Negative electrode sheet and secondary battery

Abstract

A negative electrode sheet and a secondary battery, relating to the technical field of secondary batteries. The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one side surface of the negative electrode current collector. The negative electrode active material layer comprises a negative electrode active material and a first binder. The negative electrode active material layer comprises negative electrode active material particles. The first binder comprises granular fluorine-based binders; and the number of granular fluorine-based binders covering the surface of a single negative electrode active material particle in the negative electrode sheet is S, wherein S satisfies: 5≤S≤300. A secondary battery comprises the negative electrode sheet, wherein the first binder comprises PVDF particles; the number of PVDF particles covering the surface of a single negative electrode active material particle is S, wherein S satisfies 51≤S≤300; and the average particle size of the PVDF particles is D1, wherein 50 nm≤D1≤350 nm; the electrolyte comprises a first additive, i.e., fluoroethylene carbonate; and the mass content of the fluoroethylene carbonate in the electrolyte is η1, wherein 1%≤η1≤20%.

Description

A negative electrode and a secondary battery

[0001] Cross-references to related applications

[0002] This application claims priority to Chinese Patent Application No. 202510381707X, filed on March 28, 2025, entitled "A Secondary Battery", and Chinese Patent Application No. 2025103910505, filed on March 31, 2025, entitled "A Negative Electrode and Battery", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application belongs to the field of secondary battery technology, specifically relating to a negative electrode sheet and a secondary battery. Background Technology

[0004] Battery technology has consistently pursued high energy density, long cycle life, and stable performance under varying temperature conditions. In conventional lithium-ion batteries, the electrolyte viscosity increases at low temperatures, causing a sharp decrease in ion diffusion rate. This results in the battery being unable to provide sufficient power when high current output is required, such as for rapid vehicle starting, severely impacting the normal operation of equipment. This situation significantly limits the widespread adoption and efficient operation of related power tools in low-temperature environments, making the development of batteries with excellent low-temperature rate discharge performance an urgent priority.

[0005] When aqueous PVDF (polyvinylidene fluoride) binders are applied to the negative electrode side of lithium-ion batteries, they exhibit better flexibility and ionic conductivity at low temperatures compared to traditional oil-based PVDF, effectively improving the battery's low-temperature performance. However, the adhesion of aqueous PVDF to the surface of the negative electrode active material leads to inconsistent SEI thickness between the areas with and without aqueous PVDF, resulting in uneven SEI film formation. This weakens the battery's high-temperature performance and reduces its high-temperature cycle performance, making it difficult to achieve both high and low-temperature performance during lithium-ion battery use. Summary of the Invention

[0006] Therefore, the technical problem to be solved by this application is to overcome the shortcomings of existing batteries in terms of rate discharge performance under low-temperature conditions, thereby providing a negative electrode sheet. Another technical problem to be solved by this application is to overcome the difficulty in simultaneously achieving low-temperature and high-temperature performance when aqueous PVDF is used in the negative electrode sheet in the prior art, thereby providing a secondary battery.

[0007] Therefore, this application provides the following technical solution.

[0008] The first aspect of this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector; the negative electrode active material layer includes a negative electrode active material and a first binder; the negative electrode active material layer includes negative electrode active material particles, and the first binder includes a particulate fluorine-based binder; the amount of particulate fluorine-based binder covering the surface of a single negative electrode active material particle in the negative electrode sheet is S, wherein S satisfies: 5≤S≤300.

[0009] In one optional embodiment, the amount of particulate fluorine-based binder covering the surface of a single negative electrode active material particle in the negative electrode sheet is S, wherein S satisfies: 5≤S≤150.

[0010] In one optional embodiment, the ratio of the median particle size of the negative electrode active material particles in the negative electrode sheet to the median particle size of the particulate fluorine-based binder is 10-400; optionally, the ratio of the median particle size of the negative electrode active material particles in the negative electrode sheet to the median particle size of the particulate fluorine-based binder is 20-300.

[0011] And / or, the median particle size of the particulate fluoropolymer binder is 50 nm-300 nm;

[0012] And / or, the median particle size of the negative electrode active material particles is 3μm-20μm; optionally, the median particle size of the negative electrode active material particles is 5μm-15μm.

[0013] And / or, the negative electrode active material particles in the negative electrode active material layer include at least one of artificial graphite, natural graphite, mesophase carbon microspheres, hard carbon, and soft carbon.

[0014] In one optional embodiment, the particulate fluoropolymer binder has a weight-average molecular weight of 600,000 to 900,000.

[0015] And / or, the particulate fluoropolymer binder comprises polyvinylidene fluoride polymers; optionally, the particulate fluoropolymer binder comprises a copolymer or homopolymer formed from at least one monomer selected from vinylidene fluoride, chlorotrifluoroethylene, trifluoroethylene, tetrafluoroethylene and hexafluoropropylene.

[0016] And / or, the particulate fluoropolymer binder includes functional groups, the functional groups including at least one selected from carboxylic acid, sulfonic acid, phosphoric acid, cyano, and amino.

[0017] In one optional embodiment, the negative electrode further includes a conductive agent; the median particle size of the conductive agent particles is 100nm-800nm;

[0018] And / or, the conductive agent has a mass content of 0.2wt%-5wt% in the negative electrode active material layer; optionally, the conductive agent has a mass content of 0.2wt%-3wt% in the negative electrode active material layer.

[0019] And / or, the conductive agent includes at least one of conductive carbon black, carbon nanotubes, carbon fibers, and graphene.

[0020] In one optional embodiment, the ratio of the peak area A1 of the weight loss peak in the thermogravimetric curve of the negative electrode active material layer in the range of 250-350℃ to the peak area A2 of the weight loss peak in the thermogravimetric curve of the negative electrode active material layer in the range of 400-500℃ is 0.5-2.

[0021] Optionally, the ratio of the peak area A1 of the weight loss peak at 250-350℃ to the peak area A2 of the weight loss peak at 400-500℃ in the thermogravimetric curve of the negative electrode active material layer is 0.8-1.4.

[0022] In one optional embodiment, the negative electrode active material layer further includes a second binder, the second binder including at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and potassium carboxymethyl cellulose;

[0023] And / or, the negative electrode active material layer further includes a third binder, the third binder including at least one of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyacrylonitrile, polyacrylamide, polymethacrylate, and styrene-acrylic emulsion.

[0024] In one optional embodiment, the first binder has a mass content of 1.4wt%-6wt% in the negative electrode active material layer;

[0025] And / or, the mass content of the second binder in the negative electrode active material layer is 0-2.5 wt%; optionally, the mass content of the second binder in the negative electrode active material layer is 0.4 wt%-2 wt%.

[0026] And / or, the third binder has a mass content of 0-6 wt% in the negative electrode active material layer; optionally, the third binder has a mass content of 0.4 wt%-3 wt% in the negative electrode active material layer.

[0027] A second aspect of this application provides a battery, including a positive electrode and the aforementioned negative electrode; the positive electrode includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector, the positive active material layer includes a positive active material, the positive active material includes at least one of a first lithium iron phosphate and a second lithium iron phosphate, the first lithium iron phosphate includes first lithium iron phosphate particles, the median particle size of the first lithium iron phosphate particles is 1μm-15μm, optionally, the median particle size of the first lithium iron phosphate particles is 3μm-12μm;

[0028] And / or, the second lithium iron phosphate comprises second lithium iron phosphate particles, the median particle size of the second lithium iron phosphate particles being 100nm-500nm, optionally, the median particle size of the second lithium iron phosphate particles being 100nm-300nm.

[0029] In one optional embodiment, the ratio of the median particle size of the particulate fluorine-based binder to the median particle size of the first lithium iron phosphate particles is (10-200):1.

[0030] And / or, the molar ratio of iron to lithium in the first lithium iron phosphate is (0.94-1.00):1;

[0031] And / or, the resistivity of the first lithium iron phosphate powder is less than 100 Ω·m.

[0032] Another aspect of 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 negative active material particles and a first binder.

[0033] The first binder comprises PVDF particles; the number of PVDF particles on the surface of a single negative electrode active material particle is the coverage number S, where S satisfies 51≤S≤300; the average particle size of the PVDF particles is D1, where 50nm≤D1≤350nm;

[0034] The electrolyte includes a first additive, fluoroethylene carbonate, and the mass content of the fluoroethylene carbonate in the electrolyte is η1, where 1% ≤ η1 ≤ 20%.

[0035] In one possible implementation, the areal density of the negative electrode active layer is 3 mg / cm³. 2 ≤CW≤8mg / cm 2 , 180nm≤D1≤350nm.

[0036] In one possible implementation, the areal density of the negative electrode active layer is 8 mg / cm³. 2 <CW≤13mg / cm2 , 50nm≤D1<180nm.

[0037] In one possible implementation, the negative electrode active material comprises a silicon-based material.

[0038] In one possible implementation, the mass content of silicon element is 1% ≤ η2 ≤ 50%, based on the mass of the negative electrode active layer.

[0039] In one possible implementation, the D50 particle size of the silicon-based material is D2, where 5μm≤D2≤10μm;

[0040] In one possible implementation, the silicon-based material is selected from at least one of silicon oxide, silicon carbon, and nano-silicon.

[0041] In one possible implementation, the negative electrode active layer further includes a second binder comprising polyacrylic acid.

[0042] In one possible implementation, the mass ratio of the second adhesive to the first adhesive is η3, where η3 is 0-1.

[0043] In one possible implementation, the polyacrylic acid includes one or more of polyacrylic acid-acrylonitrile, polyacrylic acid-acrylonitrile-acrylamide, and polyacrylic acid-acrylamide.

[0044] In one possible implementation, the electrolyte further includes a second additive, vinylene carbonate.

[0045] In one possible implementation, the mass ratio of the vinylene carbonate to the fluoroethylene carbonate in the electrolyte is 0.1-0.45.

[0046] In one possible implementation, the compaction density of the negative electrode active layer is 1.3-1.85 g / cm³. 3 ;

[0047] In one possible implementation, the porosity of the negative electrode sheet is 25%-40%;

[0048] In one possible implementation, the OI value of the negative electrode is 5-15;

[0049] In one possible implementation, the PVDF particles refer to polymers containing at least vinylidene fluoride monomers.

[0050] The technical solution of this application has the following advantages:

[0051] The negative electrode sheet provided in this application includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector; the negative electrode active material layer includes a negative electrode active material and a first binder; the negative electrode active material layer includes negative electrode active material particles, and the first binder includes a particulate fluorine-based binder; the amount of particulate fluorine-based binder covering the surface of a single negative electrode active material particle in the negative electrode sheet is S, wherein S satisfies: 5≤S≤300. The first binder in this application includes a particulate fluorine-based binder. On the one hand, the particulate fluorine-based binder has a strong electrolyte absorption capacity. Covering it on the surface of the negative electrode active material particles can improve the ion conduction capacity, ensure the wetting effect of the electrolyte in the electrode and the lithium ion transport rate in the electrode, thereby improving the low-temperature rate discharge capability of the battery. On the other hand, the particulate fluorine-based binder can create a suitable gap between the negative electrode active material particles, which facilitates the rapid insertion and extraction of lithium ions. Combined with the strong electrolyte absorption capacity of the particulate fluorine-based binder, it can further improve the ion conduction capacity. The two work synergistically to improve the lithium ion transport rate and enhance the low-temperature rate discharge performance. The number of granular fluorine-based binders covering the surface of a single negative electrode active material particle in this application is 5-300, which makes the gap between the negative electrode active material particles suitable, facilitates the rapid insertion and extraction of lithium ions, improves ion transport capability, helps to improve the wettability of the electrode sheet, enhances the lithium ion transport rate in the electrode sheet, optimizes the lithium ion transport path, and thus improves the low-temperature rate discharge performance of the battery; at the same time, it can also ensure good adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material layer and the current collector, keep the active material layer in close contact with the current collector, reduce the phenomenon of active material expansion and active material layer shedding during cycling, reduce cell failure, and improve battery cycle performance.

[0052] This application discloses a secondary battery comprising 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 comprises negative active material particles and a first binder. The first binder comprises PVDF particles. The number of PVDF particles on the surface of a single negative active material particle is a coverage number S, where S satisfies 51 ≤ S ≤ 300. The average particle size of the PVDF particles is D1, where 50 nm ≤ D1 ≤ 350 nm. The electrolyte includes a first additive, fluoroethylene carbonate, and the mass content of the fluoroethylene carbonate in the electrolyte is η1, where 1% ≤ η1 ≤ 20%. This application improves the electrolyte content at this location by controlling the average particle size of the PVDF particles to satisfy 50 nm ≤ D1 ≤ 350 nm, thereby improving the FEC content and film thickness at this location, reducing interfacial impedance, and enhancing the high-temperature cycle performance of the battery. This application optimizes the distribution of PVDF particles on the surface of the negative electrode active material particles in the negative electrode sheet, and combined with the optimized electrolyte, it can work well with PVDF to form a stable, dense and low-resistance SEI film, thereby improving the high-temperature cycle and storage performance of the battery, so that the battery has excellent high-temperature cycle and high-temperature storage performance while having good low-temperature performance. Attached Figure Description

[0053] 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.

[0054] Figure 1 is a SEM image of the negative electrode sheet in Example 1;

[0055] Figure 2 is a DSC diagram of the PVDF particles in Example 1. Detailed Implementation

[0056] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0057] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0058] In conventional lithium-ion batteries, the electrolyte viscosity increases at low temperatures, causing a sharp decrease in the ion diffusion rate. This results in the battery being unable to provide sufficient power when high current output is required, such as for rapid vehicle starting, severely impacting normal equipment operation. To improve the poor rate discharge performance under low-temperature conditions, this application provides the following technical solution.

[0059] The first aspect of this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector; the negative electrode active material layer includes a negative electrode active material and a first binder; the negative electrode active material layer includes negative electrode active material particles, and the first binder includes a particulate fluorine-based binder; the amount of particulate fluorine-based binder covering the surface of a single negative electrode active material particle in the negative electrode sheet is S, wherein S satisfies: 5≤S≤300. The first binder in this application includes a particulate fluorine-based binder. On the one hand, the particulate fluorine-based binder has a strong electrolyte absorption capacity. Covering the surface of the negative electrode active material particles with it can improve the ion conduction capacity, ensure the wetting effect of the electrolyte in the electrode and the lithium ion transport rate in the electrode, thereby improving the low-temperature rate discharge capacity of the battery, especially at -30℃, it can still meet the high-rate charge and discharge performance. On the other hand, the particulate fluorine-based binder can make the negative electrode active material particles have a suitable gap, which facilitates the rapid insertion and extraction of lithium ions. Combined with the strong electrolyte absorption capacity of the particulate fluorine-based binder, it can further improve the ion conduction capacity. The two work synergistically to improve the lithium ion transport rate and improve the low-temperature rate discharge performance. The granular fluorine-based binder of this application can also improve the adhesion between negative electrode active material particles. The number of granular fluorine-based binders covering the surface of a single negative electrode active material particle is 5-300, which makes the gap between the negative electrode active material particles suitable, facilitating the rapid insertion and extraction of lithium ions, improving ion transport capacity, improving the wettability of the electrode sheet, enhancing the lithium ion transport rate in the electrode sheet, optimizing the lithium ion transport path, and thus improving the low-temperature rate discharge performance of the battery. The number of granular fluorine-based binders covering the surface of a single negative electrode active material particle is 5-300, which can also ensure good adhesion between negative electrode active material particles and adhesion between the negative electrode active material layer and the current collector, keeping the active material layer in close contact with the current collector, reducing phenomena such as active material expansion and active material layer shedding during cycling, reducing cell failure, and improving battery cycle performance.

[0060] If the surface of a single negative electrode active material particle is covered with too much granular fluorine-based binder, although it can improve the adhesion between the negative electrode active materials and the wettability of the electrode in the electrolyte, the fluorine-based binder itself is a polymer with insulating properties. Excessive use will increase the internal resistance of the cell and affect the conductivity of the negative electrode active material particles, thus reducing the conductivity of the cell and worsening the lithium-ion transport rate, thereby affecting the low-temperature rate discharge performance. If the coverage is too small, the granular fluorine-based binder cannot absorb enough electrolyte, i.e., it has poor electrolyte retention capacity, and therefore cannot form sufficient electrolyte on the surface of the negative electrode, reducing the wettability and ion transport performance of the electrode, thus failing to effectively improve the low-temperature rate discharge performance of the battery. At the same time, if there is too little granular fluorine-based binder, the adhesion between the negative electrode active materials will be insufficient, causing problems such as expansion and detachment of the active materials during cycling, thereby affecting cycle performance.

[0061] The term "coverage" in this application refers to the attachment or connection of particulate fluorine-based binder to the surface of negative electrode active material particles in at least one of the forms of points, lines, or surfaces.

[0062] An exemplary method for testing the coverage quantity in this application is as follows: The electrode sheets in the battery cell are disassembled, soaked in DMC solvent for 24 hours, and then dried in a 100°C oven for 24 hours. A 2cm × 2cm electrode sheet is taken, and four sampling points are recorded on the electrode sheet to obtain four SEM images at a magnification of 5000. Five negative electrode active material particles are taken from each SEM image. The coverage quantity of granular fluorine-based binder on each negative electrode active material particle is counted, and the average value is taken. Note that the granular fluorine-based binder is a relatively rounded sphere. When fluorine-based binder particles and conductive agent particles are mixed, they are distinguished by EDS (Electrode Sequencing). A particle EDS image is obtained; particles containing fluorine are fluorine-based binder particles, and this is used for differentiation. For example, the coverage quantity S is 5, 10, 20, 40, 80, 100, 120, 140, 150, 180, 200, 220, 240, 260, 280, 300 or within any two of the above values.

[0063] As an optional implementation, the amount of particulate fluorine-based binder covering the surface of a single negative electrode active material particle in the negative electrode sheet is S, wherein S satisfies: 5≤S≤150.

[0064] The number of granular fluorine-based binders covering the surface of a single negative electrode active material particle in this application can be selected from 5 to 150, which can further improve the adhesion between negative electrode active material particles, the adhesion between the negative electrode active material layer and the current collector, reduce the expansion and shedding of active material, and improve cycle performance; at the same time, it can also enable lithium ion insertion and extraction to reach the optimal level, further improve lithium ion transport capability, improve electrode wettability and lithium ion transport rate, further optimize lithium ion transport path, and help improve the low-temperature rate performance of the battery.

[0065] As an optional implementation, the ratio of the median particle size of the negative electrode active material particles in the negative electrode sheet to the median particle size of the particulate fluorinated binder is 10-400; optionally, the ratio is 20-300; exemplaryly, the ratio is 10, 20, 40, 60, 80, 100, 140, 180, 200, 240, 280, 300, 340, 380, 400, or within any two of the above values.

[0066] And / or, the median particle size of the particulate fluoropolymer binder is 50nm-300nm; for example, the median particle size of the particulate fluoropolymer binder is 50nm, 70nm, 90nm, 110nm, 140nm, 170nm, 200nm, 230nm, 270nm, 300nm or within any two of the above values;

[0067] And / or, the median particle size of the negative electrode active material particles is 3μm-20μm; optionally, the median particle size of the negative electrode active material particles is 5μm-15μm; for example, 5μm, 7μm, 9μm, 11μm, 13μm, 15μm or within any two of the above values;

[0068] And / or, the negative electrode active material particles in the negative electrode active material layer include at least one of artificial graphite, natural graphite, mesophase carbon microspheres, hard carbon, and soft carbon.

[0069] The ratio of the median particle size of the negative electrode active material particles to the median particle size of the granular fluorine-based binder described in this application is 10-400. Within this ratio range, it is beneficial for the granular fluorine-based binder to cover the surface of the negative electrode active material particles. This not only improves the wettability and electrolyte retention of the electrode in the electrolyte, further enhancing ion conduction capability, but also helps reduce transmission resistance, polarization, and the phenomenon that excessive coverage by the granular fluorine-based binder affects ion conduction capability and reduces the internal resistance of the cell, thereby improving low-temperature rate discharge performance; it also enhances the binding force between the negative electrode active material particles, reducing the movement and aggregation of negative electrode active material particles during high-rate charge and discharge, further improving cycle stability. If the ratio of the median particle size of the active material particles in the negative electrode to the median particle size of the particulate fluorine-based binder is too large, the small size of the particulate fluorine-based binder will result in fewer bonding sites between the active material particles, leading to weaker bonding forces. During high-rate charge and discharge, the active material particles are prone to migration and aggregation, affecting the rate performance and cycle stability of the battery. If the ratio is too low, the large particle size of the particulate fluorine-based binder will increase the lithium-ion transport path and enhance its transport resistance. In particular, the polarization phenomenon of the battery will be more severe when the battery is charged and discharged at high rates in low-temperature environments, thus failing to meet the requirements for high-power applications of the battery.

[0070] The median particle size of the particulate fluorine-based binder in this application is 50nm-300nm. When the median particle size of the particulate fluorine-based binder meets the above range, a more suitable number of particulate fluorine-based binders are obtained for the same mass. This is beneficial for the uniform distribution of the particulate fluorine-based binder on the negative electrode active material particles, reducing the possibility of localized high or low content of particulate fluorine-based binder, further ensuring the adhesion between the negative electrode active material particles, and thus improving battery life. Simultaneously, when the median particle size of the particulate fluorine-based binder meets the above range, it also prevents small particles of particulate fluorine-based binder from migrating to the electrode surface during the drying process. This further ensures the adhesion performance of the active material layer and facilitates ion insertion and extraction, further improving low-temperature rate discharge performance.

[0071] The median particle size of the negative electrode active material particles in this application is 3μm-20μm. Meeting this range is beneficial for the particulate fluorine-based binder to cover the surface of the negative electrode active material particles, better providing electrolyte retention capacity, and improving ion transport, thereby enhancing the battery rate performance. The median particle size of the negative electrode active material particles is 3μm-20μm. Optimizing the particle size in this application helps improve the processing performance of the battery slurry and provides a suitable contact area between the negative electrode active material and the electrolyte. This reduces the possibility of violent reactions due to excessive contact area between the negative electrode active material and the electrolyte, thereby improving battery storage performance, especially the storage performance of the battery under high temperature conditions.

[0072] The median particle size D50 is tested using methods known in the art. For example, the median particle size D50 is tested using a laser particle size analyzer according to GB / T19077-2016 "Particle size analysis - Laser diffraction method". The dispersion medium and the sample to be tested are placed in the analyzer, and the instrument is turned on for testing.

[0073] As an optional implementation, the weight-average molecular weight of the particulate fluoropolymer adhesive is 600,000 to 900,000; for example, the weight-average molecular weight of the particulate fluoropolymer adhesive is 600,000, 700,000, 800,000, 900,000 or within any two of the above values.

[0074] And / or, the particulate fluoropolymer binder comprises polyvinylidene fluoride polymers; optionally, the particulate fluoropolymer binder comprises a copolymer or homopolymer formed from at least one monomer selected from vinylidene fluoride, chlorotrifluoroethylene, trifluoroethylene, tetrafluoroethylene and hexafluoropropylene.

[0075] And / or, the particulate fluoropolymer binder includes functional groups, the functional groups including at least one selected from carboxylic acid, sulfonic acid, phosphoric acid, cyano, and amino.

[0076] The particulate fluorine-based binder of this application has a weight-average molecular weight of 600,000-900,000, which is beneficial for improving the adhesion between negative electrode active material particles, reducing active material layer shedding, helping to reduce battery failure, and improving cycle performance. A weight-average molecular weight within this range also helps the battery reduce DCIR and improve power performance. The weight-average molecular weight is tested using methods known in the art. For example, the weight-average molecular weight is tested using gel permeation chromatography.

[0077] The particulate fluorine-based binder includes polyvinylidene fluoride (PVDF) polymers. These particulate PVDF polymers possess strong liquid absorption capacity and abundant CF bonds, effectively enhancing lithium-ion transport rates and improving the battery's low-temperature rate discharge performance. Furthermore, they contribute to improving the adhesion between negative electrode active material particles and between the negative electrode active material layer and the current collector, reducing the likelihood of negative electrode active material detachment and improving battery cycle performance. The particulate fluorine-based binder of this application includes functional groups, including at least one of carboxylic acid, sulfonic acid, phosphoric acid, cyano, and amino groups. These functional groups help improve the adhesion between the negative electrode active material layer and the current collector, as well as between the active material and the negative electrode active material. Introducing these functional groups also helps improve the ion transport capacity of the amorphous phase region, thereby improving low-temperature rate discharge performance.

[0078] As an optional implementation, the polydispersity index of the particulate fluoropolymer binder is not higher than 0.2; and / or,

[0079] The absolute value of the Zeta potential of the particulate fluorinated binder polymer is 30mV-80mV. It should be noted that the polydispersity index and Zeta are performance parameters of polyvinylidene fluoride polymer emulsions.

[0080] This application controls the polydispersity index (PDI) of the granular fluoropolymer binder to be no higher than 0.2, which is beneficial for the uniformity of polymer latex particle size, improves the stability of material properties, and reduces demulsification and uneven dispersion during slurry preparation. For example, the PDI of the granular fluoropolymer binder is 0.05, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, 0.2, or within any two of these values. This application also controls the absolute value of the zeta potential of the granular fluoropolymer binder to be between 30mV and 80mV, which can give the polymer emulsion better stability, help improve the dispersion stability of polyvinylidene fluoride polymers in the negative electrode, and improve the bonding strength between the granular fluoropolymer binder and the negative electrode active material particles.

[0081] Polydispersity index (PDI) and zeta potential are tested using methods known in the art. For example, the polymer PDI can be directly obtained using a dynamic light scattering (DLS) instrument; the zeta potential is obtained by measuring the zeta potential of the emulsion using a zeta potential analyzer.

[0082] As an optional implementation, the negative electrode further includes a conductive agent; the median particle size of the conductive agent particles is 100nm-800nm; for example, the median particle size of the conductive agent particles is 100nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm, 800nm ​​or within any two of the above values.

[0083] And / or, the conductive agent has a mass content of 0.2wt%-5wt% in the negative electrode active material layer; optionally, the conductive agent has a mass content of 0.2wt%-3wt% in the negative electrode active material layer; exemplaryly, the conductive agent has a mass content of 0.2wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt% in the negative electrode active material layer, or within any two of the above values.

[0084] And / or, the conductive agent includes at least one of conductive carbon black, carbon nanotubes, carbon fibers, and graphene.

[0085] The conductive agent in this application has a median particle size of 100nm-800nm, which helps to form a denser conductive network. Combined with the electrolyte's ability to bind particulate fluorine-based binders, this further enhances electron transport capabilities, thus improving the battery's low-temperature rate discharge performance. The conductive agent content in this application is 0.2wt%-5wt%, promoting electron transport and enabling faster ion transport within the dense conductive network, reducing battery side reactions, and ultimately improving the battery's initial efficiency and high-temperature lifespan.

[0086] As an optional implementation, the ratio of the peak area A1 of the weight loss peak in the thermogravimetric curve of the negative electrode active material layer in the range of 250-350℃ to the peak area A2 of the weight loss peak in the thermogravimetric curve of the negative electrode active material layer in the range of 400-500℃ is 0.5-2.

[0087] Optionally, the ratio of the peak area A1 of the weight loss peak at 250-350℃ to the peak area A2 of the weight loss peak at 400-500℃ in the thermogravimetric curve of the negative electrode active material layer is 0.8-1.4. For example, the ratio of A1 to A2 is 0.5, 0.7, 0.9, 1.1, 1.4, 1.6, 1.8, 2, or falls within any two of the above values.

[0088] In this application, the ratio of A1 to A2 is 0.5-2. A1 is mainly generated by the weight loss of the second binder, and A2 is mainly generated by the weight loss of the first binder. The ratio of the area of ​​the weight loss peaks generated by the first and second binders within the above range can further improve the adhesion between the negative electrode active material layer and the current collector, suppress the expansion of the electrode during charging, discharging, and high-temperature storage, and is beneficial for stabilizing the electrode structure, thus improving the long-term cycle stability and storage performance of the battery. Controlling the peak area ratio of these two weight loss peaks also helps to improve the wetting effect of the electrode and enhance ion conduction, thereby further improving the low-temperature rate discharge performance of the battery. Controlling the weight loss peak area ratio to meet the above range can also improve the dispersion uniformity of the slurry, reduce sedimentation, and improve processing problems such as coating and scraping, which is beneficial for preventing battery failure and reducing internal resistance, further improving the battery's charge-discharge performance, rate performance, and low-temperature performance. The ratio of A1 to A2 can be selected as 0.8-1.4.

[0089] As an optional implementation, the negative electrode active material layer further includes a second binder, which includes at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and potassium carboxymethyl cellulose.

[0090] And / or, the negative electrode active material layer further includes a third binder, the third binder including at least one of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyacrylonitrile, polyacrylamide, polymethacrylate, and styrene-acrylic emulsion.

[0091] This application incorporates a second binder into the negative electrode sheet to further enhance dispersion, resulting in more uniform and dispersed active materials. A third binder further improves adhesion and cycle life. The first binder exhibits good low-temperature performance and suitable adhesion, improving the battery's low-temperature discharge performance. Introducing the second and third binders on top of the first binder further improves the adhesion between active materials and between the negative electrode active material layer and the current collector, thus contributing to increased cycle life.

[0092] As an optional implementation, the mass content of the first binder in the negative electrode active material layer is 1.4wt%-6wt%; for example, the mass content of the first binder in the negative electrode active material layer is 1.4wt%, 1.8wt%, 2.4wt%, 3wt%, 4wt%, 5wt%, 6wt%, or within any two of the above values.

[0093] And / or, the mass content of the second binder in the negative electrode active material layer is 0-2.5 wt%; optionally, the mass content of the second binder in the negative electrode active material layer is 0.4 wt%-2 wt%; exemplaryly, the mass content of the second binder in the negative electrode active material layer is 0.2 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.4 wt%, 1.6 wt%, 1.8 wt%, 2 wt%, 2.5 wt%, or within any two of the above values; specifically, 0 refers to the case where the second binder is not present.

[0094] And / or, the mass content of the third binder in the negative electrode active material layer is 0-6 wt%; optionally, the mass content of the third binder in the negative electrode active material layer is 0.4 wt%-3 wt%. For example, the mass content of the third binder in the negative electrode active material layer is 0.2 wt%, 0.4 wt%, 0.6 wt%, 1 wt%, 1.4 wt%, 1.8 wt%, 2.4 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, or within any two of the above values; specifically, 0 refers to the case where the third binder is not present.

[0095] The amount of the first binder, meeting the above-mentioned range, can improve the battery's internal resistance, low-temperature power performance, and long-term high-temperature (e.g., 45°C) cycle performance. The amount of the second binder in this application, meeting the above-mentioned range, is beneficial for improving the uniformity of dispersion of conductive agents and negative electrode active materials in the slurry, reducing problems such as agglomeration and sedimentation, and improving processing performance such as coating and scraping, which is beneficial for improving electrical performance. The amount of the second binder, meeting the above-mentioned range, makes the viscosity and solid content of the slurry more suitable, improving the toughness and other properties of the electrode sheet. The third binder has better adhesion than the first binder, but its low-temperature performance is worse. The amount of the third binder in this application, meeting the above-mentioned range, can further increase the adhesion between the negative electrode active material layer and the current collector without affecting the battery's low-temperature discharge capability, suppressing expansion, stabilizing the electrode structure, and helping to reduce battery internal resistance and long-term cycle performance. This application, by controlling the amounts of the first, second, and third binders in the negative electrode active material layer, can optimize the battery's internal resistance, low-temperature power performance, and long-term cycle performance.

[0096] A second aspect of this application provides a battery, including a positive electrode and the aforementioned negative electrode; the positive electrode includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector, the positive active material layer includes a positive active material, the positive active material includes at least one of a first lithium iron phosphate and a second lithium iron phosphate, the first lithium iron phosphate includes first lithium iron phosphate particles, the median particle size of the first lithium iron phosphate particles is 1μm-15μm, optionally, the median particle size of the first lithium iron phosphate particles is 3μm-12μm;

[0097] And / or, the second lithium iron phosphate comprises second lithium iron phosphate particles, the median particle size of the second lithium iron phosphate particles being 100nm-500nm, optionally, the median particle size of the second lithium iron phosphate particles being 100nm-300nm.

[0098] The lithium iron phosphate particles of this application have a median particle size of 1μm-15μm. When used in conjunction with a particulate fluorine-based binder, it helps maintain the electrolyte retention at the interface between the positive and negative electrodes within a suitable range, ensuring that the lithium insertion / extraction rates of the battery are matched and achieving synergistic efficiency. When used in conjunction with carbon-based active materials coated with a particulate fluorine-based binder, it can better exert conductivity and ion conduction capabilities, improving the battery's low-temperature rate discharge performance. It should be noted that the lithium iron phosphate particles are spherical.

[0099] For example, the median particle size of the first lithium iron phosphate particle is 1μm, 3μm, 5μm, 7μm, 9μm, 11μm, 13μm, 15μm or within any two of the above values; the median particle size of the second lithium iron phosphate particle is 100nm, 200nm, 300nm, 400nm, 500nm or within any two of the above values.

[0100] The median particle size of the second lithium iron phosphate particles in this application is 100nm-500nm, which has a shorter lithium-ion diffusion path and can improve the solid-phase lithium-ion diffusion kinetics. The particulate fluorine-based binder has high electrolyte affinity and high electrolyte retention capacity. Combined with the short-range diffusion characteristics of the second lithium iron phosphate, it can optimize the matching degree of interfacial lithium-ion insertion / extraction kinetics, forming a low-impedance composite particle conduction network, thereby improving the rate discharge performance of the battery under low-temperature conditions.

[0101] As an optional implementation, the ratio of the median particle size of the granular fluorinated binder to the median particle size of the first lithium iron phosphate particles is (10-200):1; for example, the ratio of the median particle size of the granular fluorinated binder to the median particle size of the first lithium iron phosphate particles is 10:1, 20:1, 40:1, 80:1, 100:1, 140:1, 160:1, 200:1 or within any two of the above values.

[0102] And / or, the molar ratio of iron to lithium in the first lithium iron phosphate is (0.94-1.00):1;

[0103] And / or, the resistivity of the first lithium iron phosphate powder is less than 100 Ω·m;

[0104] This application regulates the molar ratio of lithium iron phosphate in the first lithium iron phosphate, which helps to reduce the battery's internal resistance and improve battery performance. The ratio of the median particle size of the granular fluorine-based binder to the median particle size of the first lithium iron phosphate particles meets the aforementioned range. The first lithium iron phosphate can reduce the lithium-ion transport distance and increase the lithium-ion transport rate. Combined with the granular fluorine-based binder, which has sufficient electrolyte retention capacity, the two work together to match the lithium-ion extraction and insertion rates, further improving the battery's low-temperature rate discharge performance.

[0105] The molar ratio of iron to lithium in lithium iron phosphate and its resistivity are obtained by methods known in the art. For example, the molar ratio is calculated by measuring the mass of iron and lithium in lithium iron phosphate using ICP testing. The resistivity is then measured using a powder resistivity analyzer.

[0106] As an optional implementation, the positive electrode active material layer further includes a conductive agent and / or a binder. The conductive agent includes at least one selected from conductive carbon black (SP), carbon nanotubes (CNTs), carbon fiber (VGCF), and graphene. The binder includes at least one selected from polyvinylidene fluoride (PVDF), polyimide (PI), polyacrylic acid (PAA), sodium polyacrylate (PAA-Na), and lithium polyacrylate (PAA-li). The amounts of conductive agent and binder added are those known in the art.

[0107] As an optional implementation, the battery further includes an electrolyte, which may be a solid or liquid electrolyte. Optionally, the electrolyte is a liquid electrolyte.

[0108] Optionally, the electrolyte comprises a solvent and an electrolyte salt; the solvent comprises at least one of carbonates such as ethylene carbonate, fluoroethylene carbonate, propylene carbonate, methyl ethyl carbonate, dimethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, and diethyl carbonate; and at least one of carboxylic acid esters such as ethyl formate, ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, methyl butyrate, ethyl butyrate, and 1,4-butyrolactone. The electrolyte salt comprises at least one of lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(oxalato)borate, lithium difluorooxalato)borate, lithium tetrafluoroborate, lithium difluorodioxalato)borate, lithium trifluoromethanesulfonate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

[0109] As an optional implementation, the battery further includes a separator. Optionally, the thickness of the separator is 3μm-12μm.

[0110] Optionally, the diaphragm includes a base membrane; the base membrane is selected from at least one of polyethylene, polypropylene, polyvinylidene fluoride, copolymer of polyvinylidene fluoride and hexafluoropropylene, polyethylene terephthalate, polyimide, glass fiber, and nonwoven fabric.

[0111] As an optional implementation, at least one surface of the base film is further provided with a porous layer, the porous layer comprising inorganic particles and a binder; the inorganic particles are selected from at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, calcium oxide, zirconium oxide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate; the binder comprises at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, carboxymethyl cellulose salt, polyvinylpyrrolidone, and polymethyl methacrylate.

[0112] As an optional embodiment, the diaphragm further includes an adhesive layer disposed on one side of the base membrane and / or the porous layer; the adhesive layer includes an adhesive, which includes at least one selected from polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, carboxymethyl cellulose salt, polyvinylpyrrolidone, and polymethyl methacrylate.

[0113] When aqueous PVDF is used in a negative electrode, it is difficult to achieve both low-temperature and high-temperature performance. Based on this, 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 negative active material particles and a first binder.

[0114] 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, where S satisfies 51≤S≤300; the average particle size D1 of the PVDF particles is 50nm≤D1≤350nm.

[0115] The electrolyte includes a first additive, fluoroethylene carbonate (FEC), and the mass content of fluoroethylene carbonate in the electrolyte is η1, where 1% ≤ η1 ≤ 20%.

[0116] If the average particle size D1 of PVDF particles is too small, such as less than 50 nm, the smaller the particle size, the larger the specific surface area. For the same amount of added PVDF, the more particles there are, the more electrolyte is absorbed, resulting in a high concentration of the film-forming additive FEC at that location. This leads to an excessively thick film at that location, increasing impedance and negatively impacting the battery's high-temperature performance. Conversely, if the particle size exceeds 350 nm, the opposite occurs, resulting in an excessively thin SEI film at that location with poor thermal stability, also detrimental to the battery's high-temperature performance. This application addresses this by controlling the average particle size of PVDF particles to satisfy 50 nm ≤ D1 ≤ 350 nm, thereby improving the electrolyte content at that location, which in turn improves the FEC content and film thickness, reduces interfacial impedance, and enhances the battery's high-temperature cycling performance. This application also optimizes the distribution of PVDF particles on the surface of the negative electrode active material particles (i.e., the number of PVDF particles covering the surface of a single negative electrode active material particle), and combined with an optimized electrolyte, it can effectively coordinate with PVDF to form a stable, dense, and low-impedance SEI film, thus improving the battery's high-temperature cycling and storage performance. If the coverage number S is too small, it means that the PVDF particles are not sufficiently dispersed on the surface of the negative electrode active particles, which can easily lead to uneven or insufficient electrolyte content on the surface of the negative electrode active particles. For example, if it is less than 51, the amount of electrolyte absorbed is small, which means that the concentration of the corresponding film-forming additive FEC at this position is low, resulting in poor thermal stability and making it unfavorable for high temperature. If the value of S is too large, the amount of electrolyte absorbed is too high, resulting in a high concentration of the film-forming additive FEC at that location, leading to a thicker film and increased impedance. This application discloses an electrolyte formulation (the mass content of FEC in the electrolyte satisfies 1% ≤ η1 ≤ 20%) and the PVDF particle coverage number S on the surface of the negative electrode active material particles. In this application, S satisfies 51 ≤ S ≤ 300. On the one hand, this limited range ensures an effective bonding network; on the other hand, because PVDF particles have a certain swelling effect on the electrolyte, this application finds that when the PVDF particle coverage number S, the average particle size D1, and the mass content η1 of fluoroethylene carbonate in the electrolyte simultaneously meet the aforementioned ranges, the electrolyte content on the surface of the negative electrode active particles can be appropriate and uniform, ensuring that the electrolyte additives that can participate in the reaction can reach the optimal level. This allows the battery to have good low-temperature performance as well as excellent high-temperature cycling and high-temperature storage performance. For example, S can be 51, 60, 90, 120, 150, 180, 210, 240, 270, or 300. D1 can be 50nm, 60nm, 80nm, 100nm, 120nm, 150nm, 200nm, 220nm, 250nm, 280nm, 300nm, or 350nm. η1 can be 1%, 2%, 3%, 4%, 5%, 8%, 10%, 12%, 15%, 18%, or 20%.

[0117] In one possible implementation, the areal density of the negative electrode active layer is 3 mg / cm³. 2 ≤CW≤8mg / cm2 The density of the negative electrode active layer is 180nm≤D1≤350nm. When the areal density of the negative electrode active layer is low, PVDF with larger particle size can effectively bind the negative electrode active material and maintain structural stability. Larger PVDF particles can provide higher porosity to the negative electrode sheet. Under high temperature conditions, the electrolyte is consumed quickly, and higher porosity is beneficial for electrolyte residue in the negative electrode sheet, which is conducive to lithium ion transport, thereby improving the high temperature performance of the battery.

[0118] In one possible implementation, the areal density of the negative electrode active layer is 8 mg / cm³. 2 <CW≤13mg / cm 2 For a given anode density, 50nm ≤ D1 < 180nm. Higher areal density results in a thicker negative electrode active layer. Generally, thicker electrodes require higher stability of the bonding network. When the areal density of the negative electrode active layer is high, PVDF selects smaller particles, resulting in more bonding sites per unit volume. Under high temperatures, the battery expands during charging and discharging. Excessive expansion can damage the bonding network, meaning the conductive network (bound by the binder) will also break down, leading to deterioration of high-temperature performance. Therefore, more bonding sites improve adhesion, resulting in better structural stability, lower expansion, and better high-temperature performance.

[0119] In one possible implementation, the negative electrode active material comprises a silicon-based material;

[0120] Optionally, based on the mass of the negative electrode active layer, the silicon content is 1% ≤ η2 ≤ 50%; for example, η2 can be 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50%. Within this range, energy density and processability can be balanced. If the silicon content is too high, the processability of the negative electrode slurry is poor, and the battery expands too much, resulting in a high risk of failure.

[0121] Optionally, the particle size D2 of the silicon-based material is 5μm ≤ D2 ≤ 10μm; for example, D2 can be 5μm, 6μm, 7μm, 8μm, 9μm, or 10μm. If the particle size of the silicon-based material is too small, the specific surface area is larger, the slurry may be more alkaline, the slurry stability is poor, and processing is too difficult. In addition, small particle size requires more binder, which is not conducive to the construction of the bonding network. If the silicon-based material particles are too large, silicon is prone to expansion. After expansion, the risk of conductive network failure is greater than that of small particle size, which is not conducive to the overall capacity.

[0122] Optionally, the silicon-based material is selected from at least one of silicon oxide, silicon carbon, and nano-silicon.

[0123] In one possible implementation, the negative electrode active layer further includes a second binder comprising polyacrylic acid (PAA);

[0124] Optionally, the mass ratio of the second binder to the first binder is η3, where η3 is 0-1; for example, η3 can be 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1. Polyacrylic acid-based solution binders have excellent adhesion. When used in conjunction with aqueous PVDF, they can strengthen the bonding network of the negative electrode sheet, enhance its stability, and provide stronger suppression of electrode expansion at high temperatures, thereby further improving high-temperature performance. Furthermore, solution binders can achieve both bonding and uniform coating of the negative electrode active material. They also have good affinity for the electrolyte, resulting in a more uniform distribution of the electrolyte on the surface of the negative electrode active material, thus consolidating the formation of the SEI film. However, excessively high content of the second binder can lead to deterioration of low-temperature performance. η3 is limited to 0-1, meaning the amount of the second binder is less than half of the total binder content, to avoid severe deterioration of the battery's low-temperature performance, ensuring the battery maintains good performance at both high and low temperatures.

[0125] Optionally, the polyacrylic acid includes one or more of polyacrylic acid-acrylonitrile, polyacrylic acid-acrylonitrile-acrylamide, and polyacrylic acid-acrylamide.

[0126] In one possible implementation, the electrolyte further includes a second additive, vinylene carbonate (VC);

[0127] Optionally, the mass ratio of vinylene carbonate to fluoroethylene carbonate in the electrolyte is 0.1-0.45. Examples include 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, or 0.45. FEC is beneficial for forming inorganic components of LiF, which helps improve SEI film stability. VC can form polyVC, further inhibiting the continuous reaction between the electrolyte and the negative electrode, reducing side reactions, and improving high-temperature performance. However, a high FEC content can generate HF, damaging the SEI film, accelerating electrolyte consumption, and causing severe high-temperature gas generation. High VC content also leads to high impedance, reduced kinetics, and poor power output. The synergistic effect of VC and FEC can significantly improve high-temperature performance while maintaining power output, hence this range is preferable.

[0128] In this application, the mass content of the first additive fluoroethylene carbonate and the second additive vinylene carbonate in the electrolyte can be obtained by methods conventional in the art, such as gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS), or liquid chromatography (LC).

[0129] In one possible implementation, the compaction density of the active layer is 1.3-1.85. Exemplary values ​​could be 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or 1.85.

[0130] In one possible implementation, the porosity of the negative electrode is 25%-40%. Exemplary examples include 25%, 30%, 35%, or 40%.

[0131] If the compaction density is too low and the porosity is too high, the electrical contact will be insufficient and the electron conduction will be poor, which will not be conducive to charge transfer. If the compaction density is too high and the porosity is too low, the liquid retention of the electrode will be low and the ion transport will be slow, which will not be conducive to high and low temperature performance. A compaction density of 1.3-1.85 and a porosity of 25%-40% can make the battery take into account both high and low temperature performance.

[0132] In one possible implementation, the OI value of the negative electrode is 5-15; for example, the OI value can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15. A low OI value means that the particles of the negative electrode active material are not arranged in an orderly manner and lack a clear orientation to promote electron conduction, which reduces the overall conductivity of the negative electrode. Simultaneously, a low orientation index may lead to insufficient bonding between the auxiliary active material and the auxiliary current collector, increasing powder shedding during battery cycling and thus shortening battery life. The OI value of 5-15 in this application ensures both the conductivity of the negative electrode and the battery life.

[0133] The OI value is the ratio of the diffraction intensity I(004) of the (004) crystal plane to the diffraction intensity I(110) of the (110) crystal plane of the negative electrode active material, which is I(004) / I(110).

[0134] In one possible implementation, the PVDF particles refer to polymers that contain at least vinylidene fluoride monomer (VDF).

[0135] The PVDF particles in this application may be homopolymers of VDF or copolymers formed from VDF and comonomers. Exemplarily, the comonomers include one or more of hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), and terefluoroethylene (TrFE).

[0136] In one possible implementation, the PVDF particles have a crystallinity of 45%-55%.

[0137] PVDF particles with a crystallinity in the range of 45%-55% have a good balance of liquid absorption capacity, adhesion and flexibility; if the crystallinity is too low, the adhesion is insufficient, the electrode toughness is poor, the processing performance is insufficient, the side reactions such as gas generation at high temperature of the battery are aggravated, and the failure risk is high; if the crystallinity is too high, the electrolyte is difficult to penetrate into the crystallization area, the liquid absorption capacity is poor, and the low temperature performance of the battery is poor.

[0138] The method for testing crystallinity includes: preparing 5-10 mg of PVDF dry adhesive, placing it in a crucible, letting it stand for 5 minutes, and then performing the first stage of heating: heating from 30℃ to 210℃ at a rate of 10℃ / min; holding at 210℃ for 2 minutes; then performing the second stage of cooling: cooling to 30℃ at a rate of 10℃ / min; holding at 30℃ for 2 minutes; and then performing the third stage of heating: heating to 210℃ at a rate of 10℃ / min. The normalized enthalpy of melting during the third stage of heating is used to calculate the crystallinity. 104.7 J / g is the theoretical enthalpy of melting for 100% crystallization of PVDF.

[0139] Optionally, the secondary battery is a lithium-ion battery.

[0140] In one possible implementation, the negative electrode active material includes artificial graphite, optionally primary particulate graphite, which is obtained by single-particle graphitization of petroleum coke, needle coke, or pitch coke.

[0141] In one possible implementation, the negative electrode current collector is a single-sided smooth copper foil, a double-sided smooth copper foil, or a porous copper foil.

[0142] This application also provides a method for preparing a secondary battery, comprising:

[0143] Preparation of negative electrode:

[0144] (1) The negative electrode active material, the first conductive agent, the first binder, the second binder, the thickener, and the first solvent are mixed evenly to obtain a negative electrode slurry;

[0145] (2) The negative electrode slurry is coated on the surface of the negative electrode current collector and then baked to obtain the negative electrode sheet.

[0146] Preparation of positive electrode sheet: The positive electrode active material, the second conductive agent, the third binder and the second solvent are mixed to prepare a positive electrode slurry. The positive electrode slurry is coated on the positive electrode current collector and dried to obtain a positive electrode sheet.

[0147] The positive electrode, negative electrode, and separator are assembled into a cell using the industry-standard winding or stacking method. The cell is then encapsulated with an aluminum-plastic film and subsequently undergoes baking, electrolyte injection, formation, and secondary sealing processes to obtain a lithium-ion battery.

[0148] In one possible implementation, the mass ratio of negative electrode active material: first binder PVDF particles: second binder polyacrylic acid-acrylamide-acrylonitrile: first conductive agent: thickener is 90-98:1-5:1-5:0-5:1-5. For example, it is 95:1.8:1.5:0.5:1.2.

[0149] Optionally, the first conductive agent includes one or more of graphite, carbon black, acetylene black, and graphene.

[0150] Optionally, the thickener includes one or more of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose.

[0151] Optionally, the first solvent may include water.

[0152] Optionally, the positive electrode active material includes at least one of lithium iron phosphate (LFP), lithium nickel cobalt manganese oxide (NCM), lithium cobalt oxide (LCO), and lithium manganese iron phosphate (LFMP).

[0153] 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 fiber, and graphene.

[0154] Optionally, the third adhesive includes at least one of polyvinylidene fluoride (PVDF) and polyimide.

[0155] Optionally, the positive current collector is aluminum foil or carbon-coated aluminum foil.

[0156] Optionally, the second solvent includes one or more of N-methylpyrrolidone (NMP) and N-ethylpyrrolidone.

[0157] Example I-1

[0158] This embodiment provides a battery, comprising: a positive electrode sheet: lithium iron phosphate (powder resistivity 25 Ω·m), polyvinylidene fluoride (PVDF), conductive carbon black (SP, particle size 10-40 nm), and carbon nanotubes (tube diameter 5-10 nm, tube length 10-50 μm) are stirred in a mass ratio of 92:4:3:1, and after uniform mixing, NMP is added to prepare a positive electrode active material slurry with a solid content of 55 wt%. The slurry is uniformly coated on both sides of an aluminum foil, dried, and compacted using a roller press to obtain the positive electrode sheet. The lithium iron phosphate has a lithium iron phosphate molar ratio of 1:1, and the median particle size of the lithium iron phosphate particles is 7 μm.

[0159] Negative electrode sheet: The negative electrode active material (graphite), the first binder (particulate fluorine-based binder, a copolymer of vinylidene fluoride and hexafluoropropylene), and the conductive agent carbon black are mixed at a mass ratio of 94:4:2 and stirred at high speed to obtain a uniformly dispersed mixture. Water is added to prepare a negative electrode active material slurry. The slurry is uniformly coated on both sides of a copper foil, and then dried and rolled to obtain the negative electrode sheet. Specifically, the number of particulate fluorine-based binders covering the surface of a single negative electrode active material particle is S = 5, the median particle size of the particulate fluorine-based binder is 100 nm, the median particle size of the negative electrode active material particles is 6 μm, the weight-average molecular weight of the particulate fluorine-based binder is 700,000, and the median particle size of the conductive agent is 300 nm.

[0160] The electrolyte comprises EC, DEC, FEC, and lithium hexafluorophosphate; wherein the mass ratio of EC to DEC is 1:1; the content of FEC in the electrolyte is 10 wt%, and the content of lithium hexafluorophosphate is 12 wt%.

[0161] The membrane consists of a 7μm thick PP base membrane and a 2μm thick porous layer disposed on one side of the base membrane. The porous layer consists of PVDF and alumina in a mass ratio of 5:95.

[0162] Examples I-2 to I-5

[0163] Examples I-2 to I-5 provide a battery that is basically the same as that in Example I-1, except that the coverage quantity S is different. Specific parameters are shown in Table I-1.

[0164] Examples I-6 to I-12

[0165] Examples I-6 to I-12 provide a battery that is basically the same as that in Example I-1, except that the ratio of the median particle size of the negative electrode active material particles to the median particle size of the particulate fluorine-based binder is different. Specific parameters are shown in Table I-1.

[0166] Examples I-13 to I-18

[0167] Examples I-13 to I-18 provide a battery that is basically the same as that in Example I-1, except that the median particle size of the negative electrode active material particles and the median particle size of the particulate fluorine-based binder are different. Specific parameters are shown in Table I-1.

[0168] Examples I-19 to I-23

[0169] Examples I-19 to I-23 provide a battery that is basically the same as that in Example I-1, with the main differences being the different binders and the different ratios of A1 to A2. Specific parameters are shown in Table I-1.

[0170] The negative electrode active material layer in Examples I-19 to I-23 includes a first binder and a second binder, wherein the second binder is CMC; the mass ratios of the negative electrode active material, conductive agent, first binder, and second binder in Examples I-19 to I-23 are 94:2:3.1:0.9, 94:2:2.7:1.3, 94:2:2.5:1.5, 94:2:2.2:1.8, and 94:2:1.8:2.2, respectively.

[0171] Examples I-24 to I-28

[0172] Examples I-24 to I-28 provide a battery that is basically the same as that in Example I-1, except that the median particle size of the first lithium iron phosphate is different. Specific parameters are shown in Table I-1.

[0173] Examples I-29 to I-30

[0174] Examples I-29 to I-30 provide a battery that is basically the same as that in Example I-1, except that the ratio of the median particle size of the particulate fluorine-based binder to the median particle size of the first lithium iron phosphate particles is different. Specific parameters are shown in Table I-1.

[0175] Comparative Example I-1

[0176] This comparative example provides a battery that is basically the same as that in Example I-1, but with a coverage quantity S of 3.

[0177] Comparative Example I-2

[0178] This comparative example provides a battery that is basically the same as that in Example I-1, but with a coverage quantity S of 310.

[0179] Table I-1 Parameters in each embodiment and comparative example Note: " / " in the table indicates that the data does not exist.

[0180] Test case

[0181] This test case provides the battery performance obtained from the above embodiments and comparative examples, as detailed below:

[0182] Room temperature cycling performance: At (25±2)℃, the cell was left to stand for 30 minutes, then discharged at 3C to the lower limit voltage, and left to stand for 30 minutes. It was then charged at 3C to the upper limit voltage, and cut off at 0.05C. This cycle was repeated 800 times at 25℃, followed by a 3C discharge to the lower limit voltage. The cell was then charged at 3C to the upper limit voltage, cut off at 0.05C, and left to stand for 30 minutes. The 3C charge-discharge cycle was repeated, and the capacity decay was observed after 800 cycles. Using the highest capacity from the first 5 cycles as a benchmark, the capacity retention rate after 800 cycles was calculated as: (800-cycle capacity / highest capacity) × 100%.

[0183] High-temperature cycling performance: The battery was allowed to stand for 30 minutes at (45±2)℃, then discharged at 3C to the lower limit voltage, and allowed to stand for 30 minutes. It was then charged at 3C to the upper limit voltage, and stopped at 0.05C. The battery was allowed to stand for 30 minutes at 45℃, then discharged at 3C to the lower limit voltage. It was allowed to stand for 30 minutes, then charged at 3C to the upper limit voltage, and stopped at 0.05C. This cycle was repeated for 800 cycles. The capacity decay of the cell after 800 cycles was observed. Using the highest capacity from the first 5 cycles as a benchmark, the capacity retention rate after 800 cycles was calculated as: (800-cycle capacity / highest capacity) × 100%.

[0184] Cold start performance: Under (25±2)℃ conditions, discharge at 1C standard constant current until the discharge termination voltage is 2.2V, rest for 30 minutes, then charge at 1C standard constant current and constant voltage until the charging limit voltage is 3.65V, with a cutoff current of 0.05C. Rest for 30 minutes, discharge at 1C standard constant current until the discharge termination voltage is 2.2V to obtain the actual cell capacity C0, rest for 30 minutes; charge at 1C standard constant current and constant voltage until the charging limit voltage is 3.65V, with a cutoff current of 0.05C. Discharge at 1C0 for 30 minutes, which is 50% SOC. After standing at (25±2)℃ for 2 hours, place the cell in a -28℃ constant temperature chamber and maintain the constant temperature for 4 hours; test the terminal voltage value of 3C constant current discharge for 2 seconds. By comparing the data of this terminal voltage, the low-temperature power performance of the battery is evaluated. The higher the voltage, the better the performance. The unit is V.

[0185] The test results are shown in Table I-2.

[0186] Table I-2 Test results for each embodiment and comparative example

[0187] Cold start refers to the startup process of equipment in a low-temperature environment, which requires the battery to provide high power output in a short time. Cold start is often used as one of the key indicators for evaluating the low-temperature rate performance of a battery. The higher the cold start voltage, the better the low-temperature rate performance.

[0188] Based on the above results, this application's control of the granular fluorine-based binder coverage on the surface of individual negative electrode active material particles to 5-300 particles can improve ion transport capability, enhance electrode wettability, increase lithium-ion transport rate within the electrode, optimize lithium-ion transport paths, and improve battery performance at low rates, resulting in a higher cold-start voltage. A coverage quantity of 5-300 particles can also reduce active material expansion and active layer shedding during cycling, reducing cell failure and improving battery cycle performance, resulting in higher capacity retention.

[0189] Example II-1

[0190] This embodiment provides a lithium-ion battery, the preparation method of which includes:

[0191] Preparation of the positive electrode sheet: PVDF binder and conductive carbon black were added to NMP and stirred until homogeneous. Then, lithium iron phosphate, the positive electrode active material, was added and stirred to obtain a uniformly dispersed positive electrode slurry. The solid components included 96.7 wt% lithium iron phosphate and 1.8 wt% PVDF, with a solid content of 61 wt% and a viscosity of 10100 mPa·s. The positive electrode slurry was uniformly coated on both sides of an aluminum foil, dried at 100-130℃ for 4 hours, and then compacted using a roller press to a compaction density of 2.4 g / cm³. 3The positive electrode is obtained, and the resistance of the positive electrode is 240mΩ.

[0192] Preparation of the negative electrode sheet: Aqueous PVDF emulsion (2518L aqueous PVDF emulsion selected from Sinochem Lantian Fluorine Materials Co., Ltd.), graphite, silicon carbide (D50 of 7 μm), thickener sodium carboxymethyl cellulose (CMC), polyacrylic acid binder (LA136DL from Sichuan Indile Materials Technology Group Co., Ltd.), and conductive carbon black were mixed and dispersed in deionized water to obtain a negative electrode slurry. The solid components included 90.2 wt% graphite, 5% silicon carbide (silicon element content of 50.1% by mass), 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 was 43.5 wt%, and the viscosity was 4610 mPa·s. The negative electrode slurry was uniformly coated on both sides of copper foil, dried at 70-100℃ for 5 h, and compacted with a roller press. The compaction density of the negative electrode active layer was 1.65 g / cm³. 3 The resulting negative electrode sheet has an areal density of 6.8 mg / cm³. 2 The PVDF particle coverage number S is 186, and the porosity of the negative electrode is 30%. The SEM image of the negative electrode is shown in Figure 1. The DSC image of the PVDF particles is shown in Figure 2. From Figure 2, the crystallinity is calculated to be 48.1%.

[0193] After welding tabs to the positive and negative electrode sheets, they are wound together with the separator to form a battery cell, packaged, and then injected with electrolyte (the first additive FEC content in the electrolyte is 4.3%, and the second additive VC content is 1.5%). The specific composition is shown in the table below. After hot pressing and secondary sealing, a lithium-ion battery is obtained.

[0194] Table II-1 Electrolyte Composition in Example II-1

[0195] The contents of each component in Table II-1 are measured values ​​obtained by gas chromatography-mass spectrometry (GC-MS).

[0196] The testing methods for each parameter are as follows:

[0197] 1. Number of PVDF particles S on the surface of a single negative electrode active material particle: Observe the morphology of the negative electrode sheet by 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 the negative electrode active material particle. Count the number of PVDF particles on the surface of 10 negative electrode active material particles, and record them as S1, S2, S3...S10 respectively. Finally, obtain S = (S1 + S2... + S10) / 10.

[0198] 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 residual electrolyte, dried, and then observed under a SEM (scanning electron microscope). This particle size is obtained by observation and measurement using a SEM. Typically, different regions are selected, with five regions randomly chosen. The particle size of at least 10 particles in each region is measured, and the average of at least 50 values ​​is taken to obtain D1.

[0199] 3. Areal density of the negative electrode active layer: Take a negative electrode sheet, punch out a circular sheet of a fixed size using a punching machine, weigh it, and subtract the weight of the copper foil (which can be calculated based on the density, thickness, and area of ​​copper) to get the weight of the active layer. Finally, divide by the area and then divide by two to get the areal density of the active layer on one side of the electrode sheet.

[0200] 4. Mass content of silicon in the negative electrode active layer: The content of C and Si elements is quantitatively determined by energy dispersive spectroscopy in SEM, thereby determining the mass content of Si.

[0201] 5. Compacted density of the negative electrode active layer: The compacted density is obtained by dividing the areal density of the negative electrode active layer by the thickness of the negative electrode active layer.

[0202] 6. Porosity of the negative electrode: The porosity is calculated by analyzing the amount of gas adsorbed onto the surface of a porous material under different pressures, utilizing the phenomenon of gas (such as nitrogen or carbon dioxide) adsorption. Steps: Place the negative electrode sample in a vacuum environment; introduce gas under different pressures and measure the amount of gas adsorbed; calculate the pore volume and porosity based on the adsorption curve.

[0203] 7. OI value of negative electrode: When XRD diffraction pattern test is performed on the horizontally placed negative electrode sample, the diffraction signal of the (110) crystal plane can be collected from the graphite layer structure perpendicular to the negative electrode, and the diffraction signal of the (004) crystal plane comes from the graphite layer structure parallel to the negative electrode. Therefore, the orientation of the graphite electrode can be described by the ratio of the (004) diffraction peak intensity (or integrated area) to the (110) diffraction peak intensity (or integrated area). OI = I(004) / I(110).

[0204] The parameters for each embodiment and comparative example are shown in Tables II-2 to II-3.

[0205] Table II-2

[0206] Table II-3

[0207] Secondary battery performance testing methods:

[0208] 1. High-temperature performance:

[0209] At 45℃, after 500 charge / discharge cycles at 1C / 1C, calculate the battery capacity retention rate and battery expansion rate after 500 cycles.

[0210] The residual capacity and recovery capacity of the battery were measured after 60 days at 60℃ and 100% SOC, and the percentage of residual capacity and recovery capacity were calculated.

[0211] Residual capacity refers to the amount of electricity a battery retains after being stored at full charge (100% SOC) for 60 days and then discharged to empty. The percentage of residual capacity is the percentage of residual capacity obtained by dividing the residual capacity by the initial full charge. The percentage of capacity retention is the amount of electricity a battery that has been discharged to empty and then fully charged is divided by the initial full charge.

[0212] 2. Low temperature performance:

[0213] -30℃ Power Discharge: 1) Under (25±2)℃ environment, standard constant current discharge to discharge termination voltage, rest for 30min; then standard constant current and constant voltage charging to charging limit voltage, cut-off current 0.05C, rest for 30min; standard constant current discharge to discharge termination voltage, obtain the actual cell capacity C0; rest for 30min; 2) Standard constant current and constant voltage charging to charging limit voltage, cut-off current 0.05C, discharge with 1C0 for 30min, which is 50% SOC; 3) -30℃ rest for 4h; 4) Test 10C0 discharge for 30s, rest for 10min, then discharge with 10C0 again for 30s, sampling accuracy 10ms. Record the voltage value when the discharge time is 2S (lower limit of low temperature discharge 1.2V).

[0214] The test results are shown in Table II-4.

[0215] Table II-4

[0216] As can be seen from the comparison of the examples and comparative examples in Table II-4, the capacity of the secondary battery of this application is significantly increased at 45°C and the battery expansion rate is reduced; the percentage of residual capacity and the percentage of recovered capacity at 60°C are increased. The battery of this application has good low-temperature performance and its high-temperature performance is significantly improved.

[0217] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A negative electrode sheet, characterized in that, The electrode includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector; the negative electrode active material layer includes a negative electrode active material and a first binder; the negative electrode active material layer includes negative electrode active material particles, and the first binder includes a particulate fluorine-based binder; the amount of particulate fluorine-based binder covering the surface of a single negative electrode active material particle in the negative electrode sheet is S, wherein S satisfies: 5≤S≤300.

2. The negative electrode sheet according to claim 1, characterized in that, The amount of particulate fluorine-based binder covering the surface of a single negative electrode active material particle in the negative electrode sheet is S, where S satisfies: 5≤S≤150.

3. The negative electrode sheet according to claim 1, characterized by The ratio of the median particle size of the negative electrode active material particles in the negative electrode sheet to the median particle size of the particulate fluorine-based binder is 10-400; optionally, the ratio of the median particle size of the negative electrode active material particles in the negative electrode sheet to the median particle size of the particulate fluorine-based binder is 20-300. And / or, the median particle size of the particulate fluoropolymer binder is 50 nm-300 nm; And / or, the median particle size of the negative electrode active material particles is 3μm-20μm; optionally, the median particle size of the negative electrode active material particles is 5μm-15μm. And / or, the negative electrode active material particles in the negative electrode active material layer include at least one of artificial graphite, natural graphite, mesophase carbon microspheres, hard carbon, and soft carbon.

4. The negative electrode sheet according to any one of claims 1 to 3, wherein The particulate fluoropolymer binder has a weight-average molecular weight of 600,000 to 900,000. And / or, the particulate fluoropolymer binder comprises polyvinylidene fluoride polymers; optionally, the particulate fluoropolymer binder comprises a copolymer or homopolymer formed from at least one monomer selected from vinylidene fluoride, chlorotrifluoroethylene, trifluoroethylene, tetrafluoroethylene and hexafluoropropylene. And / or, the particulate fluoropolymer binder includes functional groups, the functional groups including at least one selected from carboxylic acid, sulfonic acid, phosphoric acid, cyano, and amino.

5. The negative electrode sheet according to any one of claims 1 to 3, wherein The negative electrode sheet also includes a conductive agent; the median particle size of the conductive agent particles is 100nm-800nm; And / or, the conductive agent has a mass content of 0.2wt%-5wt% in the negative electrode active material layer; optionally, the conductive agent has a mass content of 0.2wt%-3wt% in the negative electrode active material layer. And / or, the conductive agent includes at least one of conductive carbon black, carbon nanotubes, carbon fibers, and graphene.

6. The negative electrode sheet according to any one of claims 1 to 3, wherein The ratio of the peak area A1 of the weight loss peak in the thermogravimetric curve of the negative electrode active material layer in the range of 250-350℃ to the peak area A2 of the weight loss peak in the thermogravimetric curve of the negative electrode active material layer in the range of 400-500℃ is 0.5-2. Optionally, the ratio of the peak area A1 of the weight loss peak at 250-350℃ to the peak area A2 of the weight loss peak at 400-500℃ in the thermogravimetric curve of the negative electrode active material layer is 0.8-1.

4.

7. The negative electrode sheet according to any one of claims 1 to 3, wherein The negative electrode active material layer further includes a second binder, which includes at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and potassium carboxymethyl cellulose. And / or, the negative electrode active material layer further includes a third binder, the third binder including at least one of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyacrylonitrile, polyacrylamide, polymethacrylate, and styrene-acrylic emulsion.

8. The negative electrode sheet according to claim 7, characterized by The first binder has a mass content of 1.4wt%-6wt% in the negative electrode active material layer; And / or, the mass content of the second binder in the negative electrode active material layer is 0-2.5 wt%; optionally, the mass content of the second binder in the negative electrode active material layer is 0.4 wt%-2 wt%. And / or, the third binder has a mass content of 0-6 wt% in the negative electrode active material layer; optionally, the third binder has a mass content of 0.4 wt%-3 wt% in the negative electrode active material layer.

9. A battery, characterized by The device includes a positive electrode sheet and a negative electrode sheet as described in any one of claims 1-8; the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector, the positive active material layer includes a positive active material, the positive active material includes at least one of lithium iron phosphate and lithium iron phosphate, the first lithium iron phosphate includes first lithium iron phosphate particles, the median particle size of the first lithium iron phosphate particles is 1μm-15μm, optionally, the median particle size of the first lithium iron phosphate particles is 3μm-12μm; And / or, the second lithium iron phosphate comprises second lithium iron phosphate particles, the median particle size of the second lithium iron phosphate particles being 100nm-500nm, optionally, the median particle size of the second lithium iron phosphate particles being 100nm-300nm.

10. The battery of claim 9, wherein the electrolyte comprises a lithium salt. The ratio of the median particle size of the granular fluorine-based binder to the median particle size of the first lithium iron phosphate particles is (10-200):1; And / or, the molar ratio of iron to lithium in the first lithium iron phosphate is (0.94-1.00):1; And / or, the resistivity of the first lithium iron phosphate powder is less than 100 Ω·m.

11. 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 electrode current collector and a negative electrode active layer disposed on at least one side of the negative electrode current collector, the negative electrode active layer including negative electrode active material particles and a first binder; The first binder comprises PVDF particles; the number of PVDF particles on the surface of a single negative electrode active material particle is the coverage number S, where S satisfies 51≤S≤300; the average particle size of the PVDF particles is D1, where 50nm≤D1≤350nm; The electrolyte includes a first additive, fluoroethylene carbonate, and the mass content of the fluoroethylene carbonate in the electrolyte is η1, based on the total mass of the electrolyte, where 1% ≤ η1 ≤ 20%.

12. The secondary battery according to claim 11, characterized by The areal density of the negative electrode active layer is 3 mg / cm³. 2 ≤CW≤8mg / cm 2 180nm≤D1≤350nm; or, the areal density of the negative electrode active layer is 8mg / cm³. 2 <CW≤13mg / cm 2 , 50nm≤D1<180nm.

13. The secondary battery according to claim 11 or 12, characterized by The negative electrode active material includes at least one of graphite and silicon-based materials; And / or, the negative electrode active layer further includes a second binder, the second binder comprising polyacrylic acid; And / or, the electrolyte further includes a second additive, vinylene carbonate; And / or, the compaction density of the negative electrode active layer is 1.3-1.85 g / cm³. 3 ; And / or, the porosity of the negative electrode is 25%-40%; And / or, the OI value of the negative electrode is 5-15; And / or, the PVDF particles refer to polymers containing at least vinylidene fluoride monomers.

14. The secondary battery according to claim 13, characterized in that, Based on the mass of the negative electrode active layer, the mass content of silicon element is 1% ≤ η2 ≤ 50%; and / or The silicon-based material has a D50 particle size of D2, where 5μm≤D2≤10μm; and / or The silicon-based material is selected from at least one of silicon oxide, silicon carbon, and nano-silicon.

15. The secondary battery according to claim 13, characterized by The mass ratio of the second adhesive to the first adhesive is η3, where η3 is 0-1; and / or The polyacrylic acid includes one or more of polyacrylic acid-acrylonitrile, polyacrylic acid-acrylonitrile-acrylamide, and polyacrylic acid-acrylamide; and / or The mass ratio of the vinylene carbonate to the fluoroethylene carbonate in the electrolyte is 0.1-0.45.

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