Preparation method of battery diaphragm with thermal runaway prevention function

Abstract

The invention discloses a preparation method of a battery diaphragm with a thermal runaway prevention function. The preparation method comprises the following steps: dissolving an ethylene-vinyl acetate copolymer and poly (butylene succinate) in an organic solvent to serve as a fiber shell precursor solution; polyvinylidene fluoride is dissolved in an organic solvent to serve as a fiber inner core precursor solution; respectively pumping the inner core precursor solution and the outer shell precursor solution into an inner injector and an outer injector for electrostatic spinning; and drying after spinning to obtain the battery diaphragm with the thermal runaway prevention function. By constructing the spinning fiber shell with the gradient melting point, the diaphragm is partially closed at the shutdown temperature of 120 DEG C or below, and the high-temperature performance of the battery is reserved to the maximum extent; when the shutdown temperature is reached, the hole is quickly and completely closed to block reaction, thermal runaway is prevented in time, and the high-temperature performance of the battery is furthest reserved.

Description

Technical Field

[0001] The invention belongs to the technical field of materials, and in particular relates to a method for preparing a battery separator. Background Art

[0002] The batteries of new energy electric vehicles will continue to heat up during use, affecting the electrochemical balance and causing a significant decrease in charging and discharging performance. The continuous accumulation of heat can even raise the battery temperature to over 100°C, causing thermal runaway. Therefore, the safety performance of lithium-ion batteries needs to be improved urgently.

[0003] Current thermal management methods all act on the outside of the battery and cannot quickly respond to and regulate the thermal effects inside the battery. Some polymers also deform when they undergo phase changes. If phase change materials are introduced into electrochemically inert diaphragms, they can quickly respond to thermal effects inside the battery. When the battery temperature rises to the melting point, the polymer melts and changes from a solid state to a viscous flow state, which can block pores through flow deformation. Ideally, a sharp increase in resistance in a short period of time can help block various abuses associated with high currents.

[0004] Due to the large porosity, mild preparation conditions and flexible operating space, some studies have used electrospinning to prepare diaphragms to adjust the battery pores to prevent thermal runaway. The diaphragm melt deformation closure usually selects polymers with a melting point of around 120°C, which can successfully retain the high temperature performance of the battery. However, most of the current polymer diaphragms take more than 20 minutes to melt and close, and those with high porosity even take 1 hour, and the closure is slow and uncontrollable. If the heat accumulates rapidly, the skeleton will melt before the diaphragm is completely closed, which may cause the battery to short-circuit and thermal runaway.

[0005] Therefore, how to achieve timely closure of the diaphragm to prevent thermal runaway while retaining the high temperature performance of the battery has become a technical problem that needs to be solved urgently. Summary of the invention

[0006] The purpose of the present invention is to provide a method for preparing a battery diaphragm with a thermal runaway prevention function. The resulting diaphragm has good performance at room temperature, gradually closes the pores below the shutdown temperature, and quickly and completely closes the pores when the shutdown temperature is reached to block the reaction, thereby maximizing the retention of the high temperature performance of the battery.

[0007] In order to achieve the above purpose, the technical solutions adopted are as follows:

[0008] A method for preparing a battery separator with a thermal runaway prevention function comprises the following steps:

[0009] (1) Ethylene-vinyl acetate copolymer (EVA) and polybutylene succinate (PBS) are dissolved in an organic solvent as a fiber shell precursor solution;

[0010] (2) polyvinylidene fluoride (PVDF) is dissolved in an organic solvent as a fiber core precursor solution;

[0011] (3) pumping the inner core and outer shell precursor solutions into the inner and outer syringes respectively for electrospinning;

[0012] (4) After spinning, drying is performed to obtain a battery separator with a thermal runaway prevention function.

[0013] According to the above scheme, the organic solvent in step (1) is chloroform or dichloromethane.

[0014] According to the above scheme, the ethylene-vinyl acetate copolymer in step (1) is one or any combination of EVA12 (mp: 100°C), EVA14 (mp: 95°C), EVA16 (mp: 90°C), EVA18 (mp: 85°C), and EVA19 (mp: 80°C).

[0015] According to the above scheme, the mixed concentration of ethylene-vinyl acetate copolymer and polybutylene succinate in the fiber shell precursor solution of step (1) is 0.2-0.5 g / mL; the mass ratio of ethylene-vinyl acetate copolymer to polybutylene succinate is 1:9-9:1.

[0016] According to the above scheme, in the optimized scheme, the ethylene-vinyl acetate copolymer used in step (1) is EVA19 (80° C.), and the usage ratio with PBS is 1:1.

[0017] According to the above scheme, in the optimized scheme, step (1) uses ethylene-vinyl acetate copolymer EVA19 (80° C.) and the usage ratio of PBS is 5:3.

[0018] According to the above scheme, the organic solvent in step (2) is dimethylformamide or dichloromethane.

[0019] According to the above scheme, the concentration of polyvinylidene fluoride in the fiber core precursor solution in step (2) is 0.01-0.1 g / mL.

[0020] According to the above scheme, the electrospinning process parameters used in step (3) are:

[0021] The voltage was 18-20 kV; the distance between the needle and the collector was 5-15 cm; the feed rates inside and outside the syringe were 0.15-0.6 mL / h and 0.3-1.0 mL / h, respectively.

[0022] According to the above scheme, the drying condition in step (4) is vacuum drying at 20-30°C for 6-12 hours.

[0023] The present invention uses coaxial electrospinning technology to prepare a gradient phase change diaphragm whose components are polyvinylidene fluoride @ ethylene-vinyl acetate copolymer / polybutylene succinate (PVDF@EVA / PBS). PVDF with a melting point of 170°C has good thermal stability and good electrochemical properties, and can be used as a spinning skeleton. The PBS melting point of 120°C is the warning temperature for thermal runaway. At this temperature, the pores can be completely closed and the current can be completely cut off to prevent thermal runaway. EVA with different contents of VA has different melting points. EVA with a gradient melting point can be selected and mixed with PBS to make it melt in a gradient and gradually close the pores, thereby retaining the high temperature performance of the battery.

[0024] The innovation of the present invention is to propose the construction of a polymer fiber shell with a gradient melting point. Compared with one-time closed cells, the single-stage closed cell time in the gradient phase change is shortened; and the more gradients there are, the shorter the single-stage closed cell time is, and the controllability of the membrane closed cell process is enhanced. Therefore, according to the changes in temperature and discharge performance during the operation of the high-temperature battery, the polymer content, melting point temperature and gradient during the "gradient phase change" process are adjusted to optimize the porosity size and change rate of the high-temperature diaphragm. Partial closure below the shutdown temperature (120°C) maximizes the retention of the battery's high temperature performance; when the shutdown temperature is reached, the cells are quickly and completely closed to block the reaction and prevent thermal runaway in time.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1) The present invention constructs a textile shell with a gradient melting point, and the diaphragm is partially closed below the shutdown temperature (120°C), thereby maximizing the retention of the high-temperature performance of the battery; when the shutdown temperature is reached, the pores are quickly and completely closed to block the reaction, thereby preventing thermal runaway in time.

[0027] 2) The fiber core of the present invention adopts high melting point polyvinylidene fluoride, and the diaphragm has good thermal stability and electrochemical properties.

[0028] 3) The present invention adopts coaxial electrospinning to prepare the diaphragm, which has a simple method, mature technology and a large amount of low-priced spinning fiber shell material, and has commercial application potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 : Scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the PVDF@EVA / PBS membrane obtained in Example 1.

[0030] Figure 2 : DSC melting curve of the PVDF@EVA / PBS membrane obtained in Example 1.

[0031] Figure 3 : SEM images of the PVDF@EVA / PBS membrane obtained in Example 1 after being treated at different temperatures.

[0032] Figure 4 : The porosity of the diaphragms obtained by changing the EVA / PBS ratio in Example 1 and the comparative example after heat treatment at 80°C and the discharge specific capacity of the battery assembled with the heat-treated diaphragms.

[0033] Figure 5 : Battery impedance and closure time of the diaphragms obtained by changing the EVA / PBS ratio in Example 1 and the comparative example at the shutdown temperature (120°C). DETAILED DESCRIPTION

[0034] The following examples further illustrate the technical solution of the present invention, but are not intended to limit the scope of protection of the present invention.

[0035] The specific embodiment provides a method for preparing a battery separator with thermal runaway prevention function:

[0036] (1) Ethylene-vinyl acetate copolymer (EVA) and polybutylene succinate (PBS) are dissolved in an organic solvent as a precursor solution for the fiber shell;

[0037] (2) polyvinylidene fluoride (PVDF) dissolved in an organic solvent as a precursor solution for the fiber core;

[0038] (3) pumping the inner core and outer shell precursor solutions into the inner and outer syringes respectively for electrospinning;

[0039] (4) After spinning, the desired film is obtained by drying.

[0040] Specifically, the organic solvent in step (1) is chloroform or dichloromethane.

[0041] Specifically, in step (1), different segments of EVA can be selected according to the melting point requirements, such as EVA-14 (80°C), EVA-16 (90°C). The multi-gradient phase transition can also be made by mixing EVA with different melting points with PBS, wherein the PBS content is 10% to 90%.

[0042] Specifically, in step (1), the mixed concentration of ethylene-vinyl acetate copolymer and polybutylene succinate in the fiber shell precursor solution of step (1) is 0.2-0.5 g / mL; the mass ratio of ethylene-vinyl acetate copolymer to polybutylene succinate is 1:9-9:1.

[0043] Specifically, the organic solvent in step (2) is dimethylformamide or dichloromethane.

[0044] Specifically, in step (2), the concentration of polyvinylidene fluoride is 0.01-0.1 g / mL.

[0045] Specifically, the electrospinning conditions in step (3) are: 18-20 kV voltage, the distance between the needle and the collector is 5-15 cm, and the feed rates inside and outside the syringe are 0.15-0.6 mL / h and 0.3-1.0 mL / h, respectively.

[0046] Specifically, the drying condition in step (4) is vacuum drying at 20-30° C. for 6-12 hours.

[0047] Example 1

[0048] A battery separator with a thermal runaway prevention function, wherein the preparation method thereof comprises the following steps:

[0049] 1) Weigh 5 g of ethylene-vinyl acetate copolymer (EVA19, 80° C.) and 3 g of polybutylene succinate and dissolve them in 10 mL of chloroform to obtain a precursor solution of a spinning shell;

[0050] 2) Weigh 0.4 g of polyvinylidene fluoride and dissolve it in 10 mL of dimethylformamide to obtain a precursor solution for the spinning fiber core;

[0051] 3) pumping the solutions of step 1) and 2) into the outer needle and inner needle of the syringe respectively, and electrospinning was performed under the conditions of 20 kV voltage, 15 cm distance between the needle and the collector, and 0.6 mL / h and 1.0 mL / h feeding rates inside and outside the syringe respectively;

[0052] 4) After spinning, vacuum drying was performed at 25°C for 12 hours to obtain the PVDF@EVA / PBS membrane.

[0053] Figure 1 The SEM image of the PVDF@EVA / PBS membrane obtained in this example is shown in FIG. Figure 1 It can be seen that the pores of the diaphragm are relatively large, and due to the low PVDF content, no obvious core-shell structure is observed.

[0054] Figure 2 is the DSC melting curve of the PVDF@EVA / PBS membrane obtained in this example. Figure 2 It can be seen that the diaphragm contains PVDF, EVA and PBS, and the content of PVDF is relatively low.

[0055] Figure 3 The SEM image of the PVDF@EVA / PBS membrane obtained in this example after being treated at the melting point of EVA-14 (80°C) and PBS (120°C) for 5 minutes. Figure 3 It can be seen that the diaphragm can be gradually closed under gradient temperature.

[0056] Example 2

[0057] A battery separator with a thermal runaway prevention function, wherein the preparation method thereof comprises the following steps:

[0058] 1) Weigh 3 g of EVA12 and EVA19 and 2 g of PBS and dissolve them in 10 mL of chloroform to obtain a fiber shell precursor solution;

[0059] 2) Weigh 0.4 g of polyvinylidene fluoride and dissolve it in 10 mL of dimethylformamide to obtain a precursor solution for the fiber core;

[0060] 3) pumping the solutions of step 1) and 2) into the outer needle and inner needle of the syringe respectively, and electrospinning was performed under the conditions of 20 kV voltage, 15 cm distance between the needle and the collector, and 0.6 mL / h and 1.0 mL / h feeding rates inside and outside the syringe respectively;

[0061] 4) After spinning, vacuum drying was performed at 25°C for 12 hours to obtain the PVDF@EVA / PBS membrane.

[0062] Adding EVA12 can increase the two-gradient phase change to three-gradient, which can improve the controllability of the diaphragm phase change closed hole, thereby shortening the 120°C diaphragm closed hole time and improving the high temperature performance of the battery.

[0063] Example 3

[0064] A battery separator with a thermal runaway prevention function, wherein the preparation method thereof comprises the following steps:

[0065] 1) Weigh 6 g EVA19 and 2 g PBS in 10 mL dichloromethane to obtain a precursor solution for the spinning shell;

[0066] 2) Weigh 0.4 g of polyvinylidene fluoride and dissolve it in 10 mL of dimethylformamide to obtain a precursor solution for the fiber core;

[0067] 3) pumping the solutions of step 1) and 2) into the outer needle and inner needle of the syringe respectively, and electrospinning was performed under the conditions of 20 kV voltage, 15 cm distance between the needle and the collector, and 0.6 mL / h and 1.0 mL / h feeding rates inside and outside the syringe respectively;

[0068] 4) After spinning, vacuum drying was performed at 25°C for 12 hours to obtain the PVDF@EVA / PBS membrane.

[0069] Reducing the proportion of PBS in the shell can shorten the closed-cell time at 120°C, but it also weakens the high-temperature performance of the battery.

[0070] Example 4

[0071] A battery separator with a thermal runaway prevention function, wherein the preparation method thereof comprises the following steps:

[0072] 1) Weigh 4 g EVA19 and 4 g PBS in 10 mL dichloromethane to obtain a precursor solution for the spinning shell;

[0073] 2) Weigh 0.4 g of polyvinylidene fluoride and dissolve it in 10 mL of dimethylformamide to obtain a precursor solution for the spinning fiber core;

[0074] 3) pumping the solutions of step 1) and 2) into the outer needle and inner needle of the syringe respectively, and electrospinning was performed under the conditions of 20 kV voltage, 15 cm distance between the needle and the collector, and 0.6 mL / h and 1.0 mL / h feeding rates inside and outside the syringe respectively;

[0075] 4) After spinning, vacuum drying was performed at 25°C for 12 hours to obtain the PVDF@EVA / PBS membrane.

[0076] Reducing the proportion of EVA in the outer shell can improve the high-temperature performance of the battery, but it also increases the closed-cell time of the battery at 120°C and increases the risk of thermal runaway.

[0077] Example 5

[0078] A battery separator with a thermal runaway prevention function, wherein the preparation method thereof comprises the following steps:

[0079] 1) Weigh 5 g of EVA19 and 3 g of PBS and dissolve them in 10 mL of chloroform to obtain a precursor solution for the spinning shell;

[0080] 2) Weigh 0.8 g of polyvinylidene fluoride and dissolve it in 10 mL of dichloromethane to obtain a precursor solution for the fiber core;

[0081] 3) pumping the solutions of step 1) and 2) into the outer needle and inner needle of the syringe respectively, and electrospinning was performed under the conditions of 19 kV voltage, 15 cm distance between the needle and the collector, and 0.3 mL / h and 1.0 mL / h feeding rates inside and outside the syringe respectively;

[0082] 4) After spinning, vacuum drying was performed at 25°C for 12 hours to obtain the PVDF@EVA / PBS membrane.

[0083] Reducing the feeding speed of the syringe inner tube can better form the core-shell structure, but at the same time, the electrospinning voltage needs to be reduced. At the same time, in order to ensure that the content of the spinning fiber core remains unchanged, the quality of PVDF in step 2 needs to be improved.

[0084] Comparative Example

[0085] PVDF@EVA / PBS membranes with different EVA / PBS ratios were prepared. The specific preparation method was the same as in Example 1, except that the mass ratios of EVA19 / PBS were adjusted to 1:1, 1:3, and 0:1.

[0086] Application Examples

[0087] The battery discharge performance before complete closure and the closure time at shutdown temperature are two key points that need to be examined when using gradient phase change membranes to prevent thermal runaway. Therefore, membranes of different EVA / PBS were prepared by adjusting the EVA / PBS ratio in Example 1, heated at 80°C to achieve partial pore closure, and batteries were prepared using the heat-treated membranes. The relationship between the membrane porosity and the battery discharge specific capacity was studied. Furthermore, the partially closed membrane was continued to be heated at 120°C to achieve complete closure, and the influence of the membrane components on the closure time at 120°C was studied.

[0088] Figure 4 The porosity of the diaphragm and the discharge capacity of the assembled battery after the diaphragm with different EVA / PBS ratios in Example 1 and the comparative example was heat treated at 80°C. By adjusting the EVA / PBS ratio, the diaphragm porosity and battery discharge capacity at 80°C can be controlled. When the EVA / PBS ratio is 5 / 3, the diaphragm porosity is 30%, and the battery can still maintain a high specific capacity of 118 mAh g -1 This indicates that as long as the porosity is greater than a certain value, its substantial reduction will not seriously affect the discharge specific capacity.

[0089] Figure 5 The closed-cell battery impedance and closed-cell time at 120°C after heat treatment at 80°C for the diaphragms with different EVA / PBS ratios in Example 1 and the comparative example. The high resistance of 29 to 32 ohms indicates that the diaphragm is completely closed. As the EVA / PBS ratio increases, the closed-cell time decreases; when the ratio is 1 / 1, the closed-cell time is reduced to half that of the pure PBS membrane. The diaphragm with EVA / PBS ratio of 5 / 3 can achieve complete closed-cell operation within 6 minutes at 120°C and maintain 118 mAh g -1 If more gradients are designed to optimize the gradient phase change, the pore closure process will be more controllable. That is, partial pore closure below the shutdown temperature (120°C) is achieved to maximize the retention of the battery's high-temperature performance; when the shutdown temperature is reached, the pores are quickly and completely closed to block the reaction and prevent thermal runaway in time.

[0090] Obviously, the above embodiments are merely examples for clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from this are still within the protection scope of the invention.

Claims

1. A method for preparing a battery separator having a thermal runaway prevention function, characterized in that The steps include: (1) Ethylene-vinyl acetate copolymer and polybutylene succinate are dissolved in an organic solvent as a fiber shell precursor solution; (2) polyvinylidene fluoride is dissolved in an organic solvent as a fiber core precursor solution; (3) pumping the inner core and outer shell precursor solutions into the inner and outer syringes respectively for electrospinning; (4) After spinning, drying is performed to obtain a battery separator with a thermal runaway prevention function.

2. The method for preparing a battery separator having a thermal runaway prevention function as claimed in claim 1, characterized in that The organic solvent in step (1) is chloroform or dichloromethane.

3. The method for preparing a battery separator having a thermal runaway prevention function as claimed in claim 1, characterized in that The ethylene-vinyl acetate copolymer in step (1) is one or any combination of EVA12, EVA14, EVA16, EVA18, and EVA19.

4. The method for preparing a battery separator having a thermal runaway prevention function as claimed in claim 1, characterized in that In step (1), the mixed concentration of ethylene-vinyl acetate copolymer and polybutylene succinate in the fiber shell precursor solution is 0.2-0.5 g / mL; the mass ratio of ethylene-vinyl acetate copolymer to polybutylene succinate is 1:9-9:

1.

5. The method for preparing a battery separator having a thermal runaway prevention function as claimed in claim 1, characterized in that In step (1), ethylene-vinyl acetate copolymer EVA19 is used, and the mass ratio of EVA19 to PBS is 1:

1.

6. The method for preparing a battery separator having a thermal runaway prevention function as claimed in claim 1, characterized in that In step (1), ethylene-vinyl acetate copolymer EVA19 is used, and the mass ratio of EVA19 to PBS is 5:

3.

7. The method for preparing a battery separator having a thermal runaway prevention function as claimed in claim 1, characterized in that The organic solvent in step (2) is dimethylformamide or dichloromethane.

8. The method for preparing a battery separator having a thermal runaway prevention function as claimed in claim 1, characterized in that The concentration of polyvinylidene fluoride in the fiber core precursor solution in step (2) is 0.01-0.1 g / mL.

9. The method for preparing a battery separator having a thermal runaway prevention function as claimed in claim 1, characterized in that The electrospinning process parameters in step (3) are: The voltage was 18-20 kV; the distance between the needle and the collector was 5-15 cm; the feed rates inside and outside the syringe were 0.15-0.6 mL / h and 0.3-1.0 mL / h, respectively.

10. The method for preparing a battery separator having a thermal runaway prevention function according to claim 1, characterized in that The drying condition in step (4) is vacuum drying at 20-30° C. for 6-12 hours.

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