The Study On The Preparation And Lithium-Ion Battery Performance Of Lithium-Rich Manganese Based Material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂

Lithium-rich manganese-based cathode materials with ultrahigh specific capacity are expected to promote the energy density of lithium-ion batteries to a new level.However,the material still faces some obstacles in practical application,including low initial Coulombic efficiency and serious voltage hysteresis caused by severe structural reorganization and oxygen release in the first charge-discharge process;capacity and voltage decay due to phase transitions and irreversible oxygen redox reactions;poor rate performance due to the low reaction kinetics of the electrode.In response to the above problems,this thesis takes the low-cost and structurally stable Li1.2Mn0.54Ni0.13Co0.13O2 as the research object,and the performance of the materials was modified by doping,coating,high conductivity material compositing and surface treatment methods,and several lithium-rich manganese-based materials with improved initial Coulombic efficiency,rate and cycle performance have been obtained.The main research contents are as follows:(1)The Li1.2Mn0.54Ni0.13Co0.13O2 material with Co doping and NaxCoO2 coating was prepared by sol-gel method along with a subsequent wet chemical treatment process.Co doping can reduce the resistivity of the material and NaxCoO2 with Na vacancies coating can provide more lithium-ion diffusion channels in the material.The synergistic effect of coating and doping can also improve the ion reaction kinetics on both the surface and inside of the material,resulting in excellent electrochemical performance of the material:the discharge capacity is 126.9 mAh g-1 at 5 C current density,and the capacity retention of 81.2%can be maintained after 100 cycles at 0.2 C.The Differential Scanning Calorimeter results show that the Co-doped and NaxCoO2-coated Li-rich Mnbased materials have high thermal stability.(2)Nb2CTx MXene and Li1.2Mn0.54Ni0.13Co0.13O2 were compounded together by hydrothermal reaction method to obtain a unique heterostructure lithium-rich manganese-based cathode material for lithium-ion batteries.This unique structure can not only effectively buffer the volume expansion and structure collapse caused by Li+intercalation/deintercalation,but also optimize the conductive network of the lithium rich material,making the material with excellent electrochemical performance:the initial Coulombic efficiency of the heterogeneous material can reach 85.5%at 0.1 C,higher than 77.4%of the pristine sample.The discharge capacity at 0.5 C is 191.2 mAh g-1,which can be maintained at 160.3 mAh g-1 after 200 cycles.In addition,the specific discharge capacity of the material at 5 C is 128.9 mAh g-1,showing an excellent rate performance and cycling stability.(3)High performance Li1.2Mn0.54Ni0.13Co0.13O2 cathode material was obtained through thiourea induced interface engineering along with a proper anion and oxygen vacancy optimization methods,and sulfur doping and in-situ coherent spinel phase growth on the surface of the material were synchronously realized.Compared with the untreated material,the polarization and resistance of the modified material reduced significantly,whereas the lithium-ion diffusion kinetics and structural stability improved significantly.Moreover,the irreversible oxygen release and phase transition during cycling are effectively suppressed.Thus,the modified material shows excellent electrochemical performance especially long cycle performance:the discharge capacities at 0.1 C and 5 C are 272.1 mAh g-1 and 162.1 mAh g-1,respectively.At 1 C current density,the capacity retention after 600 cycles is about 82.1%.In addition,the specific discharge capacity of the pocket full battery composed of the modified material cathode and commercial graphite anode can reach 185.3 mAh g-1 at 1 C,and the capacity retention rate is about 81.7%after 140 cycles.(4)Li1.2Mn0.54Ni0.13Co0.13O2 material with Li3PO4 and spinel phase double coating layer was prepared by NH4H2PO4 assisted pyrolysis method,and oxygen vacancy and PO43were synchronously induced into the material.The lithium fast ion conductor and spinel phase material coating can improve the diffusion ability of lithium-ions and alleviate the dissolution of transition metal ions effectively.In addition,the oxygen vacancy and PO43-doping can simultaneously stabilize the surface oxygen and reduce the irreversible release of oxygen.Therefore,the initial Coulomb efficiency of the material is significantly improved,from 81.7%to 94%.In addition,the initial discharge specific capacity of the electrode at 0.1 C can reach 299.6 mAh g-1,and the discharge capacity can maintain 187.8 mAh g-1 at 5 C,the capacity retention rate is 91.1%after 200 cycles at 1 C.