WO2019192475A1

WO2019192475A1 - Coating slurries for preparing separators, separators for electrochemical devices and preparation methods therefor

Info

Publication number: WO2019192475A1
Application number: PCT/CN2019/081007
Authority: WO
Inventors: Alex Cheng; Lianjie WANG; Yongle Chen; Zhixue Wang; Cancan Huang; Xianyou JIANG
Original assignee: Shanghai Energy New Materials Technology Co., Ltd.
Priority date: 2018-04-03
Filing date: 2019-04-02
Publication date: 2019-10-10
Also published as: CN108565381B; CN108565381A

Abstract

Disclosed herein are a coating slurry for preparing a separator, comprising at least one copolymer having a crystallinity degree ranging from 30%to 70%and at least one solvent in which the at least one copolymer is dissolved, wherein the at least one copolymer comprises a first structural unit and at least one second structural unit; a method for preparing the coating slurry disclosed herein; a method for preparing separators with the coating slurry disclosed herein; a separator for an electrochemical device prepared by the method disclosed herein; as well as an electrochemical device comprising the separator.

Description

COATING SLURRIES FOR PREPARING SEPARATORS, SEPARATORS FOR ELECTROCHEMICAL DEVICES AND PREPARATION METHODS THEREFOR

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of priority to Chinese Application No. 201810289099. X, filed on April 3, 2018, the content of which is incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to electrochemistry field, and especially relates to coating slurries for preparing separators, separators for electrochemical devices and preparation methods therefor, as well as electrochemical devices comprising the separator.

BACKGROUND

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

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

A separator is generally formed by a polymeric microporous membrane. For example, polyolefin-based microporous membranes have been widely used as separators in lithium secondary batteries because of their favorable chemical stability and excellent physical properties. However, they may have low crystallinity degree, high swelling ratio, bad adhesive property, and low melting temperatures, leading to high internal resistance, poor conductivity, and fracture under a high temperature. Various techniques for improving the chemical and physical properties of polyolefin-based separators have been disclosed, including, for example, forming a porous coating layer on a polyolefin microporous membrane to prepare a coated separator. The porous coating layer may improve the adhesive property and/or heat-resistance of the coated separator. The preparation method and composition of the porous coating layer can affect many properties of the coated separator. The porous coating layer usually comprises at least one polymer, e.g., polyvinylidene fluoride (PVDF) homopolymers or copolymers. Nowadays, PVDF-co-HFP is a commonly used PVDF copolymer, but such coating layer and the coated separator prepared using PVDF-co-HFP may have low crystallinity degree, high swelling ratio, and bad adhesive property, leading to high internal resistance, poor cycle performance, and other poor electrochemical chemical device properties. Therefore, there is a need to develop advanced coating layers, coated separators and preparation methods therefor to meet the increasing demand on separators having improved properties, such as good adhesiveness, high crystallinity degree, and low swelling ratio, to be used in various high-performance electrochemical devices.

SUMMARY OF THE INVENTION

The present disclosure provides a coating slurry for preparing a separator for an electrochemical device. The coating slurry comprises at least one copolymer having a crystallinity degree ranging, for example, from 30%to 70%, and at least one solvent in which the at least one copolymer is dissolved, wherein the at least one copolymer comprises a first structural unit and at least one second structural unit.

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

The present disclosure further provides a separator for an electrochemical device prepared by the method disclosed herein. Specifically, 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.

DETAILED DESCRIPTION

The present disclosure provides some exemplary embodiments of a coating slurry for preparing a separator for electrochemical devices. In some embodiments of the present disclosure, the coating slurry disclosed herein comprises at least one copolymer and at least one solvent, wherein the at least one copolymer is dissolved in the at least one solvent. The coating slurry may be prepared by mixing the at least one copolymer and the at least one solvent.

The at least one copolymer disclosed herein may have a crystallinity degree ranging, for example, from 30%to 70%, such as from 40%to 60%. Crystallization of polymers is a process associated with partial alignment of their molecular chains. These chains fold together and form ordered regions called lamellae, which compose larger spheroidal structures named spherulites. Crystallization affects optical, mechanical, thermal and chemical properties of the polymer. The crystallinity degree can be determined by different analytical methods. As disclosed herein, the crystallinity degree of the at least one copolymer is measured by a differential scanning calorimetry (DSC) . The at least one copolymer disclosed herein can swell in an nonaqueous electrolyte, and it may have a swelling ratio, which is also called as swelling degree, ranging, for example, from 5%to 30%, such as from 10%to 20%. The swelling ratio disclosed herein is a parameter to characterize the weight change of the at least one copolymer after swelling, which can be calculated by:

swelling ratio (%) = (Ws -Wd) /Wd × 100,

wherein Wd is the weight of the at least one copolymer before swelling, and Ws is the weight of the swollen copolymer obtained by immersing the at least one copolymer in a nonaqueous electrolyte for seven days, wherein the nonaqueous electrolyte is a mixture of ethylene carbonate (EC) , diethyl carbonate (DEC) , and dimethyl carbonate (DMC) in a weight ratio of EC: DEC: DMC=1: 1: 1.

If a polymer having a crystallinity degree lower than 30%and a swelling ratio higher than 30%is used in a coating slurry for preparing a separator, the prepared separator may have a large increase in its dimension when impregnated by a nonaqueous electrolyte, leading to a decrease in adhesiveness. When the separator having weak adhesiveness or binding property is used to prepare an electrochemical device, the electrochemical device may have high internal resistance and bad cycle life. Such issues can be solved by employing a polymer material having relatively high crystallinity degree and relatively low swelling ratio in the coating slurry for preparation of the separator. The coating slurry of the present disclosure comprises at least one copolymer having relatively high crystallinity degree and relatively low swelling ratio, so the separator prepared by the coating slurry may have strong binding property.

Different types of the at least one copolymer in the coating slurry may affect the adhesiveness of the coating layer formed by the coating slurry. In some embodiments of the present disclosure, the at least one copolymer disclosed herein may comprise a first structural unit and at least one second structural unit. In some embodiments, the first structural unit may be derived from tetrafluoroethylene (TFE) . In some embodiments, the at least one second structural unit derived from an entity chosen from vinylidene fluoride, acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, methacrylates, 2-chloroethyl vinyl ether, 2-ethylhexyl acrylate, hydroxyethyl methacrylate, butyl acrylate, butyl methacrylate, trimethylolpropane triacrylate (TMPTA) . In some embodiments, the at least one second structural unit is a second structural unit derived from vinylidene fluoride. In some embodiments, the at least one copolymer disclosed herein may be chosen from polyvinylidene fluoride-co-tetrafluoroethylene (PVDF-co-TFE) , polytetrafluoroethylene-co-acrylic acid (PTFE-co-PAA) , polytetrafluoroethylene-co-methyl methacrylate (PTFE-co-PMMA) , and PVDF-co-CTFE.

The weight percentage of the first structural unit in the copolymer may affect the crystallinity degree and swelling ratio of the copolymer. In some embodiments, the weight percentage of the first structural unit in the copolymer may range, for example, from 0.1 wt%to 20 wt%, such as from 3 wt%to 15 wt%, and further such as from 5 wt%to 10 wt%.

The at least one copolymer disclosed herein may be in a form of particles. The particle size of the at least one copolymer can affect the properties of the coating layer made from the coating slurry disclosed herein. In some embodiments, the at least one copolymer disclosed herein is in a form of particles, which have a particle size ranging, for example, from 0.1 μm to 20 μm, such as from 2 μm to 10 μm.

In some embodiments of the present disclosure, the coating slurry further comprises at least one homopolymer chosen, for example, from PVDF homopolymer, meta-aramid, and para-aramid. If meta-aramid or para-aramid is included in the coating slurry, the separator made from the coating slurry may have improved heat-resistance.

The at least one solvent used in the coating slurry may depend on the type of the at least one copolymer used herein. For example, the at least one solvent may have a solubility parameter similar to that of the at least one 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. The at least one solvent disclosed herein may be chosen, for example, from N-methyl pyrrolidone (NMP) , dimethylacetamide (DMAC) , N, N-dimethylformamide (DMF) , dimethyl sulfoxide (DMSO) , and acetone.

In some embodiments of the present disclosure, the coating slurry may further comprise at least one inorganic filler. When the coating slurry containing the at least one inorganic filler is used to prepare a separator, the at least one inorganic filler may be embedded in the coating layer of the separator and fixed by the at least one copolymer. 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 in a high temperature environment. 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 weight percentage of the at least one inorganic filler present in the coating slurry may be controlled in a specific range, as it may affect the pore structure (e.g., pore size, porosity, and uniformity of pores) , thickness and surface density of the coating layer formed from the coating slurry. In some embodiments, the weight ratio of the at least one copolymer and the at least one inorganic filler in the coating slurry ranges, for example, from 5%to 50%, such as from 10%to 40%.

Various inorganic particles can be used as the at least one inorganic filler in the coating slurry, including, 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. Examples of the at least one inorganic filler include aluminum trioxide (Al ₂O ₃) , boehmite (γ-AlOOH) , silicon dioxide (SiO ₂) , zirconium dioxide (ZrO ₂) , titanium oxide (TiO ₂) , cerium oxide (CeO ₂) , calcium carbonate (Ca ₂O ₃) , calcium oxide (CaO) , zinc oxide (ZnO) , magnesium oxide (MgO) , cerium titanate (CeTiO ₃) , calcium titanate (CaTiO ₃) , barium titanate (BaTiO ₃) , lithium phosphate (Li ₃PO ₄) , lithium titanium phosphate (LTPO) , lithium aluminum titanium phosphate (LATP) , lithium nitride (Li ₃N) , barium sulfate (BaSO ₄) , and lithium lanthanum titanate (LLTO) . In addition, the at least one inorganic filler disclosed herein may have an average particle size ranging, for example, from 0.1 to 20 μm, such as from 1 to 10 μm.

The present disclosure further provides some exemplary embodiments of a method for making a separator for an electrochemical device using the coating slurry disclosed herein. In one embodiment, the method disclosed herein comprises:

(A) preparing a coating slurry disclosed herein, comprising at least one copolymer and at least one solvent;

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

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

In step (A) , the coating slurry may be prepared by mixing the at least one copolymer and the at least one solvent to obtain a mixture. The coating slurry may be a solution when the at least one copolymer is dissolved in the at least one solvent. Any suitable techniques may be used to enhance the solubility of the at least one copolymer in the at least one solvent or shorten the dissolution time. For example, stirring the mixture, shearing the mixture, or raising the temperature of the at least one solvent and/or the temperature of the mixture (e.g., the temperature may be raised to a range of from 5℃ to 80℃, such as from 20 ℃ to 50 ℃) . In some embodiments, the at least one copolymer disclosed herein may comprise a first structural unit and at least one second structural unit, and may have a crystallinity degree ranging, for example, from 30%to 70%, and a swelling ratio ranging, for example, from 5%to 30%.

In the case that the coating slurry further comprises at least one inorganic filler as disclosed in some embodiments above, the coating slurry prepared in step (A) may be a suspension as the at least one inorganic filler disperses in the coating slurry. The coating slurry containing the at least one inorganic filler may be prepared by mixing the at least one copolymer, the at least one inorganic filler, and the at least one solvent to obtain a mixture. In some other embodiments, the coating slurry containing the at least one inorganic filler may be prepared by:

(A1) mixing the at least one copolymer and a first solvent to obtain a first mixture;

(A2) mixing the at least one inorganic filler and a second solvent to obtain a second mixture; and

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

In steps (A1) and (A2) , each of the first solvent and the second solvent may be NMP, DMAC, DMF, DMSO, acetone, or a mixture thereof. The compositions of the first solvent and the second solvent may be the same or different. In some embodiments, the first solvent and the second solvent may have the same composition, which may achieve a more stable coating slurry.

In step (A1) , in some embodiments, the first mixture may comprise, for example, from 3 wt%to 20 wt%, such as from 5 wt%to 15 wt%, of the at least one copolymer of the total weight of the first mixture.

In step (A2) , in some embodiments, the second mixture may comprise, for example, from 0.5 to 4 parts by weight of the at least one inorganic filler and from 8 to 50 parts by weight of the second solvent. In some embodiments, the second mixture comprises from 2 to 3 parts by weight of the at least one inorganic filler and from 20 to 30 parts by weight of the second solvent.

In step (A3) , during the mixing, the weight ratio of the first mixture and the second mixture may range, for example, from 90: 10 to 50: 50, such as from 80: 20 to 60: 40.

In step (B) , the coating slurry prepared in step (A) 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 5 to 100 m/min, such as from 15 to 80 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 by sequence.

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

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 coating slurry may be subjected to a vacuum oven for a predetermined time period so as to remove the at least one solvent from the wet coating layer. The pressure and temperature of the vacuum oven may depend on the amount and type of solvent to be removed. Phase inversion process is an alternative method to remove the at least one solvent, which may be initiated by exposing the wet coating layer to a poor solvent or non-solvent of the at least one copolymer, 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 at least one 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 at least one solvent is removed. In some embodiments, step (C) comprises immersing the coated porous base membrane in a poor solvent or non-solvent having a temperature ranging, for example, from 0℃ to 90℃, such as from 30℃ to 80℃, for a predetermined time period, for example, from 1 to 30 minutes, such as from 5 to 20 minutes. To remove the at least one 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. Step (C) may further comprise taking the coated porous base membrane out from the poor solvent or non-solvent and removing a residue of the at least one solvent and/or the poor solvent or non-solvent therefrom. Residues of the at least one solvent and/or the poor solvent or non-solvent may be removed by any method known in the art, 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, in a predetermined speed. The three-stage oven may have a temperature ranging from 45 to 55℃ in its first stage, a temperature ranging from 55 to 65℃ in its second stage, and a temperature ranging from 50 to 60℃ in its third stage. In an example, the three-stage oven has temperatures of 50℃, 60℃, and 55℃ in its first, second, and third stages respectively.

By such a 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 above comprises a porous base membrane and a coating layer being formed on at least one side of the porous base membrane. The coating layer may also comprise inorganic fillers that are embedded in the coating layer and fixed by the at least one copolymer.

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 not in direct 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, i.e., the coating layer is formed directly on at least one surface of the porous base membrane. In some embodiments, 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 is not in direct contact 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 pass from one surface side to the other surface side of the coating layer. The average size of the pores within the coating layer may range, for example, from 0.1 to 100 μm, such as from 1 to 10 μm. The porosity of the coating layer may range, for example, from 10%to 60%, such as from 20%to 40%. The coating layer may have an air permeability ranging, for example, from 50 to 400 sec/100ml, such as from 100 to 250 sec/100ml. 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 2 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 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 method known in the art or 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 method known in the art, such as electro-blowing, electro-spinning, or melt-blowing, or 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 disclosed herein comprises a coating layer on at least one side of the porous base membrane, comprising at least one copolymer of relatively high crystallinity degree and relatively low swelling ratio. With the presence of the at least one copolymer in the coating layer, the separator can have excellent adhesive property and good contact interface with the electrodes, even when it is impregnated with a nonaqueous electrolyte. Thus the electrochemical devices employing the separator of the present disclosure may have improved mechanical strength, low internal resistance, and improved cycle performance. The separator disclosed herein can have an increased air permeability. The air permeability increase of the separator may range, for example, from 5 to 200 sec/100ml, such as from 10 to 100 sec/100ml. In addition, inclusion of the inorganic fillers in the separator of the present disclosure can enhance the thermal shrinkage property of the separator, leading to a more stable performance under a high temperature. 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 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, comprising 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 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.

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

A PVDF-co-TFE containing 2 wt%structural units derived from TFE and 98 wt%structural units derived from vinylidene fluoride was prepared. 1.52 kg of the PVDF-co-TFE was dissolved in 17.48 kg of DMAC to obtain a first mixture having a solid content of 8 wt%. 0.08 kg of alumina powder was dispersed in 0.92 kg of DMAC to obtain a second mixture. The first mixture and the second mixture were mixed to obtain a coating slurry.

A PE membrane having a thickness of 12 μm was used as a porous base membrane. The coating slurry prepared above was coated on one surface 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 a three-stage oven having temperatures of 50℃, 60℃ and 55℃, respectively, in the first, the second and the third stage thereof to obtain a separator. The coating layer had a thickness of 2 μm, and the separator had a thickness of 14 μm.

A lithium-ion battery was made by placing the above prepared separator between lithium manganese oxide (LiMn ₂O ₄) as positive electrode and artificial graphite as negative electrode, and injecting an electrolyte of mixture of ethylene carbonate (EC) , diethyl carbonate (DEC) , dimethyl carbonate (DMC) in a weight ratio of EC: DEC: DMC=in a weight ratio of 1: 1: 1.

Example 2

A PVDF-co-TFE containing 5 wt%structural units derived from TFE and 95 wt%structural units derived from vinylidene fluoride was prepared. 1.44 kg of the PVDF-co-TFE was dissolved in 16.56 kg of DMAC to obtain a first mixture having a solid content of 8 wt%. 0.16 kg of alumina powder was dispersed in 1.84 kg of DMAC to obtain a second mixture. The first mixture and the second mixture were mixed to obtain a coating slurry.

The same procedures as set forth above in Example 1 were used to prepare a separator using the above prepared coating slurry, and a lithium-ion battery.

Example 3

A PVDF-co-TFE containing 8 wt%structural units derived from TFE and 92 wt%structural units derived from vinylidene fluoride was prepared. 1.2 kg of the PVDF-co-TFE was dissolved in 13.8 kg of DMAC to obtain a first mixture having a solid content of 8 wt%. 0.4 kg of alumina powder was dispersed in 4.6 kg of DMAC to obtain a second mixture. The first mixture and the second mixture were mixed to obtain a coating slurry.

Example 4

A PVDF-co-TFE containing 15 wt%structural units derived from TFE and 85 wt%structural units derived from vinylidene fluoride was prepared. 0.8 kg of the PVDF-co-TFE was dissolved in 9.2 kg of DMAC to obtain a first mixture having a solid content of 10 wt%. 0.8 kg of alumina powder was dispersed in 9.2 kg of DMAC to obtain a second mixture. The first mixture and the second mixture were mixed to obtain a coating slurry.

Example 5

A PTFE-co-PAA containing 8 wt%structural units derived from TFE and 92 wt%structural units derived from acrylic acid was prepared. 1.6 kg of the PTFE-co-PAA was dissolved in 18.4 kg of DMAC to obtain a mixture having a solid content of 8 wt%. The mixture was used as a coating slurry.

Example 6

A PVDF-co-TFE containing 2 wt%structural units derived from TFE and 98 wt%structural units derived from vinylidene fluoride was prepared. 1.44 kg of the PVDF-co-TFE was dissolved in 16.56 kg of DMAC to obtain mixture having a solid content of 8 wt%. The mixture was used as coating slurry.

Comparative Example

A PVDF-co-HFP containing 8 wt%structural units derived from hexafluoropropylene (HFP) and 92 wt%structural units derived from vinylidene fluoride was prepared. 1.12 kg of the PVDF-co-HFP was dissolved in 12.88 kg of DMAC to obtain a first mixture having a solid content of 8 wt%. 0.48 kg of alumina powder was dispersed in 5.52 kg of DMAC to obtain a second mixture. The first mixture and the second mixture were mixed to obtain a coating slurry.

The copolymers, the separators, the lithium-ion batteries of Examples 1-5 and Comparative Example were tested using the following methods.

Crystallinity degree of the copolymer was tested using a differential scanning calorimetry with a model number of TA DSC Q2000.

Swelling ratio of the copolymer was tested according to the following method. The copolymer was dissolved in DMAC firstly, and then the DMAC was removed by extraction with water to obtain a porous membrane. The porous membrane was cut into a sample and the sample was weighed. The sample was then immersed in a nonaqueous electrolyte mixture of ethylene carbonate (EC) , diethyl carbonate (DEC) , dimethyl carbonate (DMC) in a weight ratio of EC: DEC: DMC=1: 1: 1 for seven days to obtain a swollen sample, which was weighed. The swelling ratio of the copolymer was calculated by:

swelling ratio (%) = (Ws -Wd) /Wd × 100,

wherein Wd is the weight of the sample before swelling, and Ws is the weight of the swollen sample.

Interface adhesiveness of the separator was tested according to the following method. The separator was cut into samples of 25 mm width and 100 mm length; two samples of the separator were stacked and hot pressed at 1 MPa, 100℃ with a speed of 10 m/min in a hot press machine. The tensile force (unit: N) required for separating the two stacked samples was measured. The adhesive force (N/m) = the tensile force /0.025m.

Air permeability of the PE porous base membrane and the separator were tested using Labthink BTY-Den Gas Permeability Tester and the method of positive pressure. The air permeability increase was calculated by:

air permeability increase (s/100cc) = air permeability of the separator -air permeability of the PE porous base membrane.

Internal resistance of the lithium-ion battery was tested using an AC voltage drop method. The lithium-ion battery was applied with a current of 1 KHz frequency and 50 mA. The voltages of the lithium-ion battery were sampled. The internal resistance of the lithium-ion battery was calculated through an Operational Amplifier circuit after rectification and filtering.

Cycle performance of the lithium-ion battery was tested according to the following method. At room temperature, 500 cycles of charging and discharging at 0.5C respectively were performed on the lithium-ion battery. The capacity retention rate was calculated using the following formula:

capacity retention rate (%) = (capacity after 500 cycles/capacity before the cycle test at room temperature) ×100%.

Table 1 summarizes the testing results of the copolymers, the separators and the lithium-ion batteries that were prepared in Examples 1-6 and Comparative Example.

Table 1.

As shown in Table 1, in Examples 1-6, the copolymer used for preparing the separators had higher crystallinity degree and lower swelling ratio than that of the copolymer used in Comparative Example. The separators prepared in Examples 1-6 had much better interface adhesiveness than that in Comparative Example. The lithium-ion batteries prepared in Examples 1-6 had much lower internal resistance and better cycle performance than that in Comparative Example. The separator prepared in Comparative Example had weak interface adhesive property, resulting in high internal resistance and poor cycle performance of the corresponding lithium-ion battery.

In Examples 1-4, inorganic fillers were added into the coating slurry during the preparation of the separators. As the inorganic fillers present in the coating layer can improve the porous structure of the coating layer, the separators prepared in Examples 1-4 had better air permeability. In Examples 5 and 6, the separator was prepared without adding inorganic fillers, so it had poor air permeability comparing to those in Examples 1-4.

Claims

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

at least one copolymer comprising a first structural unit and at least one second structural unit, wherein the at least one copolymer has a crystallinity degree ranging from 30%to 70%; and

at least one solvent in which the at least one copolymer is dissolved.
The coating slurry according to claim 1, wherein the at least one copolymer has a swelling ratio ranging from 5%to 30%in a nonaqueous electrolyte mixture of ethylene carbonate (EC) , diethyl carbonate (DEC) , and dimethyl carbonate (DMC) with a weight ratio of EC: DEC: DMC=1: 1: 1.
The coating slurry according to claim 1, wherein the first structural unit is derived from tetrafluoroethylene and the at least one second structural unit is derived from an entity chosen from vinylidene fluoride, acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, methacrylates, 2-chloroethyl vinyl ether, 2-ethylhexyl acrylate, hydroxyethyl methacrylate, butyl acrylate, butyl methacrylate, trimethylolpropane triacrylate (TMPTA) .
The coating slurry according to claim 3, wherein the at least one copolymer comprises from 0.1 wt%to 20 wt%of the first structural unit derived from tetrafluoroethylene based on the total weight of the at least one copolymer.
The coating slurry according to claim 3, wherein the at least one copolymer is polyvinylidene fluoride-co-tetrafluoroethylene.
The coating slurry according to claim 1, wherein the at least one copolymer is in a form of particles having a particle size ranging from 0.1 μm to 20 μm.
The coating slurry according to claim 1, wherein the coating slurry comprises from 3 wt%to 20 wt%of the at least one copolymer.
The coating slurry according to claim 1, wherein the at least one solvent is chosen from N-methyl pyrrolidone, dimethylacetamide, N, N-dimethylformamide, dimethyl sulfoxide, and acetone.
The coating slurry according to claim 1, further comprising at least one homopolymer.
The coating slurry according to claim 1, further comprising at least one inorganic filler.
The coating slurry according to claim 10, wherein the at least one inorganic filler is chosen from oxides, hydroxides, sulfides, nitrides, carbides, carbonates, sulfates, phosphates, and titanates comprising at least one of metallic and semiconductor elements chosen from Si, Al, Ca, Ti, B, Sn, Mg, Li, Co, Ni, Sr, Ce, Zr, Y, Pb, Zn, Ba, and La.
The coating slurry according to claim 10, wherein the at least one inorganic filler is 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 coating slurry according to claim 10, wherein the weight ratio of the at least one copolymer and the at least one inorganic filler in the coating slurry ranges from 5%to 50%.
A method for preparing the coating slurry of claim 10, comprising:

mixing the at least one copolymer and a first solvent to obtain a first mixture;

mixing the at least one 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 for an electrochemical device, comprising:

preparing a coating slurry of claim 1;

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

removing the at least one solvent from the wet coating layer.
The method according to claim 15, wherein the at least one solvent is removed from the wet coating layer by:

immersing the coated porous base membrane in a poor solvent or non-solvent of the at least one copolymer; and

removing a residue of the at least one 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 prepared by the method of claim 15, comprising:

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.