CN111082129B

CN111082129B - Electrochemical device and electronic device

Info

Publication number: CN111082129B
Application number: CN201911350641.9A
Authority: CN
Inventors: 崔辉; 冯鹏洋; 张水蓉; 董佳丽
Original assignee: Dongguan Amperex Technology Ltd
Current assignee: Dongguan Amperex Technology Ltd
Priority date: 2019-12-24
Filing date: 2019-12-24
Publication date: 2021-01-12
Anticipated expiration: 2039-12-24
Also published as: CN111082129A

Abstract

The present application relates to an electrochemical device and an electronic device. Specifically, the present application provides an electrochemical device comprising a positive electrode, a negative electrode, and an electrolytic solution, wherein: the negative electrode includes a negative electrode current collector and a negative electrode active material disposed on the negative electrode current collector, the electrolyte includes fluoroethylene carbonate, and the electrochemical device satisfies the following relationship: k is more than or equal to 17.55₁‑K₂‑1.63K₃ ²+11.27K₃Less than or equal to 20.80, wherein: k₁Is represented by m²A specific surface area per unit mass of the negative electrode active material is 1.0. ltoreq. K₁≤2.0；K₂Represents the content of said fluoroethylene carbonate required per Ah capacity in g/Ah and 0.05. ltoreq.K₂Less than or equal to 0.25; and K₃Represents a weight value of the anode active material required per Ah capacity in g/Ah. The electrochemical device of the present application has improved kinetic and cycling performance.

Description

Electrochemical device and electronic device

Technical Field

The present application relates to the field of energy storage, and in particular, to an electrochemical device and an electronic device.

Background

Electrochemical devices (such as lithium ion batteries) are widely applied to the fields of wearable devices, smart phones, unmanned planes, and even electric vehicles due to their advantages of high energy density, long cycle life, and no memory effect, and have become the most promising new green chemical power source in the world today. With the expansion of the application of lithium ion batteries and the development of modern information technology, people put forward more performance requirements on lithium ion batteries, such as higher energy density, rapid charge and discharge, and the like.

In view of the above, there is a need for an improved electrochemical device that can achieve rapid charging and discharging while maintaining excellent electrochemical performance.

Disclosure of Invention

The present application seeks to solve at least one of the problems presented in the related art, at least to some extent, by providing an electrochemical device and an electronic device.

According to one aspect of the present application, there is provided an electrochemical device including a positive electrode, a negative electrode, and an electrolyte, wherein:

the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the negative electrode active material layer containing a negative electrode active material,

the electrolyte includes fluoroethylene carbonate (FEC), and

the electrochemical device satisfies the following relationship: k is more than or equal to 17.55₁-K₂-1.63K₃ ²+11.27K₃≤20.80，

Wherein:

K₁is represented by m²A specific surface area per unit mass of the negative electrode active material is 1.0. ltoreq. K₁≤2.0；

K₂Represents the content of fluoroethylene carbonate required per Ah capacity in g/Ah and 0.015. ltoreq. K₂Less than or equal to 0.36; and is

K₃Represents a weight value of the anode active material required per Ah capacity in g/Ah.

According to an embodiment of the present application, the negative active material includes first particles which are secondary particles containing not less than 2 primary particles, and the number of the secondary particles is 50% to 90% of the total amount of the negative active material particles.

According to embodiments of the present application, the Dv50 of the primary particles of the secondary particles is from 2 μ ι η to 10 μ ι η and the Dv50 of the secondary particles is from 5 μ ι η to 20 μ ι η.

According to an embodiment of the present application, the anode active material includes second particles, the second particles being primary particles, the second particles having a Dv50 of 3 μm to 10 μm.

According to an embodiment of the present application, the Dv50 of the negative electrode active material is 9 μm to 25 μm.

According to an embodiment of the present application, the anode active material layer has a porosity of 15% to 40%.

According to an embodiment of the application, the negative electrode further comprises a conductive layer having a thickness of 0.5 μm to 1.5 μm.

According to an embodiment of the present application, the electrolyte further comprises an organic solvent, the organic solvent comprising Ethyl Propionate (EP).

According to the embodiment of the application, the content value W and the K of the ethyl propionate required per Ah capacity are measured in g/Ah₁Ratio W to K of₁Is 0.12 to 1.14.

According to the examples of the present application, the ratio W: P of the content value W of the ethyl propionate required per Ah capacity in g/Ah to the porosity value P of the anode active material layer in percentage is 0.6 to 6.3.

According to another aspect of the present application, there is provided an electronic device comprising an electrochemical device according to the present application.

Additional aspects and advantages of the present application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of embodiments of the present application.

Drawings

Fig. 1 shows a Scanning Electron Microscope (SEM) morphology of a negative active material in an electrochemical device of the present application.

FIG. 2 shows EIS curves according to example 34 of the present application.

Detailed Description

Embodiments of the present application will be described in detail below. The embodiments of the present application should not be construed as limiting the present application.

In the detailed description and claims, a list of items joined by the term "at least one of may mean any combination of the listed items. For example, if items a and B are listed, the phrase "at least one of a and B" means a only; only B; or A and B. In another example, if items A, B and C are listed, the phrase "at least one of A, B and C" means a only; or only B; only C; a and B (excluding C); a and C (excluding B); b and C (excluding A); or A, B and C. Item a may comprise a single element or multiple elements. Item B may comprise a single element or multiple elements. Item C may comprise a single element or multiple elements.

The present application provides an electrochemical device, which includes a positive electrode, a negative electrode and an electrolyte, wherein:

the electrolyte includes fluoroethylene carbonate, and

Wherein:

In the electrochemical device, the FEC is easy to form a film on the negative electrode, and the relation between the specific surface areas of the FEC and the negative electrode active material can be determined through the relational expression, so that a Solid Electrolyte Interface (SEI) film can be better formed, the stability of the SEI film in the later period of the cycle is improved, the formation of side reaction products is reduced, and the cycle performance of the electrochemical device is finally improved. In addition, optimizing the relationship between the electrolyte composition (i.e., FEC) and the negative electrode active material contributes to an improvement in high compacted density (e.g., compacted density of 1.78 g/cm)³Above) the electrolyte of the lower negative active material, thereby improving the dynamic properties of the electrochemical device, such as direct current resistance and charge transfer resistance.

Based on the above-described embodiments, the negative electrode, the electrode solution, the positive electrode, the separator, the electrochemical device, and the electronic device, which can be used in the present application, are described in detail below.

Negative electrode

The particle size and specific surface area of the negative electrode active material (e.g., graphite) may affect the intercalation and deintercalation paths of lithium ions, thereby affecting the kinetics of the lithium ion battery. The negative electrode active material with small particle size and excellent isotropy can reduce the specific surface area of a base surface as much as possible and increase the specific surface area of the end surface under the condition that the total specific surface area is not changed, wherein the end surface is a surface in the Lc direction, the base surface is a surface in the La direction, La is the average size of the negative electrode active material along the a axis direction, and Lc refers to the thickness of the negative electrode active material layer stacked along the c axis direction perpendicular to the negative electrode active material layer. The increase of the specific surface area of the end face can enable lithium ions to directly enter between the negative active material layers from the defect positions of the end face, so that the rapid lithium ion migration is realized, the impedance of the lithium ion battery is reduced, and the dynamics of the lithium ion battery is improved. During the cycle, the deintercalation of lithium ions is more likely to expand in the Lc direction, and the small particle size with excellent syntropy inhibits the interlayer expansion of the negative electrode active material, thereby achieving the improvement of cycle performance.

According to an embodiment of the present application, the negative active material includes first particles, which are secondary particles containing not less than 2 primary particles. In some embodiments, the secondary particles comprise 3 to 10 primary particles. In some embodiments, the amount of the secondary particles is 50% to 95% of the total amount of the anode active material particles. In some embodiments, the amount of the secondary particles is 60% to 90% of the total amount of the anode active material particles.

According to embodiments of the present application, the Dv50 of the primary particles of the secondary particles is from 2 μ ι η to 10 μ ι η and the Dv50 of the secondary particles is from 5 μ ι η to 20 μ ι η. In some embodiments, the Dv50 of the primary particles in the secondary particles is 3 μ ι η, 4 μ ι η, 5 μ ι η, 6 μ ι η, 7 μ ι η, 8 μ ι η, or 9 μ ι η. In some embodiments, the Dv50 of the secondary particles is 8 μm to 18 μm. In some embodiments, the Dv50 of the secondary particles is 10 μm to 15 μm.

According to an embodiment of the present application, the anode active material includes second particles, the second particles being primary particles, the second particles having a Dv50 of 3 μm to 10 μm. In some embodiments, the Dv50 of the second particles is 4 μ ι η, 5 μ ι η, 6 μ ι η, 7 μ ι η, 8 μ ι η, or 9 μ ι η.

Fig. 1 shows a Scanning Electron Microscope (SEM) morphology of a negative active material in an electrochemical device of the present application. As shown in the figure, the negative active material in the electrochemical device of the present application includes primary particles and secondary particles, and the combination of the primary particles and the secondary particles can make the negative active material have a smaller particle size, a higher specific surface area on the end surface, and a lower specific surface area on the basal plane, thereby reducing the charging resistance of the lithium ion battery.

According to an embodiment of the present application, the Dv50 of the negative electrode active material is 9 μm to 25 μm. In some embodiments, the negative active material has a Dv50 of 10 μm to 20 μm. In some embodiments, the anode active material has a Dv50 of 12 μm to 15 μm.

According to an embodiment of the application, the weight ratio of the second particles to the first particles is from 1:9 to 1: 1. In some embodiments, the weight ratio of the second particles to the first particles is 1:8, 1:7, 1:6, 1:5, 1:4, 1:3, or 1: 2.

According to an embodiment of the present application, the negative active material has a porosity of 15% to 40%. In some embodiments, the negative active material has a porosity of 20% to 35%. In some embodiments, the negative active material has a porosity of 25% to 30%.

According to an embodiment of the present application, the negative electrode active material has a polygonal-rim-like spherical shape.

According to an embodiment of the present application, the negative active material has an OI value of 2.0 to 5.0, wherein an X-ray diffraction pattern of the negative active material includes an 004 diffraction line pattern and a 110 diffraction line pattern, and a ratio C004/C110 of a C-axis length C004 of a unit cell length obtained from the 004 diffraction line pattern to an a-axis length C110 of a unit cell length obtained from the 110 diffraction line pattern is the OI value of the negative active material. In some embodiments, the anode material has an OI value of 3.0 to 4.0. The OI value in the range can obtain the cathode active material with more excellent performance and better matching with the electrolyte.

According to an embodiment of the present application, the negative active material is selected from natural graphite, artificial graphite, mesophase micro carbon spheres (abbreviated as MCMB), hard carbon, soft carbon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO₂Spinel-structured lithiated TiO₂-Li₄Ti₅O₁₂And Li-Al alloy. Non-limiting examples of carbon materials include crystalline carbon, amorphous carbon, and mixtures thereof. The crystalline carbon may be natural graphite or artificial graphite in an amorphous form or in a form of a flake, a platelet, a sphere or a fiber. The amorphous carbon may be soft carbon, hard carbon, mesophase pitch carbide, calcined coke, or the like.

According to an embodiment of the application, the negative electrode further comprises a conductive layer having a thickness of 0.5 μm to 1.5 μm. In some embodiments, the conductive layer has a thickness of 0.8 μm to 1.2 μm. In some embodiments, the conductive layer has a thickness of 1.0 μm. In some embodiments, the conductive material of the conductive layer may comprise any conductive material as long as it does not cause a chemical change. Non-limiting examples of the conductive material include carbon-based materials (e.g., natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, carbon nanotubes, graphene, etc.), metal-based materials (e.g., metal powders, metal fibers, etc., such as copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives), and mixtures thereof. In some embodiments, the conductive material of the conductive layer comprises carbon nanotubes having a diameter of 1nm to 30 nm. In some embodiments, the carbon nanotubes have a diameter of 1nm to 25 nm. In some embodiments, the carbon nanotubes have a diameter of 2nm to 20 nm. In some embodiments, the conductive material of the conductive layer comprises graphene, and the number of layers of graphene is 1 to 700. In some embodiments, the number of layers of graphene is from 1 to 500. In some embodiments, the number of layers of graphene is from 1 to 300.

According to an embodiment of the present application, the anode further comprises a binder selected from at least one of: polyvinylidene fluoride, copolymers of vinylidene fluoride-hexafluoropropylene, polyamides, polyacrylonitrile, polyacrylates, polyacrylic acids, polyacrylates, sodium carboxymethylcellulose, polyvinylpyrrolidone, polyvinyl ethers, polymethyl methacrylate, polytetrafluoroethylene and polyhexafluoropropylene, styrene-butadiene rubber and acrylates and epoxy resins.

The negative current collector used herein may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrates coated with conductive metals, and combinations thereof.

Electrolyte solution

According to an embodiment of the present application, the electrolyte further comprises an organic solvent comprising ethyl propionate.

According to an embodiment of the present application, the organic solvent further comprises Ethylene Carbonate (EC), Propylene Carbonate (PC), butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate (DEC), methyl n-propyl carbonate, ethyl n-propyl carbonate, di-n-propyl carbonate, bis (fluoromethyl) carbonate, bis (difluoromethyl) carbonate, bis (trifluoromethyl) carbonate, bis (2-fluoroethyl) carbonate, bis (2, 2-difluoroethyl) carbonate, bis (2,2, 2-trifluoroethyl) carbonate, 2-fluoroethyl methyl carbonate, 2, 2-difluoroethyl methyl carbonate, 2,2, 2-trifluoroethyl methyl carbonate, methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, sec-butyl acetate, isobutyl acetate, tert-butyl acetate, At least one of methyl propionate, propyl propionate, isopropyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, methyl isobutyrate, ethyl isobutyrate, methyl valerate, ethyl valerate, methyl trifluoroacetate, ethyl trifluoroacetate, propyl trifluoroacetate, butyl trifluoroacetate, and 2,2, 2-trifluoroethyl trifluoroacetate, γ -butyrolactone, or γ -valerolactone.

According to an embodiment of the present application, the organic solvent is contained in an amount of 70% to 95% by total weight of the electrolyte.

According to the embodiment of the present application, the content of the ethyl propionate is 5% to 80% by total weight of the organic solvent.

According to the embodiment of the application, the content value W and the K of the ethyl propionate required per Ah capacity are measured in g/Ah₁Ratio W to K of₁Is 0.12 to 1.14. In some embodiments, K is W₁Is 0.15 to 1.10. In some embodiments, K is W₁Is 0.3 to 1.0. In some embodiments, K is W₁Is 0.5 to 0.8.

According to the embodiment of the present application, the ratio W: P of the content value W of the ethyl propionate required per Ah capacity in g/Ah to the porosity value P of the anode active material in percentage is 0.6 to 6.3. In some embodiments, W: P is 0.8 to 6.0. In some embodiments, W: P is 1.0 to 5.0. In some embodiments, W: P is 2.0 to 4.0. In some embodiments, W: P is 2.5 to 3.0.

According to an embodiment of the present application, the electrolyte includes a lithium salt. Electrolytes that may be used in the electrolytes of embodiments of the present application include, but are not limited to: inorganic lithium salts, e.g. LiClO₄、LiAsF₆、LiPF₆、LiBF₄、LiSbF₆、LiSO₃F、LiN(FSO₂)₂Etc.; organic lithium salts containing fluorine, e.g. LiCF₃SO₃、LiN(FSO₂)(CF₃SO₂)、LiN(CF₃SO₂)₂、LiN(C₂F₅SO₂)₂Cyclic 1, 3-hexafluoropropane disulfonimide lithium, cyclic 1, 2-tetrafluoroethane disulfonimide lithium, LiN (CF)₃SO₂)(C₄F₉SO₂)、LiC(CF₃SO₂)₃、LiPF₄(CF₃)₂、LiPF₄(C₂F₅)₂、LiPF₄(CF₃SO₂)₂、LiPF₄(C₂F₅SO₂)₂、LiBF₂(CF₃)₂、LiBF2(C2F5)2、LiBF₂(CF₃SO₂)₂、LiBF₂(C₂F₅SO₂)₂(ii) a The dicarboxylic acid complex-containing lithium salt may, for example, be lithium bis (oxalato) borate, lithium difluorooxalato borate, lithium tris (oxalato) phosphate, lithium difluorobis (oxalato) phosphate, lithium tetrafluoro (oxalato) phosphate, or the like. The electrolyte may be used alone or in combination of two or more. For example, in some embodiments, the electrolyte comprises LiPF₆And LiBF₄Combinations of (a) and (b). In some embodiments, the electrolyte comprises LiPF₆Or LiBF₄An inorganic lithium salt and LiCF₃SO₃、LiN(CF₃SO₂)₂、LiN(C₂F₅SO₂)₂And the like, a combination of fluorine-containing organic lithium salts. In some embodiments, the concentration of the electrolyte is in the range of 0.8 to 3mol/L, such as in the range of 0.8 to 2.5mol/L, in the range of 0.8 to 2mol/L, in the range of 1 to 2mol/L, again such as 1mol/L, 1.15mol/L, 1.2mol/L, 1.5mol/L, 2mol/L, or 2.5 mol/L.

According to an embodiment of the application, the electrolyte further comprises an additive selected from one or more of the following: LiBF4, vinyl sulfate (DTD), Vinylene Carbonate (VC), Vinyl Ethylene Carbonate (VEC), Succinonitrile (SN), Adiponitrile (ADN), 1, 3-Propane Sultone (PS), or lithium dioxalate borate.

Positive electrode

In an electrochemical device according to the present application, a positive electrode includes a positive electrode current collector and a positive electrode active material disposed on the positive electrode current collector. The specific kind of the positive electrode active material is not particularly limited and may be selected as desired.

For example, in some embodiments, the positive electrode active material includes a compound that reversibly intercalates and deintercalates lithium ions. In some embodiments, the positive active material may include a composite oxide containing lithium and at least one element selected from cobalt, manganese, and nickel. In still other embodiments, the positive electrode active material is selected from lithium cobaltate (LiCoO)₂) Lithium nickel manganese cobalt ternary material and lithium manganateLiMn₂O₄) Lithium nickel manganese oxide (LiNi)_0.5Mn_1.5O₄) Lithium iron phosphate (LiFePO)₄) One or more of them.

In some embodiments, the positive electrode active material layer may have a coating layer on the surface, or may be mixed with another compound having a coating layer. The coating may include at least one coating element compound selected from an oxide of the coating element, a hydroxide of the coating element, an oxyhydroxide of the coating element, an oxycarbonate (oxycarbonate) of the coating element, and an oxycarbonate (hydroxycarbonate) of the coating element. The compounds used for the coating may be amorphous or crystalline. The coating element contained in the coating layer may include Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, F, or a mixture thereof. The coating layer may be applied by any method as long as the method does not adversely affect the properties of the positive electrode active material. For example, the method may include any coating method well known to those of ordinary skill in the art, such as spraying, dipping, and the like.

In some embodiments, the positive active material layer further comprises a binder, and optionally further comprises a positive conductive material.

The binder improves the binding of the positive electrode active material particles to each other, and also improves the binding of the positive electrode active material to the current collector. Non-limiting examples of binders include polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1, 1-difluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy, nylon, and the like.

The positive electrode active material layer includes a positive electrode conductive material, thereby imparting conductivity to the electrode. The positive electrode conductive material may include any conductive material as long as it does not cause a chemical change. Non-limiting examples of the positive electrode conductive material include carbon-based materials (e.g., natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, etc.), metal-based materials (e.g., metal powder, metal fiber, etc., including, for example, copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives), and mixtures thereof.

The positive electrode current collector for the electrochemical device according to the present application may be aluminum (Al), but is not limited thereto.

Isolation film

In some embodiments, a separator is provided between the positive and negative electrodes to prevent short circuits. The material and shape of the separation film that can be used for the embodiment of the present application are not particularly limited, and may be any of the techniques disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic substance or the like formed of a material stable to the electrolyte of the present application.

For example, the release film may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, a film or a composite film with a porous structure, and the material of the substrate layer is at least one selected from polyethylene, polypropylene, polyethylene terephthalate and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be used.

At least one surface of the substrate layer is provided with a surface treatment layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance.

The inorganic layer comprises inorganic particles and a binder, wherein the inorganic particles are selected from one or more of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium oxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide and barium sulfate. The binder is selected from one or a combination of more of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene and polyhexafluoropropylene.

The polymer layer comprises a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride and poly (vinylidene fluoride-hexafluoropropylene)

Electrochemical device

The electrochemical device of the present application includes any device in which electrochemical reactions occur, and specific examples thereof include all kinds of primary batteries, secondary batteries, fuel cells, solar cells, or capacitors. In particular, the electrochemical device is a lithium secondary battery including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

Electronic device

The present application further provides an electronic device comprising an electrochemical device according to the present application.

The use of the electrochemical device of the present application is not particularly limited, and it can be used for any electronic device known in the art. In some embodiments, the electrochemical device of the present application can be used in, but is not limited to, notebook computers, pen-input computers, mobile computers, electronic book players, cellular phones, portable facsimile machines, portable copiers, portable printers, headphones, video recorders, liquid crystal televisions, portable cleaners, portable CDs, mini-discs, transceivers, electronic organizers, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, mopeds, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large batteries, lithium ion capacitors, and the like.

Taking a lithium ion battery as an example and describing the preparation of the lithium ion battery with reference to specific examples, those skilled in the art will understand that the preparation method described in the present application is only an example, and any other suitable preparation method is within the scope of the present application.

Examples

The following describes performance evaluation according to examples and comparative examples of lithium ion batteries of the present application.

Preparation of lithium ion battery

1. Preparation of the negative electrode

According to the arrangement of each embodiment and each comparative example, graphite, Styrene Butadiene Rubber (SBR) serving as a binder and sodium carboxymethyl cellulose (CMC) serving as a thickener are fully stirred and mixed in a proper amount of deionized water solvent according to the weight ratio of 97.4:1.2:1.4 to form uniform negative electrode slurry; coating the slurry on a negative current collector (copper foil or nickel foil), drying, cold pressing, cutting into pieces, and welding a tab to obtain a negative electrode, wherein the OI value of graphite is 3.25; and the surface of the negative current collector is provided with a conductive layer containing carbon nano tubes, and the tube diameters of the carbon nano tubes are 1nm to 20 nm.

2. Preparation of the electrolyte

At water content<In a 10ppm argon atmosphere glove box, Ethylene Carbonate (EC), Propylene Carbonate (PC) and diethyl carbonate (DEC) are uniformly mixed according to the mass ratio of 3:3:4 to obtain a non-aqueous solvent, and then fully dried LiPF₆Dissolving in the non-aqueous solvent, adding fluoroethylene carbonate with a certain mass, and further adding the other additives to obtain the basic electrolyte. The base electrolyte is suitable for use in the examples of the invention listed in tables 1 and 2. The electrolyte solvent systems of the examples listed in tables 3 and 4 are specifically given.

3. Preparation of the Positive electrode

The positive electrode active material lithium cobaltate (LiCoO)₂) The conductive agent acetylene black and the binder polyvinylidene fluoride (PVDF) are fully stirred and mixed in a proper amount of N-methyl pyrrolidone (NMP) solvent according to the weight ratio of 96:2:2 to form uniform anode slurry; coating the slurry on an aluminum foil of a positive current collector, drying, cold pressing, cutting into pieces, and welding tabs to obtain the positive electrode.

4. Preparation of the separator

Polyethylene (PE) porous polymer films were used as separators.

5. Preparation of lithium ion battery

And sequentially stacking the anode, the isolating membrane and the cathode to enable the isolating membrane to be positioned between the anode and the cathode to play an isolating role, then winding, placing the wound isolating membrane into an outer packaging foil, injecting the electrolyte, and carrying out vacuum packaging, standing, formation, shaping and other procedures to finish the preparation of the lithium ion battery.

Second, testing method

1. Scanning Electron Microscope (SEM) testing method of negative electrode active material

SEM scanning is carried out on the sample by using a JSM-6360LV model of JEOL company and an X-ray energy spectrometer matched with the JSM-6360LV model so as to analyze the morphological structure and element distribution of the sample.

2. Method for testing direct current impedance (DCR) of lithium ion battery

The lithium ion battery is charged to 4.2V by constant current of 1.5C, and then charged to 0.05C by constant voltage. After standing for 30min, the mixture was discharged at a current of 0.1C for 10s (the corresponding voltage value U1 was recorded every 0.1 s), and discharged at a current of 1C for 360s (the corresponding voltage value U2 was recorded every 0.1 s). The charging and discharging steps were repeated 5 times. "1C" is a current value at which the battery capacity is completely discharged within 1 hour. The DCR of the lithium ion battery at 50% state of charge (SOC) is calculated as follows:

R＝(U2-U1)/(1C-0.1C)。

3. test method of Electrochemical Impedance Spectroscopy (EIS) of lithium ion battery

And placing the three-electrode lithium ion battery in an electrochemical workstation, and connecting the working electrode and the reference electrode in sequence. And applying a sine wave alternating current signal with the amplitude of 5mV and the frequency of 500KHz-30mHz to the electrochemical system to obtain the corresponding electrical signal feedback in the frequency domain range, so as to obtain the EIS curve of the lithium ion battery. As shown in fig. 2, the right intersection of the first semi-arc and the x-axis is the charge transfer resistance value, Rct.

4. Method for testing normal-temperature cycle performance of lithium ion battery

The lithium ion battery is put into a thermostat at 25 ℃, charged to 4.4V at a constant current of 1.5C, charged to 0.05C at a constant voltage of 4.4V, and discharged to 3.0V at a constant current of 1.0C, which is marked as a charging and discharging cycle process. The test of the charge and discharge cycles was performed 800 times in the above manner. The capacity retention of the lithium ion battery was calculated by the following formula:

capacity retention rate is residual discharge capacity/initial discharge capacity × 100%.

Third, test results

Table 1 shows specific surface area value K per unit mass of the negative electrode active material₁FEC value K₂And weight value K of negative electrode active material₃Influence on the capacity retention of lithium ion batteries. In each of examples and comparative examples shown in Table 1, the negative active material graphite included second particles having a Dv50 of 2.5 μm and first particles having a Dv50 of 22 μm, the weight ratio of the second particles to the first particles was 0.2, W: K₁Is 0.1, and W: P is 6.5, and the primer layer thickness is 1 μm.

TABLE 1

Examples/comparative examples	K₁-K₂-1.63K₃ ²+11.27K₃	K₁	K₂	K₃	Capacity retention rate
						Example 1	19	1.5	0.05	4.54	70.1％
Example 2	19	1.5	0.1	4.53	70.2％
						Example 3	19	1.5	0.15	4.52	70.9％
Example 4	19	1.5	0.25	4.49	71.2％
						Example 5	17	1.5	0.15	5.00	70.0％
Example 6	18	1.5	0.15	4.78	70.5％
						Example 7	20	1.5	0.15	4.18	72.2％
Example 8	20.8	1.5	0.15	3.45	73.0％
						Example 9	19	1	0.15	4.36	71.5％
Example 10	19	1.2	0.15	4.43	71.1％
						Example 11	19	1.8	0.15	4.60	69.4％
Example 12	19	2	0.15	4.65	68.9％
						Comparative example 1	17	1.5	0.15	4.99	49.50％
Comparative example 2	19	3	0.15	4.88	61.20％
						Comparative example 3	19	1.5	0.5	4.41	50.10％

As shown in Table 1, comparative example 1 does not satisfy 17.55. ltoreq. K₁-K₂-1.63K₃ ²+11.27K₃Less than or equal to 20.80, and the capacity retention rate is poor. Comparative examples 2 and 3 although satisfying 17.55. ltoreq. K₁-K₂-1.63K₃ ²+11.27K₃Less than or equal to 20.80, but K₁Or K₂Outside the desired range of the present application (i.e., 1.0. ltoreq. K)₁K is less than or equal to 2.0 and less than or equal to 0.05₂0.25) or less), the capacity retention rate of the lithium ion battery is not effectively improved. Examples 1 to 12 not only satisfied 17.55. ltoreq.K₁–K₂-1.63K₃ ²+11.27K₃Not more than 20.80, and not more than 1.0 and not more than K₁K is not less than 2.0 and not more than 0.05₂Less than or equal to 0.25, and the capacity retention rate of the lithium ion battery is remarkably improved.

When K is present as in examples 1 to 4₁When the K content is gradually increased within the range of 1.0-2.0, the capacity retention rate of the lithium ion battery is slightly increased, because the K content is increased₁、K₂And K₃The relationship between them is timed with the FEC content (i.e. K) in the electrolyte₂) Increase of (A), K₃Will vary accordingly. When K is₂/K₃When the capacity retention rate of the lithium ion battery is kept unchanged, the capacity retention rate of the lithium ion battery does not change obviously. When the content of FEC in the electrolyte is too high (e.g., K in comparative example 3)₂0.5), the content of the corresponding solvent in the electrolyte is reduced, and the lithium salt in the electrolyte is difficult to dissociate; meanwhile, FEC is easy to decompose, the formation of HF in electrolyte is accelerated, the acidity of the electrolyte is increased, HF attacks the positive electrode interface, the dissolution of transition metal is accelerated, the performance of a battery cell is damaged, and the capacity retention rate of the lithium ion battery is reduced.

When K is present as in examples 5 to 8₁And K₂Keep a certain timing, K₁-K₂-1.63K₃ ²+11.27K₃When the K is gradually increased in the range of 17.55-20.80₃Gradually decreases, i.e., the content of FEC corresponding to a unit of the negative electrode active material increases. The FEC has strong electron-withdrawing groups, has higher reduction potential, and can be reduced on the surface of the negative electrode to form a stable passivation film prior to the solvent, so that the reductive decomposition of the electrolyte solvent on the negative electrode is inhibited, the damage of an SEI film in the circulation process is relieved, and the consumption of the electrolyte in the lithium ion battery is reduced. Thus, the capacity retention rate of the lithium ion battery is remarkable.

When K is present as in examples 9 to 12₁-K₂-1.63K₃ ²+11.27K₃And K₂Within the desired range of the application and with a constant time, with K₁The increase in the content increases the specific surface area of the negative electrode active material, increases contact with the electrolyte during the cycle, and increases the SEI film that needs to be formed. However, the destruction of SEI film during the cycle cannot be alleviated because K2 is not increasedAnd repair, the consumption of the electrolyte is accelerated. Thereby, the capacity retention rate of the lithium ion battery gradually decreases.

Table 2 shows the effect of the first and second particles of the negative active material on the DCR of the lithium ion battery. The examples shown in table 2 are based on a further development of example 3, i.e. differ only by the parameters in table 2.

TABLE 2

As shown in example 27, when Dv50 of the second particle of the anode active material was more than 10 μm, the DCR of the lithium ion battery was high. As shown in example 17, when Dv50 of the second particles of the anode active material was less than 3 μm, the DCR of the lithium ion battery was high; as shown in example 22, when Dv50 of the first particle of the anode active material was less than 5 μm, the DCR of the lithium ion battery was high; this is because the particles of the negative electrode active material are small, and the negative electrode is easily pulverized after coating, so that the transport path of lithium ions is lengthened, resulting in an increase in DCR of the lithium ion battery. As shown in example 28, when Dv50 of the first particles of the anode active material was greater than 20 μm, the particles of the anode active material were larger, resulting in an increase in DCR of the lithium ion battery.

As shown in examples 13 to 16, while Dv50 of the first and second particles of the negative electrode active material was maintained for a certain time, as the weight ratio of the second particles to the first particles was gradually decreased in the range of 1:9 to 1:1, the relative content of the first particles was increased and the specific surface area of the end faces was increased, so that lithium ions could directly enter between graphite layers from the end face defect sites, achieving rapid lithium ion migration, thereby reducing the impedance of the lithium ion battery and improving the kinetics of the lithium ion battery.

As shown in examples 18 to 21, when the Dv50 of the first particle and the weight ratio of the second particle to the first particle of the negative electrode active material were kept constant, as the Dv50 of the second particle was gradually increased in the range of 3 μm to 10 μm, the negative electrode active material contained more particles having larger particle diameters, so that the isotropy of the graphite layer was decreased, the basal plane specific surface area was increased, the end plane specific surface area was decreased, the path of lithium ions entering between the graphite layers was lengthened, the migration of lithium ions was slowed, and thus the impedance of the lithium ion battery was increased, and the kinetics was decreased.

As shown in examples 23 to 26, when Dv50 of the second particles of the negative electrode active material and the weight ratio of the second particles to the first particles were kept constant, as Dv50 of the first particles was gradually increased in the range of 5 μm to 20 μm, the specific surface area of the end faces of the negative electrode active material was increased, the specific surface area of the basal plane was decreased, the path of lithium ions entering the graphite interlayer was shortened, and thereby the impedance of the lithium ion battery was increased, and the kinetics was decreased.

Table 3 shows the effect of EP content in the electrolyte on DCR and capacity retention of the lithium ion battery. The examples shown in table 3 are modifications of example 3, i.e. differ only by the parameters in table 3, the EC, PC, DEC and EP contents in table 3 being calculated on the basis of the total weight of the solvent system.

TABLE 3

When W: K is as in example 34₁When the average molecular weight is more than 1.14, the capacity retention rate of the lithium ion battery is low. When W: K is as shown in example 35₁When the average cell size is less than 0.12, the DCR of the lithium ion battery is large.

As shown in examples 29-33, K is added with W₁The DCR of the lithium ion battery is gradually reduced when the DCR is gradually increased within the range of 0.12-1.14, the dynamic performance is improved, but the capacity retention rate is gradually reduced. This is because EP itself has a low viscosity, and as the content thereof increases, the migration resistance of lithium ions in the electrolyte decreases, and during charge and dischargeConcentration polarization brought by the electrolyte is reduced, so that the dynamics of the lithium ion battery is improved. However, EP is very easy to oxidize (the oxidation potential is much lower than that of the traditional carbonate solvent), and is easy to decompose and generate gas in the circulation process, so that the capacity retention rate is reduced.

Table 4 shows the effect of the porosity of the negative active material on Rct and capacity retention of the lithium ion battery. The examples shown in table 4 are modifications of example 3, i.e. differ only by the parameters in table 4, the EC, PC, DEC and EP contents in table 4 being calculated on the basis of the total weight of the solvent system.

TABLE 4

As shown in example 36, when W: P is less than 0.6, the capacity retention ratio of the lithium ion battery is low. As shown in example 42, when W: P is greater than 6.3, the electrolyte is insufficient and cannot completely infiltrate the graphite voids, resulting in a high Rct of the lithium ion battery.

As shown in examples 37-41, as W: P is gradually decreased within the range of 0.6-6.3, Rct of the lithium ion battery is decreased, the kinetic performance is improved, but the capacity retention rate is decreased. This is because the porosity of the negative electrode active material increases with a decrease in W: P, and the amount of electrolyte that can infiltrate the negative electrode active material increases, so that the contact between the electrolyte and the negative electrode active material increases, thereby improving the dynamic performance of the lithium ion battery. However, as the porosity of the negative electrode active material increases, byproducts increase during the cycle, the consumption of the electrolyte increases, and the capacity retention rate decreases.

The effect of the thickness of the conductive layer on the negative current collector on the DCR and capacity retention of the lithium ion battery is listed in table 5. The examples shown in table 5 are modifications of example 32, i.e. differ only by the parameters in table 5.

TABLE 5

	Thickness of the conductive layer (μm)	DCR(mΩ)	Capacity retention rate
				Example 32	1	46.2	69.40％
Example 43	0.4	60.4	58.40％
				Example 44	0.5	53.4	72.80％
Example 45	0.8	49.7	71.90％
				Example 46	1.5	44.5	69.30％
Example 47	1.6	40.8	55.70％
				Example 48	0	57.3	60.30％

Example 48 was prepared in the same manner as example 32, except that there was no conductive layer on the negative current collector.

From examples 43 to 48, it is understood that when the thickness of the conductive layer on the negative electrode current collector is in the range of 0.5 μm to 1.5 μm, the conductive layer functions to conduct electrons, and at the same time, the adhesion between the negative electrode active material layer and the negative electrode current collector can be increased, thereby reducing the peeling of the negative electrode active material layer during the cycle, showing that the DCR charge transfer resistance is reduced and the kinetics is improved.

Reference throughout this specification to "an embodiment," "some embodiments," "one embodiment," "another example," "an example," "a specific example," or "some examples" means that at least one embodiment or example in this application includes a particular feature, structure, material, or characteristic described in the embodiment or example. Thus, throughout the specification, descriptions appear, for example: "in some embodiments," "in an embodiment," "in one embodiment," "in another example," "in one example," "in a particular example," or "by example," which do not necessarily refer to the same embodiment or example in this application. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

Although illustrative embodiments have been illustrated and described, it will be appreciated by those skilled in the art that the above embodiments are not to be construed as limiting the application and that changes, substitutions and alterations can be made to the embodiments without departing from the spirit, principles and scope of the application.

Claims

1. An electrochemical device comprising a positive electrode, a negative electrode, and an electrolyte, wherein:

the electrolyte includes fluoroethylene carbonate, and

Wherein:

2. The electrochemical device according to claim 1, wherein the negative electrode active material includes first particles, the first particles are secondary particles, and the number of the secondary particles is 50% to 95% of the total amount of the negative electrode active material particles.

3. The electrochemical device of claim 2, wherein Dv50 of a primary particle of the secondary particles is 2 μ ι η to 10 μ ι η, and Dv50 of the secondary particles is 5 μ ι η to 20 μ ι η.

4. The electrochemical device according to claim 1, wherein the negative active material includes second particles, the second particles being primary particles, the second particles having a Dv50 of 3 μm to 10 μm.

5. The electrochemical device according to claim 1, wherein the Dv50 of the negative electrode active material is 9 μm to 25 μm.

6. The electrochemical device according to claim 1, wherein the anode active material layer has a porosity of 15% to 40%.

7. The electrochemical device of claim 1, wherein the negative electrode further comprises a conductive layer having a thickness of 0.5 μ ι η to 1.5 μ ι η.

8. The electrochemical device of claim 1, wherein the electrolyte further comprises an organic solvent comprising ethyl propionate.

9. The electrochemical device according to claim 8, wherein the ethyl propionate is contained in an amount of 5 to 80% by total weight of the organic solvent.

10. The electrochemical device of claim 8, wherein the ethyl propionate content value W and the K required per Ah capacity are in g/Ah₁Ratio W to K of₁Is 0.12 to 1.14.

11. The electrochemical device according to claim 8, wherein a ratio Wp of a content value W of the ethyl propionate required per Ah capacity in g/Ah to a porosity value P of the anode active material layer in percentage is 0.6 to 6.3.