NANOTUBES WITH COATING

Technical Field

[0001] Hierarchical carbon nanotubes are porous and can contain sulfur inside the nanotube and a polymeric coating around the outside of the nanotube. The hierarchical carbon nanotube can be used as a cathode in a battery or as a conductor in a capacitor or supercapacitor .

Brief Description of the Figures

[0002] The features and advantages of certain

embodiments will be more readily appreciated when considered in conjunction with the accompanying figures. The figures are not to be construed as limiting any of the preferred embodiments.

[0003] Fig. 1 is a schematic illustration of a method of producing a coated, sulfur-loaded hierarchical carbon nanotube according to certain embodiments.

Detailed Description of the Invention

[0004] There has been an increasing energy demand and environmental crisis that requires more environmentally friendly energy storage systems that are safe, lower in cost, and have high energy densities. Among the most promising energy storage devices, lithium-sulfur (Li-S) batteries have attracted much attention in recent years due to a high theoretical capacity of approximately 1,672 milliamp hour per gram (mAh g ¹ ) , which is over 5 times that of currently used transition metal oxide cathode materials; the lower cost and abundant sources of sulfur; and the non-poisonous and environmentally benign nature of the lithium-sulfur system.

[0005] However, the practical applications of Li-S cells are still limited by the following drawbacks: the poor electrical conductivity of sulfur (5 ^χ 10 Siemens per centimeter (S cnT ¹ ) ) limits the utilization efficiency of the active materials and rate capability; the high solubility of polysulfide intermediates in an electrolyte results in a shuttling effect in the charge/discharge process; and a large volumetric expansion of approximately 80% during the charge/discharge process, which results in rapid capacity decay and a low coulomb efficiency. The high storage capacity and cycling ability of sulfur arises from the electrochemical cleavage and re-formation of sulfur-sulfur bonds in the cathode, which is believed to proceed in two steps. First, the reduction of sulfur to lithium higher polysulfides (Li ₂ Sn, 4 < n < 8) is followed by further reduction to lithium lower polysulfides (Li ₂ S _n , 1 < n < 3) . The higher polysulfides are easily dissolved into the electrolyte, thus enabling them to penetrate through a polymer separator and react with the lithium metal anode. This reaction leads to the loss of sulfur active materials. Moreover, even if some of the dissolved polysulfides could diffuse back to the cathode during the recharge process, the sulfur particles formed on the surface of the cathode are electrochemically inactive due to the poor conductivity of the polysulfides. Such a degradation path leads to poor capacity retention, especially during long cycling (i.e., more than 100 cycles) .

[0006] In order to improve the capacity and conductivity and prevent polysulfide dissolution and shuttling, many different approaches have been developed during the past few decades to try and combat these negative effects. However, none of these strategies have been successful. As a result, there is a need and an on-going industry wide concern for energy storage devices that possess a high capacity and efficiency, and low shuttling effect, and are low cost.

[0007] It has been discovered that a porous hierarchical carbon nanotube can be loaded with sulfur or sulfur-containing compounds and then coated. The pores of the nanotube allow expansion of the sulfur during cycling or discharge, while the coating allows migration of desirable compounds into the nanotube and disallows migration of the sulfur outside of the nanotube .

[0008] According to certain embodiments, a device comprises: a hierarchical carbon nanotube; a plurality of pores extending through a wall of the nanotube to an interior of the nanotube; sulfur located within the interior of the nanotube; and a coating surrounding the wall of the nanotube.

[0009] According to certain other embodiments, a method of producing a nanotube comprises: providing a carbon nanotube; etching the carbon nanotube to create a plurality of pores to produce a hierarchical, porous carbon nanotube; introducing sulfur into an interior space of the hierarchical, porous carbon nanotube to produce a sulfur-loaded, hierarchical, porous carbon nanotube; and coating the outside surface of the sulfur-loaded, hierarchical, porous carbon nanotube with a coating.

[0010] According to certain other embodiments, the carbon nanotube is used in a battery or energy storage device.

[0011] It is to be understood that the discussion of preferred embodiments regarding the nanotube is intended to apply to all of the device and method embodiments.

[0012] Turning to the figures, Fig. 1 shows a

hierarchical carbon nanotube and method of producing the

hierarchical carbon nanotube according to certain embodiments. As can be seen, the methods can include providing a carbon nanotube. Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure . Nanotubes are members of the

fullerene structural family, with the name being derived from the long, hollow structure and walls formed by one-atom-thick sheets of carbon, called graphene . These sheets are rolled at specific and discrete chiral angles to form the nanotube having a diameter of close to 1 nanometer. The carbon nanotube can be a single-walled nanotube or a multi-walled nanotube.

[ 0013 ] The methods can include the step of etching the carbon nanotube to create a plurality of pores to produce a hierarchical, porous carbon nanotube. The plurality of pores can be formed on the wall of the hierarchical, porous carbon nanotube; thus, exposing the void making up the inside of the cylindrical-shaped nanotube. The size of the pores, the number of pores, and the spacing of the pores can vary and be dependent on the conditions during etching. The conditions that can be controlled include time, temperature, and pressure. Preferably, the etching is performed at a time in the range from about 15 minutes to 5 hours, a temperature in the range from about 400 °C to about 600 °C, and a pressure of 1 atmosphere. According to certain embodiments, the perimeters of the pores do not touch one another, rather, the pores are discrete, individual pores formed through the wall of the nanotube. The plurality of pores can be macropores, mesopores, micropores, or combinations thereof. As used herein, a "macropore" is a pore having a diameter greater than 50 nanometers (nm) . As used herein, a "mesopore" is a pore having a diameter in the range of 2 nm to 50 nm. As used herein, a "nanopore" is a pore having a diameter of less than 2 nm. According to certain embodiments, the plurality of pores has a pore size in the range of about 0.5 nm to about 50 nm. [0014] According to certain embodiments, the pore size of the plurality of pores is selected to allow expansion of sulfur ions during discharging of the ions. The pore size can prevent large volumetric expansion (i.e., expansion greater than approximately 80%) during lithiation, which can destroy the polymer shell and the formed polysulfides cannot be

encapsulated, which results in rapid capacity decay and a low Coulombic efficiency. Another advantage to having a plurality of pores in the nanotube is that the pores allow migration of ions and compounds into the interior of the nanotube, whereas a non-porous nanotube only allows migration into the interior through the open top and bottom of the nanotube. The increased ability for migration can provide an increased lifetime of an energy storage device (e.g., a battery or capacitor) utilizing the nanotube.

[0015] The methods also include introducing sulfur into an interior space of the hierarchical, porous carbon nanotube. The introduction of the sulfur can be accomplished by sulfur impregnation, for example. The sulfur can be elemental sulfur or a sulfur-containing compound, such as lithium sulfide and lithium polysulfides. After the sulfur has been introduced into the interior space (i.e., the inside of the cylindrical-shaped nanotube) of the hierarchical, porous carbon nanotube, a sulfur- loaded, hierarchical, porous carbon nanotube is produced.

[0016] The methods also include coating the outside surface of the sulfur-loaded, hierarchical, porous carbon nanotube with a coating. The coating can be a polymer. A polymer is a large molecule composed of repeating units, typically connected by covalent chemical bonds. A polymer is formed from monomers. During the formation of the polymer, some chemical groups can be lost from each monomer. The piece of the monomer that is incorporated into the polymer is known as the repeating unit or monomer residue. The backbone of the polymer is the continuous link between the monomer residues. The polymer can also contain functional groups connected to the backbone at various locations along the backbone. Polymer nomenclature is generally based upon the type of monomer

residues comprising the polymer. A polymer formed from one type of monomer residue is called a homopolymer. A copolymer is formed from two or more different types of monomer residues. The number of repeating units of a polymer is referred to as the chain length of the polymer. The number of repeating units of a polymer can range from approximately 11 to greater than 10,000. In a copolymer, the repeating units from each of the monomer residues can be arranged in various manners along the polymer chain. For example, the repeating units can be random,

alternating, periodic, or block. The conditions of the

polymerization reaction can be adjusted to help control the average number of repeating units (the average chain length) of the polymer.

[0017] The average molecular weight for a copolymer can be expressed as follows:

Avg. molecular weight= (M . W . mi * RU mi ) + (M.W.m ₂ * RU m ₂ ) .

where M. W . mi is the molecular weight of the first monomer; RU mi is the number of repeating units of the first monomer;

M. W.m ₂ is the molecular weight of the second monomer; and RU m ₂ is the number of repeating units of the second monomer. Of course, a terpolymer would include three monomers, a

tetrapolymer would include four monomers, and so on.

[0018] Polymer molecules can be cross-linked. As used herein, a "cross-link" is a connection between two or more polymer molecules. Cross-linking can increase the molecular weight of the polymer molecules.

[0019] According to certain embodiments, the polymer is a microporous coating, wherein the polymer coating allows migration of metal anions through the coating and restricts or prevents migration of sulfur cations and sulfur compounds through the coating. According to certain embodiments, the pore size of the microporous coating is selected such that sulfur ions and sulfur compounds are prevented from travelling through the pores. According to certain other embodiments, the pore size of the microporous coating is selected such that sulfur ions and sulfur compounds are prevented from travelling through the pores, but smaller sized ions (i.e., smaller in size

compared to the sulfur ions and sulfur compounds) are allowed to travel through the pores to the inside of the hierarchical, porous carbon nanotube.

[0020] The polymer can be a polyelectrolyte or an electrically conductive polymer. The polymer can be selected from the group consisting of polyaniline, polyacetylene, polyparaphenylene, polyparaphenylene sulfide, polythiophene, polypyrrole, polyisothianaphthalene, polyparaphenylene vinylene, and combinations thereof. The polymer can have a molecular weight in the range from about 10,000 to about 3,000,000. The coating can be formed on the outside of the sulfur-loaded, hierarchical, porous carbon nanotube via any coating method, for example, via in situ polymerization or other coating processes known to those skilled in the art. The conditions of the polymerization can be controlled to produce a polymer coating that is micro-porous, a desired molecular weight, and a desired thickness. The desired thickness of the coating can range from about 2 nm to about 50 nm. [0021] The following is but one example of a method for forming the coated, sulfur-loaded, hierarchical, porous carbon nanotube. The hierarchical, porous carbon nanotube can be prepared by loading 1 gram (g) of a multi-walled carbon nanotube into a tubular furnace and heated from approximately 22 °C to 550 °C at a rate of 15 °C /min under air at a rate of 100 ml/min, then maintained at that temperature for 30 min, and then cooled down to 22 °C. The hierarchical, porous carbon nanotube can then be loaded with sulfur by mixing 1 g of elemental sulfur with 0.5 g of the hierarchical, porous carbon nanotube and then sealing the mixture in an autoclave. The mixture can then be placed in an oven at 150 °C for 12 hours to allow for sufficient diffusion of melted sulfur into the pores. The sulfur-loaded, hierarchical, porous carbon nanotube can then be coated via in situ polymerization by dispersing 0.5 g of the sulfur-loaded, hierarchical, porous carbon nanotube in 30 milliliters (mL) of 1 molar hydrochloric acid. 0.2 mL of a first monomer of aniline can then be added to this mixture, then stirred in an ice bath for 6 hours. A second monomer of 0.5 g of ammonium persulfate dissolved in 20 mL of 1 molar hydrochloric acid can then be dropped into the suspension. The polymerization can then be carried out for 24 hours by stirring slowly at 0 °C. The resulting coated, sulfur-loaded, hierarchical, porous carbon nanotube particles can then be purified by repeated

centrifugation and dried in a vacuum drying oven at 60 °C for 12 hours .

[0022] According to certain embodiments, the nanotube according to the embodiments disclosed is used in a battery. The battery can include: an anode; an electrolyte; and cathode of the nanotube disclosed. [0023] The anode can be a metal, or a metal containing structure. The metal can be selected from lithium, aluminum, sodium, zinc, or nickel.

[0024] The electrolyte can be aqueous or non-aqueous. An aqueous electrolyte may be advantageous for minimizing dendrite formation during the cycling of the battery. According to certain embodiments, the electrolyte is selected from the group consisting of a carbonate, an ether, a polyether, and combinations thereof. The carbonate can be selected from ethylene carbonate, propylene carbonate, butylene carbonate, γ- butyrolactone, γ-valerolactone, N-methylmorpholine N-oxide, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate, propyl acetate, or ethyl butyrate. A carbonate is a salt of carbonic acid (H ₂ CO ₃ ) or an ester of carbonic acid having a carbonate group C (=0) (0-) ₂ - The ether can be selected from tetrahydrofuran (THF) , 1 , 3-dioxolane (DOL) , 1,2- dimethoxyethane (DME) , or tetra ( -ethylene glycol) dimethyl ether (TEGDME) . The polyether can be selected from polyethylene glycol (PEG) or polypropylene oxide (PPO) .

[0025] According to the embodiments for the battery, the plurality of pores on the hierarchical, porous carbon nanotube have a pore size sufficient to allow expansion of the sulfur during charging of the battery. During charging, ions from the anode can enter the interior of the carbon nanotube via the open top and bottom and the pores. The ions can then chemically react with the sulfur located inside the interior. During discharge of the battery, the sulfur can expand, but is

prevented from exiting the interior due to the size of the pores. The sulfur will react with lithium to form polysulfides resulting in approximately 80% volumetric expansion. The formed polysulfides are encapsulated by the polymer coating to prevent or decrease the diffusion of polysulfides from the cathode to the anode .

[0026] The battery can further include a separator that separates the anode from the cathode. The separator can be made from a material selected from non-woven fabrics (e.g., nylon, cotton polyesters, and glass) or microporous polymeric films (e.g., polyethylene, polypropylene, poly ( tetrafluoroethylene, etc . ) .

[0027] According to certain other embodiments, the nanotube according to the disclosed embodiments is used in an energy storage device. The energy storage device can be a capacitor or a supercapacitor . A capacitor generally includes two conductors separated by a non-conductive region. A

supercapacitor is a high-capacity capacitor with capacitance values much higher than other capacitors, but lower voltage limits .

[0028] The energy storage device can include: a first conductor; a second conductor, wherein the first conductor or both of the first and second conductors include the nanotube disclosed; and a non-conductive region located between the first and second conductors. If the nanotube disclosed is not used as both the first and second conductor, then the second conductor can be composed of aluminum, tantalum, silver, brass, gold, platinum, conductive carbon, or a mixture of the foregoing, as non-limiting examples. The non-conductive region can include vacuum or a dielectric. The dielectric can be selected from ceramics, glass, mica, air, paper, or a conductor depleted region. In some arrangements, an electrolyte may also be present .

[0029] The capacitor or supercapacitor can be prepared as cylindrical, flat sheet, film, double layer, disc, or tubular in shape. The disclosed embodiments can also be utilized in electrodes of hybrid energy storage devices that exhibit characteristics of both batteries and capacitors. An example of such devices is a lithium ion capacitor. The disclosed

embodiments may also be used in electrochemical double-layer capacitors as well.

[0030] Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is, therefore, evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention.

[0031] As used herein, the words "comprise," "have," "include, " and all grammatical variations thereof are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps. While compositions, systems, and methods are described in terms of "comprising, " "containing, " or "including" various components or steps, the compositions, systems, and methods also can "consist essentially of" or "consist of" the various components and steps. It should also be understood that, as used herein, "first," "second," and "third, " are assigned arbitrarily and are merely intended to differentiate between two or more conductors, etc., as the case may be, and does not indicate any sequence. Furthermore, it is to be understood that the mere use of the word "first" does not require that there be any "second, " and the mere use of the word "second" does not require that there be any "third, " etc. [0032] Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In

particular, every range of values (of the form, "from about a to about b, " or, equivalently, "from approximately a to b, " or, equivalently, "from approximately a - b") disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles "a" or "an, " as used in the claims, are defined herein to mean one or more than one of the element that it introduces. If there is any conflict in the usages of a word or term in this specification and one or more patent (s) or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted.