EP2643875A1 - An electrode for lithium ion batteries and the method for manufacturing the same - Google Patents

An electrode for lithium ion batteries and the method for manufacturing the same

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Publication number
EP2643875A1
EP2643875A1 EP11843969.4A EP11843969A EP2643875A1 EP 2643875 A1 EP2643875 A1 EP 2643875A1 EP 11843969 A EP11843969 A EP 11843969A EP 2643875 A1 EP2643875 A1 EP 2643875A1
Authority
EP
European Patent Office
Prior art keywords
carbon
silicon
electrode
carbon fiber
fiber foil
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP11843969.4A
Other languages
German (de)
French (fr)
Other versions
EP2643875A4 (en
Inventor
Jingjun Zhang
Longjie Zhou
Jun Yang
Haiping Jia
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Robert Bosch GmbH
Original Assignee
Robert Bosch GmbH
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Robert Bosch GmbH filed Critical Robert Bosch GmbH
Priority claimed from PCT/CN2011/082696 external-priority patent/WO2012068987A1/en
Publication of EP2643875A1 publication Critical patent/EP2643875A1/en
Publication of EP2643875A4 publication Critical patent/EP2643875A4/en
Withdrawn legal-status Critical Current

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Classifications

    • 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
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/04—Processes of manufacture in general
    • H01M4/0471—Processes of manufacture in general involving thermal treatment, e.g. firing, sintering, backing particulate active material, thermal decomposition, pyrolysis
    • 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
    • H01M4/134—Electrodes based on metals, Si or alloys
    • 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
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H01M4/1395—Processes of manufacture of electrodes based on metals, Si or alloys
    • 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
    • H01M4/362—Composites
    • H01M4/364—Composites as mixtures
    • 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
    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H01M4/386—Silicon or alloys based on silicon
    • 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
    • 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
    • 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

Definitions

  • the present invention relates to an electrode, in particular to an electrode for lithium ion batteries formed of a ternary composite of silicon, carbon, and carbon fiber foil.
  • the present invention also relates to the manufacture method of said electrode. Background of the invention
  • lithium ion batteries are commonly used in devices or tools, such as cell-phones, notebooks, cameras, power tools, etc.
  • Graphite is the most important cathode material for lithium ion batteries.
  • Relatively low capacity for storing lithium ions of current graphite cathodes is an important reason for the relatively low energy density of batteries.
  • a composite electrode formed by a silicone/carbon active layer and a rigid copper current collector layer.
  • cathode materials of lithium batteries with large volume effect such as silicon
  • a significant volume change would occur in the silicon/carbon active layer during the charge and discharge cycle, which produces a strong mechanical stress not only inside the active layer but also between the silicon/carbon active layer and the rigid copper current collector layer, and in turn causes powdering and scaling off of the silicon material, breaking of the electric contact between particles of the material and between the coating layer and the copper current collector, and significant decrease of the charge and discharge capacity.
  • the battery fails rapidly.
  • a cathode for lithium ion batteries which can overcome the above defects, is in urgent need, so as to solve the problems such as significant decrease of charge and discharge capacity and rapid failure of the battery and allow lithium batteries to be widely applied in hybrid electric vehicles, plug-in hybrid electric vehicles and pure electric vehicles.
  • the present invention provides an electrode for lithium ion batteries, which is composed of a ternary composite of silicon, carbon, and carbon fiber foil.
  • said carbon is elementary carbon.
  • said carbon is formed by heat treatment of organic substances capable of forming carbon after heat treatment.
  • the weight ratio of silicon and carbon in the electrode is in the range of 4.0-0.1, preferably 2.33-0.50.
  • the total weight content of silicon and carbon in the electrode is >20 , based on the total weight of the ternary composite of silicon/carbon/carbon fiber foil.
  • the present invention further provides a method for manufacturing silicon/carbon/carbon fiber foil composite electrode, comprising the steps of:
  • the organic substance capable of forming carbon after heat treatment in step A refers to any organic substance known in the art, provided that it can form carbon after heat treatment. It may be any substance selected from the group consisting of asphalt, polyvinyl chloride, polyacrylonitrile, phenolic resin, and sucrose.
  • the inert gas employed in step C is argon (Ar), the reductive gas is hydrogen (H 2 ).
  • the volume ratio of argon and hydrogen is 90-100:10-0.
  • step C of the method of the present invention the process of heating in an inert gas atmosphere or an inert gas atmosphere mixed with a reductive gas is preferably carried out at a temperature of 400-1000°C for at least 2 hours.
  • Figure la is a photograph of the carbon fiber foil
  • Figure lb is a scanning electron microscope photograph of the carbon fiber foil, with a magnification factor of 250;
  • Figure lc is a scanning electron microscope photograph of the silicon/carbon/carbon fiber foil composite electrode of the present invention, with a magnification factor of 250;
  • Figure 2 is a comparative schematic diagram illustrating the charge and discharge cycling performance of the silicon/carbon/carbon fiber foil composite electrode manufactured by the method according to the present invention (electrode No. 1), the silicon/carbon/carbon fiber foil composite electrode manufactured by prior art methods (electrode No. 2), and the silicon/carbon/copper foil composite electrode manufactured by prior art methods (electrode No. 3); and
  • Figure 3 is a comparative schematic diagram illustrating the charge and discharge cycling performance of silicon/carbon/carbon fiber foil composite electrodes with different weight contents of silicon/carbon (electrodes No. 1, 4, and 5).
  • the present invention relates to a novel electrode for lithium ion batteries, formed of a ternary composite of silicon, carbon, and carbon fiber foil (hereinafter referred to as "ternary composite"),.
  • ternary composite refers to a ternary composite formed by distribution of silicon and carbon in voids of the carbon fiber foil.
  • the carbon distributed in the carbon fiber foil may be elementary carbon in any forms or any organic compound capable of forming carbon after heat treatment.
  • silicon refers to elementary silicon, which may be, e.g., monocrystalline silicon, polycrystalline silicon, amorphous silicon, and the like. The smaller the particle of elementary silicon, the better its performances.
  • carbon refers to elementary carbon, and may be formed of an organic substance capable of forming carbon after heat treatment.
  • the organic substance capable of forming carbon after heat treatment refers to any organic substance known in the art, provided that it can form carbon after heat treatment.
  • it may be any substance selected from the group consisting of asphalt, polyvinyl chloride, polyacrylonitrile, phenolic resin, and sucrose, etc., more preferably, polyvinyl chloride (PVC).
  • the ratio of silicon and carbon can be determined, according to the performances of the final product, by those skilled in the art.
  • the content of silicon may be increased in order to increase the electric capacity of the electrode.
  • the content of carbon may be increased in order to increase the stability and cycle life of the electrode.
  • the weight ratio of silicon and carbon in the electrode according to the present invention is preferably in the range of 4.0-0.1, more preferably 2.5-0.25, most preferably, 2.33-0.50.
  • the possibility of structural breaking and powdering of the electrode may be increased if the content of silicon is too high; and the capacity of the electrode may be decreased if the content of carbon is too high.
  • the total weight content of silicon and carbon based on the total weight of the ternary composite of silicon, carbon and carbon fiber foil, may be determined, according to the final demand of the electrode, by those skilled in the art. For example, if the mechanical stability of the electrode is preferably considered, the weight content of silicon and carbon should be decreased. On the contrary, if the capacity and cycling performance of the electrode is preferably considered, the weight content of silicon and carbon may be suitably increased. Preferably, the total weight content of silicon and carbon is >20 , based on the total weight of the ternary composite of silicon, carbon, and carbon fiber foil.
  • the carbon fiber foil used in the present invention is a weaved layer of carbon fiber with a porous structure.
  • the term "carbon fiber foil” refers to a carbon fiber foil with voids, in which carbon fibers intercross and overlap each other and form a porous structure.
  • Said carbon fiber foil includes many kinds of carbon fibers manufactured from various starting materials and by various processes, such as those of the model TGP-H-030 (Toray, Japan). Referring to figure la and lb, which illustrate the structure of a carbon fiber foil according to an embodiment, it can be seen from figure lb that there are voids among the fibers.
  • the carbon fiber foil is relatively thin.
  • the carbon fiber foil may be in any shapes, such as circle, square, or irregular shapes, and can be determined as required.
  • the invention relates to a method for manufacturing silicon/carbon/carbon fiber foil composite electrodes, comprising the steps of:
  • the starting materials of silicon and the organic substance capable of forming carbon after heat treatment are firstly mixed in a solvent, if appropriate, with stirring, to form a slurry.
  • the staring material "organic substance capable of forming carbon after heat treatment” refers to any organic substance known in the art, provided that it can form carbon after heat treatment. It may be any substance selected from the group consisting of asphalt, polyvinyl chloride, polyacrylonitrile, phenolic resin, sucrose, etc., more preferably, polyvinyl chloride.
  • the solvent can be any suitable solvent, provided that it does not react with the starting materials, i.e. the organic substance capable of forming carbon after heat treatment or silicon.
  • the solvent is a volatile solvent.
  • the solvent may be, e.g., acetone, cyclohexanone, ⁇ , ⁇ -dimethylfomamide (DMF), tetrahydrofuran (THF), water, etc., most preferably, THF
  • the weight ratio of the starting materials of silicon and the organic substance capable of forming carbon after heat treatment can be determined according to the desired final product electrode.
  • the carbon in the electrode of the present invention is formed by heat treatment, such that the carbonization rate of the organic substance capable of forming carbon after heat treatment, can be calculated experimentally, and the weight of the organic substance in the starting material can be calculated from the weight of carbon in the desired final product electrode, so that the weight ratio of the starting materials of silicon and the organic substance capable of forming carbon after heat treatment can be determined by the weight ratio of silicon and carbon in the designed electrode.
  • the inventor has experimentally determined that a certain polyvinyl chloride has a carbonization rate of 17% after heat treatment, such that the weight ratio of silicon and polyvinyl chloride in the starting material can be determined by the weight ratio of silicon and carbon in the designed electrode.
  • the weight ratio of silicon and carbon is in the range of 4.0-0.1, preferably 2.5-0.25, more preferably 2.33-0.50.
  • the weight ratio of the starting materials of silicon and the organic substance capable of forming carbon after heat treatment can be selected accordingly.
  • the organic substance is polyvinyl chloride
  • the weight ratio of the starting materials of silicon and polyvinyl chloride could be 0.40
  • the weight ratio of silicon and carbon in the electrode of the present application is 2.33, accordingly.
  • the mixture is preferably stirred, e.g., by such means as mechanical stirring or ultrasonic stirring, to mix the mixture homogenously and form a slurry.
  • stirring time is not strictly restricted, it is preferably at least 20 minutes, more preferably, at least 30 minutes.
  • step B the carbon fiber foil is immersed in said slurry after the slurry has been formed, such that the slurry coats on and penetrates into the carbon fiber foil.
  • the carbon fiber foil being employed may be in any form, such as circle, square, or irregular forms, which can be determined as required.
  • step C the carbon fiber foil, which has been coated and penetrated with the slurry, is heated in an inert gas atmosphere or an inert gas atmosphere mixed with a reductive gas at a temperature of at least 400 °C , preferably 600-1000 °C , more preferably 800-1000°C , for at least 2 hours, such that the organic substance capable of forming carbon after heat treatment is completely carbonized and silicon and carbon are completely combined with the carbon fiber foil.
  • any inert gas atmosphere such as helium, neon, argon, krypton, xenon, or nitrogen, or mixed gases thereof, preferably argon, nitrogen, etc.
  • the inert gas does not contain oxygen, most preferably, an inert gas with high purity is employed, in order to prevent oxidation.
  • a mixed gas atmosphere of inert gas and a small amount of reductive gas may be employed, wherein the reductive gas is preferably H 2 .
  • the mixed gas atmosphere of inert gas and a small amount of reductive gas is a mixed gas of argon and hydrogen.
  • the ratio of the inert gas and the reductive gas is 90-100:10-0.
  • heating time in step C is not strictly restricted, it is typically 2 hours, and can be determined as required.
  • step C the carbon fiber foil, which has been coated and penetrated with the slurry, can be optionally dried before heating.
  • Said drying process can be carried out at room temperature or higher, preferably, 50-70 °C .
  • the drying time is not strictly restricted, provided the solvent is substantially volatilized, preferably, the drying process is carried out for at least 4 hours.
  • the silicon/carbon/carbon fiber foil composite electrode of the present invention has a significantly improved cycling performance.
  • the present invention provides a fundamental solution to the problem of the generation of mechanic stress between silicon carbon active layer and rigid copper foil current collector layer, and improve the cycle life of the electrode accordingly.
  • the electrode is capable of performing hundreds of lithium insertion/extraction cycles under high current density (0.5 C).
  • the conservation rate of the capacity is at least 84.2% and the specific capacity is at least 977 mAh/g.
  • Example 1 silicon/carbon/carbon fiber foil electrode manufactured according to the method of the present invention (electrode No. 1)
  • the coated and penetrated carbon fiber foil was then heated under a H 2 -Ar atmosphere (5 vol.% H 2 , 95 vol.% Ar) at 900°C for 2 h, to obtain a silicon/carbon/carbon fiber foil composite electrode (electrode No. 1) formed of the ternary composite of silicon/carbon/carbon fiber foil.
  • the mass load of the silicon/carbon on the carbon fiber foil is about 25% by weight.
  • the weight ratio of silicon and carbon in the electrode is calculated to be 2.33, on the basis of the carbonization rate of the polyvinyl chloride.
  • Comparative example 1 silicon/carbon/copper foil electrode manufactured according to prior art method (electrode No. 3)
  • the starting material silicon (the same as example 1) and PVC (the same as example 1) (the weight ratio of Si/PVC is 1:4) were mixed in THF and stirred under ultrasonication for 30 min to form a preliminary slurry. Then, the resulting preliminary slurry was sprayed onto a flat glass surface and dried at 80 °C , and the obtained precursor was heated under a H 2 -Ar atmosphere (5 vol.% H 2 , 95 vol.% Ar) at 900 °C for 2h. The resulting material was named as active material for further use.
  • a slurry was prepared using 80 wt.% the active material, 10 wt.% polyvinylidene fluoride (PVDF) binder (Aldrich), and 10 wt.% carbon black (Super P, 40 nm, Timcal) as the conducting agent, in a solution of N-methyl-2-pyrrolidone (NMP).
  • the slurry was coated on a copper foil to obtain a homogeneous layer. After coating, the homogeneous layer was dried at 80°C for 10 mins to remove the solvent of NMP. Then, a circle piece of electrode with a diameter of 12 mm was cut off from the dried layer as Electrode No. 3. It was then further dried at 100 °C for 6 h.
  • the mass load of silicon/carbon on the copper foil is about 20%.
  • the weight ratio of silicon and carbon in the electrode is calculated to be 2.33, on the basis of the carbonization rate of the polyvinyl chloride.
  • Comparative example 2 silicon/carbon/carbon fiber foil electrode manufactured according to prior art method (electrode No. 2)
  • a carbon fiber foil (a small circle with a diameter of 12 mm) was immersed in the slurry prepared in comparative example 1, and then the slurry was ultras onicated for 1 min until it coated on and penetrated into the carbon fiber foil. The foil was then further dried at 100°C for 6 h to form Electrode No. 2.
  • the mass load of silicon/carbon on the carbon fiber foil is about 60 wt.%.
  • the weight ratio of silicon and carbon in the electrode is calculated to be 2.33, on the basis of the carbonization rate of the polyvinyl chloride.
  • Example 2 and example 3 silicon/carbon/carbon fiber foil electrode manufactured according to the method of the present application (electrode No.4 and electrode No. 5)
  • Electrode No.4 and electrode No. 5 were manufactured according to a method similar to example 1, except that the weight ratio of silicon and carbon in electrode No. 4 was 1.17 and that in electrode No. 5 was 0.50; and the mass loads of silicon/carbon on the carbon fiber foil in electrode No.4 and electrode No. 5 were about 25 wt.%.
  • the electrochemical performances of electrode No.l, electrode No.4 and electrode No. 5 were showed in figure 3.
  • CR2016 coin-type cells were assembled in an argon-filled glove box (MB- 10 compact, MBRAUN) with electrodes No. 1, 2, 3, 4 and 5 as the working electrodes, respectively, metallic lithium as the counter electrode, lmol/L LiPF 6 in EC:DMC (ethylene carbonate (EC) : dimethyl carbonate (DMC), volume ratio of 1:1) as electrolyte, and ET20-26 (Entek) as separator.
  • EC:DMC ethylene carbonate (EC) : dimethyl carbonate (DMC), volume ratio of 1:1) as electrolyte
  • ET20-26 Entek
  • Figure 2 illustrates the cycling number and capacity of the cells with electrodes No.
  • the silicon/carbon/carbon fiber foil electrode manufactured by using carbon fiber foil instead of copper foil (electrode 2) in comparative example 2 has a higher capacity and cycle life.
  • the silicon/carbon/carbon fiber foil composite electrode manufactured according to the method of the present invention in example 1 has a highest electric capacity and cycle life.
  • FIG. 3 illustrates the cycling number and capacity of the cells with electrodes No. 1, 4 and 5 as the working electrodes, respectively.
  • the ratio of silicon/carbon significantly influences the performances of the electrodes.
  • the higher the silicon content in the electrode the larger the capacity of the electrode and the shorter the cycle life; the lower the silicon content, the smaller the capacity of the electrode and the longer the cycle life.

Abstract

The present invention relates to an electrode formed of a ternary composite of silicon, carbon, and carbon fiber foil, for lithium ion batteries. The present invention also relates to a method for manufacturing silicon/carbon/carbon fiber foil composite electrode for lithium itteries, comprising the steps of: A. mixing silicon and an organic substance capable of forming carbon after heat treatment, in a solvent, to form a slurry; B. immersing the carbon fiber foil in said slurry until the slurry coats on and penetrates into the carbon fiber foil; and C. heating the carbon fiber foil, which has been coated and penetrated with the slurry, in an inert gas atmosphere or an inert gas atmosphere mixed with a reductive gas at a temperature of at least 400℃ for at least 2 hours.

Description

An electrode for lithium ion batteries and the method for
manufacturing the same
Field of the invention
The present invention relates to an electrode, in particular to an electrode for lithium ion batteries formed of a ternary composite of silicon, carbon, and carbon fiber foil. The present invention also relates to the manufacture method of said electrode. Background of the invention
Nowadays, lithium ion batteries are commonly used in devices or tools, such as cell-phones, notebooks, cameras, power tools, etc. Graphite is the most important cathode material for lithium ion batteries. As more attention are paid to electric vehicles in automotive industry, the development of lithium ion batteries with high energy density has become an urgent need for electric vehicle industry. Relatively low capacity for storing lithium ions of current graphite cathodes is an important reason for the relatively low energy density of batteries.
Now, the researchers have realized that if the graphite is replaced with silicon composites, the capacity of the cathode of lithium ion batteries could be increased by many times. It has been suggested to replace graphite with silicon/carbon composites in the prior art. Conventional silicon/carbon composites, commonly manufactured by pyrolysis, mechanical mixing and high energy ball milling, or combination thereof, consist of Si particles embedded in a dense carbon matrix. However, the volume change effect of Si can only be inhibited to a limited degree by silicon/carbon composites manufactured with such method, thus only limited stability and cycle life can be offered. Structural breaking and powdering tend to occur if lithium ions are embedded into the structure of silicon material during the charge and discharge cycles. As a result, the cycling ability of the battery would be very poor.
Also disclosed in the prior art is a composite electrode formed by a silicone/carbon active layer and a rigid copper current collector layer. As for cathode materials of lithium batteries with large volume effect, such as silicon, a significant volume change would occur in the silicon/carbon active layer during the charge and discharge cycle, which produces a strong mechanical stress not only inside the active layer but also between the silicon/carbon active layer and the rigid copper current collector layer, and in turn causes powdering and scaling off of the silicon material, breaking of the electric contact between particles of the material and between the coating layer and the copper current collector, and significant decrease of the charge and discharge capacity. As a result, the battery fails rapidly.
Therefore, a cathode for lithium ion batteries, which can overcome the above defects, is in urgent need, so as to solve the problems such as significant decrease of charge and discharge capacity and rapid failure of the battery and allow lithium batteries to be widely applied in hybrid electric vehicles, plug-in hybrid electric vehicles and pure electric vehicles. Summary of the invention
According to an aspect, the present invention provides an electrode for lithium ion batteries, which is composed of a ternary composite of silicon, carbon, and carbon fiber foil.
In an embodiment, said carbon is elementary carbon.
In another embodiment, said carbon is formed by heat treatment of organic substances capable of forming carbon after heat treatment.
In an embodiment, the weight ratio of silicon and carbon in the electrode is in the range of 4.0-0.1, preferably 2.33-0.50.
In another embodiment, the total weight content of silicon and carbon in the electrode is >20 , based on the total weight of the ternary composite of silicon/carbon/carbon fiber foil.
According to another aspect, the present invention further provides a method for manufacturing silicon/carbon/carbon fiber foil composite electrode, comprising the steps of:
A. mixing silicon and an organic substance capable of forming carbon after heat treatment, in a solvent, to form a slurry;
B. immersing the carbon fiber foil in said slurry until the slurry coats on and penetrates into the carbon fiber foil; and
C. heating the carbon fiber foil, which has been coated and penetrated with the slurry, in an inert gas atmosphere or an inert gas atmosphere mixed with a reductive gas at a temperature of at least 400 °C for at least 2 hours.
In an embodiment of the method of the present invention, the organic substance capable of forming carbon after heat treatment in step A refers to any organic substance known in the art, provided that it can form carbon after heat treatment. It may be any substance selected from the group consisting of asphalt, polyvinyl chloride, polyacrylonitrile, phenolic resin, and sucrose.
In an embodiment of the method of the present invention, the inert gas employed in step C is argon (Ar), the reductive gas is hydrogen (H2). Preferably, the volume ratio of argon and hydrogen is 90-100:10-0.
In step C of the method of the present invention, the process of heating in an inert gas atmosphere or an inert gas atmosphere mixed with a reductive gas is preferably carried out at a temperature of 400-1000°C for at least 2 hours.
Brief description of the drawings
The present invention will be more apparent from the following drawings.
Figure la is a photograph of the carbon fiber foil;
Figure lb is a scanning electron microscope photograph of the carbon fiber foil, with a magnification factor of 250;
Figure lc is a scanning electron microscope photograph of the silicon/carbon/carbon fiber foil composite electrode of the present invention, with a magnification factor of 250;
Figure 2 is a comparative schematic diagram illustrating the charge and discharge cycling performance of the silicon/carbon/carbon fiber foil composite electrode manufactured by the method according to the present invention (electrode No. 1), the silicon/carbon/carbon fiber foil composite electrode manufactured by prior art methods (electrode No. 2), and the silicon/carbon/copper foil composite electrode manufactured by prior art methods (electrode No. 3); and
Figure 3 is a comparative schematic diagram illustrating the charge and discharge cycling performance of silicon/carbon/carbon fiber foil composite electrodes with different weight contents of silicon/carbon (electrodes No. 1, 4, and 5).
Detailed description of the invention
In the first aspect, the present invention relates to a novel electrode for lithium ion batteries, formed of a ternary composite of silicon, carbon, and carbon fiber foil (hereinafter referred to as "ternary composite"),.
As used herein, the term "ternary composite" refers to a ternary composite formed by distribution of silicon and carbon in voids of the carbon fiber foil. The carbon distributed in the carbon fiber foil may be elementary carbon in any forms or any organic compound capable of forming carbon after heat treatment.
As used herein, the term "silicon" refers to elementary silicon, which may be, e.g., monocrystalline silicon, polycrystalline silicon, amorphous silicon, and the like. The smaller the particle of elementary silicon, the better its performances.
As used herein, the term "carbon" refers to elementary carbon, and may be formed of an organic substance capable of forming carbon after heat treatment. The organic substance capable of forming carbon after heat treatment refers to any organic substance known in the art, provided that it can form carbon after heat treatment. Preferably, it may be any substance selected from the group consisting of asphalt, polyvinyl chloride, polyacrylonitrile, phenolic resin, and sucrose, etc., more preferably, polyvinyl chloride (PVC).
In the electrode according to the present invention, the ratio of silicon and carbon can be determined, according to the performances of the final product, by those skilled in the art. For example, the content of silicon may be increased in order to increase the electric capacity of the electrode. On the contrary, the content of carbon may be increased in order to increase the stability and cycle life of the electrode. The weight ratio of silicon and carbon in the electrode according to the present invention is preferably in the range of 4.0-0.1, more preferably 2.5-0.25, most preferably, 2.33-0.50. The possibility of structural breaking and powdering of the electrode may be increased if the content of silicon is too high; and the capacity of the electrode may be decreased if the content of carbon is too high.
In the electrode of the present invention, the total weight content of silicon and carbon, based on the total weight of the ternary composite of silicon, carbon and carbon fiber foil, may be determined, according to the final demand of the electrode, by those skilled in the art. For example, if the mechanical stability of the electrode is preferably considered, the weight content of silicon and carbon should be decreased. On the contrary, if the capacity and cycling performance of the electrode is preferably considered, the weight content of silicon and carbon may be suitably increased. Preferably, the total weight content of silicon and carbon is >20 , based on the total weight of the ternary composite of silicon, carbon, and carbon fiber foil.
The carbon fiber foil used in the present invention, as a part of the silicon/carbon/carbon fiber foil composite electrode for lithium ion batteries, unlike conventional copper foil, is a weaved layer of carbon fiber with a porous structure. In particular, as used herein, the term "carbon fiber foil" refers to a carbon fiber foil with voids, in which carbon fibers intercross and overlap each other and form a porous structure. Said carbon fiber foil includes many kinds of carbon fibers manufactured from various starting materials and by various processes, such as those of the model TGP-H-030 (Toray, Japan). Referring to figure la and lb, which illustrate the structure of a carbon fiber foil according to an embodiment, it can be seen from figure lb that there are voids among the fibers. The carbon fiber foil is relatively thin. The carbon fiber foil may be in any shapes, such as circle, square, or irregular shapes, and can be determined as required.
Referring to figure lc, it illustrates the structure of a ternary composite according to an embodiment of the present invention. In the ternary composite, silicon and carbon are coated on the carbon fiber foil and penetrates into the voids of the carbon fiber foil. According to another aspect, the invention relates to a method for manufacturing silicon/carbon/carbon fiber foil composite electrodes, comprising the steps of:
A. mixing silicon and an organic substance capable of forming carbon after heat treatment, in a solvent, to form a slurry;
B. immersing the carbon fiber foil in said slurry until the slurry coats on and penetrates into the carbon fiber foil; and
C. heating the carbon fiber foil, which has been coated and penetrated with the slurry, in an inert gas atmosphere or an inert gas atmosphere mixed with a reductive gas at a temperature of at least 400 °C for at least 2 hours.
In step A of the above mentioned method, the starting materials of silicon and the organic substance capable of forming carbon after heat treatment are firstly mixed in a solvent, if appropriate, with stirring, to form a slurry. As used herein, the staring material "organic substance capable of forming carbon after heat treatment" refers to any organic substance known in the art, provided that it can form carbon after heat treatment. It may be any substance selected from the group consisting of asphalt, polyvinyl chloride, polyacrylonitrile, phenolic resin, sucrose, etc., more preferably, polyvinyl chloride. The solvent can be any suitable solvent, provided that it does not react with the starting materials, i.e. the organic substance capable of forming carbon after heat treatment or silicon. Preferably, the solvent is a volatile solvent. The solvent may be, e.g., acetone, cyclohexanone, Ν,Ν-dimethylfomamide (DMF), tetrahydrofuran (THF), water, etc., most preferably, THF
In step A, the weight ratio of the starting materials of silicon and the organic substance capable of forming carbon after heat treatment, can be determined according to the desired final product electrode. Firstly, the carbon in the electrode of the present invention is formed by heat treatment, such that the carbonization rate of the organic substance capable of forming carbon after heat treatment, can be calculated experimentally, and the weight of the organic substance in the starting material can be calculated from the weight of carbon in the desired final product electrode, so that the weight ratio of the starting materials of silicon and the organic substance capable of forming carbon after heat treatment can be determined by the weight ratio of silicon and carbon in the designed electrode. For instance, in case that polyvinyl chloride is employed as the organic substance, the inventor has experimentally determined that a certain polyvinyl chloride has a carbonization rate of 17% after heat treatment, such that the weight ratio of silicon and polyvinyl chloride in the starting material can be determined by the weight ratio of silicon and carbon in the designed electrode.
In the electrode manufactured according to the present invention, the weight ratio of silicon and carbon is in the range of 4.0-0.1, preferably 2.5-0.25, more preferably 2.33-0.50. The weight ratio of the starting materials of silicon and the organic substance capable of forming carbon after heat treatment, can be selected accordingly. For instance, in the case that the organic substance is polyvinyl chloride, the weight ratio of the starting materials of silicon and polyvinyl chloride, could be 0.40, and the weight ratio of silicon and carbon in the electrode of the present application is 2.33, accordingly.
After the starting materials of silicon and the organic substance capable of forming carbon after heat treatment is mixed in a solvent, the mixture is preferably stirred, e.g., by such means as mechanical stirring or ultrasonic stirring, to mix the mixture homogenously and form a slurry. Although the stirring time is not strictly restricted, it is preferably at least 20 minutes, more preferably, at least 30 minutes.
In step B, the carbon fiber foil is immersed in said slurry after the slurry has been formed, such that the slurry coats on and penetrates into the carbon fiber foil. The carbon fiber foil being employed may be in any form, such as circle, square, or irregular forms, which can be determined as required.
In step C, the carbon fiber foil, which has been coated and penetrated with the slurry, is heated in an inert gas atmosphere or an inert gas atmosphere mixed with a reductive gas at a temperature of at least 400 °C , preferably 600-1000 °C , more preferably 800-1000°C , for at least 2 hours, such that the organic substance capable of forming carbon after heat treatment is completely carbonized and silicon and carbon are completely combined with the carbon fiber foil.
Any inert gas atmosphere, such as helium, neon, argon, krypton, xenon, or nitrogen, or mixed gases thereof, preferably argon, nitrogen, etc., can be employed in the process. Preferably, the inert gas does not contain oxygen, most preferably, an inert gas with high purity is employed, in order to prevent oxidation. In order to completely avoid the influence of the oxygen possibly presented in the solvent and inert gas, a mixed gas atmosphere of inert gas and a small amount of reductive gas may be employed, wherein the reductive gas is preferably H2. Preferably, the mixed gas atmosphere of inert gas and a small amount of reductive gas is a mixed gas of argon and hydrogen. Preferably, the ratio of the inert gas and the reductive gas is 90-100:10-0.
Although the heating time in step C is not strictly restricted, it is typically 2 hours, and can be determined as required.
In step C, the carbon fiber foil, which has been coated and penetrated with the slurry, can be optionally dried before heating. Said drying process can be carried out at room temperature or higher, preferably, 50-70 °C . The drying time is not strictly restricted, provided the solvent is substantially volatilized, preferably, the drying process is carried out for at least 4 hours.
As compared with prior art electrodes formed of silicon, carbon, and copper foils, the silicon/carbon/carbon fiber foil composite electrode of the present invention has a significantly improved cycling performance. The present invention provides a fundamental solution to the problem of the generation of mechanic stress between silicon carbon active layer and rigid copper foil current collector layer, and improve the cycle life of the electrode accordingly. For instance, as shown in example 2 (electrode No. 4), the electrode is capable of performing hundreds of lithium insertion/extraction cycles under high current density (0.5 C). Moreover, even if 90 cycles are performed, the conservation rate of the capacity is at least 84.2% and the specific capacity is at least 977 mAh/g.
The following examples further illustrate the invention. As used herein, unless otherwise specified, all ratios and percentages used in the present invention are on weight basis. Examples
Example 1: silicon/carbon/carbon fiber foil electrode manufactured according to the method of the present invention (electrode No. 1)
The starting material silicon (silicon powder, 50 nm, 99.5%, Nanjing Emperor Nano Material Co., Ltd., Nanjing, China) and PVC (polyvinyl chloride, Mw = -233,000 g/mol, Aldrich) (the weight ratio of Si/PVC is 1:4) were mixed in THF and stirred under ultrasonication for 30 min to form a slurry. Then carbon fiber foil (a small circle with D = 12 mm, TGP-H-030, thickness = 110 μιη, Toray) was immersed in the slurry, and the slurry was further ultrasonicated for 1 min until it coated on and penetrated into the carbon fiber foil. After being dried at 60°C for 5h, the coated and penetrated carbon fiber foil was then heated under a H2-Ar atmosphere (5 vol.% H2, 95 vol.% Ar) at 900°C for 2 h, to obtain a silicon/carbon/carbon fiber foil composite electrode (electrode No. 1) formed of the ternary composite of silicon/carbon/carbon fiber foil. The mass load of the silicon/carbon on the carbon fiber foil is about 25% by weight. The weight ratio of silicon and carbon in the electrode is calculated to be 2.33, on the basis of the carbonization rate of the polyvinyl chloride.
Comparative example 1 : silicon/carbon/copper foil electrode manufactured according to prior art method (electrode No. 3)
The starting material silicon (the same as example 1) and PVC (the same as example 1) (the weight ratio of Si/PVC is 1:4) were mixed in THF and stirred under ultrasonication for 30 min to form a preliminary slurry. Then, the resulting preliminary slurry was sprayed onto a flat glass surface and dried at 80 °C , and the obtained precursor was heated under a H2-Ar atmosphere (5 vol.% H2, 95 vol.% Ar) at 900 °C for 2h. The resulting material was named as active material for further use. A slurry was prepared using 80 wt.% the active material, 10 wt.% polyvinylidene fluoride (PVDF) binder (Aldrich), and 10 wt.% carbon black (Super P, 40 nm, Timcal) as the conducting agent, in a solution of N-methyl-2-pyrrolidone (NMP). The slurry was coated on a copper foil to obtain a homogeneous layer. After coating, the homogeneous layer was dried at 80°C for 10 mins to remove the solvent of NMP. Then, a circle piece of electrode with a diameter of 12 mm was cut off from the dried layer as Electrode No. 3. It was then further dried at 100 °C for 6 h. The mass load of silicon/carbon on the copper foil is about 20%. The weight ratio of silicon and carbon in the electrode is calculated to be 2.33, on the basis of the carbonization rate of the polyvinyl chloride.
Comparative example 2: silicon/carbon/carbon fiber foil electrode manufactured according to prior art method (electrode No. 2)
A carbon fiber foil (a small circle with a diameter of 12 mm) was immersed in the slurry prepared in comparative example 1, and then the slurry was ultras onicated for 1 min until it coated on and penetrated into the carbon fiber foil. The foil was then further dried at 100°C for 6 h to form Electrode No. 2. The mass load of silicon/carbon on the carbon fiber foil is about 60 wt.%. The weight ratio of silicon and carbon in the electrode is calculated to be 2.33, on the basis of the carbonization rate of the polyvinyl chloride.
Example 2 and example 3: silicon/carbon/carbon fiber foil electrode manufactured according to the method of the present application (electrode No.4 and electrode No. 5)
Electrode No.4 and electrode No. 5 were manufactured according to a method similar to example 1, except that the weight ratio of silicon and carbon in electrode No. 4 was 1.17 and that in electrode No. 5 was 0.50; and the mass loads of silicon/carbon on the carbon fiber foil in electrode No.4 and electrode No. 5 were about 25 wt.%. The electrochemical performances of electrode No.l, electrode No.4 and electrode No. 5 were showed in figure 3.
CR2016 coin-type cells were assembled in an argon-filled glove box (MB- 10 compact, MBRAUN) with electrodes No. 1, 2, 3, 4 and 5 as the working electrodes, respectively, metallic lithium as the counter electrode, lmol/L LiPF6 in EC:DMC (ethylene carbonate (EC) : dimethyl carbonate (DMC), volume ratio of 1:1) as electrolyte, and ET20-26 (Entek) as separator. Example of the electrochemical test
The charge and discharge tests were conducted on a LAND battery test system
(Wuhan Kingnuo Electronics Co. Ltd., China) at 25 °C with a current density of 0.5 mA/mg, The cut-off voltage was 0.01 V versus metallic lithium for discharge (Li insertion) and 1.2 V versus metallic lithium for charge (Li extraction).
The electrochemical performances of electrodes No. 1-3 were shown in figure 2.
Figure 2 illustrates the cycling number and capacity of the cells with electrodes No.
1-3 as the working electrodes, respectively.
As shown in figure 2, under the same manufacture condition, as compared to the silicon/carbon/copper foil electrode manufactured according to the prior art methods
(electrode 3) in comparative example 1, the silicon/carbon/carbon fiber foil electrode manufactured by using carbon fiber foil instead of copper foil (electrode 2) in comparative example 2 has a higher capacity and cycle life. Moreover, the silicon/carbon/carbon fiber foil composite electrode manufactured according to the method of the present invention in example 1 has a highest electric capacity and cycle life.
The electrochemical performances of electrodes No. 1, 4 and 5 are shown in figure 3. Figure 3 illustrates the cycling number and capacity of the cells with electrodes No. 1, 4 and 5 as the working electrodes, respectively.
As shown in figure 3, in the electrodes of the present invention, the ratio of silicon/carbon significantly influences the performances of the electrodes. The higher the silicon content in the electrode, the larger the capacity of the electrode and the shorter the cycle life; the lower the silicon content, the smaller the capacity of the electrode and the longer the cycle life.
The above examples only servers to illustrate the invention but do not restrict the scope of the invention in any manner. On the contrary, it should be understood that any embodiments and modification may be made by a person skilled in the art without departing from the spirit of the present invention upon reading the foregoing description.

Claims

What is claimed is:
1. An electrode for lithium ion batteries, formed of a ternary composite of silicon, carbon, and carbon fiber foil.
2. The electrode according to claim 1, wherein the carbon is elementary carbon.
3. The electrode according to claim 1, wherein the carbon is formed by heat treatment of an organic substance capable of forming carbon after the heat treatment.
4. The electrode according to claim 2 or 3, wherein the weight ratio of silicon and carbon is in the range of 4.0-0.1.
5. The electrode according to claim 4, wherein the weight ratio of silicon and carbon is in the range of 2.33-0.50.
6. The electrode according to any one of claims 1-3, wherein the total weight content of silicon and carbon is >20 , based on the total weight of the ternary composite of silicon, carbon, and carbon fiber foil.
7. A method for manufacturing a silicon/carbon/carbon fiber foil composite electrode for lithium ion batteries, comprising the steps of:
A. mixing silicon and an organic substance capable of forming carbon after heat treatment, in a solvent, to form a slurry;
B. immersing the carbon fiber foil in said slurry until the slurry coats on and penetrates into the carbon fiber foil; and
C. heating the carbon fiber foil, which has been coated and penetrated with the slurry, in an inert gas atmosphere or an inert gas atmosphere mixed with a reductive gas at a temperature of at least 400 °C for at least 2 hours.
8. The method according to claim 7, wherein the organic substance capable of forming carbon after heat treatment is selected from asphalt, polyvinyl chloride, polyacrylonitrile, phenolic resin, and sucrose.
9. The method of claim 7, wherein the inert gas is argon and the reductive gas is hydrogen.
10. The method of claim 9, wherein the ratio of argon and hydrogen is 90-100:10-0.
11. The method according to any one of claims 7-10, wherein the heating in an inert gas atmosphere or an inert gas atmosphere mixed with a reductive gas in step C is carried out at a temperature of 400-1000 °C for at least 2 hours.
EP11843969.4A 2010-11-25 2011-11-23 An electrode for lithium ion batteries and the method for manufacturing the same Withdrawn EP2643875A4 (en)

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Citations (3)

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Publication number Priority date Publication date Assignee Title
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Patent Citations (3)

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Publication number Priority date Publication date Assignee Title
US5225296A (en) * 1989-11-21 1993-07-06 Ricoh Company, Ltd. Electrode and method of producing the same
US20080096110A1 (en) * 2006-06-16 2008-04-24 Matsushita Electric Industrial Co., Ltd. Negative electrode and non-aqueous electrolyte secondary battery using the same
US20090269511A1 (en) * 2008-04-25 2009-10-29 Aruna Zhamu Process for producing hybrid nano-filament electrodes for lithium batteries

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Title
GUO J ET AL: "A porous silicon-carbon anode with high overall capacity on carbon fiber current collector", ELECTROCHEMISTRY COMMUNICATIONS, ELSEVIER, AMSTERDAM, NL, vol. 12, no. 7, 1 July 2010 (2010-07-01), pages 981-984, XP027087946, ISSN: 1388-2481, DOI: 10.1016/J.ELECOM.2010.05.006 [retrieved on 2010-06-14] *
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