Fluoropolymer binder for lithium-ion secondary battery cathodes

Abstract

A fluoropolymer binder composition for use in a lithium-ion secondary battery cathode is provided, which contains a tetrafluoroethylene polymer and an elastomeric fluoropolymer. A cathode composition containing the fluoropolymer binder composition together with cathode active particles and conductive carbon is also provided. The tetrafluoroethylene polymer is generally a high molecular weight non-melt processable tetrafluoroethylene homopolymer and modified tetrafluoroethylene homopolymer. The elastomeric fluoropolymer is generally a vinylidene fluoride elastomeric fluoropolymer. The cathode composition is formed by dry mixing the fluoropolymer binder, the cathode active material, and the conductive carbon by a solvent-free process and applying a shear force, which fibrillates the tetrafluoroethylene polymer. The cathode composition has a fluoropolymer binder uniformly dispersed in the main cathode active material, and has elasticity such that a thin film of the cathode composition can be formed into a cylindrical shape without breaking, enabling their usefulness as a lithium-ion battery cathode electrode film.

Description

[Technical field]

[0001] The present invention relates to a fluoropolymer binder composition for use in lithium-ion secondary battery cathodes and a method for making the same. Summary of the Invention [Means for solving the problem]

[0002] The present invention provides a fluoropolymer binder composition for use in a lithium ion secondary battery cathode, comprising: i.) a tetrafluoroethylene polymer; and ii.) an elastomeric fluoropolymer (alternatively referred to herein as a fluoroelastomer). The tetrafluoroethylene polymer is generally selected from high molecular weight non-melt processable tetrafluoroethylene homopolymers and high molecular weight non-melt processable modified tetrafluoroethylene homopolymers. The elastomeric fluoropolymer is generally a vinylidene fluoride elastomeric fluoropolymer. The binder composition is formed by a solvent-free process, generally by dry mixing the tetrafluoroethylene polymer with the elastomeric fluoropolymer, and, in one embodiment, applying shear forces to the composition, which fibrillates the tetrafluoroethylene polymer.

[0003] The present invention further provides a cathode composition for use in a lithium ion secondary battery cathode, comprising the fluoropolymer binder composition described above, further containing cathode active particles and conductive carbon. The cathode composition has elasticity such that a calendered film of the composition suitable for use as a cathode electrode film in a lithium ion battery can be formed into a tubular shape without breaking.

[0004] The present invention further provides a method for producing the above cathode composition for use in a lithium-ion secondary battery cathode, comprising mixing an elastomeric fluoropolymer, a cathode active particle, a tetrafluoroethylene polymer, and a conductive carbon to form a cathode composition. In one embodiment, the method comprises mixing an elastomeric fluoropolymer with a cathode active particle to form a composition of fluoroelastomer and cathode active material, and then mixing the composition with a tetrafluoroethylene polymer and a conductive carbon to form a cathode composition. The mixing step of the method is carried out without a solvent and generally comprises dry mixing a composition of fluoroelastomer, cathode active particle, tetrafluoroethylene polymer, and conductive carbon, and applying a shear force to the composition, whereby the shear force is sufficient to fibrillate the tetrafluoroethylene polymer. [Brief description of the drawings]

[0005] [Figure 1] 1 is a fluorine content distribution plot obtained from LIBS microscopy performed on films of the present composition of cathode active material, tetrafluoroethylene polymer, and elastomeric fluoropolymer. [Diagram 2] 1 is a fluorine content distribution plot obtained from LIBS microscopy performed on films of the present composition of cathode active material, tetrafluoroethylene polymer, and elastomeric fluoropolymer. [Diagram 3] 1 is a fluorine content distribution plot obtained from LIBS microscopy performed on films of the present composition of cathode active material, tetrafluoroethylene polymer, and elastomeric fluoropolymer. [Figure 4] 1 is a fluorine content distribution plot obtained from LIBS microscopy performed on films of the present composition of cathode active material, tetrafluoroethylene polymer, and elastomeric fluoropolymer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] In one embodiment, the invention is a fluoropolymer binder composition for use in a lithium ion secondary battery cathode, the fluoropolymer binder composition comprising: i.) a tetrafluoroethylene polymer; and ii.) an elastomeric fluoropolymer.

[0007] In another embodiment, the invention is a cathode composition for use in a lithium ion secondary battery cathode comprising: i.) a tetrafluoroethylene polymer; ii.) an elastomeric fluoropolymer; iii.) cathode active particles; and iv.) conductive carbon.

[0008] In one embodiment, the cathode composition has elasticity such that a calendered film of the composition suitable for use as a cathode electrode film in a lithium ion battery can be formed into a tubular shape without breaking.

[0009] In one embodiment, the tetrafluoroethylene polymer has a molecular weight of at least about 1.8×10 11 In another embodiment, the tetrafluoroethylene polymer has a melt creep viscosity of at least about 2.0×10 11 In another embodiment, the tetrafluoroethylene polymer has a melt creep viscosity of at least about 3.0×10 11 In a preferred embodiment, the tetrafluoroethylene polymer has a melt creep viscosity of at least about 4.0×10 11It has a melt creep viscosity of poise. As used herein, melt creep viscosity (MCV) is measured by the method described in Ebnesajjad, Sina, (2015), Fluoroplastics, Volume 1-Non-Melt Processible Fluoropolymers-The Definitive User's Guide and Data Book (2nd Edition), Appendix 5, Melt Creep Viscosity of Polytetrafluoroethylene, pp. 660-661, with reference to U.S. Pat. No. 3,819,594.

[0010] The tetrafluoroethylene polymer is a polymer containing repeating units of tetrafluoroethylene monomer, also referred to in the art as TFE, and has a molecular weight of at least about 1.8×10 11It has a melt creep viscosity of poise. At such high melt viscosity, the polymer does not flow in the molten state and is therefore not a melt processable polymer. In one embodiment, the tetrafluoroethylene polymer is a tetrafluoroethylene homopolymer containing only repeating units of tetrafluoroethylene monomer, also known in the art as polytetrafluoroethylene and abbreviated as PTFE. In another embodiment, the tetrafluoroethylene polymer is a "modified" PTFE, which refers to a copolymer of TFE and a low concentration of comonomer such that the melting point of the resulting polymer is not substantially reduced below the melting point of PTFE. The concentration of such comonomer is less than 1% by weight, preferably less than 0.5% by weight. A minimum amount of at least about 0.05% by weight is used to have a significant effect. Examples of comonomers for modified PTFE include perfluoroolefins, particularly hexafluoropropylene (HFP) or perfluoro(alkyl vinyl ether) (PAVE), where the alkyl group contains 1 to 5 carbon atoms, with perfluoro(ethyl vinyl ether) (PEVE) and perfluoro(propyl vinyl ether) (PPVE) being preferred. Further examples of comonomers for modified PTFE include chlorotrifluoroethylene (CTFE) and perfluorobutylethylene (PFBE), or other similar monomers that introduce bulky side groups into the polymer chain.

[0011] In a preferred embodiment, the tetrafluoroethylene polymer is fibrillizable, meaning that the tetrafluoroethylene polymer is capable of forming nano-sized (i.e., <100 nm wide) fibrils in at least one dimension that can vary in length from submicrometers to several micrometers when the tetrafluoroethylene polymer is subjected to shear, for example, during the present mixing method for producing a cathode composition.

[0012] In one embodiment, the elastomeric fluoropolymer has a Mooney viscosity of at least about 20 MU measured under condition ML1+10 (121°C) according to ASTM D1646. In another embodiment, the elastomeric fluoropolymer has a Mooney viscosity of at least about 30 MU measured under condition ML1+10 (121°C) according to ASTM D1646. In another embodiment, the elastomeric fluoropolymer has a Mooney viscosity of at least about 40 MU measured under condition ML1+10 (121°C) according to ASTM D1646. In another embodiment, the elastomeric fluoropolymer has a Mooney viscosity of at least about 50 MU measured under condition ML1+10 (121°C) according to ASTM D1646. In another embodiment, the elastomeric fluoropolymer has a Mooney viscosity of at least about 60 MU measured under condition ML1+10 (121°C) according to ASTM D1646. In a preferred embodiment, the elastomeric fluoropolymer has a Mooney viscosity of at least about 70 MU measured under condition ML1+10 (121° C.) according to ASTM D1646.

[0013] The elastomeric fluoropolymer is an elastomer that provides the cathode composition with elasticity such that a calendered film of the composition can be formed into a tubular shape without breaking, for example, a tubular shape having a radius of curvature sufficient for utility as a cathode electrode film in a lithium ion battery.

[0014] Examples of elastomeric fluoropolymers include vinylidene fluoride / hexafluoropropylene (VDF / HFP), vinylidene fluoride / hexafluoropropylene / tetrafluoroethylene (VDF / HFP / TFE), vinylidene fluoride / perfluoro(methyl vinyl ether) / tetrafluoroethylene (VDF / PMVE / TFE), tetrafluoroethylene / perfluoro(methyl vinyl ether) (TFE / PMVE), tetrafluoroethylene / propylene (TFE / P), and ethylene / tetrafluoroethylene / perfluoro(methyl vinyl ether) (E / TFE / PMVE).

[0015] Alternatively, the elastomeric fluoropolymers useful in the present invention may be (A) VDF-based elastic fluoropolymer, wherein VDF is (i) C2-C8 perfluoroolefins such as tetrafluoroethylene (TFE) and hexafluoropropylene (HFP); (ii) hydrogen-containing C2-C8 olefins such as vinyl fluoride (VF), trifluoroethylene, hexafluoroisobutene, perfluoroalkylethylenes of the formula CH2=CH-Rf, where Rf is a C1-C6 perfluoroalkyl group; (iii) C2-C8 fluoroolefins containing at least one of iodine, chlorine, and bromine, such as chlorotrifluoroethylene (CTFE); (iv) (per)fluoroalkyl vinyl ethers (PAVE) of the formula CF2=CFORf, where Rf is a C1-C6 (per)fluoroalkyl group, preferably -CF3, -C2F5, -C3F7; (v) (per)fluoro-oxy-alkyl vinyl ethers of the formula CF2=CFOX, where X is a C1-C12 ((per)fluoro)-oxyalkyl containing a catenary oxygen atom, for example a perfluoro-2-propoxypropyl group; (vi) (per)fluorodioxole; (vii) (per)fluoro-methoxy-vinyl ethers having the formula: CF2=CFOCF2ORf2, where Rf2 is selected from the group consisting of C1-C6 (per)fluoroalkyl; C5-C6 cyclic (per)fluoroalkyl, and C2-C6 (per)fluoroalkyl containing at least one catenary oxygen atom, and Rf2 is preferably -CF2CF3, -CF2CF2OCF3, or -CF3; (viii) VDF-based elastic fluoropolymers copolymerized with at least one additional comonomer selected from the group consisting of C2-C8 non-fluorinated olefins, such as ethylene and propylene; and (2) It can be described as a TFE-based elastomeric fluoropolymer, in which the TFE is copolymerized with at least one additional comonomer selected from the group consisting of (i)-(vii) described immediately above.

[0016] In a preferred embodiment, the elastomeric fluoropolymer is a vinylidene fluoride copolymer, more preferably a copolymer of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene.

[0017] In one embodiment, the elastomeric fluoropolymer binder is crosslinked, which not only improves the mechanical properties of the polymer, but also helps to provide good contact between the components of the cathode composition.

[0018] In a preferred embodiment, the fluoropolymer binder composition has a viscosity of at least about 4.0×10 11 The elastomeric fluoropolymers include tetrafluoroethylene homopolymers having a melt creep viscosity of poise, and are copolymers of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene having a Mooney viscosity of at least about 17 MU measured under condition ML1+10 (121° C.) according to ASTM D1646.

[0019] In one embodiment, the weight ratio of tetrafluoroethylene polymer to elastomeric fluoropolymer in the binder and cathode composition is from about 4:1 to about 1:4. In a preferred embodiment, the weight ratio of tetrafluoroethylene polymer to the elastomeric fluoropolymer is about 1:1. The amount of elastomeric fluoropolymer in the cathode composition can be determined within this range by one skilled in the art, and is preferably the minimum amount required for a film of the cathode composition to function adequately in a cathode having the radius of curvature required for a given lithium ion battery.

[0020] In one embodiment of the cathode composition, the amount of fluoropolymer binder is from about 2 to about 10 weight percent, the amount of cathode active particles is from about 90 to about 97 weight percent, and the amount of conductive carbon is from about 0.1 to about 5 weight percent, based on the combined weight of the fluoropolymer binder, the cathode active particles, and the conductive carbon.

[0021] In one embodiment, based on the combined weight of the fluoropolymer binder, the cathode active particles, and the conductive carbon, the amount of tetrafluoroethylene polymer in the cathode composition is about 2 weight percent, the amount of elastomeric fluoropolymer is about 2 weight percent, the amount of cathode active particles is about 95 weight percent, and the amount of conductive carbon is about 1 weight percent. In one embodiment, based on the combined weight of the fluoropolymer binder, the cathode active particles, and the conductive carbon, the amount of tetrafluoroethylene polymer in the cathode composition is about 1 weight percent, the amount of elastomeric fluoropolymer is about 1 weight percent, the amount of cathode active particles is about 97 weight percent, and the amount of conductive carbon is about 1 weight percent.

[0022] The cathode active particles are selected from electrochemical cathode active materials known in the art. Examples of cathode active particles include metal oxides, metal sulfides, or lithium metal oxides. In a preferred embodiment, the cathode active particles comprise a lithium transition metal oxide. Examples of lithium metal oxides include lithium nickel manganese cobalt oxide (NMC), lithium manganese oxide (LMO), lithium iron phosphate (LiFePO4), lithium cobalt oxide (LCO), lithium titanate (LTO), and / or lithium nickel cobalt aluminum oxide (NCA). In some embodiments, the cathode active material is, for example, a layered transition metal oxide (LiCoO2 (LCO), Li(NiMnCo)O2 (NMC), LiNi 0.8 Co 0.15 Al 0.05 O2(NCA), etc.), spinel-type manganese oxide (LiMn2O4(LMO, LiMn 1.5Ni 0.5 O4 (LMNO), or olivine (LiFePO4, etc.), LiNiO2, LiNi 1-x Co x O2, LiNi 0.85 Co 0.1 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, LiMn2O4, and combinations thereof.

[0023] The conductive carbon is selected from conductive carbon materials known in the art. Examples of conductive carbon materials useful in the present invention include carbon black, porous carbon, carbon nanotubes, carbon fibers, VGCF (vapor grown carbon fibers), graphene sheets, acetylene black, and combinations thereof.

[0024] In one embodiment, the invention is a method for making a cathode composition for use in a lithium ion secondary battery cathode, the method comprising mixing an elastomeric fluoropolymer, cathode active particles, a tetrafluoroethylene polymer, and a conductive carbon to form the cathode composition.

[0025] In one embodiment, prior to mixing with any other components of the cathode composition, the elastomeric fluoropolymer is masticated at a high enough temperature (e.g., about 80°C to about 150°C) to impart flowability to the elastomeric fluoropolymer. The inventors have discovered that an elastomeric fluoropolymer in a less viscous and semi-fluid state and then mixed with the cathode active particles, conductive carbon, and tetrafluoroethylene polymer results in a cathode composition having all components substantially uniformly dispersed. Without being bound by theory, the inventors believe that in forming the cathode composition, the elastomeric fluoropolymer in a semi-fluid or fluid state acts as a viscous lubricant that is compatible with the tetrafluoroethylene polymer, improving the mixing of the cathode composition components (as shown in the experimental results by shorter mixer torque peak times) and improving the dispersion of the cathode composition components (as shown in the experimental results by improved laser-induced breakdown spectroscopy (LIBS) results).

[0026] In one embodiment, the present invention is a method for producing a cathode composition for use in a lithium-ion secondary battery cathode, comprising: i.) mixing an elastomeric fluoropolymer with cathode active particles to form a composition of fluoroelastomer and cathode active material; and ii.) mixing the composition of fluoroelastomer and cathode active material with a tetrafluoroethylene polymer to form a cathode composition. In one embodiment of this method, the starting form of the elastomeric fluoropolymer is a pellet or sheet. In one embodiment, prior to mixing the elastomeric fluoropolymer with the cathode active material in step i.), the elastomeric fluoropolymer pellet or sheet is masticated at a high temperature (e.g., about 80°C to about 150°C) sufficient to impart flowability to the elastomeric fluoropolymer, resulting in the pellets or sheets combined into one homogeneous, continuous, semi-fluid mass of the elastomeric fluoropolymer, thereby resulting in better mixing and substantially uniform distribution of the elastomeric fluoropolymer throughout the cathode active material.

[0027] In an alternative embodiment, in mixing step i.), the composition of fluoroelastomer and cathode active material is a concentrate containing about 1 to about 5 weight percent fluoroelastomer and about 90 to about 97 weight percent cathode active particles, and then in mixing step ii.), additional elastomeric fluoropolymer is added.

[0028] In a preferred embodiment, the mixing step is carried out without the presence of a solvent, or, alternatively, by dry mixing, and the components of the cathode composition are in a dry form, such as an aggregate, granule, bead, pellet, sheet, or powder.

[0029] In one embodiment, the mixing step ii.) is carried out by applying a shear force to the composition of fluoroelastomer, cathode active particles, tetrafluoroethylene polymer, and conductive carbon, whereby the shear force is sufficient to fibrillate the tetrafluoroethylene polymer.

[0030] In one embodiment, the temperature of the mixing step is from about 80° C. to about 150° C. In another embodiment, the temperature of the mixing step is about 120° C.

[0031] In one embodiment, the mixing step is carried out in a mixer, the fill rate is about 75% to about 95%, and the maximum mixing torque occurs within about 0.5 to about 3 minutes from the start of mixing. Mixers useful in the present method can be readily determined by one of ordinary skill in the art and include mixers capable of generating high shear forces and generally applicable for uniformly mixing plastic and rubber materials.

[0032] In this method, the degree of dispersion of the tetrafluoroethylene polymer and the fluoroelastomer in the cathode composition obtained from the mixing step can be determined by laser-induced breakdown spectroscopy (LIBS). It is desirable that the degree of dispersion of the tetrafluoroethylene polymer and the fluoroelastomer in the cathode composition is high enough to enable the cathode composition to be suitable for use as a cathode electrode film in a lithium-ion battery.

[0033] In a preferred embodiment, the degree of dispersion of the tetrafluoroethylene polymer and the fluoroelastomer in the cathode composition provides the cathode composition with sufficient elasticity such that a calendered film of the cathode composition can be formed into a tubular shape without failure, e.g., breaks, cracks, crazing, or other structural discontinuities that could result in poor cathode performance. EXAMPLES

[0034] material a. Cathode active material: LMO (lithium manganese oxide) dry powder, supplied by Posco Es Materials b. PTFE1 (P1): Tetrafluoroethylene homopolymer dry powder, 4.0 × 10 11 Melt Creep Viscosity in Poise, supplied by Chemours FC LLC c. PTFE2 (P2): Modified tetrafluoroethylene dry powder, 1.8 × 10 11 Melt Creep Viscosity in Poise, supplied by Chemours FC LLC d. PTFE3 (P3): Modified tetrafluoroethylene dry powder, 1.3 × 10 10 Melt Creep Viscosity in Poise, supplied by Chemours FC LLC e. Fluoroelastomer 1 (F1): VDF / HFP copolymer pellets or sheets, Mooney viscosity 65MU measured under condition ML1+10 (100°C) according to ASTM D1646, supplied by Chemours FC LLC f. Fluoroelastomer 2 (F2): VDF / HFP / TFE copolymer pellets or sheets, Mooney viscosity 75MU measured under condition ML1+10 (121°C) according to ASTM D1646, supplied by Chemours FC LLC g. Fluoroelastomer 3 (F3): VDF / TFE / PMVE copolymer pellets or sheets, Mooney viscosity 65MU measured under condition ML1+10 (121°C) according to ASTM D1646, supplied by Chemours FC LLC h. Fluoroelastomer 4 (F4): VDF / HFP / TFE copolymer pellets or sheets, Mooney viscosity 23MU measured under condition ML1+10 (121°C) according to ASTM D1646, supplied by Chemours FC LLC

[0035] procedure: A quantity of elastomeric fluoropolymer (rubbery / elastic pellets or sheets) was added to the Brabender mixer for mastication at 120°C for 1 minute at 60 RPM to produce a less viscous, semi-fluid continuous mass of elastomeric fluoropolymer. A quantity of cathode active particles was then added to the Brabender mixer to mix with the semi-fluid elastomeric fluoropolymer and distribute the elastomeric fluoropolymer evenly throughout the cathode active particle matrix. The mixing process was carried out at 120°C for 0.5 minutes at 60 RPM. A quantity of tetrafluoroethylene polymer binder powder was then added to the Brabender mixer to mix with the previously formed mixture of cathode active particles and elastomeric fluoropolymer to distribute the tetrafluoroethylene polymer binder powder evenly throughout the cathode active particle matrix and fibrillate the tetrafluoroethylene polymer. Mixing was carried out at 60 RPM at 120°C for a time sufficient to observe MPT (time to peak torque (min)) and MPTQ (maximum peak torque (kgf.m)), which are evidence of fibrillation of the tetrafluoroethylene polymer, and then mixing was continued under these conditions for an additional 5 minutes. The formed cathode mixture was then removed from the mixer, cooled to room temperature, and observed visually (by naked eye) and by SEM (scanning electron microscope) to evaluate the degree of mixing, dispersion of the elastomeric fluoropolymer and the tetrafluoroethylene polymer throughout the cathode active particle continuous phase, and to evaluate the degree of fibrillation of the tetrafluoroethylene polymer. A portion of the so-formed cathode mixture was taken and pressed at a temperature between 120°C and 200°C to form a film having a thickness of 200 micrometers. The resulting film was then observed visually (by naked eye), by SEM (scanning electron microscope), and by laser-induced breakdown spectroscopy (LIBS) microscope.

[0036] For purposes of enabling analysis of dispersion of the cathode composition components by SEM and LIBS, conductive carbon was omitted from the compositions of these examples because it would complicate and obscure sample analysis by SEM and / or LIBS, however, when included in the cathode compositions, conductive carbon did not otherwise substantially change the experimental mixing, dispersion, and fibrillation results observed.

[0037] result Table 1. List of abbreviations: Ex = Example number; C = cathode active material (wt%); P1 = PTFE1 (wt%); P2 = PTFE2 (wt%); P3 = PTFE3 (wt%); F1 = Fluoroelastomer1 (wt%); F2 = Fluoroelastomer2 (wt%); F3 = Fluoroelastomer3 (wt%); F1 = Fluoroelastomer4 (wt%); FF = Mixer fill factor (% of mixer volume); MPT = Maximum time to peak torque (min); MPTQ = Maximum peak torque (kgf.m); ST = Mixer set temperature (°C); CT = Mixer chamber measured temperature (°C).

[0038] [Table 1]

[0039] In all examples shown in Table 1, the RPM (revolutions per minute) setting of the mixer was 60 RPM.

[0040] Example 11 is a composite of 97% by weight of cathode active material (LMO), 2% by weight of PTFE1 (P1) (tetrafluoroethylene homopolymer dry powder, 4.0×10 11(relatively high melt creep viscosity in poise), and 1 wt. % of fluoroelastomer 2 ((F2) (VDF / HFP / TFE copolymer, ASTM The composition has a relatively high Mooney viscosity of 75MU measured under condition ML1+10 (121°C) according to D1646. The mixer fill is 95%, the mixer peak torque maximum time is 0.5 minutes, the mixer maximum peak torque is 15.6kgf.m, the mixer set temperature is 150°C, and the mixer chamber measured temperature is 139.2°C. FIG. 1 is a fluorine content distribution plot obtained from LIBS microscopy performed on a film of the composition of Example 11. The plot shows that the fluoropolymer binder component of this composition is effectively uniformly dispersed in the main cathode active material. A 200 micrometer thick film of the composition of Example 11 was found to have sufficient elasticity such that the film can be formed into a tubular shape without breaking, and thus has utility as a cathode electrode film in a lithium ion battery.

[0041] Example 12 is identical to Example 11, except that the mixer fill rate is 85%, the mixer peak torque maximum time is 1 minute, the mixer maximum peak torque is 15.4 kgf.m, the mixer set temperature is 120°C, and the mixer chamber measured temperature is 112.9°C. FIG. 2 is a fluorine content distribution plot obtained from LIBS microscopy performed on a film of the composition of Example 12. The plot shows that the fluoropolymer binder component of this composition is very effectively uniformly dispersed in the main cathode active material. A 200 micrometer thick film of the composition of Example 12 was found to have sufficient elasticity such that the film can be formed into a tubular shape without breaking, and thus has utility as a cathode electrode film in a lithium ion battery.

[0042] Example 1 is a composite of 97 wt. % cathode active material (LMO) and 3 wt. % PTFE1 (P1) (tetrafluoroethylene homopolymer dry powder, 4.0×10 11A comparative composition of Example 1 having a relatively high melt creep viscosity of 1.0 poise. The mixer fill is 85%, the mixer peak torque maximum time is 3 minutes, the mixer maximum peak torque is 23.5 kgf.m, the mixer set temperature is 150°C, and the mixer chamber measured temperature is 143.0°C. FIG. 3 is a fluorine content distribution plot obtained from LIBS microscopy performed on a film of the composition of Example 1. The plot shows that the fluoropolymer binder component of this composition is not effectively dispersed in the main cathode active material in the dry mixing process of the present invention in the absence of an elastomeric fluoropolymer. A 200 micrometer thick film of the composition of Example 1 was found to have insufficient elasticity such that the film could not be formed into a tubular shape without breaking, and therefore is unlikely to have utility as a cathode electrode film in a lithium ion battery.

[0043] Example 13 is a composite of 97 wt. % cathode active material (LMO) and 2 wt. % PTFE1 (P1) (tetrafluoroethylene homopolymer dry powder, 4.0×10 1113. Comparative composition of Example 13 composition with relatively high melt creep viscosity of 1000 poise, and 1 wt.% fluoroelastomer 4 ((F4): VDF / HFP / TFE copolymer pellet or sheet, Mooney viscosity 23 MU measured under condition ML1+10 (121°C) according to ASTM D1646). Mixer filling rate is 85%, mixer peak torque maximum time is 1.5 minutes, mixer maximum peak torque is 12.4 kgf.m, mixer set temperature is 120°C, and mixer chamber measured temperature is 116.1°C. FIG. 4 is a fluorine content distribution plot obtained from LIBS microscopy performed on the film of the composition of Example 13. This plot shows that the fluoropolymer binder component of this composition is not effectively dispersed in the main cathode active material in the dry mixing process of the present invention by using an elastomeric fluoropolymer with a relatively low Mooney viscosity. A 200 micrometer thick film of the composition of Example 13 was found to have insufficient elasticity such that the film could not be formed into a tubular shape without breaking, and therefore is unlikely to have utility as a cathode electrode film in a lithium ion battery.

Claims

1. 1. A fluoropolymer binder composition for use in a lithium ion secondary battery cathode, comprising: i.) a tetrafluoroethylene polymer; and ii.) an elastomeric fluoropolymer.

2. The tetrafluoroethylene polymer has a viscosity of at least about 1.8×10 11 The binder composition of claim 1 having a melt creep viscosity in poise.

3. The tetrafluoroethylene polymer has a viscosity of at least about 2.0×10 11 The binder composition of claim 1 having a melt creep viscosity in poise.

4. The tetrafluoroethylene polymer has a viscosity of at least about 3.0×10 11 The binder composition of claim 1 having a melt creep viscosity in poise.

5. The tetrafluoroethylene polymer has a viscosity of at least about 4.0×10 11 The binder composition of claim 1 having a melt creep viscosity in poise.

6. 10. The binder composition of claim 1, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 20 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

7. 10. The binder composition of claim 1, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 30 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

8. 10. The binder composition of claim 1, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 40 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

9. 10. The binder composition of claim 1, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 50 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

10. 10. The binder composition of claim 1, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 60 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

11. 10. The binder composition of claim 1, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 70 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

12. 10. The binder composition of claim 1, wherein the elastomeric fluoropolymer is a vinylidene fluoride elastomeric fluoropolymer.

13. 13. The binder composition of claim 12, wherein the vinylidene fluoride elastomeric fluoropolymer comprises a copolymer of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene.

14. 2. The binder composition of claim 1, wherein the tetrafluoroethylene polymer is a tetrafluoroethylene homopolymer.

15. 2. The binder composition of claim 1, wherein the tetrafluoroethylene polymer is selected from the group consisting of high molecular weight non-melt processable tetrafluoroethylene homopolymers and high molecular weight non-melt processable modified tetrafluoroethylene homopolymers.

16. The tetrafluoroethylene polymer has a viscosity of at least about 4.0×10 11 2. The binder composition of claim 1, wherein the elastomeric fluoropolymer is a tetrafluoroethylene homopolymer having a melt creep viscosity of 100 poise, and the elastomeric fluoropolymer is a copolymer of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene having a Mooney viscosity of at least about 17 MU measured under condition ML1+10 (121°C) according to ASTM D1646.

17. The binder composition of claim 1, wherein the weight ratio of the tetrafluoroethylene polymer to the elastomeric fluoropolymer is from about 4:1 to about 1:

4.

18. The binder composition of claim 1 , wherein the tetrafluoroethylene polymer is fibrillated.

19. The binder composition of claim 1 formed by a solvent-free process.

20. The binder composition of claim 1 , wherein the composition is formed by dry blending the tetrafluoroethylene polymer and the elastomeric fluoropolymer.

21. 2. The binder composition of claim 1, wherein the composition is formed by dry blending the tetrafluoroethylene polymer and the elastomeric fluoropolymer and subjecting the composition to shear forces, thereby fibrillating the tetrafluoroethylene polymer.

22. 1. A cathode composition for use in a lithium ion secondary battery cathode, comprising: i.) a fluoropolymer binder comprising a mixture of a tetrafluoroethylene polymer and an elastomeric fluoropolymer; ii.) cathode active particles; and iii.) conductive carbon, wherein the cathode composition has elasticity such that a calendered film of the composition suitable for use as a cathode electrode film in a lithium ion battery can be formed into a tubular shape without fracture.

23. 23. The cathode composition of claim 22, wherein the cathode active particles comprise a lithium transition metal oxide.

24. The tetrafluoroethylene polymer has a viscosity of at least about 1.8×10 11 The binder composition of claim 1 having a melt creep viscosity in poise.

25. The tetrafluoroethylene polymer has a viscosity of at least about 2.0×10 11 23. The cathode composition of claim 22 having a melt creep viscosity of poise.

26. The tetrafluoroethylene polymer has a viscosity of at least about 3.0×10 11 23. The cathode composition of claim 22 having a melt creep viscosity of poise.

27. The tetrafluoroethylene polymer has a viscosity of at least about 4.0×10 11 23. The cathode composition of claim 22 having a melt creep viscosity of poise.

28. 23. The cathode composition of claim 22, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 20 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

29. 23. The cathode composition of claim 22, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 30 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

30. 23. The cathode composition of claim 22, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 40 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

31. 23. The cathode composition of claim 22, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 50 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

32. 23. The cathode composition of claim 22, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 60 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

33. 23. The cathode composition of claim 22, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 70 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

34. 23. The cathode composition of claim 22, wherein the elastomeric fluoropolymer is a vinylidene fluoride elastomeric fluoropolymer.

35. 35. The cathode composition of claim 34, wherein the vinylidene fluoride elastomeric fluoropolymer comprises a copolymer of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene.

36. 23. The cathode composition of claim 22, wherein the tetrafluoroethylene polymer is a tetrafluoroethylene homopolymer.

37. 23. The cathode composition of claim 22, wherein the tetrafluoroethylene polymer is selected from the group consisting of high molecular weight non-melt processable tetrafluoroethylene homopolymers and high molecular weight non-melt processable modified tetrafluoroethylene homopolymers.

38. The tetrafluoroethylene polymer has a viscosity of at least about 4.0×10 11 23. The cathode composition of claim 22, wherein the elastomeric fluoropolymer is a tetrafluoroethylene homopolymer having a melt creep viscosity of poise, and the elastomeric fluoropolymer is a copolymer of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene having a Mooney viscosity of at least about 17 MU measured in accordance with ASTM D1646 under condition ML1+10 (121°C).

39. 23. The cathode composition of claim 22, wherein the amount of the fluoropolymer binder in the cathode composition is from about 5 to about 10 weight percent, the amount of the cathode active particles is from about 90 to about 97 weight percent, and the amount of the conductive carbon is from about 0.1 to about 1 weight percent, based on the combined weight of the fluoropolymer binder, the cathode active particles, and the conductive carbon.

40. 23. The cathode composition of claim 22, wherein the amount of the tetrafluoroethylene polymer is about 2 weight percent, the amount of the elastomeric fluoropolymer is about 1 weight percent, the amount of the cathode active particles is about 96 weight percent, and the amount of the conductive carbon is about 1 weight percent, based on the combined weight of the fluoropolymer binder, the cathode active particles, and the conductive carbon.

41. 23. The cathode composition of claim 22, wherein the tetrafluoroethylene polymer is fibrillated.

42. 23. The cathode composition of claim 22 formed by a solvent-free process.

43. 23. The cathode composition of claim 22, wherein the composition is formed by dry mixing the tetrafluoroethylene polymer, the elastomeric fluoropolymer, the cathode active particles, and the conductive carbon.

44. 23. The cathode composition of claim 22, wherein the composition is formed by dry-mixing the tetrafluoroethylene polymer, the elastomeric fluoropolymer, the cathode active particles, and the conductive carbon, and subjecting the composition to a shear force, whereby the tetrafluoroethylene polymer is fibrillated.

45. 1. A method for making a cathode composition for use in a lithium-ion secondary battery cathode, the method comprising: mixing an elastomeric fluoropolymer, cathode active particles, a tetrafluoroethylene polymer, and a conductive carbon to form the cathode composition.

46. 46. ​​The method of claim 45, wherein prior to the mixing step, the elastomeric fluoropolymer is masticated at a temperature high enough to impart flowability to the elastomeric fluoropolymer.

47. 47. The method of claim 46, wherein the elevated temperature is from about 80°C to about 150°C.

48. 46. ​​The method of claim 45, comprising: i.) mixing the elastomeric fluoropolymer with the cathode active particles to form a fluoroelastomer and cathode active material composition; and ii.) mixing the fluoroelastomer and cathode active material composition with a tetrafluoroethylene polymer and conductive carbon to form the cathode composition.

49. 49. The method of claim 48, wherein prior to said mixing step i.), said elastomeric fluoropolymer is masticated at a temperature high enough to impart flowability to said elastomeric fluoropolymer.

50. 50. The method of claim 49, wherein the elevated temperature is from about 80°C to about 150°C.

51. 49. The method of claim 48, wherein the fluoroelastomer and cathode active material composition of step i.) is a concentrate containing about 1 to about 5 weight percent fluoroelastomer and about 90 to about 97 weight percent cathode active material, and additional elastomeric fluoropolymer is added in step ii.).

52. 46. ​​The method of claim 45, wherein the cathode composition contains from about 1 to about 5 weight percent tetrafluoroethylene polymer, from about 1 to about 5 weight percent elastomeric fluoropolymer, from about 90 to about 97 weight percent cathode active material, and from about 0.1 to about 1 weight percent conductive carbon.

53. 46. ​​The method of claim 45, wherein the weight ratio of said tetrafluoroethylene polymer to said elastomeric fluoropolymer is from about 4:1 to about 1:

4.

54. 46. ​​The method of claim 45, wherein the mixing step is carried out in the absence of a solvent.

55. 46. ​​The method of claim 45, wherein the mixing step is carried out by dry mixing.

56. 46. ​​The method of claim 45, wherein the mixing step is carried out by applying a shear force to the composition of fluoroelastomer, cathode active particles, tetrafluoroethylene polymer, and conductive carbon, whereby the shear force is sufficient to fibrillate the tetrafluoroethylene polymer.

57. 46. ​​The method of claim 45, wherein the mixing step is carried out at a temperature of from about 80°C to about 150°C.

58. 46. ​​The method of claim 45, wherein the mixing step is carried out at a temperature of about 120°C.

59. 46. ​​The method of claim 45, wherein the mixing step is carried out in a mixer, the fill factor is about 75% to about 95%, and maximum mixing torque occurs within about 0.5 to about 3 minutes of mixing initiation.

60. 46. ​​The method of claim 45, wherein the degree of dispersion of the tetrafluoroethylene polymer and the fluoroelastomer in the cathode composition, as determined by laser-induced breakdown spectroscopy (LIBS), is sufficiently high to enable the cathode composition to be suitable for use as a cathode electrode film in a lithium-ion battery.

61. 46. ​​The method of claim 45, wherein the cathode composition has elasticity such that a calendered film of the composition can be formed into a tubular shape without breaking.

62. 46. ​​The method of claim 45, wherein the cathode active particles comprise a lithium transition metal oxide.

63. The tetrafluoroethylene polymer has a viscosity of at least about 1.8×10 11 46. ​​The method of claim 45, having a melt creep viscosity in poise.

64. The tetrafluoroethylene polymer has a viscosity of at least about 2.0×10 11 46. ​​The method of claim 45, having a melt creep viscosity in poise.

65. The tetrafluoroethylene polymer has a viscosity of at least about 3.0×10 11 46. ​​The method of claim 45, having a melt creep viscosity in poise.

66. The tetrafluoroethylene polymer has a viscosity of at least about 4.0×10 11 46. ​​The method of claim 45, having a melt creep viscosity in poise.

67. 46. ​​The method of claim 45, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 30 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

68. 46. ​​The method of claim 45, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 40 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

69. 46. ​​The method of claim 45, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 50 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

70. 46. ​​The method of claim 45, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 60 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

71. 46. ​​The method of claim 45, wherein the elastomeric fluoropolymer has a Mooney viscosity of at least about 70 MU measured according to ASTM D1646 under condition ML1+10 (121°C).

72. 46. ​​The method of claim 45, wherein the elastomeric fluoropolymer is a vinylidene fluoride elastomeric fluoropolymer.

73. 73. The method of claim 72, wherein the vinylidene fluoride elastomeric fluoropolymer comprises a copolymer of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene.

74. 46. ​​The method of claim 45, wherein the tetrafluoroethylene polymer is a tetrafluoroethylene homopolymer.

75. The tetrafluoroethylene polymer has a viscosity of at least about 4.0×10 11 46. ​​The method of claim 45, wherein the elastomeric fluoropolymer is a tetrafluoroethylene homopolymer having a melt creep viscosity of poise, and the elastomeric fluoropolymer is a copolymer of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene having a Mooney viscosity of at least about 17 MU measured under condition ML1+10 (121°C) in accordance with ASTM D1646.

Images