A flame-retardant solid-state electrolyte membrane and a solid-state pouch battery based thereon

Abstract

The application discloses a kind of flame-retardant solid electrolyte membrane and based on its solid soft package battery, in the both sides of polyimide fiber skeleton film coating contains the inorganic solid electrolyte slurry of first binder polyethylene oxide and second binder and dry, to obtain.The solid electrolyte membrane prepared in the application has good ionic conductivity, adapts the electrochemical window of ternary positive electrode material, excellent mechanical properties and flame-retardant performance, and has significant effect on improving the safety performance and service life of solid soft package battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion batteries, in particular to a flame-retardant solid-state electrolyte membrane, a preparation method thereof and a preparation method of a related solid-state soft-pack battery. BACKGROUND

[0002] In recent years, with the urgent demand for driving range of electric vehicles and large-scale energy storage devices of renewable energy power generation, lithium ion batteries with high safety, high capacity, high power and long life have become a research hotspot. Replacing the flammable and explosive organic electrolyte in the traditional lithium ion battery with a solid-state electrolyte is considered an important strategy to achieve high safety and high energy density of lithium ion batteries. However, considering the many practical difficulties faced by the large-scale production of all-solid-state batteries, and the urgent demand for high safety and high energy density power batteries for new energy vehicles, the semi-solid-state battery with a small amount of electrolyte as a transition scheme for the development of solid-state batteries is undoubtedly a suitable choice. Therefore, the development and industrialization of semi-solid-state lithium ion battery system related technology is imminent.

[0003] For the separator in the semi-solid-state battery, the existing technology generally adopts a separator surface coated with an inorganic solid-state electrolyte coating to prepare a solid-state electrolyte composite separator, so as to fix a small amount of electrolyte in the composite separator skeleton structure, reduce the free solvent, achieve a high ionic conductivity and an electrochemical window compatible with high-voltage ternary positive electrode materials, reduce the risk of electrolyte leakage, and improve the safety performance of the battery. However, the problem of the current composite separator is that the interface of the inorganic solid-state electrolyte is still not uniform, the "point-to-point" solid-solid contact between the electrolyte and the electrode leads to poor compatibility of the battery interface, the increase of the interface resistance increases the diffusion resistance of lithium ions, and the life and power performance of the battery are affected to a certain extent. Especially when assembling a multi-layer soft-pack battery, the increase of the contact area exacerbates the contact problem between the electrolyte separator and the electrode. In view of the above problems, a common strategy is to add a polymer-lithium salt system adhesive buffer layer on the surface of the inorganic solid-state electrolyte to avoid the contact between the rough inorganic solid-state electrolyte coating and the electrode. However, the increase of the thickness of the composite separator will undoubtedly affect the energy density of the battery, and in addition, most of the polymer-lithium salt system still has a certain viscosity after drying, making it difficult to roll the separator. Another strategy is to reduce the content of inorganic solid-state electrolyte and prepare an organic-inorganic composite separator mainly composed of a polymer adhesive, which improves the interface contact, but at the same time sacrifices the ionic conductivity and mechanical strength of the composite separator, and its oxidation stability is also difficult to adapt to high-voltage positive electrode materials.

[0004] To address this, the present invention proposes to modify the interface between the electrolyte membrane and the electrode using a specific binder and corresponding treatment strategy. While ensuring a high-concentration inorganic solid electrolyte coating, the binder in the inorganic solid electrolyte coating is melted by high-temperature hot pressing and then solidified at room temperature to promote the integration of electrolyte and electrode, thereby overcoming the problem of poor contact between electrolyte and electrode. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention aims to provide a flame-retardant solid electrolyte membrane, its preparation method, and a related method for preparing solid-state pouch batteries. This method involves coating both sides of a polyimide fiber skeleton membrane with an inorganic solid electrolyte slurry and then drying it to obtain the solid electrolyte membrane. The non-flammability of the polyimide skeleton and the inorganic solid electrolyte itself endows the electrolyte membrane with excellent flame-retardant properties; the inorganic solid electrolyte serves as the ion-conducting host, and its wide electrochemical window allows the composite membrane to be well-suited for most cathode materials, including ternary cathode materials (LiNi). x Co y Mn z O2 (x+y+z=1), lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4), etc.; the binder, polyethylene oxide, has a low melting point. During the assembly of the soft-pack battery, it is melted by high-temperature hot pressing and then solidified at room temperature to promote a better bond between the electrolyte membrane and the electrode, enhancing the electrolyte / electrode interface contact; the excellent thermal stability of the polyimide fiber skeleton membrane ensures that the electrolyte membrane does not experience pore blockage or deformation during high-temperature hot pressing. Simultaneously, to alleviate the problems of poor mechanical properties and high viscosity of the polyethylene oxide-lithium salt system, another second binder is blended with polyethylene oxide in equal amounts to enhance the mechanical properties of the inorganic solid electrolyte coating, reduce the surface viscosity of the coating, and improve the processing performance of the electrolyte membrane. The electrolyte membrane of this invention has strong liquid absorption and liquid retention capabilities. When assembling the soft-pack battery, only a small amount of electrolyte is needed to achieve ion conduction performance comparable to that of a liquid battery. The resulting battery system exists in a quasi-solid state, reducing the risk of electrolyte leakage. Combined with the excellent flame-retardant properties of the electrolyte membrane, the safety performance of the soft-pack battery is significantly improved.

[0006] To achieve its objectives, the present invention employs the following technical solution:

[0007] A flame-retardant solid electrolyte membrane is characterized in that: the flame-retardant solid electrolyte membrane is obtained by coating both sides of a polyimide fiber skeleton membrane with an inorganic solid electrolyte slurry containing a first binder, polyethylene oxide, and a second binder, and then drying it. The inorganic solid electrolyte slurry comprises the following raw materials by mass percentage: 2%–25% inorganic solid electrolyte powder, 1%–2% polyethylene oxide (first binder), 1%–2% second binder, 2%–4% lithium salt, and 67%–94% organic solvent.

[0008] Furthermore, the inorganic solid electrolyte powder is at least one of lithium aluminum titanium phosphate (LATP), lithium aluminum germanium phosphate (LAGP), lithium lanthanum zirconium oxide (LLZO), lithium lanthanum zirconium tantalum oxide (LLZTO), lithium lanthanum zirconium niobium oxide (LLZNO), and lithium lanthanum titanium oxide (LLTO).

[0009] Further: the first adhesive, polyethylene oxide, has a weight-average molecular weight of 100,000 to 1,000,000; the second adhesive is at least one of polyvinylidene fluoride, poly(vinylidene fluoride-co-hexafluoropropylene), polymethyl methacrylate, and polyacrylonitrile, and the weight-average molecular weight of the second adhesive is 300,000 to 1,000,000.

[0010] Furthermore, the lithium salt is at least one of LiClO4, LiTFSI, LiFSI, LiBOB, LiC2O4F2B, LiN(SO2CF3)2, LiCF3SO3 and LiN(SO2CF2CF3)2.

[0011] Further, the organic solvent is at least one selected from N-methylpyrrolidone, N-dimethylformamide, N-dimethylacetamide, acetonitrile, dimethyl sulfoxide, and tetrahydrofuran.

[0012] Furthermore, the thickness of the coating layer formed by coating the inorganic solid electrolyte slurry onto the polyimide fiber skeleton membrane is 25μm to 100μm, and the thickness of the inorganic solid electrolyte layer obtained after drying is 1μm to 4μm, and the thickness of the flame-retardant solid electrolyte membrane is 10μm to 30μm.

[0013] The preparation process of the flame-retardant solid electrolyte membrane of the present invention includes the following steps:

[0014] (1) First, add the inorganic solid electrolyte powder to the organic solvent and sonicate it in an ice bath for 30 min to 1 h to make the powder uniformly dispersed in the organic solvent; then add the first binder polyethylene oxide and the second binder, and heat and stir at 60℃ to 100℃ for 2 h to 24 h to obtain a uniform slurry with high viscosity; finally, add lithium salt and continue to heat and stir at 60℃ to 100℃ for 2 h to 12 h to form a uniformly dispersed inorganic solid electrolyte slurry.

[0015] (2) The inorganic solid electrolyte slurry obtained in step (1) is coated on one side of the polyimide fiber skeleton membrane, and then placed in a forced-air drying oven at 60℃~100℃ for 2h~12h to allow the solvent to evaporate completely. Then, an inorganic solid electrolyte slurry is coated on the other side of the polyimide fiber skeleton membrane, and placed in a forced-air drying oven at 60℃~100℃ for 2h~24h to allow the solvent to evaporate completely, thus obtaining a flame-retardant solid electrolyte membrane coated with inorganic solid electrolyte on both sides.

[0016] The present invention also provides a solid-state pouch battery, comprising a positive electrode, a negative electrode, and a separator, wherein: the positive electrode material is a ternary material LiNi. x Co y Mn z O2 (x+y+z=1), lithium cobalt oxide LiCoO2 or lithium iron phosphate LiFePO4; the negative electrode material is silicon-carbon composite material or graphite; the separator is the above-mentioned flame-retardant solid electrolyte membrane.

[0017] The preparation method of the solid-state soft-pack battery of the present invention is as follows: the flame-retardant solid electrolyte membrane is assembled with the positive electrode and the negative electrode to form a soft-pack cell, and then hot-pressed at 100℃~150℃ for 15min~30min (the first binder, polyethylene oxide, is melted by hot pressing and then solidified at room temperature to promote the integration of the electrolyte membrane with the positive and negative electrode sheets), and then a small amount of electrolyte is injected and sealed to form a solid-state soft-pack battery.

[0018] Compared with existing technologies, the beneficial effects of this invention are reflected in:

[0019] The solid electrolyte membrane provided by this invention uses two different polymers as binders. The first binder, polyethylene oxide, has a melting point of around 60°C. During high-temperature hot pressing, it can melt to promote contact between the electrolyte membrane and the electrode. However, when mixed with an equal amount of lithium salt, its mechanical properties are significantly weakened, and it creates a high degree of viscosity on the surface of the inorganic solid electrolyte coating, which is detrimental to the winding of the electrolyte membrane and the assembly process of the pouch battery. The second binder, when blended with an equal amount of polyethylene oxide, can significantly enhance the mechanical properties of the inorganic solid electrolyte coating, while reducing the surface viscosity of the coating and improving the processing performance of the electrolyte membrane. Furthermore, the polyimide fiber skeleton membrane possesses excellent thermal stability. Compared to other common separators such as polyethylene membranes, polypropylene membranes, and cellulose membranes, the polyimide fiber skeleton membrane does not experience closed-cell or deformation phenomena during high-temperature hot pressing, ensuring the usability of the electrolyte membrane. The electrolyte membrane of this invention possesses strong liquid absorption and retention capabilities. When assembling a pouch battery, only a small amount of electrolyte is needed to achieve ion conductivity comparable to that of a liquid battery. Furthermore, the resulting battery system exists in a quasi-solid state, reducing the risk of electrolyte leakage. Combined with the excellent flame-retardant properties of the electrolyte membrane, the safety performance of the pouch battery is significantly improved. Therefore, the solid electrolyte membrane prepared by this invention exhibits good ionic conductivity, an electrochemical window suitable for ternary cathode materials, and excellent mechanical and flame-retardant properties. This method has a significant effect on improving the safety performance and lifespan of solid-state pouch batteries. Attached Figure Description

[0020] Figure 1 This is a SEM image of the surface of the polyimide fiber skeleton membrane in Example 1.

[0021] Figure 2This is a SEM image of the surface of the solid electrolyte membrane prepared in Example 1.

[0022] Figure 3 The impedance diagram is shown for the solid electrolyte membrane prepared in Example 1.

[0023] Figure 4 This is an electrochemical window diagram of the solid electrolyte membrane prepared in Example 1.

[0024] Figure 5 This is a graph showing the flame retardant properties of the solid electrolyte membrane prepared in Example 1.

[0025] Figure 6 This is a photograph of the solid-state pouch cell assembled using the solid electrolyte membrane prepared in Example 1.

[0026] Figure 7 This is a charge-discharge curve of the solid-state pouch cell assembled using the solid electrolyte membrane prepared in Example 1. Figure 7 a) and cycle performance diagram ( Figure 7 b). Detailed Implementation

[0027] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides detailed implementation methods and specific operation processes. However, the protection scope of the present invention is not limited to the following embodiments.

[0028] Example 1

[0029] A method for preparing a flame-retardant solid electrolyte membrane includes the following steps:

[0030] 3.2g of lithium aluminum titanium phosphate (LATP, 300nm in diameter) powder was added to 20g of N-N dimethylformamide and sonicated in an ice bath for 30min to uniformly disperse the LATP powder in the solvent. Then, 0.4g of polyethylene oxide and 0.4g of polyvinylidene fluoride were added and the mixture was magnetically stirred at 60℃ for 12h. Then, 0.8g of LiClO4 was added and the mixture was magnetically stirred at 60℃ for another 6h to obtain a homogeneous electrolyte slurry.

[0031] A polyimide fiber skeleton membrane (~16 μm) was laid flat on a glass plate. The above-mentioned electrolyte slurry was uniformly coated onto the polyimide membrane and then dried in an 80°C oven for 6 hours to allow the solvent to evaporate completely. The polyimide membrane (~18 μm) was then removed, and the LATP-coated side was laid flat on a glass plate. Another layer of the above-mentioned electrolyte slurry was coated on the uncoated side, and the membrane was dried in an 80°C oven for 12 hours to allow the solvent to evaporate completely, resulting in a solid electrolyte membrane (~20 μm) coated with LATP on both sides.

[0032] The obtained solid electrolyte membrane was cut into sheets (size: 46mm × 60mm), and four solid electrolyte membranes, two silicon-carbon negative electrode sheets (size: 42mm × 56mm), and one positive electrode sheet (size: 40mm × 54mm) were assembled into a pouch cell. Among them:

[0033] The preparation method of the positive electrode sheet is as follows: The positive electrode material (LiNi) is prepared by... 0.88 Co 0.06 Mn 0.06 O2), acetylene black, and polyvinylidene fluoride (weight average molecular weight 600,000) were ground at a mass ratio of 98.3:0.5:1.2 and dispersed in N-methylpyrrolidone organic solvent (solid content: 60%) and stirred for 12 hours to obtain a uniform slurry. The slurry was then coated onto aluminum foil (loading: 45.2 g / cm³). 2 The aluminum foil was dried in an 80℃ forced-air oven for 12 hours. After removal, the above slurry was coated on the other side of the aluminum foil (loading: 45.2 g / cm³). 2 The material is placed in an 80℃ forced-air oven and dried for 12 hours to obtain a double-sided coated positive electrode material, which is then rolled and cut into sheets (size: 40mm×54mm).

[0034] The preparation method of the silicon-carbon anode sheet is as follows: Silicon-carbon anode material, acetylene black, polyacrylic acid (PAA), and styrene-butadiene rubber latex (SBR) are ground and dispersed in water (solid content: 40%) at a mass ratio of 93.5:2.0:3.5:1.0, and stirred for 12 hours to obtain a uniform slurry. Then, the slurry is coated onto copper foil (loading: 18.1 g / cm³). 2 The copper foil was dried in an 80°C oven for 12 hours. After removal, the same paste (loading: 18.1 g / cm³) was applied to the other side of the copper foil. 2 The material is placed in an 80℃ forced-air oven and dried for 12 hours to obtain a double-sided coated silicon-carbon anode material, which is then rolled and cut into sheets (size: 42mm×56mm).

[0035] The assembled soft-pack cells were hot-pressed at 130℃ for 30 minutes, then encapsulated with aluminum-plastic film, injected with 0.3g of commercial electrolyte, and sealed to obtain a solid-state soft-pack battery, which was then subjected to electrochemical performance testing.

[0036] Figure 1 This is a SEM image of the surface of the polyimide fiber skeleton membrane. It can be seen that the pore size of the polyimide membrane ranges from 2 to 20 μm.

[0037] Figure 2 The image shows a SEM image of the surface of the solid electrolyte membrane. It can be seen that LATP powder is uniformly dispersed on the surface of the polyimide membrane, and the gaps between the LATP powder particles also help to promote the rapid dispersion of the electrolyte within the membrane.

[0038] Figure 3 The impedance diagram of the prepared solid electrolyte membrane is shown. It can be seen that the bulk impedance of the electrolyte membrane measured by the blocked electrode method is 1.3 Ω, and the ionic conductivity is calculated to be 1.34 × 10⁻⁶. -3 The S / cm ratio is greater than that of currently widely used polymer electrolytes.

[0039] Figure 4 This is an electrochemical window diagram of the prepared solid electrolyte membrane. It can be seen that the oxidative decomposition voltage of the electrolyte membrane is as high as 5V, proving that it is suitable for cathode materials with higher operating voltages.

[0040] Figure 5 The image shows the flame retardant properties of the prepared solid electrolyte membrane. It can be seen that the flame extinguishes immediately within 1 second after the electrolyte membrane is ignited, fully demonstrating the excellent flame retardant properties of the solid electrolyte membrane.

[0041] Figure 6 This is a photograph of a solid-state pouch cell assembled using the prepared solid electrolyte membrane.

[0042] Figure 7 Charge-discharge curves of solid-state pouch cells assembled using the prepared solid electrolyte membrane. Figure 7 a) and cycle performance diagram ( Figure 7 b). The cathode is a ternary material (LiNi). 0.88 Co 0.06 Mn 0.06 The negative electrode is a silicon-carbon composite material (O2). The operating voltage of the soft-pack battery is 2.8–4.3V, and the cycling conditions are 25℃ and 0.5C. It can be seen that the initial discharge capacity is 175.6mAh, and the discharge capacity after 150 cycles is 160.0mAh, with a capacity retention rate of 91.1% and a coulombic efficiency close to 100%.

[0043] Example 2

[0044] A method for preparing a flame-retardant solid electrolyte membrane includes the following steps:

[0045] 7.2g of lithium lanthanum zirconium tantalum oxide (LLZTO, 300nm diameter) powder was added to 20g of N-N dimethylformamide and sonicated in an ice bath for 30min to uniformly disperse the LLZTO powder in the solvent. Then, 0.4g of polyethylene oxide and 0.4g of poly(vinylidene fluoride-co-hexafluoropropylene) were added and magnetically stirred at 60℃ for 12h. Then, 0.8g of LiClO4 was added and magnetic stirring was continued at 60℃ for 6h to obtain a homogeneous electrolyte slurry.

[0046] A polyimide fiber skeleton membrane (~16 μm) was laid flat on a glass plate. The electrolyte slurry was then uniformly coated onto the polyimide membrane and dried in a forced-air oven at 80°C for 6 hours to allow the solvent to evaporate completely. The polyimide membrane (~18 μm) was then removed and laid flat on a glass plate with the LLZTO-coated side facing down. Another layer of the electrolyte slurry was coated onto the uncoated side, and the membrane was again dried in a forced-air oven at 80°C for 12 hours to allow the solvent to evaporate completely, resulting in a solid electrolyte membrane (~20 μm) coated with LLZTO on both sides.

[0047] Example 3

[0048] A method for preparing a flame-retardant solid electrolyte membrane includes the following steps:

[0049] 1.87g of lithium lanthanum zirconium oxide (LLZO, 300nm in diameter) powder was added to 20g of N-N dimethylformamide and sonicated in an ice bath for 30min to uniformly disperse the LLZO powder in the solvent. Then, 0.4g of polyethylene oxide and 0.4g of polyacrylonitrile were added and the mixture was magnetically stirred at 60℃ for 12h. Then, 0.8g of LiTFSI was added and the mixture was magnetically stirred at 60℃ for another 6h to obtain a homogeneous electrolyte slurry.

[0050] A polyimide fiber skeleton membrane (~16 μm) was laid flat on a glass plate. The electrolyte slurry was then uniformly coated onto the polyimide membrane and dried in a forced-air oven at 80°C for 6 hours to allow the solvent to evaporate completely. The polyimide membrane (~18 μm) was then removed, and the side coated with LLZO was laid flat on the glass plate with the LLZO coating facing down. Another layer of the electrolyte slurry was coated on the uncoated side, and the membrane was dried in a forced-air oven at 80°C for 12 hours to allow the solvent to evaporate completely, resulting in a solid electrolyte membrane (~20 μm) coated with LLZO on both sides.

[0051] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make corresponding adjustments and improvements without departing from the principle of the present invention, and these adjustments and improvements should also be considered within the scope of protection of the present invention.

Claims

1. A solid-state pouch cell, comprising a positive electrode, a negative electrode, and a separator, characterized in that: The cathode material is lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4), or ternary material LiNi. x Co y Mn z O2, where x + y + z = 1; the negative electrode material is silicon-carbon composite material or graphite; the separator is a flame-retardant solid electrolyte membrane; The flame-retardant solid electrolyte membrane is obtained by coating both sides of a polyimide fiber skeleton membrane with an inorganic solid electrolyte slurry containing a first binder, polyethylene oxide, and a second binder, and then drying it. The inorganic solid electrolyte slurry comprises the following raw materials by mass percentage: 2%–25% inorganic solid electrolyte powder, 1%–2% polyethylene oxide as the first binder, 1%–2% as the second binder, 2%–4% lithium salt, and 67%–94% organic solvent; The first adhesive, polyethylene oxide, has a weight-average molecular weight of 100,000 to 1,000,000; the second adhesive is at least one of polyvinylidene fluoride, poly(vinylidene fluoride-co-hexafluoropropylene), polymethyl methacrylate, and polyacrylonitrile, and the weight-average molecular weight of the second adhesive is 300,000 to 1,000,000. The solid-state soft-pack battery is prepared by assembling the flame-retardant solid electrolyte membrane with the positive and negative electrodes into a soft-pack cell, then hot-pressing it at 100℃~150℃ for 15min~30min, injecting a small amount of electrolyte, and sealing it to form a solid-state soft-pack battery.

2. The solid-state pouch battery according to claim 1, characterized in that: The inorganic solid electrolyte powder is at least one of lithium titanium aluminum phosphate, lithium germanium aluminum phosphate, lithium lanthanum zirconium oxide, lithium lanthanum zirconium tantalum oxide, lithium lanthanum zirconium niobium oxide, and lithium lanthanum titanium oxide.

3. A solid-state pouch battery according to claim 1, characterized in that: The lithium salt is at least one of LiClO4, LiTFSI, LiFSI, LiBOB, LiC2O4F2B, LiN(SO2CF3)2, LiCF3SO3 and LiN(SO2CF2CF3)2.

4. A solid-state pouch battery according to claim 1, characterized in that: The organic solvent is at least one selected from N-methylpyrrolidone, N-dimethylformamide, N-dimethylacetamide, acetonitrile, dimethyl sulfoxide, and tetrahydrofuran.

5. A solid-state pouch battery according to claim 1, characterized in that: The thickness of the coating layer formed by coating the inorganic solid electrolyte slurry onto the polyimide fiber skeleton membrane is 25μm to 100μm, and the thickness of the inorganic solid electrolyte layer after drying is 1μm to 4μm. The thickness of the flame-retardant solid electrolyte membrane is 10μm to 30μm.

6. A solid-state pouch battery according to claim 1, characterized in that: The preparation process of the flame-retardant solid electrolyte membrane includes the following steps: (1) First, add the inorganic solid electrolyte powder to the organic solvent and sonicate it in an ice bath for 30 min to 1 h to make the powder uniformly dispersed in the organic solvent; then add the first binder polyethylene oxide and the second binder, and heat and stir at 60℃ to 100℃ for 2 h to 24 h to obtain a uniform slurry with high viscosity; finally, add lithium salt and continue to heat and stir at 60℃ to 100℃ for 2 h to 12 h to form a uniformly dispersed inorganic solid electrolyte slurry. (2) The inorganic solid electrolyte slurry obtained in step (1) is coated on one side of the polyimide fiber skeleton membrane, and then placed in a forced-air drying oven and dried at 60℃~100℃ for 2h~12h to allow the solvent to evaporate completely. Then, an inorganic solid electrolyte slurry is coated on the other side of the polyimide fiber skeleton and placed in a forced-air drying oven and dried at 60℃~100℃ for 2h~24h to allow the solvent to evaporate completely, thus obtaining a flame-retardant solid electrolyte membrane coated with inorganic solid electrolyte on both sides.

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