Secondary battery and battery module, battery pack, and electric device including the secondary battery

Abstract

The application discloses a secondary battery, a battery module, a battery pack and a power consumption device containing the secondary battery. The secondary battery comprises a positive electrode sheet and an electrolyte. The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode film layer comprises one or more of lithium-containing phosphate with an olivine structure and modified compounds thereof. The electrolyte comprises an organic solvent, and the organic solvent comprises a first solvent shown in formula 1. R 11 is one of hydrogen, C1-C4 alkyl and C1-C4 haloalkyl, R 12 is one of C1-C4 alkyl and C1-C4 haloalkyl. The mass percentage of the first solvent is greater than or equal to 20% based on the mass of the electrolyte. The mass of the electrolyte / the rated capacity of the secondary battery is less than or equal to 3.6 g / Ah. The secondary battery provided by the application can maintain high energy density, and has good kinetic performance, high capacity retention rate and low volume expansion rate.

Description

[0001] This application is a divisional application of the patent application with the application number 202180006893.9 (international application number PCT / CN2021 / 109903), the application name of “Secondary battery and battery module, battery pack and power utilization device containing the secondary battery”, the application date of 2021.07.30. TECHNICAL FIELD

[0002] The application belongs to the technical field of secondary batteries, and particularly relates to a secondary battery and a battery module, a battery pack and a power utilization device containing the secondary battery. BACKGROUND

[0003] The secondary battery has the characteristics of good cycle performance, stable electrochemical performance and low price, and therefore occupies a large market share. With the increasing requirement of customers on the energy density of the battery, it is crucial to develop a battery with higher energy density and longer cycle life. At present, an effective way to improve the energy density of the battery is to improve the space utilization rate in the battery, for example, to improve the coating weight and the compaction density of the pole piece, but the disadvantage brought by this is that the transmission path of the active ion becomes longer and the diffusion rate of the active ion becomes slower, so that the deintercalation of the active ion between the positive electrode and the negative electrode of the battery becomes difficult, and the kinetic performance of the battery becomes poor. SUMMARY

[0004] The purpose of the application is to provide a secondary battery and a battery module, a battery pack and a power utilization device containing the secondary battery, aiming to make the secondary battery have good kinetic performance, higher capacity retention rate and lower volume expansion rate while maintaining high energy density.

[0005] The first aspect of the application provides a secondary battery, which comprises a positive pole piece and an electrolyte. The positive pole piece comprises a positive current collector and a positive film layer arranged on at least one surface of the positive current collector, and the positive film layer comprises one or more of lithium-containing olivine structure phosphates and modified compounds thereof. The electrolyte comprises an organic solvent, and the organic solvent comprises a first solvent represented by formula 1.

[0006]

[0007] In formula 1, R 11 and R 12 are independently one of C1-C4 alkyl and C1-C4 halogenated alkyl.

[0008] The secondary battery of the application satisfies: the mass of the first solvent / the rated capacity of the secondary battery≥0.7 g / Ah, and the mass of the electrolyte / the rated capacity of the secondary battery≤3.5 g / Ah.

[0009] When the secondary battery of this application satisfies the condition that the mass / rated capacity of the first solvent is ≥0.7 g / Ah and the mass / rated capacity of the electrolyte is ≤3.5 g / Ah, the electrolyte inside the battery can effectively wet the positive and negative electrode plates, fully leveraging the effect of the first solvent in improving the battery's kinetic performance. Simultaneously, the irreversible consumption of active ions inside the battery can be controlled within a small range. Therefore, the secondary battery can maintain high energy density while exhibiting good kinetic performance, high capacity retention, and low volume expansion rate.

[0010] Optionally, the mass of the first solvent / the rated capacity of the secondary battery is ≥1.0 g / Ah.

[0011] Optionally, the mass of the electrolyte / rated capacity of the secondary battery is 2.8 g / Ah to 3.3 g / Ah.

[0012] In any embodiment of this application, the secondary battery further satisfies the following condition: the content of the first solvent / (thickness of the positive electrode film × 3 + 0.008 / porosity of the positive electrode film) ≥ 1. The content of the first solvent refers to the mass percentage of the first solvent based on the total mass of the organic solvent; the thickness of the positive electrode film represents the thickness of the positive electrode film on one side of the positive electrode current collector, measured in mm. In this case, the problem of poor battery kinetic performance caused by the increase in the coating weight and compaction density of the positive electrode film can be effectively solved.

[0013] Optionally, the secondary battery also satisfies the following condition: the content of the first solvent / (thickness of the positive electrode film × 3 + 0.008 / porosity of the positive electrode film) ≥ 1.5.

[0014] In any embodiment of this application, the electrolyte further includes additives.

[0015] Optionally, the reduction potential of the additive is ≥0.8V (vs Li+ / Li). In this case, the additive can preferentially participate in the formation of the SEI film on the surface of the negative electrode active material through electrochemical reduction reaction before the organic solvent, and promote the formation of a stable SEI film on the surface of the negative electrode active material, preventing further reaction between the negative electrode active material and the electrolyte, and reducing the degree of damage to the SEI film caused by the protons generated by the reduction of the first solvent.

[0016] In any embodiment of this application, the additive includes an organic additive, which comprises one or more of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, vinyl sulfate, and 1,3-propane sulpholactone. The organic additive preferentially undergoes electrochemical reduction on the surface of the negative electrode active material to form a high-performance SEI film, reducing the damage to the SEI film caused by protons generated during the reduction of the first solvent. The reduction products of the organic additive are mainly organic components, which can improve the mechanical stability of the SEI film and thus enhance the cycle performance of the battery.

[0017] In any embodiment of this application, the additive includes inorganic additives, which include one or more of tris(trimethylsilyl)phosphate, lithium bis(oxalato)borate, lithium difluorodi(oxalato)phosphate, lithium tetrafluoro(oxalato)phosphate, lithium difluoro(oxalato)borate, and lithium difluorophosphate. Inorganic additives can preferentially undergo electrochemical reduction on the surface of the negative electrode active material to form a high-performance SEI film, thereby reducing the damage to the SEI film caused by protons generated during the reduction of the first solvent. The reduction products of the inorganic additives are mainly inorganic components, which can improve the thermal stability of the SEI film and are beneficial to improving the high-temperature performance of the battery.

[0018] In any embodiment of this application, the mass percentage of organic additives is ≥50% based on the total mass of the additives. Optionally, the mass percentage of organic additives is 70%-100%.

[0019] In any embodiment of this application, the mass percentage of inorganic additives is ≤50% based on the total mass of the additives. Optionally, the mass percentage of inorganic additives is 0%-30%.

[0020] In any embodiment of this application, the secondary battery further includes a negative electrode sheet, which includes a negative electrode active material.

[0021] In any embodiment of this application, the secondary battery further satisfies: (liquid retention coefficient × additive content) / (relative mass of negative electrode × specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 1. In this case, the additive can effectively protect the negative electrode interface and suppress the problem of aggravated side reactions at the negative electrode interface caused by the instability of the first solvent, resulting in higher capacity retention and lower volume expansion rate during the use of the secondary battery. The liquid retention coefficient refers to the ratio of the mass of the electrolyte to the rated capacity of the secondary battery, expressed in g / Ah; the additive content refers to the mass percentage of the additive based on the total mass of the electrolyte; the relative mass of the negative electrode refers to the ratio of the mass of the negative electrode active material in the negative electrode sheet to the rated capacity of the secondary battery, expressed in g / Ah; the specific surface area of ​​the negative electrode active material is expressed in m². 2 / g calculation.

[0022] Optionally, the secondary battery also satisfies: (liquid retention coefficient × additive content) / (relative mass of negative electrode × specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 2.

[0023] In any embodiment of this application, the thickness of the positive electrode film is ≥0.07mm. Optionally, the thickness of the positive electrode film is 0.07mm-0.14mm. With a moderate thickness of the positive electrode film, the transport path of active ions in the positive electrode film is moderate, and the battery can maintain good dynamic performance without excessively sacrificing the battery's energy density.

[0024] In any embodiment of this application, the porosity of the positive electrode film is ≤50%. Optionally, the porosity of the positive electrode film is 5%-50%. With a moderate porosity, the transport resistance of active ions in the positive electrode film is moderate, and the battery can maintain good kinetic performance without excessively sacrificing the battery's energy density.

[0025] In any embodiment of this application, the relative mass of the negative electrode is ≤2.1 g / Ah. Optionally, the relative mass of the negative electrode is 1.2 g / Ah-2.1 g / Ah.

[0026] In any embodiment of this application, the specific surface area of ​​the negative electrode active material is 0.5 m². 2 / g-6.0m 2 / g. Optionally, the specific surface area of ​​the negative electrode active material is 1.0 m². 2 / g-3.2m 2 / g.

[0027] In any embodiment of this application, the content of the first solvent is ≥10%. Optionally, the content of the first solvent is 10%-80%. A moderate content of the first solvent can, on the one hand, fully exert its effect on improving the battery dynamic performance, and on the other hand, avoid excessive damage to the negative electrode interface.

[0028] In any embodiment of this application, the content of the additive is ≥3%. Optionally, the content of the additive is 3%-9%. A moderate content of the additive can, on the one hand, fully exert the protective effect on the negative electrode interface, and on the other hand, avoid the formation of an excessively thick SEI film on the surface of the negative electrode active material, thereby increasing the film-forming resistance of the negative electrode.

[0029] In any embodiment of this application, in formula 1, R 11 and R 12 Each of the following is an independent compound: methyl, ethyl, propyl, butyl, fluoromethyl, fluoroethyl, fluoropropyl, and fluorobutyl. R 11 and R 12 When selected from the above-mentioned groups, the viscosity of the electrolyte can be kept within a suitable range, and the electrolyte can have higher conductivity.

[0030] In any embodiment of this application, the first solvent represented by Formula 1 is selected from one or more of the following compounds:

[0031]

[0032] In any embodiment of this application, the organic solvent further includes one or more of the second solvent shown in Formula 2 and the third solvent shown in Formula 3, wherein in Formula 2, R 21 It is one of H, methyl, and ethyl, and in formula 3, R = , R 32 Each of the following is an independent compound: methyl, ethyl, or propyl.

[0033]

[0034] In any embodiment of this application, the mass percentage of the second solvent is ≥10% based on the total mass of the organic solvent. Optionally, the mass percentage of the second solvent is 10%-80%. The second solvent has a higher dielectric constant, which is beneficial to the dissociation of lithium salt. Adding the second solvent to the electrolyte is beneficial to increasing the conductivity of the electrolyte.

[0035] In any embodiment of this application, the mass percentage of the third solvent is ≥0% based on the total mass of the organic solvent. Optionally, the mass percentage of the third solvent is 5%-80%. The third solvent has a lower dielectric constant and a weaker ability to dissociate lithium salts, but it has lower viscosity and better fluidity. Adding the third solvent to the electrolyte can increase the conductivity of the electrolyte by increasing the migration rate of active ions.

[0036] In any embodiment of this application, the second solvent represented by Formula 2 is selected from one or two of the following compounds:

[0037]

[0038] In any embodiment of this application, the third solvent represented by Formula 3 is selected from one or more of the following compounds:

[0039]

[0040] In any embodiment of this application, the lithium phosphate with olivine structure includes one or more of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and their respective modified compounds.

[0041] A second aspect of this application provides a battery module that includes the secondary battery of the first aspect of this application.

[0042] A third aspect of this application provides a battery pack, which includes one of the secondary battery of the first aspect of this application and the battery module of the second aspect of this application.

[0043] The fourth aspect of this application provides an apparatus comprising at least one of the secondary battery of the first aspect of this application, the battery module of the second aspect of this application, and the battery pack of the third aspect of this application.

[0044] The battery module, battery pack, and power device of this application include the secondary battery provided in this application, and therefore have at least the same advantages as the secondary battery. Attached Figure Description

[0045] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some implementation methods of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0046] FIG. 1 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0047] FIG. 2 This is an exploded view of one embodiment of the secondary battery of this application.

[0048] FIG. 3 This is a schematic diagram of one embodiment of the battery module of this application.

[0049] FIG. 4 This is a schematic diagram of one embodiment of the battery pack of this application.

[0050] FIG. 5 yes FIG. 4 The exploded diagram.

[0051] FIG. 6 This is a schematic diagram of one embodiment of the electrical device that uses a secondary battery as a power source according to this application. Detailed Implementation

[0052] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the secondary battery, battery module, battery pack, and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0053] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0054] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0055] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0056] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0057] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0058] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: 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).

[0059] Secondary battery

[0060] Secondary batteries, also known as rechargeable batteries or storage batteries, are batteries that can be recharged after being discharged to activate the active materials and continue to be used.

[0061] Typically, a secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing active ions to pass through. The electrolyte, located between the positive and negative electrodes, conducts the active ions.

[0062] In the secondary battery of this application, 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. For example, the positive current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0063] In the secondary battery of this application, the positive electrode film layer includes a positive electrode active material. As an example, the positive electrode active material may include one or more of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds. Examples of lithium transition metal oxides include, but are not limited to, one or more of lithium cobalt oxides, lithium nickel oxides, lithium manganese oxides, lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, lithium nickel cobalt manganese oxides, lithium nickel cobalt aluminum oxides, and their modified compounds. Examples of lithium-containing phosphates with an olivine structure include, but are not limited to, lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and their respective modified compounds. These positive electrode active materials may be used alone or in combination of two or more.

[0064] In some embodiments, the positive electrode active material may include at least one or more of lithium phosphates with an olivine structure and their modified compounds.

[0065] In some embodiments, the positive electrode active material may be only one or more of lithium phosphates with an olivine structure and their modified compounds.

[0066] In the secondary battery of this application, the modified compounds of the above-mentioned positive electrode active materials can be used to modify the positive electrode active materials by doping, surface coating, or doping and surface coating at the same time.

[0067] In the secondary battery of this application, the positive electrode film typically comprises a positive electrode active material, an optional binder, and an optional conductive agent. The positive electrode film is typically formed by coating a positive electrode slurry onto a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is typically formed by dispersing the positive electrode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring until homogeneous. The solvent may be N-methylpyrrolidone (NMP), but is not limited to this. As an example, the binder used for the positive electrode film may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene-propylene terpolymer, ethylene-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. As an example, the conductive agent used for the positive electrode film may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0068] In the secondary battery of this application, the positive electrode current collector can be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil can be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material can be selected from one or more of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymer material base layer can be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0069] In the secondary battery of this application, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector. For example, the negative current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative current collector.

[0070] The negative electrode active material can be any negative electrode active material known in the art for use in secondary batteries. As examples, the negative electrode active material may include one or more of natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. Silicon-based materials may include one or more of elemental silicon, silicon oxide, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include one or more of elemental tin, tin oxide, and tin alloys. This application is not limited to these materials; other conventionally known materials that can be used as negative electrode active materials for secondary batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0071] In the secondary battery of this application, the negative electrode film layer typically comprises a negative electrode active material, an optional binder, an optional conductive agent, and other optional additives. The negative electrode film layer is typically formed by coating a negative electrode slurry onto a negative electrode current collector, followed by drying and cold pressing. The negative electrode slurry coating is typically formed by dispersing the negative electrode active material, an optional conductive agent, an optional binder, and other optional additives in a solvent and stirring until homogeneous. The solvent may be N-methylpyrrolidone (NMP) or deionized water, but is not limited to these. As an example, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. As an example, the binder may include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). Other optional additives may include thickeners (e.g., sodium carboxymethyl cellulose, CMC-Na), PTC thermistor materials, etc.

[0072] In the secondary battery of this application, the negative electrode current collector can be a metal foil or a composite current collector. As an example of a metal foil, copper foil can be used. The composite current collector may include a polymer material substrate and a metal material layer formed on at least one surface of the polymer material substrate. As an example, the metal material can be selected from one or more of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymer material substrate can be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0073] In the field of batteries, the electrolyte can be selected from at least one of solid electrolytes, gel electrolytes, and liquid electrolytes (i.e., electrolyte solutions). The secondary battery of this application uses a liquid electrolyte, i.e., an electrolyte solution.

[0074] In the secondary battery of this application, the electrolyte may include an organic solvent and optional additives. The organic solvent in this application is a non-aqueous organic solvent, which may include the first solvent shown in Formula 1.

[0075]

[0076] In Equation 1, R 11 and R 12 Each is independently one of a C1-C4 alkyl group or a C1-C4 haloalkyl group, R 11 and R 12 They can be the same or different. Alkyl and haloalkyl substituents can be straight-chain or branched-chain structures. The number of halogen atoms in a haloalkyl substituent can be one or more; when a haloalkyl substituent contains multiple halogen atoms, these halogen atoms can be the same or different.

[0077] Typically, high energy density and good kinetic performance in a battery cannot be simultaneously achieved. Increasing the coating weight and compaction density of the positive electrode film can improve the battery's energy density, but it also leads to poor battery kinetic performance. Adding the first solvent shown in Formula 1 to the electrolyte can improve the kinetic performance of both the electrolyte and the battery. This is because the first solvent shown in Formula 1 has the advantages of low viscosity and high dielectric constant, which can greatly improve the conductivity of the electrolyte.

[0078] Generally, the higher the electrolyte content in a battery, the better its performance. However, the inventors discovered that when the positive electrode includes lithium phosphate with an olivine structure and its modified compounds, within a certain range, increasing the electrolyte injection volume improves the wettability of the electrolyte inside the battery and also improves the battery's kinetic performance. However, further increasing the electrolyte injection volume has little effect on improving the battery's kinetic performance. Furthermore, further increasing the electrolyte injection volume also increases the content of organic solvents inside the battery. Some of these organic solvents consume active ions, resulting in accelerated battery capacity decay and increased gas production.

[0079] The inventors also discovered that in batteries primarily using lithium phosphates with an olivine structure and their modified compounds as positive electrode active materials, when the mass / rated capacity of the first solvent is ≥0.7 g / Ah and the mass / rated capacity of the electrolyte is ≤3.5 g / Ah, the battery can maintain high energy density while exhibiting good kinetic performance, high capacity retention, and low volume expansion rate. This may be because batteries primarily using lithium phosphates with an olivine structure and their modified compounds as positive electrode active materials have relatively low energy density. To further improve the energy density, an effective approach is to increase the utilization rate of the battery's internal space, for example, by increasing the coating weight and compaction density of the electrode sheets. In this case, the residual space inside the battery becomes very small, and the amount of gas generated inside the battery tends to increase as the electrolyte content increases. Therefore, batteries primarily using lithium phosphates with an olivine structure and their modified compounds as positive electrode active materials are more sensitive to the electrolyte content. Meanwhile, to improve the kinetic performance of the electrolyte and the battery, the aforementioned first solvent is often used in the electrolyte. However, the first solvent has poor oxidation resistance and is easily oxidized and decomposed, leading to an increase in gas production inside the battery. Therefore, batteries that mainly use lithium phosphates with olivine structures and their modified compounds as positive electrode active materials must strictly control the content of the electrolyte and the first solvent inside.

[0080] Through extensive research, the inventors discovered that when the ratio of the mass of the first solvent to the rated capacity of the secondary battery is ≥0.7 g / Ah and the ratio of the mass of the electrolyte to the rated capacity of the secondary battery is ≤3.5 g / Ah, the electrolyte inside the battery can effectively wet the positive and negative electrode plates, fully leveraging the role of the first solvent in improving the battery's kinetic performance. Simultaneously, the irreversible consumption of active ions inside the battery can be controlled within a small range. Therefore, the secondary battery can maintain high energy density while exhibiting good kinetic performance, high capacity retention, and low volume expansion rate.

[0081] "A battery primarily using lithium phosphates with an olivine structure and their modified compounds as the positive electrode active material" indicates that the positive electrode active material can be solely lithium phosphates with an olivine structure and their modified compounds, or it can be a combination of lithium phosphates with an olivine structure and their modified compounds with other positive electrode active materials. That is, in addition to lithium phosphates with an olivine structure and their modified compounds, the positive electrode active material also includes other positive electrode active materials, such as lithium transition metal oxides and their modified compounds. In some embodiments, based on the total mass of the positive electrode active material, the mass percentage of lithium phosphates with an olivine structure and their modified compounds is ≥50%. Optionally, the mass percentage of lithium phosphates with an olivine structure and their modified compounds is 80%-100%.

[0082] In the secondary battery of this application, the mass of the electrolyte refers to the mass of the electrolyte inside the finished battery, rather than the mass of the electrolyte injected during the battery manufacturing process.

[0083] In the secondary battery of this application, the rated capacity of the secondary battery refers to the capacity discharged by a fully charged battery at room temperature when discharged with a current of 1 / 3×I1(A) to reach the termination voltage, where I1 represents the 1-hour rate discharge current. For details, please refer to the national standard GB / T 31484-2015 Requirements and Test Methods for Cycle Performance of Power Batteries for Electric Vehicles.

[0084] In the secondary battery of this application, the ratio of the mass of the electrolyte to the rated capacity of the secondary battery can also be called the electrolyte retention coefficient.

[0085] In some embodiments, the mass of the first solvent / rated capacity of the secondary battery can be ≥0.7 g / Ah, >0.8 g / Ah, ≥0.9 g / Ah, ≥1.0 g / Ah, ≥1.1 g / Ah, ≥1.2 g / Ah, ≥1.3 g / Ah, ≥1.4 g / Ah, ≥1.5 g / Ah, ≥1.6 g / Ah, ≥1.7 g / Ah, ≥1.8 g / Ah, or ≥1.9 g / Ah.

[0086] In some embodiments, the mass of the first solvent / rated capacity of the secondary battery can be 0.7 g / Ah-2.0 g / Ah, 0.8 g / Ah-2.0 g / Ah, 0.9 g / Ah-2.0 g / Ah, 1.0 g / Ah-2.0 g / Ah, 1.1 g / Ah-2.0 g / Ah, 1.2 g / Ah-2.0 g / Ah, 1.5 g / Ah-2.0 g / Ah, 0.7 g / Ah-1.9 g / Ah, 0.8 g / Ah-1.9 g / Ah, or 0.9 g / Ah-1. 9g / Ah, 1.0g / Ah-1.9g / Ah, 1.1g / Ah-1.9g / Ah, 1.2g / Ah-1.9g / Ah, 1.5g / Ah-1.9g / Ah, 0.7g / Ah-1.6g / Ah, 0.8g / Ah-1.6g / Ah, 0.9g / Ah-1.6g / Ah, 1.0g / Ah-1.6g / Ah, 1.1g / Ah-1.6g / Ah, 1.2g / Ah-1.6g / Ah, or 1.5g / Ah-1.6g / Ah.

[0087] In some embodiments, the mass of the electrolyte / rated capacity of the secondary battery may be ≤3.5g / Ah, ≤3.4g / Ah, ≤3.3g / Ah, ≤3.2g / Ah, ≤3.1g / Ah, ≤3.0g / Ah, ≤2.9g / Ah, ≤2.8g / Ah, ≤2.7g / Ah, ≤2.6g / Ah, ≤2.5g / Ah, ≤2.4g / Ah, ≤2.3g / Ah, ≤2.2g / Ah, ≤2.1g / Ah, or ≤2.0g / Ah.

[0088] In some embodiments, the mass of the electrolyte / rated capacity of the secondary battery can be 2.4 g / Ah-3.5 g / Ah, 2.5 g / Ah-3.5 g / Ah, 2.6 g / Ah-3.5 g / Ah, 2.7 g / Ah-3.5 g / Ah, 2.8 g / Ah-3.5 g / Ah, 2.9 g / Ah-3.5 g / Ah, 3.0 g / Ah-3.5 g / Ah, 3.1 g / Ah. g / Ah-3.5g / Ah, 2.4g / Ah-3.3g / Ah, 2.5g / Ah-3.3g / Ah, 2.6g / Ah-3.3g / Ah, 2.7g / Ah-3. 3g / Ah, 2.8g / Ah-3.3g / Ah, 2.9g / Ah-3.3g / Ah, 3.0g / Ah-3.3g / Ah, or 3.1g / Ah-3.3g / Ah.

[0089] In some implementations, in Equation 1, R 11 and R 12 It can be independently one of methyl, ethyl, propyl, butyl, fluoromethyl, fluoroethyl, fluoropropyl, and fluorobutyl, R 11 and R 12 They can be the same or different. The number of fluorine atoms can be one or more. R 11 and R 12 When selected from the above-mentioned groups, the viscosity of the electrolyte can be kept within a suitable range, and the electrolyte can have higher conductivity.

[0090] In some embodiments, as an example, the first solvent shown in Formula 1 may be selected from one or more of the following compounds:

[0091]

[0092] The first solvent shown in Formula 1 has the advantages of low viscosity and high dielectric constant, which can improve the conductivity of the electrolyte and thus, to some extent, alleviate the problem of poor battery kinetic performance caused by the increase in the coating weight and compaction density of the positive electrode film. However, the inventors also found that the first solvent shown in Formula 1 is unstable at the negative electrode and is easily reduced to generate protons and other products, increasing the amount of gas produced by the battery. The protons generated by the reduction of the first solvent shown in Formula 1 can also decompose components such as lithium carbonate and lithium alkoxy in the SEI film, making the stability of the SEI film worse. As a result, the SEI film needs to be constantly repaired during battery use. This repair process consumes more active ions, increases the irreversible consumption of active ions, and causes the battery capacity to decay rapidly. In addition, the products after the reduction of the first solvent shown in Formula 1 accumulate on the surface of the negative electrode active material, which can prolong the transport path of active ions and reduce the transport speed of active ions, thereby deteriorating the kinetic performance of the negative electrode sheet and the battery kinetic performance.

[0093] Through extensive research, the inventors discovered a solution to the above problems, which not only ensures high energy density and good dynamic performance of the battery, but also ensures good cycle performance and storage performance.

[0094] To improve battery energy density, the coating weight and compaction density of the positive electrode film are typically increased. This leads to an increasing demand on the electrolyte's kinetic performance. A thicker positive electrode film results in a longer transport path for active ions, further increasing the demand on electrolyte kinetic performance; conversely, lower porosity leads to greater resistance during active ion transport, also increasing the demand on electrolyte kinetic performance. Therefore, these two parameters of the positive electrode film are independent of each other, yet both influence electrolyte kinetic performance. The inventors, through extensive research and experimentation, obtained coefficients for these two independent demands and used their sum to represent the total demand on electrolyte kinetic performance. The content of the first solvent is strongly correlated with electrolyte kinetic performance; a higher content results in better performance. The inventors found that the content of the first solvent can characterize the level of electrolyte kinetic performance. Only when the electrolyte kinetic performance level is greater than or equal to the total demand on electrolyte kinetic performance from the positive electrode film can the problem of poor battery kinetic performance resulting from increased coating weight and compaction density of the positive electrode film be effectively mitigated.

[0095] Through extensive research, the inventors discovered that when the secondary battery also satisfies the condition that the content of the first solvent / (thickness of the positive electrode film × 3 + 0.008 / porosity of the positive electrode film) ≥ 1, the problem of poor battery dynamic performance caused by the increase of the coating weight and compaction density of the positive electrode film can be effectively solved.

[0096] In some embodiments, the secondary battery may satisfy the following conditions: the content of the first solvent / (thickness of the positive electrode film × 3 + 0.008 / porosity of the positive electrode film) ≥ 1.1, the content of the first solvent / (thickness of the positive electrode film × 3 + 0.008 / porosity of the positive electrode film) ≥ 1.2, the content of the first solvent / (thickness of the positive electrode film × 3 + 0.008 / porosity of the positive electrode film) ≥ 1.3, and the content of the first solvent / (thickness of the positive electrode film × 3 + 0.008 / porosity of the positive electrode film) ≥ 1.3. Porosity) ≥ 1.4, First solvent content / (thickness of positive electrode film × 3 + 0.008 / porosity of positive electrode film) ≥ 1.5, First solvent content / (thickness of positive electrode film × 3 + 0.008 / porosity of positive electrode film) ≥ 1.6, First solvent content / (thickness of positive electrode film × 3 + 0.008 / porosity of positive electrode film) ≥ 1.7, or First solvent content / (thickness of positive electrode film × 3 + 0.008 / porosity of positive electrode film) ≥ 1.8.

[0097] The content of the first solvent refers to the mass percentage of the first solvent based on the total mass of the organic solvents.

[0098] The thickness of the positive electrode film layer refers to the thickness of a single-sided positive electrode film layer on the positive electrode current collector, measured in mm. When the positive electrode film layer is disposed on either of the two opposing surfaces of the positive electrode current collector, the thickness of the positive electrode film layer = the thickness of the positive electrode sheet - the thickness of the positive electrode current collector; when the positive electrode film layer is disposed on both opposing surfaces of the positive electrode current collector, the thickness of the positive electrode film layer = (thickness of the positive electrode sheet - thickness of the positive electrode current collector) / 2. In this application, thickness measurement can be performed using a micrometer, such as a Mitutoyo 293-100 micrometer with an accuracy of 0.1 μm. In this application, "thickness of the positive electrode film layer" refers to the thickness of a single-sided positive electrode film layer, and "total thickness of the positive electrode film layer" represents the sum of the thicknesses of all positive electrode film layers when the positive electrode film layer is disposed on both opposing surfaces of the positive electrode current collector.

[0099] The porosity of the positive electrode film is expressed as the ratio of the pore volume (i.e., the true volume) of the positive electrode sheet to its apparent volume. The porosity of the positive electrode film can be tested using GB / T24586-2009, "Methods for Determination of Apparent Density, True Density and Porosity of Iron Ore." This test method includes the following steps: Thirty small circular pieces with a diameter of 14 mm are cut from the positive electrode sheet. Based on the principle of gas adsorption, an inert gas (e.g., helium or nitrogen) is used as the medium to test the true volume of these 30 small circular pieces with a diameter of 14 mm. The apparent volume of the positive electrode sheet is calculated based on the area, thickness, and number of the small circular pieces. The ratio of the true volume to the apparent volume is the porosity of the positive electrode film.

[0100] The surface of lithium phosphates and their modified compounds with olivine structure contains polar functional groups and has a large specific surface area, making them extremely prone to water absorption. The drying process before battery electrolyte filling cannot completely remove the moisture. During battery use, this moisture will gradually diffuse into the electrolyte and react with it. At the same time, the presence of moisture will also damage the SEI film (Solid Electrolyte Interface Membrane) on the surface of the negative electrode active material, affecting the stability of the negative electrode interface.

[0101] A reduced positive electrode film thickness shortens the transport path of active ions, improving the kinetic performance of the secondary battery. Simultaneously, moisture in the positive electrode film is more easily removed during the drying process, reducing the damaging effect of moisture on the negative electrode interface. However, a reduced positive electrode film thickness also lowers the energy density of the secondary battery. In some embodiments, the thickness of the positive electrode film can be ≥0.07 mm. For example, the thickness of the positive electrode film can be ≥0.075 mm, ≥0.08 mm, ≥0.085 mm, ≥0.09 mm, ≥0.10 mm, ≥0.11 mm, ≥0.12 mm, ≥0.13 mm, ≥0.14 mm, or ≥0.15 mm.

[0102] In some embodiments, the thickness of the positive electrode film may optionally be 0.07 mm to 0.14 mm. More specifically, the thickness of the positive electrode film may be 0.09 mm to 0.12 mm.

[0103] Increased porosity of the positive electrode film layer allows for easier wetting of the film by the electrolyte, reducing the resistance to ion transport within the film and improving the kinetic performance of the secondary battery. Simultaneously, moisture in the positive electrode film layer is more easily removed during the drying process, minimizing its damaging effect on the negative electrode interface. However, increased porosity of the positive electrode film layer can decrease the energy density of the secondary battery. In some embodiments, the porosity of the positive electrode film layer can be ≤50%. For example, the porosity can be ≤48%, ≤45%, ≤42%, ≤40%, ≤38%, ≤37%, ≤35%, ≤32%, or ≤30%.

[0104] In some embodiments, optionally, the porosity of the positive electrode film layer may be 5%-50%. More specifically, the porosity of the positive electrode film layer may be 20%-40%.

[0105] In some embodiments, the single-sided coating weight of the positive electrode film can be ≥20 mg / cm³. 2 Optionally, the coating weight on one side of the positive electrode film can be 23 mg / cm³. 2 -29mg / cm 2 More specifically, the single-sided coating weight of the positive electrode film can be 25 mg / cm³. 2 -29mg / cm2 .

[0106] In some embodiments, the compaction density of the positive electrode film layer can be ≥2.1 g / cm³. 3 Optionally, the compaction density of the positive electrode film can be 2.1 g / cm³. 3 -2.8g / cm 3 More specifically, the compaction density of the positive electrode film can be 2.3 g / cm³. 3 -2.6g / cm 3 .

[0107] In some embodiments, the mass percentage of the first solvent (i.e., the content of the first solvent) may be ≥10% based on the total mass of the organic solvent. For example, the content of the first solvent may be ≥15%, ≥20%, ≥25%, ≥30%, ≥35%, ≥40%, ≥45%, ≥50%, ≥55%, or ≥60%.

[0108] In some embodiments, the content of the first solvent may be 10%-80%, 15%-80%, 20%-80%, 25%-80%, 30%-80%, 35%-80%, 40%-80%, 45%-80%, 50%-80%, 10%-75%, 15%-75%, 20%-75%, 25%-75%, 30%-75%, 35%-75%, 40%-75%, 45%-75%, 50%-75%, 10%-70%, 1 5%-70%, 20%-70%, 25%-70%, 30%-70%, 35%-70%, 40%-70%, 45%-70%, 50%-70%, 10%-65%, 15%-65%, 20%-65%, 25%-65%, 30%-65%, 35%-65%, 40%-65%, 45%-65%, 50%-65%, 1 0%-60%, 15%-60%, 20%-60%, 25%-60%, 30%-60%, 35%-60%, 40%-60%, 45%-60%, 50%-60%, 10%-55%, 15%-55%, 20%-55%, 25%-55%, 30%-55%, 35%-55%, 40%-55%, 45%-55%, or 50%-55%.

[0109] In some embodiments, the organic solvent may further include one or more of the second solvent shown in Formula 2 and the third solvent shown in Formula 3. In Formula 2, R 21 It is one of H, methyl, and ethyl; in formula 3, R 31 R 32 Each is independently one of methyl, ethyl, or propyl, R 31 and R 32They can be the same or different.

[0110]

[0111] In the secondary battery of this application, the organic solvent may include, in addition to the first solvent shown in Formula 1, the second solvent shown in Formula 2, or the third solvent shown in Formula 3, or both the second solvent shown in Formula 2 and the third solvent shown in Formula 3. In some embodiments, the organic solvent may include, in addition to the first solvent shown in Formula 1, the second solvent shown in Formula 2.

[0112]

[0113] In some embodiments, as an example, the second solvent shown in Formula 2 may be selected from one or both of the following compounds:

[0114]

[0115] In some embodiments, as an example, the third solvent shown in Formula 3 may be selected from one or more of the following compounds:

[0116]

[0117] The second solvent has a higher dielectric constant, which is beneficial for the dissociation of the lithium salt. In some embodiments, the mass percentage of the second solvent may be ≥10% based on the total mass of the organic solvent. Optionally, the mass percentage of the second solvent may be 10%-80%. More specifically, the mass percentage of the second solvent may be 20%-50%.

[0118] The third solvent has a lower dielectric constant and weaker ability to dissociate lithium salts, but it has lower viscosity and better flowability, which can increase the migration rate of active ions. In some embodiments, the mass percentage of the third solvent may be ≥0% based on the total mass of the organic solvent. Optionally, the mass percentage of the third solvent may be 5%-80%. More specifically, the mass percentage of the third solvent may be 5%-20%.

[0119] In the secondary battery of this application, the electrolyte may further include additives, which may be film-forming additives. In some embodiments, the reduction potential of the additive may satisfy ≥0.8V (vs Li). + / Li).

[0120] The SEI film on the surface of the negative electrode active material provides electronic insulation while allowing active ions to freely enter and exit, and also prevents further reaction between the negative electrode active material and the electrolyte. Therefore, the properties of the SEI film on the surface of the negative electrode active material affect the performance of the negative electrode sheet and the battery performance, such as the battery's cycle performance and storage performance. The additive in this application has a reduction potential ≥0.8V (vs Li). + Li can preferentially participate in the formation of the SEI film on the surface of the negative electrode active material through electrochemical reduction reaction, and promote the formation of a stable SEI film on the surface of the negative electrode active material, preventing further reaction between the negative electrode active material and the electrolyte, and reducing the degree of damage to the SEI film caused by the protons generated by the reduction of the first solvent.

[0121] In some embodiments, the additive may include one or more of organic and inorganic additives.

[0122] In the secondary battery of this application, the additive may also include only organic additives.

[0123] In some embodiments, as an example, the organic additive may include one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl ethylene carbonate (VEC), ethylene sulfate (DTD), and 1,3-propane sultone (PS). The organic additive is an electrochemically reducing additive with a higher reduction potential than the organic solvent. Therefore, it can preferentially undergo electrochemical reduction on the surface of the negative electrode active material to form a high-performance SEI film, thereby reducing the damage to the SEI film caused by protons generated during the reduction by the first solvent. The reduction products of the organic additive are mainly organic components, which can improve the mechanical stability of the SEI film, thus contributing to improved battery cycle performance.

[0124] In some embodiments, as an example, the inorganic additive may include one or more of tris(trimethylsilyl)phosphate (TMSP), lithium bis(oxalate)borate (LiBOB), lithium bis[ethanedioato(2-)-KO1,κO2]difluorophosphate(1-) (LiDFOP), lithium tetrafluoro(oxalato)phosphate (LiTFOP), lithium difluoro(oxalato)borate (LiDFOB), and lithium difluorophosphate (LiPO2F2). The inorganic additive is an electrochemically reducing additive, and its reduction potential is higher than that of the organic solvent. Therefore, it can preferentially undergo electrochemical reduction on the surface of the negative electrode active material to form a high-performance SEI film, thereby reducing the degree of damage to the SEI film caused by protons generated during the reduction of the first solvent. The reduction products of inorganic additives are mainly inorganic components. These inorganic components can improve the thermal stability of the SEI film, which is beneficial to improving the high-temperature performance of the battery.

[0125] In some embodiments, the additive may optionally include one or more of vinylene carbonate, fluoroethylene carbonate, 1,3-propane sulpholactone, vinyl sulfate, lithium difluorophosphate, and tris(trimethylsilyl)phosphate. The reduction products of vinylene carbonate, fluoroethylene carbonate, 1,3-propane sulpholactone, and vinyl sulfate can serve as organic components in the SEI film, improving its mechanical stability and tolerance to volume changes in the negative electrode active material during battery cycling. The reduction products of lithium difluorophosphate and tris(trimethylsilyl)phosphate can serve as inorganic components in the SEI film, improving its thermal stability and preventing decomposition even at high temperatures.

[0126] In some embodiments, the mass percentage of the organic additive may be ≥50% based on the total mass of the additive. For example, the mass percentage of the organic additive may be ≥55%, ≥60%, ≥65%, ≥70%, ≥75%, ≥80%, ≥85%, ≥90%, ≥95%, or 100%.

[0127] In some embodiments, the mass percentage of the organic additive may be 50%-100%, 55%-100%, 60%-100%, 65%-100%, 70%-100%, 75%-100%, 80%-100%, 85%-100%, 90%-100%, or 95%-100%.

[0128] In some embodiments, the mass percentage of the inorganic additive may be ≤50% based on the total mass of the additives. For example, the mass percentage of the inorganic additive may be ≤45%, ≤40%, ≤35%, ≤30%, ≤25%, ≤20%, ≤15%, ≤10%, ≤5%, or 0%.

[0129] In some embodiments, the mass percentage of the inorganic additive may be 0%-50%, 0%-45%, 0%-40%, 0%-35%, 0%-30%, 0%-25%, 0%-20%, 0%-15%, 0%-10%, or 0%-5%.

[0130] Increasing the additive content increases the amount of additive available for consumption during the secondary battery formation process, leading to a thicker SEI film formed on the surface of the negative electrode active material and an increase in negative electrode film-forming impedance. However, the thicker SEI film provides stronger protection for the negative electrode active material, improves the stability of the negative electrode interface, and increases the SEI film's tolerance to side effects caused by the first solvent. In some embodiments, the mass percentage of the additive (i.e., the additive content) can be ≥3% based on the total mass of the electrolyte. For example, the additive content can be ≥3.5%, ≥4%, ≥4.5%, ≥5%, ≥5.5%, ≥6%, ≥6.5%, ≥7%, ≥7.5%, ≥8%, ≥9%, ≥10%, ≥11%, or ≥12%.

[0131] In the secondary battery of this application, "additive content" refers to the sum of the content of organic additives and the content of inorganic additives.

[0132] In some embodiments, the additive may optionally be present in a concentration of 3%-9%. More specifically, the additive may be present in a concentration of 5%-8%.

[0133] This application does not specifically limit the type of lithium salt, which can be selected according to actual needs. In some embodiments, as examples, the lithium salt may include LiPF6, LiBF4, LiAsF6, Li(FSO2)2N, LiCF3SO3, LiClO4, LiN(C x F 2x+1 SO2)(C y F 2y+1 One or more of SO2, where x and y are positive integers.

[0134] The smaller the mass percentage of lithium salt (e.g., less than 10%), the fewer active ion transport units are inside the battery. During high-rate charging, the negative electrode potential drops rapidly, making it easier for dendrites to grow on the negative electrode surface, increasing the irreversible consumption of active ions. Continuous dendrite growth may also puncture the separator, causing internal short circuits between the positive and negative electrodes. A smaller mass percentage of lithium salt also results in a relatively higher content of free organic solvents (especially the primary solvent) in the electrolyte, making the SEI film on the surface of the negative electrode active material unstable and more susceptible to proton decomposition. In some embodiments, the mass percentage of lithium salt can be ≥10% based on the total mass of the electrolyte. For example, the mass percentage of lithium salt can be ≥11%, ≥12%, ≥13%, ≥14%, ≥15%, ≥16%, ≥17%, or ≥18%.

[0135] The conductivity of the electrolyte is affected by the total number of migratable active ions and their migration rate. Increasing the mass percentage of lithium salt increases the total number of migratable active ions, but simultaneously increases the electrolyte viscosity and slows down the migration rate of active ions. Therefore, there is an optimal mass percentage of lithium salt. In some embodiments, the mass percentage of lithium salt can be 10%-20%. Optionally, the mass percentage of lithium salt can be 12%-17%.

[0136] Additives promote the formation of a stable SEI film on the surface of the negative electrode active material. The larger the specific surface area of ​​the negative electrode active material, the greater the demand for additives; the primary solvent also consumes additional additives. Through extensive research and experimentation, the inventors obtained coefficients for these two additive consumption behaviors and used the sum of these two consumption behaviors to represent the additive demand level; the additive retention level can be expressed as the liquid retention coefficient × the additive content. Only when the additive retention level is greater than or equal to the additive demand level can a good negative electrode interface be guaranteed, thereby effectively suppressing side reactions at the negative electrode interface.

[0137] Through extensive research, the inventors discovered that when a secondary battery also satisfies the following condition: (liquid retention coefficient × additive content) / (relative mass of negative electrode × specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 1, the additive can effectively protect the negative electrode interface and suppress the problem of aggravated side reactions at the negative electrode interface caused by the instability of the first solvent. Therefore, the secondary battery has a higher capacity retention rate and a lower volume expansion rate during use.

[0138] The electrolyte retention coefficient refers to the ratio of the electrolyte mass to the rated capacity of the secondary battery, expressed in g / Ah; the additive content refers to the mass percentage of the additive based on the total mass of the electrolyte; the relative mass of the negative electrode refers to the ratio of the mass of the negative electrode active material in the negative electrode sheet to the rated capacity of the secondary battery, expressed in g / Ah; the specific surface area of ​​the negative electrode active material is expressed in m².2 / g calculation.

[0139] In some embodiments, the secondary battery may further satisfy the following: (liquid retention coefficient × additive content) / (relative mass of negative electrode × specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 1.2, (liquid retention coefficient × additive content) / (relative mass of negative electrode × specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 1.4, (liquid retention coefficient × additive content) / (relative mass of negative electrode × specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 1.6, (liquid retention coefficient × additive content) / (relative mass of negative electrode × specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 1.8 ... ×Specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 2, (liquid retention coefficient × additive content) / (relative mass of negative electrode × specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 2.2, (liquid retention coefficient × additive content) / (relative mass of negative electrode × specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 2.4, (liquid retention coefficient × additive content) / (relative mass of negative electrode × specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 2.8, or (liquid retention coefficient × additive content) / (relative mass of negative electrode × specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 3.2.

[0140] In some embodiments, the relative mass of the negative electrode may be ≤2.1 g / Ah. For example, the relative mass of the negative electrode may be ≤2.08 g / Ah, ≤2.05 g / Ah, ≤2.0 g / Ah, ≤1.95 g / Ah, ≤1.9 g / Ah, ≤1.85 g / Ah, ≤1.8 g / Ah, ≤1.75 g / Ah, ≤1.7 g / Ah, ≤1.65 g / Ah, ≤1.6 g / Ah, ≤1.55 g / Ah, or ≤1.5 g / Ah.

[0141] In some embodiments, the relative mass of the negative electrode may optionally be 1.2 g / Ah to 2.1 g / Ah. More specifically, the relative mass of the negative electrode may be 1.5 g / Ah to 1.8 g / Ah.

[0142] In the secondary battery of this application, the specific surface area of ​​the negative electrode active material can be 0.5 m². 2 / g-6.0m 2 / g. For example, the specific surface area of ​​the negative electrode active material can be 1.0 m². 2 / g-5.0m 2 / g, 1.0m 2 / g-4.0m 2 / g, 1.0m 2 / g-3.2m 2 / g, 1.0m 2 / g-2.8m 2 / g, 1.2m 2 / g-5.0m 2 / g, 1.2m 2 / g-4.0m 2 / g, 1.2m 2 / g-3.0m 2 / g, 1.2m 2 / g-2.8m 2 / g, 1.5m 2 / g-5.0m 2 / g, 1.5m 2 / g-4.0m 2 / g, 1.5m 2 / g-3.0m 2 / g, 2.0m 2 / g-5.0m 2 / g, 2.0m 2 / g-4.0m 2 / g, or 2.0m 2 / g-3.0m 2 / g.

[0143] The secondary battery of this application also includes a separator. The separator is disposed between the positive and negative electrode plates, serving a separating function. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected. In some embodiments, the material of the separator can be selected from one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer may be the same or different. In some embodiments, a ceramic coating or a metal oxide coating may also be provided on the separator.

[0144] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0145] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0146] In some implementations, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0147] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. FIG. 1 This is an example of a square-structured secondary battery 5.

[0148] In some implementations, refer to FIG. 2 The outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. Positive electrode sheets, negative electrode sheets, and a separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to requirements.

[0149] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be multiple, the specific number of which can be adjusted according to the application and capacity of the battery module.

[0150] FIG. 3 This is battery module 4, used as an example. (See reference...) FIG. 3 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0151] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0152] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0153] FIG. 4 and FIG. 5 This is battery pack 1 as an example. (See reference...) FIG. 4 and FIG. 5The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0154] Methods for preparing secondary batteries

[0155] In some embodiments, the method for preparing the secondary battery of this application may include the following steps:

[0156] S10, assembling the positive electrode, separator, negative electrode and electrolyte to form a secondary battery;

[0157] S20. The secondary batteries are tested, and those that meet the requirements of mass of the first solvent / rated capacity of the secondary battery ≥ 0.7 g / Ah and mass of electrolyte / rated capacity of the secondary battery ≤ 3.5 g / Ah are selected.

[0158] The secondary batteries obtained by the method of this application can maintain high energy density while exhibiting good kinetic performance, high capacity retention, and low volume expansion rate.

[0159] In some embodiments, in step S10, as an example, the positive electrode sheet, separator, and negative electrode sheet can be formed into an electrode assembly through a winding process or a stacking process. The electrode assembly is placed in an outer packaging, dried, and then injected with electrolyte. After vacuum sealing, standing, formation, shaping, and other processes, a secondary battery is obtained.

[0160] In some embodiments, in step S20, optionally, secondary batteries that satisfy the condition that the mass of the first solvent / rated capacity of the secondary battery is ≥1.0 g / Ah are selected.

[0161] In some embodiments, in step S20, optionally, secondary batteries that meet the requirements of electrolyte mass / rated capacity of secondary batteries being 2.8 g / Ah-3.3 g / Ah are selected.

[0162] In some embodiments, the preparation method further includes the step of: S30, screening out secondary batteries that satisfy the condition that the content of the first solvent / (thickness of the positive electrode film × 3 + 0.008 / porosity of the positive electrode film) ≥ 1. At this point, not only are high energy density and good kinetic performance of the battery ensured, but also a good negative electrode interface is ensured, enabling the battery of this application to have good cycle performance and storage performance.

[0163] In some embodiments, the preparation method further includes the step of: S40, screening out secondary batteries that satisfy the following conditions: the content of the first solvent / (thickness of the positive electrode film × 3 + 0.008 / porosity of the positive electrode film) ≥ 1 and (liquid retention coefficient × additive content) / (relative mass of the negative electrode × specific surface area of ​​the negative electrode active material × 0.012 + liquid retention coefficient × content of the first solvent × 0.03) ≥ 1. In this case, the problems of poor battery kinetic performance caused by the increased coating weight and compaction density of the positive electrode film, as well as the damage to the negative electrode interface caused by the first solvent, can be effectively solved.

[0164] Electric device

[0165] This application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack described in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., 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.

[0166] The electrical device can be equipped with a secondary battery, battery module, or battery pack according to its usage requirements.

[0167] FIG. 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the device's requirements for high power and high energy density, a battery pack or battery module can be used.

[0168] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use rechargeable batteries as their power source.

[0169] Embodiment

[0170] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on weight, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0171] Example 1

[0172] Preparation of positive electrode sheet

[0173] Lithium iron phosphate (LiFePO4), polyvinylidene fluoride (PVDF), and acetylene black (HAM) were mixed at a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) was then added to adjust the viscosity, and the mixture was thoroughly stirred according to methods known in the art to form a positive electrode slurry. The positive electrode slurry was uniformly coated onto an aluminum foil current collector, and after drying and cold pressing, a positive electrode sheet was obtained. The porosity of the positive electrode film was 30%, and the thickness of the positive electrode film was 0.1 mm.

[0174] Preparation of negative electrode sheet

[0175] The negative electrode active material graphite (with a specific surface area of ​​2m²) 2 The following ingredients are mixed in a mass ratio of 97:1:1:1:1: and then deionized water is added as a solvent. The mixture is stirred thoroughly according to methods known in the art to form a negative electrode slurry. The negative electrode slurry is then uniformly coated onto a copper foil current collector, dried, and cold-pressed to obtain a negative electrode sheet.

[0176] Preparation of electrolyte

[0177] In an argon-atmosphere glove box with a water content of <10ppm, the first solvent compound 1-1, the second solvent compound 2-1, and the third solvent compound 3-1 were mixed uniformly at a mass ratio of 50:30:20 to obtain an organic solvent. 15% by mass of lithium hexafluorophosphate (LiPF6) was slowly added as a lithium salt, and the mixture was stirred thoroughly until it was completely dissolved. After the mixture returned to room temperature, 3% of vinylene carbonate (VC), 2% of fluoroethylene carbonate (FEC), 0.5% of 1,3-propanesulfonyl lactone (PS), and 0.5% of lithium difluorodioxanol phosphate (LiDFOP) were added sequentially to obtain the electrolyte.

[0178] Preparation of separator

[0179] A polyethylene (PE) film coated with a nano-alumina layer is used as the isolation membrane.

[0180] Preparation of secondary battery

[0181] The positive electrode, separator, and negative electrode are stacked and wound in sequence to obtain an electrode assembly. The electrode assembly is then placed in an outer aluminum-plastic film packaging, dried, and injected with electrolyte. After encapsulation, settling, formation, aging, secondary encapsulation, and capacity testing, a secondary battery is obtained. The rated capacity of the first solvent / secondary battery is 1.6 g / Ah, the electrolyte retention coefficient of the secondary battery is 3.1 g / Ah, and the relative mass of the negative electrode is 1.7 g / Ah.

[0182] Examples 2-30 and Comparative Examples 1-8

[0183] The preparation methods of Examples 2-30 and Comparative Examples 1-8 are similar to those of Example 1, except that the relevant parameters in the preparation of the positive electrode, negative electrode and electrolyte are adjusted. For details of the parameters, please refer to Table 1 and Table 2.

[0184]

[0185]

[0186] Table 2

[0187]

[0188] Test section

[0189] (1) Initial DC internal resistance (DCR) test

[0190] At room temperature, the battery is charged at a constant current of 0.5C to 3.65V, and then charged at a constant voltage until the current is 0.05C. The battery is then discharged at a constant current of 0.5C for 30 minutes to adjust the battery to 50% SOC. The voltage of the battery at this time is recorded as U1. The battery is then discharged at a constant current of 4C for 30 seconds, with a sampling time of 0.1 seconds. The voltage at the end of the discharge is recorded as U2. The initial DCR of the battery is represented by the discharge DCR at 50% SOC. The initial DCR of the battery is (U1-U2) / 4C.

[0191] (2) 60℃ Cyclic Performance Test

[0192] At 60℃, the battery is charged at a constant current of 0.5C to 3.65V, then charged at a constant voltage to a current of 0.05C. The battery is then allowed to stand for 5 minutes, followed by a constant current discharge of 1 / 3C to 2.5V. This constitutes the battery's first charge-discharge cycle, and the discharge capacity recorded is the first cycle discharge capacity. The battery is subjected to 1000 charge-discharge cycles using the same method, and the discharge capacity after 1000 cycles is recorded. The capacity retention rate (%) after 1000 cycles at 60℃ is calculated as follows: (Discharge capacity after 1000 cycles / Discharge capacity of the first cycle) × 100%.

[0193] (3) Volume expansion rate test at 60℃

[0194] At 60℃, the battery is charged at a constant current of 0.5C to 3.65V, and then charged at a constant voltage until the current is 0.05C. The volume of the battery at this point is measured using the water displacement method and recorded as V0. The battery is then placed in a constant temperature chamber at 60℃ and stored for 30 days. After that, the volume of the battery is measured using the water displacement method and recorded as V1. The volume expansion rate of the battery after 30 days of storage at 60℃ is calculated as [(V1-V0) / V0]×100%.

[0195] The specific parameters of Examples 1-30 and Comparative Examples 1-8 are detailed in Tables 1 and 2, and the test results are detailed in Table 3.

[0196] In Table 3, Formula I = Content of the first solvent / (Thickness of the positive electrode film × 3 + 0.008 / Porosity of the positive electrode film), and Formula II = (Liquid retention coefficient × Content of additives) / (Relative mass of negative electrode × Specific surface area of ​​negative electrode active material × 0.012 + Liquid retention coefficient × Content of the first solvent × 0.03).

[0197] Table 3

[0198]

[0199] Comparing Examples 1-30 and Comparative Examples 1-8 reveals that only when the battery simultaneously satisfies the conditions of a first solvent mass / secondary battery rated capacity ≥ 0.7 g / Ah and an electrolyte mass / secondary battery rated capacity ≤ 3.5 g / Ah can the secondary battery maintain high energy density while exhibiting low initial DCR, high high-temperature cycle capacity retention, and low high-temperature storage volume expansion rate. When the first solvent mass / secondary battery rated capacity does not meet the condition ≥ 0.7 g / Ah, the content of the first solvent inside the battery is too low, failing to effectively improve the battery's kinetic performance, resulting in a high initial DCR. When the electrolyte mass / secondary battery rated capacity does not meet the condition ≤ 3.5 g / Ah, some organic solvent in the electrolyte continuously consumes active ions, leading to a continuous increase in irreversible consumption of active ions, preventing the battery from simultaneously achieving low initial DCR, high high-temperature cycle capacity retention, and low high-temperature storage volume expansion rate.

[0200] Comparing Examples 1-7 reveals that the batteries exhibited low initial DCR, high high-temperature cycle capacity retention, and low high-temperature storage volume expansion. Furthermore, the performance of the batteries varied slightly depending on the type of first solvent in the electrolyte, due to the inherent characteristics of each solvent. Among these first solvents, compound 1-1 had the lowest viscosity and the highest dielectric constant, resulting in the lowest initial DCR for the prepared battery. However, compound 1-1 exhibited the worst stability at the negative electrode, more easily disrupting the interface stability and causing continuous film formation on the surface of the active material, leading to the greatest irreversible lithium-ion consumption. The products generated by the reduction and decomposition of compound 1-1 at the negative electrode also accumulated at the interface, further prolonging the lithium-ion transport path and slowing down the lithium-ion transport rate. Therefore, among Examples 1-7, Example 1 exhibited the worst negative electrode interface condition, the fastest high-temperature cycle capacity decay, and the largest high-temperature storage volume expansion.

[0201] By comparing Examples 8-9, it can be found that the lower the content of the first solvent, the less damage to the negative electrode interface, the higher the high-temperature cycle capacity retention rate of the battery, and the lower the high-temperature storage volume expansion rate. However, at the same time, the lower the content of the first solvent, the weaker the improvement on the battery's dynamic performance, and the larger the battery's initial DCR.

[0202] Comparing Examples 10-11 reveals that reducing the thickness of the positive electrode film leads to a decrease in the initial DCR, an increase in high-temperature cycle capacity retention, and a reduction in high-temperature storage volume expansion. This is primarily because a reduced positive electrode film thickness shortens the lithium-ion transport path, improving the battery's kinetic performance and reducing the initial DCR. The olivine-structured lithium phosphate used in the positive electrode film contains polar functional groups and has a large specific surface area, making it highly susceptible to water absorption. The drying process before electrolyte injection cannot completely remove moisture from the positive electrode film. During battery use, moisture in the positive electrode film gradually diffuses into the electrolyte and reacts with it; simultaneously, the presence of moisture damages the SEI film on the surface of the negative electrode active material, affecting the stability of the negative electrode interface. Therefore, reducing the thickness of the positive electrode film makes it easier to remove moisture during the drying process, reducing the damage to the negative electrode interface, and consequently slowing down the high-temperature cycle capacity decay and reducing high-temperature storage gas generation.

[0203] Comparing Examples 12-13 reveals that increased porosity of the positive electrode film leads to a decrease in the initial DCR, an increase in high-temperature cycle capacity retention, and a reduction in high-temperature storage volume expansion. Increased porosity in the positive electrode film allows for easier electrolyte wetting, reducing resistance to lithium-ion transport and thus improving battery kinetics and lowering the initial DCR. Furthermore, increased porosity makes it easier to remove moisture from the positive electrode film during the drying process, reducing the damaging effect of moisture on the negative electrode interface. Consequently, high-temperature cycle capacity decay slows down, and high-temperature storage gas generation decreases.

[0204] Comparing Examples 16-17 reveals that increasing the additive content leads to a higher initial DCR, increased high-temperature cycle capacity retention, and a lower high-temperature storage volume expansion rate. This is primarily because increasing the additive content results in a greater amount of additive consumed during the formation process, leading to a thicker SEI film formed on the surface of the negative electrode active material and increased negative electrode film resistance, thus increasing the initial DCR. Correspondingly, the increased additive content enhances the protective effect of the SEI film on the negative electrode active material, improves the stability of the negative electrode interface, and increases tolerance to side effects caused by the first solvent. Consequently, the high-temperature cycle capacity decay slows down, and the high-temperature storage gas production decreases.

[0205] Comparing Examples 18-21 reveals that reducing the relative mass of the negative electrode and the specific surface area of ​​the negative electrode active material increases the initial DCR of the battery, improves high-temperature cycle capacity retention, and reduces high-temperature storage volume expansion. This is primarily because reducing the relative mass of the negative electrode and the specific surface area of ​​the negative electrode active material reduces the amount of additives required to form an SEI film on the surface of the negative electrode active material. This significantly reduces the battery's additive requirements, resulting in a more abundant amount of additives within the battery. Consequently, the high-temperature cycle capacity decay slows down, and high-temperature storage gas generation decreases. The reduced additive requirements also lead to an additive overload within the battery, increasing the amount of additives available for SEI film formation on the surface of the negative electrode active material during the formation process. This results in a thicker SEI film, thus increasing the initial DCR of the battery.

[0206] Comparing Examples 22-25 reveals that the electrolyte kinetic performance levels generally meet the overall requirements of the positive electrode film for electrolyte kinetic performance, resulting in low initial DCR for the batteries. However, when certain parameters change, causing the additive retention level to fall below the required level, the additive's protection of the negative electrode interface becomes insufficient. The first solvent then undergoes side reactions at the negative electrode interface, consuming a certain amount of lithium ions. Consequently, the battery's high-temperature cycle capacity decay accelerates. The first solvent also damages the SEI film on the surface of the negative electrode active material, necessitating continuous repair of the SEI film during battery use. This repair process generates a certain amount of gas, leading to a greater high-temperature storage volume expansion rate.

[0207] As the specific surface area of ​​the negative electrode active material increases, the amount of additives required to form an SEI film on its surface increases. Since the total amount of additives is constant, the SEI film formed on the surface of the negative electrode active material will be relatively thinner, thus reducing the initial DCR of the battery. However, in Example 22, the increased specific surface area of ​​the negative electrode active material results in an additive retention level lower than the required level. The additives cannot adequately form an SEI film on the surface of the negative electrode active material, failing to adequately protect the negative electrode interface. Consequently, the high-temperature cycle capacity decay of the battery is significantly faster, and the high-temperature storage volume expansion rate is significantly larger.

[0208] With a reduction in the additive content, the number of additives that can participate in the formation of an SEI film on the surface of the negative electrode active material decreases, resulting in a lower negative electrode film-forming impedance and thus a lower initial DCR of the battery. However, in Example 23, due to the reduced additive content, the additive retention level is lower than the required level, and the additive cannot fully form an SEI film on the surface of the negative electrode active material, failing to adequately protect the negative electrode interface. Consequently, the battery's high-temperature cycle capacity decay is significantly faster, and the high-temperature storage volume expansion rate is significantly larger.

[0209] When the battery's liquid retention coefficient decreases, the number of additives that can participate in the formation of an SEI film on the surface of the negative electrode active material decreases, and the negative electrode film-forming impedance becomes lower. Therefore, the battery's initial DCR also decreases. However, in Example 24, because the liquid retention coefficient is reduced, the additive retention level is lower than the additive requirement level. At this time, the additive can no longer adequately protect the negative electrode interface. Therefore, the battery's high-temperature cycle capacity decay is significantly faster, and the high-temperature storage volume expansion rate is significantly larger.

[0210] With an increase in the relative mass of the negative electrode, the SEI film formed on the surface of the negative electrode active material becomes relatively thinner due to a fixed total amount of additives. Therefore, the initial DCR of the battery decreases. However, in Example 25, the increase in the relative mass of the negative electrode results in a lower additive retention level than the required level. The additives cannot fully form an SEI film on the surface of the negative electrode active material and cannot adequately protect the negative electrode interface. Consequently, the high-temperature cycle capacity decay of the battery increases significantly, and the high-temperature storage volume expansion rate increases significantly.

[0211] Comparative studies of Examples 26-28 reveal that when the retention level of the additive exceeds its required level, the negative electrode interface remains effectively protected even with the presence of the first solvent in the electrolyte. Therefore, the battery maintains a high high-temperature cycle capacity retention rate and a low high-temperature storage volume expansion rate. However, when certain parameters change, causing the electrolyte kinetic performance to fall short of the positive electrode film's overall requirements for electrolyte kinetic performance, the battery's initial DCR increases significantly.

[0212] In Example 26, the reduced content of the first solvent caused the electrolyte kinetic performance level to fall short of the total requirements of the positive electrode film for electrolyte kinetic performance, thus failing to effectively improve the kinetic performance of the electrolyte and the battery, resulting in a significant increase in the initial DCR of the battery.

[0213] Increasing the thickness of the positive electrode film increases the energy density of the battery. However, in Example 27, the increased thickness of the positive electrode film causes the electrolyte kinetic performance to fall short of the overall requirements of the positive electrode film for electrolyte kinetic performance. As a result, the transport path of lithium ions in the positive electrode film becomes longer, and the initial DCR of the battery increases significantly.

[0214] The reduced porosity of the positive electrode film increases the energy density of the battery. However, in Example 28, the reduced porosity of the positive electrode film results in the electrolyte kinetic performance not meeting the overall requirements of the positive electrode film for electrolyte kinetic performance. The electrolyte is less able to wet the positive electrode film, and the transport resistance of lithium ions in the positive electrode film increases. Therefore, the initial DCR of the battery will increase significantly.

[0215] Comparing Examples 29-30 reveals that the battery exhibits a higher initial DCR, lower high-temperature cycle capacity retention, and higher high-temperature storage volume expansion rate. This is primarily due to two factors: First, the increased thickness and decreased porosity of the positive electrode film make it more difficult for the electrolyte to wet the film, resulting in a longer lithium-ion transport path and increased transport resistance. Consequently, the electrolyte kinetic performance does not meet the overall requirements of the positive electrode film for electrolyte kinetic performance, leading to a higher initial DCR. Second, the lower content of electrolyte additives results in a lower additive retention level than required, preventing the additives from adequately protecting the negative electrode interface. Consequently, the battery experiences faster high-temperature cycle capacity decay and increased gas production during high-temperature storage.

[0216] Comparing Examples 14-15, it can be observed that as the lithium salt mass percentage increases, the initial DCR of the battery first decreases and then increases. This is mainly due to the change in the conductivity of the electrolyte. The conductivity of the electrolyte is affected by the total number of migrateable lithium ions and the lithium ion migration rate. An increase in the lithium salt mass percentage increases the total number of migrateable lithium ions, but simultaneously increases the electrolyte viscosity, and the lithium ion migration rate slows down. Therefore, there is an optimal value for the lithium salt mass percentage, at which the initial DCR of the battery is lowest. Furthermore, a higher lithium salt mass percentage results in a lower content of free organic solvents in the electrolyte, and a lower content of the primary solvent that causes side reactions at the negative electrode interface. Consequently, the high-temperature cycle capacity decay of the battery slows down, and the high-temperature storage gas production decreases.

[0217] As shown in Table 3, the batteries in Examples 1-22 satisfy both Formula I and Formula II, achieving a first solvent mass / secondary battery rated capacity ≥ 0.7 g / Ah and an electrolyte mass / secondary battery rated capacity ≤ 3.5 g / Ah. This results in batteries exhibiting low initial DCR, high high-temperature cycling capacity retention, and low high-temperature storage volume expansion. On one hand, the electrolyte kinetics meet the overall requirements of the positive electrode film for battery kinetics, leading to a low initial DCR. On the other hand, the electrolyte additives effectively protect the negative electrode interface, suppressing the exacerbation of negative electrode interface side reactions caused by the instability of the first solvent. Consequently, the high-temperature cycling capacity decay slows down, and the high-temperature storage gas production decreases.

[0218] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, comprising a positive electrode and an electrolyte; The positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, wherein the positive electrode film layer includes one or more of lithium phosphates with an olivine structure and their modified compounds. The electrolyte includes an organic solvent, and the organic solvent includes the first solvent shown in Formula 1; ； In Equation 1, R 11 It is one of hydrogen, C1-C4 alkyl, or C1-C4 haloalkyl, R 12 It is one of C1-C4 alkyl groups and C1-C4 haloalkyl groups; Based on the mass of the electrolyte, the mass percentage of the first solvent is greater than or equal to 29%; And the secondary battery satisfies: The electrolyte mass / rated capacity of the secondary battery is 2.5 g / Ah - 3.5 g / Ah. The porosity of the positive electrode film is 5%-50%. The secondary battery further includes a negative electrode sheet, which comprises a negative electrode active material having a specific surface area of ​​1.5 m². 2 / g-2.8m 2 / g.

2. The secondary battery according to claim 1, wherein, Based on the mass of the electrolyte, the mass percentage of the first solvent is 29%-61.3%.

3. The secondary battery according to claim 1, wherein, The mass of the electrolyte relative to the rated capacity of the secondary battery is 2.9 g / Ah - 3.3 g / Ah.

4. The secondary battery according to any one of claims 1 to 3, wherein, The secondary battery also satisfies the following condition: the content of the first solvent / (thickness of the positive electrode film × 3 + 0.008 / porosity of the positive electrode film) ≥ 1. The content of the first solvent refers to the percentage by mass of the first solvent based on the total mass of the organic solvents. The thickness of the positive electrode film is expressed in mm as the thickness of the single-sided positive electrode film on the positive current collector.

5. The secondary battery according to claim 4, wherein, The content of the first solvent / (thickness of the positive electrode film × 3 + 0.008 / porosity of the positive electrode film) = 1~1.

84.

6. The secondary battery according to any one of claims 1 to 3, wherein, The electrolyte also includes additives, the reduction potential of which is ≥0.8V (vs Li). + / Li).

7. The secondary battery according to claim 6, wherein, The content of the additive is ≥3%, and the content of the additive refers to the mass percentage of the additive based on the total mass of the electrolyte.

8. The secondary battery according to claim 7, wherein, The content of the additive is 3% to 9%.

9. The secondary battery according to claim 7, wherein, The additives include organic additives, which include one or more of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, and 1,3-propane sulpholactone.

10. The secondary battery according to claim 9, wherein, The organic additives include vinylene carbonate, fluoroethylene carbonate, and at least one selected from vinyl ethylene carbonate, vinyl sulfate, and 1,3-propane sulpholactone.

11. The secondary battery according to claim 9 or 10, wherein, The additives also include inorganic additives, which include one or more of lithium dioxaburate borate, lithium difluorodioxaburate phosphate, lithium tetrafluorooxaburate phosphate, and lithium difluorooxaburate borate.

12. The secondary battery according to claim 11, wherein, Based on the total mass of the additives, the organic additives comprise ≥50% by mass.

13. The secondary battery according to claim 11, wherein, Based on the total mass of the additives, the organic additives account for ≥70% by mass.

14. The secondary battery according to claim 11, wherein, Based on the total mass of the additives, the mass percentage of the organic additives is 83.3%-94.4%.

15. The secondary battery according to claim 11, wherein, Based on the total mass of the additives, the inorganic additives comprise ≤50% by mass.

16. The secondary battery according to claim 11, wherein, Based on the total mass of the additives, the mass percentage of the inorganic additives is ≤30%.

17. The secondary battery according to claim 11, wherein, Based on the total mass of the additives, the inorganic additives account for 5.6%-16.7% of the mass.

18. The secondary battery according to claim 1, wherein, The secondary battery also satisfies: (liquid retention coefficient × additive content) / (relative mass of negative electrode × specific surface area of ​​negative electrode active material × 0.012 + liquid retention coefficient × content of first solvent × 0.03) ≥ 1. The electrolyte retention factor is the ratio of the mass of the electrolyte to the rated capacity of the secondary battery, expressed in g / Ah. The additive content refers to the mass percentage of the additive based on the total mass of the electrolyte. The relative mass of the negative electrode refers to the ratio of the mass of the negative electrode active material in the negative electrode sheet to the rated capacity of the secondary battery, expressed in g / Ah.

19. The secondary battery according to claim 18, wherein, (Liquid retention coefficient × Additive content) / (Relative mass of negative electrode × Specific surface area of ​​negative electrode active material × 0.012 + Liquid retention coefficient × Content of first solvent × 0.03) = 2~3.

27.

20. The secondary battery according to claim 1, wherein, The thickness of the positive electrode film is ≥0.07mm.

21. The secondary battery according to claim 20, wherein, The thickness of the positive electrode film is 0.07mm-0.14mm.

22. The secondary battery according to claim 20, wherein, The thickness of the positive electrode film is 0.07mm-0.12mm.

23. The secondary battery according to claim 18, wherein, The relative mass of the negative electrode is ≤2.1 g / Ah; and / or, The additive content is ≥2.5% based on the total mass of the electrolyte.

24. The secondary battery according to claim 23, wherein, The relative mass of the negative electrode is 1.2 g / Ah - 1.9 g / Ah; and / or, The additive content is 2.5%-9% based on the total mass of the electrolyte.

25. The secondary battery according to claim 1, wherein, In Equation 1, R 11 It is one of hydrogen, methyl, ethyl, propyl, butyl, fluoromethyl, fluoroethyl, fluoropropyl, and fluorobutyl, R 12 It is one of methyl, ethyl, propyl, butyl, fluoromethyl, fluoroethyl, fluoropropyl, and fluorobutyl.

26. The secondary battery according to claim 1, wherein, The first solvent shown in Formula 1 is selected from one or more of the following compounds:

27. The secondary battery according to claim 1, wherein, In Equation 1, R 11 It is one of hydrogen, methyl, ethyl, propyl, and butyl, R 12 It is one of methyl, ethyl, propyl, and butyl.

28. The secondary battery according to claim 1, wherein, The organic solvent also includes one or more of the second solvent shown in Formula 2 and the third solvent shown in Formula 3. In Formula 2, R 21 R is one of H, methyl, or ethyl, and in formula 3, R 31 R 32 Each of the following can be independently selected: methyl, ethyl, or propyl. 。 29. The secondary battery according to claim 28, wherein, The second solvent shown in Formula 2 is selected from one or two of the following compounds: 。 30. The secondary battery according to claim 28, wherein, The third solvent shown in Formula 3 is selected from one or more of the following compounds: 。 31. The secondary battery according to claim 28, wherein, The organic solvent includes compound 2-1, compound 3-1, and one of compounds 1-1 to 1-8 selected below:

32. The secondary battery according to claim 28, wherein, Based on the total mass of the organic solvents, the mass percentage of the second solvent is ≥10%; and / or, Based on the total mass of organic solvents, the mass percentage of the third solvent is ≥0%.

33. The secondary battery according to claim 32, wherein, Based on the total mass of the organic solvents, the mass percentage of the second solvent is 10% to 80%; and / or, The third solvent has a mass percentage of 5% to 80% based on the total mass of the organic solvents.

34. The secondary battery according to claim 33, wherein, The second solvent has a mass percentage of 20% to 50% based on the total mass of the organic solvents; and / or, the third solvent has a mass percentage of 5% to 20% based on the total mass of the organic solvents.

35. The secondary battery according to claim 1, wherein, The olivine-structured lithium phosphates include one or more of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and their respective modified compounds.

36. The secondary battery according to any one of claims 1 to 3, wherein, The single-sided coating weight of the positive electrode film is 23 mg / cm³. 2 ~29mg / cm 2 , and / or The compaction density of the positive electrode film is 2.1 g / cm³. 3 ~2.8g / cm 3 .

37. The secondary battery according to claim 36, wherein, The single-sided coating weight of the positive electrode film is 25 mg / cm³. 2 ~29mg / cm 2 , and / or The compaction density of the positive electrode film is 2.3 g / cm³. 3 ~2.6g / cm 3 .

38. A battery module comprising the secondary battery according to any one of claims 1-37.

39. A battery pack comprising a secondary battery according to any one of claims 1-37 and a battery module according to claim 38.

40. An electrical device comprising at least one of the secondary battery according to any one of claims 1-37, the battery module according to claim 38, and the battery pack according to claim 39.

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