Structure Design And Performance Regulation Of Ni-High And Li-Rich Cathode Materials For High Energy Density Lithium-Ion Batteries

Rechargeable Li-ion batteries have been successfully applied in portable electronic devices and have promoted the rapid development of electric vehicles and smart grid.These fast-developing fields has put higher requirements on the energy density of Li-ion battery cathode materials.Currently,the capacities of the commercially available cathode materials,such as layered LiCoO₂,olivine LiFePO₄ and spinel LiMn₂O₄,are still far from satisfactory.In order to further improve the charge storage capability of cathode materials,tremendous efforts have been devoted to the exploration of Ni-high and Li-rich cathode materials.However,Ni-high cathode material has such disadvantages as serious Li/Ni mixing,high surface residual lithium and poor thermal stability;In parallel,Li-rich cathode material suffers from high initial irreversible capacity,fast voltage decay,poor cycling performance and low rate performance.Aimed to address the above problems of Ni-high and Li-rich cathode materials,this thesis mainly focuses on the structural modifications of the cathode materials to improve their electrochemical performances.The main contents of the thesis are summarized as follows.(1)An effective“etching-induced coating strategy”is designed and successfully applied to preparation?-LiAlO₂ coated Ni-high LiNi_0.8Co_0.1Mn_0.1O₂ cathode material.Through Al Cl₃ hydrolysis,protons are produced,which can etch the surface of Ni_0.8Co_0.1Mn_0.1(OH)₂ hydroxide precursors and provide freshly exposed surface for the anchoring of Al(OH)₃.?-LiAlO₂ coating layer is finally achieved by heating this Al(OH)₃-coated Ni_0.8Co_0.1Mn_0.1(OH)₂ precursor with the lithium source.Electrochemical tests indicate that the 2.2 wt%?-LiAlO₂ coated LiNi_0.8Co_0.1Mn_0.1O₂cathode material can provide a high rate capacity of 135.2 m Ah g^-1 at 10 C;and the capacity retention rate of 200 cycles at 0.5 C is as high as 85.8%.More importantly,the thermal stability of the 2.2 wt%?-LiAlO₂ coated LiNi_0.8Co_0.1Mn_0.1O₂ cathode material is also significantly improved.(2)A novel“PVP bridged coating method”is proposed for the preparation of?-LiAlO₂ coated Li-rich Li_1.2Ni_0.182Co_0.08Mn_0.538O₂ cathode materials,in which the PVP molecules can bridge the gap between the coating shell and the electrode material core through hydrogen bonding and functional groups.Utilizing this strategy,a uniform Li-ion conductor?-LiAlO₂ nanolayer is successfully formed on the surface of the Li-rich Li_1.2Ni_0.182Co_0.08Mn_0.538O₂ cathode material,which leads to an excellent rate capability(177.0 m Ah g^-1 delivered at 5 C),a superior cycling stability(89.3%capacity retention achieved after 100 cycles at 10 C)and a higher thermal stability(the value of exothermic peak from 220.9?to 234.1?)for the composite material.In addition,galvanostatic intermittent titration measurements demonstrate that the?-LiAlO₂ coated sample exhibits a higher Li⁺diffusion coefficient compared with the pristine one and density functional theory calculation reveals that the?-LiAlO₂ coating layer can decrease the Li⁺diffusion barrier.(3)A new Li-rich cation-disordered rock-salt structured cathode material of Li_1.2Ti_0.59Mn_0.22O₂ with the stoichiometry of Ti⁴⁺as high as 0.59 is designed based on“Ti⁴⁺/Mn²⁺double ion coupling”.The reduced capacity contribution caused by the electrochemically inactive Ti⁴⁺is balanced by the Mn²⁺/Mn⁴⁺two-electron redox couple.The protocol material of Li_1.2Ti_0.59Mn_0.22O₂ is synthesized using solid-state reaction method,whose structure and electrochemical performances are systematically compared with the low-ratio d⁰ TM counterpart of Li_1.2Ti_0.37Mn_0.44O₂.Results demonstrate that Li_1.2Ti_0.59Mn_0.22O₂ delivers a high initial discharge capacity of 278.3 m Ah g^-1 and better cycling retention of 70.2%after 30 cycles at 10 m A g^-1 compared to the Li_1.2Ti_0.37Mn_0.44O₂.Galvanostatic intermittent titration technique and density functional theory calculation also further confirm that the Li_1.2Ti_0.59Mn_0.22O₂ material possesses a higher Li⁺diffusion coefficient and lower Li⁺diffusion energy barrier.