CN114824164A - Lithium ion battery cathode, preparation method thereof and lithium ion battery - Google Patents
Lithium ion battery cathode, preparation method thereof and lithium ion battery Download PDFInfo
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- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/133—Electrodes based on carbonaceous material, e.g. graphite-intercalation compounds or CFx
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- H01M10/00—Secondary cells; Manufacture thereof
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- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/139—Processes of manufacture
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- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/362—Composites
- H01M4/366—Composites as layered products
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- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
- H01M4/583—Carbonaceous material, e.g. graphite-intercalation compounds or CFx
- H01M4/587—Carbonaceous material, e.g. graphite-intercalation compounds or CFx for inserting or intercalating light metals
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- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
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- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
- H01M4/624—Electric conductive fillers
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- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
- H01M2004/027—Negative electrodes
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- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Abstract
The invention relates to a lithium ion battery negative electrode, a preparation method thereof and a lithium ion battery, wherein the negative electrode comprises a negative electrode current collector and a plurality of negative electrode active material layers which are stacked on the negative electrode current collector, and the negative electrode active material layers contain negative electrode active substances, a conductive agent and a binder; the content of the binder in the negative active material layer in direct contact with the negative current collector is 2-3 wt%, the content of the conductive agent is 1.0-2.0 wt%, the content of the binder in each negative active material layer along the direction far away from the negative current collector is gradually reduced, and the content of the conductive agent is gradually increased. When the lithium ion battery cathode is used for a lithium ion battery, the battery impedance can be reduced, and the cycle stability of the battery is improved.
Description
Technical Field
The application relates to the field of new energy, in particular to a lithium ion battery cathode, a preparation method thereof and a lithium ion battery.
Background
The transformation from the traditional fuel vehicle to the new energy vehicle is tendency, and the rapid increase of the new energy vehicle demands puts higher and higher requirements on the power battery. The battery performance indexes such as high energy density, high safety, ultra-long service life and the like force the development of the battery to expand from an apparent layer to a more microscopic layer, and the research and the design of a fine pole piece are in the forefront. Along with the continuous improvement of the performance requirements of the battery, the electrode is continuously thickened, and the electrode structure needs more refined design and representation to ensure the low-impedance diffusion and transmission of lithium ions and electrons in the charging and discharging process of the battery, so that the charging and discharging efficiency of the battery is improved. The charging and discharging performance of the existing high energy density power battery needs to be further improved.
Disclosure of Invention
The invention aims to provide a lithium ion battery cathode, a preparation method thereof and a lithium ion battery, which can reduce battery impedance and improve the cycle stability of the battery when being used for the lithium ion battery.
In order to achieve the above object, a first aspect of the present invention provides a lithium ion battery negative electrode including a negative electrode current collector and a plurality of negative electrode active material layers stacked in sequence on the negative electrode current collector, the plurality of negative electrode active material layers containing a negative electrode active material, a conductive agent, and a binder;
the content of the binder in the negative active material layer in direct contact with the negative current collector is 2-3 wt%, the content of the conductive agent is 1-2 wt%, the content of the binder in each negative active material layer along the direction far away from the negative current collector is gradually reduced, and the content of the conductive agent is gradually increased.
In a second aspect, the present invention provides a method of preparing a negative electrode for a lithium ion battery, the method comprising: coating a plurality of negative active material sizing agents on a current collector in sequence to obtain a pole piece coated with a plurality of negative active material sizing agent layers, and drying and rolling the pole piece to obtain the pole piece;
wherein the negative electrode active material slurry contains the negative electrode active material, the conductive agent, and the binder; the content of the conductive agent is 1-2 parts by weight and the content of the binder is 2-3 parts by weight relative to 100 parts by weight of the negative electrode active material, the content of the binder in each negative electrode active material slurry layer along the direction far away from the negative electrode current collector is gradually reduced, and the content of the conductive agent is gradually increased.
In a third aspect, the invention provides a lithium ion battery comprising the negative electrode provided in the first aspect of the invention.
Through the technical scheme, the conductive network in the lithium ion battery cathode is optimized in a layered mode, so that the lithium ion battery has lower resistance and the cycle stability is ensured.
Additional features and advantages of the invention will be set forth in the detailed description which follows.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention and not to limit the invention. In the drawings:
fig. 1 is a schematic structural diagram of an embodiment of an electrode of a lithium ion battery according to the present invention.
Detailed Description
The following detailed description of embodiments of the invention refers to the accompanying drawings. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.
As shown in fig. 1, a first aspect of the present invention provides a lithium ion battery negative electrode, including a negative electrode current collector and a plurality of negative electrode active material layers sequentially stacked on the negative electrode current collector, the plurality of negative electrode active material layers containing a negative electrode active material, a conductive agent, and a binder; the content of the binder in the negative active material layer in direct contact with the negative current collector is 2-3 wt%, the content of the conductive agent is 1-2 wt%, the content of the binder in each negative active material layer along the direction far away from the negative current collector is gradually reduced, and the content of the conductive agent is gradually increased.
The lithium ion battery cathode has a better conductive agent-binder layered distribution network, can effectively reduce the transmission impedance of electrons in the charging and discharging process and improve the charging and discharging performance of the battery, and can form high-strength connection among particles of a cathode active material and between the particles of the active material and a current collector, thereby improving the stability of the electrode in the charging and discharging process and improving the cycle performance of the battery.
In one embodiment of the present invention, t as defined by the following formula (1) satisfies 0.01< t.ltoreq.0.5, and d as defined by the following formula (2) satisfies 1. ltoreq. d.ltoreq.15;
t=(α i-1 -α i )f i /(f i-1 ×α i-1 ) Formula (1)
d=(β i ×S i )/[(β i -β i-1 )S i-1 ]Formula (2)
Wherein t is a dimensionless quantity defined by formula (1), and d is a dimensionless quantity defined by formula (2); i is any integer from 2 to n, and n is the number of the anode active material layers;
α i represents the ratio of the weight of the binder of the i-th negative electrode active material layer to the weight of the negative electrode active material in the direction away from the negative electrode current collector, in%,% β i Denotes the ratio of the weight of the conductive agent of the i-th negative electrode active material layer to the weight of the negative electrode active material in the direction away from the negative electrode current collector, in%, f i Denotes a peeling force required for peeling the i-th layer of the negative electrode active material layer from the i-1 th layer of the negative electrode active material layer in a direction away from the negative electrode current collector, and has a unit of N/m, and f when i =2 i-1 Showing the negative electrode active material of the 1 st layer in a direction away from the negative electrode current collectorThe stripping force required for stripping the material layer from the negative electrode current collector is f when i is more than 2 i-1 Denotes a peeling force required for peeling the i-1 st layer negative electrode active material layer from the i-2 nd layer negative electrode active material layer in a direction away from the negative electrode current collector, and has a unit of N/m, S i Represents the conductivity of the i-th negative electrode active material layer in the direction away from the negative electrode current collector, and has a unit of Ω · cm.
The inventor of the present disclosure finds in research that when t is greater than or equal to 0.01 and less than or equal to 0.5, the negative active material particles can form an optimal bonding effect, thereby avoiding the situations of capacity reduction, lithium precipitation during charging, even formation of a lithium dendrite piercing diaphragm, and the like of the battery caused by the falling of the negative active material particles in the battery manufacturing process and the long-term circulation process, and further avoiding the situation that the impedance of the battery is too large, so as to generate an optimal bonding-conductive network, ensure the safe operation of the battery, and effectively prolong the service life of the battery.
When d is more than or equal to 1 and less than or equal to 15, the contact impedance of the upper layer (close to the side of the diaphragm) of the negative active material particles can be effectively reduced, the risk of lithium precipitation in the charging process can be avoided, meanwhile, the thermal runaway of the battery in the charging and discharging process can be effectively prevented, the cycle performance and the safety performance of the battery can be improved, the conductive agent is matched with the binder, the proportion of the active material is not influenced, an optimal conductive network can be formed, and the performance of the pole piece can reach the optimal state.
In one embodiment of the present invention, 0.04. ltoreq. t.ltoreq.0.38, preferably 0.2. ltoreq. t.ltoreq.0.3, and 2.08. ltoreq. d.ltoreq.13.8, preferably 3. ltoreq. d.ltoreq.10, and when within the above range, it is possible to further improve the cycle performance and stability of the battery and improve the charge and discharge performance of the battery.
In one embodiment of the present invention, 0<α j +β j ≤5，0.2≤f j ≤4，0.2≤S j Less than or equal to 10, wherein j is any integer from 1 to n; alpha is alpha j Represents the ratio of the weight of the binder of the jth anode active material layer to the weight of the anode active material in the direction away from the anode current collector, in%, [ beta ] j A conductive agent indicating the jth negative active material layer in a direction away from the negative current collectorThe ratio of the weight of (b) to the weight of the negative electrode active material, in%, when j =1, f j Denotes a peeling force required for peeling the 1 st layer negative active material layer from the negative current collector in a direction away from the negative current collector, and f is not less than 2 j Denotes a peeling force in units of N/m, S required for peeling the jth layer of the negative electrode active material layer from the jth-1 layer of the negative electrode active material layer in a direction away from the negative electrode current collector j Represents the electrical conductivity of the jth anode active material layer in the direction away from the anode current collector, and has a unit of Ω · cm.
In the present invention, the peel force of each layer was tested by the following method: after disassembling the battery product, taking out the negative pole piece, washing the negative pole piece with one or more of organic solvents such as dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate and the like, naturally airing the negative pole piece in a clean environment to prepare a pole piece to be tested, and sequentially testing the stripping force of each negative pole active material layer of the pole piece to be tested according to the sequence from the ith negative pole active material layer to the 1 st negative pole active material layer. Conductivity S of ith anode active material layer i The negative pole piece is taken out after a battery product is disassembled, washed for a plurality of times by one or more of organic solvents such as dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate and the like, naturally dried in a clean environment to obtain the pole piece to be tested, and subjected to conductivity test according to the sequence from the ith negative pole active material layer to the 1 st negative pole active material layer. When the lithium ion battery negative electrode is in the range, the lithium ion battery negative electrode has better electrical connection and physical connection among layers of the pole piece, so that the transmission impedance of electrons in the charging and discharging process can be further reduced, the charging and discharging performance of the battery can be improved, higher-strength connection can be formed among particles of a negative electrode active material and between the particles of the active material and a current collector, the stability of the electrode in the charging and discharging process can be further improved, and the cycle performance of the battery can be improved.
In one embodiment of the present invention, the alpha is 3. ltoreq. alpha j +β j ≤5，0.2≤f j ≤2，0.2≤S j Less than or equal to 5, when inWithin the above range, the cycle performance and stability of the lithium ion battery containing the negative electrode of the present invention can be further improved, and the charge and discharge performance and cycle performance of the battery can be effectively improved.
In a specific embodiment of the invention, n is more than or equal to 2 and less than or equal to 5, so as to meet the precision requirement of equipment for preparing the cathode, and be beneficial to ensuring that the lithium ion battery has better cycle performance and stability.
According to the present invention, the thickness of the anode active material layer and the thickness of the single anode active material layer may be varied within a wide range, respectively. In a preferred embodiment, the total thickness of the plurality of anode active material layers is 30 to 200 μm, more preferably 50 to 120 μm, and the thickness of the single anode active material layer is 6 to 150 μm, more preferably 25 to 60 μm. When the thickness of each layer is within the above range, the cycle performance and stability of the battery can be further improved, and the charge and discharge performance of the battery can be improved. The thickness of each anode active material layer may be the same or different, and preferably the same.
In one embodiment of the present invention, the single layer of the negative electrode active material layer has an areal density of 0.3 to 15g/cm 2 Preferably 3 to 15g/cm 2 Better conductive agent binder networks can be obtained in high energy density battery products.
According to the present invention, the negative electrode collector may be any negative electrode collector known to those skilled in the art, and may be selected from, for example, a copper foil, a polymer film having a metal passivation layer, a metal film having a metal passivation layer, and the like. In one embodiment of the present invention, the negative electrode current collector is selected from a copper foil, an aluminum foil, a PP film with a metal passivation layer, a PE film with a metal passivation layer, or a metal film with a metal passivation layer; the thickness of the negative electrode current collector may also vary within a wide range, and may be, for example, 3 to 15 μm, preferably 4.5 to 8 μm.
According to the present invention, the negative active material may also be any kind of negative active material known to those skilled in the art, and may be selected from one or more of graphite, hard carbon, soft carbon, mesocarbon microbeads, silicon-carbon composites, and silicon-and germanium-based materials, for example. In one embodiment, the negative electrode may include, but is not limited to, one or a combination of graphite negative electrode, silicon negative electrode, lithium titanate negative electrode, germanium-based negative electrode, tin-based negative electrode, hard carbon negative electrode, soft carbon negative electrode, and mesocarbon microbead negative electrode.
According to the present invention, the binder may also be any kind of binder known to those skilled in the art as an electrode auxiliary material and an additive for physical connection between active materials and/or between active materials and a current collector, and may be selected from one or more of styrene-butadiene rubber, polyacrylic acid, polyamide, polyvinyl alcohol, polyethylene imine and polyimide, for example.
According to the present invention, the conductive agent may also be selected from any kind of conductive agent known to those skilled in the art, and may be a combination including, but not limited to, one or more of a branched conductive agent, a one-dimensional chain conductive agent, a two-dimensional sheet conductive agent, a polymer conductive agent, a carbon black conductive agent, a graphite conductive agent, and the like, and more preferably, may be selected from one or more of superconducting carbon black, acetylene black, ketjen black, carbon nanotubes, carbon fibers, sheet graphite, graphene, polyacetylene, polythiophene, polypyrrole, and polyaniline.
In one embodiment of the present invention, the negative active material layer further contains a dispersant, which may be conventionally used by those skilled in the art, and may include, for example, but not limited to, one or more of carboxymethyl cellulose, polyacrylamide, sodium dodecyl sulfate, and methyl amyl alcohol.
In a second aspect, the present invention provides a method for preparing the negative electrode of the lithium ion battery provided in the first aspect, the method comprising: coating a plurality of negative electrode active material sizing agents on a negative electrode current collector in sequence to obtain a pole piece coated with a plurality of negative electrode active material sizing agent layers, and drying and rolling the pole piece to obtain the pole piece; wherein the negative electrode active material slurry contains a negative electrode active material, a conductive agent and a binder; the conductive agent is directly coated on the negative active material slurry on the negative current collector, the using amount of the conductive agent is 1-2 parts by weight and the using amount of the binder is 2-3 parts by weight relative to 100 parts by weight of the negative active material, the content of the binder in each negative active material slurry layer along the direction far away from the negative current collector is gradually reduced, and the content of the conductive agent is gradually increased.
In a preferred embodiment of the present invention, the conductive agent is used in an amount of 1 to 2 parts by weight, and the binder is used in an amount of 2 to 3 parts by weight.
The coating method of the slurry is not particularly limited, and in one embodiment, the negative active material slurry may be sequentially coated; in another embodiment, different anode active material slurries are simultaneously coated using a multi-layer doctor blade.
According to the present invention, the total thickness and the areal density of the anode active material slurry layer can be varied within a wide range, and in one embodiment of the present invention, the total thickness of the anode active material slurry layer is 30-200 μm, preferably 100-180 μm; the surface density is 0.3-12g/cm 2 Preferably 0.3 to 10g/cm 2 。
Drying according to the invention is well known to the person skilled in the art and can be carried out, for example, in a forced air drying cabinet, the conditions of which can include: the temperature is 70-120 deg.C, and the time is 5-45 min; preferably, the temperature is 85-105 ℃, and the time is 15-30 min; the rolling is well known to those skilled in the art, and may be, for example, one-time rolling, multiple-time rolling, hot roll rolling, etc., preferably one-time rolling, and more preferably, the rolling employs one-time rolling to press the electrode sheet to a bulk density of 1.3 to 1.6g/cm3, preferably 1.45 to 1.6g/cm 3. The drying and rolling may be performed in an apparatus conventionally employed by those skilled in the art, and the present disclosure is not particularly limited thereto.
In a specific embodiment of the present invention, the negative active material slurry further contains a solvent and optionally a dispersant, wherein the solvent is one or more selected from deionized water, ethanol, dipropylene glycol butyl ether, propylene glycol and propylene carbonate methyl ether, and the dispersant is one or more selected from carboxymethyl cellulose, polyacrylamide, sodium dodecyl sulfate and methyl amyl alcohol; the solvent is used in an amount of 6 to 120 parts by weight and the dispersant is used in an amount of 0 to 5 parts by weight with respect to 100 parts by weight of the negative electrode active material.
In a third aspect, the invention provides a lithium ion battery comprising the negative electrode provided in the first aspect of the invention.
According to the invention, the lithium ion battery further comprises a positive electrode, a diaphragm and an electrode solution. In a specific embodiment, the positive electrode may be any kind of positive electrode suitable for a secondary battery in the art, and specifically may include, but is not limited to, one or more of a lithium iron phosphate positive electrode, a nickel cobalt manganese ternary positive electrode, a nickel cobalt aluminum ternary positive electrode, a lithium cobaltate positive electrode sheet, and a lithium manganate positive electrode. The separator may be any one or more of various separators suitable for secondary batteries in the art, and specifically, may include, but is not limited to, polyethylene, polypropylene, non-woven fabric, and multi-layer composite film thereof. The electrolyte comprises electrolyte salt and an organic solvent, wherein the specific types and the compositions of the electrolyte salt and the organic solvent are not specifically limited, and can be selected according to actual requirements, and the electrolyte salt and the organic solvent are not specifically limited in the invention.
The invention is further illustrated by the following examples, but is not to be construed as being limited thereto.
Example 1
S1, mixing graphite, carboxymethyl cellulose (CMC), conductive carbon black (SP), Styrene Butadiene Rubber (SBR) and water according to the proportion of 100:1.6: 1.0: 3.0: 128 to prepare a first anode active material slurry a 1;
s2, mixing graphite, CMC, SP, SBR and water according to the proportion of 100:1.6: 2: 2: preparing a second anode active material slurry A2 in a weight ratio of 128;
s3, uniformly coating the first negative electrode active material slurry and the second negative electrode active material slurry on a copper foil with the thickness of 6 microns by using double scrapers, drying at 105 ℃ for 20min, and rolling for one time until the volume density is 1.5g/cm 3 After forming, die cutting and forming are carried out, the battery is assembled with a positive plate with the size of 60 multiplied by 70mm and the capacity of 1.5Ah, a PP diaphragm with the thickness of 14 mu m and an aluminum plastic film, and lithium hexafluorophosphate serving as solute is injected after bakingAnd the solvent is carbonate electrolyte to prepare the lithium ion battery.
Wherein, the coating thickness of the first cathode active material slurry layer is separately adjusted to about 40 μm and the surface density is about 0.6g/cm during the blade coating process 2 Fixing the film thickness of the first layer of scraper, simultaneously scraping with two layers of scrapers, adjusting the total thickness of the two negative electrode active material layers to about 80 μm, and adjusting the total surface density to about 1.2g/cm 2 . Wherein the first negative electrode active material layer has a thickness of 40 μm and an area density of 0.6g/cm 2 (ii) a The second negative electrode active material layer had a thickness of 40 μm and an area density of 0.6g/cm 2 (ii) a The first negative active material layer was in direct contact with the copper foil, and the content of the binder in the first active material layer was 3 wt%, and the content of the conductive agent was 1 wt%.
Example 2
A lithium ion battery was prepared in the same manner as in example 1, except that, in step S1, the weight ratio of graphite, CMC, SP, SBR, and water in the first negative electrode active material slurry B1 was 100:1.6: 1: 3: 128;
in step S2, the weight ratio of graphite, CMC, SP, SBR, and water in the second negative electrode active material slurry B2 is 100:1.6:1.5: 2: 128.
example 3
A lithium ion battery was manufactured in the same manner as in example 1, except that, in step S1, the weight ratio of graphite, CMC, SP, SBR, and water in the first negative electrode active material slurry C1 was 100:1.6: 1: 3: 128;
in step S2, the weight ratio of graphite, CMC, SP, SBR, and water in the second negative electrode active material slurry C2 is 100:1.6: 2: 2.5: 128.
example 4
A lithium ion battery was manufactured in the same manner as in example 1, except that, in step S1, the weight ratio of graphite, CMC, SP, SBR, and water in the first negative electrode active material slurry D1 was 100:1.6: 1.1: 2.2: 128;
in step S2, the weight ratio of graphite, CMC, SP, SBR, and water in the second negative electrode active material slurry D2 is 100:1.6: 1.2: 2.1: 128.
example 5
A lithium ion battery was manufactured in the same manner as in example 1, except that, in step S1, the weight ratio of graphite, CMC, SP, SBR, and water in the first negative electrode active material slurry E1 was 100:1.6: 1.4: 2.7: 128;
in step S2, the weight ratio of graphite, CMC, SP, SBR, and water in the second negative electrode active material slurry E2 is 100:1.6: 1.6: 2.3: 128.
example 6
A lithium ion battery was prepared in the same manner as in example 1, except that, in step S1, the weight ratio of graphite, CMC, SP, SBR, and water in the first negative electrode active material slurry F1 was 100:1.6: 1.4: 3: 128;
in step S2, the weight ratio of graphite, CMC, SP, SBR, and water in the second negative electrode active material slurry F2 is 100:1.6: 1.6: 2: 128.
example 7
A lithium ion battery was prepared in the same manner as in example 1, except that, in step S1, the weight ratio of graphite, CMC, SP, SBR, and water in the first negative electrode active material slurry G1 was 100:1.6: 1: 2.7: 128;
in step S2, the weight ratio of graphite, CMC, SP, SBR, and water in the second negative electrode active material slurry G2 is 100:1.6: 2: 2.3: 128.
example 8
A lithium ion battery was prepared in the same manner as in example 1, except that, in step S1, the weight ratio of graphite, CMC, SP, SBR, and water in the first negative electrode active material slurry H1 was 100:1.6: 1.2: 2.9: 128;
in step S2, the weight ratio of graphite, CMC, SP, SBR, and water in the second negative electrode active material slurry H2 is 100:1.6: 1.3: 2.7: 128.
example 9
A lithium ion battery was prepared in the same manner as in example 1, except that, in step S1, the weight ratio of graphite, CMC, SP, SBR, and water in the first negative electrode active material slurry I1 was 100:1.6: 1.8: 2.2: 128;
in step S2, the weight ratio of graphite, CMC, SP, SBR, and water in the second negative electrode active material slurry I2 is 100:1.6: 2: 2.1: 128.
example 10
A lithium ion battery was manufactured in the same manner as in example 1, except that, in step S1, the weight ratio of graphite, CMC, SP, SBR, and water in the first negative electrode active material slurry J1 was 100:1.6: 1.2: 2.6: 128;
in step S2, the weight ratio of graphite, CMC, SP, SBR, and water in the second negative electrode active material slurry J2 is 100:1.6: 1.9: 2.3: 128.
comparative example 1
S1, preparing the negative active slurry a from graphite, CMC, SP, SBR and water according to the weight ratio of 100:1.6:1.5:3.2: 128.
S2, uniformly coating the prepared negative active slurry a on a copper foil with the thickness of 6 microns by using a scraper, adjusting the height of the scraper, controlling the thickness of a coating film to be about 80 microns and the surface density to be about 120g/dm 2 Then drying at 105 ℃ for 20min, performing primary rolling forming, performing die cutting forming, assembling the die cutting forming with a positive plate with the size of 60 multiplied by 70mm and the capacity of 1.5Ah, a PP diaphragm with the thickness of 14 mu m and an aluminum plastic film into a battery, injecting electrolyte with lithium hexafluorophosphate as a solute and carbonic ester as a solvent after baking, and preparing the lithium ion battery.
Comparative example 2
A lithium ion battery was manufactured in the same manner as in example 1, except that, in step S1, the weight ratio of graphite, CMC, SP, SBR, and water in the first negative electrode active material slurry b1 was 100:1.6: 2: 2: 128;
in step S2, the weight ratio of graphite, CMC, SP, SBR, and water in the second negative electrode active material slurry b2 is 100:1.6: 1: 3: 128.
comparative example 3
A lithium ion battery was manufactured in the same manner as in example 1, except that, in step S1, the weight ratio of graphite, CMC, SP, SBR, and water in the first negative electrode active material slurry c1 was 100:1.6: 2: 3: 128;
in step S2, the weight ratio of graphite, CMC, SP, SBR, and water in the second negative electrode active material slurry c2 is 100:1.6: 1: 2: 128.
comparative example 4
A lithium ion battery was manufactured in the same manner as in example 1, except that, in step S1, the weight ratio of graphite, CMC, SP, SBR, and water in the first negative electrode active material slurry d1 was 100:1.6: 1: 2: 128;
in step S2, the weight ratio of graphite, CMC, SP, SBR, and water in the second negative electrode active material slurry d2 is 100:1.6: 2: 3: 128.
test example
And (3) disassembling the battery product, taking out the negative plate, washing the negative plate by using one or more of dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate and other organic solvents, and naturally airing the washed negative plate in a clean environment to obtain the pole piece to be tested.
(1) Pole piece peeling force f i Test method (2)
And (3) using a universal tensile testing machine to test the stripping force of the pole piece to be tested. Cutting a pole piece to be tested into corresponding sizes and specifications, sticking dressing on one side of the pole piece by using a transparent adhesive tape with a proper specification, shaping by using a shaping roller flat adhesive tape, stripping a crack of 10mm in advance, fixing stripping surfaces by using double-sided adhesive tapes respectively, clamping by using a clamp of an electronic universal tensile testing machine, selecting a standard testing mode, carrying out tensile stripping test on the pole piece to be tested from a surface layer to an inner layer step by step, recording the stripping force value of the test, testing 4 sample pieces on each group of pole pieces, and taking the average value as the stripping force testing value of the group; the scotch tape with the active material adhered thereto after the test was left for use. The part taken out is cut into proper size to prepare SEM sample and XRD sample, EDS analysis and XRD component analysis are carried out to determine the content ratio of the conductive agent and the binding agent of the active material layer, and the test result is shown in Table 1.
(2) Method for testing conductivity S of pole piece
And (3) preparing the transparent adhesive tapes with the active substances in different layers reserved in the step (1) into proper samples, and testing the conductivity of the pole piece to be tested by adopting an RTS-8 four-probe resistance instrument. Cutting the sample wafer into corresponding sizes, placing the sample wafer on a sample carrying table, setting an experimental mode as a single-point test, testing 5 points on each sample wafer, taking the average value as a conductivity test value of the pole piece, and obtaining a test result shown in table 1.
TABLE 1
Test example 2
The soft-package batteries (capacity 2000mAh) prepared by the electrodes prepared in the examples and the comparative examples were tested for battery impedance of 30s when charged under the condition of 1.5C and capacity retention rate of 500 cycles of constant current charge-discharge cycle at normal temperature of 0.5C by using a blue test cabinet, and two batteries were tested for each group, and the results are shown in table 2 below.
TABLE 2
As can be seen from table 2, t and d of examples 1 to 10 are within the range of the present application, the dc internal resistance of the battery is the smallest, and the cycle capacity decays the slower, because the conductive network is formed the best under the distribution of the conductive agent and the binder, the battery satisfies the lower resistance while the cycle stability is ensured, and the battery performance is superior.
Comparative example 1 is a single-layer coated electrode, and the network of the conductive agent binder is not optimized, so the internal resistance and the retention rate of the cycle capacity of the battery are not as good as those of examples 1-10; the t value and the d value of the comparative example 2 do not meet the optimal range of the conductive network, so that the direct current internal resistance of the battery is the maximum, the circulation stability of the battery is poorer, and the performance of the battery is the worst; the t value of the comparative example 3 meets the optimization range, the d value does not meet the range, and the conductive agent is not distributed uniformly, so that the direct-current internal resistance is larger; comparative example 4 only d value satisfied the range, and thus stability during cycle was insufficient and capacity retention rate was low.
The preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, however, the present invention is not limited to the specific details of the above embodiments, and various simple modifications can be made to the technical solution of the present invention within the technical idea of the present invention, and these simple modifications are within the protective scope of the present invention.
It should be noted that the various technical features described in the above embodiments can be combined in any suitable manner without contradiction, and the invention is not described in any way for the possible combinations in order to avoid unnecessary repetition.
In addition, any combination of the various embodiments of the present invention is also possible, and the same should be considered as the disclosure of the present invention as long as it does not depart from the spirit of the present invention.
Claims (10)
1. The lithium ion battery cathode is characterized in that the cathode comprises a cathode current collector and a plurality of cathode active material layers which are sequentially stacked on the cathode current collector, wherein the plurality of cathode active material layers contain cathode active materials, conductive agents and binders;
the content of the binder in the negative active material layer in direct contact with the negative current collector is 2-3 wt%, the content of the conductive agent is 1-2 wt%, the content of the binder in each negative active material layer along the direction far away from the negative current collector is gradually reduced, and the content of the conductive agent is gradually increased.
2. The lithium ion battery negative electrode according to claim 1, wherein t defined as following formula (1) satisfies 0.01< t ≦ 0.5, and d defined as following formula (2) satisfies 1 ≦ d ≦ 15;
t=(α i-1 -α i )f i /(f i-1 ×α i-1 ) Formula (1)
d=(β i ×S i )/[(β i -β i-1 )S i-1 ]Formula (2)
Wherein i is an arbitrary integer of 2 to n, and n is the number of the anode active material layers;
α i represents the ratio of the weight of the binder of the i-th negative electrode active material layer to the weight of the negative electrode active material in the direction away from the negative electrode current collector, in%,% β i Denotes the ith direction along the direction away from the negative electrode current collectorThe ratio of the weight of the conductive agent to the weight of the negative electrode active material in each negative electrode active material layer, in%, f i Denotes a peeling force required for peeling the i-th layer of the negative electrode active material layer from the i-1 th layer of the negative electrode active material layer in a direction away from the negative electrode current collector, and has a unit of N/m, and f when i =2 i-1 Denotes a peeling force required for peeling the 1 st layer of the negative electrode active material layer from the negative electrode current collector in a direction away from the negative electrode current collector, and when i > 2, f i-1 Expressing a peeling force required for peeling the i-1 st layer of the negative electrode active material layer from the i-2 nd layer of the negative electrode active material layer in a direction away from the negative electrode current collector, in units of N/m, S i Represents the conductivity of the i-th negative electrode active material layer in the direction away from the negative electrode current collector, and has a unit of Ω · cm.
3. The lithium ion battery negative electrode of claim 2, wherein t is 0.2. ltoreq. t.ltoreq.0.3, and d is 3. ltoreq. d.ltoreq.10.
4. The lithium ion battery negative electrode of claim 2, wherein 0 < a j +β j ≤5，0.2≤f j ≤4，0.2≤S j Less than or equal to 10, wherein j is any integer from 1 to n; alpha is alpha j Represents the ratio of the weight of the binder of the jth negative electrode active material layer to the weight of the negative electrode active material in the direction away from the negative electrode current collector, and has a unit of% j Represents a ratio of the weight of the conductive agent to the weight of the anode active material layer jth in the direction away from the anode current collector, in%, when j =1, f j Denotes a peeling force required for peeling the 1 st layer negative active material layer from the negative current collector in a direction away from the negative current collector, and f is not less than 2 j Denotes a peeling force in units of N/m, S required for peeling the jth layer of the negative electrode active material layer from the jth-1 layer of the negative electrode active material layer in a direction away from the negative electrode current collector j Represents the electrical conductivity of the jth anode active material layer in the direction away from the anode current collector, and has a unit of Ω · cm.
5. The lithium ion battery negative electrode of claim 4, wherein the 3 ≦ α j +β j ≤5，0.2≤f j ≤2，0.2≤S j ≤5。
6. The lithium ion battery negative electrode of claim 2, wherein 2 ≦ n ≦ 5.
7. The negative electrode for a lithium ion battery according to any one of claims 1 to 6, wherein the total thickness of the plurality of negative electrode active material layers is 30 to 200 μm, and the thickness of a single negative electrode active material layer is 6 to 150 μm.
8. The lithium ion battery negative electrode according to any one of claims 1 to 6, wherein an areal density of a single layer of the negative electrode active material layer is 0.3 to 15g/cm 2 。
9. A method of making the negative electrode of the lithium ion battery of any of claims 1-8, comprising: coating a plurality of negative electrode active material sizing agents on a negative electrode current collector in sequence to obtain a pole piece coated with a plurality of negative electrode active material sizing agent layers, and drying and rolling the pole piece to obtain the pole piece;
wherein the negative electrode active material slurry contains the negative electrode active material, the conductive agent, and the binder; the conductive agent is directly coated on the negative active material slurry on the negative current collector, the using amount of the conductive agent is 1-2 parts by weight and the using amount of the binder is 2-3 parts by weight relative to 100 parts by weight of the negative active material, the content of the binder in each negative active material slurry layer along the direction far away from the negative current collector is gradually reduced, and the content of the conductive agent is gradually increased.
10. A lithium ion battery, characterized in that it comprises a negative electrode according to any one of claims 1 to 8.
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