Binder additives for batteries

Abstract

Dry-processed electrodes eliminate energy / capital-intensive solvent drying processes, but indispensable polytetrafluoroethylene (PTFE) binders suffer from electrochemical reduction by lithium due to inherently low energy level of their lowest unoccupied molecular orbital (LUMO). The invention introduces a strategy to elevate the LUMO through the addition of amine-containing additives. By mixing the additives with PTFE, initial Coulombic efficiency and mechanical durability are improved without requiring modifications to existing manufacturing lines.

Description

Atorney Docket: 130466.00318BINDER ADDITIVES FOR BATTERIESCROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 689,627 filed August 30, 2024. which is fully incorporated herein.BACKGROUND OF THE INVENTION

[0002] High energy density and production cost remain the major challenges in the commercialization of lithium-ion batteries. A dry process for electrode manufacturing has attracted attention in terms of cost savings and environmental benignity7. The elimination of the solvent process has been accompanied by a reduction of manufacturing costs. Furthermore, the dry manufacturing process is environmentally friendly as it does not require toxic organic solvents such as N-methylpyrrohdone (NMP). To date, many dry processed film manufacturing technologies have been reported including dry spraying deposition, 3D printing, melt stretching, powder compression, vapor deposition, and polymer fibrilization.

[0003] For the polymer fibrilization method, polytetrafluoroethylene (PTFE) is a widely used binder due to its good mechanical properties, corrosion resistance, and easy fibrilization. Due to the high electronegativity' of the fluorine (F) atoms of PTFE, the repulsive Van der Waals force between PTFE molecules is strong, which slips easily and forms PTFE fibrils under the shear force. Electrodes using fibrilized PTFE binder not only have the advantage of a dry process, but also have better electrical properties with its low tortuosity' compared to conventional slurry' casting electrodes. Unlike wet-processed binder which covers the surface of the active material, PTFE, which binds particles with its fibrils does not block the surface and has a lower charge transfer resistance, facilitating fast charging. Additionally, the conventional process with slurry' casting procedure leads uneven distribution of binder while evaporating the solvents. The binder gradient induces poor mechanical properties and decreased electric properties of electrodes, interfering to achieve the high-loading electrodes.

[0004] However, PTFE has an intrinsic problem of being easily' reduced by reacting with lithium due to its low lowest unoccupied molecular orbital (LUMO). As this phenomenon occurs, PTFE leaves only the sp2 carbon backbone through reductive defluorination.146474137v I 130466 / 00318Atorney Docket: 130466.00318

[0005] In particular, the problem is more severe in the anode, which shows poor initial Coulombic efficiency and leads to fast capacity decay due to the reduced binder. The lithium used in this reaction leads to capacity loss, and the reduced PTFE contributes to making the electrode brittle. Mitigating reduction of PTFE has been accomplished by coating the graphite active material with non-electroconductive polymers. Indeed, polymer coated anode shows higher initial Coulombic efficiency and specific capacity. In addition, the cyclabihty of the electrodes were increased by blocking the provision of electrons to PTFE and preventing breakage of PTFE fibrils. However, the coating process of graphite particles still requires a wet process involving a solvent. More importantly, wet coating of insulating binders on the active material diminishes the advantages of the fibrous binders by minimizing their contact areas.

[0006] As a result of the above limitations, the inventors have developed a strategy' to mitigate the inherent challenge of PTFE reduction by modulating its LUMO with additives to suppress its electrochemical decomposition. Therefore, the development of a binder additive with superior electrochemical and mechanical stability for use in the dry process manufacturing of electrodes, is highly desirable.SUMMARY OF THE INVENTION

[0007] An embodiment of the invention is directed to an electrode fabricated by dryprocess manufacturing, the process comprising PTFE as a binder and at least one binder additive that modifies the LUMO of PTFE. In an embodiment of the invention, the binder additive comprises at least one amine group that interacts with the fluorine groups in PTFE. Another embodiment of the invention is directed to a method that uses a PTFE binder additive for manufacturing a dry-processed stable anode in lithium-ion batteries. In an embodiment of the invention, a binder additive prevents the reduction of PTFE and physically hinders electron transfer between lithium and PTFE. In an embodiment of the invention, a binder additive is hydrogen-bonded to PTFE.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 shows the comparison of initial Coulombic efficiency (ICE) in half cells in the presence and absence of binder additives; and

[0009] FIG. 2 shows the charge / dis charge profiles of half cells.246474137v I 130466 / 00318Atorney Docket: 130466.00318DETAILED DESCRIPTION

[0010] Embodiments of the invention are directed toward mitigating the inherent challenge of PTFE reduction by modulating its LUMO with binder additives to suppress its electrochemical decomposition. In certain embodiments, a binder additive contains an amine group capable of forming hydrogen bonds with PTFE through a solvent- free, dry mixing process. In PTFE, the LUMO originates from antibonding combinations of the C2s and C2py orbitals, along with mixed F2px orbitals having anti-bonding character in the C-F bond. Hydrogen bonding can influence the electron density' and enable the engineering of the band structure. When the amine group interacts with PTFE, it redistributes electron density, thereby modulating the electronic band structure. The electrochemical decomposition of PTFE in the absence of the additive leads to fiber breakage and increases the electrode thickness, adversely affecting the cycle performance. Aspects of the invention pave the way for mitigating the reduction of fibrous PTFE binders and enabling high energydensity, cost-effective lithium-ion batteries.

[0011] In certain embodiments, the hydrogen-bonded binder additive is selected from the group consisting of tyramine, benzimidazole, dopamine, melamine, tyrosine, phenylenediamine, 5-amino-2-(4-aminophenyl)benzimidazole, 2-naphthylamine, 4- aminobiphenyl, 4,4'-oxydianiline, 4,4'-methylenedianiline, 4-aminophenoL 2- aminobenzimidazole, 2-aminoanthracene, anthracene-2,6-diamine, anthracene-9,10-diamine, l-amino-2-naphtol, 2-aminofluorene, or a combination thereof.

[0012] In certain embodiments, the hydrogen-bonded binder additive is ty ramine. Tyramine (l-hydroxy-4-ethylaminobenzene, Ty) is an organic nitrogenous compound which forms in food.

[0013] In other embodiments, molecules with amine groups such as melamine (Mm) and benzimidazole (BI) that form hydrogen bonds with PTFE, can also be used as binder additive.

[0014] An embodiment of the invention is directed to an electrode that is fabricated by a dry -processing method comprising PTFE and a binder additive. In certain embodiments, the weight ratio of PTFE and the binder additive ranges between about 10: 1 to about 1: 1. In a further embodiment, a fluorine atom of PTFE forms a hydrogen bond with a functional group of the binder additive. In certain embodiments, the functional group of the346474137v I 130466 / 00318Atorney Docket: 130466.00318 binder additive with which the fluorine of PTFE forms a hydrogen bond is an amine group.In other embodiments, the functional group of the binder additive is a hydroxyl group.

[0015] In certain embodiments, the electrode further comprises poly vinylidene fluoride (PVDF).

[0016] Embodiments of the invention are also directed to methods for fabricating a dry-processed electrode, comprising the steps of: mixing PTFE binder and at least one binder additive; applying shear force to the mixture to fibrillate and mix uniformly and causing the at least one binder additive to adhere to the PTFE binder: adding an anode active material to the mixture; and fabricating a free-standing dry electrode. In certain embodiments, the step of mixing comprises mixture grinding, mixing, chopping, hot calendaring, ball-milling or a combination thereof.

[0017] In certain embodiments, the anode active material used in the manufacture of the electrodes is selected from natural graphite, artificial graphite, carbon nanotubes, carbon nanorods, carbon nanofibers, graphene, porous carbon nanostructures, hollow carbon spheres, transition metal oxides, transition metal chalcogenides, transition metal oxalates, transition metal carbides, transition metal nitrides, transition metal phosphides, transition metal hydroxides, silicon-based compounds, alloy compounds, metal organic framework, or a combination thereof.

[0018] In certain embodiments, hydrogen bonding between the binder additive and PTFE reduces decomposition of PTFE. By preventing the reduction of PTFE, lithium loss is minimized. In an experiment using half cells and 5wt% of PTFE, the initial Coulombic efficiency of PTFE@Ty was 88.8%, compared to 82.5% when Ty was not used, as shown by Density Functional Theory (DFT) calculations (FIG. 1). DFT calculations also show that Mm exhibited lower binding energy compared to Ty. However, Mm was also effective in preventing the reduction of PTFE. Additionally, commercial PVDF binder has been found to be effective in preventing the electrochemical decomposition of PTFE through solvent-free mixing, by inhibiting electron transfer from graphite to PTFE.

[0019] In the half cells experiment, when comparing the charge / discharge profiles of the first cycle (FIG. 2), the peak resulting from the decomposition of PTFE during the charging process disappeared. Additionally, observing the discharge capacity and the potentials of the remaining plateaus corresponding to the lithiation / delithiation of graphite446474137v I 130466 / 00318Atorney Docket: 130466.00318 during charging and discharging, the Ty acted solely to prevent the reduction of PTFE without any side effects such as other sub-reactions or increased resistance leading to higher overpotential.

[0020] The DFT calculations were carried out with the ORCA 4.2. 1 computational package. The binding energies (Eb) between PTFE and additives were calculated using the following formula.

[0021] where E[0[alis the energy' of the configuration of the additive absorbed on PTFE, EPTFEand Eadd are the energies of PTFE and additive.

[0022] PTFE powder is obtained after removing the remaining solvent of polytetrafluoroethylene (PTFE) preparation (60 wt % dispersion in H2O) by stirring in deionized water at 70 °C for a day and rinsing several times with water. PTFE is mixed with Ty using a chopper at a ratio of 5: 1 wt%, and then mixed with mortar and pestle to break Ty into smaller pieces and ensure good adhesion to PTFE by applying shear force. A graphite anode was prepared with PTFE@Ty by applying shear force using mortar and pestle until a mechanically robust film is obtained. The film is then spread to the desired thickness and loading using a hand roller and cut using a 3 / 8-inch punch.

[0023] In certain embodiments, PTFE is mixed wi th Ty at a ratio ranging from 1: 1 wt% to 10: 1 wt%. In an embodiment, the ratio ofPTFE to Ty is 5: 1 wt%.

[0024] The PTFE used consists of elliptical-shaped particles with fibrous structures in between, while tyramine particles ranges from tens to hundreds of nanometers up to several microns, with larger particles exhibiting a layered structure. Dry' mixing PTFE and tyramine under shear force increases fiber formation, with some fibers clustering into a film-like structure. Nitrogen in tyramine is evenly distributed rather than clustered with slightly higher densities in the regions where F in PTFE is present. Ty interacts with and modifies the surface of PTFE, resulting in stronger binding of C-F sites.

[0025] The dry -processed anodes for half cells are fabricated with natural graphite and binder (without additive) at a weight ratio of 98:2. With the additive, the dry- processed anode is fabricated with a ratio of graphite, binder, and additive as 97.6:2:0.4. The546474137v I 130466 / 00318Atorney Docket: 130466.00318 dry-processed anodes for full cells were manufactured with weight ratios of 96:2:2 (graphite: carbon nanotube (CNT): binder). With the additive, the ratio of graphite, CNT. binder, and additive was 95:2:2: 1 for dry -processed anodes for full cells.

[0026] Dry-processed films are spread to desired thicknesses and loadings using a hand roller. In certain embodiments, the film is cut into a circular shape using a 3 / 8- inch diameter punch. The thickness and electrode density of dry-processed anodes with an areal capacity of 4.4 mAh cm’2were about 80 pm and 1.4 g cm3, respectively. An additional adhesive layer was applied to the Cu current collector to improve the adhesion between the freestanding dry-processed film and the current collector to provide conditions similar to those used in the wet process for the purpose of comparison. The adhesive was prepared by mixing polyvinylidene fluoride (PVDF, Mw -534,000,) and carbon black in N-methyl-1,2- pyrrolidine with a weight ratio of PVDF: carbon black = 3:2, and a solid content of 5 wt%. The adhesive layer was uniformly coated on the copper current collector. The dry electrode was placed on the adhesive layer and dried in an oven at 60 °C for 6 hours.

[0027] Coin cells are assembled with a CR2025 battery casing. A carbonate- based electrolyte for half cells and full cells was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPFe) and 0.05 M lithium difluoro(oxalate)borate (LiDFOB) in a solution of ethyl methyl carbonate (EMC) and 4-fluoro-l,3-dioxolan2-one (fluoroethylene carbonate or FEC) with a volume ratio of 3: 1. The half cells are assembled with 65-pm-thick lithium foils and AI2O3 coated polyethylene separators, and charged using constant currentconstant voltage (CC-CV) charging mode for cycling (0.1C cut-off). A wet-processed LiNio.s Mno.1Coo.1O2 (NMC81 1) cathode with an areal capacity of 4.0 mAh cm’2has a total loading of 23. 18 mg cm’2and a total active material loading of 20.86 mg cm’2(±0.2 mg cm’2). The cathode was fabricated with a weight ratio of 90:5:5 for the active material, carbon black, and PVDF. For the wet-processed anode, a binder was prepared by mixing carboxymethylcellulose and styrene-butadiene rubber at a 1 : 1 weight ratio. Water was used as a solvent with a solid content of 30 wt%. The slurry was applied onto a copper foil using a doctor blade, and then dried overnight at 60°C. The electrode was densified using a calendaring machine to obtain a electrode density similar to that of the dry-processed electrodes. In the full cells, the electrolyte was prepared by mixing 1.2 M LiPFe in a 95:5 volume ratio of EMC:FEC, along with 1% vinylene carbonate (VC) and 0.5% LiDFOB. The N / P ratio was 1.1 and CC-CV charging mode (0.1C cut-off) for cycling.646474137v I 130466 / 00318Atorney Docket: 130466.00318

[0028] The reduction of PTFE by lithium is evidenced by changes in the voltage profile and a decrease in ICE. This decomposition of PTFE results in a lower delithiation capacity and a reduced ICE. When the PTFE content is reduced, the voltage shoulder becomes less prominent due to the reduced amount of material available for the side reactions. However, compared to the wet-processed electrode lacking PTFE, a slight shoulder remains. In contrast, the incorporation of tyramine (PTFE@Ty) mitigates this undesirable effect, yielding a voltage profile comparable to that of a conventional anode fabricated using the wet slurry method. Notably, tyramine did not introduce any adverse effects, such as unwanted side reactions or increased resistance leading to higher overpotential. While reducing the PTFE content enhances ICE, it can lead to poor cycling performance, meaning it cannot be reduced indefinitely.

[0029] In certain embodiments, integrating carbon nanotubes (CNT) enhances cycle life. A CNT network provides the structural reinforcement to PTFE fibers, alleviating the capacity drop caused by mechanical degradation. Addition of CNT also enhances electrical conductivity. Preventing structural degradation also curtails electrochemical side reactions that consume reversible lithium.

[0030] Embodiments of the invention are directed to lithium-ion rechargeable batteries containing electrodes comprising PTFE and binder additives as discussed herein.

[0031] Although various embodiments of the present disclosure have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the present disclosure is not limited to the embodiments disclosed herein, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit of the disclosure as set forth herein.

[0032] The term ‘'substantially7’ is defined as largely but not necessarily wholly what is specified, as understood by a person of ordinary skill in the art. In any disclosed embodiment, the terms “substantially”, “approximately”, “generally”, and “about” may be substituted with “within [a percentage] of’ what is specified, where the percentage includes 0.1, 1. 5, and 10 percent.

[0033] The foregoing outlines features of several embodiments so that those skilled in the art may beter understand the aspects of the disclosure. Those skilled in the art should appreciate that they may readily use the disclosure as a basis for designing or746474137v I 130466 / 00318Attorney Docket: 130466.00318 modifying other processes and structures for carry ing out the same purposes and / or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the disclosure. The scope of the invention should be determined only by the language of the claims that follow. The term “comprising'’ within the claims is intended to mean “including at least” such that the recited listing of elements in a claim are an open group. The terms “a”, “an”, and other singular terms are intended to include the plural forms thereof unless specifically excluded.846474137v I 130466 / 00318

Claims

Atorney Docket: 130466.00318CLAIMSWhat is claimed is:

1. An electrode fabricated by dry -processing comprising polytetrafluoroethylene and at least one binder additive, wherein the weight ratio of polytetrafluoroethylene and the at least one binder additive ranges between about 10: 1 to about 1 : 1.

2. The electrode of claim 1 , wherein a fluorine atom of polytetrafluoroethylene forms a hydrogen bond with a functional group of the at least one binder additive.

3. The electrode of claim 1, wherein the at least one binder additive comprises at least one of a hydroxyl group or an amine group.

4. The electrode of claim 1, wherein the at least one binder additive is selected from the group consisting of tyramine, benzimidazole, dopamine, melamine, tyrosine, phenylenediamine, 5-amino-2-(4-aminophenyl)benzimidazole, 2-naphthylamine, 4- aminobiphenyl, 4,4'-oxydianiline, 4.4'-methylenedianiline, 4-aminophenol, 2- aminobenzimidazole, 2-aminoanthracene. anthracene-2,6-diamine. anthracene-9.10-diamine, l-amino-2-naphtol, 2-aminofluorene, or a combination thereof.

5. The electrode of claim 1 further comprising poly vinylidene fluoride.

6. The electrode of claim 1 further comprising carbon nanotubes.

7. The electrode of claim 1. wherein the carbon nanotubes form a network.

8. A method for fabricating a dry -processed electrode, comprising the steps of: mixing polytetrafluoroethylene binder and at least one binder additive; applying shear force to the mixture to fibrillate and mix uniformly and causing the at least one binder additive to adhere to the polytetrafluoroethylene binder; adding an anode active material to the mixture; and fabricating a free-standing dry electrode.946474137v I 130466 / 00318Attorney Docket: 130466.003189. The method of claim 8, wherein said step of mixing comprises mixture grinding, mixing, chopping, hot calendaring, ball-milling or a combination thereof.

10. The method of claim 8, wherein said the anode active material is selected from natural graphite, artificial graphite, carbon nanotubes, carbon nanorods, carbon nanofibers, graphene, porous carbon nanostructures, hollow carbon spheres, transition metal oxides, transition metal chalcogenides, transition metal oxalates, transition metal carbides, transition metal nitrides, transition metal phosphides, transition metal hydroxides, silicon-based compounds, alloy compounds, metal organic framework, or a combination thereof.

11. The method of claim 8 further comprising adding carbon nanotubes.

12. The method of claim 8, wherein the at least one binder additive is selected from the group consisting of tyramine, benzimidazole, dopamine, melamine, tyrosine, phenylenediamine. 5-amino-2-(4-aminophenyl)benzimidazole, 2-naphthylamine. 4- aminobiphenyl, 4,4'-oxydianiline, 4,4'-methylenedianiline, 4-aminophenol, 2- aminobenzimidazole, 2-aminoanthracene, anthracene-2,6-diamine, anthracene-9,10-diamine, l-amino-2-naphtol, 2-aminofluorene, or a combination thereof.

13. A lithium-ion rechargeable battery comprising the electrode of claim 1.1046474137v I 130466 / 00318

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