A gradient designed polymer-based lithium solid-state electrolyte, a preparation method thereof and a lithium metal solid-state battery

Abstract

The application discloses a gradient designed polymer-based lithium solid-state electrolyte, a preparation method thereof and a lithium metal solid-state battery. By bonding a silylated quaternary ammonium salt cation active agent and a fast ion conductor, effective dispersion of the two is realized, and meanwhile, a fast ion conductor-rich layer-modified interface layer-polymer-rich layer is constructed through a gradient, lithium ion flux distribution is controlled, and lithium ion uniform deposition is guided, so that lithium dendrite growth is inhibited, and the lithium metal solid-state battery designed based on the same has excellent electrochemical performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium metal batteries, in particular to a gradient designed polymer-based lithium solid-state electrolyte, a preparation method thereof and a lithium metal solid-state battery. BACKGROUND

[0002] With the transformation of global energy structure towards clean and low-carbon, high-energy density energy storage technology has become a research hotspot. Lithium metal batteries are considered as the core solution for the next generation of power batteries due to their high theoretical specific capacity (3860 mAh / g) and low redox potential (-3.04 V vs. SHE). However, lithium metal anodes face severe challenges in practical applications - uncontrollable growth of lithium dendrites. These dendrites not only pierce the separator and cause short circuits, leading to thermal runaway and even explosion of the battery, but also accelerate the loss of active lithium, significantly reducing the cycle life and safety of the battery. Although researchers have alleviated the dendrite problem through strategies such as optimizing electrode structure, regulating electrolyte composition (such as introducing fluorinated solvents or additives), and designing three-dimensional current collectors, the inherent limitations of liquid electrolytes (such as low mechanical strength and high volatility) still make it difficult to fundamentally solve the dendrite growth problem. Solid-state electrolytes are an ideal choice for inhibiting lithium dendrites due to their high mechanical strength and non-flammable characteristics. Inorganic solid-state electrolytes have attracted widespread attention due to their high ionic conductivity and mechanical strength, but their inherent high rigidity inevitably reduces the interface contact and increases the interface impedance, which seriously hinders the practical application of inorganic SSEs. In recent years, polymer / inorganic composite solid-state electrolytes have received increasing attention due to their simple preparation, high flexibility, and wide application scenarios. However, traditional polymer solid-state electrolytes have problems such as insufficient ionic conductivity and low lithium ion transference number, which limit the dendrite suppression effect. The fundamental reason is that the lithium ion transport channels in the polymer matrix are not uniform, which cannot achieve uniform lithium deposition. The modification methods currently proposed mainly include artificial solid electrolyte interface (SEI) construction, filler modification, and interface design. Although filler modification can improve the performance of polymer electrolytes, the agglomeration and uneven dispersion of fillers limit their further development. Interface design is considered an ideal modification method by optimizing the interface properties between the electrolyte and the electrode. Therefore, how to improve the ion transport uniformity and dendrite suppression ability of composite solid-state electrolytes through interface design has become a key breakthrough in current research. SUMMARY

[0003] In order to solve the problem of uneven lithium deposition of the existing composite solid-state electrolyte, the present application aims to provide a gradient designed polymer-based lithium solid-state electrolyte and its preparation method and lithium metal solid-state battery, by bonding silanized quaternary ammonium salt cationic active agent and fast ion conductor, realizing effective dispersion of the two, and by gradient construction of fast ion conductor-rich layer-modified interface layer-polymer-rich layer, realizing regulation of lithium ion flux distribution and guiding uniform lithium ion deposition, so as to inhibit lithium dendrite growth, and the lithium metal solid-state battery designed based on this has excellent electrochemical performance.

[0004] In order to achieve the above technical purpose, the present application adopts the following technical scheme:

[0005] A preparation method of a gradient designed polymer-based lithium solid-state electrolyte, comprising the following steps:

[0006] (1) Under an inert atmosphere, disperse quaternary ammonium salt surfactant in DMF, then add 3-aminopropyl triethoxysilane, and reflux at 65-75 DEG C to obtain a bifunctional surfactant;

[0007] (2) Under the condition of 75-85 DEG C, soak the fast ion conductor in hydrogen peroxide for pretreatment, and then ultrasonically disperse the pretreated fast ion conductor in anhydrous ethanol containing the bifunctional surfactant, and stir at 55-65 DEG C to obtain a fast ion conductor modified by the bifunctional surfactant;

[0008] (3) Disperse the fast ion conductor modified by the bifunctional surfactant, olefin polymer and lithium salt in a polar solvent, denoted as A liquid; disperse the olefin polymer and lithium salt in a polar solvent, denoted as B liquid; use a stepwise spin coating method to coat on a polytetrafluoroethylene plate, wherein the first layer is coated with A liquid, after drying to form a film, the second layer is coated with a mixture of A liquid and B liquid, after drying to form a film, the third layer is coated with B liquid, after drying to form a film, and the polymer-based lithium solid-state electrolyte is obtained.

[0009] Preferably, in step (1), the quaternary ammonium salt surfactant is selected from one or more of cetyltrimethylammonium chloride (CTAC), dodecyltrimethylammonium chloride (DTAC), octadecyltrimethylammonium chloride (OTAC), and cetyltrimethylammonium bromide (CTAB). Taking CTAC as an example, the following reaction occurs:

[0010] CTAC + NH2(CH2)3Si(OC2H5)3 → [CTA]⁺[⁻OOC(CH2)3Si(OC2H5)3] + HCl

[0011] The quaternary ammonium salt surfactant is silanized, which can adjust the hydrophilicity and hydrophobicity of the surfactant surface, the silane group can form a chemical bond with the surface of the fast ionic conductor, and is adsorbed on the surface of the fast ionic conductor, and the long-chain organic group can form steric hindrance on the particle surface, thereby improving the dispersion stability and preventing particle agglomeration.

[0012] Preferably, in step (1), the mass-volume ratio of the quaternary ammonium salt surfactant to DMF is 0.5-1 g:1 ml; the mass-volume ratio of the quaternary ammonium salt surfactant to 3-aminopropyl triethoxysilane is 2-5 g:1 ml.

[0013] Preferably, in step (2), the fast ionic conductor is selected from one or more of LLZO, LLTO, LLZTO, LATP, NASICON type. The fast ionic conductor is soaked to remove the surface carbonate and expose the -OH group, realizing hydroxylation. In the subsequent stirring reaction process, -Si(OC2H5)3 is hydrolyzed to -Si(OH)3 and condensed with the -OH on the surface of the fast ionic conductor, forming a unique Si-O-T (T is Zr, Ta, Al, Ti) chemical bond. The hydrophobic tail of the surfactant can extend outward, reducing the hydrophilicity of the surface of the fast ionic conductor, and improving the dispersion of the fast ionic conductor in the solvent by steric hindrance effect, effectively preventing the agglomeration of the fast ionic conductor and the surfactant.

[0014] Preferably, in step (2), the mass ratio of the fast ionic conductor to the bifunctional surfactant is 1:0.05-0.2; the concentration of the bifunctional surfactant in the anhydrous ethanol containing the bifunctional surfactant is 0.5-2 wt%.

[0015] Preferably, in step (3), the mass ratio of the bifunctional surfactant modified fast ionic conductor, the olefin polymer and the lithium salt in the A liquid is 70:20:10; the mass ratio of the olefin polymer and the lithium salt in the B liquid is 90:10.

[0016] Preferably, in step (3), the olefin polymer is selected from one or more of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene) copolymer, poly(vinylidene fluoride-trifluoroethylene) copolymer, poly(vinylidene fluoride-tetrafluoroethylene) copolymer, poly(vinylidene fluoride-trifluoroethylene-trifluorochloroethylene) copolymer.

[0017] The lithium salt is selected from one or more of LiCl, LiF, LiBr, LiI, Li2SO4, LiNO3, LiPF6, LiTFSI, LiFSI, LiDFOB.

[0018] The polar solvent is selected from one or more of tetrahydrofuran, N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, acetone, 2,2,2-trifluoro-N,N-dimethylacetamide.

[0019] Preferably, in step (3), the coating thickness of A liquid, the mixture of A liquid and B liquid, and B liquid is 100 mu m, wherein the volume ratio of A liquid and B liquid in the mixture of A liquid and B liquid is 1:1.

[0020] Taking LLZO as an example, the gradient structure designed polymer electrolyte of the application can be recorded as: LLZO-rich layer→modified interface layer→polymer-rich layer (close to lithium metal), adopting a gradual transition mode, the LLZO-rich layer provides a fast lithium ion transmission path, the modified interface layer bridges the ceramic / polymer through Si-O-Zr covalent bonds, effectively reducing the interface impedance between different components, and the high-flexibility polymer-rich layer adapts to the volume change of the lithium negative electrode, this design can reduce the transmission resistance of ions between different layers, effectively reduce the interface polarization, realize high-speed ion transmission, and effectively improve the interface compatibility. At the same time, effectively reduce the stress concentration at the interface, improve the mechanical stability and chemical stability of the interface. Therefore, the gradient structure design realizes the regulation of lithium ion flux distribution, guides the uniform deposition of lithium ions, and thus realizes the effect of inhibiting lithium dendrites.

[0021] The application further provides a gradient designed polymer-based lithium solid-state electrolyte prepared by the preparation method.

[0022] The application further provides a lithium metal solid-state battery, which comprises a positive electrode sheet, a gradient designed polymer-based lithium solid-state electrolyte and a lithium metal negative electrode.

[0023] In the application, the active material of the positive electrode sheet can be a conventional commercial lithium battery positive electrode material, and the lithium metal negative electrode can be a lithium metal, a lithium-copper composite tape or other alloyed lithium metal negative electrode, and there is no special requirement.

[0024] The application has the following beneficial technical effects:

[0025] 1. In the application, the silanized quaternary ammonium salt surfactant is condensed with the -OH groups on the surface of the fast ion conductor to form a unique Si-O-T (T is Zr, Ta, Al or Ti) chemical bond, the hydrophobic tail of the surfactant can extend outward, the hydrophilicity of the LLZO surface is reduced, the dispersibility of the fast ion conductor in the organic solvent is improved by using the steric hindrance effect, and the agglomeration of the fast ion conductor and the surfactant is effectively prevented.

[0026] 2、The application can realize the effect of inhibiting lithium dendrite growth by designing a fast ion conductor-rich layer-modified interface layer-polymer-rich layer, the fast ion conductor-rich layer can provide a high-speed ion channel, the modified interface layer realizes stress buffering and bonding of the fast ion conductor (through Si-O-T) and hydrogen bond in the polymer, forms a covalent bond network bridge, the polymer-rich layer realizes close electrode contact, and the lithium ion flux distribution is regulated and guided to be uniformly deposited through the design, so that the effect of inhibiting lithium dendrite growth is realized.

[0027] 3、The polymer-based lithium solid-state battery of the application is matched with the gradient designed polymer-based lithium solid-state electrolyte, has high ion conductivity (2.71 mS / cm), a wide electrochemical window, high ion transference number (0.59), and can be stably cycled for 1500 h in a lithium-lithium symmetric battery, the full battery can be stably cycled for 1500 cycles at a room temperature 1 C rate, the capacity retention rate is >80%, the soft package battery assembled by matching the modified composite solid-state electrolyte can realize more than 200 stable cycles, and has industrial application value. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The room temperature ion conductivity diagram of the button cell assembled for the embodiment 1 and the comparative example 1 of the application is shown.

[0029] Figure 2 The room temperature electrochemical window test result diagram of the button cell assembled for the embodiment 1 and the comparative example 1 of the application is shown.

[0030] Figure 3 The ion transference number diagram of the lithium metal symmetric battery assembled for the embodiment 1 and the comparative example 1 of the application is shown.

[0031] Figure 4 The room temperature stability test diagram of the lithium metal symmetric battery assembled for the embodiment 1 and the comparative example 1 of the application is shown.

[0032] Figure 5 The room temperature electrochemical performance of the button cell assembled in the embodiment 1 of the application at 2 C is shown.

[0033] Figure 6 The room temperature electrochemical performance of the lithium metal soft package battery assembled in the embodiment 1 of the application at 0.2 C is shown. DETAILED DESCRIPTION

[0034] Embodiment 1

[0035] (1) Synthesis of bifunctional surfactant: 10 g of cetyltrimethylammonium chloride (CTAC) was dissolved in 15 mL of DMF, stirred under nitrogen protection, 3 mL of 3-aminopropyltriethoxysilane was added, and refluxed at 70 °C for 12 h. After cooling, centrifugation, and washing with ethanol and deionized water for 3 times, respectively, the silanized Si-CTAC was obtained by vacuum drying, and the reaction equation is as follows:

[0036] CTAC + NH2(CH2)3Si(OC2H5)3 → [CTA]⁺[⁻OOC(CH2)3Si(OC2H5)3] + HCl

[0037] (2) Atomic-level bonding of LLZO surface: LLZO powder was soaked in 30% H2O2, treated at 80 °C for 2 h to remove surface carbonate and expose -OH groups for hydroxylation. 1 g of LLZO was dispersed in 10 g of anhydrous ethanol containing 1 wt% Si-CTAC, ultrasonically treated for 1 h, and then stirred at 60 °C for 12 h to hydrolyze -Si(OC2H5)3 to -Si(OH)3 and condense with the surface -OH of LLZO. The powder was collected by centrifugation and dried in vacuum to obtain CTAC-Si-LLZO, forming a Si-O-Zr chemical bond;

[0038] (3) Gradient structure design: A solution of CTAC-Si-LLZO / PVDF / LiTFSI (mass ratio 70:20:10) was dissolved in 10 mL of THF solution; B solution: PVDF / LiTFSI (mass ratio 90:10) was dissolved in 10 mL of THF solution. A ladder-type spin coating method was used to coat on a polytetrafluoroethylene plate: first layer: coating A solution (100 μm), drying at 60 °C for 2 h to form a film; second layer: coating A:B = 1:1 (volume ratio) mixed solution (100 μm), drying at 60 °C for 2 h to form a film; third layer: coating B solution (100 μm), drying at 60 °C for 12 h to remove residual solvent, to obtain a gradient structure designed polymer electrolyte: LLZO-rich layer → modified interface layer → polymer-rich layer.

[0039] (4) Preparation of positive electrode: lithium nickel cobalt manganese (LiNi 0.8 Co 0.1 Mn 0.1 O2) as active material, conductive agent (acetylene black), and binder [polyvinylidene fluoride (PVDF)] were mixed in a mass ratio of 80:10:10, coated on a carbon-coated aluminum foil, and dried in a vacuum drying oven at 80 °C for 24 h to remove solvent.

[0040] (5) Selection of negative electrode: 450 μm metal lithium sheet was used as the negative electrode.

[0041] (6) Assembly of lithium solid-state battery: Assemble the symmetric battery in the order of negative electrode shell, gasket (304 stainless steel material, diameter x thickness: 15.6 mm x 0.5 mm), gradient structure designed polymer-based solid-state electrolyte, gasket (15.6 mm x 0.5 mm), and positive electrode shell, and conduct ion conductivity test of the solid-state electrolyte. Assemble the battery in the order of negative electrode shell, lithium sheet (15.6 mm x 0.45 mm), gradient structure designed polymer-based solid-state electrolyte, lithium sheet (15.6 mm x 0.45 mm), and positive electrode shell, and conduct ion transference number and lithium stability test of the solid-state electrolyte. Assemble the battery in the order of negative electrode shell, lithium sheet (15.6 mm x 0.45 mm), gradient structure designed polymer-based solid-state electrolyte, positive electrode sheet, and positive electrode shell, and conduct room temperature impedance and charge-discharge performance test.

[0042] (7) Assembly of lithium soft-pack battery: Cut the positive electrode sheet into 43 x 56 mm, the negative electrode sheet into 45 x 58 mm, and the gradient structure designed polymer-based solid-state electrolyte into 50 x 65 mm, and stack and seal in the order of positive electrode, solid-state electrolyte, and negative electrode in an inert atmosphere, and place in a cabinet for soft-pack battery cycle performance test.

[0043] It is detected that the gradient structure designed polymer-based solid-state electrolyte has high ion conductivity, reaching 2.71 mS / cm, the lithium metal symmetric battery assembled has high ion transference number, reaching 0.59, the Li||Li symmetric battery assembled can be stably cycled for 1500 h, the lithium metal solid-state battery assembled can be stably cycled for 1500 cycles at room temperature 1C rate, the capacity retention rate is >80%, the soft-pack battery assembled realizes more than 200 cycles of cycle, and has great industrial application value.

[0044] Example 2

[0045] The same as example 1, the only difference is that 10 g of dodecyltrimethylammonium chloride (DTAC) is dissolved in 15 mL of DMF, stirred under nitrogen protection, 3 mL of 3-aminopropyltriethoxysilane is added, and the reaction is carried out at 70°C for 12 h. After cooling, centrifugation, and washing with ethanol and deionized water for 3 times respectively, vacuum drying gives silanized Si-DTAC, and the reaction equation is as follows:

[0046] DTAC + NH2(CH2)3Si(OC2H5)3 → [DTA]⁺[⁻OOC(CH2)3Si(OC2H5)3] + HCl.

[0047] Example 3

[0048] The same as Example 1, the only difference is that 10 g of cetyltrimethylammonium bromide (CTAB) is dissolved in 15 mL of DMF, stirred under nitrogen protection, 3 mL of 3- aminopropyltriethoxysilane is added, and the reaction is carried out at 70°C for 12 h. After cooling, centrifugation, washing with ethanol and deionized water for 3 times respectively, and vacuum drying, silanized Si-CTAB is obtained, and the reaction equation is as follows:

[0049] CTAB + NH2(CH2)3Si(OC2H5)3 → [CTA]⁺[⁻OOC(CH2)3Si(OC2H5)3] + HBr.

[0050] Comparative Example 1

[0051] The same as Example 1, the only difference is that step (1) is omitted, and silanization of quaternary ammonium salt surfactant is not carried out, and the specific steps are as follows:

[0052] (1) Preparation of 1wt% CTAC anhydrous ethanol solution: 0.1 g of CTAC powder is dissolved in 9.9 g of anhydrous ethanol, and ultrasonic stirring is carried out for 2 h to obtain a 1wt% CTAC anhydrous ethanol solution;

[0053] (2) The LLZO powder is soaked in 30% H2O2, treated at 80°C for 2 hours to remove the surface carbonate and expose the -OH group to achieve hydroxylation. 1 g of LLZO is dispersed in anhydrous ethanol containing 1wt% CTAC, ultrasonic treatment is carried out for 1 h, and then stirring is carried out at 60°C for 12 h. The powder is collected by centrifugation and vacuum dried;

[0054] The subsequent steps are the same as Example 1.

[0055] Comparative Example 2

[0056] The same as Example 1, the only difference is that step (1) is omitted, and quaternary ammonium salt surfactant is not introduced, and 3-aminopropyltriethoxysilane is used alone, and the specific steps are as follows:

[0057] (1) The LLZO powder is soaked in 30% H2O2, treated at 80°C for 2 hours to remove the surface carbonate and expose the -OH group to achieve hydroxylation. 1 g of LLZO is dispersed in a mixed solution of anhydrous ethanol and 3-aminopropyltriethoxysilane, ultrasonic treatment is carried out for 1 h, and then stirring is carried out at 60°C for 12 h. The powder is collected by centrifugation and vacuum dried;

[0058] The subsequent steps are the same as Example 1.

[0059] Comparative Example 3

[0060] The same as Example 1, the only difference is that imidazoline cationic surfactant is used, and the specific steps are as follows:

[0061] (1) 10 g of 3-methyl-2-thioximidazoline-1-carboxylic acid ethyl ester was dissolved in 15 mL of DMF, stirred under nitrogen protection, 3 mL of 3- aminopropyltriethoxysilane was added, and refluxed at 70°C for 12 h. After cooling, centrifugation, washing with ethanol and deionized water for 3 times respectively, and vacuum drying;

[0062] The subsequent steps were the same as Example 1.

[0063] Comparative Example 4

[0064] The same as Example 1, the only difference was that an amine cationic surfactant was used, and the specific steps were as follows:

[0065] (1) 10 g of 3-methyl-2-thioximidazoline-1-carboxylic acid ethyl ester was dissolved in 15 mL of DMF, stirred under nitrogen protection, 3 mL of 3- aminopropyltriethoxysilane was added, and refluxed at 70°C for 12 h. After cooling, centrifugation, washing with ethanol and deionized water for 3 times respectively, and vacuum drying;

[0066] The subsequent steps were the same as Example 1.

[0067] Comparative Example 5

[0068] The same as Example 1, the only difference was that in step (3), only A liquid (300 μm) was coated on a polytetrafluoroethylene plate, dried at 60°C for 12 h to remove residual solvent, and a composite solid-state electrolyte was obtained for assembling a battery.

[0069] Comparative Example 6

[0070] The same as Example 1, the only difference was that in step (3), a two-layer structure design was adopted, the first layer: coating A liquid (150 μm), drying at 60°C for 2 h to form a film; the second layer: coating B liquid (150 μm), drying at 60°C for 12 h to remove residual solvent, and a composite solid-state electrolyte was obtained for assembling a battery.

[0071] Comparative Example 7

[0072] The same as Example 1, the only difference was that in step (3), only A:B = 1:1 mixed liquid (300 μm) was coated on a polytetrafluoroethylene plate, dried at 60°C for 12 h to remove residual solvent, and a composite solid-state electrolyte was obtained for assembling a battery.

[0073] Table 1 Performance test results of each example and comparative example

[0074] Group Room temperature ionic conductivity (mS / cm) Ionic transference number Electrochemical window (Vs. Li + (V) Full cell impedance value at room temperature (Ω) Symmetric cells at 0.1 mA / cm 2 Stable cycle length Capacity retention rate at room temperature 1C cycle 1500 cycles (%) Soft package battery stable cycle number at room temperature 0.2C Example 1 2.71 0.59 4.8 34 1500 81.34 206 Example 2 1.64 0.55 4.5 61 1150 73.41 137 Example 3 1.96 0.56 4.7 45 1370 78.63 184 Comparative Example 1 0.981 0.32 4.3 91 628 31.24 51 Comparative Example 2 0.175 0.1 3.6 317 83 0 3 Comparative Example 3 0.177 0.21 4.2 248 417 13.49 27 Comparative Example 4 0.556 0.17 4.1 283 267 7.91 12 Comparative Example 5 0.981 0.35 4.4 175 891 45.18 84 Comparative Example 6 1.385 0.41 4.5 124 1087 58.42 97 Comparative Example 7 1.171 0.39 4.4 147 911 51.71 90

[0075] From Table 1, when a single functional quaternary ammonium salt cationic surfactant is used in Comparative Example 1 (not silanized), the ion transference number is 0.32, the electrochemical window is 4.3 V, the full cell impedance at room temperature is 91 Ω, the Li||Li button symmetric cell is cycled for 628 h, and the capacity retention rate of the NCM811 positive electrode solid-state lithium metal button cell assembled at room temperature under the condition of 2 C is cycled for 1500 cycles is 31.24%. It is shown that the quaternary ammonium salt cationic surfactant not silanized cannot realize effective dispersion of the fast ion conductor and the surfactant, thereby leading to increased interfacial impedance and affecting the electrochemical performance.

[0076] It is known from Comparative Example 2 that the single silane additive cannot be effectively adsorbed into the fast ion conductor, the ion transference number is 0.1, the electrochemical window is only 3.6 V, the full cell impedance at room temperature reaches 317 Ω, and the Li||Li button symmetric cell can only be cycled for 83 h. It is shown that the silanized additive cannot interact and bond with the fast ion conductor, and the interfacial impedance is huge.

[0077] It is known from Comparative Examples 3 and 4 that the non-quaternary ammonium salt cationic surfactant additive cannot undergo condensation reaction with the silanized additive to form a silicon-oxygen bond, and exhibits very poor electrochemical performance.

[0078] It is known from Comparative Examples 5, 6 and 7 that the polymer electrolyte designed with a single layer and a double layer structure cannot realize high-speed transmission of lithium ions due to the interfacial impedance between the components, and the interface compatibility is poor.

[0079] In Example 1, the dual functional surfactant is bonded with the fast ion conductor to realize effective dispersion of the two, and the fast ion conductor-rich layer→modified interface layer→polymer-rich layer is designed to effectively reduce the interfacial impedance between different interface layers, realize high room temperature ionic conductivity (2.71 mS / cm), high ion transference number (t Li + =0.59), wide electrochemical window (4.8 V), low interfacial impedance (34 Ω), Li||Li button symmetric cell realizes stable cycling for 1500 h, the capacity retention rate of the NCM811 positive electrode solid-state lithium metal button cell assembled at room temperature under the condition of 2 C is cycled for 1500 cycles is 81.34%, and in addition, the soft pack battery assembled using the composite solid-state electrolyte designed with a gradient structure realizes stable cycling for 200 cycles at 0.2 C. It is known from Examples 2 and 3 that different quaternary ammonium salt surfactants can all exhibit excellent ion conductivity and electrochemical performance, and the design exhibits innovation.

Claims

1. A method for preparing a gradient-designed polymer-based lithium solid electrolyte, characterized in that, Includes the following steps: (1) Under an inert atmosphere, quaternary ammonium salt surfactants are dispersed in DMF, and then 3-aminopropyltriethoxysilane is added. The mixture is refluxed at 65~75℃ to obtain bifunctional surfactants. The quaternary ammonium salt surfactant is selected from one or more of hexadecyltrimethylammonium chloride, dodecyltrimethylammonium chloride, octadecyltrimethylammonium chloride, and hexadecyltrimethylammonium bromide; The mass-to-volume ratio of the quaternary ammonium salt surfactant to DMF is 0.5~1g:1ml; the mass-to-volume ratio of the quaternary ammonium salt surfactant to 3-aminopropyltriethoxysilane is 2~5g:1ml. (2) At 75~85℃, the fast ion conductor was pretreated by immersing it in hydrogen peroxide. The pretreated fast ion conductor was then ultrasonically dispersed in anhydrous ethanol containing a bifunctional surfactant and stirred at 55~65℃ to obtain a bifunctional surfactant-modified fast ion conductor. (3) The bifunctional surfactant-modified fast ion conductor, olefin polymer and lithium salt are dispersed in a polar solvent and denoted as solution A; the olefin polymer and lithium salt are dispersed in a polar solvent and denoted as solution B; the solution is coated on a polytetrafluoroethylene plate by a step-by-step spin coating method. The first layer is coated with solution A and dried to form a film. The second layer is coated with a mixture of solution A and solution B and dried to form a film. The third layer is coated with solution B and dried to form a film. The polymer-based lithium solid electrolyte is obtained.

2. The preparation method according to claim 1, characterized in that, In step (2), the fast ion conductor is selected from one or more of the following types: LLZO, LLTO, LLZTO, LATP, and NASICON.

3. The preparation method according to claim 1, characterized in that, In step (2), the mass ratio of the fast ion conductor to the bifunctional surfactant is 1:0.05~0.2; the concentration of the bifunctional surfactant in the anhydrous ethanol containing the bifunctional surfactant is 0.5~2wt%.

4. The preparation method according to claim 1, characterized in that, In step (3), the mass ratio of the bifunctional surfactant-modified fast ion conductor, olefin polymer, and lithium salt in solution A is 70:20:10; and the mass ratio of olefin polymer and lithium salt in solution B is 90:

10.

5. The preparation method according to claim 1, characterized in that, In step (3), the olefin polymer is selected from one or more of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene) copolymer, poly(vinylidene fluoride-trifluoroethylene) copolymer, poly(vinylidene fluoride-tetrafluoroethylene) copolymer, and poly(vinylidene fluoride-trifluoroethylene-trifluorochloroethylene) copolymer; The lithium salt is selected from one or more of LiCl, LiF, LiBr, LiI, Li2SO4, LiNO3, LiPF6, LiTFSI, LiFSI, and LiDFOB; The polar solvent is selected from one or more of tetrahydrofuran, N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, acetone, and 2,2,2-trifluoro-N,N-dimethylacetamide.

6. The preparation method according to claim 1, characterized in that, In step (3), the coating thickness of liquid A, the mixture of liquid A and liquid B, and liquid B is 100 μm. The volume ratio of liquid A to liquid B in the mixture of liquid A and liquid B is 1:

1.

7. A gradient-designed polymer-based lithium solid electrolyte prepared by the preparation method according to any one of claims 1-6.

8. A lithium metal solid-state battery, characterized in that, It includes a positive electrode, a gradient-designed polymer-based lithium solid electrolyte as described in claim 7, and a lithium metal negative electrode.

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