cathode performance analyses were carried out in detail in this study.