Synthesis And Modification Of LiNi_0.8Co_0.1Mn_0.1O₂ Rich Nickel Cathode Materials For Lithium-ion Batteries

With the continuous improvement of new energy vehicles'requirements for mileage and energy density of power battery,higher requirements are put forward for the performance of high-nickel ternary cathode materials.Therefore,aiming at the LiNi_0.8Co_0.1Mn_0.1O₂?NCM811?rich nickel cathode materials for lithium-ion batteries,the cathode material NCM811 was modified to obtain excellent electrochemical performance by optimizing the synthesis process.The structure,morphology and electrochemical properties of the prepared and modified NCM811cathode materials were characterized and analyzed,providing technical support for t he industrialization and application of NCM811.The main contents and results are as follows:?1?The precursor Ni_0.8Co_0.1Mn_0.1?OH?₂ was synthesized by a single-tank continuous process.Through experiments,the optimal process conditions of precursor synthesis are determined to solve the problem of low production efficiency in the batch production process.The results show that the precursor of spherical like Ni_0.8Co_0.1Mn_0.1?OH?₂ with uniform morphology and nanometer thickness can be obtained by adjusting the stirring speed,reaction temperature,reaction pH value and the amount of complexing agent.The NCM811 particles,which were sintered at high temperature in an oxygen atmosphere after lithium addition,remained spherical like.Moreover,NCM811 particles are secondary particles accumulated by 300-800 nm primary grains.The material crystals are completely grown with the I_?003?/I_?104?value of 1.69,having a higher degree of crystallinity and a vibrating density of 2.1 g cm^-3.The initial discharge specific capacity and Coulombic efficiency of NCM811 are 198.7 mAh g^-1 and 85.9%at 0.1C in the range of 2.7-4.3 V,respectively.After 20 cycles,the discharge specific capacity and capacity retention are 184.8 mAh g^-1 and 90.3%,respectively.?2?In view of the key and common technical problems existing in the domestic preparation process of high-nickel type nickel cobalt manganese ternary precursor,we adopt the four-tank series continuous synthesis and classification precipitation technology to improve the solid content of the reaction process,achieving the precise control of nucleation and growth in the synthesis process of high-nickel type ternary precursor,and effectively solving the difficult particle size control and irregular micro morphology of product in the preparation process of high-nickel type ternary precursor.The results show that the as-prepared materials have excellent electrochemical properties.The initial specific discharge capacity of material can reach 202 mAh g^-1 and 188 mAh g^-1 at 0.1C and 0.5C in the voltage range of 2.7-4.3 V,respectively.It has good stability and consistency,and has a certain guiding effect on actual production.?3?The effect of sintering process on the morphology,structure and electrochemical performance of NCM811 ternary cathode materials was studied.By investigating the internal correlation between different precursor parameters and ca thode material parameters,we found the inherent law between the key parameters of precursor and the performance of cathode material,and obtained the optimum sintering process:the lithium ratio is 1.06,and the sintering process is as follows:in the oxygen atmosphere of 45 L h^-1,the material is heated from room temperature to 750°C with the heating rate of 5 ^oC min^-1 in the first stage,and hold for 4 hours at750°C.In the second stage,the temperature is raised from 750°C to 845°C with the heating rate is 1°C min^-1,and hold for 12 hours at 845°C.Finally,the reaction temperature is naturally cooled to room temperature.The NCM811 cathode material prepared is excellent.The initial discharge capacity of the NCM811 can reach 190 mAh g^-1 and 180 mAh g^-1 at 0.1 C and 0.2 C in the voltage range of 2.7-4.3 V,respectively.The capacity retention rate is 94.5%after 100 cycles.?4?Aiming at the problems of serious Li⁺/Ni²⁺mixed discharge of NCM811 material and the possibility of side reactions of the residual alkali on the surface and the electrolyte,Al ₂O₃ was selected as the coating material to modify NCM811.Through the comparison of different Al ₂O₃coating amount,it is found that the NCM811 with 2wt.%Al₂O₃ coating amount gets the best electrochemical performance.The results show that the material has the the lowest degree of cation mixing,delivering the initial Coulombic efficiency of 89.8%in the voltage range of 2.7-4.3V at 0.1C,which is significantly higher than that of the uncoated sample?84.5%?.After 100cycles at 0.5 C,the specific discharge capacity is 162.2 mAh g^-1 with the capacity retention of82.67%.This is mainly due to the thin and dense Al ₂O₃ coating formed on surface of the containing 2wt%Al₂O₃ sample can not only effectively reduce the mixed disorder degree of Li⁺/Ni²⁺in the material,but also inhibit the transformation of the material surface from layered structure to spinel phase and salt rock phase,and effectively inhibit the side reaction between the residual alkali on the surface of the material and the corrosion of the surface layer of the material by HF in the electrolyte,ultimately improving the electrochemical properties of the mater ial.The synthesis method is simple,effective and suitable for industrial production.?5?The relationship between the structural evolution of NCM811 cathode material and the battery performance under different cycle ratio was studied.More lithium vacanc y will be produced in the material at low cycle rate of 1 C,which will lead to the transition from layered structure to rock salt phase,which can inhibit the side reactions on the surface of the material to a certain extent,maintain the integrity of the material in the cycle process,and improve the electrochemical performance of the material.However,at a high cycle rate of 2 C,due to insufficient lithium vacancy in the material,the material will be transformed from a layered structure to an unstable spinel structure,which will seriously destroy the structure of the material,hinder the transmission of lithium ions,and reduce the cycle stability of the material.The electrochemical test results further prove the above conclusion.After 300 cycles,t he capacity retention ratio of NCM811 cathode material is 78.23%at 1C,which is much higher than that of65.76% at 2 C.