WO2019085899A1

WO2019085899A1 - Methods for preparing polymer solutions, separators, electrochemical devices and products thereof

Info

Publication number: WO2019085899A1
Application number: PCT/CN2018/112707
Authority: WO
Inventors: Alex Cheng; Yongle Chen; Wenke YANG; Lianjie WANG; Zhixue Wang; Fangbo HE
Original assignee: Shanghai Energy New Materials Technology Co., Ltd.
Priority date: 2017-10-31
Filing date: 2018-10-30
Publication date: 2019-05-09

Abstract

Disclosed here area polymer solution, comprising an aromatic polymers having a structure of –C(=O) -N– and an intrinsic viscosity ranging from 0.5 dL/g to 1.45 dL/g, wherein the aromatic polymer is obtained by reacting aromatic diamines with compounds having acyl groups in a reaction system that comprises a solvent having a water content ranging from 3000 ppm to 5000 ppm; a method for preparing a separator with the polymer solution, separators prepared by the method disclosed herein, and electrochemical devices comprising the separator.

Description

METHODS FOR PREPARING POLYMER SOLUTIONS, SEPARATORS, ELECTROCHEMICAL DEVICES AND PRODUCTS THEREOF

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of priority to Chinese Application No. 201711044607. X, filed on October 31, 2017, and Chinese Application No. 201810223944.3, filed on March 19, 2018, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to electrochemistry field, and especially relates to polymer solutions that can be used for making separators, separators for electrochemical devices, and electrochemical devices comprising the separator disclosed herein, as well as preparation methods thereof.

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 developed and used. 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 at least one polymer and/or inorganic particles. Coated separators usually have improved safety as they may have lower thermal shrinkage due to the heat-resistant properties of the polymer and/or inorganic particles in the coating layer. Some heat-resistant aromatic polymers, such as aromatic polyamides and aromatic polyimides, can be used in the coating layer. However, the heat-resistant aromatic polymers may be hardly soluble in an organic solvent, so it can be difficult to prepare a coating slurry containing the heat-resistant aromatic polymers for preparation of a coating layer of a coated separator. Therefore, there is a need to develop methods for preparing a coating slurry containing heat-resistant aromatic polymers, which can be used to prepare heat-resistant coated separators for electrochemical devices.

SUMMARY OF THE INVENTION

The present disclosure provides a polymer solution, comprising an aromatic polymer with a structure of –C (=O) N–and an intrinsic viscosity ranging from 0.5 dL/g to 1.45 dL/g, wherein the aromatic polymer is formed by reacting aromatic diamines with compounds having acyl groups in a reaction system that comprises a solvent having a water content ranging from 3000 ppm to 5000 ppm.

The present disclosure further provides a method of preparing the polymer solution disclosed herein.

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

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.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a diagram of relationship between the mole ratio of reactants and intrinsic viscosity of the aromatic polymers in Examples A1-A9 and Comparative Examples A1-A4.

DETAILED DESCRIPTION

The present disclosure provides some exemplary embodiments of the polymer solution and methods for preparing such polymer solutions. The polymer solution disclosed herein can be used to prepare a coating slurry for preparing separators used in electrochemical devices. In one embodiment of the present disclosure, the method for preparing a polymer solution disclosed herein comprises reacting aromatic diamines with compounds having acyl groups in a reaction system to produce aromatic polymers containing a structure of –C (=O) -N–. The reaction system disclosed herein comprises reactants, i.e., the aromatic diamines and the compounds having acyl groups, and further comprises a solvent having a water content ranging, for example, from 3000 ppm to 5000 ppm. The polymer solution prepared by the method disclosed herein comprises the aromatic polymers containing a structure of –C (=O) -N–that are dissolved in the solvent.

The aromatic diamines disclosed herein may be one or more chosen, for example, from hydroxydiphenylamines, 1, 2-phenylenediamines, 1, 3-phenylenediamines, p-phenylenediamines (PPD) , 3, 3'-benzophenone diamines, 3, 3'-methylene dianilines, 3, 3'-diaminodiphenyl sulfones, 1, 2-naphthylenediamines, 1, 3-naphthylenediamines, 1, 4-naphthylenediamines, 1, 5-naphthylenediamines, 1, 6-naphthylenediamines, 1, 7-naphthylenediamines, 1,8-naphthylenediamines, 2, 3-naphthylenediamines, 2, 6-naphthylenediamines, and 3, 3'-diphenyl diamines. In one embodiment, the aromatic diamines disclosed herein are PPD.

The compounds having acyl groups disclosed herein may, for example, be diacylhalides, dianhydrides, or diisocyanates. In one embodiment, aromatic diacyl halides are used in the reaction with the aromatic diamines.

The diacyl halides disclosed herein may be one or more chosen, for example, from o-phthaloyl dichlorides, terephthaloyl chlorides (TPC) , pyromellitic dichlorides, 3, 3’, 4, 4’-diphenylsulfone tetracarboxylic acid dichlorides, 3, 3’, 4, 4’-dibenzophenone tetracarboxylic acid dichlorides, 2, 2’- (3, 4-dicarboxyphenyl) hexafluoropropane dichlorides, 3, 3’, 4, 4’-diphenyl tetracarboxylic acid dichlorides, 1, 2-phenylene dicarboxylic acid dichlorides, 1, 3-phenylene dicarboxylic acid dichlorides, 1, 4-phenylene dicarboxylic acid dichlorides, 1, 2-naphthylene dicarboxylic acid dichlorides, 1, 3-naphthylene dicarboxylic acid dichlorides, 1, 4-naphthylene dicarboxylic acid dichlorides, 1, 5-naphthylene dicarboxylic acid dichlorides, 1, 6-naphthylene dicarboxylic acid dichlorides, 1, 7-naphthylene dicarboxylic acid dichlorides, 1, 8-naphthylene dicarboxylic acid dichlorides, 2, 3-naphthylene dicarboxylic acid dichlorides, 2, 6-naphthylene dicarboxylic acid dichlorides, 3, 3'-biphenylene dicarboxylic acid dichlorides, 3, 3'-dibenzophenone dicarboxylic acid dichlorides, and 3, 3'-diphenylsulfone dicarboxylic acid dichlorides. In one embodiment, the diacyl halides disclosed herein are TPC. The reaction between the aromatic diamines and the diacyl halides may produce an aromatic polymer having an amido structure, i.e., –C (=O) -NH–.

The dianhydrides disclosed herein may be one or more chosen, for example, from pyromellitic dianhydrides, 3, 3’, 4, 4’-diphenylsulfone tetracarboxylic dianhydrides, 3, 3’, 4, 4’-dibenzophenone tetracarboxylic dianhydrides, 2, 2’- (3, 4-dicarboxyphenyl) hexafluoropropane dianhydrides, and 3, 3’, 4, 4’-biphenyl tetracarboxylic dianhydrides. The reaction between the aromatic diamines and the dianhydrides may produce an aromatic polymer having an imide structure, i.e., –C (=O) -N-C (=O) –.

The diisocyanates disclosed herein may be one or more chosen, for example, from 1, 2-phenylene diisocyanates, 1, 3-phenylene diisocyanates, 1, 4-phenylene diisocyanates, 1, 2-naphthylene diisocyanates, 1, 3-naphthylene diisocyanates, 1, 4-naphthylene diisocyanates, 1, 5-naphthylene diisocyanates, 1, 6-naphthylene diisocyanates, 1, 7-naphthylene diisocyanates, 1, 8-naphthylene diisocyanates, 2, 3-naphthylene diisocyanates, 2, 6-naphthylene diisocyanates, 3, 3’-naphthylene diisocyanates, 3, 3’-dibenzophenone diisocyanates, and 3, 3’-diphenylsulfone diisocyanates. The reaction between the aromatic diamines and the diisocyanates may produce an aromatic polymer having a urea structure, i.e., –HN-C (=O) -NH–.

In some embodiments, the mole ratio of the aromatic diamines and the compounds having acyl groups may range, for example, from 0.95: 1 to 1: 1, such as from 0.96: 1 to 1: 1, and further such as from 0.97: 1 to 1: 1.

In some embodiments, the reactants, i.e., the aromatic diamines and the compounds having acyl groups, may have a weight percentage ranging, for example, from 2 wt%to 30 wt%, such as from 4 wt%to 12 wt%, and further such as from 5 wt%to 8 wt%, in the reaction system, relative to the total amount of the solvent in the reaction system.

In some embodiments, the solvent disclosed herein is one or more organic solvent chosen, for example, from N-methyl-2-pyrrolidone (NMP) , N, N-dimethylacetamide (DMAC) , N, N-dimethylformamide (DMF) , dimethyl sulfoxide (DMSO) and triethyl phosphate (TEP) .

In some embodiments, the solvent disclosed herein may have a water content ranging, for example, from 3000 ppm to 5000 ppm, such as from 3000 ppm to 4500 ppm, and further such as from 3000 ppm to 4000 ppm. When a solvent having a water content lower than 3000 ppm is used in the reaction system, the different mole ratios of the aromatic diamines and the compounds having acyl groups within a range of, for example, from 0.95 to 1, may cause huge variations in the intrinsic viscosity of the obtained aromatic polymers, and the obtained aromatic polymers may have a high intrinsic viscosity that is not appropriate to be used. Specifically, the aromatic polymers having a high intrinsic viscosity may be easily precipitated from the polymer solution, so it can be difficult to prepare a stable polymer solution for preparing a coating slurry that can be used in preparation of a separator. In addition, if a polymer solution having some insoluble matters is used to prepare a coating slurry for forming a coating layer of a separator, the coating layer may be uneven, wrinkled, and/or containing particles on its surface, resulting in a poor appearance. When a solvent having a water content higher than 5000 ppm is used in the reaction system, the obtained aromatic polymers may have a low intrinsic viscosity and a weak heat-resistance that may not be suitable to be used in the preparation of a heat-resistant separator. When a solvent having a water content ranging from 3000 ppm to 5000 ppm is used in the reaction system, certain side reactions, such as precipitation, may be avoided and the resulting aromatic polymers may have a suitable intrinsic viscosity and good heat-resistance and may not precipitate from the polymer solution easily.

In some embodiments, the solvent in the reaction system further comprises chlorides of alkali metal or alkaline-earth metal. The chlorides of alkali metal or alkaline-earth metal disclosed herein may be, for example, one or more of magnesium chlorides (MgCl ₂) , calcium chlorides (CaCl ₂) , sodium chloride (NaCl) and potassium chloride (KCl) . In one example, CaCl ₂is used in the solvent of the reaction system. The chlorides of alkali metal or alkaline-earth metal disclosed herein may be present with a weight percentage ranging, for example, from 10 wt%to 12 wt%, such as from 11 wt%to 12 wt%, relative to a total weight of the solvent.

In some embodiments, the reaction of the aromatic diamines and the compounds having acyl groups is carried out in a temperature ranging, for example, from 10℃ to 30℃, such as from 15℃ to 25℃.

Intrinsic viscosity is one of the commonly used parameters to characterize a polymer and reflects the capability of the polymer in solution to enhance the viscosity of the solution. The unit of intrinsic viscosity can be deciliters per gram (dL/g) or milliliter per gram (mL/g) . A method for the measurement of intrinsic viscosity is, for example, using a capillary tube viscometer.

In some embodiments, the aromatic polymers in the polymer solution disclosed herein may have an intrinsic viscosity ranging, for example, from 0.5 dL/g to 1.5 dL/g, such as from 0.6 dL/g to 1.3 dL/g, and further such as from 0.8 dL/g to 1.2 dL/g. If an aromatic polymer having an intrinsic viscosity lower than 0.5 dL/g is used to prepare a coating layer of the separator, the resulting coating layer may have a low elasticity modulus and weak heat-resistance due to the low molecule weight of the aromatic polymer. So an aromatic polymer having an intrinsic viscosity lower than 0.5 dL/g may not be suitable to be used to prepare a heat-resistant separator. If an aromatic polymer having an intrinsic viscosity higher than 1.5 dL/g is produced in the reaction system disclosed herein, it may precipitate in the solvent due to high molecule weight, so it may not be able to obtain a stable polymer solution or a stable coating slurry thereafter for making a coated separator.

In some embodiments, the aromatic polymers in the polymer solution are aromatic polyamides, for example, wholly aromatic polyamides. The aromatic polyamides disclosed herein may be para aromatic polyamides or meta aromatic polyamides. In one embodiment, the aromatic polyamides disclosed herein are para aromatic polyamides that have good mechanical strength and can form a suitable porous structure in preparation of a separator. Specific examples of the aromatic polyamides disclosed herein include poly (p-phenylene terephthalamide) (PPTA) , poly (m-phenylene isophthalamide) (PMIA) , poly (p-benzamide) , poly (m-benzamide) , poly (4, 4’-benzanilide terephthalamide) , poly (p-phenylene-4, 4’-biphenylene dimethyl amide) , poly (m-phenylene-4, 4’-biphenylene dimethyl amide) , poly (p-phenylene-2, 6-naphthalenedicarboxamide) , poly (m-phenylene-2, 6-naphthalenedicarboxamide) , poly (2-chloro-p-phenylene terephthalamide) , copolymer of p-phenylene terephthalamide and 2, 6-dichloro-p-phenylene terephthalamide, copolymer of m-phenylene terephthalamide and 2, 6-dichloro-p-phenylene terephthalamide, and polyphenylene sulfone terephthalamide. In one embodiment, the aromatic polymers disclosed herein are PPTAs.

The present disclosure also provides some exemplary embodiments of the use of the polymer solution disclosed herein in preparation of separators for electrochemical devices. In one embodiment, the method for preparing the separator disclosed herein comprises:

(A) preparing a coating slurry with the polymer solution disclosed herein;

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

(C) removing the solvent from the wet coating layer.

In step (A) , the coating slurry is prepared using the polymer solution disclosed herein. In some embodiments, the polymer solution can be used as the coating slurry directly. In some other embodiments, the preparation of the coating slurry may comprise one or more of the following steps:

(A1) concentrating the polymer solution through, for example, removing at least a part of solvent from the polymer solution;

(A2) diluting the polymer solution through, for example, adding an additional solvent into the polymer solution;

(A3) removing the chlorides from the polymer solution through, for example, washing the polymer solution with water or alcohol; and

(A4) precipitating the aromatic polymers from the polymer solution, washing the aromatic polymers with water; and dissolving the aromatic polymers in a solvent.

In some embodiments, the coating slurry prepared in the step (A) further comprises an inorganic filler. In such a case, the coating slurry may be a suspension as the inorganic filler disperses in the coating slurry. The coating slurry disclosed herein may be prepared by adding the inorganic filler into the polymer solution disclosed herein, or, mixing the polymer solution disclosed herein with a mixture comprising the inorganic filler and a second solvent. The second solvent may be one or more chosen, for example, from NMP, DMAC, DMSO, DMF, and TEP. The inorganic filler disclosed herein may comprise, for example, 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 0.005 to 1 μm, such as from 0.01 to 0.2 μm. When the coating slurry containing the inorganic filler is used to prepare a coating layer of a separator, the inorganic filler in the coating layer can help enhance 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 a high temperature. Furthermore, the presence of the inorganic filler may also facilitate, for example, the formation of pores in the coating layer, the increase of the physical strength of the coating layer, and/or the increase in an impregnation rate of a liquid electrolyte. In some embodiments, the coating slurry disclosed herein comprises the aromatic polymers having a weight percentage ranging, for example, from 20 wt%to 40 wt%, the inorganic filler having a weight percentage ranging, for example, from 2 wt%to 10 wt%, and the solvent having a weight percentage ranging, for example, from 40 wt%to 60 wt%, relative to the total weight of the coating slurry.

In step (B) , the coating slurry disclosed herein is applied onto 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 1 to 300 m/min, such as from 5 to 50 m/min. In the case that both sides of the porous base membrane are coated with the coating slurry, the both sides can be coated simultaneously or can be coated by sequence.

In step (C) , the solvent in the wet coating layer 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 solvent is removed, a dry coating layer having a porous structure can be formed.

In some embodiments, the solvent may be removed through a combination of thermal evaporation and vacuum evaporation. For example, the porous base membrane coated with the coating slurry 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 solvent, which may be initiated by exposing the wet coating layer to a poor solvent or non-solvent of the aromatic polymers disclosed herein, such as deionized water. When the wet coating layer is exposed to the poor solvent or non-solvent, most of the organic 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 having a temperature ranging, for example, from 30℃ to 80℃, for a first predetermined time period ranging, for example, from 1 to 10 minutes. Specifically, the poor solvent or non-solvent disclosed herein may have a temperature of 30℃, 35℃, 40℃, 45℃, 50℃, 60℃, 70℃, or 80℃. The first predetermined time period may be 1 minute, 2, minutes, 4 minutes, 5 minutes, 7 minutes, or 10 minutes. To remove the solvent from the wet coating layer more efficiently, a flowing poor solvent or non-solvent may be used, or passing the coated porous base membrane through a tank of poor solvent or non-solvent at a predetermined speed. The step (C) may further comprise taking the coated membrane out from the poor solvent or non-solvent and removing residues of the 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 comprise hot wind blowing or placing the coated membrane in a closed oven or an open oven. The thermal evaporation may be carried out at a temperature ranging, for example, from 50℃ to 90℃, for a second predetermined period ranging, for example, from 5 to 10 minutes. Specifically, the temperature of the thermal evaporation may be 50℃, 60℃, 70℃, 80℃, or 90℃. The second predetermined time period may be 5 minute, 6, minutes, 7 minutes, 8 minutes, 9 minutes, or 10 minutes.

Through the method set forth above, a dry and porous coating layer may be formed on at least one side of the porous base membrane. The separator prepared by the method disclosed above comprises a porous base membrane and a coating layer being formed on at least one side of the porous base membrane. In the case where inorganic fillers are included in the coating slurry, the prepared coating layer comprises aromatic polymers and inorganic fillers that are embedded in the coating layer and fixed by the aromatic polymers.

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 indirectly 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 indirect 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. The separator may have a three-layer structure when both surfaces of the porous base membrane are coated with the coating layer. In some other embodiments, the coating layer indirectly contacts with the porous base membrane, i.e., 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 0.02 to 2 μm, such as from 0.03 to 1 μm. The porosity of the coating layer may range, for example, from 10%to 80%, such as from 30%to 60%. Additionally, the coating layer on one side of the porous base membrane may have a thickness ranging, for example, from 0.5 to 10 μm, such as from 1 to 4 μm, further such as from 2 to 3 μm. In some embodiments, the coating layer disclosed herein comprises, for example, from 20 wt%to 40 wt%of the aromatic polymers and from 50 wt%to 80 wt%of the inorganic filler relative to the total weight of 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, the porous base membrane disclosed herein comprises at least one material chosen, for example, from polyethylene (PE) , polypropylene (PP) , polybutylene, polypentene, polymethylpentene (TPX) , thermoplastic polyurethane, polyimide, polyvinylidene fluoride, copolymers thereof, and mixtures thereof. The copolymers disclosed herein may be ethylene-propylene copolymer or ethylene-ɑ-alkene copolymer. In one embodiment, the porous base membrane is made from PE. The PE disclosed herein may be low density polyethylene (LDPE) , high density polyethylene (HDPE) , or ultra-high molecular weight polyethylene (UHMWPE) having a weight average molecule weight (M _w) of, for example, higher than 4×10 ⁵. The porous base membrane can be a polymeric microporous membrane prepared by, for example, a melting-extruding-stretching process, or a thermally induced phase separation (TIPS) process. The porous base membrane can also be a non-woven fabric membrane. The porous base membrane may have a porosity ranging, for example, from 30%to 60%, such as from 35%to 50%. In addition, the porous base membrane may have a single-layer structure or a multi-layer structure. 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 porous base membrane disclosed herein can be prepared according to a method known in the art, 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.

In the present disclosure, a stable coating slurry containing heat-resistant aromatic polymers is prepared using a polymer solution containing the heat-resistant aromatic polymers. The polymer solution is prepared in situ through reactions between aromatic diamines and compounds having acyl groups in a reaction system. In preparing the polymer solution, some reaction conditions, e.g., water content of the solvent in the reaction system and temperature, are controlled to obtain a polymer solution that is suitable to be used to prepare a coating slurry. A stable coating slurry comprising heat-resistant aromatic polymers can be prepared effectively using the methods disclosed herein.

The separator disclosed herein comprises a porous base membrane and a coating layer disposed on at least one side of the porous base membrane. Because of the presence of the aromatic polymers in the coating layer, the separator disclosed herein has good heat-resistance. In some embodiments, the separator disclosed herein has a breakdown temperature as high as 400℃, which means the separator does not break until the temperature reaches 400℃ or higher. In addition, the separator disclosed herein has good air permeability, especially when inorganic filler is included in the coating layer. Therefore, the electrochemical devices employing the 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 or 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 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 process known in the art, such as a winding process or a lamination (stacking) and folding process.

As disclosed herein, the term “a” or “an” means one or more.

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 A1

A 5000ml separable glass reaction kettle having a stirring blade, a thermometer, a nitrogen inlet and a powder inlet was used. After sufficiently dried by flowing nitrogen into the kettle, the kettle was charged with 2455.2 g NMP and 305.8 g CaCl ₂. The CaCl ₂ was placed in vacuum at 250℃ for drying for four hours before use. The temperature of the mixture of NMP and CaCl ₂ was raised to 100℃, so that CaCl ₂ was completely dissolved in NMP. Then the temperature of the mixture of NMP and CaCl ₂ was reduced to 25℃, and the water content of the mixture was adjusted to 3500 ppm. 79.2 g PPD was then added into the kettle and dissolved in the mixture. While the mixture in the kettle was stirred at a temperature of 20±2℃, 152.5 g TPC was added into the kettle in three times with a 10 minutes interval. At the end, the mole ratio of PPD and TPC was 0.975. After the addition of TPC, the mixture in the kettle was continued to be stirred for two hours at a temperature of 20±3℃ to obtain a PPTA solution. The obtained PPTA showed optical anisotropy.

Example A2

The same procedures as set forth above in Example A1 were used to prepare a PPTA solution except that 150.95 g TPC was used and the mole ratio of PPD and TPC was 0.985.

Example A3

The same procedures as set forth above in Example A1 were used to prepare a PPTA solution except that 149.44 g TPC was used and the mole ratio of PPD and TPC was 0.995.

A PE membrane having a thickness of 14 μm was used as a porous base membrane. The PPTA solution prepared above was coated on one side of the porous base membrane through a gravure coating process at a speed of 20 m/min. The coated membrane was immersed in water, and then dried by oven. A separator having a thickness of 18 μm was obtained. The coating layer on one side of the PE membrane has a thickness of 4 μm.

Example A4

The same procedures as set forth above in Example A1 were used to prepare a PPTA solution except that the water content of the mixture of NMP and CaCl ₂ was adjusted to 4000 ppm.

Example A5

The same procedures as set forth above in Example A1 were used to prepare a PPTA solution except that the water content of the mixture of NMP and CaCl ₂ was adjusted to 4000 ppm, 150.95 g TPC was used, and the mole ratio of PPD and TPC was 0.985.

Example A6

The same procedures as set forth above in Example A1 were used to prepare a PPTA solution except that the water content of the mixture of NMP and CaCl ₂ was adjusted to 4000 ppm, 149.44 g TPC was used, and the mole ratio of PPD and TPC was 0.995.

The same procedures as set forth above in Example A3 were used to prepare a separator for an electrochemical device.

Example A7

The same procedures as set forth above in Example A1 were used to prepare a PPTA solution except that the water content of the mixture of NMP and CaCl ₂ was adjusted to 4500 ppm.

Example A8

The same procedures as set forth above in Example A1 were used to prepare a PPTA solution except that the water content of the mixture of NMP and CaCl ₂ was adjusted to 4500 ppm, 150.95 g TPC was used, and the mole ratio of PPD and TPC was 0.985.

Example A9

The same procedures as set forth above in Example A1 were used to prepare a PPTA solution except that the water content of the mixture of NMP and CaCl ₂ was adjusted to 4500 ppm, 149.44 g TPC was used, and the mole ratio of PPD and TPC was 0.995.

Comparative Example A1

The same procedures as set forth above in Example A1 were used to prepare a PPTA solution except that the water content of the mixture of NMP and CaCl ₂ was adjusted to 1000 ppm.

Comparative Example A2

The same procedures as set forth above in Example A1 were used to prepare a PPTA solution except that the water content of the mixture of NMP and CaCl ₂ was adjusted to 1000 ppm, 150.95 g TPC was used, and the mole ratio of PPD and TPC was 0.985.

Comparative Example A3

The same procedures as set forth above in Example A1 were used to prepare a PPTA solution except that the water content of the mixture of NMP and CaCl ₂ was adjusted to 1000 ppm, 149.44 g TPC was used, and the mole ratio of PPD and TPC was 0.995.

Comparative Example A4

The same procedures as set forth above in Example A1 were used to prepare a PPTA solution except that the water content of the mixture of NMP and CaCl2 was adjusted to 6000 ppm.

Testing Methods

The water content of the solvent was measured by a Karl-Fisher Titrator with a method known in the art.

The intrinsic viscosity of aromatic polymer was measured and calculated using the following method. 0.5 g aromatic polymer was dissolved in 100 ml sulfuric acid having a concentration of from 96%to 98%to prepare a polymer solution. Another sulfuric acid having a concentration of from 96%to 98%was used as a blank control. A capillary tube viscometer was used to measure the flowing time of the polymer solution and the blank control respectively at 25℃ with a method known in the art. The intrinsic viscosity of the aromatic polymer was calculated using the following formula:

Intrinsic Viscosity (unit: dL/g) = ln (T/T ₀) /C,

wherein T is the flowing time (unit: s) of the polymer solution, T ₀ is the flowing time (unit: s) of the blank control, C is the concentration of the aromatic polymer in the polymer solution (unit: g/dL) .

The viscosity of coating slurry was measured by a digital viscometer (NDJ-5S) with a method known in the art at a temperature of 40±5 ℃.

The appearance of the coating layer of the separator was observed with naked eyes to check if it has an even surface.

The amounts of reactants, the reaction conditions, and the intrinsic viscosity of PPTA in Examples A1-A9 and Comparative Examples A1-A4 were recorded in Table 1.

Table 1.

Figure 1 is a diagram showing the relationship between the mole ratio of reactants (i.e., the aromatic diamines and the compounds having acyl groups) and the intrinsic viscosity of the aromatic polymers in Examples A1-A9 and Comparative Examples A1-A4. As shown in Figure 1, when the water content of solvent is 3500 ppm (i.e., Examples A1-A3) , 4000 ppm (i.e., Examples A4-A6) , or 4500 ppm (i.e., Examples A7-A9) , the fitting line of the mole ratio of reactants and the intrinsic viscosity of the aromatic polymers has a small slope, indicating the intrinsic viscosity does not have a big change when the mole ratio of reactants is controlled in a specific range, i.e., from 0.975 to 0.995. When the water content of solvent is 1000 ppm (i.e., Comparative Examples A1-A3) , the fitting line of the mole ratio and the intrinsic viscosity has a big slope, indicating that the intrinsic viscosity has a big variation when different mole ratios (i.e., 0.975, 0985, and 0.995) of reactants are used.

In Comparative Example A4, the solvent has a high water content, i.e., 6000 ppm. The synthesized aromatic polymer has a low intrinsic viscosity, resulting in weak heat-resistance. The separator prepared in Comparative Example A4 may not meet the heat-resistance requirement of certain electrochemical device.

The viscosities of PPTA solutions and appearances of coating layers of the separators prepared in Examples A3, A6, and A7, and Comparative Examples A2 and A4 were listed in Table 2.

Table 2.

As shown in Table 2, the PPTA solutions prepared in Examples A3, A6, and A7 had viscosities in a range of from 50 mPa·s to 500 mPa·s, and the coating layers prepared by these PPTA solutions had even surface. In Comparative Examples A2, the PPTA solution had a high viscosity (i.e., 1320 mPa·s) , the coating layer of the prepared separator had a few particles on its surface. In Comparative Examples A4, the PPTA solution had a low viscosity (i.e., 34 mPa·s) so the aromatic polymer had a low molecule weight, although the coating layer had an even surface, the heat-resistance of the coating layer was weak due to the low molecule weight of the aromatic polymer.

Example B1

1.8 g CaCl ₂ was added into 30 g NMP to prepare a CaCl ₂ solution. 1 g Al ₂O ₃ powder (particle size: 0.02um) was added into the CaCl ₂ solution. The obtained mixture was stirred for 1 hour and then 35 g PPTA solution that was prepared according to the process set forth in Example A1 was added in to form a coating slurry.

The resulting coating slurry was coated on one side of a PE base membrane having a thickness of 14 μm through a roller coating process using an automatic coating machine at a speed of 80 m/min. The coated membrane was immersed in a water bath of 30℃ for 2 minutes, and then dried to obtain a separator for an electrochemical device. The coating layer had a thickness of 4 μm.

Example B2

The same procedures as set forth above in Example B1 were used to prepare a separator for an electrochemical device except that a water bath of 40℃ was used.

Example B3

The same procedures as set forth above in Example B1 were used to prepare a separator for an electrochemical device except that a water bath of 50℃ was used.

Example B4

The same procedures as set forth above in Example B1 were used to prepare a separator for an electrochemical device except that a water bath of 60℃ was used.

Example B5

The same procedures as set forth above in Example B1 were used to prepare a separator for an electrochemical device except that a water bath of 70℃ was used.

Example B6

The same procedures as set forth above in Example B1 were used to prepare a separator for an electrochemical device except that a water bath of 80℃ was used.

Comparative Example B1

The same procedures as set forth above in Example B1 were used to prepare a separator for an electrochemical device except that the coated membrane was placed in a constant temperature and humidity chamber having a temperature of 60℃ and a humidity of 60%for 2 minutes, and then washed by water at room temperature.

Comparative Example B2

The same procedures as set forth above in Example B1 were used to prepare a separator for an electrochemical device except that a water bath of room temperature was used.

For each of the separators prepared in Examples B1-B6 and Comparative Examples B1 and B2, the air permeability was measured using an air permeability testing machine (model of EGBO-55/65-1/1M R) . Three samples were tested for each separator and an average value was calculated. In addition, the thermomechanical property of each separator obtained in Examples B1-B6 and Comparative Examples B1 and B2 was tested using a thermomechanical analysis machine (TMA) with a stationary method (additional force: 0.025 N, rate of temperature increase: 0.5℃/min) . The testing results were shown in Table 3.

Table 3.

As shown in Table 3, the separators prepared in Examples B1-B6 had better air permeability than those of the separators prepared in Comparative Examples B1 and B2, while maintaining the similar thermomechanical properties. These results indicate that the solvent removal process played an important role in the properties of the resulting separators. Immersing the coated membrane in water bath of a constant temperature that is higher than the room temperature and lower than 90 ℃ can assist in the formation of a porous structure in the coating layer of the separator to provide better air permeability, while maintaining similar thermomechanical properties (such as heat-resistance) .

Claims

A polymer solution, comprising an aromatic polymer having a structure of –C (=O) -N–and an intrinsic viscosity ranging from 0.5 dL/g to 1.45 dL/g, wherein the aromatic polymer is obtained by reacting aromatic diamines with compounds having acyl groups in a reaction system that comprises a solvent having a water content ranging from 3000 ppm to 5000 ppm.
The polymer solution according to claim 1, wherein the mole ratio of the aromatic diamines and the compounds having acyl groups ranges from 0.95 to 1.
The polymer solution according to claim 1, wherein the aromatic diamines and the compounds having acyl groups have a weight percentage ranging from 2 wt%to 30 wt%in the reaction system, relative to the total weight of the solvent.
The polymer solution according to claim 1, wherein the aromatic diamines are one or more chosen from hydroxydiphenylamines, 1, 2-phenylenediamines, 1, 3-phenylenediamines, p-phenylenediamines, 3, 3'-benzophenone diamines, 3, 3'-methylene dianilines, 3, 3'-diaminodiphenyl sulfones, 1, 2-naphthylenediamines, 1, 3-naphthylenediamines, 1, 4-naphthylenediamines, 1, 5-naphthylenediamines, 1, 6-naphthylenediamines, 1, 7-naphthylenediamines, 1, 8-naphthylenediamines, 2, 3-naphthylenediamines, 2, 6-naphthylenediamines, and 3, 3'-diphenyl diamines.
The polymer solution according to claim 1, wherein the compounds having acyl groups are one or more chosen from acyl halides, dianhydrides, and diisocyanates.
The polymer solution according to claim 5, wherein the acyl halides are one or more chosen from o-phthaloyl dichlorides, terephthaloyl chlorides, pyromellitic dichlorides, 3, 3’, 4, 4’-diphenylsulfone tetracarboxylic acid dichlorides, 3, 3’, 4, 4’-dibenzophenone tetracarboxylic acid dichlorides, 2, 2’- (3, 4-dicarboxyphenyl) hexafluoropropane dichlorides, 3, 3’, 4, 4’-diphenyl tetracarboxylic acid dichlorides, 1, 2-phenylene dicarboxylic acid dichlorides, 1, 3-phenylene dicarboxylic acid dichlorides, 1, 4-phenylene dicarboxylic acid dichlorides, 1, 2-naphthylene dicarboxylic acid dichlorides, 1, 3-naphthylene dicarboxylic acid dichlorides, 1, 4-naphthylene dicarboxylic acid dichlorides, 1, 5-naphthylene dicarboxylic acid dichlorides, 1, 6-naphthylene dicarboxylic acid dichlorides, 1, 7-naphthylene dicarboxylic acid dichlorides, 1, 8-naphthylene dicarboxylic acid dichlorides, 2, 3-naphthylene dicarboxylic acid dichlorides, 2, 6-naphthylene dicarboxylic acid dichlorides, 3, 3'-biphenylene dicarboxylic acid dichlorides, 3, 3'-dibenzophenone dicarboxylic acid dichlorides, and 3, 3'-diphenylsulfone dicarboxylic acid dichlorides.
The polymer solution according to claim 5, wherein the dianhydridesareone or more chosen from pyromellitic dianhydrides, 3, 3’, 4, 4’-diphenylsulfone tetracarboxylic dianhydrides, 3, 3’, 4, 4’-dibenzophenone tetracarboxylic dianhydrides, 2, 2’- (3, 4-dicarboxyphenyl) hexafluoropropane dianhydrides, and 3, 3’, 4, 4’-biphenyl tetracarboxylic dianhydrides.
The polymer solution according to claim 5, wherein the diisocyanates are chosen from 1, 2-phenylene diisocyanates, 1, 3-phenylene diisocyanates, 1, 4-phenylene diisocyanates, 1, 2-naphthylene diisocyanates, 1, 3-naphthylene diisocyanates, 1, 4-naphthylene diisocyanates, 1, 5-naphthylene diisocyanates, 1, 6-naphthylene diisocyanates, 1, 7-naphthylene diisocyanates, 1, 8-naphthylene diisocyanates, 2, 3-naphthylene diisocyanates, 2, 6-naphthylene diisocyanates, 3, 3’-naphthylene diisocyanates, 3, 3’-dibenzophenone diisocyanates, and 3, 3’-diphenylsulfone diisocyanates.
The polymer solution according to claim 1, wherein the reaction system further comprises chlorides of alkali metal or alkaline-earth metal.
The polymer solution according to claim 1, wherein the solvent is one or more chosen from N-methyl-2-pyrrolidone, N, N-dimethylacetamide, N, N-dimethylformamide, dimethyl sulfoxide and triethyl phosphate.
The polymer solution according to claim 1, wherein the aromatic polymer has an intrinsic viscosity ranging from 0.5 dL/g to 1.4 dL/g.
A method for preparing a separator for an electrochemical device, comprising:

preparing a polymer solution through reacting aromatic diamines with compounds having acyl groups in a reaction system to produce aromatic polymers containing a structure of –C (=O) -NH–and having an intrinsic viscosity ranging from 0.5 dL/g to 1.45 dL/g, wherein the reaction system comprises a solvent having a water content ranging from 3000 ppm to 5000 ppm;

preparing a coating slurry with the polymer solution;

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

removing the solvent from the wet coating layer.
The method according to claim 12, wherein the coating slurry further comprises an inorganic filler.
The method according to claim 12, wherein the coating slurry has a viscosity ranging from 50 mPa·s to 500 mPa·s.
The method according to claim 12, wherein the solvent is removed from the wet coating layer through immersing the coated membrane in a water bath having a constant temperature that is higher than room temperature and lower than 90 ℃.
The method according to claim 15, wherein the constant temperature ranges from 30 ℃ to 80 ℃.
A separator for an electrochemical device, wherein the separator is prepared by the method of claim 12 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 17 interposed between the positive electrode and the negative electrode.