Secondary battery

Abstract

The invention belongs to the technical field of secondary batteries, provides a secondary battery, and solves the problem that low-temperature performance and high-temperature performance are difficult to consider when water-based PVDF (Polyvinylidene Fluoride) is applied to a negative plate in the prior art. The secondary battery comprises a positive plate, a negative plate, a diaphragm and an electrolyte, the negative plate comprises a negative current collector and a negative active layer arranged on at least one side of the negative current collector, and the negative active layer comprises negative active substance particles and a first binder; the first binder comprises PVDF (Polyvinylidene Fluoride) particles; the number of the PVDF particles on the surface of a single negative electrode active material particle is the coverage number S, and S is greater than or equal to 51 and less than or equal to 300; the average particle size of the PVDF particles is D1, and D1 is more than or equal to 50nm and less than or equal to 350nm; the electrolyte comprises fluoroethylene carbonate serving as a first additive, the mass content eta of the fluoroethylene carbonate in the electrolyte is larger than or equal to 1% and smaller than or equal to 20%.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and particularly relates to a secondary battery. Background Art

[0002] When a water-based PVDF (polyvinylidene fluoride) binder is applied to the negative electrode side of a lithium-ion battery, compared with the traditional oil-based PVDF, it has better flexibility and ionic conductivity in a low-temperature environment, and can effectively improve the low-temperature performance of the battery. However, the water-based PVDF adheres to the surface of the negative active material, resulting in inconsistent SEI thickness formed by the negative electrode where the water-based PVDF adheres and the negative electrode where the water-based PVDF does not adhere, thus causing uneven SEI film formation, which weakens the high-temperature performance of the battery to a certain extent and results in poor high-temperature cycle performance, making it difficult to balance the high- and low-temperature performances during the use of lithium-ion batteries. Summary of the Invention

[0003] Therefore, the technical problem to be solved by the present invention is to overcome the problem that it is difficult to balance the low-temperature performance and high-temperature performance when the water-based PVDF is applied to the negative electrode sheet in the prior art, so as to provide a secondary battery.

[0004] For this purpose, the present invention provides the following technical solutions.

[0005] A secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one side of the negative electrode current collector. The negative electrode active layer includes negative electrode active material particles and a first binder;

[0006] The first binder includes PVDF particles; the number of PVDF particles on the surface of a single negative electrode active material particle is the coverage number S, and S satisfies 51 ≤ S ≤ 300; the average particle size of the PVDF particles is D1, and 50 nm ≤ D1 ≤ 350 nm;

[0007] The electrolyte includes a first additive, vinylene carbonate fluoride, and the mass content of the vinylene carbonate fluoride in the electrolyte is η1, and 1% ≤ η1 ≤ 20%.

[0008] In a possible implementation manner, the areal density of the negative electrode active layer is 3 mg / cm 2 ≤ CW ≤ 8 mg / cm 2 , and 180 nm ≤ D1 ≤ 350 nm.

[0009] In a possible implementation manner, the areal density of the negative electrode active layer is 8 mg / cm 2 < CW ≤ 13 mg / cm 2 , and 50 nm ≤ D1 < 180 nm.

[0010] In a possible implementation, the negative electrode active material includes a silicon-based material.

[0011] In a possible implementation, based on the mass of the negative electrode active layer, the mass content of silicon element is 1% ≤ η2 ≤ 50%;

[0012] In a possible implementation, the D50 particle size of the silicon-based material is D2, 5 μm ≤ D2 ≤ 10 μm;

[0013] In a possible implementation, the silicon-based material is selected from at least one of silicon oxide, silicon carbide, and nanosilicon.

[0014] In a possible implementation, the negative electrode active layer further includes a second binder, and the second binder includes polyacrylic acid.

[0015] In a possible implementation, the mass ratio of the second binder to the first binder is η3, and η3 is 0 to 1;

[0016] In a possible implementation, the polyacrylic acid includes one or more of polyacrylic acid-acrylonitrile, polyacrylic acid-acrylonitrile-acrylamide, and polyacrylic acid-acrylamide.

[0017] In a possible implementation, the electrolyte further includes a second additive, vinylene carbonate.

[0018] In a possible implementation, the ratio of the mass content of vinylene carbonate to fluoroethylene carbonate in the electrolyte is 0.1 to 0.45.

[0019] In a possible implementation, the tap density of the negative electrode active layer is 1.3 to 1.85 g / cm 3 ;

[0020] In a possible implementation, the porosity of the negative electrode sheet is 25% to 40%;

[0021] In a possible implementation, the OI value of the negative electrode sheet is 5 to 15;

[0022] In a possible implementation, the PVDF particles refer to a polymer containing at least vinylidene fluoride monomers.

[0023] The technical solution of the present invention has the following advantages:

[0024] The secondary battery of the present application includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one side of the negative electrode current collector. The negative electrode active layer includes negative electrode active material particles and a first binder; the first binder includes PVDF particles; the number of PVDF particles on the surface of a single negative electrode active material particle is the coverage number S, and S satisfies 51 ≤ S ≤ 300; the average particle size of the PVDF particles is D1, and 50 nm ≤ D1 ≤ 350 nm; the electrolyte includes a first additive, fluoroethylene carbonate, and the mass content of fluoroethylene carbonate in the electrolyte is η1, and 1% ≤ η1 ≤ 20%. By regulating the average particle size of the PVDF particles to satisfy 50 nm ≤ D1 ≤ 350 nm, the present application improves the electrolyte content at this position, that is, improves the content of FEC and the thickness of the film formation at this position, reduces the interfacial impedance, and improves the high-temperature cycling performance of the battery. The present application optimizes the distribution of PVDF particles on the surface of the negative electrode active material particles in the negative electrode sheet, and combines with the optimized electrolyte, which can well coordinate with PVDF to form a stable, dense and low-impedance SEI film, thereby improving the high-temperature cycling and storage performance of the battery, and enabling the battery to have excellent high-temperature cycling and high-temperature storage performance while having good low-temperature performance. Description of the Drawings

[0025] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0026] Figure 1 SEM image of the negative electrode sheet for Example 1;

[0027] Figure 2 DSC diagram of the PVDF particles for Example 1. Detailed Embodiments

[0028] The following embodiments are provided to better further understand the present invention. They are not limited to the best embodiments, and do not limit the content and protection scope of the present invention. Any product that is the same as or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior art features falls within the protection scope of the present invention.

[0029] For those embodiments in which specific experimental steps or conditions are not specified, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For the reagents or instruments whose manufacturers are not specified, they are all conventional reagent products that can be obtained through commercial purchase.

[0030] When the aqueous PVDF is applied to the negative electrode sheet, it is difficult to balance the low-temperature performance and the high-temperature performance. Based on this, the present application provides a secondary battery, including a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one side of the negative electrode current collector. The negative electrode active layer includes negative electrode active material particles and a first binder;

[0031] The first binder includes PVDF particles; the number of PVDF particles on the surface of a single negative electrode active material particle is the coverage number S, and S satisfies 51 ≤ S ≤ 300; the average particle size D1 of the PVDF particles, 50 nm ≤ D1 ≤ 350 nm;

[0032] The electrolyte includes a first additive, fluoroethylene carbonate (FEC), and the mass content η1 of fluoroethylene carbonate in the electrolyte satisfies 1% ≤ η1 ≤ 20%.

[0033] If the average particle size D1 of the PVDF particles is too small, such as less than 50 nm, the smaller the particle size, the larger the specific surface area. At the same mass addition amount, the number of PVDF particles is larger, and the corresponding amount of electrolyte absorbed is more. That is, the concentration of the film-forming additive FEC at this position is high, resulting in too thick film formation at this position, increased impedance, and being unfavorable to the high-temperature performance of the battery. After it is greater than 350 nm, it is just the opposite, resulting in too thin SEI film at this position and poor thermal stability, being unfavorable to the high-temperature performance of the battery. In this application, by regulating the average particle size of the PVDF particles to satisfy 50 nm ≤ D1 ≤ 350 nm, the electrolyte content at this position is improved, that is, the content of FEC at this position and the thickness of the film formation are improved, the interfacial impedance is reduced, and the high-temperature cycle performance of the battery is enhanced. In this application, the distribution of PVDF particles on the surface of the negative active material particles in the negative electrode sheet (that is, the coverage number of PVDF particles on the surface of a single negative active material particle) is optimized, and combined with the optimized electrolyte, it can well coordinate with PVDF to form a stable, dense and low-impedance SEI film, thereby enhancing the high-temperature cycle and storage performance of the battery. If the coverage number S is too small, it means that the dispersibility of PVDF particles on the surface of the negative active particles is insufficient, which easily leads to uneven or too little electrolyte content on the surface of the negative active particles. For example, if it is less than 51, the corresponding amount of absorbed electrolyte is small, that is, the concentration of the film-forming additive FEC at this position is low and the thermal stability is poor, being unfavorable to high temperature. If S is too large, the corresponding amount of absorbed electrolyte is too high, and the concentration of the film-forming additive FEC at this position is high, that is, the thicker the film formation and the increased impedance. This application discloses the electrolyte formula (the mass content of FEC in the electrolyte satisfies 1% ≤ η1 ≤ 20%) and the coverage number S of PVDF particles on the surface of the negative active material particles. In this application, S satisfies 51 ≤ S ≤ 300. On the one hand, an effective bonding network can be ensured within this limited range. Secondly, because PVDF particles have a certain swelling effect on the electrolyte, it is found in this application that when the coverage number S of PVDF particles, the average particle size D1, and the mass content η1 of fluorinated ethylene carbonate in the electrolyte simultaneously satisfy the foregoing ranges, the electrolyte content on the surface of the negative active particles can be appropriate and uniform, and it can be ensured that the electrolyte additives participating in the reaction can reach the optimal level, enabling the battery to have excellent high-temperature cycle and high-temperature storage performance while having good low-temperature performance. Exemplarily, S can be 51, 60, 90, 120, 150, 180, 210, 240, 270 or 300. D1 can be 50 nm, 60 nm, 80 nm, 100 nm, 120 nm, 150 nm, 200 nm, 220 nm, 250 nm, 280 nm, 300 nm or 350 nm. η1 can be 1%, 2%, 3%, 4%, 5%, 8%, 10%, 12%, 15%, 18% or 20%.

[0034] In a possible implementation manner, the areal density of the negative active layer is 3 mg / cm 2 ≤ CW ≤ 8 mg / cm2 When 180 nm ≤ D1 ≤ 350 nm, that is, when the areal density of the negative electrode active layer is relatively low, larger-sized particles of PVDF are selected, which can effectively bond the negative electrode active material and maintain structural stability. Larger PVDF particles can provide a higher porosity for the negative electrode sheet. Under high-temperature conditions, the electrolyte is consumed quickly, and a higher porosity is beneficial to the residual electrolyte within the negative electrode sheet, facilitating lithium-ion transport, thereby improving the high-temperature performance of the battery.

[0035] In a possible implementation, the areal density of the negative electrode active layer is 8 mg / cm 2 <CW ≤ 13 mg / cm 2 When 50 nm ≤ D1 < 180 nm, the greater the areal density, the thicker the negative electrode active layer. Generally, when the electrode sheet is thicker, a higher requirement for maintaining the stability of the bonding network is needed. When the areal density of the negative electrode active layer is relatively high, smaller-sized particles of PVDF are selected, which have more bonding sites per unit volume. When the battery expands during charge and discharge in a high-temperature environment, excessive expansion may lead to the destruction of the bonding network, that is, the conductive network (formed by bonding with the binder) will also be damaged, and thus the high-temperature performance will deteriorate. Therefore, more bonding sites can improve the bonding, resulting in better structural stability, lower expansion, and better high-temperature performance.

[0036] In a possible implementation, the negative electrode active material includes a silicon-based material;

[0037] Optionally, based on the mass of the negative electrode active layer, the mass content of silicon element is 1% ≤ η2 ≤ 50%; for example, η2 can be 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50%. Within this range, the energy density and processability can be balanced. If the content of silicon element is too high, the processability of the negative electrode slurry is relatively poor, and in addition, the battery expands too much, and the risk of failure is very high.

[0038] Optionally, the particle size D2 of the silicon-based material is 5 μm ≤ D2 ≤ 10 μm; for example, D2 can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm. When the particle size of the silicon-based material is too small, the specific surface area is larger, the alkalinity of the slurry may be stronger, the stability of the slurry is poor, and the processing is too difficult. In addition, smaller particle sizes require more binders, which is not conducive to the construction of the bonding network. When the particles of the silicon-based material are too large, silicon is prone to expansion, and the risk of conductive network failure after expansion is greater than that when the particle size is small, which is not conducive to the overall capacity utilization.

[0039] Optionally, the silicon-based material is selected from at least one of silicon oxide, silicon carbide, and nano-silicon.

[0040] In a possible implementation, the negative electrode active layer further includes a second binder, and the second binder includes polyacrylic acid (PAA);

[0041] Optionally, the mass ratio of the second binder to the first binder is η3, and η3 is 0-1; illustratively, η3 can be 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1. The polyacrylic acid type solution binder has good adhesion. The introduction and use with water-based PVDF can strengthen the bonding network of the negative electrode sheet, enhance the stability of the negative electrode sheet, and suppress the expansion of the electrode sheet at high temperature. Stronger ability, thereby further improving the high temperature performance. In addition, the solution-type binder can achieve bonding while uniformly coating the negative electrode active material. It also has good affinity for the electrolyte, making the distribution state of the electrolyte on the surface of the negative electrode active material more uniform, thereby consolidating the formation of the SEI film. However, too high a content of the second binder will lead to deterioration of low-temperature performance. η3 is limited to 0-1, that is, the amount of the second binder is less than half of the total amount of the binder, avoiding serious deterioration of the low-temperature performance of the battery, so that the battery can take into account both high and low temperature performance.

[0042] Optionally, the polyacrylic acid includes one or more of polyacrylic acid-acrylonitrile, polyacrylic acid-acrylonitrile-acrylamide, and polyacrylic acid-acrylamide.

[0043] In a possible implementation, the electrolyte further includes a second additive of vinylene carbonate (VC);

[0044] Optionally, the ratio of the mass content of vinylene carbonate to fluoroethylene carbonate in the electrolyte is 0.1 to 0.45. Exemplarily, it can be 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4 or 0.45. FEC is beneficial to the formation of inorganic components of LiF, which is beneficial to improving the stability of the SEI film. VC can form poly-VC, further inhibiting the continuous reaction between the electrolyte and the negative electrode, reducing side reactions, and improving high-temperature performance. However, when the FEC content is high, HF will be generated, which will destroy the SEI film and accelerate the consumption of the electrolyte, and the high-temperature gas production will be serious. The high VC content has a large impedance, which will reduce the kinetics and the power is poor; through the synergistic effect of VC and FEC, the high-temperature performance can be significantly improved while taking into account the power, so this range is preferred.

[0045] In a possible implementation, the compaction density of the active layer is 1.3 to 1.85, and can be 1.3, 1.4, 1.5, 1.6, 1.7, 1.8 or 1.85.

[0046] In a possible implementation, the porosity of the negative electrode sheet is 25% to 40%, and can be 25%, 30%, 35% or 40%.

[0047] If the compaction density is too low and the porosity is too high, the electrical contact is insufficient and the electron conduction is poor, which is not conducive to charge transfer. If the compaction is too high and the porosity is too low, the liquid retention capacity of the electrode sheet is low and the ion transport is slow, which is not conducive to high and low temperature performance. When the compaction density is between 1.3 and 1.85 and the porosity is between 25% and 40%, the battery can take into account both high and low temperature performance.

[0048] In a possible implementation, the OI value of the negative electrode sheet is 5-15; for example, the OI value can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15. If the OI value is too small, it means that the arrangement of the negative active material particles is not orderly enough and there is no clear orientation to promote electron conduction, which will reduce the overall conductivity of the negative electrode sheet. At the same time, a lower orientation index may lead to insufficient binding force between the accessory active material and the accessory current collector, increasing the powder falling phenomenon during the cycling of the battery, thereby shortening the battery life. In this application, the OI value is between 5 and 15, which can ensure the conductivity of the negative electrode sheet and the battery life.

[0049] The OI value is the ratio I(004) / I(110) of the diffraction intensity I(004) of the (004) crystal plane of the negative active material to the diffraction intensity I(110) of the (110) crystal plane.

[0050] In a possible implementation, the PVDF particles refer to polymers containing at least vinylidene fluoride monomer (VDF).

[0051] The PVDF particles in this application can be homopolymers of VDF or copolymers formed by VDF and comonomers. For example, the comonomers include one or more of hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), and trifluoroethylene (TrFE).

[0052] In a possible implementation, the crystallinity of the PVDF particles is 45%-55%.

[0053] PVDF particles with a crystallinity in the range of 45%-55% have good liquid absorption capacity, adhesion and flexibility; if the crystallinity is too low, the adhesion is insufficient, the toughness of the electrode sheet is poor, the processing performance is insufficient, and side reactions such as high temperature gas generation in the battery are aggravated and the failure risk is high; if the crystallinity is too high, it is difficult for the electrolyte to penetrate into the crystalline region, the liquid absorption capacity is poor, and the low temperature performance of the battery is poor.

[0054] The test method for crystallinity includes: preparing 5-10 mg of PVDF dry glue, putting it into a crucible, and after standing for 5 min, performing the first-stage heating: heating from 30 °C to 210 °C at a rate of 10 °C / min; maintaining at 210 °C for 2 min; then performing the second-stage cooling: cooling to 30 °C at a rate of 10 °C / min; maintaining at 30 °C for 2 min; then performing the third-stage heating: heating to 210 °C at a rate of 10 °C / min. The crystallinity is calculated by taking the normalized melting enthalpy value during the third-stage heating process, and 104.7 J / g is the theoretical melting enthalpy value for 100% crystallization of PVDF.

[0055] Optionally, the secondary battery is a lithium-ion battery.

[0056] In a possible implementation manner, the negative electrode active material includes artificial graphite, preferably primary particle graphite, and the artificial graphite is obtained by graphitizing single particles of petroleum coke, needle coke or pitch coke.

[0057] In a possible implementation manner, the negative electrode current collector is a single-sided polished copper foil, a double-sided polished copper foil or a porous copper foil.

[0058] This application also provides a preparation method for a secondary battery, including:

[0059] Preparing a negative electrode sheet:

[0060] (1) Mixing the negative electrode active material, the first conductive agent, the first binder, the second binder, the thickening agent, and the first solvent evenly to obtain a negative electrode slurry;

[0061] (2) Coating the negative electrode slurry on the surface of the negative electrode current collector, and obtaining the negative electrode sheet after baking.

[0062] Preparing a positive electrode sheet: Mixing the positive electrode active material, the second conductive agent, the third binder, and the second solvent to prepare a positive electrode slurry, and coating the positive electrode slurry on the positive electrode current collector, and obtaining the positive electrode sheet after drying.

[0063] Assembling the positive electrode sheet, the negative electrode sheet and the separator into an electric core by the commonly used winding or stacking method in the industry, then encapsulating with an aluminum-plastic film, and then successively passing through baking, injecting electrolyte, forming, and second sealing processes to obtain a lithium-ion battery.

[0064] In a possible implementation manner, the mass ratio of the negative electrode active material: the first binder PVDF particles: the second binder polyacrylic acid-acrylamide-acrylonitrile: the first conductive agent: the thickening agent = 90-98:1-5:1-5:0-5:1-5. Exemplarily, it is 95:1.8:1.5:0.5:1.2.

[0065] Optionally, the first conductive agent includes one or more of graphite, carbon black, acetylene black, and graphene.

[0066] Optionally, the thickening agent includes one or more of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose.

[0067] Optionally, the first solvent includes water.

[0068] Optionally, the positive electrode active material includes at least one of lithium iron phosphate (LFP), ternary lithium nickel cobalt manganese oxide (NCM), lithium cobalt oxide (LCO), and lithium iron manganese phosphate (LFMP).

[0069] Optionally, the second conductive agent includes at least one of conductive carbon black, conductive graphite, multi-walled carbon nanotubes, single-walled carbon nanotubes, conductive carbon fibers, and graphene.

[0070] Optionally, the third binder includes at least one of polyvinylidene fluoride (PVDF) and polyimide.

[0071] Optionally, the positive electrode current collector is aluminum foil or carbon-coated aluminum foil.

[0072] Optionally, the second solvent includes one or more of N-methylpyrrolidone (NMP) and N-ethylpyrrolidone.

[0073] Example 1

[0074] This example provides a lithium-ion battery, and its preparation method includes:

[0075] Preparing a positive electrode sheet: Adding binder PVDF and conductive carbon black into NMP, after stirring evenly, adding positive electrode active material lithium iron phosphate, and obtaining a uniformly dispersed positive electrode slurry after stirring. The solid components include 96.7 wt% of lithium iron phosphate and 1.8 wt% of PVDF. The solid content of the positive electrode slurry is 61 wt%, and the viscosity is 10100 mPa·s. Coating the positive electrode slurry evenly on both sides of the aluminum foil, drying at 100 - 130 °C for 4 h, and compacting it using a roll press. The compacting density is 2.4 g / cm 3 , obtaining a positive electrode sheet, and the resistance of the positive electrode sheet is 240 mΩ.

[0076] Preparation of the negative electrode sheet: An aqueous PVDF emulsion (2518L aqueous PVDF emulsion from Sinochem Lantian Fluoromaterials Co., Ltd.), graphite, silicon carbide (D50 is 7 μm), thickening agent sodium carboxymethyl cellulose (CMC), polyacrylic acid binder (LA136DL from Sichuan Yindi Le Materials Technology Group Co., Ltd.), and conductive carbon black are mixed and dispersed in deionized water to obtain a negative electrode slurry. The solid components include 90.2 wt% graphite, 5% silicon carbide (the mass ratio of silicon element in silicon carbide is 50.1%), 1.5 wt% CMC, 1.8 wt% conductive carbon black, 1 wt% PVDF particles, and 0.5% SBR. The solid content of the negative electrode slurry is 43.5 wt%, and the viscosity is 4610 mPa·s. The negative electrode slurry is evenly coated on both sides of the copper foil, dried at 70 - 100 °C for 5 h, and compacted by a roll press. The compaction density of the negative electrode active layer is 1.65 g / cm 3 , and a negative electrode sheet is obtained. The areal density of the negative electrode active layer is 6.8 mg / cm 2 , the coverage number S of PVDF particles is 186, and the porosity of the negative electrode sheet is 30%; The SEM image of the negative electrode sheet is as shown in Figure 1 . The DSC image of PVDF particles is as shown in Figure 2 . It can be known from Figure 2 that the calculated crystallinity is 48.1%.

[0077] After welding the electrode tabs of the positive electrode sheet and the negative electrode sheet, they are wound with a separator into an electric core, encapsulated, and then an electrolyte is injected (the content of the first additive FEC in the electrolyte is 4.3%, and the content of the second additive VC is 1.5%). The specific components are as shown in the following table. After hot pressing and forming and secondary sealing, a lithium-ion battery is obtained.

[0078] Table 1 Composition of the electrolyte in Example 1

[0079] EC PC EMC EA DTD FEC VC LiFSI LiPF6 18.5％ 5％ 38％ 16.2％ 0.5％ 4.3％ 1.5％ 6％ 10％

[0080] The test methods for each parameter are as follows:

[0081] 1. The number S of PVDF particles on the surface of a single negative electrode active material particle: Observe the morphology of the negative electrode sheet through a scanning electron microscope, count the number of PVDF particles on one side of a single negative electrode active material particle, multiply by 6 (6 sides of the negative electrode active material particle) to obtain S1 of this negative electrode active material particle. A total of 10 negative electrode active material particles' surface PVDF particle numbers are counted, denoted as S1, S2, S3... S10 respectively. Finally, S = (S1 + S2... + S10) / 10.

[0082] 2. Average particle size D1 of PVDF particles: After disassembling the battery, the negative electrode sheet is rinsed with DMC (dimethyl carbonate) solvent to remove the residual electrolyte, dried, and then observed under an SEM scanning electron microscope. This particle size is obtained by observing and measuring through an SEM scanning electron microscope. Usually, different regions are selected, 5 regions are randomly selected, at least 10 particle sizes are measured in each region, and then at least 50 obtained values are averaged to obtain D1.

[0083] 3. Areal density of the negative active layer: Take the negative electrode sheet, punch out circular pieces of a fixed size with a punching machine, weigh them, and subtract the weight of the copper foil (which can be calculated based on the density, thickness, and area of copper) to obtain the weight of the active layer. Finally, divide by the area and then divide by two to obtain the areal density of the active layer on one side of the electrode sheet.

[0084] 4. Mass content of silicon element in the negative active layer: The contents of C and Si elements are measured quantitatively by energy spectrum in SEM, and thus the mass content of Si is measured.

[0085] 5. Apparent density of the negative active layer: Divide the areal density of the negative active layer by the thickness of the negative active layer to obtain the apparent density.

[0086] 6. Porosity of the negative electrode sheet: Utilize the phenomenon that gas (such as nitrogen or carbon dioxide) is adsorbed on the surface of porous materials at different pressures, and calculate the porosity by analyzing the gas adsorption amount. Steps: Put the negative electrode sheet sample into a vacuum environment; introduce gas at different pressures and measure the gas adsorption amount; calculate the pore volume and porosity according to the adsorption curve.

[0087] 7. OI value of the negative electrode sheet: When performing XRD diffraction pattern testing on the horizontally placed negative electrode sheet sample, the diffraction signal of the (110) crystal plane that can be collected comes from graphite with a layer structure perpendicular to the negative electrode sheet, and the diffraction signal of the (004) crystal plane comes from graphite with a layer structure parallel to the negative electrode sheet. Therefore, the orientation of the graphite electrode can be described by the ratio of the (004) diffraction peak intensity (or integral area) to the (110) diffraction peak intensity (or integral area), OI = I(004) / I(110).

[0088] The parameters of each example and comparative example are shown in Table 2 - Table 3.

[0089] Table 2

[0090]

[0091]

[0092] Table 3

[0093]

[0094]

[0095] Secondary battery performance test method:

[0096] 1. High-temperature performance:

[0097] At 45°C, charge and discharge at 1C / 1C for 500 cycles, and calculate the capacity retention rate and the expansion rate of the battery after 500 cycles.

[0098] At 60°C and 100% SOC, measure the residual capacity and the recovery capacity of the battery for 60 days, and calculate the percentage of the residual capacity and the percentage of the recovery capacity.

[0099] The residual capacity refers to the amount of electricity obtained by storing the battery at full charge (i.e., 100% SOC) for 60 days and then discharging it to empty. The percentage obtained by dividing the residual capacity by the initial full charge is the percentage of the residual capacity; the amount of electricity obtained by recharging the battery discharged to empty divided by the initial full charge is the recovery capacity retention rate.

[0100] 2. Low-temperature performance:

[0101] -30°C power discharge: 1). In an environment of (25±2)°C, discharge at a standard constant current to the discharge cut-off voltage, and leave it standing for 30 min; then charge at a standard constant current and constant voltage to the charge limit voltage, with a cut-off current of 0.05C, and leave it standing for 30 min; discharge at a standard constant current to the discharge cut-off voltage to obtain the actual capacity C0 of the battery cell; leave it standing for 30 min; 2). Charge at a standard constant current and constant voltage to the charge limit voltage, with a cut-off current of 0.05C, and discharge at 1C0 for 30 min., which is 50% SOC; 3). Stand still at -30°C for 4 h; 4). Test the 10C0 discharge for 30 s, stand still for 10 min and then discharge at 10C0 for 30 s again, with a sampling accuracy of 10 ms. Record the voltage value at 2S of the discharge time (the low-temperature discharge lower limit is 1.2V).

[0102] The test results are shown in Table 4.

[0103] Table 4

[0104]

[0105]

[0106]

[0107] It can be seen from the comparison of the examples and the comparative examples in Table 4 that the secondary battery of the present application has a significant increase in capacity and a decrease in the battery expansion rate at 45°C; the percentage of the residual capacity and the percentage of the recovery capacity at 60°C increase. The battery of the present application has good low-temperature performance while the high-temperature performance is significantly improved.

[0108] Obviously, the above embodiments are merely examples for clear illustration and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or alterations can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here. And the obvious changes or alterations derived therefrom still fall within the protection scope of the present invention.

Claims

1. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, characterized in that: The negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer disposed on at least one side of the negative electrode current collector, wherein the negative electrode active layer comprises negative electrode active material particles and a first binder; The first binder includes PVDF particles; the number of PVDF particles on the surface of a single negative electrode active material particle is a coverage number S, S satisfies 51≤S≤300; the average particle size of the PVDF particles is D1, 50nm≤D1≤350nm; The electrolyte includes a first additive, fluoroethylene carbonate, and the mass content of the fluoroethylene carbonate in the electrolyte is η1, 1%≤η1≤20%.

2. The secondary battery according to claim 1, characterized in that: The surface density of the negative electrode active layer is 3 mg / cm 2 ≤CW≤8mg / cm 2 , 180nm≤D1≤350nm.

3. The secondary battery according to claim 1, characterized in that: The surface density of the negative electrode active layer is 8 mg / cm 2 <CW≤13mg / cm 2 , 50nm≤D1<180nm.

4. The secondary battery according to any one of claims 1 to 3, characterized in that: The negative electrode active material includes at least one of graphite and a silicon-based material.

5. The secondary battery according to claim 4, characterized in that: Based on the mass of the negative electrode active layer, the mass content of silicon is 1%≤η2≤50%; and / or The D50 particle size of the silicon-based material is D2, 5 μm ≤ D2 ≤ 10 μm; and / or The silicon-based material is selected from at least one of silicon oxygen, silicon carbon, and nano silicon.

6. The secondary battery according to any one of claims 1 to 3, characterized in that: The negative electrode active layer further includes a second binder, and the second binder includes polyacrylic acid.

7. The secondary battery according to claim 6, characterized in that: The mass ratio of the second binder to the first binder is η3, and η3 is 0 to 1; and / or The polyacrylic acid includes one or more of polyacrylic acid-acrylonitrile, polyacrylic acid-acrylonitrile-acrylamide, and polyacrylic acid-acrylamide.

8. The secondary battery according to any one of claims 1 to 3, characterized in that: The electrolyte further includes a second additive, vinylene carbonate.

9. The secondary battery according to claim 8, characterized in that: The mass content ratio of the vinylene carbonate to the fluoroethylene carbonate in the electrolyte is 0.1 to 0.

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10. The secondary battery according to any one of claims 1 to 3, characterized in that: The compaction density of the negative electrode active layer is 1.3 to 1.85 g / cm 3 ; and / or The porosity of the negative electrode sheet is 25% to 40%; and / or The OI value of the negative electrode sheet is 5 to 15; and / or The PVDF particles refer to a polymer containing at least a vinylidene fluoride monomer.

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