Negative electrode sheet and its preparation method, secondary battery and battery pack

Abstract

This application provides a negative electrode sheet and its preparation method, a secondary battery, and a battery pack. The negative electrode sheet includes a negative current collector, and at least one side of the negative current collector has a negative active material layer. The negative active material layer includes a negative active material and a binder. The binder includes a copolymer formed by polymerizing acrylonitrile and acrylic acid. Based on the content of the copolymer, the molar percentage of acrylonitrile is 80% to 90%, and the molar percentage of acrylic acid is 10% to 20%.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a negative electrode sheet and its preparation method, a secondary battery, and a battery pack. Background Technology

[0002] Secondary batteries (such as lithium-ion batteries) have advantages such as high energy density, low self-discharge, and light weight, and are therefore widely used in energy storage devices and other fields.

[0003] As a key structural component of secondary batteries, the negative electrode plays a crucial role in their performance. With increasing demands on secondary battery performance, particularly in wind power and solar power energy storage applications, improving the peel strength and flexibility of the negative electrode to further enhance the cycle performance of the secondary battery has become a pressing issue. Summary of the Invention

[0004] To address the aforementioned technical problems, this application discloses a negative electrode sheet and its preparation method, a secondary battery, and a battery pack, in order to improve the peel strength and flexibility of the negative electrode sheet, thereby improving the cycle performance of the secondary battery.

[0005] In a first aspect, this application provides a negative electrode sheet, including a negative electrode current collector, wherein at least one side of the negative electrode current collector has a negative electrode active material layer, the negative electrode active material layer including a negative electrode active material and a binder;

[0006] The adhesive comprises a copolymer formed by polymerizing acrylonitrile and acrylic acid, wherein the molar percentage of acrylonitrile is 80% to 90% and the molar percentage of acrylic acid is 10% to 20% based on the content of the copolymer.

[0007] In some embodiments of this application, the molar ratio of cyano groups to amide groups in the infrared spectral test results of the negative electrode active material layer is 1.5 to 3:1.

[0008] In some embodiments of this application, the negative electrode active material layer further includes a lithium-containing emulsifier, wherein the mass percentage of the lithium-containing emulsifier in the negative electrode active material layer is 0.1% to 0.3%.

[0009] In some embodiments of this application, the lithium-containing emulsifier includes at least one of lithium dodecyl sulfate, lithium dodecylbenzene sulfonate, and lithium hexadecylbenzene sulfonate.

[0010] In some embodiments of this application, the molar ratio of cyano groups to carboxyl groups in the infrared spectral test results of the negative electrode active material layer is 2.0 to 7:1.

[0011] In some embodiments of this application, the negative electrode active material includes a carbon material, which includes at least one of artificial graphite, natural graphite, and hard carbon.

[0012] Secondly, this application provides a method for preparing a negative electrode sheet as described in the first aspect, comprising the following steps:

[0013] Acrylonitrile and acrylic acid are mixed to obtain a reaction raw material, wherein the molar percentage of acrylonitrile is 80%~90% and the molar percentage of acrylic acid is 10%~20%.

[0014] The first portion of the reactants is added to the reaction solution, and the mixture is gradually heated and stirred to carry out the polymerization reaction; wherein the reaction solution includes a lithium-containing emulsifier.

[0015] After the reaction solution is heated to the target temperature, the second part of the reaction raw materials is added to the reaction solution, and the polymerization reaction is continued to obtain a precipitate. After washing and neutralizing the precipitate, the binder is obtained.

[0016] The negative electrode active material, conductive agent, binder and solvent at a temperature of 60℃~80℃ are mixed and stirred to form a negative electrode slurry. The negative electrode slurry is coated on at least one surface of the negative electrode current collector and dried to obtain the negative electrode sheet.

[0017] In some embodiments of this application, the reaction solution includes a lithium-containing emulsifier, and the amount of the lithium-containing emulsifier added is 7% to 10% based on the mass of the reaction raw materials.

[0018] In some embodiments of this application, the mass ratio of the reaction raw materials described in the first part to the reaction raw materials described in the second part is 1:1 to 2.

[0019] In some embodiments of this application, the stirring speed is 2000 rpm to 3000 rpm.

[0020] Thirdly, this application provides a secondary battery, including the negative electrode sheet described in the first aspect, or the negative electrode sheet prepared by the preparation method described in the second aspect.

[0021] Fourthly, this application provides a battery pack including a housing and at least one secondary battery as described in the third aspect, the secondary battery being housed within the housing.

[0022] Fifthly, this application provides an electrical device including the secondary battery described in the third aspect or the battery pack described in the fourth aspect.

[0023] Compared with the prior art, this application has at least the following beneficial effects:

[0024] This application provides a negative electrode sheet, its preparation method, a secondary battery, and a battery pack. The negative electrode sheet's negative electrode active material layer includes a negative electrode active material and a binder. The binder includes a copolymer formed by polymerizing acrylonitrile and acrylic acid. Based on the copolymer content, the molar percentage of acrylonitrile is 80%~90%, and the molar percentage of acrylic acid is 10%~20%. Compared to existing negative electrode sheets, the negative electrode sheet of this application has a higher acrylonitrile content in the negative electrode binder, which improves the flexibility of the negative electrode sheet. Furthermore, the higher acrylonitrile content improves the dispersibility of the negative electrode active material, thereby increasing the peel strength of the negative electrode sheet. The negative electrode sheet of this application exhibits excellent flexibility and peel strength, and the thickness uniformity and rebound uniformity after rolling are improved, thus enabling the secondary battery with the negative electrode sheet of this application to have better cycle performance. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of the structure of a residential energy storage system according to one embodiment of this application;

[0027] Figure 2 This is a schematic diagram of the energy storage system according to one embodiment of this application.

[0028] Explanation of reference numerals in the attached drawings: 1-Energy storage device, 2-Power conversion device, 3-First user load, 4-Second user load, 400-Energy storage system, 410-High voltage cable, 420-First power conversion device, 430-Second power conversion device. Detailed Implementation

[0029] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0030] In this application, the terms "upper," "lower," "left," "right," "front," "rear," "top," "bottom," "inner," "outer," "vertical," "horizontal," "lateral," and "longitudinal" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for the purpose of better describing this application and its embodiments, and are not intended to limit the indicated device, element, or component to having a specific orientation, or to be constructed and operated in a specific orientation.

[0031] Furthermore, in addition to indicating location or positional relationship, some of the aforementioned terms may also have other meanings. For example, the term "above" may also be used in some cases to indicate a certain dependency or connection relationship. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0032] Furthermore, the terms "installation," "setup," "equipped with," "connection," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral structure; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium, or an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0033] Furthermore, the terms "first," "second," etc., are primarily used to distinguish different devices, components, or parts (which may be the same or different in specific type and construction), and are not intended to indicate or imply the relative importance or quantity of the indicated devices, components, or parts. Unless otherwise stated, "a plurality of" means two or more.

[0034] It should be noted that this application uses lithium-ion batteries as an example of secondary batteries to explain the application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0035] This application provides a negative electrode sheet, which includes a negative current collector. At least one side of the negative current collector has a negative active material layer, which includes a negative active material and a binder. The binder comprises a copolymer formed by polymerizing acrylonitrile and acrylic acid. Based on the content of the copolymer, the molar percentage of acrylonitrile is 80% to 90%, and the molar percentage of acrylic acid is 10% to 20%. For example, the molar percentage of acrylonitrile is 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, or 90%, and the molar percentage of acrylic acid is 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 90%.

[0036] In the negative electrode sheet of this application, the acrylonitrile content in the negative electrode binder is higher, which improves the flexibility of the negative electrode sheet. Furthermore, the higher acrylonitrile content improves the dispersibility of the negative electrode active material, thereby increasing the peel strength of the negative electrode sheet. The negative electrode sheet of this application exhibits excellent flexibility and peel strength, and the thickness uniformity and rebound uniformity after rolling are improved, thus enabling lithium-ion batteries using the negative electrode sheet of this application to have better cycle performance.

[0037] In some embodiments of this application, the molar ratio of cyano groups to amide groups in the infrared spectroscopy test results of the negative electrode active material layer is 1.5 to 3:1. For example, the molar ratio of cyano groups to amide groups is 1.5:1, 1.8:1, 2:1, 2.2:1, 2.3:1, 2.5:1, or 3:1. Since the binder of this application includes a copolymer formed by the polymerization of acrylonitrile and acrylic acid, the cyano groups and amide groups in the negative electrode active material layer can be introduced through the binder. The molar ratio of cyano groups to amide groups can be measured by infrared spectroscopy testing of the negative electrode active material layer of this application. By controlling the molar ratio of cyano groups to amide groups in the negative electrode active material layer within the above range, this application is beneficial to improve the flexibility of the negative electrode sheet while improving its peel strength, thereby improving the cycle performance of the lithium-ion battery.

[0038] In some embodiments of this application, the negative electrode active material layer further includes a lithium-containing emulsifier, the mass percentage of which is 0.1% to 0.3%. For example, the mass percentage of the lithium-containing emulsifier in the negative electrode active material layer is 0.1%, 0.2%, 0.25%, or 0.3%. When the negative electrode sheet contains a lithium-containing emulsifier, it is beneficial to improve the kinetic performance of the lithium-ion battery, thereby further improving the cycle performance of the lithium-ion battery.

[0039] This application does not impose any particular limitation on the method of controlling the content of lithium-containing emulsifier in the negative electrode active material layer, as long as the purpose of this application can be achieved. It is understood that, given a fixed binder content in the negative electrode active material layer, a higher content of lithium-containing emulsifier in the binder will result in a higher content of lithium-containing emulsifier in the negative electrode active material layer; similarly, given a fixed content of lithium-containing emulsifier in the binder, a higher content of binder in the negative electrode active material layer will result in a higher content of lithium-containing emulsifier in the negative electrode active material layer. Based on this, by way of example, the content of lithium-containing emulsifier in the negative electrode active material layer can be controlled by adjusting the content of lithium-containing emulsifier in the binder, and / or by adjusting the content of the binder in the negative electrode active material layer.

[0040] In some embodiments of this application, the lithium-containing emulsifier includes at least one selected from lithium dodecyl sulfate, lithium dodecylbenzene sulfonate, and lithium hexadecylbenzene sulfonate. The aforementioned lithium-containing emulsifiers are beneficial for improving the kinetic performance of lithium-ion batteries, thereby further improving the cycle performance of lithium-ion batteries.

[0041] In some embodiments of this application, the molar ratio of cyano groups to carboxyl groups in the infrared spectral test results of the negative electrode active material layer is 2.0 to 7:1. For example, the molar ratio of cyano groups to carboxyl groups is 2.0:1, 2.5:1, 3:1, 4:1, 5:1, 6:1, or 7:1. By controlling the molar ratio of cyano groups to carboxyl groups in the negative electrode active material layer within the above range, it is beneficial to obtain a negative electrode sheet with excellent flexibility and peel strength.

[0042] This application does not impose any particular limitation on the molecular weight of the aforementioned copolymer, as long as it achieves the purpose of this application. For example, the number average molecular weight of the copolymer can be 500,000 to 1,000,000. By controlling the number average molecular weight of the copolymer within the above range, it is beneficial to obtain a binder with excellent bonding properties, thereby improving the peel strength of the negative electrode sheet and the cycle performance of the lithium-ion battery.

[0043] In some embodiments of this application, the negative electrode active material includes a carbon material, which includes at least one of artificial graphite, natural graphite, and hard carbon. In the negative electrode slurry, the binder of this application exhibits better dispersibility on the surface of the aforementioned negative electrode active material, thereby also resulting in better dispersibility of the negative electrode active material in the negative electrode slurry. This better dispersibility facilitates greater contact between the negative electrode active material particles and the negative electrode current collector. Furthermore, the better dispersibility slows down the rise of the binder in the negative electrode active material layer during the drying process, further promoting greater contact between the binder and the negative electrode current collector, thereby improving the interfacial adhesion performance between the negative electrode active material layer and the negative electrode current collector. The combined effect of these two factors improves the peel strength of the negative electrode sheet.

[0044] This application also provides a method for preparing the negative electrode sheet according to any of the above embodiments, comprising the following steps:

[0045] Step A: Acrylonitrile and acrylic acid are mixed to obtain the reaction raw materials. The molar percentage of acrylonitrile in the reaction raw materials is 80%~90%, and the molar percentage of acrylic acid is 10%~20%.

[0046] Step B: Add the first part of the reaction raw materials to the reaction solution, gradually heat and stir to carry out the polymerization reaction; wherein, the reaction solution includes a lithium-containing emulsifier;

[0047] Step C: After the reaction solution is heated to 65℃~75℃, the second part of the reaction raw materials is added to the reaction solution. After stirring and polymerization, a precipitate is obtained. The precipitate is washed and neutralized to obtain the binder.

[0048] Step D: Mix the negative electrode active material, conductive agent, binder and solvent at a temperature of 60℃~80℃ and stir to form a negative electrode slurry. Coat the negative electrode slurry onto at least one surface of the negative electrode current collector and dry it to obtain a negative electrode sheet.

[0049] In step A, this application does not have any particular restrictions on the source of raw materials such as acrylonitrile and acrylic acid. They can be commercially available acrylonitrile and acrylic acid, as long as they can achieve the purpose of this application.

[0050] In step B, the reactants can be added to the reaction solution in several batches, for example, in two batches, to better control the reaction rate, increase the yield, and reduce waste of reactants. The reaction initiation temperature of this application can be 45℃~55℃, which is beneficial for controlling the initial reaction rate. The reaction solution of this application includes a lithium-containing emulsifier. On the one hand, the addition of the lithium-containing emulsifier is beneficial for improving the dispersibility of acrylonitrile in the reaction solution. Acrylonitrile is a poorly soluble monomer, so when its dispersibility is improved, it can polymerize more fully with acrylic acid. On the other hand, the inventors unexpectedly discovered that when the negative electrode contains a lithium-containing emulsifier, the kinetic performance of the lithium-ion battery becomes exceptionally good. This may be because the lithium-containing emulsifier can provide an additional lithium-ion transport path during lithium-ion transport. The reaction solution of this application may also include deionized water and ammonium persulfate, wherein deionized water can be used as a solvent, and ammonium persulfate can be used as a reaction initiator. The mass concentration of ammonium persulfate in the reaction solution can be 15%~25%. During the reaction, nitrogen gas can also be introduced to purge air from the reaction apparatus. The reaction apparatus of this application can be a reaction vessel.

[0051] In step C of this application, the target temperature can be 65℃~75℃, which is beneficial for better control of the reaction rate. Higher reaction temperatures also allow for a more complete polymerization reaction, thereby increasing the yield of the binder. The second part of acrylonitrile can be added dropwise. As the reaction proceeds, a precipitate is formed in the reaction system; this precipitate is the copolymer of acrylonitrile and acrylic acid. After washing and neutralizing the precipitate, the binder can be obtained.

[0052] The inventors discovered that the binder in this application has a higher proportion of acrylonitrile, making it more difficult to dissolve in solvents (such as water). Based on this discovery, in step D, by controlling the temperature of the solvent at 60°C to 80°C, some of the cyano groups (-CN) in the binder are hydrolyzed to form amide groups. The amide groups can increase the number of hydrogen bonds and improve the degree of crosslinking of the binder, thereby improving the dispersibility of the binder in the negative electrode slurry. This is beneficial for improving the flexibility of the negative electrode sheet while also increasing its peel strength.

[0053] This application allows the precipitate to be washed 2 to 5 times with deionized water to remove unreacted monomers and small molecule impurities. Neutralization can be performed using sodium carbonate or sodium hydroxide solution, with a concentration of 10 wt% to 50 wt%. Furthermore, neutralization can be stopped by measuring the pH value of the reaction system and stopping once the target pH value is reached. The target pH value can be any value between 6.8 and 7.2, preferably pH=7.0.

[0054] The preparation of the negative electrode sheet may also include processes such as cold pressing, slitting, cutting, and welding of electrode tabs. This application does not have any particular restrictions, as long as the negative electrode sheet can be obtained.

[0055] The method for preparing the negative electrode sheet of this application involves adding the reaction raw materials in batches and controlling the temperature of the solvent at 60℃~80℃, so that the cyano (-CN) in the binder is partially hydrolyzed to form amide groups, thereby improving the dispersibility of the binder in the solvent and thus facilitating the preparation of a negative electrode sheet with excellent peel strength and flexibility.

[0056] In some embodiments of this application, the reaction solution includes a lithium-containing emulsifier, and the amount of lithium-containing emulsifier added is 7% to 10% based on the mass of the reaction raw materials. For example, the content of the lithium-containing emulsifier is 7%, 8%, 8.5%, 9%, or 10%. By controlling the content of the lithium-containing emulsifier within the above range, it is beneficial to control the content of lithium-containing emulsifier in the negative electrode active material layer. Within the scope of this application, when the negative electrode sheet contains a lithium-containing emulsifier, it is beneficial to improve the kinetic performance of the lithium-ion battery, thereby further improving the cycle performance of the lithium-ion battery. The lithium-containing emulsifier of this application includes at least one of lithium dodecyl sulfate, lithium dodecylbenzene sulfonate, and lithium hexadecylbenzene sulfonate.

[0057] In some embodiments of this application, the mass ratio of the first part of the reactants to the second part of the reactants is 1:1 to 2. For example, the mass ratio of the first part of the reactants to the second part of the reactants is 1:1, 1:1.2, 1:1.5, 1:1.8, or 1:2. This facilitates better control of the reaction rate, increases the yield, reduces waste of reactants, and lowers production costs.

[0058] In one alternative embodiment, the stirring speed is 2000 rpm to 3000 rpm, which is beneficial for further improving the dispersibility of the binder in the negative electrode slurry. The stirring in this application can be performed using a planetary mixer.

[0059] The negative electrode active material layer of this application can be disposed on one or both surfaces of the negative electrode current collector in the thickness direction. In this application, the negative electrode active material layer is disposed on the surface of the negative electrode current collector; that is, the negative electrode active material layer can be disposed on a portion of one surface of the negative electrode current collector, or it can be disposed on the entire surface of one surface of the negative electrode current collector. This application does not have any particular limitation on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it can be, but is not limited to, copper foil, copper alloy foil, nickel foil, or composite current collectors. In this application, there is no particular limitation on the thickness of the negative electrode current collector, as long as it can achieve the purpose of this application; for example, a thickness of 4μm to 12μm. The single-sided thickness of the negative electrode active material layer of this application can be 80μm to 200μm, and the compaction density can be 1.4g / cm³. 3 ~1.8g / cm 3 .

[0060] This application also provides a secondary battery, including the negative electrode sheet described in any of the above embodiments.

[0061] The secondary battery of this application may also include a positive electrode, a separator, and an electrolyte, wherein the separator is located between the positive electrode and the negative electrode and serves as a separator.

[0062] This application does not impose any particular limitation on the positive electrode sheet, as long as it achieves the purpose of this application. For example, the positive electrode sheet typically includes a positive current collector and a positive active material layer. The positive active material layer can be disposed on one surface or on two surfaces in the thickness direction of the positive current collector. In this application, the positive active material layer is disposed on the surface of the positive current collector, that is, the positive active material layer can be disposed on a portion of a surface of the positive current collector or on the entire surface of a surface of the positive current collector. This application does not impose any particular limitation on the positive current collector, as long as it achieves the purpose of this application, such as including but not limited to aluminum foil, aluminum alloy foil, or composite current collectors. In this application, there is no particular limitation on the thickness of the positive current collector, as long as it achieves the purpose of this application, such as a thickness of 8μm to 13μm. The single-sided thickness of the positive active material layer in this application can be 100μm to 200μm.

[0063] In this application, the positive electrode active material layer includes a positive electrode active material. This application does not have any particular restrictions on the positive electrode active material, as long as it can achieve the purpose of this application. For example, it may include at least one of lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, and lithium manganese iron phosphate.

[0064] In this application, the positive electrode active material layer may also include a positive electrode binder. This application does not have any particular limitation on the positive electrode binder, as long as it can achieve the purpose of this application. For example, it may include, but is not limited to, at least one of fluorinated resin, polypropylene resin, fiber-type binder, rubber-type binder or polyimide-type binder.

[0065] This application does not impose any particular limitation on the diaphragm; those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved. For example, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane can be selected.

[0066] The secondary battery of this application also includes a casing. This application does not impose any particular restrictions on the casing, and those skilled in the art can choose one according to actual needs, as long as it can achieve the purpose of this application. For example, the casing may include an aluminum-plastic film.

[0067] This application does not impose any particular limitation on the preparation method of the secondary battery. Any preparation method known in the art can be used, as long as it can achieve the purpose of this application. For example, the preparation method of the secondary battery includes, but is not limited to, the following steps: stacking the positive electrode, separator and negative electrode in sequence, and winding and folding them as needed to obtain a bare cell with a wound structure; placing the bare cell in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain the secondary battery.

[0068] This application also provides a battery pack, including a housing and at least one secondary battery as described in any of the above embodiments, the secondary battery being housed within the housing. The battery pack with this secondary battery exhibits excellent performance, which is beneficial for its use. Housed within the housing, the battery is secured and protected, thus extending the battery pack's lifespan. It is understood that the battery pack may contain one or more secondary batteries, and when the battery pack contains multiple secondary batteries, these batteries can be connected in at least one manner, such as parallel or series connection.

[0069] This application also provides an electrical device including a secondary battery or battery pack as described in any of the above embodiments, which is beneficial for improving the product competitiveness and performance of the electrical device. In an optional embodiment, the electrical device includes an electrical device body, and the secondary battery or battery pack is used to supply power to the electrical device body. In an optional embodiment, the electrical device body includes a positive terminal and a negative terminal, the positive terminal of the secondary battery or battery pack is used to electrically connect to the positive terminal of the electrical device body, and the negative terminal of the secondary battery or battery pack is used to electrically connect to the negative terminal of the electrical device body, so as to supply power to the electrical device.

[0070] The electrical equipment covered by this application may include, but is not limited to: containers, household energy storage systems, electric vehicles, electric cars, ships, spacecraft, electric toys, and power tools, etc. Among them, spacecraft include, for example, airplanes, rockets, space shuttles, and spacecraft, etc. Electric toys include, for example, stationary or mobile electric toys, specifically, electric car toys, electric ship toys, and electric airplane toys, etc. Power tools include, for example, metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, specifically, electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers.

[0071] Please see Figure 1 , Figure 1 This is a schematic diagram of the structure of a residential energy storage system according to one embodiment of this application, and this application... Figure 1 The implementation plan is illustrated using the residential energy storage scenario in user-side energy storage as an example. The energy storage device in this application is not limited to the residential energy storage scenario.

[0072] This application provides a residential energy storage system, which includes a power conversion device 2 (photovoltaic panel), a first user load 3 (streetlight), a second user load 4 (e.g., household appliances such as air conditioners), and an energy storage device 1. The energy storage device 1 is a small energy storage box that can be wall-mounted to an outdoor wall. Specifically, the photovoltaic panel can convert solar energy into electrical energy during periods of low electricity prices, and the energy storage device 1 is used to store this electrical energy and supply it to streetlights and household appliances during peak electricity prices, or to provide power during power outages / power failures.

[0073] Please see Figure 2 , Figure 2 This is a schematic diagram of the structure of an energy storage system 400 according to one embodiment of this application, and this application Figure 2 The implementation plan is illustrated using the shared energy storage scenario on the power generation / distribution side as an example. The energy storage device 1 in this application is not limited to the power generation / distribution side energy storage scenario.

[0074] This application provides an energy storage system 400, which includes a high-voltage cable 410, a first power conversion device 420, a second power conversion device 430, and the energy storage device 1 provided in this application. During power generation, the first power conversion device 420 and the second power conversion device 430 convert other forms of energy into electrical energy, which is then connected to the high-voltage cable 410 and supplied to the power consumption side of the distribution network. When the power load is low and the first power conversion device 420 and the second power conversion device 430 generate excess power, the excess power is stored in the energy storage device 1, reducing wind and solar curtailment rates and improving the absorption of new energy power generation. When the power load is high, the power grid issues an instruction to transmit the power stored in the energy storage device 1, along with the high-voltage cable 410, in a grid-connected mode to supply power to the power consumption side. This provides various services such as peak shaving, frequency regulation, and backup for the power grid operation, fully leveraging the peak shaving function of the power grid, promoting peak shaving and valley filling, and alleviating the power supply pressure on the power grid.

[0075] Optionally, the first power conversion device 420 and the second power conversion device 430 can convert at least one of solar energy, light energy, wind energy, thermal energy, tidal energy, biomass energy and mechanical energy into electrical energy.

[0076] The number of energy storage devices 1 can be multiple, and the multiple energy storage devices 1 can be connected in series or in parallel. The multiple energy storage devices 1 are supported and electrically connected by an isolation plate (not shown in the figure). In this embodiment, "multiple" means two or more. An energy storage box can also be provided on the outside of the energy storage device 1 to house the energy storage device 1.

[0077] Optionally, the energy storage device 1 may include, but is not limited to, a single battery cell, a battery module, a battery pack, or a battery system. The actual application form of the energy storage device 1 provided in this application embodiment may be, but is not limited to, the listed products, and may also be other application forms. This application embodiment does not strictly limit the application form of the energy storage device 1. This application embodiment only uses a multi-cell battery as an example for illustration. When the energy storage device 1 is a single battery cell, the energy storage device 1 may be at least one of cylindrical batteries, prismatic batteries, etc.

[0078] Example

[0079] The following examples, embodiments, and comparative examples illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below.

[0080] Example 1-1

[0081] <Preparation of Adhesive>

[0082] Acrylonitrile and acrylic acid were weighed and mixed at a molar ratio of 80:20 to obtain the reaction raw materials. Deionized water was added to the reactor at 50°C, and then the first part of the weighed reaction raw materials was added to the reactor. Nitrogen gas was purged for 2 hours, and then 20 wt% ammonium persulfate was added as an initiator. The reactor was then gradually heated to 70°C, and the second part of the reaction raw materials was added dropwise. After reacting for 20 hours, a precipitate was obtained, wherein the mass ratio of the first part of the reaction raw materials to the second part of the reaction raw materials was 1:1. The obtained precipitate was washed three times with deionized water to remove unreacted monomers and small molecule impurities to obtain a copolymer. Sodium carbonate with a concentration of 25 wt% was slowly added to the obtained copolymer while stirring continuously until the pH value of the mixture reached 7.0. Then, the addition of the first solution was stopped, neutralization was stopped, and the binder was obtained.

[0083] <Preparation of Negative Electrode Sheets>

[0084] Artificial graphite (anode active material), sodium carboxymethyl cellulose (CMC) thickener, conductive carbon black (Super-P), and the prepared binder were weighed at a mass ratio of 96:2:1:1. The weighed artificial graphite, CMC, and conductive carbon black were then mixed with half of the binder, and deionized water at 70°C was added. The mixture was kneaded to form a paste. The remaining half of the binder was then added to the paste, followed by deionized water at 70°C. The mixture was dispersed at high speed (2200 rpm) using a planetary mixer to prepare a negative electrode slurry with a solid content of 50 wt%, and stirred until homogeneous. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil current collector. After drying, cold pressing, slitting, cutting, and welding of tabs, the negative electrode sheet was obtained. The single-sided thickness of the negative electrode active material layer was 70 μm.

[0085] <Preparation of the positive electrode>

[0086] Lithium iron phosphate (LiFePO4), conductive carbon black (Super-P), and PVDF binder were mixed in a mass ratio of 94:3:3. Then, N-methylpyrrolidone (NMP) was added as a solvent to prepare a positive electrode slurry with a solid content of 60 wt%, which was stirred evenly. The positive electrode slurry was then uniformly coated onto one surface of a 10 μm thick aluminum foil current collector. After drying, cold pressing, slitting, cutting, and welding of tabs, the positive electrode sheet was obtained. The single-sided thickness of the positive electrode active material layer was 100 μm.

[0087] <Preparation of Electrolyte>

[0088] In an argon-atmosphere glove box with a moisture content ≤1ppm, ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 1:1:1. Lithium salt LiPF6 was then added and dissolved in the solvent. After thorough mixing, an electrolyte was obtained. The molar concentration of LiPF6 in the electrolyte was 1 mol / L.

[0089] <Preparation of the diaphragm>

[0090] A porous polyethylene (PE) film with a thickness of 16 μm was used as the separator.

[0091] <Preparation of Lithium-ion Batteries>

[0092] The positive electrode, separator, and negative electrode prepared above are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The resulting cells are then wound to obtain a bare battery cell. The bare battery cell is placed in an aluminum-plastic film packaging bag, vacuum dried, and then injected with electrolyte. After vacuum sealing, settling, and formation processes, a lithium-ion battery is obtained.

[0093] Examples 1-2 to 1-5

[0094] Except for adjusting the ratio of acrylonitrile to acrylic acid according to Table 1 in the <Preparation of Adhesive> section, the rest is the same as in Example 1-1.

[0095] Examples 1-6 to Examples 1-7

[0096] Except for adjusting the mass ratio of the first and second parts of the reaction raw materials according to Table 1 in the <Preparation of Adhesive>, the rest is the same as in Example 1-1.

[0097] Examples 1-8 to 1-9

[0098] Except for adjusting the water temperature of deionized water and the stirring speed of the stirrer according to Table 1 in the <Preparation of Negative Electrode Sheet>, the rest is the same as in Example 1-1.

[0099] Example 2-1

[0100] Except for the preparation of the adhesive, which differs from Example 1-1, the rest is the same as Example 1-1.

[0101] <Preparation of Adhesive>

[0102] Acrylonitrile and acrylic acid were weighed at a molar ratio of 80:20 and mixed to obtain the reaction raw materials. Then, lithium dodecyl sulfate was weighed at 7 wt% of the reaction raw materials. Deionized water was added to the reactor at 50°C, and then the first part of the weighed reaction raw materials was added to the reactor. Nitrogen gas was purged for 2 hours. Then, 20 wt% ammonium persulfate as an initiator and the weighed lithium dodecyl sulfate were added to the reactor. The reactor was then gradually heated to 70°C, and the second part of the reaction raw materials was added dropwise. After reacting for 20 hours, a precipitate was obtained. The mass ratio of the first part of the reaction raw materials to the second part of the reaction raw materials was 1:1. The obtained precipitate was washed three times with deionized water to remove unreacted monomers and small molecule impurities to obtain a copolymer. Sodium carbonate at a concentration of 30 wt% was slowly added to the obtained copolymer while stirring continuously. The addition of the first solution was stopped when the pH value of the mixture reached 7.0, and neutralization was stopped to obtain the binder.

[0103] Examples 2-2 to 2-5

[0104] Except for the section on "Preparation of Binder", where the type and amount of lithium-containing emulsifier added to the reaction solution are adjusted according to Table 3 to change the content of lithium-containing emulsifier in the negative electrode active material layer as shown in Table 3, the rest is the same as in Example 2-1.

[0105] Comparative Examples 1 to 2

[0106] Except for adjusting the ratio of acrylonitrile to acrylic acid according to Table 1 in the <Preparation of Adhesive> section, the rest is the same as in Example 1-1.

[0107] Comparative Examples 3 to 4

[0108] Except for adjusting the water temperature of deionized water and the stirring speed of the stirrer according to Table 1 in the <Preparation of Negative Electrode Sheet>, the rest is the same as in Example 1-1.

[0109] Test methods and equipment:

[0110] Tests on the molar ratios of cyano groups to amide groups and cyano groups to carboxyl groups in the negative electrode active material layer:

[0111] The negative electrode active material layer is scraped off the surface of the negative electrode sheet with a scraper to obtain a sample. The sample is then placed in an infrared spectrometer to test the content of cyano, amide, and carboxyl groups in the sample, thereby obtaining the molar ratio of cyano to amide and the molar ratio of cyano to carboxyl groups.

[0112] Lithium-containing emulsifier content test in the negative electrode active material layer:

[0113] Weigh an appropriate amount of negative electrode sample, add it to a container, and then place it in an oven for preheating at 80°C for 1 hour. Inject the preheated sample into an elemental analyzer (model SPECTRO ARCOS), start the instrument, set the parameters to the standard parameters of furnace 1, and begin the test. Finally, record the results to obtain the Li element content in the negative electrode, thereby obtaining the content of lithium emulsifier.

[0114] Peel strength test:

[0115] One side of the negative electrode sheet of the sample is adhered to a steel plate with double-sided tape. The steel plate is mounted on a fixed fixture at the bottom of the high-speed rail tensile testing machine. After confirming that the bottom layer of the sample is firmly bonded to the steel plate, approximately 1 cm is torn along the interface between the negative electrode sheet and the double-sided tape at one end of the sample's length. This torn corner of the negative electrode sheet is then clamped onto a movable fixture at the top of the high-speed rail tensile testing machine. The sample preparation is now complete. Testing: Tension angle 90°, tension speed 20 mm / min, until the interface between the negative electrode sheet and the double-sided tape is completely separated. After separation, the negative electrode material layer is adhered to the surface of the double-sided tape, and the other side of the separated interface is at least partially exposed of the negative electrode current collector. Record the average load force (N) during the tensile process, divide it by the sample width to obtain the peel strength between the negative electrode active material layer and the negative electrode current collector, and then take the average value.

[0116] Flexibility test:

[0117] ① Cut the negative electrode sheet into a sample 60cm long and 40cm wide using a cutting knife. ② Place the sample into the fixture of the flexibility tester (model PY-H613). ③ Make the negative electrode sheet at a 90° angle with the test head, and press the test head vertically into the negative electrode sheet to a depth of 8±0.5 mm. Record the maximum pressure value displayed by the sensor of the test head, which is the flexibility value, in mN.

[0118] Thickness rebound rate test:

[0119] The thickness of the negative electrode sheet obtained after the cold pressing process is measured using a micrometer and recorded as H0. After the lithium-ion battery with this negative electrode sheet is fully charged, that is, after the state of charge (SOC) = 100%, the negative electrode sheet is removed from the lithium-ion battery, and its thickness is measured at this time and recorded as H1. The thickness rebound rate of the negative electrode sheet = (H1 - H0) / H0 × 100%.

[0120] Cyclic performance test:

[0121] The test temperature was 25℃. The lithium-ion battery was charged at a constant current of 1x (C) to 3.65V, then charged at a constant voltage to the cutoff current of 0.05C. After resting for 30 minutes, it was discharged at a constant power of 1C to 2.5V. This was recorded as one charge-discharge cycle, and the capacity obtained in this step was taken as the initial discharge capacity C0. The same conditions were applied for 1000 cycles, and the discharge capacity of the 1000th cycle was recorded. Cycle capacity retention rate = (Discharge capacity of the 1000th cycle / Initial discharge capacity C0) × 100%.

[0122] Table 1. Preparation parameters of Examples 1-1 to 1-9 and Comparative Examples 1 to 4

[0123]

[0124] Table 2 Performance data of Examples 1-1 to 1-9 and Comparative Examples 1 to 4

[0125]

[0126] In Table 2, " / " indicates that the corresponding performance data could not be measured.

[0127] Referring to Table 2, it can be seen from Examples 1-1 to 1-5 and Comparative Examples 1 to 2 that when the relative content of acrylonitrile in the copolymer is low (e.g., Comparative Examples 1 and 2), the peel strength, flexibility, and thickness rebound rate of the negative electrode sheet are all poor, and the cycle performance of the lithium-ion battery is also low. This may be because the relative content of acrylonitrile affects the relative content of cyano groups, thereby affecting the content of amide groups formed by partial cyano hydrolysis, thus reducing the relative content of amide groups. From Examples 1-1 to 1-5 and Comparative Examples 3 to 4, it can be seen that when the temperature of deionized water is too low during the preparation of the negative electrode slurry (e.g., Comparative Example 3), due to the limitations of this application... The binder in Comparative Example 3 has low solubility at room temperature and forms spherical shapes in the negative electrode slurry, making it difficult to disperse the negative electrode active material at low temperatures. This results in the negative electrode slurry of Comparative Example 3 being unable to achieve homogenization, making it difficult to prepare the negative electrode sheet. When the temperature of deionized water is too high during the preparation of the negative electrode slurry (e.g., in Comparative Example 4), the peel strength, flexibility, and thickness rebound rate of the negative electrode sheet are all poor, and the cycle performance of the lithium-ion battery is also low. This may be because excessively high water temperatures easily lead to accelerated evaporation of deionized water, making it more difficult to control the solid content of the negative electrode slurry. In contrast, the negative electrode sheet of this application has higher peel strength, while also exhibiting good flexibility and thickness rebound rate, thus improving the capacity retention of the lithium-ion battery. It is evident that this application improves the peel strength, flexibility, thickness uniformity, and rebound uniformity of the negative electrode sheet, thereby improving the cycle performance of the lithium-ion battery.

[0128] The mass ratio of the first to the second part of the reaction raw materials, the temperature of the deionized water, and the stirring speed of the mixer also typically affect the performance of the binder. As can be seen from Examples 1-1 and 1-6 to 1-9, by adjusting the above preparation parameters within the range of this application, it is beneficial to obtain a negative electrode sheet with good peelability and flexibility, while maintaining the acrylonitrile and acrylic acid content in the copolymer within the range of this application.

[0129] Table 3 Preparation parameters for Examples 1-1 and 2-1 to 2-5

[0130]

[0131] Note: In Table 3, " / " indicates that the relevant preparation parameters do not exist.

[0132] Table 4 Performance data of Examples 1-1, 2-1 to 2-5

[0133]

[0134] Referring to Table 4, it can be seen from Examples 1-1 and 2-1 to 2-5 that by introducing lithium-containing emulsifiers into the negative electrode active material layer and controlling the type and content of lithium-containing emulsifiers within the scope of this application, the flexibility of the negative electrode sheet is further improved while the peel strength of the negative electrode sheet does not change significantly. This is beneficial to further improving the cycle performance of the lithium-ion battery, and the lithium-ion battery exhibits better kinetic performance.

[0135] The above provides a detailed description of a negative electrode sheet and its preparation method, as well as a secondary battery and battery pack disclosed in this application. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the technical solutions and core inventive points of the embodiments of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A method for preparing a negative electrode sheet, characterized in that, Includes the following steps: Acrylonitrile and acrylic acid are mixed to obtain a reaction raw material, wherein the molar percentage of acrylonitrile is 80%~90% and the molar percentage of acrylic acid is 10%~20%. The first part of the reaction raw materials is added to the reaction solution, and the temperature is gradually increased and stirred to carry out the polymerization reaction. The reaction solution includes deionized water and ammonium persulfate. After the reaction solution is heated to the target temperature, the second part of the reaction raw materials is added to the reaction solution, and the polymerization reaction is continued to obtain a precipitate. After the precipitate is washed and neutralized, the binder is obtained. The negative electrode active material, conductive agent, binder and deionized water at a temperature of 60℃~80℃ are mixed and stirred to form a negative electrode slurry. The negative electrode slurry is coated on at least one surface of the negative electrode current collector and dried to obtain the negative electrode sheet.

2. The preparation method according to claim 1, characterized in that, The reaction solution also includes a lithium-containing emulsifier, and the amount of lithium-containing emulsifier added is 7% to 10% based on the mass of the reaction raw materials.

3. The preparation method according to claim 1, characterized in that, The mass ratio of the reaction raw materials described in Part I to those described in Part II is 1:1~2.

4. The preparation method according to claim 1, characterized in that, The stirring speed is 2000rpm~3000rpm.

5. A negative electrode sheet prepared by the preparation method according to any one of claims 1 to 4, characterized in that, The present invention includes a negative electrode current collector, wherein at least one side of the negative electrode current collector has a negative electrode active material layer, the negative electrode active material layer comprising a negative electrode active material and a binder; The adhesive comprises a copolymer formed by polymerizing acrylonitrile and acrylic acid, wherein the molar percentage of acrylonitrile is 80% to 90% and the molar percentage of acrylic acid is 10% to 20% based on the content of the copolymer.

6. The negative electrode sheet according to claim 5, characterized in that, The negative electrode active material layer also includes a lithium-containing emulsifier, and the mass percentage of the lithium-containing emulsifier in the negative electrode active material layer is 0.1% to 0.3%.

7. The negative electrode sheet according to claim 6, characterized in that, The lithium-containing emulsifier includes at least one of lithium dodecyl sulfate, lithium dodecylbenzene sulfonate, and lithium hexadecylbenzene sulfonate.

8. The negative electrode sheet according to claim 5, characterized in that, In the infrared spectral test results of the negative electrode active material layer, the molar ratio of cyano groups to carboxyl groups is 2.0~7:

1.

9. The negative electrode sheet according to claim 5, characterized in that, The negative electrode active material includes carbon materials, and the carbon materials include at least one of artificial graphite, natural graphite and hard carbon.

10. A secondary battery, characterized in that, The negative electrode sheet prepared by the method described in any one of claims 1 to 4 includes the negative electrode sheet prepared by the method described in claims 1 to 4.

11. A battery pack, characterized in that, It includes a housing and at least one secondary battery as described in claim 10, the secondary battery being housed within the housing.

12. An electrical appliance, characterized in that, Includes the secondary battery of claim 10 or the battery pack of claim 11.

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