Negative electrode, electrochemical device and electronic device

Abstract

The present invention provides a negative electrode, an electrochemical device and an electronic device, wherein the negative electrode comprises a negative electrode active material layer, wherein the negative electrode active material layer comprises at least two types of negative electrode active materials and at least two types of binders, wherein the at least two types of binders include lithium carboxymethyl cellulose. The negative electrode of the present invention has good low temperature performance, and the electrochemical device prepared by using the negative electrode has a high capacity retention rate at both 0°C and -20°C.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries and relates to a negative electrode, an electrochemical device and an electronic device. Background Art

[0002] Lithium-ion battery is a secondary battery (rechargeable battery) that mainly relies on the movement of lithium ions between the positive and negative electrodes to work. + Insertion and deinsertion back and forth between the two electrodes: During charging, Li + It is deintercalated from the positive electrode and then embedded into the negative electrode through the electrolyte, and the negative electrode is in a lithium-rich state; the opposite is true during discharge. Lithium-ion batteries have the advantages of high energy density, high voltage platform, long cycle life, and no memory effect. They have been widely used in new energy vehicles, mobile phones, power tools, electric bicycles and other fields.

[0003] Generally, lithium-ion batteries need to be used in a suitable temperature environment. If the temperature is too low (for example, below 0°C), the migration impedance in the lithium-ion battery will increase and the migration speed will decrease, resulting in a lower capacity retention rate of the lithium-ion battery at low temperatures, increased internal resistance of the battery, lower battery output power, and shorter cycle life. In severe cases, lithium metal precipitation on the negative electrode plate may occur, leading to safety problems. Summary of the invention

[0004] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a negative electrode, an electrochemical device and an electronic device. The present invention adopts a mixed negative electrode active material and a specific binder for use with the negative electrode active material. The two act synergistically to improve the transmission capacity of active ions (such as lithium ions) at low temperatures, improve the low temperature performance of the negative electrode, and enable the electrochemical device to have a good capacity retention rate at low temperatures (-20°C).

[0005] To achieve this object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a negative electrode, comprising a negative electrode active material layer, the negative electrode comprising a negative electrode active material layer, the negative electrode active material layer comprising at least two types of negative electrode active materials and at least two types of binders, wherein the at least two types of binders include lithium carboxymethyl cellulose.

[0007] In the present invention, "two types" refers to two substances that are not completely the same, that is, the two substances have different chemical formulas, or the two substances have the same chemical formula but different structures.

[0008] The negative electrode of the present invention adopts a mixed negative electrode active material, which uses two or more active materials in combination, and is combined with a specific mixed binder so that the negative electrode active material and the binder act synergistically to improve the transmission capacity of active ions (such as lithium ions) at low temperatures, thereby improving the low temperature performance of the negative electrode.

[0009] The binder of the present invention is a mixed binder, including lithium carboxymethyl cellulose and a second binder. The lithium carboxymethyl cellulose has good low-temperature performance. When the lithium carboxymethyl cellulose is combined with other binders and mixed with the mixed negative electrode active material in the present invention, the two act synergistically and have better lithium ion transmission capacity, which can further improve the low-temperature performance of the negative electrode.

[0010] Preferably, the negative electrode active material includes a combination of at least two of natural graphite, artificial graphite, hard carbon or silicon, for example, it can be a combination of natural graphite and artificial graphite, a combination of artificial graphite and hard carbon, a combination of hard carbon and silicon, a combination of artificial graphite, hard carbon and silicon, a combination of natural graphite, artificial graphite, hard carbon and silicon, etc.

[0011] Preferably, the at least two types of negative electrode active materials are a first active material and a second active material, the first active material is artificial graphite, and the second active material is hard carbon. Under the same energy density, a mixture of artificial graphite and hard carbon has better low-temperature performance.

[0012] Preferably, the mass ratio of the first active material to the second active material is (8:2) to (9.5:0.5), for example, 9.5:0.5, 9:1, 8.5:1.5 or 8:2, etc. Within this range, the combination of the two active materials has the best effect.

[0013] As a preferred technical solution of the negative electrode of the present invention, the at least two types of binders are a first binder and a second binder, the first binder is lithium carboxymethyl cellulose, and the second binder is any one or a combination of at least two of styrene-butadiene rubber, styrene-acrylic emulsion, polyacrylic acid, polyacrylonitrile or sodium alginate, for example, it can be a combination of styrene-butadiene rubber and styrene-acrylic emulsion, a combination of styrene-acrylic emulsion and polyacrylic acid, a combination of styrene-butadiene rubber and sodium alginate, or a combination of styrene-acrylic emulsion, polyacrylic acid and polyacrylonitrile, preferably styrene-acrylic emulsion.

[0014] In the present invention, lithium carboxymethyl cellulose and styrene acrylic emulsion are preferably used in combination. On the one hand, the viscosity and stability of the slurry can be improved to ensure the stability and coating of the slurry during the coating process; on the other hand, the Li-ionized groups on the surface of the binder can serve as the transmission sites of Li ions to shorten the transmission path of Li ions, and the surface polar functional groups can be effectively adsorbed on the graphite surface and fully adsorb the electrolyte, thereby reducing the transmission impedance of Li ions, thereby reducing the internal resistance of the battery and improving the low-temperature performance.

[0015] Preferably, the mass ratio of the first binder to the second binder is (1:3) to (3:1), for example, it can be 1:1, 1:2, 1:3, 1.2:1.8, 1.4:1.6, 1.5:1.2, 2:1 or 3:1, etc.

[0016] In the present invention, the lithium carboxymethyl cellulose is present in a suitable proportion in the binder, which can further reduce the amount of binder added, reduce the internal resistance of the battery, and improve the low-temperature transmission of lithium ions.

[0017] Preferably, based on the mass of the negative electrode active material layer, the total amount of the at least two types of binders is 0.8wt% to 4wt%, for example, it can be 0.8wt%, 1wt%, 1.2wt%, 1.4wt%, 1.6wt%, 1.8wt%, 2wt%, 2.2wt%, 2.4wt%, 2.6wt%, 2.8wt%, 3wt%, 3.2%, 3.4%, 3.6%, 3.8% or 4wt%, etc.

[0018] The present invention preferably uses a lower binder content. The specific binder and the negative electrode active material are matched with a lower binder content, which can further improve the lithium ion transmission capacity at low temperatures, reduce the low-temperature internal resistance of the battery, and improve the low-temperature performance.

[0019] As a preferred technical solution of the negative electrode of the present invention, the negative electrode active material layer also includes a conductive agent.

[0020] Preferably, the conductive agent includes at least one of conductive carbon black, conductive graphite, multi-walled carbon nanotubes, single-walled carbon nanotubes, graphene and carbon fibers, and is preferably a combination of at least two of them, for example, it can be a combination of conductive carbon black and conductive graphite, a combination of conductive graphite and multi-walled carbon nanotubes, a combination of single-walled carbon nanotubes and graphene, a combination of graphene and carbon fibers, a combination of conductive carbon black, single-walled carbon nanotubes and graphene, a combination of conductive graphite, single-walled carbon nanotubes, graphene and carbon fibers, etc., preferably a combination of at least two.

[0021] Preferably, the conductive agent is conductive carbon black and single-walled carbon nanotubes. The use of this mixed conductive agent can construct a multi-dimensional conductive network, reduce the electrode sheet resistance, reduce the internal resistance of the battery cell, and improve the electrochemical performance of the negative electrode.

[0022] Preferably, the content of the conductive agent is 0.2wt% to 2wt%, for example, it can be 0.2wt%, 0.4wt%, 0.6wt%, 0.8wt%, 1wt%, 1.2wt%, 1.4wt%, 1.6wt%, 1.8wt% or 2wt%.

[0023] Preferably, the mass ratio of the negative electrode active material, the conductive agent and the binder is (93 to 98): (0.2 to 2): (0.8 to 4), wherein the content of the negative electrode active material (93 to 98) can be, for example, 93, 93.5, 94, 94.5, 95, 95.5, 96, 96.5, 97 or 98, etc., the content of the conductive agent (0.2 to 2) can be, for example, 0.2, 0.5, 1, 1.2, 1.5, 1.8 or 2, etc., and the content of the binder (0.8 to 4) can be, for example, 0.8, 1, 1.5, 2, 2.5, 3 or 4, etc. Within this ratio range, the negative electrode performance is optimal.

[0024] The present invention does not limit the preparation method of the negative electrode. For example, a certain proportion of negative electrode active material, conductive agent and binder can be mixed in a solvent to obtain negative electrode slurry, and then the prepared negative electrode slurry is evenly coated on the surface of the current collector and dried to obtain the negative electrode.

[0025] Preferably, the solvent comprises water.

[0026] Preferably, the current collector comprises copper foil.

[0027] In a second aspect, the present invention provides an electrochemical device, wherein the negative electrode of the electrochemical device adopts the negative electrode according to the first aspect.

[0028] The negative electrode in the electrochemical device of the present invention adopts the negative electrode described in the first aspect, and the prepared electrochemical device has good electrochemical performance even at low temperatures.

[0029] In some embodiments, the electrochemical device includes a lithium ion battery, but the present application is not limited thereto.

[0030] Preferably, the electrochemical device further comprises a positive electrode, a separator and an electrolyte.

[0031] Preferably, the positive electrode comprises a current collector and a positive electrode active material layer disposed on the current collector.

[0032] Exemplarily, the current collector of the positive electrode may be an aluminum foil.

[0033] Preferably, the positive electrode active material layer includes a positive electrode active material, a binder and a conductive agent.

[0034] In some embodiments, the types of positive electrode active materials include but are not limited to ternary positive electrode materials, lithium iron phosphate, lithium cobalt oxide and lithium manganese oxide, etc.; the ternary positive electrode material can be, for example, a nickel-cobalt-manganese ternary positive electrode material or a nickel-cobalt-aluminum ternary positive electrode material.

[0035] Exemplarily, the binder of the positive electrode includes polyvinylidene fluoride (PVDF).

[0036] Preferably, the conductive agent of the positive electrode includes conductive carbon black and / or conductive carbon tubes.

[0037] Preferably, the mass ratio of the positive electrode active material, the binder and the conductive agent is (95 to 98):(1 to 3):(1 to 3), wherein the content of the positive electrode active material (95 to 98) can be, for example, 95, 95.5, 96, 96.5, 97 or 98, etc., the content of the binder (1 to 3) can be, for example, 1, 1.5, 2, 2.5 or 3, etc., and the content of the conductive agent (1 to 3) can be, for example, 1, 1.5, 2, 2.5 or 3, etc.

[0038] The present invention does not limit the preparation method of the positive electrode. Exemplarily, the positive electrode active material, the conductive agent and the binder are added to the solvent in a certain ratio and stirred, and the fluidity of the slurry is adjusted to obtain the positive electrode slurry. The prepared positive electrode slurry is then coated on the current collector, dried and rolled to obtain the positive electrode.

[0039] Preferably, the electrolyte comprises a lithium salt and a non-aqueous solvent.

[0040] Preferably, the lithium salt includes any one of LiPF6, LiBF4, LiCH3SO3 or LiSiF6, or a combination of at least two thereof, such as a combination of LiPF6 and LiBF4, a combination of LiCH3SO3 and LiSiF6, a combination of LiBF4 and LiCH3SO3, or a combination of LiPF6, LiBF4, LiCH3SO3 and LiSiF6, etc.

[0041] Preferably, the non-aqueous solvent includes any one of a carbonate compound, a carboxylate compound and an ether compound or a combination of at least two of them, for example, it can be a combination of a carbonate compound and a carboxylate compound, a combination of a carboxylate compound and an ether compound, or a combination of a carbonate compound, a carboxylate compound and an ether compound, etc.

[0042] In some embodiments, the carbonate compound includes any one of diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC) or ethylene carbonate (EC), or a combination of at least two thereof, for example, a combination of diethyl carbonate and dimethyl carbonate, a combination of dimethyl carbonate and dipropyl carbonate, a combination of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate, or a combination of diethyl carbonate, dimethyl carbonate, dipropyl carbonate and ethylene carbonate, etc.

[0043] In some embodiments, the carboxylate compound includes any one of methyl acetate, ethyl acetate, methyl propionate, ethyl propionate or methyl formate, or a combination of at least two thereof, such as a combination of methyl acetate and ethyl acetate, a combination of ethyl acetate and methyl propionate, a combination of ethyl propionate and methyl formate, etc.

[0044] In some embodiments, the ether compound includes any one of dibutyl ether, tetraglyme, ethoxymethoxyethane or tetrahydrofuran, or a combination of at least two thereof, for example, a combination of dibutyl ether and tetraglyme, a combination of tetraglyme and ethoxymethoxyethane, or a combination of tetraglyme, ethoxymethoxyethane and tetrahydrofuran, etc.

[0045] As a preferred technical solution of the electrochemical device described in the present invention, the diaphragm includes any one of polyethylene, polypropylene, polyvinylidene fluoride or polyimide, or a combination of at least two of them, for example, it can be a diaphragm of polyethylene and polypropylene, a diaphragm of polypropylene and polyvinylidene fluoride, or a diaphragm of polyethylene, polypropylene, polyvinylidene fluoride and polyimide, etc.

[0046] In the present invention, the method of assembling an electrochemical device using the negative electrode is a prior art, and those skilled in the art can refer to the method disclosed in the prior art for assembly. Taking a lithium-ion battery as an example, the positive electrode, the separator, and the negative electrode are wound or stacked in sequence to form a battery core, which is loaded into a battery shell, and an electrolyte is injected, formed, and packaged to obtain an electrochemical device.

[0047] In a third aspect, the present invention provides an electronic device, comprising the electrochemical device according to the second aspect.

[0048] Exemplarily, the electronic device of the present invention may be a mobile computer, a portable phone, a memory card, a liquid crystal television, a car, a motorcycle, a motor, a clock, a camera, etc.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] (1) The negative electrode of the present invention adopts a mixed negative electrode active material, which uses two or more active materials in combination, and is combined with a specific mixed binder, so that the negative electrode active material and the binder work synergistically to improve the transmission capacity of active ions (such as lithium ions) at low temperatures, thereby improving the low temperature performance of the negative electrode.

[0051] (2) The binder of the present invention is a mixed binder, including lithium carboxymethyl cellulose and a second binder. The lithium carboxymethyl cellulose has good low-temperature performance. When the lithium carboxymethyl cellulose is combined with other binders and mixed with the mixed negative electrode active material of the present invention, the two act synergistically and have better lithium ion transmission capacity, which can further improve the low-temperature performance of the negative electrode.

[0052] The numerical range described in the present invention not only includes the point values ​​listed above, but also includes any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range. DETAILED DESCRIPTION

[0053] The technical solution of the present invention is further illustrated below through specific implementation methods.

[0054] Example 1

[0055] This embodiment provides a negative electrode, which includes a current collector and a negative electrode active material layer arranged on the surface of the current collector, the current collector is copper foil, the negative electrode active material layer includes a negative electrode active material, a conductive agent and a binder, and the mass ratio of the negative electrode active material, the binder and the conductive agent is 96.2:3.3:0.5.

[0056] The negative electrode active material includes a first active material and a second active material, the first active material is artificial graphite, the second active material is hard carbon, the mass ratio of artificial graphite to hard carbon is 9:1, the binder includes a first binder and a second binder, the first binder is lithium carboxymethyl cellulose, the second binder is styrene acrylic emulsion, the mass ratio of lithium carboxymethyl cellulose to styrene acrylic emulsion is 1.5:1.8, the conductive agent is conductive carbon black, and the mass ratio of artificial graphite, hard carbon, lithium carboxymethyl cellulose, styrene acrylic emulsion and conductive carbon black in the negative electrode active material layer is 86.58:9.62:1.5:1.8:0.5.

[0057] This embodiment also provides a method for preparing the above-mentioned negative electrode, and the preparation method comprises:

[0058] Artificial graphite, hard carbon, lithium carboxymethyl cellulose, styrene acrylic emulsion and conductive carbon black in a mass ratio of 86.58:9.62:1.5:1.8:0.5 are dispersed in water, stirred evenly and then coated on the surface of copper foil, and dried to obtain the negative electrode.

[0059] This embodiment also provides an electrochemical device and a preparation method thereof, wherein the electrochemical device comprises a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode adopts the above-mentioned negative electrode, and the preparation method comprises:

[0060] (1) Preparation of positive electrode: Lithium iron phosphate, PVDF and conductive carbon black in a mass ratio of 96:2:2 were dispersed in NMP, stirred evenly and then coated on the surface of aluminum foil, and dried for 40 min to obtain a positive electrode.

[0061] (2) Assembly of the electrochemical device: The electrolyte used is a LiPF6 electrolyte with a concentration of 1 M, and the solvent in the electrolyte is EC, EMC and DMC with a mass ratio of 1:1:1; the diaphragm uses a PE-based film with a thickness of 8 μm, and then the above-mentioned positive and negative electrodes are assembled and packaged with an aluminum-plastic film to obtain the electrochemical device.

[0062] Examples 2 to 11 and Comparative Examples 1 to 2 are based on the steps of Example 1 with parameter changes, and the specific changed parameters are shown in Table 1.

[0063] Capacity retention test:

[0064] The electrochemical devices of the embodiment and the comparative example were placed in a Shenghong BTS-5V50A48CH test cabinet at 25°C for 1 / 3C charge and discharge, and the discharge capacity was recorded as C0. Then, the electrochemical device was charged at 1 / 3C at 25°C and discharged at 1 / 3C at 0°C, and the discharge capacity was recorded as C1.

[0065] 0℃ capacity retention rate = C1 / C0×100%

[0066] Similarly, the electrochemical devices of the embodiment and the comparative example were placed in a test cabinet at -20°C for testing to obtain the -20°C capacity retention rate. The test results are shown in Table 2.

[0067] Table 1

[0068]

[0069] Table 2

[0070]

[0071]

[0072] By comparing Example 1 with Examples 3 to 6, it can be seen that the selection and content of the first active material and the second active material in the present invention will affect the low-temperature performance of the negative electrode. When the negative electrode active material is a mixture of artificial graphite and hard carbon in a suitable ratio, the low-temperature stability of the negative electrode is the highest; under the same negative electrode design conditions, when artificial graphite and hard carbon are used, and when the mass ratio of artificial graphite to hard carbon is (9.5:0.5) to (8:2), the hard carbon particles can be distributed between the particles of artificial graphite, shortening the transmission path of lithium ions. , so the performance is better; therefore, the -20°C capacity retention rate of Example 1 is higher than that of Examples 3 to 6.

[0073] By comparing Example 1 with Example 7, and Example 2 with Example 8, it can be seen that the content of the binder in the present invention affects the low-temperature performance of the negative electrode. When the total amount of the binder is between 0.8wt% and 3wt%, the low-temperature performance of the negative electrode is optimal; when the content of the binder is high, the excess binder will coat the surface of the negative electrode active material, blocking the embedding of lithium ions in the negative electrode. When the content of the binder is low, the negative electrode active material will peel off from the copper foil and cannot be assembled into a battery device. Therefore, the capacity retention rate of Examples 1 to 2 is higher than that of Examples 7 and 8 at 0°C and -20°C.

[0074] By comparing Example 1 with Examples 9 to 11, it can be seen that in the negative electrode of the present invention, when the binder contains lithium carboxymethyl cellulose and styrene acrylic emulsion, the performance of the negative electrode is the best, and the lithium carboxymethyl cellulose and other binders have the most suitable ratio; when the ratio of lithium carboxymethyl cellulose is high, the excess lithium carboxymethyl cellulose will be distributed between the active particles, hindering the transmission of lithium ions, and when the ratio of lithium carboxymethyl cellulose is low, it cannot have a thickening effect, reducing the stability of the active material slurry, and cannot be assembled into a battery device. Therefore, the capacity retention rate of Example 1 is higher than that of Examples 9 to 11 at 0°C and -20°C.

[0075] By comparing Example 1 with Comparative Examples 1 and 2, it can be seen that the synergistic effect of the mixed negative electrode active material and the mixed binder in the present invention can improve the low-temperature performance of the negative electrode. When the negative electrode contains only one negative electrode active material, or the binder of the negative electrode does not contain lithium carboxymethyl cellulose, the low-temperature performance of the negative electrode will decrease. This is because under the same electrode design, a single active material cannot fully fill the negative electrode electrode space, increasing the lithium ion transmission radius, and the binder of the non-lithiation group cannot provide an effective transmission channel for lithium ion migration. Therefore, the capacity retention rates of Comparative Examples 1 and 2 at 0°C and -20°C are both worse than those of Example 1.

[0076] From the above examples 1 to 11, it can be seen that the present invention adopts a mixed negative electrode active material and uses a specific binder in combination with the negative electrode active material. The synergistic effect of the two can improve the transmission capacity of active ions (such as lithium ions) at low temperatures, improve the low temperature performance of the negative electrode, and enable the electrochemical device to have a good capacity retention rate at low temperatures (-20°C).

[0077] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A negative electrode, characterized in that: The negative electrode comprises a negative electrode active material layer, wherein the negative electrode active material layer comprises at least two types of negative electrode active materials and at least two types of binders, Based on the mass of the negative electrode active material layer, the total amount of the at least two types of binders is 2.2wt% to 3.3wt%; The at least two types of binders are a first binder and a second binder, the first binder is lithium carboxymethyl cellulose, and the second binder is styrene acrylic emulsion; The mass ratio of the first binder to the second binder is (1:3) to (3:1).

2. The negative electrode according to claim 1, characterized in that The negative electrode active material includes a combination of at least two of natural graphite, artificial graphite, hard carbon or silicon.

3. The negative electrode according to claim 1, characterized in that The at least two types of negative electrode active materials are a first active material and a second active material, and the first active material and the second active material satisfy at least one of the following conditions (a) to (b): (a) the first active material is artificial graphite, and the second active material is hard carbon; (b) The mass ratio of the first active material to the second active material is (8:2) to (9.5:0.5).

4. The negative electrode according to claim 1, characterized in that The negative electrode active material layer further includes a conductive agent.

5. The negative electrode according to claim 4, characterized in that The conductive agent satisfies at least one of the following conditions (f) to (h): (f) the conductive agent comprises at least one of conductive carbon black, conductive graphite, multi-walled carbon nanotubes, single-walled carbon nanotubes, graphene or carbon fiber; (g) the conductive agent comprises a combination of at least two of conductive carbon black, conductive graphite, multi-walled carbon nanotubes, single-walled carbon nanotubes, graphene or carbon fibers; (h) The content of the conductive agent is 0.2 wt % to 2 wt %.

6. The negative electrode according to claim 4, characterized in that The mass ratio of the negative electrode active material, the conductive agent and the binder is (93 to 98):(0.2 to 2):(0.8 to 4).

7. An electrochemical device, characterized in that: The negative electrode of the electrochemical device adopts the negative electrode according to any one of claims 1 to 6.

8. An electronic device, characterized in that: The electronic device comprises the electrochemical device according to claim 7.

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