Preparation And Characterization Of High-capacity Li-rich Manganese-based Cathode Material 0.5Li₂MnO₃·0.5LiMn_1/3Co_1/3Ni_1/3O₂

Lithium-rich manganese-based solid solution Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂?LMSS? as cathode material for lithium ion battery have been caused much attention by researchers world-widely, because of its high specific capacity, good thermal stability, low cost and friendly to environment etc. The presence of the insulating phase Li₂MnO₃ make lithium-rich material have poor conductivity, resulting that materials can not meet the high current charging and discharging rate performance.However, charging to the high pressure state, its coulombic efficiency is rather low in the first cycle owing to the disapperance of Li₂ O, which limit the practical application of lithium rich cathode material in lithium ion batteries. In addition, there are reasons for the low capacity such as material lattice defects, oxidative decomposition of the electrolyte. To solve these problems, we have made the following studies:Firstly, we used a co-precipitation method to make the lithium cathode material LMSS by optimizing the synthesis process to find the best preparation conditions,considering the concentration of ammonia, precursor pre-calcination temperature,different Ni, Co, Mn sources, the calcination temperature in the influence on the properties of LMSS. Scanning electron microscope?SEM? image?X-ray diffraction?XRD? pattern?First charge and discharge performance graphs indicate the LMSS has the best morphology and electrochemical properties when the preparation conditions are 0.3 mol/L ammonia as chelating agent, precursor calcination at 450?, and the sulphate transition metal material calcination at 850?. SEM image shows that the best LMSS with spherical particles is smooth in the surface, with 6-7 ?m of particle size. XRD image shows it has better layered structure, more complete and order crystal structure. The LMSS material can deliver the charge/discharge capacities of265.7/211.3 mAh·g^-1at the rate of 0.1C in the first cycle within the potential range2.0-4.8 V, its coulombic efficiency is up to 79.5%.Secondly, in order to improve the electrochemical properties of the lithium-rich material in addition to the question on the synthesis process, we made the research on coating and doping on the LMSS under the optimum conditions. The LMSS was surface-coated with different content of WO₃ layer to form the WO₃-LMSS. XRD measurement indicates the LMSS does not change the internal structure and form impurity phase. TEM images show that WO₃ was coated on the LMSS successfully.The coulombic efficiency of LMSS in the first cycle is only 79.5%, while 2 wt%WO₃-LMSS is up to 86.8%. After 50 cycles, the capacity retention rate of 2 wt%WO₃-LMSS increased 13.1% compared with that of the LMSS, which demonstrated that cycle performance is improved. The discharge capacity at 2 C rate is 66.7% of that at 0.1 C rate. The rate capability of the 2 wt% WO₃-LMSS is better than that of the LMSS.Thirdly, LMSS was coated with different content of V₂O₅/MoO₃ hybrid layer to prepare the LMSS-MoV material. XRD measurement indicates the. hybrid layer does not change the crystal structure. TEM, FETEM images and EDX spectra show V₂O₅/Mo O₃ was coated on the LMSS successfully. The coulombic efficiency of LMSS-2Mo V in the first cycle is 81.6% at the 0.1C rate, and the capacity retention rate is superior to the LMSS after 50 cycles, which showed the better cycle performance. EIS results show that the charge transfer resistance?Rct? of the LMSS-2Mo V is significantly less than that of the material LMSS. In a word,V₂O₅/Mo O₃ hybrid layer can solve the first charge-discharge capacity loss problem effectively and improve the electrochemical properties of lithium-rich material.Finally, the LMSS was doped with different content of Na to prepare the LMSS-S. XRD measurement shows the doped material LMSS-S exhibit a characteristic peak structure of the new phase Na0.7MnO₂.05. SEM image display the surface state of LMSS-S have changed. LMSS-S2 has the best discharge capacity in the first cycle of 240.7 mAh·g^-1. The discharge capacity decline with the continued increase in the amount of Na, the best doped content is no more than 5%. The discharge capacity of LMSS-2S is superior to that of the LMSS at different rate, and the capacity retention rate is 85.8% at the 0.1C rate after 50 cycles. A new phase was formed on the surface, the two-phase interface provides a fast diffusion paths for lithium ion and electron conduction.