WO2024092683A1

WO2024092683A1 - Positive electrode sheet, secondary battery, and electric device

Info

Publication number: WO2024092683A1
Application number: PCT/CN2022/129726
Authority: WO
Inventors: 靳超; 任苗苗; 严观福生; 叶永煌
Original assignee: 宁德时代新能源科技股份有限公司
Priority date: 2022-11-04
Filing date: 2022-11-04
Publication date: 2024-05-10

Abstract

The present application relates to a positive electrode sheet, a secondary battery, and an electric device. The positive electrode sheet comprises: a positive electrode current collector; a primer layer located on at least one side of the positive electrode current collector, the primer layer comprising a binder and a first conductive agent and a second conductive agent which have different length-to-diameter ratios, and the second conductive agent being linear; and a positive electrode active material layer located on the side of the primer layer distant from the positive electrode current collector, the positive electrode active material layer comprising a positive electrode active material, the content of nickel in the positive electrode active material being greater than or equal to 80%, and the volume average particle size Dv50 of the positive electrode active material being greater than or equal to 10 μm.

Description

Positive electrode sheet, secondary battery and electrical device

Technical Field

The present application belongs to the technical field of secondary batteries, and specifically relates to a positive electrode sheet, a secondary battery and an electrical device.

Background technique

Secondary batteries are widely used in various consumer electronic products and electric vehicles due to their outstanding features such as light weight, no pollution, and no memory effect. As the demand for power batteries gradually expands, customers' demand for power battery energy density is also increasing.

Since high-nickel ternary materials have high energy density, they are increasingly used in secondary batteries. Since high-nickel ternary materials have poor high-temperature structural stability and are prone to release oxygen at high temperatures, the cycle life of secondary batteries decays rapidly. In related technologies, when high-nickel ternary materials are used as positive electrode materials, lithium bis(fluorosulfonyl)imide (LiFSI) is usually added to the electrolyte to increase the cycle life of secondary batteries. However, the electrolyte with LiFSI added is prone to corrode aluminum foil under high pressure, which can easily cause aluminum foil to break in the later stages of secondary battery cycles, causing battery safety issues.

Therefore, in the related art, the use of high-nickel ternary materials as positive electrode materials often leads to safety issues of the secondary batteries while improving the energy density of the secondary batteries.

Summary of the invention

In view of the technical problems existing in the background technology, the present application provides a positive electrode plate, a secondary battery and an electrical device, aiming to improve the energy density of the secondary battery while improving the safety of the secondary battery.

In order to achieve the above-mentioned object, the first aspect of the present application provides a positive electrode sheet, comprising:

Positive electrode current collector;

A primer layer, located on at least one side of the positive electrode current collector, the primer layer comprising a binder and a first conductive agent and a second conductive agent having different aspect ratios, wherein the second conductive agent is linear;

The positive electrode active material layer is located on the side of the base coating away from the positive electrode current collector, the positive electrode active material layer includes a positive electrode active material, the nickel content in the positive electrode active material is ≥80%, and the volume average particle size Dv50 of the positive electrode active material is ≥10μm. Compared with the prior art, the present application includes at least the following beneficial effects:

The positive electrode sheet of the present application is provided with an undercoat layer. When the positive electrode active material layer has a high compaction density, the undercoat layer can improve or even prevent the oxide film on the surface of the aluminum foil from being crushed by the positive electrode active material, prevent the fresh aluminum foil from reacting with LiFSI in the electrolyte due to exposure, solve the problem of aluminum foil corrosion, and improve the safety of the secondary battery; in addition, a linear conductive agent is provided in the undercoat layer, and the linear conductive agent and the binder can form a thermistor in combination. When overcharged or shallowly punctured, the temperature of the battery cell increases, the binder swells, and the linear conductive agent is pulled off, the internal resistance of the undercoat layer increases, and the connection between the positive electrode active material layer and the aluminum foil is cut off, forming a short circuit, which acts as a switch, and can further improve the safety of the secondary battery. In this way, the above-mentioned positive electrode sheet is applied to the secondary battery, which can improve the safety of the secondary battery while increasing the energy density of the secondary battery.

In any embodiment of the present application, the length of the second conductive agent is 5-50 μm.

In any embodiment of the present application, the specific surface area of the second conductive agent is 100-300 m ² /g.

In any embodiment of the present application, the diameter of the second conductive agent is 1-20 nm.

In any embodiment of the present application, the second conductive agent includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and carbon fibers.

In any embodiment of the present application, the first conductive agent is spherical or quasi-spherical;

Optionally, the first conductive agent includes one or more of acetylene black, carbon black, graphite, Ketjen black and graphene.

In any embodiment of the present application, the swelling degree of the binder at 25° C. is ≥ 40%.

In any embodiment of the present application, the binder includes one or more of polyglutamic acid, styrene-butadiene rubber, olefin polymers and acrylic polymers;

Optionally, the acrylic polymer includes one or more of cross-linked polyacrylic acid, lithium polyacrylate, sodium polyacrylate and polyacrylic acid-polyacrylonitrile copolymer.

In any embodiment of the present application, the mass ratio of the binder, the first conductive agent, and the second conductive agent is (5-15):(5-10):(0.2-1).

In any embodiment of the present application, the primer layer further includes an alkaline substance.

In any embodiment of the present application, the alkaline substance includes one or more of metal oxides, hydroxides and carbonates.

In any embodiment of the present application, the alkaline substance includes one or more of sodium oxide, potassium oxide, calcium oxide, sodium hydroxide, calcium hydroxide, potassium hydroxide, calcium carbonate, sodium carbonate and potassium carbonate.

In any embodiment of the present application, the mass ratio of the binder, the first conductive agent, the second conductive agent, and the alkaline substance is (5-15):(5-10):(0.2-1):(2-5).

In any embodiment of the present application, the thickness of the primer layer is denoted as L, and the thickness of the primer layer satisfies: 0＜L≤4μm; optionally, 1≤L≤2μm.

In any embodiment of the present application, the positive electrode active material includes NCM ternary material.

In any embodiment of the present application, the positive electrode active material includes a material with a chemical formula of LiNi _x Co _y Mn _z O ₂ ;

Among them, x≥0.8, y≤0.12, z≥0.08.

In any embodiment of the present application, the surface of the positive electrode active material is coated with a carbon coating layer;

Optionally, the carbon coating layer accounts for 0.1-15% of the mass percentage of the positive electrode active material after coating.

A second aspect of the present application provides a secondary battery, comprising the positive electrode sheet as described in the first aspect of the present application.

In any embodiment of the present application, the secondary battery further includes a negative electrode sheet, and the negative electrode sheet includes:

Anode current collector;

The negative electrode active material layer is located on at least one side of the negative electrode current collector, and the negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes a carbon material and a silicon material, and the mass ratio of the carbon material to the silicon material is (75-95):(5-25).

A third aspect of the present application provides an electrical device, comprising the secondary battery of the second aspect of the present application.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solution of the present application, the following is a brief introduction to the drawings used in the present application. Obviously, the drawings described below are only some embodiments of the present application, and for ordinary technicians in this field, other drawings can be obtained based on the drawings without creative work.

FIG1 is a schematic diagram showing the change of storage gas production with storage days in Example 4 and Example 15.

FIG. 2 is a schematic diagram of an embodiment of a secondary battery.

FIG. 3 is an exploded view of FIG. 2 .

FIG. 4 is a schematic diagram of an embodiment of a battery pack.

FIG. 5 is a schematic diagram of an embodiment of an electric device using a secondary battery as a power source.

Description of reference numerals:

1. Battery pack; 2. Upper box; 3. Lower box; 4. Secondary battery; 41. Shell; 42. Electrode assembly; 43. Cover plate; 5. Electrical device.

Detailed ways

The present application is further described below in conjunction with specific implementations. It should be understood that these specific implementations are only used to illustrate the present application and are not used to limit the scope of the present application.

For simplicity, only some numerical ranges are specifically disclosed herein. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, and any upper limit can be combined with any other upper limit to form an unspecified range. In addition, each separately disclosed point or single value can itself be combined as a lower limit or upper limit with any other point or single value or with other lower limits or upper limits to form an unspecified range.

The "range" disclosed in the present application is defined in the form of a lower limit and an upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The range defined in this way can be inclusive or exclusive of end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 60-120 and 80-110 is listed for a specific parameter, it is understood that the range of 60-110 and 80-120 is also expected. In addition, if the

minimum range values

1 and 2 are listed, and if the

maximum range values

3, 4 and 5 are listed, the following ranges can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "a-b" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" represents that all real numbers between "0-5" have been fully listed herein, and "0-5" is just the abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

If there is no special explanation, the "include" and "comprising" mentioned in this application represent open-ended or closed-ended expressions. For example, the "include" and "comprising" may represent that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

In the description of this article, it should be noted that, unless otherwise specified, "above" and "below" are inclusive of the number itself, and "several" in "one or several" means two or more.

In the description herein, unless otherwise stated, the term "or" is inclusive. That is, the phrase "A or (or) B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist). Unless otherwise stated, the terms used in this application have the well-known meanings commonly understood by those skilled in the art. Unless otherwise stated, the numerical values of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the embodiments of this application).

Since high-nickel ternary materials have high energy density, they are used more and more widely in secondary batteries. However, due to the poor high-temperature structural stability of high-nickel ternary materials, oxygen is easily released at high temperatures, resulting in rapid decay of the storage life of the secondary battery and reduced safety. In the related art, when high-nickel ternary materials are used as positive electrode materials, the storage life of the secondary battery can be effectively improved and the safety can be improved by adding lithium bis(fluorosulfonyl)imide (LiFSI) to the electrolyte. However, the electrolyte containing LiFSI is easy to corrode the aluminum foil under high pressure, and it is easy to cause the aluminum foil to break brittlely in the later stage of the cycle, causing secondary battery safety accidents. In addition, the secondary battery has poor shallow puncture tolerance, and the electrolyte has poor high-voltage stability, and the risk of overcharge failure is high. The technical personnel of this application analyzed and found that it was mainly because the positive electrode material particles crushed the aluminum foil under high pressure density, the surface oxide film of the aluminum foil was destroyed, and the fresh aluminum foil was exposed. The fresh aluminum foil reacted with the electrolyte, resulting in aluminum foil corrosion.

The positive electrode plate provided in the present application includes: a positive electrode current collector, a primer layer located on at least one side of the positive electrode current collector, and a positive electrode active material layer located on a side of the primer layer away from the positive electrode current collector; the primer layer includes a binder and a first conductive agent and a second conductive agent with different aspect ratios, and the second conductive agent is linear; the positive electrode active material layer includes a positive electrode active material, the nickel content in the positive electrode active material is ≥80%, and the volume average particle size Dv50 of the positive electrode active material is ≥10μm.

It should be noted that the undercoat layer is located on at least one side of the positive electrode current collector, and the undercoat layer may be in contact with the positive electrode current collector or may not be in contact with the positive electrode current collector.

It can be understood that the nickel content in the positive electrode active material used in the above-mentioned positive electrode plate is ≥80%. When such a positive electrode plate is used in a secondary battery, LiFSI is added to the electrolyte; since the volume average particle size Dv50 of the positive electrode active material is ≥10μm, if the positive electrode active material is in direct contact with the aluminum foil, the positive electrode active material is easy to damage the oxide film on the surface of the aluminum foil and expose the fresh aluminum foil. When the LiFSI in the electrolyte contacts the aluminum foil, a reaction occurs to corrode the aluminum foil.

Without wishing to be limited to any theory, the positive electrode plate of the present application is provided with an undercoat layer. When the positive electrode active material layer has a high compaction density, the undercoat layer can improve or even prevent the oxide film on the surface of the aluminum foil from being crushed by the positive electrode active material, prevent the reaction with LiFSI in the electrolyte due to the exposure of fresh aluminum foil, solve the problem of aluminum foil corrosion, and improve the safety of the secondary battery; in addition, a linear conductive agent is provided in the undercoat layer, and the linear conductive agent and the binder can form a thermistor in combination. When overcharged or shallowly punctured, the temperature of the battery cell increases, the binder swells, and the linear conductive agent is pulled off, the internal resistance of the undercoat layer increases, and the connection between the positive electrode active material layer and the aluminum foil is cut off, forming a short circuit, which acts as a switch, and can further improve the safety of the secondary battery. In this way, the above-mentioned positive electrode plate is applied to the secondary battery, which can improve the safety of the secondary battery while increasing the energy density of the secondary battery.

As an example, the volume average particle size Dv50 of the positive electrode active material mentioned above can be conveniently measured by referring to the particle size distribution laser diffraction method of GB/T19077-2016 using a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer produced by Malvern Instruments Ltd. in the United Kingdom.

The nickel content in the above-mentioned positive electrode active material can be measured by an ICAP6300 spectrometer.

It should be noted that the bottom coating layer includes both the first conductive agent and the second conductive agent with different aspect ratios, which can make the bottom coating layer have uniform conductivity, and at the same time can be combined with the binder to form a thermistor to improve the safety of the secondary battery; in addition, the difficulty and cost of the bottom coating preparation process can be reduced. If only a linear second conductive agent is set in the bottom coating layer, the amount of the second conductive agent is not easy to control; if the amount of the second conductive agent is too small, the second conductive agent is easy to locally clump, and the dispersion effect is poor, resulting in the bottom coating layer having poor local conductivity; if the amount of the second conductive agent is too much, it is not easy to pull the second conductive agent apart when the binder is heated and swelled, and the thermistor cannot be effectively formed. If only the first conductive agent is set in the bottom coating layer, it cannot form a thermistor in combination with the binder, and the effect of improving the safety of the secondary battery cannot be achieved.

The inventors have found through in-depth research that when the positive electrode plate of the present application satisfies the above-mentioned design conditions and optionally satisfies one or more of the following conditions, the overcharge or shallow puncture safety and storage life of the secondary battery can be further improved.

In some embodiments, the length of the second conductive agent is 5-50 μm; for example, the length of the second conductive agent can be 10-50 μm, 15-45 μm, 20-40 μm, 25-35 μm or 10-30 μm, etc. When the length of the second conductive agent is controlled within the given range, it can be combined with the binder to form a thermistor, improve the safety of the secondary battery during overcharge or shallow puncture, and facilitate the uniform distribution of the second conductive agent in the primer slurry during the preparation of the positive electrode sheet.

In some embodiments, the specific surface area of the second conductive agent is 100-300m ² /g; for example, the specific surface area of the second conductive agent can be 150-300m ² /g, 150-250m ² /g, 200-250m ² /g, 200-300m ² /g or 150-200m ² /g, etc. When the specific surface area of the second conductive agent is controlled within the given range, the second conductive agent and the binder are in good contact and uniformly mixed, the second conductive agent is interspersed between the binders, and the two are firmly combined. While taking into account the good conductivity of the base coating, the base coating and the second conductive agent can form a thermistor to improve the safety of the secondary battery. When the specific surface area of the second conductive agent is less than the given range, the conductivity of the base coating is poor and cannot meet the use requirements; when the specific surface area of the second conductive agent is greater than the given range, the second conductive agent cannot be pulled apart when the binder swells at high temperature, thereby failing to form a thermistor.

In some embodiments, the diameter of the second conductive agent is 1-20 nm; for example, the diameter of the second conductive agent can be 2-20 nm, 5-18 nm, 7-15 nm, 10-15 nm or 10-13 nm, etc. When the diameter of the second conductive agent is controlled within the above range, more bridges can be provided within the effective area, thereby improving conductivity.

The length and diameter of the second conductive agent mentioned above can be measured by the following method: the length and diameter of the second conductive agent are determined by using a Hitachi transmission electron microscope HT7800 series and ImajeJ software.

The specific surface area of the second conductive agent mentioned above can be measured by using a BET specific surface analyzer.

In some embodiments, the second conductive agent includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon fibers.

In some embodiments, the first conductive agent is spherical or quasi-spherical. Compared with the second conductive agent, the first conductive agent has a smaller aspect ratio. Optionally, the first conductive agent includes one or more of acetylene black, carbon black, graphite, Ketjen black and graphene.

It should be noted that “quasi-spherical” means that the shape of the first conductive agent is close to a sphere.

In some embodiments, the swelling degree of the binder at 25° C. is ≥ 40%; for example, the swelling degree of the binder may be 40-100%, 60-150%, 100-200%, 150-250% or 200-300%, etc. When the swelling degree of the binder is controlled within the above range, the binder swells and breaks the second conductive agent during overcharging or shallow puncture.

The swelling degree of the binder mentioned above can be measured by the following method: cut five positive electrode sheets of the same size and shape, and measure the weight W1 of each sheet respectively; soak each sheet in an electrolyte, and place the electrolyte at 50-70°C; soak for 8-12 days, take out each sheet every 24 hours to dry the electrolyte on the surface of the sheet, and test the thickness and weight of each sheet every day; until the thickness and weight of each sheet remain unchanged, record the weight W2 of each sheet; calculate the swelling degree I' of each sheet according to the following formula:

Swelling degree I' = (W2-W1)/W1*100%;

The swelling degree I is obtained by taking the average value of the swelling degree I’ of each electrode.

In some embodiments, the binder includes one or more of polyglutamic acid, styrene-butadiene rubber, olefin polymers and acrylic polymers. Optionally, the acrylic polymer includes one or more of cross-linked polyacrylic acid, lithium polyacrylate, sodium polyacrylate and polyacrylic acid-polyacrylonitrile copolymer.

In some embodiments, the mass ratio of the binder, the first conductive agent, and the second conductive agent is (5-15): (5-10): (0.2-1); for example, the binder, the first conductive agent, and the second conductive agent can be (7-15): (5-9): (0.2-0.9), (5-10): (6-10): (0.3-0.8), (9-14): (7-9): (0.4-0.7) or (10-12): (6-8): (0.5-0.6), etc. When the mass ratio of the binder, the first conductive agent, and the second conductive agent is controlled within the above range, during the preparation of the primer slurry, the filtering time of the primer slurry can be made less than 90s, and no filter residue will be generated; at the same time, the internal short resistance and overcharge window of the secondary battery can be improved.

In some of the embodiments, the primer layer further includes an alkaline substance. The alkaline substance is added to the primer layer, and the alkaline substance can react with carbon dioxide, absorb excess gas, improve the life of the secondary battery without additionally affecting the energy density of the secondary battery; at the same time, the alkaline substance can react with hydrogen fluoride to improve the damage of hydrogen fluoride to the SEI film and the CEI film; in addition, when the components of the primer layer contain carboxyl functional groups, the alkaline substance can neutralize the carboxyl functional groups, thereby improving the stability of the primer slurry during the preparation process of the primer layer.

In some embodiments, the alkaline substance includes one or more of metal oxides, hydroxides and carbonates.

Optionally, the alkaline substance includes one or more of sodium oxide, potassium oxide, calcium oxide, sodium hydroxide, calcium hydroxide, potassium hydroxide, calcium carbonate, sodium carbonate and potassium carbonate.

In some embodiments, the mass ratio of the binder, the first conductive agent, the second conductive agent, and the alkaline substance is (5-15): (5-10): (0.2-1): (2-5); for example, the mass ratio of the binder, the first conductive agent, the second conductive agent, and the alkaline substance can be (7-15): (5-9): (0.2-0.9): (2-4), (5-10): (6-10): (0.3-0.8): (2-3), (9-14): (7-9): (0.4-0.7): (3-5) or (10-12): (6-8): (0.5-0.6): (4-5), etc. When the mass ratio of the binder, the first conductive agent, the second conductive agent, and the alkaline substance is controlled within the above range, the storage gas generation of the secondary battery can be reduced, the storage life can be extended, and there is no excessive HF and _CO2 residue.

In some embodiments, the thickness of the primer layer is recorded as L, and the thickness of the primer layer satisfies: 0＜L≤4μm; for example, the thickness of the primer layer can satisfy: 0.5≤L≤4μm, 0.5≤L≤3.5μm, 1≤L≤3μm, 1.5≤L≤2.5μm or 2≤L≤2.5μm, etc. When the thickness of the primer layer is controlled within the above range, the damage of the positive electrode material particles to the aluminum foil can be effectively improved without affecting the energy density of the secondary battery. Optionally, the thickness of the primer layer satisfies: 1≤L≤2μm.

In some of the embodiments, the positive electrode active material includes NCM ternary material.

In some embodiments, the positive electrode active material includes a material with a chemical formula of LiNi _x Co _y Mn _z O ₂ ; wherein x≥0.8, y≤0.12, and z≥0.08.

In some of the embodiments, the surface of the positive electrode active material is coated with a carbon coating layer; the carbon coating layer is beneficial to improving the electronic conductivity of the positive electrode active material and enhancing the rate performance of the secondary battery; and it can also improve the side reaction between the positive electrode active material and the electrolyte and stabilize the surface structure of the positive electrode active material.

Optionally, the mass percentage of the carbon coating layer to the positive electrode active material after coating is 0.1-15%; for example, it can be 0.5-15%, 1-14%, 3-12%, 5-10%, 6-8% or 0.1-5%, etc.

As an example, the carbon coating layer can be one or more of a thermal decomposition product of an organic carbon source, superconducting carbon, acetylene black, carbon black, carbon nanotubes, carbon dots, graphene, Ketjen black, and carbon nanofibers. The organic carbon source can be one or more of glucose, fructose, sucrose, maltose, starch, cellulose, polypyrrole, polyaniline, polythiophene, polyethylene dioxythiophene, polystyrene sulfonate, and polyphenylene sulfide.

The present application also provides a method for preparing a primer slurry, comprising the following steps:

Mixing a first conductive agent and a binder to prepare slurry A;

Dissolving the second conductive agent in a portion of deionized water to prepare slurry B;

dissolving the alkaline substance in the remaining deionized water to prepare slurry C;

Slurry A and slurry C were added to slurry B respectively, and mixed to prepare primer slurry.

In some of the embodiments, the mass ratio of the binder, the first conductive agent, the second conductive agent and the alkaline substance in the primer slurry is (5-15):(5-10):(0.2-01):(2-5).

As an example, when preparing slurry A, the binder is in the form of a binder aqueous solution with a solid content of 25%, and is prepared together with the first conductive agent to prepare slurry A. The sum of the mass percentages of the binder, the first conductive agent, the second conductive agent, the alkaline substance and the deionized water in the finally prepared primer slurry is 100%.

In some embodiments, the positive electrode active material may also use other conventional materials that can be used as positive electrode active materials for batteries. As an example, the positive electrode active material may also use at least one of olivine-structured lithium-containing phosphates, lithium cobalt oxides (such as LiCoO ₂ ), lithium nickel oxides (such as LiNiO ₂ ), lithium manganese oxides (such as LiMnO ₂ , LiMn ₂ O ₄ ), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, lithium nickel cobalt aluminum oxides (such as LiNi _0.85 Co _0.15 Al _0.05 O ₂ ) and modified compounds thereof. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, lithium iron phosphate (such as LiFePO ₄ (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO ₄ ), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. The weight ratio of the positive electrode active material in the positive electrode film layer is 80-100% by weight, based on the total weight of the positive electrode film layer.

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

The positive electrode active material layer may also optionally include a binder, a conductive agent, and other optional auxiliary agents.

As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin. The weight ratio of the binder in the positive electrode film layer is 0 to 20 weight%, based on the total weight of the positive electrode film layer.

As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers. The weight ratio of the conductive agent in the positive electrode film layer is 0 to 20 weight %, based on the total weight of the positive electrode film layer.

As an example, the positive electrode sheet can be prepared in the following way:

The components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry, wherein the positive electrode slurry has a solid content of 40-80 wt % and a viscosity at room temperature adjusted to 5000-25000 mPa·s;

Applying the primer slurry on the surface of the positive electrode current collector and drying it to form a primer layer;

The positive electrode slurry is coated on the surface of the primer layer, and after drying, it is cold-pressed by a cold rolling mill to form a positive electrode sheet; the positive electrode powder coating unit surface density is 150-350 mg/m ² , and the positive electrode sheet compaction density is 3.0-3.6 g/cm ³ , and can be 3.3-3.5 g/cm ³ . The calculation formula of the compaction density is:

Compacted density = coating surface density/(thickness of the electrode after extrusion - thickness of the current collector).

The above raw materials not specifically mentioned can be obtained from the market.

Secondary battery

A secondary battery is a battery that can be recharged to activate the active materials after being discharged and continue to be used.

Generally, a secondary battery includes the positive electrode sheet, the negative electrode sheet, the separator and the electrolyte provided above in the present application. During the battery charging and discharging process, active ions are embedded and released back and forth between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet to play an isolating role. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet.

In some embodiments, the negative electrode plate includes:

Anode current collector;

The negative electrode active material layer is located on at least one side of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material includes a carbon material and a silicon material. The mass ratio of the carbon material to the silicon material is (75-95):(5-25).

Optionally, the silicon material may be selected from at least one of elemental silicon, silicon-oxygen compounds, silicon-carbon compounds, silicon-nitrogen compounds and silicon alloys.

Optionally, the carbon material may be selected from at least one of artificial graphite, natural graphite, soft carbon, and hard carbon.

In some embodiments, the negative electrode active material may also include other negative electrode active materials for batteries known in the art. As an example, the negative electrode active material may also include tin-based materials and lithium titanate, etc. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more. The weight ratio of the negative electrode active material in the negative electrode film layer is 70-100% by weight, based on the total weight of the negative electrode film layer.

In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS). The weight ratio of the binder in the negative electrode film layer is 0-30% by weight, based on the total weight of the negative electrode film layer.

In some embodiments, the negative electrode film layer may further include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers. The weight ratio of the conductive agent in the negative electrode film layer is 0-20 weight %, based on the total weight of the negative electrode film layer.

In some embodiments, the negative electrode film layer may further include other additives, such as a thickener (such as sodium carboxymethyl cellulose (CMC-Na)), etc. The weight ratio of the other additives in the negative electrode film layer is 0-15% by weight, based on the total weight of the negative electrode film layer.

As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

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

As an example, the negative electrode sheet can be prepared in the following manner: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry, wherein the solid content of the negative electrode slurry is 30-70wt%, and the viscosity at room temperature is adjusted to 2000-10000mPa·s; the obtained negative electrode slurry is coated on the negative electrode collector, and after a drying process, cold pressing such as rolling, a negative electrode sheet is obtained. The negative electrode powder coating unit area density is 75-220mg/ ^m2 , and the negative electrode sheet compaction density is 1.2-2.0g/ ^m3 .

Electrolytes

The electrolyte plays the role of conducting ions between the positive electrode and the negative electrode. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

In some embodiments, the electrolyte salt may be selected from one or more of lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO ₂ F ₂ ), lithium difluorobis(oxalatophosphate) (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP). The concentration of the electrolyte salt is generally 0.5-5 mol/L.

In some embodiments, the solvent can be selected from one or more 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), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE).

In some embodiments, the electrolyte may further include additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

Isolation film

In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

In some embodiments, the isolation film has a thickness of 6-40 μm, and optionally 12-20 μm.

In some embodiments, the secondary battery of the present application is a lithium ion secondary battery.

In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

In some embodiments, the secondary battery may include an outer package that can be used to encapsulate the electrode assembly and the electrolyte.

In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

Secondary batteries can be prepared according to conventional methods in the art, for example, the positive electrode sheet, the separator, and the negative electrode sheet are wound (or stacked) in sequence, so that the separator is placed between the positive electrode sheet and the negative electrode sheet to play an isolating role, to obtain a battery cell, the battery cell is placed in an outer package, the electrolyte is injected and the package is sealed to obtain a secondary battery.

The embodiment of the present application has no particular limitation on the shape of the secondary battery, which can be cylindrical, square or any other shape. FIG2 is a secondary battery 4 of a square structure as an example.

In some embodiments, referring to FIG. 3 , the outer package may include a shell 41 and a cover plate 43. The shell 41 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate are combined to form a receiving cavity. The shell 41 has an opening connected to the receiving cavity, and the cover plate 43 can be covered on the opening to close the receiving cavity.

The positive electrode sheet, the negative electrode sheet and the separator can be wound or laminated to form an electrode assembly 42. The electrode assembly 52 is encapsulated in the housing cavity. The electrolyte is infiltrated in the electrode assembly 42. The number of electrode assemblies 42 contained in the secondary battery 4 can be one or more, which can be adjusted according to needs.

In some embodiments, secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module. In the battery module, multiple secondary batteries can be arranged in sequence along the length direction of the battery module. Of course, they can also be arranged in any other way. Further, the multiple secondary batteries can be fixed by fasteners. The battery module can also include a housing with a storage space, and multiple secondary batteries are accommodated in the storage space.

In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art according to the application and capacity of the battery pack.

In some embodiments, the secondary batteries described above may also be assembled into a battery pack, and the number of secondary batteries contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

FIG4 is a battery pack 1 as an example. The battery pack 1 may include a battery box and a plurality of secondary batteries or battery modules disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating secondary batteries or battery modules. The plurality of secondary batteries or battery modules can be arranged in the battery box in any manner.

Electrical devices

The present application also provides an electrical device, which includes at least one of the secondary battery, battery module or battery pack provided in the present application. The secondary battery, battery module or battery pack can be used as a power source for the device, or as an energy storage unit for the device. The device can be, but is not limited to, a mobile device (such as a mobile phone, a laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc.

The device can select a secondary battery, a battery module or a battery pack according to its usage requirements.

Fig. 5 is an example of an electric device 5. The electric device 5 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the device's requirements for high power and high energy density of secondary batteries, a battery pack may be used.

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

The beneficial effects of the present application are further illustrated below in conjunction with embodiments.

Example

In order to make the technical problems, technical solutions and beneficial effects solved by the present application clearer, the following will be further described in detail with reference to the embodiments and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present application and its application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the art without creative work are within the scope of protection of the present application. The materials used in the embodiments and comparative examples of the present application can all be obtained commercially.

1. Preparation of substrate with primer layer

Example 1

Mix 10g of conductive agent acetylene black (as the first conductive agent) and 4g of cross-linked polyacrylic acid solution (as a binder, with a solid content of 25%) to prepare slurry A; dissolve 0.02g of single-walled carbon nanotubes in deionized water and fully dissolve to prepare solution B; dissolve 10g of sodium hydroxide in deionized water to prepare solution C; add slurry A and solution C to solution B respectively, stir and mix thoroughly to obtain a primer slurry. Filter the primer slurry, and the residue content in the primer slurry and the filtering time of the primer slurry are shown in Table 1. The filtered primer slurry is evenly coated on the positive electrode current collector and dried to obtain a substrate with a primer layer.

Example 2-18

The preparation method of the substrate with a primer layer in Examples 2-18 is basically similar to the preparation method of the substrate with a primer layer in Example 1, and the main difference is that when preparing the substrate with a primer layer, at least one of the type and/or amount of the first conductive agent, the type and/or amount of the second conductive agent, the type and/or amount of the binder, and the type and/or amount of the alkaline substance used is different, as shown in Table 1 for details.

Comparative Example 1

The preparation method of the substrate with a primer layer in Comparative Example 1 is basically similar to the preparation method of the substrate with a primer layer in Example 5, and the main difference is that the first conductive agent of the same mass is used to replace the second conductive agent. See Table 1 for details.

Table 1

In Table 1, SWCNT represents single-walled carbon nanotubes, and MWCNT represents multi-walled carbon nanotubes. The amount of the binder is obtained by converting the amount of the binder aqueous solution added when preparing slurry A to the solid content.

It can be seen from the filtering time of the primer slurry in each embodiment in Table 1 and the content of filter residue therein that when the mass proportion of the binder in the primer slurry is 5-15%, the mass proportion of the second conductive agent is 0.2-1%, the mass proportion of the first conductive agent is 5-10%, and the mass proportion of the alkaline substance is 2-5%, the filtering time of the primer slurry is appropriate and the primer slurry does not contain filter residue.

When the mass proportion of the binder in the primer slurry is higher than 15%, the filtration time of the primer slurry is too long and the primer slurry contains filter residue, which is not conducive to the preparation of the primer slurry. The technicians analyzed the reason and found that it may be because the mass proportion of the binder in the primer slurry is too high, the viscosity of the primer slurry is too high and it is difficult to filter, and the material is easy to agglomerate and produce filter residue.

When the mass proportion of the second conductive agent in the primer slurry is greater than 1%, the filtration time of the primer slurry is too long. The technicians analyzed the reason and found that it may be because when the mass proportion of the linear second conductive agent is too large, it is difficult to disperse evenly in the primer slurry, which increases the filtration time.

When the mass proportion of alkaline substances in the primer slurry is higher than 5%, the filtration time of the primer slurry is too long and there is a lot of filter residue in the primer slurry. The technicians analyzed the reason and found that it may be because when the mass proportion of alkaline substances in the primer slurry is too high, the primer slurry has a high alkalinity and is easy to react with carbon dioxide in the air or carboxyl groups in the binder to form chemical gel, thereby prolonging the filtration time of the primer slurry and easily producing filter residue.

2. Preparation of batteries

1. Preparation of positive electrode

The positive electrode active material NCM, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) are dissolved in a solvent N-methylpyrrolidone (NMP) at a weight ratio of 96.5:1.5:2, and the positive electrode slurry is obtained after being fully stirred and mixed. The positive electrode slurry is uniformly coated on the substrate with the primer layer prepared in the above embodiments and comparative examples, and then dried, cold pressed, and cut to obtain the positive electrode sheet.

2. Preparation of negative electrode sheet

The negative electrode active material graphite, silicon, conductive agent acetylene black, high molecular polymer, and thickener sodium carboxymethyl cellulose (CMC) were dissolved in deionized water at a weight ratio of 90:5:2:2:1, and then uniformly mixed with deionized water to prepare a negative electrode slurry. The negative electrode slurry was coated on a copper foil, dried, and then cold-pressed and cut to obtain an anode electrode sheet.

3. Diaphragm: Polyethylene film (PE) is used as the isolation membrane, and the surface is coated with PVDF and alumina coating.

4. Preparation of electrolyte

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1, and then LiPF6:LiFSI (2:8) was uniformly dissolved in the above solution to obtain an electrolyte. In the electrolyte, the concentration of lithium salt is 1 mol/L.

5. Preparation of secondary batteries: stack the positive electrode sheet, separator, and negative electrode sheet in order, so that the separator is between the positive and negative electrode sheets to play an isolating role, then wind to obtain a bare cell, weld the tabs to the bare cell, and put the bare cell into an aluminum shell, bake at 80°C to remove water, then inject electrolyte and seal to obtain an uncharged battery. The uncharged battery then goes through the processes of static, hot and cold pressing, formation, shaping, capacity testing, etc. to obtain a secondary battery.

3. Battery performance test

1. Secondary battery energy density test

Each secondary battery was placed at 25°C for 30 minutes, charged to 4.25V at a rate of 1/3C, and then discharged to 2.8V at a rate of 1/3C. The released capacity is C0, and the released energy is E0. The energy density of the secondary battery was calculated based on the formula battery energy density = E0/cell volume.

2. Overcharge test

Each secondary battery was placed at 25°C for 30 minutes, charged to 4.25V at a rate of 1/3C, and then the constant voltage was reached at 0.05C. After being placed at rest for 10 minutes, the battery was charged to failure at a rate of 1/3C, and the charging time T (h) was recorded. The failure SOC was calculated based on the charging time = 100% + T/3%.

3. Shallow puncture test

The prepared secondary batteries were discharged at 25°C at a rate of 1/3C to 2.8V, left standing for 60 minutes, and charged at a rate of 1/3C to 4.25V, and the constant voltage was terminated to 0.05C; left standing for 60 minutes, part of the shell was removed, and the voltage, internal resistance and weight of the battery cell were recorded.

The nail penetration test was carried out at 25°C. The nail diameter was 1mm and the speed was 0.1mm/s. The nail was inserted at a uniform speed perpendicular to the direction of the battery plate, and the insertion position was close to the geometric center of the punctured surface. The penetration depth was 4mm and the voltage drop was monitored. The test environment was observed for 1h. If there was no failure, the penetration depth was changed to 5mm and the above steps were repeated until failure. The voltage, internal resistance and weight of the battery cell at the time of failure were recorded.

4. Internal short resistance test

Each secondary battery was charged to 4.25V at 1/3C rate at 25°C, and the constant voltage was terminated at 0.05C. After standing for 60min, the voltage was recorded as V ₀ . According to the "shallow puncture test" method, the voltage after the battery cell penetrated 4mm was recorded as V ₁ .

Each of the prepared secondary batteries was charged to 4.25V at a rate of 1/3C at 25°C, and the constant voltage was terminated to 0.05C. After standing for 60min, _V2 was discharged at 1/3C for 10s; the voltage after short circuit was _V3 ; DC pulse resistance DCR = ( _V0 - _V2 )/(1/3C); Ishort _circuit = ( _V0 - _V1 )/DCR; internal short circuit resistance Rshort _circuit = ( _V1 - _V3 )/Ishort _circuit .

5. Storage days test

Each secondary battery was stored at 60℃ and 97% SOC. Before and during storage, C0 and Cn were taken as the initial reversible capacity and actual reversible capacity of the battery, respectively. The battery was taken out for testing once every 15 days. The testing process is as follows: each secondary battery was placed at 25℃ for 30 minutes, charged to 4.25V at a rate of 1/3C, and ended at a constant voltage of 0.05C. The battery was placed for 30 minutes, and discharged to 2.8V at a rate of 1/3C, and the initial reversible capacity C0 was recorded. The battery was tested once every 15 days, and Cn (n = 1, 2, 3, 4, 5...) was recorded; when Cn/C0 ≤ 80%, n was recorded; EOL storage days = 15*n.

6. Storage gas production test

The prepared battery cells were installed with gas pipes and stored at 60°C and 97% SOC. A barometer was connected to the gas pipe to monitor the air pressure in real time and record the EOL storage gas production when Cn/C0≤80%. When Cn/C0≤80%, the battery cells were removed and the contents of _CO2 and HF were detected by gas chromatography.

The changes of storage gas production with storage days in Example 5 and Example 13 are shown in Figure 1. The difference between Example 5 and Example 13 is that the base coating of Example 13 does not contain alkaline substances, and the growth rate of storage gas production with storage days in Example 13 is significantly higher than the growth rate of storage gas production with storage days in Example 5, indicating that the addition of alkaline substances in the base coating can react with _CO2 , absorb excess gas, and slow down the growth rate of storage gas production.

The performance test results of the secondary batteries prepared in various embodiments and comparative examples are shown in Table 2 below.

Table 2

From the results of Examples 1-20 in Table 2, it can be seen that by adjusting the type/amount of the first conductive agent, the type/amount of the second conductive agent, the type/amount of the binder, and the type and amount of the alkaline substance, a primer layer can be formed on the surface of the positive electrode collector, thereby improving the safety and storage performance of the secondary battery and reducing gas production.

The difference between Example 5 and Comparative Example 1 is that the bottom coating of Comparative Example 1 does not contain single-walled carbon nanotubes; from the results of Example 5 and Comparative Example 1 in Table 2, it can be seen that the measurement results of internal short resistance, overcharge window and shallow puncture in Comparative Example 1 are worse than those of Example 5; it shows that a linear conductive agent is added to the bottom coating, and the linear conductive agent and the binder are combined to form a thermistor. When overcharged or shallowly punctured, the temperature of the battery cell increases, the binder swells, and the linear conductive agent is broken, the internal resistance of the bottom coating increases, and the connection between the positive active material layer and the aluminum foil is cut off, forming a short circuit, playing a switch role, and can further improve the safety of the secondary battery.

The difference between Example 5 and Examples 10-13 is that the amount of alkaline substances added to the primer layer is different. No alkaline substances are added to the primer layer of Example 13. The amount of alkaline substances added in Example 11, Example 10, Example 5, and Example 12 increases successively, with the amount of alkaline substances added in Example 12 being the largest, and the amount of alkaline substances added in Example 11 being the smallest. From the results of Examples 5 and Examples 10-13, it can be seen that with the increase in the amount of alkaline substances added in the primer layer, the EOL storage gas production decreases; and compared with Example 13, the EOL HF content and EOL CO ₂ content in Examples 5, 10, and 12 are significantly reduced. Compared with Examples 5, 10, and 11, the EOL storage days in Example 12 are significantly reduced; the technicians analyzed the reason, which may be due to the fact that as the mass proportion of alkaline substances in the primer layer continues to increase, CO ₂ gas is basically adsorbed, the total gas production is basically maintained constant, and HF can continue to react with the alkaline substances to reduce, but the by-products formed accumulate, the polarization loss increases, and the storage life is significantly reduced. The results of Example 5 and Examples 10-13 show that when preparing the primer layer, setting the mass proportion of the alkaline substance in the primer slurry within the range of 2-5% can improve the storage performance of the secondary battery while reducing gas generation.

The difference between Examples 1-6 is that the amount of binder added is different, and the amount of binder added in the primer layer of Examples 1 to 6 increases successively. Compared with Examples 1 and 2, the internal short resistance, overcharge window and shallow puncture results of the secondary battery are significantly improved by increasing the amount of binder added in the primer layer in Examples 3-6. The technicians analyzed that the reason may be that, on the one hand, due to the small amount of binder added, when overcharged or shallowly punctured, the binder cannot pull off the second conductive agent after being heated and swollen, and the connection between the positive electrode active material layer and the aluminum foil cannot be cut off, then the second conductive agent combined with the binder cannot play the role of a thermistor switch, resulting in poor internal short resistance, overcharge window and shallow puncture results; on the other hand, due to the small amount of binder added, the bonding force between the primer layer and the positive electrode current collector and the positive electrode active material layer is poor, and the effect of the primer layer is poor. However, in the preparation process of the primer slurry, when the mass proportion of the binder is higher than 15%, the filtration time of the primer slurry is too long and the primer slurry contains filter residue, which is not conducive to the preparation of the primer slurry.

The difference between Example 5 and Examples 7-9 is that the amount of the linear second conductive agent added is different. The amount of the second conductive agent added in Example 5 is the smallest, and the amount of the second conductive agent added in Examples 7 to 9 increases successively, and the amount of the second conductive agent added in Example 9 is the largest; the comprehensive results of internal short resistance, overcharge window, and shallow puncture in Example 5 are the best. When the mass proportion of the second conductive agent in the primer slurry exceeds 1%, as the mass proportion of the second conductive agent in the primer slurry increases, the results of internal short resistance, overcharge window, and shallow puncture will deteriorate. The technicians analyzed the reason and found that it may be due to the excessive mass proportion of the second conductive agent in the primer slurry, which increases the conductivity of the primer layer; when overcharging or shallow puncture occurs, the temperature of the battery cell increases, and the binder cannot pull the second conductive agent off after swelling. The internal resistance of the primer layer is small, and the connection between the positive active material layer and the aluminum foil cannot be cut off, so the second conductive agent combined with the binder cannot play the role of a thermistor switch. In addition, when the mass proportion of the second conductive agent in the primer slurry exceeds 1%, the filtration time of the primer slurry is longer than 90s and filter residue is generated, which is not conducive to the preparation of the primer slurry.

The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed in the present application, and these modifications or replacements should be included in the protection scope of the present application. Therefore, the protection scope of the present application shall be based on the protection scope of the claims.

Claims

A positive electrode sheet, comprising:

Positive electrode current collector;

A primer layer, located on at least one side of the positive electrode current collector, the primer layer comprising a binder and a first conductive agent and a second conductive agent having different aspect ratios, wherein the second conductive agent is linear;

The positive electrode active material layer is located on the side of the undercoat layer away from the positive electrode current collector, the positive electrode active material layer comprises a positive electrode active material, the nickel content in the positive electrode active material is ≥80%, and the volume average particle size Dv50 of the positive electrode active material is ≥10 μm.
The positive electrode sheet as claimed in claim 1, wherein the length of the second conductive agent is 5-50 μm.
The positive electrode sheet according to claim 2, wherein the specific surface area of the second conductive agent is 100-300 m2 /g.
The positive electrode sheet according to any one of claims 2 to 3, wherein the diameter of the second conductive agent is 1-20 nm.
The positive electrode plate according to any one of claims 1 to 4, wherein the second conductive agent comprises one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and carbon fibers.
The positive electrode sheet according to any one of claims 1 to 5, wherein the first conductive agent is spherical or quasi-spherical;

Optionally, the first conductive agent includes one or more of acetylene black, carbon black, graphite, Ketjen black and graphene.
The positive electrode sheet according to any one of claims 1 to 6, wherein the swelling degree of the binder at 25° C. is ≥ 40%.
The positive electrode sheet according to any one of claims 1 to 7, wherein the binder comprises one or more of polyglutamic acid, styrene-butadiene rubber, olefin polymers and acrylic polymers;

Optionally, the acrylic polymer includes one or more of cross-linked polyacrylic acid, lithium polyacrylate, sodium polyacrylate and polyacrylic acid-polyacrylonitrile copolymer.
The positive electrode sheet according to any one of claims 1 to 8, wherein the mass ratio of the binder, the first conductive agent, and the second conductive agent is (5-15):(5-10):(0.2-1).
The positive electrode sheet according to any one of claims 1 to 9, wherein the undercoat layer further comprises an alkaline substance.
The positive electrode plate as claimed in claim 10, wherein the alkaline substance comprises one or more of metal oxides, hydroxides and carbonates.
The positive electrode sheet according to any one of claims 10 to 11, wherein the alkaline substance comprises one or more of sodium oxide, potassium oxide, calcium oxide, sodium hydroxide, calcium hydroxide, potassium hydroxide, calcium carbonate, sodium carbonate and potassium carbonate.
The positive electrode sheet according to any one of claims 10 to 12, wherein the mass ratio of the binder, the first conductive agent, the second conductive agent, and the alkaline substance is (5-15):(5-10):(0.2-1):(2-5).
The positive electrode plate according to any one of claims 1 to 13, wherein the thickness of the primer layer is denoted as L, and the thickness of the primer layer satisfies: 0＜L≤4μm; optionally, 1≤L≤2μm.
The positive electrode sheet according to any one of claims 1 to 14, wherein the positive electrode active material comprises NCM ternary material.
The positive electrode sheet according to any one of claims 1 to 15, wherein the positive electrode active material comprises a material having a chemical formula of LiNi x Co y Mn z O 2 ;

Among them, x≥0.8, y≤0.12, z≥0.08.
The positive electrode sheet according to any one of claims 1 to 16, wherein the surface of the positive electrode active material is coated with a carbon coating layer;

Optionally, the carbon coating layer accounts for 0.1-15% of the mass percentage of the positive electrode active material after coating.
A secondary battery comprising the positive electrode sheet as claimed in any one of claims 1 to 17.
The secondary battery according to claim 18, wherein the secondary battery further comprises a negative electrode plate, wherein the negative electrode plate comprises:

Anode current collector;

The negative electrode active material layer is located on at least one side of the negative electrode current collector, and the negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes a carbon material and a silicon material, and the mass ratio of the carbon material to the silicon material is (75-95):(5-25).
An electrical device comprising the secondary battery as claimed in any one of claims 18 to 19.