Application Of Graphene In Li-ion Capacitive Battery

Lithium-ion battery is a kind of secondary battery with high specific energy,and supercapacitor owes outstanding advantages of specific power density and cycle life.Lithium-ion capacitive battery produced by the fusion of supercapacitor and lithium-ion battery technology has characteristics of high-energy density of lithium-ion battery and high-power density as well as long life of supercapacitor,which is a promising electrochemical energy storage techonoly.Graphene has a unique two-dimensional morphology,high electrical conductivity and large specific surface area,widely applied in the field of electrochemical energy storage.In this paper,capacitive carbon material was added into the anode of the lithium ion battery with lithium iron phosphate(LFP)as the positive electrodes and hard carbon as the negative electrodes,to construct the lithium ion capacitive battery system.The application of graphene in the lithium-ion capacitive battery has been studied and energy storage system with high energy and high power has been obtained.Using graphene as a conductive agent and lithium iron phosphate with different amounts of activated carbon as positive electrode and hard carbon as negative electrode to prepared lithium-ion capacitive batteries,the electrochemical performance difference of lithium-ion capacitive batteries between pre-lithified and non-pre-lithified and the matching of mass capacity with different activated carbon content have been discussed.It turns out that the first coulomb specific efficiency of lithium-ion capacitive battery is very low,resulting from that a large amount of positive active lithium is consumed by the solid electrolyte membrane(SEI membrane)formed in the negative electrode when lithium-ion capacitive battery is not pre-lithiated;after lithium-ion capacitor battery is pre-lithiated,not only the first Coulomb efficiency was increased,but also energy density and power density are improved.The content of activated carbon has significant influence on the matching value of mass capacity.When the content of activated carbon is 10%and 20%,the ratio of A/C(A here refers to the mass capacity of negative electrode,C refers to the mass capacity of positive electrode,the mass capacity of positive and negative electrode is expressed by the ratio of A/C,the same below)is 1.0 and 1.05,owing the better electrochemical performance than other ratios;When the content of activated carbon is 50%and 80%,the best ratio of A/C is 1.2 and 2.0,Graphene oxide(GO)was prepared by using the modified Hummers method and reduced graphene oxide(rGO)was obtained by reducing at 800?.Utilizing high conductivity and large specific surface area of rGO,rGO and lithium iron phosphate were simply ground to make positive electrode,and hard carbon was used as negative electrode to prepare graphene-type capacitive battery.When A/C value is 1.0,electrochemical performance of lithium-ion capacitive batteries with different rGO contents has been studied.At high current density of 5C,specific capacities of LrGO10-HC,LrGO20-HC and LrGO30-HC capacitive batteries are up to 104.13 mAh g-1,105.34 mAh g-1 and 90.94 mAh g-1,respectively.After 2000 cycles the capacity retention are 86.37%,88.48%and 85.01%,respectively.Therefore,when amount of rGO added is 20%,the as-prepared capacitive battery has the best cycle stability and rate performance.Hydrothermal method is used to combine GO and activated carbon YP to form a graphene/activated carbon aerogel(GO@AC1)with three-dimensional conductive network,which improves conductivity and increases ion transmission channel.LFP and GO@AC1 were used as positive electrode and hard carbon were used as negative electrode to assemble lithium-ion capacitive battery(labeled LrGO/AC-HC).During charging and discharging,its capacitance behavior can be noticeably observed.At 0.2C,1C,5C and 15C,the specific capacities based on the positive mass of LrGO/AC-HC are 168.3 mAh g-1,165 mAh g-1,136.1 mAh g-1,and 83.5 mAh g-1;while specific capacities of LrGO30-HC are 147.1 mAh g-1,149.1 mAh g-1,129.8 mAh g-1 and 83.4 mAh g-1.Therefore,compared with LrGO30-HC,rate performance of LrGO/AC-HC capacitor battery has been improved.This is because compared with graphene material,GrO@AC1 aerogel material has three-dimensional structure,larger specific surface area.The larger the specific surface area,the stronger the ability of material to adsorb ions,so that electrochemical reaction sites of composite material are increased,and rate performance is improved.