A polyvinylidene fluoride film used as a battery separator and a preparation method thereof

Abstract

The present invention relates to the technical field of polyvinylidene fluoride film preparation, and particularly to a polyvinylidene fluoride film used as a battery separator and a preparation method thereof, comprising the following steps: S1. By mass, 4 to 15 parts of polyvinylidene fluoride powder are added to 85 to 96 parts of an organic mixed solvent, and after stirring and dissolving, a uniform polyvinylidene fluoride solution is obtained; S2. The polyvinylidene fluoride solution is directionally spread on the surface of a liquid coagulation bath, and the spread solution is solidified into a film by a non-solvent-induced phase separation method, and dried at 23 to 27 °C to obtain a polyvinylidene fluoride film. The polyvinylidene fluoride film prepared by the preparation method provided by the present invention has uniform pore size and distribution and low crystallinity, can realize a uniform electroplating / stripping process of lithium / sodium ions, inhibit the growth of dendrites, and obtain a lithium / sodium ion battery with a longer service life and higher safety performance.

Description

Technical Field

[0001] The present invention relates to the technical field of polyvinylidene fluoride film preparation, and particularly relates to a polyvinylidene fluoride film used as a battery separator and a preparation method thereof. Background Art

[0002] As a key component of the battery, the main function of the separator is to isolate the positive electrode and the negative electrode to prevent internal short circuit of the battery caused by the contact of the positive and negative electrodes. In addition, the separator needs to absorb and retain the liquid electrolyte, and at the same time, the separator needs to be a porous structure to provide a channel for the migration of lithium / sodium ions. Therefore, the pore size and the pore size distribution range of the separator have an important impact on the performance of the battery. An excessively large pore size will cause serious self-discharge of the battery, while an excessively small pore size will affect the efficiency of the solvation sheath formed by lithium / sodium ions in the liquid electrolyte passing through the separator. In addition, an uneven pore size distribution or a too wide pore size distribution range will cause uneven plating / stripping of lithium ions and accelerate the growth of lithium dendrites. These performances related to the pore size of the separator will further affect the life and safety performance of the battery.

[0003] At present, the commercialized battery separator is mainly the PP separator. Although its pore size distribution range is relatively concentrated, due to the problem of the pore-forming process, the shape of its pores is elliptical with a relatively large aspect ratio (the ratio of the long diameter to the short diameter). Currently, porous separators are mainly obtained by electrospinning or doctor blading. For example, Guanghai Chen et al. reported in "AFlexible Dual-Ion Battery Based on PVDF-HFP-Modified Gel Polymer Electrolytewith Excellent Cycling Performance and Superior Rate Capability,AdvancedEnergy Materials,2018,1801219" that a PVDF-HFP polymer membrane with a pore size of about 450 nm was obtained by doctor blading. Menglin Li et al. reported in "Enhanced the mechanical strength of polyimide(PI)nanofiber separator via PAALi binder for lithium ionbattery,CompositesCommunications,2021,24,100607" that a polyimide membrane with a pore size of 510 nm and a pore size distribution range of about ±100 nm was obtained by electrospinning. The disadvantages of the separators obtained by these two methods are that the pore size is too large and the pore size distribution is not concentrated. The pore characteristics of the above-mentioned separators are not conducive to the requirements of the battery in terms of long life and high safety performance. Therefore, a separator with a suitable pore size and a concentrated pore size distribution range is very important for the life and safety performance of the battery.

[0004] The invention patent application number CN202410230391.X, “A porous polymer membrane and its preparation method,” proposes a method for rapidly preparing polymer films, which may address the poor pore size and distribution quality issues encountered by other existing membrane-forming technologies. However, one of the characteristics of the membrane-forming method adopted in this invention is its “fast film-forming speed,” which can affect the crystallinity of crystalline polymers such as polyvinylidene fluoride during the film-forming process. The faster the film-forming speed, the more likely the polymer crystallinity will be significantly improved. Taking crystalline polyvinylidene fluoride and its copolymers as an example, the crystallinity of polymer films prepared using traditional blade coating or casting methods is generally low, typically around 30%, as reported in the literature “Enhancing piezoelectric properties of PVDF-HFP composite nanofibers with cellulose nanocrystals, Materials Today Communications, 2024, 39, 108872.” However, polyvinylidene fluoride films prepared using a similar rapid film-forming method may have a very high crystallinity. The extremely rapid transition of a polymer from a dissolved state to a solid state may be similar to the process of rapidly annealing a polymer membrane in cold water after heating it at high temperatures, a process traditionally employed to improve its crystallinity, as reported in the paper "Ultrahigh β-phase content poly(vinylidene fluoride) with relaxor-like ferroelectricity for high energy density capacitors, Nature Communications, 2019, 10, 4535." The increased crystallinity of PVDF membranes is detrimental to their application as separators in lithium / sodium ion batteries, hindering their wettability with the electrolyte and the migration of lithium / sodium ions, thereby limiting improvements in battery performance. Summary of the Invention

[0005] In view of this, the present invention proposes a polyvinylidene fluoride film used as a battery separator and a preparation method thereof, in order to solve the problems in existing membrane making technology, such as the difficulty in controlling the pore size of the battery separator and the insufficient concentration of the pore size distribution range leading to dendrite growth, as well as the problem of the vicious growth of the crystallinity of the crystalline polymer membrane caused by the rapid film forming process of the polymer solution.

[0006] The technical solution of the present invention is achieved as follows:

[0007] In a first aspect, the present invention provides a method for preparing a polyvinylidene fluoride film used as a battery separator, comprising the following steps:

[0008] S1. Add 4 - 15 parts by mass of polyvinylidene fluoride powder to 85 - 96 parts of an organic mixed solvent, stir and dissolve to obtain a uniform polyvinylidene fluoride solution.

[0009] S2. Make the polyvinylidene fluoride solution spread directionally on the surface of a liquid coagulation bath, and solidify the spread solution into a film by the method of non - solvent - induced phase separation, and dry it at 23 - 27 °C to obtain a polyvinylidene fluoride film.

[0010] In the present invention, by precisely controlling the composition and film - forming conditions of the polyvinylidene fluoride solution, precise regulation of the pore size and distribution of the polyvinylidene fluoride film is achieved. At the same time, the problem of too high crystallinity caused by rapid film - forming is avoided, and the film has good electrolyte wettability and ion migration performance. Specifically, in step S1, by dissolving a specific proportion of polyvinylidene fluoride powder in an organic mixed solvent, the viscosity and phase - separation behavior of the solution can be adjusted, thereby affecting the pore size and distribution of the final film; in step S2, a porous polyvinylidene fluoride film is prepared by the non - solvent - induced phase - separation method. By making the polyvinylidene fluoride solution flow directionally and solidify into a film on the surface of a liquid coagulation bath, a film with a specific pore structure can be formed. The non - solvent - induced phase - separation method uses the exchange between a solvent and a non - solvent to induce phase separation of a polymer solution. When the polyvinylidene fluoride solution contacts the coagulation bath (usually containing water), the solvent and the non - solvent exchange rapidly, causing polyvinylidene fluoride to precipitate from the solution and form a porous structure. By controlling the phase - separation process, a porous film with uniform pore size and concentrated distribution is formed. Directional flow helps to form a uniform film structure and avoid non - uniformity of pore size and distribution; a lower drying temperature can slow down the crystallization rate of polyvinylidene fluoride, which is beneficial to avoiding too high crystallinity and thus maintaining good flexibility and electrolyte wettability of the film.

[0011] Based on the above technical solution, preferably, in step S1, the organic mixed solvent includes component A and component B. By mass percentage, component A is 95 - 99.5%, and component B is 0.5 - 5%.

[0012] Based on the above technical solution, preferably, component A is one of dimethyl sulfoxide, N,N - dimethylformamide, N,N - dimethylacetamide or N - methylpyrrolidone, and component B is petroleum ether.

[0013] Specifically, component A, as a good solvent, can effectively dissolve polyvinylidene fluoride powder, while component B, as a non-solvent, can reduce the solubility of the solution in polyvinylidene fluoride. The presence of component B can affect the crystallization behavior of polyvinylidene fluoride. By adjusting the ratio of component A and component B, the solubility of polyvinylidene fluoride in the solution can be precisely controlled, and the crystallization rate of polyvinylidene fluoride can be slowed down, which helps to avoid the problem of too high crystallinity caused by rapid film formation, thereby maintaining the good flexibility and electrolyte wettability of the film.

[0014] Based on the above technical solutions, preferably, in step S1, the stirring and dissolution temperature is 20 - 35 °C, and the time is 12 - 48 h.

[0015] Based on the above technical solutions, preferably, in step S2, the liquid coagulation bath includes component A and water, and the volume ratio of component A to water is 0:100 - 37:63. Component A can be one of dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide or N-methylpyrrolidone.

[0016] Directional flow helps to form a uniform film, and the composition of the liquid coagulation bath (such as the ratio of component A to water) directly affects the pore formation process and film formation rate. When the polyvinylidene fluoride solution contacts the coagulation bath, the solvent in the solution exchanges with the non-solvent (water) in the coagulation bath. The presence of component A can adjust this exchange rate, thereby affecting the pore formation process. By adjusting the ratio of component A and water, the pore size and distribution can be precisely controlled. A higher water content usually leads to faster phase separation and larger pore sizes; while increasing the proportion of component A can slow down the phase separation and help to form smaller and more uniformly distributed pores.

[0017] In the second aspect, the present invention provides a polyvinylidene fluoride film, which is prepared by using the preparation method described in any one of the above, and the polyvinylidene fluoride film is a porous film.

[0018] Based on the above technical solutions, preferably, the pore size of the polyvinylidene fluoride film is 42 - 234 nm, the pore size distribution range is ±2 nm - ±26 nm, the porosity is 42% - 84%, and the Gurley value of the air permeability is 196 s / 100 mL - 856 s / 100 mL.

[0019] Based on the above technical solutions, preferably, the thickness of the polyvinylidene fluoride film is 5.2 μm - 22.7 μm, and the crystallinity of the polyvinylidene fluoride film is 26% - 31%.

[0020] In the third aspect, the present invention provides an application of a polyvinylidene fluoride film, and the polyvinylidene fluoride film is used in a lithium-ion battery or a sodium-ion battery.

[0021] Based on the above technical solutions, preferably, when the polyvinylidene fluoride film is used in a lithium-ion battery, the electrolyte of the lithium-ion battery is any one of commercial electrolytes LB-266, LB-083, and LB-002; when the polyvinylidene fluoride film is used in a sodium-ion battery, the electrolyte of the sodium-ion battery is any one of commercial electrolytes NC-021, NC-022, and NC-004.

[0022] More preferably, electrolyte LB-266 has a purity of 1M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC, electrolyte LB-083 has a purity of 1M LiPF6 in DMC:EC:EMC = 1:1:1 Vol% with 5% FEC, and electrolyte LB-002 has a purity of 1M LiPF6 in DMC:EC:EMC = 1:1:1 Vol%. When the polyvinylidene fluoride film is used as a battery separator in a lithium-ion battery, any one of the above electrolytes is used as the electrolyte of the liquid lithium-ion battery, and a Li||LFP button battery is assembled. After cycling 221 to 321 times at a charge-discharge rate of 1C, the capacity retention rate reaches more than 79%.

[0023] More preferably, electrolyte NC-021 has a purity of 1M NaPF6 in EC:PC = 1:1:1 Vol%, electrolyte NC-022 has a purity of 1M NaPF6 in EC:PC = 1:1:1 Vol% with 2% FEC, and electrolyte NC-004 has a purity of 1M NaPF6 in EC:PC = 1:1:1 Vol% with 5% FEC. When the polyvinylidene fluoride film is used as a battery separator in a sodium-ion battery, any one of the above electrolytes is used as the electrolyte of the liquid sodium-ion battery, and a Na||PBAs button battery is assembled. After cycling 308 to 356 times at a charge-discharge rate of 1C, the capacity retention rate reaches more than 67%.

[0024] The polyvinylidene fluoride film used as a battery separator in the present invention and its preparation method have the following beneficial effects compared with the prior art:

[0025] (1) The pore size of the polyvinylidene fluoride film prepared by the preparation method provided in the present invention is 42nm to 234nm, and the pore size distribution range is: ±2nm to ±26nm. The pore size is adjustable within a range suitable for practical applications, and the pore size distribution range is very concentrated. It can realize a uniform electroplating / stripping process of lithium / sodium ions, inhibit the growth of dendrites, and obtain a lithium / sodium ion battery with a longer life and higher performance;

[0026] (2) Compared with commercial PP separators, the polyvinylidene fluoride film obtained in this invention has higher porosity and air permeability, which will increase the electrolyte absorption rate of the separator and is beneficial to improving the performance of the battery; the thickness of the polyvinylidene fluoride film is between 5.2 μm and 22.7 μm. Compared with commercial PP separators (with a general thickness of 25 μm), it is small in volume and light in weight, and can be used for higher battery energy density;

[0027] (3) By regulating the ratio of component A (good solvent) and component B (non-solvent) in the organic mixed solvent, and adopting the method of non-solvent induced phase separation combined with directional flow film formation, this invention realizes precise control of the pore structure of the polyvinylidene fluoride film, making the prepared film have an ideal pore size and a very narrow distribution range, and reducing the crystallinity of the polyvinylidene fluoride film to 26% - 31%, which can better absorb the electrolyte and promote ion migration. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of this invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of this invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0029] Figure 1 It is the cycle performance graph of the lithium-ion battery in Example 1 at room temperature;

[0030] Figure 2 It is the cross-sectional SEM graph and pore size distribution graph of the polyvinylidene fluoride film in Example 2

[0031] Figure 3 It is the cycle performance graph of the lithium-ion battery in Example 2 at room temperature;

[0032] Figure 4 It is the cycle performance graph of the sodium-ion battery in Example 2 at room temperature;

[0033] Figure 5 It is the cycle performance graph of the lithium-ion battery in Example 3 at room temperature;

[0034] Figure 6 It is the cycle performance graph of the lithium-ion battery in Example 4 at room temperature;

[0035] Figure 7 It is the cross-sectional SEM graph and pore size distribution graph of the polyvinylidene fluoride film in Example 5;

[0036] Figure 8 It is the cycle performance graph of the lithium-ion battery in Example 5 at room temperature;

[0037] Figure 9 Cycling performance graph of the sodium-ion battery of Example 5 at room temperature;

[0038] Figure 10 Cycling performance graph of the lithium-ion battery of Example 6 at room temperature;

[0039] Figure 11 Cycling performance graph of the lithium-ion battery of Example 7 at room temperature;

[0040] Figure 12 Cross-sectional SEM image and pore size distribution graph of the polyvinylidene fluoride film of Example 8;

[0041] Figure 13 Cycling performance graph of the lithium-ion battery of Example 8 at room temperature;

[0042] Figure 14 Cycling performance graph of the sodium-ion battery of Example 8 at room temperature;

[0043] Figure 15 Cycling performance graph of the lithium-ion battery of Example 9 at room temperature;

[0044] Figure 16 Cycling performance graph of the lithium-ion battery of Comparative Example 1 at room temperature;

[0045] Figure 17 Polyvinylidene fluoride film graphs of Comparative Examples 2, 4, and 6;

[0046] Figure 18 Cycling performance graph of the lithium-ion battery of Comparative Example 7 at room temperature;

[0047] Figure 19 Cross-sectional SEM image and pore size distribution graph of the polyvinylidene fluoride film of Comparative Example 8;

[0048] Figure 20 Cycling performance graph of the lithium-ion battery of Comparative Example 8 at room temperature;

[0049] Figure 21 Cycling performance graph of the lithium-ion battery of Comparative Example 9 at room temperature;

[0050] Figure 22 Cross-sectional SEM image and pore size distribution graph of the polyvinylidene fluoride film of Comparative Example 10;

[0051] Figure 23 Cycling performance graph of the lithium-ion battery of Comparative Example 10 at room temperature;

[0052] Figure 24 Cycling performance graph of the sodium-ion battery of Comparative Example 10 at room temperature;

[0053] Figure 25 It is the cross-sectional SEM image of the lithium-ion battery separator of Comparative Example 11;

[0054] Figure 26 It is the pore size distribution diagram of the lithium-ion battery separator of Comparative Example 11;

[0055] Figure 27 It is the cycling performance diagram of the lithium-ion battery of Comparative Example 11 at room temperature. Specific Embodiments

[0056] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0057] Example 1

[0058] This example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0059] S1. Add 4 g of polyvinylidene fluoride powder into a mixed solvent of 96 g of DMF and petroleum ether, wherein petroleum ether accounts for 5.0% of the mixed solvent, and stir at 20 °C for 12 h to dissolve to obtain a uniform polyvinylidene fluoride solution;

[0060] S2. By the method of non-solvent induced phase separation, make the polyvinylidene fluoride solution flow directionally and solidify into a film on the surface of a liquid coagulation bath (volume ratio V 水 :V DMF is = 100:0) to obtain a wet porous polyvinylidene fluoride film; dry this polyvinylidene fluoride film at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0061] The melting enthalpy ΔH f of the obtained polyvinylidene fluoride film can be measured by a differential scanning calorimeter (DSC), and then the crystallinity χ c of the prepared polyvinylidene fluoride can be calculated according to the following formula:

[0062]

[0063] In the formula, refers to the melting enthalpy (104.6 J / g) of polyvinylidene fluoride with a crystallinity of 100%.

[0064] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this embodiment is 28.5%, the average thickness is 5.2 μm; the pore size is 52 nm, the pore size distribution range is: ±8.6 nm, the porosity is 42%, and the Gurley value of the air permeability is 196 s / 100 mL. From Figure 1 it can be seen that the polyvinylidene fluoride film prepared in this embodiment is a porous film with a relatively uniform pore size distribution.

[0065] Using the above polyvinylidene fluoride film as a lithium-ion battery separator, and using a commercial electrolyte LB-266 (1 M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte, a Li||LFP button battery is assembled, as Figure 1 shown, the capacity retention rate of the lithium-ion battery prepared in this embodiment after 305 cycles at a charge-discharge rate of 1C is 79.60%.

[0066] Example 2

[0067] This embodiment provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0068] S1. Add 4 g of polyvinylidene fluoride powder to a mixed solvent of 96 g of DMF and petroleum ether, where petroleum ether accounts for 2.0% of the mixed solvent, and stir at 20 °C for 12 h to dissolve to obtain a uniform polyvinylidene fluoride solution;

[0069] S2. By the method of non-solvent induced phase separation, make the polyvinylidene fluoride solution flow directionally and solidify into a film in a liquid coagulation bath (a mixed solvent with a volume ratio of V 水 :V DMF = 100:0), to obtain a porous polyvinylidene fluoride film; dry this polyvinylidene fluoride film at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0070] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this embodiment is 30%, the thickness is 5.9 μm (as Figure 2 shown in a, Figure 2 a shows the cross-sectional SEM image of the polyvinylidene fluoride film), the pore size is 44 nm, the pore size distribution range is: ±2.7 nm, (as Figure 2 shown in b, Figure 2 b shows the pore size distribution diagram of the polyvinylidene fluoride film), the porosity is 43.5%, and the Gurley value of the air permeability is 203 s / 100 mL. As Figure 2 shown, the polyvinylidene fluoride film prepared in this embodiment is a porous film with a relatively uniform pore size distribution.

[0071] Using the above polyvinylidene fluoride film as the lithium-ion battery separator and a commercial electrolyte LB-266 (1 M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte, a Li||LFP button battery was assembled. As Figure 3 shown, for the lithium-ion battery prepared in this embodiment, the capacity retention rate after 254 cycles at a charge-discharge rate of 1C was 91.31%.

[0072] Using the above polyvinylidene fluoride film as the sodium-ion battery separator and a commercial electrolyte NC-022 (1 M NaPF6 in EC:PC = 1:1:1 Vol% with 2% FEC) as the liquid electrolyte, a Na||PBAs button battery was assembled. As Figure 4 shown, for the sodium-ion battery prepared in this embodiment, the capacity retention rate after 308 cycles at a charge-discharge rate of 1C was 67.20%.

[0073] Example 3

[0074] This embodiment provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0075] S1. Add 4 g of polyvinylidene fluoride powder to a mixed solvent of 96 g of DMF and petroleum ether, where petroleum ether accounts for 0.5% of the mixed solvent. Stir at 20 °C for 12 h to dissolve and obtain a uniform polyvinylidene fluoride solution.

[0076] S2. By the method of non-solvent induced phase separation, make the polyvinylidene fluoride solution flow directionally and solidify into a film in a liquid coagulation bath (a mixed solvent with a volume ratio of V 水 :V DMF = 100:0) to obtain a porous polyvinylidene fluoride film; dry this polyvinylidene fluoride film at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0077] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this embodiment was 31%, the thickness was 7.1 μm, the pore size was 42 nm, the pore size distribution range was ±2.1 nm, the porosity was 44.5%, and the Gurley value of the air permeability was 206 s / 100 mL.

[0078] Using the above polyvinylidene fluoride film as the lithium-ion battery separator and a commercial electrolyte LB-266 (1 M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte, a Li||LFP button battery was assembled. As Figure 5As shown, the lithium-ion battery prepared in this embodiment has a capacity retention rate of 78.09% after 261 cycles at a charge-discharge rate of 1C.

[0079] Example 4

[0080] This embodiment provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0081] S1. Add 8 g of polyvinylidene fluoride powder to a mixed solvent of 92 g of DMF and petroleum ether, where petroleum ether accounts for 5.0% of the mixed solvent, and stir at 20 °C for 48 h to dissolve to obtain a uniform polyvinylidene fluoride solution;

[0082] S2. By the method of non-solvent-induced phase separation, make the polyvinylidene fluoride solution flow directionally and solidify into a film in a liquid coagulation bath (a mixed solvent with a volume ratio of V 水 :V NMP = 82:18) to obtain a porous polyvinylidene fluoride film; dry this polyvinylidene fluoride film at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0083] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this embodiment is 27%, the thickness is 10.7 μm, the pore size is 114 nm, the pore size distribution range is: ±15.7 nm, the porosity is 56%, and the Gurley value of the air permeability is 374 s / 100 mL.

[0084] Using the above polyvinylidene fluoride film as a lithium-ion battery separator and a commercial electrolyte LB-266 (1 M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte, assemble a Li||LFP button battery. As Figure 6 shown, the lithium-ion battery prepared in this embodiment has a capacity retention rate of 83.04% after 300 cycles at a charge-discharge rate of 1C.

[0085] Example 5

[0086] This embodiment provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0087] S1. Add 8 g of polyvinylidene fluoride powder to a mixed solvent of 92 g of DMF and petroleum ether, where petroleum ether accounts for 2.0% of the mixed solvent, and stir at 20 °C for 48 h to dissolve to obtain a uniform polyvinylidene fluoride solution;

[0088] S2. By the method of non-solvent-induced phase separation, make the polyvinylidene fluoride solution flow directionally and in a liquid coagulation bath (volume ratio of V水 : V NMP = 82: 18) to form a porous polyvinylidene fluoride film; dry this polyvinylidene fluoride film at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0089] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this example is 29%, and the thickness is 11.6 μm (as Figure 7 shown in Figure 7 a, Figure 7 a shows the cross-sectional SEM image of the polyvinylidene fluoride film), the pore size is 96 nm, and the pore size distribution range is: ±1.8 nm (as Figure 7 shown in Figure 7 b,

[0090] b shows the pore size distribution diagram of the polyvinylidene fluoride film), the porosity is 58%, and the Gurley value of the air permeability is 379 s / 100 mL. It can be Figure 8 seen that the polyvinylidene fluoride film prepared in this example is a porous film with a relatively uniform pore size distribution.

[0091] Use the above polyvinylidene fluoride film as a lithium-ion battery separator, and use a commercial electrolyte LB-266 (1 M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte to assemble a Li||LFP button battery. As Figure 9 shown, the capacity retention rate of the lithium-ion battery prepared in this example after 402 cycles at a charge-discharge rate of 1C is 78.84%.

[0092] Example 6

[0093] This example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0094] S1. Add 8 g of polyvinylidene fluoride powder to a mixed solvent of 92 g of DMF and petroleum ether, where petroleum ether accounts for 0.5% of the mixed solvent, and stir at 20 °C for 48 h to dissolve to obtain a uniform polyvinylidene fluoride solution;

[0095] S2. Through the method of non-solvent-induced phase separation, make the polyvinylidene fluoride solution flow directionally and in a liquid coagulation bath (volume ratio of V水 : V NMP = 82: 18 mixed solvent), and cured into a film to obtain a porous polyvinylidene fluoride film; the polyvinylidene fluoride film was dried at room temperature to obtain a porous polyvinylidene fluoride film for later use.

[0096] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this embodiment is 30.5%, the thickness is 13.5 μm, the pore size is 91 nm, the pore size distribution range is: ±2.1 nm, the porosity is 62%, and the Gurley value of the air permeability is 387 s / 100 mL.

[0097] Using the above polyvinylidene fluoride film as a lithium-ion battery separator, with a commercial electrolyte LB-266 (1 M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte, an Li||LFP button battery was assembled. As Figure 10 shown, the capacity retention rate of the lithium-ion battery prepared in this embodiment after 321 cycles at a charge-discharge rate of 1C is 83.30%.

[0098] Example 7

[0099] This embodiment provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0100] S1. Add 15 g of polyvinylidene fluoride powder to a mixed solvent of 85 g of DMF and petroleum ether, where petroleum ether accounts for 5.0% of the mixed solvent, and stir at 35 °C for 36 h to dissolve to obtain a uniform polyvinylidene fluoride solution;

[0101] S2. By the method of non-solvent-induced phase separation, make the polyvinylidene fluoride solution flow directionally, and cure it into a film in a liquid coagulation bath (volume ratio of V 水 : V DMF = 63: 37 mixed solvent) to obtain a porous polyvinylidene fluoride film; the polyvinylidene fluoride film was dried at room temperature to obtain a porous polyvinylidene fluoride film for later use.

[0102] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this embodiment is 26%, the thickness is 20.7 μm, the pore size is 234 nm, the pore size distribution range is: ±25.8 nm, the porosity is 78%, and the Gurley value of the air permeability is 832 s / 100 mL.

[0103] Using the above polyvinylidene fluoride film as the lithium-ion battery separator, with commercial electrolyte LB-266 (1M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte, an Li||LFP button battery was assembled. As Figure 11 shown, for the lithium-ion battery prepared in this embodiment, the capacity retention rate after 507 cycles at a charge-discharge rate of 1C is 80.51%.

[0104] Example 8

[0105] This embodiment provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0106] S1. Add 15 g of polyvinylidene fluoride powder to a mixed solvent of 85 g of DMF and petroleum ether, where petroleum ether accounts for 2.0% of the mixed solvent. Stir at 35 °C for 36 h to dissolve and obtain a uniform polyvinylidene fluoride solution.

[0107] S2. By the method of non-solvent-induced phase separation, make the polyvinylidene fluoride solution flow directionally and solidify it into a film in a liquid coagulation bath (a mixed solvent with a volume ratio of V 水 :V DMF = 63:37), to obtain a porous polyvinylidene fluoride film; dry this polyvinylidene fluoride film at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0108] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this embodiment is 27.5%, the thickness is 21.6 μm (as Figure 12 shown in Figure 12 a, Figure 12 a shows the cross-sectional SEM image of the polyvinylidene fluoride film), the pore size is 214 nm, and the pore size distribution range is: ±3.8 nm (as<able> Figure 12 shown in Figure 12 b,

[0109] b shows the pore size distribution diagram of the polyvinylidene fluoride film), the porosity is 81%, and the Gurley value of the air permeability is 841 s / 100 mL. It can be Figure 13 seen that the polyvinylidene fluoride film prepared in this embodiment is a porous film with a relatively uniform pore size distribution.

[0110] Using the above polyvinylidene fluoride film as the sodium-ion battery separator, and using the commercial electrolyte NC-022 (1M NaPF6 in EC:PC = 1:1:1 Vol% with 2% FEC) as the liquid electrolyte, a Na||PBAs button battery was assembled. As Figure 14 shown, for the sodium-ion battery prepared in this embodiment, the capacity retention rate after 356 cycles at a charge-discharge rate of 1C is 82.80%.

[0111] Example 9

[0112] This embodiment provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0113] S1. Add 15 g of polyvinylidene fluoride powder to a mixed solvent of 85 g of DMF and petroleum ether, where petroleum ether accounts for 0.5% of the mixed solvent. After stirring and dissolving at 35 °C for 36 h, a uniform polyvinylidene fluoride solution is obtained;

[0114] S2. By the method of non-solvent-induced phase separation, the polyvinylidene fluoride solution is made to flow directionally and solidified into a film in a liquid coagulation bath (a mixed solvent with a volume ratio of V 水 :V DMF = 63:37), and a porous polyvinylidene fluoride film is obtained; this polyvinylidene fluoride film is dried at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0115] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this embodiment is 29%, the thickness is 22.7 μm, the pore size is 211 nm, the pore size distribution range is: ±3.1 nm, the porosity is 84%, and the Gurley value of the air permeability is 856 s / 100 mL.

[0116] Using the above polyvinylidene fluoride film as the lithium-ion battery separator, and using the commercial electrolyte LB-266 (1M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte, a Li||LFP button battery was assembled. As Figure 15 shown, for the lithium-ion battery prepared in this embodiment, the capacity retention rate after 397 cycles at a charge-discharge rate of 1C is 81.91%.

[0117] Comparative Example 1

[0118] This comparative example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0119] S1. Add 4 g of polyvinylidene fluoride powder to a mixed solvent of 96 g of DMF and petroleum ether, where petroleum ether accounts for 0.2% of the mixed solvent. Stir at 20 °C for 12 h to dissolve and obtain a uniform polyvinylidene fluoride solution.

[0120] S2. By the method of non-solvent induced phase separation, make the polyvinylidene fluoride solution flow directionally and solidify it into a film in a liquid coagulation bath (a mixed solvent with a volume ratio of V 水 :V DMF = 100:0), to obtain a porous polyvinylidene fluoride film; dry this polyvinylidene fluoride film at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0121] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this comparative example is 38%, the thickness is 8.4 μm, the pore size is 41 nm, the pore size distribution range is: ±2.0 nm, the porosity is 45%, and the Gurley value of the air permeability is 208 s / 100 mL.

[0122] Use the above polyvinylidene fluoride film as a lithium-ion battery separator, and use a commercial electrolyte LB-266 (1 M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte to assemble a Li||LFP button battery. As Figure 16 shown, the lithium-ion battery prepared in this comparative example has a capacity retention rate of 84.50% after 211 cycles at a charge-discharge rate of 1C.

[0123] Comparative Example 2

[0124] This comparative example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0125] S1. Add 4 g of polyvinylidene fluoride powder to a mixed solvent of 96 g of DMF and petroleum ether, where petroleum ether accounts for 6.0% of the mixed solvent. Stir at 20 °C for 12 h to dissolve, and the solution precipitates. As Figure 17 (a) shown, a complete polyvinylidene fluoride film was not prepared in this comparative example because the B-component organic solvent is petroleum ether, which can hinder the dissolution of polyvinylidene fluoride, and has low miscibility with the A-component organic solvent, is insoluble in water, and can greatly slow down the diffusion of the organic solvent in the polyvinylidene fluoride solution into the coagulation bath.

[0126] Comparative Example 3

[0127] This comparative example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0128] S1. Add 8 g of polyvinylidene fluoride powder to a mixed solvent of 92 g of DMF and petroleum ether, where petroleum ether accounts for 0.2% of the mixed solvent. Stir at 20 °C for 48 h to dissolve and obtain a homogeneous polyvinylidene fluoride solution;

[0129] S2. By the method of non-solvent induced phase separation, make the polyvinylidene fluoride solution flow directionally and solidify it into a film in a liquid coagulation bath (a mixed solvent with a volume ratio of V 水 :V NMP = 82:18) to obtain a porous polyvinylidene fluoride film; Dry this polyvinylidene fluoride film at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0130] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this comparative example is 36.5%, the thickness is 14.1 μm, the pore size is 82 nm, the pore size distribution range is: ±2.2 nm, the porosity is 64%, and the Gurley value of the air permeability is 391 s / 100 mL.

[0131] Comparative Example 4

[0132] This comparative example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0133] S1. Add 8 g of polyvinylidene fluoride powder to a mixed solvent of 92 g of DMF and petroleum ether, where petroleum ether accounts for 6.0% of the mixed solvent. Stir at 20 °C for 48 h to dissolve, and the solution precipitates. As Figure 17 (b) shows, in this comparative example, a complete polyvinylidene fluoride film was not prepared because the B-component organic solvent is petroleum ether, which can hinder the dissolution of polyvinylidene fluoride and has a low miscibility with the A-component organic solvent, is insoluble in water, and can greatly slow down the diffusion of the organic solvent in the polyvinylidene fluoride solution into the coagulation bath.

[0134] Comparative Example 5

[0135] This comparative example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0136] S1. Add 15 g of polyvinylidene fluoride powder to a mixed solvent of 85 g of DMF and petroleum ether, where petroleum ether accounts for 0.2% of the mixed solvent. Stir at 35 °C for 36 h to dissolve and obtain a homogeneous polyvinylidene fluoride solution;

[0137] S2. By the method of non-solvent induced phase separation, make the polyvinylidene fluoride solution flow directionally and solidify it into a film in a liquid coagulation bath (a mixed solvent with a volume ratio of V 水 :V DMFIt was solidified into a film in a mixed solvent of [[ID=]], obtaining a porous polyvinylidene fluoride film; this polyvinylidene fluoride film was dried at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0138] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this comparative example was 35%, the thickness was 23.4 μm, the pore size was 206 nm, the pore size distribution range was ±2.8 nm, the porosity was 82%, and the Gurley value of the air permeability was 841 s / 100 mL.

[0139] Comparative Example 6

[0140] This comparative example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0141] S1. Add 15 g of polyvinylidene fluoride powder to a mixed solvent of 85 g of DMF and petroleum ether, where petroleum ether accounts for 6.0% of the mixed solvent, stir at 35 °C for 36 h to dissolve, and the solution precipitates. As shown in Figure 17 (c), a complete polyvinylidene fluoride film was not prepared in this comparative example because the B-component organic solvent is petroleum ether, which can hinder the dissolution of polyvinylidene fluoride and has low miscibility with the A-component organic solvent, is insoluble in water, and can greatly slow down the diffusion of the organic solvent in the polyvinylidene fluoride solution into the coagulation bath.

[0142] Comparative Example 7

[0143] This comparative example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0144] S1. Add 4 g of polyvinylidene fluoride powder to 96 g of DMF, stir at 20 °C for 12 h to dissolve, and obtain a uniform polyvinylidene fluoride solution;

[0145] S2. Through the method of non-solvent-induced phase separation, make the polyvinylidene fluoride solution flow directionally and solidify it into a film in a liquid coagulation bath (mixed solvent with a volume ratio of V 水 : V DMF = 100:0), obtaining a porous polyvinylidene fluoride film; this polyvinylidene fluoride film was dried at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0146] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this comparative example was 51%, the thickness was 9.3 μm, the pore size was 53 nm, the pore size distribution range was ±1.9 nm, the porosity was 46%, and the Gurley value of the air permeability was 228 s / 100 mL.

[0147] Using the above polyvinylidene fluoride film as the lithium-ion battery separator, with commercial electrolyte LB-266 (1M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte, a Li||LFP button battery was assembled. As Figure 18 shown, for the lithium-ion battery prepared in this comparative example, the capacity retention rate after 297 cycles at a charge-discharge rate of 1C was 58.27%.

[0148] Comparative Example 8

[0149] This comparative example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0150] S1. Add 8 g of polyvinylidene fluoride powder to 92 g of DMF, and stir at 20 °C for 48 h to dissolve, obtaining a uniform polyvinylidene fluoride solution;

[0151] S2. Through the method of non-solvent-induced phase separation, make the polyvinylidene fluoride solution flow directionally and solidify into a film in a liquid coagulation bath (a mixed solvent with a volume ratio of V 水 :V NMP = 82:18), obtaining a porous polyvinylidene fluoride film; dry this polyvinylidene fluoride film at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0152] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this comparative example was 48%, the thickness was 14.6 μm (as shown in Figure 19 a, Figure 19 a shows the cross-sectional SEM image of the polyvinylidene fluoride film), the pore size was 148 nm, and the pore size distribution range was: ±3.3 nm (as shown in Figure 19 b, Figure 19 b shows the pore size distribution diagram of the polyvinylidene fluoride film), the porosity was 69%, and the Gurley value of the air permeability was 391 s / 100 mL. As shown in Figure 19 shown, the pore size distribution of the polyvinylidene fluoride film prepared in this comparative example was uniform.

[0153] Using the above polyvinylidene fluoride film as the lithium-ion battery separator, with commercial electrolyte LB-266 (1M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte, a Li||LFP button battery was assembled. As Figure 20 shown, for the lithium-ion battery prepared in this comparative example, the capacity retention rate after 282 cycles at a charge-discharge rate of 1C was 68.84%.

[0154] Comparative Example 9

[0155] This comparative example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0156] S1. Add 15 g of polyvinylidene fluoride powder to 85 g of DMF, and stir at 35 °C for 36 h to dissolve, obtaining a uniform polyvinylidene fluoride solution;

[0157] S2. By the method of non-solvent induced phase separation, make the polyvinylidene fluoride solution flow directionally, and solidify it into a film in a liquid coagulation bath (a mixed solvent with a volume ratio of V 水 :V DMF = 63:37), obtaining a porous polyvinylidene fluoride film; dry this polyvinylidene fluoride film at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0158] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this comparative example is 45%, the thickness is 23.7 μm, the pore size is 202 nm, the pore size distribution range is: ±3.5 nm, the porosity is 88%, and the Gurley value of the air permeability is 868 s / 100 mL.

[0159] Use the above polyvinylidene fluoride film as a lithium-ion battery separator, and use a commercial electrolyte LB-266 (1 M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte to assemble a Li||LFP button battery. As Figure 21 shown, the capacity retention rate of the lithium-ion battery prepared in this comparative example after 255 cycles at a charge-discharge rate of 1C is 78.73%.

[0160] Comparative Example 10

[0161] This comparative example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0162] S1. Add 15 g of polyvinylidene fluoride powder to 85 g of DMF, and stir at 35 °C for 48 h to dissolve, obtaining a uniform polyvinylidene fluoride solution;

[0163] S2. By the doctor blade method, scrape the obtained polyvinylidene fluoride solution into a liquid film with a thickness of 100 μm, and then place it in a liquid coagulation bath (a mixed solvent with a volume ratio of V 水 :V DMF = 67:37) to solidify into a film, obtaining a porous polyvinylidene fluoride film; dry this polyvinylidene fluoride film at room temperature to obtain a porous polyvinylidene fluoride film for standby.

[0164] After testing, the crystallinity of the polyvinylidene fluoride film prepared in this comparative example is 35%, and the thickness is 19.5 μm (as Figure 22 shown in Figure 22 a. Figure 22 a shows the cross-sectional SEM image of the polyvinylidene fluoride film). The pore size is 586 nm, and the pore size distribution range is: ±63.2 nm (as Figure 22 shown in Figure 22 b.

[0165] b shows the pore size distribution diagram of the polyvinylidene fluoride film). The porosity is 81%, and the Gurley value of the gas permeability is 856 s / 100 mL. It can be Figure 23 seen that the polyvinylidene fluoride film prepared by doctor blade coating has a variety of macroporous structures with coexisting finger-like pores and honeycomb-like pores, which is not conducive to inhibiting dendrite growth, and the concentration degree of the pore size distribution is significantly deviated.

[0166] Using the above polyvinylidene fluoride film as the lithium-ion battery separator and a commercial electrolyte LB-266 (1 M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte, a Li||LFP button battery was assembled. As Figure 24 shown, the capacity retention rate of the lithium-ion battery prepared in this comparative example after 300 cycles at a charge-discharge rate of 1C is 36.19%.

[0167] Comparative Example 11

[0168] This comparative example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0169] Using commercial PP with an Al2O3 coating as the lithium-ion battery separator (Al2O3@PP, Celgard, LLC), as Figure 25 、 Figure 26 shown, Figure 25 a shows the SEM image of the cross-section of the commercial PP separator, Figure 25 b shows the magnified SEM image of the local cross-section of the commercial PP separator. It can be Figure 25 seen that although the commercial PP membrane is a porous membrane, further observation through the magnified c image shows that the pore size distribution of the commercial PP membrane is not uniform, and there is a large difference in the sizes of macropores and micropores. Figure 26The pore size distribution diagram of a commercial PP separator is shown, and it can be seen that the concentration degree of the pore size distribution is significantly deviated. Using commercial electrolyte LB-266 (1M LiPF6 in DEC:DMC:EC = 1:1:1 Vol% with 1% VC) as the liquid electrolyte, a button battery of 79.6Li||LFP is assembled. As Figure 27 shown, for the lithium-ion battery prepared in this comparative example, the capacity retention rate after 200 cycles at a charge-discharge rate of 1C is 9.82%. It should be noted that after 190 cycles, the specific capacity shows a cliff-like decline, indicating that the life of this battery is 190 cycles.

[0170] Comparative Example 12

[0171] This comparative example provides a polyvinylidene fluoride film used as a battery separator and its preparation method, which specifically includes the following steps:

[0172] S1. Add 16 g of polyvinylidene fluoride powder to a mixed solvent of 84 g of DMF and petroleum ether, wherein petroleum ether accounts for 5.0% of the mixed solvent. Stir at 20 °C for 12 h to dissolve to obtain a uniform polyvinylidene fluoride solution;

[0173] S2. By the method of non-solvent-induced phase separation, make the polyvinylidene fluoride solution flow directionally and solidify into a film on the surface of a liquid coagulation bath (volume ratio V 水 :V DMF is = 100:0 - 63:37). There is still some liquid polymer on the surface of the polyvinylidene fluoride polymer liquid film that is not completely solidified, and a completely solidified and uniform polyvinylidene fluoride film cannot be obtained.

[0174] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a polyvinylidene fluoride film for use as a battery separator, characterized in that: The following steps are involved: S1. Add 4 to 15 parts by mass of polyvinylidene fluoride powder to 85 to 96 parts of an organic mixed solvent, and stir to dissolve to obtain a uniform polyvinylidene fluoride solution; S2. Directly spreading the polyvinylidene fluoride solution on the surface of a liquid coagulation bath, solidifying the spread solution into a film by a non-solvent-induced phase separation method, and drying at 23-27° C. to obtain a polyvinylidene fluoride film; In step S1, the organic mixed solvent includes component A and component B, wherein, by mass percentage, component A accounts for 95-99.5% and component B accounts for 0.5-5%; The component A is one of dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide or N-methylpyrrolidone, and the component B is petroleum ether.

2. The method for preparing a polyvinylidene fluoride film for use as a battery separator according to claim 1, wherein: In step S1, the stirring and dissolving temperature is 20 to 35° C., and the time is 12 to 48 hours.

3. The method for preparing a polyvinylidene fluoride film for use as a battery separator according to claim 2, wherein: In step S2, the liquid coagulation bath includes component A and water, and the volume ratio of component A to water is 0:100 to 37:

63.

4. A polyvinylidene fluoride film, characterized in that: The polyvinylidene fluoride film is prepared by the preparation method according to any one of claims 1 to 3, and the polyvinylidene fluoride film is a porous film.

5. The polyvinylidene fluoride film according to claim 4, wherein: The polyvinylidene fluoride film has a pore size of 42 to 234 nm, a pore size distribution range of ±2 nm to ±26 nm, a porosity of 42% to 84%, and a Gurley value of air permeability of 196 s / 100 mL to 856 s / 100 mL.

6. The polyvinylidene fluoride film according to claim 4, wherein: The thickness of the polyvinylidene fluoride film is 5.2 μm to 22.7 μm, and the crystallinity of the polyvinylidene fluoride film is 26% to 31%.

7. Use of a polyvinylidene fluoride film according to any one of claims 4 to 6, characterized in that: The polyvinylidene fluoride film is used for lithium ion batteries or sodium ion batteries.

8. The use of a polyvinylidene fluoride film according to claim 7, characterized in that: When the polyvinylidene fluoride film is used in a lithium-ion battery, the electrolyte of the lithium-ion battery is any one of the commercial electrolytes LB-266, LB-083 and LB-002; when the polyvinylidene fluoride film is used in a sodium-ion battery, the electrolyte of the sodium-ion battery is any one of the commercial electrolytes NC-021, NC-022 and NC-004.

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