Synthesis And Modification Of LiNi_0.5Co_0.2Mn_0.3O₂ Cathode Materials For Lithium-ion Battery

Layer-structured transitional meal oxides have been investigated intensively, due to its higher capacity, less toxicity and lower cost compared with layered LiCoO2. Among them, LiNi0.5Co0.2Mn0.3O2 has attracted much attention due to its advantage of higher capacity. However, this material exhibits poor rate capability and considerable capacity loss at high current density, obstacling its application in Li-ion batteries. Aiming to these problems, the synthesis conditions was optimized to control the good layer structure and lower cation mixture. The effects of calcination temperature, sintering time, the mole rate of Li to the precursor on the structure and electrochemical performances of LiNio.5Co0.2Mn0.3O2 were investigated. Based on this, the influences of doping and coating on the structure and electrochemical performances of LiNi0.5Co0.2Mn0.3O2 were intensively investigated.LiNio.5Co0.2Mn0.3O2 layered materials were synthesized by solid-state reaction using Li2CO3 and three transition-metal hydroxide precursors of composition Nio.5Coo.2Mno.3(OH)2. The effects of the calcination temperature, the sintering time, the amount of Li2CO3 on the initial discharge capacity and rate capacity are investigated. The electrochemical property study showed that the optimum reaction conditions are determined as follows:the calcination temperature is 880 ℃, the sintering time is 12 h, the amount of lithium is n(Li):n(Nio.5Coo.2Mno.3)(OH)2=1.05. At 2.7-4.4 V, the discharge capacities reached 195.2,158.4 and 114.9 mAh g-1 at the rates of 0.1 C,1 C and 5 C.Li[Ni0.5Co0.2Mn0.3]1-xVxO2(x=0, x=0.01, x=0.03, x=0.05) layered materials were synthesized by solid-state reaction using Li2CO3, (Ni0.5Co0.2Mn0.3)(OH)2 and V2O5. The influence of the vanadium substituted LiNi0.5Co0.2Mn0.3O2 (LNCM-532) cathode materials on the structure and electrochemical properties were investigated. XRD, EDS, XPS results show that vanadium is successfully substituted for the transitional metal in LiNi0.5Co0.2Mn0.3O2 crystal lattice. It should be explained that the Li[Ni0.5Co0.2Mn0.3]1-xVxO2 solid solution can be stably exist only if the vanadium content is relatively low. When it is beyond a value (about 2.60%), excess vanadium will exist in the surface in other form as Li3VO4. Electrochemical tests confirm that the Li[Ni0.5Co0.2Mn0.3]0.97V0.03O2 has the best rate capacity and cycling performance among all the samples. EIS measurement indicates that vanadium substitution contributes to the better lithium ion diffusion and the lower charge transfer resistance.Li2MnO3@LiNio.5Co0.2Mn0.3O2 composite has been synthesized with Li2MnOs used as coating by sol-gel. The influence of the amount of Li2MnO3 and pH on the structure and electrochemical properties of LiNio.5Co0.2Mn0.3O2 cathode materials were investigated. Through Electrochemical tests, the optimum reaction conditions are determined as follows:the pH is 6 and the amount of Li2MnO3 is 5%. The Li2MnO3 coated LiNio.5Coo.2Mno 3O2 cathode materials has the better rate capacity and cycling performance than LiNi0.5Co0.2Mn0.3O2. CV measurement indicates that Li2MnO3 coating contributes to the better lithium ion diffusion.The modification LiNi0.5Co0.2Mn0.3O2 cathode materials has good layered structure, less cation mixing, large Li ion diffusion coefficient, good rate properties and cycling performance. Therefore vanadium substitution or Li2MnO3 coating LiNi0.5Co0.2Mn0.3O2 works well in practice.