Preparation And Electrochemical Performance Of Ti/Na Doped LiNi_0.815Co_0.15Al_0.035O₂ Cathode Materials

High-nickel ternary cathode material LiNi_0.815Co_0.15Al_0.035O₂（NCA）has the advantages of high energy density,low cost,and low environmental pollution,and is very likely to become the next generation of widely used lithium-ion battery cathode materials.However,the higher nickel content leads to increased cationic mixing,poor cycle and rate performance of the material,which hinders the further development of NCA cathode materials.In order to solve the above problems,the NCA cathode materials were modified by Ti⁴⁺and Na⁺doping respectively,and the structure,morphology and electrochemical properties of the modified cathode materials were systematically studied.The main contents and results are as follows.NCA cathode materials were synthesized by high temperature solid phase method,and the effects of calcination temperature and time on their structure and electrochemical properties were investigated.The XRD test results show that with the increase of calcination temperature（750℃→825℃）,the layered structure of NCA material develops more and more perfectly.When calcined at 750℃and 21 h,the prepared NCA cathode material has better cycle stability and rate performance.At a current density of 0.5 C,after 100 cycles,the discharge specific capacity is157.84 m Ah/g,and the capacity retention rate is 89.97%.In order to improve the cycling performance of NCA,it was doped with Ti⁴⁺in different proportions by high temperature solid phase method.XRD and SEM tests showed that Ti⁴⁺doping did not affect the layered structure and spherical morphology of NCA.Rietveld refinement shows that Ti⁴⁺doping reduces the cationic mixing of NCA materials.After Ti⁴⁺doping,the NCA cycle performance improved significantly.At 0.5 C current density,after 100 cycles,the capacity retention rate of 1%Ti-NCA material was as high as 92.71%（Pure-NCA was 89.97%）.At a high current density of 1.0 C,it still has a capacity retention rate of 91.61%after 100 cycles.CV and EIS tests showed that after 100 cycles,the 1%Ti-NCA cathode material had lower polarization and higher lithium ion diffusion coefficient(8.43×10^-14cm²/S),while the Pure-NCA cathode material only had 6.92×10^-15cm²/S.Finally,Na⁺doped NCA materials were synthesized by high temperature solid phase method.XRD test and Rietveld refinement results show that Na⁺doping can effectively reduce the cation mixing of NCA materials,and expand the lithium layer spacing by occupying the lithium layer.The XPS results showed that Na⁺doping could reduce the Ni²⁺content on the surface of NCA materials,which confirmed that Na⁺doping had an inhibitory effect on cation mixing.CV and EIS test results show that Na⁺doping can improve the diffusion coefficient of lithium ions,inhibit the polarization of NCA and reduce the charge transfer impedance.2%Na-NCA material has excellent rate performance and cycle stability.At 1.0 C current density,after 100 cycles,the discharge specific capacity is 149.16 m Ah/g,and the capacity retention rate is as high as 96.49%,which is 12.02%higher than Pure-NCA（84.47%）.At 5.0 C current density,Pure-NCA is only94.02 m Ah/g,while 2%Na-NCA is as high as 121.07 m Ah/g,an increase of 27.05 m Ah/g.