Preparation And Modification Of High Nickel Single-Crystalline Cathode Materials For Lithium-Ion Batteries

Ni-rich polycrystalline layered NCM generates high internal stress due to the volume change caused by anisotropy during cycling,which easily causes intergranular cracks in the primary grains,the resulting cracks propagate along the grain boundary of the primary particle and break the secondary sphere,causing serious structural damage and capacity reduction.To address these issues,micron-grained single crystals are another good option.The structural design is reasonable,and the internal stress of the particles is easy to release,which can effectively alleviate the possible mechanical cracking.However,the formation of Ni O-type rock-salt phase and micro-cracks and the generation of Li₂CO₃ residue on the surface of high-nickel single crystal in the cycling process lead to the high impedance of Li⁺deintercalation.Due to the unique structure and limited reaction interface of individual particles,which make the diffusion of Li⁺in the bulk phase of micron-sized single-crystal NCM inefficient.In view of the problems existing in single crystal cathodes,this thesis takes single crystal Li Ni_xCo_yMn_zO₂（x=0.83）as the research object,and focuses on the effects of doping and coating on single crystal cathode materials.（1）In order to solve the problem of poor rate performance of single crystalcathode,Li Ni_0.83Co_0.05Mn_0.12O₂andB-doped Li Ni_0.83Co_0.05Mn_0.12O₂ cathode material were prepared by simple solid-phase sintering method.The introduction of boron on the structure increases the distance between lattices and promotes the rapid migration of Li⁺ions.In addition,SCNCM@B can alleviate the mixing of cations Li⁺/Ni²⁺,effectively mitigating the undesirable phase transition from the layered structure to the disordered rock-salt phase.Meanwhile,an appropriate amount of B doping can prevent the formation of intergranular cracks caused by anisotropic stress,thereby reducing the electrode/electrolyte interaction and improving the interfacial stability.As expected,the cycling performance of the SCNCM@0.6%B cathode was significantly improved even at high cut-off voltage and high temperature.In particular,it shows a superior reversible capacity of 175.32 m Ah g^-1with a capacity retention rate of 91.35%over 500 cycles in the pouch full cell.（2）Based on the performance degradation mechanism of single crystal materiais and the preliminary exploration of the interface properties,we improved the high-voltage performance of the cathode material by appropriately increasing the Co content and using surface modification.Based on single crystal cathode material Li Ni_0.83Co_0.11Mn_0.06O₂,a NASICON type fast ion conductor layer of Li_1.3In_0.3Ti_1.7（PO₄）₃（LITP）was constructed on the surface of the single crystal cathode material.LITP as the fast Li-ion conductor can accelerate the Li-ions diffusion and alleviate the electrode-electrolyte side reaction simultaneously.More importantly,LITP can serve as the Li-ions regulator,ensuring the homogeneous distribution of Li-ions and reducing the difference of Li-ions concentration at SCNCM surface.It can relieve the stress induced by the inconsistent Li-ions dispersion,which effectively decreases the degree of structural disordering and lattice mismatch at surface,eventually maintaining the high structure integrity during long-term cycling.the LITP-SCNCM cathode can achieve 196.4 m Ah g^-1 reversible capacity with a high capacity retention rate of 92.8%after 200 cycles at25℃in the range of 2.75～4.6 V.LITP modification design clearly addresses the problem of nickel-rich layered oxides operating under harsh conditions（≥4.4 V）and significantly improves the electrochemical performance.There are 42 figures,6 tables and 122 references in this thesis...