Research On Preparation And Electrochemical Performance Of Phosphate-Modified High-Nickel Nickel Cobalt Lithium Aluminate

High nickel nickel cobalt lithium aluminate(Li Ni_0.87Co_0.09Al_0.04O₂,NCA)cathode material is considered as the most promising cathode material for high energy density lithium ion batteries due to its high specific capacity.However,in the long cycle process,NCA materials have some problems,such as short cycle life,poor rate performance and unstable structure,which are mainly due to the side reaction with the electrolyte.The reaction process will aggravate the transition metal dissolution and surface structural degradation,and inert substances as reaction products will adhere to the surface,further hindering the transport of lithium ions.To solve the above problems,this thesis adopts phosphate ion conductor to cover NCA cathode material,which can not only inhibit the occurrence of side reactions,but also accelerate the transmission of surface lithium ions.The main contents include the following aspects:1.Rare earth phosphate Ce PO₄ was introduced to modify the surface of NCA material.The results reveal that the cathode material with 2 wt%coating（C2-NCA）exhibits the best electrochemical performance.The capacity retention rates of C2-NCA reach 95.2%and 83.4%at 25℃and 60℃after 100 cycles at 0.5 C,respectively,which are much higher than 85.6%and 75.1%of the Pristine-NCA.Besides,C2-NCA still exhibits capacity advantage at higher rate.2.La PO₄ was introduced to dual-function modify the NCA cathode material to optimize its electrochemical performance under the dual action of coating and doping.The results show that the 2 wt%La PO₄ modified sample（L2-NCA）exhibits better cycle stability at 0.5 C compared with the Pristine-NCA（85.2%/81.9%）at 25℃/60℃,and the capacity retention rates increased to 96.1%/90.5%.At the same time,under the high current density of 1,2 and 5 C,the rate performances of modified sample are also significantly improved.3.NASICON-type solid electrolyte phosphate Li Ti₂（PO₄）₃ was introduced to coat NCA cathode materials.Due to its unique phosphate structure,the cycle stability and rate performance of the modified samples were improved effectively.The results exhibit that the capacity retention rate of T2-NCA with 2 wt%coating amounts can reach 90.2%after100 cycles at the rate of 2 C.In addition,the specific discharge capacity can still come to153.3 m Ah·g^-1 at a high rate of 5 C.In contrast,the original samples are only 81.2%and122.4 m Ah·g^-1.4.Li_1.5Al_0.5Zr_1.5（PO₄）₃ with high ionic conductivity was introduced for surface coating modification of NCA cathode material,and the rate performance was significantly improved.The results show that the modified sample（1 wt%,LAZP1-NCA）exhibits a higher initial discharge capacity and capacity retention rate(165.2 m Ah·g^-1/88.5%)after100 cycles at 2 C,while the unmodified sample only possesses 156.8 m Ah·g^-1/80.7%.Notably,the LAZP1-NCA sample has a capacity of 150.6 m Ah·g^-1 at 5 C,much higher than the 123.5 m Ah·g^-1 of the Pristine-NCA.The above research results revral that it is feasible,effective and promising to optimize the electrochemical performance of nickel-rich NCA cathode materials by using phosphate ion conductor coating strategy.