Study On Modification Of Layered Cathode Materials By High-Valence Ions For Lithium-Ion Batteries And Their Electrochemical Performance

Lithium-ion batteries have become a research hotspot in energy storage devices due to its advantages such as high specific capacity,high energy density,and long cycle life,and are now widely used in compact electronic devices,electric vehicles,large-scale energy storage and other scenarios.However,the energy density of state-of-the-art lithium-ion batteries can hardly meet the market demand,and the electrochemical performance of current cathode materials is one of the bottlenecks which limit the energy density of lithium-ion batteries and hinder their development.high-nickel ternary cathode material Li Ni_xCo_yMn_zO₂（x+y+z=1）and lithium rich manganese based cathode material x Li₂Mn O₃·（1-x）Li MO₂（M=Ni,Co,Mn）are regarded as the most promising next-generation high-energy-density cathode materials for lithium-ion batteries because of their advantages of high specific capacity and low cost,but the stress accumulation（induced by irreversible phase transitions）together with the internal chalking in high-nickel ternary cathode materials and the poor cycling stability,severe phase transition and low first Coulomb efficiency of lithium rich manganese based cathode materials have seriously delayed their large-scale applications.To address the above issues,this thesis tries to synthesize high-valence W-doped Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）and Nb-doped Li_1.2Ni_0.2Mn_0.6O₂（N20）materials via chemistry methods to study their synthesis-structure-performance relationships.The main research contents are as follows:（1）W-doped NCM811 materials were synthesized by solution chemistry,and the synthesis process was investigated and optimized.XRD results showed that the materials had a good layered structure;the W-NCM particles were observed as spherical micron-sized particles from SEM;XPS and TEM results showed that W was successfully introduced into the lattice of NCM811 material.The doped 0.5 wt%（0.5%W-NCM）materials exhibit better electrochemical performance;it can provide a discharge specific capacity of 206.1 m Ah g^-1 at 0.1 C,and the capacity retention rate is83.1%after 100 cycles.（2）The carbonate precursors of lithium rich manganese based cathode material with unique nanoflower structures were prepared by hydrothermal method,and Nb-doped N20 materials were synthesized in combination with solution chemistry.XRD results demonstrated that the materials have good layered structures and C2/m structures corresponding to Li₂Mn O₃ components;unique nanoflower structures of secondary particles were observed from SEM;XPS and TEM results showed that Nb was successfully introduced into the N20 material lattice.The doped 0.5 wt%（0.5%Nb-N20）materials exhibited better electrochemical performance,and the material had84.8%capacity retention after 100 cycles at 1 C,which is significantly improved compared to N20 material（72.8%）...