WO2019072146A1

WO2019072146A1 - Methods for preparing coating slurries, separators, electrochemical devices and products thereof

Info

Publication number: WO2019072146A1
Application number: PCT/CN2018/109313
Authority: WO
Inventors: Alex Cheng; Lianjie WANG; Yongle Chen; Donghong XU
Original assignee: Shanghai Energy New Materials Technology Co., Ltd.
Priority date: 2017-10-09
Filing date: 2018-10-08
Publication date: 2019-04-18
Also published as: CN107732106A; CN107732106B

Abstract

The present disclosure provides a method for preparing a coating slurry for preparing a separator, comprising dissolving aramids in an acid solution to obtain a carboxylated aramid solution; mixing the carboxylated aramid solution and a first solvent to obtain a first mixture; mixing an inorganic filler and a second solvent to obtain a second mixture; and mixing the first mixture and the second mixture; a method for preparing a separator with the coating slurry disclosed herein; a separator prepared by the method disclosed herein; as well as an electrochemical device comprising the separator disclosed herein.

Description

METHODS FOR PREPARING COATING SLURRIES, SEPARATORS, ELECTROCHEMICAL DEVICES AND PRODUCTS THEREOF

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of priority to Chinese Application No. 201710930261.7, filed on October 9, 2017, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to electrochemistry field, and especially relates to methods for preparing coating slurries, methods for preparing separators for electrochemical devices with the coating slurries, coating slurries and separators prepared by the methods, as well as electrochemical devices comprising the separator disclosed herein.

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 membranes have been widely used as separators in lithium secondary batteries because of their favorable chemical stability and excellent physical properties. However, they may shrink at a high temperature, resulting in a volume change and leading to direct contact of the positive electrode and the negative electrode. To reduce thermal shrinkage of the polyolefin-based separators at a high temperature, coated separators have been proposed. A coated separator usually comprises a porous base membrane and a coating layer formed on at least one side of the porous base membrane, wherein the coating layer may comprise polymers and/or inorganic particles. Coated separators usually have improved safety as they may have lower thermal shrinkage due to the heat-resistance of the polymer and inorganic particles in the coating layer.

Aramid fibers are a class of heat-resistant and strong synthetic fibers. The chain molecules in the aramid fibers are highly oriented along the fiber axis. As a result, a higher proportion of the chemical bond contributes more to fiber strength than in many other synthetic fibers. Aramids have a high melting point of, for example, more than 500℃. If an aramid-containing coating layer is formed on at least one side of a porous base membrane to prepare a coated separator, the resulting coated separator may have excellent thermal stability or heat-resistance. However, aramids have a low solubility in most organic solvents. It is difficult to dissolve aramids in an organic solvent to prepare a coating slurry for preparing a coated separator. Therefore, there is a need to develop methods for preparing a coating slurry containing aramids, which can be used to prepare heat-resistant separators for use in electrochemical devices.

SUMMARY OF THE INVENTION

The present disclosure provides a method for preparing a coating slurry for preparing a separator, comprising: dissolving aramids in an acid solution to obtain a carboxylated aramid solution; mixing the carboxylated aramid solution and a first solvent to obtain a first mixture; mixing an inorganic filler and a second solvent to obtain a second mixture; and mixing the first mixture and the second mixture.

The present disclosure further provides a coating slurry prepared by the method disclosed herein.

The present disclosure further provides a method for preparing a separator for an electrochemical device, comprising: applying the coating slurry disclosed herein on at least one side of a porous base membrane to obtain a wet coating layer; and removing the first solvent and the second solvent from the wet coating layer.

The present disclosure further provides a separator for an electrochemical device prepared by the method disclosed herein. The separator disclosed herein comprises a porous base membrane and a coating layer being formed on at least one side of the porous base membrane, wherein the coating layer is formed using the coating slurry disclosed herein.

The present disclosure further 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.

DETAILED DESCRIPTION

The present disclosure provides some exemplary embodiments of a method for preparing a coating slurry for preparing a separator that is used in an electrochemical device. In one embodiment of the present disclosure, the method for preparing a coating slurry disclosed herein comprises:

(A) dissolving aramids in an acid solution to obtain a carboxylated aramid solution;

(B) mixing the carboxylated aramid solution and a first solvent to obtain a first mixture;

(C) mixing an inorganic filler and a second solvent to obtain a second mixture; and

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

In the step (A) , aramids are dissolved in the acid solution to obtain a carboxylated aramid solution. Aramids, which are also called as “aromatic polyamides, ” are a class of heat-resistant polymers. Two categories of aramids are para-aramids (poly-p-phenylene terephthamide, PPTA) and meta-aramids (poly-m-phenylene isophthalamide, PMIA) . In some embodiments of the present disclosure, para-aramids are used in the step (A) . The acid solution disclosed herein may comprise a protonic acid. The protonic acid may be a strong acid, e.g., sulfuric acid (H ₂SO ₄) . In one embodiment, the acid solution is concentrated sulfuric acid having a weight percentage ranging, for example, from 98%to 100%. When the aramids are added into the acid solution, carboxylation reactions may happen between the aramids and the protonic acid in the acid solution to produce carboxylated aramids. The carboxylated aramids can maintain the heat-resistant property of aramids. As the carboxylated aramids contain many carboxyl groups, they can be easily dissolved in an organic solvent, for example, the first solvent used in the step (B) .

In the step (A) , stirring may be used to mix the aramids and the acid solution. In some embodiments, an ultrasonic irradiation is applied to the mixture of the aramids and the acid solution to assist the carboxylation reactions between the aramids and the acid solution, so as to obtain a uniform carboxylated aramid solution. In some embodiments, lyotropic liquid crystal may appear with the increasing amount of aramids in the solution. The ultrasonic irradiation used herein may have a frequency ranging from 20 KHz to 60 KHz. The time length of applying the ultrasonic irradiation may range from 1 hour to 5 hours, for example, from 2 hours to 3 hours.

In the step (B) , the carboxylated aramid solution prepared in the step (A) is mixed with the first solvent to obtain the first mixture. Stirring may be used to mix the carboxylated aramid solution and the first solvent. The first solvent disclosed herein may be an organic solvent chosen, for example, from N-methyl pyrrolidone (NMP) , dimethylacetamide (DMAC) , N, N-dimethylformamide (DMF) , dimethyl sulfoxide (DMSO) , and acetone.

In one embodiment, in the step (A) , from 2 to 5 parts, such as from 2.5 to 4.5 parts, by weight of the aramids are dissolved in from 10 to 25 parts, such as from 15 to 20 parts, by weight of the acid solution to prepare a carboxylated aramid solution; in the step (B) , the above prepared carboxylated aramid solution is added into from 40 to 70 parts, such as from 45 to 65 parts, by weight of the first solvent to obtain the first mixture.

In the step (C) , the inorganic filler is dispersed in the second solvent to obtain the second mixture. The inorganic filler disclosed herein may comprise inorganic particles. The inorganic particles may include, for example, oxides, hydroxides, sulfides, nitrides, carbides, carbonates, sulfates, phosphates, titanates, and the like, comprising 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. Specific examples of the inorganic particles include alumina (Al ₂O ₃) , boehmite (γ-AlOOH) , silica (SiO ₂) , zirconium dioxide (ZrO 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) . In addition, the inorganic particles disclosed herein may have an average particle size ranging, for example, from 0.01 to 1 μm, such as from 0.02 to 0.5 μm.

In the step (C) , the second solvent may be an organic solvent chosen, for example, from NMP, DMAC, DMF, DMSO, and acetone. The first solvent and the second solvent may have the same composition, or may have different compositions.

The second mixture prepared in the step (C) may comprise from 2 to 5 parts, such as from 2.5 to 4.5 parts, by weight of the inorganic filler and from 10 to 40 parts, such as from 20 to 30 parts, by weight of the second solvent.

In the step (C) , a dispersant may be added into the second solvent to help dispersion and prevent agglomeration of the inorganic filler in the second solvent. The dispersant may be added into the second solvent together with the inorganic filler, or may be added before or after the inorganic filler is added. The dispersant disclosed herein may comprise polyethylene oxide (PEO) in the form of, for example, powder, which may have a weight average molecule weight (M _w) ranging, for example, from 1×10 ⁵ to 1×10 ⁶, such as from 2×10 ⁵ to 5×10 ⁵, and further such as 3×10 ⁵.

In the step (D) , the first mixture and the second mixture are mixed to obtain the coating slurry. In one embodiment, the first mixture is slowly added into the second mixture to obtain the coating slurry. The coating slurry disclosed herein may be a suspension as the inorganic filler disperses in the coating slurry.

In the present disclosure, the problem that it is difficult to dissolve aramids in an organic solvent to prepare a slurry can be solved through carboxylation reactions of aramids. When added into the acid solution, the hardly soluble aramids are converted into soluble carboxylated aramids. Through the methods disclosed above, a coating slurry comprising carboxylated aramids is prepared.

The coating slurry disclosed herein may be used to prepare a separator for electrochemical devices. The present disclosure further provides some exemplary embodiments of a method for preparing separators that are used in electrochemical devices. In one embodiment, the method for preparing a separator comprises:

(E) applying the coating slurry disclosed herein on at least one side of a porous base membrane to obtain a wet coating layer; and

(F) removing the first solvent and the second solvent from the wet coating layer.

In the step (E) , the coating slurry disclosed herein is applied on at least one side of the porous base membrane. Any coating method known in the art may be used to coat the porous base membrane with the coating slurry, such as roller coating, spray coating, dip coating, spin coating, or combinations thereof. Examples of the roller coating include gravure coating, silk screen coating, and slot die coating. The coating speed may be controlled in a range of, for example, from 10 to 90 m/min, such as from 15 to 16 m/min. In the case that both sides of the porous base membrane are coated with the coating slurry disclosed herein, the both sides can be coated simultaneously or can be coated by sequence.

In the step (F) , the solvent in the wet coating layer, i.e., the first solvent and the second 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 first solvent and the second solvent are removed, a dry coating layer having a porous structure can be formed.

In some embodiments, the first solvent and the second solvent may be removed through a combination of thermal evaporation and vacuum evaporation. For example, the porous base membrane coated with the coating slurry disclosed herein may be subjected to a vacuum oven for a predetermined time period so as to remove the solvent from the wet coating layer. The pressure and temperature of the vacuum oven may depend on the amount and type of solvent to be removed. Phase inversion process is an alternative method to remove the first solvent and the second solvent, which may be initiated by exposing the wet coating layer to a poor solvent or non-solvent of carboxylated aramids, such as water (e.g., deionized water) , alcohols (e.g., ethanol) , or a combination thereof. When the wet coating layer is exposed to the poor solvent or non-solvent, most of the solvent may transfer from the wet coating layer to the poor solvent or non-solvent, resulting in a porous structure in the coating layer. The phase inversion process is energy-efficient as no phase change happens when the solvent is removed. In some embodiments, the step (C) comprises immersing the coated porous base membrane in a poor solvent or non-solvent for a predetermined time period ranging, for example, from 0.5 to 3 minutes, such as from 1 to 2 minutes. To remove the solvent from the wet coating layer more efficiently, a flowing poor solvent or non-solvent may be used, or making the coated porous base membrane pass through a tank of poor solvent or non-solvent in a predetermined speed. The step (C) may further comprise taking the coated porous base membrane out from the poor solvent or non-solvent and removing residues of the first solvent, the second solvent, and/or the poor solvent or non-solvent therefrom. The residues 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, passing the coated porous base membrane through a multi-stage open oven, e.g., a three-stage oven, at a predetermined speed. The three-stage oven may have a temperature ranging, for example, from 45 to 55℃ in its first stage, a temperature ranging, for example, from 50 to 60℃ in its second stage, and a temperature ranging, for example, from 40 to 50℃ in its third stage. In an example, the three-stage oven has temperatures of 50℃, 55℃, and 45℃ in its first, second, and third stages, respectively.

Through the above method, a dry and porous coating layer may be formed on at least one side of the porous base membrane. The separator prepared by the methods disclosed herein comprises a porous base membrane and a coating layer being formed on at least one side of the porous base membrane. The coating layer comprises carboxylated aramids and inorganic fillers that are embedded in the coating layer and fixed by the carboxylated aramids.

The “at least one side” disclosed herein means the coating layer is disposed on one side or both sides of the porous base membrane, and the coating layer can be in direct contact or indirect contact with the porous base membrane. The separator disclosed herein may have a laminated structure. In some embodiments of the present disclosure, the coating layer is in direct contact with the porous base membrane, which means, the coating layer is formed on at least one surface of the porous base membrane. In such a case, the separator disclosed herein may have a two-layer structure when only one surface of the porous base membrane is coated with the coating layer disclosed herein. The separator may have a three-layer structure when both surfaces of the porous base membrane are coated with the coating layer disclosed herein. In some other embodiments, the coating layer is not in direct contact with the porous base membrane, which means that the separator disclosed herein further comprises at least one additional layer (e.g., an adhesive layer) interposed between the coating layer and the porous base 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 coating layer.

The coating layer disclosed herein has a pore structure allowing gas, liquid, or ions to 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 10 to 500 μm, such as from 20 to 300 μm. The porosity of the coating layer may range, for example, from 20%to 70%, such as from 30%to 50%. Additionally, the coating layer on one side of the porous base membrane may have a thickness ranging, for example, from 0.5 to 5 μm, such as from 1 to 4 μm.

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 an M _w ranging, for example, from 5×10 ⁴ to 2×10 ⁶, such as from 1×10 ⁵ to 1×10 ⁶. The pores within the polyolefin-based porous 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 membrane may have a porosity ranging, for example, from 25%to 50%, such as from 30%to 45%. Furthermore, the polyolefin-based porous 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 membranes 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 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. The non-woven membrane disclosed herein may be 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 method known in the art, such as electro-blowing, electro-spinning, or melt-blowing, or can be purchased directly in the market.

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 prepared by the method disclosed herein comprises a porous base membrane and a coating layer disposed on at least one side of the porous base membrane. Both the carboxylated aramids and the inorganic filler in the coating layer 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 in an environment with 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. As the separator disclosed herein can have good heat-resistance and air permeability, the electrochemical devices employing such separator may have improved safety, low internal resistance, and good cycle performance. The separators disclosed herein can have a wide range of applications and can be used for making high-energy density and/or high-power density batteries used in many stationary and portable devices, e.g., automotive batteries, batteries for medical devices, and batteries for other large devices.

The present disclosure further provides embodiments of an electrochemical device. The electrochemical device comprises a positive electrode, a negative electrode, and a separator disclosed herein that 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 the two electrodes. 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 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 cycle life as discussed above.

The electrochemical device disclosed herein may be manufactured by a method known in the art. In one embodiment, an electrode assembly is formed by placing the separator disclosed herein 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 process known in the art, such as a winding process or a lamination (stacking) and folding process.

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.

Example 1

0.2 kg para-aramids was added into 1 kg 99.5 wt%concentrated sulfuric acid. An ultrasonic irradiation was applied to the mixture of para-aramids and the concentrated sulfuric acid for 2 to 3 hours to obtain a carboxylated aramid solution. The carboxylated aramid solution was slowly added into 6 kg NMP to obtain a first mixture. 0.5 kg alumina was added and dispersed in 2.3 kg NMP to obtain a second mixture. The first mixture was added into the second mixture to obtain a coating slurry for preparing a separator.

A PE membrane having a thickness of 12 μm was used a porous base membrane. The coating slurry prepared above was coated on two sides of the PE membrane through a gravure coating process at a speed of 15 m/min. The coated PE membrane was immersed in water, and then dried by passing through a three-stage oven having temperatures of 50℃, 55℃ and 45℃, respectively, in the first, the second and the third stage thereof. A separator having a thickness of 16 μm was obtained. The coating layer on each side of the PE membrane has a thickness of 2 μm.

Example 2

0.3 kg para-aramids was added into 1.5 kg 99.5 wt%concentrated sulfuric acid. An ultrasonic irradiation was applied to the mixture of para-aramids and concentrated sulfuric acid for 2 to 3 hours to obtain a carboxylated aramid solution. The carboxylated aramid solution was slowly added into 6 kg DMAC to obtain a first mixture. 0.4 kg alumina was added and dispersed in 1.8 kg DMAC to obtain a second mixture. The first mixture was added into the second mixture to obtain a coating slurry for preparing a separator.

The same procedures as set forth above in Example 1 were used to prepare a separator using the above prepared coating slurry.

Example 3

0.4 kg para-aramids was added into 2.0 kg 100 wt%concentrated sulfuric acid. An ultrasonic irradiation was applied to the mixture of para-aramids and concentrated sulfuric acid for 2 to 3 hours to obtain a carboxylated aramid solution. The carboxylated aramid solution was slowly added into 5 kg DMF to obtain a first mixture. 0.3 kg alumina was added and dispersed in 2.3 kg DMF to obtain a second mixture. The first mixture was added into the second mixture to obtain a coating slurry for preparing a separator.

Example 4

0.5 kg para-aramids was added into 2.5 kg 100 wt%concentrated sulfuric acid. An ultrasonic irradiation was applied to the mixture of para-aramids and concentrated sulfuric acid for 2 to 3 hours to obtain a carboxylated aramid solution. The carboxylated aramid solution was slowly added into 5 kg DMAC to obtain a first mixture. 0.3 kg alumina was added and dispersed in 2.3 kg DMAC to obtain a second mixture. The first mixture was added into the second mixture to obtain a coating slurry for preparing a separator.

Comparative Example

0.56 kg polyvinylidene fluoride (PVDF) was added into 7.44 kg DMAC. The mixture of PVDF and DMAC was stirred for 1 to 2 hours to obtain a coating slurry.

The separators prepared in Examples 1-4 and Comparative Example were evaluated on air permeability and thermal shrinkage. For each separator, the air permeability was tested using a method provided in Japan Standard “JIS P8117-2009 Paper and board-Determination of air permeance” with an air permeability tester (Asahi-Seiko EGO1-55-1MR) . 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 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 testing results as follows.

Table 1.

	Air Permeability (s/100cc)	Thermal Shrinkage (%)
Example 1	258	6.7
Example 1	260	5.6
Example 1	267	4.3
Example 1	270	3.1
Comparative Example	340	40.5

As shown in Table 1, the separators prepared in Examples 1-4 have better air permeability and much lower thermal shrinkage percentages in comparison with the separator prepared in Comparative Example. This can be explained by the presence of aramid polymers and inorganic fillers in the coating layer of the separators prepared in Examples 1-4.

Claims

A method for preparing a coating slurry for preparing a separator for an electrochemical device, comprising:

dissolving aramids in an acid solution to obtain a carboxylated aramid solution;

mixing the carboxylated aramid solution and a first solvent to obtain a first mixture;

mixing an inorganic filler and a second solvent to obtain a second mixture; and

mixing the first mixture and the second mixture.
The method for preparing a coating slurry according to claim 1, wherein the aramids are para-aramids.
The method for preparing a coating slurry according to claim 1, wherein the acid solution comprises a protonic acid.
The method for preparing a coating slurry according to claim 1, wherein the acid solution is concentrated sulfuric acid having a weight percentage ranging from 98%to 100%.
The method for preparing a coating slurry according to claim 1, wherein an ultrasonic irradiation is applied to a mixture of the acid solution and the aramids.
The method for preparing a coating slurry according to claim 5, wherein the ultrasonic irradiation has a frequency ranging from 20 KHz to 60 KHz.
The method for preparing a coating slurry according to claim 5, wherein the ultrasonic irradiation is applied to the mixture of the acid solution and the aramids with a time period ranging from one hour to five hours.
The method for preparing a coating slurry according to claim 1, wherein from 2 to 5 parts by weight of aramid is dissolved in from 10 to 25 parts by weight of acid solution, and the carboxylated aramid solution is added into from 40 to 70 parts by weight of the first solvent to obtain the first mixture.
The method for preparing a coating slurry according to claim 1, wherein the inorganic filler is one or more chosen from alumina, boehmite, silica, zirconium dioxide, 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 method for preparing a coating slurry according to claim 1, wherein the second mixture comprises from 2 to 5 parts by weight of the inorganic filler and from 10 to 40 parts by weight of the second solvent.
The method for preparing a coating slurry according to claim 1, further comprising:

adding a dispersant into the second solvent.
The method for preparing a coating slurry according to claim 11, wherein the dispersant comprises polyethylene oxide having a weight average molecule weight ranging from 1×10 ⁵ to 1×10 ⁶.
The method for preparing a coating slurry according to claim 1, wherein each of the first solvent and the second solvent is chosen from N-methyl pyrrolidone, dimethylacetamide, N, N-dimethylformamide, dimethyl sulfoxide, and acetone.
A coating slurry prepared according to the method of claim 1.
A method for preparing a separator for an electrochemical device, comprising:

dissolving aramids in an acid solution to obtain a carboxylated aramid solution;

mixing the carboxylated aramid solution and a first solvent to obtain a first mixture;

mixing an inorganic filler and a second solvent to obtain a second mixture;

mixing the first mixture and the second mixture to form a coating slurry;

applying the coating slurry onto at least one side of a porous base membrane to obtain a wet coating layer; and

removing the first solvent and the second solvent from the wet coating layer.
The method according to claim 15, wherein the first solvent and the second solvent are removed from the wet coating layer by:

immersing the coated porous base membrane in a poor solvent or non-solvent of carboxylated aramids; and

removing residues of the first solvent, the second solvent and/or the poor solvent or non-solvent from the coated porous base membrane.
The method according to claim 15, wherein the porous base membrane is a polyolefin-based porous membrane or a non-woven porous membrane.
A separator for an electrochemical device, wherein the separator is prepared by the method of claim 15 and comprises:

a porous base membrane; and

a coating layer being formed on at least one side of the porous base membrane.
An electrochemical device comprising a positive electrode, a negative electrode, and a separator according to claim 18 interposed between the positive electrode and the negative electrode.