WO2019034143A1

WO2019034143A1 - Separators and electrochemical devices comprising the separator

Info

Publication number: WO2019034143A1
Application number: PCT/CN2018/101032
Authority: WO
Inventors: Alex Cheng; Jinzhen BAO; Zhixue Wang; Fangbo HE; Yongle Chen; Ting GU
Original assignee: Shanghai Energy New Materials Technology Co., Ltd.
Priority date: 2017-08-18
Filing date: 2018-08-17
Publication date: 2019-02-21

Abstract

A separator (10) for an electrochemical device comprises a porous base membrane (11) and at least one partially-coated layer (13) disposed on at least one side of the porous base membrane (11), wherein the coated area (131) and the uncoated area (133) of the at least one partially-coated layer (13) are in an alternating coating pattern, and further wherein the coated area (131) are porous and comprise at least one adhesive material.

Description

SEPARATORS AND ELECTROCHEMICAL DEVICES COMPRISING THE SEPARATOR

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of priority to Chinese Application No. 201721039422.5, filed on August 18, 2017, and Chinese Application No. 201711445260. X, filed on December 27, 2017, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to separators for electrochemical devices and electrochemical devices comprising the separator.

BACKGROUND

With the growing market of consumer electronics, electrical vehicles and energy storage, lithium secondary batteries are required to have high energy density, excellent cycle life, and stable safety. The liquid electrolyte present in the lithium secondary battery is important to the battery cycle life. If the amount of the liquid electrolytes injected into the battery is insufficient to impregnate or soak the battery components homogeneously, the likelihood of reduced cycle life and early cell failure is increased. The battery components (e.g., positive electrode, separator, negative electrode) are often compacted to achieve a high energy density. Considering the compacted structure, it can be difficult for the battery to contain a large amount of liquid electrolyte. It thus may take hours or days to impregnate the compacted battery components with the liquid electrolyte. The electrolyte impregnation process can be time consuming, resulting in a prolonged battery manufacturing process. To enhance the speed of electrolyte impregnation to the electrode assembly and shorten the impregnation time (or soakage time) , after the liquid electrolyte is injected, some standing conditions can be optimized, e.g., placing the cell horizontally, extending the standing time, and increasing the standing temperature. Nonetheless, these optimized conditions may not provide an effective way and ultimate solution to solve the problem of slow electrolyte impregnation within a battery.

The main reason for the slow electrolyte impregnation is due to insufficient number of flow channels and spaces within the battery for the liquid electrolyte. To increase the number of flow channels and spaces inside the battery, one approach is to reduce the compacting density of electrode sheets. However, the reduced compacting density may lead to a decreased capacity and a reduced energy density of the battery. Another approach is to increase the surface roughness and gaps of the electrode sheets by a special surface treatment, e.g., corona discharge. Additional flow channels and spaces for liquid electrolyte can be created after the surface treatment, but this approach may also result in a decreased battery capacity. Moreover, the surface treatment may cause the internal surfaces of the battery become worse. Therefore, the electrode sheets may be deformed after hundreds of charge/discharge cycles, resulting in a serious safety issue.

Accordingly, there is still a need to further develop a method or a product that can improve the electrolyte impregnation process, so as to increase the battery production efficiency and save cost. Meanwhile, the resulting batteries can still meet the requirements on capacity and energy density.

SUMMARY OF THE INVENTION

Disclosed herein is a separator for an electrochemical device. The separator comprises a porous base membrane and at least one partially-coated layer disposed on at least one side of the porous base membrane, wherein the coated area (s) and the uncoated area (s) of the at least one partially-coated layer are in an alternating coating pattern, and further wherein the coated area (s) are porous and comprise at least one adhesive material.

Further disclosed herein is an electrochemical device, comprising a positive electrode, a negative electrode, and a separator as disclosed herein interposed between the positive electrode and the negative electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 through 7 are schematic cross-sectional views of seven exemplary separators according to the embodiments of the present disclosure.

Figures 8A through 8G are coating patterns of seven exemplary partially-coated layers according to the embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure provides some exemplary embodiments of a separator for an electrochemical device. In one embodiment of the separator disclosed herein, a partially-coated layer is disposed on at least one side of a porous base membrane of the separator. The “at least one side” disclosed herein means the partially-coated layer is disposed on one side or both sides of the porous base membrane, and the partially-coated layer can be in direct contact or not in direct contact with the porous base membrane. When the partially-coated layer is in direct contact with the porous base membrane, the partially-coated layer is formed on at least one surface of the porous base membrane, e.g., separator 10 shown in Figure 1 and separator 20 shown in Figure 2. In Figure 1, separator 10 includes a porous base membrane 11 and a partially-coated layer 13 being formed on one surface of the porous base membrane 11. The partially-coated layer 13 comprises coated areas 131 and uncoated areas 133. In Figure 2, separator 20 includes a porous base membrane 21 and two partially-coated

layers

23 and 24 being formed on each surface of the porous base membrane 21.

In addition, as discussed above, in some embodiments of the present disclosure, the partially-coated layer may not be in direct contact with the porous base membrane, and at least one additional layer may be formed between the partially-coated layer and the porous base membrane, e.g., separator 30 shown in Figure 3.

The porous base membrane of the separator disclosed herein has numerous pores inside, through which gas, liquid, or ions can pass from one surface side to the other surface side. The average pore size of the pores in the porous base membrane may range, for example, from 10 to 100 nm, such as from 20 to 50 nm. The porous base membrane may be made of at least one material chosen, for example, from polyolefin (e.g., polyethylene (PE) , high density polyethylene (HDPE) , polypropylene (PP) ) , aramids, polyamides, polyacrylonitrile (PAN) , polyethylene terephthalate (PET) , and various nonwoven fibers. For example, a polyolefin-based membrane may be used as the porous base membrane. The polyolefin-based membrane disclosed herein may be single-layered (e.g., a single-layered PE membrane, a single-layered PP membrane) or multi-layered (e.g., a PP/PE/PP three-layer membrane) . The porous base membrane may have a thickness ranging, for example, from 0.5 to 50 μm, such as from 3 to 20 μm.

The “partially-coated layer” disclosed herein comprises coated area (s) and uncoated area (s) in an alternating coating pattern from a top view of the separator. 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 “coating pattern” disclosed herein means an arrangement of the coated area (s) and the uncoated area (s) in the partially-coated layer. 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, Figures 8A-8G illustrate some alternating coating patterns of the partially-coated layer disclosed herein, wherein the black parts are coated areas and the white parts are uncoated areas. Figure 8A illustrates an exemplary alternating coating pattern comprising discontinuous coated areas and a continuous uncoated area. Each of the coated areas is square-shaped. The continuous uncoated area is mesh-like. Conversely, Figures 8B and 8C illustrate other exemplary alternating coating patterns of the partially-coated layer disclosed herein, wherein the coated area is continuous and the uncoated areas are discontinuous or scattered. In Figure 8B, the discontinuous uncoated areas have the same size and shape with a uniform distribution. While in Figure 8C, the discontinuous uncoated areas have different sizes and shapes with a non-uniform distribution. In the direction shown by the grey line, the continuous coated area and the discontinuous uncoated areas are distributed alternately.

In some embodiments, both coated areas and uncoated areas are discontinuous. For example, the alternating coating patterns in Figures 8D and 8E comprise stripe-shaped coated areas and stripe-shaped uncoated areas which are distributed alternately and in parallel. The stripe-shaped coated area may have an equal width ranging, for example, from 0.5 mm to 50 mm, such as from 1 mm to 10 mm. The stripe-shaped uncoated areas may also have an equal width that may be less than that of the stripe-shaped coated area. The width of the stripe-shaped uncoated areas may range, for example, from 0.1 mm to 10 mm, such as from 0.5 mm to 5 mm. In addition, the stripe-shaped coated areas or uncoated areas may have a length direction that can form an angle θ with the transverse direction (TD) of the separator, wherein 0°≤θ≤90°. For example, in Figure 8D, θ=0°, which means the length direction of the stripe-shaped coated area is the same as the TD of the separator. In Figure 8E, θ=25°.

In some other embodiments, at least one of the coated areas may comprise a secondary coating pattern, i.e., a coated area comprises coated portions and uncoated portions distributed alternately. Similarly to the coating pattern discussed above, at least one of the coated portions and the uncoated portions may have a shape of dot, circle, triangle, square, diamond, rectangle, stripe, mesh or an irregular shape. For example, as shown in Figure 8F, firstly, stripe-shaped coated areas and stripe-shaped uncoated areas distribute alternately. In addition, different from the coating pattern in Figure 8E, each of the stripe-shaped coated areas in Figure 8F is not completely coated, but comprises a mesh-like uncoated portion and a plurality of square-shaped coated portions. The example shown in Figure 8G also has a secondary coating pattern, wherein each of the stripe-shaped coated areas in Figure 8G comprises a mesh-like coated portion and a plurality of square-shaped uncoated portions.

As discussed above, the alternating coating pattern disclosed herein can be regular or irregular. Figures 8A, 8B, 8D, and 8E show some exemplary regular alternating coating patterns in which either the coated areas or the uncoated areas have the same size and shape with a uniform distribution. Figure 8C shows an exemplary irregular alternating coating pattern in which the discontinuous uncoated areas have different sizes and shapes with a non-uniform distribution. Alternatively, an exemplary irregular alternating coating pattern disclosed herein can have discontinuous coated areas with different sizes and shapes and/or with a non-uniform distribution.. 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. For example, in Figure 1, the coated areas 131 are separated by uncoated areas 133.

The partially-coated layer may be formed by partially coating at least two areas of the surface (e.g., the surface of a porous base membrane or an additional layer) with a coating slurry, leaving other area (s) uncoated.

With the presence of the uncoated areas, the partially-coated layer can provide more channels and spaces for the liquid electrolyte. Thus the separator disclosed herein can be quickly impregnated by the liquid electrolyte, and the impregnation time can be shortened. Moreover, with an increased amount of liquid electrolyte in the separator or the corresponding electrochemical device disclosed herein, the electrochemical device will have improved cycle life. The area ratio of the coated areas and the uncoated areas may be controlled in a specified range to ensure the separator has good binding property to electrodes and can also be impregnated quickly by the liquid electrolyte. The uncoated areas may cover, for example, from 1%to 50%, such as from 10%to 30%, of the entire area of the partially-coated layer. The partially-coated layer of the separator disclosed herein may have the same size as that of the porous base membrane.

The coated areas of the partially-coated layer disclosed herein comprise at least one adhesive material, so that the coated areas are adhesive. A firm physical bonding can be formed between the partially-coated layer and the surface where it is formed, which can prevent the partially-coated layer from being peeled off easily. When the separator disclosed herein is used for manufacturing a cell, the binding property of the coated areas can assist the assembling process because the relative movements between the separator and the electrodes can be reduced or avoided. In addition, a strong contact interface may be formed between the separator and the electrodes in the cell due to the adhesiveness of the coated areas. The strong contact interface can also increase the cycle life of the cell.

The at least one adhesive material included in the coated areas disclosed herein may be a polymer chosen, for example, from polyvinylidene fluoride (PVDF) , polytetrafluoroethylene (PTFE) , polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP) , polyvinylidene fluoride-co-tetrafluoroethylene (PVDF-co-TFE) , polyacrylic acid (PAA) , polyacrylate, polyacrylate salt, polymethacrylate (PMA) , polymethylmethacrylate (PMMA) , polyethylene oxide (PEO) , polyester, polyolefin, aramid, polyimide (PI) , polyethersulfone, and polysulfone. The partially-coated layer may comprise, for example, from 20 wt%to 100 wt%, such as from 30 wt%to 70 wt%, of the at least one adhesive material.

The coated areas of the partially-coated layer disclosed herein may have a porous structure. Thus the separator disclosed herein can contain more liquid electrolyte and can provide more channels for ions (e.g., lithium-ions) transfer between the positive electrode and the negative electrode. If the coated areas are not porous, the ions can pass the partially-coated layer only through the uncoated areas. The coated areas having a porous structure can increase the permeability of the separator and thereby improve the performance of the corresponding electrochemical device.

In some embodiments, besides the at least one adhesive material, the coated areas of the partially-coated layer may further comprise at least one inorganic filler. The presence of the at least one inorganic filler can improve the heat resistance of the separator and reduce thermal shrinkage of the porous base membrane at a high temperature. Moreover, the presence of the at least one inorganic filler can contribute to, for example, the formation of micro pores in the coated areas, the increase of the physical strength of the coated areas, and the increase in an impregnation rate of a liquid electrolyte. The at least one inorganic filler may be stably fixed in the coated areas by the at least one adhesive material, thereby preventing the inorganic filler from being dropped off. Various inorganic particles can be used as the at least one inorganic filler disclosed herein, including, for example, an oxide, a hydroxide, a sulfide, a nitride, and a carbide, a carbonate, a sulfate, a phosphate, and a titanate, and the like of at least one of metallic and semiconductor elements, such as Si, Al, Ca, Ti, B, Sn, Mg, Li, Co, Ni, Sr, Ce, Zr, Y, Pb, Zn, Ba, and La. For example, one or more of alumina (Al ₂O ₃) , boehmite (γ-AlOOH) , silica (SiO ₂) , titanium oxide (TiO ₂) , cerium oxide (CeO ₂) , calcium oxide (CaO) , zinc oxide (ZnO) , magnesium oxide (MgO) , lithium nitride (Li ₃N) , calcium carbonate (CaCO ₃) , barium sulfate (BaSO ₄) , lithium phosphate (Li ₃PO ₄) , lithium titanium phosphate (LTPO) , lithium aluminum titanium phosphate (LATP) , cerium titanate (CeTiO ₃) , calcium titanate (CaTiO ₃) , barium titanate (BaTiO ₃) and lithium lanthanum titanate (LLTO) can be used as the inorganic filler. The at least one inorganic filler disclosed herein may have an average particle size ranging, for example, from 0.01 to 10 μm, such as from 0.5 to 5 μm.

The thickness of the partially-coated layer on one side of the porous base membrane may range, for example, from 0.3 μm to 6 μm, such as from 0.5 μm to 5μm. The thickness of the partially-coated layer depends on the thickness of the coated area (s) .

In some embodiments, the separator of the present disclosure may comprise a porous base membrane and a partially-coated layer disposed on both sides of the porous base membrane. For example, Figure 2 illustrates a separator 20 comprising a porous base membrane 21, a partially-coated layer 23 disposed on one surface of the porous base membrane 21, and another partially-coated layer 24 disposed on the other surface of the porous base membrane 21. The partially-coated

layers

23 and 24 may, for example, have the same composition, physical properties, and coating pattern. In such a case, the separator 20 may be prepared by partially coating both sides of the porous base membrane 21 with the same coating slurry and forming the same coating pattern on both surfaces thereof . The two surfaces of the porous base membrane 21 may be partially coated concurrently or with a time interval. In other embodiments, the partially-coated

layers

23 and 24 may be different in at least one of the compositions, physical properties, and coating patterns. The “physical properties” disclosed herein includes some performance parameters for membrane characterization, e.g., average pore size, porosity, air permeability, and thickness.

In some embodiments of the separator disclosed herein, the separator may further comprise a completely-coated layer disposed on at least one surface of the porous base membrane. The completely-coated layer may have the same size as that of the porous base membrane. When the completely-coated layer is formed on one surface of the porous base membrane, the entire surface of the porous base membrane is coated continuously. The completely-coated layer may also have a porous structure allowing gas, liquid, or ions pass through. When a polyolefin-based membrane is employed as the porous base membrane in the separator disclosed herein, the completely-coated layer can enhance its mechanical strength and may also improve its thermal stability when a heat-resistant material (e.g., inorganic material and heat-resistant polymer) is included in the completely-coated layer. When a nonwoven fabric membrane is employed as the porous base membrane in the separator disclosed herein, a completely-coated layer is included in the separator, so that potential direct contact of two electrodes due to the large pore size of the nonwoven fabric membrane can be avoided.

The completely-coated layer may comprise at least one organic material and/or at least one inorganic material. The at least one organic material may be chosen, for example, from polyvinylidene fluoride (PVDF) , polytetrafluoroethylene (PTFE) , polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP) , polyvinylidene fluoride-co-tetrafluoroethylene (PVDF-co-TFE) , polyacrylic acid (PAA) , polyacrylate, polyacrylate salt, polymethacrylate (PMA) , polymethylmethacrylate (PMMA) , polyethylene oxide (PEO) , polyester, polyolefin, aramid, polyimide (PI) , polyethersulfone (PES) , and polysulfone. The presence of the at least one inorganic material in the completely-coated layer can improve the heat resistance of the separator and reduce thermal shrinkage of the porous base membrane at a high temperature. Moreover, the presence of the at least one inorganic material in the completely-coated layer can contribute to, for example, the formation of micro pores in the completely-coated layer, the increase of the physical strength of the completely-coated layer, and the increase in an impregnation rate of a liquid electrolyte. The at least one inorganic material may, for example, be an oxide, a hydroxide, a sulfide, a nitride, and a carbide, a carbonate, a sulfate, a phosphate, and a titanate, and the like of at least one of metallic and semiconductor elements, such as Si, Al, Ca, Ti, B, Sn, Mg, Li, Co, Ni, Sr, Ce, Zr, Y, Pb, Zn, Ba, and La. For example, one or more of alumina (Al ₂O ₃) , boehmite (γ-AlOOH) , silica (SiO ₂) , titanium oxide (TiO ₂) , cerium oxide (CeO ₂) , calcium oxide (CaO) , zinc oxide (ZnO) , magnesium oxide (MgO) , lithium nitride (Li ₃N) , calcium carbonate (CaCO ₃) , barium sulfate (BaSO ₄) , lithium phosphate (Li ₃PO ₄) , lithium titanium phosphate (LTPO) , lithium aluminum titanium phosphate (LATP) , cerium titanate (CeTiO ₃) , calcium titanate (CaTiO ₃) , barium titanate (BaTiO ₃) and lithium lanthanum titanate (LLTO) can be used as the inorganic material. The at least one inorganic material disclosed herein may have an average particle size ranging, for example, from 0.01 to 10 μm, such as from 0.5 to 5 μm. In some embodiments, the completely-coated layer may comprise from 80 wt%to 100 wt%, such as from 90 wt%to 98 wt%, of the at least one inorganic material.

In some embodiments, the completely-coated layer may comprise two or more sub-layers, which means, the completely-coated layer itself may have a laminated structure. For example, the completely-coated layer may comprise a first sub-layer laminated with a second sub-layer, wherein the first sub-layer may comprise more than 50 wt%of the at least one inorganic material, and the second sub-layer may comprise more than 50 wt%of the at least one organic material.

The thickness of the completely-coated layer on one surface of the porous base membrane may range, for example, from 0.5 μm to 5 μm, such as from 1 μm to 3μm.

In some embodiments, the completely-coated layer may be disposed between the porous base membrane and the partially-coated layer. In some other embodiments, the completely-coated layer and the partially-coated layer may be respectively disposed on two sides of the porous base membrane. Figures 3 through 7 illustrate schematic cross-sectional views of five exemplary separators comprising a partially-coated layer and a completely-coated layer according to five embodiments of the present disclosure.

In Figure 3, the separator 30 has a three-layer structure formed by a porous base membrane 31, a partially-coated layer 33 and a completely-coated layer 35. The completely-coated layer 35 is formed on one surface of the porous base membrane 31, and the partially-coated layer 33 is formed on an outer surface of the completely-coated layer 35. Thus the completely-coated layer 35 is disposed between the porous base membrane 31 and the partially-coated layer 33.

In Figure 4, the separator 40 also has a three-layer structure formed by a porous base membrane 41, a partially-coated layer 43 and a completely-coated layer 46. But different from the separator 30 shown in Figure 3, in the separator 40, the partially-coated layer 43 and the completely-coated layer 46 are respectively disposed on two surfaces of the porous base membrane 41.

In Figure 5, the separator 50 includes a porous base membrane 51, a partially-coated layer 53 and two completely-coated

layers

55 and 56 that are disposed on two surfaces of the porous base membrane 51 respectively. On the outer surface of the completely-coated layer 55, the partially-coated layer 53 is formed. The completely-coated

layers

55 and 56 have the same composition and physical properties. In other embodiments of the present disclosure, the completely-coated

layers

55 and 56 may be different in at least one of compositions and physical properties.

In Figure 6, the separator 60 includes a porous base membrane 61, two partially-coated

layers

63 and 64, and a completely-coated layer 65. The completely-coated layer 65 is formed on one surface of the porous base membrane 61. The partially-coated

layers

63 and 64 are disposed on two sides of the porous base membrane 61 respectively. The completely-coated layer 65 is disposed between the porous base membrane 61 and the partially-coated layer 63. The partially-coated

layers

63 and 64 have the same composition, physical properties and coating pattern. In other embodiments of the present disclosure, partially-coated

layers

63 and 64 may be different in at least one of the compositions, physical properties, and coating patterns.

In Figure 7, the separator 70 includes a porous base membrane 71, two partially-coated

layers

73 and 74, and two completely-coated

layers

75 and 76, each formed on both sides of the porous base membrane 71. The completely-coated

layers

75 and 76 are respectively formed on both surfaces of the porous base membrane 71. The partially-coated-coated

layers

73 and 74 are respectively formed on the outer surface of the completely-coated layer on both sides of the porous base membrane 71. As discussed above, the completely-coated

layers

75 and 76 may have the same composition and physical properties, or may be different in at least one of the compositions and physical properties. The partially-coated

layers

73 and 74 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.

The present disclosure also provides some embodiments of methods for preparing the separator disclosed herein. In one embodiment, the method for preparing the separator 10 of Figure 1 comprises:

(A) preparing a coating slurry comprising at least one adhesive material and at least one solvent;

(B) partially coating one surface of a porous base membrane with the coating slurry prepared in step (A) to obtain a wet partially-coated layer; and

(C) removing the at least one solvent from the wet partially-coated layer to obtain the partially-coated layer.

In the operation (A) , the at least one adhesive material may be, for example, dissolved in the at least one solvent to obtain the coating slurry. The at least one solvent used herein may be chosen, for example, from N-methyl pyrrolidone (NMP) , dimethylacetamide (DMAC) , N, N-dimethylformamide (DMF) , dimethyl sulfoxide (DMSO) , and acetone. The coating slurry may further comprise at least one inorganic filler. The at least one inorganic filler that can be used herein is set forth above.

In the operation (B) , the surface of the porous base membrane is partially coated by the coating slurry, leaving continuous or discontinuous uncoated areas. After the operation (B) , a coating pattern is formed on the surface of the porous base membrane. Various coating techniques, such as gravure coating, blade coating, extrusion coating, spray coating, spin coating, and dip coating, can be used herein.

In the operation (C) , at least a portion of the at least one solvent in the wet partially-coated layer is removed therefrom to obtain the partially-coated layer. With removal of the at least one solvent, a porous structure is formed within the coated areas of the partially-coated layer. The at least one solvent may be removed via various methods, e.g., evaporating by heating, and/or, extracting using water (i.e., immersing the coated porous base membrane in water so that the at least one solvent may be transferred from the wet partially-coated layer into water) .

As discussed above, the separator disclosed herein includes a porous base membrane and a partially-coated layer disposed on at least one side of the porous base membrane. The partially-coated layer comprises coated areas and uncoated areas forming a coating pattern, so it can be easily impregnated by a liquid electrolyte. The coated areas are porous, so the partially-coated layer will not block the ions transfer between the positive electrode and the positive electrode. The coated areas comprise at least one adhesive material, so the partially-coated layer has a binding property. Thus a strong contact interface can be formed between the separator and the electrodes.

Further, the present disclosure provides embodiments of an electrochemical device. The electrochemical device comprises a positive electrode, a negative electrode, and a separator disclosed herein that is interposed between the positive electrode and the negative electrode. A liquid 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 ions (e.g., lithium ions) between the two electrodes. Such electrochemical devices include any devices in which electrochemical reactions occur. For example, the electrochemical device disclosed herein includes primary batteries, secondary batteries, fuel cells, solar cells and capacitors. In some embodiments, the electrochemical device disclosed herein is a lithium secondary battery, such as a lithium metal battery, a lithium ion battery, a lithium polymer battery, a lithium sulfur battery, and a lithium air battery.

The manufacturing process of the electrochemical device disclosed herein can be efficient. At first, as discussed above, the separator of the present disclosure can provide channels for fast impregnation of liquid electrolyte, so that the time for electrolyte impregnation can be shortened. The shortened time means a reduced cost in the manufacturing process. Secondly, as the separator of the present disclosure is adhesive (at least one side of the separator is adhesive) , a firm physical bonding can be formed between the separator and the positive electrode and/or the negative electrode, which may facilitate the manufacturing process as relative movements between the separator and the electrodes can be reduced or avoided.

The electrochemical device disclosed herein may be manufactured by a method known in the art. In one embodiment, a cell is formed by placing a 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.

With the separator of the present disclosure inside, the electrochemical device disclosed herein can exhibit improved cycle life and safety as discussed above.

Reference is 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 1-7 and Comparative Examples 1-3, lithium-ion cells were produced.

Example 1

Preparation of a positive electrode. Lithium cobalt oxide (LCO) , conductive carbon, and PVDF in a weight ratio of 97: 1.5: 1.5 were added into NMP and mixed to obtain a positive electrode slurry. A positive plate was prepared by coating with the positive electrode slurry, compacting, and slitting.

Preparation of a negative electrode. Graphite, conductive carbon, sodium carboxymethyl cellulose, and styrene-butadiene rubber (SBR) in a weight ratio of 97: 0.8: 0.7: 1.5 were added into deionized water and mixed to obtain a negative electrode slurry. A negative plate was prepared by coating with the negative electrode slurry, compacting, and slitting.

Preparation of a non-aqueous electrolyte. LiPF ₆ was added into a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (weight ratio of EC: DEC = 6: 4) to obtain a solution containing 1.0 mol/L of LiPF ₆, which was used as the non-aqueous electrolyte.

Preparation of a separator. A single-layer porous PE membrane having a thickness of 12μm was used as a porous base membrane. To prepare the separator, a completed-coated layer containing Al ₂O ₃ having a thickness of 2μm was formed on one surface of the PE membrane, and a partially-coated layer containing PVDF-HFP copolymer having a thickness of 2μm was formed on the other surface of the PE membrane. The partially-coated layer included a continuous mesh-like coated area and a plurality of irregular-shaped uncoated areas scattered in the coated area.

Cell assembly. The above prepared positive electrode, separator, and negative electrode were winded and placed in an aluminum-plastic packaging bag. Then the above prepared non-aqueous electrolyte was injected into the packaging bag. The packaging bag was kept still at room temperature for a period of time (i.e., Impregnation Time or Soaking Time) , so that the non-aqueous electrolyte could impregnate into the electrodes and the separator. The packaging bag was then sealed after an additional part was cut off. A lithium-ion cell was obtained after a hot-pressing treatment followed by a formation process.

Example 2

The same procedures for preparing the positive electrode, the negative electrode, and the non-aqueous electrolyte as set forth above in Example 1 were used herein.

Preparation of a separator. A single-layer porous PE membrane having a thickness of 12μm was used as a porous base membrane. To prepare the separator, a completed-coated layer containing Al ₂O ₃ having a thickness of 2μm was formed on one surface of the PE membrane, and a partially-coated layer containing PVDF having a thickness of 2μm was formed on the other surface of the PE membrane. The partially-coated layer included stripe-shaped coated areas alternating with stripe-shaped uncoated areas parallel to TD of the separator. Each of the stripe-shaped coated area comprised a mesh-like coated portion and a plurality of dot-like uncoated portions.

Cell assembly. The above prepared positive electrode, separator, and negative electrode were winded, hot pressed, and placed in an aluminum-plastic packaging bag. Then the above prepared non-aqueous electrolyte was injected into the aluminum-plastic packaging bag. The packaging bag was kept still at room temperature for a period of time (i.e., Impregnation Time or Soaking Time) so that the non-aqueous electrolyte could impregnate into the electrodes and the separator. The packaging bag was then sealed after an additional part was cut off. A lithium-ion cell was obtained after a formation process.

Example 3

Preparation of a separator. A single-layer porous PE membrane having a thickness of 12μm was used as a porous base membrane. To prepare the separator, a completed-coated layer containing Al ₂O ₃ having a thickness of 2μm was formed on one surface of the PE membrane, and a partially-coated layer containing PVDF having a thickness of 2μm was formed on the other surface of the PE membrane. The partially-coated layer included stripe-shaped coated areas alternating with stripe-shaped uncoated areas, the length direction of which formed an angle of 45° with the TD of the separator. Each of the stripe-shaped coated area comprised a mesh-like coated portion and a plurality of dot-like uncoated portions.

The same procedures for preparing the positive electrode, the negative electrode, the non-aqueous electrolyte, and the cell assembly as set forth above in Example 2 were used to prepare a lithium-ion cell.

Example 4

Preparation of a separator. A single-layer porous PP membrane having a thickness of 12μm was used as a porous base membrane. To prepare the separator, a completed-coated layer containing Al ₂O ₃ having a thickness of 2μm was formed on one surface of the PP membrane, and a partially-coated layer containing PVDF having a thickness of 2μm was formed on the other surface of the PE membrane. The partially-coated layer included stripe-shaped coated areas alternating with stripe-shaped uncoated areas, the length direction of which formed an angle of 30° with the TD of the separator. Each of the stripe-shaped coated area comprised a plurality of dot-like coated portions and a continuous uncoated portion.

Example 5

Preparation of a separator. A three-layer porous PE membrane having a thickness of 12μm was used as a porous base membrane. To prepare the separator, a completed-coated layer containing Al ₂O ₃ having a thickness of 2μm was formed on one surface of the PE membrane, and a partially-coated layer containing PVDF having a thickness of 1μm was formed on the other surface of the PE membrane. The partially-coated layer included a plurality of diamond-shaped coated areas and a continuous uncoated area. Each of the diamond-shaped coated area comprised a plurality of dot-like coated portions and a continuous uncoated portion.

Example 6

Preparation of a separator. A single-layer porous PE membrane having a thickness of 12μm was used as a porous base membrane. To prepare the separator, a partially-coated layer containing PVDF and Al ₂O ₃ having a thickness of 2μm was formed on both surfaces of the PE membrane. The partially-coated layer included stripe-shaped coated areas alternating with stripe-shaped uncoated areas, the length direction of which formed an angle of 45° with the TD of the separator. Each of the stripe-shaped coated area comprised a plurality of dot-like coated portions and a continuous uncoated portion.

Example 7

Preparation of a separator. A single-layer porous PP membrane having a thickness of 16μm was used as a porous base membrane. To prepare the separator, a completely-coated layer containing Al ₂O ₃ having a thickness of 2μm was formed on both surfaces of the PP membrane. On one side of the PP membrane, a partially-coated layer containing PVDF having a thickness of 1μm was formed on an outer surface of the completely-coated layer. The partially-coated layer included stripe-shaped coated areas alternating with stripe-shaped uncoated areas parallel to TD of the separator. Each of the stripe-shaped coated area comprised a mesh-like coated portion and a plurality of dot-like uncoated portions.

Example 8

Preparation of a separator. A single-layer porous PP membrane having a thickness of 14μm was used as a porous base membrane. To prepare the separator, a completely-coated layer containing Al ₂O ₃ having a thickness of 2μm was formed on both surfaces of the PP membrane. On each of both sides of the PP membrane, a partially-coated layer containing PVDF having a thickness of 1μm was formed on an outer surface of the completely-coated layer. The partially-coated layer included stripe-shaped coated areas alternating with stripe-shaped uncoated areas, the length direction of which formed an angle of 30° with the TD of the separator. Each of the stripe-shaped coated area comprised a mesh-like coated portion and a plurality of dot-like uncoated portions.

Comparative Example 1

A single-layer porous PE membrane having a thickness of 12μm was used as the separator directly. The same procedures for preparing the positive electrode, the negative electrode, the non-aqueous electrolyte, and the cell assembly as set forth above in Example 2 were used to prepare a lithium-ion cell.

Comparative Example 2

Preparation of a separator. A single-layer porous PE membrane having a thickness of 12μm was used as a porous base membrane. A completely-coated layer containing PVDF having a thickness of 2μm was formed on both surfaces of the PE membrane to obtain the separator.

Comparative Example 3

Preparation of a separator. A single-layer porous PE membrane having a thickness of 12μm was used as a porous base membrane. A partially-coated layer containing Al ₂O ₃ having a thickness of 2μm was formed on both surfaces of the PE membrane to obtain the separator.

The separators and lithium-ion cells prepared in Examples 1-8 and Comparative Examples 1-3 were tested according to the following Tests 1-6. The testing results were shown in Table 1.

Test 1 Impregnation Time

In Example 1, the time period between the non-aqueous electrolyte injection and the hot-pressing treatment was recorded as Impregnation Time. In Examples 2-8 and Comparative Examples 1-3, the time period between the non-aqueous electrolyte injection and the formation process was recorded as Impregnation Time.

Test 2 Electrolyte Retention Amount

The Electrolyte Retention Amount was calculated by:

Electrolyte Retention Amount = Weight of the lithium-ion cell after seal -weight of the lithium-ion cell before electrolyte injection + weight of the additional part of the packaging bag that was cut

Test 3 Adhesiveness of Two Surfaces of the Separator

After being fully charged, the lithium-ion cell is disassembled to check the adhesiveness of the two surfaces of the separator.

Test 4 Retention Rate after 800 Cycles

At 25℃, the lithium-ion cell is subjected to 800 cycles of charge and discharge at 1C/1C. The Retention Rate after 800 Cycles is calculated by:

Retention Rate after 800 Cycles = C1/C0×100%,

wherein C0 is the initial capacity of the lithium-ion cell, and C1 is the capacity of the lithium-ion cell after 800 cycles of charge and discharge.

Test 5 Whether Deformed after 800 Cycles

After 800 cycles of charge and discharge test in Test 4, the lithium-ion cell is observed with naked eyes to check if it is deformed.

Test 6 Nail Test

A nail having a diameter of 3 mm is penetrated into the lithium-ion cell from top with a speed of 50 mm/sand kept in the lithium-ion cell for ten minutes. The lithium-ion cell is observed with naked eyes to check if it is smoked, exploded or get on fire.

Table 1

In Comparative Example 1, a single-layer porous PE membrane was used as the separator. As the PE membrane has very limited affinity to the non-aqueous electrolyte, it is difficult to be wetted by the non-aqueous electrolyte. As shown in Table 1, it took 48 hours for electrolyte impregnation. The PE membrane had a low Electrolyte Possessing Capacity. In addition, the PE membrane could not form a strong contact surface with the electrodes as it did not have adhesive or binding property. The lithium-ion cell of Comparative Example 1 was deformed after 800 cycles charge/discharge test and exploded in Nail Test, indicating it had short cycle life and serious safety issue.

The separator prepared in Comparative Example 2 comprised a PE membrane and a completely-coated layer containing PVDF on its both sides, so it was adhesive to the positive electrode and the negative electrode and could form strong contact interfaces with the electrodes. The lithium-ion cell of Comparative Example 2 was not deformed after 800 cycles maybe because of the strong contact interfaces. As the separator did not provide any channel for electrolyte impregnation, it took a long period of time, i.e., 96 hours, to impregnate the separator with the non-aqueous electrolyte. The separator also had a low Electrolyte Possessing Capacity. The lithium-ion cell had a very low retention rate (i.e., 68%) after the 800 cycles charge/discharge test.

The separator prepared in Comparative Example 3 comprised a PE membrane and a partially-coated layer containing Al ₂O ₃ on its both sides. As the separator provided channels for electrolyte impregnation, the Impregnation Time was reduced greatly. However, as the partially-coated layer of Al ₂O ₃ was not adhesive and could not form strong contact surfaces with the electrodes, the lithium-ion cell was seriously deformed after the 800 cycles charge/discharge test.

In Example 1, during the production of the lithium-ion cell, the non-aqueous electrolyte was injected into the electrode assembly before the electrode assembly was hot pressed. As the electrodes and the separator had not been bonded before the hot pressing, there were sufficient flow channels and spaces for electrolyte impregnation, so the Impregnation Time in Example 1 was short. The electrolyte retention amount of the lithium-ion cell was as high as 13.2 g due to the existence of the scattered uncoated areas in the partially-coated layer of the separator. During the charge/discharge cycles, the non-aqueous electrolyte in the uncoated areas could spread into the cell where an additional electrolyte was needed. Therefore, the lithium-ion cell had a high Retention Rate after 800 cycles of charge and discharge. In addition, the cell was not deformed because local electrolyte insufficiency was minimized.

Each of the separators prepared in Examples 2-5 comprised a polyolefin-based membrane, a completely-coated layer containing Al ₂O ₃ formed on one surface of the polyolefin-based membrane, and a partially-coated layer containing PVDF on the other surface of the polyolefin-based membrane. Therefore, each of the separators prepared in Examples 2-5 had one adhesive surface as the partially-coated layer containing PVDF was adhesive. The separator prepared in Example 7 also had one adhesive surface. The separators prepared in Examples 6 and 8 had two adhesive surfaces so they could form strong contact surfaces with both the positive electrode and the negative electrode.

In Examples 1-8, with the presence of the partially-coated layer containing PVDF, the separators prepared in Examples 1-8 could be impregnated by the non-aqueous electrolyte quickly. Therefore, the Impregnation Time was decreased in comparison with those in Comparative Examples. The test results in Table 1 showed that all the coating patterns used in Examples 1-8 could increase the speed of the electrolyte impregnation.

The lithium-ion cells prepared in Examples 1-8 had increased retention rate after 800 charge/discharge cycles and improved safety.

Claims

A separator for an electrochemical device, comprising:

a porous base membrane; and

at least one partially-coated layer disposed on at least one side of the porous base membrane, wherein the coated area (s) and the uncoated area (s) of the at least one partially-coated layer are in an alternating coating pattern, and further wherein the coated area (s) are porous and comprise at least one adhesive material.
The separator according to claim 1, wherein either the coated areas or the uncoated areas are discontinuous.
The separator according to claim 1, wherein at least one of the coated areas and the uncoated areas has a shape of dot, circle, triangle, square, diamond, rectangle, stripe or mesh.
The separator according to claim 1, wherein either the coated area (s) or the uncoated area (s) are mesh-like.
The separator according to claim 1, wherein the coated areas and uncoated areas are stripe-shaped and distributed in parallel.
The separator according to claim 1, wherein at least one of the coated areas comprises coated portions and uncoated portions distributed alternately.
The separator according to claim 1, wherein the uncoated area (s) cover from 1%to 50%of the entire area of the partially-coated layer.
The separator according to claim 1, wherein the at least one adhesive material is a polymer chosen from polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-tetrafluoroethylene, polyacrylic acid, polyacrylate, polyacrylate salt, polymethacrylate, polymethylmethacrylate, polyethylene oxide, polyester, polyolefin, aramid, polyimide, polyethersulfone, and polysulfone.
The separator according to claim 1, wherein the partially-coated layer comprises from 20 wt%to 100 wt%of the at least one adhesive material.
The separator according to claim 1, wherein the partially-coated layer further comprises at least one inorganic filler.
The separator according to claim 10, wherein the at least one inorganic filler is chosen from alumina, boehmite, silica, titanium oxide, cerium oxide, calcium oxide, zinc oxide, magnesium oxide, lithium nitride, calcium carbonate, barium sulfate, lithium phosphate, lithium titanium phosphate, lithium aluminum titanium phosphate, cerium titanate, calcium titanate, barium titanate, and lithium lanthanum titanate.
The separator according to claim 1, wherein the partially-coated layer on one side of the porous base membrane has a thickness ranging from 0.3 μm to 6 μm.
The separator according to claim 1, wherein the porous base membrane comprises a material chosen from polyethylene, high density polyethylene, polypropylene, polyamide, polyacrylonitrile, polyethylene terephthalate, and viscose fiber.
The separator according to claim 1, further comprises:

at least one completely-coated layer disposed on at least one surface of the porous base membrane, wherein the completely-coated layer comprises at least one organic material and/or at least one inorganic material.
The separator according to claim 14, wherein the completely-coated layer comprises from 80 wt%to 100 wt%of the at least one inorganic material.
The separator according to claim 14, wherein the porous base membrane comprises nonwoven fabrics.
The separator according to claim 14, wherein the at least one organic material is chosen from polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-tetrafluoroethylene, polyacrylic acid, polyacrylate, polyacrylate salt, polymethacrylate, polymethylmethacrylate, polyethylene oxide, polyester, polyolefin, aramid, polyimide, polyethersulfone, and polysulfone.
The separator according to claim 14, 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 14, wherein the completely-coated layer on one surface of the porous base membrane has a thickness ranging from 0.5 μm to 5 μm.
An electrochemical device comprising a positive electrode, a negative electrode, and a separator according to claim 1 interposed between the positive electrode and the negative electrode.