Electrolyte flame retardant additive and lithium battery separator thereof

Abstract

The application provides an electrolyte flame-retardant additive and a lithium battery diaphragm thereof, and belongs to the technical field of electrochemistry. ‑ The anion improves the lithium ion migration number, solves the problem of deterioration of conductivity of traditional flame retardants, and realizes synchronous improvement of flame-retardant safety, electrochemical performance and high-temperature stability through the process breakthrough of the vertical HAP nanowire-PBI-COFs interlocking diaphragm and the pulse electric field induced mineralization combined with cerium salt catalysis, and reduces the problems of toxicity and conductivity deterioration of traditional phosphorus-based flame retardants and high-temperature melting failure of polyolefin diaphragms.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of electrochemistry, in particular to an electrolyte flame-retardant additive and a lithium battery separator thereof. BACKGROUND

[0002] The electrolyte flame-retardant additive is an organic compound added to the electrolyte of a lithium ion battery, which reduces the flammability of the electrolyte, improves the thermal stability and enhances the safety performance of the battery through chemical mechanisms such as free radical capture and barrier layer formation, prevents combustion or explosion caused by thermal runaway, and commonly includes phosphorus-based, boron-based, halogen-based and composite additives; the lithium battery separator is a porous film located between the positive and negative electrodes, which plays a core role in isolating the positive and negative electrodes to prevent short circuiting, while allowing lithium ions to pass through to maintain normal charging and discharging processes, and needs to have good ionic conductivity, mechanical strength and chemical stability.

[0003] The current improvement of the safety performance of lithium ion batteries faces double challenges: on the one hand, traditional phosphorus-based flame retardants such as triethyl phosphate (TEP) can reduce the flammability of the electrolyte, but significantly deteriorate the cycle stability of the battery, as it is difficult to form a stable solid electrolyte interface and positive electrolyte interface, leading to continuous decomposition of the electrolyte and uncontrollable growth of lithium dendrites; on the other hand, polyolefin-based separators are severely shrunk and damaged at high temperatures (> 400℃), aggravating the risk of thermal runaway, while existing heat-resistant separators, such as aramid-coated separators and polyimide separators, increase the mechanical strength but sacrifice the ionic conductivity due to the increased interface impedance.

[0004] In view of the above problems, the present application proposes an electrolyte flame-retardant additive and a lithium battery separator thereof, which adopts a phosphorus-free organic silicon flame-retardant system combined with a biomimetic porous structure separator, introduces a self-crosslinking organic silicon quaternary ammonium salt into the flame retardant through molecular structure design and material compounding process, and constructs a biomimetic mineralization coating on the surface of the separator, achieving the synergistic optimization of flame-retardant performance and electrochemical performance. SUMMARY

[0005] The purpose of the present application is to propose an electrolyte flame-retardant additive and a lithium battery separator thereof in view of the above problems existing in the prior art.

[0006] The purpose of the present application can be achieved by the following technical solutions:

[0007] An electrolyte flame-retardant additive, the chemical structure of which is as follows:

[0008] [(CH3O)3Si-(CH2)3-N + (CH3)2-R]X - ;

[0009] wherein R is a C2-C6 alkyl chain containing acryloyloxy group, X - is TFSI- ;

[0010] The additive has a mass fraction of 1% to 5% in the electrolyte and does not contain halogens or phosphorus.

[0011] The preparation method of the flame retardant additive includes the following steps:

[0012] S1. 3-Chloropropyltrimethoxysilane was reacted with N,N-dimethylacrylamide at 60°C for 12 hours to generate a quaternary ammonium salt intermediate;

[0013] S2. The intermediate was anion exchanged with LiTFSI in a mixed solvent of ethanol and water at a temperature of 25°C for 6 hours.

[0014] S3. The solvent is removed by vacuum distillation to obtain a white solid.

[0015] The flame retardant additive undergoes cross-linking at temperatures exceeding 180°C to generate a SiO2 / C ceramic layer, which forms a physically interlocked structure with the hydroxyapatite nanowires on the membrane surface.

[0016] The anion exchange step (S2) is carried out at room temperature with a yield of ≥92%, which is significantly better than the high-cost ion exchange resin method (yield <80%).

[0017] Traditional flame retardants often contain phosphorus / halogens. The self-crosslinking organosilicon quaternary ammonium salt provided by this invention is made from TFSI - Anionic alternatives to traditional phosphorus / halogen flame retardants, such as phosphate esters, avoid the generation of toxic gases (such as PH3 and HF) through high-temperature decomposition, thereby improving the environmental safety of the battery. Acryloyloxy groups spontaneously crosslink to form a SiO2 / C ceramic layer during thermal runaway (>180℃), covering the electrode surface and blocking oxygen diffusion, thus increasing the limiting oxygen index to over 38%.

[0018] Preferably, the R group of the flame retardant additive is 2-acryloyloxyethyl or 3-acryloyloxypropyl. The short-chain acryloyloxy group (C2-C3) has low steric hindrance, reduces the activation energy of the crosslinking reaction by 15%, and shortens the ceramic layer formation time to within 3 seconds (long-chain C6 requires 8 seconds), which can optimize the polymerization efficiency. In addition, the hydrophilic alkyl chain can improve the dispersion stability of the additive in the electrolyte and prevent precipitation.

[0019] Preferably, the thermal polymerization temperature of the additive is 180~220℃, which covers the starting point of thermal runaway of lithium battery (≈200℃) and is earlier than the melting temperature of the separator (PE 135℃ / PP 165℃). This allows for the preferential formation of a barrier layer to suppress chain reactions, and this temperature is higher than the normal operating temperature of the battery (<60℃), which helps to ensure cycle stability (capacity retention rate >90% after 500 2C charge-discharge cycles).

[0020] A lithium battery electrolyte comprises a lithium salt, an organic solvent, and the aforementioned flame retardant additive, and further comprises 0.5-2 wt% cerium nitrate (Ce(NO3)3). 3+ Catalytic crosslinking of organosilicon quaternary ammonium salts increases the thickness of the ceramic layer to 5 μm, facilitating synergistic catalytic flame retardancy and improving flame retardancy efficiency by 40%. In addition, CeO2 nanoparticles fill the cracks in the SEI film, reducing the risk of lithium dendrite puncture.

[0021] A lithium battery separator, comprising:

[0022] Polyolefin substrates and surface functional coatings:

[0023] The functional coating consists of a base layer and a surface layer;

[0024] The bottom layer is a porous adhesive layer containing PBI-COFs, with a pore size of 2~5 nm, accounting for 60% of the coating thickness;

[0025] The surface layer consists of a vertically oriented array of hydroxyapatite (HAP) nanowires with a diameter of 20–50 nm and a length of 1–3 μm.

[0026] The hydroxyapatite nanowire array on the surface of the diaphragm is used to form a physically interlocked structure with the SiO2 / C ceramic layer generated by cross-linking with the electrolyte flame retardant additive at temperatures exceeding 180°C.

[0027] HAP nanowire arrays (melting point 1650℃) exhibit a thermal shrinkage rate of <5% at 400℃ (commercial PE membranes >80%), and their puncture resistance is increased to 500 MPa (aramid membranes ≈300 MPa), demonstrating high-temperature stability. PBI-COFs refer to porous materials formed by the composite of polybenzimidazole (PBI) and covalent organic frameworks (COFs), possessing a one-dimensional proton channel structure. The one-dimensional proton channels of PBI-COFs (IEC=2.0 mmol / g) enable hydrogen bond network conduction, and the ionic conductivity at 120℃ is 0.8 mS / cm.

[0028] Preferably, the hydroxyapatite nanowires are doped with 5-10 at% Mg. 2+ Preferred growth along the c-axis, with nanowire spacing of 100–200 nm. Mg 2+ Doped HAP nanowires facilitate the strengthening of crystal structure and the reduction of interfacial impedance; Mg 2+ Ca substitution 2+ This induces lattice distortion, increasing the growth rate of HAP along the

[001] crystal orientation by two times, and achieving an array orientation degree >95% (undoped ≈80%). Furthermore, magnesium ions enhance the desolvation capability of lithium ions, reducing the interfacial impedance to 50 Ω·cm. 2 (Traditional coating >200 Ω·cm)2 ).

[0029] Preferably, the PBI-COFs porous adhesive layer comprises one-dimensional channels loaded with phosphoric acid, the surface of which is modified with sulfonic acid groups, and the ion exchange capacity is 1.8~2.2 mmol / g.

[0030] Preferably, the method for preparing the lithium battery separator includes:

[0031] S1. Immerse the base film in a solution containing Ca. 2+ PO4 3- and in PAA's simulated body fluids;

[0032] S2. Apply a pulsed electric field (10 Hz, 0.5 V / μm) to induce hydroxyapatite to grow along the

[001] crystal orientation;

[0033] S3, mineralization time 30~60 minutes, temperature 37℃, finally forming a vertical hydroxyapatite nanowire array.

[0034] 10 Hz pulsed electric field drives Ca 2+ The HAP nanowires are oriented and migrated along the electric field lines to achieve vertical orientation growth. The reaction at 37°C and ambient pressure avoids thermal damage to the base film. The process takes only 30 minutes (90% shorter than the hydrothermal method) and reduces energy consumption by 70%.

[0035] A synergistic flame retardant system includes the electrolyte and the diaphragm. When the temperature exceeds 180°C, the flame retardant additives in the electrolyte exhibit flammability in Ce. 3+ Cross-linking occurs under catalysis, forming a physically interlocked structure between the generated SiO2 / C ceramic layer and the hydroxyapatite nanowires on the membrane surface. The interlocked interface area accounts for 70%–85% of the membrane surface area. The ceramic layer is embedded in the nanowire gaps (depth ≥ 100 nm), increasing the interfacial bonding energy to 200 J / m. 2 (Physical contact only 50 J / m) 2 To avoid high-temperature stratification, the interlocking structure covers 85% of the surface area, reducing the oxygen diffusion coefficient to 1×10⁻⁶. -14 m 2 / s (a single flame-retardant layer is 1×10) -12 m 2 / s), the thermal runaway trigger temperature rises to 280℃.

[0036] Compared with existing technologies, this electrolyte flame retardant additive and its lithium battery separator have the following beneficial effects:

[0037] 1. This invention provides an electrolyte flame retardant additive and its lithium battery separator. The flame retardant additive achieves phosphorus-free flame retardancy through self-crosslinking organosilicon quaternary ammonium salt. Acryloyloxy groups can polymerize and crosslink to form a ceramic barrier layer during thermal runaway. TFSI -Anions increase the lithium-ion transference number, solving the problem of deteriorated conductivity in traditional flame retardants.

[0038] 2. The present invention provides an electrolyte flame retardant additive and its lithium battery separator. The lithium battery separator provides ultra-high temperature mechanical support through a vertical HAP nanowire array. The bottom layer is designed with sulfonic acid group modified PBI-COFs proton channels, with an ionic conductivity of 0.82 mS / cm. The shrinkage strength is significantly improved compared with conventional aramid coating.

[0039] 3. The present invention provides an electrolyte flame retardant additive and its lithium battery separator, which achieves dual barrier through the ceramic layer-nanowire interlocking structure. The cross-linked ceramic layer inhibits the diffusion of combustible gases, and the HAP nanoarray blocks thermal shock, thereby raising the thermal runaway trigger temperature to above 280°C.

[0040] 4. The present invention provides an electrolyte flame retardant additive and its lithium battery separator. Ce(NO3)3 is added to the electrolyte to catalyze the crosslinking of organosilicon quaternary ammonium salt, thereby increasing the thickness of the ceramic layer to 5 μm. At the same time, CeO2 nanoparticles repair SEI film cracks, thereby achieving a high-temperature cycling capacity retention rate of 92.1%.

[0041] In summary, this invention provides an electrolyte flame retardant additive and its lithium battery separator. The flame retardant additive achieves phosphorus-free flame retardancy through self-crosslinking organosilicon quaternary ammonium salts, TFSI - Anions increase the lithium-ion transference number, solving the problem of deteriorated conductivity in traditional flame retardants. Through a breakthrough in process technology combining vertical HAP nanowire-PBI-COFs interlocking membrane and pulsed electric field-induced mineralization with cerium salt catalysis, the flame retardant safety, electrochemical performance and high-temperature stability are improved simultaneously, mitigating the problems of toxicity and conductivity degradation of traditional phosphorus-based flame retardants and high-temperature melting failure of polyolefin membranes. Detailed Implementation

[0042] The following are specific embodiments of the present invention, which further describe the technical solution of the present invention, but the present invention is not limited to these embodiments. Specific Implementation Example 1:

[0043] First, a flame retardant additive was synthesized by reacting 3-chloropropyltrimethoxysilane with N,N-dimethylacrylamide (molar ratio 1:1.05) at 60°C under nitrogen protection for 12 hours to generate a quaternary ammonium salt intermediate. The intermediate was then anion-exchanged with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (molar ratio 1:1) in ethanol / water (volume ratio 4:1) at 25°C for 6 hours. Vacuum distillation (60°C, -0.095 MPa) yielded a white solid. Next, a cerium nitrate electrolyte was prepared, with 1M LiPF6 in EC / DEC (volume ratio 3:7) as the base solution and 5 wt% of the synthesized flame retardant + 1.5 wt% Ce(NO3)3 as the additive. The mixture was magnetically stirred in an argon glove box at 25°C for 24 hours (moisture content <10 ppm). HAP nanowire membranes were prepared by immersing the base membrane in simulated body fluid and maintaining a constant temperature of 37°C. A pulsed electric field (10 Hz, 0.5 V / μm) was applied to induce HAP growth along

[001] . The membrane was mineralized for 45 minutes, rinsed with deionized water, and vacuum dried at 60°C. Base membrane: Celgard 2400 (PP / PE / PP three-layer). Simulated body fluid: 1.5×SBF (containing Ca). 2+ 2.5 mM, PO4 3- 1.0mM, Mg 2+ 1.5 mM, PAA 0.1 wt%.

[0044] Comparative Example 1:

[0045] The difference from Specific Example 1 is that a conventional electrolyte (1M LiPF6 EC / DEC + 5% triethyl phosphate TEP) is used, and the diaphragm is Celgard 2325.

[0046] Thermal polymerization behavior analysis (DSC test): According to GB / T 19466.2-2004 "Differential Scanning Calorimetry for Plastics", a DSC 214 Polyma (Netzsch) was used, with a heating rate of 10℃ / min (25℃→300℃), N2 (50 mL / min), and a sample volume of 5.0±0.1 mg (sealed aluminum crucible). The peak temperature of the exothermic peak (T) was recorded. peak ).

[0047] Localized overheating trigger test: The device consisted of a miniature heating element (2mm in diameter, 10W power); an infrared thermal imager (FLIRA700, accuracy ±1℃); and a pouch battery (NCM622 / graphite, capacity 2Ah). Procedure: The heating element was brought into contact with the center area of ​​the separator, and the temperature was raised to 200℃ within 5 seconds; the infrared camera recorded the electrode temperature distribution over 0-60 seconds (10fps); the heat diffusion area was analyzed (defined as the area ≥150℃).

[0048] Interlocking structure characterization (SEM-EDS): After needle penetration triggers thermal runaway, the battery is quenched with liquid nitrogen; the separator is disassembled and the cross-section is cut by focused ion beam (FIB). The depth of the SiO2 / C ceramic layer embedded with HAP nanowires is measured (average value taken from ≥5 points); the distribution of Si, Ca, and P elements is mapped to verify the interlocking interface.

[0049]

[0050] The above data shows that the flame retardant preferentially polymerizes at 205℃, 15℃ earlier than the diaphragm failure, thus forming an early fire barrier; Ce 3+ Catalysis interlocks the ceramic layer with HAP nanowires, increasing the oxygen diffusion path blocking efficiency to 85%; in contrast, due to the decomposition of phosphorus-based flame retardants and membrane melting, thermal runaway cannot be suppressed. Specific Implementation Example 2:

[0051] First, a flame retardant additive was synthesized by reacting 3-chloropropyltrimethoxysilane with N,N-dimethylacrylamide (molar ratio 1:1.05) at 60°C under nitrogen protection for 12 hours to generate a quaternary ammonium salt intermediate. This intermediate was then anion-exchanged with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (molar ratio 1:1) in ethanol / water (volume ratio 4:1) at 25°C for 6 hours. Vacuum distillation (60°C, -0.095 MPa) yielded a white solid. Next, a cerium nitrate electrolyte was prepared. The base solution was 1M LiPF6 in EC / DEC (volume ratio 3:7), and the additive was 5 wt% of the synthesized flame retardant + 1.5 wt% Ce(NO3)3. The mixture was magnetically stirred in an argon glove box at 25°C for 24 hours (moisture content <10 ppm). The test cell used a Li / / stainless steel blocking electrode.

[0052] Comparative Example 2:

[0053] The difference from Specific Example 2 is that a traditional flame-retardant electrolyte containing 5% triethyl phosphate (TEP) is used, and the test battery is a Li / / stainless steel blocking electrode.

[0054] Test method:

[0055] Ionic conductivity (EIS test): Electrochemical workstation (frequency range 0.1 Hz–1 MHz); measured at 25℃ and 120℃, conductivity calculation formula: σ=L / R / Aσ (L: electrode spacing, R: impedance, A: electrode area).

[0056] High temperature stability: After storing the electrolyte at 120℃ for 48 hours, centrifuge (10,000 rpm, 10 minutes), weigh the precipitate, and calculate the precipitation rate: precipitation rate = precipitate mass / total electrolyte mass * 100%.

[0057] Li+ Transference number: calculated using the potentiostatic polarization method (±10 mV), t + .

[0058]

[0059] As shown in the table above, the organosilicon quaternary ammonium salt flame retardant additive synthesized from 3-chloropropyltrimethoxysilane and N,N-dimethylacrylamide, along with an electrolyte system containing 1.5 wt% cerium nitrate, exhibited significantly superior electrochemical performance compared to traditional triethyl phosphate (TEP) flame retardants in Li / / stainless steel blocking electrode tests. Compared to the TEP system, which had a conductivity of 6.1 mS / cm, an ionic conductivity of 8.4 mS / cm at 25℃, and maintained at 5.2 mS / cm at 120℃ (a decrease of only 38%), the TEP system showed a conductivity reduction to 2.0 mS / cm (a decrease of 67%). Furthermore, after 48 hours of storage at 120℃, the precipitation rate was zero. The TEP system, due to decomposition, produced 5.8% precipitation, far exceeding the national standard limit of ≤2.0%, and the lithium-ion transference number increased to 0.63. This is due to the TFSI... - Weakly coordinating anions reduce Li + The TEP system had a resistance level of only 0.41; this result verifies that phosphorus-free organosilicon flame retardants pass TFSI. - High degree of dissociation maintains ion concentration, Ce 3+ The synergistic mechanism of catalytic crosslinking inhibiting high-temperature decomposition and CeO2 filling SEI cracks completely solves the core contradiction between the deterioration of conductivity and high-temperature stability of traditional phosphorus-based flame retardants while ensuring flame retardant safety, providing a reliable solution for the high-temperature fast charging requirements of high-energy-density lithium batteries. Specific Implementation Example 3:

[0060] Preparation of PBI-COFs substrate: PBI was dissolved in DMAC (10 wt%), and 1,3,5-tricarboxymethyl phloroglucinol (molar ratio 2:3) was added. The mixture was reacted at 80℃ for 12 hours to generate PBI-COFs. The mixture was then coated onto the substrate and cured at 120℃ for 2 hours to form a porous layer with a pore size of 2-5 nm (thickness accounting for 60%).

[0061] HAP nanowire mineralization:

[0062] Simulated body fluid: 1.5 × SBF (containing Ca) 2+ 2.5 mM, PO4 3- 1.0 mM, Mg 2+1.5 mM, PAA 0.1wt%); immersion in the base film, applying a pulsed electric field (10 Hz, 0.5 V / μm), mineralizing at 37℃ for 45 min; rinsing with deionized water, vacuum drying at 60℃ to obtain a vertical Mg-HAP nanowire array (diameter 30±5 nm, length 2±0.5 μm). Base film: Celgard2400 (PP / PE / PP).

[0063] Comparative Example 3:

[0064] The difference from Specific Example 3 is that it uses an aramid-coated diaphragm, commercially available Solupor® AS3330 (aramid coating thickness 5μm), with random, non-oriented fibers.

[0065] Comparative Example 4:

[0066] The difference from Specific Embodiment 3 is that the diaphragm uses Celgard 2325, a single-layer PP film (25μm thick), and has no functional coating.

[0067] Test process:

[0068] High-temperature heat shrinkage test (GB / T 13542-2009): Using a constant temperature oven (accuracy ±1℃) and digital vernier calipers (accuracy 0.01mm), first cut a diaphragm sample of 100×100 mm and mark the four corner reference points; lay it flat on a ceramic plate and place it in a 400℃ oven for 30 minutes; after cooling, measure the dimensions and calculate the area shrinkage rate.

[0069] Shrinkage rate (%) = (L0) 2 -L1 2 ) / L0 2 ×100%

[0070] (L0: Initial side length; L1: Processed side length);

[0071] Ionic conductivity: Measurement of Li at 120℃ using the blocked electrode method + Electrical conductivity;

[0072] Puncture strength: Universal testing machine (needle diameter 1mm, speed 2mm / s), record the maximum puncture force.

[0073]

[0074] Results: The HAP nanowire array enabled the membrane to maintain its structural integrity at 400℃ (thermal shrinkage rate of 3.8%), while Mg... 2+ Doping increases the ionic conductivity to 0.82 mS / cm (164% higher than aramid membranes), meeting the fast charging requirements of high-energy-density batteries.

[0075] In specific embodiment three, the biomimetic mineralized membrane, through the synergistic effect of a vertical Mg-HAP nanowire array (diameter 30±5 nm, length 2±0.5 μm) induced by a pulsed electric field and an underlying sulfonic acid-modified PBI-COF porous layer (pore size 2-5 nm, IEC=2.0 mmol / g), exhibits a thermal shrinkage rate as low as 3.8% at 400℃, exceeding the 62.5% of aramid membranes and 100% of pure PP membranes by over 95%. This is attributed to the ultra-high melting point of HAP at 1650℃ and the orientational growth along the 001 crystal orientation, which suppresses thermal deformation. Its ionic conductivity at 120℃ reaches 0.82 mS / cm, a 164% improvement over the 0.31 mS / cm of aramid membranes, stemming from the proton hopping channels of PBI-COFs and the interaction between Mg and HAP. 2+ Doping reduces interfacial impedance to 50 Ω·cm 2 Dual optimization; thanks to the mechanical interlocking structure of the vertical nanowires and Mg 2+ It enhances crystal hardness, achieving a puncture resistance of up to 498 MPa, a 63% improvement over aramid's 305 MPa. This solves the cascading problems of high-temperature shrinkage, ion conduction degradation, and insufficient mechanical strength in traditional separators, providing a core barrier to support the 280℃ thermal runaway threshold of lithium batteries. Specific Implementation Example 4:

[0076] First, a flame retardant additive was synthesized by reacting 3-chloropropyltrimethoxysilane with N,N-dimethylacrylamide (molar ratio 1:1.05) at 60°C under nitrogen protection for 12 hours to generate a quaternary ammonium salt intermediate. The intermediate was then anion-exchanged with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (molar ratio 1:1) in ethanol / water (volume ratio 4:1) at 25°C for 6 hours. Vacuum distillation (60°C, -0.095 MPa) yielded a white solid. Next, a cerium nitrate electrolyte was prepared, with 1M LiPF6 in EC / DEC (volume ratio 3:7) as the base solution and 5 wt% of the synthesized flame retardant + 1.5 wt% Ce(NO3)3 as the additive. The mixture was magnetically stirred in an argon glove box at 25°C for 24 hours (moisture content <10 ppm). HAP nanowire membranes were prepared by immersing the base membrane in simulated body fluid and maintaining a constant temperature of 37°C. A pulsed electric field (10 Hz, 0.5 V / μm) was applied to induce HAP growth along

[001] . The membrane was mineralized for 45 minutes, rinsed with deionized water, and vacuum dried at 60°C. Base membrane: Celgard 2400 (PP / PE / PP three-layer). Simulated body fluid: 1.5×SBF (containing Ca). 2+ 2.5 mM, PO4 3- 1.0mM, Mg 2+ 1.5 mM, PAA 0.1 wt%. Battery assembly: Positive electrode NCM622 (loading 12 mg / cm³). 2 The negative electrode is graphite (loading 8 mg / cm³).2 ).

[0077] Comparative Example 5:

[0078] The difference from Specific Example 4 is that a conventional system is used, namely, a commercial flame retardant containing 5% triethyl phosphate (TEP), the electrolyte is the same base liquid as in Specific Example 4, with an additional 5% TEP (cerium-free salt), and the diaphragm is commercial Celgard 2325 (single-layer PP membrane, no functional coating).

[0079] Test process:

[0080] Cyclic performance: 300 cycles at a voltage window of 2.5–4.3 V, 2C rate, and 60°C.

[0081] Capacity retention rate = (300th discharge capacity / First discharge capacity) × 100%.

[0082] Needle penetration test: According to GB 38031-2020, a fully charged battery (4.3V) is penetrated by a φ3 mm steel needle at a speed of 25 mm / s. The criterion for judgment is whether it catches fire / explodes (pass rate statistics).

[0083] Thermal runaway trigger temperature (ARC test): The thermal runaway trigger temperature is determined by an adiabatic calorimeter.

[0084]

[0085] The synergistic flame-retardant system in Specific Example 4 (containing 5 wt% organosilicon quaternary ammonium salt / Ce(NO3)3 electrolyte + vertical Mg-HAP nanowire separator) exhibited safety and cycle performance in NCM622 / graphite pouch cells. At 60℃, it maintained a capacity retention of 92.1% after 300 cycles, a 17.7% improvement over the traditional TEP system's 78.3%. This improvement stems from the CeO2 nanoparticles repairing SEI cracks and TFSI. - Anion-stabilized electrode interface; 100% pass needle penetration test with no ignition / explosion, while 60% of traditional systems fail due to Ce above 180℃. 3+ The catalytically formed SiO2 / C ceramic layer and HAP nanowires have an interlocking structure with an interlocking area >70%, which suppresses the oxygen diffusion coefficient to 1×10⁻⁶. -14 m² / s, instantaneously suffocating flames; the thermal runaway trigger temperature measured by the adiabatic calorimeter (ARC) reached 283℃, a jump of 101℃ compared to the traditional system (182℃), which is attributed to the dual barrier of the ceramic layer blocking the diffusion of combustible gases and the HAP nanoarray reflecting thermal shock.

[0086] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A synergistic flame retardant system, characterized in that, Including electrolyte flame retardant additives and lithium battery separators; The general chemical structural formula of the electrolyte flame retardant additive is as follows: [(CH3O)3Si-(CH2)3-N + (CH3)2-R]X - ; Wherein, R is a C2-C6 alkyl chain containing an acryloyloxy group, and X... - For TFSI - ; The electrolyte flame retardant additive has a mass fraction of 1% to 5% in the electrolyte and does not contain halogens or phosphorus. The preparation method of the electrolyte flame retardant additive includes the following steps: S1. 3-Chloropropyltrimethoxysilane was reacted with N,N-dimethylacrylamide at 60°C for 12 hours to generate a quaternary ammonium salt intermediate; S2. The intermediate was anion exchanged with LiTFSI in a mixed solvent of ethanol and water at a temperature of 25°C for 6 hours. S3. The solvent was removed by vacuum distillation to obtain a white solid; The lithium battery separator comprises a polyolefin substrate and a surface functional coating; The surface functional coating consists of a base layer and a surface layer; The bottom layer of the surface functional coating is a porous adhesive layer containing PBI-COFs, with a pore size of 2~5 nm, accounting for 60% of the thickness of the surface functional coating; Among them, the surface functional coating consists of a vertically oriented array of hydroxyapatite nanowires with a diameter of 20~50nm and a length of 1~3μm. The electrolyte flame retardant additive, when the temperature exceeds 180°C, in Ce 3+ Crosslinking occurs under the catalysis of [agent name] to generate a SiO2 / C ceramic layer. The hydroxyapatite nanowire array on the surface of the lithium battery separator is used to form a physically interlocked structure with the SiO2 / C ceramic layer generated by crosslinking with the electrolyte flame retardant additive when the temperature exceeds 180°C.

2. The synergistic flame retardant system as described in claim 1, characterized in that, The R group of the flame retardant additive is 2-acryloyloxyethyl or 3-acryloyloxypropyl.

3. The synergistic flame retardant system as described in claim 1, characterized in that, The hydroxyapatite nanowires in the lithium battery separator contain 5-10 at% Mg. 2+ The crystal structure is preferentially oriented along the c-axis, and the nanowire spacing is 100~200 nm.

4. The synergistic flame retardant system as described in claim 1, characterized in that, The bottom layer of the surface functional coating in the lithium battery separator includes one-dimensional channels loaded with phosphoric acid, and the surface of the channels is modified with sulfonic acid groups, with an ion exchange capacity of 1.8~2.2 mmol / g.

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