Secondary battery, battery module, battery pack and power-consuming device

Abstract

This application relates to secondary batteries, battery modules, battery packs, and electrical devices. The secondary battery includes a positive electrode tab. Among them, the positive electrode tab includes a positive electrode current collector, a first positive electrode active material layer distributed on one side of the positive electrode current collector, and a second positive electrode active material layer distributed on the other side of the positive electrode current collector; wherein, the first positive electrode active material layer contains a first positive electrode active material, the active material capacity per unit coating area of the first positive electrode active material is Cap1, the second positive electrode active material layer contains a second positive electrode active material and a third positive electrode active material, the active material capacity per unit coating area of the second positive electrode active material is Cap2, the active material capacity per unit coating area of the third positive electrode active material is Cap3, and the following relationships are satisfied: (1) (Cap2 + Cap3) / Cap1 = C1, (2) 1 < C1 ≤ 1.5, and (3) 0.02 mAh / mm 2 ≤ Cap1 ≤ 0.045 mAh / mm 2 .

Description

Technical Field

[0001] The present application relates to a secondary battery, specifically to a secondary battery, for example, a lithium-ion battery, and a battery module, a battery pack, and an electrical device including the secondary battery. Background Art

[0002] With the development of new energy vehicles and energy storage services, the market has an increasingly high requirement for the service life of batteries. Currently, the methods for improving battery life mainly start from material improvement. At present, the stability of materials can be improved by the technology of gradient doping from the core to the outer core of the cathode material precursor of the battery to enhance battery life. It is also possible to improve the material and battery life by coating and doping the surface of carbon materials with an amorphous carbon coating layer containing nitrogen elements to protect the core. The existing technology has the problem of requiring additional material preparation processes, so the improvement of the positive electrode plate remains an urgent problem to be solved. Summary of the Invention

[0003] In order to solve the problems existing in the prior art, the present application designs different capacities on both sides of the positive electrode plate, pre-stores a part of active ions (for example, lithium ions) on the side of the electrode plate with a larger capacity, and slowly releases them during use to supplement the attenuation caused by the consumption of active ions, thereby extending the service life of the battery.

[0004] According to the first aspect of the present application, a secondary battery is provided. The secondary battery includes a positive electrode plate, wherein the positive electrode plate includes a positive current collector, a first positive active material layer distributed on one surface of the positive current collector, and a second positive active material layer distributed on the other surface of the positive current collector;

[0005] Wherein, the first positive active material layer contains a first positive active material, and the active material capacity per unit coating area of the first positive active material is Cap1.

[0006] The second positive active material layer contains a second positive active material and a third positive active material. The active material capacity per unit coating area of the second positive active material is Cap2, and the active material capacity per unit coating area of the third positive active material is Cap3, and the following relationship is satisfied:

[0007] (1) (Cap2 + Cap3) / Cap1 = C1

[0008] (2) 1 < C1 ≤ 1.5, and

[0009] (3) 0.02 mAh / mm 2 ≤ Cap1 ≤ 0.045 mAh / mm 2 .

[0010] In an embodiment of the present application, 1.1 ≤ C1 ≤ 1.35 or 1.15 ≤ C1 ≤ 1.25.

[0011] In an embodiment of the present application, Cap3 / Cap2 = C2, and it satisfies: 0 < C2 ≤ 0.5, 0.1 ≤ C2 ≤ 0.4 or 0.2 ≤ C2 ≤ 0.3.

[0012] In an embodiment of the present application, the secondary battery includes a negative electrode tab, and the negative electrode tab includes a negative electrode current collector, a first negative electrode active material layer distributed on one surface of the negative electrode current collector and opposite to the first positive electrode active material layer, and a second negative electrode active material layer distributed on the other surface of the negative electrode current collector;

[0013] Wherein, the first negative electrode active material layer contains a first negative electrode active material, and the active material capacity per unit coating area of the first negative electrode active material is CapC,

[0014] The second negative electrode active material layer contains a second negative electrode active material, and the active material capacity per unit coating area of the second negative electrode active material is CapD, and it satisfies the following relationship:

[0015] (1) CapD / CapC = C3, and

[0016] (2) 1 ≤ C3 ≤ C1.

[0017] In an embodiment of the present application, 1 ≤ C3 ≤ 1.15 or 1 ≤ C3 ≤ 1.05.

[0018] According to the second aspect of the present application, there is provided a battery module, which includes the secondary battery described in the present application.

[0019] According to the third aspect of the present application, there is provided a battery pack, which includes the secondary battery or the battery module described in the present application.

[0020] According to the fourth aspect of the present application, there is provided an electrical device, which includes the secondary battery, the battery module or the battery pack described in the present application, and the secondary battery or the battery module or the battery pack is used as the power source or the energy storage unit of the electrical device.

[0021] Compared with the prior art, by blending two active materials on one side of the positive electrode tab to increase the capacity, there are excess active ions remaining on the positive electrode on this side after the battery is charged, and they are slowly released during the subsequent aging process to supplement the loss of active ions during the process, thereby prolonging the service life of the battery. At the same time, in the present application, no additional material preparation process is required in the existing manufacturing process, and it will not increase the manufacturing cost.

[0022] These and other features and advantages will become apparent by reading the following detailed description and by reference to the associated drawings.It is to be understood that the foregoing general description and the following detailed description are illustrative only and are not restrictive of the aspects of what is claimed. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is the battery cycle life curve of Examples 1, 3, 4, 7 and Comparative Example 1 of the present application.

[0024] Figure 2 It is a schematic diagram of one embodiment of the secondary battery of the present application.

[0025] Figure 3 yes Figure 2 An exploded view of a secondary battery is shown.

[0026] Figure 4 It is a schematic diagram of an embodiment of a battery module of the present application.

[0027] Figure 5 It is a schematic diagram of an embodiment of the battery pack of the present application.

[0028] Figure 6 yes Figure 5 Exploded view of the battery pack shown.

[0029] Figure 7 This is a schematic diagram of one embodiment of a device using the secondary battery of the present application as a power source.

[0030] Description of reference numerals:

[0031] 1. Battery Pack

[0032] 2. Upper box

[0033] 3. Lower box

[0034] 4. Battery module

[0035] 5. Secondary battery

[0036] 51. Shell

[0037] 52. Electrode assembly

[0038] 53. Cover DETAILED DESCRIPTION

[0039] The present application is described in detail below in conjunction with the accompanying drawings, and the features of the present application will be further revealed in the following specific description.

[0040] The "ranges" disclosed herein are defined in terms of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of a particular range. The ranges defined in this way can include or exclude the end values, and can be combined arbitrarily, i.e., 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 particular parameter, ranges of 60 - 110 and 80 - 120 are also contemplated. Additionally, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1 - 3, 1 - 4, 1 - 5, 2 - 3, 2 - 4, and 2 - 5. In this application, unless otherwise specified, the numerical range "a - b" represents an abbreviated representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0 - 5" represents that all real numbers between "0 - 5" are fully listed herein, and "0 - 5" is just an abbreviated representation of these numerical combinations. Additionally, when stating that a certain parameter is an integer ≥2, it is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0041] In this application, unless otherwise specified, all the embodiments and preferred embodiments mentioned herein can be combined with each other to form new technical solutions. In this application, unless otherwise specified, all the technical features and preferred features mentioned herein can be combined with each other to form new technical solutions.

[0042] In this application, unless otherwise specified, the terms "comprising" and "including" mean open-ended or closed-ended. For example, the said "comprising" and "including" can mean that other components not listed can also be included or contained, or can mean that only the listed components are included or contained.

[0043] In the description herein, unless otherwise specified, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) while B is true (or exists); or both A and B are true (or exist).

[0044] In this application, the active material capacity per unit coating area Cap1 of the first positive electrode active material, the active material capacity per unit coating area Cap2 of the second positive electrode active material, and the active material capacity per unit coating area Cap3 of the third positive electrode active material satisfy the following relationships: (1) (Cap2 + Cap3) / Cap1 = C1, (2) 1 < C1 ≤ 1.5, and (3) 0.02 mAh / mm2 ≤Cap1≤0.045mAh / mm 2 . Thus, the active material capacities coated per unit area on both sides of the positive electrode of the secondary battery described in the present application are different from each other. The side with small capacity (A side, Cap1) can be coated with a first active material (or called a first positive electrode active material), and the side with large capacity (B side, Cap2+Cap3) can be coated with a second active material (or called a second positive electrode active material) and a third active material (or called a third positive electrode active material). In an embodiment of the present application, the first active material and the second active material may be the same or different. The second active material is different from the third active material. During the charging process, the side with large capacity (Cap2+Cap3) can retain excess active material (for example, active lithium). As the secondary battery ages, the active material will be consumed. In a low SOC state, the potential of the anode (negative electrode) corresponding to the cathode (positive electrode) on the side with small capacity rises, and the potential difference between the anodes on both sides causes the active material of the anode corresponding to the cathode on the side with large capacity to slowly deintercalate into the electrolyte, and diffuse and embed into the anode corresponding to the cathode on the side with small capacity to reduce the potential difference, thereby replenishing the active material (for example, active lithium) lost during the aging process, thereby extending the battery life.

[0045] In one embodiment of the present application, C1 (i.e., (Cap2+Cap3) / Cap1) can be within a numerical range consisting of any two of the following listed values ​​as end values: 1.00, 1.05, 1.1, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50. It should be particularly emphasized that although the above numerical values ​​are listed in parallel, it does not mean that the numerical range consisting of any two of the above numerical values ​​as endpoints can obtain equivalent or similar performance. This also applies to the numerical ranges mentioned below. Regarding the preferred embodiments of the present application, selection is made only based on the specific discussion below and specific experimental data. In an optional embodiment of the present application, 1.1≤C1≤1.35 or 1.15≤C1≤1.25.

[0046] In this application, by controlling the ratio of the second active material to the third active material included in the side with a larger capacity on the positive electrode plate, when the secondary battery is charged to the upper limit voltage, the charging capacities on both sides of the positive electrode are made consistent. On the one hand, this can avoid the precipitation of the active material on the negative electrode corresponding to the side with a larger capacity (for example, the phenomenon of lithium precipitation occurs), and on the other hand, it can also regulate the release rate of physical substances (such as lithium). In this application, the side with a larger capacity on the positive electrode plate includes a second active material (or called the second positive electrode active material) and a third active material (or called the third positive electrode active material), wherein the active material capacity per unit coating area Cap2 of the second positive electrode active material and the active material capacity per unit coating area Cap3 of the third positive electrode active material satisfy Cap3 / Cap2 = C2, and 0 < C2 ≤ 0.5. In an embodiment of this application, C2 (that is, Cap3 / Cap2) can be within the numerical range formed by any two of the following listed values as the end values: 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50. In an alternative embodiment of this application, 0.1 ≤ C2 ≤ 0.4 or 0.2 ≤ C2 ≤ 0.3.

[0047] In this application, the capacity of the negative electrode plate corresponding to the positive electrode plate on the side with a larger capacity can be controlled within a smaller range. This can reduce the weight of the secondary battery and also improve the energy density of the secondary battery. In an embodiment of this application, the negative electrode plate includes a negative electrode current collector, a first negative electrode active material layer (C surface) distributed on one surface of the negative electrode current collector and opposite to the first positive electrode active material layer (A surface), and a second negative electrode active material layer (D surface) distributed on the other surface of the negative electrode current collector and opposite to the second positive electrode active material layer (B surface); the first negative electrode active material layer contains a first negative electrode active material, the active material capacity per unit coating area of the first negative electrode active material is CapC, the second negative electrode active material layer contains a second negative electrode active material, the active material capacity per unit coating area of the second negative electrode active material is CapD, and the following relationships are satisfied: (1) CapD / CapC = C3, and (2) 1 ≤ C3 ≤ C1. In an embodiment of this application, C3 (that is, CapD / CapC) can be within the numerical range formed by any one of the following listed values as the lower limit value and the above-mentioned C1 as the upper limit value: 1.00, 1.05, 1.1, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, provided that C3 ≤ C1. In an alternative embodiment of this application, 1 ≤ C3 ≤ 1.15 or 1 ≤ C3 ≤ 1.05.

[0048] In the following, the performance improvement brought about by the differentiated capacity design on both sides of the positive electrode plate of the present application is characterized mainly based on secondary batteries, especially lithium-ion secondary batteries. However, it should be emphasized here that the differentiated capacity design on both sides of the positive electrode plate of the present application can be used for any electrical device including a carbon-based electrode, and the electrical device can benefit from it.

[0049] Secondary battery

[0050] In one embodiment of the present application, a secondary battery is provided, which can be a lithium ion secondary battery, a potassium ion secondary battery, a sodium ion secondary battery, a lithium sulfur battery, etc., and is particularly preferably a lithium ion secondary battery. The secondary battery of the present application includes a positive electrode (pole sheet), a negative electrode (pole sheet), a separator, an electrolyte / liquid, etc. During the battery charge and discharge process, active ions are embedded and removed back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet.

[0051] [Positive electrode]

[0052] In the secondary battery of the present application, the positive electrode plate includes a positive electrode current collector and a positive electrode film layer (or positive electrode active material layer) disposed on at least one surface of the positive electrode current collector and including a positive electrode active material. For example, the positive electrode current collector has two surfaces opposite to each other in the thickness direction of itself, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector. In the secondary battery of the present application, the positive electrode current collector may be a metal foil or a composite current collector, for example, the metal foil may be an aluminum foil, and the composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector 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 material substrate (such as a substrate of polypropylene PP, polyethylene terephthalate PET, polybutylene terephthalate PBT, polystyrene PS, polyethylene PE and its copolymers, etc.).

[0053] In the present application, the compaction density of the first positive electrode active material layer and the second positive electrode active material layer of the secondary battery is controlled to reduce the ion transmission path to improve the cycle life of the secondary battery, while at the same time, the particles of the active material will not be broken due to the high compaction density, resulting in an increase in the specific surface area (BET), leading to the side reaction of the outlet line and deteriorating the cycle life of the secondary battery to a certain extent. Usually, the compaction density of the first positive electrode active material layer and the second positive electrode active material layer can be 2.0-3.6g / cm 3In one embodiment of the present application, the compaction density of the first positive electrode active material layer and the second positive electrode active material layer can be within a numerical range consisting of any two of the following numerical values ​​as end values: 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5. In an optional embodiment of the present application, the compaction density of the first positive electrode active material layer and the second positive electrode active material layer can be 2.3-3.5 g / cm 3 .

[0054] In the secondary battery of the present application, the positive electrode active material (substance) may adopt the positive electrode active material for secondary batteries known in the art. For example, the positive electrode active material may include one or more of the following: lithium-containing phosphates of olivine structure, lithium transition metal oxides and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for secondary batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2(NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811)), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05O2) and its modified compounds, etc. One or more. Examples of lithium-containing phosphates with olivine structure may include, but are not limited to, lithium iron phosphate (such as LiFePO4 (LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. In an embodiment of the present application, the second positive electrode active material and the third positive electrode active material are the same or different, and are selected from lithium iron phosphate (LFP), lithium manganate (LMO), lithium nickel cobalt manganate (NCM), lithium cobalt oxide (LCO), lithium nickel cobalt aluminum oxide (NCA), and oxides containing active sodium ions, polyanionic materials or Prussian blue materials.

[0055] In some embodiments, the positive electrode film layer may also optionally include a binder. Non-limiting examples of binders that can be used for the positive electrode film layer may include one or more of the following: polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin. In an embodiment of the present application, the first positive electrode active material layer and / or the second positive electrode active material layer each independently contain a binder selected from polyvinylidene fluoride, polyacrylic acid, polytetrafluoroethylene, polyimide and combinations thereof.

[0056] In some embodiments, the positive electrode film layer may also optionally include a conductive agent. Examples of conductive agents for the positive electrode film layer may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers. In an embodiment of the present application, the first positive electrode active material layer and / or the second positive electrode active material layer each independently contain a conductive agent of graphite, carbon black, acetylene black, graphene, carbon nanotubes and a combination of the above substances.

[0057] In one embodiment of the present application, the positive electrode can be prepared in the following manner: the above-mentioned components for preparing the positive electrode, such as the positive electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as N-methylpyrrolidone) to form a uniform positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0058] [Negative electrode]

[0059] The secondary battery of the present application includes a negative electrode plate, which includes a negative electrode current collector and a negative electrode film layer (or called a negative electrode active material layer) arranged on at least one surface of the negative electrode current collector. In an embodiment of the present application, the first negative electrode active material and the second negative electrode active material are the same or different, and each independently contains natural graphite, artificial graphite, graphene, carbon nanotubes, soft carbon, hard carbon and a combination of two or more thereof.

[0060] In an embodiment of the present application, the compaction density of the first negative electrode active material layer and the second negative electrode active material layer may be 0.5-2.0 g / cm 3 In one embodiment of the present application, the compaction density of the first negative electrode active material layer and the second negative electrode active material layer can be within a numerical range consisting of any two of the following numerical values ​​as end values: 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0. In an optional embodiment of the present application, the compaction density of the first positive electrode active material layer and the second positive electrode active material layer can be 1.0-1.8 g / cm 3 .

[0061] In one embodiment of the present application, the negative electrode film layer may include a certain amount of other commonly used negative electrode active materials in addition to the negative electrode active materials described above, for example, one or more of natural graphite, other artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be selected from one or more of elemental silicon, silicon oxide, and silicon-carbon composites. The tin-based material may be selected from one or more of elemental tin, tin oxide compounds, and tin alloys.

[0062] In the secondary battery of the present application, the negative electrode membrane comprises a negative electrode active material and an optional binder, an optional conductive agent and other optional auxiliary agents, and is usually formed by coating and drying a negative electrode slurry. The negative electrode slurry coating is usually formed by dispersing the negative electrode active material and the optional conductive agent and binder in a solvent and stirring them evenly. The solvent can be N-methylpyrrolidone (NMP) or deionized water.

[0063] As an example, the conductive agent may include one or more of superconducting carbon, carbon black (eg, acetylene black, Ketjen black), carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0064] As an example, the binder may include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA) and carboxymethyl chitosan (CMCS). As an example, the binder may include one or more of styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS). Other optional additives are, for example, thickeners (such as sodium carboxymethyl cellulose CMC-Na), PTC thermistor materials, etc.

[0065] In addition, in the secondary battery of the present application, the negative electrode plate does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode plate of the present application may also include a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the first negative electrode film layer and disposed on the surface of the negative electrode current collector. In some other embodiments, the negative electrode plate of the present application may also include a covering protective layer covering the surface of the second negative electrode film layer.

[0066] In the secondary battery of the present application, the negative electrode current collector may be a metal foil or a composite current collector, for example, the metal foil may be a copper foil, a silver foil, an iron foil, or a foil composed of an alloy of the above metals. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer, and 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 material base layer (such as a base layer made of materials such as polypropylene PP, polyethylene terephthalate PET, polybutylene terephthalate PBT, polystyrene PS, polyethylene PE and copolymers thereof).

[0067] [Electrolytes]

[0068] The electrolyte plays the role of conducting ions between the positive electrode plate and the negative electrode plate. The electrolyte can be selected from at least one of a solid electrolyte and a liquid electrolyte (i.e., an electrolyte). In some embodiments, the electrolyte uses an electrolyte. The electrolyte includes an electrolyte salt and a solvent. In some embodiments, the electrolyte salt can be selected from LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bisfluorosulfonyl imide), LiTFSI (lithium bistrifluoromethanesulfonyl imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluoro oxalate borate), LiBOB (lithium dioxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluoro dioxalate phosphate) and LiTFOP (lithium tetrafluoro oxalate phosphate) One or more. In one embodiment of the present application, the solvent may be selected from one or more of the following: ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE). In one embodiment of the present application, based on the total weight of the electrolyte, the content of the solvent is 60-99% by weight, such as 65-95% by weight, or 70-90% by weight, or 75-89% by weight, or 80-85% by weight. In one embodiment of the present application, based on the total weight of the electrolyte, the content of the electrolyte is 1-40% by weight, such as 5-35% by weight, or 10-30% by weight, or 11-25% by weight, or 15-20% by weight.

[0069] In one embodiment of the present application, the electrolyte may optionally contain additives. For example, the additives may include one or more of the following: negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high temperature performance, additives that improve battery low temperature performance, etc.

[0070] [Isolation film]

[0071] In one embodiment of the present application, the secondary battery further comprises a separator, which separates the anode side of the secondary battery from the cathode side, and provides selective permeation or blocking for substances of different types, sizes and charges in the system. For example, the separator can insulate electrons, physically isolate the positive and negative active substances of the secondary battery, prevent internal short circuits and form an electric field in a certain direction, and allow the ions in the battery to pass through the separator and move between the positive and negative electrodes. In one embodiment of the present application, the material used to prepare the separator may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The separator may 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 an embodiment of the present application, the separator is selected from polyolefin separators, polyester separators, polyimide separators, polyamide separators and cellulose separators.

[0072] In one embodiment of the present application, the positive electrode sheet, the negative electrode sheet and the separator can be made into an electrode assembly / bare cell by a winding process or a lamination process.

[0073] In one embodiment of the present application, the secondary battery may include an outer package, which can be used to encapsulate the above-mentioned electrode assembly and electrolyte. In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. In other embodiments, the outer package of the secondary battery can be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.

[0074] The shape of the secondary battery of the present application can be cylindrical, square or any other shape. Figure 2 The secondary battery 5 is a square structure as an example. Figure 3 Shows Figure 2 The exploded view of the secondary battery 5 is shown in FIG. 5 . The outer package may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate. The bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity. The cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation film can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly is encapsulated in the receiving cavity, and the electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more.

[0075] In one embodiment of the present application, a plurality of secondary batteries may be assembled together to form a battery module, wherein the battery module includes two or more secondary batteries, and the specific number depends on the application of the battery module and the parameters of a single battery module.

[0076] Figure 4 4 is an example of a battery module. Figure 4 In the battery module 4, the plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of secondary batteries 5 may be fixed by fasteners.

[0077] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0078] In one embodiment of the present application, two or more of the above-mentioned battery modules can be assembled into a battery pack, and the number of battery modules contained in the battery pack depends on the application of the battery pack and the parameters of the individual battery modules. The battery pack may include a battery box and a plurality of battery modules disposed in the battery box, and the battery box includes an upper box body and a lower box body, and the upper box body can cover the lower box body and match well with it to form a closed space for accommodating the battery modules. Two or more battery modules can be arranged in the battery box in a desired manner.

[0079] Figure 5 and Figure 6 1 is a battery pack 1 as an example. Figure 5 and Figure 6 The battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3, wherein the upper box body 2 is used to cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in the battery box in any manner.

[0080] Electrical devices

[0081] In one embodiment of the present application, the electric device of the present application includes at least one of the secondary battery, battery module, or battery pack of the present application, and the secondary battery, battery module, or battery pack can be used as a power source of the electric device, and can also be used as an energy storage unit of the electric device. The electric device includes but is not limited to mobile digital devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0082] Figure 7 The device is used as an example. The device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the device's requirements for high power and high energy density, a battery pack or a battery module can be used.

[0083] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a secondary battery may be used as a power source.

[0084] In the following, the influence of the secondary battery manufactured according to the embodiment of the present application on the performance of the electrochemical device is characterized based on specific examples, but it should be pointed out that the protection scope of the present application is defined by the claims and is not limited to the above specific embodiments.

[0085] Example

[0086] Unless otherwise specified, all raw materials used in the present invention are analytically pure and all water is deionized water.

[0087] Example 1

[0088] Preparation of positive electrode

[0089] Lithium iron phosphate (LiFePO4 or LFP), conductive carbon black (Super-P), polyvinylidene fluoride (PVDF) are mixed with N, N-dimethylpyrrolidone (NMP) at a weight ratio of 96:2.5:1.5, and stirred evenly to obtain a slurry for coating the positive electrode sheet. The viscosity can be adjusted by NMP during the stirring process. Then, the slurry is evenly coated on the A side of the 13μm thick positive electrode current collector (aluminum foil) with a certain width to ensure that the unit area capacity of the A side of the positive electrode sheet is 0.036mAh / mm 2 .

[0090] LiFePO4, LiCoMnO4 0.6 Co 0.2 Mn 0.2 O2 or NCM622), conductive carbon black (Super-P), polyvinylidene fluoride (PVDF) are mixed with N, N-dimethylpyrrolidone (NMP) in a weight ratio of 78:18:2.5:1.5, stirred evenly to obtain a slurry for coating the positive electrode sheet, and evenly coated on the B side of the aluminum foil, and the unit area capacity of the B side is controlled to be 1.05 times that of the A side (i.e., C1=1.05), and the capacity ratio of NMC622 and LFP in the B side is 0.25 (i.e., C2=0.25). The positive electrode sheet is cold pressed and sliced ​​(i.e., the electrode sheet is cut and cut into the required size) to obtain a positive electrode sheet.

[0091] Preparation of negative electrode

[0092] Artificial graphite, conductive carbon black (Super-P), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR) are mixed with water at a weight ratio of 97:0.5:1:1.5, and stirred evenly to obtain a slurry for coating the negative electrode sheet. The viscosity can be adjusted by water during the stirring process. Then, the slurry is coated on both sides of a 6μm thick negative electrode current collector (copper foil) with a certain width to ensure that the capacity per unit area on both sides is the same, and the negative electrode sheet is obtained after cold pressing and slicing.

[0093] Preparation of lithium-ion secondary batteries

[0094] 7μm polyethylene (PE) is used as a separator. The positive electrode sheet, separator and negative electrode sheet are wound into a battery cell, and a lithium-ion secondary battery is obtained through ear welding, aluminum shell packaging, liquid injection, packaging formation and vacuum molding. The battery cell has a width of 148mm, a thickness of 28mm, a height of 98mm and a capacity of 40Ah. The electrolyte is a solution of 1M LiPF6 ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 1:2).

[0095] Embodiment 2-8

[0096] The lithium ion secondary batteries of Examples 2-8 were prepared in the same manner as in Example 1, except that the capacity per unit area of ​​surface B was controlled to be 1.1, 1.15, 1.2, 1.25, 1.3, 1.35 and 1.4 times that of surface A, respectively (i.e., C1 was 1.1, 1.15, 1.2, 1.25, 1.3, 1.35 and 1.4, respectively).

[0097] Comparative Example 1

[0098] The lithium ion secondary battery of Comparative Example 1 was prepared in the same manner as in Example 1, except that LFP with the same capacity (ie, C1=1) was coated on the A and B surfaces of the positive electrode sheet.

[0099] Comparative Examples 2-3

[0100] The lithium ion secondary batteries of Comparative Examples 2-3 were prepared in the same manner as in Example 1, except that the capacity per unit area of ​​the B surface was controlled to be 1.6 and 0.9 times that of the A surface, respectively (ie, C1 was 1.6 and 0.9, respectively).

[0101] Examples 9-14

[0102] Except that the capacity ratio of NMC622 and LFP in the B side of Examples 9-14 is controlled to be 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5 respectively (that is, C2 is 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5 respectively, and the equivalent weight ratio of NMC622 and LFP is 4:92, 7:89, 14:82, 21:75, 28:68, 35:61 respectively), the lithium ion secondary batteries of Examples 9-14 are prepared in the manner of Example 4.

[0103] Examples 15-18

[0104] The lithium ion secondary batteries of Examples 15-18 were prepared in the same manner as in Example 4, except that the capacity ratios of the negative electrode sheet faces D and C corresponding to the positive electrode sheet faces B and A were 1.05, 1.1, 1.15 and 1.2 respectively (i.e., C3 were 1.05, 1.1, 1.15 and 1.2 respectively).

[0105] Comparative Example 4

[0106] The lithium ion secondary battery of Comparative Example 4 was prepared in the same manner as in Example 4, except that the capacity ratio of the negative electrode sheet faces D and C corresponding to the positive electrode sheet faces B and A was 1.3 (ie, C3 was 1.3).

[0107] Comparative Example 5

[0108] The lithium ion secondary battery of Comparative Example 5 was prepared in the manner of Example 4 except that the capacity ratio of NMC622 and LFP in the B side of Comparative Example 5 was controlled to be 0.6 (ie, C2 was 0.6, and the equivalent weight ratio of NMC622 and LFP was 42:54, respectively).

[0109] Pole capacity test

[0110] The positive electrode or negative electrode was cut into small discs and assembled into button batteries in a glove box, where the electrode was lithium metal, the separator was Celgard C2400, and the electrolyte was a solution of 1.3M LiPF6 EC and DMC (volume ratio 1:2). The assembled button batteries were tested for capacity using a blue battery tester.

[0111] Energy density test

[0112] The battery cell is charged at a constant current of 0.1C to a rated voltage of 4.0V and maintained at a constant voltage for 30 minutes, and then discharged at a constant current of 0.1C to 2.5V. The energy released during constant current discharge is recorded and divided by the total mass of the battery cell to obtain the battery cell energy density.

[0113] Cyclic performance test

[0114] Place the prepared hard shell battery / secondary battery on the Xinwei machine. At 25°C, charge to the rated voltage at a constant current rate of 0.5C, let it stand for 5 minutes, then discharge to the cut-off voltage at a constant current rate of 0.5C, record the discharge capacity, and let it stand for another 5 minutes. Repeat this cycle for 1500 times, cycle performance = 1500th cycle capacity / 1st cycle capacity × 100%.

[0115] Table 1: Cycle performance of secondary batteries prepared in Examples 1-8 and Comparative Examples 1-3

[0116]

[0117]

[0118] As shown in Table 1 and Figure 1 As shown, in Examples 1-8, as the positive electrode AB surface capacity ratio increases, the excess positive electrode reduces the energy density of the battery cell, but the cycle capacity retention rate increases. After 1500 cycles, the energy density first increases and then decreases, and is the largest when C1 is 1.2, but it is improved compared to Comparative Example 1.

[0119] In Comparative Example 2, the capacity ratio of the positive electrode AB surface is 1.6, the energy density of the battery cell is the lowest, and due to the excessive capacity of the B surface, lithium deposition occurs in the corresponding negative electrode after cycling, resulting in a deterioration in the cycle capacity retention rate.

[0120] In Comparative Example 3, since the capacity of the positive electrode B side is smaller than that of the A side, the lithium replenishment effect required by the present application cannot be achieved, and the cycle capacity retention rate becomes poor.

[0121] Table 2: Cycle performance of secondary batteries prepared in Examples 9-18 and Comparative Examples 4-5

[0122]

[0123] As shown in Table 2, in Example 4 and Examples 9-11, as the proportion of NCM622 on the B side increases, the thickness of the electrode decreases, the energy density of the battery cell increases, and the cycle capacity retention rate does not change much.

[0124] In Examples 12-14 and Comparative Example 5, the proportion of NCM622 on the B side continues to increase. Since the potential rises too quickly in the later stage of charging, the A side reaches the charging cut-off voltage ahead of time, the capacity cannot be fully utilized, and the energy density of the battery cell begins to decrease. At the same time, too much NCM622 participates in the cycle, causing the cycle capacity retention rate of the secondary battery to deteriorate.

[0125] In Examples 15-18 and Comparative Example 4, the D-side (anode) capacity of the negative electrode plate increases, the thickness increases, the energy density decreases, and at the same time, the excessive anode consumes more lithium, resulting in a decrease in the cycle capacity retention rate.

[0126] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery, comprising a positive electrode sheet, wherein: The positive electrode sheet includes a positive electrode current collector, a first positive electrode active material layer distributed on one surface of the positive electrode current collector, and a second positive electrode active material layer distributed on the other surface of the positive electrode current collector; Wherein, the first positive electrode active material layer contains a first positive electrode active material, and the active material capacity per unit coating area of the first positive electrode active material is Cap1. The second positive electrode active material layer contains a second positive electrode active material and a third positive electrode active material. The active material capacity per unit coating area of the second positive electrode active material is Cap2, and the active material capacity per unit coating area of the third positive electrode active material is Cap3, and the following relationships are satisfied: (1) (Cap2 + Cap3) / Cap1 = C1, (2) 1 < C1 ≤ 1.5, and (3)0.02mAh / mm 2 ≤ Cap1 ≤ 0.045mAh / mm 2 。 2. The secondary battery according to claim 1, wherein 1.1≤C1≤1.35。 3. The secondary battery according to claim 1 or 2, wherein: 1.15≤C1≤1.25。 4. The secondary battery according to claim 1, wherein Cap3 / Cap2 = C2, and it is satisfied that: 0 < C2 ≤ 0.

5.

5. The secondary battery according to claim 4, wherein 0.1≤C2≤0.4。 6. The secondary battery according to claim 5, wherein 0.2≤C2≤0.3。 7. The secondary battery according to claim 1, wherein The secondary battery includes a negative electrode sheet, which includes a negative electrode current collector, a first negative electrode active material layer distributed on one surface of the negative electrode current collector and opposite to the first positive electrode active material layer, and a second negative electrode active material layer distributed on the other surface of the negative electrode current collector; Wherein, the first negative electrode active material layer contains a first negative electrode active material, and the active material capacity per unit coating area of the first negative electrode active material is CapC. The second negative electrode active material layer contains a second negative electrode active material, and the active material capacity per unit coating area of the second negative electrode active material is CapD, and the following relationships are satisfied: (1) CapD / CapC = C3, and (2) 1 ≤ C3 ≤ C1.

8. The secondary battery according to claim 7, wherein 1≤C3≤1.15。 9. The secondary battery according to claim 8, wherein 1≤C3≤1.05。 10. A battery module, wherein: Including the secondary battery according to any one of claims 1-9.

11. A battery pack, wherein: Including the secondary battery according to any one of claims 1-9 or the battery module according to claim 10.

12. An electrical device, wherein: Including the secondary battery according to any one of claims 1-9, or the battery module according to claim 10, or the battery pack according to claim 11, and the secondary battery or the battery module or the battery pack is used as the power source of the electrical device or the energy storage unit of the electrical device.

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