Thermally stable diaphragm material for lithium ion battery and preparation method of thermally stable diaphragm material

Abstract

The invention discloses a thermal stable diaphragm material for a lithium ion battery and a preparation method of the thermal stable diaphragm material, and the material solves the key problems of easy thermal runaway, deformation shrinkage, channel blockage and the like of the existing diaphragm under extreme working conditions by synergistically constructing a three-dimensional composite structure with high thermal stability, mechanical flexibility and ion permeability. The flexible lithium battery diaphragm has good large-scale preparation capability and engineering application prospect, cement which is a traditional building material is introduced into a flexible lithium battery diaphragm system for the first time, and a multi-scale synergistic ion guide channel system is constructed by utilizing the high thermal stability and natural porosity of the cement and combining a polymer flexible phase and an in-situ foaming process. The strategy not only realizes the morphological stability and thermal protection capability of the material under a high-temperature condition, but also significantly improves the ion permeability and interface electrochemical suitability, and breaks through the problem of performance balance between thermal stress and flexibility of the existing polyolefin and inorganic coating diaphragm. Besides, the whole system adopts low-cost raw materials, the preparation process can be completed under normal-temperature and normal-pressure conditions, high-temperature sintering, electrospinning or vacuum treatment is not needed, and the industrial popularization potential is extremely high.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of safety protection materials for new energy storage devices, and particularly relates to a heat-stable diaphragm material for lithium ion batteries and a preparation method thereof. BACKGROUND

[0002] With the wide application of lithium ion batteries in new energy vehicles, portable electronic devices and large-scale energy storage systems, the safety of the batteries has been increasingly concerned. Especially under high-rate charging and discharging, external short circuit, collision or high-temperature environment, the lithium battery may have a thermal runaway phenomenon, rapidly release a large amount of heat, and induce serious accidents such as fire or explosion. In the internal structure of the lithium battery, the diaphragm, as a key isolation component between the positive and negative electrodes, not only bears the function of the electrolyte ion transmission medium, but also plays a key role in blocking heat diffusion and preventing electrode contact short circuit under thermal stress. Therefore, the heat stability, melting inhibition ability and flame retardant properties of the diaphragm material are directly related to the overall thermal safety performance of the battery system.

[0003] Currently, polyethylene (PE) and polypropylene (PP) microporous membranes are mainly used as separators in commercial lithium-ion batteries. These materials are prepared by solvent method or stretching method, and have good mechanical strength and stable microporous structure, which can realize the effective migration of lithium ions between the positive and negative electrodes. However, due to the low melting point of polyolefin materials (about 120℃~140℃), once the temperature rises to the critical value during battery operation, the separator will undergo significant thermal shrinkage, pore collapse and even melting, resulting in direct contact between the electrodes, causing short circuit and thermal runaway. Therefore, the traditional polyolefin separators are difficult to provide sufficient thermal stability and blocking protection when facing high temperature or extreme working conditions, which has become one of the key bottlenecks restricting the improvement of the thermal safety performance of lithium batteries. In recent years, researchers have tried to develop various modification strategies to improve the stability of the separator material under high temperature conditions. Among them, the inorganic ceramic coating method is a typical means, which introduces aluminum oxide, titanium oxide and other heat-resistant inorganic coatings on the surface of the polyolefin separator, significantly improving the thermal stability and flame retardant performance of the separator. However, such ceramic enhancement layer often lacks flexibility, is prone to peeling or interfacial delamination in repeated charge and discharge cycles, and the processing process is relatively complex, making it difficult to achieve large-scale low-cost preparation. Electrospun nanofiber separators construct fiber network structure through polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF) high molecular materials, which have high thermal stability and good ion permeability. However, such separators often have low mechanical strength and are prone to aging and brittle fracture, which limits their practical application in high-energy-density batteries. In addition, gel separators and ionic liquid separators as functional alternative solutions can improve the wettability of the electrolyte interface and ion migration rate, and improve the electrochemical performance. However, their long-term stability and chemical compatibility are poor, and they are prone to electrolyte leakage or material decomposition, and the preparation cost is high, which is difficult to meet the needs of large-scale industrial production. Therefore, there is currently a lack of an ideal separator material that can achieve a comprehensive balance between thermal stability, mechanical properties, ion conductivity and processing cost.

[0004] Compared with organic polymer materials, cement-based inorganic materials have natural thermal stability and non-combustible properties. Their decomposition temperature is much higher than the working temperature range of the battery, and they will not soften or melt under high temperature conditions, which can effectively block heat diffusion and inhibit the risk of electrode short circuit. In addition, the porous structure formed during the hydration and hardening process of cement has good adjustability, and can form a network of through micro-nano channels by controlling the ratio, curing conditions or foaming process. This structure not only helps to improve the overall ion transport flux, but also has excellent thermal insulation and slow-release functions, which can delay the temperature rise rate in the early stage of thermal runaway, giving the battery system response time. More importantly, cement materials are widely available, low-cost, have good mechanical strength and dimensional stability, and can be used to build flexible reinforced structures with polymer materials to meet the multiple stress requirements during battery packaging and assembly. However, the application of cement in the battery field is still in its infancy, and related research is mostly focused on structure and building energy storage. Its functional design and engineering integration in lithium ion battery separators are still a blank, and systematic material development and mechanism verification are urgently needed.

[0005] In summary, there is still a lack of a composite material system that can achieve a multi-dimensional balance between high thermal stability, mechanical strength, ion transport performance, and low-cost processability in the current lithium ion battery separator field. Existing polymer separators have inherent shortcomings in thermal safety, while high-performance inorganic separators, although having certain thermal stability, face the practical obstacles of insufficient flexibility, complex preparation, and high cost. Under this background, if a cement-based inorganic framework with excellent thermal stability and structural stability is combined with a polymer network with good flexibility and ion affinity, the interface between the two phases is synergistically controlled, and a through ion-conducting channel structure is formed by introducing a controllable foaming process. This not only can build a stable ion migration path, but also can provide thermal blocking and electrode isolation functions at high temperatures. This strategy is expected to break through the design paradigm of traditional battery separator materials and open up a new path for low-cost, scalable, and thermally protective composite separators, providing an innovative solution for the development of high-safety energy storage devices. SUMMARY

[0006] This section is intended to summarize some aspects of the embodiments of the present application and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the abstract and title of the specification to avoid obscuring the purpose of this section, abstract and title, and such simplifications or omissions cannot be used to limit the scope of the present application.

[0007] In view of the above and / or the following problems existing in the prior art, the present application is proposed.

[0008] 1. Poor thermal stability of the separator, which can easily lead to thermal runaway Traditional polyolefin separators are prone to shrinkage and melting at high temperatures, and cannot effectively prevent positive and negative short circuits, which is one of the main causes of thermal runaway accidents. The present application uses the non-combustibility and high thermal decomposition temperature of cement materials to build a thermally stable inorganic framework, effectively improving the structural integrity and heat resistance of the separator in a high temperature environment.

[0009] 2. Ion channel structure is unstable or difficult to control The ion channel structure in existing ceramic or fiber separators depends on randomly distributed pores or electrospun fiber accumulation, and the pore size and connectivity are difficult to control. The present application uses in-situ foaming technology to build a controllable porous network in the cement-polymer matrix, realizing an ordered structure and uniform distribution of lithium ion migration channels.

[0010] 3. Mechanical strength and flexibility are difficult to balance Pure inorganic separators are usually brittle and prone to cracking, and cannot meet the operation requirements of battery winding, pressing and other operations. The present application introduces a polymer phase (such as PVDF, PVA, etc.) into the cement matrix to form an interpenetrating network structure, significantly improving flexibility and toughness while ensuring rigidity, achieving an excellent balance of mechanical properties.

[0011] 4. High preparation cost and complex process, not conducive to industrial promotion Current high-performance ceramic or gel separators usually have high preparation cost and complex process, which is not conducive to large-scale production. The present application uses cement, polymer and general foaming process, which is cheap in raw materials and simple in process, and has good sustainability and industrialization prospects.

[0012] Therefore, the purpose of the present application is to overcome the shortcomings of the prior art and provide a thermally stable separator material for lithium ion batteries.

[0013] To solve the above technical problems, the present application provides the following technical solutions: the thermally stable separator material is a flexible sheet structure material with a thickness of 50-1000 μm, which is composed of three types of functional phases, which are: (i) Inorganic framework phase, which is a rigid support network formed by hydration reaction of cement, magnesium cement, alkali-activated slag cementitious material, phosphorus gypsum or industrial solid waste-based cementitious material; (ii) Polymer flexible phase, which is a flexible ion conducting network formed by dispersing and coating biodegradable polymers such as PVDF, PVA, PAN, PEGDA, polyphosphate and biobased polymers between the inorganic framework phase; (iii) Internal pore structure phase, which is a through-porous network with a pore size of 100-500 nm induced in-situ during hydration and curing by introducing a chemical foaming agent.

[0014] Further, the internal pore structure provides a continuous and uniform lithium ion migration path, and plays a role in releasing local thermal stress and inhibiting short circuit.

[0015] As a preferred scheme of the heat-stable separator material for lithium ion batteries, the raw materials include, in terms of mass percentage of the heat-stable separator material, 40% to 80% of inorganic materials, 5% to 25% of functional polymers, 1% to 10% of chemical foaming agents, and the balance of functional additives and deionized water; The inorganic material includes one of Portland cement, magnesia cement, alkali-activated slag cementitious material, phosphorus gypsum, or industrial solid waste-based cementitious material. The functional polymer includes one of PVDF, PVA, PAN, PEGDA, polyphosphate, and biobased degradable polymer.

[0016] As a preferred scheme of the heat-stable separator material for lithium ion batteries, the chemical foaming agent includes one of hydrogen peroxide, sodium bicarbonate, starch, and cellulose.

[0017] As a preferred scheme of the heat-stable separator material for lithium ion batteries, the functional additive includes one of flame-retardant and heat-stable agent, inorganic / ceramic reinforcing filler, ion-conducting and interface wetting agent.

[0018] The flame-retardant and heat-stable agent is used to improve the high-temperature heat resistance and flame-retardant performance of the material, and includes triphenyl phosphate (TPP), DOPO derivative, or aluminum hydroxide. The inorganic / ceramic reinforcing filler is used to improve the mechanical strength and thermal dimensional stability of the separator, and includes aluminum oxide (Al2O3), silicon dioxide (SiO2), or boron nitride (h-BN). The ion-conducting and interface wetting agent is used to improve the wettability of the electrolyte and reduce the interface impedance, and includes lithium bis-trifluoromethanesulfonimide (LiTFSI), ionic liquid, or silane coupling agent.

[0019] Another object of the present application is to provide a preparation method of the heat-stable separator material for lithium ion batteries.

[0020] To solve the above technical problems, the present application provides the following technical scheme: including, The functional polymer is dissolved in the solvent to form a homogeneous polymer precursor solution; The functional polymer is stirred to ensure complete dissolution and defoaming, the stirring speed is 300-600 rpm, and the stirring time is 30-90 min. adding inorganic material powder into the homogeneous polymer precursor solution to form inorganic material-polymer composite slurry with uniform dispersion under continuous stirring; wherein the average particle size of the inorganic material powder is 1-15 μm, the stirring speed of the continuous stirring is 500-1000 rpm, and the stirring time is 30-60 min, and the dispersion efficiency can be improved by heating at 30-50 ℃ if necessary; adding chemical foaming agent into the inorganic material-polymer composite slurry to promote microbubble formation by high-speed shearing dispersion, thereby obtaining separator slurry; The separator slurry is applied on an inert substrate by blade coating, suction filtration or casting process, and is placed in an environment with temperature of 25-60 ℃ and relative humidity of 55%-80% for 6-24 h of curing, thereby forming wet film with good pore structure and adhesion. After drying, post-treatment, peeling or cutting, the heat-stable separator material for lithium ion battery is obtained.

[0021] As a preferred solution of the preparation method of the heat-stable separator material for lithium ion battery, the mass fraction of the functional polymer in the homogeneous polymer precursor solution is 5wt%-18wt%.

[0022] As a preferred solution of the preparation method of the heat-stable separator material for lithium ion battery, the high-speed shearing dispersion for promoting microbubble formation has a speed of 2000-8000 rpm for 1-5 min, and the pH of the dispersion system is controlled at 8-11.

[0023] As a preferred solution of the preparation method of the heat-stable separator material for lithium ion battery, the drying temperature of the wet film is 50 ℃-90 ℃, and the drying time is 2-8 h.

[0024] As a preferred solution of the preparation method of the heat-stable separator material for lithium ion battery, the post-treatment includes one or more of hot pressing, heat treatment or surface modification.

[0025] As a preferred solution of the preparation method of the heat-stable separator material for lithium ion battery, the hot pressing has a temperature of 90-130 ℃, a pressure of 0.5-3 MPa, and a time of 1-5 min. The heat treatment has a temperature of 100-150 ℃ and a time of 10-60 min. The surface modification includes spraying or immersing the dried wet film in 0.05-0.5wt% PVDF solution or 0.1-1wt% PVA solution for 5-60 s, and then drying at 60-90 ℃ for 10-30 min; or, Curing at 80-110°C for 10-30 min after impregnating the dried wet film with 0.1-1 wt% SiO2 sol or 0.05-0.8 wt% Al2O3 sol for 10-120 s; or, Drying at 60-80°C for 10-20 min after spraying 0.05-0.3 wt% silane coupling agent.

[0026] Advantages of the present application: (1) In terms of thermal stability, the cement-based skeleton has a thermal decomposition temperature much higher than that of polyolefin materials and non-combustible properties, which can maintain the integrity of the form under extreme conditions up to 200°C or above, without melting, shrinking or pore collapse, effectively blocking the positive and negative contact to prevent short circuit, thereby significantly delaying or even blocking the occurrence of thermal runaway process.

[0027] (2) In terms of ion conduction performance, the present application builds a porous network structure with good throughness and adjustable pore size distribution in the material by introducing in-situ foaming process, forming a stable and continuous lithium ion migration channel. Although the overall film thickness increases to the range of 50-1000 μm, due to the presence of porous channels, the ion diffusion resistance is significantly reduced, thereby maintaining high electrical conductivity, overcoming the problem of poor ion conductivity of traditional inorganic materials with large thickness.

[0028] (3) In terms of mechanical properties, the cement inorganic phase provides high structural strength and dimensional stability, while the flexible polymer network improves the toughness and anti-cracking ability of the separator during winding, cell assembly and long-term charge and discharge process. The synergistic effect of the two realizes the balance of strength and flexibility, meeting the requirements of lithium battery actual processing and assembly conditions.

[0029] (4) The preparation process adopted by the present application does not require high-temperature sintering, electrospinning or vacuum treatment, and the overall process is completed at room temperature and normal pressure. The material sources are widely available and the process is simple, which significantly reduces the production energy consumption and manufacturing cost. Cement, polymer and foaming agent are all conventional industrial raw materials, which have good environmental friendliness and industrialization promotion prospect.

[0030] (5) Compared with traditional PE / PP microporous membrane, ceramic coated membrane or electrospun nanofiber membrane, etc. mainstream separator technology, the present application not only realizes the qualitative improvement in thermal stability and flame retardancy, but also breaks through the technical contradiction that safety and flexibility cannot be achieved at high temperature, providing a new material solution path for building high-safety and high-energy-density battery systems. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description only constitute some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort based on these drawings. Among them: Figure 1 Preparation flow chart of the thermal stable separator material for lithium ion battery of the present application.

[0032] Figure 2 Overall structure schematic diagram of the cement-polymer composite separator prepared by the present application.

[0033] Figure 3 Microscopic section structure diagram of the cement-polymer composite separator prepared by the present application.

[0034] Figure 4 Lithium ion transmission mechanism and thermal safety principle schematic diagram of the cement-polymer composite separator prepared by the present application after being used for assembling lithium ion battery.

[0035] Figure 5 Actual appearance diagram of the cement-polymer composite separator prepared by the present application. DETAILED DESCRIPTION

[0036] In order to make the above objectives, features and advantages of the present application more apparent and comprehensible, the specific embodiments of the present application will be described in detail in the following with reference to the description of the embodiments.

[0037] In the following description, a lot of specific details are set forth in order to provide a thorough understanding of the present application, but the present application can also be implemented in other ways different from those described herein, and those skilled in the art can make similar generalizations without departing from the connotation of the present application, therefore the present application is not limited to the specific embodiments disclosed below.

[0038] Secondly, the "one embodiment" or "embodiment" referred to herein means that the specific features, structures or characteristics can be included in at least one implementation of the present application. The "in one embodiment" appearing in different places in the specification does not mean the same embodiment, nor is the embodiment alone or selectively exclusive of other embodiments.

[0039] The raw materials used in the present application are commercially available in the art without special description, and specifically: The polyvinylidene fluoride is Solef5130 or equivalent specification, and the molecular weight is about 4.5x10 5 g / mol, and the melting point is 165-170℃; The polyacrylonitrile is Aldrich product, and the molecular weight is about 150k, and the purity is not less than 99%; Polyvinyl alcohol, type 1799, degree of polymerization 1700±50, degree of alcoholysis 98%~99%; The silicate cement used is of grade P.O42.5R conforming to the national standard GB175-2020, with a D50 of approximately 6μm. The manufacturer is not limited, but must meet this standard. The activity of lightly calcined magnesium oxide is not less than 65%, and the particle size D50 is 5-10µm. The chemical foaming agent is analytical grade sodium bicarbonate (NaHCO3, content ≥99.5%) and anhydrous citric acid (content ≥99.5%) or 30wt% hydrogen peroxide solution; The lithium salt additive is battery-grade lithium bis(trifluoromethanesulfonyl)imide, with a moisture content not exceeding 20 ppm; The silane coupling agent used is KH-550 or KH-560, with an industrial-grade purity of not less than 98%. Apart from the specific models mentioned above, other solvents and additives such as NMP, DMF, Pluronic F127, CMC, and boron nitride powder can be commercially available products that meet analytical grade or battery grade standards, without needing to specify a particular brand.

[0040] Example 1 Reference Figure 1 The flowchart illustrates that this embodiment provides a thermally stable separator material for lithium-ion batteries, its preparation method, and its application. Specifically: 1) Weigh the raw materials according to the following mass percentages: 60 wt% silicate cement, 15 wt% functional polymer (polyvinylidene fluoride), 6 wt% foaming agent (4 wt% sodium bicarbonate and 2 wt% citric acid), the balance being 19 wt% functional additives and deionized water (3 wt% lithium bis(trifluoromethanesulfonyl)imide, 0.5 wt% silane coupling agent KH-550, 0.5 wt% Pluronic F127 surfactant, the balance being deionized water); 2) The functional polymer polyvinylidene fluoride was dissolved in anhydrous N-methylpyrrolidone and stirred at 400 rpm for 60 min to ensure complete dissolution, forming a homogeneous polymer precursor solution with a functional polymer concentration of 12 wt%. 3) Add cement powder to the homogeneous polymer precursor liquid and stir for 45 minutes at a system temperature of 30℃ and a rotation speed of 700 rpm to form a uniformly dispersed cement-polymer composite slurry. 4) Add NaHCO3 / citric acid aqueous solution to cement-polymer composite slurry, shear at 4000 rpm for 3 min and adjust pH to 9 to form stable microbubbles to obtain diaphragm slurry; 5) The obtained separator slurry was cast on a polytetrafluoroethylene substrate with a wet film thickness of 300 pm, and cured at 40 °C in an environment with a relative humidity of 65% for 12 h to complete the foaming, polymer curing and cement hydration; After curing, the sample was dried by hot air at 80 °C for 6 h, and then hot-pressed at 110 °C and 1.0 MPa for 3 min. After cooling, the porous cement-polymer composite separator with a thickness of about 122 ± 6 pm was peeled off from the substrate and cut to obtain a hot-stable separator material for lithium ion batteries.

[0041] The cement-polymer composite separator prepared in this example was tested for relevant performance, and the results showed that: The thickness of the separator obtained by casting and hot-pressing was 122 ± 6 pm, the apparent density was 0.47 g cm-3, the porosity was 45 ± 3%, the pore size distribution measured by a liquid pore size analyzer was concentrated in the range of 150-320 nm, and the D50 was 230 nm.

[0042] The Gurley air permeability test (ASTM D726) result was 420 s / 100 mL.

[0043] The liquid absorption rate of the separator in 1 M LiPF6 / EC:DEC = 1:1 electrolyte was 138%, and the contact angle was 21°, showing excellent electrolyte wettability and rapid infiltration ability.

[0044] The ionic conductivity of the separator measured by electrochemical impedance spectroscopy (EIS, 25 °C) was 1.02 mS / cm, close to that of a commercial polyolefin separator and with lower interfacial impedance.

[0045] In terms of thermal stability, the dimensional shrinkage of the separator was only 1.3% after being kept at 200 °C in air for 30 min, and was still less than 2% at 220 °C, which was much lower than the more than 20% shrinkage of conventional polyolefin separators at 150 °C.

[0046] The UL-94 vertical burning test result reached the VTM-0 level, showing excellent flame retardant performance.

[0047] In the mechanical test, the puncture strength of the separator was 520 gf, the tensile strength was 42 MPa, and the elongation at break was 9%, fully meeting the winding and assembly requirements of lithium battery separators.

[0048] The separator was assembled into a standard 2032 button type LiFePO4||graphite battery, and the capacity retention rate was 92% after 300 cycles at 1C rate, and the capacity retention rate was 88% at 3C rate, and no short circuit or capacity drop occurred after 10 times of continuous charge and discharge under thermal shock at 150 °C.

[0049] The above results fully demonstrate the significant advantages of the thermal stable porous composite separator described in the application in terms of high temperature safety, ion transport and electrochemical performance, and the effectiveness of its industrial application.

[0050] Figure 2 The schematic diagram of the overall structure of the cement-polymer composite separator prepared by the application shows the spatial distribution relationship of the cement skeleton, polymer network and porous ion-conducting channels, and embodies the microstructure characteristics of three-phase synergy. The separator is composed of a three-dimensional inorganic skeleton formed by cement hydration products, which provides mechanical support at high temperature. The flexible polymer network embedded therein forms a continuous ionophilic interface and enhances the wettability of the electrolyte. The nanoscale pores (pore size about 100-500 nm) penetrating the thickness direction construct low-bending ion channels, realizing fast lithium ion migration and uniform electrolyte distribution. This multiphase synergistic structure has high temperature dimensional stability, excellent ion conductivity and good mechanical strength, which is the key to realizing battery thermal safety and high efficiency electrochemical performance.

[0051] Figure 3 The microstructure diagram of the composite separator of the application, showing the inorganic framework formed between the cement hydration products (such as C-S-H gel), the embedded polymer flexible chain segments, and the pore channels penetrating the entire thickness direction. The figure emphasizes the pore size range (about 100-500 nm) and connectivity.

[0052] Figure 4 The schematic diagram of the lithium ion transport mechanism and thermal safety principle of the cement-polymer composite separator prepared by the application after being used for lithium ion battery assembly. The separator is composed of a three-dimensional inorganic skeleton formed by cement hydration products, a flexible polymer network embedded therein, and a connected nanochannel penetrating the thickness direction, which together form a stable three-phase system. During battery operation, lithium ions in the electrolyte migrate rapidly along the connected channels under the driving of an external electric field. The polymer phase provides a continuous ionophilic interface to reduce the interfacial impedance and maintain high ionic conductivity. The inorganic skeleton provides rigid support at high temperature, maintaining the size and structural integrity of the separator even above 200°C, thereby preventing the positive and negative electrodes from contacting and short-circuiting during thermal runaway. The result of three-phase synergy is to realize a low-bending high-efficiency lithium ion channel, excellent electrolyte wettability, and outstanding high-temperature dimensional stability, so that the battery can maintain safe and stable electrochemical performance under normal charging and discharging and sudden thermal shock conditions.

[0053] Figure 5 The actual appearance diagram of the cement-polymer composite separator prepared by the application. As can be seen, the separator is in the form of a square sheet with a thickness of about 0.1 mm, a slightly fine granular surface and a slight curl, showing the rough texture and good flexibility characteristic of cement-based materials. It can be cut into the required size according to the needs of battery assembly.

[0054] Example 2 1) The raw materials are weighed according to the following mass percentage: 50wt% of Portland cement, 20wt% of functional polymer (polyvinylidene fluoride), 4.5wt% of foaming agent (3wt% of sodium bicarbonate and 1.5wt% of citric acid), and the rest is functional additives and deionized water (4wt% of lithium bis-trifluoromethanesulfonimide, 0.5wt% of silane coupling agent KH-560, 0.5wt% of Pluronic F127 surfactant, and the rest is deionized water); 2) The functional polymer polyvinylidene fluoride is dissolved in anhydrous N-methyl pyrrolidone, and stirred at 400 rpm for 60 min to ensure complete dissolution, forming a homogeneous polymer precursor solution with a functional polymer concentration of 10wt%; 3) Add cement powder to the homogeneous polymer precursor solution, and stir at a system temperature of 30°C and a rotation speed of 700 rpm for 45 min to form a uniformly dispersed cement-polymer composite slurry; 4) Add NaHCO3 / citric acid aqueous solution to the cement-polymer composite slurry, shear at 3500 rpm for 3 min, and adjust the pH to 9 to form stable microbubbles, obtaining a diaphragm slurry; 5) The obtained diaphragm slurry is cast on a polytetrafluoroethylene substrate with a wet film thickness of 280μm, and is cured at 40°C and a relative humidity of 65% for 12h to complete foaming, polymer curing and cement hydration; After curing, hot air drying at 80°C for 6h, and hot pressing at 115°C and 1.2MPa for 3min, the porous cement-polymer composite diaphragm with a thickness of about 110μm is obtained by peeling off the substrate and cutting, which is a heat stable diaphragm material for lithium ion batteries.

[0055] The cement-polymer composite diaphragm prepared in this example is tested for relevant performance, and the results show that: The diaphragm obtained by casting and hot pressing has a porosity of about 46%, a D50 of 210nm, a Gurley air permeability test (ASTM D726) result of 430s / 100mL, a liquid absorption rate in 1MLiPF6 / EC:DEC=1:1 electrolyte of 140%, an ionic conductivity of 0.96mS / cm, a size shrinkage rate of the diaphragm after 30min in air at 200°C of only 1.5%, a puncture strength of the diaphragm of 510gf, and a capacity retention rate of 91% after 300 cycles at 1C rate.

[0056] Example 3 1) The raw materials are weighed according to the following mass percentage: 70wt% of Portland cement, 10wt% of functional polymer (polyacrylonitrile), 5wt% of foaming agent (30wt% mass concentration of hydrogen peroxide solution), and the balance of functional additives and deionized water (3wt% of lithium bis-trifluoromethanesulfonimide, 0.3wt% of silane coupling agent KH-550, 0.5wt% of Pluronic F127 surfactant, and the balance of deionized water); 2) The functional polymer polyacrylonitrile was dissolved in anhydrous N-methyl pyrrolidone, and stirring was performed at 400 rpm for 60 min to ensure complete dissolution, forming a homogeneous polymer precursor solution with a functional polymer concentration of 8wt%; 3) The cement powder was added to the homogeneous polymer precursor solution, and stirring was performed at a system temperature of 30°C and a rotation speed of 600 rpm for 50 min to form a uniformly dispersed cement-polymer composite slurry; 4) The hydrogen peroxide solution was added to the cement-polymer composite slurry, and shearing was performed at 3000 rpm for 2 min and the pH was adjusted to 9 to form stable microbubbles, obtaining a diaphragm slurry; 5) The obtained diaphragm slurry was cast on a polytetrafluoroethylene substrate at a wet film thickness of 320 μm, and the foaming, polymer curing, and cement hydration were completed in an environment of 45°C and a relative humidity of 60% for 12 h; After curing, hot air drying was performed at 85°C for 6 h, and then hot pressing was performed at 120°C and 1.0 MPa for 3 min, and after cooling, the porous cement-polymer composite diaphragm with a thickness of about 125 μm was peeled off from the substrate and cut to obtain a porous cement-polymer composite diaphragm, which was a heat-stable diaphragm material for lithium ion batteries of the present embodiment.

[0057] The cement-polymer composite diaphragm prepared in the present embodiment was subjected to relevant performance tests, and the results showed that: The diaphragm obtained by casting and hot pressing had a porosity of about 48%, a D50 of 240 nm, a Gurley air permeability test (ASTM D726) result of 410 s / 100 mL, a liquid absorption rate in 1 M LiPF6 / EC:DEC=1:1 electrolyte of 135%, an ionic conductivity of 1.05 mS / cm, a size shrinkage rate of the diaphragm after being kept in an air atmosphere at 200°C for 30 min of only 1.2%, a diaphragm puncture strength of 530 gf, and a capacity retention rate of 92% after 300 cycles at a 1C rate.

[0058] Comparative Example 1 The difference between the present comparative example and Example 1 was that the raw material formula was adjusted as follows: 90wt% of Portland cement (P.O42.5R Portland cement, D50 of about 6 μm), 5wt% of functional polymer (Solef5130 type polyvinylidene fluoride, molecular weight of about 4.5×10 5The remaining raw material proportions and step processes were consistent with Example 1 to obtain the composite film of the present comparative example.

[0059] The slurry obtained from the raw materials of the present comparative example had poor fluidity and a collapsed pore structure, and the film had uneven thickness after film formation. The test results showed that the porosity was only 28%, the pore size distribution was severely large and discontinuous, the electrolyte absorption rate was less than 80%, the ionic conductivity decreased to 0.35 mS / cm, the heat shrinkage rate reached 7% at 200°C x 30 min, and the puncture strength was less than 250 gf, which could not meet the use requirements of lithium battery separators.

[0060] Comparative Example 2 The present comparative example differed from Example 1 in that the concentration of polyvinylidene fluoride in the raw material formula was adjusted to 25 wt%, and the remaining raw material proportions and step processes were consistent with Example 1 to obtain the composite film of the present comparative example.

[0061] The present comparative example had a high polymer content and a large film density, and the pore size D50 decreased to 80 nm and the Gurley value exceeded 3000 s / 100 mL. The electrolyte was difficult to infiltrate, the electrolyte absorption rate was only 65%, the ionic conductivity decreased to 0.28 mS / cm, the internal resistance of the battery increased significantly after assembly, and the cycle capacity rapidly decayed.

[0062] Comparative Example 3 The present comparative example differed from Example 1 in that NaHCO3 / citric acid aqueous solution was added to the cement-polymer composite slurry in step 4) and stirred at 500 rpm for 5 min to obtain a separator slurry. The remaining step processes were consistent with Example 1 to obtain the composite film of the present comparative example.

[0063] The present comparative example did not undergo high-speed shearing during film formation, lacked sufficient microbubble generation, had uneven pore structure and was prone to collapse, and finally the separator porosity decreased to 32%, the heat shrinkage rate increased to 5%, and the ionic conductivity was 0.42 mS / cm, which could not meet the dual requirements of high-temperature safety and high ionic conductivity.

[0064] Comparative Example 4 The present comparative example differed from Example 1 in that the curing condition of step 5) was adjusted to 20°C and 35% relative humidity for 4 h of static curing; and the remaining step processes were consistent with Example 1 to obtain the composite film of the present comparative example.

[0065] In the present comparative example, the hydration and foaming reactions were insufficient, the film strength and pore structure formation were incomplete, the puncture strength decreased to 180 gf, the porosity was only 30%, the heat shrinkage rate increased to 6% at 200°C, and the electrochemical performance was significantly substandard.

[0066] Comparative Example 5 The difference between the present comparative example and Example 1 is that the foaming agent system in the raw material is omitted, and the rest of the raw material ratio and process steps are referred to Example 1 to obtain the composite film of the present comparative example.

[0067] The product of the present comparative example has a thickness of about 115 μm, a porosity of only 29%, a liquid pore size D50 of about 80 nm, a Gurley air permeability of more than 3200 s / 100 mL, and an electrolyte absorption rate of 68%, and an ionic conductivity of only 0.30 mS / cm due to the lack of through-pores formed in situ. Although the heat shrinkage rate can still be maintained at 2.5% under the condition of 200°C x 30 min, the initial internal resistance significantly increases after battery assembly, and the capacity retention rate is only 78% after 200 cycles at 1C rate, which is a significant decrease compared to the retention rate of more than 92% of the three-phase system separator of the present application under the same conditions, and the comprehensive technical effect of high ion conductivity and thermal safety cannot be achieved.

[0068] The performance of the composite films of Comparative Examples 1-3 and Comparative Examples 1-5 is shown in Table 1.

[0069] Table 1

[0070] As can be seen from Table 1, the porosity of Examples 1-3 is stable at 45%-48%, the pore size D50 is concentrated at 210-240 nm, the Gurley air permeability is controlled at 410-430 s / 100 mL, forming an ideal structure of high porosity + uniform through-porous nanochannel, the ionic conductivity reaches 0.96-1.05 mS / cm, and the electrolyte absorption rate is also maintained at 135%-140%, ensuring full electrolyte infiltration and efficient ion migration. Relying on the high thermal stability of the cement-based inorganic skeleton, the heat shrinkage rate of Examples 1-3 at 200°C is only 1.2%-1.5%, which is much lower than that of the comparative examples; the puncture strength of the products of Examples 1-3 reaches 510-530 gf, meeting the mechanical requirements of lithium battery winding and pressing; the capacity retention rate is 91%-92% after 300 cycles at 1C rate, reflecting long-term electrochemical stability, while the capacity retention rate of Comparative Example 5 decreases to 78% after only 200 cycles, and Comparative Example 2 shows rapid capacity decay.

[0071] The performance defects of Comparative Examples 1-5 are all due to unbalanced formulation or process omission. Comparative Example 1 has a cement ratio far exceeding the optimal range, resulting in insufficient polymer flexibility, a porosity of only 28%, a puncture strength of <250 gf, an ionic conductivity of 0.35 mS / cm, and a decrease in thermal stability due to uneven structure.

[0072] The polymer concentration of Comparative Example 2 is too high, resulting in a too large density of the film body, a pore size D50 of only 80 nm, a Gurley air permeability >3000 s / 100 mL, an electrolyte absorption rate of only 65%, and a low ion conductivity of 0.28 mS / cm, which finally leads to a rapid decay of the cycle capacity.

[0073] Comparative Example 3 does not use the high-speed shearing process, the pore structure is uneven and prone to collapse, the porosity is only 32%, the thermal shrinkage at 200°C increases to 5%, and the ion conductivity decreases to 0.42 mS / cm, which cannot meet the dual requirements of ion transmission and thermal stability.

[0074] Comparative Example 4 does not meet the standard curing conditions, resulting in insufficient cement hydration and foaming reaction, and Comparative Example 5 omits the foaming agent due to the lack of in-situ foaming mechanism, which cannot form a through hole.

[0075] The above data show that the three-phase structure of the present application is not simply superimposed, but is a functional synergy achieved through formulation and process. The complexity and effectiveness of the technical solution far exceed the existing single performance optimization approach. Only under specific formulation and process conditions can the technical effects of the present application be achieved.

[0076] In summary, the cement-polymer composite porous separator material provided by the present application, with its unique three-phase synergistic structure design and in-situ pore formation preparation method, significantly improves the thermal safety, structural stability and ion transmission performance of lithium ion batteries under extreme temperatures, overcomes the key bottleneck of traditional separator materials in thermal runaway protection, and has the following significant technical advantages and beneficial effects: First, in terms of thermal stability, the cement-based skeleton has a much higher thermal decomposition temperature and non-combustible property than polyolefin materials, which can maintain its integrity at extreme conditions of up to 200°C or higher without melting, shrinking or pore collapse, effectively blocking the positive and negative electrodes from contacting, preventing short circuits, and thus significantly delaying or even stopping the occurrence of thermal runaway.

[0077] Second, in terms of ion conduction performance, the present application introduces an in-situ foaming process to construct a porous network structure with good connectivity and adjustable pore size distribution inside the material, forming a stable and continuous lithium ion migration channel. Despite the increase in overall film thickness to the range of 50-1000 μm, the presence of the porous channel significantly reduces the ion diffusion resistance, thereby maintaining a high conductivity and overcoming the problem of poor ion conductivity of traditional inorganic materials.

[0078] Third, in terms of mechanical properties, the cement inorganic phase provides high structural strength and dimensional stability, while the flexible polymer network improves the toughness and crack resistance of the separator during winding, cell assembly and long-term charging and discharging. The synergistic effect of the two meets the requirements of strength and flexibility, and meets the requirements of actual processing and assembly conditions of lithium batteries.

[0079] In addition, the preparation process adopted by the present application does not require high-temperature sintering, electrospinning or vacuum treatment, and the overall process is completed at normal temperature and pressure, the material source is extensive, the process is simple, and the production energy consumption and manufacturing cost are significantly reduced. Cement, polymer and foaming agent are all conventional industrial raw materials, which have good environmental friendliness and industrialization promotion prospect.

[0080] Finally, compared with traditional PE / PP microporous membrane, ceramic coated membrane or electrospinning nanofiber membrane and other mainstream separator technology, the present application not only realizes the qualitative improvement in thermal stability and flame retardancy, but also breaks through the technical contradiction that safety and flexibility cannot be compatible at high temperature, and provides a new material solution for building high safety and high energy density battery system.

[0081] In summary, the present application realizes significant optimization from multiple aspects such as material system construction, pore structure design, thermal protection performance, preparation process and engineering adaptability, has significant technical progress and practical application value, and is especially suitable for new generation power battery and energy storage battery system with high thermal safety requirement.

[0082] It should be noted that the above examples are only used to illustrate the technical solutions of the present application and not to limit it, although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application, and all should be covered in the scope of the claims of the present application.

Claims

1. A thermally stable separator material for lithium ion batteries, characterized by: The heat-stable separator material is a flexible sheet structure material with a thickness of 50-1000 μm, which is composed of three types of functional phases, respectively: (i) Inorganic skeleton phase, which is a rigid support network formed by hydration reaction of silicate cement, magnesia cement, alkali-activated slag cementitious material, phosphogypsum or industrial solid waste-based cementitious material; (ii) Polymer flexible phase, which is a flexible ion-conducting network constructed by dispersing a functional polymer selected from PVDF, PVA, PAN, PEGDA, polyphosphate, and biobased degradable polymer between the inorganic skeleton phase; (ⅲ) Internal pore structure phase, which is a through-porous network with a pore size of 100-500 nm induced in situ during hydration and curing by introducing a chemical foaming agent.

2. The thermally stable separator material for lithium ion batteries of claim 1, wherein: The raw materials of the heat-stable separator material include, in terms of mass percentage of the heat-stable separator material, 40-80 wt% of inorganic material, 5-25 wt% of functional polymer, 1-10 wt% of chemical foaming agent, and the balance of functional additives and deionized water; The inorganic material includes one of silicate cement, magnesia cement, alkali-activated slag cementitious material, phosphogypsum or industrial solid waste-based cementitious material. The functional polymer includes one of PVDF, PVA, PAN, PEGDA, polyphosphate, and biobased degradable polymer.

3. The thermally stable separator material for lithium ion batteries of claim 2, wherein: The chemical foaming agent includes one of hydrogen peroxide, sodium bicarbonate, starch, and cellulose.

4. The thermally stable separator material for lithium ion batteries of claim 2, wherein: The functional additives include one of flame retardant and heat stabilizer, inorganic / ceramic reinforcing filler, ion-conducting and interfacial wetting agent.

5. The method of claim 1 to 4, wherein the method is characterized by: The method includes, The functional polymer is dissolved in a solvent to form a homogeneous polymer precursor solution; The inorganic material powder is added to the homogeneous polymer precursor solution to form a uniformly dispersed inorganic material-polymer composite slurry under continuous stirring; The chemical foaming agent is added to the inorganic material-polymer composite slurry to promote microbubble formation by high-speed shear dispersion, obtaining a separator slurry; The separator slurry is applied on an inert substrate by blade coating, suction filtration or casting process, and is placed in an environment with a temperature of 25-60℃ and a relative humidity of 55%-80% for 6-24h of curing, forming a wet film with good pore structure and adhesion. After drying, post-treatment, peeling or cutting, the heat-stable separator material for lithium ion batteries is obtained.

6. The method for preparing the thermally stable separator material for lithium-ion batteries as described in claim 5, characterized in that: The mass fraction of the functional polymer in the homogeneous polymer precursor solution is 5wt%-18wt%.

7. The method for preparing the thermally stable separator material for lithium-ion batteries as described in claim 5, characterized in that: The high-speed shear dispersion promotes microbubble formation, wherein the rotation speed of high-speed shear dispersion is 2000-8000 rpm, the time is 1-5 min, and the pH of the dispersion system is controlled at 8-11.

8. The method for preparing the thermally stable separator material for lithium-ion batteries as described in claim 5, characterized in that: The drying temperature of the wet film is 50℃-90℃, and the drying time is 2-8h.

9. The method for preparing the thermally stable separator material for lithium-ion batteries as described in claim 5, characterized in that: The post-treatment includes one or more of hot pressing, heat treatment or surface modification.

10. The method for preparing the thermally stable separator material for lithium-ion batteries as described in claim 9, characterized in that: The temperature of the hot pressing is 90-130℃, the pressure is 0.5-3 MPa, and the time is 1-5 min; The temperature of the heat treatment is 100-150℃, and the heat treatment time is 10-60 min; The surface modification comprises spraying or dipping the dried wet film with 0.05-0.5wt% PVDF solution or 0.1-1wt% PVA solution, drying at 60-90°C for 10-30min after 5-60s; or, impregnating the dried wet film with 0.1-1wt% SiO2 sol or 0.05-0.8wt% Al2O3 sol, curing at 80-110°C for 10-30min after 10-120s; or, spraying 0.05-0.3wt% silane coupling agent, drying at 60-80°C for 10-20min.

Images