CN114914396A

CN114914396A - Electrochemical device and electronic device

Info

Publication number: CN114914396A
Application number: CN202210838972.2A
Authority: CN
Inventors: 陶威
Original assignee: Ningde Amperex Technology Ltd
Current assignee: Ningde Amperex Technology Ltd
Priority date: 2022-07-18
Filing date: 2022-07-18
Publication date: 2022-08-16
Anticipated expiration: 2042-07-18
Also published as: CN114914396B

Abstract

Embodiments of the present application provide an electrochemical device and an electronic device. The electrochemical device comprises a negative pole piece, wherein the negative pole piece comprises a negative pole current collector, a first layer and a second layer, and the first layer is positioned between the negative pole current collector and the second layer; wherein the first layer comprises a first negative active material and the second layer comprises a second negative active material; the first negative electrode active material and the second negative electrode active material each include a functional group; the functional group comprises at least one of tertiary amine functional group, amino group, nitro group, cyano group, carboxyl group or fluoro group; the mass percentage content of the functional groups in the first layer is a first content, the mass percentage content of the functional groups in the second layer is a second content, and the first content is less than the second content. Therefore, the second layer ensures that the negative pole piece has excellent dynamic performance, the quick charging capacity is ensured, and the first layer ensures the high capacity of the negative pole piece, so that the compatibility of the high energy density and the quick charging performance of the electrochemical device is realized.

Description

Electrochemical device and electronic device

Technical Field

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

Background

With the development of electrochemical energy storage technology, higher and higher requirements are placed on the energy density and the fast charge performance of electrochemical devices (e.g., lithium ion batteries). However, the energy density and the fast charge performance of the electrochemical device are difficult to be compatible, and generally, the dynamic performance of the pole piece is improved to a certain extent to deteriorate the capacity, so that further improvement in this respect is expected.

Disclosure of Invention

The application provides an electrochemical device, which comprises a negative pole piece, wherein the negative pole piece comprises a negative pole current collector, a first layer and a second layer, and the first layer is positioned between the negative pole current collector and the second layer; wherein the first layer comprises a first negative active material and the second layer comprises a second negative active material; the first negative electrode active material and the second negative electrode active material each include a functional group; the functional group comprises at least one of tertiary amine functional group, amino group, nitro group, cyano group, carboxyl group or fluoro group; the mass percentage content of the functional groups in the first layer is a first content, the mass percentage content of the functional groups in the second layer is a second content, and the first content is less than the second content.

In some embodiments, the first content is 0.01% to 0.1% and the second content is 0.1% to 2%. In some embodiments, the functional group is an amino group, the first content is 0.01% to 0.05%, and the second content is 0.1% to 0.2%. In some embodiments, the first negative active material and the second negative active material each independently comprise at least one of graphite or hard carbon. In some embodiments, the first negative active material and the second negative active material each further independently comprise a silicon-based material. In some embodiments, the silicon-based material in the first layer is present in an amount of 1% to 30% by mass. In some embodiments, the silicon-based material in the second layer is present in an amount of 0.1 to 4% by mass. In some embodiments, the ratio of the mass of the second layer to the total mass of the first and second layers is 5% to 80%. In some embodiments, the ratio of the mass of the second layer to the total mass of the first and second layers is 5% to 40%.

Embodiments of the present application also provide an electronic device including the electrochemical device described above.

By adopting the design of the double-layer active material layer in the negative pole piece, the mass percentage content of the functional group in the lower first layer is less than that of the functional group in the upper second layer, wherein the functional group comprises at least one of tertiary amine functional group, amino group, nitro group, cyano group, carboxyl group or fluorine group, thus the second layer ensures that the negative pole piece has excellent dynamic performance, the quick charging capability is ensured, the first layer ensures the high capacity of the negative pole piece, and the compatibility of the high energy density and the quick charging performance of the electrochemical device is realized.

Drawings

Fig. 1 shows a schematic cross-sectional view of a negative electrode tab taken along a width direction and a thickness direction according to some embodiments.

Fig. 2 shows schematic cross-sectional views of negative electrode pole pieces taken along the width direction and the thickness direction according to further embodiments.

Detailed Description

The following examples are presented to enable those skilled in the art to more fully understand the present application and are not intended to limit the present application in any way.

In general, when the surface kinetic properties of the negative electrode sheet are insufficient, surface lithium deposition is easily caused. The means for improving the kinetic performance generally affect the energy density of the electrochemical device to varying degrees. By adopting the double-layer coating technology, the active material layer on the upper layer is rich in functional groups of lone pair electrons, the dynamic performance of the electrochemical device is favorably improved, and the high-capacity active material layer on the lower layer is favorable for ensuring the high energy density of the electrochemical device.

Fig. 1 and 2 show schematic cross-sectional views of a negative electrode tab taken along a width direction and a thickness direction, according to some embodiments. Some embodiments of the present application provide an electrochemical device including a negative electrode tab. As shown in fig. 1 and 2, in some embodiments, the negative electrode tab includes a negative electrode collector 10, a first layer 11, and a second layer 12, the first layer 11 being located between the negative electrode collector 10 and the second layer 12. It should be understood that although the first and

second layers

11 and 12 are illustrated in fig. 1 and 2 as being located on only one side of the negative electrode collector 10, this is merely exemplary and the first and

second layers

11 and 12 may be present on both sides of the negative electrode collector 10.

In some embodiments, the first layer 11 includes a first negative active material 111 and the second layer 12 includes a second negative active material 121. In some embodiments, the first anode active material 111 and the second anode active material 121 each include a functional group 112. In some embodiments, functional group 112 includes at least one of a tertiary amine functional group, an amino group, a nitro group, a cyano group, a carboxyl group, or a fluoro group. In some embodiments, the functional group 112 is present in the first layer 11 at a first amount by mass and the functional group 112 is present in the second layer 12 at a second amount by mass, the first amount being less than the second amount. These functional groups are electronegative and can effectively enhance the lithium-philic property of the second layer 12, thereby enhancing the coulomb force of the second layer 12 with lithium ions, accelerating the migration and speed of the lithium ions, and enhancing the dynamic performance of the electrochemical device. And the content of the functional group 112 in the lower first layer 11 is small in mass percentage so as to be advantageous in exerting the high capacity characteristic of the first layer 11, and the first efficiency of the first layer 11 is higher than that of the second layer 12. By using the first layer 11 and the second layer 12 in combination, high dynamic performance and high energy density of the electrochemical device are achieved.

In some embodiments, functionalization can be performed by suspending functional groups 112 by the action of acid-base etching, hydrothermal methods, and the like. The functionalization may also be performed by a substitution reaction (for example, substitution of a certain carbon atom for a corresponding functional group). Both of these effects may enhance the electronegativity of the second negative active material in the second layer 12, enhancing the kinetic level.

In some embodiments, the first content is 0.01% to 0.1%. In some embodiments, by setting the mass percentage content of the functional groups 112 in the first layer 11 to this range, it is possible to maximize the capacity of the first layer 11 and eliminate the adverse effect of the excessive functional groups 112 on the energy density of the electrochemical device.

In some embodiments, the second content is 0.1% to 2%. If the second content is too small, the effect of the second layer 12 in improving the kinetic properties is relatively limited; if the second content is too large, the effect of the second layer 12 on improving the dynamic properties is no longer significantly increased. In some embodiments, the second content is 0.1%, 0.5%, 1%, 1.2%, 1.5%, 1.8%, 2%, or other suitable value. In some embodiments, the functional group is an amino group, the first content is 0.01% to 0.05%, and the second content is 0.1% to 0.2%.

In some embodiments, the first anode active material 111 and the second anode active material 121 each independently include at least one of graphite or hard carbon. In some embodiments, the first negative active material 111 and the second negative active material 121 also each independently include a silicon-based material. The silicon-based material can improve the energy density of the electrochemical device. In some embodiments, the silicon-based material comprises at least one of silicon, a silicon oxy material, a silicon carbon material, or a silicon oxy carbon material. In some embodiments, the first anode active material 111 and the second anode active material 121 may be the same or different.

In some embodiments, the silicon-based material in the first layer 11 is present in an amount of 1% to 30% by mass. If the content of the silicon-based material in the first layer 11 is too small in percentage by mass, the effect of the silicon-based material in enhancing the energy density of the electrochemical device is relatively small; if the content of the silicon-based material in the first layer 11 is too large in percentage by mass, the volume expansion of the silicon-based material caused during the cycle is too large, which is disadvantageous for the structural stability of the first layer 11.

In some embodiments, the silicon-based material in the second layer 12 is present in an amount of 0.1 to 4% by mass. If the content of the silicon-based material in the second layer 12 is too small in percentage by mass, the effect of the silicon-based material in enhancing the energy density of the electrochemical device is relatively small; if the content of the silicon-based material in the second layer 12 is too large in percentage by mass, the volume expansion of the silicon-based material caused during the cycle is too large, which is disadvantageous for the structural stability of the second layer 12. In addition, excess silicon-based material may also adversely affect the conductive properties of second layer 12 due to its relatively weak conductivity.

In some embodiments, the ratio of the mass of the second layer 12 to the total mass of the first layer 11 and the second layer 12 is 5% to 80%. In some embodiments, the ratio of the mass of the second layer 12 to the total mass of the first layer 11 and the second layer 12 is 5% to 40%. In some embodiments, the ratio of the mass of the second layer 12 to the total mass of the first and

second layers

11, 12 has less impact on the dynamic performance of the negative electrode sheet, however, has greater impact on the overall capacity of the electrochemical device, and by reducing this ratio, it is beneficial to increase the overall energy density of the electrochemical device. For example, the ratio in fig. 2 is smaller than that in fig. 1, which is advantageous for improving the overall energy density of the electrochemical device. Of course, if the ratio is too small, it is also not advantageous for the second layer 12 to sufficiently exert the effect of improving the dynamic properties. In some embodiments, the ratio of the mass of the second layer 12 to the total mass of the first layer 11 and the second layer 12 is 5%, 10%, 15%, 20%, 30%, 40%, or other suitable value.

In some embodiments, negative current collector 10 may employ at least one of a copper foil, a nickel foil, or a carbon-based current collector. In some embodiments, the first layer 11 and the second layer 12 may also each independently comprise a binder comprising at least one of styrene-butadiene rubber, polyacrylic acid, polyacrylate, polyimide, polyamideimide, polyvinylidene fluoride, polytetrafluoroethylene, aqueous acrylic resin, or polyvinyl formal. In some embodiments, the mass ratio of the first negative electrode active material and the binder in the first layer 11 is 90 to 99: 1-10. In some embodiments, the mass ratio of the second anode active material and the binder in the second layer 12 is 90-99: 1-10. It will be appreciated that this is merely exemplary and that other suitable materials or other suitable mass ratios may be included. In some embodiments, the first layer 11 and the second layer 12 may also include a dispersant, such as, for example, carboxymethyl cellulose, sodium carboxymethyl cellulose, and the like.

In some embodiments, the electrochemical device includes an electrode assembly that may include a positive pole piece, a negative pole piece, and a separator disposed between the positive and negative pole pieces. In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer. In some embodiments, the positive active material layer may be disposed on one or both sides of the positive current collector.

In some embodiments, the positive electrode current collector may be an aluminum foil, but other positive electrode current collectors commonly used in the art may also be used. In some embodiments, the thickness of the positive electrode current collector may be 1 μm to 50 μm. In some embodiments, the positive electrode active material layer may be coated only on a partial area of the positive electrode collector.

In some embodiments, the positive electrode active material layer may include a positive electrode active material, a conductive agent, and a binder. In some embodiments, the positive active material may include at least one of lithium cobaltate, lithium iron phosphate, lithium aluminate, lithium manganate, or lithium nickel cobalt manganate. In some embodiments, the conductive agent of the positive electrode sheet may include at least one of conductive carbon black, flake graphite, graphene, or carbon nanotubes. In some embodiments, the binder in the positive electrode sheet may include at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, a styrene-acrylate copolymer, a styrene-butadiene copolymer, a polyamide, polyacrylonitrile, a polyacrylate, a polyacrylic acid, a polyacrylate, sodium carboxymethyl cellulose, polyvinyl acetate, polyvinylpyrrolidone, a polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. In some embodiments, the mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode active material layer is (80-99): (0.1-10): (0.1-10), but this is merely an example and any other suitable mass ratio may be employed.

In some embodiments, the separator comprises at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the polyethylene includes at least one selected from high density polyethylene, low density polyethylene, or ultra high molecular weight polyethylene. Particularly polyethylene and polypropylene, which have a good effect on preventing short circuits and can improve the stability of the battery through a shutdown effect. In some embodiments, the thickness of the isolation film is in the range of about 3 μm to 20 μm.

In some embodiments, the surface of the separator may further include a porous layer disposed on at least one surface of the separator, the porous layer including inorganic particles selected from alumina (Al) and a binder ₂ O ₃ ) Silicon oxide (SiO) ₂ ) Magnesium oxide (MgO), titanium oxide (TiO) ₂ ) Hafnium oxide (HfO) ₂ ) Tin oxide (SnO) ₂ ) Cerium oxide (CeO) ₂ ) Nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO) ₂ ) Yttrium oxide (Y) ₂ O ₃ ) At least one of silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. In some embodiments, the pores of the separator film have a diameter in the range of about 0.01 μm to 1 μm. The binder of the porous layer is at least one selected from polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethylcellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene or polyhexafluoropropylene. The porous layer on the surface of the isolating membrane can improve the heat resistance, the oxidation resistance and the electrolyte infiltration performance of the isolating membrane and enhance the adhesion between the isolating membrane and the pole piece.

In some embodiments, the electrochemical device comprises a lithium ion battery, but the application is not so limited. In some embodiments, the electrochemical device further comprises an electrolyte comprising at least one of fluoroether, fluoroether carbonate, or ether nitrile. In some embodiments, the electrolyte further includes a lithium salt including lithium bis (fluorosulfonyl) imide and lithium hexafluorophosphate, the concentration of the lithium salt is 1 to 2mol/L, and the mass ratio of lithium bis (fluorosulfonyl) imide to lithium hexafluorophosphate is 0.06 to 5. In some embodiments, the electrolyte may further include a non-aqueous solvent. The non-aqueous solvent may be a carbonate compound, a carboxylate compound, an ether compound, other organic solvent, or a combination thereof.

The carbonate compound may be a chain carbonate compound, a cyclic carbonate compound, a fluoro carbonate compound, or a combination thereof.

Examples of the chain carbonate compound are diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), Methyl Propyl Carbonate (MPC), Ethyl Propyl Carbonate (EPC), Methyl Ethyl Carbonate (MEC), and combinations thereof. Examples of the cyclic carbonate compound are Ethylene Carbonate (EC), Propylene Carbonate (PC), Butylene Carbonate (BC), Vinyl Ethylene Carbonate (VEC), or a combination thereof. Examples of the fluoro carbonate compound are fluoroethylene carbonate (FEC), 1, 2-difluoroethylene carbonate, 1, 2-trifluoroethylene carbonate, 1,2, 2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1, 2-difluoro-1-methylethylene carbonate, 1, 2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, or a combination thereof.

Examples of carboxylate compounds are methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ -butyrolactone, decalactone, valerolactone, mevalonic lactone, caprolactone, methyl formate, or combinations thereof.

Examples of the ether compound are dibutyl ether, tetraglyme, diglyme, 1, 2-dimethoxyethane, 1, 2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, or a combination thereof.

Examples of other organic solvents are dimethylsulfoxide, 1, 2-dioxolane, sulfolane, methyl sulfolane, 1, 3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphate esters or combinations thereof.

Embodiments of the present application also provide an electronic device including the electrochemical device described above. The electronic device of the embodiment of the present application is not particularly limited, and may be any electronic device known in the art. In some embodiments, the electronic device may include, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable phone, a portable facsimile machine, a portable copier, a portable printer, a headphone, a video recorder, a liquid crystal television, a handheld cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notebook, a calculator, a memory card, a portable recorder, a radio, a backup power source, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, an unmanned aerial vehicle, a lighting fixture, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a large household battery, a lithium ion capacitor, and the like.

In the following, some specific examples and comparative examples are listed to better illustrate the present application, wherein a lithium ion battery is taken as an example.

Comparative example 1

Preparing a negative pole piece: the current collector adopts copper foil, the negative active material adopts artificial graphite, the binder adopts styrene butadiene rubber, and the dispersant adopts carboxymethyl cellulose. Mixing artificial graphite, styrene butadiene rubber and sodium carboxymethylcellulose according to the mass percentage of 98: 1: 1, dispersing the mixture in deionized water to form slurry, uniformly stirring, coating the slurry on a copper foil, drying to form a negative active material layer, and carrying out cold pressing and stripping to obtain the negative pole piece. Wherein the thickness of the negative electrode active material layer is 120 μm, and the content of the functional group in the negative electrode active material layer is less than 0.01% by mass.

Preparing a positive pole piece: mixing a positive electrode active material lithium cobaltate, conductive carbon black and a binder polyvinylidene fluoride (PVDF) according to the mass percentage of 94.8: 2.8: and 2.4, fully stirring and uniformly mixing the mixture in an N-methyl pyrrolidone solvent system, and coating the mixture on an aluminum foil to obtain a positive active material layer, wherein the thickness of the positive active material layer is 80 microns. And drying and cold pressing to obtain the positive pole piece.

Preparing an isolating membrane: stirring polyacrylate to form uniform slurry, coating the slurry on the two side surfaces of the porous base material (polyethylene), and drying to form the isolating membrane.

Preparing an electrolyte: in the environment with the water content of less than 10 ppm, lithium hexafluorophosphate and a nonaqueous organic solvent (ethylene carbonate (EC), diethyl carbonate (DEC), Propylene Carbonate (PC), Propyl Propionate (PP), Vinylene Carbonate (VC) = 20: 30: 20: 28: 2, mass percentage ratio) are mixed according to the mass percentage ratio of 8: 92 was formulated to form an electrolyte having a lithium salt concentration of 1 mol/L.

Preparing a lithium ion battery: and sequentially stacking the positive pole piece, the isolating film and the negative pole piece in sequence to enable the isolating film to be positioned between the positive pole piece and the negative pole piece to play an isolating role, and winding to obtain the electrode assembly. And (3) placing the electrode assembly in an outer packaging aluminum-plastic film, dehydrating at 80 ℃, injecting the electrolyte, packaging, and carrying out technological processes such as formation, degassing, shaping and the like to obtain the lithium ion battery.

The preparation of only the negative electrode sheet in the comparative example 2 is different from that of the comparative example 1, specifically, the artificial graphite is adjusted to be the artificial graphite connected with the amino functional group, and the mass percentage content of the amino functional group in the negative electrode active material layer is 0.5%.

Example 1 preparation of only a negative electrode sheet was different from comparative example 1, specifically, the negative active material layer was double-coated, the thickness of the first layer close to the negative current collector was 60 μm, the thickness of the second layer far from the negative current collector was 60 μm, the artificial graphite in the second layer was linked with an amino functional group, the content of the amino functional group in the second layer was 0.5% by mass, and the content of the amino functional group in the first layer was 0.01% by mass. Examples 2-3 differ from example 1 only in the content of functional groups in the first layer and in the second layer. Example 4 differs from example 1 only in that the functional group is changed from amino to carboxyl. Example 5 differs from example 4 in that the thickness of the first layer close to the negative electrode current collector was 96 μm and the thickness of the second layer far from the negative electrode current collector was 24 μm. The difference between example 6 and example 4 is that the content of the carboxyl group in the second layer was 0.2% by mass. Example 7 differs from example 4 in that the first layer contains 4% by mass of silicon. Examples 8 to 10 differ from example 7 in the mass percentage content of carboxyl groups in the second layer and/or the ratio of the mass of the second layer to the total mass of the first layer and the second layer. Example 11 differs from example 4 in that the ratio of the mass of the second layer to the total mass of the first layer and the second layer is 40%.

In addition, in the present application, the following method may be employed to measure the corresponding parameters.

And (3) testing the efficiency for the first time:

charging the lithium ion battery at a constant current of 1C at 45 ℃ until the voltage rises to a rated voltage of 4.45V, then charging at the constant voltage until the current is reduced to 0.02C, and recording the charging capacity Qc; standing for 5 min, then performing 0.2C constant current discharge on the lithium ion battery, and stopping and recording the discharge capacity Qd when the voltage is reduced to 3V; wherein first efficiency = Qc/Qd.

And (3) energy density testing:

the test conditions are 25 ℃, 3C is charged to 4.45V, then the battery is placed for 30 min, then 1C is discharged to 3V, the battery is placed for 10 min, the test is cycled, and after the last cycle is 1000 times, the volume = length × width × thickness of the lithium ion battery, and the energy density = capacity/volume of the lithium ion battery is calculated by using the capacity and volume of the lithium ion battery.

The capacity of the lithium ion battery means that the lithium ion battery is directly charged to rated voltage of 4.48V at constant current of 0.5C at 25 ℃, then constant voltage charging is carried out until the current is reduced to 0.02C, and at the moment, the lithium ion battery is placed for 30 min; then, discharging the lithium ion battery at 0.2C until the discharge voltage reaches 3V; wherein capacity = current time; the capacity = & · t · d of the lithium ion battery; volume =5.0mm 80mm 61mm of the lithium ion battery.

Lithium separation test:

and under the test condition of 25 ℃, carrying out 3C constant current charging on the lithium ion battery until the voltage is increased to the rated voltage of 4.45V, then transferring to constant voltage for charging until the current is reduced to 0.02C, standing for 20 min, and observing a fully charged negative electrode interface after disassembling the lithium ion battery.

And (3) testing the type and content of functional groups:

cutting the negative pole piece by plasma laser beams to obtain a cross section; different areas of the cross-section are scanned.

Functional group testing: for the asymmetric functional group structure, the method is characterized by an infrared spectrometer (FT-IR), and the functional group structure is determined according to the comparison of the absorption peak position of the functional group with a standard card; and for the symmetrical functional group structure, a Raman spectrometer (Raman) is adopted for characterization, and the type of the functional group can be obtained according to the position of a Raman peak.

Testing the content of functional groups: and cutting the negative pole piece by using a plasma laser beam to obtain a section. EDS scanning is carried out on different areas of the cross section, distribution diagrams of the types and the contents of elements (different functional groups have characteristic elements, for example, fluorine element can represent fluorine group) in different thickness areas can be obtained, a straight line parallel to the current collector is drawn along the position of color mutation on the distribution diagrams, a first layer is arranged below the straight line along the thickness direction of the pole piece, and a second layer is arranged above the straight line. And (3) scraping the powder to obtain the powder of the upper layer and the lower layer, wherein due to different thermal decomposition temperatures of different functional groups, the functional groups are represented by an infrared spectrometer or a Raman spectrometer, and then the mass percentage content of the functional groups in the powder is analyzed by thermal re-analysis.

And (3) impedance testing:

and (3) placing the lithium ion battery at 25 ℃, charging the lithium ion battery to 4.48V rated voltage at a constant current of 1C, charging the lithium ion battery to 0.02C at a constant voltage, standing for 30 min, discharging for 60min at 1C after the voltage is stabilized, recording the voltage V1 at the moment, standing for 2h, and recording the voltage V2 at the moment. Impedance R = (V2-V1)/1C.

Table 1 shows the respective parameters and evaluation results of examples 1 to 11 and comparative examples 1 to 2.

TABLE 1

Comparing examples 1 to 6 with comparative example 1, it can be seen that, compared to a common negative active material layer, by adopting a double-layer negative active material layer design, in which the second layer contains a specific functional group, the surface lithium deposition of the negative electrode sheet can be improved, the impedance can be reduced, and the dynamic performance of the electrochemical device can be improved without much influence on the energy density.

As can be seen from comparing examples 1 to 6 and comparative example 2, by adopting a two-layer negative active material layer design in which the mass percentage content of the functional group in the second layer is higher than that in the first layer, with respect to the negative active material layers each having a functional group as a whole, while ensuring the first efficiency of the electrochemical device, the energy density of the electrochemical device can be increased, and the impedance can be reduced to some extent, and the kinetic performance of the electrochemical device can be improved.

As can be seen from comparing examples 4, 5, and 11, the energy density of the electrochemical device increases as the ratio of the mass of the second layer to the total mass of the first layer and the second layer decreases.

As can be seen from comparison of examples 4 and 6, when the content of the functional group in the first layer was the same and the content of the functional group in the second layer was decreased by mass%, the impedance of the electrochemical device was increased and the energy density of the electrochemical device was increased. The same conclusion can be reached by comparing examples 8 to 10.

As can be seen from comparing examples 4 and 7, the energy density of the electrochemical device increases after the silicon-based material is added to the first layer.

The above description is only a preferred embodiment of the application and is illustrative of the principles of the technology employed. It will be appreciated by those skilled in the art that the scope of the disclosure herein is not limited to the particular combination of features described above, but also encompasses other combinations of features described above or equivalents thereof. For example, the above features and the features having similar functions disclosed in the present application are mutually replaced to form the technical solution.

Claims

1. An electrochemical device, comprising:

the negative pole piece comprises a negative pole current collector, a first layer and a second layer, wherein the first layer is positioned between the negative pole current collector and the second layer;

wherein the first layer comprises a first negative active material and the second layer comprises a second negative active material; the first negative electrode active material and the second negative electrode active material each include a functional group; the functional group comprises at least one of tertiary amine functional group, amino group, nitro group, cyano group, carboxyl group or fluoro group; the mass percentage content of the functional group in the first layer is a first content, the mass percentage content of the functional group in the second layer is a second content, and the first content is smaller than the second content.

2. The electrochemical device of claim 1, wherein the first content is 0.01% to 0.1%, and the second content is 0.1% to 2%.

3. The electrochemical device of claim 1, the functional group being an amino group, the first content being 0.01% to 0.05%, the second content being 0.1% to 0.2%.

4. The electrochemical device of claim 1, wherein the first negative active material and the second negative active material each independently comprise at least one of graphite or hard carbon.

5. The electrochemical device according to claim 4, wherein the first negative active material and the second negative active material further each independently comprise a silicon-based material.

6. The electrochemical device according to claim 5, wherein the silicon-based material in the first layer is contained in an amount of 1 to 30% by mass.

7. The electrochemical device according to claim 5, wherein the silicon-based material in the second layer is contained in an amount of 0.1 to 4% by mass.

8. The electrochemical device according to claim 1, wherein a ratio of the mass of the second layer to the total mass of the first layer and the second layer is 5% to 80%.

9. The electrochemical device according to claim 8, wherein a ratio of the mass of the second layer to the total mass of the first layer and the second layer is 5% to 40%.

10. An electronic device comprising the electrochemical device according to any one of claims 1 to 9.