Preparation And Modification Of LiNi_1-x-yCo_xMn_yO₂ Cathode Materials For Lithium-ion Batteries And Their Electrochemical Performances

With the diversified and portable development of consumer electronics, especially to guarantee the high endurance mileage for new energy vehicles, rechargeable lithium-ion batteries have been challenging the new requirements of higher energy density and longer cycle life. Because of the highlighted advantages such as large specific capacity and superior cycling stability, it is of great significance to the microstructure design and the modification for LiNi1-x-yCoxMnyO2 (x+y≤0.5) cathode materials with better electrochemical performances for accommodating the increasingly diverse ranges of applications. In this paper, the main points are summarized as follows:1. A stepwise co-precipitation route in which precipitate feedstock with different ratios of Ni, Mn elements in three steps has been successfully adopted to synthesize the precursor of Ni0.5Co0.2Mn0.3(OH)2,after mixing with Li2CO3 and post-heat treatment, the LiNi0.5Co0.2Mn0.3O2 spherical cathode materials with a gradient element distribution (nickel-rich in centre and manganese-rich around surface) from the centre to surface have been prepared. The growth mechanism and influence factors of Ni0.5Co0.2Mn0.3(OH)2 crystals have been explored refinedly. The results indicate that the as-prepared precursor and product have well-distributed spherical morphology with the size of 8-10 μm when reaction time t=12 h,pH=10.5, stirring speed is 1000 rpm, and chelating agent concentration is 3:4. The obtained gradient concentration sample exhibits high rate capability, good cycle performance and efficient fast charge and discharge performance. It delivers initial specific discharge capacity of 166.3 mAh g-1 at 0.2 C rate, even at 20 C rate, the specific discharge capacity reach up to 104.1 mAh g-1, and with capacity retention of 95.8% after 200 cycles at 0.5 C.Furthermore, at the fast charge and discharge rate of 20 C/20 C, it possesses discharge capacity of 85.4 mAh g-1 and the capacity decline only to 16.1% at 5 C/5 C over 500 cycles. The enhanced electrochemical performances of the as-prepared sample are attributed to its unique gradient concentration structured architecture, nickel-poor in outer layer favors for alleviating side reaction between the high reactivity of Ni4+ and electrolyte, impeding the collapse of the structure during charge and discharge. The diffusion pathway formed in transport of Ni and Mn during the high-temperature calcination, efficiently promote the diffusion coefficient of lithium-ion between the centre and the edge of electrode materials.2. The facile and general impregnation strategy which based on precipitation conversion in ethanol-water solution (the volume ratio of water/ethanol is 0:1,1:1 or 1:0) has been presented to coat Al(OH)3 layer for the Ni0.8Co0.1Mn0.1(OH)2 precursor,which was pre-fabricated via co-precipitation method, and followed by mixing with Li2CO3 and post-annealing treatment to fabricate the LiNi0.8Co0.1Mn0.1O2@Al2O3 cathode materials. The electrochemical properties and synthesis mechanism of the materials of preparing in different solution systems and different concentration of Al3+have been studied particularly. The electrochemical results suggest that Al2O3 coated materials in ethanol solution exhibit improved rate capability and cycle stability, and the as-prepared materials in water solution show distinctly higher initial coulombic efficiency and specific discharge capacity, while the ethanol-water solution system could synergize the advantages of ethanol solution with water solution. Specifically,the as-prepared sample, which was prepared in ethanol-water solution with Al(NO3)3 concentration at 0.2 M, delivers initial specific charge and discharge capacity of 225.2 and 185.9 mAh g-1 respectively at 0.2 C with a increased initial coulombic efficiency of 82.5% and the specific discharge capacity still respectively remain 167.2 and 133.2 mAh g-1 at 2 C and 20 C, and with capacity retention of 89.0% and 82.6% at 0.5 C after 200 and 300 cycles, respectively. Compared with the pristine sample, the Al2O3 layer contributes to segregating the electrolyte and protecting the active component inside, enhancing the structural stability of the electrode materials. Moreover, the Al2O3 layer is availably conducive to decreasing the residual alkali in the surface of electrode, and improving the electrochemical performance.