Preparation And Performance Study Of Gradient NCA Cathode Materials For Li-ion Batteries

NCA cathode material with high nickel series has good ternary synergistic effect,high specific capacity,good cycling performance,low cost and low toxicity,which is considered as one of the alternative materials of Li Co O₂.However,the poor thermal stability of high nickel materials under the state of charge leads to serious capacity degradation and poor synthesis conditions,which hinders their large-scale commercial application.In view of the problems existing in the high nickel ternary anode materials,this paper starts with the core design,shell design and sintering process of the precursor,solves the problem of capacity attenuation of the high nickel ternary anode materials,and selects the best precursor synthesis conditions and sintering process parameters.In the core design part,the effects of core composition,complexing agent concentration,p H value and precipitation temperature on the structure and properties of the material were studied.In the process of exploring the optimal composition of the core Ni_xCo_1-x（OH）₂,the sample with the highest nickel content in the core has the worst cycle performance.The nickel content should not be too large,otherwise it will seriously affect the cycle performance of the material,and determine x≤0.9.When the ammonia concentration is 0.8mol/L,the secondary particles of the sample are closer to spherical,the particle size is the largest,the particle size is 8～16μm,uniformly dispersed,and the sample surface has fewer voids.Proper ammonia concentration（0.3～0.6mol/L）is conducive to the growth of precursors.The ammonia concentration is 0.6mol/L,and the sample has the best cycle performance.Proper ammonia concentration is conducive to the growth of precursors,so as to achieve the purpose of improving the performance of gradient materials.The p H value of the core reaction and the temperature have a great influence on the electrochemical performance of the positive electrode material.In order to ensure that the material has high electrochemical performance,it is appropriate to control the p H of the core reaction at11.50～12.00.The precipitation temperature should be below 60℃.By using different methods to prepare(Ni_0.88Co_0.12)_a(Ni_0.80Co_0.15Al_0.05)_1-a（OH）₂,the electrochemical performance of the material prepared by the two-stage method is better than that of the continuous method,indicating that the impurities remaining in the synthetic core Influencing the performance of the material,we should pay attention to washing and filtering the core in further research.The composition of the core and shell will greatly affect the final performance of the material.Li(Ni_0.88Co_0.12)_0.1(Ni_0.75Co_0.15Al_0.1)_0.9O₂has the worst cycle performance,indicating that the shell with too high Al content will damage the cycle performance of the gradient material.A value of 0.3～0.5 is more appropriate.Increasing the precipitation reaction time of the outer shell is beneficial to the development of the outer layer crystal and the deposition and coating is more uniform,and the electrochemical performance of the gradient positive electrode material is improved.According to experimental research,in order to ensure higher capacity and cycle performance,the outer layer reaction time should not be less than 4h.The cation mixing degree of the graded positive grade material becomes more and more serious with the increase of the sintering temperature,and the material appears powdering phenomenon,and the sintering temperature is determined to be 700℃.The lithium ratio of 1.05 results in a positive electrode material with good cycle performance and high specific discharge capacity.Compared with the homogeneous Li Ni_0.8Co_0.15Al_0.05O₂material,the element distribution of the gradient Li Ni_0.8Co_0.15Al_0.05O₂material presents a gradient distribution from inside to outside.The homogeneous NCA material has a high first discharge specific capacity of 206.3m Ah/g,and the gradient material has a high first coulombic efficiency of90.71%and 91.06%.During discharge,as the number of cycles increases,the discharge specific capacity shows a downward trend,but the loss capacity of homogeneous materials is more.After a 100-week cycle,the gradient material prepared by the two-stage method has a capacity retention rate of 84.78%.Further research found that after 100 weeks of charge and discharge cycles of the gradient Li Ni_0.8Co_0.15Al_0.05O₂material prepared by the two-stage method,particles in the positive electrode sheet were less broken and cracks were less,corresponding to the electrochemical impedance diagram,the charge transfer resistance of the material The increase is also minimal,so battery capacity decay is slower.Therefore,the gradient cathode material prepared by the two-step method can take into account both high specific discharge capacity and cycle performance.The preparation method is simple and easy to control,and has good application prospects.