Preparation And Modification Of Li-rich Layered Oxide Cathode Materials

Considering the urgent need of high energy density lithium ion battery and the limitation of environment and resources in our country,the synthesis and modification of high capacity and low cost lithium-rich manganese-based cathode material Li_1.2Ni_0.2Mn_0.6O₂ were studied in this thesis.The structure and properties of as-prepared materials were analyzed by using TEM,SEM,XRD,DSC and charge and discharge tests etc.methods.The main results are as follows:?1?The spherical Li_1.2Ni_0.2Mn_0.6O₂ material was prepared successfully by coprecipitation method.The effects of precipitant,lithium mixing ratio,calcination temperature and calcination time on the structure,morphology and electrical performances of the materials were studied,and the best preparation conditions were optimized.The precursor of Ni_0.2Mn_0.6?OH?_1.6prepared by using NaOH as precipitant was uniformly mixed with lithium salt at a lithium ratio of 1.05,and then calcined at 850°C for 12 h.The obtained Li_1.2Ni_0.2Mn_0.6O₂ material has a particle size about 5-15?m,a better layer structure and crystallinity,smaller interfacial impedance and better charge and discharge performance.It exhibits discharge capacity of312.2 mAh·g^-1 at 0.05 C rate and coulombic efficiency of 71.2%in the first cycle.Its capacity still retains 94.6%after 0.2 C rate charge-discharge for50 cycles.?2?Al₂O₃ coated Li_1.2Ni_0.2Mn_0.6O₂ materials were prepared successfully by microemulsion method,and the effects of oil-water ratio and coating amount on the structure and electrochemical performances of the materials were studied.The results show that the oil-water ratio and the coating amount mainly affect the uniformity and thickness of the coating layer respectively.When the coating ratio is 1.0%and the oil-water ratio is200:3,the Al₂O₃ coated material can effectively prevent direct contact with the electrolyte,inhibit the increase of interface impedance,and significantly enhance the structurally and thermally properties,as well as cycling and fast charging performances of the material.It exhibits discharge capacity of 337.5 mAh·g^-1 at 0.05 C rate and coulombic efficiency of 82.0%in the first cycle.Its capacity still retains 84.9%after1.0 C rate charge-discharge for 200 cycles.Its thermal reaction temperature in the 4.8 V charge state is increased to 273.06°C,and the heat release is reduced by 72%.?3?P⁵⁺doped,Na⁺and S^2-co-doped Li_1.2Ni_0.2Mn_0.6O₂ materials were prepared successfully by high temperature solid state reaction respectively.P⁵⁺doping can improve the structural stability,increase the lithium layer spacing and the number of lithium holes,and thus significantly improve the rate performance.The capacity of the prepared Li_1.1P_0.02Ni_0.2Mn_0.6O₂material reaches 206.3 mAh·g^-1 at 2 C rate and 173.0 mAh·g^-1 at 5 C rate.Its thermal stability is also significantly improved.However,the spherical particles of Li_1.2-2xNa_2xNi_0.2Mn_0.6O_2-xS_x material prepared by Na⁺and S^2-co-doping have a certain degree of damage.In addition,as the doping amount increases,the lithium layer spacing of the sample decreases first and then increases,and the discharge capacity at different rate also decreases first and then gradually increases.In general,the Na⁺and S^2-co-doping doesn't exert the synergistic effect of the two doping elements,and thus cannot improve remarkably the performance of Li_1.2Ni_0.2Mn_0.6O₂material.