Preparation And Properties Of Nickel-rich Ternayr Cathode Materials

Nickel-rich ternary lithium nickel cobalt aluminum oxide(LiNi_0.8Co_0.15Al_0.05O₂,NCA)cathodes are favored by many fields such as electric vehicles due to its high specific capacity,low cost,and stable structure.At present,NCA cathode still has the disadvantages of low initial coulombic efficiency,insufficient stability,and poor cycle performance,which greatly restricts its commercial development.Therefore,NCA cathode were selected as the research object in this paper,according to the above existing disadvantages,the NCA material were modified by doping,surface coating and adding organics substances to improve its electrochemical performance.The NCA material was modified by high-temperature solid-phase doping with manganese to obtain LiNi_0.8Co_（0.2-2x）Al_xMn_xO₂（x=0,0.005,0.01,0.025,0.04,0.05）materials.Co-doping of cobalt,aluminum,and manganese inhibits the lithium-nickel mixing of material,thereby improving its cycle performance and rate performance.The results show that the obtained materials all haveα-NaFeO₂ structure,and the degree of lithium-nickel mixing decreases with the increase of manganese and aluminum elements and the decrease of cobalt.When x is 0.01,the material has the lowest degree of lithium-nickel mixing,the smallest charge transfer resistance and a larger lithium ion diffusion coefficient,and owns the best electrochemical performance and stability.At 0.1 C,the specific discharge capacity can reach 175.2 mAh·g^-1,and at 1 C there is still a discharge capacity of 110.9 mAh·g^-1,and the capacity retention rate after 50 cycles at 0.2 C is 92.7%.Using the oxalate co-precipitation-high temperature solid phase method,the precursor of NCA was preferentially formed,and then LiNi_1/3Co_1/3Mn_1/3O₂ precursors with different contents were precipitated on the surface,finally the precursor was calcined with lithium salt to obtain the coated material Li（NCA）_1-x（NCM）_xO₂（x=0.1,0.2,0.3,0.4,0.5）.The results show that LiNi_1/3Co_1/3Mn_1/3O₂ is successfully coated on the surface of NCA,and with the increase of x,the coating gradually becomes uniform and thickens.The coated materials are allα-Na FeO₂ structures with good layered structure.The lithium-nickel mixing degree of the material shows a tendency of decreasing first and then increasing with the increase of x,and the minimum value is obtained at x=0.3.The different coating contents of LiNi_1/3Co_1/3Mn_1/3O₂ materials can reduce the surface electrochemical degradation and the bulk mechanical degradation of the NCA material during the cycle to different extents,and inhibit the increase of the charge transfer resistance and the reduction of the lithium ion diffusion coefficient,thereby making the coated material have better cycle performance.After cycling 50 cycles at 0.2 C,the materials all have a small capacity decay rates of 7.5%,7.4%,2.9%,5.8%,and 6.2%,respectively.When x=0.3,the material has the best cycle performance due to its better layered structure,minimum lithium-nickel mixing degree and charge transfer resistance,and maximum lithium ion diffusion coefficient,so the optimum coating amount is x=0.3.The oxalic acid co-precipitation-high temperature solid phase method was used to improve the electrochemical performance of NCA materials by adding different organic substances（polyvinylpyrrolidone（PVP）,polyvinyl alcohol（PVA）,sodium lauryl sulfate（SDS）,hexadecyltrimethylammonium bromide（CTAB））during the precipitation process.The results show that the organic substance PVA mainly through the spatial effect,electrostatic and hydrogen bond effects,promotes the growth of NCA-PVA toward smaller size,uniform particle,thus effectively improves the discharge specific capacity and rate performance of the material.At 0.1 C,the specific discharge capacity of the material increases from 143.4 to 184.8 mAh·g^-1.At 5 C,the discharge specific capacity also remains79.4 mAh·g^-1.The organic substance PVP mainly improves the morphology of precursor by spatial effect and electrostatic effect,and the rod-shaped material NCA-PVP with better layered structure and lower lithium-nickel mixing degree is obtained,and it has higher first charge-discharge efficiency and better cycle performance.At 0.1 C,the charge and discharge efficiency increases from 78.3%to 89.2%,after 50 cycles at 0.2 C,the capacity retention rate is 94.3%.The ionic organic substances SDS and CTAB rely on the role of electrostatic effect to affect the morphology of the material,and finally the NCA-SDS material with a larger particle size range,severe lithium-nickel mixing degree and the NCA-CTAB material having the growth trend of the rod-like structure and good layered structure are obtained.NCA-SDS has a smaller specific discharge capacity(104.6 mAh·g^-1,0.1 C),poorer cycling performance and rate performance.While compared with NCA,the first charge-discharge efficiency and capacity retention rate of NCA-CTAB are improved.When cycled at 0.1 C,the first charge-discharge efficiency of NCA-CTAB and NCA are 82.7%and 78.3%,respectively.After cycling at 0.2 C after 50 cycles,the capacity retention of NCA-CTAB is92.8%,which is higher than 89.0%of the NCA material.