Phase Transition Mechanism Exploration And Structural Control Of High Nickel Ternary Cathode Materials

With the rapid development of the electric vehicle industry,longer battery duration of electric vehicle power is gradually required.Therefore,cathode materials of lithium-ion battery with high specific capacity are regarded as the current research hotspot.Among them,layered Ni-rich ternary cathode materials have attracted extensive attention.Though with high specific capacity,due to the excessively high nickel content of this material,problems such as phase transition,cracks,and side reactions are likely to occur during cycling process,resulting in poor cycle stability and safety performance.Here,density functional theory calculations are used to deeply analyze the primary cause of the failure of Ni-rich ternary cathode materials,and structural stability improvement has been achieved by building fast ion conductor cladding on the surface of Ni-rich cathode.Meanwhile,a"chemical-mechanical"coupling strategy is proposed to improve the structure/interface stability.Finally,the purpose that stabilize the structure and improve the cycle performance of Ni-rich cathode is achieved.The research contents and related conclusions are as follows:（1）The phase transition and cracks of Ni-rich ternary materials during charge and discharge are alleviated by constructing a fast ion conductor coating layer on the particle surface.Li_1.4Al_0.4Ti_1.6（PO₄）₃ coating modified Li Ni_0.83Co_0.10Mn_0.07O₂ material is designed reasonably.Meanwhile,characterization techniques such as high-resolution transmission electron microscopy and X-ray powder diffraction are used,which prove the positive effect of the coating layer on the inhibition of material phase transitions and side reactions.Additionally,the results of galvanostatic intermittent titration and in-situ X-ray powder diffraction show that this method can improve the lithium-ion transport kinetics at the material surface interface.In addition,the analysis of the evolution characteristics of the material structure demonstrated that the phase transition of Ni-rich materials and the side reactions with the electrolytes are suppressed by the Li_1.4Al_0.4Ti_1.6（PO₄）₃ coating layer,and the protection of the surface interface as well as the bulk phase structure has been achieved.Greatly,the modified material can maintain a complete structure and a relatively smooth surface even after 500 cycles at 5 C in the voltage range of 2.5～4.3V.（2）A"chemical-mechanical"coupling method is proposed to make rational regulation of the structure/interface of Ni-rich cathode materials.Density functional calculation predicts that the phase transition trend and structural stability of the Li Nb O₃ coated Li Ni_0.88Co_0.10Mn_0.02O₂ material can be modified.The difference between the formation energies of the H3phase and the H2 phase decreases from 0.15 e V in the pristine sample to0.064 e V in the modified sample（3%LNO）,indicating that the stress field of the Li Nb O₃ coating layer makes the phase change of Li Ni_0.88Co_0.10Mn_0.02O₂ relieved during cycling,therefore,a stable structure and excellent cycling performance are to be obtained.After the coating with Li Nb O₃,mechanically,the phase transition of the material is suppressed by the stress field of the coating layer,and the crystal structure is strengthened due to the doping of a small amount of Nb⁵⁺in the bulk,which is conducive to improving the structural stability and consistent with the calculation results.Chemically,the physical barrier brought by the coating layer could protect electrodes from electrolyte erosion,and is conducive to the rapid and reversible transmission of lithium ions.Greatly,excellent electrochemical properties（74%capacity retention after 300cycles at 2 C within 2.5～4.3 V）and structural stability（morphology remains intact after 500 cycles at 5 C within 2.5～4.3 V）are successfully achieved.This"chemical-mechanical"coupling control method makes the synthesis of crack-free/strongly stable high nickel cathode materials a reality.There are 73 figures,15 tables and 137 references...