Study On The Synthesis And Performance Of NCA Used As Cathode Material For Lithium-ion Battery

Developing the new electrode material with high performance and low cost has become the research direction in the field of lithium-ion battery.LiCoO₂,a conventional cathode material for lithium-ion batteries,greatly limits its wide application due to its safety and price shortcomings.As one of the many substitutes for LiCoO₂ electrode material,the ternary positive electrode material LiNi_0.8Co_0.15Al_0.05O₂（NCA）not only possesses very similar electrochemical properties,but also has excellent safety performance and relatively low material cost,which is considered as the most promising cathode material for the development and application of alternative LiCoO₂ electrode material.At present,the material has been gradually commercialized and has been applied in a wide range of applications,especially in the field of electric vehicles（EV）and hybrid electric vehicles（HEV）.In this thesis,the research progress of NCA cathode material was reviewed in detail.The preparation conditions of NCA cathode material by sol-gel method were investigated.The effects of coating amount of LiCoO₂ and Cr content on the properties of NCA were investigated.The precursor of NCA cathode material was synthesized by sol-gel method and the desired cathode material was obtained by heat treatment.The effects of different calcination.temperature,calcination time,lithium ratio,coated LiCoO₂ content and Cr content on the properties of NCA were discussed.The optimal technical parameters were obtained.The structure and morphology of the materials were characterized by means of XRD,SEM and other tests.The electrochemical performance of the material was tested by blue cell test system and electrochemical workstation.（1）LiNi_0.8Co_0.15Al_0.05O₂ powder material with fine particles,uniform dispersion and single structure can be obtained when the calcination temperature is 800℃,the calcination time is 24h,and the lithium ratio is x=1.05.It was found that the initial discharge capacity reached 213.4 mAh/g at 0.1 C current rate,and the capacity retention rate was 91.9%after 50 cycles.The first discharge capacity was 117.0 mAh/g at 5 C rate,which shows good rate capability.（2）A layer of LiCoO₂ "protective shell" was formed on the surface of NCA by molten salt method to form NCA-LiCoO₂ composites with different coating amounts.The effects of LiCoO₂ coating on NCA properties were investigated.The results show that it exhibits the best electrochemical performance when the mass fraction of LiCoO₂ is 3%.The initial discharge capacity of NCA-LiCoO₂ at 0.1 C is 210.3 mAh/g,and the capacity loss is very small compared with NCA 213.4 mAh/g.The first discharge specific capacity of NCA-LiCoO₂ at 5 C is 134.8 mAh/g,which has been significantly improved when compared with the NCA 117.0 mAh/g.After AC impedance test,the results showed that the Rct value of NCA-LiCoO₂ was 58Ω and that of NCA was 79Ω,indicating that the coating LiCoO₂ can reduce the charge transfer resistance of the material and improve its electrochemical performance.（3）LiNi_0.8-xCo_0.15Al_0.05CrxO₂（x=0.00,0.01,0.02 and 0.03）was synthesized by sol-gel method in a pure oxygen atmosphere at a sintering temperature 800℃,sintering time 24 h,lithium ratio x=1.05.After testing,it can be seen that when x=0.01,the material had the best electrochemical performance,the initial discharge capacity at 0.1 C reached 219.5 mAh/g,and the capacity retention after 50 cycles was 97.1%,comparing to NCA 213.4 mAh/g.From the result we know that,LiNi_0.79Co_0.15Al_0.05Cr_0.01O₂ shows significant improvement in both rate performance and cycle performance.Especially at 5 C,the initial discharge capacity of LiNi_0.79Co_0.15Al_0.05Cr_0.01O₂ reached 147.7 mAh/g,which was significantly higher than that of NCA 117.0 mAh/g.After AC impedance test,the result could be seen that,the Rct of the Cr-doped material element was 38Ω,comparing to NCA 79Ω,it has been greatly improved.The experiment showed that,doping Cr element played a positive role in improving the performance of NCA.