Preparation,Modification And High-Voltage Performance Of Single-Crystal Ni-Base Layered Cathode Materials

Increasing the Ni-content and cut-off voltage of the cathode material Li Ni_1-x-yCo_xMn_yO₂（NCM）to increase energy density and lowering the Co-content in the material system to reduce cost have become the current mainstream in order to meet the rising demand for energy density and the cost pressure of power batteries.However,the cyclic performance and thermal stability of materials appear to be trending in the direction of"one loses the other"due to the growth in material energy density,which also makes material preparation more complex.The superposition of Ni-rich,high-voltage and low/no Co effect makes the problems of harmful phase transition,interfacial side reaction,transition metal ion dissolution and micro-crack formation more prominent.Therefore,in view of the above problems,this paper mainly selects three materials,traditional Ni-medium Co-medium Li(Ni_0.5Co_0.2Mn_0.3)O₂（NCM523）,Ni-rich Co-poor Li(Ni_0.8Co_0.11Mn_0.06)O₂（NCM811）and Ni-rich Co-free Li(Ni_0.8Mn_0.2)O₂（NM82）,to study their preparation,modification and high voltage performance.The main research contents are as follows:（1）NCM523 monocrystalline materials with Na-doping and Li₃BO₃/Al₂O₃ co-coating were synthesized by coordinated internal and external regulation to address the issues of structural degradation,interfacial side reaction and micro-crack in NCM523 monocrystalline material.The materials were then studied at various cut-off voltages（coin-type cells:4.3/4.4/4.5/4.6 V,and pouch-type cells:4.2/4.3/4.4 V）and their physical properties were analyzed by XRD,SEM,TEM and XPS.The findings indicate that the material delivers a discharge capacity of 201.8 m Ah g^-1 within the voltage range of2.95-4.6 V of coin-type coin,and hold at 158.9 m Ah g^-1 after 200 cycles at1C.After 1600 cycles within the voltage range of 2.8-4.4 V at 1C of pouch-type cell,it still maintains 81.2%discharge capacity retention rate.By using CV curve,GITT,and COMSOL,the kinetic characteristics of NCM and NNCM@AB materials were investigated.Moreover,by examining the crystal structure,morphology,surface degradation of the particles,and chemical composition of the pole of cathode after cycling,the high voltage failure and modification mechanism of NCM523 single-crystal material were explored.（2）In view of the inherent structural/interfacial instability problems of NCM811 single-crystal materials,a dual modification strategy was proposed,which is modified by Zr-doping and B₂O₃-coating to enhance the reaction kinetics.The findings indicate that the material delivers a discharge capacity of 206.0 m Ah g^-1 within the voltage range of 2.95-4.6 V at 1C of coin-type coin,and hold 83.9%discharge capacity retention rate after 200 cycles.After1400 cycles within the voltage range of 2.8-4.4 V at 1C and 25℃of pouch-type cell,it still maintains 83.5%discharge capacity retention rate.Meanwhile,the dynamic properties of the modified materials were studied by CV,EIS,GITT,and COMSOL.Finally,the mechanism of Co-modification on the cycling performance was discussed.（3）In order to better understand the failure mechanism of Ni-rich Co-free single crystal materials under high voltage conditions,NCM811 single crystal materials with low cobalt and NM82 single crystal materials without cobalt were systematically studied and compared.Due to the discovery that Co significantly inhibits Li/Ni antisite defects.NCM811 outperforms NM82 in terms of cycling,power,and high-temperature storage performance.Additionally,it is discovered that NM has higher safety performance than NCM and that the detrimental effects of the Li/Ni antisite defects in NM82are manageable at low current densities.Finally,it is discussed how Ni-rich,Co-free single-crystal materials fail under high voltage.（4）A new Ni-rich Co-free single crystal material Li Ni_0.8Mn_0.18Fe_0.02O₂（NMF）was devised and prepared by coprecipitate method in order to address the issue of the NM82 Ni-rich Co-free single-crystal materials poor cycling performance under high voltage.It is systematically compared with Li Ni_0.83Co_0.11Mn_0.06O₂（MNCM）and Li Ni_0.83Co_0.11Mn_0.06O₂（SNCM）,two commercially common cathode materials.The findings demonstrate that the NMF cathode material excellent thermal stability and cycling performance（both at coin-type half-cell and pouch-type full-cell）in addition to a material cost reduction of more than 15%.It also shows an energy density（at high-temperature and high-voltage）that is nearly equal to that of MNCM and SNCM materials under normal conditions.More importantly,the cyclic decay mechanism of these three materials was investigated by in-situ characterization and density functional theory（DFT）calculations.It is found that NMF has high voltage plateau,good phase stability and less oxygen release,which can greatly alleviate reversible H2-H3 harmful phase transition,reduce internal strain and reduce the formation of cracks.（5）Wet Al-doping and Zr B₂-coating were internally and externally modified in order to address the issue that NM82 Ni-rich Co-free single-crystal material has poor cyclic performance at utra-high voltage due to substantial Li/Ni antisitedefect/interfaceinstability.Asingle-crystal Li Ni_0.8Mn_0.19Al_0.01O₂·0.04Zr B₂（NMAZB）cathode that can withstand extremely high voltages has been produced.The synergistic effect of the two decreases the Li/Ni antisite defects and interfacial impedance,greatly enhancing the crystal structure and mechanical integrity of the material,and thus showing excellent electrochemical performance under ultra-high voltage（coin-type half-cell within the range of 2.95-4.65 V and pouch-type full-cell of 2.8-4.55 V）.Notably,for pouch-type full-cell,NMAZB maintained a capacity retention rate of 87.2%after 500 cycles within the voltage range of2.8-4.55 V range.There are 119 figures,27 tables and 362 references in this thesis.