Synthesis And Modification Of LiNi_0.5Co_0.2Mn_0.3O₂ As Cathode Material For Lithium-ion Batteries

As a new generation secondary batteries, lithium-ion batterieshave been widely applied in many fields such as portable electronics, automobiles, stationary energy storage and aerospace engineeringfor their high energy density, low self-discharge rate, long cycle-life, memory-effect free, high output voltage and low toxicity.Currently, commercialized cathode materials of lithium-ion batteries include LiCoO₂,LiMn2O₄ and LiFePO₄ etc. As the application field of Lithium-ion battery getting wider, the requirements of its energy density of cathode material, voltage range, rate performance and cost of production have been raised. Based on the layered structures of LiCoO₂,LiMnO₂,LiNiO₂, the LiNi_xCo_yMn_zO₂ cathode materials with lower material cost and theoretical capacity of 278 mAh?g-1, display superior electrochemical performances and were considered as a new promising generation cathode materials for lithium-ion batteries.In this thesis, LiNi_0.5Co_0.2Mn_0.3O₂ cathode materials were selectedfor the purposes of obtainingbetter optimized conditions for manufacturing and better electrochemical performance. LiNi_0.5Co_0.2Mn_0.3O₂ cathode materials weremodified by Li₃PO₄ electrolyte through two methods, and the electrochemical performance of the modified materialsachieveda betterperformance. As a result, this paper proposes a new idea and method for the commercialized application of the LiNi_xCo_yMn_zO₂ cathode materials.At first, the precursor of Ni_0.5Co_0.2Mn_0.3（OH）₂ was synthesized via hydroxide co-precipitation method and characterized by XRD, SEM,TG/DTA and particle size distribution.The optimum proceeding are as follow: the reaction was conducted under 50℃ with stirring speed of 2000 rpm, and the amount of surfactant was 0.8 wt%, the pH value of the end point was 11.6. The obtained Ni_0.5Co_0.2Mn_0.3（OH）₂ powders are spheroidal particles and have a narrow and uniform distribution of particle size with D50 = 1.466 μm.Then, Ni_0.5Co_0.2Mn_0.3（OH）₂was mixed with lithium salt(Li /(Ni_0.5Co_0.2Mn_0.3)=1.03/ 1) and calcinated at 550℃ for 5 hours and 900℃ for 10 hours. The structural characteristicsand electrochemical performances of the LiNi_0.5Co_0.2Mn_0.3O₂ powders were measured by XRD, SEM, ICP-OES, TG-DTA, particle size analysisand electrochemical test methods. The initial discharge capacity of the LiNi_0.5Co_0.2Mn_0.3O₂ cathode materials was 181.3 mAh?g-1 in the voltage range of 2.2 4.6V at 0.1C rate. After 30 cycles, the capacity retention ratio was 67.5%.Lithium phosphate was used as electrolyte additives to improve the electrochemical performance of the batteriesconsidering its good conductivity. The results showed that the cells with 2 wt% lithium phosphate in 1 mol?L-1 LiPF6/（EC+DMC+MC）（1: 1: 1 vol.） electrolyte exhibited good performance. The initial discharge capacity was 194.8 mAh?g-1 in the voltage range of 2.2 4.6 V at 0.1 C rate. After 30 cycles, the discharge capacity retained 166.2 mAh?g-1 and discharge capacity retention ratio was 85.3 %.The Li₃PO₄-doped Li(Ni_0.5Co_0.2Mn_0.3)O₂ powders were synthesized via rheological phase method. The results showed that the materials doping with 1 wt% Li₃PO₄ obtained good performance. The initial discharge capacity was 188.6 mAh?g-1 in 2.2 4.6 V voltage range at 0.1 C rate and the capacity remained 92.9 % after 30 cycles.