WO2007022672A1 - A cathode, a lithium ion battery comprising such a cathode and processes for preparation thereof - Google Patents

A cathode, a lithium ion battery comprising such a cathode and processes for preparation thereof Download PDF

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Publication number
WO2007022672A1
WO2007022672A1 PCT/CN2006/000759 CN2006000759W WO2007022672A1 WO 2007022672 A1 WO2007022672 A1 WO 2007022672A1 CN 2006000759 W CN2006000759 W CN 2006000759W WO 2007022672 A1 WO2007022672 A1 WO 2007022672A1
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WIPO (PCT)
Prior art keywords
cathode
active substance
adhesive
lithium ion
based polymer
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PCT/CN2006/000759
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French (fr)
Inventor
Feng Xiao
Jianchang Zhang
Huajun Sun
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Byd Company Limited
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Publication of WO2007022672A1 publication Critical patent/WO2007022672A1/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
    • 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
    • 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
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/621—Binders
    • 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
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/621—Binders
    • H01M4/622—Binders being polymers
    • 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
    • H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
    • 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
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
    • H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
    • 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
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
    • H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
    • 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

  • a CATHODE 5 A LITHIUM ION BATTERY COMPRISING SUCH A CATHODE AND PROCESSES FOR PREPARATION THEREOF
  • the present invention relates to a cathode, a lithium ion battery comprising such a cathode and processes for preparation thereof.
  • a lithium ion battery mainly comprises an electrode core and an electrolyte, both sealed in a battery shell.
  • Said electrode core comprises electrodes and a membrane.
  • Said battery electrode comprises a cathode and an anode and said cathode comprises a current collector and a cathode material coated on and/or filled in the current collector.
  • Said cathode material comprises a cathode active substance, a conductive additive and an adhesive.
  • the process for preparing the cathode comprises coating a slurry comprising a cathode active substance, a conductive additive, an adhesive and a solvent on and/or filled it in the current collector, and then drying and optionally rolling the resulted current collector.
  • the lithium ion secondary battery uses polyvinylmetadifiuoride (PVDF) as an adhesive, and N-dimethylpyrrolidone (NMP), N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO) and the like as a solvent of PVDF.
  • PVDF polyvinylmetadifiuoride
  • NMP N-dimethylpyrrolidone
  • DMF N,N-dimethyl formamide
  • DMSO dimethyl sulfoxide
  • the cathode active substance separates from the current collector.
  • PVDF is corrosive and expensive.
  • the above organic compounds used as solvents such as NMP, DMF and DMSO possess rather high boiling points and are difficult to remove from the sheet during the process of the preparation of the sheet. Therefore, a relatively high temperature such as of 120-135 ° C is required to obtain a cathode after the slurry is coated on the current collector, which brings about much inconvenience to the operation.
  • said organic solvents are harmful to human body and undesirably pollute the environment.
  • water-based adhesive possess the advantages of no pollution, low cost, incombustibility and safety in use.
  • the water-based adhesive was mainly used in the cathode in this art.
  • JP Application Laid-Open Hei 1993-74461 discloses a water-based adhesive using styrene-butadiene rubber (SBR) as an adhesive.
  • SBR styrene-butadiene rubber
  • CN 1507093 A discloses a cathode used for the non-aqueous electrolyte secondary battery, which uses a carbon material able to reversibly intercalate and de-intercalate lithium ion and an adhesive as the cathode material, characterized in that said carbon material is graphite and the adhesive is at least one selected from adhesive of ethylene-acrylic acid copolymer, adhesive of ethylene-acrylate copolymer, adhesive of ethylene-methyl acrylate copolymer, adhesive of ethylene-methacrylic acid copolymer and adhesive of ethylene-methacrylate copolymer with the content of ethylene being above 70% by weight and below 95% by weight.
  • CN 1622366 A discloses a cathode of a re-chargeable lithium ion battery, which comprises a current collector and a cathode active substance layer coated on the current collector.
  • the cathode active substance layer comprises a cathod active substance and a cellulose-based polymer adhesive with the esterification degree of the cellulose-based polymer being 1.3 or more and the molecular weight of the same being 100000 or more.
  • Said cellulose-based polymer adhesive is one or more selected from carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropylmethyl cellulose, hydroxypropylethyl cellulose.
  • CMC carboxymethyl cellulose
  • methyl cellulose methyl cellulose
  • ethyl cellulose hydroxypropylmethyl cellulose
  • hydroxypropylethyl cellulose methylpropyl cellulose
  • Such adhesives will not make the cathode active substance covered, but make the cathode active substance separate from the current collector in the repeated charge-discharge cycles because of their relatively poor adhesiveness compared with that of polyvinyl metadifluoride, whereby resulting in a low utilization ratio of the cathode active substance, a poor discharge performance and a poor cycle performance of the battery.
  • the object of the present invention is to overcome the disadvantages of the prior lithium ion battery, i.e. with poor cycle performance and poor high rate discharge performance, and to provide a cathode and a lithium ion battery comprising such a cathode with advantages of excellent cycle performance and improved high rate discharge performances.
  • Another object of the present invention is to provide the processes for preparation thereof.
  • the cathode of the lithium ion battery provided by the present invention comprises a current collector and a cathode material coated on and/or filled in the current collector, and said cathode material comprises a cathode active substance, a conductive additive and an adhesive, wherein said adhesive comprises an enol-based polymer.
  • the lithium ion battery provided by the present invention comprises an electrode core and a non-aqueous electrolyte, both sealed in a battery shell, said electrode core comprises a cathode, an anode, and a membrane, wherein said cathode is the cathode provided by the present invention.
  • the process for preparing the cathode of the lithium battery comprises coating a slurry comprising a cathode active substance, a conductive additive, an adhesive and a solvent on and/or filling it in a current collector, drying and optionally rolling the resulted current collector, wherein said adhesive comprises an enol-based polymer.
  • the process for preparing the lithium ion battery provided by the present invention comprises sealing an electrode core and an electrolyte in a battery shell, said electrode core comprising a cathode, an anode and a membrane, wherein said cathode is the cathode provided by the present invention.
  • the enol-based polymer used as an adhesive of the cathode comprised by the lithium ion battery provided by the present invention possesses excellent adhesiveness as well as does not make the cathode active substance covered, it can effectively enhance the utilization ratio of the cathode active substance. Furthermore, because the conventional adhesives comprise a large amount of active groups, which are comprised more or less by all adhesives, and the excessive active groups therein will react with other substances such as the electrolyte and the like or decompose itself under the strong oxidative environment of the cathode, it is impossible for the battery to normally develop its capacity.
  • the adhesive of enol-based polymer provided by the present invention comprises less active groups, thereby eliminates the effect of the active groups and avoids the loss of the battery capacity initiated by the adhesive. Meanwhile, the excellent bonding performance of the hydroxyl in the enol-based polymer with the water-decomposed groups formed on the surface of metal is able to guarantee the excellent adhesivity between the metal current collector and the cathode active substance, as a result, the cathode active substance will not separate from the current collector in the repeated charge-discharge cycles, thereby guarantee the cycle performance and high rate discharge performance.
  • the solvent used in the preparation of the cathode slurry of the lithium ion battery is water, whereby lowering the temperature required by the evaporation of the solvent, eliminating the pollution to the environment while preparing the sheet as well as making the operation more convenient.
  • the cathode slurry is prepared by mixing an enol-based polymer with a cellulose-based polymer and/or a sodium salt, potassium salt or ammonium salt thereof
  • the cellulose-based polymer and the sodium salt, potassium salt or ammonium salt thereof used as a thickener possess relatively high ionic conductivity and dispersibility to the whole slurry at molecular level and are able to form a uniform thin layer on the coated cathode sheet among the particles of the cathode active substance, among the particles of the cathode active substance and the particles of the conductive additive and among the particles of the cathode active substance and the electrolyte, whereby enhancing the ionic conductivity between solid and liquid.
  • the high rate discharge performance of the battery is improved.
  • said enol-based polymer may be one or more selected from polyvinyl alcohol (PVA), polypropenol and polyisobutenol.
  • PVA polyvinyl alcohol
  • Polyvinyl alcohol is preferred for its high solubility in water and excellent adhesiveness.
  • the content of the adhesive of enol-based polymer may be that of the cathode adhesive of the prior art, and is preferably 0.5-10% by weight, more preferably 0.5-5% by weight based on the weight of the cathode active substance.
  • the polymerization degree of the enol-based polymer determines the viscosity of the polymer solution and the viscosity is directly proportional to the polymerization degree. If the polymerization degree is too low, the adhesiveness of the polymer is worsened, whereby affecting the uniform dispersion of the cathode active substance and leading to the poor cycle performance of the battery. On the contrary, if the polymerization degree is too high, the polymer solution is too viscous, and the cathode material is difficult to be uniformly coated on the current collector. Therefore, although the enol-based polymer with a higher or lower polymerization degree is usable, the polymerization degree thereof is preferably 1700-5000, more preferably 1900-3000.
  • the alcoholysis degree of the enol-based polymer is also a factor affecting the viscosity of the polymer solution.
  • Said alcoholysis degree of the enol-based polymer means the reduction (alcoholysis) degree of a polyvinyl acetate when the polyvinyl acetate is alcoholyzed (or reducted) to polyvinyl alcohol. The higher the alcoholysis degree is, the higher the reduction degree of the polyvinyl acetate to the polyvinyl alcohol is.
  • the alcoholysis degree of polyvinyl alcohol is lower than 70%, the solubility of the polymer in water is lowered, while the polyvinyl alcohol with an alcoholysis degree higher than 99% can only be dissolved in the water of a temperature higher than 95 °C.
  • the product with an alcoholysis degree of 80-95% has the highest water-solubility, which facilitates the formation of a stable reticular structure in water, maximizes the adhesiveness and simultaneously ensures the uniform dispersion of the cathode active substance.
  • the alcoholysis degree of the enol-based polymer used in the present invention is preferably 70-99%, more preferably 80-95%.
  • said adhesive according to the present invention also preferably comprises a cellulose-based polymer and/or the sodium salt potassium salt or ammonium salt thereof.
  • the content of said cellulose-based polymer and/or the sodium salt, potassium salt or ammonium salt thereof is 0-5% by weight based on the weight of the cathode active substance.
  • Said cellulose base polymer is one or more selected from carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropylmethyl cellulose (HPMC) and hydroxypropylethyl cellulose.
  • CMC carboxymethyl cellulose
  • HPMC hydroxypropylmethyl cellulose
  • Said sodium salt, potassium salt or ammonium salt of the cellulose-based polymer can enhance the solubility of the cellulose-based polymer in water.
  • the content of the cellulose-based polymer and/or the sodium salt or potassium salt or ammonium salt thereof as a thickener affects the viscosity of the cathode material. If the content of the cellulose-based polymer and/or the sodium salt, potassium salt or ammonium salt thereof is too large, e.g., more than 5%, the cathode material slurry will be more viscous and unsuitable for the coating material. Therefore, the amount of the cellulose-based polymer and/or the sodium salt, potassium salt, or ammonium salt thereof accounts for 0-5% by weight, preferably 0.1-2% by weight based on the cathode active substance.
  • the cathode active substance used for the cathode of the lithium ion battery according to the present invention is not specially limited and may be the conventional cathode active substance able to reversibly intercalate and de-intercalate lithium ion in the art.
  • the cathode active substance is one or more selected from Li x Ni /-y Co ⁇ 2 (wherein 0.9 ⁇ x ⁇ l.l and 0 ⁇ y ⁇ 1.0), Li; +3 M b Mn 2-J )O 4 (wherein -0.
  • M is one of lithium, boron, magnesium, aluminum, titanium, chromium, iron, cobalt, nickel, copper, zinc, gallium, yttrium, fluorine, iodine and sulfur), and Li 111 Mn 2-0 B n O 2 (where B is a transition metal, 0.9 ⁇ m ⁇ l .l and O ⁇ n ⁇ l.0).
  • Said conductive additive is not specially limited and may be an conventional cathode conductive additive of the art, such as one or more of ketjen carbon black, acetylene black, furnace black, carbon fiber VGCF, nano-graphite, graphite and conductive graphite, and the content thereof is preferably 0.01-20% by weight, more preferably 0.5-10% by weight based on the weight of the cathode active substance.
  • the cathode current collector may be a conventional cathode current collector in the lithium ion battery such as punching metal, metal foil, reticular metal and foam metal.
  • said cathode current collector may be selected from aluminum foil, copper foil and various punched steel belts, and aluminum foil is used as the cathode current collector in the embodiment of the present invention.
  • the process for preparing the cathode comprises coating a slurry comprising a cathode active substance, a conductive additive, an adhesive and a solvent on and/or filling it in the current collector, drying and optionally rolling the resulted collector, wherein said adhesive comprises an enol-based polymer.
  • Said solvent is water.
  • the amount of the solvent is such that said slurry possesses viscosity and flowability and can be coated on and/or filled in said current collector.
  • the content of said solvent is 20-70% by weight, preferably 30-60% by weight based on the weight of the cathode active substance.
  • the lithium ion battery provided by the present invention comprises an electrode core and a non-aqueous electrolyte, both sealed in a battery shell, wherein said electrode core comprises a cathode, an anode and a membrane. Because the present invention only relates to the improvement on the cathode and there are no special limits to the cathode, the membrane and non-aqueous electrolyte of the battery, various cathodes, membranes and non-aqueous electrolytes suitable for the lithium ion battery can all be used in the present invention.
  • Said membrane possesses an electrical insulating performance and a liquid maintaining performance, and is located between the cathode and the anode and contained in the battery case together with the cathode, anode and non-aqueous electrolyte.
  • Said membrane may be selected from various membranes used for the lithium ion battery, e.g., it may be one or more selected from polyolefin microporous/porous membrane, polyethylene blanket, glass fiber blanket, ultrafine glass fiber paper and multilayer composite microporous membrane of polypropylene and polyethylene membrane (PP/PE composite membrane). The location, property and species of said membrane are known to the person skilled in the art.
  • the anode comprises a current collector and an anode material coated on and/or filled in the current collector, and said anode material comprises an anode active substance and an anode adhesive.
  • Said anode active substance is not specially limited and may be any conventional anode active substance able to reversibly intercalate and de-intercalate lithium ion, such as one or more of natural graphite, synthetic graphite, petroleum coke, organic cracked carbon, mesophase carbon micro sphere, carbon fiber, tin alloy, and silicon alloy, with synthetic graphite being preferred.
  • Said anode can also comprises a conductive additive, which is not specially limited and may be a conventional one of the this field such as one or more selected from ketjen carbon black, acetylene black, furnace black, carbon fiber VGCF, conducting carbon black and conducting graphite.
  • a conductive additive which is not specially limited and may be a conventional one of the this field such as one or more selected from ketjen carbon black, acetylene black, furnace black, carbon fiber VGCF, conducting carbon black and conducting graphite.
  • the content of said conductive additive is 0-15% by weight, preferably 2-10% by weight based on the weight of the cathode active substance.
  • said adhesive may be one or more of fluorine-containing resins and polyolefin compounds such as polyvinylmetadifluoride (PVDF), polyterafluoroethylene (PTFE) and styrene-butadiene rubber (SBR).
  • PVDF polyvinylmetadifluoride
  • PTFE polyterafluoroethylene
  • SBR styrene-butadiene rubber
  • the content of said anode adhesive may be vary from 0.01% to 8% by weight, preferably from 0.02% to 5% by weight according to the species of the adhesive.
  • said anode adhesive is a mixture of a fiber-based polymer and rubber latex such as styrene-butadiene rubber (SBR).
  • SBR styrene-butadiene rubber
  • the amounts of said fiber-based polymer and styrene-butadiene rubber are known to the person skilled in the art.
  • the anode current collector may be a conventional one in the lithium ion battery such as punching metal, metal foil, reticular metal and foam metal, and the copper foil is used as the anode current collector in the embodiments of the present invention.
  • Said process for preparing the anode may be the conventional process known to the person skilled in the art. For example, it may comprise mixing an anode active substance, a conductive additive, and an anode adhesive with a solvent to prepare a mixture, coating the mixture on and/or filling the mixture in a current collector, drying and optionally rolling the resulted current collector to obtain said anode.
  • said solvent may be one or more selected from N-dimethylpyrrolidone (NMP), N,N-dimethyl formamide (DMF), N,N-diethyl formamide (DEF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), water and alcohols, and when said anode adhesive is a water-soluble adhesive, said solvent is preferably water.
  • the amount of the solvent is such that said slurry possesses viscosity and flowability and can be coated on said current collector.
  • the content of said solvent is 100-150% by weight based on the weight of the anode active substance.
  • Said non-aqueous electrolyte is not specially limited and may be a conventional nonaqueous electrolyte comprising a lithium salt as electrolyte and a non-aqueous solvent.
  • the electrolyte lithium salt is one or more selected from lithium hexafluorophosphate (LiPF 6 ), lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium halide, lithium chloroaluminate and lithium fluorohydrocarbylsulphonate.
  • the organic solvent is preferably a mixed solvent of a chain ester and a cyclic ester, wherein the chain ester is at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC) 5 methylethyl carbonate (EMC), methylpropyl carbonate (MPC), dipropyl carbonate (DPC), and other chain organic esters comprising fluorine, sulfur and unsaturated bonds; and the cyclic ester may be at least one of ethenyl carbonate (EC), propenyl carbonate (PC), vinylidene carbonate (VC), ⁇ -butyrolactone ( ⁇ -BL), sultone, and other cyclic organic esters comprising fluorine, sulfur and unsaturated bonds.
  • the amount of the electrolyte is preferably 1.5-4.9g/Ah, and the concentration thereof is preferably 0.5-2.9mol/l.
  • the process for preparing the lithium ion battery according to the present invention the other procedures are known to the person skilled in the art except that said cathode is prepared according to the process provided by the present invention.
  • the lithium ion battery can be prepared by winding up the prepared cathode, anode with a membrane to form an electrode core, sealing the electrode core and an electrolyte into a battery case. In such a way, the lithium ion battery according to the invention is completed.
  • LiCoO 2 as the cathode active substance and 4.5 g polyvinyl alcohol as the adhesive were added to 40 g water, mixed with 3.2 g acetylene black as the conductive additive, and then stirred in a vacuum stirring machine to form a uniform cathode slurry.
  • An aluminum foil as the current collector was uniformly coated with the above slurry and then dried at 150 ° C, rolled and cut to prepare a cathode with size of 540 mm x43.5 mm, wherein 5.8 g LiCoO 2 as the active substance was comprised.
  • LiCoO 2 as the cathode active substance
  • polypropenol as the adhesive (a polymerization degree of 2500 and an alcoholysis degree of 85%) were added to 50 g water, mixed with 3.2 g acetylene black as the conductive additive, and then stirred in a vacuum stirring machine to form a uniform cathode slurry.
  • a copper foil was uniformly coated with the above slurry, and then dried at 90°C, rolled and cut to prepare an anode with size of 500 mm x44 mm, wherein 2.6 g natural graphite as the active substance was comprised.
  • LiCoO 2 as the cathode active substance
  • 1.05 g polyvinyl alcohol as the adhesive a polymerization degree of 3000 and an alcoholysis degree of 85%
  • 1 g hydroxypropylmethyl cellulose and 0.5 g sodium carboxymethyl cellulose were added to 40 g water, mixed with 3.15 g acetylene black as the conductive additive, and then stirred in a vacuum stirring machine to form a uniform cathode slurry.
  • a copper foil as the current collector was uniformly coated with the above slurry, and then dried at 90 0 C, rolled and cut to prepare an anode with size of 500 mm x44 mm, wherein 2.6 g natural graphite as the active substance was comprised.
  • LiCoO 2 as the cathode active substance 100 g LiCoO 2 as the cathode active substance, 1.8 g polyvinyl alcohol as the adhesive (a polymerization degree of 2500 and an alcoholysis degree of 95%), and 1.8 g carboxymethyl cellulose were added to 40 g water, mixed with 3.2 g acetylene black as the conductive additive, and then stirred in a vacuum stirring machine to form a uniform cathode slurry.
  • An aluminum foil was uniformly coated with the above slurry, and then dried at 15O 0 C, rolled and cut to prepare a cathode with size of 540 mm x43.5 mm, wherein 5.8 g LiCoO 2 as the active substance was comprised.
  • a copper foil was uniformly coated with the above slurry, and then dried at 90 "C, rolled and cut to prepare an anode with size of 500x44 mm, wherein 2.6 g natural graphite as the active substance was comprised.
  • LiCoO 2 as the cathode active substance
  • 3.O g polyvinyl alcohol as the adhesive a polymerization degree of 1900 and an alcoholysis degree of 85%
  • 0.3 g ammonium carboxymethyl cellulose as the thickening agent were added to 40 g water, mixed with 3.2 g acetylene black as the conductive additive, and then stirred in a vacuum stirring machine to form a uniform cathode slurry.
  • a reference cathode and a reference lithium ion battery ACl using said cathode were prepared according to Example 1 except that the adhesive used in the cathode was PVDF (commodity of Atofie Co., 761 # PVDF) and the solvent was N-methyl-2-pyrrolidone (NMP).
  • PVDF composition of Atofie Co., 761 # PVDF
  • NMP N-methyl-2-pyrrolidone
  • Rate of capacity preservation (discharge capacity after 500 cycles/discharge capacity after one cycle)xlOO%
  • the batteries provided by the present invention possess more excellent high rate discharge performance and improved cycle performance.

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  • Chemical Kinetics & Catalysis (AREA)
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Abstract

A cathode of a lithium ion battery and a lithium ion battery comprising such a cathode are disclosed. Said cathode comprises a current collector and a cathode material coated on and/or filled in the current collector, said cathode material comprises a cathode active substance, a conductive additive and an adhesive, wherein said adhesive comprises an enol-based polymer. The lithium ion battery comprising such a cathode possesses excellent cycle performance and improved high rate discharge performance.

Description

A CATHODE5 A LITHIUM ION BATTERY COMPRISING SUCH A CATHODE AND PROCESSES FOR PREPARATION THEREOF
FIELD QF THE INVENTION The present invention relates to a cathode, a lithium ion battery comprising such a cathode and processes for preparation thereof.
BACKGROUND OF THE INVENTION A lithium ion battery mainly comprises an electrode core and an electrolyte, both sealed in a battery shell. Said electrode core comprises electrodes and a membrane. Said battery electrode comprises a cathode and an anode and said cathode comprises a current collector and a cathode material coated on and/or filled in the current collector. Said cathode material comprises a cathode active substance, a conductive additive and an adhesive. The process for preparing the cathode comprises coating a slurry comprising a cathode active substance, a conductive additive, an adhesive and a solvent on and/or filled it in the current collector, and then drying and optionally rolling the resulted current collector.
Presently, the lithium ion secondary battery uses polyvinylmetadifiuoride (PVDF) as an adhesive, and N-dimethylpyrrolidone (NMP), N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO) and the like as a solvent of PVDF. On the one hand, being a fiber, PVDF will make the cathode active substance covered and difficult to exert its functionality, whereby decreasing the utilization ratio of the cathode active substance and thus the capacity of the battery will become smaller. The adhesivity of polyvinylmetadifiuoride will worsen if the amount of the PVDF decreases. As a result, the cathode active substance separates from the current collector. Beside, PVDF is corrosive and expensive. On the other hand, the above organic compounds used as solvents such as NMP, DMF and DMSO possess rather high boiling points and are difficult to remove from the sheet during the process of the preparation of the sheet. Therefore, a relatively high temperature such as of 120-135 °C is required to obtain a cathode after the slurry is coated on the current collector, which brings about much inconvenience to the operation. Besides, said organic solvents are harmful to human body and undesirably pollute the environment.
Compared with the above organic solvent-based adhesive, water-based adhesive possess the advantages of no pollution, low cost, incombustibility and safety in use. Up to now, the water-based adhesive was mainly used in the cathode in this art. For example, JP Application Laid-Open Hei 1993-74461 discloses a water-based adhesive using styrene-butadiene rubber (SBR) as an adhesive. Since there are double bonds in the main chain of styrene-butadiene rubber and the cathode is in a strong oxidative environment during the process of charge-discharge, when such an adhesive is used in the cathode, the double bonds of styrene-butadiene rubber will react with the organic compounds in the electrolyte during the process of charge-discharge and worsen the performance of the battery. CN 1507093 A discloses a cathode used for the non-aqueous electrolyte secondary battery, which uses a carbon material able to reversibly intercalate and de-intercalate lithium ion and an adhesive as the cathode material, characterized in that said carbon material is graphite and the adhesive is at least one selected from adhesive of ethylene-acrylic acid copolymer, adhesive of ethylene-acrylate copolymer, adhesive of ethylene-methyl acrylate copolymer, adhesive of ethylene-methacrylic acid copolymer and adhesive of ethylene-methacrylate copolymer with the content of ethylene being above 70% by weight and below 95% by weight. These adhesives are polyacrylate type water-based adhesives suitable for the cathode but still not suitable for the cathode (LiCoO2) because of such as polarization of the electrode, CN 1622366 A discloses a cathode of a re-chargeable lithium ion battery, which comprises a current collector and a cathode active substance layer coated on the current collector. The cathode active substance layer comprises a cathod active substance and a cellulose-based polymer adhesive with the esterification degree of the cellulose-based polymer being 1.3 or more and the molecular weight of the same being 100000 or more. Said cellulose-based polymer adhesive is one or more selected from carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropylmethyl cellulose, hydroxypropylethyl cellulose. Such adhesives will not make the cathode active substance covered, but make the cathode active substance separate from the current collector in the repeated charge-discharge cycles because of their relatively poor adhesiveness compared with that of polyvinyl metadifluoride, whereby resulting in a low utilization ratio of the cathode active substance, a poor discharge performance and a poor cycle performance of the battery. SUMMARY OF THE INVENTION
The object of the present invention is to overcome the disadvantages of the prior lithium ion battery, i.e. with poor cycle performance and poor high rate discharge performance, and to provide a cathode and a lithium ion battery comprising such a cathode with advantages of excellent cycle performance and improved high rate discharge performances. Another object of the present invention is to provide the processes for preparation thereof.
The cathode of the lithium ion battery provided by the present invention comprises a current collector and a cathode material coated on and/or filled in the current collector, and said cathode material comprises a cathode active substance, a conductive additive and an adhesive, wherein said adhesive comprises an enol-based polymer.
The lithium ion battery provided by the present invention comprises an electrode core and a non-aqueous electrolyte, both sealed in a battery shell, said electrode core comprises a cathode, an anode, and a membrane, wherein said cathode is the cathode provided by the present invention.
The process for preparing the cathode of the lithium battery provided by the present invention comprises coating a slurry comprising a cathode active substance, a conductive additive, an adhesive and a solvent on and/or filling it in a current collector, drying and optionally rolling the resulted current collector, wherein said adhesive comprises an enol-based polymer.
The process for preparing the lithium ion battery provided by the present invention comprises sealing an electrode core and an electrolyte in a battery shell, said electrode core comprising a cathode, an anode and a membrane, wherein said cathode is the cathode provided by the present invention.
Because the enol-based polymer used as an adhesive of the cathode comprised by the lithium ion battery provided by the present invention possesses excellent adhesiveness as well as does not make the cathode active substance covered, it can effectively enhance the utilization ratio of the cathode active substance. Furthermore, because the conventional adhesives comprise a large amount of active groups, which are comprised more or less by all adhesives, and the excessive active groups therein will react with other substances such as the electrolyte and the like or decompose itself under the strong oxidative environment of the cathode, it is impossible for the battery to normally develop its capacity. The adhesive of enol-based polymer provided by the present invention comprises less active groups, thereby eliminates the effect of the active groups and avoids the loss of the battery capacity initiated by the adhesive. Meanwhile, the excellent bonding performance of the hydroxyl in the enol-based polymer with the water-decomposed groups formed on the surface of metal is able to guarantee the excellent adhesivity between the metal current collector and the cathode active substance, as a result, the cathode active substance will not separate from the current collector in the repeated charge-discharge cycles, thereby guarantee the cycle performance and high rate discharge performance. Besides, the solvent used in the preparation of the cathode slurry of the lithium ion battery is water, whereby lowering the temperature required by the evaporation of the solvent, eliminating the pollution to the environment while preparing the sheet as well as making the operation more convenient.
In addition, according to an embodiment of the present invention, when the cathode slurry is prepared by mixing an enol-based polymer with a cellulose-based polymer and/or a sodium salt, potassium salt or ammonium salt thereof, the cellulose-based polymer and the sodium salt, potassium salt or ammonium salt thereof used as a thickener possess relatively high ionic conductivity and dispersibility to the whole slurry at molecular level and are able to form a uniform thin layer on the coated cathode sheet among the particles of the cathode active substance, among the particles of the cathode active substance and the particles of the conductive additive and among the particles of the cathode active substance and the electrolyte, whereby enhancing the ionic conductivity between solid and liquid. As a result, the high rate discharge performance of the battery is improved.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the cathode of the lithium ion battery provided by the present invention, said enol-based polymer may be one or more selected from polyvinyl alcohol (PVA), polypropenol and polyisobutenol. Polyvinyl alcohol is preferred for its high solubility in water and excellent adhesiveness. The content of the adhesive of enol-based polymer may be that of the cathode adhesive of the prior art, and is preferably 0.5-10% by weight, more preferably 0.5-5% by weight based on the weight of the cathode active substance.
The polymerization degree of the enol-based polymer determines the viscosity of the polymer solution and the viscosity is directly proportional to the polymerization degree. If the polymerization degree is too low, the adhesiveness of the polymer is worsened, whereby affecting the uniform dispersion of the cathode active substance and leading to the poor cycle performance of the battery. On the contrary, if the polymerization degree is too high, the polymer solution is too viscous, and the cathode material is difficult to be uniformly coated on the current collector. Therefore, although the enol-based polymer with a higher or lower polymerization degree is usable, the polymerization degree thereof is preferably 1700-5000, more preferably 1900-3000.
The alcoholysis degree of the enol-based polymer is also a factor affecting the viscosity of the polymer solution. Said alcoholysis degree of the enol-based polymer means the reduction (alcoholysis) degree of a polyvinyl acetate when the polyvinyl acetate is alcoholyzed (or reducted) to polyvinyl alcohol. The higher the alcoholysis degree is, the higher the reduction degree of the polyvinyl acetate to the polyvinyl alcohol is. If the alcoholysis degree of polyvinyl alcohol is lower than 70%, the solubility of the polymer in water is lowered, while the polyvinyl alcohol with an alcoholysis degree higher than 99% can only be dissolved in the water of a temperature higher than 95 °C. The product with an alcoholysis degree of 80-95% has the highest water-solubility, which facilitates the formation of a stable reticular structure in water, maximizes the adhesiveness and simultaneously ensures the uniform dispersion of the cathode active substance. Although an enol-based polymer with a higher or lower alcoholysis degree is usable, the alcoholysis degree of the enol-based polymer used in the present invention is preferably 70-99%, more preferably 80-95%.
Because the cellulose-based polymer or the sodium salt, potassium salt, or ammonium salt thereof possesses very high ionic conductivity, can disperse throughout the whole slurry at molecular level, and is able to form a uniform thin layer among the particles of the cathode active substance, among the particles of the cathode active substance and the particles of the conductive additive, and among the particles of the cathode active substance and the electrolyte on the coated cathode sheet, the ionic conductivity between solid and liquid and the discharge performance of the battery are enhanced. Therefore, said adhesive according to the present invention also preferably comprises a cellulose-based polymer and/or the sodium salt potassium salt or ammonium salt thereof. The content of said cellulose-based polymer and/or the sodium salt, potassium salt or ammonium salt thereof is 0-5% by weight based on the weight of the cathode active substance. Said cellulose base polymer is one or more selected from carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropylmethyl cellulose (HPMC) and hydroxypropylethyl cellulose. Said sodium salt, potassium salt or ammonium salt of the cellulose-based polymer can enhance the solubility of the cellulose-based polymer in water.
The content of the cellulose-based polymer and/or the sodium salt or potassium salt or ammonium salt thereof as a thickener affects the viscosity of the cathode material. If the content of the cellulose-based polymer and/or the sodium salt, potassium salt or ammonium salt thereof is too large, e.g., more than 5%, the cathode material slurry will be more viscous and unsuitable for the coating material. Therefore, the amount of the cellulose-based polymer and/or the sodium salt, potassium salt, or ammonium salt thereof accounts for 0-5% by weight, preferably 0.1-2% by weight based on the cathode active substance. The cathode active substance used for the cathode of the lithium ion battery according to the present invention is not specially limited and may be the conventional cathode active substance able to reversibly intercalate and de-intercalate lithium ion in the art. Preferably, the cathode active substance is one or more selected from LixNi /-yCoθ2 (wherein 0.9<x<l.l and 0<y<1.0), Li; +3MbMn2-J)O4 (wherein -0. l≤a<0.2, O≤b≤l .0, and M is one of lithium, boron, magnesium, aluminum, titanium, chromium, iron, cobalt, nickel, copper, zinc, gallium, yttrium, fluorine, iodine and sulfur), and Li111Mn2-0BnO2 (where B is a transition metal, 0.9<m≤l .l and O≤n≤l.0).
Said conductive additive is not specially limited and may be an conventional cathode conductive additive of the art, such as one or more of ketjen carbon black, acetylene black, furnace black, carbon fiber VGCF, nano-graphite, graphite and conductive graphite, and the content thereof is preferably 0.01-20% by weight, more preferably 0.5-10% by weight based on the weight of the cathode active substance.
The cathode current collector may be a conventional cathode current collector in the lithium ion battery such as punching metal, metal foil, reticular metal and foam metal. In particular, said cathode current collector may be selected from aluminum foil, copper foil and various punched steel belts, and aluminum foil is used as the cathode current collector in the embodiment of the present invention.
The process for preparing the cathode provided by the present invention comprises coating a slurry comprising a cathode active substance, a conductive additive, an adhesive and a solvent on and/or filling it in the current collector, drying and optionally rolling the resulted collector, wherein said adhesive comprises an enol-based polymer. Said solvent is water. The amount of the solvent is such that said slurry possesses viscosity and flowability and can be coated on and/or filled in said current collector. Generally, the content of said solvent is 20-70% by weight, preferably 30-60% by weight based on the weight of the cathode active substance. The methods and conditions of drying and rolling have been known to the person skilled in the art. The lithium ion battery provided by the present invention comprises an electrode core and a non-aqueous electrolyte, both sealed in a battery shell, wherein said electrode core comprises a cathode, an anode and a membrane. Because the present invention only relates to the improvement on the cathode and there are no special limits to the cathode, the membrane and non-aqueous electrolyte of the battery, various cathodes, membranes and non-aqueous electrolytes suitable for the lithium ion battery can all be used in the present invention.
Said membrane possesses an electrical insulating performance and a liquid maintaining performance, and is located between the cathode and the anode and contained in the battery case together with the cathode, anode and non-aqueous electrolyte. Said membrane may be selected from various membranes used for the lithium ion battery, e.g., it may be one or more selected from polyolefin microporous/porous membrane, polyethylene blanket, glass fiber blanket, ultrafine glass fiber paper and multilayer composite microporous membrane of polypropylene and polyethylene membrane (PP/PE composite membrane). The location, property and species of said membrane are known to the person skilled in the art.
The constitution of said anode is known to the person skilled in the art. Generally, the anode comprises a current collector and an anode material coated on and/or filled in the current collector, and said anode material comprises an anode active substance and an anode adhesive. Said anode active substance is not specially limited and may be any conventional anode active substance able to reversibly intercalate and de-intercalate lithium ion, such as one or more of natural graphite, synthetic graphite, petroleum coke, organic cracked carbon, mesophase carbon micro sphere, carbon fiber, tin alloy, and silicon alloy, with synthetic graphite being preferred. Said anode can also comprises a conductive additive, which is not specially limited and may be a conventional one of the this field such as one or more selected from ketjen carbon black, acetylene black, furnace black, carbon fiber VGCF, conducting carbon black and conducting graphite. The content of said conductive additive is 0-15% by weight, preferably 2-10% by weight based on the weight of the cathode active substance.
The species and content of said anode adhesive are known to the person skilled in the art. For example, said adhesive may be one or more of fluorine-containing resins and polyolefin compounds such as polyvinylmetadifluoride (PVDF), polyterafluoroethylene (PTFE) and styrene-butadiene rubber (SBR). Generally, based on weight of the anode active substance, the content of said anode adhesive may be vary from 0.01% to 8% by weight, preferably from 0.02% to 5% by weight according to the species of the adhesive.
Preferably, said anode adhesive is a mixture of a fiber-based polymer and rubber latex such as styrene-butadiene rubber (SBR). The amounts of said fiber-based polymer and styrene-butadiene rubber are known to the person skilled in the art.
The anode current collector may be a conventional one in the lithium ion battery such as punching metal, metal foil, reticular metal and foam metal, and the copper foil is used as the anode current collector in the embodiments of the present invention.
Said process for preparing the anode may be the conventional process known to the person skilled in the art. For example, it may comprise mixing an anode active substance, a conductive additive, and an anode adhesive with a solvent to prepare a mixture, coating the mixture on and/or filling the mixture in a current collector, drying and optionally rolling the resulted current collector to obtain said anode. Wherein said solvent may be one or more selected from N-dimethylpyrrolidone (NMP), N,N-dimethyl formamide (DMF), N,N-diethyl formamide (DEF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), water and alcohols, and when said anode adhesive is a water-soluble adhesive, said solvent is preferably water. The amount of the solvent is such that said slurry possesses viscosity and flowability and can be coated on said current collector. Generally, the content of said solvent is 100-150% by weight based on the weight of the anode active substance. The methods and conditions of drying and rolling are known to the person skilled in the art.
Said non-aqueous electrolyte is not specially limited and may be a conventional nonaqueous electrolyte comprising a lithium salt as electrolyte and a non-aqueous solvent. For example, the electrolyte lithium salt is one or more selected from lithium hexafluorophosphate (LiPF6), lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium halide, lithium chloroaluminate and lithium fluorohydrocarbylsulphonate. The organic solvent is preferably a mixed solvent of a chain ester and a cyclic ester, wherein the chain ester is at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC)5 methylethyl carbonate (EMC), methylpropyl carbonate (MPC), dipropyl carbonate (DPC), and other chain organic esters comprising fluorine, sulfur and unsaturated bonds; and the cyclic ester may be at least one of ethenyl carbonate (EC), propenyl carbonate (PC), vinylidene carbonate (VC), γ-butyrolactone (γ-BL), sultone, and other cyclic organic esters comprising fluorine, sulfur and unsaturated bonds. The amount of the electrolyte is preferably 1.5-4.9g/Ah, and the concentration thereof is preferably 0.5-2.9mol/l.
As for the process for preparing the lithium ion battery according to the present invention, the other procedures are known to the person skilled in the art except that said cathode is prepared according to the process provided by the present invention. Generally, the lithium ion battery can be prepared by winding up the prepared cathode, anode with a membrane to form an electrode core, sealing the electrode core and an electrolyte into a battery case. In such a way, the lithium ion battery according to the invention is completed.
The present invention will be described in detail through the following exemplified examples.
Example 1 (1) Preparation of the cathode
100 g LiCoO2 as the cathode active substance and 4.5 g polyvinyl alcohol as the adhesive (a polymerization degree of 1900 and an alcoholysis degree of 80%) were added to 40 g water, mixed with 3.2 g acetylene black as the conductive additive, and then stirred in a vacuum stirring machine to form a uniform cathode slurry. An aluminum foil as the current collector was uniformly coated with the above slurry and then dried at 150°C, rolled and cut to prepare a cathode with size of 540 mm x43.5 mm, wherein 5.8 g LiCoO2 as the active substance was comprised. (2) Preparation of the anode 100 g natural graphite as the anode active substance, a mixture of 1 g carboxymethyl cellulose and 3 g styrene-butadiene rubber as the adhesive, and 4 g carbon black as the conductive additive were mixed with 120 g water, and then stirred in a vacuum stirring machine to form a uniform cathode slurry. A copper foil was uniformly coated with the above slurry, and then dried at
90 "C, rolled and cut to prepare a anode with size of 500 mm x44 mm, wherein 2.6 g natural graphite as the active substance was comprised.
(3) Assembly of the battery
The above cathode and anode were wound up with a polypropylene membrane into a square electrode core of the lithium ion battery, and then sealed in a battery she]] together with a non aqueous electrolyte of LiPF6 dissolved in a mixed solvent of EC/DMC=1 : 1 in the concentration of 1 mol/1 in an amount of 3.8 g/Ah to prepare a lithium ion battery Al .
Example 2
(I) Preparation of the cathode
100 g LiCoO2 as the cathode active substance and 4.5 g polypropenol as the adhesive (a polymerization degree of 2500 and an alcoholysis degree of 85%) were added to 50 g water, mixed with 3.2 g acetylene black as the conductive additive, and then stirred in a vacuum stirring machine to form a uniform cathode slurry.
An aluminum foil was uniformly coated with the above slurry, and then dried at 150°C, rolled and cut to prepare a cathode with size of 540 mm x43.5 mm, wherein 5.8 g LiCoO2 as the active substance was comprised.
(2) Preparation of the anode 100 g anode active substance natural graphite, a mixture of 1 g carboxymethyl cellulose and 3 g styrene-butadiene rubber as the adhesive, and 4 g carbon black as the conductive additive were added to 100 g water, and then stirred in a vacuum stirring machine to form a uniform anode slurry.
A copper foil was uniformly coated with the above slurry, and then dried at 90°C, rolled and cut to prepare an anode with size of 500 mm x44 mm, wherein 2.6 g natural graphite as the active substance was comprised.
(3) Assembly of the battery
The above cathode and anode were wound up with a polypropylene membrane into a square electrode core of the lithium ion battery, and then sealed in a battery shell together with a non aqueous electrolyte of LiPF6 dissolved in a mixed solvent of EC/DMC=l .i in the concentration of 1 mo 1/1 in an amount of 3.8 g/Ah to prepare a lithium ion battery A2.
Example 3
(1) Preparation of the cathode
100 g LiCoO2 as the cathode active substance, 1.05 g polyvinyl alcohol as the adhesive (a polymerization degree of 3000 and an alcoholysis degree of 85%), 1 g hydroxypropylmethyl cellulose and 0.5 g sodium carboxymethyl cellulose were added to 40 g water, mixed with 3.15 g acetylene black as the conductive additive, and then stirred in a vacuum stirring machine to form a uniform cathode slurry.
An aluminum foil as the current collector was uniformly coated with the above slurry and then dried at 150°C, rolled and cut to prepare a cathode with size of 540 mm x43.5 mm, wherein 5.8 g LiCoO2 as the active substance was comprised.
(2) Preparation of the anode
100 g natural graphite as the anode active substance, a mixture adhesive comprising 1 g carboxymethyl cellulose and 3 g styrene-butadiene rubber as the adhesive, and 4 g carbon black as the conductive additive were mixed with 120 g water, and then stirred in a vacuum stirring machine to form a uniform anode slurry.
A copper foil as the current collector was uniformly coated with the above slurry, and then dried at 900C, rolled and cut to prepare an anode with size of 500 mm x44 mm, wherein 2.6 g natural graphite as the active substance was comprised.
(3) Assembly of the battery The above cathode and anode were wound up with a polypropylene membrane into a square electrode core of the lithium ion battery, and then sealed in a battery shell together with a non-aqueous electrolyte of LiPF5 dissolved in a mixed solvent of ECZDMC=I :1 in the concentration of 1 mol/1 in an amount of 3.8 g/Ah to prepare a lithium ion battery A3.
Example 4
(1) Preparation of the cathode
100 g LiCoO2 as the cathode active substance, 1.8 g polyvinyl alcohol as the adhesive (a polymerization degree of 2500 and an alcoholysis degree of 95%), and 1.8 g carboxymethyl cellulose were added to 40 g water, mixed with 3.2 g acetylene black as the conductive additive, and then stirred in a vacuum stirring machine to form a uniform cathode slurry. An aluminum foil was uniformly coated with the above slurry, and then dried at 15O0C, rolled and cut to prepare a cathode with size of 540 mm x43.5 mm, wherein 5.8 g LiCoO2 as the active substance was comprised.
(2) Preparation of the anode
100 g natural graphite as the anode active substance, a mixture comprising 1 g carboxymethyl cellulose and 3 g styrene-butadiene rubber as the adhesive and 4 g carbon black as the conductive additive were added to 12O g water, and then stirred in a vacuum stirring machine to form a uniform anode slurry.
A copper foil was uniformly coated with the above slurry, and then dried at 90 "C, rolled and cut to prepare an anode with size of 500x44 mm, wherein 2.6 g natural graphite as the active substance was comprised.
(3) Assembly of the battery
The above cathode and anode were wound up with a polypropylene membrane into a square electrode core of the lithium ion battery, and then sealed in a battery shell together with a non-aqueous electrolyte of LiPF6 dissolved in a mixed solvent of EC/DMC=1:1 in the concentration of 1 mol/1 in an amount of 3.8 g/Ah to prepare a lithium ion battery A4.
Example 5
(1) Preparation of the cathode 100 g LiCoO2 as the cathode active substance, 2.12 g polyisobutenol (a polymerization degree of 1900 and an alcoholysis degree of 80%) as the adhesive and 1.06 g carboxymethyl cellulose were added to 50 g water, mixed with 3.2 g acetylene black as the conductive additive, and then stirred in a vacuum stirring machine to form a uniformcathode slurry. An aluminum foil was uniformly coated with the above slurry, and then dried at 150°C, rolled and cut to prepare a cathode with size of 540 mmx43.5 mm, wherein 5.8 g LiCoO2 as the active substance was comprised.
(2) Preparation of the anode 100 g anode active substance natural graphite, a mixed adhesive comprising 1 g carboxymethyl cellulose and 3 g styrene-butadiene rubber and 4 g carbon black as the conductive additive were added to 12O g water, and then stirred in a vacuum stirring machine to form a uniform anode slurry. A copper foil was uniformly coated with the above slurry, and then dried at
90°C, rolled and cut to prepare an anode with size of 500 mmx44 mm, wherein 2.6 g natural graphite as the active substance was comprised.
(3) Assembly of the battery
The above cathode and anode were wound up with a polypropylene membrane into a square electrode core of the lithium ion battery, and then sealed in a battery shell together with a non-aqueous electrolyte of LiPF6 dissolved in a mixed solvent of EC/DMC=1 :1 in the concentration of 1 mol/1 in an amount of 3.8 g/Ah to prepare a lithium ion battery A5.
Example 6
(1) Preparation of the cathode
100 g LiCoO2 as the cathode active substance, 3.O g polyvinyl alcohol as the adhesive (a polymerization degree of 1900 and an alcoholysis degree of 85%), and 0.3 g ammonium carboxymethyl cellulose as the thickening agent were added to 40 g water, mixed with 3.2 g acetylene black as the conductive additive, and then stirred in a vacuum stirring machine to form a uniform cathode slurry.
An aluminum foil was uniformly coated with the above slurry, and then dried at 1500C, rolled and cut to prepare a cathode with size of 540 mmx43.5 mm, wherein 5.8 g LiCoO2 as the active substance was comprised. (2) Preparation of the anode
100 g natural graphite as the anode active substance, a mixed adhesive comprising 1 g carboxymethyl cellulose and 3 g styrene-butadiene rubber and 4 g carbon black as the conductive additive were added to 12O g water, and then stirred in a vacuum stirring machine to form a uniform anode slurry. A copper foil was uniformly coated with the above slurry, and then dried at
90°C, rolled and cut to prepare an anode with size of 500x44 mm, wherein 2.6 g natural graphite as the active substance was comprised. (3) Assembly of the battery The above cathode and anode were wound up with a polypropylene membrane into a square electrode core of the lithium ion battery, and then sealed in a battery shell together with LiPF6 dissolved in a mixed solvent of EC/DMC=1 :1 in the concentration of 1 mol/1 in an amount of 3.8 g/Ah to prepare a lithium ion battery A6.
Comparative example 1
A reference cathode and a reference lithium ion battery ACl using said cathode were prepared according to Example 1 except that the adhesive used in the cathode was PVDF (commodity of Atofie Co., 761#PVDF) and the solvent was N-methyl-2-pyrrolidone (NMP).
Performances Test
1. Test of High Rate Discharge Performance
C2c/Co.2c: The high rate discharge performance was expressed as the ratio of the discharge capacity when the battery was discharged from 4.2 V to 3.0 V respectively with the current of 2 C and 0.2 C. The results are shown in Table 1
2. Test of Cycle Performance The batteries were charged to 4.2 V with a current of 1C at 230C and then charged with a constant voltage until the cut-off current was 0.05 C. Subsequently, the batteries were laid-aside for 10 min. Then the batteries were discharged to 3.0 V with a current of 1C and stood for 5 min. The above procedures were repeated for 500 times to obtain the capacity when the battery were discharged to 3.0 V with a current of 1C after 500 cycles, and the rate of capacity preservation was calculated according to the following formula:
Rate of capacity preservation=(discharge capacity after 500 cycles/discharge capacity after one cycle)xlOO%
The results are shown in Table 1. Table 1
Figure imgf000016_0001
It can be seen from the results in Tables 1, compared with the reference battery, the batteries provided by the present invention possess more excellent high rate discharge performance and improved cycle performance.

Claims

What we claim is:
L A cathode of a lithium ion battery comprising a current collector and a cathode material coated on and/or filled in the current collector; and said cathode material comprising a cathode active substance, a conductive additive and an adhesive, wherein said adhesive comprises an enol-based polymer.
2. The cathode according to claim 1 , wherein said enol-based polymer is one or more selected from polyvinyl alcohol, polypropenol and polyisobutenol.
3. The cathode according to claim 1, wherein the polymerization degree of said enol-based polymer is 1700-5000.
4. The cathode according to claim 1, wherein the alcoholysis degree of said enol-based polymer is 70-99%.
5. The cathode according to claim 1, wherein the content of said enol-based polymer is 0.5-10% by weight based on the weight of the cathode active substance.
6. The cathode according to claim 1, wherein said adhesive also comprises a cellulose-based polymer and/or a sodium salt, potassium salt or ammonium salt thereof in a content of 0-5% by weight based on the weight of the cathode active substance.
7. A process for preparing the cathode of claim 1, which comprises coating a slurry comprising a cathode active substance, a conductive additive, an adhesive, and a solvent on and/or filling it in the current collector, drying and optionally rolling the resulted current collector, wherein said adhesive comprises an enol-based polymer.
8. The process according to claim 7, wherein said adhesive also comprises a cellulose-based polymer and/or a sodium salt, potassium salt or ammonium salt thereof in a content of 0-5% by weight based on the weight of the cathode active substance.
9. A lithium ion battery, which comprises an electrode core and a non-aqueous electrolyte, both sealed in a battery shell, and said electrode core comprises a cathode, an anode and a membrane, wherein said cathode is a cathode of any one of claims 1-6.
10. A process for preparing the lithium ion battery of claim 9, which comprises sealing an electrode core and an electrolyte in a battery shell, and said electrode core comprising a cathode, an anode and a membrane, wherein said cathode is a cathode of any one of claims 1-6.
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