Study On The Precast And Cycle Stability Of Elastic SEI Film For Metal Lithium Anode

Lithium-ion battery is an indispensable energy storage tool for computers,mobile phones,power tools,energy storage,automobiles and other fields.However,with the evolution of technology,its weight and volumetric energy density can no longer meet the requirements,and its flammable and explosive organic electrolyte also makes batteries have serious safety risks.Metal lithium anode is the ultimate goal of high-energy lithium-ion batteries.Its highest theoretical specific capacity and lowest electrode potential have made it a research hotspot again in recent years.In order to improve the cycle life,fire and explosion caused by lithium dendrites,volume change and SEI film instability during charging and discharging.We proposes to study a precast SEI film that takes into account strength and elasticity in this paper.The SEI film enables it to perform dynamic displacement along with the movement of the metal lithium surface,ensuring that the"interface distance"is as small as possible,thereby preventing the formation of lithium dendrites,improving the cycle stability and safety of the battery.In this paper,we firstly use PVDF-HFP as the matrix to discuss the influence of LiTFSI,LiFSI,LiBOB,LiNO₃,LiPF₆,LiOH,LiF,LiPO₂F₂ and LiDFOB on the performance of mixed matrix precast SEI film.The test results show that when LiBOB is added,the precast SEI film prepared by it has the best cycle performance,which is 2.8 times that of the absence of lithium salt.When different types and contents of lithium salts are added to PVDF-HFP matrix,the morphology will be different,which will affect its tensile strength,elongation,ionic conductivity and electrolyte absorption rate,and further affect its cycle performance.We have studied the influence of adding electrolyte to the precast SEI film with the mixed matrix of PVDF-HFP and LiBOB.Through the utilization of bis 1,3-dioxolane,ethylene glycol dimethyl ether,propylene carbonate,diethyl carbonate,dimethyl carbonate,ethyl methyl carbonate and other solvents,and LiTFSI,LiFSI,LiBOB,LiPF₆,LiPO₂F₂,LiDFOB and LiNO₃ and other lithium salts as solutes,the preferred electrolyte system is:the solvent is 1,3-dioxolane and 1,2-Dimethoxyethane in a volume ratio of 1:1,LiTFSI with 1.0 mol·L^-1 and LiNO₃ with0.40 mol·L^-1 are solutes.On the basis of the above,We have studied the conventional copper foil of 10?m thickness,the copper foil etched by 2 mol·L^-1 hydrochloric acid,the copper foil sprayed with gold and platinum by ion sputtering method,graphene coated carbon black copper foil as the substrates.Observed by electron microscopy,the morphology of the particles on the surface of the substrate has an"inducing"effect on the precast SEI film and the morphology of the deposited metallic lithium primary crystal grains.When the carbon-coated copper foil is used as the substrate,the precast SEI film on it has the highest number of cycles.At 0.5 m A·cm^-2 and 0.5 m Ah·cm^-2,the number of cycles is as high as 820,which is 9 times of without precast SEI film on pure copper foil.In order to further improve the strength and cycle performance of the precast SEI film,we have studied the influence of using aluminum oxide nanospheres,titanium dioxide nanotubes,fumed silica,silicon carbide nanowhiskers and garnet-type fast ion conductor lanthanum zirconate lithium(LLZO)as inorganic additives,and dioctyl phthalate,dinonyl phthalate,diethylene glycol dibenzoate,polymethyl methacrylate and polyethylene oxide as organic additives are blended with PVDF-HFP and LiBOB.The results show that,after mixed with different types and contents of additives,the basic structure of the matrix remains unchanged,but the grain size,inter-grain connection strength and pore size and other properties will change after the composite.It comprehensively affects the ionic conductivity,tensile strength,elongation,and stretching vibration strength of some functional groups in the infrared spectrum of the composite material,thereby affecting the cycle performance of the material.Test result display,in 0.70 g PVDF-HFP and 0.004 mol LiBOB,when 0.8%silicon carbide nanowhiskers and 2%PEO are added,the coin cell with precast SEI film has the best cycle performance.It can be cycled 961 times at 0.5 m A·cm^-2 and0.5 m Ah·cm^-2,and the stable cycle efficiency is as high as 99.1%or more.Through electrochemical performance,SEI film morphology and composition analysis during the cycle,the failure mechanism of the precast SEI film is proposed,and improvement measures are proposed for this failure mode.At the end of the cycle of coin cell with preset SEI film,its delithiation current is significantly reduced and the impedance is significantly increased.The original strong framework collapsed,forming loosely connected Li₂CO₃,LiF and collapsed PVDF-HFP mixture.When the composite oxide prepared by aluminum sol,fumed silica and diboron trioxide is used as the protective layer,the coulombic efficiency of the precast SEI film is quickly stabilized at about 98.8%,and at 1.0 m A·cm^-2 and 1.0 m Ah·cm^-2,the number of cycles is 782,this is an increase of 73%compared to no protective layer.In order to verify the effect of the precast SEI film,we made a full battery for testing.The S/C composites are used as the cathode,at 0.5C/0.5C,when Lithium metal without the precast SEI film as the cathode,the first discharge/charge efficiency of the battery is 93.7%and the cycle number is 161.And when Lithium metal with the precast SEI film as the cathode,the first discharge/charge efficiency of the battery is 96.0%,and the cycle number is 389.