Secondary battery and electronic device

Abstract

The invention provides a secondary battery and electronic equipment, and particularly relates to the technical field of secondary batteries. The secondary battery comprises a positive pole piece, a negative pole piece, a diaphragm and an electrolyte, the negative pole piece comprises a negative current collector and at least two negative active material layers, and each negative active material layer comprises a negative active material, a conductive agent and a binder. The negative electrode active material comprises a silicon-based material, the mass content of the silicon-based material in the negative electrode active material in the negative electrode active material layer close to the negative electrode current collector is greater than or equal to that of the negative electrode active material layer far away from the negative electrode current collector, and the content of the binder in the negative electrode active material layer is sequentially reduced from close to far away from the negative electrode current collector. By designing the contents of the silicon-based material and the binder in different active substance layers, the stripping force of the negative pole piece can be ensured not to deteriorate on the basis of improving dynamics, and the high-performance fast-charging negative pole piece is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a secondary battery and an electronic device. Background Art

[0002] In recent years, with the rapid development of mobile electronic devices and the booming of new energy vehicles, the market has put forward higher requirements for the energy density, cycle life and fast charging performance of lithium-ion batteries. The theoretical specific capacity of graphite negative electrode materials is relatively low, only 372mAh·g -1 , it is difficult to meet the high energy density requirements of new lithium-ion batteries.

[0003] Although silicon-based materials have extremely high theoretical specific capacity, they have a high volume expansion during the charge and discharge process. Therefore, mixing graphite materials and silicon-based materials can improve the specific energy density of negative electrode materials and alleviate the volume expansion of silicon-based materials during lithium insertion and extraction, which is expected to prepare high energy density and long cycle life lithium-ion battery negative electrode materials. However, for silicon-containing negative electrodes, the kinetic performance is usually difficult to meet the increasing demand for fast charging, and there is also the problem of significantly reduced stripping force, which will deteriorate the cycle life, rate, internal resistance and other properties of lithium-ion batteries.

[0004] Therefore, it is necessary to provide a secondary battery and an electronic device to solve the above problems. Summary of the invention

[0005] In view of the above shortcomings of the prior art, the present invention provides a secondary battery and an electronic device to improve the problem of deterioration of the peeling force of the multi-layer coated silicon-containing negative electrode in the prior art.

[0006] To achieve the above-mentioned purpose and other related purposes, the present invention provides a secondary battery, which comprises a positive electrode plate, a negative electrode plate, a separator and an electrolyte, wherein the negative electrode plate comprises a negative electrode collector and at least two layers of negative electrode active material layers, wherein the negative electrode active material layer is arranged on the surface of at least one side of the negative electrode collector, and at least two layers of the negative electrode active material layers are arranged in sequence along the thickness direction of the negative electrode collector; each layer of the negative electrode active material layer comprises a negative electrode active material, a conductive agent and a binder, wherein the negative electrode active material comprises a silicon-based material, and the mass proportion of the silicon-based material in the negative electrode active material in the negative electrode active material in the negative electrode active material layer close to the negative electrode collector is greater than or equal to the mass proportion of the silicon-based material in the negative electrode active material in the negative electrode active material layer far from the negative electrode collector; the binder content in the negative electrode active material layer decreases from close to to far from the negative electrode collector.

[0007] In an example of the present invention, the negative electrode sheet includes n layers of negative electrode active material layers. The negative electrode active material layers from the one close to the negative electrode current collector to the one far from the negative electrode current collector are the 1st, 2nd, …, i-th, …, n-th layers respectively. The mass of the silicon-based material in the i-th negative electrode active material layer accounts for a i % of the mass of the negative electrode active material, and the mass content of the binder in the i-th negative electrode active material layer is x i %, then 2.2 + a i / 10 + (1 - i / n) * 0.8 - 0.3 ≤ x i ≤ 2.2 + a i / 10 + (1 - i / n) * 0.8 + 0.3, where n ≥ 2 and a i ≤ 15.

[0008] In an example of the present invention, the mass content x i % of the binder in the i-th negative electrode active material layer satisfies: 2.2 + a i / 10 + (1 - i / n) * 0.8 - 0.1 ≤ x i ≤ 2.2 + a i / 10 + (1 - i / n) * 0.8 + 0.1.

[0009] In an example of the present invention, 2 ≤ n ≤ 3.

[0010] In an example of the present invention, the silicon-based material includes at least one of silicon, silicon-carbon material, and silicon oxide compound; and / or, the negative electrode active material further includes a carbon material, and the carbon material includes at least one of artificial graphite, natural graphite, soft carbon, hard carbon, and mesophase carbon microspheres.

[0011] In an example of the present invention, the tap density of the negative electrode active material layer decreases successively from the side close to the negative electrode current collector to the side far from the negative electrode current collector, and the porosity of the negative electrode active material layer increases successively from the side close to the negative electrode current collector to the side far from the negative electrode current collector.

[0012] In an example of the present invention, the tap density of the negative electrode sheet is 1.5 - 1.7 g / cm 3 ; and / or, the adhesion of the negative electrode sheet is 14 - 18 N / m.

[0013] In an example of the present invention, the conductive agent includes at least one of conductive carbon black, conductive graphite, carbon fiber, carbon nanotube, and graphene; and / or, the binder includes at least one of polyacrylic acid, polyvinylidene fluoride, styrene-butadiene rubber, and sodium carboxymethyl cellulose.

[0014] In an example of the present invention, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes one or more of lithium nickel cobalt manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese aluminum oxide, lithium iron phosphate, and lithium manganese iron phosphate.

[0015] The present invention also provides an electronic device, which includes the secondary battery described in any one of the above.

[0016] The present invention adopts a multi-layer coating technology and controls the content of each substance in each active material layer of the negative electrode plate, so that the proportion of the silicon-based material in the negative electrode active material in the active material layer close to the current collector is greater than or equal to the proportion of the silicon-based material in the negative electrode active material in the active material layer far from the current collector, and the binder content in the active material layer close to the current collector is greater than the binder content in the active material layer far from the current collector, thereby realizing the hierarchical gradient lithium intercalation of the negative electrode plate, improving the fast charging ability of the secondary battery, and at the same time improving the problem of reducing the peeling force of the multi-layer coated electrode plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0018] Figure 1 It is a schematic structural diagram of the negative electrode plate of the secondary battery of the present invention in an embodiment;

[0019] Figure 2 It is a comparative diagram of the cycle performance curves of the embodiment of the present invention and the comparative example;

[0020] Figure 3 It is a comparative diagram of the charge and discharge curves of the embodiment of the present invention and the comparative example;

[0021] Figure 4 It is a comparative diagram of the rate charge performance curves of the embodiment of the present invention and the comparative example;

[0022] Figure 5 It is a comparative diagram of the fast charge cycle performance of the embodiment of the present invention and the comparative example;

[0023] Figure 6 It is a comparative diagram of the interfaces after disassembling the batteries of the embodiment of the present invention and the comparative example.

[0024] REFERENCE SIGNS

[0025] 100, negative electrode plate; 110, negative current collector; 120, negative active material layer. Detailed implementation manners

[0026] The following describes the implementation manners of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments, and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0028] Unless otherwise stated or there is a contradiction, the terms or phrases used herein have the following meanings:

[0029] As used herein, "a plurality of", "multiple types", "multiple times", etc., unless otherwise specified, mean greater than 2 or equal to 2 in quantity. For example, "one or more types" means one type or two or more types.

[0030] As used herein, "preferred", "better", "more preferable" are only used to describe implementation manners or embodiments with better effects, and it should be understood that they do not constitute a limitation on the protection scope of the present invention. If "preferred" appears in a technical solution for multiple times, unless otherwise specified and there is no contradiction or mutual restriction relationship, each "preferred" is independent of each other.

[0031] As used herein, "further", "even further", "especially", etc. are used for descriptive purposes and represent differences in content, but should not be understood as a limitation on the protection scope of the present invention.

[0032] As used herein, when it comes to a numerical range, unless otherwise specified, the distribution of the optional numerical values within this numerical range is considered continuous, and includes the two numerical endpoints (i.e., the minimum value and the maximum value) of this numerical range, as well as each numerical value between these two numerical endpoints. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be combined.

[0033] The specific energy of a secondary battery is closely related to the specific capacity of the negative electrode active material. Generally, the higher the specific capacity of the negative electrode active material, the more beneficial it is to improve the specific energy of the secondary battery. At present, graphite-based negative electrode materials are mostly used in commercial lithium-ion batteries. However, the theoretical specific capacity of graphite-based negative electrode materials is relatively low, making it difficult to meet the requirements of new lithium-ion batteries for high energy density. Although silicon-based materials have a relatively high theoretical specific capacity and can achieve the purpose of improving the specific energy of secondary batteries when used as negative electrode active materials, the volume expansion of silicon-based materials is relatively large during the charging process. As a result, the large expansion stress generated inside the silicon-based materials will damage the structure of the silicon-based materials. This structural damage of the silicon-based materials will not only destroy the electrical contact between the silicon-based materials but also may cause the negative electrode film to peel off from the negative electrode current collector, making the process of ion deintercalation and intercalation unable to proceed smoothly; moreover, the irreversibility of ions during the deintercalation and intercalation processes will increase, not only reducing the first charge-discharge efficiency of the secondary battery but also affecting the cycle performance and safety performance of the secondary battery. At the same time, due to the large volume expansion of silicon-based materials during the charge-discharge process, the SEI film on the surface of the negative electrode film will continuously break and repair, consuming a large amount of ions, resulting in an increasing degree of ion irreversibility and also affecting the cycle performance of the secondary battery.

[0034] Mixing graphite-based materials and silicon-based materials can not only improve the specific energy density of the negative electrode material but also alleviate the volume expansion of silicon-based materials during the lithium deintercalation and intercalation processes, promising to prepare negative electrode materials for lithium-ion batteries with high energy density and long cycle life. However, for silicon-containing negative electrodes, the kinetic performance usually fails to meet the increasing demand for fast charging.

[0035] The inventors of this application found in their research that by using a double-layer coating or multi-layer coating technology to adopt a design with a high silicon content and poor kinetics in the active material layer close to the current collector and a design with a low silicon content and better kinetics in the active material layer far from the current collector, it is possible to achieve a hierarchical gradient lithium intercalation of lithium ions during the charging process and improve the fast charging ability of the Si-containing negative electrode. However, in practical applications, the negative electrode sheets with double-layer or multi-layer coatings have the problem of a significant reduction in the peel strength, and the reduction in the peel strength will deteriorate the performance of the lithium-ion battery such as the cycle life, rate performance, and internal resistance, making the double-layer or multi-layer coated negative electrodes unable to achieve the expected effect.

[0036] Based on this, this application provides a secondary battery and an electronic device including the secondary battery. The negative electrode sheet containing silicon is prepared by using a multi-layer coating technology. By designing the content of silicon-based materials and binders in different active material layers, on the basis of improving the kinetics, the peel strength of the negative electrode sheet is ensured not to deteriorate, thereby obtaining a high-performance fast-charging negative electrode sheet.

[0037] In a first aspect of the present invention, a secondary battery is provided, which includes a positive electrode plate, a negative electrode plate, a separator, and an electrolyte. The separator is disposed between the positive electrode plate and the negative electrode plate to play an isolation role; the electrolyte plays a role of conducting ions between the positive electrode plate and the negative electrode plate. In this application, the secondary battery can be a lithium-ion secondary battery, a sodium-ion secondary battery, or the like.

[0038] Please refer to Figure 1 , the negative electrode plate 100 includes a negative electrode current collector 110 and at least two layers of negative electrode active material layers 120. The at least two layers of negative electrode active material layers 120 are disposed on at least one surface of the negative electrode current collector 110 and are sequentially arranged along the thickness direction of the negative electrode current collector 110. That is, the negative electrode current collector 110 has two surfaces opposite to each other in its own thickness direction, which are respectively denoted as the first surface and the second surface. The negative electrode active material layer 120 can be sequentially stacked along the thickness direction of the negative electrode current collector 110 on any one of the first surface and the second surface of the negative electrode current collector 110, or can be disposed on both the first surface and the second surface.

[0039] The above-mentioned negative electrode current collector 110 can be selected from conventional current collector types in the art. As an example, the negative electrode current collector 110 can be a copper foil, and the thickness of the copper foil is 4-15 μm. Further, the thickness of the copper foil is 5-10 μm. Further still, the thickness of the copper foil is 8 μm; the negative electrode current collector 110 can also be a composite current collector, which uses a polymer insulating resin material as the "sandwich" layer, and copper is deposited on both the upper and lower surfaces. The polymer resin can be polyethylene terephthalate (PET), polypropylene (PP), polyimide (PI), polystyrene (PS), polyamide (PA), etc.

[0040] Each layer of the negative electrode active material layer 120 includes a negative electrode active material, a conductive agent, and a binder. Among them, the negative electrode active material is the main substance participating in the electrochemical reaction, and the specific energy of the secondary battery is closely related to the specific capacity of the negative electrode active material. To improve the energy density of the battery, the negative electrode active material of the present invention includes a silicon-based material with a relatively high theoretical specific capacity and a carbon material with excellent comprehensive performance. Moreover, the mass ratio of the silicon-based material in the negative electrode active material in the negative electrode active material layer 120 close to the negative electrode current collector 110 is greater than or equal to the mass ratio of the silicon-based material in the negative electrode active material in the negative electrode active material layer 120 far from the negative electrode current collector 110, so that rapid insertion of lithium ions into the outer active material layer during the charging process can be achieved, hierarchical gradient lithium intercalation can be realized, and the kinetics of the electrode plate can be improved.

[0041] The binder is used to bind the negative electrode active material and the conductive agent, and provides a certain amount of bonding force for the negative electrode active material layer 120 to bond it to the negative electrode current collector. In this application, a design with a higher binder content is adopted in the negative electrode active material layer 120 close to the negative electrode current collector 110, and a design with a lower binder content is adopted in the negative electrode active material layer 120 far from the negative electrode current collector 110. Figure 1 The curve above represents water evaporation), the binder will undergo a "floating" process from close to the negative electrode current collector layer to far away from the current collector layer. Therefore, the design of using a high content of binder in the active material layer close to the negative electrode current collector can significantly improve the problem of deterioration of the peeling force of the negative electrode sheet, thereby ensuring that the peeling force will not deteriorate on the basis of improving the dynamics, and obtaining a high-performance fast-charging negative electrode sheet 110.

[0042] Conductive agents can improve electronic conductivity. In order to ensure that the battery has good charge and discharge performance, they collect microcurrents between active materials and between active materials and current collectors to reduce the contact resistance of the battery and accelerate the movement rate of electrons. In addition, conductive agents can also improve the processability of electrodes, promote the infiltration of electrolytes into electrodes, and effectively increase the migration rate of lithium ions in battery materials, thereby improving the charge and discharge efficiency of the battery and the service life of the battery.

[0043] In some embodiments, the silicon-based material includes silicon, silicon-carbon material, silicon oxide (SiO x , 0<x≤2), that is, the silicon-based material can be any one of the substances listed above, or any two or more. As an example, the silicon-based material is silicon alone, or a silicon oxide compound, or a combination of a silicon-carbon material and a silicon oxide compound, and the like. Further preferably, the silicon-based material includes silicon-carbon materials and / or silicon oxide compounds. The carbon material includes at least one of artificial graphite, natural graphite, soft carbon, hard carbon, and mesophase carbon microbeads, that is, the carbon material can be any one or more of the substances listed above, such as soft carbon, or a combination of artificial graphite and hard carbon, or a combination of natural graphite, soft carbon, and hard carbon, and the like; further, the carbon material is artificial graphite, or natural graphite, or a combination of artificial graphite and natural graphite.

[0044] The binder includes at least one of polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC). For example, the binder is polyvinylidene fluoride, or a combination of styrene butadiene rubber and carboxymethyl cellulose, etc. Further, the binder is polyacrylic acid.

[0045] The conductive agent includes at least one of conductive carbon black (SP), conductive graphite, carbon fiber, carbon nanotube, and graphene. Optionally, the conductive agent is conductive carbon black; optionally, the conductive agent is a composition of carbon fiber and conductive carbon black, optionally, the conductive agent is a composition of carbon nanotube and graphene, and so on.

[0046] In one embodiment, the negative electrode sheet includes n layers of negative electrode active material layers arranged in sequence along the thickness direction of the negative electrode current collector 110. The negative electrode active material layers 120 from the one close to the negative electrode current collector 110 to the one far from the negative electrode current collector 110 are respectively denoted as the first layer, the second layer,..., the i-th layer,..., the n-th layer. Correspondingly, the mass ratio of the silicon-based material in each negative electrode active material layer in the negative electrode active material is a1%, a2%,..., a i %,..., a n %, then, a1 ≥ a2,..., ≥ a i ..., ≥ a n , where n ≥ 2, 0 < a n ≤ 15; the contents of the binder in each negative electrode active material layer are respectively: x1%, x2%,..., x i %,..., x n %, then, x1 > x2,..., > x i ..., > x n .

[0047] Furthermore, the binder content in the i-th negative electrode active material layer satisfies: 2.2 + a i / 10 + (1 - i / n) * 0.8 - 0.3 ≤ x i ≤ 2.2 + a i / 10 + (1 - i / n) * 0.8 + 0.3, where 2 ≤ n ≤ 3; furthermore, the binder content in the i-th negative electrode active material layer satisfies 2.2 + a i / 10 + (1 - i / n) * 0.8 - 0.1 ≤ x i ≤ 2.2 + a i / 10 + (1 - i / n) * 0.8 + 0.1.

[0048] In one embodiment, the compaction density of the n-layer negative electrode active material layer 120 decreases successively from the side close to the negative electrode current collector 110 to the side far from the negative electrode current collector 110. Correspondingly, the porosity in the negative electrode active material layer 120 increases successively from the side close to the negative electrode current collector 110 to the side far from the negative electrode current collector 110. To achieve the above object, a method of secondary rolling can be adopted to prepare the negative electrode plate, that is, after coating one layer of the negative electrode active material layer, rolling is carried out first, and then the next layer of the negative electrode active material layer 120 is coated after the rolling is completed. Since the negative electrode active material layer 120 close to the negative electrode current collector 110 undergoes more rolling times and the negative electrode active material layer 120 far from the negative electrode current collector 110 undergoes fewer rolling times, the compaction density of the negative electrode active material layer 120 close to the negative electrode current collector 110 is high and the porosity is low, while the compaction density of the negative electrode active material layer 120 far from the negative electrode current collector 110 is low and the porosity is relatively high. Further, the overall compaction density of the negative electrode plate of the present application is 1.5-1.7 g / cm 3 , for example, the compaction density is 1.5 g / cm 3 , 1.6 g / cm 3 or 1.7 g / cm 3 , and so on. Those skilled in the art can select within the above range according to actual needs. The adhesion between the negative electrode active material layer and the negative electrode current collector of the negative electrode plate of the present invention can reach 14-18 N / m.

[0049] The positive electrode plate, separator and electrolyte of the secondary battery can all be set according to the conventional selection in the art. Hereinafter, a lithium ion secondary battery will be taken as an example for detailed description.

[0050] The positive electrode plate includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. That is, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode active material layer can be provided on any one or both of the two surfaces of the positive electrode current collector. The positive electrode current collector can be aluminum foil, and the thickness of the aluminum foil is 5-20 μm. Further, the thickness of the aluminum foil is 10-15 μm. Further still, the thickness of the aluminum foil is 12 μm; the positive electrode current collector can also be a composite current collector, and the composite current collector uses a polymer insulating resin material as the "sandwich" layer, and aluminum is deposited on its upper and lower surfaces. The polymer resin can be polyethylene terephthalate PET, polypropylene PP, polyimide PI, polystyrene PS, polyamide PA, etc.

[0051] The positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. Among them, the positive electrode active material can be selected from one or more combinations of lithium nickel cobalt manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese aluminum oxide, lithium iron phosphate, and lithium manganese iron phosphate. The positive electrode conductive agent is selected from one of conductive carbon black (SP), acetylene black, nano metal powder, graphene, carbon nanotube, carbon nanofiber, or a composition of two or more kinds mixed in any proportion. The positive electrode binder is selected from one or more mixtures of polyvinylidene fluoride (PVDF), polyvinylidene fluoride - hexafluoropropylene, polytetrafluoroethylene, sodium carboxymethyl cellulose, and styrene - butadiene rubber. Preferably, the positive electrode active layer further includes a lithium supplement agent, and the lithium supplement agent includes but is not limited to Li6CoO4, Li2NiO2, and Li5FeO4.

[0052] In some embodiments, the electrolyte includes a lithium salt and an organic solvent. As an example, the lithium salt can be selected from one or several of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluoro(dioxalato)phosphate (LiDFOP), and lithium tetrafluoro(oxalato)phosphate (LiTFOP). The organic solvent can be selected from one or several of fluoroethylene carbonate (FEC), 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), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), and ethyl butyrate (EB).

[0053] In some embodiments, additives can also be added to the electrolyte. For example, the electrolyte can include a negative electrode film - forming additive, a positive electrode film - forming additive, an additive for improving the overcharge performance of the battery, an additive for improving the high - temperature performance of the battery, an additive for improving the low - temperature performance of the battery, etc.

[0054] The separator separates the positive electrode plate from the negative electrode plate, preventing internal short circuit of the battery, and at the same time enabling active ions to pass through the separator and move between the positive and negative electrodes. In the present application, there is no particular limitation on the type of the separator, and any well-known porous structure separator with good chemical stability and mechanical stability can be selected. As an example, the material of the separator can be selected from polyethylene (PE) film, polypropylene (PP) film, polyvinylidene fluoride film, and one or several of multilayer composite films containing one or more of them. The separator can be a single-layer separator or a multilayer composite separator, without particular limitation. When the separator is a multilayer composite separator, the materials of each layer can be the same or different, without particular limitation.

[0055] In some embodiments, the positive electrode plate, the negative electrode plate, and the separator can be made into an electrode assembly by a winding process or a stacking process.

[0056] The secondary battery further includes a housing, which can be used to encapsulate the electrode assembly and the electrolyte as described above. In some embodiments, the housing of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc.; or it can be a soft package, such as an aluminum plastic film, etc.

[0057] The present application has no particular limitation on the shape of the secondary battery, and it can be cylindrical, square, or any other shape. In one embodiment, the secondary battery is a square shell battery, and the housing of the battery includes a housing body and a cover plate. Wherein, the housing body includes a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose a receiving cavity. The housing body has an opening communicating with the receiving cavity, and the cover plate can be covered on the opening to close the receiving cavity, and the electrode assembly formed by winding or stacking the positive electrode plate, the negative electrode plate, and the separator is encapsulated in the receiving cavity. The electrolyte infiltrates into the electrode assembly. The number of electrode assemblies contained in the lithium-ion battery can be one or more, and those skilled in the art can select according to specific actual needs.

[0058] The second aspect of the present invention provides an electronic device, which includes the secondary battery described above in the present invention. The secondary battery can be used in the electronic device in the form of a single battery, a battery module, or a battery pack.

[0059] The electronic device of the present invention includes, but is not limited to, mobile phones, tablets, laptop computers, electric toys, battery cars, new energy vehicles, ships, spacecraft, etc. Among them, the electric toys can include fixed or mobile electric toys, such as game consoles, electric vehicle toys, electric ship toys, and electric aircraft toys, etc., and the spacecraft can include airplanes, rockets, space shuttles, and spaceships, etc. The new energy vehicle can be a pure electric vehicle, a hybrid vehicle, or an extended-range vehicle, etc.

[0060] The technical solution of the present invention will be described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the examples can be purchased commercially.

[0061] Example 1

[0062] This example provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte.

[0063] Among them, the negative electrode sheet includes a negative electrode current collector and a first negative electrode active material layer and a second negative electrode active material layer sequentially arranged from the inside to the outside along the thickness direction of the negative electrode current collector. In this example, the negative electrode current collector is a copper foil current collector with a thickness of 8 μm. The active material of the first negative electrode active material layer is silicon oxide and artificial graphite, and the mass of silicon oxide accounts for 10% of the total mass of silicon oxide and artificial graphite. The mass content of the binder PAA in the first negative electrode active material layer is 3.6%; the active material of the second negative electrode active material layer is silicon oxide and artificial graphite, and the mass of silicon oxide accounts for 10% of the total mass of silicon oxide and artificial graphite. The mass content of the binder PAA in the second negative electrode active material layer is 3.0%.

[0064] The preparation process of the secondary battery is as follows:

[0065] (1) Preparation of the positive electrode sheet:

[0066] Mix the positive electrode active material LiNi 0.9 Co 0.05 Mn 0.05 O2, the binder PVDF, and the conductive agent acetylene black in a mass ratio of 98:1:1, and add the solvent N-methylpyrrolidone (NMP). Stir evenly with a vacuum mixer to obtain a positive electrode slurry; evenly coat the positive electrode slurry on a 16-μm aluminum foil current collector, dry it at room temperature, transfer it to an oven, and dry it at 120 °C for 6 h. Then, obtain the positive electrode sheet through cold pressing and slitting.

[0067] (2) Preparation of the negative electrode sheet:

[0068] Mix the negative electrode active material (silicon oxide (SiO): artificial graphite = 10:90), conductive agent acetylene black, binder PAA, and thickener CMC-Na in a mass ratio of 94.4:1:3.6:1, add deionized water, and obtain the first negative electrode slurry under the action of a vacuum mixer; uniformly coat the first negative electrode slurry on an 8-μm copper foil current collector, dry it at room temperature, transfer it to an oven for drying, and then obtain the first layer of negative electrode active material through cold pressing; then mix the negative electrode active material (silicon oxide (SiO): artificial graphite = 10:90), conductive agent acetylene black, binder PAA, and thickener CMC-Na in a mass ratio of 95:1:3:1, add deionized water, and obtain the second negative electrode slurry under the action of a vacuum mixer. Uniformly coat the second negative electrode slurry on the first layer of negative electrode active material layer, dry it at room temperature, transfer it to an oven for drying, and obtain the negative electrode plate through cold pressing and slitting.

[0069] (3) Preparation of the electrolyte:

[0070] In a glove box with an argon atmosphere with a water content of <10 ppm, mix ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) evenly in a volume ratio of 20:20:60 to obtain an organic solvent; dissolve the fully dried lithium salt (LiPF6) in the above organic solvent, and obtain the electrolyte after mixing evenly. Among them, the concentration of LiPF6 is 1 mol / L.

[0071] (4) Preparation of the separator:

[0072] Select a 12-μm-thick polypropylene separator.

[0073] (5) Preparation of the battery:

[0074] Stack the positive electrode plate, separator, and negative electrode plate prepared above in sequence, with the separator in the middle of the positive and negative electrode plates to play a role in isolation; then wrap it with an aluminum-plastic film, transfer it to a vacuum oven for drying at 120°C, inject 3.0 g / Ah of the electrolyte and then seal it. After standing, hot and cold pressing, forming, clamping, and grading, etc., finally prepare a soft-pack battery (i.e., a lithium-ion secondary battery) with a capacity of 1 Ah.

[0075] The specific steps and conditions of forming are as follows: After injecting the electrolyte, maintain a hot pressing environment of 0.1 MPa, charge at 45°C at 0.02C for 17 min in a static state, stand for 5 min and then charge to 0.3 Ah at 0.02C. After that, cut off the air bag and vacuum package it, and stand at room temperature for 48 h to complete the forming of the electrolyte.

[0076] Example 2

[0077] The difference between this embodiment and Embodiment 1 is that: in the first layer of the negative electrode active material layer of the negative electrode sheet, the mass content of the binder PAA is 3.7%, and in the second layer of the negative electrode active material layer, the mass content of the binder PAA is 3.2%.

[0078] Embodiment 3

[0079] The difference between this embodiment and Embodiment 1 is that: in the first layer of the active material layer of the negative electrode sheet, the mass of the silicon oxide compound accounts for 12% of the total mass of the negative electrode active material in this layer, and the mass content of the binder PAA in the first layer of the active material layer is 3.8%; in the second layer of the negative electrode active material layer, the mass of the silicon oxide compound accounts for 8% of the total mass of the negative electrode active material in this layer, and the mass content of the binder PAA in the second layer of the negative electrode active material layer is 3%.

[0080] Embodiment 4

[0081] The difference between this embodiment and Embodiment 1 is that: the negative electrode sheet includes a first layer of negative electrode active material layer, a second layer of negative electrode active material layer, and a third layer of negative electrode active material layer which are sequentially arranged from the inside to the outside along the thickness direction of the negative electrode current collector;

[0082] In the first layer of the negative electrode active material layer, the mass of the silicon oxide compound accounts for 10% of the total mass of the negative electrode active material in this layer, and the mass content of the binder PAA in the first layer of the active material layer is 3.83%;

[0083] In the second layer of the negative electrode active material layer, the mass of the silicon oxide compound accounts for 8% of the total mass of the negative electrode active material in this layer, and the mass content of the binder PAA in the second layer of the active material layer is 3.57%;

[0084] In the second layer of the negative electrode active material layer, the mass of the silicon oxide compound accounts for 6% of the total mass of the negative electrode active material in this layer, and the mass content of the binder PAA in the second layer of the active material layer is 3.1%.

[0085] Comparative Example 1

[0086] The difference between this comparative example and Embodiment 1 is that: in the first layer of the negative electrode active material layer, the mass content of the binder is 3%, and in the second layer of the negative electrode active material layer, the mass content of the binder is 3%.

[0087] Comparative Example 2

[0088] The difference between this comparative example and Embodiment 1 is that: in the first layer of the negative electrode active material layer, the mass content of the binder is 3.6%, and in the second layer of the negative electrode active material layer, the mass content of the binder is 3.6%.

[0089] Comparative Example 3

[0090] The difference between this comparative example and Example 1 is that: the mass content of the binder in the first layer of the negative electrode active material layer is 3%, and the mass content of the binder in the second layer of the negative electrode active material layer is 3.6%.

[0091] Comparative Example 4

[0092] The difference between this comparative example and Example 1 is that: the mass of the silicon oxide compound in the first layer of the negative electrode active material layer accounts for 8% of the total mass of the negative electrode active material in this layer, and the mass content of the binder in the first layer of the negative electrode active material layer is 3%; the mass of the silicon oxide compound in the second layer of the negative electrode active material layer accounts for 12% of the total mass of the negative electrode active material in this layer, and the mass content of the binder in the second layer of the negative electrode active material layer is 3.6%.

[0093] Comparative Example 5

[0094] The difference between this comparative example and Example 1 is that: the mass of the silicon oxide compound in the first layer of the negative electrode active material layer accounts for 12% of the total mass of the negative electrode active material in this layer, and the mass content of the binder in the first layer of the negative electrode active material layer is 3%; the mass of the silicon oxide compound in the second layer of the negative electrode active material layer accounts for 8% of the total mass of the negative electrode active material in this layer, and the mass content of the binder in the second layer of the negative electrode active material layer is 3.6%.

[0095] Performance tests were respectively carried out on the secondary batteries and negative electrode plates of Examples 1 to 4 and Comparative Examples 1 to 5. For the test results, see Table 1 and Figures 3 to 6 , and the test process is as follows:

[0096] (1) Pole piece peel force / adhesion force: Take the rolled negative electrode plate, and cut the pole piece with a customized knife die to obtain a test pole piece; take a flat thin steel plate, with a length of about 200 - 300 mm and a width of about 40 - 60 mm. First, stick a double-sided tape in the center of the steel plate (the length is greater than the sample test length and is the same width as the pole piece), smooth it forcefully to ensure that the double-sided tape is closely attached to the center of the steel plate; uncover the double-sided tape, and attach the test pole piece to the tape, ensuring that the pole piece and the tape are perfectly matched. According to the steps specified by the instrument, set the test width, the pole piece peel length is 50 - 150 mm, and the peel speed is 50 - 100 mm / min, and then start the test, and record the peel force when the negative electrode active material layer is peeled off from the negative electrode current collector.

[0097] (2) Fast charging cycle performance: The battery cells adopt the same fast charging process steps, charging from the fast charging lower limit voltage to the fast charging upper limit voltage. After standing for 10 min, discharge at 1C to the fast charging lower limit voltage and stand for 10 min. Repeat the above test process until the fast charging capacity decays to 80%, and record the number of cycles.

[0098] (3) Disassembly of fast charging interface: Using the fast charging cycle process, perform a 200 cls fast charging cycle test on Example 1 and Comparative Example 1. After the cycle is completed, disassemble the cell interface.

[0099] (4) Cycling performance: The cell is charged at a constant current and constant voltage of 1C to the maximum voltage. After charging is completed, let it stand for 10 min, and then discharge it at 1C to the minimum voltage. After discharging is completed, let it stand for 10 min. The cell is repeatedly tested in cycles until the capacity decays to 80%.

[0100] (5) Rate performance: The cell is charged at a specific rate from the minimum voltage at a constant current to the maximum voltage, and the charging capacity is recorded.

[0101] Table 1: Test results of the peel strength of the negative electrode sheets and the fast charging ability of the batteries for Examples 1 to 4 and Comparative Examples 1 to 5

[0102]

[0103]

[0104] It should be noted that the judgment criteria for the fast charging ability in Table 1 are as follows:

[0105] Excellent: At 25°C, the fast charging time t for charging the battery from 10% SOC (state of charge, which can be understood as the percentage of the remaining battery charge) to 80% SOC satisfies: t ≤ 20 min;

[0106] Good: At 25°C, the fast charging time t for charging the battery from 10% SOC to 80% SOC satisfies: 20 < t ≤ 22 min;

[0107] Average: At 25°C, the fast charging time t for charging the battery from 10% SOC to 80% SOC satisfies: 22 < t ≤ 24 min;

[0108] Poor: At 25°C, the fast charging time t for charging the battery from 10% SOC to 80% SOC satisfies t > 25 min.

[0109] As can be seen from Table 1, the negative electrode sheets of Examples 1 to 4 all adopt the method of gradually decreasing the content of the binder in the active material layer. The peeling force of the negative active material layer in the negative electrode sheet has been significantly improved compared with Comparative Example 1 and Comparative Examples 3 to 5. This is because during the drying process of the electrode sheet, the binder will float up as the moisture evaporates, resulting in a decrease in the binder in the active material layer at the bottom layer (close to the negative current collector). In the initial state of each embodiment of the present application, the binder in the bottom active material layer is greater than that in the upper layer. Even if part of the binder in the bottom layer floats up during the drying process, there is still enough binder in the bottom active material layer to provide sufficient bonding force; while in the comparative examples, the content of the binder in each active material layer is the same or the content in the bottom layer is less than that in the upper layer, resulting in a decrease in the peeling force of the electrode sheet; in Comparative Example 2, although the upper and lower layers adopt the same content of the binder, because the content of the binder is sufficient, even if part of the binder in the bottom layer floats up, there is still enough binder to provide the bonding force. Therefore, the peeling force of Comparative Example 2 is comparable to that of the examples, but because of the large amount of binder, it will affect the transmission of lithium ions, resulting in a decrease in the energy density and further affecting its electrical performance.

[0110] In addition, the negative electrode sheets of Examples 1 to 4 all adopt that the content of the silicon-based material in the bottom active material layer is greater than or equal to the content of the silicon-based material in the upper active material layer, which can enable the rapid insertion of lithium ions in the outer active material layer, realize hierarchical gradient lithium intercalation, improve the kinetics of the electrode sheet, and further optimize the fast charging ability of the battery. Comparative Example 4 adopts the opposite method (the content of the silicon-based material in the bottom layer is less than the content of the silicon-based material in the upper layer), and the fast charging ability of the battery is poor. Although Comparative Example 5 adopts the same silicon-based material design method, due to the low content of the binder in the bottom layer, the peeling force is reduced, resulting in a decrease in the fast charging ability of the battery.

[0111] To verify the electrical performance of the secondary battery of the present application, the inventors of the present application selected the secondary battery of Example 1 to conduct cycle, rate performance, and fast charging performance tests on it respectively and compared it with the comparative examples. The test results are shown in Figures 2 to 6 .

[0112] See Figure 2 and Figure 3 , the secondary battery of Example 1 has significantly improved cycle performance compared with the secondary battery of the comparative example; compared with Comparative Example 1, the capacity retention rate of the secondary battery of Example 1 after 1000 cls of cycling has increased from 84% to 88%, and the cycle life cutoff at 80% SOH (state of health, which can be understood as the percentage of the current capacity of the battery to the factory capacity) has increased from 1200 cls to 1400 cls.

[0113] See Figure 4, the secondary battery of Example 1 shows a significantly improved rate performance compared to the secondary battery of the comparative example, demonstrating excellent kinetic performance, indicating that the combination of this patent can effectively improve the kinetic performance of lithium-ion batteries.

[0114] See Figure 5 , comparing Example 1 and Comparative Example 1, the fast charge cycle performance of Example 1 is significantly improved, and the fast charge cycle capacity retention rate is much greater than that of Comparative Example 1.

[0115] See Figure 6 , where Figure 6 (a) is the disassembly interface of Example 1, Figure 6 (b) is the disassembly interface of Comparative Example 1. Comparing the disassembly interfaces after fast charge cycling of Example 1 and Comparative Example 1, the interface of Example 1 is flat on the surface, without obvious wrinkles or black edges, while the interface of Comparative Example 1 has slight wrinkles on the surface and a black edge at the edge, indicating that no lithium deposition occurs on the negative electrode during the fast charge process of Example 1, and further indicating that the electrode sheet of this application can achieve hierarchical gradient lithium intercalation, improving the kinetics of the electrode sheet.

[0116] The present invention adopts a multi-layer coating technique to control the content of each substance in the active materials of each layer in the negative electrode sheet, so that the content of the silicon-based material in the active material layer close to the current collector is greater than or equal to the content of the silicon-based material in the active material layer far from the current collector, and the content of the binder in the active material layer close to the current collector is greater than the content of the binder in the active material layer far from the current collector, thereby realizing hierarchical gradient lithium intercalation of the negative electrode sheet, improving the fast charge ability of the secondary battery, and at the same time improving the problem of reducing the peel strength of the multi-layer coated electrode sheet. Therefore, the present invention effectively overcomes some practical problems in the prior art and thus has high utilization value and practical significance.

[0117] The above embodiments are only illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes completed by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A secondary battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, characterized in that The negative electrode plate includes: A negative electrode current collector; At least two negative electrode active material layers, disposed on at least one surface of the negative electrode current collector, and the at least two negative electrode active material layers are sequentially disposed along the thickness direction of the negative electrode current collector; Wherein, the negative electrode active material layer includes a negative electrode active material, a conductive agent, and a binder, the negative electrode active material includes a silicon-based material, and the mass ratio of the silicon-based material in the negative electrode active material in the negative electrode active material layer close to the negative electrode current collector is greater than or equal to the mass ratio of the silicon-based material in the negative electrode active material in the negative electrode active material layer far from the negative electrode current collector; The content of the binder in the negative electrode active material layer decreases sequentially from the side close to the negative electrode current collector to the side far from the negative electrode current collector.

2. The secondary battery according to claim 1, characterized in that, The negative electrode plate includes n negative electrode active material layers. The negative electrode active material layers from the side close to the negative electrode current collector to the side far from the negative electrode current collector are the 1st, 2nd, …, i-th, …, n-th layers respectively. The mass ratio of the silicon-based material in the i-th negative electrode active material layer to the mass of the negative electrode active material is ai%, and the mass content of the binder in the i-th negative electrode active material layer is xi%. Then 2.2 + ai / 10 + (1 - i / n) * 0.8 - 0.3 ≤ xi ≤ 2.2 + ai / 10 + (1 - i / n) * 0.8 + 0.3, where n ≥ 2 and ai ≤ 15.

3. The secondary battery according to claim 2, characterized in that, The mass content xi% of the binder in the i-th negative electrode active material layer satisfies: 2.2 + ai / 10 + (1 - i / n) * 0.8 - 0.1 ≤ xi ≤ 2.2 + ai / 10 + (1 - i / n) * 0.8 + 0.

1.

4. The secondary battery according to claim 2 or 3, characterized in that, 2≤n≤3。 5. The secondary battery according to claim 1, characterized in that, The silicon-based material includes at least one of silicon, silicon-carbon material, and silicon oxide; and / or, the negative electrode active material further includes a carbon material, and the carbon material includes at least one of artificial graphite, natural graphite, soft carbon, hard carbon, and mesophase carbon microspheres.

6. The secondary battery according to claim 1, characterized in that, The compaction density of the negative electrode active material layer decreases sequentially from the side close to the negative electrode current collector side to the side far from the negative electrode current collector side, and the porosity of the negative electrode active material layer increases sequentially from the side close to the negative electrode current collector to the side far from the negative electrode current collector.

7. The secondary battery according to claim 1, characterized in that, The compaction density of the negative electrode sheet is 1.5 to 1.7 g / cm 3 ; and / or, the adhesion of the negative electrode sheet is 14 to 18 N / m.

8. The secondary battery according to claim 1, characterized in that, The conductive agent includes at least one of conductive carbon black, conductive graphite, carbon fiber, carbon nanotube, and graphene; and / or, the binder includes at least one of polyacrylic acid, polyvinylidene fluoride, and styrene-butadiene rubber.

9. The secondary battery according to claim 1, characterized in that, The positive electrode plate includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes one or more of lithium nickel cobalt manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese aluminate, lithium iron phosphate, and lithium manganese iron phosphate.

10. An electronic device, characterized in that, The electronic device includes the secondary battery according to any one of claims 1 to 9.

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