On The Doping Modification And Electrochemical Performance Of Li-Rich Mn-based Li_1.2Mn_0.54Ni_0.13Co_0.13O₂

Lithium-ion battery has become the preferred energy device for energy storage and electric vehicles due to its high energy density,high working voltage and low cost.Layered Li-rich Mn-based cathode materials have attracted widely attention because of their high discharge capacity.However,the inferior cycling stability,rate capability and the low initial coulombic efficiency of Li-rich Mn-based cathode materials severely limit their commercial application.In this paper,F^-or Fe³⁺doping Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode materials（LMNC）were prepared by sol-gel method,and XRD,SEM,HRTEM,XPS,EIS,CV,ICP-OES and galvanostatic charge-discharge test were used to investigate the effects of F^-or Fe³⁺doping on the crystal structure,phase transition behavior during charge/discharge cycling and electrochemical performance of LMNC cathode materials.The main research contents and results are as follows:（1）F^-or Fe³⁺doping LMNC is composed of hexagonal Li MO₂ and monoclinic Li₂Mn O₃ components.All the cathode materials have well layered structure,crystallinity,dispersion and low Li⁺/Ni²⁺mixing.The particle sizes of the materials are 200-500 nm.The substitution of O^2-by F^-and the substitution of Co³⁺by Fe³⁺can stabilize the Li₂Mn O₃ phase,inhibit the loss of lattice oxygen,the phase transition,and the side reactions between electrolyte and cathode material,suppress the Jahn-Teller distortion effect,expand the Li layer spacing and acelerate Li⁺migration.（2）The polyvinylidene fluoride（PVDF）doping LMNC materials synthesized by solid-phase reaction show better electrochemical performance than the ammonium fluoride doping materials prepared by liquid-phase reaction.The discharge capacity of Li_1.2Mn_0.54Ni_0.13Co_0.13O_1.95F_0.05（LMNC-0.05F_P）is195.4 m Ah g^-1 at 1C and the capacity retention is 87.4%after 100 cycles,which is higher than 190.7 m Ah g^-1 and 64.8%of the undoped material（LMNC-RAW）.The Li⁺diffusion coefficient of LMNC-0.05F_P is 5.86×10^-16 cm²·s^-1,which is about 3.2 times to that of LMNC-RAW.（3）The LMNC materials prepared with three carboxyl groups of citric acid as complexing agent can reduce the mixing degree of Li⁺/Ni²⁺and increase the Li⁺diffusion coefficient by an order of magnitude,while exhibiting better electrochemical properties than the materials prepared with only one carboxyl group of ethanoic acid.The Li⁺diffusion coefficients of LMNC-RAW_C and Li_1.2Mn_0.54Ni_0.13Co_0.12Fe_0.01O₂（LMNC-0.01Fe _C）are 1.08×10^-15 cm²·s^-1 and1.02×10^-14 cm²·s^-1,the discharge capacities are 272.4m Ah g^-1 and 265.1m Ah g^-1at 0.1C,and the discharge capacities are 109.2m Ah g^-1 and 126.1m Ah g^-1 at 5C in the rate performance test,respectively.The capacity retention of LMNC-0.01Fe _Cis 87.1%after 100 cycles at 1C and it is 73.7%after 200 cycles.