| With the development of energy vehicles,higher standards have been proposed for lithium-ion batteries,such as energy density,service life and thermal stability.Ni-rich layered cathode materials,LiNi0.8Co0.1Mn0.iO2 and LiNi0.8Co0.15Al0.05O2 are considered to be the promising and competitive cathode materials in the current power battery system,owing to their high energy density and low cost.As the nickel content increases,the specific capacity of Ni-rich ternary materials will increase accordingly,but it has also caused many problems such as increased production progress difficulty,poor cycle stability and poor thermal stability,which seriously limit its further commercial application.The main research work of this article is as follows:1.The residual alkali and residual lithium on the surface of the LiNi0.8Co0.1Mn0.1O2 cathode material prepared by the co-precipitation method are likely to cause side reactions with the electrolyte.In addition,the thermal stability and safety stability of Ni-rich cathode materials are relatively poor.Surface coating modification is a common method to improve its performance.Due to the weak interaction between the zirconium salt hydrolyzed product and the surface of the LiNi0.8Co0.1Mn0.1O2 cathode material in the conventional coating process,it is difficult to ensure the uniformity coating of zirconium oxide on the cathode material surface.We designed a pretreatment of the precursor by citric acid,using the strong covalent bond between citric acid and transition metal hydroxide to achieve uniform ZrO2 coating on the surface of the LiNi0.8Co0.1Mn0.1O2 cathode material.The thesis studied the effect of ZrO2 coating amount on the electrochemical stability of LiNi0.8Co0.1Mn0.1O2 cathode materials such as cycle stability and rate performance.The results show that this method can form a ZrO2 coating with a thickness of about 7 nm on the surface of LiNi0.8Co0.1Mn0.1O2 cathode material,which can significantly improve the electrochemical performance of LiNi0.8Co0.1Mn0.1O2 cathode material.Among them,the capacity retention of 1 wt%ZrO2 coated cathode material after 300 cycles at 1 C increased from 57.1%to 98.7%,and the thermal runaway starting temperature also increased from 281.7? to 290.1?.2.Due to the large difference in Ksp of Ni(OH)2,Co(OH)2,Al(OH)3,Al3+precipitation occurs quickly when using the conventional hydroxide co-precipitation method,resulting in uneven distribution of elements in the synthetic LiNi0.8Co0.15Al0.05O2cahode material.So,we use NaAlO2 as a raw material to reduce the precipitation rate of Al3+and achieve the co-precipitation of Ni2+,Co2+,and Al3+.The paper studies the influence of pH value and calcination temperature on the structure of LiNi0.8Co0.15Al0.05O2 cathode material,and optimizes the synthesis process conditions.The experimental results show that the optimal process conditions for the synthesis of LiNi0.8Co0.15Al0.05O2 cathode material are pH at 11.0 and calcination temperature of 750?.The LiNi0.8Co0.15Al0.05O2 cathode material prepared under this condition had a first discharge capacity of 178.2 mAh g-1 at a current density of 0.5 C and a capacity retention of 83.6%after 100 cycles.3.LiNi0.8Co0.15Al0.05O2 cathode material and LiNi0.8Co0.1Mn0.1O2 cathode material have similar problems of fast capacity decay,poor safety and stability.Lithium zirconium phosphate is a fast ion conductor,which can effectively increase the diffusion rate of lithium ions without reducing the material capacity after coating.We used sodium pyrophosphate as the raw material to coat the surface of LiNi0.8Co0.15Al0.05O2 cathode material with lithium zirconium phosphate by wet reaction.The capacity retention of the modified material after 400 cycles at 0.5 C was 46.9%increased to 58.8%.In addition,we also mixed and modified LNCM811 and LNCA in different proportions.The research results show that the discharge specific capacity and cycle stability of 75%LiNi0.8Co0.1Mn0.1O2 and 25%LiNi0.8Co0.15Al0.05O2 mixed composition is the best. |
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