CN110660948B

CN110660948B - Isolation membrane, preparation method thereof and electrochemical device containing isolation membrane

Info

Publication number: CN110660948B
Application number: CN201810712595.1A
Authority: CN
Inventors: 盛长亮; 黄华锋; 黄起森
Original assignee: Contemporary Amperex Technology Co Ltd
Current assignee: Contemporary Amperex Technology Co Ltd
Priority date: 2018-06-29
Filing date: 2018-06-29
Publication date: 2022-06-10
Anticipated expiration: 2038-06-29
Also published as: CN110660948A

Abstract

The application relates to the field of energy storage, in particular to an isolating membrane, a preparation method thereof and an electrochemical device containing the isolating membrane. The isolating membrane comprises a porous substrate and a discontinuous inorganic layer which is arranged on the surface of at least one side of the porous substrate and only partially covers the surface, wherein the inorganic layer does not contain a binder, and the inorganic layer at least partially covers the surface of the pores in the substrate in the inner area of the substrate corresponding to the surface part covered by the inorganic layer. The isolating membrane has strong flexibility, the inorganic layer is not easy to have brittle fracture or the membrane layer is not easy to fall off during winding and bending, the mechanical processing performance is good, meanwhile, the isolating membrane has good wetting property with electrolyte and low thermal shrinkage rate, the electrical core using the isolating membrane has good dynamic performance and cycle performance, and the long-term reliability of the electrical core is favorably improved.

Description

Isolation membrane, preparation method thereof and electrochemical device containing isolation membrane

Technical Field

The invention relates to the field of energy storage, in particular to an isolating membrane, a preparation method thereof and an electrochemical device containing the isolating membrane.

Background

In the internal structure of the battery, the isolation film is used as a key component, is usually a porous polymer film, has the characteristics of electronic isolation and ion conduction, and is used for ensuring that ions can be normally transmitted between a positive electrode and a negative electrode without short circuit. In recent years, in order to solve the problems of high thermal shrinkage and poor electrolyte wettability of a single polymer separator, a composite separator in which a ceramic coating is applied to the surface of a polymer substrate is becoming a key technology for improving the safety performance of a battery. However, the composite isolating membrane still has some problems and needs further development and research.

The ceramic coating of most isolating membranes is obtained by mixing inorganic particles and a binder to form slurry, coating and drying. Although the particle dispersion effect can be improved by adding the surfactant, the problems of particle agglomeration, uneven component distribution and wettability of the substrate and the coating in the coating are difficult to completely solve. On the other hand, since the ceramic layers on both sides of the substrate usually reach several micrometers, the thickness of the separator film increases by at least 50% or more, resulting in a decrease in the energy density of the overall cell. Meanwhile, the ceramic layer and the polymer substrate are mainly bonded by the adhesive, the bonding force of different areas is obviously affected by the distribution of the adhesive, and the cracking and aging of the surface of the ceramic layer, the change of porosity, the falling of ceramic particles and the like are easily caused in the coating process, long-term circulation or abuse of batteries and the like, so that the problems of poor ion conducting performance, piercing of an isolating membrane and the like are caused, the risks of poor consistency of a battery core, reduction of long-term reliability and the like are caused, and even the safety problem is easily caused in severe cases.

Disclosure of Invention

Therefore, a separator which has improved heat shrinkage performance and improved wettability with an electrolyte solution, has good mechanical properties, is less likely to be brittle or come off, and can be used for a long period of time is desired.

In view of this, the invention proposes a separator comprising a substrate having a porous structure, on the surface of at least one side of which an inorganic layer is provided that only partially covers said surface, said inorganic layer consisting of several discontinuous regions, said inorganic layer containing an inorganic dielectric material and being free of a binder; and the inorganic layer covers at least a part of the surface of the internal pores of the substrate in the area corresponding to the surface part covered by the inorganic layer.

The second aspect of the present invention provides a method for preparing the above-mentioned first aspect of the present invention, which at least comprises the following steps: providing a substrate having a porous structure; preparing a binder-free discontinuous inorganic layer partially covering at least one surface of the substrate by vapor deposition; the inorganic layer covers at least a part of the surface of the internal pores of the substrate in the area corresponding to the surface part covered by the inorganic layer.

The third aspect of the present invention also provides an electrochemical device comprising the separator.

The technical scheme of the invention has at least one or more of the following beneficial effects:

the isolating membrane has strong flexibility, the inorganic layer is not easy to brittle fracture or fall off during winding and bending, the mechanical processing performance is good, and meanwhile, the isolating membrane has high wettability with electrolyte and low thermal shrinkage rate, so that a battery cell using the isolating membrane has good dynamic performance and cycle performance, and the long-term reliability of the battery cell is improved.

Drawings

FIG. 1 is a schematic view of a separator according to an embodiment of the present invention.

FIG. 2 shows a 5X 10 separator according to an embodiment of the present invention³A photograph of a scanning electron microscope at magnification.

Fig. 3 shows experimental results of a separator wettability test for a certain example of the present invention and a comparative example.

Detailed Description

First, the separator according to the first aspect of the present invention is explained.

A separator according to a first aspect of the present invention comprises a substrate having a porous structure, and a discontinuous inorganic layer provided on a surface of at least one side of the substrate so as to cover only a part of the surface, the inorganic layer containing an inorganic dielectric material and being free of a binder; the inorganic layer covers at least a part of the surface of the internal pores of the substrate in the corresponding internal area of the substrate covered by the inorganic layer.

In the present invention, the "non-continuous inorganic layer" means that two different regions, a covered region covered with the inorganic layer and a non-covered region not covered with any inorganic layer, are clearly distributed on the surface of the separator, and the covered regions are discrete and not connected to each other. Compared with a pure polymer isolation membrane without an inorganic layer or a traditional organic/inorganic composite isolation membrane with a continuous or basically continuous dense inorganic layer or loose inorganic layer covered on the surface of a polymer substrate, the isolation membrane has the following outstanding characteristics.

Firstly, the surface of at least one side of the isolation film substrate is provided with the inorganic layer which is distributed discontinuously, so that on one hand, the surface polarity of the polymer substrate can be changed by utilizing the insulativity and the hydrophilicity of the inorganic material on the premise of ensuring the electronic insulation, and the wettability of the isolation film to the electrolyte is improved; meanwhile, the inorganic layer is distributed in a discontinuous manner, so that the problems of poor insulating property and even safety caused by the falling of a local inorganic film layer due to the fact that the inorganic layer is easy to break or damage in the winding or processing process because of high brittleness in a continuous state are avoided.

And secondly, the invention does not depend on the adhesion of the adhesive and the porous substrate, thereby avoiding the falling off of inorganic particles caused by the swelling failure of the adhesive in the circulation process of the inorganic layer on the surface of the isolating membrane.

Third, although only a part of the surface of the separator in the present invention is covered with the inorganic layer, since the inorganic layer is not only distributed on the surface of the substrate but also extends into the substrate to cover at least a part of the surface of the internal pores of the substrate, a certain proportion of the surface of the substrate (including the outer surface and the inner wall of the internal pores) is tightly covered with the inorganic layer. Thus, when the separator is heated, the covered area on the surface of the base material is subjected to reverse acting force from all directions of the inorganic layer to inhibit further shrinkage, so that the heat shrinkage performance of the separator is improved, and the safety performance of the battery cell can be effectively improved while the mechanical strength and the machining performance of the separator are ensured to be good.

In a preferred embodiment of the separator of the present invention, the inorganic layer extends along the surface of the substrate at the surface portion covered by the inorganic layer, and the pore morphology of the substrate is still maintained. That is, the inorganic layer as a whole is composed of a plurality of discrete regions of a shape like a Chinese character 'mi' that are not connected to each other. Therefore, the inorganic layers arranged on the surface of the substrate and at least one part of the inner wall of the pore extend along the surface of the substrate, the shape of the pore of the substrate is kept without blocking the pore, the air permeability of the isolating membrane can be improved, ion shuttling in the circulation process is facilitated, and the dynamic performance of the battery cell is improved.

In another preferred embodiment of the separator of the present invention, the inorganic layer is a discontinuous island-like or sheet-like film layer distributed on the surface of the substrate. That is, the inorganic layer substantially covers the pores of the substrate at the surface portion covered by the inorganic layer, so that the inorganic layer is composed of a plurality of isolated islands or a sheet-like film layer as a whole. For example, fig. 1 shows a schematic surface view of a separator having an island distribution inorganic layer according to the present invention, wherein a discontinuous island distribution inorganic layer only partially covering a surface of a substrate having a porous structure is disposed on the surface.

As a preferred embodiment of the separation film of the present invention, part of the surface of the substrate and at least a part of the inner wall of the hole of the corresponding inner area of the surface are covered with the inorganic layer, and the thickness of the part of the substrate in which the inner hole surface of the substrate is covered with the inorganic layer is 0.4% to 40% of the total thickness of the substrate. Therefore, although only a part of the surface of the separator is covered with the inorganic layer, the inorganic layer is not only distributed on the surface of the substrate but also extends into the substrate to a certain depth, so that a certain proportion of the surface of the substrate (including the outer surface and the inner wall of the internal pores) is tightly coated with the inorganic layer. When the isolating film is heated, the covered area on the surface of the base material is subjected to reverse acting forces from all directions of the inorganic layer to inhibit further shrinkage, so that the isolating film hardly undergoes thermal shrinkage (< 3%) in the transverse direction and the longitudinal direction after being placed at 90 ℃ for 1 hour, and the safety performance of the battery cell can be effectively improved while the mechanical strength and the machining performance of the isolating film are good.

In a preferred embodiment of the separator of the present invention, the number of the discontinuous regions of the inorganic layer per unit area of the surface of the substrate is 100/mm²2000 pieces/mm². The distribution density of the non-continuous covering inorganic layer per unit area of the substrate may affect the thermal shrinkage and the mechanical processability of the separator. If the number of discontinuous regions of the inorganic layer per unit area of the substrate surface is too low (less than 100/mm)²) The area covered by the inorganic layer on the surface of the base material is too sparse, the thermal shrinkage rate of the isolating membrane is greatly influenced by the base material, and the large-area shrinkage occurs when the electric core generates heat abnormally, so that the cathode and the anode are in direct contact with a short circuit to release heat in a large quantity, and thermal runaway is caused. If the number of discontinuous regions of the inorganic layer per unit area of the substrate surface is too high (more than 2000/mm)²) The area of the substrate surface covered by the inorganic layer is too dense, and the film breaking caused by bending, winding and other operations in the subsequent processing and using processes and the like can not be effectively avoidedLocal falling off.

As a preferred embodiment of the separator of the present invention, the inorganic layer is distributed on the surface of the substrate in the form of a discontinuous island-like or sheet-like film layer, and the ratio of the total area covered by the inorganic layer outside the substrate to the entire area outside the substrate is 0.4 to 0.9. The inorganic layer on the surface of the base material does not contain a binder, the main component is an inorganic dielectric material, and the flexibility, the machining performance, the heat shrinkage rate and the hydrophilicity of the isolating film are closely related to the total covered area of the inorganic layer on the outer surface of the base material. If the area covered by the inorganic layer on the outer surface of the substrate is too large (the ratio of the area relative to the whole outer layer is higher than 0.9), the film layer may be broken and partially peeled off due to the operations of subsequent processing, bending, winding and the like during use. However, if the area covered by the inorganic layer on the surface of the substrate is too small (the ratio of the area to the entire outer layer is less than 0.4), although the flexibility and the machinability of the separator can be ensured, the inorganic layer on the surface of the substrate is too small, and therefore, the shrinkage of the substrate due to heat cannot be effectively suppressed, and the wettability of the interface is also poor.

As a preferred embodiment of the separator of the present invention, the ratio of air permeability (Gurley) G1 of the separator to air permeability G2 of the substrate satisfies: 1< G1/G2 ≦ 3, preferably satisfying: 1< G1/G2< 1.4. According to the invention, the discontinuous inorganic layers are arranged on the surface of the substrate with the porous structure and the inner wall of at least one part of pores, and the ratio of the air permeability of the formed isolation membrane to the air permeability of the substrate is not higher than 3, so that the situation that the surface of the substrate is excessively sealed after the inorganic material layer is arranged can be avoided, the insulating property of the isolation membrane after the composition is improved, the form of the original substrate is basically maintained, the rapid transmission of ions can be ensured, and the safety and the dynamic performance are good.

As a preferred embodiment of the separator of the present invention, the separator has a heat shrinkage of less than 3%, preferably less than 2%, in the transverse and longitudinal directions after standing alone at 90 ℃ for 1 hour. The thermal shrinkage performance of the isolating film is closely related to the safety performance of the battery cell, and because only part of the area of the substrate of the isolating film is provided with the inorganic layer, if the thermal shrinkage rate of the isolating film is too high at 90 ℃, the substrate is more prone to shrinkage failure under the abnormal conditions of higher temperature such as thermal runaway and the like, the risk of effective electronic insulation cannot be increased, and the safety performance of the battery cell cannot be guaranteed.

As a preferred embodiment of the separator of the present invention, the separator has tensile strengths in the transverse and longitudinal directions of not less than 2000kgf/cm²More preferably not less than 2100kgf/cm². In the manufacturing process of the battery core, usually, the positive plate, the isolation film and the negative plate are sequentially stacked and wound (winding process), or the positive plate, the negative plate and the isolation film are cut to target sizes and sequentially stacked (lamination process), and in the processing process, the rolled isolation film needs to be stretched and flattened under a certain tensile force, so that a wrinkle area is avoided. Because the inorganic material has a low elastic modulus and is easily broken under the action of tensile force, if the transverse and longitudinal tensile strength of the isolating membrane is too low, the tensile resistance of the discontinuous inorganic layer arranged on the surface of the isolating membrane is poor, and the discontinuous inorganic layer is easily broken or falls off in the processing and using processes, so that the safety performance of the battery cell is deteriorated.

As a preferred embodiment of the separator of the present invention, the inorganic layer has a thickness of 5nm to 1000nm, and the inorganic layer may have a thickness of 1000nm, 990nm, 950nm, 900nm, 850nm, 800nm, 750nm, 720nm, 700nm, 680nm, 650nm, 600nm, 550nm, 500nm, 490nm, 450nm, 430nm, 400nm, 380nm, 350nm, 300nm, 280nm, 250nm, 200 nm; the lower limit of the thickness of the inorganic layer may be 5nm, 8nm, 10nm, 12nm, 15nm, 18nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm, 55nm, 60nm, 65nm, 70nm, 75nm, 80nm, 85nm, 90nm, 95nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190 nm. The range of the inorganic layer may consist of any number of the above upper or lower limits. If the inorganic layer on the surface of the base material is too thin, the thermal shrinkage rate of the isolating film cannot be effectively reduced, and the electrolyte wettability of the isolating film is not obviously improved; if the inorganic layer is too thick, pores on the surface or inside of the substrate are filled with the inorganic layer, which brings certain adverse effect on the air permeability of the isolating membrane, further reduces the rate capability of a battery using the isolating membrane, increases the thickness and mass of the isolating membrane, and reduces the volume energy density and mass energy density of the battery.

Preferably, the thickness of the inorganic layer is 10nm to 500 nm; further preferably, the thickness of the inorganic layer is 20nm to 200 nm. Within the range, the wettability of the isolating membrane and the electrolyte, the thermal shrinkage rate and the air permeability at 90 ℃ are not influenced, the thickness and the mass of the isolating membrane are slightly increased, and the mass energy density and the volume energy density of the battery cell are improved.

In a preferred embodiment of the separator of the present invention, the inorganic layer of the present invention is formed by random stacking of nanoclusters (the nanoclusters are crystalline or amorphous nanoparticles), and the gaps between the plurality of nanoclusters microscopically constitute pores for shuttling ions. The pores have an average pore diameter of 0.1nm to 20 nm. If the average pore diameter of the pores is too small, the air permeability of the isolating membrane is easily too low, the ion transmission performance is affected, and the dynamic performance of the battery using the isolating membrane is poor; if the average pore diameter of the pores is too large, on the basis of ensuring the porosity, the inorganic layer structure is too loose, the collapse of the film structure is easy to happen, so that partial film falls off, the mechanical property of the isolating film is poor, and the reliability of the battery is reduced in long-term use.

As a preferable embodiment of the separator of the present invention, AlO is contained in the inorganic layer_x(1.45 ≦ x ≦ 1.55) as the inorganic dielectric material. The specific amount of X can be calculated by accurately analyzing the element contents of Al and O in the inorganic layer material by adopting an X-ray photoelectron spectroscopy (XPS) technology. Due to the poor heat resistance of the polymer substrate, the substrate is preferably not heated or heated at a temperature much lower than the melting point during the vapor deposition process, and the energy available for the gaseous precursors to react to form new species is low, resulting in deviation of the stoichiometry of the prepared inorganic layer, resulting in slightly poor electrical and mechanical properties of the inorganic layer. When the partial stoichiometric ratio or the super stoichiometric ratio of alumina is too high (x)<1.45 or x>1.55) of insulating material, the insulating properties and chemical stability of which are impairedWhen the metal oxide is used for a protective layer on the surface of a separator, the metal oxide is easily corroded and decomposed in an electrolyte environment, so that the thermal shrinkage rate of the separator is improved, and the mechanical strength of the separator is reduced.

As a preferable embodiment of the separator of the present invention, AlO is contained in the inorganic layer_xIs 50 to 100%, preferably 80 to 100%, of the mass of the inorganic layer. The inorganic layer contains AlO _xIn addition, other inorganic substances may be contained.

In a preferred embodiment of the barrier film of the present invention, the inorganic layer further contains at least one of an oxide of Si, a nitride of Si, an oxide of Ti, a nitride of Ti, an oxide of Zn, a nitride of Zn, an oxide of Mg, a nitride of Mg, an oxide of Zr, a nitride of Zr, an oxide of Ca, a nitride of Ca, an oxide of Ba, and a nitride of Ba. Specific examples are: silicon oxide, titanium oxide, magnesium oxide, zirconium oxide, calcium oxide, zinc oxide, barium oxide, and silicon nitride.

As a preferred embodiment of the separator of the present invention, the inorganic layer is prepared by Vapor Deposition (Vapor Deposition). The inorganic layer prepared by the vapor phase method has stable structure and property, the film layer of the formed inorganic layer has good uniformity, and the thickness deviation can be controlled within +/-5%. And the molecules of the inorganic layer are combined with the porous base material through chemical bonds, so that the inorganic layer and the porous base material have strong bonding force and are not easy to peel off, and the bonding force of the inorganic layer and the porous base material is not lower than 30N/m when tested by a tape method. Therefore, the thickness of the inorganic layer can be reduced to a certain degree by adopting a gas phase method, the bonding force with the porous substrate is improved, and the good wettability and air permeability of the isolating film and the electrolyte and low thermal shrinkage at 90 ℃ are ensured.

As a preferred embodiment of the separation film of the present invention, Vapor Deposition methods include an Atomic Layer Deposition (ALD), a Chemical Vapor Deposition (CVD), a Physical Vapor Deposition (PVD), and a Thermal Evaporation (Thermal Evaporation). Preferably, a plasma-assisted thermal evaporation deposition method, a reactive ion beam sputtering deposition method, an electron beam evaporation method, a magnetron sputtering method, a plasma arc plating method can be used.

As a preferred embodiment of the separator of the present invention, the material of the substrate is selected from at least one of Polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), aramid, polyethylene terephthalate (PET), Polytetrafluoroethylene (PTFE), Polyacrylonitrile (PAN), Polyimide (PI), polyamide, polyester, and natural fiber.

As a preferable embodiment of the separator of the present invention, the thickness of the substrate is 3 μm to 20 μm, preferably 5 μm to 15 μm, and more preferably 5 μm to 12 μm. The inorganic layer can be uniformly and densely coated on the surface of the base material fiber, so that the thickness of the base material does not need to be increased to improve the heat shrinkage resistance and the mechanical strength of the isolating membrane.

As a preferable embodiment of the separator of the present invention, the porosity of the substrate is 20% to 60%, preferably 30% to 50%, and more preferably 35% to 45%.

It will be appreciated by a person skilled in the art that the different preferred embodiments of the invention described above, which have been described above, set forth some preferred features of the solution according to the invention from different aspects and different points of view, can be combined with each other and thus have a combined advantageous technical effect. Accordingly, the present invention covers various combinations of parameters or features (e.g., substrate material, substrate thickness, inorganic layer material, inorganic layer distribution and distribution density, inorganic layer thickness and micro-morphology, etc.) referred to in the above preferred embodiments, and all combinations are considered to be disclosed in the present specification and fall within the scope of the present invention.

Next, a method for producing a separator according to the second aspect of the present invention is explained.

Step 1) providing a substrate having a porous structure;

and 2) preparing a discontinuous inorganic layer which partially covers the surface and does not contain the binder on the surface of at least one side of the substrate by adopting a vapor deposition method, wherein the inorganic layer at least covers a part of the surface of the internal pores of the substrate in the internal area of the substrate corresponding to the surface part covered by the inorganic layer.

The discontinuous inorganic layer may be formed on the substrate by using a patterned mask or baffle over the substrate. Alternatively, in the vapor deposition method using charged ions (such as magnetron sputtering, plasma enhanced chemical vapor deposition, etc.), a conductive substrate having holes locally or a magnet capable of generating a micro magnetic field locally is provided on the back surface of the substrate, so that a discontinuous inorganic layer can be formed on the substrate in a random distribution.

For example, the method of preparing the inorganic layer in the present invention is described by taking a plasma-assisted thermal evaporation deposition technique as an example. In the plasma-assisted thermal evaporation deposition, the heating source is an electron beam, and the heating target is a corresponding element (such as Al, Si, Mg and the like) except oxygen in the inorganic layer. Under vacuum condition, using oxygen-containing activated gas (such as oxygen, ozone, oxygen ion, nitric oxide, nitrogen dioxide, carbon dioxide, water vapor, etc.) as reactive gas, and controlling substrate temperature, adjusting heating current (such as 50A-300A), and vacuum degree of process chamber (such as 10A)^-1To 10^-3Pa), oxygen flow (e.g., 200 seem to 500 seem), plasma power (e.g., 300W to 600W), and process time, the deposition rate of the inorganic layer on the surface of the porous substrate can be adjusted, further adjusting the thickness, composition, and micro-topography of the inorganic layer. By using a patterned mask in the above-described plasma-assisted thermal evaporation deposition technique, a regularly distributed discontinuous inorganic layer can be formed on the substrate surface.

As a preferred embodiment of the preparation method of the present invention, in step 1, the Vapor Deposition method includes an Atomic Layer Deposition method (ALD), a Chemical Vapor Deposition method (CVD), a Physical Vapor Deposition method (PVD), and a Thermal Evaporation method (Thermal Evaporation Deposition). Preferably, a plasma-assisted thermal evaporation deposition method, a reactive ion beam sputtering deposition method, an Electron Beam Evaporation Method (EBEM), a Magnetron sputtering method (Magnetron sputtering), and a plasma arc plating method can be used.

As a preferred embodiment of the preparation method of the present invention, before the vapor deposition method is performed, the substrate is subjected to a surface pretreatment, which may be specifically one or more selected from plasma activation, corona pretreatment, chemical pretreatment, and electron beam pretreatment, and is preferably subjected to a plasma or electron beam pretreatment.

Again, an electrochemical device according to the third aspect of the invention is explained. The electrochemical device of the present invention may be one of a lithium ion secondary battery, a lithium primary battery, a sodium ion battery, and a magnesium ion battery, but is not limited thereto. The structure and composition of these electrochemical devices are the same as those known in the art, except that the separator of the present invention is used; such structures and compositions are also well known in the art.

For example, a lithium ion secondary battery according to the present invention generally includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte enclosed in a case; wherein the separator is the separator of the first aspect of the present invention. The material of the release film substrate is not particularly limited, and may be a polymer release film, for example, one selected from polyethylene, polypropylene, and an ethylene-propylene copolymer. Plasma-assisted thermal evaporation deposition techniques may be employed to form a discontinuous inorganic layer on a substrate. The thickness of the separator film can be formed to constitute a lithium ion secondary battery according to standard techniques. The present invention has no special requirement for the positive electrode active material used in the lithium ion secondary battery, and can be various common positive electrode active materials, such as layered lithium transition metal oxide, lithium-rich manganese-based oxide, lithium iron phosphate, lithium cobaltate, or doped or coated positive electrode active materials thereof.

The following examples and comparative examples further illustrate the separator of the present invention, its preparation method, and a lithium ion secondary battery using a lithium ion secondary battery as an example. These examples are merely illustrative of the present invention, and the present invention is not limited to the following examples. The technical scheme of the invention is to be modified or replaced equivalently without departing from the scope of the technical scheme of the invention, and the technical scheme of the invention is covered by the protection scope of the invention.

Examples and comparative examples

Preparation of the separator

The isolating film of each embodiment adopts a polyethylene film as a substrate.

Then, a plasma-assisted thermal evaporation deposition technology is adopted, and a mask or a baffle with a certain shape is arranged above the base material to prepare the isolation film with the surface containing the discontinuous inorganic layer. Wherein the heating source is electron beam, the heating target is Al, oxygen is used as reaction gas under vacuum condition, the temperature of the substrate is controlled to be less than 100 ℃, and the heating current (50A to 300A) and the vacuum degree (10) of the process chamber are adjusted^-1To 10^-3Pa), oxygen flow (200sccm to 500sccm), plasma power (300W to 600W) and process time, and the deposition rate of the inorganic layer on the surface of the porous substrate is adjusted, so as to further adjust the thickness, the composition and the microscopic morphology of the inorganic layer.

Preparation of positive pole piece

Mixing a positive electrode active material (LiNi)_0.8Co_0.1Mn_0.1O₂) The conductive agent acetylene black (SP) and the binder polyvinylidene fluoride (PVDF) are mixed, and the weight ratio of the mixture of the conductive agent acetylene black (SP) to the binder polyvinylidene fluoride (PVDF) is 96: 2: 2. adding solvent N-methyl pyrrolidone, and mixing and stirring uniformly to obtain the anode slurry. And uniformly coating the positive electrode slurry on a positive electrode current collector aluminum foil, drying at 85 ℃, then carrying out cold pressing, trimming, cutting into pieces and slitting, and drying for 4 hours at 85 ℃ under a vacuum condition to obtain a positive electrode piece.

Preparation of negative pole piece

Mixing a negative electrode active substance (artificial graphite), a conductive agent acetylene black, a binder Styrene Butadiene Rubber (SBR), a thickening agent sodium carboxymethyl cellulose (CMC) according to a weight ratio of 96: 1: 2: 1, adding solvent deionized water, and stirring and mixing uniformly to obtain the cathode slurry. And uniformly coating the negative electrode slurry on a copper foil of a negative electrode current collector, drying at 80-90 ℃ after coating, cold pressing, trimming, cutting into pieces, slitting, and drying for 4 hours at 110 ℃ under a vacuum condition to obtain a negative electrode pole piece.

Preparation of the electrolyte

Preparing a basic electrolyte, wherein the basic electrolyte comprises dimethyl carbonate (DMC), Ethyl Methyl Carbonate (EMC) and Ethylene Carbonate (EC), and the mass ratio of the dimethyl carbonate to the ethylene carbonate is 5: 2: 3. then, an electrolyte salt lithium hexafluorophosphate was added so that the concentration of lithium hexafluorophosphate in the electrolyte solution was 1 mol/L.

Preparation of lithium ion secondary battery

The negative pole piece, the isolation film and the positive pole piece are sequentially stacked, the isolation film is positioned between the positive pole piece and the negative pole piece, the coating on the surface of one side of the isolation film faces the positive pole piece, and then the isolation film is wound into a square bare cell with the thickness of 8mm, the width of 60mm and the length of 130 mm. And (2) filling the bare cell into an aluminum foil packaging bag, baking for 10h at 75 ℃, injecting a non-aqueous electrolyte, carrying out vacuum packaging, standing for 24h, charging to 4.2V by using a constant current of 0.1C (160mA), then charging to 0.05C (80mA) by using a constant voltage of 4.2V until the current is reduced to 0.05V, then discharging to 3.0V by using a constant current of 0.1C (160mA), repeating the charging and discharging for 2 times, and finally charging to 3.8V by using a constant current of 0.1C (160mA), thus completing the preparation of the lithium ion secondary battery.

Examples 1 to 16

The separators 1 to 16 were prepared by the above method, and the specific parameters of the separators (including the substrate material, porosity, thickness of the inorganic layer, ratio of coverage area of the inorganic layer, number of discontinuous inorganic layer regions per unit area of the substrate surface, and the like) are shown in table 1 (separators 1 to 16 are respectively represented by S1 to S16).

The positive electrode sheet, the negative electrode sheet, and the electrolyte were prepared by the above method, and assembled with the separators 1 to 16, respectively, to prepare lithium ion secondary batteries, to obtain battery examples 1 to 16 (indicated by the batteries 1 to 16, respectively).

Comparative examples D1-D2

The separator of comparative example D1 directly used the substrate used in example 1 without any inorganic layer deposited.

The separator of comparative example D2 was prepared using a similar process as example 1, but with process parameters adjusted (without using mask and backside magnetic field induction techniques and depositing a thicker inorganic layer) to yield a continuous dense inorganic layer that completely covered the entire surface of the substrate.

Specific parameters of the separators of comparative examples D1, D2 are also shown in table 1.

Using the separators of comparative examples D1 and D2, respectively, comparative examples D1 and D2 of lithium ion secondary batteries were prepared in the same manner as in the respective examples.

Performance test method and results of examples and comparative examples

Finally, the test process and test results of the isolation film and the lithium ion secondary battery are described.

(1) Micro-morphology of inorganic layer of barrier film

The microscopic morphology of the inorganic layer on the resulting example separator was observed using an electron microscope. Observation by an electron microscope proves that the inorganic layer deposited by the process is formed by disordered stacking of nano particles, a plurality of particle gaps form pores, and the average pore diameter of the pores is 0.1nm to 20 nm.

FIG. 2 shows that the surface of the separator S1 of example 1 is at 5X 10³A photograph of a representative scanning electron microscope at magnification. As can be seen from fig. 2: the surface of the separation film is provided with a plurality of inorganic layers which extend along the surface of the substrate and still maintain the pore shape of the substrate, so that the inorganic layers form a plurality of discrete areas similar to the Chinese character 'mi'; it can also be seen that these inorganic layers are formed by disordered packing of nanocluster particles with minute pores formed therebetween.

(2) Film-substrate cohesion test

And (3) uniformly sticking a 3M double-sided adhesive tape on a stainless steel plate at room temperature and normal pressure, uniformly sticking a test sample on the double-sided adhesive tape with the width of 2cm, stripping the sample and the steel plate by using a high-speed rail tensile machine, and reading the maximum tensile force F (N) according to a data graph of the tensile force and the displacement, wherein the measured bonding force is F/0.02.

The film-based bonding force of the inorganic layers on the surfaces of the isolating films 1 to 16 is not lower than 30N/m.

(3) Heat shrinkage test of separator

The test specimen was cut into a square specimen having a length of 100mm and a width of 100mm, and the Machine Direction (MD) and the Transverse Direction (TD) were marked, after which the lengths in the MD and TD directions were measured with a projection tester and were marked as L1 and L2, after which the test specimen was put into a forced air oven at 90 ℃, taken out after one hour, and the lengths in the MD and TD directions were measured again with a projection tester and were marked as L3 and L4.

Heat shrinkage in MD ═ (L1-L3)/L1 × 100%;

the heat shrinkage in the TD direction was (L2-L4)/L2 × 100%.

The test data for each example and comparative example separator is shown in table 2.

(4) Tensile Strength test of Release films

A test sample with a fixed thickness T is respectively punched and cut into pieces of 100mm multiplied by 15mm along MD (length direction)/TD (width direction) by a cutting die, then the pieces are perpendicular to a chuck of a high-speed rail tension machine, the initial height of the chuck is 5cm up and down, the tensile rate is set to be 50mm/min, and the maximum tensile force is measured to be F.

Tensile strength F/9.8/(15mm × T).

(5) Air permeability test of barrier film

The test sample was made to a size of 4cm × 4cm under an environment of a temperature of 15 to 28 ℃ and a humidity of less than 80%, and measured using a Gurleytost (100cc) method using an Air-permeability tester (Air-permeability-tester), and the Air permeability value was directly obtained.

The test data for each example and comparative example separator are shown in table 2.

(6) Wettability of isolation film

The test sample was placed on a water contact angle tester, a drop of deionized water was dropped at a height of 1cm from the separator, and the drop of water dropped on the surface of the sample was photographed by an optical microscope and a high-speed camera.

The results of the experiment are shown in FIG. 3. In fig. 3, a photograph of the separator D1 after dripping water is shown, and it is understood from the photograph, that water droplets are less likely to wet the separator D1; fig. B is a photograph of the separator S3 after dripping water, and it can be seen from fig. B that the water drops have infiltrated the separator S3 and completely wetted the separator.

(7) Test for needling Strength

A sheet-like sample of the separator was prepared, fixed under a test jig, and subjected to puncturing using a high-speed iron tensile machine and a puncturing jig, using a puncturing needle having a diameter of 1mm on a puncturing tester, at a speed of 50mm/min, and the top puncturing force F after data stabilization was measured, and the puncturing strength (in units gf) was calculated as F/9.8 x 1000.

(8) Capacity test of lithium ion secondary battery

In a 25 ℃ constant temperature box, charging at a constant current of 1C multiplying power until the voltage is 4.2V, then charging at a constant voltage of 4.2V until the current is 0.05C, and then discharging at a constant current of 1C multiplying power until the voltage is 2.8V, wherein the obtained discharge capacity is the battery capacity.

The test data for each of the example and comparative example cells is shown in table 3.

(9) Normal temperature cycle performance test of lithium ion secondary battery

Charging at 25 deg.C with constant current of 0.7C to 4.35V, charging at constant voltage of 4.35V to 0.05C, and discharging at constant current of 1C to 3.0V, which is a charge-discharge cycle, and repeating the charge-discharge cycle 500 times.

The capacity retention rate after 1000 cycles was equal to the discharge capacity after 1000 cycles/discharge capacity after the first cycle × 100%.

(10) Direct current impedance (DCR) growth rate test after high-temperature storage of battery

The DCR test process of the battery is as follows: at 25 ℃, the battery was discharged at a rate of 0.3C for 10s by adjusting the state of charge (SOC) to 20% of the full charge capacity, and the DCR of the battery was (U1-U2)/I, where the voltage before discharge was U1 and the voltage after discharge was U2.

The DCR growth rate test process after the battery storage comprises the following steps: after the battery was fully charged, the battery was stored at 60 ℃ for 180 days, and then the dc impedance before and after the storage of the battery was measured according to the above-described method. Wherein, the DC impedance before the battery is stored is recorded as DCR₁And the DC impedance after battery storage is recorded as DCR₂Then, the batteryDCR increase rate (%) ═ DCR (DCR)₂-DCR₁)/DCR₁×100％。

The test results for each of the example and comparative example cells are specifically shown in table 3.

Table 2: physical properties of the separator

Table 3: electrochemical performance of battery using separator

The experimental results show that the thermal shrinkage rate of the separator D1 without the porous inorganic layer at 90 ℃ is obviously higher than that of the S1-S16 separator in the invention, the puncture strength is obviously lower than that of the S1-S16 separator in the invention, and the wettability to the electrolyte is also obviously lower than that of the separator in the invention; meanwhile, the cycle performance and the high-temperature storage performance of the lithium ion secondary battery prepared by the isolating membrane are obviously lower than those of the isolating membrane. The isolation film D2 with the continuous dense inorganic layer is provided, although the thermal shrinkage performance of the isolation film is not large compared with the performance difference of the isolation film with the discontinuous inorganic layer, the penetration depth of the inorganic layer in the base material is deep, and a large number of continuous dense inorganic layers are stacked in holes on the surface of the isolation film, so that the air permeability of the isolation film is poor, therefore, the battery D2 with the continuous inorganic layer isolation film is adopted, because the volume inside the battery core expands, when the interface between the pole piece and the isolation film is dislocated, the transmission distance of ions is increased, and meanwhile, the permeability of the isolation film to the ions is poor, so that the cycle performance and the high-temperature storage performance of the isolation film are deteriorated.

In addition, the battery capacity of the lithium ion secondary battery using the separator of the present invention was not significantly changed from that of the comparative example.

The above results show that: the isolating membrane not only keeps the advantages of low thermal shrinkage rate and the like of the traditional composite isolating membrane with the continuous inorganic ceramic coating, but also avoids the defects of large brittleness, poor mechanical processability, influence on cell density, reduction in cell cycle performance and the like of the continuous ceramic coating. The isolating membrane has good mechanical processing performance (strong flexibility, and the inorganic layer is not easy to brittle failure or fall off during winding and bending), and meanwhile, the isolating membrane has high wettability with electrolyte and low thermal shrinkage rate, so that a battery cell using the isolating membrane has good dynamic performance and cycle performance, and the long-term reliability of the battery cell is improved.

Although the present invention has been described with respect to the preferred embodiments, it is not intended to be limited to the embodiments disclosed, and many modifications and variations are possible to those skilled in the art without departing from the spirit of the invention.

Claims

1. A separator comprising a substrate having a porous structure,

The surface of at least one side of the base material is provided with an inorganic layer only partially covering the surface, the inorganic layer consists of a plurality of discontinuous areas, the inorganic layer is distributed on the surface of the base material in the form of discontinuous island-shaped or sheet-shaped film layers, the inorganic layer is formed on the base material in a mode of a patterned mask or baffle by adopting a vapor deposition method, and the number of the discontinuous areas of the inorganic layer on the surface of the base material is 100/mm²To 2000 pieces/mm²；

The inorganic layer contains an inorganic dielectric material and does not contain a binder; and is

The inorganic layer covers at least a part of the surface of the internal pores of the substrate in the area corresponding to the surface part covered by the inorganic layer.

2. The separator of claim 1, wherein the inorganic layer extends along the surface of the substrate at the surface portion covered by the inorganic layer and still maintains the pore morphology of the substrate.

3. The separator according to claim 1, wherein the thickness of the part of the substrate in which the surface of the pores inside the substrate is covered with the inorganic layer is 0.4 to 40% of the total thickness of the substrate.

4. The separator of claim 1, wherein the ratio of the gas permeability G1 of the separator to the gas permeability G2 of the substrate satisfies: 1< G1/G2 < 3.

5. The release film of claim 1, wherein the release film has a thermal shrinkage of less than 3% in the transverse and longitudinal directions after standing alone at 90 ℃ for 1 hour.

6. The release film of claim 5, wherein the release film has a thermal shrinkage of less than 2% in the transverse and longitudinal directions after standing alone at 90 ℃ for 1 hour.

7. The release film of claim 1, wherein the release film has a tensile strength of not less than 2000 kgf/cm in the transverse and longitudinal directions²。

8. The separator of any of claims 1-7, wherein the inorganic layer has a thickness of 5nm to 1000 nm.

9. The separator according to claim 8, wherein the inorganic layer has a thickness of 10nm to 500 nm.

10. The separator according to claim 9, wherein the inorganic layer has a thickness of 20nm to 200 nm.

11. The separator of any of claims 1 and 3-7, wherein the non-continuous inorganic layer covers a total area outside the substrate relative to the entire substrate outside area in a ratio of 0.4 to 0.9.

12. The separator according to any one of claims 1 to 7, wherein said inorganic layer is a porous structure with a stack of nanoclusters, said nanoclusters having pores formed therebetween for shuttling ions, said pores having an average pore diameter of 0.1nm to 20 nm.

13. The separator of any of claims 1-7, wherein the inorganic layer comprises AlO_xAs the inorganic dielectric material, x is more than or equal to 1.45 and less than or equal to 1.55.

14. The separator of claim 13, wherein AlO in the inorganic layer_xIs 50 to 100% of the mass of the inorganic layer.

15. The separator of claim 14, wherein AlO in the inorganic layer_xAccounts for 80 to 100% of the mass of the inorganic layer.

16. The barrier film according to claim 13, wherein said inorganic layer further contains at least one of an oxide of Si, a nitride of Si, an oxide of Ti, a nitride of Ti, an oxide of Zn, a nitride of Zn, an oxide of Mg, a nitride of Mg, an oxide of Zr, a nitride of Zr, an oxide of Ca, a nitride of Ca, an oxide of Ba and a nitride of Ba.

17. The separator according to any of claims 1-7, wherein the material of said substrate is selected from at least one of Polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), aramid, polyethylene terephthalate (PET), Polytetrafluoroethylene (PTFE), Polyacrylonitrile (PAN), Polyimide (PI), polyamide, polyester, natural fiber and non-woven fabric.

18. The isolation membrane of claim 17, wherein the substrate has a thickness of 3 μm to 20 μm.

19. The isolation membrane of claim 18, wherein the substrate has a thickness of 5 μm to 15 μm.

20. The isolation membrane of claim 19, wherein the substrate has a thickness of 5 μm to 12 μm.

21. The separator of claim 17, wherein the substrate has a porosity of 20% to 60%.

22. The separator of claim 21, wherein the substrate has a porosity of 30% to 50%.

23. The separator of claim 22, wherein the substrate has a porosity of 35% to 45%.

24. A method for preparing the separator according to any of claims 1 to 23, comprising at least the steps of:

1) providing a substrate having a porous structure;

2) preparing a non-continuous inorganic layer which partially covers the surface of at least one side of the base material and does not contain a binder on the surface by adopting a vapor deposition method through a patterned mask or baffle;

25. The method of claim 24, wherein step 1) comprises surface pre-treating the substrate, wherein the surface pre-treating comprises at least one of plasma activation, corona pre-treatment, chemical pre-treatment, and electron beam pre-treatment.

26. The method of claim 25, wherein the surface pretreatment is a plasma activation or electron beam pretreatment.

27. The method of claim 24, wherein the step 2) of partially covering the surface of the substrate with the inorganic layer comprises disposing a shaped mask or baffle over the substrate during the vapor deposition of the inorganic layer.

28. An electrochemical device comprising the separation membrane of any one of claims 1 to 23.