CN114665146A

CN114665146A - Electrochemical device, electronic device and method for preparing negative pole piece

Info

Publication number: CN114665146A
Application number: CN202210345403.4A
Authority: CN
Inventors: 王莹莹; 刘涛
Original assignee: Ningde Amperex Technology Ltd
Current assignee: Ningde Amperex Technology Ltd
Priority date: 2022-03-31
Filing date: 2022-03-31
Publication date: 2022-06-24

Abstract

Embodiments of the present application provide electrochemical devices, electronic devices, and methods of making negative electrode sheets. The electrochemical device comprises a negative pole piece, wherein the negative pole piece comprises a negative active material layer, and the negative active material layer comprises a negative active material, a porous dispersing agent and a binder. By adopting the porous dispersing agent in the negative electrode plate, the infiltration of the electrolyte in the negative electrode plate can be improved, the cycle performance of the electrochemical device can be improved while high energy density is obtained, and the bonding between the negative electrode active materials in the negative electrode active material layer can be improved relative to the pores which are not formed in the negative electrode active material layer by the porous dispersing agent.

Description

Electrochemical device, electronic device and method for preparing negative pole piece

Technical Field

The present application relates to the field of electrochemical energy storage, and in particular, to electrochemical devices, electronic devices, and methods of making negative electrode sheets.

Background

With the development of electrochemical energy storage technology, increasingly higher requirements are placed on the energy density and cycling performance of electrochemical devices (e.g., lithium ion batteries). In order to improve the energy density of the electrochemical device, the compaction density of the negative electrode plate tends to increase, but the increase of the compaction density of the negative electrode plate can reduce the porosity of the negative electrode plate, so that the infiltration of electrolyte is not facilitated, and the cycle capacity of the electrochemical device is reduced. In addition, a pore structure is formed between the materials in the anode active material layer, which is disadvantageous to the adhesion between the anode active material particles in the anode active material layer, and easily causes the falling-off of the anode active material particles with the cycle of the electrochemical device.

Disclosure of Invention

The application provides an electrochemical device, electrochemical device includes the negative pole piece, and the negative pole piece includes negative pole active material layer, and negative pole active material layer includes negative pole active material, porous dispersant and binder.

In some embodiments, the negative active material comprises graphite. In some embodiments, the porous dispersant comprises a porous structure of at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, lithium hydroxypropyl carboxymethyl cellulose, sodium hydroxypropyl carboxymethyl cellulose, lithium hydroxyethyl carboxymethyl cellulose, sodium hydroxyethyl carboxymethyl cellulose, or hydroxyethyl carboxymethyl cellulose. In some embodiments, the binder comprises 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 negative active material, the porous dispersant, and the binder in the negative active material layer is 96.6 to 98.6: 0.7-1.7: 0.7-1.7. In some embodiments, the pores in the porous dispersant have a pore diameter of 50nm to 500 nm. In some embodiments, the porosity of the negative active material layer is 25% to 40%.

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

The embodiment of the application also provides a method for preparing the negative pole piece, which comprises the following steps: mixing a negative electrode active material, a dispersing agent, a pore-forming agent and a binder with deionized water according to a preset ratio to obtain a negative electrode slurry; and forming the negative electrode slurry on a negative electrode current collector, and drying to volatilize the pore-forming agent to obtain the negative electrode plate. In some embodiments, the pore-forming agent comprises at least one of N-methylpyrrolidone, dimethylacetamide, dimethylformamide, or dimethylsulfoxide. In some embodiments, the pore former has a boiling point that is greater than 40 ℃ higher than the boiling point of water.

In some embodiments, the mass ratio of the negative active material, the dispersant, the pore former, and the binder is 96.6-98.6: 0.7-1.7: 0.01-0.3: 0.7-1.7. In some embodiments, the negative active material comprises graphite. In some embodiments, the dispersing agent comprises at least one of sodium carboxymethylcellulose, lithium carboxymethylcellulose, lithium hydroxypropylcarboxymethylcellulose, sodium hydroxypropylcarboxymethylcellulose, hydroxypropyl carboxymethylcellulose, lithium hydroxyethylcarboxymethylcellulose, sodium hydroxyethylcarboxymethylcellulose, or hydroxyethylcarboxymethylcellulose. In some embodiments, the binder comprises at least one of styrene butadiene rubber, polyacrylic acid, polyacrylate, polyimide, polyamideimide, polyvinylidene fluoride, polytetrafluoroethylene, aqueous acrylic resin, or polyvinyl formal.

According to the electrochemical device, the porous dispersing agent is adopted in the negative pole piece, the infiltration of the electrolyte in the negative pole piece can be improved, the high-energy density is obtained, meanwhile, the cycle performance of the electrochemical device is improved, and the bonding between the negative pole active materials in the negative pole active material layer can be improved relative to the pores which are not formed in the negative pole active material layer by the porous dispersing agent.

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 order to increase the porosity of the negative electrode sheet, there are generally the following methods: forming holes by using a mold, namely needling the surface of the negative pole piece by using the mold with a fixed shape and position to form a plurality of holes; sintering and pore-forming, namely decomposing the binder and the like in the negative pole piece by adopting a high-temperature treatment process to form a plurality of pores; and (3) carrying out laser pore-forming, namely converting light energy into heat energy by adopting laser, so that the binder and the like in the negative pole piece are decomposed to form a plurality of pores. However, the die hole forming requires additional equipment investment and additional processes, reduces the production efficiency, and requires fixing the size of the negative pole piece, so that the flexibility is not sufficient and the die is expensive; the sintering and pore-forming also needs to add new equipment investment and new working procedures, so that the production efficiency is reduced, and the energy consumption is high, thereby being not beneficial to environmental protection; the laser pore-forming also needs to add new equipment investment and new working procedures, reduces the production efficiency, has high energy consumption and is not beneficial to environmental protection.

Generally, pores may be formed in the anode active material layer using a pore former to improve wetting of the electrolyte in the anode active material layer. However, since a pore structure is formed between the materials in the anode active material layer, which is disadvantageous to the adhesion between the anode active material particles in the anode active material layer, the falling off of the anode active material particles, for example, the falling off of the surface layer of the anode active material layer, is easily caused along with the cycle of the electrochemical device, so that the anode active material layer having the surface layer of the pore structure is lost, affecting the cycle performance of the electrochemical device.

Some embodiments of the present application provide an electrochemical device including a negative electrode tab. In some embodiments, the negative electrode tab comprises a negative active material layer. In some embodiments, the negative active material layer includes a negative active material, a porous dispersant, and a binder. In some embodiments, the negative active material can be used for intercalation and deintercalation of lithium ions, the dispersant facilitates slurry dispersion during preparation of the negative electrode sheet, and the binder is used for binding of material components in the negative active material layer to form a stable negative active material layer.

By adopting the porous dispersing agent, the porosity of the negative pole piece can be improved, the infiltration of the electrolyte in the negative pole piece is improved, and the high energy density is obtained while the cycle performance of the electrochemical device is improved. In addition, the pore structure in the dispersant does not substantially affect the adhesion between the anode active materials with respect to pores that are not formed in the anode active material layer, so that the adhesion between the anode active materials in the anode active material layer can be improved, thereby improving the cycle performance of the electrochemical device.

In some embodiments, the negative active material comprises graphite. In some embodiments, the negative active material layer may also be a silicon-based material. 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 porous dispersant comprises a porous structure of at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, lithium hydroxypropyl carboxymethyl cellulose, sodium hydroxypropyl carboxymethyl cellulose, lithium hydroxyethyl carboxymethyl cellulose, sodium hydroxyethyl carboxymethyl cellulose, or hydroxyethyl carboxymethyl cellulose. The dispersant and the pore-forming agent can form a two-phase structure with the dispersant as a continuous phase and the pore-forming agent as a dispersed phase, and the dispersant with a porous structure is formed after the pore-forming agent is evaporated.

In some embodiments, the binder comprises 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 negative active material, the porous dispersant, and the binder in the negative active material layer is 96.6 to 98.6: 0.7-1.7: 0.7-1.7. In some embodiments, if the mass content of the dispersant in the anode active material layer is too small, uniform dispersion of the slurry is not facilitated; if the mass content of the dispersant in the negative electrode active material layer is too large, the energy density of the electrochemical device may be adversely affected.

In some embodiments, the pores in the porous dispersant have a pore diameter of 50nm to 500 nm. In some embodiments, the pore size of the pores in the porous dispersant can be measured, for example, by electron microscopy. In some embodiments, the pores have a pore depth of 5nm to 500 nm. In some embodiments, the shape of the holes is circular, elliptical, semicircular, or irregular. In some embodiments, the porosity of the negative active material layer is 25% to 40%. If the porosity of the negative electrode active material layer is too small, the infiltration of the electrolyte is not facilitated, thereby affecting the cycle performance of the electrochemical device; if the porosity of the negative electrode active material layer is too large, it is not advantageous to increase the energy density of the electrochemical device.

In some embodiments, the present application also provides a method of making a corresponding negative electrode tab, the method comprising: and mixing the negative active material, the dispersing agent, the pore-forming agent and the binder with deionized water according to a preset ratio to obtain negative slurry. In some embodiments, the types and respective mass ratios of the negative electrode active material, the dispersant, and the binder, such as those described above, are not repeated here. In some embodiments, the mass ratio of the negative active material, the dispersant, the pore former, and the binder is 96.6-98.6: 0.7-1.7: 0.01-0.3: 0.7-1.7. In some embodiments, the method further comprises: and forming the negative electrode slurry on a negative electrode current collector, and drying to volatilize the pore-forming agent to obtain the negative electrode piece. In some embodiments, the negative electrode slurry may be formed on the negative electrode current collector by coating.

In some embodiments, the pore-forming agent comprises at least one of N-methyl pyrrolidone (NMP), Dimethylacetamide (DMAC), Dimethylformamide (DMF), or Dimethylsulfoxide (DMSO). In some embodiments, the pore former has a boiling point that is greater than the boiling point of water by greater than 40 ℃, further greater than 50 ℃, further still, the pore former has a boiling point between 150 ℃ and 300 ℃, and further still, the pore former has a boiling point between 150 ℃ and 230 ℃. In this way, during the drying or drying process, the moisture in the negative active material layer may be evaporated first, and then the pore-forming agent and the dispersant may form a two-phase structure with the dispersant as a continuous phase and the pore-forming agent as a dispersed phase, and when the pore-forming agent is volatilized or evaporated, the dispersant forms a porous structure, i.e., a porous dispersant. If the boiling point of the pore-forming agent is close to the boiling point of water, for example, the boiling point of the pore-forming agent is 120 ℃, the boiling point of the mixture of the pore-forming agent and water may be lower than 100 ℃, and when the water evaporates, most of the pore-forming agent is also evaporated, and thus the above-mentioned two-phase structure with the dispersant as the continuous phase and the pore-forming agent as the dispersed phase cannot be formed, and thus the porous dispersant cannot be obtained. The formed porous dispersing agent is beneficial to the rapid diffusion of electrolyte in a negative pole piece, the migration speed of lithium ions is accelerated, the concentration polarization generated in the rapid charge and discharge process is reduced, and the electrochemical performance of an electrochemical device is improved.

In some embodiments, the pore former is a non-alcoholic pore former, an alcoholic pore former is easily blended with the dispersant, a small amount of alcoholic pore former is blended with the local dispersant, which is not conducive to the formation of uniform pores in the porous dispersant, and is likely to cause local collapse of the dispersant.

In some embodiments, the dispersing agent comprises at least one of sodium carboxymethylcellulose, lithium carboxymethylcellulose, lithium hydroxypropylcarboxymethylcellulose, sodium hydroxypropylcarboxymethylcellulose, hydroxypropyl carboxymethylcellulose, lithium hydroxyethylcarboxymethylcellulose, sodium hydroxyethylcarboxymethylcellulose, or hydroxyethylcarboxymethylcellulose. In some embodiments, the binder comprises at least one of styrene butadiene rubber, polyacrylic acid, polyacrylate, polyimide, polyamideimide, polyvinylidene fluoride, polytetrafluoroethylene, aqueous acrylic resin, or polyvinyl formal.

In some embodiments of the present application, a negative electrode plate including a porous dispersant is prepared by a non-solvent evaporation method, a pore-forming agent (non-alcohol solvent) is added to a dispersant solution, the slurry is coated, and then dried, the boiling point of the pore-forming agent is higher than that of water, the water is evaporated first, after the water is evaporated, droplets of the pore-forming agent are distributed in a film formed by the dispersant, a two-phase structure in which the dispersant is a continuous phase and the pore-forming agent is a dispersed phase is formed, and then the temperature is continuously raised, the pore-forming agent is evaporated to become a source of pores, and the negative electrode plate including the porous dispersant is formed.

In some embodiments, the negative electrode tab may further include a negative electrode current collector, the negative electrode active material layer being located on one or both sides of the negative electrode current collector. In some embodiments, the negative electrode current collector may employ at least one of a copper foil, a nickel foil, or a carbon-based current collector. In some embodiments, the adhesive force between the negative electrode active material layer and the negative electrode current collector is 8N/m to 20N/m. In some embodiments, testing the adhesion between the negative active material layer and the negative current collector was performed under the following conditions: the coating weight is 80mg/1540.25mm²To 200mg/1540.25mm²The compacted density is 1.40g/cm³To 1.80g/cm³. In some embodiments, the anode active material layer has an ionic conductivity of 0.1S/cm to 0.4S/cm. If the ionic conductivity of the negative active material layer is too small, the kinetic performance of the electrochemical device is affected; if the ionic conductivity of the anode active material layer is too large, the ionic conductivity of the anode active material layer is too high, which may greatly increase the material cost.

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 binder₂O₃) 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)₂O₃) 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 and 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 comprises a lithium salt comprising 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 sodium carboxymethylcellulose. Mixing artificial graphite, styrene butadiene rubber and sodium carboxymethylcellulose according to the mass percentage of 97.4: 1.3: 1.3 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 coating amount of the negative electrode active material layer is 150mg/1540.25mm²The compacted density is 1.74g/cm³。

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: under the environment that the water content is less than 10ppm, lithium hexafluorophosphate and a nonaqueous organic solvent (ethylene carbonate (EC), diethyl carbonate (DEC), Propylene Carbonate (PC), Propyl Propionate (PP), Vinylene Carbonate (VC), wherein the mass percentage ratio of lithium hexafluorophosphate to the Vinylene Carbonate (VC) is 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.

Only the preparation of the negative electrode sheet in comparative examples 2 to 11 is different from that in comparative example 1, specifically, the content ratio, coating amount or compaction density of each component of the negative electrode active material layer is different.

Example 1 only the preparation of the negative electrode sheet was different from that of comparative example 1, and specifically, a pore-forming agent Dimethylformamide (DMF) was further added to the slurry of the negative electrode active material layer in an amount of 0.015% by mass. Examples 2 to 5 are different from example 1 in the amount of DMF added as a pore-forming agent. Examples 6 to 8 and comparative examples 12 to 13 are different from example 3 in the kind of pore former used, wherein the pore former in comparative examples 12 to 13 can form a pore structure between the materials of the anode active material layer. Examples 9 to 11 are different from example 3 in the point that the compaction density of the anode active material layer is different. Examples 12 to 15 are different from example 3 in the coating amount of the anode active material layer. Examples 16 to 23 are different from example 3 in the kind of the dispersant used. Examples 24 to 26 are different from example 3 in the content of each component of the anode active material layer.

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

1) Adhesion test between the negative active material layer and the negative current collector: the brand of an instrument used for testing the adhesive force between the negative active material layer and the negative current collector is Instron, the model is 33652, a negative pole piece (with the width of 30mm multiplied by the length (100mm to 160mm)) is taken and fixed on a steel plate by using double-sided gummed paper (with the model: 3M9448A, with the width of 20mm multiplied by the length (90mm to 150mm)), a paper tape with the same width as the negative pole piece is fixed with one side of the negative pole piece by using gummed paper, a limiting block of a tensile machine is adjusted to a proper position, the paper tape is turned upwards and slides for 40mm, the sliding speed is 50mm/min, and the adhesive force between the negative active material layer and the negative current collector is tested under 180 degrees (namely, the negative active material layer and the negative current collector are stretched in the opposite direction).

2) Test of compacted density of negative electrode active material layer:

taking a lithium ion battery which is completely discharged, disassembling a negative pole piece, cleaning, drying, weighing the negative pole piece (the two sides of a negative current collector are coated with negative active material layers) with a certain area S by using an electronic balance, recording the weight as W1, and measuring the thickness T1 of the negative pole piece by using a ten-thousandth micrometer. The negative active material layer was washed off using a solvent, dried, and the weight of the negative current collector was measured as W2, and the thickness of the negative current collector, T2, was measured using a ten-thousandth ruler. The weight W0 and the thickness T0 of the negative electrode active material layer provided on the negative electrode current collector side and the compacted density of the negative electrode active material layer were calculated by the following formulas:

W0＝(W1-W2)/2

T0＝(T1-T2)/2

the compacted density of the negative electrode active material layer was W0/(T0 × S).

3) Porosity test of the negative active material layer: the porosity of the negative active material layer was tested by gas displacement method: punching more than 50 negative electrode plates with radius d by using the same die, respectively measuring the thickness h (as 2) of each negative active material layer, subtracting the thickness of a negative current collector from the thickness of the negative electrode plate and dividing by 2), and loading into a true density tester (AccuPyc II 1340) sampleIn the sample cup, He is used for filling the negative active material layer in a closed sample cabin, so that the true volume V of the negative active material layer is measured, and finally, the porosity P of the negative active material layer is obtained through the following formula: p ═ 1-V/π d²×50×h)×100％。

4) Testing the wettability of the negative pole piece: the infiltration performance of the negative pole piece is represented by the absorption capacity of the negative pole piece on the electrolyte and the amount of the electrolyte absorbed in unit time.

Comparing the absorption speed of different negative pole pieces with the same weight on the same amount of electrolyte, the negative pole piece absorbing the electrolyte is firstly strong, 20 mug of electrolyte is dripped into the negative pole piece to test the corresponding infiltration time, and the pole piece with short infiltration time is strong in infiltration.

And (3) comparing the absorption storage amount of the electrolyte when different negative pole pieces with the same weight are soaked in the electrolyte, taking out the negative pole pieces at the same time after soaking for the same time, wherein the negative pole pieces with the heavier weight have strong absorption capacity, taking 5.0g of negative pole pieces respectively, soaking in the electrolyte for testing the absorption capacity of the electrolyte, and the negative pole pieces with the high absorption capacity of the electrolyte have strong wettability.

5) Ion conductivity of the negative electrode active material layer: baking the lithium ion battery for 4 hours at 60 ℃ before use; preparing a cutting die, 23mm by 35mm by 2 mm; the manufactured negative pole piece is cut into small pieces with the size larger than that of the cutting die by ceramic scissors and then placed on the cutting die for punching. And taking out the charged negative pole piece by using tweezers for later use. Then making the negative pole piece into a single-layer symmetrical battery; placing the symmetrical cell in an electrochemical workstation to test EIS, setting the frequency to be 0.5 Hz-200 kHz, and the disturbance voltage to be 10 mV; calculating the ionic resistance Rion of the negative pole piece through the intersection point of two straight lines in an Electrochemical Impedance Spectroscopy (EIS);

the ionic conductivity is reon x a (area of negative electrode sheet)/d (thickness of negative electrode sheet).

6) Method for testing capacity retention after cycling:

at 45 ℃, the lithium ion battery is charged to 4.45V at a constant current of 1C, then charged at a constant voltage until the current is 0.05C, and then discharged to 3.0V at a constant current of 1C, which is the first cycle. The lithium ion battery was cycled 400 and 800 times according to the above conditions. The capacity retention after cycling of the lithium ion battery was calculated by the following formula:

capacity retention rate after cycles (discharge capacity corresponding to the number of cycles/discharge capacity of the first cycle) × 100%.

7) The test method of the discharge percentage comprises the following steps:

at 25 ℃, the cells were charged to 4.45V at a 0.5C constant current, cut off at a constant voltage to 0.05C, then put to a 3.0V cut off at a 0.5C constant current, and recorded as 25 ℃ discharge capacity. Charging to 4.45V at a constant current of 0.5C and stopping at a constant voltage of 0.05C at 25 ℃, then placing the lithium ion battery in a thermostat at-20 ℃, standing for 2 hours, discharging to 3.0V at a constant current of 0.5C, and recording the discharge capacity at-20 ℃. The percent discharge of the lithium ion battery is calculated by the following formula:

the discharge percentage was (-20 ℃ discharge capacity/25 ℃ discharge capacity) × 100%.

Tables 1 and 2 show the respective parameters and evaluation results of examples 1 to 26 and comparative examples 1 to 13.

TABLE 1

TABLE 2

As can be seen from comparison between example 1 and comparative example 1, when a negative electrode sheet is prepared, the porosity of the negative active material layer can be increased, the infiltration amount of the electrolyte can be improved, the ionic conductivity and the discharge percentage of the negative active material layer can be increased, and the cycle capacity retention rate of the lithium ion battery can be improved by forming the porous dispersant with even a small amount of pore-forming agent. In addition, the porosity of the negative active material layer is improved by the porous dispersing agent, and meanwhile, the binding power of the negative pole piece is hardly lost.

In examples 1 to 5, as the amount of the pore-forming agent used increased, the porosity of the anode active material layer increased first and then decreased, this is because the number of pores formed is small when the amount of the pore-forming agent is small, the pore-forming agent agglomerates to form macropores as the amount of the pore-forming agent increases, the dispersant agglomerates to settle, the number of effective pores formed decreases, and in example 5, the cohesive force between the negative active material layer and the negative current collector is reduced by 2N/m due to poor agglomeration and dispersion of the dispersing agent, therefore, the amount of the pore-forming agent is preferably 0.01% to 0.3%, most preferably the amount used in example 3, at the moment, the porous dispersing agent has uniform pore distribution and the pore diameter of 50-500nm, so that effective bonding sites are not reduced, the bonding force is not deteriorated, the porosity is also highest, the corresponding negative pole piece has the best wettability and the highest ionic conductivity, and the low-temperature discharge percentage of the assembled lithium ion battery is also highest.

In examples 6 to 8, different types of pore formers were used to obtain a negative electrode sheet with high porosity, which also had excellent wettability, improved ionic conductivity, increased percentage of discharge, and improved retention of cycle capacity.

Comparing comparative examples 1 to 4, examples 3 and examples 9 to 11, it can be seen that, after the pore-forming agent is used to form the porous dispersant for different compaction densities, the porosity of the negative electrode sheet is obviously increased compared with the corresponding comparative examples, the wettability of the negative electrode sheet is enhanced, the ionic conductivity and the discharge percentage are obviously improved, and the retention rate of the cycle capacity is improved.

Comparing comparative examples 5 to 8 and examples 12 to 15, it can be seen that, for different coating amounts of the negative active material layer, after the pore-forming agent is used to form the porous dispersing agent, the porosity of the negative pole piece is obviously increased compared with the comparative examples, the wettability of the negative pole piece is enhanced, the ionic conductivity and the discharge percentage are obviously improved, and the retention rate of the cycle capacity is improved.

In examples 16 to 23, different types of dispersants are used, so that a negative electrode sheet with high porosity and high wettability can be obtained, the ionic conductivity and the discharge percentage are both significantly improved, and the retention rate of the circulating capacity is improved.

Comparing comparative examples 9 to 11 with examples 24 to 26, it can be seen that the use of dispersants of different mass contents in the negative active material layer can improve the porosity of the negative electrode sheet, the wettability is significantly improved, the ionic conductivity and the discharge percentage are improved, and the retention rate of the cycle capacity of the lithium ion battery is improved.

As can be seen from comparing examples 6 to 8 and comparative examples 12 to 13, by forming a porous dispersant, rather than forming a pore structure between materials in the negative electrode active material layer as in comparative examples 12 to 13 (using acetone and tetrahydrofuran as pore formers, respectively), the adhesion of the negative electrode sheet can be improved, thereby improving the capacity retention rate after 800 cycles. This is because the formation of the pore structure between the materials in the negative electrode active material layer is not favorable for the adhesion between the negative electrode active material particles in the negative electrode active material layer, and the falling off of the negative electrode active material particles is easily caused with the cycle of the electrochemical device (for example, after 600 cycles or more), for example, the falling off of the surface layer of the negative electrode active material layer is caused, so that the negative electrode active material layer having the surface layer of the pore structure is lost, and the cycle capacity retention rate of the electrochemical device is influenced.

The foregoing description is only exemplary of the preferred embodiments 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 technical 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 active material layer, and the negative active material layer comprises a negative active material, a porous dispersing agent and a binder.

2. The electrochemical device of claim 1, wherein the negative active material comprises graphite.

3. The electrochemical device according to claim 1, wherein the porous dispersant comprises at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, lithium hydroxypropyl carboxymethyl cellulose, sodium hydroxypropyl carboxymethyl cellulose, lithium hydroxyethyl carboxymethyl cellulose, sodium hydroxyethyl carboxymethyl cellulose, or hydroxyethyl carboxymethyl cellulose in a porous structure.

4. The electrochemical device of claim 1, wherein the binder comprises at least one of styrene butadiene rubber, polyacrylic acid, polyacrylate, polyimide, polyamideimide, polyvinylidene fluoride, polytetrafluoroethylene, aqueous acrylic resin, or polyvinyl formal.

5. The electrochemical device according to claim 1, wherein a mass ratio of the anode active material, the porous dispersant, and the binder in the anode active material layer is 96.6 to 98.6: 0.7-1.7: 0.7-1.7.

6. The electrochemical device according to claim 1, wherein the pore diameter of the pores in the porous dispersant is 50nm to 500 nm.

7. The electrochemical device according to claim 1, wherein the porosity of the negative active material layer is 25 to 40%.

8. An electronic device comprising the electrochemical device according to any one of claims 1 to 7.

9. A method of making a negative electrode sheet, comprising:

mixing a negative electrode active material, a dispersing agent, a pore-forming agent and a binder with deionized water according to a preset ratio to obtain a negative electrode slurry;

forming the negative electrode slurry on a negative electrode current collector, and drying to volatilize the pore-forming agent to obtain the negative electrode piece;

wherein the pore-forming agent comprises at least one of N-methyl pyrrolidone, dimethylacetamide, dimethylformamide or dimethyl sulfoxide, and the boiling point of the pore-forming agent is higher than that of water by more than 40 ℃.

10. The method of claim 9, wherein at least one of:

the mass ratio of the negative electrode active material to the dispersant to the pore-forming agent to the binder is 96.6-98.6: 0.7-1.7: 0.01-0.3: 0.7 to 1.7;

the negative active material includes graphite;

the dispersant comprises at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, lithium hydroxypropyl carboxymethyl cellulose, sodium hydroxypropyl carboxymethyl cellulose, lithium hydroxyethyl carboxymethyl cellulose, sodium hydroxyethyl carboxymethyl cellulose or hydroxyethyl carboxymethyl cellulose;

the binder comprises at least one of styrene butadiene rubber, polyacrylic acid, polyacrylate, polyimide, polyamide imide, polyvinylidene fluoride, polytetrafluoroethylene, water-based acrylic resin or polyvinyl formal.