Research On Water-washing And Surface Coating Of LiNi_0.90Co_0.055Mn_0.045O₂ Ternary Cathode Material

Nickel-rich ternary cathode material has become one of the most widely used cathode materials for power lithium ion batteries because of its high energy density,low cost,and complying with the market development needs.The manufacture of electrodes,storage and electrochemical properties of cathode materials will be affected if much of alkaline lithium compounds remains on the surface of particles.Generally,a process with water-washing is used to dissolve and remove the residual lithium compounds,and a following secondary sintering is carried out to modify the particle surface and to improve the electrochemical performance of materials.LiNi_0.90Co_0.055Mn_0.045O₂ is used as the research object in this thesis,and the effects of water-washing on the structure,morphology,and electrochemical properties of the materials are explored.The results show that the residual lithium on the surface of the material dissolves in water,and the lattice lithium in the material rapidly dissolves in water during washing.At the same time,the specific surface area and charge transfer impedance of the material increase,and electrochemical performance declines.The correlation between the physicochemical properties and electrochemical properties of the water-washed materials is analyzed.The reason for the properties deterioration of the materials before and after water-washing is mainly attributed to the dissolution of lattice lithium.The effects of different sintering temperature on material’s properties and electrochemical performance are investigated.The results show that the Li⁺/Ni²⁺disordering of the material increases after the secondary sintering.The specific surface area of the material significantly increases,and the cycleability and the rate capability seriously deteriorate when the material is sintered at relatively low temperature.In contrast,the specific surface area of the material reduces,and the electrochemical performance is obviously improved when the material is sintered at relatively high temperature.However,the charge transfer impedance of the material is still higher than that of the original LiNi_0.90Co_0.055Mn_0.045O₂.In order to reduce the residual lithium and improve electrochemical performance,LiNi_0.90Co_0.055Mn_0.045O₂ is coated with Fe PO₄.The effects of coating content on the material’s properties and electrochemical performance are investigated.The results show that the content of residual lithium,especially Li₂CO₃,is significantly reduced after coating with Fe PO₄ at 700°C.The content of residual Li₂CO₃ and Li OH for the material coated with 1.2 wt.%Fe PO₄ is 0.141 wt.%and 0.336 wt.%,respectively.Initial discharge specific capacity at 25℃and at 0.1C is 203.6 m Ah g^-1,and the capacity retention after 100 cycles is 88.0%when cycled at2.8～4.3V and at 1C,The cycling stability of LiNi_0.90Co_0.055Mn_0.045O₂ is improved by coating with Fe PO₄.