Modification Of Lithium And Manganese-enriched Layered Solid-solution Li_1.2Ni_0.2Mn_0.6O₂ Cathode Materials

The lithium-rich layered solid-solution cathode material x Li₂MnO₃·?1-x?LiMO₂?0<x<1,M=Ni,Co,Ni0.5Mn0.5,etc?receives a great deal of attention owing to high specific capacity,low cost,environment-friendly and excellent safety.Despite of all these advantages,there are several defects which restrict the further development of this material seriously,such as the large irreversible capacity loss of the first cycle,the poor cycle stability and rate performance.In order to satisfy the demands of commercialization,it needs to enhance the electrochemical properties by modifying research.The lithium-rich layered solid-solution cathode material Li_1.2Ni_0.2Mn_0.6O₂ was studied in this thesis.First,Li_1.2Ni_0.2Mn_0.6O₂ was doped by the Cr,Co and Ti with different doping elements to replace part of Ni and Mn element,respectively.The doped cathode material Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂,which has better performance,was obtained.Then fast ionic conductor LiAlO₂ was used to improve the electrochemical performance of Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂.The morphology,structure and electrochemical properties of the materials were studied by X-ray diffraction,scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectroscopy,impedance test and charge/discharge test.The main points of work are shown as follows:?1?Li_1.2Ni_0.2Mn_0.6O₂ has been doped with Cr/Co by co-precipitation method and doped with Ti by ball-milling method.The result shows that there are different modified effects by doping with different elements.Compared with pristine Li_1.2Ni_0.2Mn_0.6O₂,the 8 wt% Cr-doped Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂ has the best electrochemical performance.The first discharge capacity can reach 246.6 mAh·g^-1,and the first coulomb efficiency can reach 76.44%.The value of voltage is 2.97 V and the capacity retention ratio is 95.45% after 50 cycles at 0.2 C rate.The discharge capacity is 110.2 mAh·g^-1 at 5 C rate.The XPS and dQ/dV curves show that Cr doping could enhance the structural stability of lithium-rich layered cathode material and alleviate the speed of layer structure transforming to the spinel-like structure.Four-probe test and lithium ion diffusion coefficient results show that Cr-doping can improve electronic conductivity and ionic conductivity of cathode materials,effectively.?2?Based on the above results,Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂ cathode material has been coated with LiAlO₂?1,3,5,8 wt %?by sol-gel method.The results show that LiAlO₂ just modified the surface of cathode material without changing the layered structure.The 3 wt% LiAlO₂-coated Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂ material has the best electrochemical performance.Its initial discharge capacity is 266.8 mAh·g^-1,and the coulomb efficiency reaches 79.37%.The discharge capacity is 234.2 mAh·g^-1 and the discharge capacity remains 96.7% after 50 cycle at 0.2 C.Its discharge capacity still can reach 130.5 mAh·g^-1 at 5 C rate.XPS results indicate that LiAlO₂ coating can suppress the generation of Mn3+.EIS results show that the 3 wt% LiAlO₂ coated sample has the lowest charge transfer impedance.And the ion diffusion coefficient of 3 wt% LiAlO₂ coated sample is 1.22×10-13 cm²·s^-1,while Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂ is 6.70×10-14 cm²·s^-1.These results suggest that LiAlO₂ coating can probably be an effective way in improving the electrochemical properties of lithium-rich layered cathode material Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂.This can be ascribed to LiAlO₂ coating can obstruct the direct contact between the cathode material and electrode.And restrain the erosion of the electrolyte to the cathode material during the cycling process.In addition,it also can promote the Li+ diffusion through the electrode interface.