Research On Performance Of Fluorocarbonate Electrolyte For High Specific Energy Lithium Metal Battery

The theoretical specific capacity of metallic lithium reaches 3860m Ah/g,an d it has the lowest redox potential.As a negative electrode,it can provide sufficient lithium ions,which is of great significance for improving the energy density of the battery.In practical applications,lithium metal batteries using high-load positive electrodes are due to Electrolyte consumption,electrode/electrolyte interface problems,etc.cause serious battery degradation.This topic aims to improve the capacity decay of high specific energy lithium metal batteries from the perspective of electrolytes,from the solvent and lithium salt aspects,respectively,to reduce the dendritic growth and volume expansion of the lithium metal anode under deep charge and discharge,and to increase the high specific energy.The cycle life of lithium metal batteries.As a cyclic carbonate,fluoroethylene carbonate（FEC）can decompose on the surface of lithium metal to form a stable interface layer,and at the same time,it has a high solvation ability.This paper studies the protective effects of different ratios of FEC/DEC solvents on lithium metal anodes that are deeply charged and discharged.The linear scan（LSV）test shows that the electrochemical window of the fluorinated solvent system is significantly improved compared with the traditional carbonate electrolyte.Comparing FEC/DEC and conventional carbonate solvents on Li|Libatteries and Li|Cu batteries,it is proved that the electrolyte can reduce the interfacial impedance and reduce the overpotential of Li⁺deposition.Assembling Li|NCM811（24.5 mg/cm²）full battery,the capacity is seriously attenuated under the EC solvent system,and after using the fluorinated solvent,the battery is cycled stably for 50 cycles,the CE reaches 86%,and the cycle performance under high temperature and lean electrolyte Promoted.The use of fluorinated solvents can effectively reduce the deposition resistance.It can form a stable SEI on the negative surface of lithium metal and increase the Li⁺diffusion coefficient.Through the characterization methods such as AC impedance spectroscopy,scanning electron microscope,infrared spectroscopy,X-ray diffraction,and Raman spectroscopy,It proves the protective effect of the fluorinated solvent system on the lithium metal negative electrode and the higher voltage resistance capacity during the cycle.At the same time,this electrolyte can inhibit the elution of transition metal ions of the high-load NCM positive electrode,reduce the change of the lattice constant,and improve Electrochemical reversibility.Lithium salt component affects electrochemical performance.Lithium difluorooxalate borate（LiDFOB）has good film-forming properties,changes the morphology of lithium dendrites,and has a passivation effect on the aluminum current collector,and at the same time,it can form a good interface film on the positive electrode.,Reduce the dissolution of metal ions.However,its electrical conductivity is low,and it decomposes under high pressure to produce gas.Lithium tetrafluoroborate（LiBF₄）has weak film-forming properties,but its electrical conductivity is high,which can stabilize LiDFOB.In this paper,a mixed salt system of LiDFOB and LiBF₄ is configured to replace LiPF₆,which is easy to decompose and has poor thermal stability,and improves the cycle stability of lithium metal anodes.After testing,the double-salt system has a higher electrochemical stability window,and its conductivity is slightly lower than that of LiPF₆,but it will not affect the electrochemical performance of the battery.Assembling Li|Libattery,it is proved that compared with LiPF₆,the dual-salt system can reduce the interface load transfer resistance of the lithium metal negative electrode and reduce the impedance increase in the resting state.The deposition and stripping efficiency of the dua l-salt system Li|Cu battery can be maintained at 90%.And the Li⁺stripping overpotential is 24.4 m V.Assemble Li|NCM811（24.5 mg/cm²）full battery,the double salt system is cycled to 70 cycles,the capacity retention rate is still as high as 82.6%,at th is time the capacity retention rate of the LiPF₆ system is 45.5%,and the high temperature and poorness of the battery under the double salt system The electrolyte circulation performance has been improved.By means of physical characterization,the stability and physicochemical properties of the double salt system on the positive and negative electrodes during the cycle are studied,and it is proved that it can control the morphology of lithium dendrites during the cycle,form a stable interfacial film,and reduce the volume of the lithium metal negative electrode,reduce the F content in the positive electrode and the by-products of the interface layer,reduce the change of lattice and eliminate the internal stress.