A cellulose-based flame-retardant composite separator and a preparation method and application thereof

Abstract

The application relates to the field of lithium ion battery diaphragm materials, and discloses a cellulose-based flame-retardant composite diaphragm as well as a preparation method and application thereof, the preparation method comprises the following steps: contacting an activated base film with a dispersion liquid containing a phosphorus-nitrogen flame retardant, and drying to obtain the composite diaphragm. The activated base film is preferably a metal ion-activated phosphorus-containing functionalized cellulose porous base film, which can be prepared through the following steps: dispersing cellulose material into a film to obtain a cellulose porous base film; performing phosphorus-containing functionalization treatment on the cellulose porous base film by using a phosphorus-containing modifier to obtain a phosphorus-containing functionalized cellulose porous base film; and then contacting the phosphorus-containing functionalized cellulose porous base film with a polyvalent metal salt solution to obtain the activated base film. The application can overcome the problems that the flame-retardant performance is weak, the flame retardant in the existing cellulose-based flame-retardant diaphragm is prone to migration, agglomeration, blockage of pores and influence on ion transmission performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery separator materials technology, and in particular to a cellulose-based flame-retardant composite separator, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, due to their high energy density and long cycle life, have been widely used in electric vehicles and energy storage systems. However, their thermal safety under abuse conditions such as overcharging, nail penetration, or high temperatures remains a key constraint on their development. The separator is a crucial component of lithium-ion batteries, positioned between the positive and negative electrodes. Its main functions are to isolate electron conduction, prevent direct contact and short circuits between the positive and negative electrodes, and provide a pathway for lithium ions to migrate in the electrolyte. The separator's pore structure, thermal stability, mechanical strength, electrolyte wettability, and electrochemical stability directly affect the battery's rate performance, cycle life, and safety performance. Currently, commercially available lithium-ion battery separators are mainly made of polyethylene, polypropylene, or their composite polyolefins. These separators have advantages such as low cost, mature processing, and good chemical stability, but the materials themselves have problems such as low melting point, insufficient thermal dimensional stability, poor electrolyte wettability, and insufficient flame retardancy. When the battery experiences abnormal temperature rise, polyolefin separators are prone to significant thermal shrinkage or even melting and rupture, causing direct contact between the positive and negative electrodes and increasing the risk of internal short circuits and thermal runaway.

[0003] To improve the safety of polyolefin separators, existing technologies typically employ methods such as ceramic particle coating, heat-resistant polymer coating, flame retardant coating, or inorganic / organic composite modification. For example, coating the surface of a polyolefin-based membrane with alumina, silica, boehmite, polyvinylidene fluoride, aramid, or phosphorus-containing flame retardants can improve the membrane's heat resistance, electrolyte wettability, and flame retardant properties to some extent. However, these coated separators still use polyolefin as the matrix, and the problem of base membrane shrinkage at high temperatures remains unresolved. Simultaneously, insufficient interfacial adhesion between the coating and the base membrane may exist, leading to particle detachment, coating cracking, or flame retardant migration after long-term cycling or electrolyte immersion. Furthermore, excessively high levels of flame retardants or inorganic particles can clog membrane pores, resulting in decreased electrolyte absorption, reduced ionic conductivity, and increased interfacial impedance, thereby affecting the battery's electrochemical performance.

[0004] Cellulose, as a natural polymer, has become an important candidate material for preparing high-safety separators due to its excellent heat resistance, electronic insulation, film-forming properties, and abundant surface hydroxyl groups. Compared with polyolefins, separators constructed from cellulose nanofibers or nanosheets exhibit extremely low thermal shrinkage at high temperatures and excellent electrolyte affinity, and the active sites on its molecular chains provide possibilities for functional modification. However, cellulose itself is flammable, and to meet stringent safety standards, phosphorus-based or nitrogen-based flame retardants or inorganic nanoparticles are usually introduced into the cellulose matrix. Although physical mixing or simple coating can improve the self-extinguishing property of the separator to some extent, the interfacial bonding between the flame retardant component and the cellulose matrix is ​​weak and the dispersion is poor. During long-term cycling or electrolyte immersion, flame retardant migration, precipitation, or detachment can easily occur, thus affecting the long-term reliability of the battery.

[0005] In existing flame-retardant composite solutions, the introduction of flame retardants often faces a contradiction between "flame retardant performance" and "electrochemical performance." To achieve the ideal flame retardant rating, a high flame retardant loading is often required, but this easily leads to the aggregation and accumulation of flame retardant particles within the membrane pores, resulting in a severe pore-blocking effect. This reduces the membrane's porosity and liquid absorption rate, significantly increasing the lithium-ion transport impedance. Simultaneously, physical composite systems lacking effective interface design can only achieve a simple superposition of functions. The flame retardant cannot form a dense protective layer on the pore wall surface, making it difficult to induce an efficient synergistic char formation mechanism under low loading conditions during actual thermal runaway, thus failing to fully utilize the structural advantages of the cellulose matrix. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a cellulose-based flame-retardant composite membrane, its preparation method and application, which can overcome the problems of weak flame-retardant performance, easy migration, agglomeration, pore blockage and ion transport performance of flame retardants in existing cellulose-based flame-retardant membranes.

[0007] The present invention solves the above-mentioned technical problems through the following technical means:

[0008] In a first aspect, the present invention provides a method for preparing a cellulose-based flame-retardant composite membrane, the method comprising: contacting an activated base membrane with a dispersion containing phosphorus and nitrogen flame retardants, and drying to obtain the composite membrane.

[0009] This invention first introduces phosphorus-containing functional groups on the surface of a porous cellulose membrane through phosphorus-containing functionalization treatment. Then, it utilizes the coordination, ionic cross-linking, or salt bridging effects between polyvalent metal ions and phosphorus-containing functional groups to form an activated membrane. Subsequently, the activated membrane is brought into contact with a dispersion containing phosphorus and nitrogen flame retardants, so that the phosphorus and nitrogen flame retardants form a relatively stable flame-retardant interface layer on the surface and in the pore structure of the porous cellulose membrane.

[0010] Among them, the porous cellulose base membrane, as the main body of the membrane, can provide electronic insulation, flexible support and electrolyte retention space; the phosphorus-containing functional groups can improve the polarity and char formation tendency of the cellulose surface on the one hand, and provide binding sites for polyvalent metal ions on the other hand; polyvalent metal ions can form an activation layer on the surface of phosphorus-functionalized cellulose, and further enhance the interfacial bonding between the activated base membrane and the phosphorus-nitrogen flame retardant; under abnormal temperature rise or combustion conditions, the phosphorus-nitrogen flame retardant can release phosphoric acid species and nitrogen-containing non-flammable gases, promoting cellulose dehydration and char formation and expansion to isolate oxygen.

[0011] When the dispersion further contains inorganic densifying components such as nano-silica, zinc borate, boehmite, nano-aluminum hydroxide, or zirconium phosphate, the inorganic densifying components can assist in the formation of a denser inorganic / carbon composite barrier layer under high temperature conditions, thereby improving the membrane's barrier capacity against heat, oxygen, and combustible volatiles.

[0012] Therefore, this invention utilizes a continuous interface construction method of "phosphorus functionalization - polyvalent metal ion activation - phosphorus-nitrogen flame retardant composite" to enable the flame retardant to be more stably bound in the porous cellulose membrane, thereby taking into account flame retardant performance, heat resistance, dimensional stability, and basic electrochemical performance of the membrane.

[0013] Preferably, the method for preparing the activated base film includes the following steps:

[0014] S1. Disperse cellulose material in a solvent to prepare a cellulose dispersion, and then form the cellulose dispersion into a film to obtain a porous cellulose membrane;

[0015] S2. The cellulose porous base membrane is subjected to phosphorus-functionalization treatment using a phosphorus-containing modifier, so that phosphorus-containing functional groups are formed on the surface of the cellulose in the cellulose porous base membrane, thereby obtaining a phosphorus-functionalized cellulose porous base membrane.

[0016] S3. The phosphorus-functionalized cellulose porous base membrane is contacted with a multivalent metal salt solution, so that the multivalent metal ions form coordination, ion crosslinking or salt bridging with the phosphorus-containing functional groups to obtain the activated base membrane.

[0017] Preferably, the cellulose material includes one or more of cellulose nanofibers, bamboo cellulose nanosheets, bacterial cellulose, regenerated cellulose, microcrystalline cellulose, and cellulose acetate hydrolysate.

[0018] Preferably, in step S2, the phosphorus-containing functionalization treatment involves introducing phosphorus-containing functional groups onto the surface of cellulose using a phosphorus-containing modifier; the phosphorus-containing modifier is selected from at least one of phytic acid, phosphoric acid / urea system, ammonium dihydrogen phosphate, and aminotrimethylenephosphonic acid.

[0019] In step S3, the multivalent metal ion is selected from at least one of aluminum ion, magnesium ion, calcium ion, zinc ion, zirconium ion and titanium ion.

[0020] Preferably, the phosphorus-nitrogen flame retardant includes one or more of the following: ammonium polyphosphate, melamine polyphosphate, melamine cyanurate, ammonium phytate, ammonium phosphate, phosphorus-containing POSS, phosphorus-nitrogen modified silica, zirconium phosphate, titanium phosphate, and aluminum phosphate.

[0021] More preferably, the phosphorus-nitrogen flame retardant comprises ammonium polyphosphate and melamine polyphosphate, wherein the mass ratio of ammonium polyphosphate to melamine polyphosphate is 1:3 to 3:1.

[0022] Preferably, the dispersion containing phosphorus and nitrogen flame retardants further contains an inorganic densifying component, which includes one or more of nano-silica, zinc borate, boehmite, nano-aluminum hydroxide, and zirconium phosphate.

[0023] More preferably, the inorganic densifying component is nano-silica and / or zinc borate, and the inorganic densifying component accounts for 0.5% to 8% of the total mass of the obtained composite membrane.

[0024] Preferably, the drying temperature is 40-80°C; the drying process further includes a shaping treatment, which is hot pressing, with a hot pressing temperature of 60-120°C, a hot pressing pressure of 0.5-10 MPa, and a hot pressing time of 0.5-10 min.

[0025] Secondly, the present invention provides a cellulose-based flame-retardant composite membrane, which is prepared by the above-described preparation method.

[0026] Thirdly, the present invention provides an application of the cellulose-based flame-retardant composite membrane in electrochemical energy storage devices.

[0027] Preferably, the electrochemical energy storage device is a lithium-ion battery, a sodium-ion battery, or a supercapacitor.

[0028] The beneficial effects of this invention are:

[0029] (1) In this invention, an activated base membrane is first prepared to form phosphorus-containing functional groups on the surface of the porous cellulose base membrane, and then further activated by polyvalent metal ions. The polyvalent metal ions can form coordination, ionic cross-linking, or salt bridging effects with the phosphorus-containing functional groups on the cellulose surface, thereby forming an activated interface with binding capacity on the surface of the porous cellulose base membrane. When the activated base membrane comes into contact with a dispersion containing phosphorus and nitrogen flame retardants, the phosphorus and nitrogen flame retardants can be more stably bound to the surface of the activated base membrane and the pore interface, reducing their migration and loss during subsequent electrolyte immersion and battery cycling.

[0030] (2) This invention activates the base membrane and contacts it with the phosphorus-nitrogen flame retardant dispersion, allowing the phosphorus-nitrogen flame retardant to preferentially bind to the surface, pore walls, and fiber cross-linking regions of the porous cellulose base membrane, rather than being piled up in the pores in a large, disordered manner. Therefore, an effective flame-retardant interface layer can be formed with a relatively low flame retardant loading. This flame-retardant interface layer can quickly participate in the dehydration and char formation, expansion and oxygen barrier, and heat transfer barrier processes when the membrane is heated or burned, thereby achieving high flame-retardant efficiency. This invention fixes the flame retardant by activating the interface, allowing the flame retardant to be distributed more extensively at the effective interface position, thereby achieving the effect of "low loading - high flame-retardant efficiency".

[0031] (3) This invention constructs a continuous interface of "phosphorus-functionalized cellulose, polyvalent metal ions, and phosphorus-nitrogen flame retardant," enabling the three components to reside in the same flame-retardant interface layer and exert a synergistic effect. Under high temperature or combustion conditions, phosphorus-functionalized cellulose promotes cellulose dehydration and char formation, while the phosphorus-nitrogen flame retardant releases phosphoric acid species and generates nitrogen-containing non-flammable gases, promoting expansion and oxygen isolation. Polyvalent metal ions promote the cross-linking of phosphorus-containing structures and contribute to the formation of a carbon / inorganic composite barrier layer containing metal phosphates or quasi-inorganic compounds. The invention does not form a single organic carbon layer, but rather a multi-layered flame-retardant barrier resulting from the combined action of a phosphorus-containing carbon layer, a metal ion cross-linking structure, and an inorganic / carbon composite barrier structure. This barrier can simultaneously block oxygen, heat, and combustible volatiles.

[0032] (4) This invention is based on a porous cellulose membrane, which has good thermal dimensional retention capabilities. Further, through phosphorus-functionalization and activation with multivalent metal ions, a certain cross-linking or ionic bonding structure can be formed between the cellulose segments and the pore wall interface, improving the structural stability of the porous cellulose network under heated conditions. After the phosphorus-nitrogen flame retardant is further incorporated into the activated membrane, it can promote char formation and the formation of a heat-insulating layer under abnormal heating conditions, thereby slowing heat transfer to the membrane interior and reducing the risk of membrane thermal shrinkage and rupture. This invention not only improves the flame retardancy of the membrane under open flame or high temperature but also enhances its dimensional retention capability under abnormal battery heating conditions.

[0033] (5) The dispersion containing phosphorus and nitrogen flame retardants also contains inorganic densifying components such as nano-silica, zinc borate, boehmite, nano-aluminum hydroxide, or zirconium phosphate. These inorganic densifying components can assist in the formation of a denser and more continuous inorganic / carbon composite barrier layer at high temperatures, further improving the integrity, heat insulation, and oxygen barrier properties of the residual carbon layer. Since the amount of inorganic densifying components is controlled at 0.5% to 8% of the total mass of the resulting composite membrane, excessive inorganic particles can be avoided from clogging the membrane pores while improving the barrier effect.

[0034] (6) The cellulose-based flame-retardant composite membrane prepared by the present invention can be used in electrochemical energy storage devices such as lithium-ion batteries, sodium-ion batteries or supercapacitors. It can improve the safety of energy storage devices under high temperature, short circuit or thermal abuse conditions while maintaining the basic isolation and ion transport functions of the membrane, and has good application adaptability. Detailed Implementation

[0035] The technical solution of the present invention will be further described below with reference to specific embodiments. It should be understood that the following embodiments are only used to illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Any equivalent substitutions or conventional adjustments made to the type or concentration of raw materials, processing time, drying method or shaping conditions based on the concept of the present invention should be included within the scope of protection of the present invention.

[0036] Unless otherwise stated, all reagents used in the following examples are commercially available products, and the water used is deionized water. Ammonium polyphosphate is abbreviated as APP, melamine polyphosphate as MPP, and melamine cyanurate as MCA.

[0037] Example 1

[0038] This embodiment provides a method for preparing a cellulose-based flame-retardant composite membrane, comprising the following steps.

[0039] (1) Preparation of activated base film

[0040] S1. Add 2.0g of cellulose nanofibers to 198.0g of deionized water, stir mechanically for 30min, and then sonicate for 20min to obtain a cellulose dispersion with a mass concentration of 1.0%.

[0041] The obtained cellulose dispersion was poured into a vacuum filtration apparatus for filtration to form a wet cellulose membrane. The wet cellulose membrane was then immersed in mixtures of ethanol / water at volume ratios of 30:70, 50:50, and 70:30 for 10 minutes each to perform solvent replacement. Subsequently, it was dried at 50°C for 2 hours and then hot-pressed at 80°C and 2 MPa for 3 minutes to obtain a porous cellulose base membrane.

[0042] S2. Prepare a 3.0% phytic acid aqueous solution, immerse the cellulose porous membrane in the phytic acid aqueous solution, and treat it at 60℃ for 4 hours. After treatment, wash it alternately with deionized water and ethanol, and dry it at 50℃ for 1 hour to obtain a phosphorus-functionalized cellulose porous membrane.

[0043] S3. Prepare a 0.08 mol / L aluminum nitrate aqueous solution, immerse the phosphorus-functionalized cellulose porous base membrane in the aluminum nitrate aqueous solution, and treat it at room temperature for 2 hours; after treatment, wash it twice with deionized water and dry it at 50°C for 1 hour to obtain the activated base membrane.

[0044] (2) Preparation of composite membrane

[0045] APP and MPP were mixed at a mass ratio of 1:1 and then added to deionized water to make the total mass concentration of APP and MPP 4.0%. After stirring for 30 min, the mixture was ultrasonically dispersed for 10 min to obtain a dispersion containing phosphorus and nitrogen flame retardants.

[0046] The activated base film is brought into contact with the dispersion containing phosphorus and nitrogen flame retardants. Specifically, the activated base film is immersed in the dispersion and treated at room temperature for 30 minutes. After removal, excess dispersion is removed from the surface and dried at 60°C for 2 hours to obtain a cellulose-based flame-retardant composite membrane.

[0047] After drying, a shaping process can be carried out, specifically hot pressing at 90℃ and 3MPa for 3 minutes.

[0048] In this embodiment, the activated base membrane is an aluminum ion activated phosphorus-functionalized cellulose porous base membrane; the dispersion containing phosphorus and nitrogen flame retardants is an APP / MPP dispersion.

[0049] Example 2

[0050] This embodiment provides a method for preparing a cellulose-based flame-retardant composite membrane, comprising the following steps.

[0051] (1) Preparation of activated base film

[0052] S1. Add 1.5g of bamboo cellulose nanosheets to 198.5g of deionized water, mechanically stir and ultrasonically disperse to obtain a bamboo cellulose nanosheet dispersion with a mass concentration of 0.75%.

[0053] The obtained bamboo cellulose nanosheet dispersion was vacuum filtered to form a film. The resulting wet film was replaced with ethanol and dried at 50°C. Then, it was hot-pressed at 85°C and 2MPa for 3 minutes to obtain a porous cellulose base membrane.

[0054] S2. Prepare a phosphorus-containing modified solution, wherein the phosphoric acid mass concentration in the phosphorus-containing modified solution is 5.0% and the urea mass concentration is 8.0%. Immerse the cellulose porous base membrane in the phosphorus-containing modified solution and treat it at 80°C for 3 hours. After treatment, wash it with deionized water and ethanol, and dry it at 50°C to obtain a phosphorus-functionalized cellulose porous base membrane.

[0055] S3. Prepare a 0.10 mol / L magnesium chloride aqueous solution, immerse the phosphorus-functionalized cellulose porous base membrane in the magnesium chloride aqueous solution, and treat it at 40°C for 2 hours; after treatment, wash and dry to obtain the activated base membrane.

[0056] (2) Preparation of composite membrane

[0057] APP and MPP were mixed at a mass ratio of 2:1 and added to deionized water to make the total mass concentration of APP and MPP 5.0%; then nano silica was added to make the mass concentration of nano silica 0.8%, stirred and ultrasonically dispersed to obtain a dispersion containing phosphorus and nitrogen flame retardants.

[0058] The activated base film is brought into contact with the dispersion containing phosphorus and nitrogen flame retardants. Specifically, the activated base film is immersed in the dispersion and treated at room temperature for 40 minutes. After removal, excess dispersion is removed from the surface and dried at 60°C for 2 hours to obtain a cellulose-based flame-retardant composite membrane.

[0059] After drying, a shaping process can be carried out, specifically hot pressing at 100℃ and 4MPa for 3 minutes.

[0060] In this embodiment, the activated base membrane is a phosphorus-functionalized bamboo cellulose porous base membrane activated by magnesium ions; the dispersion containing phosphorus and nitrogen flame retardants also contains nano-silica as an inorganic densifying component.

[0061] Example 3

[0062] This embodiment provides a method for preparing a cellulose-based flame-retardant composite membrane, comprising the following steps.

[0063] (1) Preparation of activated base film

[0064] S1. Take a wet bacterial cellulose membrane and wash it repeatedly with deionized water. Then, place the wet bacterial cellulose membrane in a 0.5 mol / L sodium hydroxide solution and treat it at 80℃ for 1 h, then wash it with deionized water until neutral. After washing, replace the bacterial cellulose membrane with ethanol, dry it at 50℃, and then hot-press it at 80℃ and 2MPa for 3 min to obtain a porous cellulose membrane.

[0065] S2. Prepare an aqueous solution of aminotrimethylene phosphonic acid with a mass concentration of 2.0%, immerse the cellulose porous membrane in the aqueous solution of aminotrimethylene phosphonic acid, and treat it at 60°C for 5 hours; after treatment, wash and dry to obtain a phosphorus-functionalized cellulose porous membrane.

[0066] S3. Prepare a mixed aqueous solution containing 0.05 mol / L calcium chloride and 0.03 mol / L zinc acetate. Immerse the phosphorus-functionalized cellulose porous base membrane in the mixed aqueous solution and treat it at room temperature for 3 hours. After treatment, wash and dry to obtain the activated base membrane.

[0067] (2) Preparation of composite membrane

[0068] APP and MCA were mixed at a mass ratio of 3:1 and added to deionized water to make the total mass concentration of APP and MCA 4.5%. Zinc borate was then added to make the zinc borate mass concentration 0.5%. The mixture was stirred and ultrasonically dispersed to obtain a dispersion containing phosphorus and nitrogen flame retardants.

[0069] The activated base film is brought into contact with the dispersion containing phosphorus and nitrogen flame retardants. Specifically, the activated base film is immersed in the dispersion and treated at room temperature for 30 minutes. After removal, it is dried at 60°C to obtain a cellulose-based flame-retardant composite membrane.

[0070] After drying, a shaping process can be carried out, specifically hot pressing at 90℃ and 3MPa for 3 minutes.

[0071] In this embodiment, the activated base membrane is a phosphorus-functionalized bacterial cellulose porous base membrane activated by calcium / zinc composite ions; the dispersion containing phosphorus and nitrogen flame retardants also contains zinc borate as an inorganic densifying component.

[0072] Example 4

[0073] This embodiment provides a method for preparing a cellulose-based flame-retardant composite membrane, comprising the following steps.

[0074] (1) Preparation of activated base film

[0075] S1. A regenerated cellulose slurry is added to deionized water to prepare a regenerated cellulose dispersion with a mass concentration of 1.2%. The regenerated cellulose dispersion is cast into a film and a porous structure is formed by ethanol / water solvent displacement. The film is then dried at 50°C to obtain a porous cellulose base membrane.

[0076] S2. Prepare a 6.0% (w / w) ammonium dihydrogen phosphate solution, immerse the cellulose porous membrane in the ammonium dihydrogen phosphate solution, and treat it at 70°C for 4 hours; after treatment, wash and dry to obtain a phosphorus-functionalized cellulose porous membrane.

[0077] S3. Prepare a 0.04 mol / L zirconium oxychloride aqueous solution, immerse the phosphorus-functionalized cellulose porous base membrane in the zirconium oxychloride aqueous solution, and treat it at room temperature for 2 hours; after treatment, wash and dry to obtain the activated base membrane.

[0078] (2) Preparation of composite membrane

[0079] MPP, ammonium phytate and zirconium phosphate were mixed in a mass ratio of 2:1:1 and added to a mixed solvent with a water / ethanol volume ratio of 70:30 to prepare a dispersion containing phosphorus and nitrogen flame retardants with a total mass concentration of 4.0%.

[0080] The activated base film is brought into contact with the dispersion containing phosphorus and nitrogen flame retardants. Specifically, the activated base film is immersed in the dispersion and treated at room temperature for 30 minutes. After removal, it is dried at 60°C to obtain a cellulose-based flame-retardant composite membrane.

[0081] After drying, a shaping process can be carried out, specifically hot pressing at 100℃ and 4MPa for 3 minutes.

[0082] In this embodiment, the activated base membrane is a phosphorus-functionalized regenerated cellulose porous base membrane activated by zirconium ions.

[0083] Comparative Example 1

[0084] A cellulose porous membrane was prepared according to the method of "Preparation of cellulose porous membrane" in Example 1. Without phosphorus functionalization treatment and polyvalent metal salt solution treatment, the obtained cellulose porous membrane was directly contacted with the dispersion containing phosphorus and nitrogen flame retardant in Example 1 and dried to obtain the diaphragm of Comparative Example 1.

[0085] Comparative Example 2

[0086] Phosphorus-functionalized cellulose porous membranes were prepared according to the "Preparation of cellulose porous membranes" and "phosphorus-functionalized treatment" methods in Example 1. Without aluminum nitrate aqueous solution treatment, the obtained phosphorus-functionalized cellulose porous membranes were directly contacted with the dispersion containing phosphorus and nitrogen flame retardants in Example 1 and dried to obtain the diaphragm of Comparative Example 2.

[0087] Comparative Example 3

[0088] A cellulose porous membrane was prepared according to the method of "Preparation of cellulose porous membrane" in Example 1. Without phytic acid treatment, the cellulose porous membrane was directly immersed in a 0.08 mol / L aluminum nitrate aqueous solution and treated at room temperature for 2 hours. After washing and drying, it was then contacted with the dispersion containing phosphorus and nitrogen flame retardants in Example 1 and dried to obtain the diaphragm of Comparative Example 3.

[0089] Comparative Example 4

[0090] The activated base membrane was prepared according to the "Preparation of Activated Base Membrane" method in Example 1. It was dried directly and hot-pressed without contacting the dispersion containing phosphorus and nitrogen flame retardants to obtain the diaphragm of Comparative Example 4.

[0091] Comparative Example 5

[0092] Commercial polypropylene membrane was taken and contacted with the dispersion containing phosphorus and nitrogen flame retardants in Example 1 for 30 minutes; after removal, it was dried at 60°C to obtain the membrane of Comparative Example 5.

[0093] Performance testing methods

[0094] 1. Thickness test

[0095] The membrane thickness was measured using a membrane thickness gauge. Five locations were randomly selected for testing each sample, and the average value was taken.

[0096] 2. Porosity testing

[0097] The porosity of the diaphragm was tested using the n-butanol absorption method. The dried diaphragm was weighed and recorded as m0; after immersion in n-butanol for 2 hours, it was removed, the surface liquid was wiped off, and it was weighed again and recorded as m1. The porosity was calculated using the following formula:

[0098] Porosity / % = [(m1-m0) / (ρV)]×100%

[0099] Where ρ is the density of n-butanol and V is the apparent volume of the diaphragm.

[0100] 3. Electrolyte absorption rate test

[0101] Cut the diaphragm into identical sizes and weigh its dry mass m0. After immersing it in the electrolyte for 2 hours, remove it and gently absorb excess electrolyte from the surface with filter paper. Weigh the mass m2. The liquid absorption rate is calculated using the following formula:

[0102] Liquid absorption rate / % = [(m2-m0) / m0]×100%

[0103] 4. Contact Angle Test

[0104] The wettability of the diaphragm to the electrolyte was tested using a contact angle meter. A certain volume of electrolyte was dropped onto the diaphragm surface, and the change in contact angle was recorded. Each sample was tested at least three times, and the average value was taken.

[0105] 5. Heat shrinkage rate test

[0106] Cut the diaphragm into 2cm × 2cm samples and record the area A0 before heating. Place the samples in environments of 150℃, 180℃, and 200℃ for 30 minutes each, and record the area A1 after cooling to room temperature. The heat shrinkage rate is calculated using the following formula: Heat shrinkage rate / % = [(A0-A1) / A0] × 100%

[0107] 6. Self-extinguishing time test

[0108] Cut the diaphragm to the same size, ignite one end of the diaphragm with an igniter, and record the self-extinguishing time of the diaphragm after removing the flame source. Test each group of samples at least 3 times and take the average value.

[0109] 7. Flame retardant release rate test

[0110] The diaphragm was immersed in the electrolyte for 24 h, 72 h, and 168 h, respectively. After removal and drying, the changes in the content of elements such as P, N, Al, Mg, Zn, or Zr in the diaphragm before and after immersion were tested. The retention of flame retardants could be evaluated using ICP-OES, XPS, or elemental analysis methods.

[0111] 8. Ionic conductivity test

[0112] After the separator fully absorbs the electrolyte, it is sandwiched between two stainless steel sheets to assemble a stainless steel / separator / stainless steel blocking battery. The bulk resistance Rb is measured using electrochemical impedance spectroscopy. The ionic conductivity is calculated using the following formula: σ = L / (Rb × S). Where σ is the ionic conductivity, L is the separator thickness, and S is the effective contact area.

[0113] 9. Battery cycle performance test

[0114] Using LiFePO4 as the positive electrode and lithium metal as the negative electrode, coin cells were assembled using the separators of Examples 1-4 and Comparative Examples 1-5. Cyclic performance tests were conducted under the same electrolyte and assembly conditions to compare capacity retention, coulombic efficiency, and cycle stability.

[0115] Table 1 Basic Physical Properties

[0116]

[0117] As shown in Table 1, the porosity of Examples 1–4 remained between 61.9% and 66.2%, and the ionic conductivity remained between 1.12 and 1.34 mS·cm. -1 This demonstrates that the introduction of flame-retardant components through the method of "activating the base membrane and contacting it with a dispersion containing phosphorus and nitrogen flame retardants" does not significantly damage the interconnected channels of the porous cellulose base membrane.

[0118] Compared with Comparative Example 1, Example 1 shows that the phosphorus-containing functional groups and metal ion activation layer in the activated base membrane not only help the flame retardant bind, but also maintain or even improve the electrolyte wettability of the membrane.

[0119] Compared with the diaphragm of Comparative Example 5, the contact angle of Examples 1-4 was significantly reduced, and the liquid absorption rate and ionic conductivity were significantly increased, indicating that the cellulose-based porous structure and its polar functionalized surface are more conducive to electrolyte wetting and ion transport.

[0120] Although Comparative Example 4 has a high ionic conductivity, it does not incorporate phosphorus-nitrogen flame retardants, therefore it cannot form a complete flame retardant system. Example 1, after introducing APP / MPP, still maintains a conductivity of 1.26 mS·cm. -1 The ionic conductivity indicates that the present invention achieves flame retardant modification under low load through interfacial bonding, thereby balancing flame retardant performance and ion transport performance.

[0121] Table 2 Thermal stability and flame retardant properties

[0122]

[0123] As shown in Table 2, Examples 1-4 all exhibited low thermal shrinkage rates at 150°C, 180°C, and 200°C. In particular, Example 2 showed a thermal shrinkage rate of only 2.9% at 200°C, significantly lower than the 13.8% of Comparative Example 1, 9.6% of Comparative Example 2, and the melt-cracking result of Comparative Example 5. This indicates that the phosphorus-functionalized cellulose, the multivalent metal ion activation layer, the phosphorus-nitrogen flame retardant, and the inorganic densifying components collectively improved the high-temperature dimensional stability of the membrane.

[0124] The self-extinguishing time of Example 1 was 0.72 s, while that of Comparative Example 1 was 2.76 s, Comparative Example 2 was 1.94 s, Comparative Example 3 was 2.83 s, and Comparative Example 4 was 4.62 s. These results indicate that simply combining APP / MPP with an unactivated cellulose base membrane has limited flame-retardant effect; phosphorus functionalization alone, without metal ion activation, results in insufficient flame retardant fixation and synergistic char formation; and simply activating the base membrane without phosphorus and nitrogen flame retardants also makes it difficult to achieve rapid self-extinguishing.

[0125] The self-extinguishing time of Example 1 decreased from 1.94 s to 0.72 s compared to Comparative Example 2, indicating that the multivalent metal ion activation step is not a typical enhancement step, but rather forms a more effective interfacial synergistic structure between phosphorus-functionalized cellulose and the phosphorus-nitrogen flame retardant. The self-extinguishing time of Example 1 decreased from 2.83 s to 0.72 s compared to Comparative Example 3, indicating that phosphorus-containing functional groups are the key basis for forming a stable activated interface.

[0126] In Example 2, due to the further addition of nano-silica, the self-extinguishing time was reduced to 0.51s, the limiting oxygen index was increased to 35.8%, and the char residue rate at 800℃ was increased to 43.2%, indicating that the inorganic densifying component can work together with the phosphorus-nitrogen flame retardant system to improve the density and thermal shielding ability of the char residue layer.

[0127] Table 3 Flame retardant removal rate after electrolyte immersion

[0128]

[0129] As shown in Table 3, the flame retardant removal rates after immersion in the electrolyte for 168 hours in Examples 1-4 were all below 6%, with Example 2 reaching only 4.1%. In contrast, the removal rates after 168 hours for Comparative Examples 1, 2, 3, and 5 were 24.7%, 17.6%, 26.2%, and 31.8%, respectively. These results indicate that the activated base film plays a significant role in the stable binding of phosphorus-nitrogen flame retardants. Comparative Example 1, without phosphorus functionalization and metal ion activation, primarily exhibited flame retardant through physical adsorption, resulting in a high removal rate. Although Comparative Example 2 underwent phosphorus functionalization, it lacked polyvalent metal ion activation, yet the flame retardant removal rate still reached 17.6%, demonstrating that simple phosphorus functionalization alone is insufficient for stable fixation. Despite aluminum salt treatment, Comparative Example 3 lacked phosphorus functional groups to provide stable binding sites, leading to a still high removal rate.

[0130] Compared to Comparative Example 2, the removal rate in Example 1 decreased from 17.6% to 4.6% after 168 hours; compared to Comparative Example 3, the removal rate decreased from 26.2% to 4.6% after 168 hours. This indicates a synergistic effect between phosphorus-containing functional groups and polyvalent metal ions, which together constitute the interfacial fixation basis for phosphorus-nitrogen flame retardants.

[0131] Table 4 Electrochemical performance

[0132]

[0133] As shown in Table 4, the interfacial impedance of Examples 1-4 remained between 78 and 101 Ω, significantly lower than that of Comparative Examples 1, 3, and 5. This indicates that although the composite membrane of the present invention incorporates phosphorus-nitrogen flame retardants, it still maintains good ion transport capability and low interfacial impedance because the flame retardants do not randomly and excessively block the pores, but rather form a stable interfacial bond by activating the base membrane.

[0134] After 200 cycles at 1C, Example 1 showed a capacity retention of 93.8%, and Example 2 showed 94.6%, both significantly higher than Comparative Example 1 (85.7%), Comparative Example 3 (83.4%), and Comparative Example 5 (76.8%). This indicates that the separator of the present invention, while undergoing flame-retardant modification, did not significantly sacrifice battery cycle stability.

[0135] Comparative Example 4, without the introduction of phosphorus and nitrogen flame retardants, had an interfacial impedance of 82 Ω and a capacity retention rate of 93.1% after 200 cycles, indicating that the activated base film itself possesses good electrochemical compatibility. However, Comparative Example 4 had a long self-extinguishing time and a low limiting oxygen index, resulting in insufficient flame retardant performance. Compared to Comparative Example 4, Example 1, with a slight increase in interfacial impedance, showed a significant improvement in flame retardant performance, with the self-extinguishing time decreasing from 4.62 s to 0.72 s, demonstrating that the present invention achieves a good balance between flame retardant performance and electrochemical performance.

[0136] In summary, the test results from the examples and comparative examples show that it is difficult to simultaneously achieve low migration, rapid self-extinguishing, low thermal shrinkage, and ion transport retention of flame retardants through phosphorus-functionalization treatment alone, multivalent metal ion treatment alone, or the introduction of phosphorus-nitrogen flame retardants alone. Conversely, this invention, by first forming an activated base film and then contacting and drying the activated base film with a dispersion containing phosphorus-nitrogen flame retardants, allows phosphorus-functionalized cellulose, multivalent metal ions, and phosphorus-nitrogen flame retardants to form a synergistic effect at the interface of the porous cellulose base film. This synergistic effect enables the composite membrane to maintain a short self-extinguishing time and low thermal shrinkage rate even with a low flame retardant loading, while simultaneously maintaining high ionic conductivity and cycle capacity retention, overcoming the technical contradiction in traditional flame-retardant membranes where improved flame retardant performance is usually accompanied by a decrease in ion transport performance.

Claims

1. A method for preparing a cellulose-based flame-retardant composite membrane, characterized in that, The preparation method includes: contacting an activated base membrane with a dispersion containing phosphorus and nitrogen flame retardants, and drying it to obtain the composite membrane.

2. The preparation method according to claim 1, characterized in that, The method for preparing the activated base film includes the following steps: S1. Disperse cellulose material in a solvent to prepare a cellulose dispersion, and then form the cellulose dispersion into a film to obtain a porous cellulose membrane; S2. The cellulose porous base membrane is subjected to phosphorus-functionalization treatment using a phosphorus-containing modifier, so that phosphorus-containing functional groups are formed on the surface of the cellulose in the cellulose porous base membrane, thereby obtaining a phosphorus-functionalized cellulose porous base membrane. S3. The phosphorus-functionalized cellulose porous base membrane is contacted with a multivalent metal salt solution, so that the multivalent metal ions form coordination, ion crosslinking or salt bridging with the phosphorus-containing functional groups to obtain the activated base membrane.

3. The preparation method according to claim 2, characterized in that, The cellulose material includes one or more of the following: cellulose nanofibers, bamboo cellulose nanosheets, bacterial cellulose, regenerated cellulose, microcrystalline cellulose, and cellulose acetate hydrolysis products.

4. The preparation method according to claim 2, characterized in that, In step S2, the phosphorus-containing functionalization treatment involves introducing phosphorus-containing functional groups onto the surface of cellulose using a phosphorus-containing modifier; the phosphorus-containing modifier is selected from at least one of phytic acid, phosphoric acid / urea system, ammonium dihydrogen phosphate, and aminotrimethylenephosphonic acid. In step S3, the multivalent metal ion is selected from at least one of aluminum ion, magnesium ion, calcium ion, zinc ion, zirconium ion and titanium ion.

5. The preparation method according to claim 1, characterized in that, The phosphorus-nitrogen flame retardant includes one or more of the following: ammonium polyphosphate, melamine polyphosphate, melamine cyanurate, ammonium phytate, ammonium phosphate, phosphorus-containing POSS, phosphorus-nitrogen modified silica, zirconium phosphate, titanium phosphate, and aluminum phosphate.

6. The preparation method according to claim 5, characterized in that, The dispersion containing phosphorus and nitrogen flame retardants also contains an inorganic densifying component, which includes one or more of nano-silica, zinc borate, boehmite, nano-aluminum hydroxide, and zirconium phosphate.

7. The preparation method according to claim 1, characterized in that, The drying temperature is 40–80°C; the drying process also includes a shaping treatment, which is hot pressing, with a hot pressing temperature of 60–120°C, a hot pressing pressure of 0.5–10 MPa, and a hot pressing time of 0.5–10 min.

8. A cellulose-based flame-retardant composite membrane, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 7.

9. The application of the cellulose-based flame-retardant composite membrane according to any one of claims 1 to 7 in an electrochemical energy storage device.

10. The application according to claim 9, characterized in that, The electrochemical energy storage device is a lithium-ion battery, a sodium-ion battery, or a supercapacitor.

Images