CN114927643A - Negative pole piece and preparation method and application thereof - Google Patents

Negative pole piece and preparation method and application thereof Download PDF

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Publication number
CN114927643A
CN114927643A CN202210426192.7A CN202210426192A CN114927643A CN 114927643 A CN114927643 A CN 114927643A CN 202210426192 A CN202210426192 A CN 202210426192A CN 114927643 A CN114927643 A CN 114927643A
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negative electrode
electrode layer
layer
negative
binder
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CN114927643B (en
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李峥
冯玉川
沈志鹏
陈凯
何泓材
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Suzhou Qingtao New Energy S&T Co Ltd
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Suzhou Qingtao New Energy S&T Co Ltd
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Priority to PCT/CN2022/100402 priority patent/WO2023201871A1/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • 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
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • 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
    • H01M4/139—Processes of manufacture
    • H01M4/1393—Processes of manufacture of electrodes based on carbonaceous material, e.g. graphite-intercalation compounds or CFx
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • 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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Materials Engineering (AREA)
  • Manufacturing & Machinery (AREA)
  • Inorganic Chemistry (AREA)
  • Battery Electrode And Active Subsutance (AREA)

Abstract

The invention provides a negative pole piece and a preparation method and application thereof. The negative pole piece comprises a current collector, a first negative pole layer and a second negative pole layer, wherein the first negative pole layer is positioned between the current collector and the second negative pole layer; wherein the negative electrode active material in the first negative electrode layer comprises graphite and a silicon material, and the negative electrode active material in the second negative electrode layer comprises graphite. According to the invention, the first negative electrode layer is arranged on the side, close to the current collector, of the negative electrode piece, and comprises graphite and silicon, so that the energy density of the whole negative electrode is improved, the capacity of the negative electrode is improved, and meanwhile, the expansion of silicon materials is further inhibited due to the existence of the second graphite negative electrode layer on the side far away from the current collector, so that the negative electrode piece is adapted to the existing lithium ion battery system which is commercially used in a large scale.

Description

Negative pole piece and preparation method and application thereof
Technical Field
The invention belongs to the technical field of lithium ion batteries, and relates to a negative pole piece and a preparation method and application thereof.
Background
With lithium-ion batteriesThe graphite is widely used in large-scale commercialization, and the capacity, the cycle performance and the safety performance of the lithium ion battery are widely concerned, wherein the graphite is used as a material which is earlier used in the negative electrode of the lithium ion battery, and is suitable for Li due to good conductivity, high crystallinity and good layered structure + Break-out/make-in. At present, graphite is mostly used as a negative electrode material of batteries which are put into use in larger equipment such as automobiles and the like, so that the problem of low capacity exists, in order to improve the capacity performance of the batteries, technicians propose that a silicon carbon material is used for replacing the graphite as the negative electrode of a lithium ion battery, but a silicon-based material has the problems of easy expansion, poor cycle life and the phenomena of negative electrode material collapse and falling off from a current collector after multiple cycles. And most of the research related to the silicon carbon cathode is still in the laboratory stage at present,
CN101087021A discloses a preparation method of artificial graphite cathode material for lithium ion battery, which comprises the following steps: crushing coal-series or petroleum-series needle coke, preheating, adding a modifier and a catalyst, drying, granulating, and carrying out heat treatment at the temperature of 800-3000 ℃ for 1-48 hours, wherein the specific capacity of the prepared graphite material is only 350mAh/g and is far less than the theoretical specific capacity of graphite, namely 370 mAh/g.
CN105118974A discloses a silicon-based negative electrode material and a preparation method thereof, and as electrostatic spinning equipment is introduced to blend the silicon material into carbon nanofibers, the problems of volume expansion and silicon carbon particle breakage of the silicon carbon material are effectively solved, and meanwhile, the later regeneration phenomenon of an SEI film is effectively reduced, the mechanical strength of the negative electrode material is effectively improved by utilizing the structure of the nanofibers, and the problems of low efficiency and poor consistency of the electrostatic spinning equipment make the silicon-based negative electrode material produced by the manner of the nanofibers difficult to realize industrial mass production. The patent purposefully provides a production process with simple preparation process and easy mass production and conversion, and the artificial graphite and SiO silicon carbon composite cathode material has the cathode material with high capacity, high multiplying power and high conductivity.
Therefore, how to increase the capacity of the graphite negative electrode and reduce the expansion of the negative electrode active material, and adapt to the existing lithium ion battery system used in large scale in commercialization is a technical problem to be solved urgently.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a negative pole piece and a preparation method and application thereof. According to the invention, the first negative electrode layer is arranged on the side, close to the current collector, of the negative electrode piece, and comprises graphite and silicon, so that the energy density of the whole negative electrode is improved, the capacity of the negative electrode is improved, and meanwhile, the expansion of silicon materials is further inhibited due to the existence of the second graphite negative electrode layer on the side far away from the current collector, so that the negative electrode piece is adapted to the existing lithium ion battery system which is commercialized and used in a large scale.
In order to achieve the purpose, the invention adopts the following technical scheme:
in a first aspect, the present invention provides a negative electrode plate, which includes a current collector, a first negative electrode layer and a second negative electrode layer, wherein the first negative electrode layer is located between the current collector and the second negative electrode layer;
the negative electrode active material in the first negative electrode layer comprises graphite and a silicon material, and the negative electrode active material in the second negative electrode layer comprises graphite; the graphite in the first negative electrode layer is carbon-coated graphite;
the first negative electrode layer comprises a first binder and a first conductive agent; the first binder is polyacrylonitrile binder.
The present invention is not particularly limited in the kind of silicon negative electrode active material, and any known silicon negative electrode active material can be used in the present application without departing from the inventive concept of the present application; by way of illustrative example only, and not by way of any limitation to the scope of protection, the silicon negative electrode active material includes elemental silicon, a silicon oxy compound, a silicon-based material subjected to coating treatment, and the like.
The present invention is not particularly limited in the method and type of carbon coating, and any known carbon coating method capable of increasing electron conductance can be used in the present application without departing from the inventive concept of the present application; the electronic conductivity of the graphite coated by the carbon is obviously improved, and the problem that the whole internal resistance of the cathode layer is influenced by the low electronic conductivity of the first cathode layer is solved.
It is understood that polyacrylonitrile binder refers to a class of polymers obtained by free polymerization of acrylonitrile monomer as a binder, and may be, for example, LA132 and/or LA133 binders.
In the actual production process, even if a small amount of silicon-oxygen compound is added into the graphite negative electrode of the lithium ion battery, the silicon-oxygen compound expands in the charging and discharging process, so that the whole negative electrode layer is caused to fall off from the current collector. Surprisingly, the polyacrylonitrile-based binder can effectively inhibit the expansion of the siloxane compound, has stronger bonding effect with a current collector compared with other binders, can effectively prevent the falling of the cathode layer and prolong the cycle life of the battery.
Meanwhile, the high-tensile negative current collector is selected and matched with the polyacrylonitrile binder, so that the current collector can be ensured to adapt to the volume change of a silica material.
The graphite provided by the invention is selected from one of natural graphite and artificial graphite. Further, the artificial graphite is selected from one of single-particle artificial graphite, secondary-particle artificial graphite, and a composite of single-particle artificial graphite and secondary-particle artificial graphite.
For multilayer negative electrode systems, the silicon material is selected to be added to the first negative electrode layer close to the current collector because the negative electrode active material layer close to the current collector is subjected to the binding force of the current collector thereto and therefore has an expansion rate generally smaller than that of the negative electrode active material layer on the side away from the current collector. The expansion due to the addition of silicon is avoided by the interaction of the negative electrode layer with the current collector.
In the invention, if the second negative electrode layer is not arranged in the negative electrode piece, the technical problem of unstable interface between the silica material and the electrolyte can occur.
Preferably, the second negative electrode layer includes a second binder and a second conductive agent therein.
Preferably, the second binder is an aqueous binder.
Preferably, the adhesion force of the first adhesive to the current collector is greater than the adhesion force of the second adhesive to the current collector.
In the invention, the binder in the first negative electrode layer is selected to be of a type with stronger adhesion to the current collector so as to further overcome the expansion caused by the introduction of silicon in the first negative electrode layer. In contrast, due to the small expansion of the pure graphite negative electrode, the second negative electrode layer may continue to use a conventional binder, for example, a conventional aqueous binder that may be readily known to those skilled in the art in the second negative electrode layer, including but not limited to polyvinyl alcohol, polyacrylic acid, polyethylene glycol, polyacrylamide, styrene butadiene rubber, or hydroxymethyl cellulose, and the like.
In the actual production process, even if a small amount of silicon-oxygen compound is added into the graphite negative electrode of the lithium ion battery, the silicon-oxygen compound expands in the charging and discharging process, so that the whole negative electrode layer is caused to fall off from the current collector. Surprisingly, the polyacrylonitrile-based binder can effectively inhibit the expansion of the siloxane compound, has stronger bonding effect with a current collector compared with other binders, can effectively prevent the falling of the cathode layer and prolong the cycle life of the battery.
Further preferably, the mass ratio of the binder in the second negative electrode layer is greater than the mass ratio of the binder in the first negative electrode layer.
Preferably, the mass ratio of the binder in the first negative electrode layer is 3 to 5 wt%, for example, 3 wt%, 4 wt%, or 5 wt%.
Preferably, the mass ratio of the binder in the second negative electrode layer is 4 to 8 wt%, for example, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, or the like.
Preferably, the current collector of the first negative electrode layer is made of a high tensile material, and the current collector is matched with the high expansion performance of the first negative electrode layer, so that the expansion of the first negative electrode layer caused by the introduction of silicon can be further resisted.
The tensile strength of the current collector is more than or equal to 350N/cm 2 E.g. 360N/cm 2 、380N/cm 2 、400N/cm 2 、450N/cm 2 、480N/cm 2 Or 500N/cm 2 And the like.
The invention selects the high tensile negative current collector and matches the polyacrylonitrile binder, thereby ensuring that the current collector can adapt to the volume change of the silica material.
Preferably, the mass of the silicon material in the first negative electrode layer is 3-5% of the mass of the negative electrode active material in the first negative electrode layer.
In the invention, if the addition amount of the silica material is too much, the expansion rate of the negative electrode material in the charging and discharging process is too high, the charging and discharging performance of the lithium battery is reduced, the cycle retention rate is greatly reduced, and if the addition amount is too little, the effect of increasing the battery capacity cannot be achieved.
In the invention, the graphite in the first negative electrode layer is carbon-coated graphite, so that the conductivity of the first negative electrode layer can be improved.
Preferably, the silicon material comprises a silicon oxygen material.
Preferably, the mass ratio of the first conductive agent in the first negative electrode layer is greater than the mass ratio of the second conductive agent in the second negative electrode layer.
Preferably, the first conductive agent includes any one of CNT, VGCF, super P, carbon black, acetylene black, or graphene or a combination of at least two thereof.
Preferably, the mass ratio of the CNT and/or VGCF in the first conductive agent is 15-25 wt%.
In the present invention, the choice of second conductive agent includes, but is not limited to, carbon-based materials, powdered nickel or other metal particles, or conductive polymers, for example, carbon-based materials may include particles of carbon black, graphite, superP, acetylene black (such as KETCHENTM black or denka black), carbon fibers and nanotubes, graphene, and the like; the conductive polymer includes polyaniline, polythiophene, polyacetylene, polypyrrole, poly (3, 4-ethylenedioxythiophene) polysulfonylstyrene, and the like.
In the invention, the mass ratio of the first conductive agent in the first negative electrode layer is more than that of the first conductive agent in the conventional negative electrode layer, so that the electron transmission in the charge and discharge process can be better realized, the reduction of the conductivity of the negative electrode layer caused by the addition of the silica material is prevented, and on the other hand, the electron transmission path is damaged due to the expansion of the silica material, so that the electron transmission is influenced.
Preferably, the thickness of the first negative electrode layer is 5 to 40% of the thickness of the second negative electrode layer, for example, 5%, 8%, 10%, 13%, 15%, 18%, 20%, 23%, 25%, 28%, 30%, 33%, 35%, 38%, 40, etc., preferably 16 to 25%.
According to the invention, the thickness of the first negative electrode layer is 16-25% of the thickness of the second negative electrode layer, so that the technical effects of improving the energy density and the cycle retention rate of the battery can be better realized, and if the thickness of the first negative electrode layer is too thick, the expansion coefficients of two sides of the first negative electrode layer close to the current collector and two sides of the first negative electrode layer far away from the current collector are different, so that the falling of a pole piece and the peeling between the first negative electrode layer and the second negative electrode layer are easily caused, and if the thickness of the first negative electrode layer is too small, the effect of improving the energy density is not obvious.
Preferably, the thickness of the first negative electrode layer is 16 to 55 μm, for example, 16 μm, 18 μm, 20 μm, 23 μm, 25 μm, 28 μm, 30 μm, 33 μm, 35 μm, 38 μm, 40 μm, 43 μm, 45 μm, 48 μm, 50 μm, 53 μm, or 55 μm.
Preferably, the thickness of the second negative electrode layer is 170-210 μm, such as 170 μm, 175 μm, 180 μm, 185 μm, 190 μm, 195 μm, 200 μm, 205 μm or 210 μm.
Preferably, the coating surface density of the first negative electrode layer is 30-50 g/m 2 E.g. 30g/m 2 、33g/m 2 、35g/m 2 、38g/m 2 、40g/m 2 、43g/m 2 、45g/m 2 、48g/m 2 Or 50g/m 2 And the like.
Preferably, the surface density of the second negative electrode layer is 150-170 g/m 2 E.g. 150g/m 2 、155g/m 2 、160g/m 2 、165g/m 2 Or 170g/m 2 Etc. of
In a second aspect, the present invention provides a method for preparing a negative electrode plate according to the first aspect, where the method for preparing the negative electrode plate includes:
coating the slurry of the first negative layer on the surface of a current collector to obtain a first negative layer, and coating the slurry of the second negative layer on the surface of the first negative layer to obtain the negative pole piece.
And preparing the slurry of the first negative electrode layer and the slurry of the second negative electrode layer by conventional technical means.
Illustratively, the preparation method of the cathode layer slurry comprises the following steps: and mixing the negative electrode active material, the binder, the solvent and the conductive agent to obtain negative electrode layer slurry.
In a third aspect, the present invention further provides a lithium ion battery, where the lithium ion battery includes the negative electrode tab according to the first aspect.
The lithium ion battery provided by the present invention may be a liquid battery or a solid battery, and is not particularly limited.
When the lithium ion battery is a liquid lithium ion battery, the lithium ion battery comprises a negative pole piece, a positive pole piece, a diaphragm and electrolyte as described in the first aspect.
The positive pole piece, the diaphragm and the electrolyte in the liquid lithium ion battery are all known and easily obtained by the technical personnel in the field, and the corresponding substances and the preparation method of the complete lithium ion battery can be obtained by assembly.
When it is a solid lithium ion battery, it comprises a negative electrode sheet, a positive electrode sheet and a solid electrolyte layer as described in the first aspect.
The positive pole piece and the solid electrolyte layer in the solid lithium ion battery are both easily known and available by the technical personnel in the field, and the corresponding substances which can be assembled to obtain the complete lithium ion battery and the preparation method are both applicable.
Compared with the prior art, the invention has the following beneficial effects:
according to the negative pole piece provided by the invention, the first negative pole layer is arranged on one side close to the current collector and simultaneously comprises graphite and silicon, so that the energy density of the whole negative pole is improved, the capacity of the negative pole is improved, and the existence of the second graphite negative pole layer on one side far away from the current collector further inhibits the expansion of silicon materials through the mutual matching and synergistic action of the type and the amount of the current collector and the binder, the energy density of the traditional graphite-based negative pole lithium battery and the cycle life of the battery, so that the first-circle expansion rate of the negative pole material is reduced, the cycle life of the battery is prolonged, and the negative pole piece is adapted to the existing lithium ion battery system which is commercially used on a large scale, and the preparation method is simple and does not need complicated preparation steps. According to the battery provided by the invention, the gram capacity of the negative electrode under 0.33C can reach more than 375mAh/g, the expansion rate of the first circle of the negative electrode is less than 38%, the capacity retention rate after 500 circles of circulation can reach more than 85.8%, the thickness of the negative electrode layer is further adjusted, the gram capacity of the negative electrode under 0.33C can reach more than 375mAh/g, the expansion rate of the first circle of the negative electrode is less than 30%, and the capacity retention rate after 500 circles of circulation can reach more than 93.4%.
Detailed Description
The technical solution of the present invention is further illustrated by the following specific examples. It should be understood by those skilled in the art that the examples are only for the understanding of the present invention and should not be construed as the specific limitations of the present invention.
The negative electrode for the lithium battery comprises a current collector, a first negative electrode layer and a second negative electrode layer, wherein the first negative electrode layer comprises 95-97 wt% of a graphite material coated with carbon and 3-5 wt% of a silica compound serving as active materials, and a polyacrylonitrile-based binder serving as a first binder, and the second negative electrode layer adopts 100% of graphite serving as an active material, and comprises a second binder and a second conductive agent.
It is understood that the above percentages are in proportion to the negative active material and not in proportion to the negative electrode layer, for example, 95 to 97 wt% means that the carbon-coated graphite material accounts for 95 to 97 wt% of the negative active material in the first negative electrode layer.
It is known that silicon has a higher capacity, but is limited in use due to its greater expansion properties; in this application, with the help of the adhesion of the mass flow body to first negative pole layer, the expansion performance of first negative pole layer has effectively been restrained, so, even add the silicon negative pole active material of high expansion performance in first negative pole layer, can not cause negative pole layer whole to take place unpredictable volume expansion yet. The problem of contradiction between expansion performance and energy density at present is solved through the arrangement of the multilayer negative electrode.
The polyacrylonitrile-based binder mentioned in the present invention includes, but is not limited to, the copolymer of monomer acrylonitrile and its simple modification of the copolymer, such as simple functional group substitution, functional group position change, and the polymer with unchanged binding property resulted from the change of the functional group number and the monomer number.
In one embodiment of the present application, the first negative electrode layer includes: the negative electrode comprises 90-97 wt% of a negative electrode active material consisting of 95-97 wt% of a graphite material and 3-5 wt% of a silica compound, 3-5 wt% of a polyacrylonitrile binder in the first negative electrode layer, and 1-5 wt% of a first conductive agent, and more specifically, the first conductive agent further comprises 15-20 wt% of CNT and/or VGCF. When the component proportion of each component meets the range, the energy density of the graphite-based negative electrode lithium battery can be effectively improved, the original excellent cycle retention rate of the lithium battery is not reduced, when the weight ratio of the silica compound in the first negative electrode layer active material is more than 5 wt%, the expansion of the silica compound in the charging and discharging process is difficult to inhibit, the performance of the battery is deteriorated, and when the weight ratio of the silica compound in the first negative electrode layer active material is less than 3 wt%, the energy density of the lithium battery is not obviously improved.
Preferably, the first conductive agent includes at least one or both of CNT or VGCF. Compared with other conductive agents, the CNT and VGCF fibrous structures have a certain length-diameter ratio, so that a linear conductive channel is formed in the first cathode layer when the silica compound expands, the transmission efficiency of lithium ions and electrons is improved, and the capacity performance and the cycle performance of the battery are ensured not to be degraded.
The present application does not specifically limit the specific types of the CNT and the VGCF, and any known CNT and VGCF products with a relatively long aspect ratio can be used in the present application without departing from the inventive concept of the present application, and as an illustrative example, the aspect ratio of the CNT and the VGCF is > 1000; preferably, the aspect ratio of CNT and VGCF is > 2000.
In one embodiment of the present invention, the second negative electrode layer includes: a second negative electrode active material, 4-8 wt% of a binder, and 1-5 wt% of a second conductive agent, specifically, the second negative electrode layer includes: graphite is used as the only active material, 4-8 wt% of binder and 1-3 wt% of second conductive agent.
It is understood that the second negative electrode active material may be a mixture of one or more negative electrode active materials, but should have less expansion than the first negative electrode layer; therefore, as a preferred embodiment, the second negative electrode active material is entirely composed of a graphite negative electrode active material.
In a preferred embodiment, the adhesion of the first binder to the current collector is greater than that of the second binder, and the content of the binder in the second negative electrode layer is greater than that of the binder in the first negative electrode layer;
according to the invention, the negative electrode system with high binder content is adopted, and the first negative electrode layer and the current collector have stronger interaction through the adjustment of the binder content and the type, so that the problem of pole piece falling caused by silicon expansion is avoided. The content of the second binder is larger than that of the first binder in the invention, so that the second negative electrode layer is far away from the current collector. The content of the binder in the second negative electrode layer is increased, and the binder and the current collector jointly play a role in clamping the first negative electrode layer, so that the expansion and structural change of the first negative electrode layer are further inhibited.
In one embodiment of the invention, the projected area of the second negative electrode layer on the current collector is greater than or equal to the projected area of the first negative electrode on the current collector.
The graphite material in the first negative electrode active material and the second negative electrode active material is not particularly limited, and any known graphite material that can be used as a negative electrode active material can be used in the present application without departing from the inventive concept of the present application, and the graphite material may be selected from one of natural graphite and artificial graphite, by way of illustrative example only, and without any limitation to the scope of protection. Further, the artificial graphite is selected from one of single-particle artificial graphite, secondary-particle artificial graphite, and a composite of single-particle artificial graphite and secondary-particle artificial graphite.
The graphite in the first negative electrode layer is carbon-coated graphite, the carbon-coated graphite can be favorable for compensating the reduction of the conductivity of the whole negative electrode layer caused by the addition of the silicon-oxygen compound, and meanwhile, the conductive agent can form a conductive network when the silicon-oxygen compound expands, so that the phenomenon that the conductive path is damaged caused by the expansion of silicon is effectively avoided.
Carbon coating of graphite is known in the art, for example by coating the graphite surface with a carbonaceous coating. The actual carbon coating method is not particularly required by the present application, and any known carbon coating manner or structure can be used in the present application without departing from the inventive concept of the present application.
According to the invention, the first negative electrode layer is arranged on the side, close to the current collector, of the negative electrode piece, and comprises graphite and silicon, so that the energy density of the whole negative electrode is improved, the capacity of the negative electrode is improved, and meanwhile, the graphite negative electrode with smaller expansion is adopted on the side far away from the current collector, so that the expansion of the first negative electrode layer is further inhibited, and the negative electrode piece is adapted to the existing lithium ion battery system which is commercialized and used in a large scale.
In the invention, the adhesion exists between the negative electrode layer and the current collector, so that the expansion rate of the negative electrode active material layer close to the current collector is generally smaller than that of the negative electrode active material layer far away from the current collector, and therefore, the silicon material is selected to be added into the first negative electrode layer close to the current collector. The expansion due to the addition of silicon is avoided by the interaction of the negative electrode layer with the current collector.
As a preferred embodiment, the current collector for the negative electrode is a current collector with high tensile strength, and in some embodiments, may be a copper foil; preferably, the tensile strength of the negative electrode current collector is 350N/cm or more 2 . E.g., 350N/cm 2 、400N/cm 2 、450N/cm 2 And the like.
The first negative electrode layer includes a first binder and a first conductive agent;
the first negative electrode layer includes a first binder and a first conductive agent;
the first binder is polyacrylonitrile-based binder;
the first conductive agent comprises one or a combination of several of carbon black, super-P, CNT, VGCF, acetylene black and graphene.
In some embodiments, the first conductive agent comprises at least 15 wt% to 25 wt% CNTs and/or VGCF.
The second negative electrode layer comprises a second binder and a second conductive agent;
in the invention, the binder in the first negative electrode layer is selected to be of a type having stronger adhesion with the current collector so as to further overcome the expansion of the first negative electrode layer caused by the introduction of silicon. In contrast, due to the small expansion of the pure graphite negative electrode, the second negative electrode layer may continue to use a conventional binder, for example, a conventional aqueous binder that may be readily known to those skilled in the art in the second negative electrode layer, including but not limited to polyvinyl alcohol, polyacrylic acid, polyethylene glycol, polyacrylamide, styrene butadiene rubber, or hydroxymethyl cellulose, etc.
In the present invention, the choice of second conductive agent includes, but is not limited to, carbon-based materials, powdered nickel or other metal particles, or conductive polymers, for example, carbon-based materials may include particles of carbon black, graphite, superP, acetylene black (such as KETCHENTM black or denka black), carbon fibers and nanotubes, graphene, and the like; the conductive polymer includes polyaniline, polythiophene, polyacetylene, polypyrrole, poly (3, 4-ethylenedioxythiophene) polysulfonylstyrene, and the like.
Preferably, the thickness of the first negative electrode layer is 5 to 40% of the thickness of the second negative electrode layer, for example, 5%, 8%, 10%, 13%, 15%, 18%, 20%, 23%, 25%, 28%, 30%, 33%, 35%, 38%, 40, or the like, and preferably 16 to 25%.
In the invention, the thickness of the first negative layer is within 16-25% of the thickness of the second negative layer, so that the technical effects of improving the energy density and the cycle retention rate of the battery can be better realized, and if the thickness of the first negative layer is too thick, the expansion coefficients of two sides of the first negative layer close to the current collector and two sides of the first negative layer far away from the current collector are different, so that the falling of a pole piece and the peeling between the first negative layer and the second negative layer are easily caused, and if the thickness of the first negative layer is too small, the effect of improving the energy density is not obvious.
Preferably, the thickness of the first negative electrode layer is 16 to 55 μm, for example, 16 μm, 18 μm, 20 μm, 23 μm, 25 μm, 28 μm, 30 μm, 33 μm, 35 μm, 38 μm, 40 μm, 43 μm, 45 μm, 48 μm, 50 μm, 53 μm, or 55 μm.
Preferably, the thickness of the second negative electrode layer is 170-210 μm, such as 170 μm, 175 μm, 180 μm, 185 μm, 190 μm, 195 μm, 200 μm, 205 μm or 210 μm.
In a second aspect, the present invention provides a method for preparing a negative electrode sheet as described in the first aspect, where the method includes:
coating the slurry of the first negative layer on the surface of a current collector to obtain a first negative layer, and coating the slurry of the second negative layer on the surface of the first negative layer to obtain the negative pole piece.
Preferably, the surface density of the first negative electrode layer is 30-50 g/m 2 E.g. 30g/m 2 、35g/m 2 、40g/m 2 、45g/m 2 Or 50g/m 2 And the like.
Preferably, the surface density of the second negative electrode layer is 150-170 g/m 2 E.g. 150g/m 2 、155g/m 2 、160g/m 2 、165g/m 2 Or 170g/m 2 Etc.;
the present invention provides a lithium battery including the above negative electrode.
The lithium battery comprises a positive electrode, a negative electrode, an electrolyte and a battery shell comprising the structure.
The positive electrode includes a current collector and a positive electrode active material layer formed on the current collector;
the positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, or a metal current collector surface-treated with carbon or other substances may be used.
The positive electrode collector may generally have a thickness of 3 to 500 μm.
The positive electrode collector may have fine irregularities formed on a surface thereof to improve adhesion of a positive electrode active material. For example, positive electrode current collectors in various shapes such as films, sheets, foils, nets, porous bodies, foams and non-woven fabrics may be used.
The positive electrode active material layer may include a positive electrode active material.
The positive electrode active material is a compound capable of reversibly intercalating and deintercalating lithium, and specifically, may include a lithium transition metal composite oxide containing lithium and at least one transition metal selected from the group consisting of nickel, cobalt, manganese and aluminum; preferably, it may be lithium and a transition metal such as nickel, cobalt or manganese.
More specifically, the lithium transition metal composite oxide may be a lithium manganese-based oxide (e.g., LiMnO) 2 、LiMn 2 O 4 Etc.), lithium cobalt oxides (e.g., LiCoO) 2 Etc.), lithium nickel-based oxides (e.g., LiNiO) 2 Etc.), lithium nickel manganese-based oxides (e.g., LiNi) 1-y Mn y O 2 (wherein 0)<y<1)、LiMn 2-z Ni z O 4 (wherein 0)<z<2) Etc.), lithium nickel cobalt based oxides (e.g., LiNi) 1- y1 Co y1 O 2 (wherein 0)<y 1 <1) Etc.), lithium manganese cobalt based oxides (e.g., LiCo) 1-y2 Mn y2 O 2 (wherein 0)<y 2 <1)、LiMn 2-z1 Co z1 O 4 (wherein 0)<z 1 <2) Etc.), lithium nickel manganese cobalt oxides (e.g., Li (Ni) p Co q Mn r1 )O 2 (wherein 0)<p<1，0<q<1，0<r1<1, p + q + r1 ═ 1), or lithium nickel cobalt transition metal (M) oxide (e.g., Li (Ni) p2 Co q2 Mn r3 A S2 )O 2 (wherein M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, p 2 、q 2 、r 3 And s 2 Each is an atomic fraction of an independent element, and 0<p 2 <1、0<q 2 <1、0<r 3 <1、0<s 2 <1、p 2 +q 2 +r 3 +s 2 1) and the like), and may contain any one thereof or two or more thereof. Of these, the capacity of the battery can be increasedAnd stability, the lithium transition metal composite oxide may be LiCoO 2 、LiMnO 2 、LiNiO 2 Lithium nickel manganese cobalt oxide (e.g., Li (Ni) 0.6 Mn 0.2 Co 0.2 )O 2 、Li(Ni 0.5 Mn 0.3 Co 0.2 )O 2 、Li(Ni 0.7 Mn 0.15 Co 0.15 )O 2 Or LiNi 0.8 Mn 0.1 Co 0.1 )O 2 Etc., or lithium nickel cobalt aluminum oxides (e.g., Li (Ni) 0.8 Co 0.15 Al 0.05 )O 2 Etc.) and the like. The lithium transition metal composite oxide may be Li (Ni) when considering a significant improvement effect according to control of the type and content ratio of constituent elements forming the lithium transition metal composite oxide 0.6 Mn 0.2 Co 0.2 )O 2 、Li(Ni 0.5 Mn 0.3 Co 0.2 )O 2 、Li(Ni 0.7 Mn 0.15 Co 0.15 )O 2 Or Li (Ni) 0.8 Mn 0.1 Co 0.1 )O 2 And the like, and either one or a mixture of two or more thereof may be used.
The amount of the cathode active material contained in the cathode active material layer may be 80 wt% to 99 wt%, preferably 92 wt% to 98.5 wt%.
The positive electrode active material layer may further include a positive electrode binder and/or a positive electrode conductive material, in addition to the above-described positive electrode active material.
The positive electrode binder is used to bind components such as an active material, a conductive material, and a current collector together, and specifically, may include at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, styrene-butadiene rubber, and fluororubber, preferably polyvinylidene fluoride.
The amount of the cathode binder contained in the cathode active material layer may be 1 wt% to 20 wt%, preferably 1.2 wt% to 10 wt%.
The conductive material is mainly used to assist and improve conductivity in the secondary battery, and is not particularly limited as long as it has conductivity without causing chemical changes. Specifically, the conductive material may include graphite, such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon powders, aluminum powders, and nickel powders; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives, and carbon black may be preferably contained from the aspect of improving conductivity.
The specific surface area of the positive electrode conductive material may be 80m 2 G to 200m 2 Per g, preferably 100m 2 G to 150m 2 /g。
The amount of the positive electrode conductive material contained in the positive electrode active material layer may be 1 wt% to 20 wt%, preferably 1.2 wt% to 10 wt%.
The thickness of the positive electrode active material layer may be 30 to 400 μm, preferably 50 to 110 μm.
The positive electrode may be manufactured by coating a positive electrode slurry including a positive electrode active material and optionally a positive electrode binder, a positive electrode conductive material, and a positive electrode slurry forming solvent on a positive electrode current collector, followed by drying and roll-pressing.
The positive electrode slurry forming solvent may contain an organic solvent such as N-methyl-2-pyrrolidone (NMP), and the amount may be such that a preferred viscosity is obtained when a positive electrode active material is contained and optionally a positive electrode binder, a positive electrode conductive material, and the like. For example, the amount of the positive electrode slurry forming solvent contained in the positive electrode slurry may be such that the concentration of solids containing the positive electrode active material, and optionally containing the positive electrode binder and the positive electrode conductive material, is 50 wt% to 95 wt%, preferably 70 wt% to 90 wt%.
The type of electrolyte is not particularly limited, and any known electrolyte material can be used in the present application without departing from the inventive concept of the present application. By way of illustrative example, the electrolyte may be a liquid electrolyte, a solid electrolyte, or a mixture of a solid electrolyte and a liquid electrolyte.
When the electrolyte adopts liquid electrolyte, a diaphragm is also arranged in the battery system.
The separator mainly functions to separate the negative electrode and the positive electrode and to provide a moving path for lithium ions. Any separator may be used without particular limitation so long as it is a separator commonly used in secondary batteries. In particular, a separator having excellent wettability with an electrolytic solution and low resistance to ion movement in an electrolyte is preferable. Specifically, a porous polymer film, for example, a porous polymer film made using a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, or having a laminated structure of two or more layers thereof may be used. Also, typical porous nonwoven fabrics, for example, nonwoven fabrics formed of glass fibers having a high melting point, polyethylene terephthalate fibers, and the like, may be used. In addition, a coating separator including a ceramic component or a polymer material may be used to ensure heat resistance or mechanical strength, and may be selectively used in a single layer or a multi-layer structure.
In addition, the electrolyte used in the present invention may be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a melt-type inorganic electrolyte, etc., which may be used in the manufacture of a secondary battery, but is not limited thereto.
Specifically, the electrolyte may include an organic solvent and a lithium salt.
Any organic solvent may be used without particular limitation so long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, ester solvents such as methyl acetate, ethyl acetate, γ -butyrolactone, and ∈ -caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), Ethyl Methyl Carbonate (EMC), Ethylene Carbonate (EC), and Propylene Carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a linear, branched or cyclic C2-C20 hydrocarbon group and may contain double-bonded aromatic rings or ether linkages); amides such as dimethylformamide; dioxolanes, such as 1, 3-dioxolane; or sulfolane. Among the above solvents, carbonate-based solvents are preferable, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ion conductivity and high dielectric constant, and a low-viscosity linear carbonate-based compound (e.g., ethylene carbonate, dimethyl carbonate, or diethyl carbonate) that can increase the charge/discharge performance of a battery is more preferable. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte may be excellent.
Any compound may be used as the lithium salt without particular limitation so long as it can provide lithium ions used in a lithium secondary battery. In particular, LiPF 6 、LiClO 4 、LiAsF 6 、LiBF 4 、LiSbF6、LiAlO 4 、LiAlCl 4 、LiCF 3 SO 3 、LiC 4 F 9 SO 3 、LiN(C 2 F 5 SO 3 ) 2 、LiN(C 2 F 5 SO 2 ) 2 、LiN(CF 3 SO 2 ) 2 、LiCl、LiI、LiB(C 2 O 4 ) 2 Etc. may be used as the lithium salt. The lithium salt may be used in a concentration range of 0.1 to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity to exhibit excellent properties, and lithium ions can be efficiently moved.
As an embodiment, the electrolyte may be a solid electrolyte, and the solid electrolyte particles may comprise one or more components of a polymer, an oxide solid electrolyte, a sulfide solid electrolyte, a halide solid electrolyte, a borate solid electrolyte, a nitride solid electrolyte, or a hydride solid electrolyte. When polymer particles are used, the lithium salt should be used for the recheck. As an embodiment, the polymer-based component may comprise one or more polymeric materials selected from the group comprising: polyethylene glycol, polyethylene oxide (PEO), poly (p-phenylene oxide) (PPO), poly (methyl methacrylate) (PMMA), Polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), polyvinyl chloride (PVC), and combinations thereof. It will be appreciated that the high ionic conductivity of the polymeric material is advantageous for the performance of the overall solid state electrolyte material, and preferably the polymeric material should have an ionic conductivity of greater than or equal to 10 "4S/cm.
As an embodiment, the oxide particles may include one or more of garnet ceramics, LISICON-type oxides, NASICON-type oxides, and perovskite-type ceramics. As illustrative examples, the garnet ceramic may be selected from the group comprising: li 6.5 La 3 Zr 1.75 Te 0.25 O 12 、Li 7 La 3 Zr 2 O 12 、Li 6.2 Ga 0.3 La 2.95 Rb 0.05 Zr 2 O 12 、Li 6.85 La 2.9 Ca 0.1 Zr 1.75 Nb 0.25 O 12 、Li 6.25 Al 0.25 La 3 Zr 2 O 12 、Li 6.75 La 3 Zr 1.75 Nb 0.25 O 12 、Li 6.75 La 3 Zr 1.75 Nb 0.25 O 12 And combinations thereof. LISICON-type oxides may be selected from the group comprising: li 14 Zn(GeO 4 ) 4 、Li 3+x (P 1-x Si x ) O4 (wherein 0<x<1)、Li 3+x Ge x V 1-x O 4 (wherein 0)<x<1) And combinations thereof. NASICON type oxides may be formed from LiMM' (PO) 4 ) 3 Definitions, wherein M and M' are independently selected from Al, Ge, Ti, Sn, Hf, Zr, and La. Preferably, the NASICON-type oxide may be selected from the group comprising: li 1+x Al x Ge 2-x (PO 4 ) 3 (LAGP) (wherein 0. ltoreqx≤2)、Li 1+x Al x Ti 2-x (PO 4 ) 3 (LATP) (where 0. ltoreq. x. ltoreq.2), Li 1+x Y x Zr 2-x( PO 4 ) 3 (LYZP) (where x is 0. ltoreq. x.ltoreq.2), Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 、LiTi 2 (PO 4 ) 3 、LiGeTi(PO 4 ) 3 、LiGe 2 (PO 4 ) 3 、LiHf 2 (PO 4 ) 3 And combinations thereof. The one or more perovskite-type ceramics may be selected from the group comprising: li 3.3 La 0.53 TiO 3 、LiSr 1.65 Zr 1.3 Ta 1.7 O 9 、Li 2x-y Sr 1-x Ta y Zr 1-y O 3 (where x is 0.75y and 0.60)<y<0.75)、Li 3/8 Sr 7/16 Nb 3/ 4 Zr 1/4 O 3 、Li 3x La (2/3-x) TiO 3 (wherein 0)<x<0.25), and combinations thereof. Preferably, the one or more oxide-based materials can have a thickness of greater than or equal to about 10 -5 S/cm to less than or equal to about 10 -1 Ion conductivity of S/cm.
The sulfide solid state electrolyte is selected from one or more sulfide-based materials of the group comprising: li 2 S-P 2 S 5 、Li 2 S-P 2 S 5 MSx (where M is Si, Ge and Sn and 0. ltoreq. x. ltoreq.2), Li 3.4 Si 0.4 P 0.6 S 4 、Li 10 GeP 2 S 11.7 O 0.3 、Li 9.6 P 3 S 12 、Li 7 P 3 S 11 、Li 9 P 3 S 9 O 3 、Li 10.35 Si 1.35 P 1.65 S 12 、Li 9.81 Sn 0.81 P 2.19 S 12 、Li 10 (Si 0.5 Ge 0.5 )P 2 S 12 、Li(Ge 0.5 Sn 0.5 )P 2 S 12 、Li(Si 0.5 Sn 0.5 )PsS 12 、Li 10 GeP 2 S 12 (LGPS)、Li 6 PS 5 X (where X is Cl, Br or I), Li 7 P 2 S 8 I、Li 10.35 Ge 1.35 P 1.65 S 12 、Li 3.25 Ge 0.25 P 0.75 S 4 、Li 10 SnP 2 S 12 、Li 10 SiP 2 S 12 、Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 、(1-x)P 2 S 5-x Li 2 S (wherein 0.5. ltoreq. x. ltoreq.0.7) and combinations thereof.
The halide solid state electrolyte may include one or more halide based materials selected from the group consisting of: li 2 CdCl 4 、Li 2 MgCl 4 、Li 2 CdI 4 、Li 2 ZnI 4 、Li 3 OCl、LiI、Li 5 ZnI 4 、Li 3 OCl 1-x Brx (where 0)<x<1) And combinations thereof.
The borate solid electrolyte is selected from one or more borate-based materials from the group comprising: li 2 B 4 O 7 、Li 2 O-(B 2 O 3 )-(P 2 O 5) And combinations thereof.
The nitride solid state electrolyte may be selected from one or more nitride based materials from the group comprising: li 3 N、Li 7 PN 4 、LiSi 2 N 3 LiPON, and combinations thereof.
The hydride solid-state electrolyte may be selected from one or more hydride-based materials from the group comprising: li 3 AlH 6 、LiBH 4 、LiBH 4 LiX (where X is one of Cl, Br and I), LiNH 2 、Li 2 NH、LiBH 4 -LiNH 2 And combinations thereof.
As a particular embodiment, the solid electrolyte may be a quasi-solid electrolyte comprising a mixture of the non-aqueous liquid electrolyte solution and the solid electrolyte system detailed above, e.g., comprising one or more ionic liquids and a solid electrolyte systemOne or more metal oxide particles (such as alumina (Al) 2 O 3 ) And/or silicon dioxide (SiO) 2 ))。
Example 1
This embodiment provides a negative pole piece, the negative pole piece that provides based on above-mentioned embodiment:
wherein the thickness of the first negative electrode layer is 40 μm, the thickness of the second negative electrode layer is 200 μm, the first negative electrode layer is positioned between the current collector and the second negative electrode layer, and the thickness of the first negative electrode layer is 20% of the thickness of the second negative electrode layer;
the first negative electrode layer is made of SiO, carbon-coated artificial graphite, polyacrylonitrile and carbon nanotubes (the mass ratio of SiO to carbon-coated artificial graphite is 4:96), and the second negative electrode layer is made of artificial graphite, conductive carbon black and polyacrylic acid;
the preparation method of the negative pole piece comprises the following steps:
(1) weighing a silica compound (SiO) and graphite with a carbon coating layer according to a mass ratio of 4:96, uniformly mixing to obtain a first mixed negative electrode active substance, dispersing and mixing the first mixed negative electrode active substance, a carbon nano tube and polyacrylonitrile with deionized water according to a mass ratio of 90:6:4 to obtain a first negative electrode layer slurry, and coating the single side of the first negative electrode layer slurry on a copper foil (the tensile strength is 350N/cm) 2 ) Drying and rolling the surface to obtain a first negative electrode layer;
(2) and dispersing the artificial graphite, the conductive carbon black and the polyacrylic acid in deionized water according to the mass ratio of 92:2:6 to obtain second negative electrode layer slurry, coating the second negative electrode layer slurry on the surface of the first negative electrode layer, and drying and rolling to obtain the negative electrode piece.
Example 2
The present embodiment provides a negative electrode plate, based on the negative electrode plate provided in the above specific embodiment:
the thickness of the first negative electrode layer is 30 micrometers, the thickness of the second negative electrode layer is 180 micrometers, the first negative electrode layer is positioned between the current collector and the second negative electrode layer, and the thickness of the first negative electrode layer is 16.7 of the thickness of the second negative electrode layer;
the first negative electrode layer is made of SiO, carbon-coated artificial graphite, polyacrylonitrile and carbon fiber (the mass ratio of SiO to carbon-coated artificial graphite is 3:97), and the second negative electrode layer is made of artificial graphite, conductive carbon black, styrene-butadiene rubber and sodium carboxymethylcellulose;
the preparation method of the negative pole piece comprises the following steps:
(1) weighing a silica compound (SiO) and graphite with a carbon coating layer according to a mass ratio of 3:97, uniformly mixing to obtain a first mixed negative electrode active substance, dispersing and mixing the first mixed negative electrode active substance, a carbon nano tube and polyacrylonitrile with deionization according to a mass ratio of 92:3:5 to obtain a first negative electrode layer slurry, and coating the single side of the first negative electrode layer slurry on a copper foil (the tensile strength is 350N/cm) 2 ) Drying and rolling the surface to obtain a first negative electrode layer;
(2) and dispersing the artificial graphite, the conductive carbon black, the styrene-butadiene rubber and the sodium carboxymethyl cellulose in deionized water according to the mass ratio of 92:2:3:3 to obtain second negative electrode layer slurry, coating the second negative electrode layer slurry on the surface of the first negative electrode layer, and drying and rolling to obtain the negative electrode piece.
Example 3
The present embodiment provides a negative electrode plate, based on the negative electrode plate provided in the above specific embodiment:
wherein, the first negative electrode (thickness is 52 μm, the second negative electrode layer thickness is 208 μm, the first negative electrode layer is located between the current collector and the second negative electrode layer, the first negative electrode layer thickness is 1/4;
the first negative electrode layer is made of SiO, carbon-coated artificial graphite, polyacrylonitrile and carbon nanotubes (the mass ratio of SiO to carbon-coated artificial graphite is 5:95), and the second negative electrode layer is made of artificial graphite, conductive carbon black and polyacrylic acid.
The remaining preparation process remains the same as in parameter example 1.
Example 4
The present example is different from example 1 in that the thickness of the first negative electrode layer in the present example is 100 μm, and the thickness of the first negative electrode layer is 50% of the thickness of the second negative electrode layer.
The remaining preparation process remains the same as in parameter example 1.
Example 5
The present example is different from example 1 in that the mass ratio of SiO to carbon-coated artificial graphite in the present example is 8: 92.
The remaining preparation methods and parameters were in accordance with example 1.
Example 6
The present example differs from example 1 in that the binder of the first negative electrode layer in this example is CMC + SBR.
The remaining preparation methods and parameters were in accordance with example 1.
Comparative example 1
The present comparative example is different from example 1 in that the negative electrode layer is only one layer and the negative electrode active material is pure artificial graphite.
The preparation process and parameters were kept the same as in step (2) of example 1.
Comparative example 2
The present comparative example is different from example 1 in that the negative electrode layer in the present comparative example is only one layer, and the negative electrode active material composition is the same as the first negative electrode layer composition in example 1.
The preparation process and parameters were kept the same as in step (2) of example 1.
Preparing a battery:
positive electrode piece: weighing NCM811, polyvinylidene fluoride and conductive carbon black according to the mass ratio of 90:5:5, dispersing in NMP to prepare positive slurry, coating the positive slurry on the surface of an aluminum foil, drying and rolling to form a positive pole piece, and die-cutting to form a corresponding shape.
The negative electrode plates provided in examples 1-6 and comparative examples 1-2 were used as negative electrodes, the prepared positive electrode plate was used as a positive electrode, a polyolefin separator and ethylene carbonate were used as an electrolyte, and LiCF was added to the electrolyte 3 SO 3 And obtaining the battery.
Electrochemical performance tests were performed on the batteries provided in examples 1 to 6 and comparative examples 1 to 2 under the following test conditions: the results of the charge and discharge tests at 25 ℃ and a constant current of 0.33C are shown in Table 1.
TABLE 1
Figure BDA0003608569140000221
From the data results of example 1 and example 4, it is clear that the excessive thickness of the first negative electrode layer affects the first-pass expansion rate of the negative electrode, and the first-pass expansion rate of the negative electrode increases, so that the capacity of the battery decreases and the cycle retention rate decreases during continuous charge and discharge.
From the data results of example 1 and example 5, it is known that, although the mass ratio of the silicon material in the first negative electrode layer is too large, the energy density of the battery can be effectively improved, but the first-turn expansion rate of the negative electrode is also affected, and the cycle retention rate of the battery is reduced.
From the data results of example 1 and example 6, it is found that when the adhesive force of the binder in the first negative electrode layer is the same as that in the second negative electrode layer, the first-pass expansion rate is increased, and the cycle performance of the battery is deteriorated.
From the data results of the embodiment 1 and the comparative example 1, compared with a pure graphite cathode, the cathode provided by the invention has the advantage that the capacity is obviously improved on the basis of keeping a lower expansion rate and better cycle stability.
From the data results of example 1 and comparative example 2, it can be seen that the negative electrode provided by the present invention has a higher capacity, a significantly reduced expansion rate, and a better cycle stability, compared to the negative electrode structure using a single layer.
In summary, in the negative electrode sheet provided by the invention, the first negative electrode layer is arranged on the side close to the current collector, and the first negative electrode layer simultaneously comprises graphite and silicon, so that the energy density of the whole negative electrode is improved, the capacity of the negative electrode is improved, and the expansion of the silicon material is further inhibited due to the existence of the second graphite negative electrode layer on the side far away from the current collector, so that the negative electrode sheet is adapted to the existing lithium ion battery system which is commercially used in a large scale, and the preparation method is simple and does not need complex preparation steps. According to the battery provided by the invention, the gram capacity of the negative electrode under 0.33C can reach more than 375mAh/g, the expansion rate of the first circle of the negative electrode is less than 38%, the capacity retention rate after 500 circles of circulation can reach more than 85.8%, the thickness of the negative electrode layer is further adjusted, the gram capacity of the negative electrode under 0.33C can reach more than 375mAh/g, the expansion rate of the first circle of the negative electrode is less than 30%, and the capacity retention rate after 500 circles of circulation can reach more than 93.4%.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention, and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (10)

1. A negative pole piece is characterized by comprising a current collector, a first negative pole layer and a second negative pole layer, wherein the first negative pole layer is positioned between the current collector and the second negative pole layer;
the negative electrode active material in the first negative electrode layer comprises graphite and a silicon material, and the negative electrode active material in the second negative electrode layer comprises graphite; the graphite in the first negative electrode layer is carbon-coated graphite;
the first negative electrode layer comprises a first binder and a first conductive agent; the first binder is polyacrylonitrile binder.
2. The negative electrode sheet according to claim 1, wherein the second negative electrode layer comprises a second binder and a second conductive agent;
preferably, the second binder is an aqueous binder;
preferably, the adhesion force of the first adhesive to the current collector is greater than the adhesion force of the second adhesive to the current collector;
preferably, the mass ratio of the first conductive agent in the first negative electrode layer is greater than that of the second conductive agent in the second negative electrode layer;
preferably, the first conductive agent includes any one or a combination of at least two of CNT, VGCF, super P, carbon black, acetylene black, or graphene;
preferably, the mass ratio of the CNT and/or VGCF in the first conductive agent is 15-25 wt%.
3. The negative electrode tab of claim 2, wherein the mass fraction of the binder in the second negative electrode layer is greater than the mass fraction of the binder in the first negative electrode layer.
4. The negative electrode plate as claimed in claim 3, wherein the mass ratio of the binder in the first negative electrode layer is 3-5 wt%;
preferably, the mass ratio of the binder in the second negative electrode layer is 4-8 wt%.
5. The negative electrode sheet of claim 1, wherein the tensile strength of the current collector is 350N/cm or more 2 。
6. The negative pole piece of claim 5, wherein the mass of the silicon material in the first negative pole layer is 3-5% of the mass of the negative active material in the first negative pole layer;
preferably, the silicon material comprises a silicon oxygen material.
7. The negative electrode plate as claimed in claim 6, wherein the thickness of the first negative electrode layer is 5-40%, preferably 16-25% of the thickness of the second negative electrode layer;
preferably, the thickness of the first negative electrode layer is 16-55 μm;
preferably, the thickness of the second negative electrode layer is 170-210 μm.
8. The negative electrode tab according to any one of claims 1 to 7, wherein the first negative electrode layer has a coating surface density of 30 to 50g/m 2 。
9. The preparation method of the negative electrode plate as claimed in any one of claims 1 to 8, wherein the preparation method comprises the following steps:
coating the slurry of the first negative electrode layer on the surface of a current collector to obtain a first negative electrode layer, and coating the slurry of the second negative electrode layer on the surface of the first negative electrode layer to obtain the negative electrode piece.
10. A lithium ion battery, characterized in that the lithium ion battery comprises the negative electrode sheet according to any one of claims 1 to 8.
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