Secondary battery and electric device

Abstract

The invention discloses a secondary battery and an electric device, and belongs to the technical field of batteries. The secondary battery provided by the invention comprises a positive pole piece, a negative pole piece, a diaphragm and an electrolyte, the negative pole piece comprises a negative current collector, a colloid conductive layer arranged on at least one surface of the negative current collector, and a negative active material layer arranged on the colloid conductive layer and far away from the surface of the negative current collector; the colloid covering layer is arranged on the surface, far away from the negative current collector, of the negative active material layer; the colloid conductive layer comprises first aluminum sol and a conductive agent; and the colloid covering layer comprises second aluminum sol. The secondary battery provided by the invention has good cycle performance and heat resistance.

Description

Technical Field

[0001] The present application relates to the field of battery technology, in particular to a secondary battery and an electrical device. Background Art

[0002] The performance of lithium-ion battery anodes directly impacts the battery's energy density, cycle life, and safety. Graphite has become the mainstream anode material due to its high theoretical capacity, low cost, and stability. However, it has long suffered from issues such as insufficient interfacial bonding and peeling strength with the current collector (copper foil), high-temperature interfacial failure, and continuous lithium ion consumption by the SEI membrane, resulting in reduced cycle life.

[0003] In order to improve the above problems, the existing technology mainly focuses on the following solutions: one is to roughen the copper foil (such as chemical etching, electrodeposition technology) or modify the surface of the current collector, but the roughening of the copper foil may introduce interface defects and accelerate the electrolyte penetration corrosion; the surface current collector is modified, and the modified substance is easily decomposed at high temperature and cannot pass the hot box test. For example, 201811403483.4 coated the copper foil surface with a polydopamine coating to enhance the adhesion, but the decomposition temperature of polydopamine is less than 200°C, which affects the hot box pass rate; the second is to improve the battery performance by adding inorganic particles as a reinforcing phase to the negative electrode graphite slurry, but such inorganic particles are generally micron-sized and micro-nano-sized particles, which will lead to lithium ion transmission. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and an electrical device to solve the problem in the prior art that the interface bonding between the negative electrode material and the current collector in the negative electrode plate is insufficient in peeling force, and when used in a secondary battery, the negative electrode plate will suffer from high-temperature interface failure and the SEI film will be continuously consumed, resulting in a decrease in the cycle life of the secondary battery.

[0005] To achieve the above objectives, in a first aspect, the present application provides a secondary battery, comprising a negative electrode sheet, a positive electrode sheet, a separator, and an electrolyte, wherein the negative electrode sheet comprises a negative electrode current collector, a colloidal conductive layer disposed on at least one surface of the negative electrode current collector, a negative electrode active material layer disposed on the colloidal conductive layer away from the surface of the negative electrode current collector, and a colloidal covering layer disposed on the surface of the negative electrode active material layer away from the surface of the negative electrode current collector;

[0006] The colloidal conductive layer includes a first aluminum sol and a conductive agent;

[0007] The colloidal coating includes a second aluminum sol.

[0008] As an embodiment of the present application, the secondary battery satisfies 2≤M≤8;

[0009] Wherein, M is the mass ratio of the conductive agent to the first aluminum sol in the colloidal conductive layer.

[0010] As an embodiment of the present application, the secondary battery satisfies 25≤H≤80;

[0011] Where, H = H1 / H2;

[0012] H1 is the thickness of the single-side negative electrode active material layer in the half-charge state, in μm;

[0013] H2 is the thickness of the unilateral colloidal covering layer, in μm.

[0014] As an embodiment of the present application, the thickness H1 of the single-side negative electrode active material layer in the half-charge state is 30-70 μm.

[0015] As an embodiment of the present application, the thickness H2 of the single-sided colloidal covering layer is 0.5-2 μm.

[0016] As an embodiment of the present application, the thickness of the single-sided colloidal conductive layer is 0.5-2 μm.

[0017] As an embodiment of the present application, in the colloidal conductive layer, the average particle size of the first aluminum sol is 1-100 nm.

[0018] As an embodiment of the present application, in the colloidal covering layer, the average particle size of the second aluminum sol is 1-50 nm.

[0019] As an embodiment of the present application, the conductive agent includes at least one of conductive carbon black and carbon nanotubes.

[0020] As an embodiment of the present application, the colloidal conductive layer further includes an organic binder. Based on the total mass of the colloidal conductive layer, the mass percentage of the organic binder is 8-12%.

[0021] In a second aspect of the present application, the present application provides an electrical device, which includes the secondary battery described in the present application.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] The present application provides a negative electrode sheet in a secondary battery by introducing a colloidal conductive layer between the negative electrode current collector and the negative electrode active material layer, and at the same time providing a colloidal covering layer on the surface of the negative electrode active layer away from the negative electrode current collector, i.e. forming a "sandwich" structure, and at the same time limiting the colloidal conductive layer to include a first aluminum sol and a conductive agent, and the colloidal covering layer to include a second aluminum sol. There is good interaction between the colloidal conductive layer and the colloidal covering layer, which can effectively increase the peeling force of the negative electrode sheet and reduce the resistance of the negative electrode sheet, thereby improving the cycle performance of the prepared secondary battery, and at the same time can also improve the high-temperature stability of the secondary battery and increase the hot box pass rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a SEM image of the colloidal conductive layer in Example 1;

[0025] Figure 2 This is the EDS image of the colloidal conductive layer in Example 1;

[0026] Figure 3 is a SEM image of the negative electrode active material layer in Example 1;

[0027] Figure 4 is a SEM image of the colloidal covering layer in Example 1;

[0028] Figure 5 This is the EDS image of the colloidal covering layer in Example 1;

[0029] Figure 6 Schematic diagram of the layered structure of the negative electrode sheet in Example 1. DETAILED DESCRIPTION

[0030] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0031] In this application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0032] In this application, when referring to numerical ranges, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges subsumed therein.

[0033] The reagents and instruments used in this application without indicating the manufacturer are all conventional products that can be purchased from the market.

[0034] In one embodiment of the present application, in a first aspect, a secondary battery is provided, the secondary battery comprising a negative electrode sheet, a positive electrode sheet, a separator, and an electrolyte, the negative electrode sheet comprising a negative electrode current collector, a colloidal conductive layer disposed on at least one surface of the negative electrode current collector, a negative electrode active material layer disposed on the colloidal conductive layer away from the surface of the negative electrode current collector, and a colloidal covering layer disposed on the surface of the negative electrode active material layer away from the surface of the negative electrode current collector;

[0035] The colloidal conductive layer includes a first aluminum sol and a conductive agent;

[0036] The colloidal coating includes a second aluminum sol.

[0037] The present application provides a negative electrode sheet in a secondary battery by introducing a colloidal conductive layer between the negative electrode current collector and the negative electrode active material layer, and at the same time providing a colloidal covering layer on the surface of the negative electrode active layer away from the negative electrode current collector, i.e. forming a "sandwich" structure, and at the same time limiting the colloidal conductive layer to include a first aluminum sol and a conductive agent, and the colloidal covering layer to include a second aluminum sol. There is good interaction between the colloidal conductive layer and the colloidal covering layer, which can effectively increase the peeling force of the negative electrode sheet and reduce the resistance of the negative electrode sheet, thereby improving the cycle performance of the prepared secondary battery, and at the same time can also improve the high-temperature stability of the secondary battery and increase the hot box pass rate.

[0038] Specifically, the present application introduces a colloidal conductive layer on the surface of the negative electrode current collector, and the colloidal conductive layer includes a first aluminum sol and a conductive agent. On the one hand, the conductive agent can provide excellent conductivity, and the first aluminum sol can provide the colloidal conductive layer with good structural support. The conductive agent and the first aluminum sol interact with each other to form a stable three-dimensional network structure, which can not only maintain high conductivity and reduce the resistance of the negative electrode sheet, but also enhance the mechanical strength and heat stability of the layered structure, and reduce the risk of thermal degradation in the negative electrode sheet; on the other hand, the first aluminum sol is a porous structure, which can form a stable three-dimensional network structure and is also conducive to electrolyte penetration and ion transport, thereby effectively improving the capacity retention rate of the secondary battery.

[0039] Specifically, the present application introduces a colloidal covering layer on the surface of the negative electrode active layer away from the negative electrode current collector, and the colloidal covering layer includes a second aluminum sol, which can form an O-Al-O three-dimensional structure with high surface activity on the surface of the negative electrode plate. It not only has a certain adhesion, can increase the peeling force of the negative electrode plate, and improve the temperature resistance of the secondary battery; it also has excellent liquid retention performance, can effectively stabilize the SEI film layer of the negative electrode plate, reduce the growth and thickening of the SEI film during the cycle of the secondary battery, thereby improving the cycle performance of the secondary battery.

[0040] In one embodiment, the secondary battery satisfies 2≤M≤8;

[0041] Wherein, M is the mass ratio of the conductive agent to the first aluminum sol in the colloidal conductive layer.

[0042] It should be noted that the test method for the mass ratio of the conductive agent and the first aluminum sol in the colloidal conductive layer is: disassemble the negative electrode plate from the secondary battery, use tape to stick off the colloidal covering layer and the negative active layer material on the surface of the negative electrode plate, leaving only the colloidal conductive layer, and completely dissolve the colloidal conductive layer in deionized water on the copper foil to obtain solution a, dry the solution a at 100-110°C to obtain solid b with a weight of M1, calcine the solid b at 250-300°C to obtain solid c with a weight of M2, M1-M2 is the mass of the binder, calcine the solid c at 500-600°C to obtain solid d with a weight of M3; the weight of M2-M3 is the weight of the conductive agent; M3 is the dry weight of the first aluminum sol.

[0043] Exemplarily, M may be any point value or any two point range value between 2-8, such as 2, 3, 4, 5, 6, 7, 8, etc.

[0044] In one embodiment, the secondary battery satisfies 3≤M≤4. For example, M can be 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, etc.

[0045] The research in this application found that by further selecting the mass ratio M of the conductive agent and the first aluminum sol in the colloidal conductive layer to be 2-8, especially 3-4, a more stable three-dimensional network structure can be formed. While improving the peeling force of the negative electrode sheet and reducing the resistance of the negative electrode sheet, it can also improve the heat resistance and cycle performance of the secondary battery prepared subsequently.

[0046] In one embodiment, the secondary battery satisfies 25≤H≤80;

[0047] Where, H = H1 / H2;

[0048] H1 is the thickness of the single-side negative electrode active material layer in the half-charge state, in μm;

[0049] H2 is the thickness of the unilateral colloidal covering layer, in μm.

[0050] Exemplarily, H can be any point value or any two-point range value between 25-80, such as 25, 28, 30, 32, 35, 38, 40, 42, 45, 48, 50, 52, 55, 58, 60, 62, 65, 68, 70, 72, 75, 78, 80, etc.

[0051] In one embodiment, the secondary battery satisfies 40≤H≤50. For example, H can be 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, etc.

[0052] The research in this application found that by further selecting the ratio of the thickness of the single-sided negative electrode active material layer in the half-charge state to the thickness of the single-sided colloidal covering layer to be 25-80, especially 40-50, the peeling force of the obtained negative electrode pole piece is greater, the heat resistance of the secondary battery prepared subsequently is better, the hot box pass rate is higher, and the cycle performance of the obtained secondary battery is also better.

[0053] In one embodiment, the thickness H1 of the single-side negative electrode active material layer in the half-charge state is 30-70 μm.

[0054] It should be noted that the testing method for the thickness H1 of the single-sided negative electrode active material layer in the half-charged state is to disassemble the secondary battery in the half-charged state (50% SOH, wherein the half-charged state is the half-charged state after the capacity is divided and formed during the preparation process of the secondary battery) to obtain the negative electrode sheet, and test the total thickness of the negative electrode sheet in the half-charged state. The thickness of the half-charged negative electrode active material layer = (total thickness of the half-charged negative electrode sheet - copper foil thickness - colloidal conductive layer thickness - colloidal covering layer thickness) / 2, wherein the copper foil thickness, the colloidal conductive layer thickness and the colloidal covering layer thickness can be the thickness observed and measured using the interface polishing-scanning electron microscope (CP-SEM) in the uncharged negative electrode sheet state.

[0055] Exemplarily, the thickness H1 of the single-sided negative electrode active material layer in the half-charge state can be any point value or any two-point range value between 30-70 μm, such as 30 μm, 32 μm, 35 μm, 38 μm, 40 μm, 42 μm, 45 μm, 48 μm, 50 μm, 52 μm, 55 μm, 58 μm, 60 μm, 62 μm, 65 μm, 68 μm, 70 μm, etc.

[0056] In one embodiment, the thickness H1 of the negative electrode active material layer on one side in the half-charge state is 40-50 μm. For example, H1 can be 40 μm, 41 μm, 42 μm, 43 μm, 44 μm, 45 μm, 46 μm, 47 μm, 48 μm, 49 μm, 50 μm, etc.

[0057] The present application has found that by further selecting the thickness of the negative electrode active material layer on one side in the half-charged state to be within the above range, the capacity retention rate of the obtained secondary battery can be improved and the service life can be extended.

[0058] In one embodiment, the thickness H2 of the single-sided colloid covering layer is 0.5-2 μm.

[0059] It should be noted that the test method for the thickness H2 of the single-sided colloidal covering layer is to disassemble the negative electrode sheet from the secondary battery in an empty state or a half-charged state (50% SOH), and then use a cross-sectional polishing-scanning electron microscope (CP-SEM) to observe and measure the thickness of the colloidal covering layer.

[0060] It should be noted that the thickness H2 of the single-sided colloidal covering layer in the half-charged state or the empty state and the fully-charged state has a negligible difference in thickness, that is, the test can be performed in the half-charged state or the empty state.

[0061] Exemplarily, the thickness H2 of the single-sided colloidal covering layer may be any point value or any two point range values ​​between 0.5-2 μm, such as 0.5 μm, 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2 μm, etc.

[0062] In one embodiment, the thickness H2 of the single-sided colloid coating layer is 0.8-1.2 μm. For example, H2 can be 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, etc.

[0063] The present application has found that by selecting the thickness of the single-sided colloidal covering layer within the above range, the peeling force of the negative electrode sheet can be greatly improved, and the hot box pass rate of the obtained secondary battery is higher.

[0064] In one embodiment, the thickness of the single-sided colloidal conductive layer is 0.5-2 μm.

[0065] It should be noted that the test method for the thickness of the single-sided colloidal conductive layer is

[0066] The negative electrode sheet was disassembled from the secondary battery in an empty state or a half-charged state (50% SOH), and then the thickness of the single-side colloidal layer was observed and measured using a cross-sectional polishing-scanning electron microscope (CP-SEM).

[0067] It should be noted that the thickness H2 of the single-sided colloidal covering layer in the half-charged state or the empty state has a negligible difference in thickness, that is, the test can be performed in the half-charged state or the empty state.

[0068] Exemplarily, the thickness of the single-sided colloidal conductive layer may be any point value or any two point range values ​​between 0.5-2 μm, such as 0.5 μm, 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2 μm, etc.

[0069] In one embodiment, the thickness of the single-sided colloidal conductive layer is 0.8-1.2 μm, for example, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, etc.

[0070] The present application study found that by selecting the thickness of the single-sided colloidal conductive layer within the above range, the ionic conductivity and structural stability of the secondary battery can be better improved, thereby improving the hot box pass rate and cycle performance of the secondary battery.

[0071] In one embodiment, in the colloidal conductive layer, the average particle size of the first aluminum sol is 10-100 nm.

[0072] It should be noted that the testing method for the average particle size of the first aluminum sol is: disassemble the negative electrode plate from the secondary battery, use tape to stick off the colloidal covering layer and the negative electrode active material layer on the surface of the negative electrode plate, leaving only the colloidal conductive layer, use SEM or TEM with EDS to observe the particle size of the first aluminum sol, and calculate the average particle size.

[0073] Exemplarily, in the colloidal conductive layer, the average particle size of the first aluminum sol can be any point value or any two point range values ​​between 10-100 nm, for example, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, etc.

[0074] In one embodiment, the average particle size of the first aluminum sol is 30-70 nm, for example, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, etc.

[0075] The research in this application found that the average particle size of the first aluminum sol not only affects its own mechanical properties, but also affects the permeability of the three-dimensional network structure formed by it and the conductive agent to the electrolyte and the ion transport performance. When the average particle size of the first aluminum sol is further selected to be within the above range, the structural stability of the colloidal conductive layer can be better improved, and the heat resistance and capacity retention performance of the secondary battery can be improved.

[0076] In one embodiment, in the colloidal covering layer, the average particle size of the second aluminum sol is 10-50 nm.

[0077] Exemplarily, in the colloidal covering layer, the average particle size of the second aluminum sol can be any point value between 10-50nm or any two point range values, such as 10nm, 15nm, 18nm, 20nm, 22nm, 25nm, 28nm, 30nm, 32nm, 35nm, 38nm, 40nm, 46nm, 50nm, etc.

[0078] In one embodiment, the average particle size of the second aluminum sol in the colloidal coating layer is 20-30 nm, for example, 20 nm, 22 nm, 24 nm, 26 nm, 28 nm, 30 nm, etc.

[0079] The research in this application found that the average particle size of the second aluminum sol in the colloidal covering layer affects its adhesion and liquid retention performance. When the average particle size of the second aluminum sol in the colloidal covering layer is further selected to be within the above range, the resulting negative electrode sheet has a greater peeling force and the cycle performance of the secondary battery is better.

[0080] It should be noted that the average particle size of the second aluminum sol in the colloidal covering layer is obtained through transmission electron microscopy (TEM) observation and testing.

[0081] In one embodiment, the conductive agent includes at least one of conductive carbon black and carbon nanotubes.

[0082] In one embodiment, the colloidal conductive layer further includes an organic binder. Based on the total mass of the colloidal conductive layer, the mass percentage of the organic binder is 8-12%.

[0083] It should be noted that the test method for the mass percentage of the organic binder based on the total mass of the colloidal conductive layer is as follows: disassemble the negative electrode plate from the secondary battery, use tape to stick off the colloidal covering layer and the negative electrode active layer material on the surface of the negative electrode plate, leaving only the colloidal conductive layer, and completely dissolve the colloidal conductive layer in deionized water on a copper foil to obtain a solution a, dry the solution a at 100-110°C to obtain a solid b with a weight of M1, calcine the solid b at 250-300°C to obtain a solid c with a weight of M2, M1-M2 is the mass of the binder, and the ratio of the mass of the binder to the total mass of the colloidal conductive layer is the mass percentage of the organic binder.

[0084] For example, based on the total mass of the colloidal conductive layer, the mass percentage of the organic binder may be any value between 8-12% or any two values ​​within a range, such as 8%, 9%, 10%, 11%, 12%, etc.

[0085] The present application has found that adding the organic binder in the above mass percentage range can ensure that the addition amounts of the first aluminum sol and the conductive agent in the colloidal conductive layer are within a certain range, thereby achieving excellent comprehensive effects of the secondary battery.

[0086] In one embodiment, the method for preparing the negative electrode sheet comprises the following steps:

[0087] (1) mixing a first aluminum sol, a conductive agent, and an organic binder to form a colloidal conductive layer slurry, then coating the colloidal conductive layer slurry on at least one surface of the negative electrode current collector by dip coating or gravure coating, and then drying to form a colloidal conductive layer;

[0088] (2) uniformly mixing the negative electrode active material, the negative electrode conductive agent and the negative electrode binder and dispersing them in water to form a negative electrode active material layer slurry, and then coating the negative electrode active material layer slurry on the surface of the colloidal conductive layer away from the negative electrode current collector in step (1) by extrusion coating and drying to form a negative electrode active material layer;

[0089] (3) coating the second aluminum sol on the surface of the negative electrode active material layer away from the negative electrode current collector in step (2) by dip coating and then drying to obtain a negative electrode sheet.

[0090] In one embodiment, the negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder.

[0091] The present application has no particular limitation on the negative electrode active material, and any negative electrode active material commonly used in the art can be used. For example, the negative electrode active material can be graphite.

[0092] The present application has no particular restrictions on the negative electrode conductive agent, and any negative electrode conductive agent commonly used in the art can be used. For example, the negative electrode conductive agent can be at least one of conductive carbon black, carbon nanotubes, and graphene.

[0093] The present application has no particular limitation on the negative electrode binder, and any negative electrode binder commonly used in the art can be used. For example, the negative electrode binder can be at least one of sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyvinyl alcohol.

[0094] In one embodiment, 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.

[0095] In one embodiment, the positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder.

[0096] The present application has no particular restrictions on the positive electrode active material, and any positive electrode active material commonly used in the art is applicable. For example, the positive electrode active material can be at least one of lithium iron phosphate, lithium cobalt oxide, binary positive electrode materials, ternary positive electrode materials, and quaternary positive electrode materials.

[0097] The present application has no particular restrictions on the positive electrode conductive agent, and any positive electrode conductive agent commonly used in the art is applicable. For example, the positive electrode conductive agent can be at least one of conductive carbon black (SP), carbon fiber (CF), acetylene black, conductive graphite, graphene, carbon nanotubes, and carbon microspheres.

[0098] The present application has no particular restrictions on the positive electrode binder, and any positive electrode binder commonly used in the art is applicable. For example, the positive electrode binder can be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyvinyl alcohol, polyolefin, styrene-butadiene rubber, fluorinated rubber, polyurethane, and sodium alginate.

[0099] In one embodiment, the electrolyte includes an organic solvent and a lithium salt.

[0100] The present application has no particular limitation on the organic solvent, and any organic solvent commonly used in the art can be used. For example, the organic solvent can be a carbonate organic solvent.

[0101] Illustratively, the carbonate organic solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).

[0102] In one embodiment, the organic solvent comprises at least one cyclic carbonate and at least one chain carbonate, wherein the mass ratio of the cyclic carbonate to the chain carbonate is 1:(1-4).

[0103] The present application has no particular restrictions on lithium salts, and any lithium salt commonly used in the art is acceptable. For example, the lithium salt may be at least one of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalatoborate) (LiBOB), lithium difluorooxalatoborate (LiODFB), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium fluoride (LiF), and lithium trifluoromethanesulfonate (LiCF3SO3).

[0104] In one embodiment, the molar concentration of the lithium salt in the electrolyte is 0.5-2 mol / L.

[0105] The present application has no particular limitation on the separator, and any separator conventionally used in the art is applicable. For example, the separator can be any one of a single-layer polymer porous separator, a multi-layer polymer porous separator, an inorganic porous separator, and a polymer-inorganic composite porous separator.

[0106] In a second aspect of the present application, the present application provides an electrical device comprising the secondary battery described in the present application.

[0107] Example 1

[0108] An embodiment of the present application provides a secondary battery, wherein a method for preparing the secondary battery comprises the following steps:

[0109] (1) Preparation of negative electrode sheet

[0110] S1. 2 parts by mass of the first aluminum sol, 7 parts by mass of conductive carbon SP, and 1 part by mass of polyvinyl alcohol (PVA) are mixed to form a colloidal conductive layer slurry, which is then coated on both sides of a copper foil having a thickness of 6 μm by gravure coating at a speed of 40 m / min. After coating, the slurry is placed in an oven for drying at a temperature of 100° C. to obtain a colloidal conductive layer having a thickness of 1 μm on one side.

[0111] Among them, the SEM image of the formed colloidal conductive layer is as follows Figure 1 As shown, the EDS diagram is as follows Figure 2 As shown;

[0112] S2. Mix the negative electrode active material (graphite), the negative electrode binder sodium carboxymethyl cellulose, the negative electrode binder styrene-butadiene rubber, and the conductive carbon black in a mass ratio of 96.1:1.2:1.7:1, and disperse the mixture in water to form a negative electrode active material layer slurry. Then, apply the mixture to the surface of the colloidal conductive layer formed in step S1 away from the negative electrode current collector. After coating, place the mixture in an oven for drying at a temperature of 100° C. to obtain a negative electrode active material layer. The thickness of the single-side negative electrode active material layer is 40 μm.

[0113] Among them, the SEM image of the negative electrode active material layer is as follows Figure 3 As shown;

[0114] S3. Applying the second aluminum sol to the surface of the negative electrode active material layer formed in step S2 away from the negative electrode current collector by dip coating. After coating, the surface is placed in an oven for drying at a temperature of 90° C. to obtain a colloidal coating layer. The thickness of the colloidal coating layer on one side is 1 μm. After the colloidal coating layer is formed, the negative electrode sheet is obtained.

[0115] Among them, the SEM image after forming the colloidal covering layer is as follows Figure 4 As shown, the EDS diagram is as follows Figure 5 As shown; at the same time, the layered structure diagram of the prepared negative electrode sheet is as shown Figure 6 As shown;

[0116] (2) Preparation of positive electrode sheet

[0117] The positive electrode material (lithium iron phosphate), positive electrode conductive agent (conductive carbon black), positive electrode conductive agent (carbon nanotubes), and positive electrode binder (polyvinylidene fluoride) are fully stirred and mixed in N-methylpyrrolidone at a mass ratio of 97.5:0.7:0.6:1.2, and then coated on aluminum foil. After drying, rolling, and slitting, the positive electrode sheet is obtained;

[0118] (3) Preparation of electrolyte

[0119] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of 1:1:1 to obtain a mixed solvent. After mixing evenly, 1 mol / L lithium hexafluorophosphate was added and dissolved completely, and the mixture was stirred evenly to obtain an electrolyte.

[0120] (5) Preparation of secondary batteries

[0121] The prepared positive electrode sheet, negative electrode sheet, diaphragm (ordinary wet-process ceramic diaphragm) and other battery components are assembled, and a secondary battery is obtained through processes such as shaping, baking, packaging, liquid injection, formation, and capacity division.

[0122] Examples 2-5

[0123] The embodiment of the present application provides a secondary battery, which differs from embodiment 1 in that the addition amounts of the first aluminum sol and the conductive agent in the colloidal conductive layer are adjusted to achieve the parameters in Table 1.

[0124] Examples 6-13

[0125] This embodiment of the present application provides a secondary battery, which differs from Example 1 in that the thicknesses of the negative electrode active material layer and the colloidal covering layer are adjusted to achieve the parameters in Table 1.

[0126] Examples 14-15

[0127] The embodiment of the present application provides a secondary battery, which differs from the embodiment 1 in that the thickness of the colloidal conductive layer is adjusted to achieve the parameters in Table 1.

[0128] Examples 16-18

[0129] This embodiment of the present application provides a secondary battery, which differs from Example 1 in that the average particle size of the first aluminum sol is adjusted to achieve the parameters in Table 1.

[0130] Examples 19-21

[0131] This embodiment of the present application provides a secondary battery, which differs from Example 1 in that the average particle size of the second aluminum sol is adjusted to achieve the parameters in Table 1.

[0132] Comparative Example 1

[0133] The comparative example of the present application provides a secondary battery, which differs from Example 1 in that no conductive agent is added to the colloidal conductive layer to achieve the parameters in Table 1.

[0134] Comparative Example 2

[0135] The comparative example of the present application provides a secondary battery, which differs from Example 1 in that the first aluminum sol is not added to the colloidal conductive layer to achieve the parameters in Table 1.

[0136] Comparative Example 3

[0137] The comparative example of the present application provides a secondary battery, which differs from Example 1 in that no colloidal conductive layer is introduced to achieve the parameters in Table 1.

[0138] Comparative Example 4

[0139] The comparative example of the present application provides a secondary battery, which differs from Example 1 in that no colloidal covering layer is introduced to achieve the parameters in Table 1.

[0140] Comparative Example 5

[0141] The comparative example of the present application provides a secondary battery, which differs from Example 1 in that the colloidal conductive layer and the colloidal covering layer are not introduced to achieve the parameters in Table 1.

[0142] Comparative Example 6

[0143] The comparative example of the present application provides a secondary battery, which differs from Example 1 in that the aluminum sol in the colloidal conductive layer and the colloidal covering layer is replaced by silica sol (average particle size of 60 nm) to achieve the parameters in Table 1.

[0144] In the secondary batteries prepared in the examples and comparative examples, the mass ratio M of the conductive agent to the first aluminum sol in the colloidal conductive layer, the mass percentage W of the conductive agent to the first aluminum sol in the colloidal conductive layer, the average particle size S1 of the first aluminum sol, the thickness H1 of the single-sided negative electrode active material layer in the half-charge state, the thickness H2 of the single-sided colloidal covering layer, H, the thickness H3 of the single-sided colloidal conductive layer, and the average particle size S2 of the second aluminum sol are shown in Table 1.

[0145] Table 1

[0146]

[0147]

[0148] The performance of the negative electrode sheets and secondary batteries prepared in the examples and comparative examples are shown in Table 2. The testing method includes the following steps:

[0149] 1) Resistance test: After step S1 in the preparation of the negative electrode sheet (introduction of the colloidal conductive layer), the product is subjected to resistance testing using a Yuanneng resistance tester.

[0150] 2) Peeling force test: The negative electrode is subjected to a peeling force test. The test is as follows:

[0151] (1) Cutting the negative electrode sheet: Take the rolled negative electrode sheet and cut it with a die cutter to a length of 200 mm and a width of 20 mm;

[0152] (2) Paste and fix the negative electrode: Take a flat thin steel plate, about 250mm long and 50mm wide; first stick a strip of double-sided tape in the center of the steel plate (the length should be greater than the sample test length and the same width as the electrode), smooth it firmly, and ensure that the double-sided tape is tightly attached to the center of the steel plate. Remove the double-sided tape and attach the electrode to the tape, making sure that the electrode and the tape are completely matched and attached;

[0153] (3) Installation test: There are two upper and lower clamps on the tensile testing machine. Insert the steel plate with the fixed electrode into the lower clamp and fix it vertically; insert the unadhesive electrode into the upper clamp and fix it so that the electrode attached to the adhesive tape is 180 degrees from the electrode fixed by the upper clamp. After fixing the test sample, first calibrate and reset, set the test width, the electrode peeling length to 120mm, and the peeling speed to 50mm / min, and then start the test to obtain the peel strength curve. Measure 5 samples and take the average value to obtain the peeling force of the negative electrode.

[0154] 3) Cycling performance test: The secondary battery was subjected to charge and discharge cycles at 0.5C at 60°C. The capacity retention rate was calculated as follows: N-cycle capacity retention rate (%) = (Nth cycle discharge capacity / first discharge capacity) * 100%.

[0155] 4) Heat Resistance Test: Fully charge the lithium-ion battery at 0.5C constant current and constant voltage at 25°C. Inspect the battery's appearance to ensure it is in normal operating condition. Place the fully charged battery in an oven and heat it at a rate of 5±2°C / min until it reaches 130°C. Hold the temperature constant for 1 hour and observe the battery's condition. The battery's passing criteria for the hot oven test are: no fire or explosion.

[0156] Table 2

[0157]

[0158]

[0159] As can be seen from Table 2, when the technical solution provided by the present application is adopted, the obtained resistance is within a suitable range, the peel force is high, and the heat resistance and cycle performance are excellent; specifically, the obtained product has a resistance between 7.54-55.56 mΩ, a peel force above 18.88 N / m, a capacity retention rate above 88.5%, and a hot box pass rate above 8 / 10;

[0160] As can be seen from Examples 1-5, the mass ratio of the first aluminum sol to the conductive agent in the colloidal conductive layer affects the overall performance of the product. When the value of M is within the range of 2-8, the resistance increase after the colloidal conductive layer is applied is small, the hot box pass rate is high, and when combined with the colloidal cover layer, it can improve the negative electrode sheet peeling force and secondary battery cycle performance.

[0161] As can be seen from Example 1 and Examples 6-13, the range of values ​​for the thickness H1 of the negative electrode active material layer and the thickness H2 of the colloidal covering layer, as well as their mass ratio, also affect the overall performance of the product. When the above parameters are further selected within the ranges given in this application, the peeling force of the negative electrode sheet and the capacity retention rate of the secondary battery are improved, and the interaction with the colloidal covering layer is conducive to improving the electrical performance.

[0162] As can be seen from Example 1 and Examples 16-21, the average particle size of the aluminum sol is related to the peeling force of the negative electrode sheet and the capacity retention rate of the secondary battery. The liquid retention performance and surface contact points of the aluminum sol with an excessively large average particle size are not as good as those with a smaller average particle size. However, when the average particle size is too small, the bonding force decreases and the capacity retention rate decreases. This may be because the excessively small average particle size easily causes agglomeration, which affects the capacity retention rate. When the average particle size of the aluminum sol is further selected within the range given in this application, the overall performance of the obtained product is even better.

[0163] It can be seen from Example 1 and Comparative Example 1 that when the conductive agent is not included in the colloidal conductive layer, the resistance of the resulting colloidal conductive layer increases sharply, and the cycle performance shows a certain downward trend; it can be seen from Example 1 and Comparative Example 2 that when the colloidal conductive layer does not include the first aluminum sol, the heat resistance of the resulting product decreases significantly, and the hot box pass rate is only 6 / 10; it can be seen from Example 1 and Comparative Example 3 that when the colloidal conductive layer is not introduced, the heat resistance of the resulting product decreases significantly, and the hot box pass rate is only 8 / 10; it can be seen from Example 1 and Comparative Example 4 that when When the colloidal covering layer is not introduced, the heat resistance of the obtained product is significantly reduced, the hot box pass rate is only 7 / 10, and the peeling force of the negative electrode sheet is also significantly reduced; it can be seen from Example 1 and Comparative Example 5 that when neither the colloidal conductive layer nor the colloidal covering layer is introduced, the heat resistance of the obtained product is significantly reduced, the hot box pass rate is only 3 / 10, and the peeling force of the negative electrode sheet is also significantly reduced, and the cycle performance of the secondary battery is also significantly reduced; it can be seen from Example 1 and Comparative Example 6 that when silica sol is used instead of aluminum sol, the heat resistance of the obtained product shows a certain downward trend.

[0164] Finally, it should be noted that the above embodiments are intended to illustrate the technical solutions of the present application rather than to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A secondary battery comprising a negative electrode sheet, a positive electrode sheet, a separator and an electrolyte, characterized in that: The negative electrode plate includes a negative electrode current collector, a colloidal conductive layer disposed on at least one surface of the negative electrode current collector, a negative electrode active material layer disposed on the colloidal conductive layer away from the surface of the negative electrode current collector, and a colloidal covering layer disposed on the surface of the negative electrode active material layer away from the surface of the negative electrode current collector; The colloidal conductive layer includes a first aluminum sol and a conductive agent; The colloidal coating includes a second aluminum sol.

2. The secondary battery according to claim 1, wherein The secondary battery satisfies 2≤M≤8; Wherein, M is the mass ratio of the conductive agent to the first aluminum sol in the colloidal conductive layer.

3. The secondary battery according to claim 1, wherein The secondary battery satisfies 25≤H≤80; Where, H = H1 / H2; H1 is the thickness of the single-side negative electrode active material layer in the half-charge state, in μm; H2 is the thickness of the unilateral colloidal covering layer, in μm.

4. The secondary battery according to claim 3, wherein The thickness H1 of the single-side negative electrode active material layer in the half-charge state is 30-70 μm; And / or, the thickness H2 of the single-sided colloid covering layer is 0.5-2 μm.

5. The secondary battery according to claim 1, wherein The thickness of the single-sided colloidal conductive layer in the half-electric state is 0.5-2 μm.

6. The secondary battery according to claim 1, wherein In the colloidal conductive layer, the average particle size of the first aluminum sol is 10-100 nm.

7. The secondary battery according to claim 1, wherein In the colloidal covering layer, the average particle size of the second aluminum sol is 10-50 nm.

8. The secondary battery according to claim 1, wherein The conductive agent includes at least one of conductive carbon black and carbon nanotubes.

9. The secondary battery according to claim 1, wherein The colloidal conductive layer further comprises an organic binder. Based on the total mass of the colloidal conductive layer, the mass percentage of the organic binder is 8-12%.

10. An electrical device, characterized in that: The electric device comprises the secondary battery according to any one of claims 1 to 9.

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