Study On Improvement Of Rate Performance Of LiFePO₄ Cathode Material By Graphene

Lithium-ion batteries?LIBs?with high energy density,long cycle life and good safety are the most widely used energy storage system.Lithium iron phosphate?LiFePO₄?is an olivine-structure cathode material for LIBs with stable cycle performance,good safety and low cost.However,the defect of poor conductivity imposes a great limitation on its rate performance.In this paper,a two-dimensional material,graphene,was used as composite additive to improve the electrochemical performance of LiFePO₄ cathode based on the improvement of electronic conductivity and the adsorption of double-layers,etc.A series of LiFePO₄/graphene composite cathode materials with excellent performance were prepared.The main research results are as follows:The LiFePO₄ material was synthezed via a hydrothermal method and the graphene material was prepared by thermal reduction of graphene oxide.The graphene was used as the conductive additive to fabricate the LiFePO₄ cathode and the effect of the amount of graphene on the rate capability of the cathode was studied.With the increasing graphene content,the high-rate performance of the LiFePO₄ cathodes was improved significantly.The cathode with 30 wt%graphene shows high capacities up to 103.1 mAh g^-1 and 68 mAh g^-1 during discharging with extremely high rates of 30 C and 50 C,respectively.Besides,good cycling performance at high rate is also achieved.The cathode with 30 wt%graphene displays a capacity retention of higher than 80%after 1000 cycles at 30 C.These results not only indicate that the graphene could be a promising candidate as a conductive agent,but also provide a new insight for designing LiFePO₄ electrodes with brilliant high-rate performance via a simple method.The three-dimensional carbon-coated LiFePO₄/graphene composite material with double-layer conductive network was prepared with a solvothermal method.The graphene has high conductivity and high specific surface,improving the electronic conductivity of the composite and providing some double layer adsorption for the composite,respectively.Therefore,the composite has both the double layer adsorption of supercapacitor and the electrochemical energy storage behavior of the lithium ion battery,leading to excellent rate performance.The cathode can maintain 96.3%reversible capacity after 300 cycles at 30 C,and a reversible capacity of 96.3 mAh g^-1 can be reached at a current density of 50 C.A LiFePO₄/AC/graphene composite cathode material with porous structure and high conductivity was prepared by mixing the LiFePO₄ precursor with graphene and a high-specific surface AC,AC greatly improves the porosity and specific surface area(367 m2 g^-1)of the composite.The graphene sheets construct the three-dimensional conductive network for fast ion diffusion and electron transfer,and provide some capacitive behavior for the composites.The XPS of the LiFePO/AC/graphene shows the N content of 2.45%,which can further provide capacitance for the electrode.With the buffer of capacitive material?AC and graphene?,the composite material can endure an ultrahigh current rate.The capacity of the cathode reaches 66 mAh g'1 at 100 C.Besides,the brilliant cycleability was confirmed by the capacity retention of 82%after 3000 cycles at 100 C.A LiFePO₄/graphene composite cathode material was prepared by in-situ growing the LiFePO₄ particles on the surface of the N-doped graphene sheets.In the composites,graphene sheets are closely associated with the LiFePO₄ particles.The LiFePO₄ particles alleviate the restacking phenomenon of the graphene sheets.The N-doped graphene construct a continuous conductive network,enhancing the conductivity of the composite and the wettability with the electrolyte,and providing some pseudopotential.The reversible capacity of the composite at 1 C is 140.2 mAh g^-1,and the specific capacity of 54.6 mAh g^-1 is still remained under the high current density of 100 C.It indicates that the material can be used as the electrode material of the capacitive lithium ion batteries for instantaneous heavy current discharging.The full batteries with the LiFePO₄/graphene cathode and commercial lithium titanate anode were fabricated,which exhibit good rate performance.At 30 C,the initial capacity is about 71 mAh g^-1 and maintains at 58 mAh g^-1 after 1000 cycles.At 50C,the reversible capacity retains at 64.9 mAh g^-1.