Secondary battery and electric device

Abstract

The invention discloses a secondary battery and an electric device, and belongs to the technical field of batteries. According to the secondary battery provided by the invention, the positive electrode active material with a proper Dv50 range and the fluorine-containing polymer with a specific swelling ratio range are selected as the positive electrode binder, and the lithium fluorosulfonate is added into the electrolyte, so that the technical characteristics have a mutual synergistic effect, pole piece structure collapse caused by electrolyte permeation can be effectively reduced, and the service life of the secondary battery is prolonged. Side reaction at high temperature and electrolyte decomposition gas production are reduced, and the high-temperature cycle performance of the secondary battery is improved; and meanwhile, the lithium ion diffusion time can be reduced, the contact resistance is reduced, and the rate capability of the secondary battery is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a secondary battery and a power utilization device. BACKGROUND

[0002] As the mainstream dynamic energy storage technology, secondary batteries are widely used in electric vehicles and energy storage fields. However, the high-temperature cycle stability and high-rate performance of the existing secondary batteries still have the following bottlenecks: on the one hand, the traditional electrolyte is prone to decomposition above 45℃, which leads to the thickening of the SEI film and the increase of the interface impedance, and the binder may cause the collapse of the pore structure of the electrode sheet, which aggravates the risk of capacity attenuation of the secondary battery. On the other hand, the existing positive active material reduces the particle size of the positive active material to improve the kinetics (such as Dv50≤0.5μm), but the excessively fine particle size will lead to the reduction of the porosity of the electrode sheet, and thus the electrolyte is not fully infiltrated, which reduces the rate performance of the secondary battery. SUMMARY

[0003] The present application aims to solve the technical problem that the existing technology cannot simultaneously achieve good high-temperature cycle performance and rate performance of the secondary battery, and proposes a secondary battery and a power utilization device with excellent high-temperature cycle performance and rate performance.

[0004] To achieve the above-mentioned purpose, the first aspect of the present application provides a secondary battery, comprising a positive electrode sheet and an electrolyte, the positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, the positive electrode active material layer comprising a positive electrode active material and a positive electrode binder, and the electrolyte comprising an additive, wherein the Dv50 of the positive electrode active material is 0.5μm-8μm.

[0005] The positive electrode binder comprises a fluorine-containing polymer, and the swelling rate of the fluorine-containing polymer at 60℃ is ≤20%.

[0006] The additive comprises lithium fluorosulfonate (LiSO3F).

[0007] As an embodiment of the present application, the molar concentration of the lithium fluorosulfonate in the electrolyte is 0.05mol / L-0.3mol / L.

[0008] As an embodiment of the present application, the secondary battery satisfies 0.014≤λ≤0.258,

[0009] wherein,

[0010] S is the swelling rate of the fluorine-containing polymer, dimensionless;

[0011] C F mol / L is the molar concentration of the lithium fluorosulfonate in the electrolyte.

[0012] D 50 μm is the Dv50 of the positive electrode active material.

[0013] As an embodiment of the present application, the Dv90 and Dv50 of the positive electrode active material satisfy Dv90 / Dv50≤20.

[0014] As an embodiment of the present application, the fluorine-containing polymer includes at least one of polyvinylidene fluoride, polyvinylidene fluoride-trifluoroethylene copolymer, polyvinylidene fluoride-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer.

[0015] As an embodiment of the present application, the positive electrode active material layer further includes a positive electrode conductive agent, and the mass ratio of the positive electrode conductive agent to the positive electrode binder is (0.5-2):1.

[0016] As an embodiment of the present application, the positive electrode conductive agent includes carbon nanotubes and carbon black.

[0017] As an embodiment of the present application, the porosity of the positive electrode tab is 20%-35%.

[0018] As an embodiment of the present application, the compaction density of the positive electrode tab is 2 g / cm 3 -3 g / cm 3 .

[0019] As an embodiment of the present application, the electrolyte further includes a lithium salt, and the lithium salt includes lithium bisfluorosulfonylimide.

[0020] As an embodiment of the present application, the electrolyte further includes an organic solvent, and the organic solvent includes ethylene carbonate, methyl ethyl carbonate and diethyl carbonate.

[0021] In a second aspect, the present application provides a power utilization device including the secondary battery.

[0022] Compared with the prior art, the present application has the following beneficial effects:

[0023] The secondary battery provided by the present application can effectively reduce the collapse of the tab structure caused by electrolyte penetration, reduce the side reaction and electrolyte decomposition gas at high temperature, and improve the high-temperature cycle performance of the secondary battery by selecting a positive electrode active material with a suitable Dv50 range, a fluorine-containing polymer with a specific swelling rate range as a positive electrode binder, and adding lithium fluorosulfonate in the electrolyte. The above technical features synergistically act together. At the same time, the lithium ion diffusion time can be reduced, the contact resistance can be reduced, and the rate performance of the secondary battery can be improved. DETAILED DESCRIPTION

[0024] In order to make the purposes, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of the present application.

[0025] In the present application, the technical features described in an open way include both a closed technical solution consisting of listed features and an open technical solution containing the listed features.

[0026] In the present application, if no special description is made, the numerical interval is regarded as continuous and includes the minimum value and the maximum value of the range and each value between the minimum value and the maximum value. Further, when the range refers to an integer, each integer between the minimum value and the maximum value of the range is included. In addition, when multiple ranges are provided to describe a feature or a characteristic, the ranges can be combined. In other words, unless otherwise specified, all the ranges disclosed herein should be understood as including any and all sub-ranges falling within the range.

[0027] In one embodiment of the present application, the present application provides a secondary battery, comprising a positive electrode sheet and an electrolyte, the positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, the positive electrode active material layer comprising a positive electrode active material and a positive electrode binder, and the electrolyte comprising an additive, wherein the Dv50 of the positive electrode active material is 0.5 μm to 8 μm.

[0028] The positive electrode binder comprises a fluorine-containing polymer, and the swelling rate of the fluorine-containing polymer at 60°C is ≤20%.

[0029] The additive comprises lithium fluorosulfonate.

[0030] The secondary battery provided by the present application can effectively reduce the collapse of the electrode sheet structure caused by the penetration of the electrolyte, reduce the side reaction and electrolyte decomposition gas at high temperature, improve the high-temperature cycle performance of the secondary battery, and reduce the lithium ion diffusion time, reduce the internal resistance of the secondary battery, and improve the rate performance of the secondary battery by selecting a positive electrode active material with a suitable Dv50 range, a fluorine-containing polymer with a specific swelling rate range as a positive electrode binder, and adding lithium fluorosulfonate in the electrolyte.

[0031] Specifically, in the first aspect, the Dv50 of the selected positive electrode active material is 0.5-8 μm, which can effectively shorten the diffusion path of lithium ions, reduce the contact resistance, and improve the discharge efficiency of the secondary battery at high rates, i.e., improve the rate performance of the secondary battery. Moreover, the positive electrode active material in the Dv50 particle size range has less impact on the porosity of the electrode sheet, i.e., can reduce the impact on the electrolyte infiltration, and further ensure the rate performance of the secondary battery. In the second aspect, the selected positive electrode binder is a fluorine-containing polymer in a specific swelling rate range, and the swelling characteristics of the fluorine-containing polymer in the swelling rate range can effectively reduce the deformation of the electrode sheet structure caused by electrolyte penetration under high temperature conditions. Moreover, the presence of C-F bonds in the fluorine-containing polymer can provide chemical inertness and reduce side reactions with the electrolyte, thereby improving the high-temperature cycle performance of the secondary battery. In the third aspect, lithium fluorosulfonate is added to the electrolyte, which can preferentially decompose at the positive electrode interface to generate LiF-Li2SO x a composite SEI film, inhibits electrolyte decomposition and metal ion dissolution at high temperatures, and thus improves the high-temperature cycle performance of the secondary battery. In the fourth aspect, the fluorine-containing polymer in the specific swelling rate range and the lithium fluorosulfonate in the electrolyte have a certain synergistic effect, can form a "physical-chemical" double passivation interface effect, reduce the generation of HF gas, reduce electrolyte decomposition gas, and improve the comprehensive performance of the secondary battery.

[0032] It should be noted that the test method for the Dv50 of the positive electrode active material is as follows: the secondary battery is disassembled to obtain a positive electrode sheet, the positive electrode sheet is subjected to powder scraping treatment, the scraped powder is soaked in excess N-methyl pyrrolidone (NMP) and stirred, and then tested by a laser particle size analyzer to obtain the Dv50 of the positive electrode active material.

[0033] It should be noted that the test method for the swelling rate of the fluorine-containing polymer at 60°C is as follows: the secondary battery is disassembled to obtain a positive electrode sheet, the positive electrode sheet is subjected to powder scraping treatment, the scraped powder is soaked in excess N-methyl pyrrolidone (NMP), and then subjected to multiple suction filtration. The filtrate is collected and dried at 60-70°C to a constant weight (the mass change rate is ≤0.1% after 1 h of testing, which is considered as constant weight), and the fluorine-containing polymer is placed at 60°C and subjected to swelling rate test by mass method.

[0034] Exemplarily, the Dv50 of the positive electrode active material can be any point value or any two-point range value between 0.5 μm and 8 μm, such as 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, etc.

[0035] In one embodiment, the Dv50 of the positive electrode active material is 1.5 μm to 3 μm. For example, it can be 1.5 μm, 1.8 μm, 2 μm, 2.2 μm, 2.4 μm, 2.6 μm, 2.8 μm, 3 μm, etc.

[0036] For example, the swelling rate of the fluoropolymer at 60°C can be any point value or any two points within the range of ≤20%, such as 5% to 20%, 10% to 20%, 15% to 20%, or 5%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, etc.

[0037] In one embodiment, the swelling rate of the fluoropolymer at 60°C is 8% to 15%. For example, it can be % , 9% , 10% , 11% , 12% , 13% , 14% , 15% , etc.

[0038] In one embodiment, the molar concentration of lithium fluorosulfonate in the electrolyte is 0.05 mol / L to 0.3 mol / L.

[0039] It should be noted that the method for testing the molar concentration of lithium fluorosulfonate in the electrolyte is as follows: the secondary battery is disassembled to obtain the electrolyte, which is then detected by ion chromatography. Specifically, this is achieved by separating the anions in the electrolyte (wherein the anion in lithium fluorosulfonate is SO3F). - The concentration of the solution is detected by conductivity; the mobile phase is potassium hydroxide or sodium hydroxide gradient elution, the column temperature is 30℃, and the concentration of lithium fluorosulfonate standard solution of known concentration is quantitatively substituted into the standard curve based on the peak area or peak height for calculation.

[0040] For example, the molar concentration of lithium fluorosulfonate in the electrolyte can be any point value or any two points between 0.05 mol / L and 0.3 mol / L, such as 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, etc.

[0041] In one embodiment, the molar concentration of lithium fluorosulfonate in the electrolyte is 0.08 mol / L to 0.15 mol / L. For example, it can be 0.08 mol / L, 0.1 mol / L, 0.11 mol / L, 0.12 mol / L, 0.13 mol / L, 0.14 mol / L, 0.15 mol / L, etc.

[0042] This study found that the molar concentration of lithium fluorosulfonate in the electrolyte affects its preferential decomposition at the positive electrode to form LiF-Li2SO4. xThe ability of the composite SEI film affects the ability to inhibit electrolyte decomposition and metal ion dissolution at high temperatures; at the same time, it also affects its synergistic effect with fluoropolymers, thus affecting the ability to inhibit electrolyte side reactions; when the molar concentration of lithium fluorosulfonate in the electrolyte is further selected to be 0.05 mol / L to 0.3 mol / L, especially 0.08 mol / L to 0.15 mol / L, the high-temperature cycling performance of the secondary battery is better.

[0043] In one embodiment, the secondary battery satisfies 0.014 ≤ λ ≤ 0.258.

[0044] in,

[0045] S is the swelling ratio of the fluoropolymer, which is dimensionless;

[0046] C F mol / L is the molar concentration of lithium fluorosulfonate in the electrolyte;

[0047] D 50 μm represents the Dv50 of the positive electrode active material.

[0048] For example, λ can be any point value or any two-point range value between 0.014 and 0.258, such as 0.014, 0.024, 0.034, 0.044, 0.064, 0.084, 0.104, 0.124, 0.144, 0.164, 0.184, 0.204, 0.224, 0.258, etc.

[0049] This study found that further control of the swelling ratio of the fluoropolymer, the molar concentration of lithium fluorosulfonate in the electrolyte, and the Dv50 of the positive electrode active material can satisfy [the desired parameters]. When the high temperature cycling performance of the secondary battery is better, the rate performance is also better.

[0050] Specifically, in the first aspect, 1-S and C F The product of these two factors reflects their combined effect in inhibiting electrolyte penetration and stabilizing the SEI film; for example, when the swelling ratio of fluoropolymers decreases, the adhesive stability of fluoropolymers tends to increase to some extent; simultaneously, if C... F An increase in S value indicates an increase in the density of the formed SEI film; this means that a low S value can maintain the integrity of the electrode conductive network, while the LiSO3 from the decomposition of lithium fluorosulfonate can improve the ionic conductivity of the SEI film; that is, 1-S and C... F The product of these factors can collectively reduce interfacial impedance, resulting in a significant synergistic effect. Secondly, when the Dv50 of the positive electrode active material decreases, it can offset the porosity loss caused by the decrease in the swelling ratio of the fluoropolymer, maintaining the Li... +Diffusion within the system; and the introduction of lithium fluorosulfonate can also suppress side reactions by competitively decomposing and removing trace amounts of HF in the electrolyte; therefore, it is necessary to reduce the amount of lithium fluorosulfonate added or increase the swelling rate of the fluoropolymer to keep λ within a certain range, thereby effectively maintaining a low interfacial impedance and improving the high-temperature cycle performance and rate performance of the secondary battery.

[0051] In one embodiment, the secondary battery satisfies 0.058 ≤ λ ≤ 0.113. For example, it can be 0.058, 0.06, 0.062, 0.065, 0.068, 0.07, 0.072, 0.075, 0.078, 0.08, 0.082, 0.085, 0.088, 0.09, 0.092, 0.094, 0.096, 0.098, 0.100, 1.102, 0.104, 0.106, 0.108, 0.110, 0.113, etc.

[0052] This study found that when the secondary battery is further selected to satisfy 0.058≤λ≤0.113, the overall performance of the secondary battery is better.

[0053] In one embodiment, the Dv90 and Dv50 of the positive electrode active material satisfy Dv90 / Dv50≤20.

[0054] For example, the Dv90 / Dv50 of the positive electrode active material can be any point value or any two points within a range of ≤20, such as 3 to 20, or 3, 4, 6, 8, 10, 12, 14, 16, 18, 20, etc.

[0055] In one embodiment, the Dv90 and Dv50 of the positive electrode active material satisfy 3≤Dv90 / Dv50≤20. For example, they can be 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, etc.

[0056] This study found that the Dv90 / Dv50 of the positive electrode active material affects the lithium-ion diffusion rate and the electrode pore structure, specifically affecting the porosity of the electrode. When the Dv90 / Dv50 of the positive electrode active material is further selected within the above range, both the lithium-ion diffusion rate and porosity can be balanced, reducing the kinetic limitations caused by particle agglomeration or large particles, and improving the rate performance of the secondary battery.

[0057] In one embodiment, the fluoropolymer includes at least one of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), and tetrafluoroethylene-ethylene copolymer (ETFE).

[0058] This study found that selecting the above-mentioned types of fluoropolymers can result in better overall performance of the secondary battery.

[0059] In one embodiment, the positive electrode active material layer further includes a positive electrode conductive agent, and the mass ratio of the positive electrode conductive agent to the positive electrode binder is (0.5~2):1.

[0060] It should be noted that the positive electrode active material layer also includes a positive electrode conductive agent. The test method for the mass ratio of the positive electrode conductive agent to the positive electrode binder is as follows: The secondary battery is disassembled to obtain the positive electrode sheet. The positive electrode sheet is then subjected to powder scraping. The scraped powder is tested using thermogravimetric analysis (TG) and a sulfur and carbon analyzer. TG is used to calculate the content of the positive electrode binder and the oxidation range of the positive electrode active material and the positive electrode conductive agent by utilizing the difference in thermal stability between the positive electrode conductive agent and the positive electrode binder. The carbon and sulfur analyzer tests the C content in the powder, and the carbon coating content on the surface of the positive electrode conductive agent and the positive electrode active material in the electrode sheet is deduced (if the main material is carbon coated). First, the proportion of the conductive agent and the active main material is calculated based on the ratio of the two, thus obtaining the mass ratio of the positive electrode conductive agent and the positive electrode binder.

[0061] For example, the positive electrode active material layer further includes a positive electrode conductive agent, and the mass ratio of the positive electrode conductive agent to the positive electrode binder can be any point value or any two points range between (0.5 to 2):1, such as 0.5:1, 1:1, 1.5:1, 2:1, etc.

[0062] This study found that by selecting a mass ratio of positive electrode conductive agent to positive electrode binder within the above-mentioned range in the positive electrode active material layer, both lithium-ion transport efficiency and the stability of the positive electrode structure can be taken into account, thereby resulting in better rate performance and cycle performance of the secondary battery.

[0063] In one embodiment, the positive electrode conductive agent includes carbon nanotubes and carbon black.

[0064] This application research found that by simultaneously adding carbon nanotubes and carbon black to the positive electrode conductive agent, carbon nanotubes can construct a long-range conductive network, while carbon black can enhance local conductivity and reduce the internal resistance of the secondary battery. At the same time, the two can work together with the positive electrode active material within a specific Dv50 range to shorten the lithium ion transport path, improve the lithium ion transport efficiency, and thus improve the rate performance of the secondary battery.

[0065] In one embodiment, the mass ratio of the carbon nanotubes to carbon black is (0.1 to 1): 1.

[0066] For example, the mass ratio of carbon nanotubes to carbon black is any point value or any two points within the range of (0.1 to 1):1, such as 0.1:1, 0.2:1, 0.4:1, 0.6:1, 0.8:1, 1:1, etc.

[0067] In one embodiment, the positive electrode active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, and ternary positive electrode materials.

[0068] In one embodiment, the porosity of the positive electrode sheet is 20% to 35%.

[0069] It should be noted that the test method for the porosity of the positive electrode sheet is as follows: the secondary battery is disassembled to obtain the positive electrode sheet, and then the porosity of the positive electrode sheet is obtained by mercury intrusion porosimetry.

[0070] For example, the porosity of the positive electrode sheet can be any point value or any two-point range value between 20% and 35%, such as 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 35%, etc.

[0071] In one embodiment, the compaction density of the positive electrode sheet is 2 g / cm³. 3 ~3g / cm 3 .

[0072] It should be noted that the test method for the compaction density of the positive electrode sheet is as follows: the secondary battery is disassembled to obtain the positive electrode sheet, and then a circular electrode sheet of a certain area and mass is obtained by punching. The thickness of the electrode sheet is measured, and the compaction density of the electrode sheet is calculated by using a formula.

[0073] For example, the compaction density of the positive electrode sheet can be 2 g / cm³. 3 ~3g / cm 3 The value at any point or between any two points, for example, could be 2g / cm. 3 2.2g / cm 3 2.4g / cm 3 2.6g / cm 3 2.8g / cm 3 3g / cm 3 wait.

[0074] This study found that further selecting the porosity and compaction density of the positive electrode sheet within the aforementioned ranges can reduce the subsequent electrolyte wetting time and improve the cycle performance and rate performance of the secondary battery.

[0075] In one embodiment, the method for preparing the positive electrode sheet includes the following steps:

[0076] S1. Dissolve the fluoropolymer in an organic solvent, then add the positive electrode active material and the positive electrode conductive agent, and stir to obtain the positive electrode slurry;

[0077] S2. Coat the positive electrode slurry onto one surface of the positive electrode current collector, then roll and dry to obtain the positive electrode sheet.

[0078] In one embodiment, the organic solvent in S1 includes N-methylpyrrolidone (NMP).

[0079] In one embodiment, in S1, the fluoropolymer accounts for 6% to 10% of the mass percentage of the organic solvent.

[0080] In one embodiment, in step S1, the stirring speed is 1850 rpm to 2500 rpm; the stirring time is 1 h to 3 h.

[0081] In one embodiment, in step S2, the gradient temperature drying is performed as follows: first, maintaining the temperature at 75°C to 85°C for 4 min to 6 min, then maintaining the temperature at 95°C to 105°C for 8 min to 12 min, and finally maintaining the temperature at 115°C to 125°C for 12 min to 18 min.

[0082] In one embodiment, the pressure of the roller pressing in S2 is 10T to 70T.

[0083] In one embodiment, the drying temperature in step S2 is 80°C to 100°C.

[0084] It should be noted that the porosity and compaction density of the positive electrode sheet can be adjusted by controlling the rolling pressure during the preparation process.

[0085] In one embodiment, the electrolyte further includes a lithium salt, which includes lithium bis(fluorosulfonyl)imide.

[0086] This study found that lithium bis(fluorosulfonyl)imide has excellent high-temperature stability (decomposition temperature above 200℃) and low viscosity (ionic conductivity ≥10mS / cm). Adding it as a lithium salt can effectively improve the high-temperature cycle performance of secondary batteries by combining it with other components.

[0087] In one embodiment, the molar concentration of lithium bis(fluorosulfonyl)imide is 1 mol / L to 1.5 mol / L, based on the mass of the electrolyte.

[0088] For example, the molar concentration of lithium bis(fluorosulfonyl)imide, based on the mass of the electrolyte, can be any point value or any two-point range between 1 mol / L and 1.5 mol / L, such as 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, etc.

[0089] This study found that when the molar concentration of lithium bis(fluorosulfonyl)imide is further selected within the above-mentioned range, the overall performance of the secondary battery is better.

[0090] In one embodiment, the electrolyte further includes an organic solvent, which includes ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DMC).

[0091] This application study found that EC can promote lithium salt dissociation due to its dielectric constant range (dielectric constant ε is about 90 at 25°C, and the test process is a wide-band scanning with a frequency of 1KHz to 1GHz), EMC can improve ion mobility due to its viscosity range (viscosity η is about 0.6cP at 25°C), and the addition of DEC can synergistically form a fluorine-rich passivation layer with lithium fluorosulfonate, reducing HF generation (content <50ppm). In other words, selecting the above-mentioned types of organic solvents in the system of this application can effectively improve the high-temperature cycle performance and rate performance of secondary batteries.

[0092] In one embodiment, the volume ratio of EC, EMC and DMC is (2-3):(4-6):(1-3).

[0093] This study found that when the volume ratios of EC, EMC, and DMC are further selected within the above-mentioned range, the overall performance of the resulting secondary battery is better.

[0094] In one embodiment, the secondary battery further includes a negative electrode sheet, which includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector; the negative active material layer includes a negative active substance, a negative binder, a negative conductive agent, and a negative thickener.

[0095] This application does not have any special requirements for the selection of the negative electrode active material; any negative electrode active material conventionally available in the art can be used. For example, the negative electrode active material can be any one of graphite, soft carbon, hard carbon, nano-carbon, or silicon-carbon composite.

[0096] This application does not have any special requirements for the selection of the negative electrode binder; any negative electrode binder conventionally available in the art can be used. For example, the negative electrode binder may be any one of styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), gum arabic (GA), polypyrrole (PPy), polyaniline (PANI), polyethylene dioxythiophene (PEDT), polythiophene (PTh), aramid (PPTA), etc.

[0097] This application does not have any special requirements for the selection of the negative electrode thickener; any negative electrode thickener conventionally available in the art can be used. For example, the negative electrode thickener may be any one of carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyurethane (PU), or guar gum (GG).

[0098] This application does not have any special requirements for the selection of the negative electrode conductive agent; any negative electrode conductive agent conventionally available in the art can be used. For example, the negative electrode conductive agent can be any one of carbon black (SP), graphite, carbon nanotubes (CNTs), or graphene.

[0099] A second aspect of this application provides an electrical device including the secondary battery described in this application.

[0100] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0101] Example 1

[0102] This application provides a secondary battery, the preparation method of which includes the following steps:

[0103] (1) Preparation of positive electrode sheet

[0104] S1. Dissolve the positive electrode binder (fluoropolymer: PVDF, with a swelling rate of 10% at 60℃) in N-methylpyrrolidone at a mass percentage of 8%. Then add the positive electrode active material (lithium iron phosphate, Dv50 of 1.5μm and Dv90 of 8μm) and the positive electrode conductive agent (a mixture of carbon nanotubes and carbon black in a mass ratio of 1:1) and mix evenly. The mass ratio of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent is 96:2:2. After mixing evenly, stir at 2500 rpm for 2 hours to obtain the positive electrode slurry.

[0105] S2. After coating the positive electrode slurry onto the surface of the positive electrode current collector (aluminum foil), roll it under a pressure of 30T, and then dry it at 90℃, controlling the porosity of the positive electrode sheet to be 25% and the compaction density to be 2.45 g / cm³. 3 , thus obtaining the positive electrode sheet;

[0106] (2) Preparation of negative electrode sheet

[0107] S1. After pre-stirring and kneading graphite, carbon black, and a portion of CMC for 90 minutes, add the remaining water, CMC, and SBR to obtain the negative electrode slurry; the mass ratio of graphite, carbon black, CMC, and SBR is graphite:carbon black:CMC:SBR = 97:0.5:0.8:1.7. The solid content is controlled at 46%±3%, the viscosity is 5000±500 mPa·s, and the stirring speed is controlled at 2500 rpm.

[0108] S2. Coat the negative electrode slurry onto the surface of the current collector (copper foil), with a wet film thickness of 80 μm, dry, and compact to a density of 1.25 g / cm³. 3 , thus obtaining the negative electrode sheet;

[0109] (3) Preparation of electrolyte

[0110] Ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate were mixed in a volume ratio of 3:5:2, and then lithium bis(fluorosulfonyl)imide and lithium fluorosulfonate were added to obtain an electrolyte; wherein the molar concentration of lithium bis(fluorosulfonyl)imide was 1.2 mol / L and the molar concentration of lithium fluorosulfonate was 0.15 mol / L.

[0111] (4) Preparation of secondary batteries

[0112] The prepared positive electrode sheet is formed by winding it with the negative electrode sheet and the separator, and then injecting the prepared electrolyte. After capacity testing, a secondary battery is obtained.

[0113] Examples 2-4

[0114] This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that the type of fluoropolymer is adjusted to achieve the parameters in Table 1.

[0115] Examples 5-8

[0116] This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the swelling ratio of the fluoropolymer is adjusted to achieve the parameters in Table 1.

[0117] Examples 9-11

[0118] This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the particle sizes of Dv50 and Dv90 of the positive electrode active material are adjusted to achieve the parameters in Table 1.

[0119] Examples 12-16

[0120] This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that the amount of fluoropolymer added is adjusted to achieve the parameters in Table 1.

[0121] Examples 17-18

[0122] This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the mass ratio of the positive electrode conductive agent and the positive electrode binder is adjusted to achieve the parameters in Table 1.

[0123] Examples 19-20

[0124] This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the rolling pressure during the preparation of the positive electrode sheet is adjusted to change the porosity of the positive electrode sheet in order to achieve the parameters in Table 1.

[0125] Examples 21-22

[0126] This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the rolling pressure during the preparation of the positive electrode sheet is adjusted to change the compaction density of the positive electrode sheet in order to achieve the parameters in Table 1.

[0127] Example 23

[0128] This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that lithium manganese iron phosphate is used instead of lithium iron phosphate to achieve the parameters in Table 1.

[0129] Example 24

[0130] This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that lithium difluorooxalateborate is used instead of lithium difluorosulfonylimide to achieve the parameters in Table 1.

[0131] Example 25

[0132] This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that vinylene carbonate is used instead of ethylene carbonate to achieve the parameters in Table 1.

[0133] Comparative Example 1

[0134] This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that the swelling ratio of the fluoropolymer is adjusted to achieve the parameters in Table 1.

[0135] Comparative Examples 2-3

[0136] This application provides a secondary battery as a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that the Dv50 particle size of the positive electrode active material is adjusted to achieve the parameters in Table 1.

[0137] Comparative Example 4

[0138] This application provides a secondary battery as a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that hydrogenated nitrile butadiene rubber (HNBR) is used instead of fluoropolymer to achieve the parameters in Table 1.

[0139] The Dv50, Dv90, Dv90 / Dv50 of the positive electrode active material in the examples and comparative examples, the type of fluoropolymer, swelling ratio and molar concentration in the positive electrode active material layer, λ, mass ratio m of positive electrode conductive agent and positive electrode binder, porosity and compaction density of the positive electrode sheet are shown in Table 1.

[0140] Table 1

[0141]

[0142]

[0143] The secondary batteries prepared in the examples and comparative examples underwent further performance testing, including the following aspects:

[0144] 1. 45℃ Cyclic Performance Test:

[0145] The steps are as follows: 1) Let stand for 60 minutes; 2) Charge at a constant current of 1C to 3.65V, then charge at a constant voltage of 0.05C; 3) Let stand for 10 minutes; 4) Discharge at a constant current of 1C to 2.0V; 5) Let stand for 10 minutes; 6) Repeat steps 2) to 4) 1000 times.

[0146] 2. Ratio Performance Test:

[0147] The steps are as follows: 1) Let stand for 10 minutes; 2) Charge at a constant current of 1C to 3.65V, then charge at a constant voltage to 0.05C; 3) Let stand for 5 minutes; 4) Discharge at a constant current of 0.33C to 2.0V; 5) Let stand for 5 minutes; 6) Charge at a constant current of 1C to 3.65V, then charge at a constant voltage to 0.05C; 7) Let stand for 5 minutes; 8) Discharge at a constant current of 5C to 2.0V; 9) Let stand for 5 minutes; 10) Repeat steps 6) to 9) to complete the 5C rate discharge; 11) Let stand for 10 minutes.

[0148] Battery efficiency = (Energy output during battery discharge / Theoretically stored energy of battery) * 100%.

[0149] 3. Internal resistance (DCR) test at 25℃:

[0150] The steps are as follows: 1) Let stand for 10 minutes; 2) Discharge at 1C constant current to 2.5V; 3) Let stand for 30 minutes; 4) Charge at 1C constant current to 3.65V, then charge at constant voltage to 0.05C; 5) Let stand for 30 minutes; 6) Discharge at 1C constant current to 2.5V; 7) Let stand for 30 minutes; 8) Charge at 1C constant current to 3.65V, then charge at constant voltage to 0.05C; 9) Let stand for 30 minutes; 10) Discharge at 1C constant current for 6 minutes (50% SOC); 11) Let stand for 60 minutes, and record the voltage at this time as U1; 12) Discharge at 5C constant current for 10 seconds, and record the voltage at this time as U2, and the current as I; 13) Let stand for 40 seconds; 14) Charge at 5C constant current for 10 seconds.

[0151] The formula for calculating DCR is: DCR(10s)=(U1-U2) / I.

[0152] The results are shown in Table 2.

[0153] Table 2

[0154]

[0155]

[0156] As can be seen from Table 2, when the technical solution provided in this application is adopted, the secondary battery obtained has good high-temperature cycling performance, rate performance and low internal resistance; specifically, the capacity retention rate of the obtained secondary battery after 1000 cycles at 45°C is above 89.55%, the discharge efficiency at 5C rate is above 89.54%, and the internal resistance is below 16.75mΩ.

[0157] As can be seen from Examples 1-25 and Comparative Example 1, the swelling ratio of the fluoropolymer can only achieve significant effects in the secondary battery when it is within the range of this application. As can be seen from Examples 1-25 and Comparative Examples 2-3, the secondary battery has good high-temperature cycling performance, rate performance, and low internal resistance when the Dv50 of the positive electrode active material is within the range given in this application. As can be seen from Examples 1-25 and Comparative Example 4, the overall effect of the secondary battery is better when the fluoropolymer of this application is added.

[0158] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of this application and not to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery, comprising a positive electrode and an electrolyte, wherein the positive electrode comprises a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer comprising a positive active material and a positive binder, and the electrolyte comprises additives, characterized in that, The Dv50 of the positive electrode active material is 0.5 μm to 8 μm; The positive electrode binder comprises a fluoropolymer, wherein the swelling rate of the fluoropolymer at 60°C is ≤20%; The additives include lithium fluorosulfonate.

2. The secondary battery according to claim 1, characterized in that, The molar concentration of lithium fluorosulfonate in the electrolyte is 0.05 mol / L to 0.3 mol / L.

3. The secondary battery according to claim 2, characterized in that, The secondary battery satisfies 0.014 ≤ λ ≤ 0.

258. in, S is the swelling ratio of the fluoropolymer, which is dimensionless; C F mol / L is the molar concentration of lithium fluorosulfonate in the electrolyte; D 50 μm represents the Dv50 of the positive electrode active material.

4. The secondary battery according to claim 1, characterized in that, The positive electrode active material has Dv90 and Dv50 satisfying Dv90 / Dv50≤20.

5. The secondary battery according to claim 1, characterized in that, The fluoropolymer includes at least one of polyvinylidene fluoride, polyvinylidene fluoride-trifluoroethylene copolymer, polyvinylidene fluoride-hexafluoropropylene copolymer, and tetrafluoroethylene-ethylene copolymer.

6. The secondary battery according to claim 1, characterized in that, The positive electrode active material layer also includes a positive electrode conductive agent, and the mass ratio of the positive electrode conductive agent to the positive electrode binder is (0.5~2):

1.

7. The secondary battery according to claim 6, characterized in that, The positive electrode conductive agent includes carbon nanotubes and carbon black.

8. The secondary battery according to claim 1, characterized in that, The porosity of the positive electrode sheet is 20% to 35%; And / or, the compaction density of the positive electrode sheet is 2 g / cm³. 3 ~3g / cm 3 .

9. The secondary battery according to claim 1, characterized in that, The electrolyte also includes a lithium salt, which includes lithium bis(fluorosulfonyl)imide. And / or, the electrolyte further includes an organic solvent, including ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate.

10. An electrical device, characterized in that, Includes the secondary battery as described in any one of claims 1 to 9.

Images