WO2019114692A1

WO2019114692A1 - Separators, electrochemical devices comprising the separator, and methods for making the separator

Info

Publication number: WO2019114692A1
Application number: PCT/CN2018/120282
Authority: WO
Inventors: Alex Cheng; Jinzhen BAO; Yongle Chen; Fangbo HE
Original assignee: Shanghai Energy New Materials Technology Co., Ltd.
Priority date: 2017-12-12
Filing date: 2018-12-11
Publication date: 2019-06-20
Also published as: CN108123089A

Abstract

Disclosed are a separator for an electrochemical device, comprising a nonwoven porous substrate and at least one heat-resistant layer disposed on at least one side of the nonwoven porous substrate, comprising at least one heat-resistant polymer, wherein the separator has a breakdown temperature of 450℃ or above; as well as an electrochemical device including the separator and a method for making the separator.

Description

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

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of priority to Chinese Application No. 201711319921.4, filed on December 12, 2017, the content of which is incorporated by reference.

TECHNICAL FIELD

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

BACKGROUND

A separator is a permeable membrane placed between a positive electrode and a negative electrode in an electrochemical device. The main function of the separator is to keep the two electrodes apart so as to prevent electrical short circuits while also allow the transport of ionic charge carriers that are needed to close the circuit during the passage of current. 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, energy density, rate capacity, 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 mechanical strength. However, they may have poor adhesive property, so they could not form a strong contact interface with electrodes. As polyolefin usually has a low melting point, the polyolefin-based membranes may shrink at a high temperature, resulting in a volume change and direct contact of the positive electrode and the negative electrode. Many efforts have been made to increase the adhesiveness, thermal stability, and air permeability of the polyolefin-based separators.

A common method to improve the thermal stability or heat-resistance of a polyolefin-based separator is to coat a porous base membrane with an inorganic material, e.g., alumina and boehmite. The inorganic material-coated separators may have an improved thermal stability at 120℃ , but may still shrink seriously at a higher temperature, such as 150℃ . Further, with the presence of an inorganic coating layer, the separator may be heavier than the conventional polyolefin-based separator, resulting in an increased surface density, and further resulting in a reduced energy density of the corresponding electrochemical device. In addition, the inorganic coating layer usually has poor adhesion to the electrodes. Thus, the interfaces between the inorganic material-coated separator and the electrodes may deform during charge-discharge cycles, which may shorten the life time of the electrochemical device.

Further, batteries with high-rate capacity often require separators with good air permeability. Air permeability is expressed in terms of Gurley value, i.e., the time required for a specified amount of air to pass through a specified area of the separator under a specified pressure. Due to the limitations of the materials used and manufacturing process, it may be difficult to further reduce the Gurley value of the conventional polyolefin-based separator. This problem could be solved by employing other materials with high porosity, such as nonwoven fabrics. However, nonwoven fabrics cannot be directly used as separators in electrochemical devices due to their large pores that easily lead to short circuit of a battery.

Therefore, lightweight separators with improved properties, such as high heat-resistance, high adhesiveness, and excellent air permeability, are still needed for various electrochemical devices.

SUMMARY OF THE INVENTION

The present disclosure provides a separator for an electrochemical device, comprising a nonwoven porous substrate and at least one heat-resistant layer comprising at least one heat-resistant polymer disposed on at least one side of the nonwoven porous substrate, wherein the separator has a breakdown temperature of 450 ℃ or above.

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

The present disclosure further provides a method for making the separator disclosed herein, comprising: preparing a first slurry comprising at least one heat-resistant polymer and at least one first solvent; coating the first slurry on at least one side of a nonwoven porous substrate to form a wet heat-resistant layer; and removing the at least one first solvent from the wet heat-resistant layer to form a heat-resistant layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 through 6 illustrate schematic diagrams of exemplary separators according to the embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure provides some exemplary embodiments of the separator for an electrochemical device. In one embodiment of the separator, at least one heat-resistant layer, which comprises at least one heat-resistant polymer, is formed on at least one side of a nonwoven porous substrate. Thus the separator disclosed herein has a laminated structure. The “at least one heat-resistant layer” disclosed herein means the separator may comprise one or more heat-resistant layers. Each of the heat-resistant layers comprises at least one heat-resistant polymer. The “at least one side” disclosed herein means the at least one heat-resistant layer is disposed on one side or both sides of the nonwoven porous substrate, and the heat-resistant layer can be in direct contact or not in direct contact with the nonwoven porous substrate. The separator disclosed herein exhibits a breakdown temperature of, for example, 450℃ or above, such as, ranging from 450℃ to 600℃ . The breakdown temperature disclosed herein is a temperature at which the separator breaks and cannot keep the positive electrode and the negative electrode physically apart anymore. The breakdown temperature is measured using a device TMA by thermomechanical analysis, in which the separator is mounted onto the device with the fixture therein, a constant force that is usually ranges from 0.01 to 0.05N and a linear temperature ramp are applied to the separator, wherein the temperature increases at 5℃/min. The temperature at which the separator breaks is measured and recorded as the breakdown temperature. In the present disclosure, the device TA Q400 was used to measure the breakdown temperature of the separator disclosed herein. Since the separator disclosed herein has a high breakdown temperature, the electrochemical device employing the separator can be safe at a high temperature environment, which is caused by, for example, overcharging.

The nonwoven porous substrate used here may have a better heat-resistance than a polyolefin-based substrate. In some embodiments of the separator disclosed herein, the nonwoven porous substrate comprises at least one nonwoven membrane. When the nonwoven porous substrate comprises two or more nonwoven membranes, the two or more nonwoven membranes stack up and form a laminated substrate. The term “nonwoven 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 nonwoven membrane disclosed herein may exhibit dimensional stability, i.e., thermal shrinkage of less than 5%when heated to 100℃ in about two hours. The nonwoven membrane may have a thickness ranging, for example, from 0.5 to 50 μm, such as from 0.5 to 20 μm. The nonwoven 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 nonwoven membrane may have a porosity ranging, for example, from 40%to 80%, such as from 50%to 70%. Furthermore, the nonwoven membrane may have a Gurley value of, for example, not greater than 100 sec/100ml, such as ranging from 0 to 50 sec/100ml. Because the nonwoven membrane has excellent air permeability, it is easy for ions to pass through. The non-woven membrane disclosed herein may be formed of at least one material chosen from polyethylene (PE) , high density polyethylene (HDPE) , polypropylene (PP) , polybutylene, polypentene, polymethylpentene (TPX) , polyethylene terephthalate (PET) , polyamide, polyimide (PI) , polyacrylonitrile (PAN) , polyester, polyacetal, polycarbonate, polyetherketone (PEK) , polyetheretherketone (PEEK) , polybutylene terephthalate (PBT) , polyethersulfone (PES) , polyphenylene oxide (PPO) , polyphenylene sulfide (PPS) , polyethylene naphthalene (PEN) , viscose fiber, cellulose fiber, and copolymers thereof. For example, a nonwoven membrane formed of PET can be used as the nonwoven porous substrate. The at least one nonwoven membrane disclosed herein can be prepared according to a method known in the art, such as electro-blowing, electro-spinning, or melt-blowing, or can be purchased directly in the market.

The nonwoven porous substrate may further comprise at least one coating layer formed on at least one surface of the at least one nonwoven membrane. The thickness of the at least one coating layer may range, for example, from 1 to 5 μm. The presence of the coating layer can improve the bonding strength between the nonwoven porous substrate and the at least one heat-resistant layer and can also improve the absorption of liquid electrolyte due to, for example, its porousness.

One side or both sides of the nonwoven porous substrate may be coated with the heat-resistant layer. With the presence of the heat-resistant layer, the separator disclosed herein can have improved thermal stability and reduced thermal shrinkage at a high temperature. The heat-resistant layer may be formed by coating at least one surface of the nonwoven porous substrate with a coating slurry. During the coating process, at least part of the coating slurry may penetrate into some pores of the nonwoven porous substrate, so that the pore size of these pores of the nonwoven porous substrate may be narrowed down. In such a case, the direct contact of the positive electrode and the negative electrode caused by larges pores of the nonwoven porous substrate can be prevented.

The heat-resistant layer may have a pore structure allowing ions to pass from one surface side to the other surface side. The average pore size of pores in the heat-resistant layer may range, for example, from 0.02 to 10 μm, such as from 0.1 to 5 μm. The porosity of the heat-resistant layer may range, for example, from 20%to 80%, such as from 40%to 70%. In some embodiments, the heat-resistant layer on one side of the nonwoven porous substrate may have a thickness ranging, for example, from 1 to 5 μm, such as from 2 to 4 μm.

The at least one heat-resistant layer disclosed herein comprises at least one heat-resistant polymer. The at least one heat-resistant polymer may have a melting temperature or a glass transition temperature of, for example, 450 ℃ or above, such as from 450 ℃ to 600 ℃ . Examples of the at least one heat-resistant polymer include polyamide, polyimide (PI) , polyetherimide (PEI) , polysulfone, polybenzimidazole (PBI) , polyphenylene sulfide (PPS) , polyethersulfone (PES) , polyarylsulfone (PAS) , polyketone (PK) , polyetherketone (PEK) , polyetheretherketone (PEEK) , and polydiphenyl oxide (PDPO) . For example, aromatic polyamides, i.e., aramids (e.g., para-aramid, meta-aramid) , can be used as the at least one heat-resistant polymer because of their high mechanical strength, good heat and flame resistance, and great dimensional stability. In some embodiments, a heat-resistant polymer having a low density may be used to produce a lightweight heat-resistant layer. The lightweight heat-resistant layer can reduce the total weight of the separator disclosed herein and further increase the energy density of the electrochemical device comprising the separator. In some embodiments, the heat-resistant layer disclosed herein may have a surface density ranging, for example, from 0.5 to 4 g/m ², such as from 1 to 3.5 g/m ².

The heat-resistant layer may have a binding property, which means the heat-resistant layer is adhesive. In one embodiment, the heat-resistant layer comprise a heat-resistant polymer which is adhesive, such as polyimide (PI) . In another embodiment, the heat-resistant layer further comprises at least one adhesive material, e.g., polyvinylidene fluoride (PVDF) . In the case that the heat-resistant layer is adhesive, when the separator disclosed herein contacts with an electrode, an even and uniform interface may be formed between the heat-resistant layer of the separator and the electrode, which can improve the life cycle of the corresponding electrochemical device.

In one embodiment of the present disclosure, the at least one heat-resistant layer is a completely-coated layer, which means the entire area of the surface to-be-coated (e.g., the surface of the nonwoven porous substrate or the surface of an additional layer) is coated with the heat-resistant layer, and no uncoated area exists. In such a case, the at least one heat-resistant layer on one side of the nonwoven porous substrate may have the same size as the nonwoven porous substrate.

In some other embodiments, the at least one heat-resistant layer is a partially-coated layer. The term “partially-coated layer” means the layer comprises coated area (s) and uncoated area (s) that form a coating pattern. At least one of the coated area (s) and the uncoated area (s) may have a shape of dot, circle, triangle, square, diamond, rectangle, stripe, mesh, or an irregular shape. The partially-coated layer may be formed by partially coating a surface to-be-coated (e.g., the surface of the nonwoven porous substrate or the surface of an additional layer) with a coating slurry, leaving at least one area uncoated. The coated area (s) and the uncoated area (s) may form an alternating coating pattern. The “alternating coating pattern” disclosed herein means a coating pattern in which the coated area (s) and the uncoated area (s) are distributed alternately in at least one direction within the plane of the partially-coated layer. The “alternating coating pattern” disclosed herein can be regular or irregular. In addition, the term “area (s) ” disclosed herein means one or more areas. For example, the heat-resistant layer may comprise a mesh-like coated area and a plurality of discontinuous uncoated areas. The term “discontinuous” disclosed herein means at least two coated areas that are separated by an uncoated area or at least two uncoated areas that are separate by a coated area. The partially-coated layer can provide more spaces and channels for liquid electrolyte, and also can reduce the overall weight of the separator disclosed herein. When the at least one heat-resistant layer is a partially-coated layer, its thickness depends on the thickness of the coated area (s) .

The separator disclosed herein has a laminated structure. In one embodiment, the separator disclosed herein may have a two-layer structure when only one surface of the nonwoven porous substrate is coated with the heat-resistant layer disclosed herein. For example, as shown in Figure 1, separator 10 comprises a nonwoven porous substrate 11 and a heat-resistant layer 13 formed on one surface of the nonwoven porous substrate 11. In another embodiment, the separator may have a three-layer structure when two heat-resistant layers are formed on two surfaces of the nonwoven porous substrate respectively. For example, as shown in Figure 2, separator 200 comprises a nonwoven porous substrate 21, a first heat-resistant layer 23 formed on one surface of the nonwoven porous substrate 21, and a second heat-resistant layer 24 formed on the other surface of the nonwoven porous substrate 21. The first heat-resistant layer 23 and the second heat-resistant layer 24 may, for example, have the same composition, physical properties, and coating pattern (if both are partially coated) . In other embodiments, the first heat-resistant layer 23 and the second heat-resistant layer 24 may be different in at least one of the compositions, physical properties, and coating patterns. The “physical properties” disclosed herein includes certain performance parameters for the membrane characterization, e.g., average pore size, porosity, air permeability, and thickness.

The separator disclosed herein may further comprise at least one adhesive layer. In some embodiments, the at least one adhesive layer can be formed on a surface of the nonwoven porous substrate, or, an outer surface of the heat-resistance layer such that the heat-resistant layer is disposed between the nonwoven porous substrate and the adhesive layer. In some embodiments, the at least one adhesive layer may be located at the outmost layer of the separator disclosed herein. In terms of “outmost layer, ” a separator has two outmost layers facing to the positive electrode and the negative electrode, respectively, in an electrochemical device. With the adhesive layer being the outmost layer, when the separator disclosed herein is used for manufacturing a cell, the binding property of the adhesive layer can assist the assembling process because the relative movements between the separator and the electrodes can be reduced or avoided. In addition, a good contact interface may be formed between the separator and the electrodes in the cell due to the adhesiveness of the adhesive layer. The good contact interface can reduce the internal resistance and increase the life cycle of the cell.

The at least one adhesive layer may comprise at least one organic material. The at least one organic material may be adhesive. The at least one organic material may be chosen, for example, from acrylic resin, methacrylic resin, polyolefin, aramid, polyimide (PI) , polyester, polyvinylidene fluoride (PVDF) , polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP) , polyvinylidene fluoride-co-tetrafluoroethylene (PVDF-co-TFE) , and polytetrafluoroethylene (PTFE) .

In some embodiments, the at least one adhesive layer may further comprise at least one inorganic material. Various inorganic particles can be used as the at least one inorganic material, 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 material 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 at least one inorganic material disclosed herein may have an average particle size ranging, for example, from 0.1 to 20 μm, such as from 0.1 to 10 μm.

In some embodiments, the at least one adhesive layer comprises the at least one organic material and the at least one inorganic material in a weight ratio ranging, for example, from 2: 98 to 20: 80, such as from 5: 95 to 15: 85.

In some embodiments, the at least one adhesive layer may be porous. The porous structure can provide more spaces for the storage of a liquid electrolyte, and more channels for the flow of the liquid electrolyte, thereby improving the life cycle of the electrochemical device. The pores in the adhesive layer may have an average pore size ranging, for example, from 0.02 to 10 μm, such as from 0.1 to 5 μm. The adhesive layer may have a porosity ranging, for example, from 20%to 80%, such as from 40%to70%.

The at least one adhesive layer may be a completely-coated layer or a partially-coated layer. The definitions of “completely-coated layer” and “partially-coated layer” have been discussed as set forth above. In one embodiment, the at least one adhesive layer comprises a mesh-like coated area and a plurality of discontinuous uncoated areas. The partially-coated layer can provide more spaces and channels for the liquid electrolyte, and also can reduce the overall weight of the separator disclosed herein.

The at least one adhesive layer may have a thickness ranging, for example, from 1 to 5 μm, such as from 2 to 4 μm. When the at least one adhesive layer is a partially-coated layer, its thickness depends on the thickness of the coated area (s) .

Figures 3 through 6 illustrate some exemplary separators comprising at least one heat-resistant layer and at least one adhesive layer according to the embodiments of the present disclosure.

In Figure 3, separator 30 comprises a three-layer structure formed by a nonwoven porous substrate 31, a heat-resistant layer 33 formed on one surface of the nonwoven porous substrate 31, and an adhesive layer 35 formed on the other surface of the nonwoven porous substrate 31.

In Figure 4, separator 40 has a three-layer structure, in which a heat-resistant layer 43 is disposed between a nonwoven porous substrate 41 and an adhesive layer 45.

In Figure 5, separator 50 comprises a nonwoven porous substrate 51, two heat-

resistant layers

53 and 54, and an adhesive layer 55. The two heat-

resistant layers

53 and 54 are disposed on two surfaces of the nonwoven porous substrate 51, respectively. The adhesive layer 55 is formed on an outer surface of the heat-resistant layer 53. The two heat-

resistant layers

53 and 54 have the same composition, physical properties and coating pattern. In other embodiments of the present disclosure, the two heat-

resistant layers

53 and 54 may be different in at least one of the compositions, physical properties, and coating patterns.

In Figure 6, separator 60 comprises a nonwoven porous substrate 61, two heat-

resistant layers

63 and 64, and two

adhesive layers

65 and 66. The two heat-

resistant layers

63 and 64 are formed on two surfaces of the nonwoven porous substrate 61, respectively. The two

adhesive layers

65 and 66 are formed on the outer surfaces of the two heat-

resistant layers

63 and 64 on two sides of the nonwoven porous substrate 61, respectively. The two heat-

resistant layers

63 and 64 may have the same composition and physical properties, or may be different in at least one of the compositions and physical properties. Similarly, the two

adhesive layers

65 and 66 may have the same composition, physical properties and coating patterns, or may be different in at least one of the compositions, physical properties, and coating patterns.

As discussed above, since the separator disclosed herein comprises the at least one heat-resistant layer, it can have excellent thermal stability and mechanical strength at a high temperature. The separator disclosed herein can also have good ion permeability and air permeability. When the at least one adhesive layer is included in the separator, a good contact interface can be formed between the separator and one of the electrodes. Further, the separator disclosed herein can be lightweight, which can improve the energy density of the electrochemical device comprising the same. The separator disclosed herein can have a wide range of applications and can be used for making high energy density and high power density batteries, e.g., automotive batteries, batteries for medical devices, and batteries for other large devices.

Further, the present disclosure provides an electrochemical device comprising: a positive electrode, a negative electrode, and the separator disclosed herein, which is interposed between the positive electrode and the negative electrode. An electrolyte may be further included in the electrochemical device of the present disclosure. Such electrochemical devices include any devices in which electrochemical reactions occur. For example, the electrochemical device disclosed herein includes primary batteries, lithium secondary batteries, lead-acid batteries, sodium-sulfur batteries, fuel cells, solar cells and capacitors. In some embodiments, the electrochemical device disclosed herein is a lithium secondary battery, such as a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, a lithium-sulfur battery, and a lithium-air battery.

With the separator of the present disclosure inside, the electrochemical device disclosed herein can exhibit improved safety at a high temperature as discussed above. The electrochemical devices of the present disclosure can also have an improved energy density as the separators disclosed herein can be lightweight.

The electrochemical device disclosed herein may be manufactured by a method known in the art. In one embodiment, a cell is formed by placing the separator of the present disclosure between a positive electrode and a negative electrode to obtain an electrode assembly and injecting a liquid electrolyte 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.

Further disclosed herein are some exemplary embodiments of the method for making the separator for an electrochemical device disclosed herein. In one embodiment, it is a wet coating method, which, for example, comprises:

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

(B) coating the first slurry on at least one surface of a nonwoven porous substrate to form a wet heat-resistant layer; and

(C) removing the at least one first solvent from the wet heat-resistant layer to form a heat-resistant layer.

In step (A) , a mixture of the at least one heat-resistant polymer and the at least one first solvent may be stirred to obtain the first slurry. As discussed above, the at least one heat-resistant polymer may be chosen, for example, from polyamide, PI, PEI, polysulfone, PBI, PPS, PES, PAS, PK, PEK, PEEK and PDPO. In some embodiments, the at least one heat-resistant polymer in the first slurry may have a weight percentage ranging, for example, from 1wt%to 50wt%, such as from 5wt%to 30wt%. The at least one first solvent used in step (A) may have a solubility parameter similar to that of the at least one heat-resistant polymer used, and a low boiling point, because such solvent can facilitate uniform mixing and coating process and needs to be removed in the following step. Examples of the at least one first solvent that may be used herein include N, N-dimethylformamide (DMF) , dimethylacetamide (DMAC) , N-methyl pyrrolidone (NMP) , dimethyl sulfoxide (DMSO) , acetone, diethyl ether, propyl ether, cyclohexane, tetrahydrofuran (THF) , and a mixture thereof.

In some embodiments, to improve the solubility of the at least one heat-resistant polymer in the at least one first solvent, at least one solubilizer may be added into the at least one first solvent. The at least one solubilizer can be chosen, for example, from lithium chloride (LiCl) , calcium chloride (CaCl ₂) and dodecylbenzene sulfonic acid (DBSA) . The weight ratio of the at least one heat-resistant polymer to the solubilizer may be controlled, ranging, for example, from 100: 1 to 100: 10, such as from 100: 5 to 100: 10.

In step (B) , in order to coat the first slurry on at least one surface of the nonwoven porous substrate, any method known in the art may be used, such as dip coating, die coating, roll coating, comma coating and combinations thereof. When the first slurry is coated onto both surfaces of the nonwoven porous substrate, the two surfaces can be coated concurrently or one by one. Additionally, the at least one surface of the nonwoven porous substrate may be completely coated with the first slurry to form a completely-coated layer, or may be partially coated with the first slurry to form a partially-coated layer.

In step (C) , the at least one first solvent can be removed from the wet heat-resistant layer through a method known in the art, such as a thermal evaporation, a vacuum evaporation, a phase inversion process, or combinations thereof. In some embodiments, the at least one first solvent may be removed through a thermal evaporation. For example, the coated nonwoven porous substrate may be placed in an oven having a temperature, for example, ranging from 70℃ to 90℃ , such as 80℃ , so as to evaporate the at least one first solvent. In some other embodiments, the at least one first solvent may be removed through a combination of thermal evaporation and vacuum evaporation. For example, the coated nonwoven porous substrate may be placed in a vacuum oven having a temperature, for example, ranging from 50℃ to 80℃ , such as 80℃ , and a vacuum degree ranging from 0 to 95 kPa, so as to evaporate the at least one first solvent. Phase inversion process is an alternative method to remove the at least one first solvent, which may be initiated by immersing the coated nonwoven porous substrate in a poor solvent or non-solvent of the at least one heat-resistant polymer, such as water (e.g., deionized water) , alcohols (e.g., ethanol) , or combinations thereof. In the poor solvent or non-solvent, polymer-polymer self-interactions are preferred, and the polymer coils may contract and eventually precipitate. The poor solvent or non-solvent can precipitate the at least one heat-resistant polymer from the first slurry, thereby forming a heat-resistant layer on the at least one surface of the nonwoven porous substrate. In an example, the coated nonwoven porous substrate may be immersed in water, so that the at least one first solvent can be transferred into water. Residues of the at least one first solvent and/or the poor solvent may be removed by any method known in the art, for example, heating or vacuum drying. As a result, a dry heat-resistant layer can form on at least one surface of the nonwoven porous substrate.

In some embodiments, the method for making the separator of the present disclosure may further comprise:

(D) preparing a second slurry comprising at least one organic material and at least one second solvent;

(E) coating the second slurry on at least one surface of the heat-resistant layer and the nonwoven porous substrate to form a wet adhesive layer; and

(F) removing the at least one second solvent from the wet adhesive layer to form an adhesive layer.

In step (D) , to prepare the second slurry, the at least one organic material and the at least one second solvent may be mixed and stirred to obtain the second slurry. As discussed above, the at least one organic material may be chosen, for example, from acrylic resin, methacrylic resin, polyolefin, polyamide, PI, polyester, PVDF, PVDF-co-HFP, PVDF-co-TFE, and PTFE. In some embodiments, the at least one organic material in the second slurry may have a weight percentage ranging, for example, from 1wt%to 60wt%, such as from 10wt%to 50wt%. Examples of the at least one second solvent that may be used herein include DMF, DMAC, NMP, DMSO, acetone, diethyl ether, propyl ether, cyclohexane, THF, and a mixture thereof.

In step (E) , similar to step (B) , any method known in the art may be used to coat the second slurry on at least one surface of the heat-resistant layer and the nonwoven porous substrate to form a wet adhesive layer, such as dip coating, die coating, roll coating, comma coating and combinations thereof. It can be complete coating or partial coating.

In step (F) , similar to step (C) , the at least one second solvent can be removed from the wet adhesive layer through a method known in the art, such as a thermal evaporation, a vacuum evaporation, a phase inversion process, or combinations thereof. The same method for removing the at least one first solvent from the wet heat-resistant layer may be used to remove the at least one second solvent from the wet adhesive layer.

In some embodiments, the second slurry may further comprise at least one inorganic material. Examples of the at least one inorganic material are discussed above.

References are now made in detail to the following examples. It is to be understood that the following examples are illustrative and the present disclosure is not limited thereto. In the following Examples 1-6 and Comparative Examples 1-4, separators and lithium-ion batteries comprising the separator according to the present disclosure were prepared.

Example 1

A single-layer PET nonwoven membrane having a thickness of 16 μm and an air permeability of 0 s/100ml was used as a nonwoven porous substrate. A heat-resistant layer containing para-aramid was formed on one surface of the single-layer PET nonwoven membrane to obtain a separator. The heat-resistant layer had a thickness of 3 μm and a surface density of 2.9 g/m ².

A lithium-ion battery was prepared by placing the separator prepared above between a positive electrode (lithium cobaltate) and a negative electrode (graphite) , winding, and injecting an electrolyte (1.0 mol/L LiPF6, solvent: EC: DEC (wt%) =6: 4) .

Example 2

A single-layer polyamide nonwoven membrane having a thickness of 16 μm and an air permeability of 10 s/100ml was used as a nonwoven porous substrate. A heat-resistant layer containing para-aramid was formed on one surface of the single-layer polyamide nonwoven membrane to obtain a separator. The heat-resistant layer had a thickness of 3 μm and a surface density of 2.9 g/m ².

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

Example 3

A single-layer PET nonwoven membrane having a thickness of 16 μm and an air permeability of 0 s/100ml was used as a nonwoven porous substrate. A heat-resistant layer containing meta-aramid was formed on one surface of the single-layer PET nonwoven membrane to obtain a separator. The heat-resistant layer had a thickness of 3 μm and a surface density of 2.6 g/m ².

Example 4

A single-layer PET nonwoven membrane having a thickness of 16 μm and an air permeability of 0 s/100ml was used as a nonwoven porous substrate. A heat-resistant layer containing meta-aramid having a thickness of 2 μm was formed on one surface of the single-layer PET nonwoven membrane. An adhesive layer containing PVDF having a thickness of 2 μm was formed on the outer surface of the heat-resistant layer, such that the heat-resistant layer was disposed between the nonwoven porous substrate and the adhesive layer. A separator was thus obtained. The heat-resistant layer and the adhesive layer together had a surface density of 2.9 g/m ².

Example 5

A single-layer PET nonwoven membrane having a thickness of 16 μm and an air permeability of 0 s/100ml was used as a nonwoven porous substrate. A heat-resistant layer containing meta-aramid was formed on both surfaces of the single-layer PET nonwoven membrane to obtain a separator. The heat-resistant layers on both surfaces of the single-layer PET nonwoven membrane had an aggregated thickness of 4 μm and a surface density of 4.0 g/m ².

Example 6

A single-layer PET nonwoven membrane having a thickness of 16 μm and an air permeability of 0 s/100ml was used as a nonwoven porous substrate. A heat-resistant layer containing meta-aramid and PVDF having an aggregated thickness of 4 μm was formed on both surfaces of the single-layer PET nonwoven membrane. A separator was thus obtained. The heat-resistant layer containing meta-aramid and PVDF had a surface density of 2.9 g/m ².

Comparative Example 1

A single-layer PE membrane having a thickness of 16 μm and an air permeability of 238 s/100ml was used as a porous substrate. An inorganic layer containing alumina having a thickness of 3 μm was formed on one surface of the single-layer PE membrane to obtain a separator.

Comparative Example 2

A single-layer PP membrane having a thickness of 16 μm and an air permeability of 252 s/100ml was used as a porous substrate. An inorganic layer containing alumina having a thickness of 3 μm was formed on one surface of the single-layer PP membrane to obtain a separator.

Comparative Example 3

A single-layer PET nonwoven membrane having a thickness of 16 μm and an air permeability of 0 s/100ml was used as a porous substrate. An inorganic layer containing alumina having a thickness of 3 μm was formed on one surface of the single-layer PET nonwoven membrane to obtain a separator.

Comparative Example 4

A single-layer polyamide nonwoven membrane having a thickness of 16 μm and an air permeability of 10 s/100ml was used as a porous substrate. An inorganic layer containing boehmite having a thickness of 3 μm was formed on one surface of the single-layer polyamide nonwoven membrane to obtain a separator.

The following tests were conducted for the resulting separators in Examples 1-6 and Comparative Examples 1-4:

Test 1 Thickness of Separator

A Malvern Thickness Tester was used to measure the thickness at five points of a sample having a flat surface. An average value was calculated.

Test 2 Air Permeability of Separator

The air permeability of the separator was measured using a Digital Oken Type Air-Permeability Tester, Model: EG01-55-1 MR, Asahi Seiko Co., Ltd. The time was set as 5s and the value was set as 500.

Test 3 Surface Density of Separator

A separator having a flat surface was cut into five samples, each of which has dimension of 40mm×60mm. The five samples were weighed. For each sample, the surface density was calculated by:

Surface density (g/m ²) = M/S×10,

wherein M is the weight of the sample (mg) , S is the area of the sample (cm ²) . An average value of the surface density for the five samples was calculated.

Test 4 Thermal Shrinkage of Separator

The length direction of the separator roll was defined as the machine direction (MD) , and the width direction of the separator roll was defined as the transverse direction (TD) . For each separator, five samples, each of which has dimension of 100mm×100mm, were prepared. An 80mm×80mm square was marked at the center of each sample. The distance of two opposite borders of the square was recorded as T0. T0 = 80mm. The five samples were placed in an oven having a temperature of 150℃ for 1 hour. The distance of the two opposite borders along MD was measured as T1. The distance of the two opposite borders along TD was measured as T2. For each sample, thermal shrinkage of TD was calculated by: (T0-T1) /T0×100%. Thermal shrinkage of MD was calculated by: (T0-T2) /T0×100%. The Average thermal shrinkage values of TD and MD were calculated and recorded, respectively.

Test 5 Cycle Performance of Lithium-ion Battery at Room Temperature

At room temperature, 500 cycles of charge and discharge at 1C were performed on the lithium-ion battery. The Capacity Retention Rate of the lithium-ion battery was calculated using the following formula:

Capacity Retention Rate (%) = (capacity after 500 cycles of charge and discharge /capacity before the Cycle Performance Test) ×100%.

Test 6 Rate Discharge Performance of Lithium-ion Battery

The lithium-ion battery was fully charged (4.2V) and then discharged at different rates: 1C, 2C, 5C and 10C. For each rate of 2C, 5C, and 10C, a ratio of the discharge capacity to the discharge capacity at 1C was calculated.

Test 7 Nail Test of Lithium-ion Battery

From top front of the lithium ion battery, a steel nail having a diameter of 3 mm was pierced into the lithium ion battery at a speed of 50 mm/sand stayed for 10 minutes. The tested lithium ion battery was observed with naked eyes to check if it is on fire, smoking, or exploded.

Table 1 summarizes the test results of the separators and the lithium-ion batteries prepared in Examples 1-5 and Comparative Examples 1 to 4.

Table 1. Testing Results

In Comparative Examples 1 and 2, the conventional polyolefin-based membrane, i.e., PE membrane and PP membrane, respectively, was used as the porous substrate for the separator. An inorganic coating containing alumina was formed on the surface of the PE or PP membrane. As shown in Table 1, the separators prepared in Comparative Examples 1 and 2 had high surface densities (i.e., 15.2 g/m ² and 15.9 g/m ²) , poor air permeability (i.e., 279 s/100ml and 285 s/100ml) , and high thermal shrinkage percentages both at MD and TD (i.e., >10) .

In Comparative Examples 3 and 4, a nonwoven membrane was used as the porous substrate, and an inorganic coating layer containing alumina and boehmite respectively was formed on the surface of the nonwoven membrane. As shown in Table 1, the separators prepared in Comparative Examples 3 and 4 had good air permeability and low thermal shrinkage percentages because of the usage of nonwoven porous substrate. The lithium-ion batteries prepared in Comparative Examples 3 and 4 had good capacity retention rate of discharge at 5C. They got on fire in the Nail Test, but it was not as serious as the ones prepared in Comparative Examples 1 and 2. However, the separators prepared in Comparative Examples 3 and 4 had high surface densities, so they could not contribute to the energy density improvement of the lithium-ion batteries. In addition, the lithium-ion batteries prepared in Comparative Examples 3 and 4 had lower capacity retention rates after 500 cycles of charge and discharge than that of the lithium-ion batteries prepared in Examples 1 to 6.

In Examples 1 and 2, a heat-resistant layer containing para-aramid was formed on one surface of the nonwoven porous substrate. In Example 3, a heat-resistant layer containing meta-aramid was formed on one surface of the nonwoven porous substrate. The separators prepared in Examples 1-3 had lower surface densities and better air permeability compared with that of the separators prepared in Comparative Examples 1 to 4, as well as excellent thermal shrinkage performance. The lithium-ion batteries prepared in Examples 1-3 exhibited high capacity retention rates both at 5C discharge and 500 cycles test, and did not get on fire during the Nail Test.

In Example 4, a separator having a nonwoven porous substrate, a heat-resistant layer containing meta-aramid, and an adhesive layer containing PVDF was prepared. As the bonding strength between the separator and electrodes of the lithium-ion battery was enhanced because of the presence of the adhesive layer, the lithium-ion battery prepared in Example 4 had a higher capacity retention rate after 500 cycles of charge and discharge than that of the lithium-ion batteries prepared in other Examples or Comparative Examples.

In Examples 5 and 6, a heat-resistant layer containing meta-aramid or mixture of meta-aramid and PVDF was formed on both surfaces of the nonwoven porous substrate. The separators prepared in Examples 5 and 6 had relatively low surface density, decent air permeability, and excellent thermal shrinkage property. The lithium-ion batteries prepared in Example 5 had improved rate performance, cycle performance and safety.

Claims

A separator for an electrochemical device, comprising:

a nonwoven porous substrate; and

at least one heat-resistant layer disposed on at least one side of the nonwoven porous substrate, comprising at least one heat-resistant polymer,

wherein the separator has a breakdown temperature of 450℃ or above.
The separator according to claim 1, wherein the nonwoven porous substrate comprises at least one nonwoven membrane.
The separator according to claim 1, wherein the nonwoven porous substrate comprises at least one material chosen from polyethylene, high density polyethylene, polypropylene, polybutylene, polypentene, polymethylpentene, polyethylene terephthalate, polyamide, polyimide, polyacrylonitrile, polyester, polyacetal, polycarbonate, polyetherketone, polyetheretherketone, polybutylene terephthalate, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalene, viscose fiber, cellulose fiber, and copolymers thereof.
The separator according to claim 1, wherein the heat-resistant layer has a thickness ranging from 1 to 5 μm.
The separator according to claim 1, wherein the heat-resistant layer has a surface density ranging from 0.5 g/m ² to 4 g/m ².
The separator according to claim 1, wherein the at least one heat-resistant polymer is chosen from polyamide, polyimide, polyetherimide, polysulfone, polybenzimidazole, polyphenylene sulfide, polyethersulfone, polyarylsulfone, polyketone, polyetherketone, polyetheretherketone and polydiphenyl oxide.
The separator according to claim 1, wherein the at least one heat-resistant polymer is para-aramid or meta-aramid.
The separator according to claim 1, wherein the at least one heat-resistant layer is a completely-coated layer or a partially-coated layer.
The separator according to claim 1, wherein the at least one heat-resistant layer is a partially-coated layer comprising a mesh-like coated area and discontinuous uncoated areas.
The separator according to claim 1, wherein the separator further comprises:

at least one adhesive layer being the outmost layer of the separator, comprising at least one organic material.
The separator according to claim 10, wherein the at least one organic material is chosen from acrylic resin, methacrylic resin, polyolefin, aramid, polyimide, polyester, polyvinylidene fluoride, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-tetrafluoroethylene, and polytetrafluoroethylene.
The separator according to claim 10, wherein the at least one adhesive layer further comprises at least one inorganic material.
The separator according to claim 12, wherein the at least one inorganic material is chosen from alumina, boehmite, silica, titanium oxide, cerium oxide, calcium oxide, zinc oxide, magnesium oxide, lithium nitride, calcium carbonate, barium sulfate, lithium phosphate, lithium titanium phosphate, lithium aluminum titanium phosphate, cerium titanate, calcium titanate, barium titanate and lithium lanthanum titanate.
The separator according to claim 12, wherein the at least one adhesive layer comprises the at least one organic material and the at least one inorganic material in a weight ratio ranging from 2: 98 to 20: 80.
An electrochemical device comprising a positive electrode, a negative electrode, and a separator according to claim 1 disposed between the positive electrode and the negative electrode.
A method for making a separator for an electrochemical device according to claim 1, comprising:

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

coating the first slurry on at least one surface of a nonwoven porous substrate to form a wet heat-resistant layer; and

removing the first solvent from the wet heat-resistant layer to form a heat-resistant layer.
The method according to claim 16, wherein the at least one heat-resistant polymer is chosen from polyamide, polyimide, polyetherimide, polysulfone, polybenzimidazole, polyphenylene sulfide, polyethersulfone, polyarylsulfone, polyketone, polyetherketone, polyetheretherketone and polydiphenyl oxide.
The method according to claim 16, wherein the first solvent is chosen from N, N-dimethylformamide, dimethylacetamide, N-methyl pyrrolidone, dimethyl sulfoxide, acetone, diethyl ether, propyl ether, cyclohexane and tetrahydrofuran.
The method according to claim 16, further comprising:

preparing a second slurry comprising at least one organic material and at least one second solvent;

coating the second slurry on at least one surface of the heat-resistant layer and the nonwoven porous substrate to form a wet adhesive layer; and

removing the at least one second solvent from the wet adhesive layer to form an adhesive layer.
The method according to claim 19, wherein the second slurry further comprises at least one inorganic material.