A high-temperature rigid conversion type borate cross-linked hydrogel material for battery thermal runaway propagation protection and a preparation method thereof

Abstract

The application discloses a high-temperature rigid conversion type borate cross-linked hydrogel material for battery thermal runaway propagation protection and a preparation method thereof. The preparation raw materials of the hydrogel material include polyvinyl alcohol, boric acid and deionized water. By immersing the polyvinyl alcohol hydrogel in a boric acid solution, the boric acid diffuses into the hydrogel network and reacts with the vicinal diol structure in the polyvinyl alcohol segment to form a B-O-C covalent cross-linking structure. The high-temperature rigid conversion type borate cross-linked hydrogel material provided by the application is a flexible hydrogel at normal temperature, has good mechanical strength and toughness. Under the action of high temperature of battery thermal runaway, the material can be in-situ converted from a flexible hydrogel into a continuous and dense rigid residual barrier, and can effectively resist the thermal-mechanical impact in the thermal runaway process. The application solves the problem that the existing hydrogel protection materials are difficult to have normal temperature flexibility and high-temperature barrier strength, has a simple preparation process, and has a wide application prospect in the field of battery safety protection.

Description

Technical Field

[0001] This invention relates to the field of battery thermal runaway propagation protection, and specifically to a high-temperature rigid conversion borate ester crosslinked hydrogel material for battery thermal runaway propagation protection and its preparation method. Background Technology

[0002] With the rapid development of new energy vehicles, energy storage power stations, and intelligent equipment, the energy density of lithium-ion batteries continues to increase, making their safety performance, especially thermal runaway, increasingly prominent. Lithium-ion batteries may experience thermal runaway under conditions of mechanical, electrical, or thermal abuse, manifesting as a rapid increase in cell temperature accompanied by the release, ejection, fire, or even explosion of large amounts of flammable gas. More seriously, within a module or battery pack, the heat, flames, and high-temperature ejected material released from a single cell during thermal runaway can further impact adjacent cells, inducing continuous runaway and ultimately leading to thermal runaway propagation. Therefore, developing efficient thermal runaway propagation protection materials at the battery system level is a key issue in improving battery system safety.

[0003] Currently, materials used for protecting against the spread of thermal runaway in batteries are mainly divided into two categories: heat-insulating materials and heat-absorbing materials. Heat-insulating materials, represented by aerogels, can effectively delay heat transfer due to their extremely low thermal conductivity; however, their protection mechanism is singular, providing only passive insulation. Heat accumulation in the barrier area can still trigger thermal runaway in adjacent batteries. Heat-absorbing materials, represented by organic phase change materials and inorganic hydrated salts, can absorb some heat through phase change or thermochemical decomposition. However, organic phase change materials have low heat storage density and are flammable at high temperatures, while inorganic hydrated salts generally suffer from high rigidity and poor interfacial compatibility, making them difficult to adapt to volume changes during battery charging and discharging.

[0004] Hydrogel materials, containing a large amount of water, can absorb heat through water evaporation and also possess excellent flexibility and conformability, exhibiting unique advantages in protecting against thermal runaway propagation in batteries. However, traditional pure hydrogels collapse structurally after rapid water loss at high temperatures, resulting in loose and disordered residual carbon that cannot construct a continuous and stable high-temperature barrier, making it difficult to continuously prevent the spread of heat and high-temperature projectiles. To improve the thermal runaway protection capability of hydrogel materials, existing technologies typically introduce inorganic fillers or flame retardants to enhance the high-temperature stability and flame retardant properties of the materials. For example, Ma et al. reported an ionic hydrogel for improving the thermal safety of lithium-ion batteries. This material forms a polyacrylamide / carboxymethyl cellulose dual network structure through free radical polymerization and introduces glycerol, tannic acid, and lithium chloride, enabling the hydrogel to possess both evaporative cooling and high-temperature flame retardant functions. (JiayiYuan, et al. A multifunctional ionic hydrogel facilitating enhanced thermal safety for lithium-ion battery via overheating cooling, intelligent warning, and thermal runaway blocking [J]. Materials Today Energy, 55,2026,102171). While the aforementioned technologies can improve the thermal protection performance of hydrogel materials to some extent, they still have the following shortcomings: On the one hand, if the functional components are dispersed in the hydrogel network only in the form of physical fillers, they are prone to agglomeration or phase separation inside the material, which will cause local stress concentration, reduce the material's flexibility and deformation adaptability, and is not conducive to its long-term service in bonding between battery cells; on the other hand, the residual barrier formed after the material is subjected to high temperature usually lacks sufficient mechanical strength and rigid support capacity, making it difficult to resist the thermo-mechanical coupling effects such as battery cell expansion, gas jet, flame erosion, and module stacking, which can easily lead to cracking, peeling, or collapse, thus causing the protective barrier to fail.

[0005] Therefore, there is an urgent need to develop a novel hydrogel-based battery thermal runaway propagation protection material that not only has good flexibility at room temperature, but also provides endothermic buffering in the early stages of thermal runaway due to its high decomposition enthalpy, and transforms into a continuous and dense rigid barrier in situ under high temperature, so as to solve the problem that traditional hydrogel protective materials cannot simultaneously achieve both room temperature flexibility and high temperature barrier strength. Summary of the Invention

[0006] To overcome the shortcomings of existing hydrogel-based battery thermal runaway protection materials, this invention provides a high-temperature rigid conversion borate ester crosslinked hydrogel material and its preparation method.

[0007] The core idea of ​​this invention is to use polyvinyl alcohol hydrogel as a matrix and utilize the abundant vicinal diol structures on its molecular chain to construct a borate ester covalent cross-linked structure with boric acid through a dehydration condensation reaction. This borate ester cross-linked structure chemically combines boric acid filler with the hydrogel network, reducing disordered aggregation and phase separation of boric acid and improving the flexibility of the material. More importantly, when the battery experiences thermal runaway, this covalent cross-linked network can regulate the formation path of the residual phase through dehydration and rearrangement during high-temperature pyrolysis, guiding the B2O3 phase generated by boric acid pyrolysis to undergo in-situ chemical integration with the carbonization products, ultimately forming a continuous, dense ceramic-carbon hybrid barrier with high flexural strength, achieving a high-temperature rigid transformation from a flexible hydrogel to a rigid protective layer.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a high-temperature rigid conversion borate ester crosslinked hydrogel material. The raw materials for preparing the hydrogel material include polyvinyl alcohol, boric acid, and deionized water. The hydrogel material is a flexible hydrogel at room temperature, but can be in-situ converted to form a ceramic-carbon hybrid rigid barrier under high temperature.

[0009] This invention also provides a method for preparing the above-mentioned high-temperature rigid conversion borate ester crosslinked hydrogel material, comprising the following steps: (1) Add polyvinyl alcohol to deionized water, heat and stir until completely dissolved to obtain a homogeneous polyvinyl alcohol aqueous solution; (2) Pour the polyvinyl alcohol aqueous solution obtained in step (1) into a mold and perform physical cross-linking treatment to obtain a polyvinyl alcohol hydrogel with an initial network structure; (3) The polyvinyl alcohol hydrogel obtained in step (2) is pre-soaked in deionized water to allow the hydrogel network to fully swell and improve the mobility of polymer chain segments. (4) The polyvinyl alcohol hydrogel obtained in step (3) is immersed in boric acid solution to allow boric acid to diffuse into the hydrogel network and react with the vicinal diol structure in the polyvinyl alcohol chain segment to form a BOC covalent cross-linked network. (5) Take out the hydrogel composite material obtained in step (4), remove the excess solution on the surface, and obtain the high-temperature rigid conversion borate ester crosslinked hydrogel composite material.

[0010] Further, in step (1), the mass fraction of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 5% to 15%.

[0011] Further, in step (1), the heating temperature is 90-95℃, the stirring rate is 200-500 rpm, and the stirring time is 0.5-1 h.

[0012] Further, in step (2), the physical crosslinking treatment is a freeze-thaw process; the freezing temperature is -30 to -10°C and the freezing time is 8 to 10 h; the thawing temperature is 20 to 30°C and the thawing time is 4 to 6 h; the number of freeze-thaw cycles is 1 to 3.

[0013] Furthermore, in step (3), the pre-soaking time is 0.5 to 3 hours, preferably 1 hour.

[0014] Further, in step (4), the mass fraction of the boric acid solution is 3% to 15%, preferably 6% to 12%, and more preferably 9%.

[0015] Furthermore, in step (4), the soaking temperature is 50-80 °C and the soaking time is 6-48 h, preferably 12-24 h.

[0016] Furthermore, in step (4), boric acid not only acts as a boron-containing functional component distributed in the hydrogel network, but also acts as a covalent crosslinking agent to react with polymer segments containing vicinal diol structures to form borate ester bonds, thereby improving the mechanical strength and structural stability of the hydrogel composite precursor and providing a chemical structural basis for high-temperature rigid conversion.

[0017] This invention also provides the application of the above-mentioned high-temperature rigid conversion borate ester crosslinked hydrogel material in the protection against thermal runaway propagation in batteries.

[0018] The high-temperature rigid conversion borate ester crosslinked hydrogel material of this invention is a flexible hydrogel at room temperature, which can be prepared as a gasket and adhered between adjacent battery cells. When the battery experiences thermal runaway, the evaporation of water and the dehydration of boron-containing components in the material can absorb some heat, the polymer network further carbonizes, the boron-containing components transform into the B2O3 phase, and undergo in-situ chemical integration with the carbonized framework, ultimately forming a continuous and dense ceramic-carbon hybrid barrier. This barrier has high structural integrity and rigid support capacity, and can effectively resist the thermo-mechanical coupling effects of cell expansion, gas injection, and module stacking pressure, thereby preventing the propagation of heat, flames, and high-temperature ejected materials to adjacent batteries.

[0019] Compared with the prior art, the present invention has the following advantages and beneficial effects: (1) The high-temperature rigid conversion borate ester crosslinked hydrogel material prepared by the present invention combines boric acid with a polyvinyl alcohol network through BOC covalent bonds, which can effectively reduce the agglomeration and phase separation of boric acid filler. While maintaining the flexibility of the material, it improves the strength and toughness, with the optimal tensile strength reaching 5.59 MPa and the optimal toughness reaching 18.8 MJ·m. -3 .

[0020] (2) The high-temperature rigid conversion borate ester crosslinked hydrogel material prepared by the present invention can be transformed from a flexible hydrogel into a rigid residual barrier after thermal runaway at high temperature. Its bending strength is optimal at 1.45 MPa, which is beneficial to resist thermal shocks such as cell expansion, gas jet and thermal scouring.

[0021] (3) The high-temperature rigid conversion borate ester crosslinked hydrogel material prepared by the present invention can be attached between adjacent cells. When the battery experiences thermal runaway, it can successfully suppress the spread of thermal runaway through the synergistic effect of heat absorption buffer and high-temperature rigid barrier. Under the protection of the preferred material, the highest temperature of adjacent cells is only 128°C. Attached Figure Description

[0022] Figure 1 SEM images of the high-temperature rigid conversion borate ester crosslinked hydrogel materials obtained in Examples 1, 2 and 3, and the polyvinyl alcohol hydrogel material without borate ester crosslinking structure obtained in Comparative Example 1.

[0023] Figure 2 Fourier transform infrared spectra of the high-temperature rigid conversion borate ester crosslinked hydrogel materials obtained in Examples 1, 2 and 3, and the polyvinyl alcohol hydrogel material without borate ester crosslinking structure obtained in Comparative Example 1.

[0024] Figure 3 The X-ray diffraction patterns are shown for the high-temperature rigid conversion borate ester crosslinked hydrogel materials obtained in Examples 1, 2 and 3, and the polyvinyl alcohol hydrogel material without borate ester crosslinking structure obtained in Comparative Example 1.

[0025] Figure 4 Tensile stress-strain curves are shown for the high-temperature rigid conversion borate ester crosslinked hydrogel materials obtained in Examples 1, 2, and 3, and for the polyvinyl alcohol hydrogel material without borate ester crosslinking structure obtained in Comparative Example 1.

[0026] Figure 5 The tensile strength comparison diagram shows the high-temperature rigid conversion borate ester crosslinked hydrogel materials obtained in Examples 1, 2 and 3, and the polyvinyl alcohol hydrogel material without borate ester crosslinking structure obtained in Comparative Example 1.

[0027] Figure 6 The toughness comparison diagram shows the high-temperature rigid conversion borate ester crosslinked hydrogel materials obtained in Examples 1, 2 and 3, and the polyvinyl alcohol hydrogel material without borate ester crosslinking structure obtained in Comparative Example 1.

[0028] Figure 7 The tensile stress-strain curves of the high-temperature rigid conversion borate ester crosslinked hydrogel materials obtained in Examples 2, 4 and 5 are shown.

[0029] Figure 8 The tensile stress-strain curves of the high-temperature rigid conversion borate ester crosslinked hydrogel materials obtained in Examples 2, 6 and 7 are shown.

[0030] Figure 9 The tensile stress-strain curves of the high-temperature rigid conversion borate ester crosslinked hydrogel materials obtained in Examples 2, 8 and 9 are shown.

[0031] Figure 10 The graph shows the bending resistance test results of the residues of the high-temperature rigid conversion borate ester crosslinked hydrogel materials obtained in Examples 1, 2, 3, 4 and 5 after high-temperature treatment.

[0032] Figure 11 The graph shows a comparison of the heat storage densities of the high-temperature rigid conversion borate ester crosslinked hydrogel materials obtained in Examples 1, 2, 3, 4, and 5, and the polyvinyl alcohol hydrogel material without borate ester crosslinking structure obtained in Comparative Example 1.

[0033] Figure 12 The temperature curves of the high-temperature rigid conversion borate ester crosslinked hydrogel materials obtained in Examples 1, 2, 3, 4 and 5, and the polyvinyl alcohol hydrogel material without borate ester crosslinking structure obtained in Comparative Example 1, are compared in battery thermal runaway propagation protection. Detailed Implementation

[0034] Example 1 This embodiment provides a high-temperature rigid conversion borate ester crosslinked hydrogel material with a boric acid solution concentration of 3%, and its preparation method is as follows: (1) Weigh polyvinyl alcohol and deionized water to prepare a polyvinyl alcohol aqueous solution with a mass fraction of 10%. Add polyvinyl alcohol to deionized water and stir magnetically at 90 °C and 300 rpm for 1 h until the polyvinyl alcohol is completely dissolved to obtain a homogeneous and transparent polyvinyl alcohol aqueous solution.

[0035] (2) Pour the polyvinyl alcohol aqueous solution obtained in step (1) into a mold, freeze it at -20 ℃ for 8 h, and then melt it at 25 ℃ for 4 h. By freezing-melting, a polyvinyl alcohol physical cross-linking network is constructed to obtain polyvinyl alcohol hydrogel.

[0036] (3) The polyvinyl alcohol hydrogel obtained in step (2) is pre-soaked in deionized water for 1 h to allow water molecules to enter the interior of the polyvinyl alcohol hydrogel network, improve the mobility of polyvinyl alcohol molecular chain segments, and expose more orthodiol sites that can react with boric acid.

[0037] (4) Prepare a 3% (w / w) boric acid aqueous solution, and place the pre-soaked polyvinyl alcohol hydrogel obtained in step (3) into the boric acid aqueous solution and let it stand at room temperature for 24 h. During the soaking process, boric acid molecules diffuse into the polyvinyl alcohol hydrogel network and react with the vicinal diol structure on the polyvinyl alcohol chain segment to form a BOC covalent cross-linked structure.

[0038] (5) After soaking, remove the hydrogel material, remove the excess boric acid solution from its surface, seal and store it to obtain the high-temperature rigid conversion borate ester crosslinked hydrogel material.

[0039] Example 2 This embodiment provides a high-temperature rigid conversion borate ester crosslinked hydrogel material with a boric acid solution concentration of 9%, and its preparation method is as follows: (1) Weigh polyvinyl alcohol and deionized water to prepare a polyvinyl alcohol aqueous solution with a mass fraction of 10%. Add polyvinyl alcohol to deionized water and stir magnetically at 90 °C and 300 rpm for 1 h until the polyvinyl alcohol is completely dissolved to obtain a homogeneous and transparent polyvinyl alcohol aqueous solution.

[0040] (2) Pour the polyvinyl alcohol aqueous solution obtained in step (1) into a mold, freeze it at -20 ℃ for 8 h, and then melt it at 25 ℃ for 4 h. By freezing-melting, a polyvinyl alcohol physical cross-linking network is constructed to obtain polyvinyl alcohol hydrogel.

[0041] (3) The polyvinyl alcohol hydrogel obtained in step (2) is pre-soaked in deionized water for 1 h to allow water molecules to enter the interior of the polyvinyl alcohol hydrogel network, improve the mobility of polyvinyl alcohol molecular chain segments, and expose more orthodiol sites that can react with boric acid.

[0042] (4) Prepare a 9% (w / w) boric acid aqueous solution, and place the pre-soaked polyvinyl alcohol hydrogel obtained in step (3) into the boric acid aqueous solution and let it stand at room temperature for 24 h. During the soaking process, boric acid molecules diffuse into the polyvinyl alcohol hydrogel network and react with the vicinal diol structure on the polyvinyl alcohol chain segment to form a BOC covalent cross-linked structure.

[0043] (5) After soaking, remove the hydrogel material, remove the excess boric acid solution from its surface, seal and store it to obtain the high-temperature rigid conversion borate ester crosslinked hydrogel material.

[0044] Example 3 This embodiment provides a high-temperature rigid conversion borate ester crosslinked hydrogel material with a boric acid solution concentration of 15%, and its preparation method is as follows: (1) Weigh polyvinyl alcohol and deionized water to prepare a polyvinyl alcohol aqueous solution with a mass fraction of 10%. Add polyvinyl alcohol to deionized water and stir magnetically at 90 °C and 300 rpm for 1 h until the polyvinyl alcohol is completely dissolved to obtain a homogeneous and transparent polyvinyl alcohol aqueous solution.

[0045] (2) Pour the polyvinyl alcohol aqueous solution obtained in step (1) into a mold, freeze it at -20 ℃ for 8 h, and then melt it at 25 ℃ for 4 h. By freezing-melting, a polyvinyl alcohol physical cross-linking network is constructed to obtain polyvinyl alcohol hydrogel.

[0046] (3) The polyvinyl alcohol hydrogel obtained in step (2) is pre-soaked in deionized water for 1 h to allow water molecules to enter the interior of the polyvinyl alcohol hydrogel network, improve the mobility of polyvinyl alcohol molecular chain segments, and expose more orthodiol sites that can react with boric acid.

[0047] (4) Prepare a 15% boric acid aqueous solution, and place the pre-soaked polyvinyl alcohol hydrogel obtained in step (3) into the boric acid aqueous solution and let it stand at room temperature for 24 h. During the soaking process, boric acid molecules diffuse into the polyvinyl alcohol hydrogel network and react with the vicinal diol structure on the polyvinyl alcohol chain segment to form a BOC covalent cross-linked structure.

[0048] (5) After soaking, remove the hydrogel material, remove the excess boric acid solution from its surface, seal and store it to obtain the high-temperature rigid conversion borate ester crosslinked hydrogel material.

[0049] Example 4 This embodiment provides a high-temperature rigid conversion borate ester crosslinked hydrogel material with a boric acid solution concentration of 6%, and its preparation method is as follows: (1) Weigh polyvinyl alcohol and deionized water to prepare a polyvinyl alcohol aqueous solution with a mass fraction of 10%. Add polyvinyl alcohol to deionized water and stir magnetically at 90 °C and 300 rpm for 1 h until the polyvinyl alcohol is completely dissolved to obtain a homogeneous and transparent polyvinyl alcohol aqueous solution.

[0050] (2) Pour the polyvinyl alcohol aqueous solution obtained in step (1) into a mold, freeze it at -20 ℃ for 8 h, and then melt it at 25 ℃ for 4 h. By freezing-melting, a polyvinyl alcohol physical cross-linking network is constructed to obtain polyvinyl alcohol hydrogel.

[0051] (3) The polyvinyl alcohol hydrogel obtained in step (2) is pre-soaked in deionized water for 1 h to allow water molecules to enter the interior of the polyvinyl alcohol hydrogel network, improve the mobility of polyvinyl alcohol molecular chain segments, and expose more orthodiol sites that can react with boric acid.

[0052] (4) Prepare a 6% (w / w) boric acid aqueous solution, and place the pre-soaked polyvinyl alcohol hydrogel obtained in step (3) into the boric acid aqueous solution and let it stand at room temperature for 24 h. During the soaking process, boric acid molecules diffuse into the polyvinyl alcohol hydrogel network and react with the vicinal diol structure on the polyvinyl alcohol chain segment to form a BOC covalent cross-linked structure.

[0053] (5) After soaking, remove the hydrogel material, remove the excess boric acid solution from its surface, seal and store it to obtain the high-temperature rigid conversion borate ester crosslinked hydrogel material.

[0054] Example 5 This embodiment provides a high-temperature rigid conversion borate ester crosslinked hydrogel material with a boric acid solution concentration of 12%, and its preparation method is as follows: (1) Weigh polyvinyl alcohol and deionized water to prepare a polyvinyl alcohol aqueous solution with a mass fraction of 10%. Add polyvinyl alcohol to deionized water and stir magnetically at 90 °C and 300 rpm for 1 h until the polyvinyl alcohol is completely dissolved to obtain a homogeneous and transparent polyvinyl alcohol aqueous solution.

[0055] (2) Pour the polyvinyl alcohol aqueous solution obtained in step (1) into a mold, freeze it at -20 ℃ for 8 h, and then melt it at 25 ℃ for 4 h. By freezing-melting, a polyvinyl alcohol physical cross-linking network is constructed to obtain polyvinyl alcohol hydrogel.

[0056] (3) The polyvinyl alcohol hydrogel obtained in step (2) is pre-soaked in deionized water for 1 h to allow water molecules to enter the interior of the polyvinyl alcohol hydrogel network, improve the mobility of polyvinyl alcohol molecular chain segments, and expose more orthodiol sites that can react with boric acid.

[0057] (4) Prepare a 12% boric acid aqueous solution, and place the pre-soaked polyvinyl alcohol hydrogel obtained in step (3) into the boric acid aqueous solution and let it stand at room temperature for 24 h. During the soaking process, boric acid molecules diffuse into the polyvinyl alcohol hydrogel network and react with the vicinal diol structure on the polyvinyl alcohol chain segment to form a BOC covalent cross-linked structure.

[0058] (5) After soaking, remove the hydrogel material, remove the excess boric acid solution from its surface, seal and store it to obtain the high-temperature rigid conversion borate ester crosslinked hydrogel material.

[0059] Example 6 This embodiment provides a high-temperature rigid conversion borate ester crosslinked hydrogel material with a polyvinyl alcohol concentration of 5%, and its preparation method is as follows: (1) Weigh polyvinyl alcohol and deionized water to prepare a polyvinyl alcohol aqueous solution with a mass fraction of 5%. Add polyvinyl alcohol to deionized water and stir magnetically at 90 °C and 300 rpm for 1 h until the polyvinyl alcohol is completely dissolved to obtain a homogeneous and transparent polyvinyl alcohol aqueous solution.

[0060] (2) Pour the polyvinyl alcohol aqueous solution obtained in step (1) into a mold, freeze it at -20 ℃ for 8 h, and then melt it at 25 ℃ for 4 h. By freezing-melting, a polyvinyl alcohol physical cross-linking network is constructed to obtain polyvinyl alcohol hydrogel.

[0061] (3) The polyvinyl alcohol hydrogel obtained in step (2) is pre-soaked in deionized water for 1 h to allow water molecules to enter the interior of the polyvinyl alcohol hydrogel network, improve the mobility of polyvinyl alcohol molecular chain segments, and expose more orthodiol sites that can react with boric acid.

[0062] (4) Prepare a 9% (w / w) boric acid aqueous solution, and place the pre-soaked polyvinyl alcohol hydrogel obtained in step (3) into the boric acid aqueous solution and let it stand at room temperature for 24 h. During the soaking process, boric acid molecules diffuse into the polyvinyl alcohol hydrogel network and react with the vicinal diol structure on the polyvinyl alcohol chain segment to form a BOC covalent cross-linked structure.

[0063] (5) After soaking, remove the hydrogel material, remove the excess boric acid solution from its surface, seal and store it to obtain the high-temperature rigid conversion borate ester crosslinked hydrogel material.

[0064] Example 7 This embodiment provides a high-temperature rigid conversion borate ester crosslinked hydrogel material with a polyvinyl alcohol concentration of 15%, and its preparation method is as follows: (1) Weigh polyvinyl alcohol and deionized water to prepare a polyvinyl alcohol aqueous solution with a mass fraction of 15%. Add polyvinyl alcohol to deionized water and stir magnetically at 90 °C and 300 rpm for 1 h until the polyvinyl alcohol is completely dissolved to obtain a homogeneous and transparent polyvinyl alcohol aqueous solution.

[0065] (2) Pour the polyvinyl alcohol aqueous solution obtained in step (1) into a mold, freeze it at -20 ℃ for 8 h, and then melt it at 25 ℃ for 4 h. By freezing-melting, a polyvinyl alcohol physical cross-linking network is constructed to obtain polyvinyl alcohol hydrogel.

[0066] (3) The polyvinyl alcohol hydrogel obtained in step (2) is pre-soaked in deionized water for 1 h to allow water molecules to enter the interior of the polyvinyl alcohol hydrogel network, improve the mobility of polyvinyl alcohol molecular chain segments, and expose more orthodiol sites that can react with boric acid.

[0067] (4) Prepare a 9% (w / w) boric acid aqueous solution, and place the pre-soaked polyvinyl alcohol hydrogel obtained in step (3) into the boric acid aqueous solution and let it stand at room temperature for 24 h. During the soaking process, boric acid molecules diffuse into the polyvinyl alcohol hydrogel network and react with the vicinal diol structure on the polyvinyl alcohol chain segment to form a BOC covalent cross-linked structure.

[0068] (5) After soaking, remove the hydrogel material, remove the excess boric acid solution from its surface, seal and store it to obtain the high-temperature rigid conversion borate ester crosslinked hydrogel material.

[0069] Example 8 This embodiment provides a high-temperature rigid conversion borate ester crosslinked hydrogel material with a boric acid solution immersion time of 12 h, and its preparation method is as follows: (1) Weigh polyvinyl alcohol and deionized water to prepare a polyvinyl alcohol aqueous solution with a mass fraction of 10%. Add polyvinyl alcohol to deionized water and stir magnetically at 90 °C and 300 rpm for 1 h until the polyvinyl alcohol is completely dissolved to obtain a homogeneous and transparent polyvinyl alcohol aqueous solution.

[0070] (2) Pour the polyvinyl alcohol aqueous solution obtained in step (1) into a mold, freeze it at -20 ℃ for 8 h, and then melt it at 25 ℃ for 4 h. By freezing-melting, a polyvinyl alcohol physical cross-linking network is constructed to obtain polyvinyl alcohol hydrogel.

[0071] (3) The polyvinyl alcohol hydrogel obtained in step (2) is pre-soaked in deionized water for 1 h to allow water molecules to enter the interior of the polyvinyl alcohol hydrogel network, improve the mobility of polyvinyl alcohol molecular chain segments, and expose more orthodiol sites that can react with boric acid.

[0072] (4) Prepare a 9% boric acid aqueous solution, and place the pre-soaked polyvinyl alcohol hydrogel obtained in step (3) into the boric acid aqueous solution and let it stand at room temperature for 12 h. During the soaking process, boric acid molecules diffuse into the polyvinyl alcohol hydrogel network and react with the vicinal diol structure on the polyvinyl alcohol chain segment to form a BOC covalent cross-linked structure.

[0073] (5) After soaking, remove the hydrogel material, remove the excess boric acid solution from its surface, seal and store it to obtain the high-temperature rigid conversion borate ester crosslinked hydrogel material.

[0074] Example 9 This embodiment provides a high-temperature rigid conversion borate ester crosslinked hydrogel material with a boric acid solution immersion time of 36 h, and its preparation method is as follows: (1) Weigh polyvinyl alcohol and deionized water to prepare a polyvinyl alcohol aqueous solution with a mass fraction of 10%. Add polyvinyl alcohol to deionized water and stir magnetically at 90 °C and 300 rpm for 1 h until the polyvinyl alcohol is completely dissolved to obtain a homogeneous and transparent polyvinyl alcohol aqueous solution.

[0075] (2) Pour the polyvinyl alcohol aqueous solution obtained in step (1) into a mold, freeze it at -20 ℃ for 8 h, and then melt it at 25 ℃ for 4 h. By freezing-melting, a polyvinyl alcohol physical cross-linking network is constructed to obtain polyvinyl alcohol hydrogel.

[0076] (3) The polyvinyl alcohol hydrogel obtained in step (2) is pre-soaked in deionized water for 1 h to allow water molecules to enter the interior of the polyvinyl alcohol hydrogel network, improve the mobility of polyvinyl alcohol molecular chain segments, and expose more orthodiol sites that can react with boric acid.

[0077] (4) Prepare a 9% (w / w) boric acid aqueous solution, and place the pre-soaked polyvinyl alcohol hydrogel obtained in step (3) into the boric acid aqueous solution and let it stand at room temperature for 36 h. During the soaking process, boric acid molecules diffuse into the polyvinyl alcohol hydrogel network and react with the vicinal diol structure on the polyvinyl alcohol chain segment to form a BOC covalent cross-linked structure.

[0078] (5) After soaking, remove the hydrogel material, remove the excess boric acid solution from its surface, seal and store it to obtain the high-temperature rigid conversion borate ester crosslinked hydrogel composite material.

[0079] Comparative Example 1 This comparative example provides a polyvinyl alcohol hydrogel material without a borate ester crosslinking structure, and its preparation method is as follows: (1) Weigh polyvinyl alcohol and deionized water to prepare a polyvinyl alcohol aqueous solution with a mass fraction of 10%. Add polyvinyl alcohol to deionized water and stir magnetically for 1 h at 90 ℃ and 300 rpm to obtain a homogeneous and transparent solution.

[0080] (2) Pour the polyvinyl alcohol aqueous solution obtained in step (1) into a mold, freeze at -20 ℃ for 8 h, and then melt at 25 ℃ for 4 h to obtain polyvinyl alcohol hydrogel.

[0081] Figure 1SEM images of the materials obtained in Examples 1, 2, 3, and Comparative Example 1 are shown. Comparative Example 1 exhibits a relatively loose internal network with a distinct porous structure, with pore sizes mainly distributed in the range of approximately 50–150 μm. In Example 1, after boric acid was introduced into the network, the original porous structure significantly shrank, the pore walls thickened, the number of pores decreased, and the pore size shrank to approximately 20–50 μm. This indicates that the introduction of boric acid enhanced the interchain interactions of polyvinyl alcohol, causing the network to begin to transform from loose to dense. In Example 2, after introducing an appropriate amount of boric acid, the material as a whole formed a relatively continuous and dense network structure, with the residual pores further reduced to approximately 10–30 μm. Simultaneously, a small amount of precipitated boric acid microcrystals were observed; their size was small and their distribution was relatively dispersed, without disrupting the continuity of the matrix network. In Example 3, due to the high boric acid concentration, the number and size of the boric acid precipitates increased significantly, with some aggregates reaching approximately 50–100 μm in size, leading to a decrease in matrix continuity and even noticeable accumulation and uneven distribution in localized areas.

[0082] Figure 2 The images show the FT-IR spectra of the materials obtained in Comparative Example 1, Example 1, Example 2, and Example 3. Compared with Comparative Example 1, BOC-related vibrational peaks appeared in Examples 1–3, indicating that boric acid reacted with the vicinal diol structure in the polyvinyl alcohol segments to form a covalently cross-linked borate ester structure. Meanwhile, the BOH bending vibrational peak was more pronounced in Example 3, indicating that when the boric acid concentration was too high, some boric acid failed to react completely with the polyvinyl alcohol segments and instead existed in the hydrogel network as a boric acid aggregate phase.

[0083] Figure 3 The images show the XRD patterns of the materials obtained in Comparative Example 1, Example 1, Example 2, and Example 3. Comparative Example 1 exhibits a typical broad, diffuse peak around 19.34°, reflecting its predominantly amorphous structure. After immersion in boric acid solution, this peak gradually shifts to a higher angle, reaching 21.20°, indicating that the interchain distance of PVA decreases due to the chemical cross-linking effect of boric acid esters, and the network structure becomes more compact. Furthermore, diffraction peaks of the (002) crystal plane of boric acid crystals can be observed in Example 2, while these peaks are significantly enhanced in Example 3, indicating that excess boric acid does not continue to nucleate uniformly but tends to epitaxially grow or agglomerate on existing crystals, leading to a significant increase in crystal size. Figure 2 The results are consistent with those in the previous section.

[0084] Figure 4 This is a tensile stress-strain curve diagram of the materials obtained in Comparative Example 1, Example 1, Example 2, and Example 3. (From...) Figure 4It can be seen that the fracture strain of Comparative Example 1 is only 325%, indicating that although polyvinyl alcohol hydrogel has a certain tensile deformation capacity without the introduction of boric acid, its physical cross-linking network is weak and it is prone to fracture during stretching. After soaking in boric acid solution, the fracture strains of Examples 1, 2, and 3 increased to 847%, 714%, and 496%, respectively, all significantly higher than that of Comparative Example 1. This indicates that the borate ester cross-linking structure formed by boric acid and polyvinyl alcohol segments is beneficial to improving the tensile deformation capacity of the material. Among them, Example 1 has the highest fracture strain, indicating that the segment mobility is well preserved at a lower boric acid concentration; Example 2 has higher tensile stress while maintaining a high fracture strain, demonstrating a better balance between strength and toughness; the fracture strain of Example 3 decreased, indicating that excessive boric acid crystallization or aggregation can restrict the movement of polyvinyl alcohol segments, reducing the ductility of the material.

[0085] Figure 5 The diagram shows a comparison of the tensile strengths of the materials obtained in Comparative Example 1, Example 1, Example 2, and Example 3. The tensile strength of Comparative Example 1 was only 0.073 MPa, indicating that the physical cross-linking network of the polyvinyl alcohol hydrogel was weak without the introduction of boric acid. The tensile strength of Example 1 increased to 2.08 MPa, indicating that the BOC covalent bonds can significantly enhance the hydrogel network. The tensile strength of Example 2 reached the highest value of 5.59 MPa, an increase of approximately 76.6 times compared to Comparative Example 1, indicating that uniformly dispersed boric acid microcrystals are beneficial for improving mechanical strength while maintaining a dense network structure. The tensile strength of Example 3 was lower than that of Example 2, indicating that local agglomeration caused by excessive boric acid reduced the effective load-bearing capacity of the material.

[0086] Figure 6 This is a comparison chart of the toughness of the materials obtained in Comparative Example 1, Example 1, Example 2, and Example 3. The toughness of Comparative Example 1 is only 0.096 MJ·m. -3 This indicates that the energy dissipation capacity of polyvinyl alcohol hydrogel is relatively weak; the toughness of Example 1 was improved to 6.46 MJ·m. -3 This indicates that the borate ester crosslinking structure not only improves strength but also enhances energy dissipation during tensile testing; Example 2 exhibits the highest toughness, at 18.8 MJ·m. -3 This indicates that filling with an appropriate amount of boric acid microcrystals can help optimize the local stress transmission and energy dissipation process without disrupting the network continuity, thereby achieving a synergistic improvement in strength and ductility. The toughness of Example 3 is lower than that of Example 2, indicating that the local phase separation caused by excessive boric acid crystallization and agglomeration will form stress concentration points, induce crack initiation at lower strain, and lead to a decrease in strength and elongation at break.

[0087] Figure 7The figures show the tensile stress-strain curves of the materials obtained in Examples 2, 4, and 5. When the boric acid solution concentration was within the range of 6% to 12%, the resulting materials all exhibited high tensile strength and elongation at break. Specifically, the fracture strain and tensile strength of Example 4 were 600% and 5.61 MPa, respectively, while those of Example 5 were 514% and 4.65 MPa, respectively, indicating that this range of boric acid concentration effectively constructed the borate ester crosslinking network and improved the material's strength and toughness. Example 2 showed the best overall performance with a fracture strain and tensile strength of 714% and 5.59 MPa, respectively, demonstrating that immersion in a 9% boric acid solution achieved a better balance between crosslinking density, microcrystalline dispersion, and chain segment movement.

[0088] Figure 8 The figures show the tensile stress-strain curves of the materials obtained in Examples 2, 6, and 7. In Example 6, the polyvinyl alcohol concentration was low, resulting in an insufficient initial hydrogel network skeleton and the lowest tensile strength of only 2.02 MPa. This indicates that excessively low polyvinyl alcohol content is detrimental to the formation of an effective load-bearing network. In Example 7, the polyvinyl alcohol concentration was high, significantly increasing the tensile stress to 5.61 MPa. However, the elongation at break was 598%, lower than that of Example 2, indicating that excessively high polyvinyl alcohol concentration would make the network too dense, restricting chain segment movement and leading to decreased ductility. In contrast, Example 2 showed a fracture strain of 714% and a tensile strength of 5.59 MPa, maintaining high tensile strength while exhibiting better deformability. This suggests that a 10% polyvinyl alcohol concentration is beneficial for constructing a borate ester crosslinked hydrogel network that possesses both load-bearing and deformability capabilities.

[0089] Figure 9 The figures show the tensile stress-strain curves of the materials obtained in Examples 2, 8, and 9. Example 8, with its shorter soaking time, exhibited a tensile strength of only 3.20 MPa, significantly lower than Examples 2 and 9. This indicates that a 12-hour soaking time is insufficient for boric acid to fully diffuse and form a stable borate ester crosslinking network. The tensile curves of Examples 2 and 9 are quite similar, suggesting that the borate ester crosslinking reaction and boric acid filling effect are essentially complete after 24 hours of soaking, and extending the soaking time to 36 hours has limited effect on improving tensile properties. Therefore, 24 hours of soaking is the optimal soaking time.

[0090] Figure 10The figures show the flexural strength test results of the residues after high-temperature treatment of the materials obtained in Examples 1, 2, 3, 4, and 5. Examples 1, 2, 3, 4, and 5 were all in a flexible hydrogel state before the thermal runaway test. After the high-temperature reaction in the thermal runaway test, they all formed rigid residues with a certain flexural strength, indicating that the material of the present invention can be transformed from a flexible hydrogel into a rigid material under high temperature. Specifically, the flexural strength of the residue in Example 1 was 0.87 MPa, indicating that low-concentration boric acid immersion could impart a certain high-temperature rigidity transformation capability to the material. The flexural strengths of the residues in Examples 4 and 5 were 1.22 MPa and 1.13 MPa, respectively, indicating that when the boric acid solution concentration is in the range of 6% to 12%, the material can form a rigid barrier with good load-bearing capacity. The residue in Example 2 exhibited the highest flexural strength at 1.45 MPa, indicating that the 9% boric acid solution facilitated a synergistic reaction between the borate ester crosslinking network and the small amount of boric acid microcrystals at high temperatures. This promoted the effective integration of the boron-containing ceramic phase with the covalent framework, forming a ceramic-carbon barrier with stronger load-bearing capacity. The residue in Example 3 showed a decrease in flexural strength to 1.03 MPa, indicating that phase separation caused by excessive boric acid led to an increase in microcracks and brittle defects within the residual layer, resulting in a decline in strength.

[0091] Figure 11 This is a comparison chart of the heat storage densities of the materials obtained in Comparative Example 1, Example 1, Example 2, Example 3, Example 4, and Example 5. The decomposition enthalpy of Comparative Example 1 is 2101 J·cm⁻¹. -3 The heat primarily originates from the endothermic evaporation of water from the polyvinyl alcohol hydrogel. After soaking in boric acid solution, the decomposition enthalpies of Examples 1, 4, 2, 5, and 3 were 1856 J·cm⁻¹, respectively. -3 1683 J·cm -3 1540 J·cm -3 1480 J·cm -3 and 1474 J·cm -3 The enthalpy of water in Example 2 is lower than that in Comparative Example 1. This is because after boric acid enters the hydrogel network and forms a borate ester crosslinking structure with polyvinyl alcohol, some water is replaced by boron-containing components and a denser network structure, resulting in a decrease in the evaporable water content of the material. Although the decomposition enthalpy of Example 2 is lower than that of Comparative Example 1, Example 1, and Example 4, it has higher tensile strength, toughness, and high-temperature residual barrier flexural strength, thus its overall protective capability is superior.

[0092] Application Example 1 The materials obtained in Examples 1, 2, 3, 4, 5, and Comparative Example 1 were cut into sheets with a thickness of 3 mm that matched the side dimensions of the batteries, and then sandwiched between two square ternary lithium-ion batteries. Thermal runaway in one of the batteries was triggered by needle puncture.

[0093] like Figure 12 As shown, in Comparative Example 1, the highest temperature of adjacent batteries protected by polyvinyl alcohol hydrogel material without borate crosslinking structure reached approximately 535°C, indicating that relying solely on high decomposition enthalpy endothermic heating can also lead to battery thermal runaway propagation. In contrast, the temperatures of adjacent batteries in Examples 1, 2, 3, 4, and 5 were significantly lower than in Comparative Example 1, successfully suppressing battery thermal runaway propagation. This demonstrates that borate crosslinked hydrogel materials with high-temperature rigidity conversion capability can not only absorb heat through water evaporation and boric acid dehydration, but also form a rigid barrier under high temperatures, further blocking the transfer of heat and high-temperature ejected materials. Among these, Example 2 exhibited the lowest adjacent battery temperature at only 128°C, indicating its best performance in actual battery thermal runaway propagation protection. This is mainly attributed to Example 2 having the strongest high-temperature rigidity conversion capability, with a high-temperature residual flexural strength reaching 1.45 MPa. This allows it to maintain a more complete barrier structure after thermal runaway impact, effectively resisting damage caused by cell expansion, gas ejection, and thermal erosion, thereby continuously blocking the propagation of heat and high-temperature ejected materials to adjacent batteries.

Claims

1. A method for preparing a high-temperature rigid conversion borate ester crosslinked hydrogel material, characterized in that, Includes the following steps: (1) Add polyvinyl alcohol to deionized water, heat and stir until completely dissolved to obtain a homogeneous polyvinyl alcohol aqueous solution; (2) Pour the polyvinyl alcohol aqueous solution obtained in step (1) into a mold and perform physical cross-linking treatment to obtain a polyvinyl alcohol hydrogel with an initial network structure; (3) The polyvinyl alcohol hydrogel obtained in step (2) is pre-soaked in deionized water to allow the hydrogel network to fully swell and improve the mobility of polymer chain segments. (4) The polyvinyl alcohol hydrogel obtained in step (3) is immersed in boric acid solution to allow boric acid to diffuse into the hydrogel network and react with the vicinal diol structure in the polyvinyl alcohol chain segment to form a BOC covalent cross-linked network. (5) Take out the hydrogel composite material obtained in step (4), remove the excess solution on the surface, and obtain the high-temperature rigid conversion borate ester crosslinked hydrogel composite material.

2. The preparation method according to claim 1, characterized in that, In step (1), the mass fraction of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 5% to 15%.

3. The preparation method according to claim 1, characterized in that, In step (1), the heating temperature is 90-95℃, the stirring rate is 200-500 rpm, and the stirring time is 0.5-1 h.

4. The preparation method according to claim 1, characterized in that, In step (2), the physical crosslinking treatment is a freeze-thaw process; the freezing temperature is -30 to -10°C and the freezing time is 8 to 10 h; the thawing temperature is 20 to 30°C and the thawing time is 4 to 6 h; the freeze-thaw cycle is 1 to 3 times.

5. The preparation method according to claim 1, characterized in that, In step (3), the pre-soaking time is 0.5 to 3 hours.

6. The preparation method according to claim 1, characterized in that, In step (4), the boric acid solution has a mass fraction of 3% to 15%.

7. The preparation method according to claim 1, characterized in that, In step (4), the soaking temperature is 50-80 ℃ and the soaking time is 6-48 h, preferably 12-24 h.

8. The high-temperature rigid conversion borate ester crosslinked hydrogel material obtained by the preparation method according to any one of claims 1-7, characterized in that, The hydrogel material comprises a polyvinyl alcohol hydrogel network, a borate ester crosslinking structure, and a boric acid crystal component; the borate ester crosslinking structure is a BOC covalent crosslinking structure.

9. The high-temperature rigid conversion borate ester crosslinked hydrogel material according to claim 8, characterized in that, The hydrogel material is a flexible hydrogel at room temperature, but can be transformed in situ into a ceramic-carbon hybrid rigid barrier under high temperature.

10. The application of the high-temperature rigid conversion borate ester crosslinked hydrogel material according to claim 8 or 9 in the protection against thermal runaway propagation in batteries.

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