Preparation And Electrochemical Characteristics Of LiNi_0.8Co_0.2O₂ Cathode Material For Lithium Ion Batteries

One of the key technologies for developing high energy density lithium ion batteries is the development of cathode materials.In recent years,Ni-rich layered oxide LiNi_0.8Co_0.2O₂ has been widely investigated as the cathode material for lithium ion batteries due to its high specific capacity,favourable electrochemical performance,low pollution and cost.But LiNi_0.8Co_0.2O₂ is more demanding for the atmosphere condition in the process of preparation on account of difficulty in oxidation of Ni²⁺,it must be synthesized in pure O₂ atmosphere because LiNi_0.8Co_0.2O₂ materials synthesized by traditional preparation process of LiCoO₂ hardly have electrochemical activity.At present,the processes of the precursor calcination are basically transparent,but the preparation of precursor is largely uncontrollable due to the factors of different synthesis methods and experimental chemicals.Therefore,the quality of the final product is mainly determined by the morphology of the precursor.In this work,we use sol-gel method,hydrothermal method and solvothermal method to prepare LiNi_0.8Co_0.2O₂ precursors with different morphological structure,and the resulting LiNi_0.8Co_0.2O₂ cathode materials show excellent cycle and rate performances through optimization of precursor morphology structure.Many tests are characterized to investigate morphological structure of the material,and further tests and analysis of the electrochemical performances was conducted.Conclusions of the research were listed as follows:1.LiNi_0.8Co_0.2O₂ was prepared by sol-gel method using acetate as strating chemical,and the morphological structure of precursor and product was investigated,cycle ability and rate capability of the material within different ranges of voltage were tested.The increase of charging upper voltage limit can effectively increase initial specific capacity while cycling at 1 C rate.The capacity retention ratio is 75%after 100 cycles within the voltage range of 2.7⁴.3 V,and the retention ratio is 68%after 100 cycles within the voltage range of 2.7⁴.6 V while the retention ratio is 36%within the voltage range of2.7⁴.8 V,indicating that the ideal working voltage range of LiNi_0.8Co_0.2O₂ as cathode material of lithium ion batteries is within 2.7⁴.6 V.We also tested the quality of the powder by morphology structure and electrochemical characterization,and analyzed the issues associated with the preparation processes.2.Use hydrothermal method to prepare samples by adding urea as precipitating agent to original reaction chemicals,LiNi_0.8Co_0.2O₂ precursors with different morphology structure was synthesized at various hydrothermal temperatures,and the most ideal temperature of reaction is obtained as 160?through testing of cycle ability of final products from three different precursors.Precursor synthesized at this temperature has flower-like morphology constituted of dense linear structure,and the uniformity of overall distribution is very well,this is good for improving atmosphere conditions in the calcination process of material.The results of material's structure characterization tests show that the LiNi_0.8Co_0.2O₂ obtained by optimized morphology precursor synthesized at 160?has ideal hexagonal layered structure,the rock-salt phase NiO impurities exists at particle surface on account of cation mixing is greatly reduced and the degree of structure ordering of material is improved effectively.The results of electrochemical performance tests can reach this conclusion:cycle stability of material is very good at 1 C rate,the initial specific capacity is 155 mAh/g and the capacity retention ratio is 94%after 100 cycles;the initial specific capacity of material is 120mAh/g while cycling at 5 C rate and the capacity retention ratio could reach 75%after500 cycles;the initial specific capacity of the material is 94 mAh/g at the high rate of 10 C,and the capacity retention ratio is 47.9%after 1000 cycles.Compared with materials prepared by sol-gel method,the electrochemical performance of hydrothermally synthesized LiNi_0.8Co_0.2O₂ through optimization of precursor morphology is greatly improved,indicating that improving morphological structure of precursors can effectively enhance the quality of products.3.LiNi_0.8Co_0.2O₂ precursors with different morphologies were prepared by solvothermal method at different solvent systems and quality of the products was investigated.Precursor prepared by solvothermal method using ethylene glycol as solvent has ellipsoid morphology and uniformity is pretty ideal,this is still good for improving atmosphere conditions in the calcination process so as to enhance the quality of the product.It can be found from the cyclic performance test that the specific capacity of LiNi_0.8Co_0.2O₂ prepared by solvothermal method is significantly higher than the sample prepared by normal sol-gel method and demonstrated that the quality of product is better.Compared with samples prepared by hydrothermal method,the initial specific capacity is about the same but cycle stability of solvothermally prepared samples is relatively poor,this difference can be explained by the structural defects caused by the generation of rock-salt phase NiO in the process of precursor calcination.The experimental results show that it is effective to enhance quality of the LiNi_0.8Co_0.2O₂precursors by solvothermal method.