Synthesis And Modifycation Of NCA Cathode Materials For Li-ion Batteries

As an efficient energy storage device,lithium ion batteries have been widely used in portable electronic products and electric tools.Cathode material is an important part for lithium ion battery and its performance has important influence on the performance of lithium ion battery.At present,LiNi_0.8Co_0.15Al_0.05O₂（NCA）is regarded as one of the promising high specific energy cathode materials due to its high specific capacity,low cost.However,the poor cycle performance and thermal stability for NCA cathode materials prevent its large-scale commercial application.Based on these,the influence of new aluminium source and concentration of aluminium substitution on the structure and electrochemical performance of NCA have been studied in this thesis.At the same time,NCA cathode materials were modified by concentration-gradient-liked structure in order to further enhance the electrochemical performance and thermal stability of NCA cathode materials.Firstly,the influence of different Al source on the structure and electrochemical performance of NCA cathode materials was studied.Al（NO₃）₃ and NaAlO₂ were employed respectively as aluminum source to prepare the NCA cathode materials via the co-precipitation and high temperature solid phase sintering method.The study shows that the preparation process of the Ni_0.8Co_0.15Al_0.05（OH）_2.05 precursor from NaAl O₂ is a hydrolysis process and the reaction rate is moderate in the alkaline environment,which can effectively avoid the fast precipitation of Al³⁺.The XRD result indicates that NCA cathode materials which were prepared from NaAlO₂ as Al source have low cation mixing and well-ordered layered structure.The characterization and electrochemical measurements show that LNCA-NaAlO₂ exhibits significantly improved performances（such as tap density,initial discharge capacity and volumetric energy density,rate performance,cycle performance,electrochemical stability,microstructure stability,and storage stability）compared to the performances of those prepared from Al（NO₃）₃.Secondly,in order to enhance the electrochemical performance and thermal stability of NCA cathode materials,the concentration-gradient-liked structure was used to modify the NCA cathode materials.The concentration-gradient-liked structure cathode materials have the similar electrochemical performance with the concentration-gradient structure cathode materials,andthepreparingprocessissimpleandcontrollable.The concentration-gradient-liked LNCA(LiNi_0.8Co_0.15Al_0.05O₂)cathode material was synthesized withthemulti-shelledprecursor（Ni0.8Co0.15）0.6（Ni0.8Co0.15Al0.05）0.2（Ni0.7Co0.15Al0.15）0.1（Ni0.6Co0.15Al0.25）0.1（OH）2viaion diffusion under high temperature.The study results show that compared with homogeneous NCA,the energy density,cycle stability and thermal stability of the concentration-gradient structure NCA cathode materials have been significantly enhanced...