Research On Mechanism And Electrochemical Performance Of LiTFSI And LiNO₃ In Lithium Air Battery Organic Electrolyte System

Non-aqueous lithium-air batteries have attracted great attention due to their highest theoretical energy density among different types of rechargeable batteries.However,the application of lithium air battery faces many problems,such as high charge and discharge overpotential,poor cycle performance,and actual discharge capacity of the battery is much smaller than the theoretical value.Due to the poor conductivity of the discharge product,the discharge product accumulated on the surface of the positive electrode during the discharge process will passivate the positive electrode.It can be seen from the current research that the lithium air battery discharge process follows two mechanisms,one is the surface mechanism and the other is the solution mechanism.The discharge process is affected by factors such as electrolyte and cathode material.Therefore,this paper uses an electrolyte composed of different organic solvents and lithium salts to explore the effect of solvent and lithium salt on battery performance.The organic electrolyte generally contains an aprotic organic solvent and a lithium salt.Solvents and lithium salts can affect the morphology,particle size and yield of the product during discharge,which is directly related to the capacity of the battery.A solvent with a high DN value is advantageous for generating a discharge product from a solution route,but its stability is poor,while a solvent having a low DN value has good stability,but a discharge capacity is low.The DN of the electrolyte is affected by both the solvent and the anion in the lithium salt.Therefore,this experiment uses three solvents commonly used in lithium air batteries,from low to high DN values are TMS,TEGDME,DMSO;and two commonly used lithium salts,of which Li TFSI as low DN lithium salt,Li NO3 as high DN lithium salt.It is considered that different positive electrodes will also affect the discharge mechanism.For positive electrodes with strong oxygen adsorption,it is beneficial to discharge from the surface route;and for positive electrodes with weak oxygen adsorption,it is beneficial to discharge from the solution path.In order to study the effects of electrolyte and cathode on the electrochemical performance of the battery and the mechanism of the reaction,two cathodes were selected for comparative experiments.One is a KB cathode with weak oxygen adsorption,and the other is a Co3O4 cathode with strong oxygen adsorption.The main content of this paper can be summarized as follows:(1)The effect of three different DN organic solvents on the electrochemical performance of the battery was studied by using KB carbon-based cathode and lithium bis(trifluoromethanesulfonimide)(Li TFSI)as lithium salt.From low to high DN values are sulfolane(TMS),tetraethylene glycol dimethyl ether(TEGDME)and dimethyl sulfoxide(DMSO).Electrochemical performances such as EIS,deep discharge and constant current circulation were tested on lithium air batteries using three different electrolytes,and the discharge products were characterized by XRD and FESEM.When a low DN solvent is used,the discharge capacity fluctuates greatly with changes in current density.As the current density increases,the discharge capacity decreases.When a solvent with a high DN is used,the discharge capacity of the battery changes little with the current density,and a high discharge capacity can be obtained under a large current density condition.This is because the high DN solvent can better solvate the discharge intermediate,which is beneficial to follow the solution mechanism for the discharge process.However,compared with the low DN solvent,the electrolyte of DMSO as a solvent also has problems such as easy decomposition of the electrolyte and generation of by-products during the circulation,and thus the cycle performance of the battery is poor.(2)Comparative experiments were conducted using Li NO3 as a lithium salt.In the TMS,TEGDME solvent,a higher discharge capacity can be obtained by using a high DN lithium salt compared to a low DN lithium salt because NO3-can promote the solubility of the discharge intermediate in a low DN solvent.In the high DN solvent DMSO,the discharge capacity of the battery was not significantly improved by using the high DN lithium salt.(3)A comparative experiment was carried out by replacing the KB with a Co3O4 nanowire grown on foamed nickel by hydrothermal method.The use of high DN lithium salts in low DN solvents compared to low DN lithium salts did not significantly improve battery discharge capacity.This is because the positive electrode has a stronger adsorption for O2-,while the low DN electrolyte has a weaker O2-solvation,so the discharge process follows the surface mechanism.However,the use of a high DN solvent promotes the solvation of O2-in the electrolyte,which facilitates discharge in accordance with the solution mechanism,thereby obtaining a higher discharge capacity of the battery.In summary,this experiment explored the effects of lithium salts(Li TFSI,Li NO3)and solvents(TMS,TEGDME,DMSO)on the electrochemical properties of the discharge products of lithium air batteries.And KB cathode and Co3O4 cathode were used for comparison experiments to explore their influence on the mechanism of charge and discharge process.