Construction And Electrochemical Properties Investigation Of Lithium Nickel Manganese Oxide Cathode Material With High Performance

The popularization and development of electric vehicles and mobile phone have promoted the research in pirsing cathode material with low cost and high energy density.High voltage spinel Li Ni_0.5Mn_1.5O₄（LNMO）has the advantages of high energy density,low cost,etc.However,the poor cycling stability under high rate and high temperature limited its commercial application.To improve the cycling stability of LNMO,LNMOs with octahedral structure,hollow-sphere-like structure and sphere-nanorod-like structure were prepared by solid-state method,co-operation method and one-pot hydrothermal method,and modified it with surface coating,doping and modified surface coating,the electrochemical properties of the samples mentioned above were systemtically studied,the enhancement mechanism and lithium intercalation mechanism of the composite material were systematically studied by DFT calculation,TEM,HRTEM,In-situ-XRD,etc.LNMO with octahedral structure was prepared by solid-state method.The particle size was optimized and crystal structure stability in cycle process was improved by anneal process,leading to the superior electrochemical properties with the 1000th discharge capacity of 87.8 m Ah/g and capacity retention of 97.4%under 10 C.Furthermore,nano-sized spinel LNMO was synthesized with the addition of N,N-dimethylpyrrolidone（NMP,LNMO-N）,which exhibits larger discharge capacity under low rate with discharge capacity of 125 m Ah/g udner 0.2 C and 115.3 m Ah/g after 100 cycles under 2 C much larger than that of untreated LNMO（115.6 m Ah/g and 92.5 m Ah/g under 0.2 C and 2 C,respectively）,which is ascribed to the better electronic conductivity of LNMO-N originated from the Ni-N structure on LNMO-N surface.Hollow-sphere-like structure LNMO was synthesized by co-operation method.The electrochemical property of LNMO was significantly improved by graphene coating,with the 100th discharge capacity of 87.8 m Ah/g under 2 C at 55°C,corresponding to capacity retention of 98.8%,while that of LNMO was 41.8 m Ah/g and 87%,respectively.XRD,SEM,TEM,and in-situ-XRD results reveal that RGO coating is beneficial for improving electronic conductivity of LNMO,suppressing the side reactions between electrode and electrolyte,suppressing the volume and lattice strain changes during charge and discharge process,leading to better electrochemical property of RGO-LNMO under high discharge rate and high temperature.Li_1-xNi_0.5Mn_1.5-xTi_xO₄（LNMTx O,x=0.01,0.02,0.03）,synthesized by co-operation method,exhibits better thermal stability and superior cycling stability under high rate and high temperature,such as,under 2 C the 100th discharge capacity of LNMTx O（x=0.02）was 114.4 m Ah/g with high capacity retention of 98.4%at 55°C.The superior crystal structure stability and electrochemical property are ascribed to the stronger Ti-O bands compared with Mn-O and Ni-O bands,as revealed by DFT result,the suppressed Mn³⁺dissolution,heat aggregation and two phase reaction generation during charge and discharge process.Furthermore,sphere-nanorod-like micro nano structured LNMO was prepared by one-pot hydrothermal method,and then it was modified by modified surface coating with solid state electrolyte.Electrochemical result exhibits that LNMO with sphere-nanorod-like micro nano structure shows superior electrochemical property,and the electrochemical property is further improved by modified surface coating,with the1000th discharge capacity of 120.6 m Ah/g and high capacity retention of 96.2%in half cells,and the 1st discharge capacity of 149.1 m Ah/g under 0.1 C and 97 m Ah/g under 2C in LNMO-CG/Li₂Ti O₃（LTO）full cells,while that of LNMO was 60.6 m Ah/g after 1000cycles under 20 C with capacity retention of 79%,124.5 m Ah/g under 0.1 C and 53.1m Ah/g under 2 C in LNMO/Li₂Ti O₃（LTO）full cells.The superior electrochemical property of LNMO-CG is ascribed to the special micro struature,solid state electrolyte contributes to prevent electrolyte from penetrating into the material along grain boundary,suppress structure collapse,as confirmed by SEM,TEM and in-situ-XRD.Solid state electrolyte can also provide fast lithium ion transport channel on material surface and compensate for lithium deficiency during cycling process,leading to larger discharge capacity and better cycling stability of LNMO.