CN114361381A

CN114361381A - Lithium ion battery

Info

Publication number: CN114361381A
Application number: CN202111566898.5A
Authority: CN
Inventors: 王海; 张健; 母英迪; 曾长安; 郭如德; 李素丽; 李俊义; 钱大艳
Original assignee: Zhuhai Cosmx Battery Co Ltd
Current assignee: Zhuhai Cosmx Battery Co Ltd
Priority date: 2021-12-20
Filing date: 2021-12-20
Publication date: 2022-04-15

Abstract

The invention provides a lithium ion battery, and belongs to the technical field of batteries. The battery comprises a positive plate, a negative plate, a diaphragm and electrolyte, wherein the positive plate comprises a positive current collector and a positive coating, the positive coating comprises a first coating and a second coating, the first coating is coated on the surface of the positive current collector, and the second coating is coated on the surface of the first coating; the first coating layer includes an inorganic filler, a first conductive agent, and a first binder, and the second coating layer includes a positive electrode active material, a second conductive agent, and a second binder; the thickness of the first coating is L, the thickness of the second coating is M, and L/M is less than or equal to 0.3; the contact angle theta of the electrolyte is more than or equal to 60 degrees; according to the invention, the contact angle of the electrolyte is improved, and the porosity of the polyolefin porous diaphragm substrate is improved, so that the electrolyte of the battery achieves very good wettability to the diaphragm, and the cycle performance and the safety performance of the battery are obviously improved.

Description

Lithium ion battery

Technical Field

The invention relates to a lithium ion battery and application thereof, belonging to the technical field of batteries.

Background

The lithium ion battery has the advantages of high working voltage, large specific energy density, long cycle life, low self-discharge rate, no memory effect, small environmental pollution and the like, is widely applied to various electronic consumer product markets, and is an ideal power source for future electric vehicles and various electric tools.

The electrolyte of the lithium ion battery which is commercialized at present is completely liquid, and the liquid electrolyte in the battery is dispersed in each gap of a diaphragm, a pole piece and a battery shell and mainly plays a role in transferring lithium ions.

Research shows that when the wettability of the electrolyte is poor, the gaps in the internal part of the battery are not fully filled with the electrolyte, so that the cycle performance of the battery is poor, and even the lithium separation phenomenon of the battery in the charging process is caused, so that the safety problem is caused.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a battery, the electrolyte in the battery has very good wettability to a positive plate, and the addition of the electrolyte can obviously improve the cycle performance and the safety performance of the battery.

The purpose of the invention is realized by the following technical scheme:

a battery comprises a positive plate, a negative plate, a diaphragm and electrolyte, wherein the positive plate comprises a positive current collector and a positive coating, the positive coating comprises a first coating and a second coating, the first coating is coated on the surface of the positive current collector, and the second coating is coated on the surface of the first coating; the first coating layer includes an inorganic filler, a first conductive agent, and a first binder, and the second coating layer includes a positive electrode active material, a second conductive agent, and a second binder; the thickness of the first coating is L, the thickness of the second coating is M, and L/M is less than or equal to 0.3; the contact angle theta of the electrolyte is more than or equal to 60 degrees.

In the present invention, the "contact angle of the electrolyte" refers to the contact angle of the electrolyte on the surface of the glass slide, which is an important parameter for measuring the wettability of the electrolyte on the surface of the positive plate, as shown in fig. 1, and is the included angle between the electrolyte and the glass slide. The contact angle of the electrolyte on the surface of the glass slide is in positive correlation with the contact angle and wettability of the electrolyte on the surface of the positive plate, namely, the larger the contact angle of the electrolyte on the surface of the glass slide is, the better the wettability of the electrolyte on the positive plate is.

The inventor of the invention researches and discovers that when the L/M in the battery is less than or equal to 0.3 and the contact angle theta of the electrolyte is more than or equal to 60 degrees, the electrolyte has excellent wettability to a positive plate and good fluidity and can be well filled into a gap inside the battery; when the L/M in the battery is greater than 0.3 or the contact angle theta of the electrolyte is less than 60 degrees, the wettability of the electrolyte to the positive plate is poor, the fluidity of the electrolyte is poor, and the electrolyte cannot be sufficiently filled into the gap inside the battery; further research shows that when the L/M in the battery is greater than 0.3, even if the wetting angle theta of the electrolyte is greater than or equal to 60 degrees, the wetting property of the electrolyte to the positive plate is insufficient, so that the problem of lithium precipitation of the battery in the circulating process is caused, and safety accidents are caused when the lithium precipitation is serious.

According to the invention, the contact angle theta of the electrolyte is greater than or equal to 60 degrees, and exemplarily, the contact angle theta of the electrolyte is 60 degrees, 70 degrees, 80 degrees, 90 degrees, 100 degrees, 110 degrees, 120 degrees, 130 degrees, 140 degrees, 150 degrees, 160 degrees, 170 degrees or 179 degrees.

According to the present invention, the electrolyte includes a lithium salt, a non-aqueous organic solvent, and an additive including a nitrogen-containing compound.

According to the invention, the structural formula of the nitrogen-containing compound is shown as the formula (1):

wherein R is substituted or unsubstituted alkyl, substituted or unsubstituted ester group, substituted or unsubstituted alkenyl, substituted or unsubstituted aryl; m is at least one of hexafluorophosphate, tetrafluoroborate, difluorine oxalato borate, bisoxalato borate, bisfluorosulfonyl imide and bistrifluoromethanyl imide; when containing a substituent group, the substituent group is an alkyl, halogen, or alkoxy group.

According to the invention, R is-C_1-6Alkyl, -C_1-6alkylene-COO-C_1-6Alkyl, -C_2-6Alkenyl, -C_6-12And (4) an aryl group. Preferably, R is-C_1-3Alkyl, -C_1-3alkylene-COO-C_1-3Alkyl, -C_2-4Alkenyl, -C_6-8And (4) an aryl group.

According to the invention, the nitrogen-containing compound may be specifically at least one of the following two substances:

the cation in the nitrogen-containing compound provided by the invention can reduce the surface tension of the electrolyte, so that the contact angle of the electrolyte is increased, and the wettability of the electrolyte to a positive plate is obviously improved. In addition, the cation can produce adsorption with some active functional groups on the surface of the negative active material, such as: the surface of the graphite contains some carboxyl functional groups, and cations in the nitrogen-containing compound can generate certain adsorption effect with the carboxyl functional groups, so that the electrolyte is guided to fully wet the negative active material. Meanwhile, the nitrogen-containing compound can also form an SEI film on the surface of the negative electrode, and the SEI film has high strength and low impedance and can improve the low-temperature discharge capacity of the battery.

According to the invention, the mass percentage of the addition of the nitrogen-containing compound in the total mass of the electrolyte is B wt%, wherein B wt% is less than or equal to 2 wt%; that is, the nitrogen-containing compound is added in an amount of, by mass, B% by weight or more and 2% by weight or less, illustratively, 0.1% by weight or more and B% by weight or less and 1% by weight or less, for example, B% by weight is 0.1%, 0.15%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1% by weight.

According to the invention, the non-aqueous organic solvent is selected from carbonates and/or carboxylates.

Wherein the carbonate is selected from one or more of the following solvents: ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. The carboxylic ester is selected from one or more of the following solvents: propyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isoamyl acetate, ethyl propionate, n-propyl propionate, methyl butyrate, ethyl n-butyrate.

According to the invention, the non-aqueous organic solvent comprises a linear carbonate with a number of carbon atoms of 5 or less and/or a linear carboxylate with a number of carbon atoms of 5 or less.

Preferably, the linear carbonate with the carbon number less than or equal to 5 is selected from at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate.

Preferably, the linear carboxylic acid ester with the carbon number of less than or equal to 5 is selected from at least one of ethyl propionate and propyl acetate.

Preferably, the mass percentage of the mass of the linear carbonate with the carbon atom number of less than or equal to 5 and/or the mass of the linear carboxylate with the carbon atom number of less than or equal to 5 to the total mass of the electrolyte is greater than or equal to 10 wt%, preferably 10 to 70 wt%, for example 10 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, 60 wt% or 70 wt%. The nonaqueous organic solvent provided by the invention has smaller molecular chains, and when the content of the nonaqueous organic solvent is more than or equal to 10 wt%, the wettability of the electrolyte to the positive plate can be further improved.

According to the present invention, the lithium salt is selected from one or two or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorooxalato borate, lithium difluorosulfonimide, lithium bistrifluoromethylsulfonyl imide, lithium difluorobis-oxalato phosphate, lithium tetrafluoroborate, lithium bisoxalato borate, lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis (trifluoromethylsulfonyl) imide, lithium bis (pentafluoroethylsulfonyl) imide, lithium tris (trifluoromethylsulfonyl) methide or lithium bis (trifluoromethylsulfonyl) imide.

According to the invention, the lithium salt has a concentration of 2mol/L or less, for example 0.5mol/L, 0.6mol/L, 0.7mol/L, 0.8mol/L, 0.9mol/L, 1mol/L, 1.1mol/L, 1.2mol/L, 1.3mol/L, 1.4mol/L, 1.5mol/L, 1.6mol/L, 1.7mol/L, 1.8mol/L or 2 mol/L. The inventors found that when the concentration of the conductive lithium salt is more than 2mol/L, the contact angle of the electrolyte becomes significantly small, greatly affecting the wettability of the electrolyte.

According to the invention, the thickness L (thickness after rolling) of the first coating is 2 to 10 μm, such as 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 8 μm or 10 μm; the thickness M (thickness after rolling) of the second coating layer is 30 to 80 μ M, such as 30 μ M, 35 μ M, 40 μ M, 45 μ M, 50 μ M, 55 μ M, 60 μ M, 65 μ M, 70 μ M, 75 μ M, or 80 μ M.

According to the invention, the content of the first binder in the first coating layer is greater than the content of the second binder in the second coating layer.

According to the present invention, the positive electrode current collector is bonded to a part of the first binder, and a part of the positive electrode active material is bonded to another part of the first binder.

According to the invention, the median particle diameter D of the inorganic filler₅₀Less than the median particle diameter D of the positive electrode active material₅₀。

According to the invention, the median particle diameter D of the inorganic filler₅₀0.05 to 8 μm; and/or the median particle diameter D of the positive electrode active material₅₀10 to 20 μm.

According to the invention, the first coating comprises the following components in percentage by mass: 40-93 wt% inorganic filler, 2E-15 wt% of a first conductive agent and 5-58 wt% of a first binder. When the content of the first binder is within the range, the first binder can have a good binding effect with a positive current collector, the energy density can be reduced and the performance of a battery cell can be deteriorated due to the fact that the content of the first binder is too high, and the range and the median particle diameter D are selected₅₀The inorganic filler with the particle size of 0.05-8 mu m is combined, so that a strong and compact base coat can be formed.

Preferably, the first coating comprises the following components in percentage by mass: 60-91 wt% of inorganic filler, 3-10 wt% of first conductive agent and 8-30 wt% of first binder. Illustratively, the inorganic filler comprises 40 wt%, 45 wt%, 48 wt%, 50 wt%, 55 wt%, 58 wt%, 60 wt%, 62 wt%, 65 wt%, 68 wt%, 70 wt%, 72 wt%, 75 wt%, 78 wt%, 80 wt%, 82 wt%, 85 wt%, 88 wt%, 90 wt%, 92 wt%, 93 wt% of the components of the first coating layer by mass;

illustratively, the first conductive agent accounts for 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt% of each component in the first coating layer by mass;

illustratively, the first binder is present in the first coating in an amount of 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, 18 wt%, 20 wt%, 22 wt%, 25 wt%, 28 wt%, 30 wt%, 33 wt%, 35 wt%, 38 wt%, 40 wt%, 45 wt%, 48 wt%, 50 wt%, 55 wt%, 58 wt% based on the weight of each component.

According to the invention, the second coating comprises the following components in percentage by mass: 93-99 wt% of positive electrode active material, 0.5-5 wt% of second conductive agent and 0.5-2 wt% of second binder. The second binder is selected in this content range to provide better bonding while maintaining a higher energy density.

Preferably, the second coating comprises the following components in percentage by mass: 95-98 wt% of positive electrode active material, 1-3 wt% of second conductive agent and 1-2 wt% of second binder.

Illustratively, the mass percentage of the positive electrode active substance in each component of the second coating is 93 wt%, 94 wt%, 95 wt%, 96 wt%, 97 wt%, 98 wt%, 99 wt%;

illustratively, the second conductive agent accounts for 0.5 wt%, 1 wt%, 1.5 wt%, 1.8 wt%, 2 wt%, 2.2 wt%, 2.5 wt%, 2.8 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt% of each component in the second coating layer;

illustratively, the second binder accounts for 0.5 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2 wt% of each component in the second coating layer.

According to the invention, the first conductive agent and the second conductive agent are the same or different and are independently selected from at least one of conductive carbon black, carbon nanotubes and graphene.

According to the invention, the first binder and the second binder are the same or different and are independently selected from at least one of polyvinylidene fluoride and modified polyvinylidene fluoride.

Wherein, the polyvinylidene fluoride and the modified polyvinylidene fluoride are both products sold in the market.

According to the present invention, the crystallinity of the first binder is < 40%, because a low crystallinity is advantageous for having a good bonding effect.

According to the present invention, the crystallinity of the second binder is < 40%, because a low crystallinity is advantageous for having a good bonding effect.

According to the invention, the modified polyvinylidene fluoride is acrylate modified polyvinylidene fluoride. The acrylate group contains carboxyl, and the acrylate group can form a chemical bond with a positive current collector (such as aluminum foil) to realize strong bonding with the positive current collector.

According to the invention, the molecular weight of the polyvinylidene fluoride or modified polyvinylidene fluoride is 100-150 ten thousand, such as 110 ten thousand and 130 ten thousand. The selection of the binder with larger molecular weight can enhance the binding performance, reduce the content of the binder and enhance the energy density.

According to the invention, the inorganic filler is selected from lithium-containing transition metal oxides, in particular from one or more of Lithium Cobaltate (LCO), nickel cobalt manganese ternary material (NCM), nickel cobalt aluminum ternary material (NCA), nickel cobalt manganese aluminum quaternary material (NCMA), lithium iron phosphate (LFP), Lithium Manganese Phosphate (LMP), Lithium Vanadium Phosphate (LVP), Lithium Manganate (LMO), lithium rich manganese base;

or, the inorganic filler is selected from ceramic materials, and is specifically selected from one or more of alumina, boehmite, magnesium oxide and magnesium hydroxide;

or, the inorganic filler is selected from a mixture of at least one of lithium-containing transition metal oxides and at least one of ceramic materials.

In the present invention, the inorganic filler functions as a skeleton support.

According to the present invention, the positive active material is selected from one or more of Lithium Cobaltate (LCO), nickel cobalt manganese ternary material (NCM), nickel cobalt aluminum ternary material (NCA), nickel cobalt manganese aluminum quaternary material (NCMA), lithium iron phosphate (LFP), Lithium Manganese Phosphate (LMP), Lithium Vanadium Phosphate (LVP), and Lithium Manganate (LMO).

The positive plate in the invention is double-layer, wherein a typical scanning electron microscope image of the double-layer positive plate is shown in figure 2.

According to the invention, the inorganic filler is lithium iron phosphate, the positive electrode active material is lithium cobaltate, and after the positive electrode coating of the positive electrode plate is stripped, Co and O elements are detected in EDS (electronic discharge machining) on the surface of the positive electrode coating remained on the positive electrode current collector.

According to the invention, the positive current collector is selected from aluminium foil.

According to the invention, the thickness of the positive current collector is 8-15 μm.

The invention has the beneficial effects that:

(1) according to the invention, the contact angle of the electrolyte is improved, and the ratio of the thickness of the first coating to the thickness of the second coating in the positive plate is reduced, so that the electrolyte of the battery has very good wettability to the positive plate, and the cycle performance and the safety performance of the battery are obviously improved.

(2) In order to improve the contact angle of the electrolyte, the invention further uses the nitrogen-containing compound as an additive, and simultaneously, in order to optimize the electrolyte to the maximum extent, the wettability of the electrolyte to the positive plate is greatly improved by adjusting the relation between the content of the nitrogen-containing compound and the ratio of the thickness of the first coating to the thickness of the second coating in the positive plate.

(3) The positive plate comprises a positive current collector and a positive coating, wherein the positive coating comprises a first coating and a second coating, the first coating is coated on the surface of the positive current collector, and the second coating is coated on the surface of the first coating; according to the invention, the adhesive force between the first coating and the positive current collector is greater than that between the first coating and the second coating, and/or the adhesive force between the first coating and the positive current collector is greater than that between the positive active material particles of the second coating, and after the positive coating of the positive plate is stripped, the total mass of the positive coating left on the positive current collector accounts for more than 10% of the total mass of the positive coating on the positive current collector before stripping, so that the obtained battery has good safety performance, and the probability of battery fire failure is greatly reduced when mechanical abuse (needling and heavy impact) occurs.

Drawings

FIG. 1 is an analysis diagram of contact angle.

FIG. 2 is a schematic structural diagram of a positive plate according to a preferred embodiment of the present invention

Detailed Description

As described above, the present invention provides a battery including a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte.

< separator >

According to the present invention, the separator includes a polyolefin porous separator substrate.

According to the present invention, the polyolefin porous separator substrate has a porosity of 35% or more, such as 40% or more, and further such as 50% or more, and illustratively, the separator substrate has a porosity of 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, or 50%.

According to the present invention, the polyolefin porous membrane substrate may be at least one of a polyethylene porous membrane substrate, a polypropylene porous membrane substrate, and a polyethylene-polypropylene composite porous membrane substrate.

According to the present invention, the separator further comprises a coating layer disposed on at least one functional surface of the polyolefin porous separator substrate; that is, the separator includes a polyolefin porous separator substrate and a coating layer disposed on at least one functional surface of the polyolefin porous separator substrate.

It is understood that the separator of the present invention may be obtained by providing a coating layer on either functional surface of a polyolefin porous separator substrate, or may be obtained by providing a coating layer on both functional surfaces of a polyolefin porous separator substrate.

The coating layer comprises at least one of inorganic particles and polymers.

The inorganic particles of the present invention may be selected from inorganic particles commonly used in the art, and for example, may be selected from at least one of alumina, silica, boehmite, zinc oxide, magnesium oxide, zirconia, titania, barium oxide, calcium oxide, aluminum nitride, titanium nitride, silicon nitride, boron nitride, aluminum hydroxide, magnesium hydroxide, and barium sulfate.

The polymer of the present invention may be selected from polymers commonly used in the art, for example, may be selected from at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, sodium carboxymethylcellulose, polyacrylate, polyacrylonitrile, polyvinyl alcohol, styrene-butadiene rubber, polyurethane, ethylene-acrylic acid copolymer, polymethyl methacrylate, polyimide, aramid, polystyrene, and polyester.

In the present invention, if the coating layer includes only inorganic particles, it is referred to as an inorganic coating layer; if the coating layer comprises only a polymer, it is referred to as an organic coating layer; if the coating layer includes both inorganic particles and a polymer, it is referred to as a composite coating layer. The membrane of the invention can be obtained by arranging at least one of an inorganic coating layer, an organic coating layer and a composite coating layer on any functional surface of a polyolefin porous membrane substrate, or can be obtained by arranging at least one of an inorganic coating layer, an organic coating layer and a composite coating layer on two functional surfaces of a polyolefin porous membrane substrate.

When at least two of the inorganic coating layer, the organic coating layer and the composite coating layer are arranged on a certain functional surface of the polyolefin porous diaphragm substrate, at least two of the inorganic coating layer, the organic coating layer and the composite coating layer can be arranged in a stacked manner, and at least two of the inorganic coating layer, the organic coating layer and the composite coating layer can also be arranged in parallel on the functional surface of the polyolefin porous diaphragm substrate. In the present invention, the order of stacking is not particularly limited, and the order of arranging the layers in parallel is not particularly limited.

In some embodiments, the inorganic coating layer is disposed on a functional surface of the polyolefin porous separator substrate, and the organic coating layer and/or the composite coating layer is disposed on the functional surface of the inorganic coating layer away from the polyolefin porous separator substrate.

< negative electrode sheet >

According to the invention, the surface density of the negative plate is less than or equal to 0.012g/cm²E.g.. ltoreq.0.010 g/cm²And also for example ≦ 0.009g/cm²Illustratively, the areal density of the negative electrode sheet is 0.005g/cm²、0.006g/cm²、0.007g/cm²、0.008g/cm²、0.009g/cm²、0.010g/cm²、0.011g/cm²Or 0.012g/cm²。

According to the present invention, the thickness of the negative electrode sheet is <200 μm, and illustratively, the thickness of the negative electrode sheet is 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, or 190 μm.

According to the present invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer coated on one or both surfaces of the negative electrode current collector, the negative electrode active material layer including a negative electrode active material, a conductive agent, and a binder.

According to the invention, the negative electrode active material layer comprises the following components in percentage by mass:

90-99.6 wt% of negative electrode active material, 0.2-5 wt% of conductive agent and 0.2-5 wt% of binder.

Illustratively, the mass percentage of the negative active material is 90 wt%, 91 wt%, 92 wt%, 93 wt%, 94 wt%, 95 wt%, 96 wt%, 97 wt%, 98 wt%, 99 wt%, 99.6 wt%.

Illustratively, the conductive agent is present in an amount of 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt% by mass.

Illustratively, the binder is present in an amount of 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt% by mass.

According to the present invention, the negative active material is selected from at least one of artificial graphite, natural graphite, hard carbon, soft carbon, silica or silicon carbon negative electrode materials.

According to the invention, the conductive agent is selected from one or more of conductive carbon black, Ketjen black, conductive fibers, conductive polymers, acetylene black, carbon nanotubes, graphene, flake graphite, conductive oxides and metal particles.

According to the invention, the binder is selected from at least one of polyvinylidene fluoride and its copolymer derivative, polytetrafluoroethylene and its copolymer derivative, polyacrylic acid and its copolymer derivative, polyvinyl alcohol and its copolymer derivative, polystyrene-butadiene rubber and its copolymer derivative, polyimide and its copolymer derivative, polyethyleneimine and its copolymer derivative, polyacrylate and its copolymer derivative, and sodium carboxymethylcellulose and its copolymer derivative.

According to the present invention, the thickness of the negative electrode sheet and the thickness of the positive electrode sheet satisfy the following relationship: the thickness of the positive electrode sheet/the thickness of the negative electrode sheet is (0.93-1.48): 1.

In order to make the objects, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Brief introduction to the test method for contact angle of electrolyte:

using a contact angle testing apparatus model JC2000D1, test environment: the temperature is 20-30 ℃, and the humidity is less than or equal to 70% RH; the testing steps are as follows: putting a clean glass slide on the sample table; using a sampler to extract 1 microliter of electrolyte sample and dripping the electrolyte sample on a glass slide; after the electrolyte sample is dropped on the glass slide for 5 seconds, the picture is taken by a computer, the test result shown in figure 1 is obtained, and the size of the contact angle is analyzed.

Examples and comparative examples

The batteries of examples and comparative examples were prepared by the following steps:

1) preparation of positive plate

The first step is as follows: preparing first coating slurry, mixing 40 wt% of lithium iron phosphate (LFP), 45 wt% of modified PVDF and 15 wt% of carbon black, adding NMP, and stirring to prepare the slurry.

The second step is that: and preparing second coating slurry, mixing 97 wt% of lithium cobaltate, 1 wt% of conductive carbon black, 0.8 wt% of carbon nano tube and 1.2 wt% of PVDF1, adding NMP, and stirring to prepare the slurry.

The third step: and (3) coating the first coating slurry obtained in the first step on the surface of the positive current collector by using an extrusion coating process to form a first coating with the thickness of L microns, and coating the second coating slurry obtained in the second step on the surface of the first coating to form a second coating with the thickness of M microns.

2) Preparation of negative plate

Mixing the negative active substance artificial graphite, sodium carboxymethylcellulose (CMC-Na), styrene butadiene rubber, conductive carbon black (SP) and single-walled carbon nanotubes (SWCNTs) according to the mass ratio of 96:1.5:1.5:0.9:0.1, adding deionized water, and obtaining negative active slurry under the action of a vacuum stirrer; uniformly coating the negative active slurry on two functional surfaces of the copper foil; airing the coated copper foil at room temperature, transferring the copper foil to an oven at 80 ℃ for drying for 10h, and then performing cold pressing and slitting to obtain the negative plate, wherein the surface density of the negative plate is 0.07g/cm²Wherein the thickness of the copper foil6 mu m, and the compacted density of the negative plate is 1.78g/cm³。

3) Preparation of the electrolyte

In a glove box filled with argon (H)₂O＜0.1ppm，O₂Less than 0.1ppm), uniformly mixing the nonaqueous organic solvent according to the mass percentage to obtain a mixed solution, and then quickly adding the fully dried lithium salt with the specific concentration into the mixed solution to form a basic electrolyte; the basic electrolyte is added with nitrogen-containing compounds with different mass percentage contents to obtain the electrolyte (the specific composition of the electrolyte is shown in tables 1 and 3, wherein PC is propylene carbonate, EC is ethylene carbonate, DMC is dimethyl carbonate, EP is ethyl propionate, EMC is ethyl methyl carbonate, and DEC is diethyl carbonate).

4) Preparation of the Battery

Stacking the positive plate obtained in the step 1), the negative plate obtained in the step 2) and a diaphragm (a polyethylene porous membrane with the thickness of 12 mu m) in the order of the positive plate, the diaphragm and the negative plate, and then winding to obtain a battery cell; placing the battery core in an aluminum foil package, injecting the electrolyte obtained in the step 3) into the package, and carrying out vacuum packaging, standing, formation, shaping, sorting and other processes to obtain the battery, wherein specific preparation parameters are shown in table 1.

The following tests were performed on the batteries obtained in the above examples and comparative examples, respectively, and the test results are shown in table 2.

1) Cycle performance test

The batteries obtained in examples and comparative examples were subjected to charge-discharge cycles at 25 ℃ for 100 weeks at a rate of 1C in the range of 3.0V to 4.4V, and the discharge capacity at 1 week and the discharge capacity at 100 weeks were tested; the capacity at week 100 was divided by the capacity at week 1 to obtain the cycle capacity retention.

2) Safety testing

After the circulation, the mixture is charged to 4.4V at constant current and constant voltage according to the multiplying power of 1C, and the current is cut off at 0.05C. Then, the mixture was stored at 130 ℃ for 30 min. And (5) observing whether the battery is ignited or not.

3) Low temperature discharge performance test

The batteries obtained in examples and comparative examples were charged and discharged at room temperature at a rate of 1C for 5 timesCycling, then charging to 4.45V state at 1C rate, recording 1C capacity Q₀. Laying the battery at-20 deg.C for 4h, discharging to 3V at 0.2C rate, and recording discharge capacity Q₃Calculating to obtain the discharge capacity retention rate at-20 ℃;

the low-temperature discharge capacity retention rate is calculated by the following formula:

table 1 cell compositions and performance test results of examples and comparative examples

Table 2 results of performance test of batteries of examples and comparative examples

As can be seen from table 2, when the contact angle of the electrolyte solution was <60 °, or, L/M >0.3, the cycle performance and safety performance of the battery were drastically reduced.

Furthermore, the nitrogen-containing compound can remarkably improve the wetting performance of the electrolyte on the positive plate, further improve the cycle performance of the battery and improve the low-temperature discharge performance of the battery.

The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiment. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A battery comprises a positive plate, a negative plate, a diaphragm and electrolyte, and is characterized in that the positive plate comprises a positive current collector and a positive coating, the positive coating comprises a first coating and a second coating, the first coating is coated on the surface of the positive current collector, and the second coating is coated on the surface of the first coating; the first coating layer includes an inorganic filler, a first conductive agent, and a first binder, and the second coating layer includes a positive electrode active material, a second conductive agent, and a second binder; the thickness of the first coating is L, the thickness of the second coating is M, and L/M is less than or equal to 0.3; the contact angle theta of the electrolyte is more than or equal to 60 degrees.

2. The battery according to claim 1, wherein the first coating layer has a thickness L of 2 to 10 μ M, and the second coating layer has a thickness M of 30 to 80 μ M.

3. The battery of claim 1, wherein the electrolyte comprises a lithium salt, a non-aqueous organic solvent, and an additive comprising a nitrogen-containing compound;

the structural formula of the nitrogen-containing compound is shown as the formula (1):

4. The battery of claim 3, wherein R is-C_1-6Alkyl, -C_1-6alkylene-COO-C_1-6Alkyl, -C_2-6Alkenyl, -C_6-12And (4) an aryl group.

5. The battery of claim 4, wherein R is-C_1-3Alkyl, -C_1-3alkylene-COO-C_1-3Alkyl, -C_2-4Alkenyl, -C_6-8And (4) an aryl group.

6. The battery according to any one of claims 3 to 5, wherein the nitrogen-containing compound is at least one of the following two substances:

7. the battery according to any one of claims 1 to 5, wherein the nitrogen-containing compound is added in an amount of B wt% based on the total mass of the electrolyte, wherein B wt% is 2 wt% or less.

8. The cell according to any one of claims 1 to 5, wherein the non-aqueous organic solvent is selected from carbonates and/or carboxylates;

the carbonate is selected from one or more of the following solvents: ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate; the carboxylic ester is selected from one or more of the following solvents: propyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isoamyl acetate, ethyl propionate, n-propyl propionate, methyl butyrate, ethyl n-butyrate.

9. The battery according to claim 8, wherein the non-aqueous organic solvent comprises a linear carbonate having a carbon number of 5 or less and/or a linear carboxylate having a carbon number of 5 or less.

10. The battery according to claim 9, wherein the mass of the linear carbonate having a carbon atom number of 5 or less and/or the linear carboxylate having a carbon atom number of 5 or less accounts for 10 to 70 wt% of the total mass of the electrolyte.