Study On Performance Of Electrolyte Additives And NiO-COF Cathode Material For Lithium Carbon Dioxide Battery

The theoretical energy density of lithium-air batteries is 3458 Wh·kg^-1,which is much higher than that of general lithium-ion batter ies,so it has broad application prospects in many fields.Lithium carbon dioxide battery(Li-CO₂ battery)is a rechargeable lithium battery developed from a lithium-air battery,which has similar advantages:simple structure,low cost,and high energy density.Lithium carbon dioxide batteries can fix carbon dioxide and apply carbon dioxide as a greenhouse gas in energy storage devices,thereby reducing fossil fuel consumption and reducing the"greenhouse effect."However,the lithium-carbon dioxide battery still has several unresolved problems,such as high charge and discharge overpotential,poor battery cycle performance,slow reaction rate of the cathode,and anode lithium dendrites,which hindered its practical progress.In view of the current problems,this paper systematically studied the effect of lithium iodide and potassium iodide as electrolyte additives on the performance of lithium carbon dioxide batteries and synthesized a new type of NiO-COF material as a cathode material from two aspects of electrolyte additives and cathode material modification.This paper aimed at improving the performance of lithium carbon dioxide batteries.We have carried out the following three aspects of research:1.The lithium iodide(LiI)additive was studied as a redox mediator in the electrolyte of lith ium carbon dioxide batteries,which significantly reduced the overpotential during the charge and discharge of the battery.Combined with SEM,XRD and other analysis,it can be seen that the additive lithium iodide can promote the oxidative decomposition of discharge products,avoiding the accumulation of reaction products on the cathode,which is beneficial to the extension of battery life.In addition,the I^-reversible redox reaction can be used as a liquid-phase"redox mediator".Through ex-situ UV testing,we confirmed that I₃^-can chemically oxidize lithium carbonate,the discharge product of lithium-carbon dioxide batter ies.This study provides a new way to improve the performance of lithium-carbon dioxide batteries.The discharge capacity of a lithium carbon dioxide battery at a current density of 50 m A g^-1 was increased from6700 m A h g^-1 to 9500 m A h g^-1.Lithium iodide(Li I)additive improves the reactivity of the cathode and the cycle stability of the battery.When the battery is limited to 500 m A h g^-1 and the current density is 100m A g^-1,the cycle number is 70 weeks.2.Potassium iodide(KI)was used as an electrolyte additive to study the improvement of lithium carbon dioxide battery performance.The battery is charged and discharged with a cut-off capacity of 500m A h g^-1 at a current density of 100 m A g^-1,which can be cycled steadily for 70 weeks.The introduction of potassium iodide in the lithium carbon dioxide battery effectively reduces the charging polar ization voltage of the battery by about 1.2 V.Through the characterization analysis of the cathode,it can be seen that the addition of KI is benefic ial to the formation of loose and porous cathode morphology,promotes the oxidative decomposition of the discharge product lithium carbonate,and avoids the accumulation of discharge product blocking the pores of the cathode,thus inhibiting the rapid decline of battery performance.The reversible redox reaction of I^-makes it possible to act as a liquid-phase"redox mediator".I^-is first oxidized to I₃^-,then I₃^-reacts with lithium carbonate and is reduced to I^-by itself.The introduction of the electrolyte additive potassium iodide(KI)in lithium carbon dioxide batter ies can reduce the battery's charge and discharge overpotential and improve the battery's cycle performance.KI acts as an electrolyte redox mediator,effectively improving the electrochemical performance of lithium carbon dioxide batteries.3.Covalent organic framework(COF)materials are a class of organic porous materials formed by covalent bonding,which have a structure and morphology very similar to those of graphene,and their porous structure makes them useful for excellent catalyst supports.Most of the synthesized COF is obtained by solvothermal synthesis.It takes a long time to form a well-structured crystal structure.Microwave heating can shorten the reaction time.In this paper,a new type of NiO-COF material was prepared by using microwave-assisted solvothermal method.By characterizing the morphology of the synthesized NiO-COF material,the results show that we have successfully synthesized a uniform morphology NiO-COF mater ial.The prepared NiO-COF material and carbon nanotubes are mixed in a certain ratio and coated on the carbon paper,as the cathode of the rechargeable lithium carbon dioxide battery.By testing the cycle performance of the battery,the performance of the battery is more excellent than the cathode made of pure carbon tube.It can be stably cycled for 50 weeks under the conditions of current density of 100 m A g^-1 and constant capacity of 500 m A h g^-1.The cathode material has the advantages of NiO-COF material and carbon nanotube material.On the one hand,it exerts the excellent electrical conductivity of carbon nanotubes.At the same time,the COF structure provides a supporting framework for the cathode material that provides channels for carbon dioxide and electrolyte,and storage sites for reactants,which is beneficial to the accumulation of reaction products.These advantages make the electrochemical performance of the material significantly improved.