A low-temperature resistant lithium battery separator and its preparation method

Abstract

The present invention belongs to the field of lithium-ion battery technology, and specifically relates to a low-temperature-resistant lithium battery separator and a method for preparing the same. The preparation method comprises: adding cellulose to an aqueous solution of N-methylmorpholine oxide, stirring uniformly to obtain a cellulose solution; then adding a solution of a modifier A to the cellulose solution, mixing uniformly to obtain a modified cellulose solution; adding a binder to the modified cellulose solution to obtain a modified cellulose coating slurry; and finally, coating the modified cellulose coating slurry on both surfaces of a membrane substrate, vacuum drying, to obtain a low-temperature-resistant lithium battery separator. The separator obtained by the present invention has the advantages of high ionic conductivity and high lithium battery cycle stability at low temperatures.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a low-temperature-resistant lithium battery separator and a preparation method thereof. Background Art

[0002] Lithium batteries offer numerous advantages, including high specific energy, long cycle life, and environmental friendliness. They are gradually replacing lead-acid, nickel, and nickel-cadmium batteries and are widely used in portable electronic devices. Lithium batteries primarily consist of four components: positive and negative electrode materials, an electrolyte, a separator, and a battery casing. The operating principle of lithium-ion batteries is based on the intercalation, intercalation, and migration of lithium ions between the positive and negative electrodes, enabling the charge and discharge process. During charging, electrical energy is converted into chemical energy. An oxidation reaction occurs at the positive electrode, releasing lithium ions from the positive electrode material. These ions then migrate through the separator in the electrolyte from the positive electrode to the negative electrode. Simultaneously, lithium ions intercalate into the negative electrode, causing a reduction reaction. During discharge, an oxidation reaction occurs at the negative electrode, releasing lithium ions back into the electrolyte. These ions then travel through the separator with the electrolyte, where they are reabsorbed by the positive electrode material and undergo a reduction reaction. During the charge and discharge cycle, lithium ions flow between the positive and negative electrodes, storing and releasing electrical energy. A single charge and discharge process is considered a cycle, and the battery continuously provides electrical energy during this continuous charge and discharge process.

[0003] As one of the key components of lithium batteries, the main function of the diaphragm is to isolate the positive and negative electrodes to prevent the battery from short-circuiting due to contact between the positive and negative electrodes; and to provide ion transmission channels, allowing lithium ions to move freely between the positive and negative electrodes of the battery. The diaphragm needs to have a certain mechanical strength to maintain the stability of the battery structure and prevent the growth of lithium dendrites penetrating the diaphragm and causing battery short-circuiting and other hazards. At present, the materials of battery diaphragms are mainly polymer diaphragms, such as polypropylene (PP) and polyethylene (PE), which are widely used in various batteries. Although polymer diaphragms have the advantages of low cost and good chemical stability, they have significant defects in high energy density, wide temperature range, and high safety battery applications. PP / PE, as a non-polar polymer, is not compatible with polar electrolytes (such as ) poor compatibility, resulting in high interfacial impedance, which in turn leads to low ionic conductivity and poor cycling stability of the separator. Furthermore, conventional secondary batteries suffer from poor charge and discharge capacity, a low discharge plateau, and poor cycling performance in low-temperature environments. Therefore, with the continuous development and innovation of battery technology and the expansion of its application areas, the requirements for separator performance are becoming increasingly stringent. Modifying the separator has become a necessary measure to improve battery safety and ion transport, extend battery life, and adapt to a wider range of operating environments. Summary of the Invention

[0004] The present invention aims to provide a low-temperature-resistant lithium battery separator and a method for preparing the same, in order to address the problems of low ionic conductivity and poor cycle stability of polymer separators in the prior art. To address the above technical issues, the present invention provides the following technical solutions:

[0005] A method for preparing a low-temperature resistant lithium battery separator comprises the following steps:

[0006] Step 1 Preparation of modified cellulose solution

[0007] Add cellulose to N-methylmorpholine oxide aqueous solution and stir evenly to obtain cellulose solution; dissolving in an organic solvent to obtain a modifier A solution, then adding the modifier A solution to the cellulose solution, and mixing uniformly to obtain a modified cellulose solution;

[0008] Step 2 Preparation of low temperature resistant lithium battery separator

[0009] Add a binder to the modified cellulose solution obtained in step 1, stir and mix evenly to obtain a modified cellulose coating slurry; apply the modified cellulose coating slurry on both surfaces of the membrane substrate, and vacuum dry to obtain a low-temperature resistant lithium battery separator.

[0010] In some embodiments, the organic solvent in step 1 is selected from one or both of DMSO and DMF.

[0011] In some embodiments, the mass ratio of cellulose to modifier A in step 1 is (2-5):1; the volume ratio of the N-methylmorpholine oxide aqueous solution to the organic solvent is (3-5):1; and the mass fraction of the N-methylmorpholine oxide aqueous solution is 50-80%.

[0012] In some embodiments, the membrane substrate in step 2 is a polyethylene membrane, a polypropylene membrane or a polyethylene / polypropylene double-layer membrane; the thickness of the membrane substrate is 10-30 μm, the pore size is 0.03-0.1 μm, and the thickness of the coating is 2-5 μm.

[0013] In some embodiments, the binder in step 2 is selected from one or more of polyvinyl alcohol, polyethylene oxide, polyacrylic acid, polyvinylidene fluoride and sodium carboxymethyl cellulose.

[0014] In some embodiments, the mass ratio of the cellulose to the binder is (1-5):1.

[0015] In some embodiments, the vacuum drying in step 2 is performed at a temperature of 120-150° C. and for a time of 30-60 min.

[0016] The present invention also protects the low-temperature resistant lithium battery separator prepared by the above method.

[0017] The present invention has achieved the following beneficial effects:

[0018] The present invention uses modifier A to modify cellulose, which can greatly improve the ionic conductivity of the separator and the cycle stability of the lithium battery at low temperature. The main reason is that the modifier A of the present invention has a large π conjugated system and contains a large number of strong polar groups (F and carbonyl). The large π conjugated system and strong polar groups can promote the Li + dissociation and Li + Desolvation of Li + The uniform transmission of lithium ions effectively inhibits the growth of lithium dendrites, thereby improving the ionic conductivity of the separator and the cycle performance of lithium batteries at low temperatures. DETAILED DESCRIPTION

[0019] The following will be combined with the tables in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0020] The present invention does not limit the sources of the raw materials used. Unless otherwise specified, the raw materials used in the present invention are common commercial products in the technical field.

[0021] Basic Preparation Example 1 Preparation of Modifier A

[0022] ,

[0023] Under a nitrogen atmosphere, 1,4-dimethoxyanthraquinone (Compound 1) (0.1 mol), aryl borate (Compound 2) (0.25 mol), catalyst RuH2(CO)(PPh3)3 (0.01 mol), and toluene (200 mL) were added to a reactor. The temperature was raised to 100°C and stirred for 12 hours. After completion, the reaction solution was passed through a basic alumina column to remove the remaining aryl borate, yielding a crude product. The crude product was further purified by silica gel column chromatography (n-hexane:chloroform = 1:4) to obtain 35.1 g of modifier A, with a yield of 81.3% and a HPLC purity of 99.5%.

[0024] LC-MS (ESI): [M+H] + =433.1,

[0025] 1H NMR (400 MHz, CDCl3): δ 8.35-8.28 (m, 2H), 8.12(s, 2H), 7.74-7.70(m, 2H), 7.25 (d, 4H), 6.71-6.64 (m, 2H). Example 1

[0026] Step 1 Preparation of modified cellulose solution

[0027] Cellulose (25.0 g, cotton linters, produced by Hubei Chemical Fiber Group Co., Ltd. (Xiangyang)) was added to a 60% by mass NMMO (N-methylmorpholine oxide) aqueous solution (150 mL), and stirred to obtain a cellulose solution. Modifier A (10.0 g) obtained in Basic Preparation Example 1 was dissolved in DMF (50.0 mL) to obtain a modifier A solution. The modifier A solution was then added to the cellulose solution, and stirred at 30°C for 2.0 h to obtain a modified cellulose solution.

[0028] Step 2 Preparation of low temperature resistant lithium battery separator

[0029] Add the binder polyvinylidene fluoride (Solef ® PVDF 6010, produced by Solvay Company, USA) (20.0 g) was stirred and mixed evenly to prepare a modified cellulose coating slurry; the modified cellulose coating slurry was coated on both surfaces of a polyethylene (PE) porous membrane with a thickness of 20 μm and a pore size of 0.05 μm, and the thickness of each coating was 5 μm. The membrane was vacuum dried at 150°C for 30 min to prepare a lithium battery separator. Example 2

[0030] Step 1 Preparation of modified cellulose solution

[0031] Cellulose (20.0 g, cotton linters, produced by Hubei Chemical Fiber Group Co., Ltd. (Xiangyang)) was added to a 50% by mass NMMO (N-methylmorpholine oxide) aqueous solution (150 mL), and stirred to obtain a cellulose solution. Modifier A (5.0 g) obtained in Basic Preparation Example 1 was dissolved in DMSO (50.0 mL) to obtain a modifier A solution. The modifier A solution was then added to the cellulose solution, and stirred at 40°C for 2.0 h to obtain a modified cellulose solution.

[0032] Step 2 Preparation of low temperature resistant lithium battery separator

[0033] Add the binder polyvinylidene fluoride (Solef ®PVDF 6010, produced by Solvay Company, USA) (15.0 g) was stirred and mixed evenly to prepare a modified cellulose coating slurry; the modified cellulose coating slurry was coated on both surfaces of a polyethylene (PE) porous membrane with a thickness of 20 μm and a pore size of 0.05 μm, and the thickness of each coating was 5 μm. The membrane was vacuum dried at 150°C for 30 min to prepare a lithium battery separator. Example 3

[0034] Step 1 Preparation of modified cellulose solution

[0035] Cellulose (25.0 g, cotton linters, produced by Hubei Chemical Fiber Group Co., Ltd. (Xiangyang)) was added to a 50% by mass NMMO (N-methylmorpholine oxide) aqueous solution (150 mL), and stirred to obtain a cellulose solution. Modifier A (8.0 g) obtained in Basic Preparation Example 1 was dissolved in DMF (50.0 mL) to obtain a modifier A solution. The modifier A solution was then added to the cellulose solution, and stirred at 30°C for 2.0 h to obtain a modified cellulose solution.

[0036] Step 2 Preparation of low temperature resistant lithium battery separator

[0037] Add binder polyvinyl alcohol (15.0 g) to the modified cellulose solution obtained in step 1, stir and mix evenly to obtain a modified cellulose coating slurry; apply the modified cellulose coating slurry on both surfaces of a polyethylene (PE) porous membrane with a thickness of 20 μm and a pore size of 0.05 μm, and the thickness of each coating is 5 μm. Vacuum dry at 150°C for 30 min to obtain a lithium battery separator. Example 4

[0038] On the basis of Example 1, the polyethylene (PE) porous membrane was replaced with a polypropylene porous membrane. The specific operation was as follows:

[0039] Step 1 Preparation of modified cellulose solution

[0040] Cellulose (25.0 g, cotton linters, produced by Hubei Chemical Fiber Group Co., Ltd. (Xiangyang)) was added to a 60% by mass NMMO (N-methylmorpholine oxide) aqueous solution (150 mL), and stirred to obtain a cellulose solution. Modifier A (10.0 g) obtained in Basic Preparation Example 1 was dissolved in DMF (50.0 mL) to obtain a modifier A solution. The modifier A solution was then added to the cellulose solution, and stirred at 30°C for 2.0 h to obtain a modified cellulose solution.

[0041] Step 2 Preparation of low temperature resistant lithium battery separator

[0042] Add the binder polyvinylidene fluoride (Solef ® PVDF 6010, produced by Solvay Company, USA) (20.0 g) was stirred and mixed evenly to prepare a modified cellulose coating slurry; the modified cellulose coating slurry was coated on both surfaces of a polypropylene porous membrane with a thickness of 20 μm and a pore size of 0.05 μm, and the thickness of each coating was 5 μm. The membrane was vacuum dried at 150°C for 30 min to prepare a lithium battery separator.

[0043] Comparative Example 1

[0044] Based on Example 1, the modifier A is omitted and the specific operation is as follows:

[0045] Step 1 Preparation of cellulose solution

[0046] Cellulose (25.0 g, cotton linters, produced by Hubei Chemical Fiber Group Co., Ltd. (Xiangyang)) was added to a 60% (mass fraction) NMMO (N-methylmorpholine oxide) aqueous solution (150 mL), and then DMF (50.0 mL) was added. The mixture was stirred at 30°C for 2.0 h to obtain a cellulose solution.

[0047] Step 2 Preparation of low temperature resistant lithium battery separator

[0048] Add the binder polyvinylidene fluoride (Solef ® PVDF 6010, produced by Solvay Company, USA) (20.0 g) was stirred and mixed evenly to prepare a cellulose coating slurry; the above cellulose coating slurry was coated on both surfaces of a polyethylene (PE) porous membrane with a thickness of 20 μm and a pore size of 0.05 μm, and the thickness of each coating was 5 μm. The membrane was vacuum dried at 150°C for 30 min to prepare a lithium battery separator.

[0049] Performance Testing

[0050] The lithium battery separators prepared in Examples 1-4 and Comparative Example 1 were subjected to relevant performance tests, and the test methods are as follows:

[0051] 1) Ionic conductivity test:

[0052] The bulk resistance was measured using the AC impedance method (CHI660E, Shanghai Chenhua Instrument Co., Ltd.). The measurement system consisted of a stainless steel sheet (SS) || diaphragm || stainless steel sheet (SS). The sinusoidal amplitude was 10 mV, and the sweep frequency ranged from 1 to 1 × 10⁵ Hz. The ionic conductivity was calculated using the formula: ; where σ is the ionic conductivity (S·cm -1 ); d is the thickness of the diaphragm (cm); R is the bulk resistance (Ω); S is the effective area (cm2 ).

[0053] 2) Battery performance test

[0054] Lithium battery assembly: The cathode material, LiFePO4, conductive carbon black, and polyvinylidene fluoride (PVDF), were mixed in a mass ratio of 8:1:1. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to form a slurry, which was evenly coated onto aluminum foil and then vacuum-dried at 80°C for 12 h. The dried LiFePO4 cathode was cut into 8 mm diameter positive electrode discs. Lithium-ion batteries were assembled using the cathode, separator, lithium metal anode, and electrolyte (1 M LiPF6 dissolved in a 1:1, by volume, solution of ethylene carbonate (EC) and diethyl carbonate (DEC)). All batteries were assembled in an argon-filled glove box. The assembled LiFePO4 / Li batteries (2032-type button cells) were activated.

[0055] Low-temperature cycling performance: Battery cycling tests were conducted using a NEWARE system (MIHW-200-160CH, Shenzhen Newwell Electronics Co., Ltd.). After 300 cycles at -20°C, with a charge / discharge current density of 0.5 C / 0.5 C and a voltage range of 2.5 to 4 V, the capacity retention was calculated as: capacity retention = 300th cycle discharge specific capacity / first cycle discharge specific capacity.

[0056] The results are shown in Table 1:

[0057] ,

[0058] As shown in Table 1, the modification of cellulose by using modifier A in the present invention can greatly improve the ionic conductivity of the separator and the cycle stability of the lithium battery at low temperature. The main reason is that the modifier A of the present invention has a large π conjugated system and contains a large number of strong polar groups (F and carbonyl). The large π conjugated system and strong polar groups can promote the Li + dissociation and Li + Desolvation of Li + The uniform transmission of lithium ions effectively inhibits the growth of lithium dendrites, thereby improving the ionic conductivity of the separator and the cycle performance of lithium batteries at low temperatures.

[0059] The above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for preparing a low-temperature resistant lithium battery separator, comprising the following steps: Step 1 Preparation of modified cellulose solution Add cellulose to N-methylmorpholine oxide aqueous solution and stir evenly to obtain cellulose solution; dissolving in an organic solvent to obtain a modifier A solution, then adding the modifier A solution to the cellulose solution, and mixing uniformly to obtain a modified cellulose solution; Step 2 Preparation of low temperature resistant lithium battery separator Add a binder to the modified cellulose solution obtained in step 1, stir and mix evenly to obtain a modified cellulose coating slurry; apply the modified cellulose coating slurry on both surfaces of the membrane substrate, and vacuum dry to obtain a low-temperature resistant lithium battery separator.

2. The preparation method according to claim 1, characterized in that The organic solvent in step 1 is selected from one or both of DMSO and DMF.

3. The preparation method according to claim 1, wherein In step 1, the mass ratio of cellulose to modifier A is (2-5):1; the volume ratio of the N-methylmorpholine oxide aqueous solution to the organic solvent is (3-5):1; and the mass fraction of the N-methylmorpholine oxide aqueous solution is 50-80%.

4. The preparation method according to claim 1, wherein The membrane substrate in step 2 is a polyethylene membrane, a polypropylene membrane or a polyethylene / polypropylene double-layer membrane; the thickness of the membrane substrate is 10-30 μm, the pore size is 0.03-0.1 μm, and the thickness of the coating is 2-5 μm.

5. The preparation method according to claim 1, characterized in that The binder in step 2 is selected from one or more of polyvinyl alcohol, polyethylene oxide, polyacrylic acid, polyvinylidene fluoride and sodium carboxymethyl cellulose.

6. The preparation method according to claim 1, wherein The mass ratio of the cellulose to the binder is (1-5):

1.

7. The preparation method according to claim 1, characterized in that The vacuum drying temperature in step 2 is 120-150° C. and the time is 30-60 min.

8. A low-temperature resistant lithium battery separator prepared by the preparation method according to any one of claims 1 to 7.

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