Secondary battery, battery module, battery pack, and electric device

Abstract

Provided are a secondary battery (5), a battery module (4), a battery pack (1), and an electric device. The secondary battery (5) comprises: an electrode assembly (52) and an electrolyte for infiltrating the electrode assembly (52); wherein the electrode assembly (52) comprises a negative electrode sheet, a separator, and a positive electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer on at least one surface of the negative electrode current collector, the tortuosity of the negative electrode sheet is τ, the porosity of the negative electrode material layer is ε, the Bruggeman exponent of the negative electrode material is α, the conductivity of the electrolyte is σ, 2.3≤τ≤7, the σ ranges from 8 mS / cm to 14 mS / cm, the porosity ε of the negative electrode material layer ranges from 25% to 45%, and the α ranges from 1.5 to 2.2. The secondary battery (5) has excellent fast charging performance.

Description

TECHNICAL FIELD

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

[0002] Secondary batteries have reliable working performance, and advantages such as no pollution and no memory effect, and are widely used. For example, as environmental protection issues are increasingly valued, new energy vehicles are increasingly popular, and the demand for power type secondary batteries will show explosive growth. However, as the application range of secondary batteries becomes more and more extensive, the performance of secondary batteries is also challenged.

[0003] As the pace of life is getting faster, in addition to the strong demand for the cycle life of secondary batteries, fast charging performance is also a problem that people must consider. SUMMARY

[0004] The present application is made in view of the above-mentioned problems, and aims to provide a secondary battery, a battery module, a battery pack and a power consuming device, which has excellent fast charging performance.

[0005] To achieve the above-mentioned purpose, the first aspect of the present application provides a secondary battery, comprising: an electrode assembly and an electrolyte for infiltrating the electrode assembly; wherein the electrode assembly comprises a negative electrode sheet, a separator and a positive electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer on at least one surface of the negative electrode current collector, the tortuosity of the negative electrode sheet is τ, the porosity of the negative electrode material layer is ε, the Bruggeman index of the negative electrode material is α, the conductivity of the electrolyte is σ, 2.3≤τ≤7, the range of σ is 8mS / cm to 14mS / cm, the porosity of the negative electrode material layer ε is 25% to 45%, and the α is 1.5-2.2.

[0006] By satisfying the above relationship between the tortuosity τ of the negative electrode sheet and the conductivity σ of the electrolyte, the secondary battery can obtain excellent fast charging characteristics and inhibit lithium precipitation of the negative electrode.

[0007] In some embodiments, the negative electrode material is graphite. In this way, the tortuosity of the negative electrode sheet can be more accurately estimated.

[0008] In some embodiments, the electrolyte contains at least one of a cyclic ester, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethyl acetate, methyl acetate, methyl formate, ethyl formate, methyl propionate, propyl formate, ethyl propionate, and propyl acetate, and the cyclic ester contains at least one of ethylene carbonate and propylene carbonate. In this way, the conductivity of the electrolyte can be easily adjusted.

[0009] In some embodiments, the lithium salt of the electrolyte comprises at least one of lithium hexafluorophosphate (LiPF6) and lithium fluorosulfonylimide, and the concentration of the lithium salt is 0.5-1.5 mol / L. This further increases the conductivity of the electrolyte and suppresses lithium plating at the negative electrode.

[0010] In some embodiments, the fluorinated sulfonylimide lithium includes at least one selected from lithium bis(fluorosulfonylimide), lithium fluoro(trifluoromethyl)sulfonylimide, lithium bis(trifluoromethyl)sulfonylimide, lithium bis(pentafluoroethyl)sulfonylimide, and lithium fluorine (perfluorobutylsulfonylimide), preferably lithium bis(fluorosulfonylimide). This further increases the conductivity of the electrolyte and suppresses lithium deposition at the negative electrode.

[0011] In some embodiments, the thickness of the negative electrode material layer in the negative electrode sheet is 30 μm-400 μm. This increases the coating amount of the negative electrode active material, thereby improving the energy density of the secondary battery.

[0012] A second aspect of this application is to provide a battery module comprising a secondary battery as described in the first aspect of this application.

[0013] A third aspect of this application is to provide a battery pack that includes the battery module described in the second aspect of this application.

[0014] A fourth aspect of this application is to provide an electrical device comprising at least one of the secondary battery described in the first aspect of this application, the battery module described in the second aspect of this application, and the battery pack described in the third aspect of this application.

[0015] According to this application, the fast charging capability of secondary batteries can be improved and lithium plating on the negative electrode can be suppressed. Attached Figure Description

[0016] Figure 1 This is a schematic diagram representing the tortuosity τ of the electrode.

[0017] Figure 2 This is a top-side SEM image of an example negative electrode material layer used to illustrate the calculation method for electrode tortuosity τ.

[0018] Figure 3 This is a cross-sectional SEM image of an example negative electrode material layer used to illustrate the calculation method of electrode tortuosity τ.

[0019] Figure 4 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0020] Figure 5 yes Figure 4 An exploded view of a secondary battery according to one embodiment of this application is shown.

[0021] Figure 6 This is a schematic diagram of a battery module according to one embodiment of this application.

[0022] Figure 7 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0023] Figure 8 yes Figure 7 An exploded view of a battery pack according to one embodiment of this application is shown.

[0024] Figure 9 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0025] Explanation of reference numerals in the attached figures:

[0026] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Casing; 52 Electrode assembly; 53 Top cover assembly Detailed Implementation

[0027] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, its manufacturing method, positive electrode sheet, secondary battery, battery module, battery pack, and electrical 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 a full understanding of this application by those skilled in the art and are not intended to limit the subject matter of the claims.

[0028] 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 understood that ranges of 60–110 and 80–120 are also expected. 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 "a–b" 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.

[0029] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0030] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0031] 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.

[0032] 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).

[0033] In one embodiment of this application, a secondary battery is provided, comprising: an electrode assembly and an electrolyte for wetting the electrode assembly; wherein the electrode assembly includes a negative electrode, a separator, and a positive electrode, the negative electrode including a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector, and the tortuosity of the negative electrode is τ, then τ satisfies the following equation I.

[0034]

[0035] Where ε is the porosity of the negative electrode material layer, and α is the Bruggeman exponent of the negative electrode material.

[0036] The τ and the conductivity σ of the electrolyte satisfy the following equation II.

[0037] (2τ) 0.5 +6≤σ≤(2τ) 0.5 +10 type II.

[0038] Although the mechanism is not yet clear, the inventors of this application unexpectedly discovered after processing a large amount of experimental data that by satisfying the above relationship between the tortuosity τ of the negative electrode and the conductivity σ of the electrolyte, the secondary battery can obtain excellent fast charging characteristics.

[0039] The inventors of this application hypothesize that two factors limit the fast-charging capability of secondary batteries: 1) the transport kinetics of lithium ions in the negative electrode; and 2) the liquid-phase diffusion capability of lithium ions in the electrolyte. If these two properties are mismatched—for example, if lithium ion liquid-phase transport is fast but the negative electrode material has poor kinetics—lithium ions migrating to the negative electrode surface cannot diffuse into the negative electrode interior in time, leading to the direct reduction of lithium ions to metallic lithium on the anode surface, i.e., "anode lithium deposition," which poses a safety risk. Conversely, if the negative electrode material has good kinetics but the liquid-phase diffusion capability of lithium ions in the electrolyte is weak, lithium ions released from the positive electrode cannot reach the negative electrode in time, resulting in a reduction in the charging capacity of the secondary battery. Therefore, only when the kinetics of the negative electrode material and the electrolyte are matched can good fast-charging performance be achieved.

[0040] Here, as Figure 1 As shown, the tortuosity τ of the electrode represents the ratio of the lithium ion migration path ΔL to the electrode thickness Δx. The tortuosity τ is closely related to the electrode porosity ε; for different anode materials, 0.5(ε) can be used. -α To estimate the tortuosity τ.

[0041] The Bruggeman exponent α can be calculated using SEM images of the anode material layer obtained with Wolfram Mathmatica software. The following is a reference... Figure 2 , 3 This explains how to calculate the Bruggeman exponent α.

[0042] First, SEM images of the top surface and cross-section of the negative electrode material layer are obtained for the fabricated negative electrode sheet. Figure 2 This is an example of a SEM image of the top surface of the negative electrode material layer. Figure 3This is an example of a cross-sectional SEM image of the negative electrode material layer.

[0043] Secondly, using Wolfram Mathmatica software, the major and minor axes of 50 to 90 active particles were manually marked in the SEM images of the top surface and cross-section of the negative electrode material layer, and the outline of each active particle was fitted.

[0044] Finally, using Wolfram Mathmatica software, based on the SEM images of the top surface and cross-section of the negative electrode material layer with the outline of the active particles obtained above, the Bruggeman exponent α in the planar direction of the negative electrode material layer was calculated. x α y and the Bruggeman exponent α in the normal direction (section direction) z The above equation (ε) -α In this context, α represents the Bruggeman exponent α in the normal direction (section direction). z .

[0045] In addition, the porosity of the electrode prepared above was tested, and the test method for electrode porosity was carried out in accordance with GB / T24586-2009.

[0046] In some implementations, the tortuosity τ of the negative electrode sheet and the conductivity σ of the electrolyte can further satisfy: (2τ) 0.5 +6≤σ≤(2τ) 0.5 +8. This allows for further matching of the kinetic properties of the negative electrode material and the electrolyte, resulting in better fast-charging performance.

[0047] In some embodiments, the tortuosity τ satisfies: 2.3 ≤ τ ≤ 7. By keeping the tortuosity τ within the above range, the kinetic performance of the negative electrode material is improved, thereby further enhancing the fast-charging characteristics of the secondary battery.

[0048] In some embodiments, the conductivity σ ranges from 8 mS / cm to 14 mS / cm. By ensuring the electrolyte conductivity σ is within this range, the kinetic performance of the electrolyte can be improved, thereby further enhancing the fast-charging characteristics of the secondary battery.

[0049] In some embodiments, the porosity ε of the negative electrode material layer is between 25% and 45%. With a porosity within this range, the kinetic performance of the negative electrode material can be improved, thereby further enhancing the fast-charging characteristics of the secondary battery.

[0050] In some embodiments, the negative electrode material of the negative electrode sheet is graphite, and α is 1.5 to 2.2. This allows for a more accurate estimation of the tortuosity of the negative electrode sheet. It should be noted that the negative electrode material is not limited to graphite and can also be other commonly used negative electrode materials. The Bruggeman exponent α is also applicable to negative electrode materials other than graphite.

[0051] In some embodiments, the electrolyte comprises at least one of cyclic esters, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethyl acetate, methyl formate, ethyl formate, methyl propionate, propyl formate, ethyl propionate, and propyl acetate, wherein the cyclic ester comprises at least one of ethylene carbonate and propylene carbonate. This allows for easy adjustment of the electrolyte's conductivity.

[0052] In some embodiments, the lithium salt of the electrolyte comprises at least one of lithium hexafluorophosphate (LiPF6) and lithium fluorosulfonylimide, and the concentration of the lithium salt is 0.5-1.5 mol / L. This further increases the conductivity of the electrolyte and suppresses lithium plating at the negative electrode.

[0053] In some embodiments, the fluorinated sulfonylimide lithium includes at least one selected from lithium bis(fluorosulfonylimide), lithium fluoro(trifluoromethyl)sulfonylimide, lithium bis(trifluoromethyl)sulfonylimide, lithium bis(pentafluoroethyl)sulfonylimide, and lithium fluorine (perfluorobutylsulfonylimide), preferably lithium bis(fluorosulfonylimide). This further increases the conductivity of the electrolyte and suppresses lithium deposition at the negative electrode.

[0054] In some embodiments, the coating thickness of the negative electrode material layer in the negative electrode sheet is 30 μm-400 μm. This increases the coating amount of the negative electrode active material, thereby improving the energy density of the secondary battery.

[0055] In addition, the secondary battery, battery module, battery pack and power device of this application will be described below with appropriate reference to the accompanying drawings.

[0056] [Rechargeable Battery]

[0057] In one embodiment of this application, a secondary battery is provided.

[0058] Typically, a secondary battery consists of a negative electrode, a positive electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0059] [Positive electrode plate]

[0060] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including the positive electrode active material of the first aspect of this application.

[0061] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0062] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0063] In some embodiments, the positive electrode active material may be a known battery positive electrode active material. As an example, the positive electrode active material may include at least one of the following: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, lithium nickel cobalt manganese oxides (such as LiNi1 / 3Co1 / 3Mn1 / 3O2 (also abbreviated as NCM333), LiNi0.5Co0.2Mn0.3O2 (also abbreviated as NCM523), LiNi0.5Co0.25Mn0.25O2 (also abbreviated as NCM211), and LiNi0.6Co At least one of 0.2Mn0.2O2 (also abbreviated as NCM622), LiNi0.8Co0.1Mn0.1O2 (also abbreviated as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi0.85Co0.15Al0.05O2) and their modified compounds. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also abbreviated as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0064] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0065] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0066] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0067] [Negative electrode plate]

[0068] 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, the negative electrode film layer including a negative electrode active material.

[0069] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0070] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0071] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0072] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0073] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0074] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0075] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0076] [Isolation membrane]

[0077] As for the aforementioned separator, this application does not have any particular limitations. Any known porous structure separator with electrochemical and mechanical stability can be selected according to actual needs. For example, it can be a single-layer or multi-layer film containing one or more of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride.

[0078] Electrolyte

[0079] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0080] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt, a solvent, and additives.

[0081] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0082] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0083] In some embodiments, the electrolyte includes additives, including fluoroethylene carbonate and / or vinylene carbonate. In addition, it may include other additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0084] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0085] 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.

[0086] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a 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; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0087] 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. For example, Figure 4 This is an example of a square-structured secondary battery 5.

[0088] In some implementations, refer to Figure 5The outer packaging may include a housing 51 and a cover 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 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 using a winding or stacking process. The electrode assembly 52 is encapsulated within 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 selected by those skilled in the art according to specific practical needs.

[0089] Battery module

[0090] 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 one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0091] Figure 6 This is battery module 4, used as an example. (See reference...) Figure 6 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.

[0092] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0093] Battery pack

[0094] 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 one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0095] Figure 7 and Figure 8 This is battery pack 1 as an example. (See reference...) Figure 7 and Figure 8 The 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, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0096] Electric device

[0097] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided 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 include, 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.

[0098] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0099] Figure 9 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 high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0100] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0101] Embodiment

[0102] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0103] <Example 1>

[0104] Preparation method

[0105] (1) Preparation of electrolyte

[0106] In an argon-filled glove box (water content <10 ppm, oxygen content <1 ppm), with a total mass of 100 parts, a non-aqueous organic solvent comprising 25.5 parts ethylene carbonate and 59.5 parts methyl ethyl carbonate was mixed thoroughly. Then, 15 parts lithium hexafluorophosphate (LiPF6) were slowly added to the non-aqueous organic solvent. After the lithium salt was completely dissolved, the target electrolyte was obtained. The room temperature conductivity of the target electrolyte was tested.

[0107] The room temperature conductivity of the electrolyte was tested in accordance with HG-T 4067-2015.

[0108] (2) Preparation of the positive electrode:

[0109] LiNi, the positive electrode active material 0.5 Co 0.2 Mn 0.3 A positive electrode slurry is prepared in N-methylpyrrolidone (NMP) using O2, conductive agent Super P, and binder polyvinylidene fluoride (PVDF). The solid content of the positive electrode slurry is 50 wt%, and the solid component is LiNi. 0.5 Co 0.2 Mn 0.3 The mass ratio of O2, Super P, and PVDF is 95:2:3. The positive electrode slurry is coated onto the current collector aluminum foil and dried at 85°C. After cold pressing, the foil is trimmed, cut, and slit. Then, it is dried under vacuum at 85°C for 4 hours to produce the positive electrode sheet.

[0110] (3) Preparation of negative electrode material - graphite A: Graphite with an average particle size Dv50 of 15 μm was prepared as the negative electrode material. The Bruggeman index α of this graphite is 1.9. Those skilled in the art can adjust the Bruggeman index α of graphite by adjusting the average particle size parameter of the graphite.

[0111] It should be noted that graphite B and graphite C can also be obtained by those skilled in the art by adjusting the particle size of graphite.

[0112] (4) Preparation of negative electrode:

[0113] The graphite used as the negative electrode active material was mixed evenly with the conductive agent Super P, the thickener CMC, and the binder styrene-butadiene rubber (SBR) in deionized water to prepare a negative electrode slurry. The solid content of the negative electrode slurry was 30 wt%, and the mass ratio of graphite, Super P, CMC, and SBR in the solid components was 94:3:3. The negative electrode slurry was coated onto the current collector copper foil and dried at 85°C. Then, it was cold-pressed, trimmed, cut into sheets, and slit. Finally, it was dried under vacuum at 120°C for 12 hours to prepare the negative electrode sheet. Here, the pressure of the cold pressing roller was 38T, resulting in a compaction density of 1.24 g / cm³. 3 The electrode has a porosity of 45%.

[0114] It should be noted that the porosity of different negative electrode material (graphite) layers can be adjusted by controlling the compaction density of the electrode sheet by controlling the pressure of the cold pressing roller.

[0115] (5) Preparation of lithium-ion batteries:

[0116] A 16μm polyethylene film (PE) is used as the separator. The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrodes to isolate them. The cells are then wound to obtain a bare cell, tabs are welded on, and the bare cell is placed in an outer package. The electrolyte prepared above is injected into the dried cell, followed by encapsulation, settling, formation, shaping, and capacity testing to complete the preparation of the lithium-ion battery (the thickness of the soft-pack lithium-ion battery is 4.0mm, the width is 60mm, and the length is 140mm).

[0117] ② Performance evaluation

[0118] (i) Room temperature conductivity of the electrolyte

[0119] The electrolyte prepared in (1) above was subjected to conductivity testing. The room temperature conductivity of the electrolyte was tested in accordance with HG-T4067-2015.

[0120] (ii) Porosity test of the electrode sheet

[0121] The porosity of the electrode obtained in (3) above was tested, and the method for testing the porosity of the electrode was in accordance with GB / T24586-2009.

[0122] (iii) Fast charging performance test

[0123] For the battery prepared in (4) above, the lithium-ion battery is charged to 4.25V at 1C constant current at 25°C, then charged to 0.05C at 4.25V constant voltage, and then discharged to 2.8V at 1C constant current. The discharge capacity D0 of the battery is recorded. Then, it is charged to 4.25V at 4C constant current, then charged to 0.05C at 4.25V constant voltage, and then discharged to 2.8V at 1C constant current. The discharge capacity D1 is recorded. The discharge capacity retention rate is: D1 / D0.

[0124] (iv) Lithium plating at the negative electrode

[0125] After formation, the battery is charged at 25°C with a constant current of 2C to 4.25V, then charged at 4.25V with a constant voltage until the current is less than 0.05C, and then discharged at 1C to 2.8V. After 10 cycles, it is charged at 2C with a constant current to 4.25V, and then charged at 4.25V with a constant voltage until the current is less than 0.05C to obtain a fully charged battery.

[0126] The battery was disassembled after 10 cycles to observe the lithium plating on the negative electrode.

[0127] The evaluation criteria after observation are as follows.

[0128] No lithium deposition: The inner ring area of ​​the fully charged negative electrode surface is golden yellow. When wiped with a dust-free paper, no gray metallic lithium powder is left on the paper.

[0129] Slight lithium plating: The inner ring area of ​​the fully charged negative electrode surface is dark yellow. When wiped with lint-free paper, gray metallic lithium powder is found on the paper.

[0130] Gray spots: The inner ring area of ​​the fully charged negative electrode surface is partially gray, with no golden yellow showing through.

[0131] Severe lithium plating: The entire surface of the fully charged negative electrode in the inner ring area is gray, with no golden yellow showing through.

[0132] In this application, the preferred order of lithium plating on the negative electrode is no lithium plating > gray spots > slight lithium plating > severe lithium plating.

[0133] Examples 2 to 8, Comparative Examples 1 to 2

[0134] In Examples 2 to 8 and Comparative Examples 1 to 2, except for changing the type and content of each raw material in the electrolyte as shown in Table 1, and changing parameters such as the particle size of the negative electrode material and the compaction density of the negative electrode sheet as shown in Table 2, the same preparation method as in Example 1 was used to obtain each secondary battery.

[0135] [Table 1]

[0136]

[0137]

[0138] The test results in Table 2 show that, for different negative electrode materials, the condition (2τ) is satisfied. 0.5 +6≤σ≤(2τ) 0.5 Compared to Comparative Examples 1 and 2, Examples 1 to 8 of +10 showed significantly improved fast-charging performance and a marked improvement in negative electrode lithium plating. This indicates that matching the kinetics of the negative electrode with those of the electrolyte achieves good fast-charging performance and avoids negative electrode lithium plating. In Comparative Example 1, the electrolyte conductivity was too low, causing lithium ions to migrate too slowly in the liquid phase. Lithium ions released from the positive electrode could not be immediately embedded into the negative electrode, instead accumulating at the positive electrode. This led to a rapid rise in the positive electrode potential, reaching the cutoff potential, resulting in low discharge capacity retention. Furthermore, electrons transferred to the negative electrode in the external circuit, lowering the negative electrode potential. By the time lithium ions migrated to the negative electrode, the lithium plating potential might have already been reached, leading to lithium plating. In Comparative Example 2, the electrolyte conductivity was too high. During charging, lithium ions released from the positive electrode quickly reached the negative electrode, but the poor negative electrode kinetics prevented the lithium ions accumulated at the negative electrode interface from quickly embedding into the negative electrode. As the negative electrode potential decreased, the lithium ions at the interface were reduced to metallic lithium, becoming "dead lithium," resulting in poor fast-charging performance.

[0139] Furthermore, a comparison of Examples 1 to 5 and Example 6 shows that (2τ) is satisfied. 0.5+6≤σ≤(2τ) 0.5 Examples 1 to 5 of +8 can achieve better fast charging performance and further suppress lithium plating on the negative electrode.

[0140] 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, wherein, include: Electrode assembly and electrolyte for wetting the electrode assembly; wherein, The electrode assembly includes a negative electrode sheet, a separator, and a positive electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector. Let the tortuosity of the negative electrode sheet be τ, ε be the porosity of the negative electrode material layer, α be the Bruggeman index of the negative electrode material, the conductivity of the electrolyte be σ, 2.3≤τ≤7, the range of σ is 8mS / cm to 14mS / cm, the porosity ε of the negative electrode material layer is 25% to 45%, and the α is 1.5 to 2.2; τ satisfies the following equation I, τ=0.5(e) -α formula I; The τ and the conductivity σ of the electrolyte satisfy the following equation II. (2τ) 0.5 +6 ≤ σ ≤ (2τ) 0.5 +10 Equation II.

2. The secondary battery according to claim 1, wherein, The secondary battery satisfies one or more of the following conditions: 4.9≤τ≤7; The range of σ is from 9 mS / cm to 10 mS / cm; The porosity ε of the negative electrode material layer is 25% to 39%.

3. The secondary battery according to claim 1 or 2, wherein, The negative electrode material is graphite.

4. The secondary battery according to claim 1 or 2, wherein, The electrolyte comprises at least one cyclic ester selected from ethylene carbonate and propylene carbonate, and at least one selected from dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethyl acetate, methyl acetate, methyl formate, ethyl formate, methyl propionate, ethyl propionate, and propyl acetate.

5. The secondary battery according to claim 1 or 2, wherein, The lithium salt in the electrolyte comprises at least one of lithium hexafluorophosphate and lithium fluorosulfonyl imide, and the concentration of the lithium salt is 0.5-1.5 mol / L.

6. The secondary battery according to claim 5, wherein, The fluorinated sulfonylimide lithium includes at least one of lithium bis(fluorosulfonylimide), lithium fluoro(trifluoromethyl)sulfonylimide, lithium bis(trifluoromethyl)sulfonylimide, lithium bis(pentafluoroethyl)sulfonylimide, and lithium fluoro(perfluorobutylsulfonylimide).

7. The secondary battery according to claim 5, wherein, The lithium fluorinated sulfonyl imide includes lithium difluorosulfonyl imide.

8. The secondary battery according to claim 1 or 2, wherein, In the negative electrode sheet, the thickness of the negative electrode material layer is 30μm-400μm.

9. A battery module, wherein, The secondary battery includes any one of claims 1 to 8.

10. A battery pack, wherein, It includes at least one of the secondary batteries selected from any one of claims 1 to 8 or the battery module described in claim 9.

11. An electrical appliance, wherein, It includes at least one selected from the secondary battery of any one of claims 1 to 8, the battery module of claim 9, or the battery pack of claim 10.

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