Sulfur-based electrodes mechanically stabilized by long carbon nanotube filaments

Abstract

Described are electrodes for electrochemical cells. The electrodes have particles of an active material bundled into agglomerates with electrically conductive, inter-agglomerate, carbon-nanotube filaments. Larger electrically conductive filaments, yams of carbon nanotubes, are threaded between the agglomerates. The inter-agglomerate filaments serve as feeder lines, short-range conductors that connect to the particles of active material within the agglomerates. The longer extra- agglomerate yarns serve as trunk lines, long-range conductors that connect the feeder lines to a current collector. The inter- and extra-agglomerate materials additionally serve as mechanical interlocking binders that give structure to the electrodes. The particles, agglomerates, and filaments can be combined and formed into electrodes using a dry process.

Description

SULFUR-BASED ELECTRODES MECHANICALLYSTABILIZED BY LONG CARBON NANOTUBE FILAMENTSRodrigo Villegas Salvatierra, Mahesh Banda, Venkata Pavan Raja Manuri, Sasidharan Prakasan, Binbin QiCROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Application 63 / 514,058 filed 17 July 2023 and entitled "Sulfur-Based Electrodes Mechanically Stabilized by Long Carbon Nanotube Filaments," which is incorporated herein by reference.BACKGROUND

[0002] An electric battery includes one or more electrochemical cells. Each cell includes a positive electrode (cathode) and a negative electrode (anode) physically separated by an ion conductor (electrolyte). When a cell is discharged to power an external circuit, the anode supplies negative charge carriers (electrons) to the cathode via the external circuit and positive charge carriers (cations) to the cathode via the internal electrolyte. During charging, an external power source drives electrons from the cathode to the anode and cations from the cathode through the electrolyte and to the anode.

[0003] Electrodes commonly combine active and passive materials, active material being what reacts chemically to store and produce electrical energy when the cell charges and discharges. The passive material binds the active material and can contribute to thermal and electrical conductivity. Active and passive materials can be combined using wet- or dryelectrode processes. Wet processes deposit electrode materials as a slurry. Managing solvents and the area and time required for drying are disadvantageous. Dry-electrode processes avoid these issues. In typical dry-electrode processing, a mixture of an active material, a conductive additive, and a polymeric binder are processed to form a powder. The powder is compressed and laminated over a conductive surface, such as a current collector of a metal foil.SUMMARY

[0004] This application describes wet and dry processes for making cathode electrodes for electrochemical cells. Sulfur is mixed with a monomer or polymer, along with tiny conductive filaments and larger conductive yams. The mixture is then heated, causing the sulfur to react with the monomer or polymer, resulting in small particles of sulfurized-carbon that stick to the filaments and yarns. After heating, the mixture forms agglomerates of the particles and filaments. These agglomerates are broken up to expose the filaments, which canbe done while mixing them with a binder, a solid electrolyte, or both. The filaments are thinner than the particles, while the yams are thicker than the particles. The filaments are between 50 and 100 microns long, and the yarns are longer than 100 microns.

[0005] The pyrolyzed mixture is pressed into a thin, dry film, which can be free-standing or laminated to a current collector. The film has tiny pores that can be filled with additional sulfur by exposing it to sulfur vapor. At this stage the sulfur in the sulfur-carbon particles makes up 20-50% of the weight, and the extra sulfur adds another 6-30%.

[0006] Electrodes formed using these methods have particles of sulfurized-carbon bundled into agglomerates with electrically conductive, inter-agglomerate, carbon-nanotube filaments. Larger electrically conductive filaments, yams of carbon nanotubes, are threaded between the agglomerates. The inter- agglomerate filaments serve as feeder lines, short-range conductors that connect to the particles of active material within the agglomerates. The longer extra- agglomerate yams serve as trunk lines, long-range conductors that can connect the feeder lines to a current collector. The inter- and extra-agglomerate materials additionally serve as mechanical interlocking binders that give stmcture to the electrodes. This binding effect can reduce or eliminate the use of polymer binders.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The subject matter disclosed is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

[0008] Figure 1 is a flowchart 100 depicting a dry-electrode process for creating a cathode of an electrochemical cell in accordance with one embodiment.

[0009] Figure 2 is a SEM image of a 590 x 512-micron area of a free-standing SC film 200 in accordance with one embodiment.

[0010] Figure 3 is a bar chart 300 in which each bar relates a number of yams 220 of a range of lengths in the image of Figure 2.

[0011] Figure 4 is a SEM image 400 of SC film 200 of Figure 2 under higher magnification.

[0012] Figure 5 is a SEM image 500 of SC film 200 of Figures 2 and 4 under still higher magnification.

[0013] Figure 6 is a SEM image 600 of SC film 200 of Figures 2, 4, and 5 under still higher magnification.

[0014] Figure 7 is a SEM image 700 of SC film 200 of Figures 2, 4, 5, and 6 under still higher magnification.

[0015] Figure 8 depicts an energy- storage device 800 that uses SC film 200, formed as detailed above, as an active cathode material.DETAILED DESCRIPTION

[0016] Figure 1 is a flowchart 100 depicting a dry-electrode process for creating a cathode of an electrochemical cell in accordance with one embodiment. First, at step 105, powders of sulfur, a polymer, carbon nanomaterials including carbon nano tubes (CNTs), and CNT yams are mixed in a mass ratio of e.g. 22:11:5:5 using e.g. an impeller, ball milling, or jet milling. In this example, the sulfur powder is mixed with a powder of a polymer with an average molecular weight of 150,000 Dalton; CNTs between fifty and one hundred microns long and between five and one hundred nanometers wide; and CNT yams hundreds or thousands of microns long. CNT yams can consist of multiple carbon nanotubes that are bundled in parallel and can ramify into interconnected branches that form a web-like structure. The mixing yields a fluffy powder with CNT yarns and agglomerates of the multiwall CNTs intermixed with the sulfur and polymer. The sulfur can be more than 50% of the powder by mass.

[0017] The fluffy powder is pyrolyzed (at step 110) in a furnace at temperatures between 300 and 500°C. Pyrolyzation carbonizes the polymer precursor by driving off constituent hydrogen and nitrogen and incorporating the sulfur. In some preparations, the polymer precursor is from the family of polyacrylonitrile (PAN). The filaments — multi-wall CNTs and CNT yarns — tend to fuse to the resulting sulfurized-carbon (SC) particles and agglomerates. The bulk of the smaller CNTs is entrapped within the SC powder particles, whereas the bulk of the CNT yarns are outside the agglomerates. The CNT yarns can ramify during mixing and the resultant CNT branches and fragments can be entrapped within the agglomerates with the other smaller CNTs. Some of the smaller CNTs trapped within SC particles have anchor segments that extend beyond their particles, and some of those form crosses, hooks, loops, kinks, and elbows that interlace with anchor segments from adjacent particles.

[0018] Next, in step 115, the pyrolyzed material is mixed using a high-shear mixer. The mixing loosens the agglomerates and exposes CNTs and yarn extensions that are fused to the particles and agglomerates. The resultant “fuzz” on and between the SC particles and what remains of the agglomerates gives the powder a fluffy, clumpy consistency and a very highangle of repose. This fluffy powder can be further ground and sieved to select agglomerates and collections of agglomerates of a pre-determined size or sizes. The mixing process of step 115 can add a polymer binder, carbon additives, or both, that account for between one and ten percent of the total mass of the fluffy powder. In one embodiment, the mixing step of 115 adds three weight percent of a polymer binder, such as poly vinylidene fluoride (PVDF). The concentration of the polymer binder can be less than two weight percent or the binder can be omitted entirely.

[0019] Cathode materials with little or no polymeric binder can advantageously include more active sulfur. As used herein, a binder is of a polymer if made of chains of small molecules called monomers. By this definition, the CNTs and filaments that bind SC particles and agglomerates are not polymers. Examples of polymers include polyvinylidene fluoride (PVDF), poly acrylic acid (PAA), carboxymethyl cellulose / styrene butadiene rubber (CMC / SBR), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), and polytetrafluoroethylene (PTFE). Some polymers, per- and polyfluoroalkyl substances (PFAS) for example, are persistent, can be toxic, and may be banned. Eliminating polymer binders can therefore be beneficial from regulatory and environmental perspectives.

[0020] In other methods of preparation, the mixing step of 115 can add additional material, such as a solid electrolyte powder or powders. The solid electrolyte can be at least one of garnet-type oxides, lithium-based NASICON solids, LISICON solids, Li-containing antiperovskite, lithium thiophosphates, lithium phosphate oxynitrides, glassy sulfide compounds, and compounds thereof. The electrolyte powder can be 20wt% to 60wt% of the total mass.

[0021] The pyrolyzed powder from step 115 is compressed (120) between parallel rollers to produce a self-supporting SC film in which the CNT yams form an electrically conductive, three-dimensional network that aids mechanical stability. The SC film can be free-standing. The yam fragments range from hundreds to thousands of microns in length and lack well- defined aspect ratios. Some films include yarn fragments that extend entirely through the thickness of the film. The smaller CNTs have relatively well-defined aspect ratios and tend to extend within individual SC agglomerates, sometimes bridging agglomerates, providing connectivity that is short ranged in comparison to the network of CNT yarn.

[0022] In another embodiment, the fluffy SC powder is conveyed into a roll nip and pressed between two parallel rollers operating at different rotational speeds. A dry film of interconnected agglomerates is formed on the faster rotating roll. The film can be transferred from this faster roll to a target substrate by lamination. A similar and suitable dry-electrodeprocess is described in B. Schumm and S. Kaskel, "Dry battery electrode processing, what’s next?", Next Energy, Volume 1, Issue 2, 2023, https: / / doi.org / 10.1016Zj.nxener.2023.100009, which is incorporated herein by reference.

[0023] The free-standing film can be stacked or rolled for later use, or can be immediately transferred to a substrate, such as a current collector for an electrochemical cell. For example, the free-standing film can be laminated on one or both sides of an aluminum current collector to form a cathode (125). The resulting laminate is then compacted, smoothed, and finished by calendaring (130). CNT anchor segments that extend beyond their particles to form entangled loops and kinks in the powder appear as superimposed crossing filaments in the calendared cathode material.

[0024] Forming the film prior to lamination and calendaring allows the compression, lamination, and calendaring steps to be optimized separately. For example, compaction at a pressure optimized for high cathode density may be too high for effective lamination. Other embodiments combine one or more of steps 120, 125, and 130. The laminated and calendared cathode material is cut into electrodes (135) that are combined with other cell components during cell assembly (140).

[0025] Most of the sulfur atoms in the SC cathode material are bonded to an adjacent carbon atom via a relatively strong, covalent, carbon-sulfur bond. In some embodiments, essentially all of the sulfur atoms in the agglomerates are bonded to carbon either directly or via chains of covalently bonded sulfur atoms, though some may be unbonded during cycling (i.e., moving charge carriers into and out of the cathode). The amount of elemental sulfur is less than 20wt% of the total sulfur and the mass ratio of the carbon covalently bonded to the sulfur to the carbon nanomaterial is greater than one, and greater than ten in some embodiments.

[0026] The pyrolyzed powder from step 110 can also be used in a wet process. For example, pyrolyzed powder can be mixed with a powdered carbon, such as acetylene black, a polymer binder, and an organic solvent or water to form a slurry. The binder and carbon additive can compose from 5 to 20wt% of the solid mass. The slurry is then spread over one or both sides of e.g. an aluminum foil that will serve as a current collector. The slurry-coated foil is then dried and compressed.

[0027] Some embodiments extend the lithium storage capacity of sulfurized-carbon cathodes with enhanced porosity and molecular sulfur trapped within the pores. The resultant electrode material has two distinct forms of sulfur with different chemical structure, bonding, and physical properties. The first form of sulfur occurs as the sulfur atoms in the micro- andmesoporous sulfurized carbon, sulfur atoms that are covalently bonded to the carbonaceous matrix either directly or via other sulfur atoms. This bonded sulfur can be incorporated into the carbon structure in several forms, including thiophene-like structures (where sulfur atoms are part of carbon rings), as polysulfides (chains of sulfur atoms bonded to carbon), or as terminal groups attached to the carbon lattice. The second form of sulfur occurs as molecular sulfur, commonly as a ring of eight sulfur atoms (Ss), though other allotropes exist predominantly from S2 to S20. These sulfur molecules are adsorbed to the sulfurized carbon and trapped within the pores. The capture and adsorption of this elemental trapped sulfur slow the diffusion of soluble LiPS and thus enhance the cycle life of sulfur-based electrochemical cells that incorporate these electrodes. Sulfur beyond what can be trapped in the pore volume tends to deposit on external surfaces of the carbon materials and so does not benefit from the stabilizing effects of pore constraints.

[0028] Figure 2 is a SEM image of a 590 x 512-micron area of a free-standing SC film 200 in accordance with one embodiment. The material is a compacted solid in which much of the internal structure is obscured by compacted SC agglomerates 205. CNT filaments 210, notable due to their relatively small and uniform cross sections, are exposed in some areas 215. Larger CNT yams 220 are highlighted using dashed lines that roughly trace their paths. CNT yams 220 are longer, wider, and less uniform than CNT filaments 210. The CNT filaments and yarns extend both in the plane of the film, as shown, and normal to the plane, collectively forming a conductive, three-dimensional framework in physical and electrical contact with SC particles formed into agglomerates 205.

[0029] In the example of SC film 200, CNT yams 220 have a maximum width of at least three microns, with some being considerably larger. The distributions of CNT filaments and yams are spread out in the 3D space of film 200 with vertical filaments, yarns, and sections of filaments and yarns obscured. Yarns 220 of CNT filaments can range from hundreds to thousands of microns in length and have no defined aspect ratio. Yarns 220 extend between more local networks of CNT filaments 210. The bulk of CNT filaments 210 are intraagglomerate filaments, whereas the bulk of CNT yarns 220 are extra-agglomerate filaments that extend past and are fused to multiple agglomerates.

[0030] The dimensions of the particles, agglomerates, intra-agglomerate filaments, and extra-agglomerate filaments can be specified relative to one another. The intra-agglomerate filaments are within the agglomerates and thus have an average intra-agglomerate-filament width less than the average agglomerate size, less than the average particle size in some embodiments. The extra- agglomerate filaments have an average extra-agglomerate-filamentwidth greater than the average intra-agglomerate-filament width and an average extraagglomerate length greater than the average agglomerate size. The average extra- agglomerate-filament length is greater than five times the average agglomerate size in some embodiments.

[0031] Figure 3 is a bar chart 300 in which each bar relates a number of yams 220 of a range of lengths in the image of Figure 2. The 590 x 512-micron sample area gives a sense of the relative proportions of yarn lengths in film 200. Most yams 220 are between fifty and one hundred fifty microns. Many are considerably longer than the CNT precursors used in the method of Figure 1 to form film 200.

[0032] Figure 4 is a SEM image 400 of SC film 200 of Figure 2 under higher magnification. Agglomerates 205 of particles are seen with CNTs 210 within and between the agglomerates. Segments of the larger CNT yams 220 tend to extend far longer than the average size of agglomerates 205, physically and electrically connecting the agglomerates and their constituent SC particles and CNTs. In the context of particles and agglomerates, "size" refers to the largest dimension.

[0033] Figure 5 is a SEM image 500 of SC film 200 of Figures 2 and 4 under still higher magnification. A collection of CNTs 210 encircled at lower right extends from agglomerates 205. The bulk of the CNT filaments are embedded within agglomerates 205, whereas the bulk of CNT yarns 220 are threaded between them. Electrically, CNTs 210 serve as feeder lines, short-range conductors that connect SC particles within agglomerates 205 to CNT yarns 220. The CNT yarns, in turn, serve as trunk lines, long-range conductors that connect the feeder lines to a current collector (e.g. a metal film). The resultant three-dimensional conductive framework binds the SC particles and agglomerates while providing low-resistance electrical connectivity between the SC particles and the current collector. The three-dimensional framework also mechanically supports the electrode structure.

[0034] Figure 6 is a SEM image 600 of SC film 200 of Figures 2, 4, and 5 under still higher magnification. SC particles 605 are easily discernable constituents of agglomerates 205. CNTs extend through particles 605 generally but are not visible. The widths of CNT yam fragments 220 are greater than the average size of particles 605 and are on the order of the average size of agglomerates 205. CNTs 210, on the order of ten nanometers in diameter in the depicted sample, are difficult to image and thus hidden from view. Yam fragments 220 can partially disentangle during mixing 115 leaving smaller fragments that get incorporated into agglomerates 205 with CNTs 210. However, the bulk of the material introduced as yamsin mixing 105 remains outside the agglomerates. Some of the yams can be seen to comprise carbon filaments that branch and recombine at joints.

[0035] The conductive filaments have bimodal distributions of lengths and widths selected in proportion to the sizes of agglomerates 205 and particles 605. The CNTs are relatively thin so that mixing and milling tend to combine the CNTs with SC particles 605 to form agglomerates 205. Yarns 220 are much thicker than the CNTs and tend to remain outside of particles 605 and agglomerates 205 during mixing and milling.

[0036] Figure 7 is a SEM image 700 of SC film 200 of Figures 2, 4, 5, and 6 under still higher magnification. SC particles 605 are easily discernable constituents of an agglomerate 205. A CNT yarn fragment 220 extends past an agglomerate 205. A graphic 750 at bottom highlights SC particles 605 and CNTs 210.

[0037] The conductive filaments are multi-wall CNTs and CNT yams in the forgoing embodiments. Other embodiments can include more or different combinations of conductive filaments, including e.g. SC fibers. Conductive additives of e.g. graphene, graphite, nanoplatelets, nanorods, and nanoribbons can be used with or instead of CNTs and / or CNT yams. Selenium can be used with or instead of sulfur to produce particles and agglomerates of sulfurized / selenized carbon or selenized carbon.

[0038] Sulfur-based cathodes can be limited in use by the depletion of active material from the cathode electrode, known as the shuttle effect. The shuttle of active materials happens because specific products of discharge, lithium polysulfides (LiPS), are soluble in electrolyte and thus leach out from the physical structure of the cathode material. This effect is enhanced by the poor adsorption of sulfur crystals over a conductive matrix, typically of carbon. In the above-described sulfurized-carbon synthesis, sulfur molecules react with decomposing carbon-based materials, forming new bonds between sulfur molecules and the derived carbon material. The resulting sulfurized carbon presents excellent stability even in carbonate-based electrolytes, which are not compatible with elemental-sulfur-based cathodes because of their inherent reactivity.

[0039] Sulfurized carbon materials are limited by the number of active sites available to bind with sulfur. Under pyrolysis, carbon-based precursors can convert to either a mixture of graphitic or amorphous carbon materials. Localized sp2 carbon crystalline domains can be present in both materials. From the point of view of sulfur loading, the growth of sp2 crystals in lateral size can limit the active sites to bind with sulfur, and thus the gravimetric capacity of such sulfurized carbon cathodes.

[0040] Some embodiments extend the lithium storage capacity of SC cathodes with enhanced porosity and molecular sulfur trapped within the pores. The resultant electrode material has two distinct forms of sulfur with different chemical structure, bonding, and physical properties. The first form of sulfur occurs as the sulfur atoms in the micro- and mesoporous sulfurized carbon, sulfur atoms that are covalently bonded to the carbonaceous matrix either directly or via other sulfur atoms. This bonded sulfur can be incorporated into the carbon structure in several forms, including thiophene-like structures (where sulfur atoms are part of carbon rings), as polysulfides (chains of sulfur atoms bonded to carbon), or as terminal groups attached to the carbon lattice. The second form of sulfur occurs as molecular sulfur, commonly as a ring of eight sulfur atoms (Ss), though other allotropes exist predominantly from S2 to S20. These sulfur molecules are adsorbed to the sulfurized carbon and trapped within the pores. The capture and adsorption of this elemental trapped sulfur slow the diffusion of soluble LiPS and thus enhance the cycle life of sulfur-based electrochemical cells that incorporate these electrodes. Sulfur beyond what can be trapped in the pore volume tends to deposit on external surfaces of the carbon materials and so does not benefit from the stabilizing effects of pore constraints.

[0041] Some embodiments modify the method of Figure 1 to create SC materials rich in micro- and mesopores for storing trapped sulfur. Adding molecules to the polymer precursor, which might include variants of polyacrylonitrile, and synthesis process conditions, which includes mixing methods and treatment of the mixture precursors, induce the resulting sulfurized carbon material includes micro- and mesopores. The formation of micro- and mesopores can be facilitated in sulfurized carbon by modifying precursor polymers with comonomers (molecules that are added to the chemical structure of the polymer). Chemicals, such as salts and solvents, can also be added to precursor polymers to expand cathode material and increase pore volume.

[0042] Sulfur can be loaded into the pores of sulfurized carbon in the heating program used for the precursor pyrolysis. By adjusting the temperature of the synthesis, sulfur vapors rich in smaller molecules (S2 to S4) can facilitate the loading of sulfur species in the newly formed pores of the sulfurized carbon. Pressure can further assist in the sulfur loading. After loading, the sulfur atoms covalently bonded to carbon atoms make up from twenty to fifty percent of the mass of the conductive framework and the SC particles, while the sulfur molecules are from six to thirty percent of this mass. Sulfur can also be added electrochemically using additives, such as lithium polysulfides, present in an electrolyte.

[0043] Sulfur utilization is important for understanding the efficiency, capacity, and overall performance of sulfur-based cathodes. Sulfur utilization refers to the fraction of sulfur that is actively participating in electrochemical reactions during cell discharge. The fraction of sulfur that participates in the reaction undergoes the following reaction: S + 2Li — > LLS. High sulfur utilization is desirable as it indicates that a larger amount of the sulfur is being used to store and deliver energy, leading to higher specific capacities. Sulfur utilization can be measured indirectly as a function of the specific capacity of a cell that incorporates the cathode. The specific capacity, typically in milliamp-hours per gram, is compared to the theoretical capacity of sulfur to determine the utilization rate. In some embodiments, more than 70% of the sulfur, from both the sulfurized carbon and the trapped molecules, is utilized.

[0044] Figure 8 depicts an energy- storage device 800 that uses SC film 200, formed as detailed above, as an active cathode material. Device 800 includes a cathode 805 separated from an anode 810 via an electrolyte 815 with a separator (not shown) of, e.g., a porous polymer. Anode 810 includes a current collector 820 of, e.g., copper physically and electrically connected to an anode layer 825 of lithium metal or a combination of lithium metal or lithium ions and some form of porous carbon. Cathode 805 includes a current collector 830 of, e.g., aluminum physically and electrically connected to a cathode layer 835 of an active material formed in the manner of SC film 200 of Figure 2.

[0045] Cathode layer 835 is a composite that includes distinct agglomerates (not shown) of sulfurized-carbon particles distributed in a binder and interconnected by sp2-bonded carbon nanomaterials (not shown), such as CNTs, in the manner detailed previously. The material is innervated with CNT yarns 220 that provide long-range electrical conductivity and structural support for the agglomerates. Some of yarns 220 contact cathode current collector 830. The binder physically and electrically connects yarns 220 to the agglomerates and their constituent CNTs.

[0046] Lithium in anode layer 825 is oxidized (electron loss) during cell discharge to power a load 860 external to the cell. Electrons pass from anode layer 825 to cathode layer 835 via current collectors 820 and 830 and load 860, and lithium cations (Li+) from anode layer 825 pass to cathode layer 835 via electrolyte 815 where they are reduced (electron gain) within cathode layer 835 as lithium polysulfide salts. Charging reverses this process by stripping lithium ions and electrons from cathode layer 835 and returning them to anode layer 825. Embodiments of anode layer 825 are detailed in U.S. Patent Application No. 17 / 684,239 to Raji et al. and entitled "Carbon Nanotube Carpet on and Grown from Copper," which was filed 1 March 2022 and is incorporated herein.

[0047] The conductive filaments are multi- wall CNTs and CNT yams in the forgoing embodiments. Other embodiments can include more or different combinations of conductive filaments, including e.g. SC fibers. Conductive additives of e.g. graphene, graphite, nanoplatelets, nanorods, and nanoribbons can be used with or instead of CNTs and / or CNT yams. Selenium can be used with or instead of sulfur to produce particles and agglomerates of sulfurized / selenized carbon or selenized carbon.

[0048] Returning to the topic of binders, SC particles and agglomerates incorporating CNT filaments as mechanical anchors can bind together with little or no polymeric additives that would otherwise serve as binders. In embodiments that include polymeric binders, a polymeric structure used with binding intent is of an organic polymer in which molecules (monomers) form chains. Examples of such polymers include carboxymethyl cellulose (CMC) and salts thereof, CMC combinations with SBR of different molecular weights, PAA and salts thereof, polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene oxide (PEO) of different molecular weights, fluoropolymers containing monomers such as tetrafluoroethylene and vinylidene difluoride, such as PVDF and PTFE, epoxy resins, polymers containing siloxane units, such as polydimethylsiloxane (PDMS), polyacrylic latex solutions, and mixtures thereof. Polymeric structures with binding intent can adopt different shapes, such as round particles or fibers bridging active materials. Monomers chained to form polymer binders can be bonded to one another via only one chemical bond, a single, double or triple covalent bond.

[0049] Eliminating polymer binders can be advantageous. Some polymeric binder may nevertheless be included to assist the binding effects of the CNT framework. The percentage of polymeric structure with binding intent can compose up from 0.2 to 2% in mass of all active electrode mass, which includes the 3D-CNT framework, active material and polymeric structure but excludes any separate current collector (e.g., a metal foil) or solid or liquid electrolyte. In this context, "binding intent" refers to a physical or chemical attachment between the polymeric structure and different materials in the electrode, including the SC particles and the 3D CNT-framework, leading to a mechanical stability and cohesivity among the SC particles, the agglomerates, and the 3D CNT-framework, enabling free-standing electrodes without metal-based current collectors. In another embodiment, some polymeric binders may be included to create cohesivity between the above-mentioned particles and also a traditional metal-foil-based current collector.

[0050] The foregoing discussion focuses on electrochemical cells that employ lithium ions as charge carriers. Other alkali metals (e.g. sodium and potassium) can also be used.Other variations of these embodiments will be obvious to those of ordinary skill in the art. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description. Only those claims specifically reciting “means for” or “step for” should be construed in the manner required under the sixth paragraph of 35 U.S.C. § 112.

Claims

CLAIMSWhat is claimed is:

1. An electrode comprising: particles of an active material and having an average particle size; agglomerates of electrically conductive, intra-agglomerate filaments and the particles, the agglomerates having an average agglomerate size and the intra- agglomerate filaments having an average intra-agglomerate-filament width less than the average agglomerate size; and electrically conductive extra-agglomerate filaments having an average extra- agglomerate-filament width greater than the average intra-agglomerate-filament width and an average extra- agglomerate-filament length greater than the average agglomerate size.

2. The electrode of claim 1, wherein the intra-agglomerate filaments comprise carbon nano tubes.

3. The electrode of claim 1, the intra-agglomerate filaments having an average intraagglomerate-filament length less than the average agglomerate size.

4. The electrode of claim 1, wherein the extra- agglomerate filaments comprise carbon nano tubes.

5. The electrode of claim 1, wherein the active materials of the particles consist essentially of sulfur and carbon.

6. The electrode of claim 1, wherein the average extra-agglomerate-filament length is greater than five times the average agglomerate size.

7. The electrode of claim 1, wherein the average intra-agglomerate-filament width is less than the average particle size.

8. The electrode of claim 1, wherein the intra-agglomerate filaments, the extraagglomerate filaments, and the particles form a layer of an active material of an active electrode mass, and wherein the layer of the active material is less than two weight percent polymer.

9. The electrode of claim 8, wherein the electrode essentially lacks polymer among and between the intra- agglomerate and extra-agglomerate filaments.

10. The electrode of claim 8, wherein the layer of the active material is greater than two tenths of one weight percent of the polymer.

11. An electrode comprising: particles of an active material; electrically conductive filaments; agglomerates of the particles fused to a first set of the filaments, the first set of the filaments having an average filament length, an average filament width, and filament ends extending beyond the particles as anchor segments between adjacent ones of the particles; and yarns of a second set of the filaments having an average yam length greater than the average filament length and an average yam width greater than the average filament width, wherein the yarns extend between the agglomerates.

12. The electrode of claim 11, wherein the average filament length is between fifty and one hundred microns.

13. The electrode of claim 12, wherein the average filament width is less than one micron.

14. The electrode of claim 13, wherein the average yarn width is greater than three microns.

15. The electrode of claim 14, wherein the average yarn length is greater than one hundred microns.

16. The electrode of claim 11, wherein the active material consists primarily of carbon and sulfur.

17. The electrode of claim 16, wherein the active material consists primarily of sulfurized carbon.

18. The electrode of claim 16, wherein the sulfur and carbon consist mostly of sulfur atoms covalently bonded to carbon atoms.

19. The electrode of claim 18, wherein the sulfur atoms are primarily in chains of the sulfur atoms covalently bonded to the carbon atoms.

20. The electrode of claim 11 , wherein the active material comprises at least one of sulfurized carbon, sulfurized / selenized carbon, selenized carbon, sulfurized-carbon fibers, sulfurized-carbon nanoparticles, sulfurized-carbon agglomerates, and compounds and mixtures thereof.

21. The electrode of claim 11, wherein the filaments comprise at least one of carbon nanotubes, carbon nanoribbons, and carbon nanorods.

22. The electrode of claim 21, wherein the filaments branch at joints.

23. The electrode of claim 11, wherein the anchor segments interlace as combinations of at least one of crosses, hooks, loops, kinks, and elbows.

24. The electrode of claim 11, wherein the filaments have an average length-to- width ratio of from ten to one million between the anchor segments.

25. The electrode of claim 11, further comprising second particles.

26. The electrode of claim 25, wherein the second particles comprise a second material.

27. The electrode of claim 25, wherein the second particles comprise an electrolyte.

28. The electrode of claim 27, wherein the electrolyte comprises at least one of garnettype oxides, lithium-based NASICON solids, LISICON solids, Li-containing antiperovskite, lithium thiophosphates, lithium phosphate oxynitrides, glassy sulfide compounds, and compounds thereof.

29. The electrode of claim 27, wherein the second particles are 20wt% to 60wt% of the electrode.

30. The electrode of claim 11, further comprising a conductive additive.

31. The electrode of claim 30, wherein the conductive additive comprises at least one of carbon nanotubes, graphene, graphite, nanoplatelets, and nanorods.

32. The electrode of claim 11, wherein the filaments are fused between the particles within the agglomerates.

33. A method of manufacturing an electrode for an electrochemical cell, the method comprising: producing a mixture by mixing sulfur with at least one of a monomer and a polymer, conductive filaments having an average filament length and an average filament width, and conductive yams having an average yam length greater than the average filament length and an average yam width greater than the average filament width; and pyrolyzing the mixture to react the sulfur with the at least one of the monomer and the polymer to form particles of sulfuri zed-carbon fused to the filaments and the yams.

34. The method of claim 33, wherein the pyrolyzed mixture includes agglomerates of the particles and the filaments, the method further comprising loosening the agglomerates to expose the filaments.

35. The method of claim 34, wherein the average filament width is less than the size of the particles and the average yam width is greater than the size of the particles.

36. The method of claim 34, wherein the loosening comprises mixing the pyrolyzed mixture with a binder.

37. The method of claim 34, wherein the loosening comprises mixing the pyrolyzed mixture with a solid electrolyte.

38. The method of claim 33, further comprising compressing the pyrolyzed mixture into a dry film.

39. The method of claim 38, further comprising laminating the dry film to a current collector.

40. The method of claim 33, wherein the pyrolyzed mixture includes pores, the method further comprising exposing the pyrolyzed mixture to sulfur vapors to load the pores with second sulfur.

41. The method of claim 40, wherein the sulfur of the sulfurized carbon makes up from twenty to fifty percent by mass of the pyrolyzed mixture loaded with the second sulfur and the second sulfur makes up from six to thirty percent by mass of the pyrolyzed mixture loaded with the second sulfur.

42. The method of claim 33, wherein the average filament length is between fifty and one hundred microns.

43. The method of claim 42, wherein the average yarn length is greater than one hundred microns.

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