EP4548412A1 - Binder combination for a secondary cell - Google Patents

Binder combination for a secondary cell

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Publication number
EP4548412A1
EP4548412A1 EP23736650.5A EP23736650A EP4548412A1 EP 4548412 A1 EP4548412 A1 EP 4548412A1 EP 23736650 A EP23736650 A EP 23736650A EP 4548412 A1 EP4548412 A1 EP 4548412A1
Authority
EP
European Patent Office
Prior art keywords
binder
electrode
secondary cell
anode
silicon
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.)
Pending
Application number
EP23736650.5A
Other languages
German (de)
French (fr)
Inventor
Prasad Mangesh KORDE
Parth Omprakash DAYAMA
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.)
Northvolt AB
Original Assignee
Northvolt AB
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 Northvolt AB filed Critical Northvolt AB
Publication of EP4548412A1 publication Critical patent/EP4548412A1/en
Pending 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
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/621—Binders
    • H01M4/622—Binders being polymers
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
    • C08L5/00—Compositions of polysaccharides or of their derivatives not provided for in groups C08L1/00 or C08L3/00
    • C08L5/08—Chitin; Chondroitin sulfate; Hyaluronic acid; Derivatives thereof
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/06—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of other non-metallic substances
    • H01B1/12—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of other non-metallic substances organic substances
    • H01B1/124—Intrinsically conductive polymers
    • H01B1/128—Intrinsically conductive polymers comprising six-membered aromatic rings in the main chain, e.g. polyanilines, polyphenylenes
    • 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
    • H01M4/133—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
    • H01M4/13—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
    • H01M4/36—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
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/483—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides for non-aqueous cells
    • 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
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/10—Primary casings; Jackets or wrappings
    • H01M50/116—Primary casings; Jackets or wrappings characterised by the material
    • H01M50/121—Organic material
    • 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
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/027—Negative electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2220/00—Batteries for particular applications
    • H01M2220/20—Batteries in motive systems, e.g. vehicle, ship, plane
    • 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 disclosure relates to an electrode binder for an electrode in a secondary cell. More particularly, the present disclosure relates to a binder combination comprising a chitosan-grafted-polyaniline copolymer in combination with carboxymethylcellulose or styrene-butadiene rubber, an electrode comprising the binder combination, a secondary cell comprising such electrode, as well as a vehicle comprising such secondary cell.
  • Rechargeable batteries having high energy density and discharge voltage are a vital component in portable electronic devices and are a key enabler for the electrification of transport and large-scale storage of electricity. To reach higher energy densities, new types of batteries are being developed.
  • Li-ion batteries typically consist of stacks of secondary cells, wherein each cell is composed of a cathode comprising a cathode current collector, an electrolyte, an anode comprising an anode current collector, and optionally a separator positioned between the anode and cathode.
  • the anode is made of graphite-based materials
  • the cations extracted from the cathode material diffuse from the cathode material through the electrolyte and intercalate into the graphite material at the anode during charging. During discharge, this process is reversed.
  • silica In an effort to increase the energy density of the secondary cells, development is ongoing to replace a part of the graphite with silica to increase the capacity of the electrode since silica demonstrates a higher maximum theoretical capacity than graphite.
  • a challenge with silica is that it swells and contracts during charge and discharge. This causes a mechanical stress in the electrode material, which may initiate cracks and also impair the adhesion of the electrode active material to the current collector leading to electrode disintegration. These circumstances may shorten the performance and lifetime of the secondary cell.
  • binders used for electrodes in batteries are poly-(vinylidene fluoride) (PVDF), copolymers of vinylidene difluoride (VdF) and hexafluoropropene (HFP) monomers (Kynar or KynarFlex), carboxymethylcellulose (CMC) and its sodium salt (CMC-Na), poly(acrylic acid) (PAA) and its sodium salt (PAA-Na), and styrene-butadiene rubber (SBR).
  • PVDF poly-(vinylidene fluoride)
  • VdF vinylidene difluoride
  • HFP hexafluoropropene
  • An object of the present invention is to provide an electrode binder for an electrode in a secondary cell, wherein the binder may provide an improved electric conductivity and a good mechanical strength to maintain the structure of the electrode.
  • a further object is the provision of an anode electrode with an improved electrochemical performance and low swelling.
  • the present invention provides a binder combination for an electrode in a secondary cell, wherein the binder comprises a chitosan-grafted-polyaniline copolymer in combination with carboxymethylcellulose or styrene-butadiene rubber.
  • Fig. 1 shows the swelling of three different anodes during charging.
  • Fig. 2 and fig. 3 show the ionic resistance of three different anodes.
  • Fig. 4 shows the peel strength of three different anodes. DETAILED DESCRIPTION OF THE INVENTION
  • the present invention relates to an electrode binder for an electrode in a secondary cell, wherein the binder comprises a chitosan-grafted-polyaniline copolymer (CS-g-PANI) and a polymer selected from any one of styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), or a combination of these.
  • CS-g-PANI chitosan-grafted-polyaniline copolymer
  • SBR styrene-butadiene rubber
  • CMC carboxymethylcellulose
  • the electrode binder is a binder for an anode.
  • a chitosan-grafted-polyaniline copolymer is formed by graft polymerization of chitosan and polyaniline and has a high degree of cross-linking.
  • Chitosan is a polysaccharide obtained by deacetylation of chitin from crustaceans and insects, and contributes to the mechanical stability.
  • Polyaniline is a conducting polymer derived from aniline monomers and helps to improve the electric conductivity of the binder.
  • the ratio of chitosan to polyaniline in the chitosan-grafted-polyaniline copolymer used herein is from about 30:70 to about 70:30, or from about 40:60 to about 60:40 (mass ratio chitosan:polyaniline).
  • the ratio of chitosan to polyaniline in the chitosan-grafted-polyaniline copolymer used herein is about 50:50.
  • the chitosan-grafted-polyaniline copolymer adheres strongly to silicon and may provide for less degradation of a silicon-based electrode.
  • Carboxymethyl cellulose is a water-soluble derivative of cellulose wherein some of the hydroxyl groups in the anhydroglucose units of cellulose have been substituted with carboxymethyl groups (-CH2-COOH).
  • the average number of substituted hydroxyl groups per anhydroglucose unit is defined as the degree of substitution (DS).
  • the degree of substitution of the carboxymethyl cellulose is not more than 3, such as from 1 to 3.
  • the carboxymethyl cellulose affects the flow behavior and processing properties of the binder.
  • the binder comprises a combination of a chitosan-grafted polyaniline copolymer, a styrene-butadiene rubber and carboxymethylcellulose. Such a binder prevents swelling of a silicon-based electrode.
  • the binder according to the present invention provides a much lower ionic resistance (Rion) compared to individual binders comprising either only SBR and CMC or only CS-g-PANI. This is shown in example 2 and in figures 2 and 3. Due to the fact that the binder according to the present invention is a combination of SBR and CMC with CS-g-PANI, it is expected to have an ionic resistance somewhere in between a binder comprising either only SBR and CMC or only CS-g-PANI. However, the inventors have shown that the Rion is significantly reduced compared to the individual binders. A lower Rion value means that ions diffuse faster in the electrode, leading to better fast charging capabilities of the cell.
  • the binder according to the present invention provides an improved peel strength compared to the corresponding individual binders comprising either only SBR and CMC or only CS-g-PANL
  • the peel strength is related to the adhesion of the binder to the rest of the electrode. A higher peel strength reduces the risk of delamination of the electrode.
  • the inventors have found that the peel strength of the presently invented binder far supersedes the peel strength of the individual binders. This is shown in example 3 and in figure 4.
  • the binder comprises from about 20% to about 65% (w/w) of a chitosan- grafted polyaniline copolymer, from about 20% to about 35% (w/w) styrene-butadiene rubber and from about 10% to about 35% (w/w) carboxymethylcellulose, up to a total of component parts of 100%.
  • the binder comprises about 35% to about 45% (w/w), more preferably about 40% (w/w) of a chitosan-grafted-polyaniline.
  • the polymers used in the binder of the present invention are water-processable and thus more environmentally benign and are also relatively inexpensive in comparison with many other binder polymers requiring cumbersome processing, involving inter alia the use of hazardous and high-boiling solvents, which use are often associated with higher costs.
  • the electrode is a silicone-graphite electrode comprising graphite and a silicon-based material.
  • the silicon-based material is selected from any one of silicon, silicon alloy, silicon oxide (SiO x ), and lithiated SiO x , wherein x is from 1 to 2; or a combination of at least two of these.
  • the silicon-based material is selected from silicon or silicon oxide.
  • the amount of silicon or silicon-oxide in the silicone-graphite electrode is at least about 10% (w/w), or at least about 30% (w/w), or at about least 40% (w/w), and up to at most about 50% (w/w), or at most about 60% (w/w).
  • the silicone-graphite electrode is adhered to a current collector.
  • the current collector may be in the form of a foil, or in the form of a mesh.
  • the foil is coated with a material that has been chosen from the group consisting of C, Si, Sn, Al, Zn, Ag, In, Mg.
  • the electrode is an anode.
  • the present invention relates to a secondary cell comprising an anode, a cathode, an electrolyte, and optionally a separator, characterized in that the secondary cell further comprises a binder in the anode, wherein the binder comprises a chitosan-grafted polyaniline copolymer; and a polymer selected from any one of a styrene-butadiene rubber and carboxymethylcellulose, or a combination of these.
  • the electrolyte used in the secondary cell according to the present invention is a liquid electrolyte comprising at least one lithium salt and at least one or more solvents selected from the group consisting of carbonate solvents and their fluorinated equivalents, diCi-4 ethers and their fluorinated equivalents and ionic liquids.
  • the lithium salt is preferably one or more selected from the group consisting of lithium hexafluorophosphate (Li PFe), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium (fluorosulfonyl) (trifluoromethanesulfonyl) imide (LiFTFSI), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), lithium (pentafluoroethanesulfonyl)(trifluoromethanesulfonyl)imide (LiPTFSI), lithium trifluoromethanesulfonate (LiOTf), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorobis(oxalato)phosphate (LiDF
  • the solvent is selected from the group consisting of 1,2-dimethoxyethane (DME), /V-propyl-A/- methylpyrrolidinium bis(fluorosulfonyl)imide (PYR13-FSI), /V-propyl-/V-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13-TFSI), 1-butyl-l-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14-FSI), 1-butyl-l-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14-TFSI), l-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIM-FSI), l-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIM-FSI), l
  • the term "about” refers to a value or parameter herein that includes (and describes) embodiments that are directed to that value or parameter per se. For example, description referring to "about 50” includes description of "50.” Numeric ranges are inclusive of the numbers defining the range. Generally speaking, the term “about” refers to the indicated value of the variable and to all values of the variable that are within the experimental error of the indicated value (e.g., within the 95% confidence interval for the mean) or within 10 percent of the indicated value, whichever is greater.
  • the anode was comprised of 96 weight% graphite:silicone (70:30 w/w), 2.5 weight% binder and 1.5 weight% conductive material, and with cupper foil as current collector.
  • the conductive material was comprised of 3% multi walled carbon nanotube (MWCNT), 1.61% CMC and 95.39% water.
  • the binder parameters were as defined in table 1 below.
  • the size difference between the charged and discharged state for the three anodes were investigated.
  • the thickness of the anodes was measured in their uncharged state.
  • the anodes were then assembled into coin half cells with a capacity of 8.19 mAh that were charged to 5 mV at C/10 with a constant voltage (CV) step current of C/200. After charging, the cells were disassembled, and the thicknesses of the anodes were measured. The results are shown in table 2 and in figure 1.
  • the swelling of the anode during charging is reduced to 58% when the binder according to sample 1 is used, compared to 93% and 71% for the reference binders, respectively.
  • the peel strength of the anodes was measured using a tensile testing machine (Mecmesin Multitest 2.5 dV).
  • a double-stranded tape was securely attached to a microscope glass slide, leaving 2.5 cm of the slide uncovered.
  • the opposite cover paper of the tape was then removed, and the glue-side was pressed onto the anode material.
  • the extra anode material was cut off around the glass slide, leaving 1 cm anode material from the short edge of the glass slide.
  • the anode material was then peeled off half-way from the glass slide.
  • the glass slide end with the anode material peeled off was attached to the bottom grip of the tensile tester, and the peeled-off anode material was attached to the top grip of tensile tester.
  • the tensile tester was then started and the results for the different anodes are shown in table 4 and in figure 4.
  • Table 4 The ionic resistance of the three difference anodes. As shown in table 4, the binder according to sample 1 has a significantly higher peel strength compared to the two reference binders, respectively.

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Abstract

An electrode binder, wherein the binder comprises a chitosan-grafted polyaniline copolymer; and a polymer selected from any one of a styrene-butadiene rubber and carboxymethylcellulose, or a combination of these; as well as an electrode comprising such binder; a secondary cell comprising such binder; and a vehicle comprising such secondary cell.

Description

BINDER COMBINATION FOR A SECONDARY CELL
FIELD OF THE INVENTION
The present disclosure relates to an electrode binder for an electrode in a secondary cell. More particularly, the present disclosure relates to a binder combination comprising a chitosan-grafted-polyaniline copolymer in combination with carboxymethylcellulose or styrene-butadiene rubber, an electrode comprising the binder combination, a secondary cell comprising such electrode, as well as a vehicle comprising such secondary cell.
TECHNICAL BACKGROUND
Rechargeable batteries having high energy density and discharge voltage, in particular Li-ion batteries, are a vital component in portable electronic devices and are a key enabler for the electrification of transport and large-scale storage of electricity. To reach higher energy densities, new types of batteries are being developed.
State of the art Li-ion batteries typically consist of stacks of secondary cells, wherein each cell is composed of a cathode comprising a cathode current collector, an electrolyte, an anode comprising an anode current collector, and optionally a separator positioned between the anode and cathode.
One of the limiting factors of the Li-ion battery is its anode. In secondary cells where the anode is made of graphite-based materials, the cations extracted from the cathode material diffuse from the cathode material through the electrolyte and intercalate into the graphite material at the anode during charging. During discharge, this process is reversed.
In an effort to increase the energy density of the secondary cells, development is ongoing to replace a part of the graphite with silica to increase the capacity of the electrode since silica demonstrates a higher maximum theoretical capacity than graphite. However, a challenge with silica is that it swells and contracts during charge and discharge. This causes a mechanical stress in the electrode material, which may initiate cracks and also impair the adhesion of the electrode active material to the current collector leading to electrode disintegration. These circumstances may shorten the performance and lifetime of the secondary cell.
Attempts have been made to use a binder to achieve better adherence of the electrode active material and conducting agent to the current collector and improve the mechanical strength of the anode. Conventional binders used for electrodes in batteries are poly-(vinylidene fluoride) (PVDF), copolymers of vinylidene difluoride (VdF) and hexafluoropropene (HFP) monomers (Kynar or KynarFlex), carboxymethylcellulose (CMC) and its sodium salt (CMC-Na), poly(acrylic acid) (PAA) and its sodium salt (PAA-Na), and styrene-butadiene rubber (SBR). Polyvinylidene fluoride and styrene-butadiene rubber possess a weak interaction with silicon and are ineffective in reducing volume changes of silicon material. Sodium carboxymethylcellulose and poly(acrylic acid) are rather brittle.
Hence, there is a need for improving the mechanical integrity of the electrodes and at the same time improving the electric conductivity.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electrode binder for an electrode in a secondary cell, wherein the binder may provide an improved electric conductivity and a good mechanical strength to maintain the structure of the electrode. A further object is the provision of an anode electrode with an improved electrochemical performance and low swelling.
The present invention provides a binder combination for an electrode in a secondary cell, wherein the binder comprises a chitosan-grafted-polyaniline copolymer in combination with carboxymethylcellulose or styrene-butadiene rubber.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows the swelling of three different anodes during charging. Fig. 2 and fig. 3 show the ionic resistance of three different anodes. Fig. 4 shows the peel strength of three different anodes. DETAILED DESCRIPTION OF THE INVENTION
In a first aspect, the present invention relates to an electrode binder for an electrode in a secondary cell, wherein the binder comprises a chitosan-grafted-polyaniline copolymer (CS-g-PANI) and a polymer selected from any one of styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), or a combination of these.
Preferably, the electrode binder is a binder for an anode.
A chitosan-grafted-polyaniline copolymer is formed by graft polymerization of chitosan and polyaniline and has a high degree of cross-linking. Chitosan is a polysaccharide obtained by deacetylation of chitin from crustaceans and insects, and contributes to the mechanical stability. Polyaniline is a conducting polymer derived from aniline monomers and helps to improve the electric conductivity of the binder. Preferably the ratio of chitosan to polyaniline in the chitosan-grafted-polyaniline copolymer used herein is from about 30:70 to about 70:30, or from about 40:60 to about 60:40 (mass ratio chitosan:polyaniline). Preferably the ratio of chitosan to polyaniline in the chitosan-grafted-polyaniline copolymer used herein is about 50:50. The chitosan-grafted-polyaniline copolymer adheres strongly to silicon and may provide for less degradation of a silicon-based electrode.
Styrene-butadiene rubber is an elastomeric binder derived from styrene and butadiene. Preferably the ratio of styrene to butadiene in the styrene-butadiene rubber used herein is from about 10:90 to about 30:70 (molar parts styrene:butadiene), preferably from about 24:76 to about 26:74. The styrene-butadiene rubber is flexible and may suppress the formation of cracks in the electrode material.
Carboxymethyl cellulose is a water-soluble derivative of cellulose wherein some of the hydroxyl groups in the anhydroglucose units of cellulose have been substituted with carboxymethyl groups (-CH2-COOH). The average number of substituted hydroxyl groups per anhydroglucose unit is defined as the degree of substitution (DS). Preferably the degree of substitution of the carboxymethyl cellulose is not more than 3, such as from 1 to 3. The carboxymethyl cellulose affects the flow behavior and processing properties of the binder. Preferably, the binder comprises a combination of a chitosan-grafted polyaniline copolymer, a styrene-butadiene rubber and carboxymethylcellulose. Such a binder prevents swelling of a silicon-based electrode.
The inventors have surprisingly found that a binder comprising a combination of CS-g-PANI, SBR and CMC provides the electrode with extraordinary features. This is due to synergistic effects between the components of the binder. With regards to swelling, the binder according to the present invention performs exceedingly well compared to binders comprising either only SBR and CMC or only CS-g-PANI. Figure 1 shows the difference in swelling between three electrodes, each carrying a different binder. The combination of SBR, CMC and CS-g-PANI, produces a reduction in swelling that is even lower compared to a binder comprising only CS-g-PANI or a binder comprising a combination of SBR and CMC only. When using an electrode with a binder comprising only SBR and CMC, the SBR migrates to the surface of the electrode, effectively removing the beneficial effects that the SBR would have had on the swelling if it stayed in place inside the electrode. Surprisingly, using CS-g-PANI together with SBR and CMC prevents this migration, leading to a better utilization of the swelling reducing capability of SBR. At the same time, CS-g-PANI reduces the electrode swelling by itself. The combination of these two effects provides for a binder that significantly reduces the swelling of an electrode during charging. This is shown in example 1 and in figure 1.
Moreover, the binder according to the present invention provides a much lower ionic resistance (Rion) compared to individual binders comprising either only SBR and CMC or only CS-g-PANI. This is shown in example 2 and in figures 2 and 3. Due to the fact that the binder according to the present invention is a combination of SBR and CMC with CS-g-PANI, it is expected to have an ionic resistance somewhere in between a binder comprising either only SBR and CMC or only CS-g-PANI. However, the inventors have shown that the Rion is significantly reduced compared to the individual binders. A lower Rion value means that ions diffuse faster in the electrode, leading to better fast charging capabilities of the cell. Already a reduction in Rion of 0.1 Ohm is considered a significant improvement. Furthermore, the binder according to the present invention provides an improved peel strength compared to the corresponding individual binders comprising either only SBR and CMC or only CS-g-PANL The peel strength is related to the adhesion of the binder to the rest of the electrode. A higher peel strength reduces the risk of delamination of the electrode. The inventors have found that the peel strength of the presently invented binder far supersedes the peel strength of the individual binders. This is shown in example 3 and in figure 4.
In one embodiment the binder comprises from about 20% to about 65% (w/w) of a chitosan- grafted polyaniline copolymer, from about 20% to about 35% (w/w) styrene-butadiene rubber and from about 10% to about 35% (w/w) carboxymethylcellulose, up to a total of component parts of 100%. Preferably, the binder comprises about 35% to about 45% (w/w), more preferably about 40% (w/w) of a chitosan-grafted-polyaniline.
The polymers used in the binder of the present invention are water-processable and thus more environmentally benign and are also relatively inexpensive in comparison with many other binder polymers requiring cumbersome processing, involving inter alia the use of hazardous and high-boiling solvents, which use are often associated with higher costs.
Another aspect of the invention is an electrode comprising the binder according to the first aspect of the present invention. Preferably, the electrode is a silicone-graphite electrode comprising graphite and a silicon-based material. In one embodiment, the silicon-based material is selected from any one of silicon, silicon alloy, silicon oxide (SiOx), and lithiated SiOx, wherein x is from 1 to 2; or a combination of at least two of these. Preferably, the silicon-based material is selected from silicon or silicon oxide. Preferably, the amount of silicon or silicon-oxide in the silicone-graphite electrode is at least about 10% (w/w), or at least about 30% (w/w), or at about least 40% (w/w), and up to at most about 50% (w/w), or at most about 60% (w/w).
In one embodiment, the silicone-graphite electrode is adhered to a current collector. The current collector may be in the form of a foil, or in the form of a mesh. In one embodiment, the foil is coated with a material that has been chosen from the group consisting of C, Si, Sn, Al, Zn, Ag, In, Mg. Preferably, the electrode is an anode.
In a further aspect, the present invention relates to a secondary cell comprising an anode, a cathode, an electrolyte, and optionally a separator, characterized in that the secondary cell further comprises a binder in the anode, wherein the binder comprises a chitosan-grafted polyaniline copolymer; and a polymer selected from any one of a styrene-butadiene rubber and carboxymethylcellulose, or a combination of these.
The electrolyte used in the secondary cell according to the present invention is a liquid electrolyte comprising at least one lithium salt and at least one or more solvents selected from the group consisting of carbonate solvents and their fluorinated equivalents, diCi-4 ethers and their fluorinated equivalents and ionic liquids. The lithium salt is preferably one or more selected from the group consisting of lithium hexafluorophosphate (Li PFe), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium (fluorosulfonyl) (trifluoromethanesulfonyl) imide (LiFTFSI), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), lithium (pentafluoroethanesulfonyl)(trifluoromethanesulfonyl)imide (LiPTFSI), lithium trifluoromethanesulfonate (LiOTf), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorobis(oxalato)phosphate (LiDFOP), lithium tetrafluoro(oxalato)phosphate (LiTFOP), lithium tetrafluoroborate (UBF4), lithium nitrate (LiNOs) lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI). In one embodiment, the solvent is selected from the group consisting of 1,2-dimethoxyethane (DME), /V-propyl-A/- methylpyrrolidinium bis(fluorosulfonyl)imide (PYR13-FSI), /V-propyl-/V-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13-TFSI), 1-butyl-l-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14-FSI), 1-butyl-l-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14-TFSI), l-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIM-FSI), l-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM-TFSI), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), ethylene carbonate (EC), and propylene carbonate (PC), and their fluorinated equivalents. In a further aspect, the present invention relates to a vehicle comprising a secondary cell according to the previous aspect of the present invention.
As used herein, the term "about" refers to a value or parameter herein that includes (and describes) embodiments that are directed to that value or parameter per se. For example, description referring to "about 50" includes description of "50." Numeric ranges are inclusive of the numbers defining the range. Generally speaking, the term "about" refers to the indicated value of the variable and to all values of the variable that are within the experimental error of the indicated value (e.g., within the 95% confidence interval for the mean) or within 10 percent of the indicated value, whichever is greater.
All aspects and embodiments disclosed herein can be combined with any other aspect and/or embodiment disclosed herein.
EXAMPLES
GENERAL CONDITIONS
Three anodes, having different binder compositions, were used. The anode was comprised of 96 weight% graphite:silicone (70:30 w/w), 2.5 weight% binder and 1.5 weight% conductive material, and with cupper foil as current collector. The conductive material was comprised of 3% multi walled carbon nanotube (MWCNT), 1.61% CMC and 95.39% water. The binder parameters were as defined in table 1 below.
Table 1. Definition of the binders used.
Example 1. Swelling
The size difference between the charged and discharged state for the three anodes were investigated. The thickness of the anodes was measured in their uncharged state. The anodes were then assembled into coin half cells with a capacity of 8.19 mAh that were charged to 5 mV at C/10 with a constant voltage (CV) step current of C/200. After charging, the cells were disassembled, and the thicknesses of the anodes were measured. The results are shown in table 2 and in figure 1.
Table 2. Results of the swelling experiment.
* Average of three runs. Excluding the cupper foil in the measurement.
As shown in table 2, the swelling of the anode during charging is reduced to 58% when the binder according to sample 1 is used, compared to 93% and 71% for the reference binders, respectively.
Example 2. Ionic resistance
The ionic resistance of the anodes was measured using electrochemical impedance spectroscopy (EIS). Anode symmetric cells were assembled in coin format and EIS (scan from 200 kHz to 100 mHz, sinus amplitude 10 mV) was performed to find the electronic resistance (Rion). The results are shown in table 3 and in figures 2 and 3.
Table 3. Ionic resistance of the three different anodes.
Compared to a binder comprising only CS-g-PANI, the ionic resistance was lowered by 1.08 Ohm using a binder according to sample 1. Considering the fact that a reduction in Rion of 0.1 Ohm would be significant, this is an unexpected improvement. As shown in table 3, the ionic resistance is reduced to 18,14 Ohm, which is much lower than the two reference binders, respectively. Example 3. Peel strength
The peel strength of the anodes was measured using a tensile testing machine (Mecmesin Multitest 2.5 dV). A double-stranded tape was securely attached to a microscope glass slide, leaving 2.5 cm of the slide uncovered. The opposite cover paper of the tape was then removed, and the glue-side was pressed onto the anode material. The extra anode material was cut off around the glass slide, leaving 1 cm anode material from the short edge of the glass slide. The anode material was then peeled off half-way from the glass slide. The glass slide end with the anode material peeled off was attached to the bottom grip of the tensile tester, and the peeled-off anode material was attached to the top grip of tensile tester. The tensile tester was then started and the results for the different anodes are shown in table 4 and in figure 4.
Table 4. The ionic resistance of the three difference anodes. As shown in table 4, the binder according to sample 1 has a significantly higher peel strength compared to the two reference binders, respectively.

Claims

1. An electrode binder, wherein the binder comprises a chitosan-grafted polyaniline copolymer; and a polymer selected from any one of a styrene-butadiene rubber, carboxymethylcellulose, or a combination of these.
2. The binder according to claim 1, wherein the binder comprises a combination of a chitosan-grafted polyaniline, a styrene-butadiene rubber and carboxymethylcellulose.
3. The binder according to claim 1 or 2, which is a binder for an anode.
4. An electrode comprising the binder according to claim 1 or 2.
5. The electrode according to claim 4, wherein the electrode is a silicon-graphite-electrode, comprising graphite and a silicon-based material.
6. The electrode according to claim 5, wherein the silicon-based material is selected from any one of Si or SiOx, or a combination thereof, wherein x is from 1 to 2.
7. The electrode according to claim 5 or 6, wherein the amount of the silicon-based material is at least about 10% (w/w).
8. The electrode according to claim 5 or 6, wherein the amount of the silicon-based material is at least about 30% (w/w).
9. The electrode according to any one of claims 4-8, which is an anode.
10. A secondary cell comprising an anode, a cathode, an electrolyte, and optionally a separator, characterized in that the secondary cell further comprises a binder in the anode, wherein the binder comprises a chitosan-grafted polyaniline copolymer; and a polymer selected from any one of a styrene-butadiene rubber and carboxymethylcellulose, or a combination of these.
11. A vehicle comprising the secondary cell according to claim 10, wherein the secondary cell further comprises a lithium nickel manganese cobalt oxide cathode.
EP23736650.5A 2022-06-30 2023-06-28 Binder combination for a secondary cell Pending EP4548412A1 (en)

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