A se i film, a lithium metal battery and a method of reducing concentration polarization of a lithium metal battery

Abstract

The application provides a SEI film, a lithium metal battery and a method for reducing concentration polarization of the lithium metal battery. The SEI film comprises a phenyl siloxane and a lithium source. The SEI film provided by the application can, through cooperation of the phenyl siloxane and the lithium source, to a certain extent, alleviate the concentration gradient caused by slow lithium ion transmission kinetics of the electrode negative electrolyte layer, reduce the concentration polarization phenomenon, so as to realize dendrite-free lithium deposition / peeling, and the SEI film also plays a physical barrier role to prevent corrosion reaction of the lithium metal and the electrolyte.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of lithium ion batteries, and relates to an SEI film, a lithium metal battery and a method for reducing concentration polarization of the lithium metal battery. BACKGROUND

[0002] Because metal lithium has super-high specific energy (3860 mAh / g) and the lowest negative electrochemical potential (-3.04 V vs. SHE), it has attracted widespread interest of researchers in recent years.

[0003] However, the volume expansion and safety problems caused by lithium dendrite growth during battery operation have become one of the biggest problems hindering the commercialization of metal lithium negative electrodes. Therefore, many researchers have made key researches on the lithium dendrite growth mechanism (mainly focusing on the nucleation and growth process) to find solutions. Chazalviel combined the diffusion law and the ion space charge equation in the solution, and believed that the ion depletion on the interface was more likely to occur in the flow battery. Because the effect of ion depletion on the electrode surface on electrodeposition was focused on in the liquid flow lithium metal battery, it was concluded that ion depletion would produce a net space charge region, thereby gradually increasing the local overpotential, and eventually causing the growth of fine dendrites. As can be seen from the above, in the nucleation and growth process, the overpotential caused by the change of ion concentration on the lithium metal electrode interface, and the lithium metal deposition caused the lithium ion concentration in the solution near the electrode interface to decrease, and there was a potential gradient caused by the concentration gradient near the interface. If the lithium ion concentration tends to zero on the interface, the interface electric field increases sharply in the form of a logarithmic function, and further causes serious dendrite growth. Therefore, it is very important to study the influence caused by concentration polarization.

[0004] Lithium metal concentration polarization refers to the process where lithium metal, under an electric field, migrates from the bulk solution to the surface of a lithium metal electrode, gains electrons, and deposits as lithium, simultaneously exhibiting concentration polarization. The concentration difference creates a potential difference (concentration polarization potential) on the lithium metal surface. After initial deposition, a relatively loose dendrite layer forms on the lithium metal surface, allowing the electrolyte to penetrate the dendrite layer and reach the dense lithium metal surface. When the growing dendrites penetrate the concentration polarization layer, a short circuit occurs within the concentration cell. At this point, the concentration cell tends to release all its charge and reach potential equilibrium between the positive and negative electrodes. This results in lithium ions depositing at the dendrite tips to reduce the ion concentration in the surrounding electrolyte. Simultaneously, lithium ions dissolve at the dendrite roots to increase the ion concentration in the surrounding electrolyte, accelerating the formation of a dead lithium layer. If concentration polarization on the lithium metal surface is not effectively suppressed, according to the Nernst equation, as Cli approaches 0, the overpotential on the lithium metal surface will increase sharply in a logarithmic manner. The voltage loss caused by concentration polarization will significantly reduce the energy density of the lithium metal full cell and decrease the actual capacity contribution of the positive electrode delithiation. Furthermore, when the potential drops below the electrolyte reduction potential, side reactions such as electrolyte decomposition will occur on the lithium metal surface, resulting in battery capacity loss.

[0005] Therefore, how to suppress concentration polarization on the lithium metal surface of lithium metal batteries is a technical problem that urgently needs to be solved. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide an SEI film, a lithium metal battery, and a method for reducing concentration polarization in lithium metal batteries. The SEI film provided by this invention, through the combination of phenyl siloxane and a lithium source, alleviates the concentration gradient caused by the slow lithium-ion transport kinetics in the negative electrode liquid layer to a certain extent, reducing concentration polarization and thus achieving dendrite-free lithium deposition / stripping. Furthermore, the SEI film also acts as a physical barrier, preventing corrosion reactions between lithium metal and the electrolyte.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides an SEI membrane comprising a phenyl siloxane and a lithium source.

[0009] The SEI film provided by this invention utilizes a combination of phenyl siloxanes and a lithium source. Phenyl siloxane compounds possess excellent electronic insulation and film-forming properties; when used with lithium, they can construct an artificial SEI film, effectively controlling lithium deposition. Furthermore, phenyl siloxanes exhibit strong lithium adsorption capacity, and the adsorption energies at multiple benzene ring sites form a gradient distribution, effectively capturing and uniformly regulating Li. +The flux can alleviate the concentration gradient caused by the slow lithium-ion transport kinetics in the negative electrode liquid layer to a certain extent, reduce the concentration polarization phenomenon, thereby achieving dendrite-free lithium deposition / stripping. In addition, the SEI film also acts as a physical barrier to prevent lithium metal from undergoing corrosion reaction with the electrolyte.

[0010] In the SEI film provided by this invention, phenyl siloxanes and lithium sources must work synergistically to construct an artificial SEI film and effectively regulate lithium deposition; if a pure phenyl siloxane film structure is used, it is impossible to capture and uniformly regulate Li. + The flux is insufficient to alleviate the concentration gradient caused by the slow lithium-ion transport kinetics in the negative electrode liquid layer, and it is unable to reduce the concentration polarization phenomenon. Uneven distribution of lithium dendrites leads to abnormal battery performance and a series of safety issues.

[0011] Preferably, the thickness of the SEI film is 50nm to 50μm, such as 50nm, 100nm, 300nm, 500nm, 800nm, 1μm, 5μm, 10μm, 20μm, 30μm, 40μm or 50μm.

[0012] Preferably, the phenyl siloxane includes any one or a combination of at least two of the following: octaphenylcyclotetrasiloxane, hexaphenyldisiloxane, octaphenyl-POSS, trisiloxyphenyl cage-like silsesquioxane, phenylmethylsiloxane copolymer, hexaphenylcyclotrisiloxane, octaphenylcyclotetrasiloxane, diphenyldimethylcyclosiloxane, polyphenyldimethylcyclosiloxane, or N-phenylamino-cage-like polysilsesquioxane.

[0013] Preferably, the lithium source includes any one or a combination of at least two of lithium trifluoromethanesulfonate, lithium, lithium silicon, or lithium phenyl.

[0014] In this invention, the lithium source can be selected from a variety of options.

[0015] In a second aspect, the present invention provides a lithium metal battery, the lithium metal battery comprising a positive electrode, a separator, a lithium metal negative electrode and an electrolyte; an SEI film as described in the first aspect is disposed between the separator and the lithium metal negative electrode.

[0016] The lithium metal battery provided by this invention comprises an SEI film as described in the first aspect (the SEI film is located between the lithium layer of the lithium metal anode and the separator) between the separator and the lithium metal anode. Through the combination of phenyl siloxanes and a lithium source, phenyl siloxane compounds possess excellent electronic insulation and film-forming properties. When used with lithium, they can construct an artificial SEI film, effectively controlling lithium deposition. Phenyl siloxanes have a strong adsorption capacity for lithium, and the adsorption energy at multiple benzene ring sites forms a gradient distribution, effectively capturing and uniformly regulating Li. +The flux can alleviate the concentration gradient caused by the slow lithium-ion transport kinetics in the negative electrode liquid layer to a certain extent, reduce the concentration polarization phenomenon, thereby achieving dendrite-free lithium deposition / stripping. In addition, the SEI film also acts as a physical barrier to prevent lithium metal from undergoing corrosion reaction with the electrolyte.

[0017] In this invention, the SEI film provided in the first aspect is placed between the separator and the lithium metal anode to form an artificial SEI film on the surface of the lithium metal anode and effectively regulate lithium deposition; if it is not placed between the two, it will cause the cell to have excessive impedance or even abnormal short circuit.

[0018] Preferably, the diaphragm comprises a porous diaphragm.

[0019] In this invention, a porous membrane is selected. The increased surface area within the pores of the membrane leads to a decrease in local current density. Compared to a smooth, non-porous membrane surface, the increased lithium-ion diffusion distance within the pores reduces the lithium deposition rate at the lithium anode. Based on the polarization concentration polarization law formula…

[0020]

[0021] In the formula, j is the current density, j limit The limiting diffusion current density is given. Since the porous surface reduces the local current density j, jlimit increases. As can be seen from the concentration polarization overpotential formula, the porous membrane can significantly reduce the concentration polarization at the lithium interface.

[0022]

[0023] In the formula, This represents the actual electrode potential; This is the standard equilibrium potential;

[0024] Where is the lithium ion concentration on the electrode surface; R is the molar gas constant; T, therefore, the use of a porous membrane structure better achieves the goal of reducing concentration polarization at the lithium metal interface.

[0025] Preferably, the porosity of the porous membrane is 20% to 75%, for example, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or 75%.

[0026] In this invention, if the porosity of the porous membrane is too small, it will not be conducive to reducing the local current density on the lithium surface, while if the porosity is too large, it will lead to a short circuit in the battery cell. Furthermore, it should be noted that the materials of the porous membranes provided in this invention are all conventional technical selections. This invention applies to all types of porous membranes that can be known to those skilled in the art within a reasonable range, including but not limited to polyacrylonitrile membranes, porous polymer membranes, PP, PE, inorganic composite membranes, and non-woven membranes.

[0027] Preferably, the SEI membrane is directly laminated to the surface of the diaphragm.

[0028] Alternatively, the SEI film may be directly composited with the surface of the lithium metal anode.

[0029] Alternatively, the SEI film may be located independently between the separator and the lithium metal anode.

[0030] In this invention, the SEI film can be prepared on the surface of a separator, on the surface of the lithium layer of a lithium metal anode, or directly and independently as a film structure between the separator and the lithium metal anode. Those skilled in the art can make adaptive selections and adjustments according to actual needs.

[0031] Preferably, when the SEI film is directly bonded to the surface of the separator or directly bonded to the surface of the lithium metal anode, the bonding method includes:

[0032] A phenyl siloxane, a lithium source, and a solvent are mixed to obtain a mixed solution. The mixed slurry is then coated onto the surface of a separator or a lithium metal anode and dried to obtain an SEI film.

[0033] Preferably, when the SEI film is independently located between the separator and the lithium metal anode, the method includes:

[0034] A phenyl siloxane, a lithium source, and a solvent are mixed to obtain a mixed solution. The mixed slurry is then formed into a film, and the film structure is disposed between the separator and the lithium metal anode.

[0035] Preferably, the molar concentration of the phenyl siloxane in the mixed solution is 0.1–100 mmol / L, for example, 0.1 mmol / L, 0.5 mmol / L, 1 mmol / L, 5 mmol / L, 10 mmol / L, 15 mmol / L, 20 mmol / L, 25 mmol / L, 30 mmol / L, 35 mmol / L, 40 mmol / L, 45 mmol / L, 50 mmol / L, 55 mmol / L, 60 mmol / L, 65 mmol / L, 70 mmol / L, 75 mmol / L, 80 mmol / L, 85 mmol / L, 90 mmol / L, 95 mmol / L, or 100 mmol / L, and more preferably 0.2–50 mmol / L.

[0036] In the mixed solution provided by the present invention, if the molar concentration is too low, below 0.1 mmol / L, it is not conducive to film formation, while if the molar concentration is too high, above 100 mmol / L, it will lead to low conductivity and insulation. Furthermore, when it is in the preferred range of 10 to 50 mmol / L, the degree of electronic insulation and the thickness of the film can be greatly improved.

[0037] Preferably, the lithium source has a mass fraction of ≤1 wt% in the mixed solution, such as 0.01 wt%, 0.05 wt%, 0.1 wt%, 0.15 wt%, 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%, 0.5 wt%, 0.55 wt%, 0.6 wt%, 0.65 wt%, 0.7 wt%, 0.75 wt%, 0.8 wt%, 0.85 wt%, 0.9 wt%, 0.95 wt%, or 1 wt%.

[0038] In this invention, the mass percentage of the lithium source in the mixed solution does not need to be too high. Too high a mass percentage will result in excess lithium, leading to dead lithium or lithium powder formation, which can cause a short circuit in the battery.

[0039] Preferably, the thickness of the SEI film is 50nm to 50μm, such as 50nm, 100nm, 300nm, 500nm, 800nm, 1μm, 5μm, 10μm, 20μm, 30μm, 40μm or 50μm.

[0040] In this invention, if the SEI film is too thin, it will be unable to suppress the excessive dissolution of the solvent, while if the film is too thick, the impedance and concentration polarization will be too large.

[0041] Preferably, the solvent includes any one or a combination of at least two of tetrahydrofuran, dimethyl carbonate, N-methylformamide, dimethylformamide, or methylpyrrolidone.

[0042] It should be noted that, apart from the technical features with special requirements provided by this invention, the remaining structures and material selections in lithium metal batteries are all conventional technical means.

[0043] Furthermore, the positive electrode includes a positive current collector and a positive active material layer located on at least one side of the positive current collector, the positive active material layer including a positive active material, a conductive agent and a binder.

[0044] The types of raw materials mentioned above are not particularly limited. Any known substance can be used in this application without departing from the inventive concept of this application.

[0045] Positive electrode active materials include, but are not limited to, lithium cobalt oxide (LiCoO2) and lithium nickel cobalt manganese oxide (LiNi). x Mn y Co 1-x-y O2 (NMC), lithium nickel cobalt aluminum oxide (LiNiCoAlO2, NCA), lithium manganese oxide (LiMn2O4), lithium manganese iron phosphate (LiMn) x Fe 1-x PO4 (abbreviated as LMFP), lithium vanadium phosphate (Li3V2(PO4)3), lithium vanadium oxide phosphate (LiVOPO4), lithium iron phosphate (LiFePO4), lithium titanate (Li2TiO3), and one or more lithium-rich manganese-based materials.

[0046] There are no particular restrictions on the positive and negative current collectors, as long as they are conductive and do not cause chemical changes in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, metal current collectors treated with carbon or other substances, and composite current collectors can be used.

[0047] Adhesives are components used to assist in the bonding of active materials, conductive materials, etc., and to the bonding of current collectors. Specifically, they may include at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, styrene-butadiene rubber, polyacrylic acid, polyacrylonitrile, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber.

[0048] Conductive agents can be used to assist and improve the conductivity in secondary batteries, and there are no particular limitations, as long as they are conductive without causing chemical changes. Specifically, they may include graphite, such as natural or artificial graphite; carbon materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; conductive fibers, such as carbon fibers and metal fibers; conductive tubes, such as carbon nanotubes; metal powders, such as fluorocarbon powders, aluminum powders, and nickel powders; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxides; and polyphenylene derivatives.

[0049] Electrolytes consist of electrolyte salts and solvents.

[0050] Electrolyte salts include, but are not limited to, one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium difluorosulfonylimide, lithium bistrifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0051] Solvents include, but are not limited to, one or more of the following: ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0052] The electrolyte also includes additives, which can be negative electrode film-forming additives, positive electrode film-forming additives, or additives that can improve certain battery performance, such as additives that improve battery overcharge performance, or additives that improve battery high-temperature or low-temperature performance. Any known type of additive can be used in this application without departing from the inventive concept. There are no special requirements for the mixing method of the additives; for example, they can be directly mixed with conductive agents, active materials, and binders to form a mixture.

[0053] Furthermore, the lithium metal anode can be a pure lithium metal end, or a composite lithium metal anode containing a current collector, or the corresponding anode structure can be modified in a manner known to those skilled in the art.

[0054] Thirdly, the present invention provides a method for preparing a lithium metal battery as described in the second aspect, wherein a positive electrode, a separator and a lithium metal negative electrode are sequentially combined, and the combined structure is injected into an electrolyte to obtain the lithium metal battery.

[0055] The SEI film is located between the separator and the lithium metal anode.

[0056] The method for preparing the lithium metal anode provided by this invention, except for the method for setting the SEI film provided in the first aspect, involves conventional techniques for the remaining preparation processes.

[0057] Fourthly, the present invention also provides a method for reducing concentration polarization in a lithium metal battery, the method comprising disposing an SEI film as described in the first aspect between a separator of the lithium metal battery and the lithium metal anode.

[0058] The method provided by this invention involves placing an SEI film as described in the first aspect between a separator and a lithium metal anode (the SEI film is located between the lithium layer of the lithium metal anode and the separator). Through the combination of phenyl siloxanes and a lithium source, phenyl siloxane compounds possess excellent electronic insulation and film-forming properties. When used with lithium, they can construct an artificial SEI film, effectively controlling lithium deposition. Phenyl siloxanes have a strong adsorption capacity for lithium, and the adsorption energy at multiple benzene ring sites forms a gradient distribution, effectively capturing and uniformly regulating Li. +The flux can alleviate the concentration gradient caused by the slow lithium-ion transport kinetics in the negative electrode liquid layer to a certain extent, reduce the concentration polarization phenomenon, thereby achieving dendrite-free lithium deposition / stripping. In addition, the SEI film also acts as a physical barrier to prevent lithium metal from undergoing corrosion reaction with the electrolyte.

[0059] Compared with the prior art, the present invention has the following beneficial effects:

[0060] The SEI film provided by this invention, through the combination of phenyl siloxanes and a lithium source, utilizes the excellent electronic insulation and film-forming properties of phenyl siloxane compounds. When used with lithium, an artificial SEI film can be constructed, effectively controlling lithium deposition. Phenyl siloxanes have a strong adsorption capacity for lithium, and the adsorption energy at multiple benzene ring sites forms a gradient distribution, effectively capturing and uniformly regulating the Li+ flux. This can alleviate the concentration gradient caused by the slow lithium-ion transport kinetics in the negative electrode liquid layer to a certain extent, reducing concentration polarization and thus achieving dendrite-free lithium deposition / stripping. Furthermore, the SEI film also acts as a physical barrier, preventing corrosion reactions between lithium metal and the electrolyte. Therefore, it improves the initial efficiency, cycle life, and coulombic efficiency of lithium metal batteries. Detailed Implementation

[0061] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments are merely illustrative of the present invention and should not be construed as limiting the invention.

[0062] Example 1

[0063] This embodiment provides a lithium metal battery, which includes a positive electrode, a separator, a lithium metal negative electrode, and an electrolyte; an SEI film is disposed between the separator and the lithium metal negative electrode, the SEI film is prepared on the surface of the separator, the SEI film includes hexaphenyldisiloxane and lithium trifluoromethanesulfonate, and the separator is a porous separator (a polyacrylonitrile separator with a porosity of 20%).

[0064] The method for preparing the lithium metal battery is as follows:

[0065] (a) Preparation of SEI membrane-diaphragm composite structure:

[0066] 1) Dissolve hexaphenyldisiloxane in N-methylformamide at a molar concentration of 0.25 mmol / L to form a uniformly dispersed, transparent mixed solution;

[0067] 2) Add 0.1 wt% lithium trifluoromethanesulfonate to the above mixed solution (the mass concentration of lithium trifluoromethanesulfonate in the mixed solution is 0.1 wt%) and stir until homogeneous;

[0068] 3) Spray the mixed solution from step 2) onto the surface of the porous membrane and dry it in a vacuum drying oven at 50°C for 16 hours to obtain an SEI membrane-membrane composite structure with an SEI membrane thickness of 50 nm.

[0069] 4) Cut into the required diaphragm size and set aside;

[0070] (b) Preparation of positive electrode sheet: Lithium iron phosphate, conductive agent SuperP, binder PVDF and carbon nanotubes (CNT) are mixed evenly in a mass ratio of 96.5:2.5:1.0:0.5 to prepare a lithium-ion battery positive electrode slurry of a certain viscosity. The slurry is coated on aluminum foil for current collector, dried at 85°C and then cold-pressed. After trimming, cutting and slitting, the slurry is dried at 85°C under vacuum for 8 hours to prepare a lithium metal battery positive electrode sheet that meets the requirements.

[0071] (c) The negative electrode uses 6μm thick copper foil purchased from the market and double-sided lithium-coated (lithium thickness is 20μm) copper-lithium composite strip; it is made into a lithium metal battery negative electrode sheet that meets the requirements after edge cutting, cutting and slitting.

[0072] (d) Preparation of lithium metal pouch battery: The positive electrode, negative electrode and separator prepared according to the above process are stacked to form a three positive and four negative lithium metal battery (wherein, an SEI film is set between the separator and the lithium metal negative electrode), with a capacity of 1500mAh. Electrolyte (1mol / L LiPF6, solvent is FEC+DMC+TTE, where the volume ratio of FEC:DMC:TTE is 1:1:1) is injected to complete the battery fabrication.

[0073] Example 2

[0074] This embodiment provides a lithium metal battery, which includes a positive electrode, a separator, a lithium metal negative electrode, and an electrolyte; an SEI film is disposed between the separator and the lithium metal negative electrode, the SEI film is prepared on the lithium layer surface of the lithium metal negative electrode, the SEI film includes hexaphenyldisiloxane and lithium trifluoromethanesulfonate, and the separator is a porous separator (PP / PE composite separator with a porosity of 25%).

[0075] The method for preparing the lithium metal battery is as follows:

[0076] (a) Preparation of SEI film-lithium metal anode composite structure:

[0077] 1) Dissolve hexaphenyldisiloxane in N-methylformamide at a molar concentration of 0.25 mmol / L to form a uniformly dispersed, transparent mixed solution;

[0078] 2) Add 0.1 wt% lithium trifluoromethanesulfonate to the above mixed solution (the mass concentration of lithium trifluoromethanesulfonate in the mixed solution is 0.1 wt%) and stir until homogeneous;

[0079] 3) The mixed solution from step 2) was sprayed onto the lithium layer surface of the lithium metal anode (the anode was a copper-lithium composite strip with a 6μm thick copper foil purchased from the market and lithium on both sides (the thickness of the lithium was 20μm); after edge cutting, cutting and slitting, it was made into a lithium metal battery anode sheet that met the requirements) and placed in a vacuum drying oven at 50°C for 16h to obtain an SEI film-lithium metal anode composite structure with an SEI film thickness of 80nm;

[0080] 4) Cut into the required negative electrode sheet size and set aside;

[0081] (b) Preparation of positive electrode sheet: Lithium iron phosphate, conductive agent SuperP, binder PVDF and carbon nanotubes (CNT) are mixed evenly in a mass ratio of 96.5:2.5:1.0:0.5 to prepare a lithium-ion battery positive electrode slurry of a certain viscosity. The slurry is coated on aluminum foil for current collector, dried at 85°C and then cold-pressed. After trimming, cutting and slitting, the slurry is dried at 85°C under vacuum for 8 hours to prepare a lithium metal battery positive electrode sheet that meets the requirements.

[0082] (c) Preparation of lithium metal pouch battery: The positive electrode, negative electrode and separator prepared according to the above process are stacked to form a lithium metal battery with three positive and four negative electrodes (wherein, an SEI film is set between the separator and the lithium metal negative electrode), with a capacity of 1500mAh. Electrolyte (1mol / L LiPF6, solvent is FEC+DMC+TTE, where the volume ratio of FEC:DMC:TTE is 1:1:1) is injected to complete the battery fabrication.

[0083] Example 3

[0084] This embodiment provides a lithium metal battery, which includes a positive electrode, a separator, a lithium metal negative electrode, and an electrolyte; an SEI film is disposed between the separator and the lithium metal negative electrode, the SEI film is independently located between the separator and the lithium metal negative electrode, the SEI film includes hexaphenyldisiloxane and lithium trifluoromethanesulfonate, and the separator is a porous separator (a double-sided ceramic separator with a porosity of 36%).

[0085] The method for preparing the lithium metal battery is as follows:

[0086] (a) Preparation of SEI membrane:

[0087] 1) Dissolve hexaphenyldisiloxane in N-methylformamide at a molar concentration of 50 mol / L to form a uniformly dispersed, transparent mixed solution;

[0088] 2) Add 0.1 wt% lithium trifluoromethanesulfonate to the above mixed solution (the mass concentration of lithium trifluoromethanesulfonate in the mixed solution is 0.1 wt%) and stir until homogeneous;

[0089] 3) Pour the mixed solution from step 2) into the membrane mold and dry it in a vacuum drying oven at 50°C for 16 hours. The final thickness is 50 μm.

[0090] 4) Cut into the required diaphragm size and set aside;

[0091] (b) Preparation of positive electrode sheet: Lithium iron phosphate, conductive agent SuperP, binder PVDF and carbon nanotubes (CNT) are mixed evenly in a mass ratio of 96.5:2.5:1.0:0.5 to prepare a lithium-ion battery positive electrode slurry of a certain viscosity. The slurry is coated on aluminum foil for current collector, dried at 85°C and then cold-pressed. After trimming, cutting and slitting, the slurry is dried at 85°C under vacuum for 8 hours to prepare a lithium metal battery positive electrode sheet that meets the requirements.

[0092] (c) The negative electrode uses 6μm thick copper foil purchased from the market and double-sided lithium-coated (lithium thickness is 20μm) copper-lithium composite strip; it is made into a lithium metal battery negative electrode sheet that meets the requirements after edge cutting, cutting and slitting.

[0093] (d) Preparation of lithium metal pouch battery: The positive electrode, negative electrode and separator prepared according to the above process are stacked to form a three positive and four negative lithium metal battery (wherein, an SEI film is set between the separator and the lithium metal negative electrode), with a capacity of 1500mAh. Electrolyte (1mol / L LiPF6, solvent is FEC+DMC+TTE, where the volume ratio of FEC:DMC:TTE is 1:1:1) is injected to complete the battery fabrication.

[0094] Example 4

[0095] The difference between this embodiment and Embodiment 1 is that in this embodiment, the phenyl siloxane is a phenylmethyl siloxane copolymer and the lithium source is phenyl lithium.

[0096] The remaining preparation methods and parameters are consistent with those in Example 1.

[0097] Example 5

[0098] This embodiment provides a lithium metal battery, which includes a positive electrode, a separator, a lithium metal negative electrode, and an electrolyte; an SEI film is disposed between the separator and the lithium metal negative electrode, the SEI film is prepared on the surface of the separator, the SEI film includes hexaphenyldisiloxane and lithium trifluoromethanesulfonate, and the separator is a porous separator (PP separator with a porosity of 45%).

[0099] The method for preparing the lithium metal battery is as follows:

[0100] (a) Preparation of SEI membrane-diaphragm composite structure:

[0101] 1) Dissolve hexaphenyldisiloxane in N-methylformamide at a molar concentration of 50 mmol / L to form a uniformly dispersed, transparent mixed solution;

[0102] 2) Add 0.5 wt% lithium trifluoromethanesulfonate to the above mixed solution (the mass concentration of lithium trifluoromethanesulfonate in the mixed solution is 0.5 wt%) and stir until homogeneous;

[0103] 3) Spray the mixed solution from step 2) onto the surface of the porous membrane and dry it in a vacuum drying oven at 50°C for 16 hours to obtain an SEI membrane-membrane composite structure with an SEI membrane thickness of 25 μm.

[0104] 4) Cut into the required diaphragm size and set aside;

[0105] (b) Preparation of positive electrode sheet: Lithium iron phosphate, conductive agent SuperP, binder PVDF and carbon nanotubes (CNT) are mixed evenly in a mass ratio of 96.5:2.5:1.0:0.5 to prepare a lithium-ion battery positive electrode slurry of a certain viscosity. The slurry is coated on aluminum foil for current collector, dried at 85°C and then cold-pressed. After trimming, cutting and slitting, the slurry is dried at 85°C under vacuum for 8 hours to prepare a lithium metal battery positive electrode sheet that meets the requirements.

[0106] (c) The negative electrode uses 6μm thick copper foil purchased from the market and double-sided lithium-coated (lithium thickness is 20μm) copper-lithium composite strip; it is made into a lithium metal battery negative electrode sheet that meets the requirements after edge cutting, cutting and slitting.

[0107] (d) Preparation of lithium metal pouch battery: The positive electrode, negative electrode and separator prepared according to the above process are stacked to form a three positive and four negative lithium metal battery (wherein, an SEI film is set between the separator and the lithium metal negative electrode), with a capacity of 1500mAh. Electrolyte (1mol / L LiPF6, solvent is FEC+DMC+TTE, where the volume ratio of FEC:DMC:TTE is 1:1:1) is injected to complete the battery fabrication.

[0108] Example 6

[0109] This embodiment provides a lithium metal battery, which includes a positive electrode, a separator, a lithium metal negative electrode, and an electrolyte; an SEI film is disposed between the separator and the lithium metal negative electrode, the SEI film is prepared on the surface of the separator, the SEI film includes hexaphenyldisiloxane and lithium trifluoromethanesulfonate, and the separator is a porous separator (PE separator with a porosity of 75%).

[0110] The method for preparing the lithium metal battery is as follows:

[0111] (a) Preparation of SEI membrane-diaphragm composite structure:

[0112] 1) Dissolve hexaphenyldisiloxane in N-methylformamide at a molar concentration of 100 mmol / L to form a uniformly dispersed, transparent mixed solution;

[0113] 2) Add 1 wt% lithium trifluoromethanesulfonate to the above mixed solution (the mass concentration of lithium trifluoromethanesulfonate in the mixed solution is 1 wt%) and stir until homogeneous;

[0114] 3) Spray the mixed solution from step 2) onto the surface of the porous membrane and dry it in a vacuum drying oven at 50°C for 16 hours to obtain an SEI membrane-membrane composite structure with an SEI membrane thickness of 50 μm.

[0115] 4) Cut into the required diaphragm size and set aside;

[0116] (b) Preparation of positive electrode sheet: Lithium iron phosphate, conductive agent SuperP, binder PVDF and carbon nanotubes (CNT) are mixed evenly in a mass ratio of 96.5:2.5:1.0:0.5 to prepare a lithium-ion battery positive electrode slurry of a certain viscosity. The slurry is coated on aluminum foil for current collector, dried at 85°C and then cold-pressed. After trimming, cutting and slitting, the slurry is dried at 85°C under vacuum for 8 hours to prepare a lithium metal battery positive electrode sheet that meets the requirements.

[0117] (c) The negative electrode uses 6μm thick copper foil purchased from the market and double-sided lithium-coated (lithium thickness is 20μm) copper-lithium composite strip; it is made into a lithium metal battery negative electrode sheet that meets the requirements after edge cutting, cutting and slitting.

[0118] (d) Preparation of lithium metal pouch battery: The positive electrode, negative electrode and separator prepared according to the above process are stacked to form a lithium metal battery with three positive and four negative electrodes (wherein, an SEI film is set between the separator and the lithium metal negative electrode), with a capacity of 1500mAh. Electrolyte (1mol / L LiPF6, solvent is EC+DEC+DMC, where the volume ratio of EC:DEC:DMC is 1:1:1) is injected to complete the battery fabrication.

[0119] Example 7

[0120] The difference between this embodiment and Embodiment 1 is that the porosity of the porous membrane in this embodiment is 17%.

[0121] The remaining preparation methods and parameters are consistent with those in Example 1.

[0122] Example 8

[0123] The difference between this embodiment and Embodiment 1 is that the porosity of the porous membrane in this embodiment is 80%.

[0124] The remaining preparation methods and parameters are consistent with those in Example 1.

[0125] Example 9

[0126] The difference between this embodiment and Embodiment 1 is that in step 1) of this embodiment, the molar concentration of hexaphenyldisiloxane in the mixed solution is 300 mmol / L.

[0127] The remaining preparation methods and parameters are consistent with those in Example 1.

[0128] Example 10

[0129] The difference between this embodiment and Embodiment 1 is that in step 2) of this embodiment, the mass concentration of lithium trifluoromethanesulfonate in the mixed solution is 5 wt%.

[0130] The remaining preparation methods and parameters are consistent with those in Example 1.

[0131] Example 11

[0132] The difference between this embodiment and Embodiment 1 is that the thickness of the SEI film in this embodiment is 30 nm.

[0133] The remaining preparation methods and parameters are consistent with those in Example 1.

[0134] Example 12

[0135] The difference between this embodiment and Embodiment 1 is that the thickness of the SEI film in this embodiment is 100 μm.

[0136] The remaining preparation methods and parameters are consistent with those in Example 1.

[0137] Example 13

[0138] The difference between this embodiment and Embodiment 1 is that the membrane in this embodiment is a non-porous structure, that is, a polymer membrane with a porosity of only 10%.

[0139] The remaining preparation methods and parameters are consistent with those in Example 1.

[0140] Comparative Example 1

[0141] The difference between this comparative example and Example 1 is that the lithium metal battery provided in this comparative example does not contain an SEI film.

[0142] In the preparation method, SEI film preparation is not performed.

[0143] The remaining preparation methods and parameters are consistent with those in Example 1.

[0144] Comparative Example 2

[0145] The difference between this comparative example and Example 1 is that the SEI film provided in this comparative example does not contain a lithium source.

[0146] The remaining preparation methods and parameters are consistent with those in Example 1.

[0147] Comparative Example 3

[0148] The difference between this comparative example and Example 13 is that the lithium metal battery provided in this comparative example does not contain an SEI film.

[0149] In the preparation method, SEI film preparation is not performed.

[0150] The remaining preparation methods and parameters are consistent with those in Example 13.

[0151] The lithium metal batteries provided in Examples 1-13 and Comparative Examples 1-3 were subjected to performance tests. The test conditions were as follows: at room temperature, the batteries were charged to 4.1V with a current of 75mA, then charged to 4.25V with a current of 150mA, and then charged at a constant voltage of 4.25V. The cutoff current was set to 75mA. The batteries were discharged to 2.8V with a current of 150mA. The initial charge and discharge capacity and efficiency were tested.

[0152] Coulombic efficiency and charge-discharge cycle retention were tested. At room temperature, the capacitor was charged to 4.25V with a current of 750mA, then charged at a constant voltage of 4.25V with a cutoff current set to 120mA. Discharge was performed at 1500mA to 2.8V, with 300 cycles. The discharge capacity CN per N cycles was obtained, and the capacity retention rate was calculated as CN / C1*100%. In the cycle test data, the ratio of the discharge capacity per N cycles to the charge capacity per N cycles is the coulombic efficiency for cycle N. The average coulombic efficiency from cycle 1 to cycle N is the average coulombic efficiency for N cycles. The test results are shown in Table 1.

[0153] Table 1

[0154]

[0155]

[0156] From Table 1, we can obtain:

[0157] The data results from Examples 1, 7, and 8 show that if the porosity of the porous membrane is too small, it will reduce the local current density on the lithium surface, while if the porosity is too large, it will cause a short circuit in the cell.

[0158] The data from Examples 1 and 9 show that an excessively high molar concentration of phenyl siloxanes is not conducive to improving conductivity.

[0159] The data results from Examples 1 and 10 show that an excessively high lithium source concentration (i.e., too much lithium source added) can lead to excess lithium, resulting in dead lithium or lithium powder formation, which can cause a short circuit in the battery.

[0160] The data from Examples 1 and 11 and 12 show that if the SEI film is too thin, it will not be conducive to the SEI film suppressing the excessive dissolution of the solvent; while if it is too thick, it will lead to excessive impedance and concentration polarization.

[0161] The data results from Examples 1 and 13 show that if the membrane is a non-porous structure, that is, does not contain a porous structure, it will affect the effect of reducing the concentration polarization of the lithium metal interface.

[0162] The data results from Example 1 and Comparative Example 1, and Example 13 and Comparative Example 3 show that without the SEI film provided by the present invention, the concentration gradient and concentration polarization phenomenon caused by the slow lithium-ion transport dynamics in the negative electrode liquid layer cannot be solved in lithium metal batteries.

[0163] The data from Example 1 and Comparative Example 2 show that if the SEI film does not contain a lithium source, an artificial SEI film cannot be constructed, and lithium deposition cannot be effectively controlled.

[0164] In summary, the SEI film provided by this invention, through the combination of phenyl siloxanes and a lithium source, utilizes the excellent electronic insulation and film-forming properties of phenyl siloxane compounds. When used in combination with lithium, an artificial SEI film can be constructed, effectively controlling lithium deposition. Furthermore, phenyl siloxanes exhibit strong lithium adsorption capacity, and the adsorption energies of multiple benzene ring sites form a gradient distribution, effectively capturing and uniformly regulating Li. + The increased flux can alleviate the concentration gradient caused by the slow lithium-ion transport kinetics in the negative electrode liquid layer to a certain extent, reducing concentration polarization and thus achieving dendrite-free lithium deposition / stripping. Furthermore, the SEI film acts as a physical barrier, preventing corrosion reactions between lithium metal and the electrolyte. This improves the initial efficiency, cycle life, and coulombic efficiency of lithium metal batteries.

[0165] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A lithium metal battery, characterized in that, The lithium metal battery includes a positive electrode, a separator, a lithium metal negative electrode, and an electrolyte; an SEI film is disposed between the separator and the lithium metal negative electrode; The diaphragm comprises a porous diaphragm; the porosity of the porous diaphragm is 20-75%; The SEI film is composed of phenyl siloxane and a lithium source; The SEI film is directly bonded to the surface of the separator, or the SEI film is independently located between the separator and the lithium metal anode; When the SEI membrane is directly laminated to the surface of the separator, the lamination method includes: A phenyl siloxane, a lithium source, and a solvent are mixed to obtain a mixed solution. The mixed solution is then coated onto the surface of a separator and dried to obtain an SEI membrane. When the SEI film is independently located between the separator and the lithium metal anode, the method includes: A phenyl siloxane, a lithium source, and a solvent are mixed to obtain a mixed solution. The mixed solution is then formed into a film, and the film structure is disposed between the separator and the lithium metal anode. The molar concentration of the phenyl siloxane in the mixed solution is 0.1~100 mmol / L; The mass fraction of the lithium source in the mixed solution is ≤1 wt%; The thickness of the SEI film is 50 nm to 50 μm.

2. The lithium metal battery according to claim 1, characterized in that, The phenyl siloxanes include any one or a combination of at least two of the following: octaphenylcyclotetrasiloxane, hexaphenyldisiloxane, octaphenyl-POSS, trisiloxyphenyl cage-like silsesquioxane, phenylmethylsiloxane copolymer, hexaphenylcyclotrisiloxane, octaphenylcyclotetrasiloxane, diphenyldimethylcyclosiloxane, polyphenyldimethylcyclosiloxane, or N-phenylamino-cage-like polysilsesquioxane.

3. The lithium metal battery according to claim 1, characterized in that, The lithium source includes any one or a combination of at least two of lithium trifluoromethanesulfonate, lithium, lithium silicon, or lithium phenyl.

4. The lithium metal battery according to claim 1, characterized in that, The molar concentration of the phenyl siloxane in the mixed solution is 0.2~50 mmol / L.

5. The lithium metal battery according to claim 1, characterized in that, The solvent includes any one or a combination of at least two of tetrahydrofuran, dimethyl carbonate, N-methylformamide, dimethylformamide, or methylpyrrolidone.

6. A method for preparing a lithium metal battery as described in any one of claims 1-5, characterized in that, The positive electrode, separator and lithium metal negative electrode are sequentially combined, and the combined structure is injected into the electrolyte to obtain the lithium metal battery. The SEI film is located between the separator and the lithium metal anode.

7. A method for reducing concentration polarization in a lithium metal battery as described in any one of claims 1-5, characterized in that, The method includes placing the SEI film between the separator of the lithium metal battery and the lithium metal anode.

Images