WO2019086034A1

WO2019086034A1 - Separators, electrochemical devices comprising the separator, and methods for producing the separator

Info

Publication number: WO2019086034A1
Application number: PCT/CN2018/113995
Authority: WO
Inventors: Alex Cheng; Lianjie WANG; Yongle Chen; Zhixue Wang; Chenbo LIAO
Original assignee: Shanghai Energy New Materials Technology Co., Ltd.
Priority date: 2017-11-06
Filing date: 2018-11-05
Publication date: 2019-05-09
Also published as: CN107958977B; CN107958977A

Abstract

Disclosed are a separator for an electrochemical device, comprising a nonwoven porous membrane and a heat-resistant layer being formed on at least one side of the nonwoven porous membrane, wherein the heat-resistant layer is formed by using a coating slurry that comprises from 1.5 to 6 parts by weight of at least one polyimide or polyimide precursor and from 0.5 to 4 parts by weight of at least one inorganic filler; as well as an electrochemical device including the separator and a method for making the separator.

Description

SEPARATORS, ELECTROCHEMICAL DEVICES COMPRISING THE SEPARATOR, AND METHODS FOR PRODUCING THE SEPARATOR

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of priority to Chinese Application No. 201711078122.2, filed on November 6, 2017.

TECHNICAL FIELD

The present disclosure relates to separators for electrochemical devices, electrochemical devices comprising the separator, and methods for making the separator.

BACKGROUND

With the growing market of energy storage, batteries and other forms of electrochemical devices are given more and more attentions. For example, lithium secondary batteries have been extensively used as energy sources in, for example, mobile phones, laptops, power tools, electrical vehicles, etc.

An electrode assembly of an electrochemical device usually comprises a positive electrode, a negative electrode, and a permeable membrane (i.e., separator) interposed between the positive electrode and the negative electrode. The positive electrode and the negative electrode are prevented from being in direct contact with each other by the separator, thereby avoiding internal short circuit. In the meanwhile, ionic charge carriers (e.g., lithium ions) are allowed to pass the separator through channels within the separator so as to close the current circuit. Separator is a critical component in an electrochemical device because its structure and properties can considerably affect the performances of the electrochemical device, including, for example, internal resistance, energy density, power density, cycle life, and safety.

A separator is generally formed by a polymeric microporous membrane. For example, polyolefin-based microporous membrane has been widely used as separators in lithium secondary batteries because of its favorable chemical stability and excellent physical properties. However, they may have poor thermal stability as the polyolefin materials may have low melting points. Some electrochemical devices (e.g., automotive batteries for electric vehicles) , which may be used in a high-temperature environment, require separators have high heat-resistance. The polyolefin-based microporous membrane may not meet such requirement. When the temperature inside of the electrochemical device rises, the polyolefin-based microporous membrane may shrink or melt, resulting in a volume change, which may lead to a direct contact of the positive electrode and the negative electrode, i.e., internal short circuit. The internal short circuit can cause some accidents, such as battery bulge, burning, or explosion.

To ensure the safety of electrochemical devices in an environment with high temperature, there is still a need to develop separators of high heat-resistance.

SUMMARY OF THE INVENTION

The present disclosure provides a separator for an electrochemical device, comprising a nonwoven porous membrane and a heat-resistant layer being formed on at least one side of the nonwoven porous membrane, wherein the heat-resistant layer is formed by using a coating slurry that comprises from 1.5 to 6 parts, such as from 1.5 to 5 parts, by weight of at least one polyimide or polyimide precursor and from 0.5 to 4 parts, such as from 1 to parts, by weight of at least one inorganic filler.

The present disclosure also provides an electrochemical device comprising a positive electrode, a negative electrode, and the separator disclosed herein, interposed between the positive electrode and the negative electrode.

The present disclosure further provides a method for making the separator disclosed herein, comprising:

preparing a coating slurry comprising at least one polyimide or polyimide precursor, at least one inorganic filler, and at least one solvent;

applying the coating slurry on at least one side of a nonwoven porous membrane to form a wet coating layer; and

removing the at least one solvent from the wet coating layer.

DETAILED DESCRIPTION

The present disclosure provides some exemplary embodiments of separators for electrochemical devices. In one embodiment, a heat-resistant layer is formed on at least one side of a nonwoven porous membrane wherein the heat-resistant layer is formed by using a coating slurry that comprises from 1.5 to 6 parts, such as from 1.5 to 5 parts, by weight of at least one polyimide or polyimide precursor and from 0.5 to 4 parts, such as from 1 to parts, by weight of at least one inorganic filler. The “at least one side” disclosed herein means the heat-resistant layer is disposed on one side or both sides of the nonwoven porous membrane, and can be in direct contact or not in direct contact with the nonwoven porous membrane. For example, the separator disclosed herein may have a laminated structure.

In some embodiments, the heat-resistant layer is in direct contact with the nonwoven porous membrane, i.e., the heat-resistant layer is formed on at least one surface of the nonwoven porous membrane. For example, the separator disclosed herein may have a two-layer structure when only one surface of the nonwoven porous membrane is coated with the heat-resistant layer. The separator may have a three-layer structure when both surfaces of the nonwoven porous membrane are coated with the heat-resistant layer.

In some other embodiments, the heat-resistant layer is not in direct contact with the nonwoven porous membrane, i.e., the separator disclosed herein further comprise at least one additional layer (e.g., an adhesive layer) interposed between the heat-resistant layer and the nonwoven porous membrane.

In yet another embodiment, the separator disclosed herein may further comprise at least one additional layer (e.g., an adhesive layer) disposed on the outer surface of the heat-resistant layer.

The nonwoven porous membrane serves as a substrate and the heat-resistant layer is formed on at least one side thereof. The term “nonwoven porous membrane” means a flat sheet including a multitude of randomly distributed fibers that form a web structure therein. The fibers generally can be bonded to each other or can be unbonded. The fibers can be staple fibers (i.e., discontinuous fibers of no longer than 10 cm in length) or continuous fibers. The fibers can be formed by a single material or a multitude of materials, either as a combination of different fibers or as a combination of similar fibers each comprised of different materials. Examples of the nonwoven porous membrane disclosed herein may exhibit dimensional stability, e.g., thermal shrinkage of less than 5%when heated to 100℃ for about two hours. The nonwoven porous membrane may have a relatively large average pore size ranging, for example, from 0.05 to 50 μm, such as from 0.5 to 5 μm, and a porosity ranging, for example, from 40%to 80%, such as from 50%to 70%. Furthermore, the nonwoven porous membrane may have an air permeability of, for example, less than 500 sec/100ml, such as ranging from 0 to 400 sec/100ml, and further such as ranging from 0 to 200 sec/100ml. Although the nonwoven porous membrane has excellent air permeability, it cannot be used as a separator in an electrochemical device directly, because its large pores may easily lead short circuit between the positive electrode and the negative electrode. The nonwoven porous membrane may be formed of at least one polymer material chosen from polyethylene terephthalate (PET) , polyethylene (PE) , high density polyethylene (HDPE) , polypropylene (PP) , polybutylene, polypentene, polymethylpentene (TPX) , polyamide, polyimide (PI) , polyacrylonitrile (PAN) , viscose fiber, polyester, polyacetal, polycarbonate, polyetherketone (PEK) , polyetheretherketone (PEEK) , polybutylene terephthalate (PBT) , polyethersulfone (PES) , polyphenylene oxide (PPO) , polyphenylene sulfide (PPS) , polyethylene naphthalene (PEN) , cellulose, and copolymers thereof. One example of the nonwoven porous membrane disclosed herein is formed of PET. PET has a relatively high melting point, so the nonwoven porous membrane made of PET may have high heat-resistance. The nonwoven porous membrane disclosed herein may have a thickness ranging, for example, from 10 to 30 μm, such as from 17 to 21 μm. The nonwoven porous membrane disclosed herein can be prepared according to a conventional method known in the art, such as electro-blowing, electro-spinning, and melt-blowing, or can be purchased directly in the market.

The nonwoven porous membrane disclosed herein may have better heat resistance than the polyolefin-based microporous membrane disclosed in the art. To further improve the heat resistance of the separator disclosed herein, the heat-resistant layer is formed on at least one side of the nonwoven porous membrane. In some embodiments, the heat-resistant layer may be formed by applying a coating slurry onto the nonwoven porous membrane through various suitable techniques, such as a roller coating, a spray coating, a dip coating, or a spin coating process. During the coating process, at least a part of the coating slurry may penetrate into the pores of the nonwoven porous membrane so as to decrease the size of some pores therein.

The heat-resistant layer disclosed herein has a porous structure, allowing gas, liquid, or ions to pass from one surface side to the other surface side thereof. The pores within the heat-resistant layer may have an average pore size ranging, for example, from 20 to 500 nm, such as from 150 to 400 nm. In one embodiment, the average pore size is 200 nm. The heat-resistant layer may have a porosity ranging, for example, from 20%to 70%, such as from 30%to 50%. Additionally, the heat-resistant layer on one side of the nonwoven porous membrane may have a thickness ranging, for example, from 3 to 8 μm, such as from 4 to 7 μm. In one embodiment, the thickness of the heat- resistant layer is 5μm. To make a separator with lightweight, the surface density of the heat-resistant layer on one side of the nonwoven porous membrane may be controlled in a range of, for example, from 1.5 to 8 g/m ², such as from 2 to 6 g/m ². In one embodiment, the surface density is 5 g/m ². The term “surface density” means the weight of unit area of the heat-resistant layer on one side of the nonwoven porous membrane.

The heat-resistant layer disclosed herein comprises at least one polyimide and at least one inorganic filler. To balance different properties, such as porosity, air permeability, and weight, of the separator disclosed herein, the weight ratio of the at least one polyimide and the at least one inorganic filler present in the coating slurry that is used to form the heat-resistant layer may be controlled in a specific range. In some embodiments, the coating slurry that is used to form the heat-resistant layer disclosed herein may comprise from 1.5 to 6 parts, such as from 1.5 to 5 parts, by weight of the at least one polyimide or polyimide precursor and from 0.5 to 4 parts, such as from 1 to 4 parts, by weight of the at least one inorganic filler. For example, the coating slurry that is used to form the heat-resistant layer disclosed herein comprises from 4.5 to 5.5 parts by weight of the at least one polyimide or polyimide precursor and from 1 to 3.5 parts by weight of the at least one inorganic filler. In another example, the coating slurry that is used to form the heat-resistant layer disclosed herein comprises 5 parts by weight of the at least one polyimide or polyimide precursor and 3 parts by weight of the at least one inorganic filler.

Polyimide is a class of polymers containing at least one imide (-C (O) -N (R) -C (O) -) in its main chain. Polyimides are either thermosetting or thermoplastic. Based on their chemical structures, polyimides can be classified into three categories: aliphatic polyimides, semi-aromatic polyimides, and aromatic polyimides. Polyimides, such as aromatic and semi-aromatic polyimides, are known for their thermal stability, good chemical resistance, excellent mechanical properties, and ultrahigh electrical insulation properties, and can be used as special engineering plastics, high performance fibers, selective permeation membranes, heat-resistant coatings, high-temperature composite materials, etc. Polyimides can maintain their properties at a temperature ranging, for example, from -200℃ to 300℃, and can be resistant to a temperature up to 400℃ or above. Hence, polyimide is a suitable material to be used in preparation of heat-resistant separators for electrochemical devices. According to the monomers from which polyimides are derived, examples of polyimides include polypyromellitimides, bisether anhydride-type polyimides, monoether anhydride-type polyimides, polyetherimides (PEI) , polyamide-imides (PAI) , polyester-imides (PESI) , and fluorinated polyimides.

Polyimides may be prepared by various methods, such as one-step method and two-step methods. The classical two-step method includes the formation of polyamic acids (PAAs) by a condensation polymerization reaction between anhydrides and diamines, followed by the conversion of PAAs to the desired polyimides via imidization. In the one-step method, polyimides can be prepared via polycondensation of dianhydrides and diamines at a high temperature. Examples of the dianhydride include pyromellitic dianhydride (PMDA) , biphenyltetracarboxylic dianhydride (BPDA) , benzophenonetetracarboxylic dianhydride (BTDA) , trimellitic anhydride (TMA) , benzoquinonetetracarboxylic dianhydride and naphthalene tetracarboxylic dianhydride. Examples of the diamine include oxydianiline (ODA) , 1, 3-bis (4-aminophenoxy) benzene (RODA) , 4, 4'-diaminodiphenyl ether (DAPE) , meta-phenylenediamine (MDA) , and 3, 3-diaminodiphenylmethane. Polyimides can also be prepared from dianhydrides and diisocyanates. Examples of the diisocyanate include methylenediphenyl 4, 4'-diisocyanate (MDI) and 1, 4-phenylene diisocyanate. In addition, other synthetic routs to prepare polyimides include, for example, synthesis via derivatized polyamic acid precursors, synthesis via polyisoimide precursors, synthesis from diester-acids and diamines, and synthesis via nucleophilic aromatic substitution reactions. The raw materials or monomers (e.g., dianhydrides, diamines, and diisocyanates) used in the preparation of polyimides and intermediate products (e.g., PAAs, derivatized PAAs, and polyisoimide) produced in the preparation process are polyimide precursors. A large variety of polyimides can be prepared from different polyimide precursors, such as those set forth above. Even subtle variations in the structure of dianhydride and diamine may have a significant effect on the properties of the final polyimides, such as degree of crystallinity, glass transition temperature, and melting point.

In some embodiments, the polyimides in the heat-resistant layer disclosed herein are soluble polyimides, which can be dissolved in at least one solvent. The at least one solvent may be chosen, for example, from dimethylacetamide (DMAC) , N-methyl pyrrolidone (NMP) , N, N-dimethylformamide (DMF) , dimethyl sulfoxide (DMSO) , acetone, diethyl ether, propyl ether, cyclohexane, and tetrahydrofuran (THF) . The soluble polyimides may be purchased from market or synthesized in advance. During the preparation of the separator disclosed herein, a coating slurry may be prepared by dissolving the soluble polyimides in the at least one solvent disclosed herein.

In some other embodiments, the polyimides in the heat-resistant layer disclosed herein may be synthesized in situ from polyimide precursors as most soluble polyimides are more commercially available in the form of uncured resins. The polyimide precursors can be dissolved in at least one solvent, such as DMAC, NMP, DMF, DMSO, acetone, diethyl ether, propyl ether, cyclohexane, THF, or a mixture thereof. The polyimides formed in situ may be soluble or not soluble in the at least one solvent. In the case that the polyimides in the heat-resistant layer are synthesized in situ, the polyimide precursors described above, such as dianhydrides and diamines, dianhydrides and diisocyanates, PAAs, etc., are used in the preparation of the separator disclosed herein. For example, the separator disclosed herein may be prepared by: dissolving the polyimide precursors in the at least one solvent to prepare a coating slurry; applying the coating slurry on at least one side of a nonwoven porous membrane to obtain a wet coating layer; and curing the polyimide precursors in the wet coating layer. The polyimide precursors may be cured by, for example, heating or irradiation treatment. One example of the polyimide precursor is polyimide resin of CAS No. 62929-02-6, 1- (4-aminophenyl) -1, 3, 3-trimethyl-2H-inden-5-amine, 5- (1, 3-dioxo-2-benzofuran-5-carbonyl) -2-benzofuran-1, 3-dione.

Besides the at least one polyimide, the at least one inorganic filler present in the heat-resistant layer can also contribute to the heat resistance of the separator disclosed herein, thereby further preventing short circuit and improving dimensional stability of an electrochemical device employing the separator in a high-temperature environment. In addition, the presence of the at least one inorganic filler may contribute to, for example, the formation of pores in the heat-resistant layer, the increase of the physical strength of the heat-resistant layer, and the increase in an impregnation rate of a liquid electrolyte. The at least one inorganic filler may be embedded in the heat-resistant layer by the at least one polyimide. Various inorganic particles can be used as the at least one inorganic filler disclosed herein, including, for example, an oxide, a hydroxide, a sulfide, a nitride, a carbide, a carbonate, a sulfate, a phosphate, a titanate, and the like of at least one of metallic and semiconductor elements, such as Si, Al, Ca, Ti, B, Sn, Mg, Li, Co, Ni, Sr, Ce, Zr, Y, Pb, Zn, Ba, and La. For example, one or more of alumina (Al ₂O ₃) , boehmite (γ-AlOOH) , silica (SiO ₂) , titanium oxide (TiO ₂) , cerium oxide (CeO ₂) , calcium oxide (CaO) , zinc oxide (ZnO) , magnesium oxide (MgO) , lithium nitride (Li ₃N) , calcium carbonate (CaCO ₃) , barium sulfate (BaSO ₄) , lithium phosphate (Li ₃PO ₄) , lithium titanium phosphate (LTPO) , lithium aluminum titanium phosphate (LATP) , cerium titanate (CeTiO ₃) , calcium titanate (CaTiO ₃) , barium titanate (BaTiO ₃) and lithium lanthanum titanate (LLTO) can be used as the at least one inorganic filler. The inorganic filler disclosed herein may have an average particle size ranging, for example, from 0.01 to 10 μm, such as from 0.02 to 2 μm.

The separator disclosed herein may have improved air permeability, because the nonwoven porous membrane has large pores and the heat-resistant layer has good porous structure due to the presence of the at least one inorganic filler. The separator disclosed herein may have an air permeability ranging, for example, from 20 to 500 s/100cc, such as from 100 to 250 s/100cc. There is no particular limitation for the thickness of the separator disclosed herein, and the thickness of the separator can be controlled in view of the requirements of electrochemical devices employing the separator, e.g., lithium-ion batteries. In some embodiments, the separator disclosed herein has a thickness ranging from 10 μm to 40 μm, such as from 10 μm to 18 μm. In one embodiment, the separator has a thickness of 12 μm.

The separator disclosed herein can have excellent thermal stability because of the presence of the nonwoven porous membrane, the at least one polyimide, and the at least one inorganic filler in the separator. In some embodiments, when measured at 150℃ for one hour, the separator disclosed herein may have a thermal shrinkage percentage of, for example, less than 2%, such as less than 1%, either in a machine direction (MD) or in a transverse direction (TD) . The low thermal shrinkage percentage indicates the separator disclosed herein has excellent thermal stability and high heat resistance.

The separator disclosed herein can have a wide range of applications and can be used, for example, in making high-energy density and/or high-power density batteries in many stationary and portable devices, e.g., automotive batteries, batteries for medical devices, and batteries for other large devices.

Further, the present disclosure provides an electrochemical device comprising: a positive electrode, a negative electrode, and a separator disclosed herein, which is interposed between the positive electrode and the negative electrode. An electrolyte may be further included in the electrochemical device disclosed herein. The separator is sandwiched between the positive electrode and the negative electrode to prevent physical contact between the two electrodes and the occurrence of a short circuit. The porous structure of the separator ensures a passage of ionic charge carriers (e.g., lithium ions) between the positive electrode and the negative electrode. In addition, the separator may provide a mechanical support to the electrochemical device. The electrochemical devices disclosed herein include any devices in which electrochemical reactions occur. For example, the electrochemical devices may be at least one of electrolytic cells, primary batteries, secondary batteries, fuel cells, solar cells and capacitors. In some embodiments, the electrochemical device disclosed herein is a lithium secondary battery, such as a lithium ion secondary battery, a lithium polymer secondary battery, a lithium metal secondary battery, a lithium air secondary battery and a lithium sulfur secondary battery.

With the separator of the present disclosure inside, the electrochemical device disclosed herein can exhibit improved safety at a high temperature as discussed above. The electrochemical devices of the present disclosure can also have a low internal resistance as the separators disclosed herein have good air permeability due, for example, to the use of nonwoven porous membrane.

The electrochemical device disclosed herein may be manufactured by a conventional method known to one skilled in the art. In one embodiment, an electrode assembly is formed by placing a separator of the present disclosure between a positive electrode and a negative electrode, and an electrolyte is injected into the electrode assembly. The electrode assembly may be formed by a conventional process, such as a winding process or a lamination (stacking) and folding process.

Further disclosed herein are embodiments of a method for making the separator of the present disclosure. In some embodiments, the method is a wet coating process. For example, the method may comprise:

(A) preparing a coating slurry comprising at least one polyimide or polyimide precursor, at least one inorganic filler, and at least one solvent;

(B) applying the coating slurry on at least one side of a nonwoven porous membrane to form a wet coating layer; and

(C) removing the at least one solvent from the wet coating layer.

In step (A) , the resulting coating slurry may be a suspension as it contains the at least one inorganic filler. The coating slurry may, for example, comprise from 1.5 to 6 parts by weight of the at least one polyimide or polyimide precursor, from 0.5 to 4 parts by weight of the at least one inorganic filler, and from 80 to 95 parts by weight of the at least one solvent. Further, for example, the coating slurry may comprise from 4.5 to 5.5 parts by weight of the at least one polyimide or polyimide precursor, from 1 to 3.5 parts by weight of the at least one inorganic filler, and from 85 to 92 parts by weight of the at least one solvent. In one embodiment, the coating slurry may comprise 5 parts by weight of the at least one polyimide or polyimide precursor, 3 parts by weight of the at least one inorganic filler, and 90 parts by weight of the at least one solvent.

In the case that polyimides are used to prepare the coating slurry, the at least one polyimide may be soluble and dissolved in the at least one solvent. Examples of the soluble polyimides include bisether anhydride-type polyimides, monoether anhydride-type polyimides, PEI, PAI, and PESI.

In the case that polyimide precursors are used to prepare the coating slurry, the at least one polyimide precursors may be chosen, for example, from dianhydrides and diamines, dianhydrides and diisocyanates, and PAAs as set forth above.

As to the types of the at least one solvent present in the coating slurry, it depends on the type of polyimides or polyimide precursors used in the coating slurry. For example, the at least one solvent may have a solubility parameter similar to that of the polyimides or polyimide precursors to be dissolved, and a low boiling point, so that such solvent can facilitate uniform mixing and coating process and needs to be removed in the following operation. The at least one solvent may, for example, be a polar solvent. Examples of the at least one solvent that may be used herein include DMAC, NMP, DMF, DMSO, acetone, diethyl ether, propyl ether, cyclohexane, THF and a mixture thereof.

To help the at least one polyimide or polyimide precursor dissolve in the at least one solvent quickly, various methods can be used. In some embodiments, the temperature of the at least one solvent may be controlled in a range of, for example, from 60 ℃ to 130 ℃. In some other embodiments, the coating slurry may further comprise at least one solubilizer to increase the solubility of the at least one polyimide or polyimide precursor. The at least one solubilizer may be chosen, for example, from lithium chloride (LiCl) , calcium chloride (CaCl ₂) , and dodecylbenzene sulfonic acid.

As to the at least one inorganic filler present in the coating slurry, as discussed above, various inorganic particles can be used, including, for example, an oxide, a hydroxide, a sulfide, a nitride, a carbide, a carbonate, a sulfate, a phosphate, a titanate, and the like of at least one of metallic and semiconductor elements, such as Si, Al, Ca, Ti, B, Sn, Mg, Li, Co, Ni, Sr, Ce, Zr, Y, Pb, Zn, Ba, and La. To help the at least one inorganic filler disperse in the coating slurry and avoid agglomeration, the coating slurry may further comprise, for example, at least one dispersant. Polyethylene oxide (PEO) may, for example, be used herein as a suitable dispersant. PEO used herein may have a weight average molecular weight (M _w) ranging, for example, from 100,000 to 1,000,000, such as from 200,000 to 500,000, in the form of powder. In one embodiment, PEO ultrafine powder having a M _w of 300,000 is used as the at least one dispersant.

In some embodiments, the coating slurry prepared in step (A) may comprise from 1.5 to 6 parts by weight of the at least one polyimide or polyimide precursor; from 0.5 to 4 parts by weight of the at least one inorganic filler; from 80 to 95 parts by weight of the at least one solvent; from 0.5 to 2.5 parts by weight of the at least one solubilizer; and from 0.5 to 2 parts by weight of the at least one dispersant. For example, the coating slurry prepared in step (A) may comprise from 4.5 to 5.5 parts by weight of the at least one polyimide or polyimide precursor; from 1 to 3.5 parts by weight of the at least one inorganic filler; from 85 to 92 parts by weight of the at least one solvent; from 0.8 to 1.8 parts by weight of the at least one solubilizer; and from 0.8 to 1.5 parts by weight of the at least one dispersant. In one embodiment, the coating slurry prepared in step (A) comprises 5 parts by weight of the at least one polyimide or polyimide precursor; 3 parts by weight of the at least one inorganic filler; 90 parts by weight of the at least one solvent; from 2 parts by weight of the at least one solubilizer; and from 0.15 parts by weight of the at least one dispersant.

In some embodiments, the coating slurry may be prepared by mixing the at least one polyimide or polyimide precursor, the at least one inorganic filler, the at least one solvent and the optional components, e.g., the at least one solubilizer and the at least one dispersant. The mixing may be processed by stirring. In some other embodiments, the coating slurry is prepared as follows:

(A1) adding the at least one polyimide or polyimide precursor into a first solvent to obtain a first slurry;

(A2) adding the at least one inorganic filler into a second solvent to obtain a second slurry; and

(A3) mixing the first slurry and the second slurry to obtain the coating slurry.

Each of the first solvent and second solvent may be chosen, for example, from DMAC, NMP, DMF, DMSO, acetone, diethyl ether, propyl ether, cyclohexane, THF, and a mixture thereof. The compositions of the first solvent and the second solvent may be the same or different.

In step (A1) , the at least one solubilizer as described above may be added into the first solvent to increase the solubility of the at least one polyimide or polyimide precursor. The first slurry may, for example, be a yellow liquid at room temperature.

In step (A2) , the at least one dispersant as described above may be added into the second solvent to help the at least one inorganic filler disperse in the second slurry.

In step (A3) , the first slurry may be slowly added into the second slurry.

In step (B) , the coating slurry prepared in step (A) is applied on at least one side of the nonwoven porous membrane. Any coating method known in the art may be used to coat the nonwoven porous membrane with the coating slurry, such as roller coating, spray coating, dip coating, spin coating, and combinations thereof. Examples of the roller coating may include gravure coating, silk screen coating, and slot die coating. In the case that both surfaces of the nonwoven porous membrane are coated with the coating slurry, the both surfaces can be coated simultaneously or by sequence.

In step (C) , the at least one solvent can be removed from the wet coating layer through a method known in the art, such as a thermal evaporation, a vacuum evaporation, a phase inversion process, or a combination thereof. When the at least one solvent is removed, a porous structure may be formed in the coating layer to obtain a heat-resistant layer.

In some embodiments, the at least one solvent may be removed through a combination of thermal evaporation and vacuum evaporation. For example, the nonwoven porous membrane coated with the coating slurry may be subjected to a vacuum oven for a predetermined time period so as to remove the at least one solvent from the wet coating layer. The pressure and temperature of the vacuum oven may depend on the type of the solvent to be removed.

Phase inversion process is an alternative method to remove the at least one solvent, which may be initiated by exposing the wet coating layer to a poor solvent of polyimides or polyimide precursors, such as water, alcohols (e.g., ethanol) , or a combination thereof. The water used herein is, for example, deionized water. When the wet coating layer is exposed to the poor solvent, most of the solvent may transfer from the coating layer to the poor solvent, resulting in a porous structure in the coating layer. The phase inversion process is energy-efficient as no phase change happens when the at least one solvent is removed. In some embodiments, step (C) may comprise immersing the coated nonwoven porous membrane in a poor solvent for a predetermined time period, for example, from 0.5 to 3 minutes, such as from 1 to 2 minutes. To remove the at least one solvent from the wet coating layer more efficiently, a flowing poor solvent may be used, or making the coated nonwoven porous membrane pass through a tank of poor solvent in a predetermined speed. Step (C) may further comprise taking the coated nonwoven porous membrane out from the poor solvent and drying the coated nonwoven porous membrane to remove a residue of the at least one solvent and/or the poor solvent. The residue of the at least one solvent and/or the poor solvent may be removed by, for example, thermal evaporation, vacuum evaporation, or a combination thereof.

The thermal evaporation disclosed herein may be carried out in a closed oven or an open oven. For example, the coated nonwoven porous membrane is passed through a multi-stage open oven, e.g., a three-stage oven, in a predetermined speed. The three-stage oven may have, for example, a temperature ranging from 45 to 55℃ in its first stage, a temperature ranging from 55 to 65℃ in its second stage, and a temperature ranging from 50 to 60℃ in its third stage. In one embodiment, the three-stage oven has temperatures of 50℃, 60℃, and 55℃ in its first, second, and third stages, respectively.

When the at least one polyimide precursor is used in step (A) , the method disclosed herein may further comprise:

(D) curing the at least one polyimide precursor to form polyimide.

Various curing methods may be used in step (D) , including, for example, heating (e.g., heating to a temperature ranging, for example, from 250 ℃ to 375℃) , radiation (e.g., ultraviolet (UV) ) , immersed in a dehydrating agent (e.g., a mixture of acetic acid and pyridine, acetic anhydride) , and a combination thereof. During the curing process, most of the polyimide precursors, for example, more than 80 wt%of the polyimide precursors, may react to form polyimides. In some embodiments, step (D) may be carried out before or after step (C) as an independent step. In some other embodiments, step (D) may be combined with step (C) . For example, after step (B) , the coated nonwoven porous membrane may be placed in an oven at a temperature ranging for example, from 250 ℃ to 375℃, so as to remove the at least one solvent and cure the polyimide precursors simultaneously. In another example, the combined step (C) and step (D) may comprise: immersing the coated nonwoven porous membrane in a poor solvent of polyimide; taking the coated nonwoven porous membrane out from the poor solvent; and applying a UV radiation to the coated nonwoven porous membrane at a temperature ranging, for example, from 50 ℃ to 70 ℃, for drying the coating layer and curing the polyimide precursors simultaneously.

Via the method set forth above, a dry and porous heat-resistant layer comprising at least one polyimide and at least one inorganic filler may be formed on at least one side of the nonwoven porous membrane. The inorganic fillers are embedded in the heat-resistant layer by polyimides. As the weight ratio of polyimides or polyimide precursors and the inorganic fillers is controlled in a specific range, the heat-resistant layer disclosed herein has good pore size, porosity and relatively low surface density. The resulting separator prepared by the above method may have not only high heat resistance, but also good air permeability.

Reference is now made in detail to the following examples that relate to preparation of the separators according to the present disclosure. It is to be understood that the following examples are illustrative only and the present disclosure is not limited thereto.

Example 1

0.1 kg LiCl, 0.1 kg PEO powder (M _w = 100,000-200,000) , and 0.1 kg Al ₂O ₃ were successively added into 3 kg NMP to obtain a mixture. The mixture was stirred to dissolve LiCl and PEO and to disperse Al ₂O ₃ in NMP. With stirring, 7 kg of 8 wt%polyimide resin solution (polyimide resin of CAS No. 62929-02-6, dissolved in DMAC) was slowly added into the mixture to obtain a coating slurry.

A PET nonwoven membrane having a thickness of 18 μm was used as a substrate. The above prepared coating slurry was coated on one surface of the PET nonwoven membrane through a gravure coating process with a speed of 12 m/min to form a wet coating layer. The coated PET nonwoven membrane was immersed in water for 1 min. After taken out from water, the coated PET nonwoven membrane was passed through a three-stage oven, each stage of which had a temperature of 50℃, 60℃, and 55℃, respectively. A UV radiation having a wave-length of 309nm was applied to the coated PET nonwoven membrane in the third stage of the three-stage oven. A separator having a thickness of 22 μm was then obtained.

Example 2

0.1 kg LiCl, 0.1 kg PEO powder (M _w = 100,000-200,000) , and 0.2 kg Al ₂O ₃ were successively added into 3.5 kg DMAC to obtain a mixture. The mixture was stirred to dissolve LiCl and PEO and to disperse Al ₂O ₃ in DMAC. With stirring, 6.5 kg of 8 wt%polyimide resin solution (polyimide resin of CAS No. 62929-02-6, dissolved in DMAC) was slowly added into the mixture to obtain a coating slurry.

A PET nonwoven membrane having a thickness of 18 μm was used as a substrate. The above prepared coating slurry was coated on one surface of the PET nonwoven membrane through a gravure coating process with a speed of 12 m/min to form a wet coating layer. The coated PET nonwoven membrane was immersed in water for 1.5 min. After taken out from water, the coated PET nonwoven membrane was passed through a three-stage oven, each stage of which had a temperature of 50℃, 60℃, and 55℃, respectively. A UV radiation having a wave-length of 303nm was applied to the coated PET nonwoven membrane in the third stage of the three-stage oven. A separator having a thickness of 22 μm was then obtained.

Example 3

0.1 kg CaCl ₂, 0.1 kg PEO powder (M _w = 100,000-200,000) , and 0.3 kg boehmite were successively added into 4 kg DMAC to obtain a mixture. The mixture was stirred to dissolve CaCl ₂ and PEO and to disperse boehmite in DMAC. With stirring, 6 kg of 8 wt%polyimide resin solution (polyimide resin of CAS No. 62929-02-6, dissolved in DMAC) was slowly added into the mixture to obtain a coating slurry.

A PET nonwoven membrane having a thickness of 18 μm was used as a substrate. The above prepared coating slurry was coated on one surface of the PET nonwoven membrane through a gravure coating process with a speed of 12 m/min to form a wet coating layer. The coated PET nonwoven membrane was immersed in water for 2 min. After taken out from water, the coated PET nonwoven membrane was passed through a three-stage oven, each stage of which had a temperature of 50℃, 60℃, and 55℃, respectively. A UV radiation having a wave-length of 334 nm was applied to the coated PET nonwoven membrane in the third stage of the three-stage oven. A separator having a thickness of 22 μm was then obtained.

Example 4

0.15 kg CaCl ₂, 0.1 kg PEO powder (M _w = 100,000-200,000) , and 0.4 kg boehmite were successively added into 4.5 kg NMP to obtain a mixture. The mixture was stirred to dissolve CaCl ₂ and PEO and to disperse boehmite in NMP. With stirring, 5.5 kg of 8 wt%polyimide resin solution (polyimide resin of CAS No. 62929-02-6, dissolved in DMAC) was slowly added into the mixture to obtain a coating slurry.

A PET nonwoven membrane having a thickness of 18 μm was used as a substrate. The above prepared coating slurry was coated on one surface of the PET nonwoven membrane through a gravure coating process with a speed of 12 m/min to form a wet coating layer. The coated PET nonwoven membrane was immersed in water for 1.5 min. After taken out from water, the coated PET nonwoven membrane was passed through a three-stage oven, each stage of which had a temperature of 50℃, 60℃, and 55℃, respectively. A UV radiation having a wave-length of 303 nm was applied to the coated PET nonwoven membrane in the third stage of the three-stage oven. A separator having a thickness of 22 μm was then obtained.

The following Comparative Example was conducted in comparison with Examples 1 to 4.

Comparative Example

0.1 kg LiCl, 0.1 kg PEO powder (M _w = 100,000-200,000) , and 0.1 kg Al ₂O ₃ were successively added into 3 kg NMP to obtain a mixture. The mixture was stirred to dissolve LiCl and PEO, and to disperse Al ₂O ₃ in NMP. With stirring, 7 kg of 8 wt%polyvinylidene fluoride (PVDF) solution (M _w of PVDF = 2000000, dissolved in NMP) was slowly added into the mixture to obtain a coating slurry.

A PE porous membrane having a thickness of 18 μm was used as a substrate. The above prepared coating slurry was coated on one surface of the PE membrane through a gravure coating process with a speed of 12 m/min to form a wet coating layer. The coated PE porous membrane was immersed in water for 1.5 min. After taken out from water, the coated PE porous membrane was passed through a three-stage oven, each stage of which had a temperature of 50℃, 60℃, and 55℃, respectively. A separator having a thickness of 22 μm was then obtained.

Air Permeability Test

For each separator, the air permeability test was carried out on three samples using an air permeability tester (Asahi-Seiko EGO1-55-1MR) according to a method set forth in Japanese Standard “JIS P8117-2009 Paper and Board-Determination of Air Permeance. ” The time in seconds required for 100 cm ³ of air to pass through the sample was recorded as the air permeability with the unit of s/100cc or s/100 cm ³.

Thermal Shrinkage Test

Three samples of each separator were measured. The thermal shrinkage of each separator was tested using the following method. A separator sample of 40 mm (Machine Direction, MD) × 60 mm (Transverse Direction, TD) was prepared. A square of 30 mm (MD) × 50 mm (TD) was marked on the separator sample. The separator sample was placed in an oven of 150℃ for one hour and then taken out from the oven for cooling down. The MD and TD length of the marked square were measured by a binary optics projector and recorded as L _MD (mm) and L _TD (mm) , respectively. The thermal shrinkage percentage was calculated by the following formula:

Table 1 summarizes the test results of the separators that were prepared according to Examples 1 to 4 and Comparative Example.

Table 1.

The results in Table 1 show that the air permeability values of the separators prepared in Examples 1-4 using the method of the present disclosure were close to that of the PVDF-coated separator prepared in Comparative Example, which could meet the air permeability requirements of most electrochemical devices in the market.

The thermal shrinkage results in Table 1 show that at the temperature of 150 ℃, serious thermal shrinkage happened to the PVDF-coated separator prepared in Comparative Example. However, the separators prepared in Examples 1-4 using the method of the present disclosure had lower thermal shrinkage percentages than that in Comparative Example, indicating they had high heat-resistance or good thermal stability.

Claims

A separator for an electrochemical device, comprising:

a nonwoven porous membrane; and

a heat-resistant layer being formed on at least one side of the nonwoven porous membrane,

wherein the heat-resistant layer is formed by using a coating slurry that comprises from 1.5 to 6 parts by weight of at least one polyimide or polyimide precursor and from 0.5 to 4 parts by weight of at least one inorganic filler.
The separator according to claim 1, wherein the coating slurry comprises from 2 to 4 parts by weight of at least one polyimide or polyimide precursor and from 2 to 3 parts by weight of at least one inorganic filler.
The separator according to claim 1, wherein the nonwoven porous membrane comprises at least one material chosen from polyethylene terephthalate, polyethylene, high density polyethylene, polypropylene, polybutylene, polypentene, polymethylpentene, polyamide, polyimide, polyacrylonitrile, viscose fiber, polyester, polyacetal, polycarbonate, polyetherketone, polyetheretherketone, polybutylene terephthalate, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalene, and cellulose.
The separator according to claim 1, wherein the at least one polyimide is chosen from aromatic polyimides and semi-aromatic polyimides.
The separator according to claim 1, wherein the at least one polyimide precursor is chosen from dianhydrides and diamines, dianhydrides and diisocyanates, and PAAs.
The separator according to claim 1, wherein the at least one inorganic filler is chosen from oxides, hydroxides, sulfides, nitrides, carbides, carbonates, sulfates, phosphates, and titanates comprising at least one of metallic and semiconductor elements.
The separator according to claim 1, wherein the at least one inorganic filler is chosen from alumina, boehmite, silica, titanium oxide, cerium oxide, calcium oxide, zinc oxide, magnesium oxide, lithium nitride, calcium carbonate, barium sulfate, lithium phosphate, lithium titanium phosphate, lithium aluminum titanium phosphate, cerium titanate, calcium titanate, barium titanate, and lithium lanthanum titanate.
The separator according to claim 1, wherein heat-resistant layer has a thickness ranging from 3 μm to 8 μm.
An electrochemical device comprising a positive electrode, a negative electrode, and a separator according to claim 1 interposed between the positive electrode and the negative electrode.
A method for making a separator of claim 1, comprising:

preparing a coating slurry comprising at least one polyimide or polyimide precursor, at least one inorganic filler, and at least one solvent;

applying the coating slurry on at least one side of a nonwoven porous membrane to form a wet coating layer; and

removing the at least one solvent from the wet coating layer.
The method according to claim 10, wherein the coating slurry is prepared by:

adding the at least one polyimide or polyimide precursor into a first solvent to obtain a first slurry;

adding the at least one inorganic filler into a second solvent to obtain a second slurry; and

mixing the first slurry and the second slurry to obtain the coating slurry.
The method according to claim 10, wherein the at least one solvent is removed by:

immersing the coated nonwoven porous membrane in a poor solvent of polyimides; and

drying the coated nonwoven porous membrane taken out from the poor solvent.
The method according to claim 12, wherein the drying comprises a three-stage heating process having a temperature ranging from 45 to 55℃ in the first stage, a temperature ranging from 55 to 65℃ in the second stage, and a temperature ranging from 50 to 60℃ in the third stage.
The method according to claim 10, wherein the at least one polyimide precursor is polyimide resin of CAS No. 62929-02-6 dissolved in DMAC.
The method according to claim 10, wherein the coating slurry comprises at least one polyimide precursor, and the method further comprises:

curing the at least one polyimide precursor to form polyimide.
The method according to claim 15, wherein the at least one polyimide precursor is cured by heating or radiation.
The method according to claim 10, wherein the at least one solvent is chosen from N-methyl pyrrolidone, dimethylacetamide, N, N-dimethylformamide, dimethyl sulfoxide, acetone, diethyl ether, propyl ether, cyclohexane, and tetrahydrofuran.
The method according to claim 10, wherein the coating slurry comprises:

from 1.5 to 6 parts by weight of the at least one polyimide or polyimide precursor;

from 0.5 to 4 parts by weight of the at least one inorganic filler; and

from 80 to 95 parts by weight of the at least one solvent.
The method according to claim 11, wherein the coating slurry further comprises at least one solubilizer and/or at least one dispersant.
The method according to claim 19, wherein the coating slurry comprises:

from 1.5 to 6 parts by weight of the at least one polyimide or polyimide precursor;

from 0.5 to 4 parts by weight of the at least one inorganic filler;

from 80 to 95 parts by weight of the at least one solvent;

from 0.2 to 2.5 parts by weight of the at least one solubilizer; and

from 0.5 to 2 parts by weight of the at least one dispersant.