Secondary battery and electrical apparatus

Abstract

A secondary battery and an electrical apparatus. The secondary battery comprises an electrolyte and a positive electrode sheet. The positive electrode sheet comprises a positive electrode active material. The positive electrode active material comprises a lithium iron phosphate material core and a carbon shell layer at least partially coated on the outer surface of the core. In the Raman spectrum of the positive electrode active material, the ratio Id / Ig of the peak intensity Id of a D peak having a Raman shift within the range of 1350±50 cm-1 to the peak intensity Ig of a G peak having a Raman shift within the range of 1580±50 cm-1 is 0.2-1.5. The electrolyte comprises 1% to 80% by mass of a carboxylate ester compound and / or 1% to 30% by mass of a fluoronated ether compound. The secondary battery produces a small amount of gas and has a good cycle life.

Description

Secondary batteries and electrical appliances

[0001] Cross-referencing

[0002] This application incorporates Chinese Patent Application No. 202411124316.1, filed on August 15, 2024, entitled "Secondary Battery and Electrical Device", which is incorporated herein by reference in its entirety. Technical Field

[0003] This application relates to the field of secondary battery technology, and more particularly to a secondary battery and an electrical device. Background Technology

[0004] In recent years, secondary batteries have been increasingly widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, military equipment, and aerospace. The market has also put forward higher requirements for the cycle life of secondary batteries. Therefore, increasing the size of battery modules has become a development trend in the battery field.

[0005] Volume expansion is a significant factor affecting the safety and lifespan of rechargeable batteries. As the size of battery modules increases, the number of active material particles and the electrolyte capacity inside the battery increase significantly. This severely exacerbates electrode expansion and electrolyte consumption during cycling, leading to battery expansion and a decrease in the cycle life of the rechargeable battery. Moreover, this problem becomes more pronounced the larger the battery module size. Summary of the Invention

[0006] Based on the above, this application provides a secondary battery, including an electrolyte and a positive electrode. The positive electrode includes a positive active material, which comprises a lithium iron phosphate core and a carbon shell at least partially covering the outer surface of the core. The Raman shift of the positive active material is within 1350 ± 50 cm⁻¹. -1 Peak intensity I of D peak within the range d With Raman displacement at 1580±50cm -1 Peak intensity I of G peak within the range g The ratio I d / I g The concentration is 0.2-1.5, and the electrolyte includes 1%-80% carboxylic acid ester compounds and / or 1%-30% fluorinated ether compounds by mass of the total electrolyte.

[0007] The peak intensity I of the D peak in the Raman spectrum of the positive electrode active material d Peak intensity I of G peak g The ratio I d / I gThe graphitization degree of the positive electrode active material can be characterized, and the lower the value, the higher the graphitization degree of the positive electrode active material, meaning that the hydrophobicity of the material as a whole is enhanced. The positive electrode active material I d / I g is within a suitable range, the water content of the positive electrode active material can be reduced, and in turn, the water content in the positive electrode plate and the electrolyte can be reduced, which is conducive to improving the stability of the positive electrode slurry and the processing performance of the electrode plate, and can also reduce the electrolyte consumption caused by the hydrolysis of electrolyte salts (such as lithium salts) in the electrolyte and the battery swelling caused by the hydrolysis of carboxylic acid ester compounds and fluorinated ether compounds, thereby improving the cycle life of the battery. d / I g The ratio within a suitable range also helps to improve the structural stability and electrical conductivity of the positive electrode active material, and to reduce the electronic internal resistance. Carboxylic acid ester compounds and fluorinated ether compounds help to reduce the viscosity of the electrolyte, improve the ion conductivity of the electrolyte, and reduce the transmission resistance of lithium ions in the electrolyte. Reasonable compounding of the positive electrode active material and the electrolyte can improve the internal resistance of the battery, reduce the volume expansion, and make the secondary battery have good energy efficiency and cycle performance.

[0008] In any embodiment, the positive electrode active material has a I d / I g of 0.9-1.2, which helps to further improve the electronic conductivity of the lithium iron phosphate material, reduce the direct current internal resistance, and improve the energy efficiency of the secondary battery; and further improve the stability of the positive electrode active material particles and reduce the water content of the battery, reduce the volume expansion of the battery, and improve the cycle life of the battery, thereby improving the service life of the secondary battery.

[0009] In any embodiment, the mass content of the carboxylic acid ester compound is 5%-80%, or optionally 5%-70%, based on the total mass of the electrolyte; or the mass content of the fluorinated ether compound is 5%-30%, or optionally 5%-10%, based on the total mass of the electrolyte.

[0010] The presence of carboxylic acid ester compounds and / or fluorinated ether compounds in the electrolyte helps to reduce the viscosity of the electrolyte, improve the ion conductivity, and reduce the migration resistance of lithium ions; at the same time, it helps to reduce the electrolyte decomposition gas, delay the volume expansion of the battery, and improve the cycle performance.

[0011] In any embodiment, the carboxylic acid ester compound has a structure shown in Formula I,

[0012] wherein R is selected from methyl, ethyl, n-propyl, 2-fluoroethyl, 2,2-difluoroethyl, 2,2,2-trifluoroethyl; and R1 is selected from methyl, ethyl, n-propyl, tert-pentanoic acid group, 2,2-difluoroethyl, 2,2,2-trifluoroethyl;

[0013] The fluorinated ether compound has a structure shown in Formula II or Formula III:

[0014] R2-O-R3 Formula II; R2-O-R4-O-R3 Formula III

[0015] wherein R2 and R3 are each independently selected from C(a)H(b)F(2a-b+1)-;

[0016] R4 is selected from -C(a’)H(b’)F(c’)-,

[0017] a and b are each an integer of 1-5, optionally an integer of 1-3, and 2a-b+1≥0,

[0018] a’ is an integer of 1-5, optionally an integer of 1-3; b’ is an integer of 0-10, optionally an integer of 0-6; c’ is an integer of 0-10, optionally an integer of 0-6; b’+c’=2a’.

[0019] In any embodiment, the carboxylic acid ester compound includes one or more of ethyl acetate, methyl acetate, methyl formate, ethyl propionate, 2,2-difluoroethyl acetate, propyl acetate, methyl propionate, propyl propionate, methyl tert-pivaloate, 2,2-difluoroethyl propionate, 2,2-difluoroethyl acetate, 2-fluoroethyl acetate, 2,2,2-trifluoroethyl acetate, 2,2,2-trifluoroethyl propionate, methyl 3,3-difluoropropionate, ethyl 3,3,3-trifluoropropionate, 2,2,2-trifluoroethyl 3,3,3-trifluoropropionate, methyl 3,3,3-trifluoropropionate, ethyl 3,3-difluoropropionate, and / or

[0020] The fluorinated ether compound includes at least one of the following compounds:

[0021] The carboxylic acid ester compound and the fluorinated ether compound have low viscosity, which can reduce the viscosity of the electrolyte, improve the ionic conductivity of the electrolyte, reduce the internal resistance of the battery, and improve the energy efficiency of the battery.

[0022] In any embodiment, the lithium iron phosphate material includes Li x Fe y M z PO4, wherein 0.95

[0023] Controlling the content of the doped M element in the positive electrode active material within a suitable range can effectively improve the structural stability of the positive electrode active material and the deintercalation speed of lithium ions in the electrocyclic process, inhibit the irreversible deformation of the active material particles and reduce the lithium ion migration resistance, so that the secondary battery has good cycle performance and service life.

[0024] In any embodiment, the secondary battery further comprises a negative electrode tab, the negative electrode tab comprising a current collector and a negative electrode film layer coated on at least one side of the current collector, the negative electrode film layer comprising a first negative electrode film layer close to the current collector and a second negative electrode film layer away from the current collector, the porosity of the second negative electrode film layer being greater than the porosity of the first negative electrode film layer, so that the electrolyte can be well infiltrated into the negative electrode tab, reducing the lithium ion migration resistance, reducing the interface resistance and improving the uniformity of the current density distribution, improving the energy efficiency and cycle life of the secondary battery.

[0025] In any embodiment, the porosity of the first negative electrode film layer is 10%-30%. In any embodiment, the porosity of the second negative electrode film layer is 20%-40%.

[0026] Controlling the porosity of the negative electrode film layer within a suitable range enables the negative electrode film layer to be effectively infiltrated by the electrolyte. This improves the transmission efficiency of lithium ions inside the negative electrode film layer and reduces the lithium ion migration resistance, so that the lithium secondary battery has excellent kinetic performance while achieving high energy efficiency.

[0027] In any embodiment, the thickness of the first negative electrode film layer is 0.4H-0.6H. In any embodiment, the thickness of the second negative electrode film layer is 0.4H-0.6H. In any embodiment, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 0.6-1.5.

[0028] Controlling the thickness of different negative electrode film layer regions can reduce the diffusion path of lithium ions in the active material, help to speed up the embedding and de-embedding speed of lithium ions, reduce the ion migration resistance, and improve the battery energy efficiency.

[0029] In any embodiment, the thickness of the first negative electrode film layer is 50μm-90μm, and / or the thickness of the second negative electrode film layer is 50μm-90μm. The secondary battery with a film layer thickness can accommodate more active material and help to adjust the lithium ion diffusion path, reduce the ion migration resistance, so that the capacity and energy efficiency of the secondary battery are improved, and the cycle performance of the secondary battery is improved.

[0030] In any embodiment, the length of the secondary battery is L, the height is H, and the thickness is T, wherein 300mm≤L≤650mm, 180mm≤H≤300mm, 50mm≤T≤100mm, 1≤L / H≤3, and 2≤H / T≤4. Not only can the industrial manufacturability of the secondary battery be achieved, but also the electronic transmission path can be adjusted, the electronic transmission resistance and the battery internal resistance can be reduced, and the energy efficiency of the battery can be improved.

[0031] In any embodiment, the single cell capacity of the secondary battery is 400Ah-1300Ah, which helps to store more electricity and provide longer cycle life and use time for the secondary battery.

[0032] The second aspect of the application provides a power consumption device comprising the secondary battery of the first aspect. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the application, the drawings required to be used in the embodiments of the application will be briefly introduced as follows. Obviously, the drawings described below are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the drawings.

[0034] FIG. 1 is a schematic diagram of a secondary battery according to an embodiment of the application.

[0035] FIG. 2 is an exploded view of the secondary battery according to an embodiment of the application shown in FIG. 1.

[0036] FIG. 3 is a schematic diagram of a battery module according to an embodiment of the application.

[0037] FIG. 4 is a schematic diagram of a battery pack according to an embodiment of the application.

[0038] FIG. 5 is an exploded view of the battery pack according to an embodiment of the application shown in FIG. 4.

[0039] FIG. 6 is a schematic diagram of a power consumption device using the secondary battery as a power supply according to an embodiment of the application.

[0040] Legend: 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0041] Hereinafter, embodiments of the positive electrode active material and the method for manufacturing the same, the positive electrode sheet, the secondary battery, the battery module, the battery pack, and the electrical device of the present application will be specifically described with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters known well, repeated description of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0042] The ranges disclosed herein are defined by their lower and upper limits. Ranges can be inclusive or exclusive of their endpoints, and are arbitrarily combinable. For example, if a range is listed as 60-120 and 80-110, it is understood that 60-110 and 80-120 are also expressly stated. Also, where a minimum range value of 1 and 2 is listed, and a maximum range value of 3, 4, and 5 is listed, then the following ranges are all expressly stated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise indicated, a numerical range "a-b" is intended to indicate any and all sub-combinations of the values between a and b, inclusive of the values a and b. For example, the numerical range "0-5" is intended to indicate that all real numbers between 0 and 5, inclusive of 0 and 5, have been listed herein. "0-5" is merely a shorthand for listing all of these numerical combinations. Also, when a parameter is stated to be an integer ≥ 2, it is equivalent to state that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0043] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0044] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0045] If not specifically stated, all steps of the present application can be performed in sequence or randomly, preferably in sequence. For example, the method comprising steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method can further comprise step (c) means that step (c) can be added to the method in any sequence, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0046] If not specifically stated, "comprising" and "including" mentioned in the present application means open or closed. For example, "comprising" and "including" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0047] If not specifically stated, in the present application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any one of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0048] At present, with the increasing demand for the capacity of secondary batteries in the market, the size and capacity of the battery cell are increasing. With the increase in the size of the battery assembly, the number of positive and negative active material particles inside the battery also increases, which significantly increases the number of active ions (e.g., sodium ions or lithium ions) during the cycle process, increases the deformation stress of the active material particles, and easily causes the active material particles to break and irreversibly deform, which causes the electrode sheet to swell and even break. In addition, the amount of electrolyte in the secondary battery also increases significantly, which increases the amount of trace water in the electrolyte as the amount of electrolyte increases, which exacerbates the electrolyte decomposition and gas generation side reactions, leading to electrolyte consumption and severe battery swelling, which is not conducive to the improvement of the internal resistance and cycle life of the secondary battery. The increase in the size of the battery assembly also increases the transmission path of the electrons and the non-uniformity of the current density, which worsens the internal resistance of the battery and reduces the energy efficiency.

[0049] [lithium secondary battery]

[0050] The first aspect of this application discloses a secondary battery, specifically comprising a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive and negative electrodes. During battery charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, disposed between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through. In some embodiments, the secondary battery is a lithium secondary battery.

[0051] In some embodiments, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer comprising a positive active material. As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the opposite surfaces of the positive current collector.

[0052] In some embodiments, in the secondary battery, the positive electrode active material comprises a lithium iron phosphate core and a carbon shell at least partially covering the outer surface of the core, and the Raman shift of the positive electrode active material is within 1350±50 cm⁻¹. -1 Peak intensity I of D peak within the range d With Raman displacement at 1580±50cm -1 Peak intensity I of G peak within the range g The ratio I d / I g The electrolyte comprises 1%-80% by mass of carboxylic acid esters and / or 1%-30% by mass of fluorinated ethers, accounting for 0.2-1.5% of the total mass of the electrolyte.

[0053] In this paper, the term "D peak" refers to the peak located at 1350±50 cm⁻¹ in Raman spectroscopy. -1 The peak corresponds to the characteristic peak of lattice defects in the carbon shell coating on the surface of the positive electrode active material.

[0054] In this paper, the term "G peak" refers to the peak located at 1350±50 cm⁻¹ in Raman spectroscopy. -1 The peak corresponds to the characteristic peak of the disordered structure in the carbon shell coating on the surface of the positive electrode active material.

[0055] In this article, the term "I" d "" refers to the peak intensity of peak D.

[0056] In this article, the term "I" g "" refers to the peak intensity of peak G.

[0057] In this article, the term "I" d / I g"" refers to the ratio of the intensity of the D peak to the intensity of the G peak, which is used to quantify the degree of graphitization of the positive electrode active material.

[0058] In this document, the term "graphitization degree" has the meaning known in the art, referring to the degree of graphitization of the carbon component, reflecting the integrity of the graphite crystal structure in the carbon-coated lithium iron phosphate of this application, especially in the carbon coating layer, that is, the degree of regularity of the arrangement of carbon atoms in the graphite structure.

[0059] Peak intensity I of D peak of positive electrode active material d Peak intensity I of G peak g The ratio I d / I g The lower the value, the higher the degree of graphitization of the positive electrode active material, meaning its structure is closer to the ideal graphite structure, its surface is smoother and more non-polar, and the overall hydrophobicity of the material is enhanced. Positive electrode active material I d / I g Within a suitable range, the water content of the positive electrode active material can be reduced, thereby reducing the water content in the positive electrode slurry. This improves the stability of the positive electrode slurry, makes the coating process more uniform, optimizes drying efficiency, and improves the processing performance of the electrode sheet. Furthermore, as the graphitization degree of lithium iron phosphate materials increases, their conductivity also improves. During charge and discharge, the positive electrode active material can improve the diffusion of lithium ions between graphitized layers, reduce overvoltage caused by insufficient conductivity in lithium iron phosphate materials and the increased lithium ion dissolution due to overvoltage, and reduce the deformation and expansion of the positive electrode active material during cycling. The enhanced conductivity also helps reduce the interfacial impedance of the positive electrode active material, lowers electronic resistance, and improves the internal resistance of the battery.

[0060] Carboxylic acid esters and / or fluorinated ethers in the electrolyte can reduce the viscosity of the electrolyte, improve its ion-conducting properties, and reduce the transport resistance of lithium ions in the electrolyte. However, these carboxylic acid esters and / or fluorinated ethers have low stability in acidic environments and are prone to hydrolysis and gas production. Controlling the positive electrode active material I... d / I g Maintaining the appropriate value helps reduce the amount of water entering the electrolyte from the positive electrode active material, lowers the HF content in the electrolyte due to the hydrolysis of electrolyte salts (e.g., lithium salts), thereby reducing the hydrolysis and gas production of carboxylic acid esters and / or fluorinated ethers, improving battery expansion and cycle life. Controlling the content of carboxylic acid esters and / or fluorinated ethers in the electrolyte helps improve the internal resistance of the secondary battery and control volume expansion, thus increasing energy efficiency.

[0061] The secondary battery provided in this application can effectively improve energy efficiency and control volume expansion, thereby increasing the cycle life of the secondary battery.

[0062] I d / I g The measurement method can use any method known in the art, for example, referring to GB / T 40219-2021, the positive electrode active material powder is pressed into a tablet, and the tablet is tested using a LabRAM HR Evolution laser micro-Raman spectrometer. Three points on the tablet are randomly selected for testing, and the average of three sets of measurement values is obtained. Among them, a solid laser with a wavelength of 523 nm is used as a light source, the beam diameter is 1.2 μm, the power is 1 mW, the measurement mode uses macro-Raman, and a CCD detector is used.

[0063] In some embodiments, the I d / I g of the positive electrode active material is 0.9-1.2. d / I g In some embodiments, the I d / I g of the positive electrode active material is 1.1-1.2. d / I g In some embodiments, the I d / I g of the positive electrode active material is 0.9-1.0.

[0064] In some embodiments, the I d / I g of the positive electrode active material is 0.2, 0.4, 0.6, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or any value between or within the range of any two of the above values.

[0065] Suitable I d / I g values help to obtain a positive electrode active material with improved graphitization degree, further improve the electronic conductivity of the lithium iron phosphate-based material, improve the structural stability and hydrophobicity of the positive electrode active material, reduce the internal resistance of the battery, reduce the water content of the battery cell, improve the energy efficiency of the secondary battery and control the volume expansion of the battery, and improve the cycle performance.

[0066] In some embodiments, the lithium iron phosphate-based material comprises Li x Fe y M z PO4, wherein 0.95 < x < 1.05, 0.96 < y < 1, 0 < z < 0.04, and M comprises one or more of Nb, Ti, V, W, and Mn.

[0067] In some embodiments, the M element is selected from Mn, Nb, Ti, V, or W.

[0068] In some embodiments, the mass percentage of the M element in the positive electrode active material is 0.03%-0.3%. In some embodiments, the mass percentage of the M element in the positive electrode active material is 0.05%-0.3%, 0.05%-0.2%, 0.1%-0.3%, or 0.15%-0.3%.

[0069] In some embodiments, the mass percentage of the M element in the positive electrode active material is 0.03%, 0.05%, 0.08%, 0.10%, 0.12%, 0.14%, 0.16%, 0.18%, 0.20%, 0.22%, 0.24%, 0.26%, 0.28%, 0.3%, or any value within a range between any two of the above values.

[0070] The conductivity of lithium iron phosphate is relatively low. Doping metal elements can increase the concentration of free electrons in the lithium iron phosphate lattice, reduce the transport barrier of lithium ions, improve the diffusion rate of lithium ions, and improve the rate performance of the battery. It can also effectively improve the structural stability of the positive electrode active material, inhibit the swelling and deformation of the positive electrode active material, and make the secondary battery have good kinetic performance and service life.

[0071] The positive electrode active material can be prepared by methods commonly used in the art. In some embodiments, the method for preparing the positive electrode active material comprises the following steps: mixing a lithium source, an iron source, a phosphorus source, an M source, and a carbon source in a certain proportion, sintering, thereby obtaining a positive electrode active material comprising a lithium iron phosphate material core and a carbon shell layer at least partially coated on the outer surface of the core.

[0072] In some embodiments, the lithium source comprises one or more of Li2CO3, LiH2PO4, and Li3PO4.

[0073] In some embodiments, the iron source comprises one or more of FeSO4, FePO4, FeCl2, FeC2O4, and Fe2O3.

[0074] In some embodiments, the phosphorus source comprises one or more of NH4H2PO4 and H3PO4.

[0075] In some embodiments, the M source comprises one or more of metal salts, metal oxides, and metal organic compounds containing Nb, Ti, V, W, and Mn elements.

[0076] In some embodiments, the carbon source comprises one or more of glucose, sucrose, starch, polyethylene glycol, phenol formaldehyde, carbon black, graphite, carbon nanotube, tannic acid, sodium dodecyl benzene sulfonate, alkyl glucoside, polyaniline, fructose, disodium ethylenediaminetetraacetate, polyvinylpyrrolidone, lecithin.

[0077] In some embodiments, the sintering temperature is 500-900 °C, for example 600 °C; and the sintering time at the sintering temperature is 8-20 h, for example 16 h.

[0078] In some embodiments, the sintering temperature is 500 °C, 600 °C, 650 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C, or a range between or any value within any two of the above.

[0079] In some embodiments, the sintering time is 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, 18 h, 19 h, 20 h, or a range between or any value within any two of the above.

[0080] In some embodiments, the positive current collector can employ a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0081] In some embodiments, the positive electrode film layer can further optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin.

[0082] In some embodiments, the positive electrode film layer can further optionally include a conductive agent. As an example, the conductive agent can include at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0083] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, in a solvent (for example, N-methyl pyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and then drying, cold-pressing and the like to obtain the positive electrode sheet.

[0084] In some embodiments, the mass content of the carboxylic acid ester compound is 5%-80% based on the total mass of the electrolyte. In some embodiments, the mass content of the carboxylic acid ester compound is 10%-80%, 15%-80%, 30%-80% or 5%-70% based on the total mass of the electrolyte.

[0085] In some embodiments, the mass content of the carboxylic acid ester compound is 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80% or any value or range between any two of the above values based on the total mass of the electrolyte.

[0086] With the content of the carboxylic acid ester compound in the above range, the electrolyte has excellent viscosity, can significantly improve the migration performance of lithium ions in the electrolyte, reduce the transmission resistance of lithium ions in the electrolyte; and can also control the degree of gas production by hydrolysis of the carboxylic acid ester compound, reduce the degree of volume expansion of the secondary battery, and make the secondary battery have excellent energy efficiency and cycle performance.

[0087] In some embodiments, the mass content of the fluorinated ether compound is 5%-30% based on the total mass of the electrolyte. In some embodiments, the mass content of the fluorinated ether compound is 5%-10% based on the total mass of the electrolyte.

[0088] In some embodiments, the mass content of the fluorinated ether compound is 1%, 5%, 10%, 15%, 20%, 25%, 30% or any value or range between any two of the above values based on the total mass of the electrolyte.

[0089] The fluorinated ether compound has the characteristics of low viscosity and high ionic conductivity, but is prone to side reactions with electrode materials or electrolyte salts to produce gas or solid byproducts, which can cover the electrode surface, affect the migration of lithium ions in the active material and electrolyte, and increase the internal resistance of the battery. The mass content of the fluorinated ether compound in the electrolyte in a suitable range can improve the migration performance of lithium ions, reduce the ionic resistance and improve the energy efficiency; and can also reduce the gas production side reaction, delay the volume expansion of the secondary battery, and improve the cycle performance and service life.

[0090] The carboxylic acid ester compound and the fluoroether compound can be used in combination to reduce the viscosity of the electrolyte and improve the ion conductivity. In some embodiments, the mass ratio of the carboxylic acid ester compound and the fluoroether compound is (1-10): 1, for example, (1-8): 1, (1-6): 1, or (3-6): 1.

[0091] In some embodiments, the carboxylic acid ester compound has a structure shown in Formula I,

[0092] wherein R is selected from methyl, ethyl, n-propyl, 2-fluoroethyl, 2,2-difluoroethyl, 2,2,2-trifluoroethyl; and R1is selected from methyl, ethyl, n-propyl, tert-pentanoic acid, 2,2-difluoroethyl, 2,2,2-trifluoroethyl.

[0093] In some embodiments, the carboxylic acid ester compound includes ethyl acetate, methyl acetate, methyl formate, ethyl propionate, 2,2-difluoroethyl acetate propyl acetate, methyl propionate, propyl propionate, methyl tert-pentanoate, 2,2-difluoroethyl propionate 2-fluoroethyl acetate 2,2,2-trifluoroethyl acetate 2,2,2-trifluoroethyl propionate methyl 3,3-difluoropropionate ethyl 3,3,3-trifluoropropionate 2,2,2-trifluoroethyl 3,3,3-trifluoropropionate methyl 3,3,3-trifluoropropionate ethyl 3,3-difluoropropionate one or more of the above.

[0094] The carboxylic acid ester compound has a short hydrocarbon chain length, a small molecular weight, and a relatively weak intermolecular force, so the viscosity of the compound is low. When the carboxylic acid ester compound contains fluorine atoms, the small size and high repulsive force of the fluorine atoms help to reduce the close contact between molecules, making the compound have a lower viscosity. The ester group and fluorine atom in the carboxylic acid ester compound increase the polarity of the molecule, which is beneficial to the ion transport performance.

[0095] In some embodiments, the fluoroether compound has a structure shown in Formula II or Formula III:

[0096] R2-O-R3Formula II, R2-O-R4-O-R3Formula III

[0097] wherein R2and R3are each independently selected from C(a)H(b)F(2a-b+1)-.

[0098] R4is selected from -C(a')H(b')F(c')-, wherein:

[0099] a and b are each an integer from 1 to 5, 2a-b+1≥0,

[0100] a' is an integer from 1 to 5; b' is an integer from 0 to 10; c' is an integer from 0 to 10; b'+c'=2a'.

[0101] In some embodiments, a is an integer from 1 to 3, as examples, a can be 1, 2, 3.

[0102] In some embodiments, b is an integer from 1 to 3, as examples, b can be 1, 2, 3.

[0103] In some embodiments, a' is an integer from 1 to 3, as examples, a' can be 1, 2, 3.

[0104] In some embodiments, b' is an integer from 0 to 6, as examples, b' can be 0, 1, 2, 3, 4, 5, 6.

[0105] In some embodiments, c' is an integer from 0 to 6, as examples, c' can be 0, 1, 2, 3, 4, 5, 6.

[0106] In some embodiments, R2and R3are the same or different.

[0107] In some embodiments, the fluoroether compound has a structure according to Formula II or Formula III:

[0108] R2-O-R3Formula II, R2-O-R4-O-R3Formula III

[0109] wherein R2and R3are each independently selected from C(a)H(b)F(2a-b+1)-;

[0110] R4is selected from -C(a')H(b')F(c')-, wherein:

[0111] a and b are each an integer from 1 to 3, 2a-b+1≥0,

[0112] a' is an integer from 1 to 3; b' is an integer from 0 to 6; c' is an integer from 0 to 6; b'+c'=2a'.

[0113] In some embodiments, the fluoroether compound includes at least one of the following compounds:

[0114] Shorter hydrocarbon chains are advantageous to reduce the viscosity of the fluorinated ether compound, and the oxygen and / or fluorine atoms in the molecule can increase the polarity of the molecule, improve ion transport, and increase the electrolyte conductivity.

[0115] In some embodiments, the electrolyte includes a compound represented by Formula II-3. In some embodiments, the electrolyte includes a compound represented by Formula II-4. In some embodiments, the electrolyte includes ethyl acetate and 2,2-difluoroethyl acetate. In some embodiments, the electrolyte includes ethyl acetate and a compound of Formula II-1 (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether). In some embodiments, the electrolyte includes ethyl acetate and a compound of Formula II-2.

[0116] In some embodiments, the electrolyte further includes an electrolyte salt. In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bistrifluoromethylsulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobisoxalate borate, lithium bisoxalate borate, lithium difluorobisoxalate phosphate, and lithium tetrafluorobisoxalate phosphate.

[0117] In some embodiments, the electrolyte can optionally further include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain properties of the battery, such as an additive capable of improving overcharge performance of the battery, an additive capable of improving high-temperature or low-temperature performance of the battery, and the like.

[0118] In some embodiments, the negative electrode sheet includes a current collector and a negative electrode film layer coated on at least one side of the current collector, the negative electrode film layer including a first negative electrode film layer proximate to the current collector and a second negative electrode film layer distal to the current collector, the second negative electrode film layer having a porosity greater than the porosity of the first negative electrode film layer.

[0119] In some embodiments, the negative electrode film layer includes a first negative electrode film layer connected to the current collector and a second negative electrode film layer distal to the current collector, the second negative electrode film layer having a porosity greater than the porosity of the first negative electrode film layer.

[0120] The first negative electrode film layer with low porosity can carry more active materials, so that lithium ions have enough active sites for insertion and extraction, which is beneficial to improve the capacity of the secondary battery and improve the cycle performance. The second negative electrode film layer with high porosity can make the negative electrode active material better infiltrate the electrolyte, reduce the lithium ion migration resistance, and reduce the battery internal resistance. The structure of the negative electrode film layer with "lower porosity on the bottom and higher porosity on the top" can well adapt to the volume expansion and shrinkage of the active material during the lithium ion insertion and extraction process, reduce the stress and crack formation of the active material particles, and reduce the side reaction of the active material and the electrolyte. The negative electrode film layer by adjusting the porosity of different regions helps to improve the internal resistance of the secondary battery, improve the cycle life, and slow down the capacity decay of the battery.

[0121] In some embodiments, the porosity of the first negative electrode film layer is 10%-30%. In some embodiments, the porosity of the first negative electrode film layer is 20%-25%. As an example, the porosity of the first negative electrode film layer is 10%, 15%, 20%, 25%, 30%, or any value between any two of the above values or within a range. Adjusting the porosity of the first negative electrode film layer can make the lithium ions have good migration performance, and also increase the carrying capacity of the active material.

[0122] In some embodiments, the porosity of the second negative electrode film layer is 20%-40%, and in some embodiments, the porosity of the second negative electrode film layer is 30%-35%. As an example, the porosity of the second negative electrode film layer is 20%, 25%, 30%, 35%, 40%, or any value between any two of the above values or within a range. Adjusting the porosity of the second negative electrode film layer can make the negative electrode film layer be effectively infiltrated by the electrolyte, thereby improving the transmission efficiency of lithium ions inside the negative electrode film layer and reducing the lithium ion migration resistance, reducing the battery internal resistance and improving the energy efficiency, and making the secondary battery have excellent cycle performance.

[0123] The test method for the porosity of the negative electrode film layer can use any known method, as an example: take the cross section of the negative electrode film layer, and use fault SEM (Scanning Electron Microscope) to observe the double-layer coating structure of the negative electrode film layer. Randomly take a cross-section SEM picture under a certain field of view, so that it contains the complete first negative electrode film layer and the second negative electrode film layer. Measure the total area S1 of the first negative electrode film layer and the total area R1 occupied by the first active material in the first negative electrode film layer, then define the porosity of the first active layer = R1 / S1 x 100%; measure the total area S2 of the second negative electrode film layer, and the total area R2 occupied by the second active material in the second negative electrode film layer, then define the porosity of the second active layer = R2 / S2 x 100%.

[0124] The thickness of the negative electrode film layer is H. In some embodiments, the thickness of the first negative electrode film layer is 0.4H-0.6H, for example, the thickness of the first negative electrode film layer can be 0.4H-0.55H, 0.5H-0.6H, 0.45H-0.55H.

[0125] In some embodiments, the thickness of the first negative electrode film layer is 0.4H, 0.45H, 0.5H, 0.55H, 0.6H, or any value within a range between any two of the above values.

[0126] In some embodiments, the thickness of the second negative electrode film layer is 0.4H-0.6H, for example, the thickness of the second negative electrode film layer can be 0.4H-0.55H, 0.5H-0.6H, 0.45H-0.55H.

[0127] In some embodiments, the thickness of the second negative electrode film layer is 0.4H, 0.45H, 0.5H, 0.55H, 0.6H, or any value within a range between any two of the above values.

[0128] In some embodiments, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 0.6-1.5, for example, 0.8-1.2. In some embodiments, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or any value within a range between any two of the above values.

[0129] Adjusting the thickness of the first negative electrode film layer and the second negative electrode film layer and the ratio of the two is conducive to the full infiltration of the negative electrode film layer, and helps to adjust the diffusion path of lithium ions in the electrode sheet, accelerate the embedding and extraction speed of lithium ions, reduce ion migration resistance, and improve battery energy efficiency.

[0130] In some embodiments, the thickness of the first negative electrode film layer is 50μm-90μm, in some embodiments, the thickness of the first negative electrode film layer is 65μm-80μm. In some embodiments, the thickness of the first negative electrode film layer is 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, or any value within a range between any two of the above values.

[0131] In some embodiments, the second negative electrode film layer has a thickness of 50 pm to 90 pm, and in some embodiments, the second negative electrode film layer has a thickness of 65 pm to 80 pm. In some embodiments, the second negative electrode film layer has a thickness of 50 pm, 55 pm, 60 pm, 65 pm, 70 pm, 75 pm, 80 pm, or any value within a range between any two of the above values.

[0132] A thicker negative electrode film layer can carry more active material, which can theoretically improve the capacity and cycle performance of the battery. However, a film layer that is too thick can increase the distance of lithium ion and electron transport and affect the electrolyte wetting of the electrode sheet, increase the internal resistance of the battery, reduce the energy efficiency, and affect the rate capability and cycle performance. Therefore, controlling the thickness of the negative electrode film layer within a suitable range helps to improve the capacity of the battery, and to adjust the diffusion path of lithium ions in the electrode sheet within an acceptable range, which is conducive to the embedding and de-embedding of lithium ions, reduces the ion migration resistance, improves the capacity and energy efficiency of the secondary battery, and further improves the cycle performance of the secondary battery. The thickness of the first negative electrode film layer and the second negative electrode film layer within a suitable range can also improve the mechanical properties of the electrode sheet, which can withstand the stress of the active material volume change on the electrode sheet during charging and discharging, help to reduce film layer cracking and peeling, and improve the safety of the battery.

[0133] The thickness of the negative electrode film layer can be tested using devices and methods known in the art. Illustratively, taking the negative electrode current collector having a negative electrode film layer coated on one surface as an example, under the full discharge state of the secondary battery, the thickness of the negative electrode sheet can be measured at at least 12 different positions along the thickness direction of the negative electrode sheet using a micrometer, and then the average value is taken as the thickness of the negative electrode sheet. Then, the thickness of the negative electrode sheet minus the thickness of the negative electrode current collector is the thickness of the negative electrode film layer.

[0134] The first negative electrode film layer and the second negative electrode film layer comprise a negative electrode active material. In some embodiments, the negative electrode active material in the first negative electrode film layer and the second negative electrode film layer is the same or different.

[0135] In some embodiments, the negative electrode active material includes, but is not limited to, one or more of conventional natural graphite, other artificial graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate. The silicon-based material can include one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy material. The tin-based material can include one or more of elemental tin, tin oxide, and tin alloy material. However, the application is not limited to these materials, and other conventional materials that can be used as battery negative electrode active materials can also be used. These negative electrode active materials can be used alone or in combination with two or more.

[0136] In some embodiments, the negative current collector can be a metal foil or a composite current collector. For example, as a metal foil, a copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer (e.g., a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0137] In some embodiments, the first negative electrode film layer and the second negative electrode film layer can further optionally include a binder. The binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0138] In some embodiments, the first negative electrode film layer and the second negative electrode film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0139] In some embodiments, the first negative electrode film layer and the second negative electrode film layer can further optionally include other additives, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0140] In some embodiments, the negative electrode sheet can be prepared by first dispersing a first negative electrode active material, a conductive agent, a binder, and any other components in a solvent (e.g., deionized water) to form a first negative electrode slurry; coating the first negative electrode slurry on a negative electrode current collector, and forming a first film layer after drying, cold pressing, and the like; then dispersing a second negative electrode active material, a conductive agent, a binder, and any other components in a solvent (e.g., deionized water) to form a second negative electrode slurry; coating the second negative electrode slurry on the negative electrode current collector, and forming a second film layer after drying, cold pressing, and the like, to obtain the negative electrode sheet.

[0141] In addition, the negative electrode sheet having a multi-layer coating structure helps to reduce the amount of binder, reduce the negative electrode interface resistance, and also helps to improve the uniformity of the distribution of the binder in the electrode sheet and the uniformity of the distribution of the current density, thereby reducing the electronic resistance, improving the internal resistance of the battery, and improving the energy efficiency.

[0142] In some embodiments, the secondary battery further includes a separator. The type of the separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0143] In some embodiments, the material of the separator film can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator film can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator film is a multi-layer composite film, the materials of the layers can be the same or different, and are not particularly limited.

[0144] In some embodiments, the positive electrode tab, the negative electrode tab, and the separator film can be used to form an electrode assembly by a winding process or a stacking process.

[0145] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0146] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, or the like can be listed.

[0147] The shape of the secondary battery is not particularly limited, and can be cylindrical, square, or any other shape. In some embodiments, the secondary battery is square. For example, FIG. 1 is a secondary battery 5 of a square structure as an example.

[0148] In some embodiments, referring to FIG. 2, the outer package can include a shell 51 and a cover plate 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be arranged on the opening to close the receiving cavity. The positive electrode tab, the negative electrode tab, and the separator film can be used to form an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and can be selected by a person skilled in the art according to specific actual needs.

[0149] In some embodiments, as shown in FIG. 1, the length of the secondary battery is L, the height is H, and the thickness is T, wherein 300 mm≤L≤650 mm, 180 mm≤H≤300 mm, 50 mm≤T≤100 mm, 1≤L / H≤3, and 2≤H / T≤4.

[0150] “L / H” represents the ratio of the length of the secondary battery to the height. “H / T” represents the ratio of the height of the secondary battery to the thickness.

[0151] In some embodiments, the length L of the secondary battery is 300 mm, 350 mm, 400 mm, 450 mm, 500 mm, 550 mm, 600 mm, 650 mm, or any value within a range between any two of the above values.

[0152] In some embodiments, the height H of the secondary battery is 180 mm, 200 mm, 220 mm, 240 mm, 260 mm, 280 mm, 300 mm, or any value within a range between any two of the above values.

[0153] In some embodiments, the thickness T of the secondary battery is 50 mm, 60 mm, 70 mm, 80 mm, 90 mm, 100 mm, or any value within a range between any two of the above values.

[0154] In some embodiments, the L / H of the secondary battery is 1, 2, 3, or any value within a range between any two of the above values.

[0155] In some embodiments, the H / T of the secondary battery is 2, 3, 4, or any value within a range between any two of the above values.

[0156] With the increase of the size of the battery assembly, the transmission path of the electrons in the electrode tab also increases, which can further deteriorate the non-uniformity of the current density in the electrode tab. Controlling the size of the secondary battery monomer assembly within a certain range can improve the energy efficiency while increasing the capacity of the secondary battery. By adjusting the size of the battery assembly, the electron transmission path in the electrode tab can be adjusted, the difference in electron transmission path in different parts of the electrode tab can be reduced, the non-uniformity of the current density in the electrode tab of the large-size battery can be improved, and the electron transmission resistance can be reduced and the battery energy efficiency can be improved. In addition, the size of the secondary battery also has industrial manufacturability.

[0157] In some embodiments, the monomer cell capacity of the secondary battery is 400 Ah-1300 Ah, for example, 500 Ah-1300 Ah, 600 Ah-1300 Ah.

[0158] In some embodiments, the monomer cell capacity of the secondary battery is 400 Ah, 500 Ah, 600 Ah, 700 Ah, 800 Ah, 900 Ah, 1000 Ah, 1100 Ah, 1200 Ah, 1300 Ah, or any value within a range between any two of the above values.

[0159] In this document, the term "monomer cell capacity" refers to the maximum amount of charge that can be stored in a single battery unit (cell).

[0160] The monomer cell capacity can be tested using devices and methods known in the art. Illustratively, the prepared secondary battery is cycled at a cycle rate of 0.25P (i.e., both the charge rate and the discharge rate are 0.25P), and the charging voltage is 2.5V to 3.65V, which is one charge-discharge process. The value of the cycled discharge capacity is recorded as the monomer cell capacity of the secondary battery.

[0161] The secondary battery provided herein can be applied to grid-side energy storage (e.g., large energy storage devices) and user-side energy storage (e.g., industrial and commercial energy storage devices, household energy storage devices). As an example, the secondary battery can be used as a light storage charging station, a micro-grid, a backup power station for a factory, a shopping mall, a hospital, or a school.

[0162] The secondary battery with a large monomer cell capacity can store more electricity, which helps to improve the cycle performance and service life of the secondary battery, and reduces the cost of the power station, and can balance the capacity, performance, manufacturing cost, and use cost of the secondary battery.

[0163] In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0164] FIG. 3 is a battery module 4 as an example. Referring to FIG. 3, in the battery module 4, a plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, other arbitrary arrangements can also be used. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0165] Optionally, the battery module 4 can further include a housing having an accommodation space, and the plurality of secondary batteries 5 are accommodated in the accommodation space.

[0166] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0167] FIGS. 4 and 5 are a battery pack 1 as an example. Referring to FIGS. 4 and 5, the battery pack 1 can include a battery box and a plurality of battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be arranged on the lower box body 3 to form an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0168] The secondary battery provided by the application can also be used as an energy storage device for an electric device, and the electric device comprises at least one of the secondary battery, the battery module or the battery pack provided by the application. The secondary battery, the battery module or the battery pack can be used as a power supply of the electric device, or can be used as an energy storage unit of the electric device. The electric device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0169] As the electric device, the secondary battery, the battery module or the battery pack can be selected according to the use requirement of the electric device.

[0170] FIG. 6 is an electric device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirement of high power and high capacity of the secondary battery for the electric device, the battery pack or the battery module can be used.

[0171] As another example of the device, the device can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinness, and the secondary battery can be used as a power supply.

[0172] Embodiment

[0173] Hereinafter, the embodiments of the application are described. The embodiments described below are exemplary and are only used to explain the application, and cannot be understood as a limitation of the application. If the specific technology or condition is not indicated in the embodiments, the technology or condition described in the literature in the art or according to the product instruction is used. If the reagent or instrument is not indicated by the manufacturer, it is a conventional product that can be obtained by market purchase.

[0174] I. Preparation of the positive active material

[0175] Preparation Example 1

[0176] 0.1 mol of lithium carbonate, 0.2 mol of ferrous oxalate and 0.2 mol of ammonium dihydrogen phosphate powders were uniformly dispersed in 50 ml of anhydrous ethanol, ball-milled at a rotation number of 350 r / min for 10 h, and the ball-milled powders were dried to obtain a mixed precursor powder. 0.25 mol of tannic acid was dispersed in 20 ml of water, 0.001 mol of sodium dodecylbenzenesulfonate and 0.01 mol of ferrocene were sequentially added, and the mixture was stirred at 80°C for 7 h. The carbon source was collected by centrifugal washing. Then, the carbon source and the precursor powder obtained above were uniformly mixed, and were placed in a sintering furnace. Inert gas was introduced, and the temperature was increased to 600°C at a rate of 2°C / min, and was kept for 16 h to obtain a lithium iron phosphate positive material. It was determined that the lithium iron phosphate positive material had an Id / I g was 1.01.

[0177] Preparation Example 2

[0178] The 0.1 mol lithium carbonate, 0.2 mol ferrous oxalate and 0.2 mol ammonium dihydrogen phosphate powders were uniformly dispersed in 50 ml of anhydrous ethanol, ball-milled at 350 r / min for 10 h, and the ball-milled powders were dried to obtain mixed precursor powders. 0.6 mol of polyaniline was dispersed in 30 ml of water, 0.008 mol of alkyl glucoside and 0.04 mol of ferric citrate were sequentially added, and the mixture was stirred at 100°C for 7 h. The carbon source was collected by centrifugal washing. Then the carbon source was uniformly mixed with the above-mentioned precursor powders, and was placed in a sintering furnace, an inert atmosphere was introduced, and the temperature was raised to 600°C at a rate of 2°C / min, and the temperature was maintained for 16 h to obtain lithium iron phosphate positive electrode material. It was determined that the Id / Ig of the lithium iron phosphate positive electrode material was 0.91. d / I g was 0.68.

[0179] Preparation Example 3

[0180] The 0.1 mol lithium carbonate, 0.2 mol ferrous oxalate and 0.2 mol ammonium dihydrogen phosphate powders were uniformly dispersed in 50 ml of anhydrous ethanol, ball-milled at 350 r / min for 10 h, and the ball-milled powders were dried to obtain mixed precursor powders. 1 mol of fructose was dispersed in 50 ml of water, 0.05 mol of disodium ethylenediaminetetraacetate and 0.1 mol of zinc lactate were sequentially added, and the mixture was stirred at 60°C for 10 h. The carbon source was collected by centrifugal washing. Then the carbon source was uniformly mixed with the above-mentioned precursor powders, and was placed in a sintering furnace, an inert atmosphere was introduced, and the temperature was raised to 600°C at a rate of 2°C / min, and the temperature was maintained for 16 h to obtain lithium iron phosphate positive electrode material. It was determined that the Id / Ig of the lithium iron phosphate positive electrode material was 0.91.

[0181] Preparation Example 4

[0182] The 0.1 mol lithium carbonate, 0.2 mol ferrous oxalate and 0.2 mol ammonium dihydrogen phosphate powders were uniformly dispersed in 50 ml of anhydrous ethanol, ball-milled at 350 r / min for 10 h, and the ball-milled powders were dried to obtain mixed precursor powders. 0.6 mol of polyaniline was dispersed in 30 ml of water, 0.008 mol of alkyl glucoside and 0.04 mol of ferric citrate were sequentially added, and the mixture was stirred at 100°C for 7 h. The carbon source was collected by centrifugal washing. Then the carbon source was uniformly mixed with the above-mentioned precursor powders, and was placed in a sintering furnace, an inert atmosphere was introduced, and the temperature was raised to 600°C at a rate of 2°C / min, and the temperature was maintained for 16 h to obtain lithium iron phosphate positive electrode material. It was determined that the Id / Ig of the lithium iron phosphate positive electrode material was 0.91. d / Ig 1.15.

[0183] Preparation Example 5

[0184] 0.1 mol of lithium carbonate, 0.2 mol of ferrous oxalate and 0.2 mol of ammonium dihydrogen phosphate were uniformly dispersed in 50 ml of anhydrous ethanol, ball-milled at a rotation speed of 350 r / min for 10 h, and the ball-milled powder was dried to obtain a mixed precursor powder. 0.2 mol of polyvinylpyrrolidone was dispersed in 30 ml of water, 0.01 mol of lecithin and 0.03 mol of nickel formate were sequentially added, and stirring reaction was performed at 80°C for 8 h. The carbon source was collected by centrifugal washing. Then, the carbon source was uniformly mixed with the above-obtained precursor powder, and was placed in a sintering furnace. An inert atmosphere was introduced, and the temperature was increased to 600°C at a rate of 2°C / min, and then was maintained for 16 h to obtain a lithium iron phosphate positive electrode material. It was determined that the Id / Ig of the lithium iron phosphate positive electrode material was 1.45. d g 1.45.

[0185] Preparation Example 6

[0186] 0.1 mol of lithium carbonate, 0.2 mol of ferrous oxalate and 0.2 mol of ammonium dihydrogen phosphate were uniformly dispersed in 50 ml of anhydrous ethanol, ball-milled at a rotation speed of 350 r / min for 10 h, and the ball-milled powder was dried to obtain a mixed precursor powder. 0.25 mol of tannic acid was dispersed in 20 ml of water, 0.001 mol of sodium dodecylbenzenesulfonate and 0.01 mol of ferrocene were sequentially added, and stirring reaction was performed at 80°C for 7 h. The carbon source was collected by centrifugal washing. Then, the carbon source was uniformly mixed with the above-obtained precursor powder, and was placed in a sintering furnace. An inert atmosphere was introduced, and the temperature was increased to 450°C at a rate of 2°C / min, and then was maintained for 16 h to obtain a lithium iron phosphate positive electrode material. It was determined that the Id / Ig of the lithium iron phosphate positive electrode material was 1.7.

[0187] Preparation of the secondary battery

[0188] Example 1

[0189] 1) Preparation of the positive electrode sheet

[0190] The positive electrode active material prepared in Preparation Example 1, the conductive agent Super P and the binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1:2, a solvent N-methylpyrrolidone was added, and stirring was performed to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated on a positive electrode current collector aluminum foil, and was subjected to drying, cold pressing and slitting to obtain a positive electrode sheet. The single-sided coating amount of the positive electrode film layer was 21.7 mg / cm 2 .

[0191] 2) Preparation of the negative electrode sheet

[0192] ​The graphite, conductive agent Super-P, dispersant sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were added into deionized water in a mass ratio of 96.4:0.4:1.0:2.2, and mixed by stirring to obtain a negative electrode slurry A1; the graphite, conductive agent Super-P, dispersant sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were added into deionized water in a mass ratio of 96.0:0.7:1.1:2.2, and mixed by stirring to obtain a negative electrode slurry A2. The negative electrode slurry A1 was coated on a negative electrode current collector copper foil, and the single-sided coating amount was 4.85 mg / cm 2 ; the negative electrode slurry A2 was coated on the A1, and the single-sided coating amount was 4.85 mg / cm 2 ; after drying, cold pressing, tab die-cutting, and slitting, a double-coated negative electrode sheet was obtained. The negative electrode slurry A1 formed a first negative electrode film layer with a porosity of 28% and a thickness of 75 μm; the negative electrode slurry A2 formed a second negative electrode film layer with a porosity of 36% and a thickness of 75 μm.

[0193] 3) Preparation of electrolyte

[0194] The 2,2-difluoroethyl acetate, ethylene carbonate (EC), and methyl ethyl carbonate (EMC) were mixed in an argon atmosphere with a water content of <10 ppm in a mass ratio of 3:3:4 to obtain an organic solvent, and then LiPF6 was dissolved in the organic solvent at a concentration of 1.0 mol / L to prepare an electrolyte.

[0195] 4) Separation film

[0196] A polypropylene film was selected as the separation film.

[0197] 5) Preparation of secondary battery

[0198] The above positive electrode sheet, separation film, and negative electrode sheet were stacked in order, with the separation film between the positive and negative electrode sheets to play a separation role, and then wound to obtain an electrode assembly; the electrode assembly was placed in an outer package, and after drying, the electrolyte was injected, and after vacuum packaging, standing, formation, and shaping processes, a lithium ion battery with a capacity of 620 Ah was obtained. The battery size was 350 mm x 210 mm x 70 mm, the length / high ratio was 1.7, and the height / width ratio was 3.0.

[0199] Examples 2-18

[0200] Example 2-8 was similar to the preparation steps of Example 1, but the types and proportions of the electrolyte solvents were adjusted.

[0201] Examples 9-12 were similar to the preparation steps of Example 1, but the positive electrode active material of Preparation Example 2-5 was used.

[0202] Example 13-16 were similar to the preparation procedure of Example 1, but the amount of 2,2-difluoroethyl acetate was adjusted.

[0203] Example 17 was similar to the preparation procedure of Example 1, but the capacity of the electrode was adjusted to 1200 Ah, the size of the battery was adjusted to 550 nm x 240 nm x 75 nm, the length / high ratio was 2.3, and the height / width ratio was 3.2.

[0204] Example 18 was similar to the preparation procedure of Example 1, but the capacity of the electrode was adjusted to 420 Ah, the size of the battery was adjusted to 300 nm x 200 nm x 60 nm, the length / high ratio was 1.5, and the height / width ratio was 3.3.

[0205] Comparative Example 1-3

[0206] Comparative Example 1 was similar to the preparation procedure of Example 1, but the graphitization degree I d / I g of the positive active material was 1.7, 2,2-difluoroethyl acetate was not added to the electrolyte, and the mass ratio of vinyl carbonate to ethyl methyl carbonate was 3:7.

[0207] Comparative Example 2 was similar to the preparation procedure of Example 1, but 2,2-difluoroethyl acetate was not added to the electrolyte, and the mass ratio of vinyl carbonate to ethyl methyl carbonate was 3:7.

[0208] Comparative Example 3 was similar to the preparation procedure of Example 2, but the graphitization degree I d / I g of the positive active material was 1.7.

[0209] The preparation parameters of the secondary batteries of Examples 1-18 and Comparative Examples 1-3 are shown in Table 1.

[0210] Table 1

[0211] The I d / I g values of the positive active materials prepared in Preparation Examples 1-6 were tested.

[0212] 1. Graphitization degree I d / I g of the positive active material

[0213] Referring to GB / T 40219-2021, the positive active material powder is pressed into a tablet, and the tablet is tested by a LabRAM HR Evolution laser micro-Raman spectrometer. Three points on the tablet are randomly selected for testing, and three sets of measurement values are obtained and averaged. Among them, a solid laser with a wavelength of 523 nm is used as a light source, the beam diameter is 1.2 μm, the power is 1 mW, the measurement mode is macro-Raman, and a CCD detector is used.

[0214] 2. Cell capacity test

[0215] At 25°C, the prepared secondary battery is tested at a cycle rate of 0.25P (i.e., the charge rate and discharge rate are both 0.25P), and the charge voltage is 2.5V to 3.65V. This is one charge and discharge process, and the value of the cycle discharge capacity is recorded as the cell capacity of the secondary battery.

[0216] 3. Cell cycle life and expansion force test method

[0217] The secondary cell is installed with a clamp and a pressure sensor, and the initial clamp force is 3000N. The pressure sensor monitors the external pressure of the cell surface (length x height). At 45°C, the cell is tested on a charge and discharge instrument for charge and discharge cycling at a cycle rate of 0.25P (i.e., the charge rate and discharge rate are both 0.25P), and the charge voltage is 2.5V to 3.65V. The capacity and maximum pressure value of each cycle during the cycle process are recorded. The maximum pressure value of the 1000th cycle is the expansion force (N) of the 1000th cycle.

[0218] The capacity retention rate of 45°C cycling is: the capacity retention rate after the 1000th cycle = (the discharge capacity after the 1000th cycle / the discharge capacity of the first cycle) x 100%.

[0219] 4. Test of battery energy efficiency

[0220] At 25°C and atmospheric pressure, the prepared secondary battery is discharged at 0.25P constant power to 2.5V, and then charged to 3.65V at 0.25P constant power. The charge energy E1 at this time is recorded, and the battery is left for 30 minutes. Then, the battery is discharged at 0.25P constant power to 2.5V, and the discharge energy E2 at this time is recorded. The energy efficiency of the cell at 0.25P rate = E2 / E1 x 100%.

[0221] III. Analysis of test results of each embodiment and comparative example

[0222] The secondary batteries prepared in the embodiments and comparative examples are tested using the above test methods, and the results are shown in Table 2 below:

[0223] Table 2

[0224] As can be seen from the comparison of Examples 1-18 and Comparative Examples 1-3, in the large capacity secondary battery, the I d / I g value of the positive electrode active material is 0.2-1.5, and the mass content of the carboxylate compound in the electrolyte is 1%-80% and / or the fluorinated ether compound accounts for 1%-30% of the total mass of the electrolyte, which can improve the energy efficiency of the secondary battery and is not less than 93%.

[0225] As can be seen from the comparison of Examples 1-18 and Comparative Example 1, the I d / I g value of the positive electrode active material is 0.2-1.5, and the mass content of the carboxylate compound in the electrolyte is 5%-80% and / or the fluorinated ether compound accounts for 5%-10% of the total mass of the electrolyte, which can significantly reduce the swelling force of the battery, significantly improve the energy efficiency and improve the cycle performance of the large capacity secondary battery, so that the cycle capacity retention rate of the secondary battery at 45°C is not less than 90%, the cycle swelling force at 45°C is not higher than 10000N, and the energy efficiency is not less than 94%.

[0226] As can be seen from the comparison of Examples 1-8 and Examples 13-18 and Comparative Example 2, the electrolyte contains 5%-80% carboxylate compound, 5%-10% fluorinated ether compound or a combination of the two, which can effectively reduce the swelling force of the battery and improve the energy efficiency of the secondary battery.

[0227] As can be seen from the comparison of Examples 9-12 and Comparative Example 3, the I d / I g value of the positive electrode active material is 0.2-1.5, which can significantly reduce the swelling force of the battery, improve the cycle capacity retention rate and energy efficiency of the secondary battery.

[0228] Examples 3-4 and Examples 7-8 use different types of carboxylate compounds for compounding, and as can be seen from the comparison with Comparative Example 2, the energy efficiency of the secondary battery can be significantly improved and the swelling force of the battery can be effectively reduced.

[0229] The fluorinated ether compound is used in the electrolyte of Examples 5-6, and as can be seen from the comparison with Comparative Example 2, the compound can significantly improve the energy efficiency of the secondary battery and effectively reduce the swelling force of the battery.

[0230] As can be seen from the comparison of Examples 1 and Examples 13-16 and Comparative Example 2, the use of 5%-80% carboxylate compound in the electrolyte helps to improve the energy efficiency of the secondary battery and effectively reduce the swelling force of the battery, and comprehensively improve the electrical performance of the secondary battery.

[0231] The capacity of the secondary batteries prepared in Examples 17-18 was 1200 Ah and 420 Ah, indicating that by proper matching of the positive active material and electrolyte, the battery expansion can be reduced, the energy efficiency can be improved, and the cycle performance can be improved, on the premise of increasing the battery capacity to 400 Ah-1200 Ah.

[0232] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration as the technical idea and exerting the same effects within the scope of the technical solution of the present application are all included in the technical scope of the present application. Furthermore, within the scope of the gist of the present application, various modifications that can be thought of by those skilled in the art, other modes constructed by combining part of the configuration elements of the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery characterized by comprising: The device includes an electrolyte and a positive electrode. The positive electrode includes a positive active material, which comprises a lithium iron phosphate core and a carbon shell at least partially covering the outer surface of the core. The Raman shift of the positive active material is within 1350 ± 50 cm⁻¹. -1 Peak intensity I of D peak within the range d With Raman displacement at 1580±50cm -1 Peak intensity I of G peak within the range g The ratio I d / I g The value is 0.2-1.

5. The electrolyte comprises 1%-80% of the carboxylic acid ester compound based on the total mass of the electrolyte and / or 1%-30% of the fluoroether compound based on the total mass of the electrolyte.

2. The secondary battery according to claim 1, characterized by I of the positive electrode active material is 0.6-1.2, optionally 0.9-1.

2. d / I g of the positive electrode active material is 0.6-1.2, optionally 0.9-1.

2.

3. The secondary battery according to claim 1 or 2, characterized by The mass content of the carboxylic acid ester compound is 5%-80%, or 5%-70%, based on the total mass of the electrolyte; or, The mass content of the fluoroether compound is 5%-30%, or 5%-10%, based on the total mass of the electrolyte.

4. The secondary battery according to any one of claims 1 to 3, characterized by, The carboxylate compound has a structure shown in Formula I, R is selected from methyl, ethyl, n-propyl, 2-fluoroethyl, 2,2-difluoroethyl, 2,2,2-trifluoroethyl; R1 is selected from methyl, ethyl, n-propyl, tert-pentanoic acid, 2,2-difluoroethyl, 2,2,2-trifluoroethyl; The fluoroether compound has a structure shown in Formula II or Formula III: R2-O-R3 Formula II; R2-O-R4-O-R3 Formula III R2 and R3 are each independently selected from C(a)H(b)F(2a-b+1)-; R4 is selected from -C(a')H(b')F(c')- a and b are each an integer of 1-5, or an integer of 1-3, 2a-b+1≥0, a' is an integer of 1-5, or an integer of 1-3; b' is an integer of 0-10, or an integer of 0-6; c' is an integer of 0-10, or an integer of 0-6; b'+c' = 2a'.

5. The secondary battery according to any one of claims 1 to 4, characterized by, The carboxylic acid ester compound comprises one or more of ethyl acetate, methyl acetate, methyl formate, ethyl propionate, 2,2-difluoroethyl acetate, propyl acetate, methyl propionate, propyl propionate, methyl tert-pentanoate, 2,2-difluoroethyl propionate, 2-fluoroethyl acetate, 2,2,2-trifluoroethyl acetate, 2,2,2-trifluoroethyl propionate, methyl 3,3-difluoropropionate, ethyl 3,3,3-trifluoropropionate, 2,2,2-trifluoroethyl 3,3,3-trifluoropropionate, methyl 3,3,3-trifluoropropionate, ethyl 3,3-difluoropropionate, and / or The fluoroether compound includes at least one of the following compounds:

6. The secondary battery according to any one of claims 1 to 5, characterized in that, The lithium iron phosphate-based material comprises a material represented by the following formula: Li x Fe y M z PO4, 0.95 < x < 1.05, 0.96 < y ≤ 1, 0 ≤ z < 0.04, M comprises one or more of Nb, Ti, V, W, and Mn, and the mass content of the M element in the positive electrode active material is 0.03%-0.3%.

7. The secondary battery according to any one of claims 1 to 6, characterized by The secondary battery further comprises a negative electrode tab, the negative electrode tab comprising a current collector and a negative electrode film layer coated on at least one side of the current collector, the negative electrode film layer comprising a first negative electrode film layer close to the current collector and a second negative electrode film layer away from the current collector, the porosity of the second negative electrode film layer being greater than the porosity of the first negative electrode film layer.

8. The secondary battery according to claim 7, characterized by The porosity of the first negative electrode film layer is 10%-30%, or the porosity of the second negative electrode film layer is 20%-40%.

9. The secondary battery according to claim 7 or 8, characterized by The thickness of the negative electrode film layer is H, the thickness of the first negative electrode film layer is 0.4H-0.6H, and the thickness of the second negative electrode film layer is 0.4H-0.6H; and / or the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 0.6-1.

5.

10. The secondary battery according to any one of claims 7 to 9, characterized by The thickness of the first negative electrode film layer is 50-90 microns, and / or the thickness of the second negative electrode film layer is 50-90 microns.

11. The secondary battery according to any one of claims 1 to 10, characterized by The length of the secondary battery is L, the height is H, and the thickness is T, wherein 300mm≤L≤650mm, 180mm≤H≤300mm, 50mm≤T≤100mm, 1≤L / H≤3, and 2≤H / T≤4.

12. The secondary battery according to any one of claims 1 to 11, characterized by The single cell capacity of the secondary battery is 400-1300 Ah.

13. An electrical device, comprising: The secondary battery of any one of claims 1-12.

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