Battery separator, method of making and battery

Abstract

The application discloses a battery separator, a preparation method and a battery. The preparation method of the battery separator comprises the following steps: coating a coating liquid on the surface of a base film, then immersing the base film coated with the coating liquid into a coagulation bath, and then performing drying to obtain the battery separator. According to the application, the coating liquid comprises 3-5% of a polymer lithium salt, 9-15% of a binder and 80-88% of a solvent in terms of mass fraction. According to the preparation method of the battery separator, a porous polymer film with an ion selective transmission function can be constructed in situ on the premise of maintaining the mechanical properties of the PE base film, and the high lithium ion migration number and the high ion conductivity can be synergistically improved.

Description

Technical Field

[0001] This invention relates to the field of battery separator technology, and more specifically, to battery separators, preparation methods, and batteries. Background Technology

[0002] During the charging and discharging process of lithium-ion batteries, charge transport depends on the co-migration of anions and cations in the electrolyte. The lithium-ion transference number is a crucial parameter. In traditional liquid electrolytes, the lithium-ion transference number is typically low, around 0.2-0.4, resulting in more than half of the current being contributed by the reverse migration of anions. This charge transport mode triggers a series of unfavorable kinetic and thermodynamic effects, becoming a key bottleneck limiting the battery's high power, long lifespan, and safety performance. Lithium-ion batteries using separators with high lithium-ion transference numbers can optimize ion transport behavior within the battery to a certain extent, thereby improving battery performance. This includes reducing concentration polarization, improving rate performance, suppressing dendrite growth, enhancing battery safety, improving energy efficiency and coulombic efficiency, improving low-temperature performance, and extending cycle life.

[0003] Currently, improving the lithium-ion transference number of the separator mainly involves modifying the separator bulk and using single-ion conductor polymer electrolyte separators. However, separator bulk modification requires the addition of inorganic fillers or the introduction of polymer monomers during the preparation of PE / PP separators, leading to a significant decrease in the excellent mechanical properties of polyolefin separators. In addition, the preparation of single-ion conductor polymer electrolyte separators requires first constructing a polymer framework and then fixing materials such as polylithium-to-styrene sulfonate and polylithium salt acrylate, resulting in complex processing, high process requirements, high cost, and low yield.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a battery separator, a preparation method, and a battery, providing a battery separator with simple processing and excellent mechanical properties.

[0006] This invention is implemented as follows: In a first aspect, the present invention provides a method for preparing a battery separator, comprising: coating a base film surface with a coating liquid, then immersing the base film coated with the coating liquid into a coagulation bath, and then drying it to obtain the battery separator; The coating liquid comprises, by mass fraction, 3% to 5% polymer lithium salt, 9% to 15% binder, and 80% to 88% solvent.

[0007] In an optional embodiment, the polymer lithium salt is selected from at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium dioxalate borate, and lithium tartrate borate. And / or, the adhesive is selected from at least one of polyvinylidene fluoride-hexafluoropropylene, polyimide and polyetheretherketone; And / or, the base film is selected from PE films.

[0008] In an optional embodiment, the solvent is selected from at least one of N,N-dimethylformamide and N-methylpyrrolidone.

[0009] In an optional embodiment, the coagulation bath comprises 40% to 60% solvent and 40% to 60% non-solvent by mass fraction, wherein the non-solvent is at least partially miscible with the solvent but does not dissolve the polymer lithium salt and the binder.

[0010] In an optional embodiment, the non-solvent is selected from at least one of methanol, ethanol, and acetone.

[0011] In an optional embodiment, the base film coated with the coating solution is immersed in the coagulation bath for 5 to 15 minutes.

[0012] In an optional implementation, the drying method is vacuum drying; And / or, the temperature of the drying step is 80~120℃.

[0013] In a second aspect, the present invention provides a battery separator prepared by the method described in any one of the foregoing embodiments, comprising a base membrane and a porous polymer membrane disposed on both sides of the base membrane.

[0014] Thirdly, the present invention provides a battery including the battery separator described in the foregoing embodiments.

[0015] In an optional embodiment, the electrolyte in the battery includes hexafluorophosphate.

[0016] The present invention has the following beneficial effects: The battery separator preparation method of this application can construct a porous polymer membrane with ion selective transport function in situ while maintaining the mechanical properties of the PE base membrane. This achieves a synergistic improvement in high lithium-ion transference number and high ion conductivity, maintains high voltage stability and capacity output at high rates, and has a low risk of dendrite puncture of the separator, thereby extending cycle life and improving safety. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0018] This invention provides a method for preparing a battery separator, comprising: coating a base film surface with a coating liquid, then immersing the base film coated with the coating liquid in a coagulation bath, and then drying it to obtain the battery separator; The coating liquid comprises, by mass fraction, 3% to 5% polymer lithium salt, 9% to 15% binder, and 80% to 88% solvent.

[0019] The battery separator preparation method of this application can construct a porous polymer membrane with ion-selective transport function in situ while maintaining the mechanical properties of the PE base membrane. This achieves a synergistic improvement in high lithium-ion transference number and high ionic conductivity, maintaining high voltage stability and capacity output at high rates. Simultaneously, the risk of dendrite puncture of the separator is low, thereby extending cycle life and improving safety. Specifically: First, the polymer lithium salt in the coating solution has a mass fraction of 3%-5%, which can provide an appropriate amount of sulfonylimide / borate anion immobilization sites. This effectively inhibits long-range anion migration while avoiding the decrease in lithium ion dissociation and carrier concentration caused by excessive salt. Combined with a binder of 9%-15% by mass, it can form a continuous ion channel network, ensuring the stability of the lithium ion concentration. + Conducted by jumping along polymer chain segments / interfaces.

[0020] Second, the solvent in the coating solution has a mass fraction of 40%-60%. After immersion in the coagulation bath, controllable phase separation occurs: moderate solvent retention delays polymer sedimentation, forming interconnected finger-like pores and a sponge-like substrate; rapid non-solvent extraction induces surface microporousization but not densification—both restricting anion migration across the membrane and maintaining sufficient porosity and electrolyte wettability, avoiding a sharp drop in ionic conductivity.

[0021] Third, this application is completed entirely at room temperature and pressure, without the need for high-temperature crosslinking or complex monomer polymerization, thus not damaging the tensile strength and thermal pore-closing characteristics of the PE base film, and the yield is significantly better than that of single-ion conductor membranes.

[0022] In an optional embodiment, the polymer lithium salt is selected from at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium dioxalateborate, and lithium tartrateborate; this facilitates lithium-ion dissociation while improving thermal stability.

[0023] In an optional embodiment, the adhesive is selected from at least one of polyvinylidene fluoride-hexafluoropropylene, polyimide, and polyetheretherketone. All three can form a homogeneous solution in a solvent and interact with lithium salts, which is beneficial for improving the mechanical support, ionic conductivity, and thermal safety of the membrane. In an optional embodiment, the base film is selected from PE film.

[0024] In an optional embodiment, the solvent is selected from at least one of N,N-dimethylformamide and N-methylpyrrolidone. Both have high solubility, moderate evaporation rate and good compatibility with coagulation bath, ensuring the stability of the coating solution and the uniformity of the coating. Their strong polarity promotes the complete dissociation of lithium salt and the extension of polymer chains, providing a prerequisite for subsequent phase separation to form interconnected channels and dense interfaces, thereby simultaneously optimizing ionic conductivity, migration number and coating adhesion, and avoiding performance degradation caused by process defects.

[0025] In an optional embodiment, the coagulation bath comprises, by mass fraction, 40%–60% solvent and 40%–60% non-solvent, wherein the non-solvent is at least partially miscible with the solvent but does not dissolve the polymer lithium salt and binder. Excessive solvent content can lead to macroporous structures and unobstructed anion migration channels; conversely, insufficient solvent content results in an overly dense porous polymer membrane and impeded lithium-ion transport. A solvent mass fraction of 40%–60% in the coagulation bath is beneficial for binding anions while maintaining high porosity and ionic conductivity.

[0026] In an optional embodiment, the non-solvent is selected from at least one of methanol, ethanol, and acetone. The non-solvent is chosen as a small molecule with a low boiling point, strong polarity, and partial miscibility with the solvent. It can rapidly extract the solvent and induce synchronous sedimentation of the polymer, forming a uniform and strongly adhesive functional layer. It does not dissolve lithium salts or binders, ensuring the complete preservation of anion anchoring sites, preventing loss of active components, and guaranteeing lithium ion migration rate and coating integrity during cycling.

[0027] In an optional embodiment, the base membrane coated with the coating solution is immersed in the coagulation bath for 5-15 minutes. Too short an immersion time can lead to insufficient phase separation, resulting in a loose coating and poor adhesion; too long an immersion time can cause excessive non-solvent penetration, leading to base membrane swelling or functional layer collapse. 5-15 minutes ensures sufficient solvent-non-solvent exchange, well-developed pore structure, and strong interfacial bonding, enabling the membrane to maintain the strength of the PE matrix while achieving a stable pore size distribution and excellent electrolyte absorption rate.

[0028] In an optional embodiment, the drying method is vacuum drying; the vacuum environment helps to accelerate the removal of residual solvent and avoid heat-induced microcracks.

[0029] In an optional embodiment, the drying step is performed at a temperature of 80-120°C, which can thoroughly remove moisture and low-boiling-point impurities while preventing polymer degradation or lithium salt decomposition.

[0030] The present invention also provides a battery separator prepared by the method described in any one of the foregoing embodiments, comprising a base membrane and a porous polymer membrane disposed on both sides of the base membrane.

[0031] This invention also provides a battery comprising the battery separator described in the foregoing embodiments.

[0032] In an optional embodiment, the electrolyte in the battery includes hexafluorophosphate.

[0033] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0034] Example 1 This embodiment provides a method for preparing a battery separator, specifically including the following steps: A coating solution is applied to the surface of a PE base film. The coating solution comprises 4% polymeric lithium salt, 12% binder, and 84% solvent. The polymeric lithium salt is lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide in a mass ratio of 1:1. The binder is polyetheretherketone. The solvent is N,N-dimethylformamide.

[0035] Next, the base film coated with the coating solution is immersed in a coagulation bath, which consists of 50% solvent and 50% non-solvent, for 10 minutes; the non-solvent is methanol.

[0036] The membrane was removed from the coagulation bath and vacuum dried at 100°C to obtain the battery separator.

[0037] Example 2 This embodiment provides a method for preparing a battery separator, specifically including the following steps: A coating solution is applied to the surface of a PE base film. The coating solution comprises 3% polymeric lithium salt, 15% binder, and 82% solvent. The polymeric lithium salt is lithium borate tartrate; the binder is polyvinylidene fluoride-hexafluoropropylene; and the solvent is at least one of N-methylpyrrolidone.

[0038] Next, the base film coated with the coating solution is immersed in a coagulation bath, which consists of 40% solvent and 60% non-solvent, for a duration of 5 minutes; the non-solvent is ethanol.

[0039] The membrane was removed from the coagulation bath and vacuum dried at a temperature of 80°C to obtain the battery separator.

[0040] Example 3 This embodiment provides a method for preparing a battery separator, specifically including the following steps: A coating solution is applied to the surface of a PE base film. The coating solution comprises 5% polymeric lithium salt, 9% binder, and 86% solvent. The polymeric lithium salt is lithium dioxaborate; the binder is polyimide; and the solvent is N,N-dimethylformamide.

[0041] Next, the base film coated with the coating solution is immersed in a coagulation bath, which consists of 60% solvent and 40% non-solvent, for 15 minutes; the non-solvent is acetone.

[0042] The membrane was removed from the coagulation bath and vacuum dried at a temperature of 120°C to obtain the battery separator.

[0043] Comparative Example 1 This comparative example provides a method for preparing a battery separator. The main difference from Example 1 is that the polymer lithium salt is replaced with an equal mass of polyacrylate lithium salt.

[0044] Comparative Example 2 This comparative example provides a method for preparing a battery separator. The main difference from Example 1 is that the adhesive is replaced with an equal mass of styrene-butadiene rubber.

[0045] Comparative Example 3 This comparative example provides a method for preparing a battery separator. The main difference from Example 1 is that the mass fraction of the polymer lithium salt is increased to 10%, and the mass fraction of the solvent is reduced accordingly.

[0046] Comparative Example 4 This comparative example provides a method for preparing a battery separator. The main difference from Example 1 is that the mass fraction of the polymer lithium salt is reduced to 1%, while the mass fraction of the solvent is increased accordingly.

[0047] Comparative Example 5 This comparative example provides a method for preparing a battery separator. The main difference from Example 1 is that the mass fraction of the adhesive is increased to 20%, and the mass fraction of the solvent is reduced accordingly.

[0048] Comparative Example 6 This comparative example provides a method for preparing a battery separator. The main difference from Example 1 is that the mass fraction of the adhesive is reduced to 5%, while the mass fraction of the solvent is increased accordingly.

[0049] Comparative Example 7 This comparative example provides a method for preparing a battery separator, which differs from Example 1 mainly in that the mass fraction of the solvent in the coagulation bath is increased to 70%.

[0050] Comparative Example 8 This comparative example provides a method for preparing a battery separator, which differs from Example 1 mainly in that the mass fraction of the solvent in the coagulation bath is reduced to 30%.

[0051] The battery separators prepared in the above embodiments and comparative examples were assembled into batteries, and the performance of the batteries was tested. The assembly and testing methods are as follows, and the test results are shown in Table 1.

[0052] Battery assembly: The battery model is LIR2025. The positive electrode is an active layer coated on the surface of aluminum foil. The active layer includes lithium cobalt oxide, acetylene black and PVDF in a mass ratio of 8:1:1. The negative electrode is a lithium sheet. The electrolyte is lithium hexafluorophosphate.

[0053] Performance testing: Ion transport number was measured using the DC polarization method. The positive electrode was replaced with a lithium sheet, a symmetrical cell was assembled, and a polarization voltage of 10 mV was applied. The initial interface impedance R0 and the interface impedance R after polarization were measured respectively. s Initial current I0 and steady-state current I s The result was obtained through calculation.

[0054] Ionic conductivity was measured using AC impedance spectroscopy, with a frequency range of 0.1 Hz to 1 MHz, a voltage amplitude of 5 mV, and a test temperature of 25 ℃.

[0055] The 5C rate voltage drop was tested using the DC internal resistance method. Under an ambient temperature of 25℃, the discharge curves for 0.2C and 5C were tested respectively. The voltage difference at a SOC of 50% was obtained from the curves, which is the test result.

[0056] Energy efficiency is measured by subjecting the battery to three complete charge-discharge cycles at 25°C with the SOC cap set to 90%. After the cycles are completed, another charge-discharge cycle is performed, and the ratio of the discharge energy to the charge energy is recorded as the test result.

[0057] Cyclic performance testing was performed using a constant current charge-discharge method, with both charge and discharge currents at 0.5C. The capacity decay threshold was 80% of the initial capacity.

[0058] Table 1

[0059] This application can achieve a lithium-ion transference number (t) + Simultaneous optimization with ionic conductivity, specifically: (1) The polymer lithium salt content is 3%-5%, which can maintain high carrier concentration and mobility while ensuring sufficient anion anchoring. If the polymer lithium salt content is too low, there will be insufficient anion fixation sites. + Limited improvement; excessive polymer lithium salt concentrations exacerbate lithium salt aggregation and ion association, inhibiting Li... + Dissociation leads to a decrease in conductivity.

[0060] (2) The composition of the coagulation bath within the scope of this application avoids both a high proportion of solvent, which would lead to the formation of a macroporous structure in the coating layer, resulting in unrestricted anion migration and a low lithium ion migration number, and a low proportion of solvent, which would lead to the formation of a dense skin structure on the surface, restricting both anion migration and lithium ion migration, resulting in low ionic conductivity of the membrane.

[0061] (3) The application selects to add polymer lithium salts because their molecular structure can fix or capture anions, so that the anions can only vibrate or rotate within a small range and cannot move in a long distance in a directional manner under an electric field. This can make the anion migration number approach 0 and can also serve as a lithium ion jump site, making the lithium ion migration number approach 1.

[0062] (4) Batteries made using the separator of the present invention have a high lithium-ion transport number. During charging and discharging, lithium ions carry a large proportion of the total current, the lithium-ion concentration gradient near the electrode interface is small, and concentration polarization is significantly reduced, which enables them to maintain high voltage stability and capacity output at high rates. At the same time, the high ion transport number can also promote the uniform deposition of lithium ions, reduce the risk of dendrites piercing the separator, extend cycle life, and improve safety.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a battery separator, characterized in that, include: A coating solution is coated on the surface of the base film, and then the base film coated with the coating solution is immersed in a coagulation bath and then dried to obtain the battery separator. The coating liquid comprises, by mass fraction, 3% to 5% polymer lithium salt, 9% to 15% binder, and 80% to 88% solvent.

2. The method for preparing the battery separator according to claim 1, characterized in that, The polymer lithium salt is selected from at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium dioxalate borate, and lithium tartrate borate. And / or, the adhesive is selected from at least one of polyvinylidene fluoride-hexafluoropropylene, polyimide and polyetheretherketone; And / or, the base film is selected from PE films.

3. The method for preparing the battery separator according to claim 1, characterized in that, The solvent is selected from at least one of N,N-dimethylformamide and N-methylpyrrolidone.

4. The method for preparing the battery separator according to claim 1, characterized in that, The coagulation bath comprises 40% to 60% solvent and 40% to 60% non-solvent by mass fraction, wherein the non-solvent is at least partially miscible with the solvent but does not dissolve the polymer lithium salt and the binder.

5. The method for preparing the battery separator according to claim 4, characterized in that, The non-solvent is selected from at least one of methanol, ethanol, and acetone.

6. The method for preparing the battery separator according to claim 1, characterized in that, The base film coated with the coating solution is immersed in the coagulation bath for 5 to 15 minutes.

7. The method for preparing the battery separator according to claim 1, characterized in that, The drying method is vacuum drying; And / or, the temperature of the drying step is 80~120℃.

8. A battery separator prepared by the method according to any one of claims 1-7, characterized in that, It includes a base membrane and a porous polymer membrane disposed on both sides of the base membrane.

9. A battery, characterized in that, Includes the battery separator as described in claim 8.

10. The battery according to claim 9, characterized in that, The electrolyte in the battery includes hexafluorophosphate.

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