High-safety regenerated cellulose-based lithium battery diaphragm and preparation method thereof

Abstract

The invention belongs to the technical field of lithium battery diaphragms, and discloses a high-safety regenerated cellulose-based lithium battery diaphragm and a preparation method thereof. The method comprises the following steps: 1) dissolving cellulose in a cellulose solvent to obtain a cellulose solution; preparing the cellulose solution into a cellulose membrane; 2) placing the cellulose membrane in an ethanol solution of antimony trichloride to obtain cellulose hydrogel containing antimony trichloride; 3) soaking the cellulose hydrogel in water, and removing the solvent to obtain cellulose hydrogel containing antimony oxychloride; and 4) drying the cellulose hydrogel containing antimony oxychloride to obtain the cellulose diaphragm. The safety of the cellulose diaphragm is improved through in-situ loading of antimony oxychloride, so that the composite diaphragm has a self-extinguishing property. According to the method disclosed by the invention, the use of a binder is avoided, and meanwhile, the diaphragm has good thermal stability. The diaphragm disclosed by the invention not only has good safety performance of the battery, but also has excellent electrochemical performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery separators, and particularly relates to a regenerated cellulose-based lithium battery separator and a preparation method thereof. Background Art

[0002] Lithium batteries, as efficient and sustainable energy storage devices, are widely used in fields such as electric vehicles, portable electronic devices, and energy storage power stations. Their high energy density, long lifespan, and relatively low environmental impact make them a key technology for addressing energy shortages. However, the safety issues of lithium batteries remain challenges that need to be urgently addressed. Especially under extreme conditions such as high temperature or overcharging, the battery is prone to thermal runaway, leading to accidents such as fires and explosions, seriously threatening personal safety and property safety. As an important component, the stability of the battery separator directly affects battery safety. Currently, many separator materials are prone to short circuits in high-temperature environments, and there is an urgent need to improve their safety and stability.

[0003] To improve battery safety, the industry has proposed various measures, but these measures have their limitations. Using flame-retardant electrolytes can reduce the risk of fire, but it may change the chemical environment of the battery, affecting the charge-discharge efficiency and cycle life. Improving electrode materials, such as selecting materials with better thermal stability, may also lead to a decrease in energy density and affect battery performance. Therefore, existing measures have not fully solved the risks of the battery in extreme environments, and new technical means are urgently needed.

[0004] The battery separator plays a crucial role in lithium batteries. On the one hand, it separates the positive and negative electrodes to prevent short circuits; on the other hand, it provides a channel for the conduction of lithium ions, directly affecting the charge-discharge performance of the battery. Currently, polyolefin separators are widely used due to their good stability and mechanical strength, but they have many disadvantages. First, polyolefin separators are derived from fossil resources, are not renewable, and lack sustainability and degradability. Second, the separators made of polypropylene (PP) and polyethylene (PE) materials have poor affinity for electrolytes, affecting electrochemical performance. More seriously, polyolefin separators have poor thermal stability and are prone to shrinkage and melting at high temperatures, resulting in battery short circuits and triggering safety accidents such as fires. In addition, such separators are highly flammable and are prone to combustion when the temperature gets out of control, endangering battery safety.

[0005] To solve the problems of thermal stability and flammability, various improvement schemes have been proposed. For example, using a matrix with better thermal stability such as polyether ether ketone to make the separator, but its price is expensive. The use of inorganic coatings can improve the safety of the separator, but it also brings problems. The coating may peel off or block the pore size, affecting lithium ion transport and reducing the electrochemical performance of the battery. Therefore, existing modification methods still have limitations, and there is an urgent need to find more effective and stable solutions.

[0006] Compared with polyolefin separators, cellulose-based separators have significant advantages. Cellulose is green, environmentally friendly, degradable, and widely sourced, meeting the requirements of sustainable development. A large number of polar hydroxyl functional groups in cellulose endow it with good electrolyte affinity. In addition, cellulose has good thermal stability, which helps to reduce the risk of short circuits at high temperatures.

[0007] However, cellulose-based separators still have deficiencies. Their flammability affects the safety of the battery. In addition, traditional cellulose separators are manufactured by the papermaking method, making it difficult to precisely control the pore size. As a result, the pore size is small, restricting ion transport and thus affecting electrochemical performance. Therefore, although cellulose-based separators have many advantages, they still need further improvement to enhance the safety and electrochemical performance of the battery. Summary of the Invention

[0008] To overcome the above problems, the present invention proposes a high-safety regenerated cellulose-based lithium battery separator and its preparation method. The present invention uses cellulose as a raw material, dissolves it in a cellulose solvent, and then casts or tapes to obtain a cellulose film. Then, it undergoes two processes of regeneration in an ethanol solution containing antimony trichloride and hydrolysis in water to obtain a cellulose hydrogel containing antimony oxychloride. Finally, it is dried to obtain a regenerated cellulose separator. This method has a simple preparation process, and the prepared cellulose separator has good electrochemical performance, thermal stability, and flame retardant effect.

[0009] The object of the present invention is achieved through the following technical solutions.

[0010] A preparation method of a high-safety regenerated cellulose-based lithium battery separator, comprising the following steps:

[0011] 1) Dissolve cellulose in a cellulose solvent to obtain a cellulose solution; prepare the cellulose solution into a cellulose film;

[0012] 2) Place the cellulose film in an ethanol solution of antimony trichloride to obtain a cellulose hydrogel containing antimony trichloride;

[0013] 3) Soak the cellulose hydrogel in water to remove the solvent and obtain a cellulose hydrogel containing antimony oxychloride;

[0014] 4) Dry the cellulose hydrogel containing antimony oxychloride to obtain a cellulose-based lithium battery separator.

[0015] In step 1), the cellulose film is obtained by casting or taping the cellulose solution.

[0016] In step 1), the depth of the groove of the mold used for casting is 0.1 - 1 mm.

[0017] In step 1), the cellulose is derived from microcrystalline cellulose, bleached hardwood pulp, bleached softwood pulp, bagasse pulp, bleached bamboo pulp, wheat straw cellulose, rice straw cellulose, cotton linter, or bacterial cellulose. The cellulose raw material does not affect the preparation of the final separator. Any cellulose raw material that can dissolve in the solvent is acceptable.

[0018] The cellulose solvent in step 1) is one of the following solvents: LiCl / DMAC (LiCl concentration is 8 wt%), 1-allyl-3-methylimidazolium chloride, or a deep eutectic solvent.

[0019] The deep eutectic solvent: is obtained by mixing a hydrogen bond acceptor and a hydrogen bond donor in a molar ratio of 0.5:1 to 5:1 and heating to a homogeneous and transparent state at 50 - 100 °C; the hydrogen bond donor is one of dimethylurea, N-methylthiourea, dimethylthiourea, diethylthiourea, or 2-pyrrolidone; the hydrogen bond acceptor is selected from one of the following general structural formulas:

[0020]

[0021] where the anion (X - ) is one of chloride ion, bromide ion, or acetate ion; R is an olefin group.

[0022] Preferably, the anion is chloride ion and R is allyl.

[0023] Preferably, the hydrogen bond donor is dimethylthiourea.

[0024] Preferably, the deep eutectic solvent is obtained by mixing the above hydrogen bond acceptor and hydrogen bond donor in a molar ratio of 1:1 to 2:1 and heating and stirring at 70 - 90 °C to obtain a homogeneous and transparent liquid.

[0025] The dissolution temperature of the cellulose in the above deep eutectic solvent in step 1) is 70 - 110 °C.

[0026] The ethanol solution of antimony trichloride in step 2) is composed of ethanol and antimony trichloride, and the content of antimony trichloride is 0.08% - 5% (mass percentage), preferably 0.08 - 0.6%.

[0027] In step 2), the cellulose membrane is immersed in the ethanol solution of antimony trichloride for 10 - 16 hours. The temperature of the ethanol solution of antimony trichloride is 20 - 70 °C.

[0028] In step 3), the immersion time is 8 - 24 hours; the removal of the solvent refers to washing with water.

[0029] The drying method described in step 4) is vacuum freeze-drying; specifically, it is pre-frozen at -40°C to -80°C for 8 to 24 hours (preferably 10 - 16 hours), and then immediately placed in a vacuum freeze-dryer for drying for 10 - 16 hours.

[0030] Compared with the prior art, the present invention has the following advantages:

[0031] 1. The raw material (cellulose) used in the present invention has a wide source and rich reserves; the preparation method of the diaphragm is simple and can be mass-produced.

[0032] 2. The regenerated cellulose-based diaphragm of the present invention has good electrolyte affinity, good thermal stability, is green and environmentally friendly, and is degradable, etc.

[0033] 3. Compared with the cellulose diaphragm with a dense structure obtained by the traditional papermaking method, the cellulose diaphragm of the present invention has a controllable pore structure. This pore structure is beneficial to ion transport.

[0034] 4. Compared with the thickened battery diaphragm prepared by using the coating composite diaphragm technology, for the regenerated cellulose-based diaphragm of the present invention, the flame retardant contained therein is in-situ generated in the internal network of cellulose, and the thickness hardly increases. And this avoids the disadvantages such as coating peeling and the use of additional binders that may exist in the coating technology. Changing the loaded substance can endow the battery diaphragm with different properties. Therefore, this method provides an idea for preparing a high-performance ultra-thin composite battery diaphragm.

[0035] 5. Aiming at the flammability of cellulose, the regenerated cellulose-based battery diaphragm is loaded with a flame retardant (antimony oxychloride), which makes the diaphragm have good self-extinguishing properties. Description of the Drawings

[0036] Figure 1 is the flame retardant performance of different diaphragms;

[0037] Figure 2 is the full-cell cycle performance test of different diaphragms. This full cell uses lithium iron phosphate as the positive electrode and lithium foil as the negative electrode;

[0038] Figure 3 is the SEM image of the diaphragms prepared in Comparative Example 1 to Comparative Example 3;

[0039] Figure 4 is the SEM image of the diaphragm prepared in Example 6;

[0040] Figure 5 is the SEM image of the diaphragms prepared in Example 1 and Example 2. Detailed Embodiments

[0041] To better illustrate the present invention, the following further describes the present invention in detail with reference to specific embodiments, but the implementation manners of the present invention are not limited thereto.

[0042] Example 1

[0043] A method for preparing a regenerated cellulose-based battery separator includes the following steps:

[0044] Step 1: Place 0.2 g of dry hardwood pulp (with a diameter of about 5 μm, a length greater than 100 μm, a degree of polymerization of 700 - 800, and a cellulose content of more than 90%) into 10 g of a deep eutectic solvent, heat to 90 °C and stir until dissolved to obtain a homogeneous cellulose solution; the deep eutectic solvent is a homogeneous liquid obtained by mixing a hydrogen bond acceptor and a hydrogen bond donor in a molar ratio of 2:1, heating and stirring at 70 - 90 °C, where the hydrogen bond donor is dimethylthiourea and the hydrogen bond acceptor is [DBNA]Cl; the structural formula of [DBNA]Cl is

[0045]

[0046] Step 2: Pour the homogeneous cellulose solution obtained in Step 1 into a mold to obtain a cellulose gel with a certain thickness (the thickness of the film is 500 μm).

[0047] Step 3: Add the cellulose gel obtained in Step 2 to absolute ethanol containing 0.1% (mass ratio) of antimony trichloride (the temperature of the solution is 25 °C), soak for 12 hours to obtain a cellulose gel containing antimony trichloride.

[0048] Step 4: Add the cellulose gel obtained in Step 3 to water, soak for 12 hours, antimony trichloride undergoes hydrolysis to form antimony oxychloride, and then the obtained cellulose gel containing antimony oxychloride is washed with water multiple times to fully remove the cellulose solvent.

[0049] Step 5: Freeze the cellulose gel obtained in Step 4 at -80 °C for 12 hours, and then immediately perform vacuum drying for 12 hours using a vacuum freeze dryer to obtain a regenerated cellulose-based battery separator.

[0050] The battery is assembled in a glove box (with both water content and oxygen content lower than 0.5 ppm) in the arrangement order of the positive electrode case, positive electrode, separator, negative electrode, gasket, spring piece, and negative electrode case to obtain a lithium battery CR2032.

[0051] Example 2

[0052] The preparation method of this example is the same as that of Example 1, except that absolute ethanol containing 1% (mass ratio) of antimony trichloride is used for regeneration, and other steps are the same.

[0053] Example 3

[0054] The preparation method of this example is the same as that of Example 1, except that the raw material broadleaf wood pulp is replaced with softwood pulp, and the other steps are the same.

[0055] Example 4

[0056] The preparation method of this example is the same as that of Example 1, except that the hydrogen bond donor is replaced with 2-pyrrolidone, and the ratio of the hydrogen bond donor to the hydrogen bond acceptor is 1:1 (molar ratio), and the other steps are the same.

[0057] Example 5

[0058] The preparation method of this example is the same as that of Example 1, except that pre-freezing is carried out in an environment of -40°C, and the other steps are the same.

[0059] Example 6

[0060] The preparation method of this example is the same as that of Example 1, except that the drying method is hot pressing drying (60°C, 1 MPa), and the other steps are the same.

[0061] Comparative Example 1

[0062] The preparation method of Comparative Example 1 is the same as that of Example 1, except that regeneration is carried out using absolute ethanol without antimony trichloride (the temperature of the solution is 25°C), and the other steps are the same.

[0063] Comparative Example 2

[0064] The preparation method of Comparative Example 2 is the same as that of Example 1, except that regeneration is carried out using absolute ethanol without antimony trichloride (the temperature of the solution is 50°C), and the other steps are the same.

[0065] Comparative Example 3

[0066] The preparation method of Comparative Example 3 is the same as that of Example 1, except that regeneration is carried out using absolute ethanol without antimony trichloride (the temperature of the solution is 80°C), and the other steps are the same.

[0067] Performance Test

[0068] Electrochemical tests and safety tests were carried out on the battery separators prepared in the above examples and comparative examples.

[0069] Table 1 shows the ionic conductivities of the battery separators prepared in Examples 1 to 6

[0070] Table 1 Ionic Conductivities of Different Separators

[0071] Item <![CDATA[Ionic conductivity mS / cm -1 > Example 1 0.44 Example 2 0.26 Example 3 0.44 Example 4 0.48 Example 5 0.48 Example 6 0.27

[0072] The pore sizes of the separators are different under different conditions. This change in pore size causes the separators to exhibit different ionic conductivities. For the separators obtained by the flat drying method, the pore structure in the separators is compressed, so ion transport is hindered, and thus they have low ionic conductivity.

[0073] Figure 1 The flame retardant properties of the different separators prepared in Examples 1-2 and Comparative Example 1.

[0074] Use an alcohol lamp to ignite the separator, then remove the flame and observe the combustion situation of the separator. From Figure 1 it can be seen that the pure cellulose separator (Comparative Example 1) burns out completely after being ignited. After Examples 1 and 2 are ignited and the flame is removed, the separators go out by themselves. That is, the cellulose separator loaded with antimony oxychloride has a self-extinguishing effect (flame retardant). Figure 1 In [reference], the white smoke generated in Example 2 is antimony trichloride decomposed from antimony oxychloride. And Example 2 also shows that the separator does not burn completely and a part of the residue is preserved. Compared with Example 1, Example 2 uses a higher concentration of antimony trichloride to soak the cellulose gel and has a higher antimony oxychloride loading. It shows that the higher the content of antimony oxychloride, the better the flame retardant effect. The loading of antimony oxychloride in Example 2 is ~40 wt%, and that in Example 1 is ~10 wt%.

[0075] Figure 2 The full cell cycle performance tests of the different separators prepared in Examples 1-2 and Comparative Example 1. This full cell uses lithium iron phosphate as the positive electrode and lithium foil as the negative electrode.

[0076] From Figure 2 it can be seen that compared with the pure cellulose separator (Comparative Example 1), the battery assembled with the separator loaded with antimony oxychloride has more excellent cycle performance. This may be because a lower content of antimony oxychloride helps to regulate the pores (the uniform distribution of antimony oxychloride particles makes the pore size more uniform). And a further increase in the content of antimony oxychloride will lead to a decrease in performance, because too much antimony oxychloride will block the pores and affect ion transport (see Figure 5 ).

[0077] Figure 5 SEM images of the separators prepared in Example 1 and Example 2.

[0078] Figure 3 SEM images of the separators prepared in Comparative Example 1-Comparative Example 3.

[0079] Figure 4 SEM images of the separator prepared in Example 6.

[0080] From Figure 3It can be seen that Comparative Example 1, Comparative Example 2, and Comparative Example 3 are all pure cellulose diaphragms. As the temperature of the regeneration coagulation bath increases, the pores gradually increase. From Figure 4 It can be seen that the diaphragm obtained by the hot pressing method has no obvious pores.

[0081] The diaphragms prepared in Example 1 and Example 2 were subjected to thermal stability tests at 25°C, 100°C, 120°C, and 140°C. The diaphragms prepared in Examples 1-2 did not shrink. The cellulose-based diaphragm has good thermal stability

[0082] It can be seen from the examples that the pure cellulose diaphragm obtained in the comparative example will burn out when ignited. However, the cellulose diaphragm coated with antimony oxychloride obtained in the examples has good self-extinguishing properties, significantly improving the safety of the battery. In a full battery assembled with lithium iron phosphate as the positive electrode and lithium foil as the negative electrode, this cellulose-based diaphragm loaded with antimony oxychloride has good cycle stability.

[0083] In summary, the preparation method of the cellulose-based battery diaphragm of the present invention is simple. The composite battery diaphragm is prepared by combining antimony oxychloride with flame retardant effect and cellulose with good thermal stability, significantly improving the safety of the battery. Since antimony oxychloride is generated in-situ in the cellulose network, it avoids the defects faced by traditional coating processes, such as coating peeling, the need for additional binders, and blocking ion transport. Compared with traditional polyolefin diaphragms, using cellulose as the matrix is beneficial to reducing the dependence on fossil resources and reducing environmental pollution. The process of dissolving and regenerating cellulose to prepare cellulose-based diaphragms can be matched with existing diaphragm preparation processes, so it has good prospects for popularization and application.

Claims

1. A preparation method of a high - security regenerated cellulose - based lithium battery separator, characterized in that: It includes the following steps: 1) Dissolve cellulose in a cellulose solvent to obtain a cellulose solution; cast or extrude the cellulose solution into a cellulose film; 2) Place the cellulose film in an ethanol solution of antimony trichloride to obtain a cellulose hydrogel containing antimony trichloride; 3) Immerse the cellulose hydrogel in water to remove the solvent, obtaining a cellulose hydrogel containing antimony oxychloride; 4) Dry the cellulose hydrogel containing antimony oxychloride to obtain a cellulose separator; In step 2), the ethanol solution of antimony trichloride is composed of ethanol and antimony trichloride, and the mass content of antimony trichloride is 0.08% - 5%; In step 2), the temperature of the ethanol solution of antimony trichloride is 20 - 70 °C; In step 4), the drying method is vacuum freeze-drying.

2. The preparation method of the high-safety regenerated cellulose-based lithium battery separator according to claim 1, characterized in that: In step 2), the ethanol solution of antimony trichloride is composed of ethanol and antimony trichloride, and the mass content of antimony trichloride is 0.08 - 0.6%; In step 4), the drying method is vacuum freeze-drying; specifically, pre-freeze at -40 °C to -80 °C for 8 - 24 hours, and then immediately place it in a vacuum freeze dryer for drying.

3. The method for preparing a highly safe regenerated cellulose-based lithium battery separator according to claim 1, characterized in that: In step 2), the soaking time of the cellulose film in the ethanol solution of antimony trichloride is 10 - 16 hours; In step 1), the cellulose solvent is one of the following solvents: LiCl / DMAC, 1-allyl-3-methylimidazolium chloride, or a deep eutectic solvent.

4. The preparation method of the high-safety regenerated cellulose-based lithium battery separator according to claim 3, wherein: The deep eutectic solvent: Mix a hydrogen bond acceptor and a hydrogen bond donor in a molar ratio of 0.5:1 to 5:1, and heat to a homogeneous and transparent state at 50 - 100 °C to obtain a deep eutectic solvent; the hydrogen bond donor is one of dimethylurea, N-methylthiourea, dimethylthiourea, diethylthiourea, 2-pyrrolidone; the hydrogen bond acceptor is selected from one of the following structural general formulas: wherein the anion X - is one of chloride ion, bromide ion, acetate ion; R is an alkenyl group.

5. The preparation method of the high-safety regenerated cellulose-based lithium battery separator according to claim 4, characterized in that: The anion is chloride, and R is allyl; The hydrogen bond donor is dimethylthiourea.

6. The preparation method of the highly safe regenerated cellulose-based lithium battery separator according to claim 3, wherein: The deep eutectic solvent is obtained by mixing a hydrogen bond acceptor and a hydrogen bond donor in a molar ratio of 1:1 to 2:1, and heating and stirring at 70 - 90 °C to obtain a homogeneous and transparent liquid.

7. The method for preparing a highly safe regenerated cellulose-based lithium battery separator according to claim 1, characterized in that: In step 1), the dissolution temperature of the cellulose is 70 - 110 °C; In step 1), the depth of the groove of the mold used for casting is 0.1 - 1 mm; In step 1), the cellulose is derived from microcrystalline cellulose, bleached hardwood pulp, bleached softwood pulp, bagasse pulp, bleached bamboo pulp, wheat straw cellulose, rice straw cellulose, cotton linter, bacterial cellulose.

8. The method for preparing a highly safe regenerated cellulose-based lithium battery separator according to claim 1, characterized in that: In step 3), the soaking time is 8 - 24 hours; the removal of the solvent refers to washing with water.

9. A highly safe regenerated cellulose-based lithium battery separator obtained by the preparation method according to any one of claims 1 - 8.

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