Application of preparation of morphology controllable basic magnesium sulfate flame retardant in lithium battery diaphragm based on seawater bitter brine

Abstract

The application discloses an application of a morphology-controllable basic magnesium sulfate flame retardant prepared based on seawater bitter brine in a lithium battery diaphragm and belongs to the technical fields of seawater bitter brine resource utilization, magnesium-based flame retardant materials and lithium battery diaphragms. The seawater bitter brine is used as a magnesium source and a sulfur source, fiber-like basic magnesium sulfate whiskers, fan-shaped basic magnesium sulfate whiskers and granular basic magnesium sulfate can be prepared by regulating and controlling the types of crystal growth habit agents, the molar ratio of magnesium ions and the crystal growth habit agents and hydrothermal reaction conditions. The fiber-like basic magnesium sulfate whiskers can be obtained under the regulation of ethylenediaminetetraacetic acid. The fiber-like basic magnesium sulfate whiskers are prepared into an oil-based ceramic slurry and are single-sidedly coated on the surface of a polyolefin porous substrate to form a thin layer of a flame retardant coating with a thickness of about 3 microns. The fiber-like basic magnesium sulfate whiskers form a continuous and loose whisker lapping network in the coating, which is beneficial to improving the diaphragm porosity, electrolyte retention rate, ion conductivity and lithium ion transference number and reducing the heat release rate peak value. The application realizes the high-value utilization of magnesium and sulfur resources in the seawater bitter brine and takes into account the electrochemical performance and thermal safety performance of the lithium battery diaphragm under the condition of a low coating thickness.

Description

Technical Field

[0001] This invention belongs to the field of flame retardant and lithium battery separator technology, specifically relating to the application of a morphology-controllable basic magnesium sulfate flame retardant prepared from seawater bittern in lithium battery separators. Background Technology

[0002] Bitter liquor is a high-salt byproduct produced during sea salt production and comprehensive utilization of seawater resources. It contains abundant elements such as magnesium, sulfur, potassium, calcium, sodium, chlorine, and bromine. Improper handling can lead to water and soil salinization and resource waste. Rational treatment and utilization of bitter liquor can not only alleviate environmental pressure but also achieve the recycling of elements such as magnesium and sulfur. Therefore, the resource utilization and high-value utilization of bitter liquor are of great significance. Magnesium, as an important element, has wide applications in flame retardants and inorganic reinforcing materials. Basic magnesium sulfate materials contain structural units such as magnesium, sulfate, hydroxyl groups, and water of crystallization. When heated, it can release water vapor to dilute combustible gases and lower the surface temperature of the material. After combustion, it can generate a highly thermally stable MgO inorganic skeleton, playing a role in heat insulation and oxygen barrier. At the same time, whisker-like basic magnesium sulfate also has a high aspect ratio, which can play a role in mechanical reinforcement and skeletal support in polymer materials.

[0003] Existing magnesium-based flame retardant materials based on bittern are mostly concentrated in systems such as basic magnesium carbonate or magnesium hydroxide. However, for basic magnesium sulfate, there is still a lack of systematic solutions on how to directly prepare fibrous whiskers, fan-shaped whiskers and granular materials from seawater bittern, and how to combine different morphologies with the thermal safety, electrochemical and interfacial stability of lithium battery separators.

[0004] The lithium-ion battery separator is a crucial component determining battery safety and lifespan. Commercial polyolefin separators suffer from inherent defects such as poor electrolyte wettability, high ion transport impedance, high-temperature shrinkage, and flammability. Existing ceramic separators typically rely on thicker coatings, sacrificing electrochemical performance for higher thermal safety. This leads to increased separator thickness, areal density, and the proportion of inactive components, which is detrimental to high-energy-density battery design.

[0005] Therefore, the ability to controllably prepare basic magnesium sulfate flame retardants with different morphologies using seawater bittern as raw material, and to simultaneously improve the electrochemical performance and thermal safety performance of lithium batteries by constructing a whisker interpenetrating network under the premise of low coating thickness, is a key technological direction for the battery separator industry to pursue both high safety and high energy density. Summary of the Invention

[0006] The purpose of this invention is to provide an application of a morphology-controllable basic magnesium sulfate flame retardant prepared from seawater bittern in lithium battery separators, in order to solve the problems of low added value of seawater bittern magnesium resources, difficulty in controlling the morphology of basic magnesium sulfate materials, and the inability to simultaneously achieve the desired coating thickness, electrochemical performance, and thermal safety performance of lithium battery separators.

[0007] To address the problems existing in the prior art, this invention discovers that in the complex ionic system of seawater bittern, the crystal growth habit control method using EDTA-2Na as described in patent CN 121584149 B cannot obtain pure-phase basic magnesium sulfate, but instead forms a mixed product of Mg(OH)2 and basic magnesium sulfate; only by using EDTA and controlling Mg... 2+ A molar ratio of 20:1 with EDTA is necessary to suppress the formation of fan-shaped whiskers and Mg(OH)2 impurity phases, resulting in basic magnesium sulfate whiskers with higher exposure of surface sulfate and oxygen-containing structural units and purer fibrous morphology. Therefore, this invention not only achieves high-value utilization of magnesium and sulfur resources in seawater bittern but also enables the obtained whiskers to form a more uniform and loosely overlapping network in a 3 μm single-sided coating. Under the same 3 μm single-sided coating conditions, compared to the whiskers prepared by patent CN 121584149 B, the whisker-modified membrane obtained by this invention exhibits superior performance in terms of porosity, electrolyte retention, ionic conductivity, lithium-ion transference number, and peak heat release rate.

[0008] This invention relates to a method for preparing fibrous basic magnesium sulfate whiskers based on seawater bittern, comprising the following steps:

[0009] Step 1: Take 150 mL of seawater bittern, add crystal growth habit agent, and stir for 30 min until well mixed. This solution is designated as solution A. The molar ratio of magnesium ions to crystal growth habit agent is controlled at 20:1.

[0010] The crystal growth habit agent mentioned is ethylenediaminetetraacetic acid (EDTA).

[0011] Step 2: Prepare a 1 mol / L sodium hydroxide solution at room temperature, and take 150 mL of the solution as solution B.

[0012] Step 3: Slowly add solution B dropwise to solution A, which is being stirred, and continue stirring for 30 minutes to form a homogeneous precursor solution. Transfer the mixed solution to a polytetrafluoroethylene-lined hydrothermal reactor and react hydrothermally at 180 °C for 10 hours. After the reactor has cooled naturally to room temperature, the product is filtered and repeatedly washed with deionized water until no Cl- is detected. - and SO4 2- The residue was removed, and the product was dried overnight in a vacuum oven at 80 °C to obtain fibrous basic magnesium sulfate whiskers.

[0013] The fibrous basic magnesium sulfate whiskers have a length of 30-68 μm, a diameter of 0.5-1.5 μm, and an aspect ratio of 35-62.

[0014] This invention relates to a method for preparing fan-shaped basic magnesium sulfate whiskers based on seawater bittern, comprising the following steps:

[0015] Step 1: Take 150 mL of seawater bittern as solution A.

[0016] Step 2: Prepare a 1 mol / L sodium hydroxide solution at room temperature, and take 150 mL of the solution as solution B.

[0017] Step 3: Slowly add solution B dropwise to solution A, which is being stirred, and continue stirring for 30 minutes to form a homogeneous precursor solution. Transfer the mixed solution to a polytetrafluoroethylene-lined hydrothermal reactor and react hydrothermally at 180 °C for 10 hours. After the reactor has cooled naturally to room temperature, the product is filtered and repeatedly washed with deionized water until no Cl- is detected. - and SO4 2- The residue was removed, and the product was dried overnight in a vacuum oven at 80 °C to obtain fan-shaped basic magnesium sulfate whiskers.

[0018] The fan-shaped basic magnesium sulfate whiskers have a length of 50-70 μm and a fan-shaped end width of 8-15 μm.

[0019] This invention relates to a method for preparing granular basic magnesium sulfate based on seawater bittern, comprising the following steps:

[0020] The fibrous basic magnesium sulfate whiskers prepared from seawater bittern were pulverized for 10 minutes using a high-speed pulverizer and then passed through a 200-mesh sieve to obtain granular basic magnesium sulfate.

[0021] The granular basic magnesium sulfate was observed to be 200-500 nm in size using SEM.

[0022] This invention relates to a method for preparing thin-film flame-retardant membranes of basic magnesium sulfate with different morphologies, comprising the following steps:

[0023] Step 1: In the prepared oily ceramic slurry, the oil-based solvent has a mass percentage of 70-80 wt%, the ceramic filler has a mass percentage of 15-20 wt%, the binder has a mass percentage of 1-2.5 wt%, the stabilizer has a mass percentage of 0.5-1 wt%, the dispersant has a mass percentage of 0.5-1 wt%, and the defoamer has a mass percentage of 0.5-1 wt%. According to the formula, mix the ceramic filler, stabilizer, and binder to obtain a dry powder. Mix at low speed for 30-60 min, with a stirring rate of 100-300 rpm / min.

[0024] Step 2: Add an oily solvent to the dry powder and stir at low speed for 30-60 minutes to obtain a mixed solution. The stirring speed is 300-500 rpm / min.

[0025] Step 3: Add dispersant to the mixed solution and disperse at high speed for 1-3 hours, with a stirring rate of 1000-1500 rpm / min.

[0026] Step 4: Add defoamer to the mixed solution, disperse at high speed for 0.5~1.5 h, and stir at a speed of 1000~1500 rpm / min to obtain oily ceramic slurry.

[0027] Step 5: Use a wire-type automatic coating machine to coat one side of the polyolefin porous substrate with oily ceramic slurry to form an oily coating. After drying, a thin flame-retardant membrane based on basic magnesium sulfate with different morphologies can be obtained. The coating thickness is about 3 μm.

[0028] The ceramic filler is one of fibrous basic magnesium sulfate whiskers, fan-shaped basic magnesium sulfate whiskers, and granular basic magnesium sulfate; the binder is polyvinylidene fluoride; the stabilizer is polyamide wax; the dispersant is sodium fatty alcohol ether sulfate; the defoamer is polyether-modified organosilicon; and the oily solvent is N-methylpyrrolidone.

[0029] The polyolefin porous substrate is a single layer of polyethylene or polypropylene, or a mixed layer of polyethylene and polypropylene, or a multilayer of polyethylene or polypropylene. A single-layer polypropylene membrane is preferred. The thickness of the polyolefin membrane is 1-30 μm, preferably 20-30 μm; the porosity of the membrane is 10%-70%, preferably 30%-50%.

[0030] Compared with the prior art, the beneficial effects of the present invention are reflected in:

[0031] 1. This invention uses seawater bittern as a magnesium and sulfur source to prepare basic magnesium sulfate materials, realizing the high-value utilization of magnesium and sulfur resources in seawater bittern. The seawater bittern raw material is abundant, the preparation process does not rely on high-purity magnesium salts and sulfate reagents, the process flow is simple, and it has environmentally friendly, resource recycling, and large-scale application value.

[0032] 2. This invention achieves controllable morphology of fibrous basic magnesium sulfate whiskers, fan-shaped basic magnesium sulfate whiskers, and granular basic magnesium sulfate through crystal growth habit agent regulation and mechanical crushing and sieving. Among them, the fibrous basic magnesium sulfate whiskers contain very few fan-shaped whiskers, and their length and aspect ratio are more suitable for single-sided thin-layer coating of about 3 μm. This enables the formation of a more uniform and loose whisker overlap network on the surface of the polyolefin separator, thereby improving the porosity, electrolyte retention, ionic conductivity, and lithium-ion transference number of the composite separator.

[0033] 3. This invention formulates an oil-based ceramic slurry using basic magnesium sulfate, binder, stabilizer, dispersant, and defoamer, and coats one side of the slurry onto the surface of a porous polyolefin substrate to form a thin flame-retardant coating of approximately 3 μm. This method eliminates the need for first coating an aqueous ceramic layer and then an oil-based adhesive layer, simplifying the coating process. Furthermore, the low water content of the oil-based ceramic slurry helps reduce the moisture content of the separator, improving the cycle stability of the lithium-ion battery. Additionally, the good adhesion between basic magnesium sulfate and polyvinylidene fluoride further enhances the coating's stability.

[0034] 4. The composite separator prepared by this invention can achieve both electrochemical performance and thermal safety performance under the premise of low coating thickness and single-sided coating. Under the same 3 μm single-sided coating conditions, compared with the basic magnesium sulfate whiskers prepared by patent CN 121584149 B, the fibrous basic magnesium sulfate whisker modified separator obtained by this invention exhibits better performance in terms of porosity, electrolyte retention rate, ionic conductivity, lithium ion transference number, and peak heat release rate. This indicates that the whiskers obtained by this invention are not simply raw material substitution products of the whiskers in the previous invention, but have substantial differences in morphological uniformity, surface elemental composition, and thin-layer coating construction ability.

[0035] 5. When the concentration of the alkali solution is 1.0 mol / L, the crystal form of the product matches the standard basic magnesium sulfate card better. Therefore, the preferred concentration of the alkali solution is 1.0 mol / L.

[0036] When the hydrothermal temperature is 180 ℃, the dissolution-recrystallization of Mg(OH)2 and SO42- 2- The participation in the reconstruction process is promoted, which is conducive to the directional growth of basic magnesium sulfate; after further heating, the local high alkalinity and rapid nucleation enhance the competitive generation of Mg(OH)2, so the product is transformed into a mixed phase of basic magnesium sulfate and Mg(OH)2. Therefore, the hydrothermal temperature is preferably 180 ℃.

[0037] When the hydrothermal time is extended to 15 hours, SO4 inside the whiskers... 2- It will be released and accompanied by dehydration and recrystallization, and the basic magnesium sulfate crystal form is destroyed, thus transforming into the more stable Mg(OH)2. Therefore, the reaction time is not necessarily better the longer it is. Considering the energy consumption issue, the hydrothermal time is preferably 10 h.

[0038] When n(Mg) 2+ When n(EDTA) is less than 20:1 or greater than 20:1, Mg(OH)2 is inevitably formed as a product. Only when n(Mg)2 is greater than 20:1 will Mg(OH)2 be formed as a product. 2+ Pure basic magnesium sulfate can only be prepared using bittern as a raw material when the ratio of EDTA to n is 20:1.

[0039] When the crystal growth habit agent is EDTA-2Na, the product prepared from seawater bittern is a mixture of Mg(OH)2 and basic magnesium sulfate, while when the crystal growth habit agent is EDTA, the product is pure phase basic magnesium sulfate. Attached Figure Description

[0040] Figure 1 SEM and EDS-mapping images of the fibrous basic magnesium sulfate whiskers prepared according to the present invention.

[0041] Figure 2 SEM image of the fan-shaped basic magnesium sulfate whiskers prepared according to the present invention.

[0042] Figure 3 SEM image of the granular basic magnesium sulfate prepared according to the present invention.

[0043] Figure 4 XRD patterns of fibrous basic magnesium sulfate whiskers, fan-shaped basic magnesium sulfate whiskers, and granular basic magnesium sulfate prepared according to the present invention.

[0044] Figure 5 SEM image of the cross-section of a thin-film flame-retardant diaphragm based on fibrous basic magnesium sulfate whiskers prepared for this invention.

[0045] Figure 6 Digital photographs of the electrolyte contact angles for comparative examples and embodiments of the present invention.

[0046] Figure 7 The graph shows the cycle performance of the LiFePO4 / separator / Li batteries assembled in the comparative examples and embodiments of this invention.

[0047] Figure 8 The graph shows the heat release rate curves of the comparative examples and embodiments of the present invention.

[0048] Figure 9 This is a graph showing the total heat release of the comparative examples and embodiments of the present invention.

[0049] Figure 10 The XRD patterns of basic magnesium sulfate prepared from seawater bittern at different alkaline concentrations according to the present invention are shown.

[0050] Figure 11 The XRD patterns of basic magnesium sulfate prepared from seawater bittern at different hydrothermal temperatures according to this invention are shown.

[0051] Figure 12 The XRD patterns of basic magnesium sulfate prepared from seawater bittern under different hydrothermal times according to the present invention are shown.

[0052] Figure 13 The XRD patterns are of basic magnesium sulfate prepared from seawater bittern under different molar ratios of magnesium ions and crystal growth habit agents according to the present invention.

[0053] Figure 14 The XRD patterns of basic magnesium sulfate prepared from seawater bittern under different crystal growth habit agents according to the present invention are shown.

[0054] Figure 15 SEM and EDS-mapping images of basic magnesium sulfate whiskers prepared based on patent CN 121584149 B. Detailed Implementation

[0055] The following examples further illustrate the above-described content of the present invention, but it should not be construed as limiting the scope of the subject matter of the present invention to the following examples. All technologies implemented based on the above-described content of the present invention fall within the scope of the present invention.

[0056] Example 1

[0057] The preparation method of fibrous basic magnesium sulfate whiskers based on seawater bittern is as follows:

[0058] Step 1: Take 150 mL of seawater bittern, add EDTA, and control n(Mg) 2+ ):n(EDTA)=20:1, stir for 30 min until well mixed, and record as solution A.

[0059] Step 2: Prepare a 1 mol / L sodium hydroxide solution at room temperature, and take 150 mL of the solution as solution B.

[0060] Step 3: Slowly add solution B dropwise to solution A, which is being stirred, and continue stirring for 30 minutes to form a homogeneous precursor solution. Transfer the mixed solution to a polytetrafluoroethylene-lined hydrothermal reactor and react hydrothermally at 180 °C for 10 hours. After the reactor has cooled naturally to room temperature, the product is filtered and repeatedly washed with deionized water until no Cl- is detected. - and SO4 2- The residue was removed, and the product was dried overnight in a vacuum oven at 80 °C to obtain fibrous basic magnesium sulfate whiskers.

[0061] The seawater bittern was sourced from Tianjin Haijing Technology Development Co., Ltd. Elemental composition analysis of the seawater bittern was performed using X-ray fluorescence spectrometry (Table 1), revealing the presence of elements such as Cl, K, S, Mg, Ca, Na, and Br. Further quantitative analysis of the main ions in the original bittern solution was conducted using inductively coupled plasma atomic emission spectrometry, measuring Mg... 2+ and SO4 2- The molar concentrations were 0.889 and 0.398 mol / L, respectively. Furthermore, the seawater bittern used in this invention is not limited to a single manufacturer; preferably, when the Mg content in the seawater bittern... 2+The molar concentration of SO4 is 0.8-1.0 mol / L. 2- When the molar concentration is 0.3-0.4 mol / L, it can be used as a raw material for the preparation of basic magnesium sulfate whiskers in this invention.

[0062] Table 1. Element Content of Seawater Bitter

[0063]

[0064] The preparation method of thin-film flame-retardant membrane based on fibrous basic magnesium sulfate whiskers is as follows:

[0065] Step 1: Mix 2.95 g of fibrous basic magnesium sulfate whiskers, 0.12 g of polyamide wax and 0.42 g of polyvinylidene fluoride to obtain a dry powder. Stir the mixture at low speed for 30-60 min, with a stirring rate of 100-300 rpm / min.

[0066] Step 2: Add 15 mL of N-methylpyrrolidone to the dry powder and stir at low speed for 30-60 min to obtain a mixed solution. The stirring speed is 300-500 rpm / min.

[0067] Step 3: Add 0.16 g of sodium fatty alcohol ether sulfate to the mixed solution and disperse at high speed for 1-3 h with a stirring rate of 1000-1500 rpm / min.

[0068] Step 4: Add 0.12 g of polyether-modified organosilicon to the mixed solution, disperse at high speed for 0.5~1.5 h, and stir at a speed of 1000~1500 rpm / min to obtain the oily ceramic slurry.

[0069] Step 5: Use a wire-type automatic coating machine to coat one side of the Celgard 2500 membrane with an oily ceramic slurry to form an oily coating. After vacuum drying at 60 °C, a thin flame-retardant membrane based on fibrous basic magnesium sulfate whiskers can be obtained. The coating thickness is about 3 μm.

[0070] Example 2

[0071] The preparation method of sector-shaped basic magnesium sulfate whiskers based on seawater bittern is as follows:

[0072] Step 1: Take 150 mL of seawater bittern and denote it as solution A.

[0073] Step 2: Prepare a 1 mol / L sodium hydroxide solution at room temperature, and take 150 mL of the solution as solution B.

[0074] Step 3: Slowly add solution B dropwise to solution A, which is being stirred, and continue stirring for 30 minutes to form a homogeneous precursor solution. Transfer the mixed solution to a polytetrafluoroethylene-lined hydrothermal reactor and react hydrothermally at 180 °C for 10 hours. After the reactor has cooled naturally to room temperature, the product is filtered and repeatedly washed with deionized water until no Cl- is detected. - and SO4 2- The residue was removed, and the product was dried overnight in a vacuum oven at 80 °C to obtain fan-shaped basic magnesium sulfate whiskers.

[0075] The preparation method of thin-film flame-retardant membrane based on fan-shaped basic magnesium sulfate whiskers is as follows:

[0076] Step 1: Mix 2.95g of fan-shaped basic magnesium sulfate whiskers, 0.12g of polyamide wax and 0.42g of polyvinylidene fluoride to obtain a dry powder. Stir the mixture at low speed for 30~60 min, with a stirring rate of 100~300 rpm / min.

[0077] Step 2: Add 15 mL of N-methylpyrrolidone to the dry powder and stir at low speed for 30-60 min to obtain a mixed solution. The stirring speed is 300-500 rpm / min.

[0078] Step 3: Add 0.16 g of sodium fatty alcohol ether sulfate to the mixed solution and disperse at high speed for 1-3 h with a stirring rate of 1000-1500 rpm / min.

[0079] Step 4: Add 0.12 g of polyether-modified organosilicon to the mixed solution, disperse at high speed for 0.5~1.5 h, and stir at a speed of 1000~1500 rpm / min to obtain the oily ceramic slurry.

[0080] Step 5: Use a wire-type automatic coating machine to coat one side of the Celgard 2500 diaphragm with an oily ceramic slurry to form an oily coating. After vacuum drying at 60 °C, a thin flame-retardant diaphragm based on fan-shaped basic magnesium sulfate whiskers can be obtained. The coating thickness is about 3 μm.

[0081] Example 3

[0082] The preparation method of granular basic magnesium sulfate based on seawater bittern is as follows:

[0083] The fibrous basic magnesium sulfate whiskers prepared from seawater bittern were pulverized for 10 minutes using a high-speed pulverizer and then passed through a 200-mesh sieve to obtain granular basic magnesium sulfate.

[0084] The preparation method of thin-layer flame-retardant membrane based on granular basic magnesium sulfate is as follows:

[0085] Step 1: Mix 2.95g of granular basic magnesium sulfate, 0.12g of polyamide wax and 0.42g of polyvinylidene fluoride to obtain a dry powder. Stir the mixture at low speed for 30~60 min, with a stirring rate of 100~300 rpm / min.

[0086] Step 2: Add 15 mL of N-methylpyrrolidone to the dry powder and stir at low speed for 30-60 min to obtain a mixed solution. The stirring speed is 300-500 rpm / min.

[0087] Step 3: Add 0.16 g of sodium fatty alcohol ether sulfate to the mixed solution and disperse at high speed for 1-3 h with a stirring rate of 1000-1500 rpm / min.

[0088] Step 4: Add 0.12 g of polyether-modified organosilicon to the mixed solution, disperse at high speed for 0.5~1.5 h, and stir at a speed of 1000~1500 rpm / min to obtain the oily ceramic slurry.

[0089] Step 5: Use a wire-type automatic coating machine to coat one side of the Celgard 2500 diaphragm with an oily ceramic slurry to form an oily coating. After vacuum drying at 60 °C, a thin flame-retardant diaphragm based on granular basic magnesium sulfate is obtained. The coating thickness is approximately 3 μm.

[0090] Example 4

[0091] The preparation methods of basic magnesium sulfate based on seawater bittern at different alkaline concentrations are as follows:

[0092] Step 1: Take 150 mL of seawater bittern, add EDTA, and control n(Mg) 2+ ):n(EDTA)=20:1, stir for 30 min until well mixed, and record as solution A.

[0093] Step 2: Prepare sodium hydroxide solutions of 0.5, 0.7, 0.8, 0.9, 1.0 and 1.1 mol / L at room temperature, and take 150 mL of each solution as solution B.

[0094] Step 3: Slowly add solutions B of different concentrations dropwise to solution A, which is being stirred, and continue stirring for 30 minutes to form a homogeneous precursor solution. Transfer the mixed solution to a polytetrafluoroethylene-lined hydrothermal reactor and react hydrothermally at 180 °C for 10 h. After the reactor has cooled naturally to room temperature, the product is filtered and repeatedly washed with deionized water until no Cl- is found. - and SO4 2- The residue was removed, and the product was dried overnight in a vacuum oven at 80 °C to obtain fibrous basic magnesium sulfate whiskers based on seawater bittern at different alkaline concentrations.

[0095] Example 5

[0096] The preparation methods of basic magnesium sulfate based on seawater bittern under different hydrothermal temperatures are as follows:

[0097] Step 1: Take 150 mL of seawater bittern, add EDTA, and control n(Mg) 2+ ):n(EDTA)=20:1, stir for 30 min until well mixed, and record as solution A.

[0098] Step 2: Prepare a 1.0 mol / L sodium hydroxide solution at room temperature, and take 150 mL of the solution as solution B.

[0099] Step 3: Slowly add solution B dropwise to solution A, which is being stirred, and continue stirring for 30 min to form a homogeneous precursor solution. Transfer the mixed solution to a PTFE-lined hydrothermal reactor and react it hydrothermally at 140, 160, 170, 180, 190, and 200 °C for 10 h, respectively. After the reactor has cooled naturally to room temperature, the product is filtered and repeatedly washed with deionized water until no Cl is present. - and SO4 2- The residue was dried overnight in a vacuum oven at 80 °C to obtain fibrous basic magnesium sulfate whiskers based on seawater bittern at different hydrothermal temperatures.

[0100] Example 6

[0101] The preparation methods of basic magnesium sulfate based on seawater bittern under different hydrothermal times are as follows:

[0102] Step 1: Take 150 mL of seawater bittern, add 2.48 g of EDTA, stir for 30 min until well mixed, and record as solution A.

[0103] Step 2: Prepare a 1.0 mol / L sodium hydroxide solution at room temperature, and take 150 mL of the solution as solution B.

[0104] Step 3: Slowly add solution B dropwise to solution A, which is being stirred, and continue stirring for 30 min to form a homogeneous precursor solution. Transfer the mixed solution to a PTFE-lined hydrothermal reactor and react it at 180 °C for 1, 4, 7, 10, 12, and 15 h, respectively. After the reactor has cooled naturally to room temperature, the product is filtered and repeatedly washed with deionized water until no Cl- is found. - and SO4 2- The residue was removed, and the product was dried overnight in a vacuum oven at 80 °C to obtain fibrous basic magnesium sulfate whiskers based on seawater bittern at different hydrothermal times.

[0105] Example 7

[0106] The following are methods for preparing basic magnesium sulfate based on seawater bittern under different molar ratios of magnesium ions to crystal growth habit agents:

[0107] Step 1: Take 150 mL of seawater bittern, add EDTA, where n(Mg) 2+ The ratios of EDTA were 0, 10:1, 20:1, 30:1, 40:1 and 50:1, respectively. The mixture was stirred for 30 min until homogeneous and denoted as solution A.

[0108] Step 2: Prepare a 1.0 mol / L sodium hydroxide solution at room temperature, and take 150 mL of the solution as solution B.

[0109] Step 3: Slowly add solution B dropwise to solution A, which is being stirred, and continue stirring for 30 min to form a homogeneous precursor solution. Transfer the mixed solution to a polytetrafluoroethylene-lined hydrothermal reactor and react it hydrothermally at 180 °C for 10 h. After the reactor has cooled naturally to room temperature, the product is filtered and repeatedly washed with deionized water until no Cl is found. - and SO4 2- The residue was removed, and the product was dried overnight in a vacuum oven at 80 °C to obtain fibrous basic magnesium sulfate whiskers based on seawater bittern with different molar ratios of magnesium ions to crystal growth habit agent.

[0110] Example 8

[0111] The following are methods for preparing basic magnesium sulfate based on seawater bittern using other crystal growth habit agents:

[0112] Step 1: Take 150 mL of seawater bittern and add disodium ethylenediaminetetraacetate (EDTA-2Na), where n(Mg) 2+ The ratio of EDTA-2Na is 20:1. Stir for 30 minutes until the mixture is homogeneous, and record this as solution A.

[0113] Step 2: Prepare a 1.0 mol / L sodium hydroxide solution at room temperature, and take 150 mL of the solution as solution B.

[0114] Step 3: Slowly add solution B dropwise to solution A, which is being stirred, and continue stirring for 30 min to form a homogeneous precursor solution. Transfer the mixed solution to a polytetrafluoroethylene-lined hydrothermal reactor and react it hydrothermally at 180 °C for 10 h. After the reactor has cooled naturally to room temperature, the product is filtered and repeatedly washed with deionized water until no Cl is found. - and SO4 2- The residue was removed, and the product was dried overnight in a vacuum oven at 80 °C to obtain fibrous basic magnesium sulfate whiskers based on seawater bittern with different molar ratios of magnesium ions to crystal growth habit agent.

[0115] Comparative Example 1:

[0116] Commercial Celgard 2500 membrane without any treatment.

[0117] Comparative Example 2:

[0118] The method for preparing a thin-film flame-retardant membrane of basic magnesium sulfate whiskers based on patent CN 121584149 B is as follows:

[0119] Step 1: Based on the preparation method described in patent CN 121584149 B, the corresponding basic magnesium sulfate whiskers were obtained.

[0120] Step 2: Mix 2.95 g of basic magnesium sulfate whiskers based on patent CN 121584149 B, 0.12 g of polyamide wax and 0.42 g of polyvinylidene fluoride to obtain a dry powder. Stir the mixture at low speed for 30-60 min, with a stirring rate of 100-300 rpm / min.

[0121] Step 3: Add 15 mL of N-methylpyrrolidone to the dry powder and stir at low speed for 30-60 min to obtain a mixed solution. The stirring speed is 300-500 rpm / min.

[0122] Step 4: Add 0.16 g of sodium fatty alcohol ether sulfate to the mixed solution and disperse at high speed for 1-3 h with a stirring rate of 1000-1500 rpm / min.

[0123] Step 5: Add 0.12 g of polyether-modified organosilicon to the mixed solution, disperse at high speed for 0.5~1.5 h, and stir at a speed of 1000~1500 rpm / min to obtain the oily ceramic slurry.

[0124] Step 6: Use a wire-type automatic coating machine to coat one side of the Celgard 2500 diaphragm with an oily ceramic slurry to form an oily coating. After vacuum drying at 60 °C, a thin flame-retardant diaphragm based on basic magnesium sulfate whiskers prepared according to patent CN 121584149 B can be obtained.

[0125] Various tests were performed on the diaphragm in the comparative examples and embodiments, and the test results are shown in Table 2 below.

[0126] Table 2

[0127]

[0128] SEM and EDS-mapping images of the fibrous basic magnesium sulfate whiskers prepared in this invention are as follows: Figure 1As shown, the fibrous basic magnesium sulfate whiskers have a length of 30-68 μm, a diameter of 0.5-1.5 μm, and an aspect ratio of 35-62; the SEM image of the fan-shaped basic magnesium sulfate whiskers prepared in this invention is shown below. Figure 2 As shown, the fan-shaped basic magnesium sulfate whiskers are relatively large, with a length of 50-70 μm and a fan-shaped end width of 8-15 μm; the SEM image of the granular basic magnesium sulfate prepared in this invention is shown below. Figure 3 As shown, the particle size of basic magnesium sulfate is 200~500 nm, and it exhibits significant self-aggregation. Figure 4 The XRD patterns of three morphologies of basic magnesium sulfate show that all three morphologies are type 512 basic magnesium sulfate, and the pulverization process did not change the crystal form of basic magnesium sulfate. The above test and characterization methods prove that fibrous basic magnesium sulfate whiskers, fan-shaped basic magnesium sulfate whiskers, and granular basic magnesium sulfate were successfully synthesized from seawater bittern.

[0129] SEM images of the cross-section of a thin-film flame-retardant membrane modified with fibrous basic magnesium sulfate whiskers are shown below. Figure 5 As shown, the flame-retardant coating is approximately 3 μm thick, and the basic magnesium sulfate is uniformly distributed on the Celgard 2500 separator. To demonstrate the advantages of the fibrous basic magnesium sulfate whiskers prepared from seawater bittern in the application of lithium-ion battery separators, Comparative Example 2—a thin-film flame-retardant separator based on basic magnesium sulfate whiskers prepared according to patent CN 121584149 B—was designed for comparison. Among these, the separator's porosity and electrolyte wettability are crucial performance indicators. As shown in Table 2, the porosity of Comparative Example 1 is only 36.06%, while Examples 1, 2, and 3 increased the separator's porosity to 61.06%, 58.65%, and 48.46%, respectively. This indicates that the introduction of thin-film coatings with different morphologies of basic magnesium sulfate can significantly improve the separator's porosity, providing more channels for lithium-ion transport. The electrolyte wettability of the membrane was evaluated by electrolyte contact angle testing. The contact angle data in Table 2 are the contact angles after the electrolyte has been in contact with the composite membrane for 60 s. Figure 6Digital photographs of the electrolyte contact with the composite separator from 0 s to 60 s show that Comparative Example 1 exhibits the largest contact angle (44.9°) at 0 s, and the contact angle decreases to 42.8° at 60 s, representing a decrease of only 2.1° during this period. This indicates that Comparative Example 1 has poor electrolyte wettability. In stark contrast, Examples 1, 2, and 3 show contact angles of only 10.0°, 14.5°, and 17.7° at 0 s, respectively. Furthermore, at 60 s, the electrolyte contact angles of all three composite separators remain at 0°, demonstrating excellent electrolyte wettability, which is beneficial for promoting lithium-ion transport. Additionally, although Comparative Example 2 has an electrolyte contact angle of 0° at 60 s, its initial electrolyte contact angle (12.8°) is still higher than that of Example 1. Therefore, the electrolyte wettability of Example 1 is superior to that of Comparative Example 2. High porosity and excellent electrolyte wettability directly affect the electrolyte retention rate of the composite membrane. The electrolyte retention rate of Comparative Example 1 was 79.6%, while the electrolyte retention rates of Examples 1, 2, and 3 were as high as 154.3%, 153.9%, and 147.5%, respectively, representing increases of 74.7%, 74.3%, and 67.9% compared to Comparative Example 1. Furthermore, the porosity and electrolyte retention rate of Comparative Example 2 were 51.26% and 136.7%, respectively, representing decreases of 9.8% and 17.6% compared to Example 1. Therefore, the fibrous basic magnesium sulfate whisker-modified composite membrane prepared based on seawater bittern exhibits the highest porosity and best electrolyte wettability, indicating that it is most beneficial for promoting Li... + Rapid transport is crucial for improving the electrochemical performance of lithium batteries.

[0130] The ionic conductivity and lithium-ion transference number were measured to further investigate the effect of different morphologies of basic magnesium sulfate whiskers on Li. + The influence of transport kinetics. As shown in Table 2, the ionic conductivity and lithium-ion transference number of Comparative Example 2, Example 1, Example 2, and Example 3 are significantly higher than those of Comparative Example 1. Taking Example 1 as an example, the ionic conductivity and lithium-ion transference number of Example 1 are increased by 160.2% and 24.2% respectively compared to Comparative Example 1, and by 19.9% ​​and 11.6% respectively compared to Comparative Example 2. In addition, the ionic conductivity and lithium-ion transference number of Example 1 are higher than those of Examples 2 and 3. At the same time, the porosity and electrolyte retention of Example 1 are also higher than those of Examples 2 and 3. This is because the fibrous basic magnesium sulfate whiskers prepared based on seawater bittern construct more whisker interpenetrating networks on the membrane, thereby providing more channels for ion transport and promoting the uniform deposition of lithium ions.

[0131] Figure 7The graphs show the cycling performance of the LiFePO4 / separator / Li batteries assembled in Comparative Example 1 and the Examples at a 1C charge-discharge rate. The LiFePO4 / separator / Li batteries assembled in Comparative Example 2 and the Examples all exhibited higher discharge capacities during 600 cycles than the LiFePO4 / separator / Li battery assembled in Comparative Example 1. Example 1 showed the highest initial discharge capacity, reaching 137.63 mAh g⁻¹. -1 Compared to Comparative Examples 1, 2, 2, and 3, the discharge capacity of Example 1 was improved by 11.5%, 8.5%, 7.9%, and 8.1%, respectively. Furthermore, throughout the entire cycle, the discharge capacity of Example 1 was higher than that of the comparative examples and other examples. Additionally, the capacity retention rate of Example 1 after 600 cycles was 92.2%, higher than that of Comparative Examples 1 (91.2%) and 2 (91.5%). This indicates that the fibrous basic magnesium sulfate whiskers, fan-shaped basic magnesium sulfate whiskers, and granular basic magnesium sulfate prepared in this invention can stabilize the lithium metal / electrolyte interface during cycling, reduce side reactions and irreversible lithium loss, and improve the electrochemical stability of the battery during cycling. Among these, the composite separator modified with fibrous basic magnesium sulfate whiskers prepared based on seawater bittern exhibits the best electrochemical performance.

[0132] Excellent heat shrinkage resistance of the separator is crucial for mitigating the safety hazards of lithium batteries, as it can prevent internal short circuits during thermal runaway. As shown in Table 2, Comparative Example 1 exhibits a shrinkage rate of 31% at 160 °C, significantly increasing the risk of internal short circuits in lithium batteries under high-temperature conditions, demonstrating poor heat shrinkage resistance. In contrast, the introduction of basic magnesium sulfate coatings with different morphologies significantly improves dimensional stability. Examples 1, 2, and 3 maintain low heat shrinkage rates at 160 °C. Therefore, even with single-sided coating and a coating thickness of only 3 μm, basic magnesium sulfate controllably prepared from seawater bittern can still significantly improve the heat shrinkage resistance of the separator.

[0133] The flame-retardant properties of the diaphragm were evaluated using a microcalorimeter. Figure 8 and 9 As shown, due to the extreme flammability of the polypropylene diaphragm, Comparative Example 1 exhibits the highest peak heat release rate (1139.6 W g). -1 ) and total heat release (43.7 kJ g) -1 This also indicates that the unmodified polyolefin membrane has an extremely high fire hazard. When a basic magnesium sulfate flame-retardant coating only 3 μm thick was introduced, the heat release rate and total heat release of the composite membrane were significantly reduced. Taking Example 1 as an example, the peak heat release rate and total heat release of Example 1 were reduced to 854.8 W g. -1 and 32.9 kJ g -1Compared to Comparative Example 1, the heat release rate was reduced by 25.0% and 24.7%. Furthermore, Example 1 exhibited a lower peak heat release rate and total heat release compared to Comparative Example 2, thus demonstrating better flame retardant performance. Even with single-sided coating and a coating thickness of only 3 μm, basic magnesium sulfate, controllably prepared from seawater bittern, significantly improved the flame retardant performance of the diaphragm.

[0134] In the synthesis of basic magnesium sulfate based on bittern, the effects of different alkali concentrations, hydrothermal temperatures, hydrothermal times, and the molar ratio of magnesium ions to crystal growth promoters on the crystal structure of the product were investigated. The XRD patterns of basic magnesium sulfate prepared from seawater bittern at different alkali concentrations are shown below. Figure 10 As shown, when the alkaline concentration is below 0.9 mol / L, the product obtained is Mg(OH)₂. When the alkaline concentration is 0.9 and 1.0 mol / L, XRD analysis shows that the product is basic magnesium sulfate. When the alkaline concentration reaches 1.1 mol / L, the product is a mixture of Mg(OH)₂ and basic magnesium sulfate. The crystal form of the product at an alkaline concentration of 1.0 mol / L matches the standard basic magnesium sulfate card better; therefore, an alkaline concentration of 1.0 mol / L is preferred. This is mainly because insufficient alkalinity results in only Mg(OH)₂, while increasing OH⁻ concentration further enhances the product's properties. - As the proportion of Mg(OH)2 increases, the system gradually shifts from being dominated by Mg(OH)2 to forming basic magnesium sulfate, indicating that appropriately increasing the alkalinity is beneficial to Mg. 2+ OH - and SO4 2- Under hydrothermal conditions, it is reconstituted to produce basic magnesium sulfate; however, when OH... - When in excess, the system becomes excessively supersaturated, and Mg... 2+ It preferentially and rapidly precipitates as Mg(OH)₂, increasing byproducts and agglomeration. The XRD patterns of basic magnesium sulfate prepared from seawater bittern at different hydrothermal temperatures are shown below. Figure 11 As shown, when the temperature is below 180 ℃, the hydrothermal temperature is too low, the system kinetics are insufficient, and the reaction mainly stops at the stage where Mg(OH)2 is easily formed, so the only product is Mg(OH)2; when the hydrothermal temperature is 180 ℃, the dissolution-recrystallization of Mg(OH)2 and SO42- 2- The remodeling process is promoted, which is beneficial to the directional growth of basic magnesium sulfate. With continued heating, the local high alkalinity and rapid nucleation enhance the competitive formation of Mg(OH)₂, thus the product transforms into a mixed phase of basic magnesium sulfate and Mg(OH)₂. Therefore, the optimal hydrothermal temperature is 180 °C. The XRD patterns of basic magnesium sulfate prepared from seawater bittern at different hydrothermal times are shown below. Figure 12As shown, when the hydrothermal time is 4 h, Mg(OH)₂ gradually reacts with sulfate ions under hydrothermal conditions, and basic magnesium sulfate begins to grow; when the hydrothermal time is 7 h, basic magnesium sulfate is mainly produced; when the hydrothermal time is 10 h-12 h, the product is completely converted into basic magnesium sulfate, therefore, appropriately extending the hydrothermal time is beneficial to promoting the nucleation of basic magnesium sulfate; when the hydrothermal time is extended to 15 h, SO₄²⁻ inside the whiskers... 2- The basic magnesium sulfate crystals will be released and undergo dehydration and recrystallization, destroying the crystal structure and transforming into the more stable Mg(OH)₂. Therefore, a longer reaction time is not necessarily better. Considering energy consumption, a hydrothermal time of 10 h is preferred. Finally, the XRD patterns of basic magnesium sulfate prepared from seawater bittern under different molar ratios of magnesium ions to crystal growth promoters are shown below. Figure 13 As shown, when n(Mg) 2+ When n(EDTA) is less than 20:1 or greater than 20:1, Mg(OH)2 is inevitably formed as a product. Only when n(Mg)2 is greater than 20:1 will Mg(OH)2 be formed as a product. 2+ Pure basic magnesium sulfate can only be prepared using bittern as a raw material when the ratio of EDTA to Mg(OH)₂ is 20:1. During the hydrothermal reaction, the formation of basic magnesium sulfate whiskers involves the dissolution of the Mg(OH)₂ precursor and the reaction of Mg(OH)₂ with Mg(OH)₂. 2+ OH - and SO4 2- The recombination process. EDTA can combine with Mg. 2+ Formation of complexes, thereby regulating the free Mg in the system 2+ The concentration of NaOH buffers the local supersaturation that occurs during the addition of NaOH, promoting the conversion of the Mg(OH)₂ precursor to basic magnesium sulfate whiskers and inhibiting the formation of fan-shaped whiskers. When n(Mg 2+ When the ratio of EDTA to free Mg is greater than 20:1, the amount of EDTA used is insufficient. 2+ Too high a concentration of Mg 2+ With OH - Mg(OH)₂ is preferentially formed, resulting in residual Mg(OH)₂ impurity phase in the product; when n(Mg 2+ When the ratio of EDTA to n is less than 20:1, EDTA excessively complexes with Mg. 2+ This allows for the effective Mg content in the growth of basic magnesium sulfate crystals. 2+ Insufficient supply inhibits the complete conversion of Mg(OH)₂ to basic magnesium sulfate, also leading to Mg(OH)₂ residue. Only when n(Mg 2+ When :n(EDTA) is 20:1, Mg 2+ Complexation release, Mg(OH)2 dissolution and transformation and SO4 2- The induced one-dimensional crystal growth reached a matching state, thus enabling the production of pure-phase fibrous basic magnesium sulfate whiskers from seawater bittern. The scanning electron microscope image is as follows. Figure 1When EDTA is not added and other synthesis conditions are the same, the product obtained is fan-shaped basic magnesium sulfate whiskers, and its scanning electron microscope image is as follows. Figure 2 Therefore, EDTA has the effect of suppressing fan-shaped whiskers.

[0135] Given that disodium ethylenediaminetetraacetate (EDTA-2Na) is used as a crystal growth habit agent in patent CN 121584149 B, in Example 8 of this invention, basic magnesium sulfate is synthesized using seawater bittern as a raw material and EDTA-2Na as a crystal growth habit agent. The XRD patterns of basic magnesium sulfate prepared from seawater bittern under different crystal growth habit agents are shown below. Figure 14 The results showed that when the crystal growth habit agent was EDTA-2Na, the product prepared from seawater bittern was a mixture of Mg(OH)2 and basic magnesium sulfate, while when the crystal growth habit agent was EDTA, the product was pure-phase basic magnesium sulfate. This indicates that in the complex ionic system of seawater bittern, the EDTA-2Na control method used in patent CN 121584149 B cannot be directly used to obtain pure-phase basic magnesium sulfate whiskers. The reason may be that EDTA and EDTA-2Na differ in their acid-base states in solution, dissociation behavior, and the complexation and release process of Mg²⁺. Seawater bittern contains Na… + Cl - K + and Br - The presence of multiple coexisting ions, such as Na+, and the introduction of EDTA-2Na will further alter the ionic strength and Na+ of the system. + Concentration and local acid-base environment affect the formation, dissolution, and conversion to basic magnesium sulfate of the Mg(OH)₂ precursor, resulting in the residue of the Mg(OH)₂ impurity phase. In contrast, EDTA can more moderately adjust the Mg concentration under the conditions of this invention. 2+ The release and dissolution-recrystallization process of Mg(OH)2 allows it to react with SO4. 2- The induced one-dimensional crystal growth is matched to obtain pure-phase fibrous basic magnesium sulfate whiskers.

[0136] Table 3

[0137]

[0138] This invention further compares the morphology and elemental composition of fibrous basic magnesium sulfate whiskers prepared from seawater bittern with those prepared from patent CN121584149 B, as shown in Table 3. The fibrous basic magnesium sulfate whiskers prepared from seawater bittern have a length of 30-68 μm, a diameter of 0.5-1.5 μm, and an aspect ratio of 35-62; while the basic magnesium sulfate whiskers prepared from patent CN 121584149 B have a length of 50-86 μm, a diameter of 0.5-1.5 μm, and an aspect ratio of 48-87. Both have a one-dimensional whisker structure, but the whiskers prepared from seawater bittern have a relatively lower length and aspect ratio, making them more suitable for uniform dispersion and overlapping in a thin coating of approximately 3 μm, avoiding localized accumulation and agglomeration of excessively long whiskers during the coating process. Meanwhile, SEM results showed that the product prepared from seawater bittern mainly exhibited a fibrous whisker morphology, with very little fan-shaped whisker content; while some fan-shaped whiskers could still be observed in the product prepared based on patent CN 121584149 B (see...). Figure 15 The fan-shaped whiskers, with their extended ends and larger dimensions, are prone to localized accumulation and pore obstruction in thin-layer coatings. In contrast, the more uniform morphology of fibrous whiskers facilitates the formation of a continuous, loosely interconnected whisker network on the surface of the polyolefin membrane, thereby improving the porosity, electrolyte retention, and ion transport capacity of the composite membrane. Furthermore, EDS elemental analysis revealed differences in surface elemental composition. The sulfur content in the fibrous basic magnesium sulfate whiskers prepared from seawater bittern was 12.7 wt%, higher than the 7.0 wt% in whiskers prepared from patent CN 121584149 B; the oxygen content also increased from 56.4 wt% to 59.4 wt%. This indicates that the whisker surface prepared from the seawater bittern system has a higher exposure degree of sulfate and oxygen-containing structural units, which enhances the affinity of the whisker surface for the electrolyte. Therefore, the fibrous basic magnesium sulfate whiskers prepared from seawater bittern are not simply a substitute for the whiskers in patent CN 121584149 B. Instead, they exhibit differences in morphological uniformity, surface elemental composition, and thin-layer coating construction ability, thus giving the composite membrane better wettability, liquid retention capacity, and electrochemical transport performance.

[0139] In summary, this invention ultimately provides an application of a morphology-controllable basic magnesium sulfate flame retardant prepared from seawater bittern in lithium-ion battery separators. It comprises: fibrous basic magnesium sulfate whiskers, fan-shaped basic magnesium sulfate whiskers, and granular basic magnesium sulfate, controllably prepared from seawater bittern, and oily ceramic slurries based on basic magnesium sulfate of different morphologies. This oily ceramic slurry is coated on one side of a polyolefin porous separator to form a 3 μm flame-retardant thin film coating. This flame-retardant thin film separator exhibits high porosity and excellent electrolyte wettability, and its ionic conductivity reaches as high as 0.64 mS / cm. -1The lithium-ion transference number is 0.77. After the membrane is placed at 160 °C for 0.5 hours, its shrinkage rate is only 19%, and the peak heat release rate and total heat release of the membrane are reduced by 25.0% and 24.7% respectively compared with commercial polyolefin membranes. In the comparative examples and embodiments, the composite membrane modified with fibrous basic magnesium sulfate whiskers prepared from seawater bittern has the highest porosity, electrolyte retention rate and best electrolyte wettability, thus exhibiting the highest ionic conductivity and lithium-ion transference number, and therefore the best electrochemical performance. At the same time, its peak heat release rate and total heat release are the lowest, thus exhibiting the best flame retardant performance. This indicates that the fibrous basic magnesium sulfate whiskers prepared from seawater bittern have the best overall performance when used in lithium battery membranes.

[0140] The specific embodiments described above illustrate the technical solution and beneficial effects of the present invention in detail. It should be understood that the above description is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An application of a morphology-controllable basic magnesium sulfate flame retardant prepared from seawater bittern in lithium battery separators, characterized in that: Using seawater bittern as raw material, fibrous basic magnesium sulfate whiskers, fan-shaped basic magnesium sulfate whiskers, and granular basic magnesium sulfate can be prepared in a controlled manner. The basic magnesium sulfate bases with different morphologies are then prepared into an oily ceramic slurry. This oily ceramic slurry is coated on one side of a polyolefin porous membrane to form a flame-retardant thin coating. After drying, a basic magnesium sulfate-modified lithium battery membrane can be obtained, which improves the electrochemical performance and flame-retardant performance of the membrane.

2. The application of the morphology-controllable basic magnesium sulfate flame retardant prepared based on seawater bittern according to claim 1 in lithium battery separators, characterized in that: The method for preparing fibrous basic magnesium sulfate whiskers based on seawater bittern is described above. The process includes the following steps: Step 1: Take seawater bittern, add ethylenediaminetetraacetic acid (EDTA), a crystal growth habitant, with a molar ratio of magnesium ions in the seawater bittern to the crystal growth habitant of 20:1, stir for 30-60 min until uniformly mixed, and record this as solution A; the concentration of magnesium ions in the seawater bittern is 0.8-1.0 mol / L, and the concentration of sulfate ions is 0.3-0.4 mol / L; Step 2: Prepare a 1 mol / L sodium hydroxide solution at room temperature, and take the sodium hydroxide solution as solution B; Step 3: Slowly add solution B dropwise to solution A, which is being stirred, with a volume ratio of 1:

1. Continue stirring for 30-60 minutes to form a homogeneous precursor solution. Transfer the mixed solution to a polytetrafluoroethylene-lined hydrothermal reactor for hydrothermal reaction. The reaction temperature is 180 °C, and the reaction time is 10 h. After the reactor cools naturally to room temperature, the product is filtered and repeatedly washed with deionized water until Cl-free. - and SO4 2- The residue was dried overnight in a vacuum oven at 60-100 ℃ to obtain fibrous basic magnesium sulfate whiskers. Step 4: The fibrous basic magnesium sulfate whiskers are formulated into an oily ceramic slurry and coated on one side of the surface of a polyolefin porous substrate to form a thin flame-retardant coating.

3. The application of the morphology-controllable basic magnesium sulfate flame retardant prepared based on seawater bittern according to claim 1 in lithium battery separators, characterized in that: The method for preparing fibrous basic magnesium sulfate whiskers based on seawater bittern is described above. The process includes the following steps: Step 1: Take 150 mL of seawater bittern, add ethylenediaminetetraacetic acid (EDTA), a crystal growth habitant, with a molar ratio of magnesium ions in the seawater bittern to the crystal growth habitant of 20:1, stir for 30-60 min until homogeneous, and record this as solution A; the concentration of magnesium ions in the seawater bittern is 0.8-1.0 mol / L, and the concentration of sulfate ions is 0.3-0.4 mol / L; Step 2: Prepare a 1 mol / L sodium hydroxide solution at room temperature, and take 150 mL of the sodium hydroxide solution as solution B; Step 3: Slowly add solution B dropwise to solution A, which is being stirred, with a volume ratio of 1:

1. Continue stirring for 30 minutes to form a homogeneous precursor solution. Transfer the mixed solution to a polytetrafluoroethylene-lined hydrothermal reactor for hydrothermal reaction. The reaction temperature is 180 °C, and the reaction time is 10 h. After the reactor cools naturally to room temperature, the product is filtered and repeatedly washed with deionized water until no Cl is present. - and SO4 2- The residue was dried overnight in a vacuum oven at 80°C to obtain fibrous basic magnesium sulfate whiskers. Step 4: The fibrous basic magnesium sulfate whiskers are formulated into an oily ceramic slurry and coated on one side of the polyolefin porous substrate to form a 3 μm thin flame-retardant coating.

4. The application of the morphology-controllable basic magnesium sulfate flame retardant prepared based on seawater bittern according to claim 2 in lithium battery separators, characterized in that: In step 3, the fibrous basic magnesium sulfate whisker product has a length of 30~68 μm, a diameter of 0.5-1.5 μm, and an aspect ratio of 35~62.

5. The application of the morphology-controllable basic magnesium sulfate flame retardant prepared based on seawater bittern according to claim 1 in lithium battery separators, characterized in that: The method for preparing sector-shaped basic magnesium sulfate whiskers based on seawater bittern includes the following steps: Step 1: Take 150 mL of seawater bittern, the magnesium ion concentration of which is 0.89 mol / L, and denote it as solution A; Step 2: Prepare a 1 mol / L sodium hydroxide solution at room temperature, and take 150 mL of the solution as solution B; Step 3: Slowly add solution B dropwise to solution A while it is being stirred, and continue stirring for 30 minutes to form a homogeneous precursor solution; transfer the mixed solution to a polytetrafluoroethylene-lined hydrothermal reactor for hydrothermal reaction; the reaction temperature of the hydrothermal reaction is 180 °C, and the reaction time is 10 h. After the reactor cools naturally to room temperature, the product is filtered and repeatedly washed with deionized water until no Cl is present. - and SO4 2- The residue was removed, and the product was dried overnight in a vacuum oven at 80 °C to obtain fan-shaped basic magnesium sulfate whiskers. Step 4: The fan-shaped basic magnesium sulfate whiskers are formulated into an oily ceramic slurry and coated on one side of the polyolefin porous substrate to form a 3 μm thin flame-retardant coating.

6. The application of the morphology-controllable basic magnesium sulfate flame retardant prepared based on seawater bittern according to claim 1 in lithium battery separators, characterized in that: The method for preparing granular basic magnesium sulfate based on seawater bittern includes the following steps: The fibrous basic magnesium sulfate whiskers prepared from seawater bittern were pulverized for 10 minutes using a high-speed pulverizer and then passed through a 200-mesh sieve to obtain granular basic magnesium sulfate.

7. The application of the morphology-controllable basic magnesium sulfate flame retardant prepared based on seawater bittern according to claim 1 in lithium battery separators, characterized in that: The method for preparing the thin-film flame-retardant membrane based on basic magnesium sulfate modification includes the following steps: Step 1: In the prepared oily ceramic slurry, the oil-based solvent has a mass percentage of 70-80 wt%, the ceramic filler has a mass percentage of 15-20 wt%, the binder has a mass percentage of 1-2.5 wt%, the stabilizer has a mass percentage of 0.5-1 wt%, the dispersant has a mass percentage of 0.5-1 wt%, and the defoamer has a mass percentage of 0.5-1 wt%. According to the formula, mix the ceramic filler, stabilizer, and binder to obtain a dry powder. Mix at a low speed of 100-300 rpm / min for 30-60 minutes. Step 2: Add an oily solvent to the dry powder and stir at low speed for 30-60 minutes to obtain a mixed solution. The stirring speed is 300-500 rpm / min. Step 3: Add dispersant to the mixed solution and disperse at high speed for 1-3 hours, with a stirring rate of 1000-1500 rpm / min; Step 4: Add defoamer to the mixed solution, disperse at high speed for 0.5~1.5 h, and stir at a speed of 1000~1500 rpm / min to obtain oily ceramic slurry; Step 5: Use a wire-type automatic coating machine to coat one side of the polyolefin porous substrate with oily ceramic slurry to form an oily coating. After drying, a thin flame-retardant membrane based on basic magnesium sulfate with different morphologies can be obtained.

8. The application of the morphology-controllable basic magnesium sulfate flame retardant prepared based on seawater bittern according to claim 7 in lithium battery separators, characterized in that: In step 1, the ceramic filler is one of fibrous basic magnesium sulfate whiskers, fan-shaped basic magnesium sulfate whiskers, or granular basic magnesium sulfate; the binder is polyvinylidene fluoride; and the stabilizer is polyamide wax. In step 2, the oily solvent is N-methylpyrrolidone; In step 3, the dispersant is sodium fatty alcohol ether sulfate; In step 4, the defoamer is a polyether-modified silicone. In step 5, the thickness of the diaphragm coating formed by the basic magnesium sulfate whiskers prepared from bittern is 3 μm.

9. The application of the morphology-controllable basic magnesium sulfate flame retardant prepared based on seawater bittern according to claim 7 in lithium battery separators, characterized in that: The fibrous basic magnesium sulfate whiskers form a continuous, loose whisker network in the coating, which is beneficial to improving the membrane porosity, electrolyte retention, ionic conductivity and lithium ion transference number, and reducing the peak heat release rate.

10. The application of the morphology-controllable basic magnesium sulfate flame retardant prepared based on seawater bittern according to claim 7 in lithium battery separators, characterized in that: The fibrous basic magnesium sulfate whisker flame retardant, when applied to lithium-ion battery separators with a single-sided coating thickness of only 3 μm, exhibits a separator porosity of 61.06%, an electrolyte retention rate of 154.3%, and an ionic conductivity of 0.640 mS / cm. -1 With a lithium-ion transport number of 0.77 and a peak heat release rate of 854.8 W / g, it exhibits a high energy efficiency ratio. -1 Total heat release: 32.9 kJ / g -1 The initial discharge capacity under 1C cycling is as high as 137.63 mAh g. -1 The capacity retention rate after 600 cycles is 92.2%.

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