Preparation method of composite diaphragm, secondary battery and electric device

Abstract

The invention provides a preparation method of a composite diaphragm, a secondary battery and an electric device. The composite membrane comprises a base membrane, the base membrane is sequentially divided into a first area, a second area and a third area in the width direction of the base membrane, the first area, the second area and the third area comprise heat-resistant coatings and adhesive coatings, and the surface density of the heat-resistant coatings in the second area is larger than that of the first area and that of the third area; and / or the surface density of the second area bonding coating is smaller than that of the first area and the third area. The surface densities of the heat-resistant coatings and the adhesive coatings in the first region, the second region and the third region on the base film are differentially coated, so that the infiltration condition of electrolyte in the middle positions of the positive and negative pole pieces of the secondary battery can be improved, and the transport impedance of active ions in the middle positions of the positive and negative pole pieces is reduced; the problem of lithium precipitation caused by poor infiltration of the middle positions of the positive and negative pole pieces is solved, and the safety performance and the cycle performance of the secondary battery are improved.

Description

Technical Field

[0001] The present application belongs to the technical field of secondary batteries, and specifically relates to a composite diaphragm preparation method, a secondary battery and an electrical device. Background Art

[0002] Secondary batteries, also known as rechargeable batteries, not only provide a strong and lasting source of power for various new energy vehicles, but also provide an efficient energy management solution for energy storage systems. However, obvious problems also limit the development of secondary batteries. In the preparation process of existing square shell batteries, the electrolyte is injected from the injection port. The infiltration path of the electrolyte in the battery is: first occupy the gaps around the aluminum shell, then infiltrate from the gaps around the aluminum shell to the electrode, and finally infiltrate the middle of the electrode. Therefore, the middle of the positive and negative electrode is a weak area for electrolyte infiltration. When the middle of the electrode is poorly infiltrated, the interface impedance is large, lithium insertion is difficult, and the dark mark in the middle of the freshly fully charged interface of the battery affects the charge and discharge rate and cycle life of the battery. Summary of the invention

[0003] The present application is made in view of the above-mentioned problems, and its purpose is to provide a composite diaphragm preparation method, a secondary battery and an electrical device to improve the wetting effect of the electrode liquid in the middle of the positive and negative electrode sheets, reduce the transport impedance of the active ions in the middle of the positive and negative electrode sheets, solve the lithium plating problem caused by poor wetting in the middle of the positive and negative electrode sheets, and improve the rate performance and cycle performance of the secondary battery.

[0004] In the first aspect of the present application, a secondary battery is provided, the secondary battery comprises a composite diaphragm, the composite diaphragm comprises a base film, the base film is divided into a first region, a second region and a third region in sequence along the width direction of the base film, the first region, the second region and the third region comprise a heat-resistant coating and an adhesive coating, the surface density of the heat-resistant coating in the second region is greater than that in the first region and the third region, and / or the surface density of the adhesive coating in the second region is less than that in the first region and the third region. Thus, the wetting effect of the middle position of the positive and negative electrode sheets in the secondary battery can be improved, and the rate performance and cycle life of the secondary battery can be improved.

[0005] In any embodiment, the surface density of the heat-resistant coating in the third region is greater than that in the first region, and / or the surface density of the adhesive coating in the third region is less than that in the first region. Thus, the wetting effect of the middle part of the positive and negative electrode sheets in the secondary battery can be further improved, and the rate performance and cycle life of the secondary battery can be improved.

[0006] In any embodiment, the widths of the first region, the second region, and the third region are 30%-40%, 20%-40%, and 30%-40% of the width of the base film, respectively. Controlling the width of the second region to 20%-40% of the width of the base film can achieve a balance between improving the wetting effect of the middle part of the positive and negative electrode sheets, increasing the cycle life and rate performance of the secondary battery, and maintaining the safety performance of the secondary battery.

[0007] In any embodiment, the surface density of the heat-resistant coating in the first region is σ1, the surface density of the heat-resistant coating in the second region is σ2, and the surface density of the heat-resistant coating in the third region is σ3, wherein 1g / cm2≤σ1≤1.85g / cm2, 1.1g / cm2≤σ2≤2.405g / cm2, and 1g / cm2≤σ3≤2.035g / cm2. The heat-resistant coating on the diaphragm can not only improve the heat resistance of the diaphragm, but also increase the hardness of the diaphragm, so that the diaphragm is not easy to deform during the charge and discharge cycle of the secondary battery. If the surface density of the heat-resistant coating on the diaphragm is too large, the negative electrode sheet will expand during the charge and discharge cycle of the secondary battery, and the diaphragm will support the negative electrode sheet to inhibit its expansion. The stress generated on the negative electrode sheet is concentrated, and the internal expansion force cannot be released, which may cause the negative electrode sheet to break, affecting the safety performance of the secondary battery; if the surface density of the heat-resistant coating is too small, the improvement effect on the wetting of the middle area of ​​the positive and negative electrode sheets of the secondary battery is not significant. Controlling the surface density of the inorganic coating in the second region at 110%-130% of the surface density of the inorganic coating in the first region can improve the wetting effect of the middle part of the positive and negative pole pieces in the secondary battery, and achieve a balance between improving the rate performance and cycle life of the secondary battery and maintaining the safety performance of the secondary battery.

[0008] In any embodiment, the surface density of the adhesive coating in the first region is σ4, the surface density of the heat-resistant coating in the second region is σ5, and the surface density of the heat-resistant coating in the third region is σ6, wherein 1.5g / cm2≤σ4≤2g / cm2, 0.9g / cm2≤σ5≤1.6g / cm2, and 1.2g / cm2≤σ6≤2g / cm2. The negative electrode sheet will expand during the charging process of the secondary battery and shrink during the discharge of the secondary battery. If there is no external restraint and restriction, the free expansion and contraction of the negative electrode sheet will cause the sheet to wrinkle, further produce purple spots and lithium precipitation, and affect the life of the secondary battery. One of the functions of the adhesive coating on the diaphragm is to bond the diaphragm and the negative electrode sheet together to prevent wrinkling caused by the free expansion of the negative electrode sheet. If the amount of adhesive coating added is too little, the bonding effect is poor, which is easy to cause wrinkling of the negative electrode sheet. If the amount of adhesive coating added is too much, the improvement effect of the electrolyte infiltration in the middle of the positive and negative electrode sheets is not significant. Controlling the surface density of the adhesive coating in the second region to 60%-80% of the surface density of the organic coating in the first region can strike a balance between preventing the negative electrode from wrinkling and improving the wetting effect in the middle of the positive and negative electrode sheets.

[0009] In any embodiment, the heat-resistant coating comprises at least one of inorganic particles or organic particles; and / or the adhesive coating comprises an organic polymer.

[0010] In any embodiment, the bond coating is located on the heat resistant coating.

[0011] The second aspect of the present application provides a method for preparing a composite diaphragm, wherein an organic polymer, an adhesive and water are mixed and stirred to obtain an organic slurry; inorganic particles or organic particles, an adhesive and water are mixed and stirred to obtain an inorganic slurry; a base film is taken, the flow rate of the inorganic slurry in the first and third regions is controlled to be smaller than that in the second region, the inorganic slurry is differentially coated on the base film, dried and allowed to stand; and / or the flow rate of the organic slurry in the first and third regions is controlled to be larger than that in the second region, the organic slurry is coated on the heat-resistant coating, dried and allowed to stand. The composite diaphragm prepared by the method of the present application has a surface density of a heat-resistant coating in the second region that is greater than that in the first and third regions, and / or a surface density of an adhesive coating in the second region that is less than that in the first and third regions. When the composite diaphragm is used in a secondary battery, it can effectively improve the infiltration effect of the electrolyte in the middle of the positive and negative electrode sheets, and improve the rate performance and cycle life of the secondary battery.

[0012] In any embodiment, the flow rate of the inorganic slurry in the third region is greater than that in the first region; and / or the flow rate of the organic slurry in the third region is less than that in the first region. The flow rate of the inorganic slurry in the third region is controlled to be greater than that in the first region, and / or the flow rate of the organic slurry in the third region is controlled to be less than that in the first region. The surface density of the heat-resistant coating in the third region of the composite diaphragm obtained is greater than the surface density of the inorganic coating in the first region, and / or the surface density of the adhesive coating in the third region is less than the surface density of the organic coating in the first region, which can further improve the wetting effect of the electrolyte in the middle of the positive and negative electrode sheets and improve the rate performance and cycle life of the secondary battery.

[0013] A third aspect of the present application provides an electric device, which includes the secondary battery in the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments of the present application. Obviously, the drawings described below are only some implementation methods of the present application, and for ordinary technicians in this field, other drawings can be obtained based on the drawings without creative work.

[0015] Figure 1 is a schematic diagram of an embodiment of the composite diaphragm of the present application;

[0016] Figure 2 is a schematic diagram of an embodiment of a secondary battery of the present application;

[0017] Figure 3 is an exploded schematic diagram of an embodiment of a secondary battery of the present application;

[0018] Figure 4 is a schematic diagram of an embodiment of a battery module of the present application;

[0019] Figure 5 is a schematic diagram of an embodiment of a battery pack of the present application;

[0020] Figure 6 yes Figure 5 An exploded schematic diagram of an embodiment of a battery pack is shown;

[0021] Figure 7 is a schematic diagram of an embodiment of an electric device including the secondary battery of the present application as a power source;

[0022] In the drawings, the drawings may not be drawn according to the actual scale. The reference numerals are explained as follows: 1 battery pack, 2 upper box, 3 lower box, 4 battery module, 5 secondary battery, 51 housing, 52 electrode assembly, 53 cover plate. DETAILED DESCRIPTION

[0023] Hereinafter, the embodiments of the composite diaphragm and its preparation method, secondary battery and electrical device of the present application are specifically disclosed with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following descriptions are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0024] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0025] If not otherwise specified, all embodiments and optional embodiments of the present application may be combined with each other to form new technical solutions, and such technical solutions should be deemed to be included in the disclosure of the present application.

[0026] Unless otherwise specified, all technical features and optional technical features of the present application may be combined with each other to form new technical solutions, and such technical solutions should be deemed to be included in the disclosure of the present application.

[0027] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0028] If there is no special explanation, the "include" and "comprising" mentioned in this application represent open-ended or closed-ended expressions. For example, the "include" and "comprising" may represent that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0029] If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0030] If not specifically stated, the terms used in this application have the commonly known meanings generally understood by those skilled in the art.

[0031] If not otherwise specified, the values ​​of the parameters mentioned in this application can be measured using various test methods commonly used in the art, for example, they can be measured according to the test methods given in this application.

[0032] In the present application, the terms "plurality" and "multiple" refer to two or more.

[0033] If not otherwise specified, in the present application, the term "active ions" refers to ions that can be intercalated and extracted between the positive electrode and the negative electrode of a secondary battery, including but not limited to lithium ions.

[0034] Secondary batteries, also known as rechargeable batteries, not only provide a strong and lasting source of power for various new energy vehicles, but also provide an efficient energy management solution for energy storage systems. However, obvious problems also limit the development of secondary batteries. In the preparation process of existing square shell batteries, the electrolyte is injected from the injection nozzle. The infiltration path of the electrolyte in the battery is that the electrolyte first occupies the gaps around the aluminum shell, and then infiltrates from the gaps around the aluminum shell to the electrode, and finally infiltrates the middle of the electrode. Therefore, the middle of the positive and negative electrode is a weak area for electrolyte infiltration. When the middle of the electrode is poorly infiltrated, the interface impedance is large, lithium insertion is difficult, and the dark mark in the middle of the interface of the battery is freshly fully charged. Lithium is deposited, which affects the charge and discharge rate and cycle life of the battery.

[0035] Based on this, the present application provides a composite diaphragm preparation method, a secondary battery and an electrical device.

[0036] In a first aspect of the present application, a secondary battery is provided, the secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. During the charge and discharge process of the secondary battery, active ions are intercalated and removed between the positive electrode sheet and the negative electrode sheet, the electrolyte plays a role in conducting active ions between the positive electrode sheet and the negative electrode sheet, and the composite separator is arranged between the positive electrode sheet and the negative electrode sheet to isolate the positive electrode sheet from the negative electrode sheet.

[0037] [Diaphragm]

[0038] The diaphragm adopts the composite diaphragm obtained by the preparation method described in the present application, and the composite diaphragm includes a base film, and the base film is divided into a first region, a second region and a third region in sequence along the width direction of the base film, and the first region, the second region and the third region include a heat-resistant coating and an adhesive coating, and the surface density of the heat-resistant coating in the second region is greater than that in the first region and the third region, and / or the surface density of the adhesive coating in the second region is less than that in the first region and the third region. The surface density of the heat-resistant coating in the second region on the base film is greater than that in the first region and the third region, and / or the surface density of the adhesive coating in the second region is less than that in the first region and the third region. In this way, the wetting effect of the middle position of the positive and negative pole pieces in the secondary battery can be improved, and the rate performance and cycle life of the battery cell can be improved.

[0039] Figure 1 is a schematic diagram of an embodiment of a composite diaphragm prepared in the present application, such as Figure 1 The composite diaphragm shown includes a base membrane, and along the width direction of the base membrane, the base membrane can be divided into a first region, a second region and a third region in sequence. The first region, the second region and the third region are coated with a heat-resistant coating and an adhesive coating by differential coating. The surface density of the heat-resistant coating in the second region of the obtained composite diaphragm is greater than that in the first region and the third region, and / or the surface density of the adhesive coating in the second region is less than that in the first region and the third region.

[0040] In this article, the term "base film" refers to the basic material layer that constitutes the separator. It is the main part of the separator and has a porous structure. It is usually made of materials with certain mechanical strength, chemical stability and insulation properties, such as polyolefins (such as PP, PE), etc. The base film plays a supporting and protective role in the separator, providing the separator with the necessary physical and chemical properties to ensure its stability and safety in applications such as batteries.

[0041] In this article, the term "heat-resistant coating" refers to a functional coating used to improve the heat resistance of the base film. The heat-resistant coating is often used in the preparation of separators to improve the heat resistance and safety of batteries.

[0042] In this article, the term "adhesive coating" refers to a functional coating used to improve the bonding performance between the diaphragm and other parts of the battery cell (such as the pole piece), and its function is to enhance the stability and reliability of the battery structure. Materials such as PVDF (polyvinylidene fluoride) are usually used as adhesive coatings for diaphragms. The bonding properties of PVDF can tightly bond the pole piece to the diaphragm, remove the air in the internal gap, increase the hardness of the battery cell, and maintain the consistency of the thickness of the battery cell.

[0043] In this article, the term "areal density" refers to the mass of the coating per unit area of ​​the base film, expressed in g / m 2 .

[0044] For ease of understanding, the principle of the above-mentioned beneficial effects of the composite diaphragm in the secondary battery of the present application is explained below: the higher the content of the inorganic coating on the diaphragm, the greater its surface energy. According to the wetting theory formula, the relationship between the wetting height h and the time t is as follows:

[0045]

[0046] Where, σ is the surface tension of the liquid, θ is the contact angle between the electrolyte and the porous medium, and r e is the effective capillary radius of the porous medium, μ is the viscosity of the electrolyte, k is the permeability of the porous medium, and is the pressure gradient in the x direction; ∈ is the porous medium

[0047] The porosity of the porous medium; B is the capillary geometry coefficient; the cosθ calculation formula is as follows:

[0048]

[0049] where γ SG is the solid surface energy, γ LG is the surface energy of the liquid, γ SL is the surface energy between solid and liquid; since the surface energy of the heat-resistant coating is relatively high, the wetting angle cosθ is large, and the wetting height h is larger at the same time t; the electrolyte will have a better wetting effect along the diaphragm. If the surface density of the heat-resistant coating in the second region of the base membrane is controlled to be greater than that in the first and third regions, the surface energy of the heat-resistant coating in the second region of the base membrane will be greater, and the electrolyte will have a better wetting effect along the second region; and the role of the adhesive coating is mainly to provide adhesion between the diaphragm and the pole piece. The smaller the surface density of the adhesive coating, the smaller the mass of the adhesive coating per unit area of ​​the base membrane, the smaller the adhesive force of the adhesive coating between the base membrane and the pole piece, and the smaller the adhesive force between the base membrane and the pole piece, the larger the gap between the base membrane and the pole piece. If the surface density of the adhesive coating in the second region of the base membrane is controlled to be less than that in the first and third regions, the adhesive force between the second region of the base membrane and the pole piece is small, and the gap between the diaphragm and the pole piece in the second region is larger. The electrolyte will preferentially infiltrate the area with a large gap, thereby improving the wetting effect of the pole piece in the second region of the base membrane.

[0050] Therefore, by dividing the base film into the first region, the second region, and the third region in sequence along the width direction, and controlling the surface density of the heat-resistant coating in the second region to be greater than the surface density of the heat-resistant coating in the first region and the third region, and / or the surface density of the adhesive coating in the second region to be less than the surface density of the adhesive coating in the first region and the third region, the problems of large interface impedance and dark mark lithium deposition caused by poor wetting in the middle of the positive and negative pole pieces of the secondary battery can be solved, thereby improving the cycle performance and rate performance of the secondary battery.

[0051] In some embodiments, the areal density of the heat-resistant coating in the third region is greater than that in the first region, and / or the areal density of the adhesive coating in the third region is less than that in the first region. After injecting electrolyte into the secondary battery, 1 / 3 of the electrolyte volume contacts the upper part of the bare JR and infiltrates downward along the gap between the JRs. The remaining 2 / 3 of the electrolyte preferentially occupies the gap between the cell housing and the bare JR and accumulates at the bottom of the battery under the action of gravity; the electrolyte at the bottom creeps upward along the gap between the JRs. Based on this design, during the infiltration process of 2 / 3 of the electrolyte at the bottom of the cell, the infiltration effect in the third region will be better, and the faster the electrolyte in the third region infiltrates and creeps into the second region, the better the infiltration effect of the electrolyte in the second region can be improved, and the better the overall infiltration effect of the secondary battery. Since the electrolyte provides a channel for ion migration, there are more ion channels available for lithium ions to migrate, which is beneficial to the transport of lithium ions and can improve the charge-discharge rate and cycle life of the secondary battery. By controlling the areal density of the heat-resistant coating in the third region to be greater than that in the first region, and / or the areal density of the adhesive coating in the third region to be less than that in the first region, in this design, the adhesion between the separator and the electrode in the third region is less than that in the first region, so the gap between the separator and the electrode in the third region is greater than the gap between the separator and the electrode in the first region; the density of the heat-resistant coating in the third region is greater than that in the first region. Since the surface energy of the heat-resistant coating is high, the electrolyte infiltration effect is better, so the infiltration effect in the third region will be better than that in the first region.

[0052] In some embodiments, the widths of the first region, the second region, and the third region are 30-40%, 20-40%, and 30%-40% of the width of the base film, respectively.

[0053] In some embodiments, the width of the first region can be selected as 30%, 35%, 40% of the width of the base film, or any value range between any two of them.

[0054] In some embodiments, the width of the second region can be selected as 20%, 25%, 30%, 35%, 40% of the width of the base film, or any value range between any two of them.

[0055] In some embodiments, the width of the third region can be selected as 30%, 35%, 40% of the width of the base film, or any value range between any two of them.

[0056] Based on the consideration of the position of the dark mark on the interface when the secondary battery is fully charged, the position of the dark mark is mainly in the middle area of ​​the pole piece, that is, the second area. If the width of the second area is less than 20% of the width of the base film, the width is too narrow and cannot improve the wetting effect of the middle position of the pole piece; if the width of the second area is greater than 40% of the width of the base film, the overall adhesion between the base film and the pole piece deteriorates, the base film restrains the negative pole piece less, and the diaphragm is prone to wrinkling. When the secondary battery is charged, the wrinkled position is prone to lithium deposition, affecting the safety performance of the secondary battery. Controlling the width of the second area within the range of 20%-40% of the width of the base film can achieve a balance between improving the wetting effect of the middle of the positive and negative pole pieces and improving the safety performance of the secondary battery.

[0057] In some embodiments, the surface density of the heat-resistant coating in the first region is σ1, the surface density of the heat-resistant coating in the second region is σ2, and the surface density of the heat-resistant coating in the third region is σ3, wherein 1g / cm2≤σ1≤1.85g / cm2, 1.1g / cm2≤σ2≤2.405g / cm2, and 1g / cm2≤σ3≤2.035g / cm2.

[0058] In some embodiments, the surface density σ1 of the heat-resistant coating in the first region may be 1 g / cm2, 1.2 g / cm2, 1.4 g / cm2, 1.6 g / cm2, 1.85 g / cm2 or any range therebetween.

[0059] In some embodiments, the surface density σ2 of the heat-resistant coating in the second region can be selected to be 1 g / cm2, 1.1 g / cm2, 1.4 g / cm2, 1.7 g / cm2, 2.0 g / cm2, 2.405 g / cm2 or any range therebetween.

[0060] In some embodiments, the surface density σ3 of the heat-resistant coating in the second region can be selected to be 1 g / cm2, 1.3 g / cm2, 1.6 g / cm2, 1.9 g / cm2, 2.035 g / cm2 or any range therebetween.

[0061] Controlling the surface density of the heat-resistant coating in the second region to 110%-130% of the surface density of the heat-resistant coating in the first region can achieve a balance between improving the wetting effect of the middle part of the positive and negative electrode sheets and improving the safety performance of the secondary battery. If the surface density of the heat-resistant coating in the second region is less than 110% of the surface density of the heat-resistant coating in the first region, the surface energy of the heat-resistant coating in the second region is not much different from that in the first region, and the effect of the electrolyte preferentially wetting the second region is not obvious, and the effect of improving the lithium precipitation of the dark marks in the middle part of the positive and negative electrode sheets is not obvious. If the surface density of the heat-resistant coating in the second region is greater than 130% of the heat-resistant coating in the first region, the negative electrode sheet will expand during the battery charging and discharging process due to the hard heat-resistant coating. When the negative electrode sheet expands, the presence of inorganic particles will inhibit the expansion of the negative electrode sheet. The higher the amount of heat-resistant coating, the greater the effect on the expansion of the negative electrode sheet, and the expansion force inside the negative electrode sheet cannot be released, resulting in stress concentration in the area with greater expansion. When the stress exceeds a certain level, the negative electrode sheet breaks, affecting the normal use of the secondary battery and even causing safety risks. The surface density of the heat-resistant coating in the second region is controlled at 110%-130% of the surface density of the heat-resistant coating in the first region, which can strike a balance between improving the electrolyte wetting effect in the middle of the positive and negative electrode sheets and maintaining the safety of the secondary battery.

[0062] In some embodiments, the surface density of the bonding coating in the first region is σ4, the surface density of the bonding coating in the second region is σ5, and the surface density of the bonding coating in the third region is σ6, wherein 1.5g / cm2≤σ4≤2g / cm2, 0.9g / cm2≤σ5≤1.6g / cm2, and 1.2g / cm2≤σ6≤2g / cm2.

[0063] In some embodiments, the surface density σ4 of the bonding coating in the first region may be 1.5 g / cm2, 1.6 g / cm2, 1.7 / cm2, 1.8 g / cm2, 1.9 g / cm2, 2.0 g / cm2 or any range therebetween.

[0064] In some embodiments, the surface density σ5 of the second region bonding coating can be selected as 0.9 g / cm2, 1.0 g / cm2, 1.1 g / cm2, 1.2 g / cm2, 1.3 g / cm2, 1.4 g / cm2, 1.5 g / cm2, 1.6 g / cm2 or any range therebetween.

[0065] In some embodiments, the surface density σ6 of the bonding coating in the third region may be 1.2 g / cm2, 1.4 g / cm2, 1.6 g / cm2, 1.8 g / cm2, 2.0 g / cm2 or any range therebetween.

[0066] The surface density of the adhesive coating in the second region is controlled at 60%-80% of the surface density of the adhesive coating in the first region, so as to achieve a balance between improving the wetting effect in the middle of the positive and negative electrode sheets and improving the safety performance of the secondary battery.

[0067] If the surface density of the bonding coating in the second region is 60% less than the surface density of the bonding coating in the first region, since the negative electrode sheet expands and contracts during the process of heavy charging and discharging, the negative electrode sheet expands during charging and shrinks during discharging. On the large surface of the battery cell, the negative electrode sheet has surplus such as gaps at the corners in the longitudinal direction compared to the transverse direction, but is shorter in the transverse direction and is in a stretched state. Therefore, the negative electrode sheet is prone to wrinkles in the longitudinal direction. The presence of the bonding coating will bond the diaphragm and the negative electrode sheet together. Under the constraint of the diaphragm, the wrinkling of the negative electrode sheet is suppressed. When the bonding coating content is insufficient, the inhibitory effect on the wrinkling of the negative electrode sheet is greatly weakened. The wrinkled position is more difficult to deintercalate lithium, affecting the performance of the battery cell. If the surface density of the bonding coating in the second region is greater than 80% of the surface density of the bonding coating in the first region, the bonding force between the diaphragm and the pole piece in the second region is not much different from the bonding force between the diaphragm and the pole piece in the first region, and the gap between the diaphragm and the pole piece in the second region is not much different from the gap between the diaphragm and the pole piece in the first region. The effect of the electrolyte preferentially infiltrating into the second region is not obvious, and the effect of improving the dark marks and lithium plating in the middle of the positive and negative pole pieces is not obvious.

[0068] In some embodiments, the heat-resistant coating comprises at least one of inorganic particles or organic particles; and the adhesion coating comprises an organic polymer.

[0069] Inorganic particles include boehmite, aluminum oxide, barium sulfate, magnesium oxide, magnesium hydroxide, silica, tin dioxide, titanium oxide, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, nickel oxide, hafnium dioxide, cerium oxide, zirconium titanate, barium titanate and magnesium fluoride; organic particles include polymethyl methacrylate (PMMA), polyimide (PI), polyphenylene sulfide (PPS).

[0070] Organic polymers used for base film adhesive coatings include polyvinylidene fluoride, polyacrylonitrile, and polyester.

[0071] In some embodiments, the bond coating is disposed on the heat resistant coating.

[0072] The above composite diaphragm is prepared by the following preparation method: first, an organic polymer, an adhesive and water are mixed and stirred to obtain an organic slurry, and then inorganic particles or organic particles, an adhesive and water are mixed and stirred to obtain an inorganic slurry; a base film is taken, the flow rate of the inorganic slurry in the first region and the third region is controlled to be smaller than that in the second region, the inorganic slurry is differentially coated on the base film to form a heat-resistant coating, dried and allowed to stand, and / or the flow rate of the organic slurry in the first region and the third region is controlled to be larger than that in the second region, the organic slurry is coated on the heat-resistant coating to form an adhesive coating, dried and allowed to stand.

[0073] The composite diaphragm prepared by the method of the present application has an inorganic coating surface density in the second region of the obtained composite diaphragm that is greater than that in the first region and the third region, and / or an organic coating surface density in the second region of the composite diaphragm is less than that in the first region and the third region. When the composite diaphragm is used in a secondary battery, it can effectively improve the wetting effect of the middle part of the positive and negative electrode sheets, and increase the charge and discharge rate and cycle life of the battery cell.

[0074] In some embodiments, the flow rate of the inorganic slurry in the third region is greater than that in the first region; and / or the flow rate of the organic slurry in the third region is less than that in the first region. The flow rate of the inorganic slurry in the third region is controlled to be greater than that in the first region, and the flow rate of the organic slurry in the third region is controlled to be less than that in the first region. The surface density of the inorganic coating in the third region of the composite diaphragm is greater than that in the first region, and / or the surface density of the organic coating in the third region is less than that in the first region. In this design, the adhesion between the diaphragm and the pole piece in the third region is less than that in the first region, and the gap between the diaphragm and the pole piece in the third region is greater than the gap between the base film and the pole piece in the first region. The surface density of the inorganic coating in the third region is greater than that in the first region. Since the surface energy of the inorganic coating is high, the electrolyte infiltration effect is better, so the infiltration effect of the third region will be better than that of the first region. After the battery is filled with electrolyte, 1 / 3 of the electrolyte contacts the upper part of the bare JR and infiltrates downward along the gap between the JRs. The remaining 2 / 3 of the electrolyte preferentially occupies the gap between the battery cell shell and the bare JRs, and is enriched at the bottom of the battery under the action of gravity; the electrolyte at the bottom climbs upward along the gap between the JRs. Based on this design, during the infiltration process of the 2 / 3 electrolyte at the bottom of the battery cell, the infiltration effect of the third area will be better. The faster the rate of the electrolyte in the third area infiltrating and climbing to the second area, the better the infiltration effect of the electrolyte in the second area, and the better the overall infiltration effect of the battery cell. Since the electrolyte provides channels for ion migration, there are more ion channels available for lithium ion migration, which is beneficial to the transportation of lithium ions and can improve the charge and discharge rate and cycle life of the battery cell.

[0075] [Positive electrode]

[0076] The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer which is optionally arranged on at least one surface of the positive electrode current collector. The positive electrode film layer comprises a positive electrode active material.

[0077] As an example, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode film layer is arranged on any one or both of the two opposite surfaces of the positive electrode current collector.

[0078] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.). .

[0079] In some embodiments, the positive electrode film layer includes a positive electrode active material. When the secondary battery is a lithium ion battery, the positive electrode active material may include at least one of the following materials: an olivine-structured lithium-containing phosphate, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi1 / 3Co1 / 3Mn1 / 3O2 (also referred to as NCM333), LiNi0.5Co0.2Mn0.3O2 (also referred to as NCM523), LiNi0.5Co0.25Mn0.25O2 (also referred to as NCM211), LiNi0.6Co0.2Mn0.2 ... At least one of NCM622), LiNi0.8Co0.1Mn0.1O2 (also referred to as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi0.85Co0.15Al0.05O2) and modified compounds thereof. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. The modified compounds of the above materials may be compounds that perform doping modification and / or surface coating modification on the materials.

[0080] In some embodiments, the positive electrode active material layer may further include a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0081] In some embodiments, the positive electrode active material layer may also optionally include a binder. As an example, the binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0082] In some embodiments, the positive electrode active material layer may also optionally include a lithium supplement. As an example, the lithium supplement may include lithium-rich compounds, binary lithium compounds, and composite materials based on conversion reactions. Among them, lithium-rich compounds such as Li2NiO2 are a typical positive electrode lithium supplement additive, which can extract and embed lithium ions during the charge and discharge process, thereby compensating for the lithium loss in the battery. In addition, there are other lithium compounds such as lithium oxide, lithium carbonate, lithium hydroxide, and composite materials prepared by specific processes, which can be used as lithium supplements.

[0083] In some embodiments, the positive electrode sheet can be prepared by the following method: dispersing the positive electrode active material, conductive agent, binder and any other components in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on the positive electrode collector, drying, cold pressing and other processes to form a positive electrode active material layer, and then spraying, secondary coating and other methods on the surface of the positive electrode active material layer to compound the lithium supplement agent with the positive electrode active material layer.

[0084] [Negative electrode]

[0085] The negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer which is optionally arranged on at least one surface of the negative electrode current collector. The negative electrode film layer comprises a negative electrode active material.

[0086] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

[0087] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0088] In some embodiments, the negative electrode film layer includes a negative electrode active material. In some embodiments, the negative electrode active material includes, but is not limited to, one or more of conventional natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may include one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy material. The tin-based material may include one or more of elemental tin, tin oxide, and tin alloy material.

[0089] In some embodiments, the negative electrode film layer may further include a negative electrode conductive agent. The present application has no particular restrictions on the type of the negative electrode conductive agent. As an example, the negative electrode conductive agent may include one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0090] In some embodiments, the negative electrode film layer may further include a negative electrode binder. The present application has no particular restrictions on the type of the negative electrode binder. As an example, the negative electrode binder may include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylic resin (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

[0091] In some embodiments, the negative electrode film layer may further include other additives. As an example, the other additives may include a thickener, such as sodium carboxymethyl cellulose (CMC), a PTC thermistor material, and the like.

[0092] The negative electrode film layer is usually formed by coating the negative electrode slurry on the negative electrode current collector, drying and cold pressing. The negative electrode slurry is usually formed by dispersing the negative electrode active material, optional conductive agent, optional binder, and other optional auxiliary agents in a solvent and stirring them uniformly. The solvent can be N-methylpyrrolidone (NMP) or deionized water, but is not limited thereto.

[0093] The negative electrode plate does not exclude other additional functional layers in addition to the negative electrode film layer. For example, in some embodiments, the negative electrode plate described in the present application also includes a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode collector and the negative electrode film layer and disposed on the surface of the negative electrode collector; in some embodiments, the negative electrode plate described in the present application also includes a protective layer covering the surface of the negative electrode film layer.

[0094] [Electrolytes]

[0095] In some embodiments, the electrolyte is an electrolyte solution, and the electrolyte solution includes an electrolyte salt and a solvent.

[0096] The type of the electrolyte salt is not specifically limited and can be selected according to actual needs. As an example, the electrolyte salt may include lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium perchlorate (LiClO 4 ) lithium hexafluoroarsenate (LiAsF 6 ), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO 2 F 2 ), one or more of lithium difluorobis(oxalate) phosphate (LiDFOP) and lithium tetrafluorooxalate phosphate (LiTFOP).

[0097] The type of the solvent is not specifically limited and can be selected according to actual needs. In some embodiments, as an example, the solvent may include ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), cyclopentane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS) and diethyl sulfone (ESE) One or more.

[0098] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain properties of the secondary battery, such as additives that improve the overcharge performance of the secondary battery, additives that improve the high temperature performance of the secondary battery, and additives that improve the low temperature power performance of the secondary battery.

[0099] The present application has no particular limitation on the type of secondary battery. For example, the secondary battery may be a lithium ion battery, a sodium ion battery, a lithium sulfur battery, or the like.

[0100] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square or any other shape. Figure 2 The secondary battery 5 is a square structure as an example.

[0101] In some embodiments, Figure 3 As shown, the outer package may include a shell 51 and a cover plate 53. The shell 51 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 51 has an opening connected to the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation film can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, which can be adjusted according to demand.

[0102] The preparation method of the secondary battery of the present application is well known. In some embodiments, the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte can be assembled to form a secondary battery. As an example, the positive electrode sheet, the separator and the negative electrode sheet can be formed into an electrode assembly through a winding process or a lamination process, and the electrode assembly is placed in an outer package, and the electrolyte is injected after drying, and the secondary battery is obtained through vacuum packaging, standing, forming, shaping and other processes.

[0103] In some embodiments of the present application, the secondary batteries according to the present application can be assembled into a battery module. The battery module can contain multiple secondary batteries, and the specific number can be adjusted according to the application and capacity of the battery module.

[0104] Figure 4 FIG. 4 is a schematic diagram of a battery module 4 as an example. Figure 4 As shown, in the battery module 4, the plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of secondary batteries 5 may be fixed by fasteners.

[0105] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0106] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0107] Figure 5 and Figure 6 FIG. 1 is a schematic diagram of a battery pack 1 as an example. Figure 5 and Figure 6 As shown, the battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3, wherein the upper box body 2 is used to cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in the battery box in any manner.

[0108] A second aspect of the present application provides an electric device, which includes the secondary battery in the first aspect.

[0109] The present application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack of the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device can be, but is not limited to, a mobile device (such as a mobile phone, a tablet computer, 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.

[0110] The electrical device may select a secondary battery, a battery module or a battery pack according to its usage requirements.

[0111] Figure 7 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.

[0112] 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.

[0113] Example

[0114] The following examples describe the disclosure of the present application in more detail, and these examples are only for illustrative purposes, as it is obvious to those skilled in the art that various modifications and variations are made within the scope of the disclosure of the present application. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further processing, and the instruments used in the examples are commercially available.

[0115] (1) Preparation of composite diaphragm

[0116] Example 1

[0117] Take PVDF and polyacrylate, add them into deionized water in a mass ratio of 9:1, and stir in a high-speed mixer to obtain a uniformly dispersed organic slurry; similarly, take Al2O3 and polyacrylate, add them into deionized water in a mass ratio of 9:1, and stir in a high-speed mixer to obtain a uniformly dispersed inorganic slurry. Take the polyethylene film as the base film, and coat the inorganic slurry on both sides of the PE base film through a coating die in an extrusion coating manner. Differential coating is achieved by controlling the flow rate of the extrusion die in different areas. The specific method is as follows: set the coating speed to 20m / min, the extrusion die is 3um away from the polyethylene film, the area where the slurry is sprayed out of the coating machine head is equal to the width of the base film, and a few layers of Teflon (or gaskets) are pasted on the discharge position of the extruded slurry position corresponding to the base film position in the first / third area specified above. After the inorganic slurry is coated on both sides of the polyethylene film, it is dried and allowed to stand (the drying temperature is between 95-110°C, the drying time is 15-20 minutes, and then allowed to stand at room temperature for 2h); similarly, the organic slurry is extruded and differentially coated on both sides of the heat-resistant coating to obtain a composite diaphragm. The surface density of the inorganic coating in the first and third regions of the obtained composite membrane is 1.5g / cm2, and the surface density of the inorganic coating in the second region is 1.8g / cm2; the surface density of the organic coating in the first and third regions is 1.85g / cm2, and the surface density of the organic coating in the second region is 1.295g / cm2.

[0118] At this time, the surface density of the inorganic coating in the second region is 1.2 times that of the inorganic coating in the first and third regions, and the surface density of the organic coating in the second region is 0.7 times that of the organic coating in the first and third regions.

[0119] Example 2

[0120] The preparation method of Example 2 is basically the same as that of Example 1, except that the surface density of the inorganic coating and the organic coating in the third region is changed, so that the surface density of the inorganic coating in the first region is less than the surface density of the inorganic coating in the third region, and the surface density of the organic coating in the first region is greater than the surface density of the organic coating in the third region, as shown in Table 1.

[0121] Embodiment 3-6

[0122] The preparation methods of Examples 3-6 are basically the same as those of Example 1, except that the surface density of the inorganic coating in the second region is changed. The surface density of the inorganic coating in the second region is 1.05 times, 1.1 times, 1.3 times, and 1.4 times that of the first region and the third region, respectively, as shown in Table 1.

[0123] Examples 7-10

[0124] The preparation methods of Examples 7-10 are basically the same as those of Example 1, except that the surface density of the organic coating in the second region is changed. The surface density of the organic coating in the second region is 0.5 times, 0.6 times, 0.8 times, and 0.9 times that of the first region and the third region, respectively, as shown in Table 1.

[0125] Examples 11-14

[0126] The preparation methods of Examples 11-14 are basically the same as that of Example 1, except that the width of the second region is changed. The width of the second region is 30%, 40%, 10%, and 50% of the width of the base film, respectively, as shown in Table 1.

[0127] Embodiment 15

[0128] The preparation method of Example 15 is basically the same as that of Example 1, except that the surface density of the inorganic coating in the first and third regions is 1.5 g / cm2, the surface density of the inorganic coating in the second region is 1.2 times that of the inorganic coating in the first and third regions, and the surface density of the organic coating in the first, second and third regions is equal, which is 1.85 g / cm2;

[0129] Example 16

[0130] The preparation method of Example 16 is basically the same as that of Example 1, except that the surface density of the inorganic coating in the first, second and third regions of the inorganic coating is equal, which is 1.5 g / cm2, and the surface density of the organic coating in the first and third regions is equal, which is 1.85 g / cm2, and the surface density of the organic coating in the second region is 0.7 times that of the first & third regions;

[0131] Embodiment 17

[0132] The preparation method of Example 17 is basically the same as that of Example 1, except that the surface density of the inorganic coating in the first region is 1.5 g / cm2, the surface density of the inorganic coating in the second region is 1.8 g / cm2, the surface density of the inorganic coating in the third region is 1.8 g / cm2, the surface density of the organic coating in the first and third regions is the same, both 1.85 g / cm2, and the surface density of the organic coating in the second region is 0.7 times that of the first and third regions;

[0133] Embodiment 18

[0134] The preparation method of Example 18 is basically the same as that of Example 1, except that the surface density of the inorganic coating in the first and third regions is the same; both are 1.5 g / cm2, the surface density of the inorganic coating in the second region is 1.2 times that of the first and third regions, the surface density of the organic coating in the first region is 1.85 g / cm2, and the surface density of the organic coating in the second and third regions is 0.7 times that of the first region;

[0135] Comparative Example 1

[0136] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that the surface densities of the inorganic coating and the organic coating in the first region, the second region, and the third region are the same, as shown in Table 1.

[0137] (2) Preparation of lithium-ion secondary batteries

[0138] (a) Positive electrode

[0139] The lithium-rich positive electrode material 0.4Li2MnO3·0.6LiNi0.5Mn0.5O2, the conductive agent acetylene black, and the binder PVDF (polyvinylidene fluoride) are mixed in a weight ratio of 94:4:2, and the solvent N-methylpyrrolidone is added, and the mixture is fully stirred to obtain a positive electrode slurry; the positive electrode slurry is coated on the surface of the positive electrode current collector aluminum foil, and after drying, cold pressing, and die-cutting, a positive electrode sheet is obtained.

[0140] (b) Negative electrode

[0141] Artificial graphite, conductive agent acetylene black, binder SBR (styrene butadiene rubber), binder CMC (sodium carboxymethyl cellulose) are mixed in a certain mass ratio of 95:1.5:3.1:0.4, and deionized water is added as a solvent, and the mixture is stirred and mixed to obtain the negative electrode slurry. The negative electrode slurry is then coated on both surfaces of the negative electrode current collector copper foil, and after drying, cold pressing, and die cutting, the negative electrode sheet is obtained.

[0142] (c) Isolation film

[0143] The composite diaphragm as described above or the composite diaphragm prepared by the method as described above.

[0144] (d) Electrolyte

[0145] In an argon atmosphere glove box with a water content of <10ppm, EC (ethylene carbonate), PC (polycarbonate), and DMC (dimethyl carbonate) are mixed in a weight ratio of 3:3:3, and then LiPF6, VC, DTD and PS are added and stirred evenly to obtain an electrolyte, in which the concentration of LiPF6 is 1 mol / L, and the mass percentages of VC, DTD, and PS in the total mass of the electrolyte are 3%, 1%, and 1%, respectively.

[0146] (e) Battery assembly

[0147] The positive electrode sheet, separator and negative electrode sheet prepared in the above steps are stacked in order, so that the separator is located between the positive electrode sheet and the negative electrode sheet and can separate the positive electrode sheet from the negative electrode sheet, and the positive electrode sheet, the negative electrode sheet and the separator are wound to obtain an electrode assembly; the electrode assembly is set in a shell, and the electrolyte is injected after drying; after the formation and standing processes, a lithium-ion battery is obtained.

[0148] Performance Testing

[0149] (1) Test of the liquid creep rate of bare cells

[0150] ①Prepare a steel ruler, electrolyte, two bare JRs, aluminum shell, scissors and plastic wrap;

[0151] ② Cut off the front 3-6% of the bare JR (cut off 10% according to the actual number of folds of the battery cell)

[0152] ③ Pour the electrolyte into the aluminum shell. The amount of electrolyte is the injection amount. Gently put the cut JR into it and start timing.

[0153] ④ Cover with plastic wrap to prevent the electrolyte from evaporating in the air, becoming thicker and affecting the crawling liquid;

[0154] ⑤ At 10, 20, 30, 40, 50, 60, 90, and 120 minutes of soaking, take out the JR and cut 4-6 folds, then cut the first and last folds, and test the remaining middle fold;

[0155] ⑥ Separate the positive and negative electrodes, measure the height of the lowest point of the negative electrode from the bottom and record it.

[0156] ⑦ According to the collected infiltration height data at different times, the relationship between the infiltration height h and the time t is as follows:

[0157]

[0158] Where, σ is the surface tension of the liquid, θ is the contact angle between the electrolyte and the porous medium, and r e is the effective capillary radius of the porous medium, μ is the viscosity of the electrolyte, k is the permeability of the porous medium, is the pressure gradient in the x direction; ∈ is the porosity of the porous medium; B is the capillary geometry coefficient; plot the infiltration height h versus the infiltration time The slope of the relationship diagram is the wetting rate of the bare JR.

[0159] (2) Lithium deposition test

[0160] Under a constant temperature of 25℃, the battery was charged to 4.4V at a constant current of 0.33C. The lithium deposition of the battery under different conditions was confirmed by disassembling the fully charged interface of fresh battery cells of different diaphragm groups. When lithium deposition occurs in the middle area of ​​the positive and negative electrodes of the secondary battery due to poor wetting, it is mostly strip-shaped or surface-shaped. The lithium deposition level can be confirmed according to the death state and the lithium deposition standard. The lithium deposition level is specified as follows:

[0161]

[0162] 3.DCR DC internal resistance test

[0163] ①Capacity test

[0164] At a constant temperature of 25°C, the battery was charged to 4.4V at a constant current of 1C, then charged at a constant voltage of 4.4V until the current dropped to 0.05C, and then discharged to 2.5V at a constant current of 1C to obtain the first-week discharge capacity (Cd1); the charge and discharge were repeated for 3 weeks, and the discharge capacity after 3 cycles was recorded as Cd3;

[0165] ②25℃ DCR ​​test

[0166] At a constant temperature of 25°C, discharge at a constant current of 0.1C for 5h, adjust the SOC of the cell to 50% SOC, discharge at a current of 10C (discharge rate = current / cd3) for 10s, and record the cell voltage before and after discharge, DCR = (voltage before discharge - voltage after discharge) / discharge current

[0167] ③-20℃ DCR ​​test

[0168] Adjust the temperature of the battery test box to -20℃, let it stand for 2h, then discharge it to 2.5V at a constant current of 1C, then charge it at a current of 0.33C for 3h, then discharge it at a current of 0.1C for 5h, adjust the SOC of the battery to 50%soc, discharge it at a current of 0.36C for 10s, and record the voltage before and after discharge, DCR = (voltage before discharge - voltage after discharge) / discharge current

[0169] 4. Lithium-ion battery cycle performance test

[0170] At a constant temperature of 25°C, charge to 4.4V at a constant current of 1C, then charge at a constant voltage of 4.4V until the current drops to 0.05C, and then discharge to 2.5V at a constant current of 1C to obtain the first cycle discharge capacity (Cd1); repeat the charge and discharge until the 500th cycle, and the discharge capacity after 500 cycles is recorded as Cdn. Cycle capacity retention rate = Cdn / Cd1.

[0171] Table 1

[0172]

[0173]

[0174]

[0175] Table 2

[0176]

[0177]

[0178] It can be seen from Example 1, Examples 15-16 and Comparative Example 1 that by differentially coating the heat-resistant coating and the adhesive coating on the first region, the second region, and the third region of the base film, and controlling the surface density of the heat-resistant coating in the second region to be greater than that in the first and third regions, and / or controlling the surface density of the organic coating in the second region to be less than that in the first and third regions, the wetting effect of the electrolyte and the lithium precipitation in the middle region of the positive and negative pole pieces of the secondary battery can be improved, the impedance of active ion transport can be reduced, and the cycle performance of the secondary battery can be improved.

[0179] From the comparison between Example 1 and Example 2, and Examples 17-18, it can be seen that on the basis of controlling the surface density of the inorganic coating in the second region to be greater than that in the first region and the third region, and / or controlling the surface density of the organic coating in the second region to be less than that in the first region and the third region, controlling the surface density of the inorganic coating in the third region to be greater than that in the first region, and / or controlling the surface density of the organic coating in the third region to be less than that in the first region, can further improve the wetting effect and lithium precipitation of the electrolyte in the middle area of ​​the positive and negative pole pieces of the secondary battery, reduce the impedance of the transport of active ions in the electrolyte, and improve the cycle performance of the secondary battery.

[0180] It can be seen from Example 1 and Examples 3-6 that as the surface density of the inorganic coating in the second region increases, the wetting effect of the electrolyte in the middle of the positive and negative pole pieces of the secondary battery becomes better, and the improvement in the lithium precipitation situation becomes better. However, when the surface density of the inorganic coating in the second region is greater than that in the first region and 1.3 times that in the third region, the impedance of the transport of active ions in the electrolyte increases and the cycle performance of the secondary battery decreases.

[0181] It can be seen from Examples 1 and 7-10 that as the surface density of the organic coating in the second region decreases, the wetting effect of the electrolyte in the middle of the positive and negative pole pieces of the secondary battery becomes better. However, when the surface density of the organic coating in the second region is less than that in the first region and is 0.6 times the surface density of the organic coating in the third region, the impedance of the transport of active ions in the electrolyte increases, the lithium plating situation worsens, and the cycle performance of the secondary battery decreases.

[0182] It can be seen from Example 1 and Examples 11-14 that as the proportion of the width of the second region to the total width of the base film increases, the wetting effect of the electrolyte in the middle of the positive and negative pole pieces of the secondary battery becomes better. However, when the width of the second region is greater than 40% of the total width of the base film, the impedance of the transport of active ions in the electrolyte increases, the lithium plating situation worsens, and the cycle performance of the secondary battery decreases.

[0183] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery, characterized in that: The secondary battery comprises a composite diaphragm, the composite diaphragm comprises a base film, the base film is sequentially divided into a first region, a second region and a third region along the width direction of the base film, the first region, the second region and the third region comprise a heat-resistant coating and an adhesive coating, and the surface density of the heat-resistant coating in the second region is greater than that in the first region and the third region; and / or The surface density of the bonding coating in the second region is smaller than that in the first region and the third region.

2. The secondary battery according to claim 1, characterized in that: The surface density of the heat-resistant coating in the third region is greater than that in the first region; and / or The surface density of the bonding coating in the third region is lower than that in the first region.

3. The secondary battery according to claim 1 or 2, characterized in that: The widths of the first region, the second region, and the third region are respectively 30%-40%, 20%-40%, and 30%-40% of the width of the base film.

4. The secondary battery according to any one of claims 1 to 3, characterized in that: The surface density of the heat-resistant coating in the first region is σ1, the surface density of the heat-resistant coating in the second region is σ2, and the surface density of the heat-resistant coating in the third region is σ3, wherein 1 g / cm 2 ≤σ1≤1.85g / cm 2 , 1.1g / cm 2 ≤σ2≤2.405g / cm 2 , 1g / cm 2 ≤σ3≤2.035g / cm 2 .

5. The secondary battery according to any one of claims 1 to 4, characterized in that: The surface density of the adhesive coating in the first region is σ4, the surface density of the heat-resistant coating in the second region is σ5, and the surface density of the heat-resistant coating in the third region is σ6, wherein 1.5 g / cm 2 ≤σ4≤2g / cm 2 , 0.9g / cm 2 ≤σ5≤1.6g / cm 2 , 1.2g / cm 2 ≤σ6≤2g / cm 2 .

6. The secondary battery according to claim 1, characterized in that The heat-resistant coating comprises at least one of inorganic particles or organic particles; and / or The bond coat includes an organic polymer.

7. The secondary battery according to claim 1, characterized in that: The positional relationship among the base film, the heat-resistant coating and the adhesive coating in the composite diaphragm is as follows: the heat-resistant coating is located on the base film, and the adhesive coating is located on the heat-resistant coating.

8. A method for preparing a composite diaphragm, characterized in that: The following steps are involved: An organic polymer, a binder and water are mixed and stirred to obtain an organic slurry; Mixing inorganic particles or organic particles, a binder and water and stirring to obtain an inorganic slurry; Take the base film, control the flow rate of the inorganic slurry in the first area and the third area to be smaller than that in the second area, differentially coat the inorganic slurry on the base film, dry and let stand; and / or The flow rate of the organic slurry in the first area and the third area is controlled to be greater than that in the second area, and the organic slurry is coated on the heat-resistant coating, dried and left to stand.

9. The method for preparing a composite diaphragm according to claim 8, characterized in that: The flow rate of the inorganic slurry in the third region is greater than that in the first region; and / or The flow rate of the organic slurry in the third zone is smaller than that in the first zone.

10. An electrical device, characterized in that: The invention comprises the secondary battery as claimed in claims 1 to 7.

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