A secondary battery and an electric device

Abstract

This application relates to the field of battery technology, and in particular to a secondary battery and an electrical device. The secondary battery includes a negative electrode sheet, which includes a negative current collector and a negative active layer disposed on at least one side of the negative current collector. The negative active layer includes a first coating and a second coating, with the first coating disposed between the second coating and the current collector. Both the first and second coatings contain a styrene-acrylate-acrylic acid copolymer, and the content of the styrene-acrylate-acrylic acid copolymer in the second coating is greater than the content of the styrene-acrylate-acrylic acid copolymer in the first coating. This application effectively improves the cycle performance and energy density of the battery while ensuring kinetic performance by controlling the content of the styrene-acrylate-acrylic acid copolymer in the second coating to be greater than the content in the first coating.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Technology

[0002] With the rapid popularization and promotion of electric vehicles, the demand for high-energy-density, long-cycle-performance rechargeable batteries is constantly increasing. In rechargeable batteries, the performance of the positive and negative electrodes is one of the key factors affecting electrochemical performance. Therefore, developing higher-performance negative electrodes, such as silicon-based negative electrodes, is one of the effective ways to further improve the performance of rechargeable batteries.

[0003] However, during charge-discharge cycles, silicon-based anodes have a high volume expansion rate, which consumes a large amount of electrolyte during the charge-discharge process, reducing the lithium-ion electron conduction medium and increasing the impedance, making it difficult to guarantee the cycle life of secondary batteries.

[0004] In view of the above, this application is hereby submitted. Summary of the Invention

[0005] The primary objective of this application is to provide a secondary battery that effectively improves the cycle performance and energy density of the battery while ensuring kinetic performance.

[0006] The second objective of this application is to provide an electrical appliance.

[0007] To achieve the above-mentioned objectives of this application, the following technical solution is adopted:

[0008] This application provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte. The negative electrode includes a negative current collector and a negative active layer disposed on at least one side of the negative current collector. The negative active layer includes a first coating and a second coating, wherein the first coating is disposed between the second coating and the current collector.

[0009] Both the first coating and the second coating contain styrene-acrylate-acrylic acid copolymer, and the content of styrene-acrylate-acrylic acid copolymer in the second coating is greater than the content of styrene-acrylate-acrylic acid copolymer in the first coating.

[0010] Furthermore, the content of the styrene-acrylate-acrylic acid copolymer in the second coating is 1.2wt% to 1.8wt%.

[0011] Furthermore, the content of the styrene-acrylate-acrylic acid copolymer in the first coating is 0.2wt% to 0.8wt%.

[0012] Furthermore, in the styrene-acrylate-acrylic acid copolymer, the weight percentage of acrylate structural units is 45% to 55%, and the weight percentage of acrylic structural units is 10% to 25%.

[0013] Furthermore, the negative electrode active layer comprises a negative electrode active material, which includes a silicon-based material and graphite, wherein the weight ratio of the silicon-based material to the graphite is (5-25):(75-95).

[0014] Furthermore, the styrene-acrylate-acrylic acid copolymer contains an alkali metal element, which includes at least one of Na, K, and Li.

[0015] Furthermore, the alkali metal content in the styrene-acrylate-acrylic acid copolymer is 0.91% to 1.52% by mass.

[0016] Furthermore, the styrene-acrylate-acrylic acid copolymer contains carbon and oxygen elements, wherein the mass percentage of carbon is 68.5% to 72% and the mass percentage of oxygen is 18.3% to 22.8%.

[0017] Furthermore, the first coating and the second coating each independently contain silicon-based materials, and the content of silicon-based materials in the second coating is greater than the content of silicon-based materials in the first coating.

[0018] This application also provides an electrical device, including a secondary battery as described above.

[0019] Compared with the prior art, the beneficial effects of this application are as follows:

[0020] The secondary battery of this application adds a styrene-acrylate-acrylic acid copolymer to the negative electrode and controls the content of the styrene-acrylate-acrylic acid copolymer in the second coating of the negative electrode to be greater than that in the first coating. In the initial stage, the styrene-acrylate-acrylic acid copolymer absorbs part of the electrolyte through its own swelling. In the later stage of the cycle, when the electrolyte concentration is low, the styrene-acrylate-acrylic acid copolymer releases the absorbed electrolyte under the drive of the concentration difference. This ensures that there is enough electrolyte on the electrode surface to allow ions to freely move back and forth between the positive and negative electrodes to achieve insertion and extraction. This effectively improves the cycle performance and energy density of the battery while ensuring kinetic performance. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0022] Figure 1 The Fourier transform infrared absorption spectrum of the styrene-methyl acrylate-acrylic acid copolymer prepared in Example 1 is shown below.

[0023] Figure 2 This is a swelling diagram of the styrene-acrylate-acrylic acid copolymer of this application;

[0024] Figure 3 This describes the release of the styrene-acrylate-acrylic acid copolymer onto the dry electrode sheet. Detailed Implementation

[0025] The technical solution of this application will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are only some embodiments of this application, not all embodiments, and are only used to illustrate this application, and should not be regarded as limiting the scope of this application. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0026] The following is a detailed description of a secondary battery and electrical device according to an embodiment of this application.

[0027] In some embodiments of this application, a secondary battery is provided, including a positive electrode, a negative electrode, a separator, and an electrolyte. The negative electrode includes a negative current collector and a negative active layer disposed on at least one side of the negative current collector. The negative active layer includes a first coating and a second coating, wherein the first coating is disposed between the second coating and the current collector.

[0028] Both the first coating and the second coating contain styrene-acrylate-acrylic acid copolymer, and the content of styrene-acrylate-acrylic acid copolymer in the second coating is greater than the content of styrene-acrylate-acrylic acid copolymer in the first coating.

[0029] In the negative electrode sheet of this application embodiment, the first coating and the second coating respectively add different amounts of styrene-acrylate-acrylic acid copolymer, with the content of styrene-acrylate-acrylic acid copolymer in the second coating being greater than that in the first coating. Since the construction of the solid electrolyte interface (SEI film) and the expansion of the negative electrode volume both lead to electrolyte loss, and the electrolyte is continuously consumed during cycling, the electrolyte concentration in the system gradually decreases in the later stages of long cycles. The styrene-acrylate-acrylic acid copolymer added to the second coating, under the concentration difference, releases the absorbent gel absorbed in the initial stage, ensuring sufficient dielectric on the electrode surface to allow ions to freely move back and forth between the positive and negative electrodes for insertion and extraction. The addition of a small amount of styrene-acrylate-acrylic acid copolymer to the first coating, in addition to contributing to the release of electrolyte in the later stages of cycling, also ensures good electronic conductivity between the negative electrode active layer and the current collector, effectively maintaining the overall high kinetic performance of the electrode. Specifically, in the styrene-acrylate-acrylic acid copolymer of this application embodiment, the acrylate structural unit is "miscible" because it has the same -COOR structure as the ester solvent in the electrolyte; the acrylic structural unit exists as an acrylate (at least one of -COONa, -COOK, and -COOLi) structure. After the styrene-acrylate-acrylic acid copolymer comes into contact with the electrolyte, the acrylate ionizes to form free alkali ions (Na+). + K + Li + The carboxylate ions and alkali ions can move freely inside the resin. The carboxylate ions fixed on the polymer chain repel each other, causing the polymer network to expand and creating a negative pressure inside the resin. This further promotes the absorption of electrolyte, thereby increasing the amount of electrolyte absorbed.

[0030] The embodiments of this application improve kinetic and cycle performance by adjusting the content of styrene-acrylate-acrylic acid copolymer in the first and second coatings of the negative electrode sheet, while ensuring the conductivity of the current collector and the electrode surface near the separator, and also taking into account the ion transfer capability between the positive and negative electrodes on the electrode surface near the separator.

[0031] In some embodiments, the styrene-acrylate-acrylic acid copolymer is present in a content of 1.2 wt% to 1.8 wt% in the second coating.

[0032] In some embodiments, typically but not limitingly, for example, the content of the styrene-acrylate-acrylic copolymer in the second coating may be a range of 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, or any combination thereof.

[0033] In some embodiments, the styrene-acrylate-acrylic acid copolymer is present in the first coating at a content of 0.2 wt% to 0.8 wt%; typically, but not limitingly, for example, the content of the styrene-acrylate-acrylic acid copolymer in the first coating may be a range of 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, or any combination thereof.

[0034] In some embodiments, the content of the styrene-acrylate-acrylic acid copolymer in the second coating is M2, and the content of the styrene-acrylate-acrylic acid copolymer in the first coating is M1, where 0.4 ≤ M2 - M1 ≤ 1.6. The difference in the content of the styrene-acrylate-acrylic acid copolymer between the upper and lower layers is within the above range, which is beneficial for further improving the cycle performance of the secondary battery.

[0035] In some embodiments, the styrene-acrylate-acrylic acid copolymer contains 45%–55% acrylate structural units and 10%–25% acrylic structural units by weight. The content of ester groups, carboxyl groups, and oxygen in the styrene-acrylate-acrylic acid copolymer affects its liquid absorption and swelling properties. When the acrylate structural units are present in a weight ratio of 45%–55% and the acrylic structural units in a weight ratio of 10%–25%, the negative electrode exhibits better liquid absorption properties, thereby improving the cycle performance of the secondary battery.

[0036] In some embodiments, the negative electrode active layer comprises a negative electrode active material, which includes a silicon-based material and graphite, wherein the weight ratio of the silicon-based material to the graphite is (5–25):(75–95). Typically, but not limitingly, for example, the weight ratio of the silicon-based material to graphite in the negative electrode active material is 5:95, 10:90, 15:85, 20:80, 25:75, or a range of any two of these.

[0037] In some embodiments, the silicon-based material includes at least one of silicon, silicon-carbon materials, and silicon oxide.

[0038] In some embodiments, the styrene-acrylate-acrylic acid copolymer contains an alkali metal element, wherein the mass percentage of the alkali metal element is 0.91% to 1.52%.

[0039] In some embodiments, the styrene-acrylate-acrylic acid copolymer contains carbon and oxygen elements, wherein the mass percentage of carbon is 68.5% to 72% and the mass percentage of oxygen is 18.3% to 22.8%.

[0040] For silicon-based anodes, the addition of styrene-acrylate-acrylic acid copolymer balances the effects of silicon expansion, further increasing the silicon-based material content in the system and thus improving energy density. In some embodiments, the first and second coatings each independently contain silicon-based materials, with the silicon-based material content in the second coating being greater than that in the first coating. This further improves the energy density of the secondary battery while maintaining superior cycle performance.

[0041] In some embodiments, the styrene-acrylate-acrylic acid copolymer can be prepared in the following manner:

[0042] (1) Acrylic acid, alkaline neutralizer and solvent are mixed to obtain a first mixture;

[0043] (2) Add acrylate and polystyrene to the first mixture, and then add an initiator to cause the acrylic acid, the acrylate and the polystyrene to undergo a polymerization reaction at 60°C to 80°C;

[0044] (3) The reaction system of step (2) is filtered and washed, and then dried under vacuum to obtain styrene-acrylate-acrylic acid copolymer.

[0045] In step (1), the alkaline neutralizing agent includes at least one of potassium hydroxide, sodium hydroxide, and lithium hydroxide, and the solvent can be deionized water. Acrylic acid can be dissolved in deionized water first, followed by the addition of the alkaline neutralizing agent and stirring until homogeneous, for example, stirring at a speed of 500 rpm to 1000 rpm for 30 to 120 minutes to obtain the first mixture. In some embodiments, the alkaline neutralizing agent can also be sodium hydroxide, potassium hydroxide, etc., dissolved in deionized water to obtain an alkaline neutralizing agent solution with a molar concentration of 10 to 15 mol / L.

[0046] In step (2), the acrylate can be methyl acrylate, ethyl acrylate, butyl acrylate, etc., and the initiator can be potassium persulfate. The initiator can be dissolved in deionized water to obtain an initiator solution, and nitrogen gas can be passed through the initiator solution to remove dissolved oxygen before adding the initiator solution to the first mixture. In some embodiments, the first mixture can be heated to 60-80°C before adding the initiator. In some embodiments, the weight ratio of acrylic acid, acrylate, and styrene monomer is 1:(5-15):(5-15). In step (3), the vacuum drying temperature is between 50-80°C. This application uses the above-mentioned styrene-acrylate-acrylic acid copolymer or the styrene-acrylate-acrylic acid copolymer prepared according to the above method, which has liquid absorption and swelling properties in the electrolyte, thereby improving battery performance.

[0047] In some embodiments, the first coating and the second coating also independently contain a negative electrode active material, a binder, and a conductive agent.

[0048] In some embodiments, the adhesive includes, but is not limited to, polyacrylic acid and / or styrene-butadiene copolymer.

[0049] In some embodiments, the conductive agent includes, but is not limited to, at least one of conductive carbon black, acetylene black, Ketjen black, carbon nanotubes, carbon fibers, and graphene.

[0050] In some embodiments, the content of the negative electrode active material in the first coating is 90 wt% to 96 wt%; typically, but not limitingly, for example, the content of the negative electrode active material in the first coating may be a range of 90 wt%, 91 wt%, 92 wt%, 93 wt%, 94 wt%, 95 wt%, 96 wt%, or any combination thereof.

[0051] In some embodiments of this application, the content of the negative electrode active material in the second coating is 90 wt% to 96 wt%; typically, but not limitingly, for example, the content of the negative electrode active material in the second coating can be a range of 90 wt%, 91 wt%, 92 wt%, 93 wt%, 94 wt%, 95 wt%, 96 wt%, or any combination thereof.

[0052] In some embodiments, the first coating contains 90 wt% to 96 wt% of the negative electrode active material, 0.2 wt% to 0.8 wt% of the styrene-acrylate-acrylic acid copolymer, 1 wt% to 5 wt% of the binder, and 2 wt% to 5 wt% of the conductive agent.

[0053] In some embodiments of this application, the second coating contains 90 wt% to 96 wt% of the negative electrode active material, 1.2 wt% to 1.8 wt% of the styrene-acrylate-acrylic acid copolymer, 1 wt% to 5 wt% of the binder, and 2 wt% to 5 wt% of the conductive agent.

[0054] In the embodiments of this application, styrene-acrylate-acrylic acid copolymer can replace part of the function of the binder. Styrene-acrylate-acrylic acid copolymer is a polymer and also belongs to the class of adhesives. It can provide the function of a binder, thereby reducing the amount of binder used, increasing the electrode load, and thus improving the energy density and kinetic performance.

[0055] In some embodiments, the negative current collector includes at least one of copper foil, carbon-coated copper foil, and composite copper foil.

[0056] In some embodiments, the positive electrode sheet includes a positive current collector and a positive active layer. In some embodiments, the positive active layer contains a positive active material, a binder, and a conductive agent. In some embodiments, the positive active material content in the positive active layer is 94 wt% to 96 wt%, the binder content is 2 wt% to 4 wt%, and the conductive agent content is 1 wt% to 3 wt%.

[0057] In some embodiments, the positive electrode active material includes, but is not limited to, ternary positive electrode materials and lithium iron phosphate positive electrode materials; the binder includes, but is not limited to, at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol, hydroxymethyl cellulose and polyvinylpyrrolidone; the conductive agent includes, but is not limited to, at least one of conductive carbon black, acetylene black, Ketjen black, carbon nanotubes, carbon fibers and graphene.

[0058] Some embodiments of this application also provide a method for preparing a secondary battery, comprising the following steps:

[0059] The slurry of the first coating and the slurry of the second coating are sequentially coated on the surface of the current collector, and after drying and rolling, a negative electrode sheet is obtained.

[0060] In some embodiments of this application, the method for preparing a secondary battery includes the following steps:

[0061] After the positive electrode, separator, and negative electrode are wound or stacked in sequence, they are then encapsulated, injected with electrolyte, and formed to obtain a secondary battery. The resulting secondary battery can be either a wound secondary battery or a stacked secondary battery.

[0062] In some embodiments of this application, an electrical device is also provided, including the aforementioned secondary battery.

[0063] To better explain the technical solutions of the embodiments of this application, further explanations and descriptions are provided below through several specific embodiments.

[0064] Examples 1-7 and Comparative Examples 1-3

[0065] The methods for preparing secondary batteries provided in Examples 1-7 and Comparative Examples 1-3 include the following steps:

[0066] The positive electrode, separator, and negative electrode are wound in sequence to obtain a bare cell; the bare cell is then encapsulated, dehydrated, injected with electrolyte, and formed to obtain a secondary battery.

[0067] The preparation method of the positive electrode sheet includes the following steps:

[0068] The positive electrode material (NCM622), conductive agent (conductive carbon black) and binder (polyvinylidene fluoride) are mixed in a weight ratio of 8:1:1 and then uniformly coated onto the positive electrode current collector using an extrusion coating die (DIE). After drying and rolling, the positive electrode sheet is obtained.

[0069] The method for preparing the negative electrode sheet includes the following steps:

[0070] The raw materials are dispersed in a solvent to obtain the slurry for the first coating layer;

[0071] The raw materials are dispersed in a solvent to obtain a slurry for the second coating.

[0072] The slurry for the first coating and the slurry for the second coating are uniformly applied to copper foil using an extrusion double coating die (DIE). After drying and rolling, a negative electrode sheet is obtained. In the negative electrode sheet, the first coating and the second coating are stacked sequentially along the thickness direction of the copper foil. The coating is double-sided, specifically, a single-sided double coating is applied and dried first, and then a double coating is applied and dried on the other side of the copper foil. Therefore, the structure of the negative electrode sheet is "second coating - first coating - copper foil - first coating - second coating".

[0073] The electrolyte consists of 12.5 wt% LiPF6 and 1 wt% LiFSI dissolved in an organic solvent. The composition and mass ratio of the organic solvent are FEC:EC:PC:EMC = 1:1:1:7, where FEC represents ethyl methyl carbonate, EC represents ethylene carbonate, PC represents propylene carbonate, and EMC represents methyl ethyl carbonate.

[0074] The preparation methods of styrene-acrylate-acrylic acid copolymers in Examples 1-4 and Comparative Examples 1-3 include the following steps:

[0075] Weigh 4g of acrylic acid and place it in a three-necked flask. Add 50mL of distilled water and slowly add 12mol / L NaOH solution dropwise while stirring in an ice-water bath. After partial neutralization, add 9g of methyl acrylate and 7g of polystyrene in sequence, and add potassium persulfate as an initiator. React at 70±5℃ for 7h. After the reaction is complete, dry under vacuum at 50℃ to constant weight, pulverize, and sieve to obtain product 1 with a certain particle size.

[0076] The difference between Example 5 and Example 4 lies in the amount of methyl acrylate and acrylic acid added in the preparation method of the styrene-acrylate-acrylic acid copolymer, as detailed below:

[0077] Weigh 2g of acrylic acid and place it in a three-necked flask. Add 50mL of distilled water and slowly add 12mol / L NaOH solution dropwise while stirring in an ice-water bath. After partial neutralization, add 10g of methyl acrylate and 8g of polystyrene in sequence, and add potassium persulfate as an initiator. React at 70±5℃ for 7h. After the reaction is complete, dry under vacuum at 50℃ to constant weight, pulverize, and sieve to obtain product 2 with a certain particle size.

[0078] The difference between Example 6 and Example 4 lies in the raw materials and their amounts used in the preparation method of the styrene-acrylate-acrylic acid copolymer, as detailed below:

[0079] Weigh 3g of acrylic acid and place it in a three-necked flask. Add 50mL of distilled water and slowly add 12mol / L NaOH solution dropwise while stirring in an ice-water bath. After partial neutralization, add 11g of butyl acrylate and 6g of polystyrene in sequence, and add potassium persulfate as an initiator. React at 70±5℃ for 7h. After the reaction is complete, dry under vacuum at 50℃ to constant weight, pulverize, and sieve to obtain product 3 with a certain particle size.

[0080] The difference between Example 7 and Example 4 lies in the raw materials and their amounts used in the preparation method of the styrene-acrylate-acrylic acid copolymer, as detailed below:

[0081] Weigh 5g of acrylic acid and place it in a three-necked flask. Add 50mL of distilled water and slowly add 12mol / L NaOH solution dropwise while stirring in an ice-water bath. After partial neutralization, add 10g of butyl acrylate and 9g of polystyrene in sequence, and add potassium persulfate as an initiator. React at 70±5℃ for 7h. After the reaction is complete, vacuum dry at 50℃ to constant weight, pulverize, and sieve to obtain product 4 with a certain particle size.

[0082] Small amounts of products 1–4 obtained during preparation were characterized accordingly:

[0083] (1) Fourier transform infrared absorption spectroscopy was used to characterize product 1, and the results are as follows: Figure 1 As shown.

[0084] according to Figure 1 The absorption peak of product 1 can be obtained as follows: 3061 cm⁻¹ -1 3027cm -1 It is the stretching vibration peak of the unsaturated CH on the benzene ring; 2956 cm⁻¹ -1 2930cm -1 2871cm -1 It is the stretching vibration peak of -CH3 and -CH2; 1727 cm⁻¹ -1 It is the C=O stretching vibration peak in methyl acrylate and acrylic acid; 1602 cm⁻¹ -1 1582cm-1 1493cm -1 1451cm -1 It is a vibrational peak of the benzene ring skeleton; 1247 cm⁻¹ -1 1156cm -1 959cm -1 942cm -1 It is the CO stretching vibration peak of the ester group in methyl acrylate; 759 cm⁻¹ -1 699cm -1 It is the out-of-plane bending vibration peak of the monosubstituted benzene ring CH; 553 cm⁻¹ -1 The peak is an out-of-plane bending vibration peak of the benzene ring. This confirms that the product is a styrene-methyl acrylate-acrylic acid copolymer.

[0085] (2) ICP elemental analysis. Table 1 shows the target compounds that were theoretically designed and synthesized, and Table 2 shows the products that were designed and synthesized based on Table 1 and analyzed by ICP.

[0086] Table 1

[0087] Structural unit weight styrene Methyl acrylate Butyl acrylate acrylic acid Target 1 35％ 45％ / 20％ Target 2 40％ 50％ / 10％ Target 3 30％ / 55％ 15％ Target 4 45％ / 50％ 25％

[0088] Table 2

[0089] element C O H Na Product 1 68.44％ 22.81％ 7.23％ 1.52％ Product 2 71.07％ 20.68％ 7.51％ 0.74％ Product 3 70.72％ 19.70％ 8.67％ 0.91％ Product 4 71.97％ 18.33％ 8.38％ 1.32％

[0090] As can be seen from Tables 1 and 2, the content of each element tested by ICP is consistent with the theoretical calculation value. Target 1 corresponds to product 1, target 2 corresponds to product 2, target 3 corresponds to product 3, and target 4 corresponds to product 4, which indirectly shows that the synthesized product is consistent with the designed target.

[0091] The swelling and release properties of the styrene-methyl acrylate-acrylic acid copolymer prepared in Example 4 were tested. The styrene-methyl acrylate-acrylic acid copolymer was immersed in an electrolyte containing 12.5 wt% LiPF6 and 1 wt% LiFSI (solvent: FEC:EC:PC:EMC = 1:1:1:7, where FEC represents ethyl methyl carbonate, EC represents ethylene carbonate, PC represents propylene carbonate, and EMC represents ethyl methyl carbonate) and allowed to stand for 24 h. The results are as follows. Figure 2 Subsequently, the styrene-acrylate-acrylic acid copolymer soaked in electrolyte was transferred to laboratory filter paper. The diffusion of the styrene-acrylate-acrylic acid copolymer and electrolyte in the filter paper was observed at regular intervals. The results... Figure 3 As shown.

[0092] from Figure 2 It can be seen that when styrene-methyl acrylate-acrylic acid copolymer is immersed in electrolyte, it will absorb electrolyte and swell.

[0093] from Figure 3 It can be seen that when the styrene-methyl acrylate-acrylic acid copolymer that has absorbed electrolyte is placed in filter paper, the electrolyte is released from the styrene-methyl acrylate-acrylic acid copolymer into the filter paper over time. The wetting area formed by the electrolyte diffusion into the filter paper gradually increases, while the volume of the styrene-methyl acrylate-acrylic acid copolymer gradually decreases. This indicates that the styrene-methyl acrylate-acrylic acid copolymer has good electrolyte absorption and swelling properties and a slow electrolyte release performance.

[0094] The raw materials and their contents for the first and second coatings of the negative electrode sheets in Examples 1-7 and Comparative Examples 1-3 are shown in Table 3. In Table 3, the negative electrode active material is artificial graphite and silicon; the content of artificial graphite in the negative electrode active material is 95 wt%, and the content of silicon is 5 wt%; the styrene-methyl acrylate-acrylic acid copolymer is the styrene-methyl acrylate-acrylic acid copolymer or styrene-butyl acrylate-acrylic acid copolymer prepared above; the binder is polyacrylic acid; and the conductive agent is conductive carbon black.

[0095] Table 3

[0096]

[0097]

[0098] Examples 8-13

[0099] The preparation methods of the secondary batteries provided in Examples 8 to 13 are the same as those in Example 4, except that the silicon content in the active material is different, as shown in Table 4.

[0100] Table 4

[0101]

[0102] Performance testing

[0103] The secondary batteries prepared in Examples 1-13 and Comparative Examples 1-3 were subjected to DCR tests at 50% SOC and constant voltage charging for 10s at 4×40mA, and the results are shown in Table 5.

[0104] The DCR test conditions at room temperature are: 50% SOC 4×40mA constant current discharge for 10s at 25℃.

[0105] The low-temperature DCR test conditions are: 50% SOC 0.36×40mA constant current discharge for 10s at -20℃.

[0106] Capacity test: 1. Let stand for 10 minutes; 2. Discharge at 1C constant current to 2.5V; 3. Let stand for 10 minutes; 4. Charge at 1C constant current to 4.4V, then charge at constant voltage to 0.05C current; 5. Let stand for 10 minutes; 6. Discharge at 1C constant current to 2.5V; 7. Let stand for 10 minutes.

[0107] Range voltage calibration; 8. Constant current charging to 4.4V with 0.33×40mA, followed by constant voltage charging to 0.05×40mA current; 9. Rest for 15min; 10. Constant current discharge with 1×40mA for 3456s (V0); 11. Rest for 5s; 12. Constant current charging with 1×40mA for 216s (V1); 13. Rest for 5s; 14. Constant current charging with 4.2×40mA for 60s (V2); 15. Rest for 5s; 16. Constant current charging with 4.08×40mA for 159s (V3); 17. Rest for 5s; 18. 3.4 9×40mA constant current charging for 258s (V4); 19. Rest for 5s; 20. 2.98×40mA constant current charging for 121s (V5); 21. Rest for 5s; 22. 2.94×40mA constant current charging for 122s (V6); 23. Rest for 5s; 24. 1×40mA constant current charging for 360s (V7); 25. Rest for 5s; 26. 0.5×40mA constant current charging for 504s (V8); 27. Rest for 15min; 28. 1×40mA constant current discharging to 2.5V; 29. ​​Rest for 10min.

[0108] 25℃ Cyclic Test: 1. Rest for 10 min; 2. Charge with a constant current of 1×40mA to V1; 3. Rest for 5 s; 4. Charge with a constant current of 4.2×40mA to V2; 5. Rest for 5 s; 6. Charge with a constant current of 4.08×40mA to V3; 7. Rest for 5 s; 8. Charge with a constant current of 3.49×40mA to V4; 9. Rest for 5 s; 10. Charge with a constant current of 2.98×40mA to V3. V5; 11. Rest for 5 seconds; 12. Charge with a constant current of 2.94 × 40 mA to V6; 13. Rest for 5 seconds; 14. Charge with a constant current of 1 × 40 mA to V7; 15. Rest for 5 seconds; 16. Charge with a constant current of 0.5 × 40 mA to V8; 17. Rest for 10 minutes; 18. Discharge with a constant current of 1 × 40 mA to V0; 19. Rest for 20 minutes; 20. Repeat steps 2 to 19 3000 times or until the capacity drops to 80% of the nominal capacity.

[0109] 45℃ Cyclic Test: 1. Rest for 10 min; 2. Charge with a constant current of 1×40mA to V1; 3. Rest for 5 s; 4. Charge with a constant current of 4.2×40mA to V2; 5. Rest for 5 s; 6. Charge with a constant current of 4.08×40mA to V3; 7. Rest for 5 s; 8. Charge with a constant current of 3.49×40mA to V4; 9. Rest for 5 s; 10. Charge with a constant current of 2.98×40mA to V3. V5; 11. Rest for 5 seconds; 12. Charge with a constant current of 2.94 × 40 mA to V6; 13. Rest for 5 seconds; 14. Charge with a constant current of 1 × 40 mA to V7; 15. Rest for 5 seconds; 16. Charge with a constant current of 0.5 × 40 mA to V8; 17. Rest for 10 minutes; 18. Discharge with a constant current of 1 × 40 mA to V0; 19. Rest for 20 minutes; 20. Repeat steps 2 to 19 3000 times or until the capacity drops to 80% of the nominal capacity.

[0110] Table 5

[0111]

[0112]

[0113] A comparison of Examples 1 to 4 shows that when the ratio of styrene-acrylate-acrylic acid copolymer in the first and second coatings of the negative electrode sheet changes, the performance also shows a certain change pattern. The overall kinetic performance remains basically the same, but as the amount of styrene-acrylate-acrylic acid copolymer in the second coating increases, the cycle performance is significantly improved.

[0114] In Examples 4-7, as the content of ester and carboxyl groups decreases, the oxygen content also decreases, which reduces the liquid absorption capacity of the electrode and the cycle performance. At the same time, due to the change in sodium content, the ionic conductivity fluctuates to some extent. A higher Na ion content results in better ionic conductivity, and the DCR decreases to some extent.

[0115] A comparison of Examples 8-13 and Example 4 shows that as the silicon content in the negative electrode active material gradually increases, the DCR and cycle performance remain basically unchanged. Adding styrene-acrylate-acrylic acid copolymer can effectively increase the amount of silicon material added. Since the silicon material has a high specific capacity, it is beneficial to further improve the overall energy density.

[0116] Those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The negative electrode sheet includes a negative current collector and a negative active layer disposed on at least one side of the negative current collector. The negative active layer includes a first coating and a second coating, wherein the first coating is disposed between the second coating and the current collector. Both the first coating and the second coating contain styrene-acrylate-acrylic acid copolymer, and the content of styrene-acrylate-acrylic acid copolymer in the second coating is greater than the content of styrene-acrylate-acrylic acid copolymer in the first coating; The content of the styrene-acrylate-acrylic acid copolymer in the second coating is 1.2wt%~1.8wt%; The content of the styrene-acrylate-acrylic acid copolymer in the first coating is 0.2wt%~0.8wt%; In the styrene-acrylate-acrylic acid copolymer, the weight percentage of acrylate structural units is 45%~55%, and the weight percentage of acrylic structural units is 10~25%.

2. The secondary battery according to claim 1, characterized in that, The content of the styrene-acrylate-acrylic acid copolymer in the second coating is M2, and the content of the styrene-acrylate-acrylic acid copolymer in the first coating is M1, where 0.4 ≤ M2 - M1 ≤ 1.

6.

3. The secondary battery according to claim 1, characterized in that, The negative electrode active layer comprises a negative electrode active material, which includes silicon-based material and graphite, wherein the weight ratio of silicon-based material to graphite is (5~25):(75~95).

4. The secondary battery according to claim 1, characterized in that, The styrene-acrylate-acrylic acid copolymer contains an alkali metal element, and the mass percentage of the alkali metal element is 0.91% to 1.52%.

5. The secondary battery according to claim 4, characterized in that, The styrene-acrylate-acrylic acid copolymer contains carbon and oxygen elements, wherein the mass percentage of carbon is 68.5% to 72% and the mass percentage of oxygen is 18.3% to 22.8%.

6. The secondary battery according to claim 1, characterized in that, The first coating and the second coating each contain silicon-based materials independently, and the content of silicon-based materials in the second coating is greater than that in the first coating.

7. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 1 to 6.

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