WO2019157695A1

WO2019157695A1 - Separator and preparation method therefor and electrochemical device comprising separator

Info

Publication number: WO2019157695A1
Application number: PCT/CN2018/076805
Authority: WO
Inventors: Yue Cheng; Jinzhen BAO; Yongle Chen; Zhi ZHUANG; Fangbo HE
Original assignee: Shanghai Energy New Materials Technology Co., Ltd.
Priority date: 2018-02-14
Filing date: 2018-02-14
Publication date: 2019-08-22

Abstract

Disclosed are a separator for an electrochemical device, comprising a porous base membrane and a coating layer being formed on at least one surface of the porous base membrane, wherein the coating layer comprises at least one heat-resistant polymer and at least one binder polymer; as well as an electrochemical device comprising the separator and a method for making the separator.

Description

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

TECHNICAL FIELD

The present disclosure relates to a separator, an electrochemical device comprising the separator, and a method 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.

Usually an electrode assembly of an electrochemical device comprises a positive electrode having a positive material (i.e., cathode) , a negative electrode having a negative material (i.e., anode) , and a permeable membrane (i.e., separator) interposed between the positive electrode and the negative electrode. The anode and the cathode are prevented from being in direct contact with each other by the separator, thereby avoiding internal short circuit. In the meanwhile, ions are allowed to pass the separator so as to close the circuit during the passage of current. Separator is a critical component in an electrochemical device because its structure and property considerably affect the performances of the electrochemical device, including 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. In recent years, the rapid expansion of electrical vehicle market results in intensive research and development on batteries of high energy density and high power density, which are usually called as “automotive batteries. ” Safety is a major issue for the automotive batteries as they operate in a closed space where the environment temperature may be very high. To ensure the safety of an automotive battery or an automotive battery pack (i.e., a large number of batteries in series and/or parallel connections) , the separators used therein are required to have excellent heat-resistance and good interface contact with the anode and/or the cathode. However, the conventional polyolefin-based separator may not meet these requirements because the polyolefin materials usually have low melting points. When the temperature inside of the battery rises, the polyolefin-based separator may shrink or melt, resulting in a volume change, which may lead to a direct contact of the anode and the cathode, i.e., internal short circuit. The internal short circuit can cause some accidents such as battery bulge, burning, explosion, etc.

Inorganic material-coated separators with improved heat-resistance performance have been disclosed. For example, a coating layer containing alumina or boehmite is formed on at least one surface of a polyolefin-based microporous membrane. Although the inorganic material-coated separators are stable at a high temperature, such as 120℃, they may also shrink at a higher temperature, such as 150℃. With the presence of a coating layer containing inorganic materials, the inorganic material-coated separator may be heavier than the conventional polyolefin-based separator, resulting in a higher surface density, and further resulting in a reduced energy density of the corresponding electrochemical device. In addition, the inorganic coating layer usually has bad adhesion to the electrodes. Thus the interfaces between the separator and the electrodes may deform during charge-discharge cycles, and lithium plating may happen, which may shorten the life time of the electrochemical device that contains the inorganic material-coated separator.

Lightweight separators of high heat-resistance, good interface contact with electrodes are still demanded for various electrochemical devices.

SUMMARY OF THE INVENTION

The present disclosure provides a separator for an electrochemical device. Specifically, the separator disclosed herein includes a porous base membrane and a coating layer being formed on at least one surface of the porous base membrane, and the coating layer comprises at least one heat-resistant polymer and at least one binder polymer.

The present disclosure also provides an electrochemical device. The electrochemical device disclosed herein includes 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 slurry comprising at least one heat-resistant polymer, at least one binder polymer, and at least one solvent; coating the slurry on at least one surface of a porous base membrane to form a coating layer; and removing the at least one solvent from the coating layer.

As used in the present disclosure, the term “slurry” means a mixture of ingredients with at least one solvent. It can, for example, be a suspension, a colloid solution, or a solution. For example, a slurry disclosed herein can be a solution in which the at least one heat-resistant polymer and the at least one binder polymer dissolve into the at least one solvent. Further, for example, a slurry disclosed herein can be a suspension when it contains at least one inorganic filler.

DETAILED DESCRIPTION

The present disclosure provides some exemplary embodiments of separators for electrochemical devices. In the embodiments of the present disclosure, a coating layer, which comprises at least one heat-resistant polymer and at least one binder polymer, is formed on at least one surface of a porous base membrane. In some embodiments, the separator may have a two-layer structure when only one surface of the porous base membrane is coated with the coating layer. In some other embodiments, the separator may have a three-layer structure when both surfaces of the porous base membrane are coated with the coating layer.

The porous base membrane serves as a substrate, at least one surface of which is coated with the coating layer. The porous base membrane disclosed herein may have a thickness ranging, for example, from 0.5 to 50 μm, such as from 0.5 to 20 μm, and further such as from 5 to 18 μm. The porous base membrane may have numerous pores inside, through which gas, liquid, or ions can pass from one surface side to the other surface side.

In some embodiments of the present disclosure, polyolefin-based porous membranes are used as the porous base membrane. Examples of polyolefin contained in the polyolefin-based porous membrane may include polyethylene (PE) , high density polyethylene (HDPE) , polypropylene (PP) , polybutylene, polypentene, polymethylpentene (TPX) , copolymers thereof, and mixtures thereof. The polyolefin disclosed herein may have a weight average molecular weight (M _w) ranging, for example, from 50,000 to 2,000,000, such as from 100,000 to 1,000,000. The pores within the polyolefin-based porous base membrane may have an average pore size ranging, for example, from 20 to 70 nm, such as from 30 to 60 nm. The polyolefin-based porous base membrane may have a porosity ranging, for example, from 25%to 50%, such as from 30%to 45%. Furthermore, the polyolefin-based porous base membrane may have an air permeability ranging, for example, from 50 to 400 sec/100ml, such as from 80 to 300 sec/100ml. In addition, the polyolefin-based porous membrane may have a single-layer structure or a multi-layer structure. A polyolefin-based porous membrane of the multi-layer structure may include at least two laminated polyolefin-based layers containing different types of polyolefin or a same type of polyolefin having different molecular weights. The polyolefin-based porous membrane disclosed herein can be prepared according to a conventional method known in the art, or can be purchased directly in the market.

In some other embodiments, a non-woven membrane may form at least one portion of the porous base membrane. The term “non-woven 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 comprise 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 non-woven membrane disclosed herein may exhibit dimensional stability, i.e., thermal shrinkage of less than 5%when heated to 100℃ for about two hours. The non-woven membrane may have a relatively large average pore size ranging, for example, from 0.1 to 20 μm, such as from 1 to 5 μm. The non-woven membrane may have a porosity ranging, for example, from 40%to 80%, such as from 50%to 70%. Furthermore, the non-woven 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. Some examples of the non-woven membrane are formed of one chosen from polyethylene (PE) , high density polyethylene (HDPE) , polypropylene (PP) , polybutylene, polypentene, polymethylpentene (TPX) , polyethylene terephthalate (PET) , 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 fiber, copolymers thereof, and mixtures thereof. In an example, a non-woven membrane formed of PET is used as the porous base membrane. The non-woven 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.

In addition to the porous base membrane, the separator disclosed herein also comprises at least one coating layer. In some embodiments, the coating layer is formed on at least one surface of the porous base membrane. In some embodiments, the coating layer may be applied onto the porous base membrane with a slurry using various suitable techniques, such as a roller coating, a spray coating, a dip coating, or a spin coating process. At least a part of the slurry may penetrate into the pores of the porous base membrane.

The coating layer disclosed herein may also have a pore structure allowing gas, liquid, or ions pass from one surface side to the other surface side of the coating layer. The average pore size of the pores within the coating layer may range, for example, from 0.1 to 5 μm, such as from 1 to 3 μm. The porosity of the coating layer may range, for example, from 30%to 70%, such as from 40%to 60%. The coating layer may have an air permeability ranging, for example, from 0 to 150 sec/100ml, such as from 10 to 50 sec/100ml. Additionally, there is no particular limitation in the thickness of the coating layer. The coating layer may have a thickness ranging, for example, from 0.5 to 5 μm, such as from 2 to 4 μm. To achieve a lightweight separator, the unit weight of the coating layer on one surface of the porous base membrane may be controlled in a range of, for example, from 1 to 6 g/m ², such as from 2 to 5 g/m ².

The coating layer disclosed herein comprises at least one heat-resistant polymer and at least one binder polymer. The weight ratio of the at least one heat-resistant polymer and the at least one binder polymer may be controlled in a range to make sure the separator of the present disclosure have both good heat-resistance performance and good contact interface with the electrodes. In some embodiments, the at least one heat-resistant polymer and the at least one binder polymer may be present in the coating layer in a weight ratio ranging, for example, from 1:99 to 99: 1, such as from 20: 80 to 80: 20, and further such as from 30: 70 to 70: 30.

The at least one heat-resistant polymer present in the coating layer may have a high melting temperature or a glass transition temperature of, for example, 200℃ or above, such as 300℃ or above. The presence of the at least one heat-resistant polymer can improve the heat resistance of the separator. Thus the separator may have a low thermal shrinkage percentage at an elevated temperature, and/or a high meltdown temperature. The meltdown temperature is a temperature at which the separator breaks and cannot keep the anode and the cathode physically apart anymore. The meltdown temperature may be higher than or equal to a shutdown temperature of the separator, at which the separator melts to block pores therein and limits ion conductivity. Accordingly, electrochemical devices employing the heat-resistant separator may have an improved heat-resistance and safety. The at least one heat-resistant polymer present in the coating layer may be chosen, for example, from aramid, polyimide (PI) , polyetherimide (PEI) , polysulfone, polybenzimidazole (PBI) , polyphenylene sulfide (PPS) , polyethersulfone (PES) , polyarylsulfone, polyketone, polyetherketone (PEK) , polyetheretherketone (PEEK) , polydiphenyl oxide, copolymers thereof, and mixtures thereof. For example, meta-aramid and para-aramid, which are two typical categories of aramids, can be used as the heat-resistant polymer because of their high strength, excellent solvent, heat and flame resistance and great dimensional stability. In addition, the heat-resistant polymer used in the coating layer disclosed herein may have an M _w ranging, for example, from 5,000 to 500,000, such as from 20,000 to 300,000.

The at least one binder polymer in the coating layer can enhance the bonding or adhesive property of the separator. For example, the binder polymer disclosed herein can provide a firm physical or chemical bonding between the coating layer and the porous base membrane, thereby preventing the coating layer from being peeled off from the porous base membrane easily. Moreover, the at least one binder polymer can help the coating layer physically or chemically bond to the electrodes (e.g., anode, cathode) when the separator is applied in an electrochemical device, resulting in a good contact interface between the separator and the electrodes. As discussed above, during the charge-discharge cycles of the electrochemical device, the interface deformation between the separator and anode or cathode may be prevented, thereby improving the cycle life and mechanical strength of the electrochemical device. Any polymer having an adhesive property may be used herein. Examples of the at least one binder polymer include polyvinylidene fluoride (PVDF) , polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) , polyvinylidene fluoride-co-trichloroethylene (PVDF-TCE) , polyacrylate, polyacrylate salt and polymethacrylate (PMA) , polymethylmethacrylate (PMMA) , polybutylacrylate, polyacrylonitrile (PAN) , polyvinylpyrrolidone (PVP) , polyvinylacetate (PVAc) , polyethylene-co-vinyl acetate (CEVA) , polyethylene oxide (PEO) , cellulose acetate (CA) , cellulose acetate butyrate (CAB) , cellulose acetate propionate (CAP) , carboxyl methyl cellulose (CMC) , copolymers thereof, and mixtures thereof. In some embodiments, PVDF or PVDF-HFP can be used as suitable binder polymer. The binder polymer used in the coating layer may have an M _w ranging, for example, from 2,000 to 100,000, such as from 5,000 to 50,000.

In some embodiments of the present disclosure, the coating layer may further comprise at least one inorganic filler. The at least one inorganic filler can contribute to the heat-resistance of the separator, thereby further preventing short circuit and improving dimensional stability of an electrochemical device employing the separator at a high temperature. Furthermore, the presence of the inorganic filler may also contribute, for example, to the formation of pores in the coating layer, the increase of the physical strength of the coating layer, and the increase in an impregnation rate of a liquid electrolyte. The at least one inorganic filler may be fixed in the coating layer by the at least one binder polymer or a combination of the at least one heat-resistant polymer and the at least one binder polymer.

The amount of the inorganic filler present in the coating layer may be controlled to balance the pore structure (e.g., pore size, uniformity of pores) , porosity, thickness and weight of the coating layer. For example, the inorganic filler may be present in the coating layer in an amount of, for example, less than 80 wt%, such as ranging from 20 w%to 50 wt%, based on the total weight of the at least one heat-resistant polymer, the at least one binder polymer, and the at least one inorganic filler. Various inorganic particles can be used as the at least one inorganic filler, including, for example, an oxide, a hydroxide, a sulfide, a nitride, and a carbide, a carbonate, a sulfate, a phosphate, and 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 inorganic filler. The inorganic filler disclosed herein may have an average particle size ranging, for example, from 0.01 to 20 μm, such as from 0.5 to 10 μm.

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, e.g., lithium-ion batteries.

The separator disclosed herein can have excellent thermal stability at high temperatures and good contact interface with the electrodes. The separator disclosed herein can also have excellent ion permeability and good mechanical strength. Further, the separator disclosed herein can also be lightweight when all or majority of the weight of the coating layer is from polymer materials. In the instances where the at least one inorganic filler is included in the coating layer, the weight percentage of the at least one inorganic filler disclosed herein is low. The lightweight separator may improve the energy density of the electrochemical device employing the separator. The separator disclosed herein can have a wide range of applications and can be used for 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 of the present disclosure. 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 anode and cathode. In addition, the separator may also provide a mechanical support to the electrochemical device. Such electrochemical devices include any devices in which electrochemical reactions occur. For example, the electrochemical device disclosed herein includes 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 metal secondary battery, a lithium ion secondary battery, a lithium polymer 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 and improved cycle life as discussed above. The electrochemical devices of the present disclosure can also have an improved energy density as the separators disclosed herein can be lightweight.

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 comprises a wet coating process. The method disclosed herein, for example, comprises:

(A) preparing a slurry comprising at least one heat-resistant polymer, at least one binder polymer and at least one solvent;

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

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

In the step (A) , in some embodiments, a slurry of the at least one heat-resistant polymer, the at least one binder polymer, and the at least one solvent is prepared. The at least one heat-resistant polymer and the at least one binder polymer may be present in the slurry in a weight ratio ranging, for example, from 1: 99 to 99: 1, such as from 20: 80 to 80: 20, and further such as from 30: 70 to 70: 30. The at least one heat-resistant polymer and the at least one binder polymer may in an amount ranging, for example, from 10 wt%to 100 wt%, such as from 20 wt%to 50 wt%, based on the total weight of the slurry.

As discussed above, the heat-resistant polymer in the slurry may have a melting temperature or a glass transition temperature of, for example, 200℃ or above, such as 300℃ or above. Examples of the at least one heat-resistant polymer include aramid, polyimide (PI) , polyetherimide (PEI) , polysulfone, polybenzimidazole (PBI) , polyphenylene sulfide (PPS) , polyethersulfone (PES) , polyarylsulfone, polyketone, polyetherketone (PEK) , polyetheretherketone (PEEK) , polydiphenyl oxide, copolymers thereof, and mixtures thereof. For example, meta-aramid and para-aramid, which are two typical categories of aramids, can be used as the at least one heat-resistant polymer because of their high strength, excellent solvent, heat and flame resistance and great dimensional stability.

As discussed above, examples of the at least one binder polymer include polyvinylidene fluoride (PVDF) , polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP) , polyvinylidene fluoride-co-trichloroethylene (PVDF-co-TCE) , polyacrylate, polyacrylate salt and polymethacrylate (PMA) , polymethylmethacrylate (PMMA) , polybutylacrylate, polyacrylonitrile (PAN) , polyvinylpyrrolidone (PVP) , polyvinylacetate (PVAc) , polyethylene-co-vinyl acetate (CEVA) , polyethylene oxide (PEO) , cellulose acetate (CA) , cellulose acetate butyrate (CAB) , cellulose acetate propionate (CAP) , carboxyl methyl cellulose (CMC) , copolymers thereof, and mixtures thereof.

The at least one solvent used in the slurry depends on the type of the polymers used to form the slurry. For example, the at least one solvent may have a solubility parameter similar to that of the heat-resistant polymer and/or the binder polymer to be dissolved, and a low boiling point, because such solvent can facilitate uniform mixing and coating process and needs to be removed in the following operation. Examples of the at least one solvent that may be used herein may include an organic solvent chosen from N, N-dimethylformamide (DMF) , dimethylacetamide (DMAC) , N-methyl pyrrolidone (NMP) , dmethyl sulfoxide (DMSO) , acetone, diethyl ether, propyl ether, cyclohexane, and tetrahydrofuran (THF) .

In some embodiments of the present disclosure, the slurry may be prepared by adding the at least one heat-resistant polymer and the at least one binder polymer simultaneously or successively into the at least one solvent to obtain a mixture, and stirring the mixture to obtain the slurry.

In some other embodiments of the present disclosure, the slurry may be prepared by:

(A1) adding the at least one heat-resistant polymer into a first solvent to obtain a first mixture;

(A2) adding the at least one binder polymer into a second solvent to obtain a second mixture; and

(A3) mixing the first mixture and the second mixture.

The slurry prepared by steps (A1) - (A3) includes the first solvent and the second solvent, which may be the same or different. Each of the first and the second solvents can include an organic solvent chosen, for example, from DMF, DMAC, NMP, DMSO, acetone, diethyl ether, propyl ether, cyclohexane, and THF.

In the methods for preparation of the slurry disclosed herein, to enhance the solubility of the heat-resistant polymer and/or the binder polymer in the at least one solvent, or shorten the dissolution time, various techniques may be used, for example, agitation, raising the temperature of the at least one solvent (for example, the temperature of the at least one solvent may range from 5℃ to 80℃, such as from 20℃ to 50℃) , increasing the amount of the solvent used, and/or adding at least one solubilizer into the solvent. The at least one solubilizer can be chosen, for example, from lithium chloride (LiCl) , calcium chloride (CaCl ₂) , and dodecylbenzene sulfonic acid (DBSA) .

In some embodiments of the present disclosure, the slurry may further include at least one inorganic filler. The at least one inorganic filler may be present in the slurry in an amount of, for example, less than 80 wt%, such as ranging from 20 w%to 50 wt%, based on the total weight of the at least one heat-resistant polymer, the at least one binder polymer, and the at least one inorganic filler. As discussed above, various inorganic particles can be used as the inorganic filler, including, for example, an oxide, a hydroxide, a sulfide, a nitride, and a carbide, a carbonate, a sulfate, a phosphate, and 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 inorganic filler. The inorganic filler disclosed herein may have an average particle size ranging, for example, from 0.01 to 20 μm, such as from 0.5 to 10 μm.

During the process of preparing the slurry containing the at least one inorganic filler, the at least one inorganic filler may be added into the at least one solvent together with at least one of the heat-resistant polymer and the binder polymer simultaneously or successively. In some other embodiments, a mixture of the inorganic filler and another solvent chosen, for example, from DMF, DMAC, NMP, DMSO, acetone, diethyl ether, propyl ether, cyclohexane, and THF, may be used for preparing the slurry.

In the step (B) , any method known in the art may be used to coat the porous base membrane with the 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. Additionally, when the slurry is coated onto the porous base membrane, either or both surfaces of the porous base membrane may be coated.

In the step (C) , the at least one solvent can be removed from the coating layer through a method known in the art, such as a thermal evaporation, a vacuum evaporation, a phase inversion process, or combinations thereof. In some embodiments, the at least one solvent may be removed through a combination of thermal evaporation and vacuum evaporation. For example, the porous base membrane coated with the slurry may be subjected to a vacuum oven for a period of time so as to remove the at least one solvent from the coating layer. 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 the heat-resistant polymer and/or the binder polymer, such as water, alcohols (e.g., ethanol) , or combinations thereof. The poor solvent precipitates the heat-resistant polymer and/or the binder polymer from the slurry. In an example, the porous base membrane coated with the slurry may be immersed in water for a predetermined time period, so that the at least one solvent may be transferred from the wet coating layer to water. The water used herein is, for example, deionized water. Residue of the at least one solvent and/or the poor solvent may be removed by, for example, vacuum drying, evaporation, etc. As a result, a dry coating layer forms on the porous base membrane. In instances where the at least one inorganic filler is included in the slurry, the inorganic particles are embedded in the porous coating layer.

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

The following Comparative Examples 1 to 4 were conducted in comparison with Examples 1 to 7 that relate to preparation of the separators and the lithium-ion batteries comprising the separator according to the present disclosure.

Comparative Example 1

A single-layer PE membrane having a thickness of 16 μm was used as a porous base membrane. A slurry of alumina and 8 wt%of polyacrylate was coated on one side of the single-layer PE membrane to form an alumina coating layer having a thickness of 3 μm.

A positive electrode was prepared by adding lithium cobaltate (LiCoO ₂) , conductive carbon, and PVDF in a weight ratio of 96: 2: 2 into NMP to obtain an anode slurry, and coating the anode slurry on an aluminum foil, followed by drying at 85℃, cold pressing, cutting into slices, cutting edges, and welding.

A negative electrode was prepared by adding graphite, conductive carbon, sodium salt of caboxy methyl cellulose and styrene-butadiene rubber (SBR) in a weight ratio of 96.5: 1: 1: 1.5 into deionized water to obtain a cathode slurry, and coating the cathode slurry on an aluminum foil, followed by drying at 85℃, cold pressing, cutting into slices, cutting edges, and welding.

An electrolyte was prepared by adding LiPF ₆ to a mixed solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) in a weight ratio of 3: 7. The LiPF ₆ has a concentration of 1.0 mol/L in the electrolyte.

A 18650 lithium-ion battery was produced by placing the above prepared separator between the above prepared positive electrode and negative electrode, and injecting the above prepared electrolyte.

Comparative Example 2

A single-layer PE membrane having a thickness of 16 μm was used as a porous base membrane. Para-aramid (M _w = 20,000) was added into NMP to prepare a slurry containing 40 wt%of para-aramid. The slurry was coated on one surface of the single-layer PE membrane through a gravure coating process to form a coating layer having a thickness of 3 μm.

Then the same procedures as set forth above in Comparative Example 1 were used to prepare a 18650 lithium-ion battery.

Comparative Example 3

A single-layer PE membrane having a thickness of 16 μm was used as a porous base membrane. PVDF (M _w = 40,000) was added into NMP to prepare a slurry containing 40 wt%of PVDF. The slurry was coated on both surfaces of the single-layer PE membrane through a roller coating process to form a coating layer. The coating layer on each surface of the single-layer PE membrane has a thickness of 3 μm.

Comparative Example 4

A non-woven membrane having a thickness of 16 μm was used as a porous base membrane. PVDF (M _w = 40,000) was added into NMP to prepare a slurry containing 40 wt%of PVDF. The slurry was coated on both surfaces of the non-woven membrane through a roller coating process to form a coating layer. The coating layer on each surface of the non-woven membrane has a thickness of 3 μm.

Example 1

A single-layer PE membrane having a thickness of 16 μm was used as a porous base membrane. Para-aramid (M _w = 20,000) and PVDF (M _w = 40,000) were added into NMP in a weight ratio of 3: 7 to prepare a slurry containing 40 wt%of para-aramid and PVDF. The slurry was coated on one surface of the single-layer PE membrane through a gravure coating process to form a coating layer having a thickness of 3 μm.

Example 2

A single-layer PE membrane having a thickness of 16 μm was used as a porous base membrane. Para-aramid (M _w = 20,000) and PVDF (M _w = 40,000) were added into DMAC in a weight ratio of 5: 5 to prepare a slurry containing 40 wt%of para-aramid and PVDF. The slurry was coated on one surface of the single-layer PE membrane through a gravure coating process to form a coating layer having a thickness of 3 μm.

Example 3

A single-layer PE membrane having a thickness of 16 μm was used as a porous base membrane. Para-aramid (M _w = 20,000) and PVDF (M _w = 40,000) were added into NMP in a weight ratio of 7: 3 to prepare a slurry containing 40 wt%of para-aramid and PVDF. The slurry was coated on one surface of the single-layer PE membrane through a gravure coating process to form a coating layer having a thickness of 3 μm.

Example 4

A non-woven membrane having a thickness of 16 μm was used as a porous base membrane. Meta-aramid (M _w = 10,000) and PVDF-HFP (M _w = 50,000) were added into NMP in a weight ratio of 5: 5 to prepare a slurry containing 40 wt%of meta-aramid and PVDF-HFP. The slurry was coated on one surface of the non-woven membrane through a roller coating process to form a coating layer having a thickness of 3 μm.

Example 5

A non-woven membrane having a thickness of 16 μm was used as a porous base membrane. Meta-aramid (M _w = 10,000) , PVDF-HFP (M _w = 50,000) and alumina (with average particle size of 1 μm) were added into DMAC in a weight ratio of 5: 4: 1 to prepare a slurry containing 45 wt%of meta-aramid and PVDF-HFP. The slurry was coated on one surface of the non-woven membrane through a roller coating process to form a coating layer having a thickness of 3 μm.

Example 6

A non-woven membrane having a thickness of 16 μm was used as a porous base membrane. Meta-aramid (M _w = 10,000) and PVDF-HFP (M _w = 50,000) were added into DMF in a weight ratio of 5: 5 to prepare a slurry containing 35 wt%of meta-aramid and PVDF-HFP. The slurry was coated on both surfaces of the non-woven membrane through a dip coating process to form a coating layer. The coating layer on each surface of the single-layer PE membrane has a thickness of 3 μm.

Example 7

A single-layer PP membrane having a thickness of 12 μm was used as a porous base membrane. Meta-aramid (M _w = 10,000) and PVDF (M _w = 40,000) were added into DMF in a weight ratio of 5: 5 to prepare a slurry containing 35 wt%of para-aramid and PVDF. The slurry was coated on both surfaces of the single-layer PP membrane through a dip coating process to form a coating layer. The coating layer on each surface of the single-layer PP membrane has a thickness of 3 μm.

Separators and lithium-ion batteries prepared in the above Comparative Examples 1 to 4 and Examples 1 to 7 were evaluated as follows.

Test 1: Thermal Shrinkage Test of Separators

The thermal shrinkage test was performed in order to measure thermal shrinkage percentages of the separators prepared in Comparative Example 1 to 4 and Examples 1 to 7. For each separator, five 100 mm × 100 mm samples were kept in an oven at a temperature of 150℃for one hour. The thermal shrinkage percentages in a machine direction (MD) and a transverse direction (TD) were measured using a binary optics projector. The average thermal shrinkage percentage of the five samples was shown in Table 1.

Test 2: Meltdown Temperature Test of Separators

The meltdown temperatures of the separators prepared in Comparative Example 1 to 4 and Examples 1 to 7 were measured using a Thermal Mechanical Analyzer (TA Instruments) with 8 mm × 4 mm samples. For each separator, three tests were repeated to obtain an average meltdown temperature, which was shown in Table 1.

Test 3: Peel Strength Test of Separators

To evaluate a peeling resistance of the coating layer of the separators prepared in Comparative Example 1 to 4 and Examples 1 to 7, the peel strength test was performed. An 8 mm × 10 cm sample was cut out and a double-sided adhesive tape (3M) was pasted on the coating layer of the sample. Subsequently, the force required for separating the coating layer from the porous base membrane was measured using a universal tensile testing machine with a speed of 50 m/min. For each separator, three samples were tested to obtain an average peeling force value, which was listed in Table 1.

Test 4: Adhesion to Positive Electrode Test of Separators

To evaluate the adhesion performance to the prepared positive electrode of the separators prepared in Comparative Example 1 to 4 and Examples 1 to 7, adhesive force was measured by the following method. The separator was hot pressed with the positive electrode at 85℃ and 1 MPa for one minute and cut into an 8 mm × 10 cm sample. The force required for separating the separator and the positive electrode was measured using a universal tensile testing machine having a speed of 50 m/min. Three samples were tested to obtain an average adhesive force for the separator. The results were shown in Table 1.

Test 5: Charge-Discharge Cycle Test of Lithium-ion Battery

The charge-discharge cycle test was performed at room temperature using the lithium-ion batteries produced in Comparative Example 1 to 4 and Examples 1 to 7. The charging condition was constant-current constant-voltage charging at 1C, while the discharging condition was constant-current discharging at 1C. For each sample battery, a capacity retention after 500 cycles was used as an index of cycle characteristics. The results were shown in Table 1.

Test 6: Heat-Resistance Test of Lithium-ion Battery

The heat-resistance of the lithium-ion batteries prepared in Comparative Example 1 to 4 and Examples 1 to 7 was evaluated by the following method. The fully charged (4.2V) lithium-ion battery was kept in an oven. The temperature of the oven rose to a temperature of 135℃ at a speed of 5℃/min, and kept for 0.5 hour. If the lithium-ion battery smoked, exploded or get on fire, it did not pass the test. Five samples were tested for each lithium-ion battery, and the pass rate is represented as M/5, wherein M is the number of samples which did not smoked, exploded or get on fire during the test. The results were shown in Table 1.

Test 7: Penetration Strength of Lithium-ion Battery

The penetration strength of the lithium-ion batteries prepared in Comparative Example 1 to 4 and Examples 1 to 7 was determined using a needle having a diameter of 3 mm. The needle was penetrated into the lithium-ion battery from top with a speed of 50 mm/sand kept in the lithium-ion battery for ten minutes. If the lithium-ion battery smoked, exploded or get on fire, it did not pass the test. Five samples were tested for each lithium-ion battery, and the pass rate is represented as N/5, wherein N is the number of samples which did not smoked, exploded or get on fire during the test. The results were shown in Table 1.

Table 1 summarizes the results of Tests 1 to 7 on the separators and lithium-ion batteries that were prepared according to Comparative Example 1 to 4 and Examples 1 to 7.

The separator prepared in Comparative Example 1 included a PE porous base membrane and an alumina coating layer on one side thereof. The separator had a high thermal shrinkage percentage at 150℃ and a low meltdown temperature (i.e., 172℃) . The alumina coating layer hardly presented any adhesion, resulting in bad contact interface with the positive electrode. Accordingly, the lithium-ion battery in Comparative Example 1 had a relatively low capacity retention after 500 cycles. In Comparative Example 2, the separator including a PE porous base membrane and a para-aramid coating layer on one side thereof had an improved heat-resistance. However, as the para-aramid coating layer had a low adhesion with the electrodes, the corresponding lithium-ion battery had a relatively low capacity retention after 500 cycles. In Comparative Example 3, the separator including a PE porous base membrane and a PVDF coating layer on one side thereof presented a low heat-resistance, but good peeling force and adhesive force. The lithium-ion battery that was prepared according to Comparative Example 3 had improved capacity retention after 500 cycles, but had safety issue. In Comparative Example 4, the separator including a non-woven base membrane and a PVDF coating layer on one side thereof had improved thermal shrinkage performance as the non-woven base membrane is more heat-resistant than the PE porous base membrane. However, the separator had a low meltdown temperature, so the corresponding battery had low heat-resistance.

In Examples 1 to 3 according to the present disclosure, PE porous base membranes were used and the coating layers included both aramid and PVDF in different weight ratios. When the weight percentage of aramid in the coating layer increased, the thermal shrinkage of the separator decreased and the meltdown temperature of the separator increased, indicating an improved heat-resistance of the separator, and the thermal shock and penetration pass rates of the corresponding lithium-ion battery increased. When the weight percentage of PVDF in the coating layer increased, both the peeling force and adhesive force increased, and the capacity retention after 500 cycles of the corresponding lithium-ion battery also increased. In Examples 4 to 6 according to the present disclosure, the non-woven porous membranes were used as porous base membrane, and the separators had improved heat-resistance than those in Examples 1 to 3. In Example 7 according to the present disclosure, the slurry for coating the porous base membrane was prepared by dissolving aramid in NMP to obtain an aramid solution, dissolving PVDF in NMP to obtain a PVDF solution, and mixing the aramid solution and the PVDF solution, which was different from the slurry preparation methods used in Examples 1 to 6. The results in Table 1 show that both the slurry preparation methods can work.

As clearly shown above, the separators prepared in Examples 1 to 7 according to the present disclosure demonstrated good heat-resistance behaviors (e.g., low thermal shrinkage percentages, high meltdown temperatures) , good stability (e.g., high peeling force values) , and good contact interface with electrodes (e.g., high adhesive force values) . The lithium-ion batteries in Examples 1 to 7 according to the present disclosure presented not only excellent charge-discharge cycle performance (e.g., capacity retention after 500 cycles) , but also improved safety (e.g., high thermal shock test pass rate, high penetration test pass rate) in comparison to those in Comparative Examples 1-4.

Claims

A separator for an electrochemical device, comprising:

a porous base membrane; and

a coating layer being formed on at least one surface of the porous base membrane, wherein the coating layer comprises at least one heat-resistant polymer and at least one binder polymer.
The separator according to claim 1, wherein the at least one heat-resistant polymer and the at least one binder polymer are present in the coating layer in a weight ratio ranging from 1: 99 to99: 1.
The separator according to claim 1, wherein the at least one heat-resistant polymer is chosen from aramid, polyimide, polyetherimide, polysulfone, polybenzimidazole, polyphenylene sulfide, polyethersulfone, polyarylsulfone, polyketone, polyetherketone, polyetheretherketone, and polydiphenyl oxide.
The separator according to claim 1, wherein the at least one binder polymer is chosen from polyvinylidene fluoride, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polyacrylate, polyacrylate salt, polymethacrylate, polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, and carboxyl methyl cellulose.
The separator according to claim 1, wherein the porous base membrane comprises a polyolefin-based porous membrane or a non-woven membrane.
The separator according to claim 1, wherein the coating layer further comprises at least one inorganic filler.
The separator according to claim 6, wherein the at least one inorganic filler is in an amount of less than 80 wt%based on the total weight of the at least one heat-resistant polymer, the at least one binder polymer, and the at least one inorganic filler.
The separator according to claim 6, wherein the at least one inorganic filler is chosen from an oxide, a hydroxide, a sulfide, a nitride, and a carbide, a carbonate, a sulfate, a phosphate, and a titanate comprising at least one of metallic and semiconductor elements.
The separator according to claim 8, wherein the at least one of metallic and semiconductor elements is chosen from Si, Al, Ca, Ti, B, Sn, Mg, Li, Co, Ni, Sr, Ce, Zr, Y, Pb, Zn, Ba, and La.
The separator according to claim 6, 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 the thickness of the coating layer ranges from 0.5 μm to 5 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 for an electrochemical device, comprising:

preparing a slurry comprising at least one heat-resistant polymer, at least one binder polymer, and at least one solvent;

coating the slurry on at least one surface of a porous base membrane to form a wet coating layer; and

removing the at least one solvent from the wet coating layer.
The method according to claim 13, wherein the at least one heat-resistant polymer and the at least one binder polymer are present in the slurry in a weight ratio ranging from 1: 99 to 99: 1.
The method according to claim 13, wherein the at least one heat-resistant polymer and the at least one binder polymer are present in the slurry in an amount ranging from 10 wt%to 100 wt%based on the total weight of the slurry.
The method according to claim 13, wherein the slurry is prepared by adding the at least one heat-resistant polymer and the at least one binder polymer into the at least one solvent.
The method according to claim 13, wherein the slurry is prepared by adding the at least one heat-resistant polymer into a first solvent to obtain a first mixture, adding the at least one binder polymer into a second solvent to obtain a second mixture, and mixing the first mixture and the second mixture.
The method according to claim 13, wherein the coating is processed by roller coating, spray coating, spin coating, or dip coating.
The method according to claim 13, wherein the at least one heat-resistant polymer is chosen from aramid, polyimide, polyetherimide, polysulfone, polybenzimidazole,.
The method according to claim 13, wherein the at least one binder polymer is chosen from polyvinylidene fluoride, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polyacrylate, polyacrylate salt, polymethacrylate, polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, and carboxyl methyl cellulose.
The method according to claim 13, wherein the at least one solvent is chosen from N, N-dimethylformamide, dimethylacetamide, N-methyl pyrrolidone, dimethyl sulfoxide, acetone, diethyl ether, propyl ether, cyclohexane, and tetrahydrofuran.
The method according to claim 13, wherein the slurry further comprises at least one inorganic filler in an amount of less than 80 wt%based on the total weight of the at least one heat-resistant polymer, the at least one binder polymer and the at least one inorganic filler.
The separator according to claim 22, 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.