Synthesis And Electrochemical Performance Study Of Graphene/LiNi_0.5Mn_1.5O₄ Materials

Li Ni0.5Mn1.5O4(LNMO)is one of the most promising cathode materials for future application due to its advantages of higher discharge voltage and energy density.However,the structural stability and rate performance of LNMO material is poor,which limits its applications in Li ion batteries.In this thesis,we synthesized LNMO material with different morphology by solid-state method,hydrothermal method to solve those problems.After that,we combined LNMO materials with the reduced graphene oxide,obtaining the RGO-LNMO composites.Compared with the bare LNMO materials,the RGO-LNMO composites show higher structural stability,better rate capacity and cycle performance,which can meet the needs of high-power density batteries.The structure and morphology of the LNMO materials were characterized by X-ray diffraction,Raman spectroscopy,Scanning electron microscopy and other test methods.Furthermore,we analyzed the electrochemical performance of LNMO material.Results show that: the octahedral LNMO material was synthesized by solid-state method and sintered at 800? for 8 h.The hollow spherical LNMO material was prepared by coprecipitation-hydrothermal method and the condition is 180? for 8 h,the micro-rods LNMO material was prepared by hydrothermalsolid-state method and sintered at 800 ? for 10 h.Moreover,we studied the synthesis mechanism and the electrochemical performance of LNMO materials.LNMO materials deliver a good electrochemical performance,remaining a capacity of 120 m Ah/g after 100 cycles at 0.1C.The RGO-LNMO composites were successfully synthesized by electrostatic self-assembly method.The electrochemical properties of RGO-LNMO composites were analyzed in detail.The results demonstrate that the RGO-LNMO composites display outstanding electrochemical performance,particularly in high rate capability.For example,after 1000 cycles under 10 C,the capacities of octahedral RGO-LNMO composite,hollow spherical RGO-LNMO composite,and micro-rod RGO-LNMO composite were 50.2 m Ah/g(2 wt%-RGO),84.5 m Ah/g(0.5 wt%-RGO),and 71.4 m Ah/g(1 wt%-RGO),with 68.3%,108% and 113% of cacacity retention,respectively.However,the bare LNMO material only displayed 18.8 mAh/g with 31.5% of capacity retention after 1000 cycles under 10 C.The lithium ion diffusion coefficient of the LNMO material with different morphology were 4.89×10-11cm-2/s,6.49×10-11cm-2/s,1.86×10-11cm-2/s,respectively.The Lithium ion diffusion coefficient of bare LNMO material just 0.61×10-11cm-2/s.Therefore,the coating of RGO on the surface of the LNMO material improves the high rate performance,and increases the Lithium ion diffusion coefficient.Based on the above analysis,it can be concluded that the appropriate weight ratio of RGO is 0.5%~2%,which could improve the electrochemical performance of LNMO.Among these materials,hollow spherical RGO-LNMO composite exhibits better cycle stability and excellent discharge capacity at high rate,though the rate capacity is poor at low rate.Micro-rod RGO-LNMO composite shows excellent cycle stability and higher discharge capacity at low rate,but the discharge capacity at high rate is low.The cycle stability and cycle performance of the octahedral-RGO-LNMO composite are bad.Thus,combining the LNMO materials with different morphology together can be expected to meet the requirement of Li ion batteries.