Study On Preparation And Doping Modification Of LiNi_0.5Mn_0.5O₂ Cathode Material For Lithium-ion Batteries

LiNi_0.5Mn_0.5O₂ is considered to be one of the most attractive cathode materials for lithium ion battery, due to its high specific capacity, good thermal stability, low cost, and nontoxicity. Unfortunately, this material has low electric conductivity and shows severe Li/Ni mixing about 10%, which results in its poor rate capability and cycling performance. In this study, Li Ni0.5Mn0.5O2 was synthesized by a sol-gel method and its electrochemical performance was improved by the preparation process optimization and the substitution of Mn with Si or Mo. In addition, the enhancement mechanism of the electrochemical performance was studied by X-ray diffraction?XRD?, Rietveld refinements, scanning electron microscopy?SEM?, specific surface area testing, cyclic voltammetry?CV?, electrochemical impedance spectra?EIS?, and galvanostatic charge/discharge tests.At first, the process parameters in synthesizing LiNi_0.5Mn_0.5O₂ by sol-gel method were optimized. It was found that LiNi_0.5Mn_0.5O₂ with a well layered structure, no impurity, and small particle size can be obtained when sintered at 850 ? for 10 h with 5% extra Li content and a fixed citric acid to transition metal ion ratio of 1:2. The material obtained at the above condition showed the optimun rate capability, whose discharge capacities were 181, 135, 75, and 44 mAh g-1 at 0.1, 1, 5, and 10 C, respectively.Then, LiNi0.5Mn0.5-xSixO2?x=0, 0.015, 0.03, and 0.05? was synthesized by sol-gel method and the Si doping effects on the structure, surface morphology, and electrochemical performance of LiNi_0.5Mn_0.5O₂ were investigated. The results revealed that the sample with x=0.015 showed the best electrochemical performance with the discharge capacities of 192, 151, 103, and 75 mAhg-1 at 0.1, 1, 5, and 10 C, respectively. The improved rate capability was attributed to the larger lattice parameters, smaller particle size, and the less severe agglomeration, which are all in favor of the insertion and de-insertion of Li+.Finally, LiNi0.5Mn0.5-xMoxO2?x=0, 0.01, and 0.02? was synthesized by sol-gel method and the effects of Mo doping on the structure, surface morphology, and electrochemical performance of LiNi_0.5Mn_0.5O₂ were investigated. It was revealed that the sample with x=0.01 delivered the highest discharge capacities of 195, 161, 110, and 86 mAhg-1 at 0.1, 1, 5, and 10 C, respectively. The improved rate capability might be attributed to the larger lattice parameters, lower ratio of Li/Ni mixing, and better electric conductivity. By lowering the sintering temperature to 800 oC, the sample with x=0.01 can deliver superior rate capability of 189, 165, 127, and 105 mAhg-1 at 0.1, 1, 5, and 10 C, respectively, due to the reduction of the particle size.