Research On Synthesis And Electrochemical Performance Of Cathode Materials For Lithium Ion Battery

Spinel LiMn₂O₄ and layered LiNi_1/3Co_1/3Mn_1/3O₂ are promising cathode materials for lithium-ion batteries due to their advantages,such as high working voltage,high safety,low toxicity and low cost.But the low initial specific capacity and poor cycling performance prevent spinel LiMn₂O₄ from commercial application.Layered LiNi_1/3Co_1/3Mn_1/3O₂ still has the problem of rapid capacity reduction during cycling.In this dissertation,the cathode materials of spinel LiMn₂O₄ were synthesized by sol-gel method,multiple doping was used to improve the stability of spinel structure and cycling performance.The layered LiNi_1/3Co_1/3Mn_1/3O₂ were synthesized by different methods,the influence of structure and electrochemical properties were investigated.The charge-discharge cycling in high voltage region was researched.Studies of cation doping modification of spinel LiMn₂O₄ were conducted using Li⁺ and Cr³⁺.The cathode materials Li_1.05Cr_xMn_1.95-xO₄(x=0, 0.05, 0.10, 0.15)were synthesized by sol-gel method.The sintering temperatures were 650℃,700℃and 750℃respectively.The synthesized materials were characterized with X-ray diffraction (XRD) and scanning electron microscope (SEM).The electrochemical performances of the materials were tested by constant-current cycling in the room temperature(25℃)and the high temperature(55℃)for 40 cycles.The results showed that the samples possessed pure spinel phase,good morphology and uniform distribution of grain size.The Li_1.05Cr_0.1Mn_1.85O₄ powder with best performance can be obtained at sintering temperature 700℃for 12h,which initial specific discharge capacity were 113.7mAh/g and 114.2mAh/g,and the capacity retention were 94.2% and 89.7% after 40 cycles.It was evident that multiple doping of Li⁺,Cr³⁺can improve the stability of spinel structure and cycling performance.The cathode materials LiAl_0.05MxMn_1.95-xO_3.95F_0.05(x=0, 0.01, 0.05; M=Li, Co, Cr, Ni)were synthesized by sol-gel method.The materials possessed pure spinel structure, LiCo_0.01Al_0.05Mn_1.94O_3.95F_0.05 exhibited the best electrochemical performance,which initial specific discharge capacity were 121.7mAh/g in the room temperature and 122.0 mAh/g in the high temperature,and the capacity retention were 92.9%and 87.5% after 40 cycles. It was evident that anion-cation multiple doping enhanced the stability of the spinel structure and improved cycling performance of the materials.Layered LiNi_1/3Co_1/3Mn_1/3O₂ were synthesized by low temperature solid-state method,sol-gel method and rheological phase reaction method.The material with better performance can be obtained at sintering temperature 800℃,synthesized by rheological method.The initial specific discharge capacities were 163.5mAh/g,172.3mAh/g,191.1mAh/g in the voltage region of 3.0⁴.4V,2.5⁴.4V and 2.5⁴.6V, and after 30 cycles the capacity retentions were 92.7%,89.6%,88.1% respectively.The materials showed good cycling performance.