Doping Modified Lithium-Rich Manganese-Based Cathode Materials And The Study On Electrochemical Performance

Rechargeable lithium-ion batteries（LIBs）are widely used in portable devices,smartphones,and electric vehicles,because of their high gravimetric/volumetric energy densities.With the increasing demands,LIBs are expected to be smaller,lighter,and more durable,in which the electrode materials with large specific capacities and superior stabilities are urgently required.Compared with traditional commercial cathode materials,Li-rich layered oxides（LLOs）with higher capacity/energy densities have emerged as the most potential next-generation cathode materials,which benefits from the reversible anion redox reaction.However,the excessive anion redox reaction also leads to oxygen release,resulting in transition metal migration,voltage decay,and energy density decline.Aiming at these disadvantages,ion doping is adopted to improve the electrochemical performance of LLOs.The effects of doping ions on the morphologies,crystal structures and chemical state are investigated by experiments and calculations.Meanwhile,the 1D nanonet Co3O4 anode material is developed,of which the special nanostructure can effectively improve the kinetics of Li ion diffusion.When it matches with as-developed LLO cathode,the electrochemical performance of full cell is re searched.By Na doping methods,the LLO（Li1.2Mn0.54Ni0.13Co0.13O2）with refined secondary grain particles is prepared.Moreover,doping Na ions increases the thickness of lithium ion interslab,which can accelerate Li ion diffusion in the crystal layer to enhance the rate performance of LLO.During discharging at the rate of 1 C(1 C=250 m A g^-1),the reversible capacity of N-LLO reaches 198m A h g^-1.DFT calculations results show that the formation energy for Li/Ni mixed defect is higher in Na doped materials,which is beneficial to reduce the formation of such defects and to further improve the electrochemical performance of the material.Through Sn and Na co-doping,LLO is well formed.The large number of doped Sn ions can not induce impurities,which conduces to the electrochemical activity of materials.In addition,the integration of Sn with Na ions can adjust layer spacings of the crystal structure.Meanwhile,Na ions will be implanted into the transition metal layers,leading to the increased layer thickness of transition metal,and the improved electrochemical stability.Cycling at the rate of 0.5 C for 200 cycles,the capacity of SN-LLO is 197 m A h g^-1.DFT calculations results show that the Mn ions migration barrier is higher after Sn doping,indicating the steric effect of Sn ion on Mn ions migration to enhance the electrochemical stability and slow down the voltage decay.The V-doped LLO with the surface phase separation is obtained through a high temperature treatment based on the volatilization property of V.T he surface phase of R-3m and the bulk phase of C2/m matches well,which keeps the good crystallinity and layered structure.The introduced V ca n reduce the valence state of Mn to adjust the charge distribution inside the crystal.Bader charge analyses show that the doped V reduces the oxidation state and possibility of O releasing,improving its electrochemical stability and suppressing the volta ge decay.Cycling at the rate of 0.5 C for 200 cycles,the capacity of V900 remains 201m A h g^-1.The one-dimensional nanonet structure Co3O4 is fabricated by coating carbonyl-β-cyclodextrin（C-β-CD）on the precursor.The EIS,GITT and CV analyses show that the material has abundant charge transport channels,and exhibits the excellent kinetics performance.There will improve its rate performance.By matching as-developed Co3O4 anode with the high-performance V-LLO（V900）,the full cell exhibits a high energy density,cycle stability,and safety.