WO2024145898A1

WO2024145898A1 - Separator and preparation method therefor, battery, and electric device

Info

Publication number: WO2024145898A1
Application number: PCT/CN2023/070863
Authority: WO
Inventors: 吉星; 郭满毅; 李婷; 胡海; 钟华
Original assignee: 宁德时代新能源科技股份有限公司
Filing date: 2023-01-06
Publication date: 2024-07-11

Abstract

Disclosed in the present application are a separator and a preparation method therefor, a battery, and an electric device. The separator of the present application comprises a base film and a coating disposed on at least one surface of the base film, wherein the coating comprises polymer particles, the polymer particles comprise first polymer particles and second polymer particles which are mixed with the first polymer particles, and the particle size D_v50 of the first polymer particles is different from that of the second polymer particles. The separator of the embodiments of the present application comprises the coating, that is, on the basis of having good air permeability, the separator has high mechanical properties such as compression resistance and hot melt responsiveness, the mechanical properties such as the compression resistance and the hot melt responsiveness are well-balanced, and the barrier property regarding precipitated metal is good. The preparation method for the separator can ensure the stability of the separator structure and other performances. The battery comprises the separator of the present application.

Description

Diaphragm and preparation method thereof, battery and electrical device

Technical Field

The present application relates to the field of battery technology, and in particular to a diaphragm and a preparation method thereof, a battery and an electrical device.

Background technique

In the context of energy conservation and emission reduction, new energy technologies are developing rapidly, among which the research breakthroughs and applications of battery technology are the most significant, and lithium-ion batteries are the batteries with the most prominent development momentum.

As battery applications become more and more extensive, especially in recent years with the rapid development of electric vehicles, power batteries have developed rapidly and the demand has increased dramatically. However, the safety performance of batteries has also received more and more attention, and the safety performance of batteries has become one of the important factors restricting the expansion of battery applications.

Among the safety issues of batteries, the most prominent one is the runaway of the exothermic reaction inside the battery. Although the existing safety modified diaphragms can improve the safety of the diaphragm's response to thermal runaway to a certain extent, it is found in actual applications that the safety modified diaphragms have an imbalance between the hot melt response rate and the pressure resistance, resulting in the safety modified diaphragms either having an unsatisfactory hot melt response rate or unstable performance such as air permeability, thus affecting the electrochemical performance of the battery.

Summary of the invention

In view of the above problems, the embodiments of the present application provide a diaphragm and a preparation method and application thereof to solve the technical problem of the unbalanced hot melt response rate and pressure resistance of the existing safety modified diaphragm.

In a first aspect, an embodiment of the present application provides a diaphragm. The diaphragm of the embodiment of the present application comprises a base film and a coating disposed on at least one surface of the base film, the coating containing polymer particles, the polymer particles comprising first polymer particles and second polymer particles mixed with the first polymer particles, the first polymer particles and the second polymer particles having different volume distribution particle sizes D _v 50.

The first polymer particles and the second polymer particles with different volume distribution particle sizes D _v 50 contained in the coating of the diaphragm of the embodiment of the present application are compounded to construct a porous structure with rich pores in the coating, so that the diaphragm has better air permeability. In addition, the coating formed by the compounded polymer particles also gives the coating higher mechanical properties such as compression resistance while having good hot melt responsiveness, and also has the effect of blocking the migration of precipitated metals, further improving the electrochemical performance and safety performance of the battery.

In some embodiments, the volume distribution particle size D _v 50 of the first polymer particles is greater than the volume distribution particle size D _v 50 of the second polymer particles, and the volume distribution particle size D _v 50 of the first polymer particles is 2 μm to 2.5 μm, and the volume distribution particle size D _v 50 of the second polymer particles is 0.3 μm to 0.6 μm. By controlling the particle sizes of the first polymer particles and the second polymer particles in this range, the first polymer particles and the second polymer particles with two particle sizes D _v 50 can be compounded to play a synergistic role, further improving the air permeability of the coating, that is, the membrane, balancing the mechanical properties such as the compressive resistance and the hot melt responsiveness of the coating, that is, the membrane, and further improving the performance of preventing the precipitation of metal permeability.

In some embodiments, the mass ratio of the first polymer particles to the second polymer particles is (1 to 1.5): 1. The composite polymer particles in this compounding ratio can further enhance the synergistic effect of the two, thereby further improving the balance of mechanical properties such as air permeability and pressure resistance of the coating, i.e., the membrane, and hot melt responsiveness, and further improving the performance of preventing metal permeability.

In some embodiments, the melting point of the first polymer particles is 115° C. to 120° C., and the melting point of the second polymer particles is 90° C. to 110° C. By controlling the melting points of the first polymer particles and the second polymer particles within this temperature range, the polymer particles can form a melting point gradient characteristic, thereby further improving the balance between the mechanical properties such as compressive strength and the hot melt responsiveness of the coating, i.e., the diaphragm.

In some embodiments, the polymer particles satisfy at least one of the following conditions (1) to (3):

(1) The morphology of the first polymer particles and the second polymer particles independently includes at least one of chain, spherical, quasi-spherical, fibrous, tubular, rod-shaped, amorphous, and pyramidal;

(2) at least one of the first polymer particles and the second polymer particles contains a nonionic group;

(3) The weight average molecular weight of at least one of the first polymer particles and the second polymer particles is from 1,000 to 10,000, and may be from 1,500 to 4,000.

In some embodiments, the material of the polymer particles includes at least one of polystyrene, polyolefin, polyimide, melamine resin, phenol resin, cellulose, polyester, polyphenylene sulfide, polyaromatic amide, polyamideimide, copolymer of butyl acrylate and ethyl methacrylate, and their respective modified polymers.

The polymer particles have any of the above characteristics, and can further improve the balance of mechanical properties such as air permeability, compressive resistance and hot melt responsiveness of the coating, that is, the diaphragm, while further reducing the permeability to precipitated metals, thereby improving the electrochemical properties and safety performance of the battery, such as the capacity retention rate.

In an embodiment, the nonionic group includes at least one of ethoxy, hydroxyl, and carboxylate groups. These types of nonionic groups can further improve the above-mentioned effects of the polymer particles, especially improve the compression resistance of the polymer particles, thereby improving the compression resistance and stability of the coating, i.e., the separator, during the battery production process and normal operation.

In some embodiments, based on the total dry weight of the coating as 100%, the content of the polymer particles in the coating is ≥28wt%, and can be optionally 28wt% to 38wt%. By controlling and adjusting the content of the polymer particles in the coating, the balance between the mechanical properties such as compression resistance and the hot melt responsiveness of the coating can be further improved, and the barrier property to precipitated metals can be further improved.

In some embodiments, the coating further comprises inorganic particle fillers. The presence of inorganic particle fillers can further improve the mechanical properties and stability of the coating, i.e., the membrane, such as air permeability and pressure resistance, and enhance the barrier property against precipitated metals.

In some embodiments, the inorganic particle filler satisfies at least one of the following conditions:

Based on the total dry weight of the coating as 100%, the content of the inorganic particle filler in the coating is 22wt% to 30wt%;

The volume distribution particle size D _v 50 of the inorganic particle filler is 1 μm to 1.5 μm;

The inorganic particle filler includes at least one of inorganic particles having a dielectric constant of 5 or more, inorganic particles having ion conductivity but not storing ions, and inorganic particles capable of electrochemical reaction.

The inorganic particle filler has any of the above characteristics, and can further improve the mechanical properties such as air permeability and compressive resistance of the coating, that is, the diaphragm, and enhance the barrier property to precipitated metals.

In some embodiments, the coating further comprises a cured binder. By adding a binder to the coating, the mechanical strength of the membrane can be improved, the mechanical properties of the membrane such as air permeability and pressure resistance can be adjusted, and the barrier effect of the coating on the precipitated metal can be improved.

In the embodiment, based on the total dry weight of the coating as 100%, the content of the binder in the coating after curing is 20 wt % to 30 wt %.

The thermal decomposition temperature of the binder is ≥160°C, and can be selected from 160°C to 300°C.

The aqueous binder having the above characteristics can enhance the mechanical strength of the diaphragm, can further adjust the mechanical properties of the diaphragm such as air permeability and pressure resistance, and improve the barrier effect on precipitated metals.

In some embodiments, the base film has a thickness of 4 μm to 7 μm; the base film of this thickness together with the coating can effectively reduce the thickness of the separator while meeting the battery application requirements.

In some embodiments, the coating has a thickness of 2 μm to 5 μm.

In some embodiments, the coating has an area density of 2 g/m ² to 8 g/m ² .

By controlling the coating thickness, area and other characteristics, the coating, i.e., the diaphragm, can be further improved in terms of mechanical properties including permeability and compressive strength to precipitated metals and hot-melt responsiveness.

In a second aspect, the present invention provides a method for preparing a diaphragm. The method for preparing a diaphragm in the present invention comprises the following steps:

providing a base film and a coating slurry containing polymer particles;

Applying the coating slurry on at least one surface of the base film, and drying the base film to form a coating to obtain a separator;

The polymer particles include first polymer particles and second polymer particles mixed with the first polymer particles, and the first polymer particles and the second polymer particles have different particle size distribution ranges.

The diaphragm prepared by the diaphragm preparation method of the embodiment of the present application has higher air permeability than the diaphragm of the embodiment of the text application, and at the same time has higher mechanical properties such as compressive resistance and thermal melt responsiveness, and the mechanical properties such as compressive resistance and thermal melt responsiveness are relatively balanced, which can further block the migration of precipitated metals and improve the capacity retention rate and safety performance of the battery.

In some embodiments, the coating slurry further comprises an inorganic particle filler and a binder; wherein the mass ratio of the organic polymer particles, the inorganic particle filler and the binder in the coating slurry is (28 to 38):(22 to 30):(20 to 30).

In some embodiments, the solid content of the coating slurry is 30 wt % to 40 wt %.

In some embodiments, the coating slurry has a viscosity of 100 mPa·s to 1000 mPa·s at 25°C.

The coating slurry with the above characteristics can improve the film-forming quality, improve the quality of the coating, and relatively balance the air permeability, pressure resistance and hot melt responsiveness, and improve the barrier metal permeability.

In some embodiments, the binder is added in the form of a binder aqueous solution, and the binder aqueous solution has at least one of the following conditions:

The viscosity of the binder aqueous solution at 25° C. is 3000 mPa·s to 10000 mPa·s;

The solid content of the binder aqueous solution is 40 wt % to 50 wt %.

The coating slurry having the above characteristics can improve the film-forming quality, the coating quality, and the air permeability, pressure resistance, and barrier properties including the permeability of precipitated metals.

In some embodiments, when the polymer particles are polymer particles containing nonionic groups, the polymer particles are prepared by a method comprising the following steps:

The polymer precursor particles, the ethoxy-containing polymer and the initiator are mixed and reacted to obtain polymer particles containing nonionic groups;

The polymer particles containing nonionic groups are emulsified with a nonionic surfactant to obtain a nonionic polymer emulsion.

By using ethoxylated polymers to carry out polymerization reaction with polymer precursor particles, the polymer particles can contain nonionic groups, especially improve the mechanical properties of the polymer particles such as compression resistance, and the particle size range of the prepared polymer particles can be effectively controlled.

In some embodiments, the polymer precursor particles, the ethoxy-containing polymer and the initiator are mixed in a ratio of (300 to 400: (90 to 125): (20 to 50).

In some embodiments, the material of the polymer precursor particles includes at least one of polystyrene, polyolefin, polyimide, melamine resin, phenol resin, cellulose, polyester, polyphenylene sulfide, polyaromatic amide, polyamideimide, and a copolymer of butyl acrylate and ethyl methacrylate.

In some embodiments, the ethoxy-containing polymer includes at least one of allyl polyethylene glycol and bisallyl polyether.

In some embodiments, the initiator includes at least one of di-tert-butyl peroxide and diisopropylbenzene peroxide.

In some embodiments, the reaction temperature is 130°C to 160°C.

In some embodiments, the polymer particles and the nonionic surfactant are emulsified in a mass ratio of (4 to 6):1.

The above-mentioned types of reactants, mixing ratios and reaction conditions can increase the non-ionic group grafting rate and content of the polymer particles and adjust the particle size, improve the air permeability of the coating formed by the polymer particles, that is, the membrane, and its own mechanical properties such as compression resistance, and can improve the relative balance between the compression resistance and hot melt responsiveness of the coating, that is, the membrane.

In the embodiment, the nonionic polymer emulsion satisfies at least any one of the following conditions (1) to (4):

(1) The viscosity of the nonionic polymer emulsion at 25° C. is 10 mPa·s to 100 mPa·s;

(2) The solid content of the nonionic polymer emulsion is 35wt% to 40wt%;

(3) The pH value of the nonionic polymer emulsion is 5 to 6;

(4) The nonionic surfactant includes a molecular structure of Wherein, _R1 is a carbon chain alkyl group, _R2 is a hydrophilic group, and n is a positive integer from 3 to 20.

In an exemplary embodiment, the carbon chain alkyl group is a long carbon chain alkyl group of C12 to C18.

In the exemplary embodiment, the R ₂ is at least one of a carboxyl group, an amine group, an aldehyde group, an alcohol group, an amino group, and an amide group.

In the exemplary embodiment, the nonionic surfactant includes at least one of fatty alcohol polyoxyethylene ether, alkylphenol polyoxyethylene ether, fatty acid polyoxyethylene ester, castor oil polyoxyethylene ether, fatty amine polyoxyethylene ether, sorbitan fatty acid ester, and polyoxyethylene sorbitan fatty acid ester.

By selecting the type of nonionic surfactant, the stability of the nonionic polymer emulsion can be improved, and its dispersibility in the coating slurry can be improved, thereby improving the dispersion uniformity and stability of the polymer particles.

In some embodiments, the aqueous binder is added in the form of an aqueous binder aqueous solution, and the aqueous binder aqueous solution has at least one of the following conditions:

The aqueous binder solution has a viscosity of 3000 to 10000 mPa·s at 25°C;

The solid content of the aqueous binder solution is 40 wt % to 50 wt %.

These aqueous binders can improve the stability of the coating slurry and the uniformity of component dispersion, and improve the mechanical properties and air permeability of the formed coating, so as to improve the relative balance between the compressive strength and hot melt responsiveness of the coating, that is, the diaphragm.

In some embodiments, the coating slurry is applied at a rate of 20 m/min to 100 m/min, and optionally 50 m/min to 100 m/min.

In some embodiments, the drying process is carried out at a temperature of 50°C to 70°C, and optionally 60°C to 70°C.

In some embodiments, the drying time is 1 min to 5 min, and can be 1 min to 3 min.

By adjusting the coating process conditions, the film quality, thickness and air permeability can be adjusted and optimized.

In a third aspect, the present application provides a battery. The battery of the present application includes a positive electrode, a negative electrode, and a separator stacked between the positive electrode and the negative electrode, wherein the separator is the separator described in the present application or a separator prepared by the separator preparation method of the present application. Since the battery of the present application includes the separator of the present application, the battery has high safety and good electrochemical properties such as capacity retention rate.

In an embodiment, the coating is disposed on one surface of the separator, and the coating faces the negative electrode. By only combining the coating on one surface of the separator, the overall thickness of the separator can be effectively reduced, thereby increasing the energy density of the battery.

In a fourth aspect, an embodiment of the present application provides an electric device, wherein the electric device comprises a battery according to an embodiment of the present application, and the battery is used to provide electric energy. The safety of the electric device according to an embodiment of the present application is significantly improved.

The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present application. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the drawings:

FIG1 is a schematic flow chart of a method for preparing a diaphragm according to an embodiment of the present application;

FIG2 is a schematic structural diagram of an implementation of a secondary battery in an embodiment of the present application;

FIG3 is an exploded schematic diagram of the secondary battery shown in FIG2 ;

FIG4 is a schematic structural diagram of an implementation scheme of a battery module according to an embodiment of the present application;

FIG5 is a schematic structural diagram of an implementation scheme of a battery pack according to an embodiment of the present application;

FIG6 is a schematic diagram of the exploded structure of the battery pack shown in FIG5 ;

FIG7 is a schematic diagram of an embodiment of an electric device including a secondary battery according to an embodiment of the present application as a power source;

FIG8 is a scanning electron microscope (SEM) image of the diaphragm in Example A1 of the present application after being heated at 130° C. for 15 minutes;

FIG9 is a scanning electron microscope (SEM) image of the diaphragm in Comparative Example A1 of the present application after being heated at 130° C. for 15 minutes.

The reference numerals in the specific implementation manner are as follows:

10. secondary battery cell, 11. housing, 12. electrode assembly, 13. cover plate;

20. Battery module;

30. battery pack, 31. box body, 32. lower box body.

Detailed ways

The following embodiments of the technical solution of the present application will be described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

In the description of the embodiments of the present application, the term "and/or" is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and/or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character "/" in this article generally indicates that the associated objects before and after are in an "or" relationship.

In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, which are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the embodiments of the present application.

In the description of the embodiments of the present application, unless otherwise clearly specified and limited, technical terms such as "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the internal connection of two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

In ion batteries such as lithium-ion batteries, the separator is one of the important components. It is set between the positive and negative pole pieces to separate the positive and negative electrodes of the battery, prevent the two poles from contacting and short-circuiting, and pass the function of Li ⁺ . Therefore, in addition to having good electrolyte wettability, liquid retention, and mechanical strength, the separator also needs to have more and more performance such as thermal stability, which has an important impact on battery safety, as more and more attention is paid to battery safety performance.

Currently, the method of improving the thermal stability and other properties of the diaphragm is to modify the diaphragm, such as doping or coating modifiers such as flame retardant additives, fiber-like materials, electrochemically inert ceramics, etc. during the preparation of the diaphragm, in order to achieve the purpose of improving the thermal stability and other properties of the diaphragm and thus improving battery safety.

Although current methods such as doping or coating can improve the thermal stability and other properties of the diaphragm to a certain extent, the inventors found in their research that when the exothermic reaction inside the battery accumulates to a certain extent, the modified diaphragm cannot timely block the continued reaction inside the battery, making it difficult to alleviate and inhibit the exothermic reaction inside the battery, and it is difficult to reduce the possibility of internal short circuit or even thermal runaway of the battery. Therefore, it is not ideal for improving the safety of battery thermal runaway.

In order to improve the role of the diaphragm in suppressing thermal runaway of the battery, the surface of the diaphragm is currently modified, such as adding a modified coating. However, the inventors found in their research that adding a modified coating to the diaphragm has an adverse effect on the air permeability of the diaphragm. Therefore, the inventors proposed a modified diaphragm, specifically adding a coating containing polymer particles to the base membrane of the modified diaphragm, which ensures good air permeability of the diaphragm, and has good air permeability during normal production and use of the battery; when overheating occurs inside the battery, the coating can also melt and close the pores of the base membrane in time to form a blocking barrier layer, reducing or completely blocking the reaction inside the battery, thereby reducing the risk of thermal runaway of the battery and improving the safety performance of the battery.

Based on the diaphragm developed in the early stage, the inventor conducted further research on the diaphragm and found that when preparing the diaphragm coating, the particle size of the polymer particles has an important influence on the air permeability, hot melt response rate and pressure resistance of the coating, as well as the barrier to the migration of precipitated metals, thereby affecting the corresponding performance of the diaphragm. In particular, the hot melt response rate and pressure resistance of the coating are unbalanced, resulting in the phenomenon that the hot melt response rate and air permeability of the coating, that is, the diaphragm, are in a state of compromise, which is difficult to take into account. Therefore, the inventor further studied and found that if polymer particles of different particle sizes are compounded so that the polymer particles are a mixture of different particle sizes, and polymer particles of different particle sizes are distributed in the coating, and the coating formed by polymer particles of different particle sizes is combined with the base film to form a diaphragm, the hot melt response rate and pressure resistance of the diaphragm can be significantly improved, and the balance between the two regulating the hot melt response rate and pressure resistance.

At the same time, the positive electrode material will undergo a crystal phase transition at high voltage or high temperature, resulting in metal precipitation. The precipitated metal includes transition metals, etc., and will migrate to the negative electrode through the diaphragm to deposit or further form dendrites, which may puncture the diaphragm and cause risks such as short circuits. The inventors found in their research that a coating formed by a mixture of polymer particles of different particle sizes can also serve as a barrier to the precipitated metal. Therefore, based on the previous diaphragm solution, the following diaphragm improvement solution is further proposed.

Diaphragm

In a first aspect, an embodiment of the present application provides a diaphragm. The diaphragm of the embodiment of the present application comprises a base film and a coating disposed on at least one surface of the base film. The coating contains polymer particles, the polymer particles comprising first polymer particles and second polymer particles mixed with the first polymer particles, and the first polymer particles and the second polymer particles have different volume distribution particle sizes D _v 50.

In the diaphragm of the embodiment of the present application, the base film contained therein constitutes the matrix of the diaphragm of the embodiment of the present application and plays the usual role of the diaphragm. The coating is arranged on the base film, which plays a modifying role on the base film, i.e., the diaphragm matrix. For example, when the battery overheats, the first polymer particles and the second polymer particles can undergo gradient melting and timely close the pores contained in the base film.

The first polymer particles and the second polymer particles with different particle sizes D _v 50 contained in the coating of the diaphragm of the embodiment of the present application are compounded to construct a porous structure with rich pores in the coating, so that the diaphragm has better air permeability in a normal working environment, and also gives the coating, that is, the diaphragm, a barrier property that can effectively reduce or even prevent the metals such as transition metals precipitated from the positive electrode active material, thereby reducing or preventing the precipitated metal from reaching the negative electrode through the diaphragm of the embodiment of the present application and depositing or forming dendrites. Since the diaphragm has a barrier property to the precipitated metal, it can alleviate or reduce the rate of the precipitated metal of the positive electrode active material, thereby improving the stability and cycle performance of the positive electrode active material. Among them, the precipitated metal includes a metal element or a metal ion.

At the same time, the coating formed by the compounded polymer particles also has higher mechanical properties such as compression resistance, thereby further improving the coating, that is, the compression resistance of the diaphragm during the battery production process and the normal working process, and improving the performance stability of the diaphragm such as air permeability. Moreover, when the diaphragm is in a thermal abnormality, the polymer particles with relatively small particle size D _v 50 in the compounded polymer particles can melt in time and close the pores of the base film. As the temperature continues to rise, the polymer particles with large particle size D _v 50 will also melt, further closing the pores of the base film, and improving the hot melt response of the coating of the diaphragm, thereby reducing or preventing the chemical reactions in the electrolyte, the migration of metal ions, and the crosstalk reaction of the positive and negative electrodes inside the battery, relieving or terminating the thermal runaway of the battery, thereby improving the safety performance of the battery. Moreover, the inventors have found that the mechanical properties such as compression resistance and the hot melt response rate of the compounded polymer particles given to the coating, that is, the diaphragm, can be balanced, that is, the diaphragm of the embodiment of the present application has good mechanical properties such as compression resistance and hot melt response performance at the same time.

Among them, the migration of the metal ions includes lithium ions and metal precipitation from active materials, etc., to prevent them from passing through the diaphragm and depositing on the surface of the electrode, especially the negative electrode, or further forming dendrites. The crosstalk reaction between the positive and negative electrodes refers to the release of oxygen when the positive electrode active material of the battery undergoes a phase change at high temperature. The oxygen can diffuse to the negative electrode through the diaphragm and react with the negative electrode, etc., and release a lot of heat; it also includes the hydrogen released from the negative electrode at high temperature, which diffuses to the positive electrode through the diaphragm and reacts with the positive electrode, etc., and releases a lot of heat.

It is precisely because the diaphragm of the embodiment of the present application has the good and relatively balanced mechanical properties such as compression resistance and hot melt response as mentioned above, and also has a barrier property against precipitated metals. Therefore, in the embodiment, the diaphragm of the embodiment of the present application can be used in a battery with high-voltage lithium cobalt oxide as the positive electrode material. In this way, even when the battery voltage reaches about 4.5V, when the hexagonal phase of the lithium cobalt oxide material transforms to the monoclinic phase and causes the dissolution of cobalt metal, the diaphragm of the embodiment of the present application will more effectively reduce or prevent the precipitated cobalt metal from passing through, thereby further reducing the risk of deposition of negative electrode cobalt metal or further formation of dendrites, thereby further improving the capacity retention rate and safety performance of the battery.

In some embodiments, the particle size D _v 50 of the first polymer particles is greater than the particle size D _v 50 of the second polymer particles, and the volume distribution particle size D _v 50 of the first polymer particles is 2 μm to 2.5 μm, and the volume distribution particle size D _v 50 of the second polymer particles is 0.3 μm to 0.6 μm. By controlling the particle sizes of the first polymer particles and the second polymer particles in this range, the compounding effect and synergistic effect between the first polymer particles and the second polymer particles with two particle sizes D _v 50 are enhanced, and on the basis of further improving the mechanical properties such as air permeability and pressure resistance of the coating, i.e., the membrane and the hot melt responsiveness, the energy effects such as mitigation or prevention of metal penetration are further improved. In an exemplary embodiment, the particle size volume distribution D _v 50 of the first polymer particles may be, but is not limited to, typical non-limiting particle sizes such as 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, and may be optionally 2.2 μm to 2.4 μm; in an exemplary embodiment, the volume distribution particle size D _v 50 of the second polymer particles may be, but is not limited to, typical non-limiting particle sizes such as 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, and may be optionally 0.4 μm to 0.5 μm.

In some embodiments, the mass ratio of the first polymer particles to the second polymer particles may be 1:1 to 1.5:1, and specifically may be a typical non-limiting ratio of 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, etc. Controlling the compounding ratio of the first polymer particles to the second polymer particles within this range can further improve the compounding effect between the first polymer particles and the second polymer particles, and further improve the synergistic effect of the two, thereby further improving the coating, i.e., the membrane's air permeability, mechanical properties such as pressure resistance, and hot melt responsiveness, and further improving the balance between mechanical properties such as pressure resistance and hot melt responsiveness and preventing the permeability of metal precipitation.

Based on the particle size range of the first polymer particles and the second polymer particles, in one embodiment, the melting point of the first polymer particles is 115°C to 120°C, and specifically can be 115°C, 116°C, 117°C, 118°C, 119°C, 120°C and other typical non-limiting temperatures; in another embodiment, the melting point of the second polymer particles is 90°C to 110°C, and specifically can be 90°C, 92°C, 94°C, 96°C, 98°C, 102°C, 104°C, 106°C, 108°C, 110°C, 112°C, 114°C, 116°C, 118°C, 120°C and other typical non-limiting temperatures. By controlling the melting points of the first polymer particles and the second polymer particles within the temperature range, the coating can have a thermal melt gradient characteristic, thereby further improving the pore sealing effect of the coating, that is, the diaphragm in a high temperature environment with abnormal heat, thereby further improving the barrier property and barrier stability of the diaphragm in abnormal heat, and further improving the safety of the battery.

In some embodiments, the morphology of at least one of the first polymer particles and the second polymer particles contained in the coating in the above embodiments may include at least one of chain, spherical, quasi-spherical, fibrous, tubular, rod-shaped, amorphous, and pyramidal. The polymer particles of these morphologies have high compressive strength and hot-melt responsiveness, and can form a porous structure with rich pores, which can improve the mechanical properties such as compressive strength and hot-melt responsiveness of the coating, i.e., the diaphragm, and the balance between the two, and at the same time, can improve the balanced air permeability of the coating, i.e., the diaphragm, and the barrier property to precipitated metals such as transition metals.

In some embodiments, the organic material of at least one of the first polymer particles and the second polymer particles in the above embodiments contains non-ionic groups. In some embodiments, the molar percentage of the non-ionic groups contained in the organic material of the polymer particles is 0.1% to 5%, and can be specifically 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% and other limited but non-limiting molar contents. In the exemplary embodiment, the non-ionic group can include at least one of ethoxy, hydroxyl, and carboxylate groups. By adding non-ionic groups to the polymer particles and further controlling the content and type of non-ionic groups in the polymer particles, the above-mentioned effects of the polymer particles can be further improved, especially the compressive resistance of the polymer particles, thereby improving the balance of mechanical properties such as compressive resistance of the coating, that is, the membrane and the hot melt responsiveness, and can also improve the dispersion uniformity of the polymer particles in the coating and the film quality of the coating. Among them, the non-ionic group can be a grafted side chain of these polymers, or it can be connected to the main chain of the polymer.

In some embodiments, the polymer of the polymer particles can be a polymer with a weight average molecular weight of 1000 to 10000, and further can be a polymer with a weight average molecular weight of 1500 to 4000. Wherein, the molecular weight can be tested by using gel chromatography (GPC) and referring to GBT27843-2011 chemical polymer low molecular weight component content detection method. In the exemplary embodiment, the material of the polymer particles can include at least one of polystyrene, polyolefin, polyimide, melamine resin, phenol resin, cellulose, polyester, polyphenylene sulfide, polyaramid, polyamide-imide, copolymer of butyl acrylate and ethyl methacrylate and their respective modified polymers. By selecting the polymer material of the polymer particles, such as selecting these polymers, the mechanical properties such as the first polymer particles and the second polymer particles in the above-mentioned embodiments can be effectively improved, such as the mechanical properties such as the compression resistance and the heat melt responsiveness and the balance of the mechanical properties such as the compression resistance and the heat melt responsiveness.

In addition, the polymer particles in the above embodiments can be solid particles, and of course can also be hollow particles. In the embodiments of the present application, solid particles can improve the compressive resistance of polymer particles relative to hollow particles, thereby improving the stability of the coating, that is, the diaphragm during battery production and normal operation, such as the polymer particles will not be deformed due to the extrusion force during battery production or normal operation, thereby causing the basement membrane to block holes. Therefore, in the embodiments of the present application, solid polymer particles are relatively preferred relative to hollow polymer particles.

In some embodiments, in the above embodiments, the total weight of the dry weight of the coating is 100%, and the content of the polymer particles, such as the total weight of the first polymer particles and the second polymer particles, in the coating can be ≥28wt%, and further can be 28wt% to 38wt%. By controlling and adjusting the content of the polymer particles in the coating, if it is controlled within this range, the mechanical properties such as compression resistance and thermal melting responsiveness of the coating, i.e., the diaphragm, and the balance of the two properties can be further improved, and the effect of the polymer particles melting and sealing the pores of the base film in the event of thermal anomaly can be improved, and the barrier properties of the coating, i.e., the diaphragm, in the event of thermal runaway can be improved.

In some embodiments, the coating contained in the diaphragm of the above embodiments also contains inorganic particle fillers. By adding inorganic particle fillers to the coating, on the one hand, the mechanical properties of the coating such as compression resistance can be improved, and the stability of the diaphragm in battery production and normal operation can be improved; on the other hand, it can play a synergistic role with the above polymer particles, further improve the pore structure of the coating to improve the mechanical properties such as air permeability and compression resistance of the coating, that is, the diaphragm, as well as mechanical properties such as heat shrinkage resistance, enhance the barrier properties including precipitated metals, and improve the liquid storage capacity of the coating, that is, the diaphragm. In addition, the inorganic particle filler is ideally formed into a mixture with the above polymer particles in the coating.

In an embodiment, the content of the inorganic particle filler in the coating layer may be 22wt% to 30wt%. By controlling the content ratio of the inorganic particle filler and the composite polymer particles in the coating layer, the synergistic effect between the two can be improved on the basis of fully volatilizing the respective effects described above, thereby further improving the balance between the mechanical properties such as compressive resistance and heat shrinkage resistance and liquid storage capacity of the diaphragm while ensuring that the diaphragm has good air permeability, mechanical properties such as compressive resistance and heat shrinkage resistance, and barrier effect on metal precipitation, and improving the balance between mechanical properties such as compressive resistance and hot melt responsiveness.

In the embodiment, the volume distribution particle size D _v 50 of the inorganic particle filler is 1 μm to 1.5 μm. By adjusting the inorganic particle filler, it is compounded with the particle size of the polymer particles containing the first polymer particles and the second polymer particles, such as controlling the D _v 50 to be in the range of 1 μm to 1.5 μm, which can play the role of coating skeleton, improve the mechanical properties of the coating such as compression resistance, and work together with the first polymer particles and the second polymer particles in the above particle size range to improve the air permeability, mechanical properties and liquid storage capacity of the diaphragm and the performance including the barrier effect on the precipitated metal.

In an embodiment, the above inorganic particle filler may include at least one of inorganic particles having a dielectric constant of 5 or more, inorganic particles having ion conductivity but not storing ions, and inorganic particles capable of electrochemical reaction.

In an exemplary embodiment, the inorganic material of the inorganic particles having a dielectric constant of 5 or more includes at least one of boehmite, aluminum oxide, zinc oxide, silicon oxide, titanium oxide, zirconium oxide, barium oxide, calcium oxide, magnesium oxide, nickel oxide, tin oxide, cerium oxide, yttrium oxide, hafnium oxide, aluminum hydroxide, magnesium hydroxide, silicon carbide, boron carbide, aluminum nitride, silicon nitride, boron nitride, magnesium fluoride, calcium fluoride, barium fluoride, barium sulfate, magnesium aluminum silicate, lithium magnesium silicate, sodium magnesium silicate, bentonite, hectorite, zirconium titanate, barium _titanate , Pb(Zr, _Ti ) _O3 , _Pb1 _- mLamZr1-nTinO3, Pb( _Mg3Nb2 _/ ₃ ) _O3 -PbTiO3, and modified inorganic particles thereof, 0＜m＜1, 0＜n＜1.

In the exemplary embodiments, the inorganic material of the inorganic particles having ion conductivity but not storing ions includes _Li3PO4 _, lithium titanium phosphate _Lix1Tiy1 ( _PO4 ) ₃ , lithium aluminum titanium _phosphate _Lix2Aly2Tiz1 ( _PO4 ₎ ₃ , (LiAlTiP) _x3Oy3 type glass, _lithium _lanthanum _titanate _Lix4Lay4TiO3 , _lithium _germanium _{thiophosphate} _{Lix5Gey5Pz2Sw} _, _lithium _nitride _Lix6Ny6 _, _SiS2 type glass _Lix7Siy7Sz3 _and _P2S5 type glass _Lix8Py8Sz _. At least one of _z4 , 0＜x1＜2, 0＜y1＜3, 0＜x2＜2, 0＜y2＜1, 0＜z1＜3, 0＜x3＜4, 0＜y3＜13, 0＜x4＜2, 0＜y4＜3, 0＜x5＜4, 0＜y5＜1, 0＜z2＜1, 0＜w＜5, 0＜x6＜4, 0＜y6＜2, 0＜x7＜3, 0＜y7＜2, 0＜z3＜4, 0＜x8＜3, 0＜y8＜3, 0＜z4＜7;

In an exemplary embodiment, the inorganic material of the electrochemically reactive inorganic particles includes at least one of lithium-containing transition metal oxides, lithium-containing phosphates, carbon-based materials, silicon-based materials, tin-based materials, and lithium-titanium compounds.

By selecting the material type of the inorganic particle filler, the above-mentioned effect of the inorganic particle filler is improved, the synergistic effect between the inorganic particle filler and the polymer particles is further improved, the balance between the mechanical properties such as mechanical stability and compressive resistance and hot melt responsiveness of the coating of the embodiment of the present application, that is, the diaphragm, is improved, the barrier effect including the precipitated metal is further improved, and the electrochemical properties and safety performance such as the capacity retention rate of the battery are improved. Among them, the inorganic particle filler of the above-mentioned inorganic material also has good liquid absorption and storage capacity, thereby improving the liquid absorption and storage capacity of the diaphragm.

Based on the above polymer particles and inorganic particle fillers, in the exemplary embodiment, the polymer contained in the polymer particles contained in the coating is low-density polyethylene, specifically low-density polyethylene particles, and further can be low-density polyethylene containing non-ionic groups. Among them, low-density polyethylene refers to high-pressure polyethylene (LDPE), specifically polyethylene with a molecular weight of less than 10,000.

The above-mentioned inorganic particles are compounded with the low-density polyethylene particles to enhance the above-mentioned synergistic effect between the above-mentioned composite polymer particles and the inorganic particle filler, thereby improving the thermal melt responsiveness, mechanical properties, compressive strength and other mechanical properties of the coating of the embodiment of the present application, that is, the balance between the thermal melt responsiveness and the stability of properties including air permeability, and further enhancing the barrier effect on precipitated metals and the liquid absorption and storage capacity, and improving the electrochemical properties and safety performance such as the capacity retention rate of the battery.

In some embodiments, the coating contained in the diaphragm of the above embodiments also contains a cured binder. By adding a binder to the coating, the bonding force between the polymer particles or the inorganic particle filler can be effectively enhanced, thereby improving the mechanical properties of the coating and enhancing the bonding force between the coating and the base film. At the same time, the binder can also play a synergistic role with the polymer particles or further with the inorganic particle filler, adjusting the balance between the mechanical properties of the coating, that is, the mechanical properties of the diaphragm, such as compressive resistance and hot melt responsiveness, improving stability including air permeability and barrier effects including precipitated metals, so as to improve the electrochemical properties and safety performance of the battery such as capacity retention rate.

In the embodiment, the content of the binder in the coating can be 20wt% to 30wt% based on the total dry weight of the coating as 100%. By controlling the content of the binder in the coating, the mechanical properties of the coating and the bonding force between the coating and the base film can be further enhanced, so as to further adjust the balance between the mechanical properties such as compression resistance and hot melt responsiveness of the coating, i.e., the diaphragm, and improve the barrier effect on the precipitated metal, and improve the electrochemical properties and safety performance of the battery such as the capacity retention rate.

In the embodiment, the thermal decomposition temperature of the solidified binder is higher than 160° C., and can be selected from 160° C. to 300° C. By selecting the decomposition temperature characteristics of the binder, the mechanical properties of the coating and the bonding force between the coating and the base film are enhanced, and the balance between the mechanical properties such as compression resistance and hot melt responsiveness of the diaphragm and the stability of the performance including the barrier of metal precipitation are improved.

In the embodiment, the binder can be selected from aqueous binders. Among them, the aqueous binder can be a water-soluble binder or an emulsion binder, etc. In the exemplary embodiment, the binder can include polyvinyl alcohol, polyacrylic acid-polyacrylonitrile copolymer, polyacrylate-polyacrylonitrile copolymer, polyether acrylate, polyurethane, polyacrylate, polycarbonate, polyethylene oxide, rubber, polyacrylic acid, polyacrylonitrile, gelatin, chitosan, sodium alginate, cyanoacrylate, polymerized cyclic ether derivatives, and at least one of hydroxyl derivatives of cyclodextrin. These binders can enhance the bonding force between polymer particles or further inorganic particle fillers, thereby improving the mechanical properties of the coating and enhancing the bonding force between the coating and the base film. Further adjust the balance between the mechanical properties such as the compressive resistance of the diaphragm and the hot melt responsiveness and improve the barrier effect of the coating on the precipitated metal to improve the electrochemical properties and safety performance of the battery such as the capacity retention rate.

In the embodiment, when the coating contains both the above inorganic particle filler and the binder, in the coating, the dry basis mass ratio of the above polymer particles, inorganic particle filler and binder can be (25 to 35): (55 to 65): (25 to 35). By controlling the content ratio of the three components in the coating, the above effects of each component and the synergistic effect between the three components are further improved, thereby further improving the balance of mechanical properties such as compression resistance and hot melt responsiveness of the coating, i.e., the diaphragm, and the barrier effect on precipitated metals, so as to improve the capacity retention rate and safety performance of the battery.

Based on the coating schemes in the above embodiments, in some embodiments, the coating thickness can be 2μm to 5μm. In the exemplary examples, the coating thickness can be 2μm, 3μm, 4μm, 5μm, and other typical but non-limiting thicknesses. By controlling the coating thickness, the content of components such as polymer particles can be controlled, thereby adjusting and improving the polymer particles to fully play the role described above, improving the electrochemical stability of the battery during production and normal operation and the barrier to the precipitated metal, avoiding migration and deposition to the negative electrode and possible formation of dendrites; when the battery has a thermal anomaly, the polymer particles in the coating of the diaphragm can melt in time and effectively close the pores of the base film, reducing or preventing the risk of thermal runaway, thereby improving the electrochemical performance and safety performance of the battery such as the capacity retention rate.

After testing, the surface density of the coating contained in the separator of each embodiment can be controlled within the range of 2g/ ^m2 to 8g/ ^m2 . By controlling and optimizing the properties of the coating, the air permeability, mechanical properties such as compression resistance, balance between mechanical properties such as compression resistance and hot melt responsiveness, and barrier properties such as metal migration of the separator can be further improved, thereby further improving the electrochemical performance and safety performance of the battery.

In addition, the coating contained in the separator of each embodiment above can be arranged on one surface of the base film, and of course can also be arranged on two opposite surfaces of the base film. Based on the consideration of battery energy density and the direction of metal precipitation and migration, in the embodiment, the coating can be arranged on one surface of the base film, that is, the coating is not arranged on the other surface of the base film.

The base film contained in the separator of each embodiment above can be selected from the organic separator of the battery. For example, in the embodiment, the conductive thickness of the base film can be 4μm to 7μm. In other embodiments, the air permeability of the base film can be 180s/100cc to 240s/100cc. In the exemplary embodiment, the material of the base film includes at least one of non-woven fabrics, polyolefins, polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene ether, polyphenylene sulfide, and polyethylene naphthalene. The base film of these characteristics and materials can be combined with the above coating and play a synergistic role on the basis of giving full play to the role of the battery separator, so that the separator has high mechanical properties such as compression resistance, balance of mechanical properties such as compression resistance and hot melt responsiveness, good air permeability, excellent barrier effect including metal precipitation, and can effectively reduce the risk of thermal runaway of the battery and improve the electrochemical properties and safety performance of the battery such as capacity retention rate.

In some embodiments, the above-mentioned base membrane can also be a modified base membrane, such as a base membrane modified with at least one modifier such as flame retardants, fiber-like materials, electrochemically inert ceramics, etc., to improve the thermal stability of the base membrane, so as to cooperate with the above-mentioned coating to further improve the mechanical properties and safety performance of the diaphragm of the embodiment of the present application.

Preparation method of diaphragm

In a second aspect, the present invention provides a method for preparing a diaphragm. The process flow of the method for preparing a diaphragm in the present invention is shown in FIG1 . The method for preparing a diaphragm in the present invention may include the following steps:

S01: providing a basement membrane;

S02: providing coating slurry;

S03: coating the coating slurry on at least one surface of the base film, and forming a coating after drying to obtain a separator.

In the preparation method of the diaphragm of the embodiment of the present application, after the coating slurry is formed into a coating on the surface of the base film, the coating is bonded to the surface of the base film. Since the preparation method of the diaphragm according to the embodiment of the present application is to prepare the diaphragm of the embodiment of the above text application, the formed coating contains polymer particles, and the polymer particles are like the polymer particles in the coating contained in the diaphragm of the embodiment of the above text application, which include first polymer particles and second polymer particles mixed with the first polymer particles, and the first polymer particles and the second polymer particles have different cumulative distribution particle sizes D _v 50.

In the diaphragm preparation method of the present application, first polymer particles and second polymer particles with different particle sizes D _v 50 are formulated into a coating slurry, and a film is formed directly on the base film, so that the prepared diaphragm has a coating on at least one surface to achieve the coating's modification effect on the base film.

Then the coating is the coating contained in the diaphragm of the above text application example. Therefore, the diaphragm prepared by the diaphragm preparation method of the present application example has better mechanical properties such as compression resistance, and relative balance between mechanical properties such as compression resistance and hot melt responsiveness as the diaphragm of the above text application example. While having good air permeability, the barrier effect on precipitated metals can also be further improved, which can effectively further improve the electrochemical properties and safety performance of the battery such as the capacity retention rate. In addition, the process parameters of the diaphragm preparation method of the present application example are controllable, and the quality and electrochemical properties of the prepared diaphragm are stable.

Step S01:

The base film in step S01 may be the base film contained in the diaphragm of the above text application embodiment, and therefore, the base film may be the base film contained in the diaphragm of the above text application embodiment. As in the embodiment, the thickness of the provided base film may be 4 μm to 7 μm, and its air permeability may be 180 s/100 cc to 240 s/100 cc, and its material may include at least one of non-woven fabric, polyolefin, polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene ether, polyphenylene sulfide, and polyethylene naphthalene.

Step S02:

Since the coating slurry in step S02 is a slurry for forming the coating contained in the above text application embodiment diaphragm, the slurry must contain polymer particles and solvents. In the embodiment, it may further contain inorganic particle fillers and binders. Therefore, the polymer particles contained in the coating slurry, or the material types and content ratios of the inorganic particle fillers and binders further contained therein are the types and content ratios of the polymer particles, inorganic particle fillers and binders contained in the coating of the above text application embodiment diaphragm. Therefore, in order to save space, the types, content ratios, etc. of polymer particles, inorganic particle fillers and binders are no longer described here.

In order to further improve the film-forming quality of the coating slurry, improve the quality of the coating as well as the air permeability, pressure resistance and barrier properties including metal precipitation, in one embodiment, the solid content of the coating slurry can be 30wt% to 40wt%; in other embodiments, the viscosity of the coating slurry at 25°C is 100mPa·s 1000mPa·s.

In some embodiments, when the coating slurry in step S02 is formed by mixing polymer particles with inorganic particle fillers and binders in a solvent; then the mass ratio of polymer particles, inorganic particle fillers, binders and water in the coating slurry can be (28 to 38): (22 to 30): (20 to 30): (25 to 35). This ratio refers to the mixing ratio of the binder to the polymer particles and inorganic particle fillers before curing. By adjusting the ratio of the three, the film-forming quality of the coating slurry is improved, and at the same time, the relative balance of mechanical properties such as air permeability, mechanical properties, and compressive resistance of the formed coating, that is, the diaphragm and the hot melt responsiveness and barrier properties including metal precipitation are enhanced, thereby improving the electrochemical properties of the battery and reducing the risk of thermal runaway of the battery.

Among them, the adhesive should be an adhesive before curing, such as the aqueous adhesive before curing described above. In the exemplary embodiment, it can be an adhesive before curing including polyvinyl alcohol, polyacrylic acid-polyacrylonitrile copolymer, polyacrylate-polyacrylonitrile copolymer, polyether acrylate, polyurethane, polyacrylate, polycarbonate, polyethylene oxide, rubber, polyacrylic acid, polyacrylonitrile, gelatin, chitosan, sodium alginate, cyanoacrylate, polymerized cyclic ether derivatives, and at least one hydroxyl derivative of cyclodextrin.

As in the embodiment, the binder is added in the form of a binder aqueous solution to effectively adjust the viscosity of the coating slurry, improve the dispersion uniformity and stability of the coating slurry components, and improve the film-forming quality of the coating slurry. When the binder is added in the form of a binder aqueous solution, in the embodiment, the viscosity of the binder aqueous solution at 25°C can be 3000mPa·s to 10000mPa·s; in other embodiments, the solid content of the binder aqueous solution is 40wt% to 50wt%. By controlling and adjusting the viscosity of the binder aqueous solution or the solid content at the same time, the positive effect of the binder on the coating slurry can be further improved, including the slurry stability and dispersibility.

In the embodiment, when the polymer particles contained in the coating slurry are polymer particles containing non-ionic groups, the polymer particles can be prepared according to a method comprising the following steps:

Step S021: mixing and reacting the polymer precursor particles, the ethoxy-containing polymer and the initiator to obtain polymer particles containing nonionic groups;

Step S022: emulsifying the polymer particles containing nonionic groups with a nonionic surfactant to obtain a nonionic polymer emulsion.

In the reaction process of step S021, the polymer precursor particles and the ethoxy-containing polymer undergo a polymerization reaction under the action of an initiator, so that the ethoxy-containing group is grafted onto the polymer particles.

In the embodiment, the polymer precursor particles, the ethoxy-containing polymer and the initiator are mixed in a weight ratio of (300 to 400):(90 to 125):(20 to 50).

In the embodiment, the reaction temperature in step S021 can be 130° C. to 140° C., and the reaction time should be sufficient, such as 2.5 h to 4 h at 130° C. to 140° C. Meanwhile, during the reaction, the molten reactants are stirred to improve the efficiency and uniformity of the reaction.

By controlling and adjusting the mixing ratio of the reactants in step S021, or further controlling the reaction conditions, on the basis of improving the polymer reaction efficiency and making the particle size of the polymer particles within the above D _v 50 range of 2 μm to 2.5 μm and 0.3 μm to 0.6 μm, the content of the ethoxylated nonionic groups grafted onto the polymer particles is increased, thereby improving the nonionic group modification effect of the polymer particles, thereby improving the above effect of the polymer particles, especially improving the mechanical properties of the polymer particles such as compression resistance and the relative balance between the mechanical properties such as compression resistance and the hot melt responsiveness.

In an exemplary embodiment, the material of the polymer precursor particles may include at least one of polystyrene, polyolefin, polyimide, melamine resin, phenol resin, cellulose, polyester, polyphenylene sulfide, polyaromatic amide, polyamideimide, and a copolymer of butyl acrylate and ethyl methacrylate.

The ethoxy-containing polymer may include at least one of allyl polyethylene glycol and bisallyl polyether.

The initiator may include at least one of di-tert-butyl peroxide and dicumyl peroxide.

The polymer precursor particles, ethoxylated polymer and initiator can undergo a polymer reaction in step S021 to improve the efficiency of grafting modification. The ethoxylated polymer can effectively provide ethoxylated nonionic groups to achieve grafting modification of the ethoxylated nonionic groups of the polymer precursor particles.

In step S022, during the emulsification process, the nonionic surfactant and the polymer particles containing nonionic groups form an emulsion in the solvent. In the embodiment, the mass ratio of the polymer particles containing nonionic groups to the nonionic surfactant can be (4 to 6): 1, and the exemplary embodiment can be 4.5: 1 for emulsification. The solvent should be water.

In addition, the mixing ratio of the nonionic surfactant and the polymer particles containing nonionic groups, as well as the amount of added water solvent, can be controlled to adjust and control the relevant properties of the nonionic polymer emulsion, such as viscosity, solid content, pH value, etc., so as to improve the dispersion uniformity and stability of the polymer particles in the coating slurry, thereby improving the above-mentioned effect of forming the coating and improving the electrochemical performance and safety performance of the prepared diaphragm. For example, in the embodiment, by controlling and conditioning the conditions of the emulsification treatment in step S022, the nonionic polymer emulsion has at least one of the following properties:

(1) The viscosity of the nonionic polymer emulsion is 10 mPa·s to 100 mPa·s;

(2) the solid content of the nonionic polymer emulsion is 35 wt % to 40 wt %;

(3) The pH value of the nonionic polymer emulsion is 5 to 6.

Among them, the mixing ratio of the non-ionic surfactant and the polymer particles can effectively adjust the viscosity and solid content of the non-ionic polymer emulsion, thereby improving the dispersibility of the polymer particles and improving the uniformity and stability of the coating slurry.

In some embodiments, the nonionic surfactant may include a molecular structure of

Wherein, _R1 is a carbon chain alkyl group. In the exemplary embodiment, when it is a carbon chain alkyl group, _R1 is a long carbon chain alkyl group of C12 to C18.

R ₂ is a hydrophilic group. In the exemplary embodiment, R ₂ is at least one of a carboxyl group, an amine group, an aldehyde group, an alcohol group, an amino group, and an amide group.

n is a positive integer from 3 to 20. In the examples, n can be a typical but non-limiting positive integer such as 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, etc.

In addition, there is no chronological order between step S01 and step S02 above, and the order can be adjusted according to actual production, or they can be performed simultaneously.

Step S03:

The purpose of coating the coating slurry on the base film in step S02 is to form a wet film on the base film, that is, the above coating before drying. The coating method may be unlimited, and any method that can form a film of the coating slurry on the base film is within the scope disclosed in the embodiments of the present application, such as but not limited to brushing, roller coating, spraying, etc. In addition, the coating process conditions may be further adjusted to adjust and optimize the film quality, thickness, air permeability, etc.

For example, in the embodiment, the coating slurry in step S02 is coated at a rate of 20 m/min to 100 m/min, and may be 50 m/min to 100 m/min. The coating rate can improve the quality of the coating, adjust the air permeability and thickness, and adjust the mechanical properties to adjust the relative balance between mechanical properties such as compression resistance and hot melt responsiveness.

In the drying process, the main purpose is to remove the solvent in the wet film. Based on the melting temperature range characteristics of the organic particles contained in the coating slurry, the drying process should be lower than the melting temperature of the organic particles, and should ensure the quality and stability of the coating. For example, in the embodiment, the drying temperature can be 50°C to 70°C, and can be 60°C to 70°C. In the exemplary embodiment, it can be but not limited to typical but non-limiting temperatures such as 50°C, 52°C, 55°C, 58°C, 60°C, 61°C, 62°C, 63°C, 64°C, 65°C, 66°C, 67°C, 68°C, 69°C, and 70°C. In the drying process temperature range, the drying time should be sufficient, such as the drying time can be 1min to 5min, and can be 1min to 3min.

Battery

In a third aspect, the present application provides a battery, which includes a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the separator is the separator of the above-mentioned application embodiment.

In the embodiment, the separator is only combined with a coating on one surface, and the surface of the separator containing the coating faces the negative electrode. By facing the coating toward the negative electrode, the separator can fully play the role described in the separator of the embodiment of the above application, thereby improving the electrochemical performance and safety of the battery. At the same time, only combining the coating on one surface of the separator can effectively reduce the overall thickness of the separator and improve the energy density of the battery.

In the exemplary embodiment, the battery of the present application can be a secondary battery. When the battery of the present application is a secondary battery, it includes a positive electrode, a separator and a negative electrode. That is, the negative electrode contained in the secondary battery is the negative electrode of the above-mentioned application embodiment, that is, the negative electrode contains the modified artificial graphite of the above-mentioned application embodiment.

In this way, the battery of the embodiment of the present application, such as a secondary battery, has a high energy density, a high rate capability and good cycle stability.

In an embodiment, the secondary battery of the present application may include any one of a battery cell, a battery module, and a battery pack.

The battery cell refers to a battery housing and a battery cell encapsulated in the battery housing. The shape of the battery cell is not particularly limited, and it can be cylindrical, square or any other shape. A square battery cell 10 is shown in FIG. 2 .

In some embodiments, as shown in FIG3 , the outer packaging of the battery cell 10 may include a shell 11 and a cover plate 13. The shell 11 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 11 has an opening connected to the receiving cavity, and the cover plate 13 is used to cover the opening to close the receiving cavity. The positive electrode, the separator and the negative electrode contained in the secondary battery of the embodiment of the present application can form an electrode assembly 12 through a winding process and/or a lamination process. The electrode assembly 12 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 12. The number of electrode assemblies 12 contained in the battery cell 10 may be one or more, which can be adjusted according to actual needs.

The preparation method of the battery cell 10 is well known. In some embodiments, the positive electrode, the separator, the negative electrode and the electrolyte can be assembled to form the battery cell 10. As an example, the positive electrode, the separator and the negative electrode can be formed into the electrode assembly 12 through a winding process or a lamination process, and the electrode assembly 12 is placed in an outer package, dried and injected with electrolyte, and then vacuum packaged, left to stand, formed, shaped and other processes are performed to obtain the battery cell 10.

The battery module is assembled from the battery cells 10 , that is, it may contain a plurality of the battery cells 10 , and the specific number can be adjusted according to the application and capacity of the battery module.

In some embodiments, FIG. 4 is a schematic diagram of a battery module 20 as an example. As shown in FIG. 4 , in the battery module 20, a plurality of battery cells 10 may be arranged in sequence along the length direction of the battery module 20. Of course, they may also be arranged in any other manner. Further, the plurality of battery cells 10 may be fixed by fasteners.

Optionally, the battery module 20 may further include a housing having an accommodation space, and the plurality of battery cells 10 may be accommodated in the accommodation space.

A battery pack is composed of the above-mentioned battery cells 10, that is, it may contain multiple battery cells 10, wherein multiple battery cells 10 may be assembled into the above-mentioned battery module 20. The specific number of battery cells 10 or battery modules 20 contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

As shown in the embodiment, FIG. 5 and FIG. 6 are schematic diagrams of a battery pack 30 as an example. The battery pack 30 may include a battery box and a plurality of battery modules 20 disposed in the battery box. The battery box includes an upper box body 31 and a lower box body 32, and the upper box body 31 is used to cover the lower box body 32 and form a closed space for accommodating the battery module 20. The plurality of battery modules 20 may be arranged in the battery box in any manner.

Electrical devices

In a fourth aspect, the present application also provides an electric device, which includes the battery of the above-mentioned embodiment of the present application, and further can be a secondary battery. The battery can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. Therefore, the electric device of the present application embodiment has a long standby or battery life.

The electrical device may be, but is not limited to, a mobile device (such as a mobile phone, a laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc. The electrical device may select a secondary battery, a battery module or a battery pack according to its use requirements.

Fig. 7 is a schematic diagram of an electric device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the requirements of the electric device for high power and high energy density, a battery pack or a battery module may be used.

As another example, the electric device may be a mobile phone, a tablet computer, a notebook computer, etc. The electric device is usually required to be light and thin, and a secondary battery may be used as a power source.

Example

Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used that do not indicate the manufacturer are all conventional products that can be obtained commercially.

1. Diaphragm and its preparation method embodiment

Example A1

The present embodiment provides a diaphragm and a preparation method thereof. The diaphragm of the present embodiment includes a PE base film and a coating bonded to one surface of the PE base film. The thickness of the PE base film is 7 μm; the thickness of the coating is 2 μm. The coating contains copolymerized polypropylene wax particles of two particle sizes: one of which has a particle size D _v 50 of 2 μm (referred to as the first polymer particles), and the other has a particle size D _v 50 of 0.3 μm (referred to as the second polymer particles), and the mass ratio of the first polymer particles to the second polymer particles is 1:1; the specific parameters such as the melting point of the polymer particles and the particle size are shown in Table 1 below.

The diaphragm preparation method of this embodiment includes the following steps:

S1. Preparation of coating slurry:

S11. Preparation of polymer particles

a. Mix the copolymerized polypropylene wax, allyl polyethylene glycol and di-tert-butyl peroxide in a ratio of 300:100:20, heat and melt, stir evenly, and keep the temperature at 160°C for 3h to obtain an intermediate product;

b. Preparation of polymer particles of different particle sizes

First polymer particles with large particle size D _v 50:

The intermediate product and nonionic emulsifier (fatty alcohol polyoxyethylene ether OS-10) are mixed in a ratio of 4.5:1, heated to 75°C, stirred to form a uniform emulsion, and water is added to adjust the viscosity to a suitable range of 10 mPa·s to 100 mPa·s (25°C, the same below), and the solid content is 35 wt% to 40 wt% to obtain a nonionic copolymerized polypropylene wax emulsion with a particle size D _v 50 of 2 μm;

Second polymer particles with small particle size D _v 50:

The intermediate product and nonionic emulsifier (fatty alcohol polyoxyethylene ether OS-10) are mixed in a ratio of 4.5:20, heated to 90° C., stirred to form a uniform emulsion, and water is added to adjust the viscosity to a suitable range of 10 mPa·s to 100 mPa·s, and the solid content to 35 wt % to 40 wt %, to obtain a nonionic copolymerized polypropylene wax emulsion with a particle size D _v 50 of 0.3 μm;

S12. Preparation of coating slurry:

The copolymerized polypropylene wax emulsion, polyacrylonitrile, boehmite and water are mixed in a ratio of first polymer particles: second polymer particles: polyacrylonitrile: boehmite: water = 12.5%: 12.5%: 25%: 20%: 30%, all materials are added into a double planetary stirring tank, and then the mixture is stirred for 3 hours at a revolution of 30 rpm and a rotation of 2000 rpm, and vacuum defoaming is performed throughout the process; the viscosity of the coating slurry is controlled at 500 mPa·s to 800 mPa·s;

S2. Film formation of coating slurry on the surface of base film:

The coating slurry in step S1 is coated on one side of a 7 μm PE base film to form a wet film, and then dried to form a 2 μm coating on one side of the PE base film; wherein the coating slurry coating speed is 80 m/min, the oven temperature is 65° C., and the total drying time is 2 min.

Example A2

This embodiment provides a membrane and a method for preparing the same. The membrane of this embodiment is identical to the membrane of embodiment A1 except that the particle size of the polymer particles contained in the coating is different. The D _v 50 of the first polymer particles is 2.5 μm, and the D _v 50 of the second polymer particles is 0.6 μm.

Specifically, in step b of step S11, the first polymer particles are mixed in a ratio of intermediate product: nonionic emulsifier (fatty alcohol polyoxyethylene ether OS-10) = 10:1, and heated to 70° C. The second polymer particles are mixed in a ratio of intermediate product: nonionic emulsifier (fatty alcohol polyoxyethylene ether OS-10) = 4.5:15, and heated to 85° C.

Example A3

This embodiment provides a diaphragm and a method for preparing the same. The diaphragm of this embodiment is identical to the diaphragm in Embodiment A1 except for the coating formula. Specifically, the first polymer particles: the second polymer particles: polyacrylonitrile: boehmite: water = 15%: 10%: 25%: 20%: 30% of the components of the coating slurry are mixed.

Example A4

This embodiment provides a diaphragm and a method for preparing the same. The diaphragm of this embodiment is identical to the diaphragm in embodiment A1 except that the polymer particles are different. Specifically, the polymer particles in this embodiment are directly made of copolymerized polypropylene wax (i.e., not copolymerized with allyl polyethylene glycol), and step S11 is not performed.

Example A5

This embodiment provides a diaphragm and a method for preparing the same. The diaphragm of this embodiment is identical to the diaphragm in embodiment A1 except that the polymer particles are different. Specifically, the polymer particles in this embodiment are directly made of low-melting-point polyamide (i.e., not copolymerized with allyl polyethylene glycol).

Example A6

This embodiment provides a diaphragm and a method for preparing the same. The diaphragm of this embodiment is identical to the diaphragm in embodiment A1 except that the coating does not contain inorganic particles. The components of the coating slurry are mixed in a ratio of first polymer particles: second polymer particles: polyacrylonitrile: boehmite: water = 22.5%: 22.5%: 25%: 0%: 30%.

Example A7

This embodiment provides a diaphragm and a method for preparing the same. The diaphragm of this embodiment is identical to the diaphragm in embodiment A1 except for the coating formula. Specifically, the components of the coating slurry are mixed in a ratio of first polymer particles: second polymer particles: polyacrylonitrile: boehmite: water = 16.7%: 8.3%: 25%: 20%: 30%.

Example A8

This embodiment provides a diaphragm and a method for preparing the same. The diaphragm of this embodiment is identical to the diaphragm in embodiment A1 except for the coating formula. Specifically, the first polymer particles: the second polymer particles: polyacrylonitrile: boehmite: water = 8.3%: 16.7%: 25%: 20%: 30% of the components of the coating slurry are mixed.

Comparative Example A1

This comparative example provides a separator, which is different from the separator in Example A1 in that the separator coating does not contain polymer particles. The other features are exactly the same.

The preparation method of the diaphragm in this comparative example refers to the preparation method of the diaphragm in Example A1, and the difference from the preparation method in Example A1 is that when preparing the coating slurry, the components of the coating slurry are mixed in a ratio of first polymer particles: second polymer particles: polyacrylonitrile: boehmite: water = 0%: 0%: 25%: 45%: 30%. The steps and process conditions are the same.

Comparative Example A2

This embodiment provides a membrane and a method for preparing the same. The membrane of this embodiment is identical to the membrane of embodiment A1 except that the particle size of the polymer particles contained in the coating is different. The D _v 50 of the first polymer particles is 2 μm, and the D _v 50 of the second polymer particles is 0.2 μm.

Specifically, in step b of step S11, the second polymer particles are mixed in a ratio of intermediate product: non-ionic emulsifier (fatty alcohol polyoxyethylene ether OS-10) = 4.5:25, and heated to 95°C.

Comparative Example A3

This embodiment provides a membrane and a method for preparing the same. The membrane of this embodiment is identical to the membrane of embodiment A1 except that the particle size of the polymer particles contained in the coating is different. The D _v 50 of the first polymer particles is 3 μm, and the D _v 50 of the second polymer particles is 0.6 μm.

Specifically, in step b of step S11, the first polymer particles are mixed in a ratio of intermediate product: non-ionic emulsifier (fatty alcohol polyoxyethylene ether OS-10) = 20:1, and heated to 70° C. The second polymer particles are mixed in a ratio of intermediate product: non-ionic emulsifier (fatty alcohol polyoxyethylene ether OS-10) = 4.5:15, and heated to 85° C.

Comparative Example A4

This embodiment provides a membrane and a method for preparing the same. The membrane of this embodiment is identical to the membrane of embodiment A1 except that the polymer particles are different. Specifically, the polymer particles of this embodiment are copolymerized polypropylene wax, step S11 is not performed, and the particle size D _v 50 is 2.5 μm.

Comparative Example A5

This embodiment provides a membrane and a method for preparing the same. The membrane of this embodiment is identical to the membrane of embodiment A1 except that the polymer particles are different. Specifically, the polymer particles of this embodiment are copolymerized polypropylene wax, step S11 is not performed, and the particle size D _v 50 is 0.3 μm.

2. Secondary battery cell example

Example B1 to Example B8 and Comparative Example B1 to Comparative Example B5;

The present embodiment B1 to embodiment B8 and comparative examples B1 to comparative examples B5 respectively provide a secondary battery monomer, each of which includes a battery cell formed by a positive electrode, a separator and a negative electrode, and also includes an electrolyte. Among them, the separators provided in embodiments A1 to embodiment A8 and comparative examples A1 to comparative examples A5 are the separators of secondary battery embodiments B1 to embodiment B8 and comparative examples B1 to comparative examples B5, respectively. Among them, the separator in the above embodiment A1 is used as the separator in the battery cell of the secondary battery embodiment B1, the separator in the embodiment A2 is used as the separator in the battery cell of the secondary battery embodiment B2, and so on, and the separator in comparative example A5 is used as the separator in the battery cell of the battery comparative example B5.

The positive electrode was prepared as follows:

Using methyl pyrrolidone (NMP) as solvent, LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , carbon nanotubes (CNT) and binder (PVDF) were mixed at a mass ratio of 97:2:1 to prepare a cathode slurry with a solid content of 85%. The cathode slurry was evenly coated on aluminum foil for double-sided coating, wherein the surface density of each single side was 0.131 g/1540.25 m ² , and after being fully dried, cold pressed and cut, a cathode electrode sheet was obtained as a positive electrode.

Negative electrode: Using water as solvent, artificial graphite, conductive agent SP, dispersant (CMC), and binder (SBR) are mixed at a mass ratio of 96:1:1:2 to prepare a negative electrode slurry with a solid content of 58%. The positive electrode slurry is evenly coated on aluminum foil and double-sided coating is performed, wherein the surface density of each single side is 0.065g/ ^1540.25m2 . After sufficient drying, cold pressing, and slitting, the negative electrode sheet is obtained as the negative electrode.

Electrolyte: At room temperature, ethylene carbonate (EC)/diethyl carbonate (DEC) were mixed in a volume ratio of 1:1, and LiPF ₆ was added to the mixed solution to obtain a solution with a concentration of 1 mol/L as the electrolyte;

Secondary battery assembly: In a low-humidity constant temperature room, the positive and negative electrode plates prepared as above are stacked and wound in the order of "positive electrode-separator-negative electrode" to form a bare cell, and then filled with electrolyte to assemble into a lithium-ion secondary battery.

Related performance tests of diaphragms and secondary battery cells

The separators and secondary battery cells provided in the above embodiments and comparative examples were tested respectively.

1. Diaphragm performance test:

1.1 The membranes in the above-mentioned Examples A1 to A8 and Comparative Examples A1 to A5 were respectively subjected to the relevant performance tests in Table 1. Among them, each test method is in accordance with the national standard (if there is no national standard, it is tested according to the industry standard method). The measured results are shown in Table 1 below. For example, the air permeability is tested using a membrane air permeability tester in accordance with the reference standard GB/T36363-2018; the melting point can be obtained by testing with a universal differential scanning calorimeter (DSC) method with reference to the standard ISO 11357-3-2018; unless otherwise specified, the volume distribution particle size Dv50 of the organic particles in the embodiments of the present application is determined by a particle size analyzer-laser diffraction method. Specifically, the standard GB/T 19077-2016 can be referred to, and the laser diffraction scattering particle size analyzer (Mastersizer3000 laser particle size analyzer) can be used for measurement according to the manufacturer's instructions.

Table 1

From Table 1, compared with Examples A1 to A8, the air permeability of Comparative Example A1 at room temperature and after heating at 120°C for 15 minutes is basically the same, which shows that when the ambient temperature of the diaphragm of the present application is lower than the melting point of the polymer particles contained in the coating, the diaphragm has good air permeability. When the ambient temperature is higher than the melting point of the polymer particles contained in the coating, the polymer particles contained in the diaphragm coating can melt and close the pores of the base film, causing the diaphragm to lose its air permeability, thereby effectively terminating the migration of metal ions or the positive and negative electrode crosstalk reaction in the battery.

Compared with Example A1, the air permeability of Example A6 at room temperature is poor. It can be seen that it is preferred to add inorganic particle fillers to increase the porosity of the coating and improve the air permeability.

Compared with Example A1, the room temperature air permeability of Example A7 and Example A8 is poor, which shows that the matching ratio of the first polymer and the second polymer needs to be optimized. If there are too few small-sized particles as in Example A7 or too many small-sized particles as in Example A8, the air permeability of the diaphragm will be affected during normal use.

Compared with Example A1, the air permeability of Comparative Example A2 at room temperature is poor, which shows that the D _v 50 of the second polymer particles needs to be optimized. If it is too small, the air permeability of the diaphragm during normal use will be affected.

Compared with Example A2, the air permeability value of Comparative Example A3 after heating at 120°C for 15 minutes is not large enough, which shows that the D _v 50 of the first polymer particles needs to be optimized. If it is too large, it will affect the blocking response speed of the diaphragm at high temperature.

Compared with Example A2, the air permeability values of Comparative Examples A4 and A5 after heating at 120° C. for 15 min are not large enough, indicating that the large and small particle size matching scheme of the present invention needs to be used to improve the response speed of the diaphragm at high temperature.

2. Secondary battery monomer performance test:

2.1 Permeability of the diaphragm after secondary battery monomer injection

After the secondary battery cells in the above-mentioned Examples B1 to B8 and Comparative Examples B1 to B5 were injected and formed under the same injection and formation conditions, the batteries were disassembled to obtain each diaphragm, and the air permeability of each diaphragm was tested according to the diaphragm permeability method in Section 1.1 above. The measured results are shown in "After injection and high temperature (85°C) aging" in Table 1.

2.2 Secondary battery cell cycle test

The secondary battery cells in the above-mentioned Examples B1 to B8 and Comparative Examples B1 to B5 were subjected to cycle tests at 25° C. and 1C/1C conditions, and the test results are shown in Table 2.

Table 2

As shown in Table 2, compared with Example A1, the capacity retention rate of Example A4 is lower. It can be seen that it is preferred to perform non-ionic group modification to improve the pressure resistance of the polymer particles, so that they will not deform and block holes during the use of the battery, affecting the cycle performance. Among them, compared with other embodiments in Examples A1 to A8, Examples A4 and A5, the mass mixing ratio of the polymer particles in the coating of the diaphragm of the present application embodiment will affect the capacity retention rate of the battery. Relatively speaking, the mass ratio of the first polymer particles (large particle size) to the second polymer particles (small particle size) is 1:1 to 1.5:1, which is relatively preferred.

The relatively low capacity retention rates of Comparative Examples A2 and A5 indicate that the particle size and dosage of the small particle polymer need to be limited, otherwise the normal use capacity of the battery will be affected.

2.3 Secondary battery cell air permeability test after cycling

After the secondary battery cells in the above-mentioned Examples B1 to B8 and Comparative Examples B1 to B5 were cycled at 25°C and 1C/1C, the batteries were disassembled to obtain each diaphragm, and the air permeability of each diaphragm was tested according to the diaphragm permeability method in Section 1.1 above. The measured results are shown in "After 1000 cycles" in Table 1.

Compared with Example B1, the air permeability of Example B4 after injection aging and circulation is poor. It can be seen that non-ionic group modification is preferred to help improve the pressure resistance of the polymer particles during battery use.

2.4 Test of metal deposition before and after secondary battery cell cycling

The secondary battery cells in the above-mentioned Examples B1 to B8 and Comparative Examples B1 to B5 were respectively disassembled before and after cycling at 25°C and 1C/1C conditions to obtain each negative electrode sheet. The metal element content on the negative electrode sheet was tested using ICP. The test results of the total metal content (excluding lithium) are shown in Table 3 below.

table 3

As shown in Table 3, compared with Examples B1 to B8, Comparative Example B1 has a large amount of metal deposition (except lithium) on the negative electrode sheet after battery cycling, which may pierce the diaphragm and cause safety problems. It can be seen that using the solution of the present invention, the combination of large and small particle sizes can construct limited pores and prevent metal ions with larger ion radius from passing through the diaphragm. From Comparative Example B4, it can be seen that the diaphragm contains only large particle size materials, and there is no such effect. And from Comparative Example B5, it can be seen that if the diaphragm contains only small particle sizes to construct limited pores, it will affect the air permeability of the diaphragm during normal use.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application, and they should all be included in the scope of the claims and specification of the present application. In particular, as long as there is no structural conflict, the various technical features mentioned in the various embodiments can be combined in any way. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions that fall within the scope of the claims.

Claims

A diaphragm, characterized in that it comprises a base film and a coating disposed on at least one surface of the base film, wherein the coating contains polymer particles, the polymer particles comprise first polymer particles and second polymer particles, and the volume distribution particle diameters D v 50 of the first polymer particles and the second polymer particles are different.
The diaphragm according to claim 1, characterized in that the volume distribution particle size D v 50 of the first polymer particles is greater than the volume distribution particle size D v 50 of the second polymer particles, and the volume distribution particle size D v 50 of the first polymer particles is 2 μm to 2.5 μm, and the volume distribution particle size D v 50 of the second polymer particles is 0.3 μm to 0.6 μm.
The diaphragm according to claim 2, characterized in that: the mass ratio of the first polymer particles to the second polymer particles is (1 to 1.5):1; and/or

The melting point of the first polymer particles is 115°C to 120°C, and the melting point of the second polymer particles is 90°C to 110°C.
The diaphragm according to any one of claims 1 to 3, characterized in that the polymer particles satisfy at least one of the following conditions (1) to (3):

(1) The morphology of the first polymer particles and the second polymer particles independently includes at least one of chain, spherical, quasi-spherical, fibrous, tubular, rod-shaped, amorphous, and pyramidal;

(2) at least one of the first polymer particles and the second polymer particles contains a nonionic group;

(3) The weight average molecular weight of at least one of the first polymer particles and the second polymer particles is from 1,000 to 10,000, and may be from 1,500 to 4,000.
The diaphragm according to any one of claims 1 to 4, characterized in that the material of the polymer particles comprises at least one of polystyrene, polyolefin, polyimide, melamine resin, phenol resin, cellulose, polyester, polyphenylene sulfide, polyaramid, polyamide-imide, copolymer of butyl acrylate and ethyl methacrylate and their respective modified polymers; and/or

The nonionic group includes at least one of an ethoxy group, a hydroxyl group, and a carboxylate group.
The diaphragm according to any one of claims 1 to 5, characterized in that, based on the total dry weight of the coating as 100%, the content of the polymer particles in the coating is ≥ 28wt%, optionally 28wt% to 38wt%; and/or

The coating also contains an inorganic particulate filler.
The diaphragm according to claim 6, characterized in that the inorganic particle filler satisfies at least one of the following conditions:

Based on the total dry weight of the coating as 100%, the content of the inorganic particle filler in the coating is 22wt% to 30wt%;

The volume distribution particle size D v 50 of the inorganic particle filler is 1 μm to 1.5 μm;

The inorganic particle filler includes at least one of inorganic particles having a dielectric constant of 5 or more, inorganic particles having ion conductivity but not storing ions, and inorganic particles capable of electrochemical reaction.
The diaphragm according to any one of claims 1 to 7, characterized in that the coating further comprises a binder.
The diaphragm according to claim 8, characterized in that the content of the binder in the coating is 20wt% to 30wt%, based on the total dry weight of the coating being 100%; and/or

The thermal decomposition temperature of the binder is ≥160°C, and can be selected from 160°C to 300°C.
The diaphragm according to any one of claims 1 to 8, characterized in that the thickness of the base film is 4 μm to 7 μm; and/or

The coating has a thickness of 2 μm to 5 μm; and/or

The coating has an area density of 2 g/m 2 to 8 g/m 2 .
A method for preparing a diaphragm, characterized in that it comprises the following steps:

providing a base film and a coating slurry containing polymer particles;

Applying the coating slurry on at least one surface of the base film, and drying the base film to form a coating to obtain a separator;

The polymer particles include first polymer particles and second polymer particles mixed with the first polymer particles, and the first polymer particles and the second polymer particles have different particle sizes D v 50.
The preparation method according to claim 11, characterized in that: the coating slurry further comprises an inorganic particle filler and a binder; wherein the mass ratio of the organic polymer particles, the inorganic particle filler and the binder in the coating slurry is (28 to 38): (22 to 30): (20 to 30); and/or

The solid content of the coating slurry is 30wt% to 40wt%; and/or

The coating slurry has a viscosity of 100 mPa·s to 1000 mPa·s at 25°C.
The preparation method according to claim 12, characterized in that the binder is added in the form of a binder aqueous solution, and the binder aqueous solution has at least one of the following conditions:

The viscosity of the binder aqueous solution at 25° C. is 3000 mPa·s to 10000 mPa·s;

The solid content of the binder aqueous solution is 40 wt % to 50 wt %.
The preparation method according to any one of claims 11 to 13, characterized in that when the polymer particles are polymer particles containing non-ionic groups, the polymer particles are prepared by a method comprising the following steps:

The polymer precursor particles, the ethoxy-containing polymer and the initiator are mixed and reacted to obtain polymer particles containing nonionic groups;

The polymer particles containing nonionic groups are emulsified with a nonionic surfactant to obtain a nonionic polymer emulsion.
The preparation method according to claim 14, characterized in that the polymer precursor particles, the ethoxy-containing polymer and the initiator are mixed in a ratio of (300 to 400): (90 to 125): (20 to 50); and/or

The material of the polymer precursor particles includes at least one of polystyrene, polyolefin, polyimide, melamine resin, phenol resin, cellulose, polyester, polyphenylene sulfide, polyaramid, polyamideimide, and copolymer of butyl acrylate and ethyl methacrylate; and/or

The ethoxy-containing polymer comprises at least one of allyl polyethylene glycol and bisallyl polyether; and/or

The initiator comprises at least one of di-tert-butyl peroxide and dicumyl peroxide; and/or

The reaction temperature is 130°C to 160°C; and/or

The polymer particles and the nonionic surfactant are emulsified in a mass ratio of (4 to 6):1.
The preparation method according to claim 14 or 15, characterized in that the non-ionic polymer emulsion satisfies at least any one of the following conditions (1) to (4):

(1) The viscosity of the nonionic polymer emulsion at 25° C. is 10 mPa·s to 100 mPa·s;

(2) The solid content of the nonionic polymer emulsion is 35wt% to 40wt%;

(3) The pH value of the nonionic polymer emulsion is 5 to 6;

(4) The nonionic surfactant includes a molecular structure of Wherein, R1 is a carbon chain alkyl group, R2 is a hydrophilic group, and n is a positive integer from 3 to 20.
The preparation method according to claim 16, characterized in that the carbon chain alkyl is a long carbon chain alkyl of C12 to C18; and/or

The R2 is at least one of a carboxyl group, an amine group, an aldehyde group, an alcohol group, an amino group, and an amide group.
The preparation method according to any one of claims 14 to 17 is characterized in that the nonionic surfactant includes at least one of fatty alcohol polyoxyethylene ether, alkylphenol polyoxyethylene ether, fatty acid polyoxyethylene ester, castor oil polyoxyethylene ether, fatty amine polyoxyethylene ether, sorbitan fatty acid ester, and polyoxyethylene sorbitan fatty acid ester.
The preparation method according to any one of claims 11 to 18, characterized in that the coating slurry is applied at a rate of 20 m/min to 100 m/min, optionally 50 m/min to 100 m/min; and/or

The drying temperature is 50°C to 70°C, and can be 60°C to 70°C; and/or

The drying time is 1 min to 5 min, and can be optionally 1 min to 3 min.
A battery, characterized in that it comprises a positive electrode, a negative electrode and a separator stacked between the positive electrode and the negative electrode, wherein the separator is the separator according to any one of claims 1 to 10 or a separator prepared by the preparation method according to any one of claims 11 to 19.
The battery according to claim 20 is characterized in that the coating is provided on one surface of the separator, and the coating faces the negative electrode.
An electrical device, characterized in that the electrical device comprises a battery as described in claim 20 or 21, and the battery is used to provide electrical energy.