Research Of Preparation Of High Voltage LiNi_0.5Mn_1.5O₄ And Coating Modification Of Al₂O₃ And La₂O₃

Lithium nickel manganate oxide(LiNi_0.5Mn_1.5O₄),a positive electrode material for lithium-ion batteries,has attracted much attention due to its large theoretical specific capacity(147 mAh g^-1)and high operating voltage（4.7 V）.It hopefully become a new type of cathode material in the field of power batteries.However,its cycle performance and rate performance are not ideal,which limits its large-scale application.The lithium nickel manganese oxide cathode material was prepared by polyvinylpyrrolidone（PVP）-assisted hydrothermal method.The effects of PVP dosage,calcination temperature and ammonium bicrbonate dosage on lithium nickel manganese oxide materials were studied.Compared with the hydrothermal method without PVP,the particle size prepared after the addition of PVP is smaller and the specific surface area is larger.It provides a larger electron exchange surface area at the electrode/electrolyte interface which can improve the rate performance of lithium nickel manganese oxide.Moreover,the larger specific surface area can reduce the stress caused by the Jahn-Teller effect improving the structural stability of lithium nickel manganese oxide and increasing its cycle life.The optimum preparation conditions are a calcination temperature of 800°C,a PVP quantity of 0.2 g and an ammonium bicarbonate dosage that is 5 times the theoretical amount.Lithium nickel manganese oxide material has the best performance in the described conditions.At a large rate of 5C and 10 C,the specific discharge capacities of the material are 120.3 mAh g^-1 and105.9 mAh g^-1,respectively.Capacity retention is 92.8%and a specific discharge capacity of 114.1 mAh g^-1 is obtained after 200 cycles at 1 C.The capacity retention rate is 86.4%,and the specific discharge capacity is 103.0 mAh g^-1 after 300 cycles at 2 C.The surface of the lithium nickel manganese oxide material was modified by a coating of Al₂O₃,La₂O₃ and a mixture of Al₂O₃ and La₂O₃（abbreviated as LAO）.The electrochemical properties of the coated material in the full cell were also briefly studied.Al₂O₃ can protect lithium nickel manganese oxide from corrosion of HF generated by decomposition of LiPF₆,which alleviates the dissolution of Mn.Al₂O₃ can also promote the electrochemical reversibility of lithium nickel manganese oxide.La₂O₃can enhance the conductivity of lithium nickel manganese oxide and facilitates charge transfer.In addition,La₂O₃ has good thermal stability under high temperature,which can improve the interface stability of lithium nickel manganese oxide under high temperature.La₂O₃ can also reduce the formation of a passivation layer.When the coating amount of LAO is 4%,the synergistic effect of both Al₂O₃ and La₂O₃ causes lithium nickel manganese oxide to have excellent cycle and rate performance as well as high temperature stability.Capacity retention is 94.0%after 200 cycles at 1 C and 93.3%after 500 cycles at 5 C.At a higher temperature of 55°C,capacity retention is 95.0%after 100 cycles at 1 C.At large rates of 10 C,15 C and 20 C,the specific discharge capacities are 107.4 mAh g^-1,94.4 mAh g^-1 and 82.7 mAh g^-1 respectively.The main reason for the full battery capacity attenuation of graphite is that the SEI film is formed on the graphite surface,which continuously consumes active lithium ions in the electrolyte,causing serious lithium loss.In addition,the transition metal of the positive electrode material dissolves,and the dissolved manganese migrates to the negative electrode and deposits on the graphite surface.This leads to an increase in impedance and promotes the growth of the SEI film on the surface of the negative electrode.In the lithium titanate full battery,oxidative decomposition of the electrolyte at high voltage with the Ni²⁺/Ni⁴⁺couple is the main reason for capacity decay.