Preparation And Modifications Study On Nano Flake Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ For Lithium-ion Batteries

Owing to high-energy density, good safety and low cost, Li-rich manganese-based solid solution Li_1.2Mn_0.54Co_0.13Ni_0.13O₂（LMSS） is considered as one of the most promising cathode materials for next-generation lithium-ion batteries. However, there are three problems including large irreversible capacity loss in the first cycle, poor rate capability and voltage fade that seriously hamper its widespread application. To improve these problems, optimization of material morphology, surface modification and bulk doping have been adopted in this study. The research contents are as follows:Firstly, we prepared nano-flake Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ by the sol-gel method, in which acetates were used as transition metals source, EDTA as a chelating agent. Surfactant CTAB was added to affect the morphology of the LMSS. The results showed that the as-prepared nano-flake LMSS was composed of nanoparticles, which is conducive to strengthen the interaction between the nanoparticles and shorten the Li+ transport distance. Furthermore, the nano-flake LMSS with a larger specific surface area which facilitates penetrating of the electrolyte, dispalyed an excellent rate capability. At 0.1 C rate, the sample delivered an initial discharge capacity of 258.5 mAh g-1 accompanying with a coulombic efficiency is 71.3% in the first cycle. After 50 cycles, the capacity retention was 80.4%. It can deliver capacities of 232.4, 203.6, 172.8 and 139.3 mAh g-1 at the rates of 0.5 C, 1 C, 2 C and 5 C, respectively. The nanocrystallization of electrode materials can improve their rate performances to some extent, but the initial coulombic efficiency and the cycling performance are not satisfied.Secondly, the as-prepared nano-flake LMSS was modified with hybrid LiV3O8/C layer. The surface-coated sample was investigated by electrochemical measurements including cyclic voltammogram（CV）, electrochemical impedance spectroscopy（EIS） and galvanostatic charge-discharge（GCD）. The results indicated that the LMSS cores were encapsulated by an outer shell, in which Li V3O8 nanoparticles were embedded within carbon matrix uniformly. Owing to the Li-host nature of LiV3O8 and electronic conductivity of carbon, the hybrid surface-coated Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ eletrodes had a high initial coulombic efficiency and an enhanced rate capability. In the first cycle, the composite material delivered a high specific capacities up to 272.9 mAh g-1 with a coulombic efficiency of 92.3% at 0.1 C rate. After 50 cycles, the capacity retention was 91.2% capacity retetion. In addition, it can deliver the specific capacities of 253.5, 220.7, 203.4 and 182.5 mAh g-1 at the rates of 0.5 C, 1 C, 2 C and 5 C, respectively. The results suggested that surface coating of hybrid LiV3O8/C layer can improve not only the initial coulombic efficiency but also the cycling and rate performances of the LMSS material.Finally, the Nb-doped LMSS（i.e. Li1.2Mn0.54-xCo0.13Ni0.13NbxO2（x=0, 0.005, 0.01, 0.02, 0.05）） cathode materials were prepared by sol-gel method. The results showed that the Nb-doped samples exhibited promoted electrochemical performances due to great improvement of the electrochemical activity, lowering delithiation potential and reducing oxygen vacancy of the Li₂MnO₃ component. When the Nb-doping amount x equaled to 0.01, the LMSS-Nb0.01 sample delivered the highest specific capacity and had the best cyclability. In the first charging process, the potential plateau around 3.8 V was prolonged, whereas that above 4.5 V disappeared for this sample. In the first cycle, its discharge capacity reached 288.9 mAh g-1 and the coulombic efficiency was 82.4% at 0.1 C rate. After 100 cycles, the capacity retention was 82.3%. In addition, it can deliver the specific capacities of 266.7, 234.3, 207.8 and 167.4 mAh g-1 at the rates of 0.5 C, 1 C, 2 C and 5 C, respectively. The results indicated that the LMSS doped with appropriate amounts of Nb element possessed stable crystal structure thus revealed a reduced loss of initial irreversible capacity, leading to enhanced rate capability and stable cyclability.