Doping/Coating Co-Modification And Theoretical Study On Lithium-Rich Manganese-Based Cathode Materials

Lithium-ion batteries,as an energy storage device,play a very important role in the field of new energy,in which cathode materials act as a key factor.In this paper,the lithium-rich manganese-based material Li_1.2Mn_0.6Ni_0.2O₂was used as the research object,and the joint modification by doping and coating was carried out.The structure and electrochemical performance of modified materials was analyzed by using XRD,SEM,AC impedance tests,et al.,and the first-principles calculation is used to theoretically explain the doping modification mechanism.The main results are as follows:（1）Li_1.2(Mn_0.6Ni_0.2)_1-xMo_xO₂ materials were synthesized successfully by high temperature solid state reaction.When x=0.03,the material exhibits a perfect layered structure,a stable lattice structure and a small charge transfer impedance.The discharge specific capacity is as high as332 m Ah×g^-1at 0.05 C in the first cycle and 150.2 m Ah×g^-1at a high rate of 5 C,respectively.The coulombic efficiency in the first cycle reaches to72.0%,and the capacity retention is 92.0%after 50 cycles at 0.5 C.In order to further improve its performance,Li_6.24Al_0.12La₃Zr_1.8Mo_0.2O₁₂coating was applied by sol-gel method.The results show that 2 wt%coating layer treated at 650℃can not only stabilize the surface structure of the based material well,inhibit the electrolyte reaction on the surface,but does not affect the Li⁺migration rate and reduce interface impedance.The discharge specific capacity of the joint modified material is up to343.6 m Ah×g^-1at 0.05C in the first cycle and 162.7 m Ah×g^-1at the high rate of 5 C,respectively.The coulombic efficiency is improved to 85.9%in the first cycle,and the capacity retention after 50 and 200 cycles of charge and discharge at 0.5 C is increased to 97.7%and 91.1%,respectively.（2）The Li_1.2-5xP_xMn_0.6Ni_0.2O₂ material was synthesized successfully by high temperature solid state reaction.When x=0.02,the interlayer distance of the lithium layer increases,the lattice oxygen release is effectively suppressed,the structural stability is enhanced,and the Li⁺diffusion rate is increased.The discharge specific capacity is as high as316.5 m Ah×g^-1at 0.05 C in the first cycle and 158.0 m Ah×g^-1at a high rate of 5C,respectively.The coulombic efficiency in the first cycle increases to 81.7%,and the capacity retention reaches 91.3%after 50 cycles at 0.5C.In order to further improve its performance,2 wt%Li_6.24Al_0.12La₃Zr_1.8Mo_0.2O₁₂coating was applied by sol-gel method.The discharge specific capacity of the modified material is up to 326.9m Ah×g^-1at 0.05 C in the first cycle and 169.3 m Ah×g^-1at a high rate of 5C,respectively.The coulombic efficiency is improved to 88.67%%in the first cycle,and the capacity retention after 50 and 200 cycles of charge and discharge at 0.5 C is increased to 97.2%and 90.5%,respectively.（3）First-principle calculation and analysis of Mo⁶⁺ doped and P⁵⁺doped Li_1.2Mn_0.6Ni_0.2O₂materials were done using VASP software.The optimized unit cell parameters based on the constructed structural model are consistent with the experimental values and have the same change trend.It is verified that both Mo⁶⁺doping and P⁵⁺doping can increase the unit cell volume,which is beneficial to improve the Li⁺diffusion.The energy band structure analysis show that the forbidden band widths of the Mo⁶⁺-doped and P⁵⁺-doped materials are reduced from 0.548 e V to 0.423e V and 0.411 e V,respectively,which means enhanced electronic conductivity.The total density of state and the partial density of state peak are enhanced by Mo⁶⁺doping and P⁵⁺doping,which is beneficial to the interaction between ions.The interaction between ions is mainly reflected in the enhancement of Mn-O covalentity and the formation of strong covalent bonds between Mo-O and P-O,which is beneficial to reduce the charge compensation of O^2-and oxygen loss,stabilize the lattice structure,and thus improve the charging-discharging performance.The calculation results deepen the understanding of the modification mechanism of Li_1.2Mn_0.6Ni_0.2O₂ by Mo⁶⁺ doping and P⁵⁺ doping.