Fluoropolymer binder for secondary lithium-ion battery anode

Abstract

The present invention provides fluoropolymer compositions and fabrication for improved electrochemical stability by (i) introducing monomers in the backbone of a TFE (tetrafluoroethylene) containing polymer and (ii) modification of the architecture of the polymer in various structures to obtain performance requirements and fibrillation characteristics for use in a dry, solvent free electrode formation process.

Description

TITLE OF THE INVENTIONFLUOROPOLYMBER BINDER FOR SECONDARY LITHIUM-ION BATTERY ANODE ELECTRODECROSS REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims the priority benefit of US provisional patent application no. 63 / 532,992, filed on August 16, 2023, the disclosure of which is hereby incorporated by reference in its entirety.FIELD

[0002] Fluoropolymer binders used in an anode electrode having improved reductive stability and performance in a secondary lithium-ion battery.BACKGROUND

[0003] It is recognized that multiple factors can contribute to the electrochemical stability of a fluoropolymer binder in an electrode. These include, but are not limited to, the composition and molecular structure. Further, the relationship of molecular weight and fibrillation is critical in the preparation of fluoropolymers for use. The control and regulation of fibrillation is critical to obtain required performance criteria in products. It is well known in the art that expanded polytetrafluoroethylene (ePTFE) can achieve a high tensile strength via long range fibrillation of extruded PTFE materials. This increased ePTFE strength is made possible via ultra-high molecular weight polymer chains. By comparison, PTFE fine powder products with molecular weights in this range will undergo fibrillation upon even gentle mixing. This shows the importance to effect fibrillation as desired to obtain the designated and required characteristics of the material.

[0004] US 8,637,144 provides commentary on various techniques to polymerize tetrafluoroethylene (“TFE”) alone or in combination. Dispersion polymerization of TFE followed by coagulation and drying produces a polytetrafluoroethylene (PTFE) resin that has come to be known as “fine powder”. Generally, sufficient dispersing agent is introduced into a water carrier such that, upon addition of tetrafluoroethylene monomer in the presence of a suitable polymerization initiator and, upon agitationand under autogenous tetrafluoroethylene pressure, the polymerization proceeds until the level of colloidally dispersed polymer particles is reached and the reaction is then stopped. In particular, the ‘144 patent provides a TFE copolymer, of the fine powder type, that is expandable, to produce expanded PTFE copolymeric products, providing combinations of various amount ranges of monomers with TFE. This procedure to make “expanded” PTFE recognizes the importance of the relationship of characteristics e.g., fibrillation, to obtain the physical and chemical qualities needed for an intended use. For applications where high-shear is required to fibri Hate PTFE to provide adhesion and cohesion for solvent free electrode fabrication, materials with such high molecular weights often fibrillate to an extent that leads to inefficient mixing with other electrode components and undesired agglomeration of PTFE particles.

[0005] PTFE binders are electrochemically stable in lithium-ion battery cathodes, but they can be reduced in the lithium-ion battery anode resulting in electrochemical and mechanical degradation [Ladislav Kavan, Chem. Rev. 1997, 97, 3061-3082; Guobao Li, Rongjian Xue, Liquan Chen, Solid State Ionics, 1996, 90, 221-225], The deterioration of binding force (fibrils) of PTFE will cause battery performance and cycling failure.

[0006] What is needed in the industry is the reductively stable and fibrillatable fluoropolymers which can be used as binders in anode electrodes of secondary lithium-ion batteries based on the recognition of multiple factors in the design of perfluoropolymer which coordinates required attributes, including but not limited to, control of fibrillation, with the composition and architecture of the fluoropolymer.SUMMARY

[0007] The present invention provides fluoropolymer compositions and fabrication for improved electrochemical stability by, e.g. (i) introducing perfluoro monomers in the backbone of a TFE (tetrafluoroethylene) containing polymer and (ii) modification of the architecture of the polymer structure to obtain performance requirements. The fibrillation characteristics of these binders allows them to be used in a dry, solvent free electrode formation process.

[0008] The present invention provides compositions and fabrication methods for a fluoropolymer for use as an anode binder in a Li-ion battery electrode having a highmolecular weight, capable of fibrillation, and comprised of TFE monomer and a second monomer that increases the lowest unoccupied molecular orbital (LLIMO) of the fluoropolymer, which results in higher reduction stability compared with nonmodified PTFE fluoropolymer for improved electrochemical stability. The fibrillation characteristics of these binders allows them to be used in a dry, solvent free electrode formation process and modification of the architecture of the co-polymer which results in improved performance characteristics.

[0009] In a first embodiment, the present invention is directed to a fluoropolymer for use as an anode binder in a Li-ion battery electrode, having a fibrillatable high molecular weight fluoropolymer comprised of TFE monomer and a comonomer. that increases the lowest unoccupied molecular orbital (LLIMO) of the fluoropolymer, which results in higher reduction stability compared with non-modified PTFE.

[0010] In another embodiment, the invention is directed to methods to manufacture a fluoropolymer for use in an electrode composition including the steps of polymerizing TFE monomer in a pressurized reactor, adding a monomer, polymerizing under required reactor conditions to ensure phase compatibility. The fluoropolymer has a molecular weight to provide fibrillation and a monomer which increases the lowest unoccupied molecular orbital (LLIMO) of the fluoropolymer which results in higher reduction stability compared with non-modified PTFE. The amounts of the TFE monomer and comonomer can vary based on fibrillation requirements and the LU MO requirements that are important concepts to obtain performance characteristics of a product, e.g. a Li-ion battery.

[0011] In another embodiment, the present invention is directed to an electrode composition for use in a secondary lithium-ion battery anode film, having anode active particles, a fibrillatable, high molecular weight fluoropolymer comprised of TFE monomer and a comonomer which increases the lowest unoccupied molecular orbital (LUMO) of the fluoropolymer which results in higher reduction stability compared with non-modified PTFE.

[0012] In another embodiment, the present invention is directed to a method for manufacturing an electrode composition for use in a lithium-ion secondary battery anode. The manufacturing is carried out substantially free from solvent. The present method includes a step of mixing, milling, or extruding electrode active particles withor without conductive additives together with high molecular weight fibrillatable fluoropolymer which increases the lowest unoccupied molecular orbital (LIIMO) of the fluoropolymer to form the present electrode compositions.

[0013] The mixing, milling or extruding step of the present method can be carried out by known processes to apply high shear forces to fine powders. For example, techniques and machinery that are envisioned for potential use to provide high shear forces include high speed mixing, jet-milling, pin milling, impact pulverization, hammer milling, and extruding. Other suitable techniques and machinery are also within the scope of this disclosure.

[0014] In another embodiment, the electrode films can be manufactured by compressing electrode compositions after mixing, milling or extruding steps with a calender machine. The electrode films can be laminated on current collector by hot press or calendering.

[0015] In another embodiment, the present invention is directed to a lithium-ion secondary battery having an anode with an anode electrode layer adhered to a metal current collector, the anode electrode layer having an anode electrode composition including anode active particles; a high molecular weight, fibrillatable fluoropolymer binder comprised of TFE monomer and a comonomer which increases the lowest unoccupied molecular orbital (LIIMO) of the core shell structure fluoropolymer which results in higher reduction stability compared with non-modified PTFE. The battery further includes a cathode and a separator between the cathode and anode, with an electrolyte in communication with the cathode, anode and separator.DETAILED DESCRIPTION OF THE DRAWNGS

[0016] FIG. 1 is cyclic voltammetry of graphite anode fabricated using PPVE-TFE fluoropolymer binders.

[0017] FIG. 2 is the first cycle coulombic efficiency of half cells with graphite anode fabricated using PPVE-TFE fluoropolymer binders.

[0018] FIG. 3 is of cycling performance of half cells with graphite anode fabricated using PPVE-TFE fluoropolymer binders.

[0019] FIG. 4 is SEM images of graphite anode fabricated using PPVE-TFE fluoropolymer binders at 2K magnification.

[0020] FIG. 5 is SEM images of graphite anode fabricated using PPVE-TFE fluoropolymer binders at 5K magnification.DETAILED DESCRIPTION

[0021] The materials and methods of the present invention provide a fluoropolymer with the physical and chemical properties required for use in a designated product. A primary concept of the present invention is a fluoropolymer containing tetrafluoroethylene with improved reduction stability for secondary lithium- ion battery anode electrode applications. The fluoropolymer has a high molecular weight, is fibrillatable, and contains both TFE monomer and a second monomer that increases the lowest unoccupied molecular orbital (LIIMO) of the fluoropolymer, which results in higher reduction stability compared with non-modified PTFE fluoropolymer for improved electrochemical stability. In addition, the fluoropolymer has a molecular weight so that the fluoropolymer fine powder samples retain the ability to undergo eventual inter-particle fibrillation without premature fibrillation during the initial mixing processes with other electrode components. The present invention recognizes the importance to effect initiation and termination of fibrillation, as required for the intended use of the fluoropolymer.

[0022] In a first embodiment, the comonomer includes a perfluoro(alkyl vinyl ether), particularly perfluoro(propyl vinyl ether) (PPVE), implemented in the backbone of the fluoropolymer. However, without departing from the spirit of the invention, the copolymer produced contains at least one other polymerized comonomer within the following group: olefins such as ethylene, propylene and isobutylene; fluorinated comonomers such as chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), vinylidene fluoride (CFH-CH2), vinylidene difluoride (CF2-CH2), hexafluoroisobutylene (HFIB), trifluoroethylene (CF2-CFH), fluorodioxoles and fluorodioxalanes; and perfluoroalkyl ethylene monomers, including perfluorobutylethylene (PFBE), perfluorohexylethylene (PFHE) and perfluorooctylethylene (PFOE), and a perfluoroalkyl vinyl ether monomer, including perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether) (PEVE) or variations thereof.

[0023] The fluoropolymer of a perfluoro(alkyl vinyl ether) (PAVE) and tetrafluoroethylene are copolymerized. In the present embodiment, the fluoropolymer is a perfluoro(alkyl vinyl ether) (PAVE), preferably, from about 0.5 to 30 wt %. Perfluoro(propyl vinyl ether) (PPVE), more preferably, about 0.5 to about 8 weight percent peril uoro(propyl vinyl ether) (PPVE).

[0024] Molecular orbital energy level of LUMO can be calculated by using Gaussian 16™ with the Hartree-Fock level of theory. Geometric optimizations were performed using the 6-311 +g basis set and solvation was accounted for using the SMD model with Ethylene Carbonate parameters. This configuration was chosen based on the qualitative and quantitative match of orbital energies found in reference (Masaki Yoshio, Ralph J. Brodd, Akiya Kozawa (Eds.), Lithium-Ion Batteries, 2009, Chapter 6). The calculation results of LUMO level of PTFE homopolymer and fluoropolymer example 2 (4.07 wt% PPVE) are shown in the Chart 1 . 4.07 wt % PPVE monomer could increase the LUMO of copolymer from 2.94 eV (PTFE homopolymer) to 3.05 eV. The LUMO is lowest unoccupied molecular orbital. Regarding electrochemical reactions, reduction is to add electrons to LUMO. As LUMO increases, electron becomes more difficult to add. And therefore, it leads to a better reduction stability.Chart 1

[0025] The fluoropolymer is substantially free from solvent, creating an electrode binder to enable more sustainable solvent-free electrode manufacturing processes. The manufacturing is carried out substantially free from solvent. For example, free from water, and organic solvents such as N-methyl-2-pyrrolidone (NMP) commonly used as carriers in battery binder manufacturing processes. Dry processing technology is more cost-efficient and greener compared to conventional solvent-based electrode casting technology, which uses toxic organic solvents. Also, it has an advantage of energy density enhancement because it avoids the electrode particles sedimentation for wet electrode during drying process.

[0026] The fluoropolymer dispersion also can be used to manufacture secondary lithium-ion battery anode electrode based on slurry-based processes.

[0027] In another embodiment, the invention is directed to a method to manufacture a fluoropolymer for use as binder in a secondary lithium-ion battery anode including the steps of copolymerizing tetrafluoroethylene and a second monomer to form the fluoropolymer dispersion, coagulating precipitate from the dispersion and drying the precipitate to form a dry fine powder. The fluoropolymer includes a monomer which increases the lowest unoccupied molecular orbital (LUMO) of the fluoropolymer, leading to higher reduction stability compared with non-modified PTFE fluoropolymer for improved electrochemical stability.

[0028] The fluoropolymer contains a perfluoro(alkyl vinyl ether) and is about 0.5 to about 8 weight percent perfluoro(propyl vinyl ether) (PPVE).

[0029] The method provides a fluoropolymer for use as binder in a secondary lithium-ion battery anode, the fluoropolymer has the physical property to provide fibrillation. The high molecular weight of the PTFE polymer and the LUMO increase based on the PAVE comonomer, provide characteristics which complement the chemical and physical properties necessary for the anode, most specifically, improved processability by fibrillation, reduced degradation, improved columbic efficiency and better cycling life.

[0030] In another embodiment, the invention is directed to a method to manufacture a fluoropolymer, including the steps of (i) polymerizing TFE to, (ii) adding a monomer which increases the lowest unoccupied molecular orbital (LUMO) of the fluoropolymer which results in higher reduction stability compared with nonmodified PTFE and is selected based on having the properties required for the polymer. The PTFE-based fluoropolymer has a molecular weight to provide fibrillation.

[0031] The monomer is a fluorinated comonomer such as chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), vinylidene fluoride (CFH-CH2), vinylidene difluoride (CF2-CH2), hexafluoroisobutylene (HFIB), trifluoroethylene (CF2-CFH), fluorodioxoles and fluorodioxalanes; and perfluoroalkyl ethylene monomers, including perfluorobutylethylene (PFBE), perfluorohexylethylene (PFHE) and perfluorooctylethylene (PFOE), perfluoroalkyl vinyl ether (PAVE) and morespecifically, perfluromethyl vinyl ether (PMVE), perfluroethyl vinyl ether (PEVE), perfluropropyl vinyl ether (PPVE), and ethylene (CH2-CH2).

[0032] Referring to FIG. 1 , the cyclic voltammetry curves are provided to illustrate the reversible oxidation-reduction reaction. The upward peak indicates the oxidation of the active species, and the downward peak indicates the species' reduction. This technique is used to appreciate redox features of electrode component in the battery.

[0033] In another embodiment the invention is directed to a method for manufacturing an electrode composition for use in a secondary lithium-ion battery anode, including mixing, milling or extruding anode active particles, conductive additives together with a fluoropolymer having a molecular weight to provide fibrillation. The electrode composition provides improved degradation of the fluoropolymer in the anode composition as compared to PTFE alone.

[0034] In another embodiment, the invention is directed to a composition for use in a secondary lithium-ion battery anode film, having anode active particles; conductive additives and a fluoropolymer binder of tetrafluoroethylene and a comonomer. The fluoropolymer binder has a molecular weight to provide fibrillation. The composition contains from about 0.2 to about 10 weight percent fluoropolymer binder and from about 90 to about 99.8 weight percent anode active particles.

[0035] The present anode active particles are selected from conventional materials known in this field, for example, a carbon-based material such as graphite, graphene, carbon nanotubes, mesocarbon microbeads (MCMB), and / or conductive carbon metal oxides, metal alloy, lithium titanate, Sn, SnC>2, SnO and silicon, silicon oxides or silicon-containing materials. Other suitable active components are also within the scope of this disclosure.

[0036] In another embodiment, the present invention is directed to a secondary lithium-ion battery having an anode with an anode electrode layer adhered to a metal current collector, the anode electrode layer having an anode electrode composition including (i) anode active particles, (ii) conductive additive (iii) fluoropolymer binder, a cathode electrode, a separator between the cathode and said anode; and an electrolyte in communication with said cathode, anode and separator wherein the fluoropolymer binder has a molecular weight to provide fibrillation.

[0037] In the present embodiment, the anode electrode composition is prepared by mixing, milling or extruding the anode active particles, conductive additives and fluoropolymer binder carried out substantially free from solvent. The battery shows much better reduction resistance compared with non-modified PTFE.

[0038] In the cathode electrode, the active component can include, for example, the lithium metal oxide can include lithium nickel manganese cobalt oxide (NMC), lithium manganese oxide (LMO), lithium iron phosphate (LiFePO4), lithium cobalt oxide (LCO), lithium nickel cobalt aluminum oxide (LNCA), lithium nickel cobalt aluminum oxides (NCA), lithium nickel manganese oxide (LMNO), or a combination thereof. Other suitable active cathode components are also within the scope of this disclosure.

[0039] The conductive additive in the anode electrode and / or the cathode electrode can include, for example, a conductive carbon agent such as carbon black, carbon nanotubes (CNT), carbon fibers, graphene, conductive graphite, and / or a combination thereof.

[0040] The current collectors can be metal bodies, e.g., foils / sheets. For example, the anode current collector can include copper foil, and the cathode current collector can include aluminum foil.

[0041] Separators of the present lithium-ion secondary batteries include conventional separators for lithium-ion secondary batteries capable of continuous operation of the present battery without performance degradation. The separator is configured to electrically insulate two electrodes adjacent to opposing sides of the separator, while permitting ionic communication between the two adjacent electrodes. The separator can comprise a suitable porous, electrically insulating material. In some embodiments, the separator can comprise a polymeric material. For example, the separator can comprise a cellulosic material (e.g., paper), a polyethylene resin, a polypropylene resin and / or mixtures thereof.

[0042] Electrolytes of the present lithium-ion secondary batteries include conventional electrolytes for lithium-ion secondary batteries capable of continuous operation of the present battery without performance degradation. The electrolyte facilitates ionic communication between the electrodes of the present battery, and is typically in contact with the cathode, anode and the separator. In one embodiment,present batteries use a suitable lithium-containing electrolyte. For example, a lithium salt, and a solvent, such as a non-aqueous or organic solvent, or fluorinated organic solvent. Generally, the lithium salt includes an anion that is redox stable. In some embodiments, the anion can be monovalent. In some embodiments, a lithium salt can be selected from hexafluorophosphate (LiPFe), lithium tetrafluoroborate (LiBFzi), lithium perchlorate(LiCIO4), lithium bis(trifluoromethansulfonyl)imide (LiN(SO2CFs)2), lithiumtrifluoromethansulfonate (USO3CF3), lithium bis(oxalate)borate (LiBOB) and combinations thereof. In some embodiments, the electrolyte can include a quaternary ammonium cation and an anion selected from the group consisting of hexafluorophosphate, tetrafluoroborate and iodide. In some embodiments, the salt concentration can be about 0.1 mol / L (M) to about 5 M, about 0.2 M to about 3 M, or about 0.3 M to about 2 M. In further embodiments, the salt concentration of the electrolyte can be about 0.7 M to about 1 M. In certain embodiments, the salt concentration of the electrolyte can be about 0.2 M, about 0.3 M, about 0.4 M, about 0.5M, about 0.6 M, about 0.7 M, about 0.8 M, about 0.9 M, about 1 M, about 1 .1 M, aboutl .2 M, or any range of values therebetween.

[0043] In some embodiments of the present lithium-ion secondary batteries, electrolytes include a liquid solvent. In further embodiments, the solvent can be an organic solvent. In some embodiments, a solvent can include one or more functional groups selected from carbonates, ethers and / or esters. In some embodiments, the solvent can comprise a carbonate. In further embodiments, the carbonate can be selected from cyclic carbonates such as, for example, ethylene carbonate (EC), propylene carbonate (PC), vinyl ethylene carbonate (VEC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), methyl(2,2,2-trifluoroethyl) carbonate (FEMC) and combinations thereof, or acyclic carbonates such as, for example, dimethyl carbonate(DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and combinations thereof. In certain embodiments, the electrolyte can comprise LiPFe, and one or more carbonates. An example electrolyte using in this invention is 1.2 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) with 5 wt% fluoroethylene carbonate (FEC), EC:DEC ratio of 1 :1 by weight.

[0044] As disclosed herein, electrochemical evaluations include half-cell evaluations using lithium metal counter electrodes examined galvanostatically. In one embodiment, in a half cell electrochemical cell containing a graphite electrodeand a lithium metal counter-electrode, galvanostatic current used can be C / 25 formation and C / 10 for cycling.Coulombic efficiency (CE) of an anode as used herein is the percentage ratio of specific charge (delithiation in a half cell) capacity to specific discharge (I ithiation in a half cell) capacity.EXAMPLESPolymerization - Coagulated Fluoropolymer; TFE and PPVE

[0045] The following example illustrates a semi-batch process for making high- molecular weight aqueous fluoropolymer PTFE dispersion to produce an approximately 35 weight % solids batch.Comparative Example A:

[0046] To a nominally 10-gallon jacketed, cylindrically-shaped stainless steel reactor with an aspect ratio of 1.5 equipped with a paddle stirrer was charged 600 grams of natural paraffin wax and 4.3 g of succinic acid and the reactor was sealed. After adding 24.1 liters of demineralized water, the reactor was heated to 65 °C, the reactor was agitated at 70 RPM then pressured to 400 PSIG with nitrogen and checked for leaks. After venting, an aqueous solution containing 0.7 g perfluoropolyether acid polymerized from hexafluoropropylene oxide with a number average molecular weight of approximately 1500 Daltons, 147 g of HFPO dimer acid, ammonium salt, and 61.8 g demineralized water was added. The agitator was stopped and the reactor was purged with TFE to greater than 25 PSIG and evacuated to at least -2.0 PSIG three times. The agitator was restarted, and the reactor was charged with TFE until a pressure of 400 PSIG was reached. To start the polymerization, 180 mL of a 0.015% (m / v) aqueous solution of potassium permanganate (KMnO4) were added at a rate of 80 mL / min, then KMnO4 injection continued at 3.5 mL / min until 4.7 kg of TFE had been charged from the start of KMnC>4 injection. Also, at 4.7 kg of TFE charged, the temperature was increased to 85 °C. After a total of 10.9 kg of TFE were added since kickoff, the TFE addition valve was closed, the agitator was stopped, and the reactor was vented slowly over 10 minutes. When the reactor pressure reached 1-2 PSIG, nitrogen was added to give a slow pressure rise to 5 PSIG. The reactor was evacuated for 1 minute thenthe nitrogen flow was stopped, and the reactor was vented. The reaction time was 130 minutes. The resulting dispersion, containing 33.63% polymer, was discharged from the reactor, and allowed to cool. The dispersion was found to have a raw dispersion particle size of 218.5 nm. After draining the dispersion, 495 g of coagulum, containing water, paraffin wax and polymer, were left behind in the reactor and gave an SSG of 2.1672.

[0047] Polymer was coagulated in a 3L vessel by diluting the dispersion to about 14 wt% solids and by adding about 3.7% by mass (dry weight) of a 20 wt% aqueous ammonium carbonate solution followed by vigorous agitation until the polymer fully separated from the water. The polymer was dried in a static oven at 150 °C for 24 hours.Example 1

[0048] The following example illustrates a semi-batch process for making an aqueous fluoropolymer dispersion to produce a PPVE-modified PTFE at approximately 40 weight % solids.

[0049] To a nominally 10-gallon jacketed, cylindrically-shaped stainless steel reactor with an aspect ratio of 1.5 equipped with a paddle stirrer was charged 700 grams of natural paraffin wax and 4.0 g of succinic acid and the reactor was sealed. After adding 24.1 liters of demineralized water, the reactor was heated to 65 °C, the reactor was agitated at 70 RPM then pressured to 380 PSIG with nitrogen and checked for leaks. After venting, an aqueous solution containing 120 g of HFPO dimer acid, ammonium salt, and 51 .3 g demineralized water was added. The agitator was stopped and the reactor was purged with TFE to greater than 25 PSIG and evacuated to at least -2.0 PSIG three times. The agitator was restarted, and the contents were heated to 76 °C before adding 25 mL of perfluoropropylvinyl ether (PPVE). The reactor was charged with TFE until a pressure of 380 PSIG was reached. To start the polymerization, 230 mL of a 0.1 % (m / v) aqueous solution of ammonium persulfate (APS) were added at a rate of 80 mL / min. After polymerization began (kickoff) as indicated by a decrease in pressure of 10 PSIG, additional TFE was fed to the reactor at a rate sufficient to maintain a constant pressure of 380 PSIG. After a total of 14.5 kg of TFE were added since kickoff, the TFE addition valve was closed, and the batch reacted down to 250 PSIG beforeventing slowly over 10 minutes. When the reactor pressure reached 1-2 PSIG, nitrogen was added to give a slow pressure rise to 5 PSIG. The reactor was evacuated for 1 minute then the nitrogen flow was stopped, and the reactor was vented. The reaction time was 93 minutes. The resulting dispersion, containing 40.53% polymer, was discharged from the reactor, and allowed to cool. The dispersion was found to have a raw dispersion particle size of 244.2 nm. After draining the dispersion, 223 g of coagulum, containing water, paraffin wax and polymer, were left behind in the reactor.

[0050] Polymer was coagulated in a 3L vessel by diluting the dispersion to about 14 wt% solids and by adding about 3.7% by mass (dry weight) of a 20 wt% aqueous ammonium carbonate solution followed by vigorous agitation until the polymer fully separated from the water. The polymer was dried in a static oven at 150 °C for 24 hours. SSG of the polymer was 2.1566. PPVE incorporation as measured by FTIR was 0.15% by mass.Example 2

[0051] The following example illustrates a semi-batch process for making high- PPVE content aqueous fluoropolymer PTFE dispersion to produce an approximately 35 weight % solids batch. Highly modified (PPVE) PTFE-type fine powder is produced from the dispersion.

[0052] To a nominally 10-gallon jacketed, cylindrically-shaped stainless steel reactor with an aspect ratio of 1 .5 equipped with a paddle stirrer is charged 600 grams of natural paraffin wax and 4.0 g of succinic acid and the reactor is sealed. After adding 24.1 liters of demineralized water, the reactor is heated to 65 °C, the reactor is agitated at 70 RPM then pressured to 400 PSIG with nitrogen and checked for leaks. After venting, an aqueous solution containing 0.2 g perfluoropolyether acid polymerized from hexafluoropropylene oxide with a number average molecular weight of approximately 1500 Daltons, 107 g of HFPO dimer acid, ammonium salt, and 45.3 g demineralized water is added. The agitator is stopped, and the reactor is purged with TFE to greater than 25 PSIG and evacuated to at least -2.0 PSIG three times. The agitator is restarted, and the contents are heated to 79 °C before adding 150 mL of perfluoropropylvinyl ether (PPVE). The reactor is charged with TFE until a pressure of 260 PSIG is reached. To start the polymerization, 300 mL of a 0.2%(m / v) aqueous solution of ammonium persulfate (APS) are added at a rate of 80 mL / min, then APS injection continues at 3.0 mL / min until the end of the batch (by which time 240.5 mL additional APS solution is added). After polymerization has begun (kickoff) as indicated by a decrease in pressure of 10 PSIG, additional TFE is fed to the reactor at a rate sufficient to maintain a constant pressure of 260 PSIG. Also after kickoff, 487 mL of PPVE are added at a rate of 6.0 mL / min. After a total of 12.7 kg of TFE are added since kickoff, the TFE addition valve is closed, the agitator is stopped, and the reactor is vented slowly over 10 minutes. When the reactor pressure reaches 1-2 PSIG, nitrogen is added to give a slow pressure rise to 5 PSIG. The reactor is evacuated for 1 minute then the nitrogen flow is stopped, and the reactor is vented. The reaction time is 81 minutes. The resulting dispersion, containing 36.58% polymer, is discharged from the reactor, and allowed to cool. The dispersion is found to have a raw dispersion particle size of 219.4 nm. After draining the dispersion, 292 g of coagulum, containing water, paraffin wax and polymer, are left behind in the reactor.

[0053] Polymer is coagulated in a 3L vessel by diluting the dispersion to about 14 wt% solids and by adding about 3.7% by mass (dry weight) of a 20 wt% aqueous ammonium carbonate solution followed by vigorous agitation until the polymer fully separates from the water. The polymer is dried in a static oven at 150 °C for 24 hours. Melting point of this polymer as measured by DSC on first heat is 329.01 °C and SSG is 2.1650. PPVE incorporation as measured by FTIR is 4.07%.Example 3

[0054] TFE-PPVE copolymer with a PPVE incorporation of 2.88% and an SSG of 2.1740 was prepared and isolated according to the procedure for Example 2 except 294 mL of PPVE are added at a rate of 4.0 mL / min after kickoff.Example 4

[0055] TFE-PPVE copolymer with a PPVE incorporation of 6.06% and an SSG of 2.1588 was prepared and isolated according to the procedure for Example 2 except 887 mL of PPVE are added at a rate of 8.0 mL / min after kickoff.Example 5

[0056] To a nominally 10-gallon jacketed, cylindrically-shaped stainless steel reactor with an aspect ratio of 1 .5 equipped with a paddle stirrer is charged 600 grams of natural paraffin wax and 4.0 g of succinic acid and the reactor is sealed. After adding 24.1 liters of demineralized water, the reactor is heated to 65 °C, the reactor is agitated at 70 RPM then pressured to 400 PSIG with nitrogen and checked for leaks. After venting, an aqueous solution containing 107 g of HFPO dimer acid, ammonium salt, and 45.3 g demineralized water is added. The agitator is stopped, and the reactor is purged with TFE to greater than 25 PSIG and evacuated to at least -2.0 PSIG three times. The agitator is restarted, and the contents are heated to 76 °C before adding 75 ml_ of perfluoro propyl vinyl ether (PPVE). The reactor is charged with TFE until a pressure of 380 PSIG is reached. To start the polymerization, 230 ml_ of a 0.1 % (m / v) aqueous solution of ammonium persulfate (APS) are added at a rate of 80 mL / min. After polymerization has begun (kickoff) as indicated by a decrease in pressure of 10 PSIG, additional TFE is fed to the reactor at a rate sufficient to maintain a constant pressure of 380 PSIG. After a total of 14.5 kg of TFE are added since kickoff, the TFE addition valve is closed, and the batch is reacted down to 250 PSIG before the agitator is stopped and the reactor is vented slowly over 10 minutes. When the reactor pressure reaches 1-2 PSIG, nitrogen is added to give a slow pressure rise to 5 PSIG. The reactor is evacuated for 1 minute then the nitrogen flow is stopped, and the reactor is vented. The reaction time is 92 minutes. The resulting dispersion, containing 40.34% polymer, is discharged from the reactor, and allowed to cool. The dispersion is found to have a raw dispersion particle size of 253.8 nm. After draining the dispersion, 1265 g of coagulum, containing water, paraffin wax and polymer, are left behind in the reactor.

[0057] Polymer is coagulated in a 3L vessel by diluting the dispersion to about 14 wt% solids and by adding about 3.7% by mass (dry weight) of a 20 wt% aqueous ammonium carbonate solution followed by vigorous agitation until the polymer fully separates from the water. The polymer is dried in a static oven at 150 °C for 24 hours. Melting point of this polymer as measured by DSC on first heat is 331 .01 °C and SSG is 2.1561. PPVE incorporation as measured by FTIR is 0.5%.Example 6

[0058] TFE-PPVE copolymer with a PPVE incorporation of 0.79% and an SSG of 2.1779 was prepared and isolated according to the procedure for Example 2 except 133 mL of PPVE are added at a rate of 2.0 mL / min after kickoff.

[0059] Table 1 provides the resultant properties and modifier content of the Examples.Table 1

[0060] As can been appreciated by results in Table 1 , the amount of modifier varies focusing the concept of the invention on improvement of reduction stability.TEST METHODSANODE

[0061] Test anodes were prepared using the present fluoropolymer compositions by the following procedures:

[0062] Mixing: 1 . Weigh out material for a 10g batch with a composition of 90% Graphite, 5% Super P conductive carbon, and 5% PTFE (or present inventive fluoropolymer composition).; 2. Combine graphite and Super P in a mortar and pestle for 15 min.; 3. Add graphite and Super P mixture and the PTFE to a 250 mL plastic bottle with 10 beads (1 bead / g material).; 4. Set roll mill speed to 55 and place the bottle into the holder and tape the top closed. Put on roller for 30 minutes.;5. After 30 minutes, remove the beads from the anode mixture and gently scrape any material stuck to the side of the bottle off.

[0063] Film Formation: 1 . Place 3 g of the mixture in a small glass mortar and pestle.; 2. Grind the material together until it forms a solid flake.; 3. Place the flake onto the hot plate at 100°C on a piece of Kapton® and roll out using a steel roller heated to 100°C. Roll until the film is uniform.

[0064] Calendering: 1. Allow the calendering rolls to heat to 50°C for at least an hour and check the temperature with a thermocouple. 2. Measure the thickness of the film and start the calendering gap at 50-100 pm below that. For example, if a film is 480 pm, start the gap distance at 400 pm.; 3. Place the film onto the rolls and turn the roller speed to 150. Pass the film through the gap 2 times. 4. Reduce the gap distance by 100 pm and pass the film through the new gap distance 2 times. 5. Repeat lowering the gap until 100 pm. Pass the film through the gap 5 times at 100 pm and 10 times at 50 pm until a final thickness of 70-80 microns was reached.

[0065] Anode Lamination: 1. Cut a piece of copper foil, wipe both sides with IPA, and allow to dry.; 2. Pour copper etchant into a glass tray. Place one side of the copper foil into the etchant for 10 seconds.; 3. Transfer copper foil to another glass tray filled with DI water and soak for a minute. Thoroughly rinse the copper foil with DI water and dry on a blue napkin.; 4. Plasma treat etched copper.; 5. Heat a hot press to 300°C. Place the anode film onto the copper foil and in between two sheets of the metal shim. Place the metal shim onto steel backing plates.; 6. Press at 5,000 lbs. for 5 minutes.; 7 Take it off the hot press and allow to cool before removing the metal shims.

[0066] Half cell configuration: Half cell containing a graphite electrode and a lithium metal counter-electrode and a Celgard separator was assembled a CR2032 coin cell. The electrolyte of 1 ,2M LiPFe in EC / DEC (3:7 by volume) +5% FEC was used.

[0067] Half cell testing: The coin cells were cycled using a Neware battery tester from 0.01 V - 1.25 V vs. Li / Li+with C / 25 formation for 2 cycles and C / 10 for cycling. 1C = 1 hour charge / discharge. All the cells were cycled at room temperature.DISCUSSION OF RESULTS AND TEST METHODS FOR COPOLYMEREMBODIMENT:Cyclic Voltammetry Measurement:

[0068] Referring to FIG. 1 , Cyclic voltammetry of Graphite Anode fabricated using PPVE-TFE fluoropolymer binder. The Cyclic Voltammetry was measured from 0.01V to 1.5 V at 0.05 mV / sec scan rate in the Biologic Potentiostat. The current is normalized with the mass of active Graphite anode material in the electrode. The graphite anode consists of 90:5:5 composition of Active Graphite, SP conductive carbon and the fluoropolymer binder. 2032 type lithium-ion coin cells were made with the anode in half cell format with lithium metal counter / reference electrode.

[0069] Cyclic voltammetry shows the oxidation and reduction features from the electrode components. The reduction signal between 0.9 to 0.3 is assigned to the degradation of the fluoropolymer and electrolyte reduction. Electrolyte reduction is similar for all the cells. The signal (reduction) around 0.1V represents the graphite lithiation. The oxidation signal around 0.2V represents the graphite de-lithiation. PTFE reduction is irreversible process while graphite reduces and oxidizes reversibly while switching negative current to positive current. PTFE reduction causes the capacity loss and decreases the binding property of PTFE thereby leading to poor battery performance. In order to quantify the fluoropolymer reduction (or degradation), the integrated area under the X-axis is calculated by multiplying voltage and normalized current and adding them in the voltage range of 0.9 and 0.3V. The percentage improvement or diminution in the degradation of the fluoropolymer in the anode composition is calculated using the integrated current in the cyclic voltammetry test. More explicitly, the % improvement or diminution in the degradation is 100 * (1- (integrated current of example) / integrated current of comparative example)).

[0070] The degradation results are shown for the fluoropolymer in the Table 2 below, and graphically illustrated in FIG. 1.Table 2

[0071] The results indicate an improvement or diminution in degradation of the fluoropolymer of the present invention, in the anode electrode, of 7 % or greater. This improvement or diminution in the fluoropolymer degradation will result in improved stability and increased performance the anode electrode when used in an electrochemical device or lithium-ion battery.

[0072] Without being bound to any particular theory, the improvement of reduction stability of PTFE can be achieved by molecule design as illustrated in FIG. 1 . The lowest unoccupied molecular orbital (LUMO) energy level is the key parameter to determine the reduction of polymer. The LUMO level of PTFE can be tuned by introducing different amounts of modifier of PPVE to the polymer back bone and make the polymer is more reluctantly to be reduced.

[0073] PTFE binders are not electrochemically stable for lithium-ion battery anode due to its low LUMO level resulting in the electrochemical and mechanical degradation. The deterioration of binding force of PTFE will eventually causes the battery cycling failure. The reduction stability of the fluoropolymer in the present invention is obtained by introducing different amounts of perfluoroalkyl vinyl ether (PAVE) monomers in the backbone of polymer, which permits the control of fibrillation, and thus improvement of battery performance.Coulombic Efficiency

[0074] Referring to FIG. 2, the anode incorporating the binder of the present invention provides increased the initial Coulombic efficiency (ICE) over the PTFE homopolymer binder (comparative example A). It is believed the electrochemicalreduction of the binder leads to a loss of cell capacity (loss of lithium due to lithiation of the polymer) as well as loss of mechanical cohesion in the electrode. Because of the reductive instability of the PTFE binders, this leads to a diminution of the ICE. PPVE co-monomer increase the LUMO of fluoropolymer and leads to better reduction stability. Therefore, the amount of PPVE co-monomer is variable but, in all amounts, the ICE of the cells with graphite anode fabricated using PPVE-TFE fluoropolymer binders of the present invention is improved.Cycling Life

[0075] Referring to FIG. 3, the addition of the PPVE monomer improvement in the battery performance in terms of cycling. The reduction stability improvement with PPVE-TFE fluoropolymer could improve the mechanical properties of electrode and therefore leads to the better cycling performance. However, no linear correlation was found between battery performance with a specific modifier amount.

[0076] Referring to FIGs 4 and 5, SEMs of the modified PTFE binder are illustrated at 2K and 5K magnification (respectively) for each of the Examples and Comparative Example. SEM images are the visualized results that validate the present invention both in regard to composition and architecture design). Fibrils are key to maintain good electrode integrity which is a must-have for good cycling, As illustrated in FIGs 4 and 5, the presence of the fibrils provide evidence of the improved strength and stability provided by the present invention.

Claims

CLAIMS1 . A fluoropolymer binder for use in an anode electrode for a secondary lithium-ion battery, wherein the fluoropolymer is of a high molecular weight, fibrillatable, and is comprised of TFE monomer and a second monomer that increases the lowest unoccupied molecular orbital (LIIMO) of the fluoropolymer, which results in higher reduction stability compared with non-modified PTFE.

2. The fluoropolymer of claim 1 , wherein the second monomer is selected from the group consisting of a perfluoro(alkyl vinyl ether) (PAVE), vinylidene difluoride (CF2-CH2), ethylene (CH2-CH2).

3. The fluoropolymer of claim 2, wherein the second monomer is a perfluoro(alkyl vinyl ether) (PAVE).

4. The fluoropolymer of claim 3, wherein the tetrafluoroethylene monomer and the second monomer are polymerized to form a dispersion, wherein the dispersion is coagulated.

5. The fluoropolymer of claim 4, wherein the second comonomer is 0.1 to about 8.0 weight percent of the fluoropolymer.

6. The fluoropolymer of claim 5, wherein the perfluoro(alkyl vinyl ether) (PAVE) is about 0.5 to about 8 weight percent perfluoro(propyl vinyl ether) (PPVE).

7. An electrode composition for use in a secondary lithium-ion battery anode film, comprising:(i) anode active particles; and(ii) conductive additives(iii) the fluoropolymer binder consisting of the fluoropolymer of claim 6, wherein the fluoropolymer binder has improved reductive resistance compared with non-modified PTFE.

8. The electrode composition of claim 7, wherein the cyclic voltammetry test degradation of the fluoropolymer in the anode composition is diminished by > / = 7% compared to PTFE homopolymer, as determined by the integrated intensity in the 0.3 to 0.9 V region using the formula 100 * (1- (integrated current of fluoropolymer) / integrated current of non-modified PTFE).

9. The electrode composition of claim 8, wherein the composition contains from about 0.5 to about 10 weight percent fluoropolymer binder and from about 90 to about 99.5 weight percent anode active particles.

10. A method for manufacturing an electrode composition for use in a secondary lithium-ion battery anode, comprising: mixing, milling or extruding anode active and conductive particles together with a fluoropolymer having a high molecular weight that is fibrillatable, and comprised of TFE monomer and a second monomer that increases the lowest unoccupied molecular orbital (LIIMO) of the fluoropolymer, which results in higher reduction stability compared with non-modified PTFE.11 . The method of claim 10, carried out substantially free from solvent.

12. An electrode composition for use in a secondary lithium-ion battery anode, manufactured by the method of claim 11.

13. A method to manufacture a fluoropolymer for use as binder in a secondary lithium-ion battery anode comprising the steps of: i. polymerizing tetrafluoroethylene and a comonomer to form an aqueous tetrafluoroethylene based dispersion, ii. coagulating the dispersion; iii. separating the fluoropolymer from the dispersion liquid; and iv. drying the fluoropolymer.

14. A fluoropolymer manufactured by the method of 13, wherein the fluoropolymer has a molecular weight to provide fibrillation.

15. The fluoropolymer of claim 14, wherein the comonomer is selected from the group consisting of a perfluoro(alkyl vinyl ether), ethylene and vinylidene difluoride.

16. The fluoropolymer of claim 15, wherein the comonomer increases the lowest unoccupied molecular orbital (LIIMO) of the fluoropolymer which results in higher reduction stability compared with non-modified PTFE.

17. A lithium-ion secondary battery comprising:(i) an anode comprising: an anode electrode layer adhered to a metal current collector, said anode electrode layer comprising an anode electrode composition comprising: anode active particles; conductive additives; fluoropolymer binder;(ii) a cathode;(iii) a separator between said cathode and said anode; and(iv) an electrolyte in communication with said cathode, anode and separator, wherein the fluoropolymer binder has a molecular weight to provide fibrillation and increases the lowest unoccupied molecular orbital (LUMO) of the fluoropolymer, which results in higher reduction stability compared with non-modified PTFE.

18. The secondary lithium-ion battery of claim 17, wherein the fluoropolymer binder is a copolymer of TFE and a second monomer.

19. The secondary lithium-ion battery of claim 18, wherein the second monomer is selected from the group consisting of PAVE, ethylene and vinylidene.

20. The secondary lithium-ion battery of claim 19, wherein the second monomer is a PAVE.21 . The secondary lithium-ion battery of claim 20, wherein the PAVE is PMVE, PEVE, or PPVE.

22. The secondary lithium-ion battery of claim 21 , wherein said anode electrode composition is prepared by mixing, milling or extruding the anode active particles, conductive additives and fluoropolymer binder.

23. The secondary lithium-ion battery of claim 22, wherein the mixing, milling or extruding is carried out substantially free from solvent.

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