Dual-modification And Electrochemical Performance Study Of LiNi_0.6Co_0.2Mn_0.2O₂ Cathode Materials For Lithium-ion Battery

Ni-rich ternary layered oxides are considered to be one of the most promising cathodes for the next-generation lithium ion batteries（LIBs）thanks to their higher reverible specific capacities and high energy/power densities,which have attracted extensive attention.Unfortunately,their inherent layered structure instability(Li⁺/Ni²⁺cation mixing)and surface instability（side reactions with electrolyte/air）during the charge/discharge processes under extended cutoff voltage and high rate conditions result in severe capacity decay,oxygen release and poor cycling life,which seriously limite the practical applications of Ni-rich ternary layered materials.To conquer these problems,various efforts have been proposed and demonstrated such as surface coating and elemental doping.However,elemental doping or surface coating alone can only improve bulk structural integrity or solid–electrolyte interface stability,which solves partial essues of Ni-rich ternary materials.Herein,this thesis develops an effective dual-modification strategy to improve overall structural stability and obtain excellent electrochemical performance via constructing Li Ni_0.6Co_0.2Mn_0.2O₂（NCM622）with elemental doping and surface coating.The main contents and results of this thesis are listed as follows:（1）We propose a simple dual sutructural modification approach for the first time for NCM622 via Li₂SnO₃ surface coating and Sn⁴⁺gradient doping during high-temperature synthesis,denoted as Sn-NCM@LSO cathode material.The gradient Sn-doping contributes to obtain well-ordered layered structures thanks to the strong Sn-O covalent bond,and it also releases the degree of Li⁺/Ni²⁺mixing,promoting Li⁺ion diffusion.Moreover,the ionic and electronic conductive Li₂SnO₃coating with three-dimensional channels for Li⁺diffusion protects active spherical particles from interfacial parasitic reactions with electrolyte/air,and remains comparatively integrated spherical morphology after long cycling.In LIB testings,Sn-NCM@LSO-2 cathode delivers a relatively slow increase of charge transfer impedance and exhibits significantly improved cycling and rate performance(with88.31%capacity retention after 100 cycles and improved reversible capacity of 136.2m Ah g^-1 at 5 C).（2）We explore an overall structural modification strategy for the first time to construct NCM622 with GdOF surface coating and F^-doping through heating treatment,labeled as F-NCM@GdOF cathode.The anion fluorine-doping can largely stabilize layered structure and suppress the release of lattice oxygen due to the high electronegativity of fluorine ion.The GdOF with excellent chemical and thermal stability can eliminate the corrosion of HF in the electrolyte,and enhance the surface stability of NCM622.Electrochemical tests show that F-NCM@GdOF-2 cathode exhibits significantly improved cycling and rate performance(with 91.01%capacity retention after 100 cycles from 3.0 to 4.5 V at 1 C and improved reversible capacity of116.1 m Ah g^-1 at 5 C)compared to the bare-NCM622.Our research shows that the dual-modification strategy combines the advantages of elemental doping and surface coating and simultaneously addresses the interfacial instability and bulk structure degradation,thereby significantly improving the electrochemical performance of NCM622 cathode;the dual-modification approach is simple,controllable and effectively,which can broaden the horizon for preparing other Ni-rich cathode materials.