Fabrication And Electrical Properties Of Aqueous Li-ion Capacitor Based On Graphene

With the continuous depletion of non-renewable energy and the increase of environmental protection,the development of a clean,high-performance energy storage device has become an urgent research target for current scientific researchers.Looking at the relatively mature energy storage devices currently on the market,supercapacitors and various types of batteries have advantages and disadvantages in terms of electrochemical performance,safety,and environmental protection,which cannot meet the needs of the modern society.As a new type of energy storage device developed in recent years,aqueous Li-ion capacitors not only have the characteristics of supercapacitors and Li-ion batteries,but also have the advantages of high safety,no pollution,and low cost.That has become one of the research hotspots in the field of energy storage.Therefore,this dissertation studies aqueous lithium ion capacitors and their electrode materials.The main contents of the research are as follows:1.Preparation and characterization of activated graphene.Using the optimized Hummers method,the graphene oxide dispersion was successfully prepared.Using it as a raw material,graphenes activated at different temperatures were prepared by activation of KOH,high temperature calcination and other methods.The morphology and structure of the products were characterized in detail.The electrode material was then prepared as an electrode sheet and its electrochemical performance was tested in 6 M potassium hydroxide electrolyte.The results show that the maximum mass specific capacities of graphene(named 400RGO and 800RGO)reduced at 400 ℃ and 800 ℃ can reach 276.7 F/g and 206.3 F/g at the current density of 1 A/g,respectively.2.Modification and characterization of lithium iron phosphate(LFP).Lithium iron phosphate particles were coated with carbon by dopamine polymerization at room temperature,and the effects of different reaction times on the morphology and properties of the coating were investigated.It was found that the thickness of the carbon layer coated on the surface of the lithium iron phosphate particles gradually increased with the reaction time at room temperature.In addition,the electrochemical performance of lithium iron phosphate with different coating time was measured in 1 M Li2SO4 electrolyte.The results show that the performance is best when the coating time is 5 h,the thickness of the carbon layer is about 6 nm,the specific capacitance can reach 377.6 F/g at the current density of 0.5 A/g,and the cycle performance is much better than that of the original lithium iron phosphate.Lithium iron phosphate was coated with graphene by the solvent evaporation method.When the graphene oxide content was 4.8%,the specific capacitance could reach 373.4 F/g,which was greatly improved compared with the original lithium iron phosphate.3.The construction and the study of electrochemical properties of aqueous Li-ion capacitors.The optimal modified lithium iron phosphate and activated graphene were used as positive and negative electrode materials respectively,and 1 M Li2SO4 was used as electrolyte to construct aqueous Li-ion capacitors.The effects of different anode materials,positive electrode material carbon coating,and the mass ratios of positive and negative electrode on their electrochemical performance were investigated.The results show that the Li-ion capacitors constructed with modified lithium iron phosphate and 400RGO with a mass ratio of 1:1.2 have excellent performance,and which specific capacitance can reach 82.8 F/g at a current density of 0.2 A/g,The specific capacitance can still reach 94%of the initial specific capacitance after 200 cycles of charge and discharge.At the same time,the energy density can reach 11.5 Wh/kg at a power density of 100 W/kg.