Composite comprising CNT fibres and an ionic conducting compound as part of an energy storage device

Abstract

The present invention relates to Composite comprising CNT fibres and an ionic conducting compound forming a homogeneous continuous phase or a two-phase bicontinuous structure and its process of obtainment by impregnation methods. Furthermore the invention relates to its use as part of an energy storage device such as an structural flexible electrochemical capacitor.

Claims

1. A composite, comprising: carbon nanotube fibres comprising a metal oxide, a transition-metal phosphate, a metal nitride, a conducting polymer or a combination thereof, an ionic conducting compound selected from a list consisting of an ionic polymer, an ionic liquid, a non-ionic polymer or a combination thereof, wherein the ionic compound impregnates the carbon nanotube fibres and wherein the ionic conducting compound forms a homogeneous continuous phase or a two-phase bicontinuous structure.

2. The composite of claim 1, wherein the carbon nanotube fibres are formed by less than 20 layers of carbon nanotubes and have a specific surface area larger than 100 m.sup.2/g and electrical conductivity above 10.sup.4 S/m and an aspect ratio greater than 10.

3. The composite of claim 1, wherein the metal oxide included in the carbon nanotube fibres is selected from the list consisting of Bi.sub.2O.sub.3, MnO.sub.2, RuO.sub.2, Fe.sub.2O.sub.3, Co.sub.3O.sub.4 and NiCoMnO.sub.4.

4. The composite of claim 1, wherein the metal oxide included in the carbon nanotube fibres adopts perovskite structure and has the chemical formula ABO.sub.3, where A and B are two metal cations of different sizes, and O is an oxygen anion that bonds to both.

5. The composite of claim 1, wherein the metal oxide included in the carbon nanotube fibres are selected from the list consisting of LaMnO.sub.3, LaNiO.sub.3 and SrRuO.sub.3.

6. The composite of claim 1, wherein the transition-metal phosphate is selected from the list consisting of LiFePO.sub.4, NiPO and Co.sub.3(PO.sub.4).sub.2.

7. The composite of claim 1, wherein the metal nitride is selected from the list consisting of VN, MoN and TiN.

8. The composite of claim 1, wherein the conducting polymer included in the carbon nanotube fibres is selected from the list consisting of polyacetylene, polypyrrole, polyaniline, poly(thiophene), poly(3,4-ethylenedioxythiophene), poly(pyrrole), polycarbazol, polyindol, polyazepine and a combination thereof.

9. The composite of claim 1, wherein the ionic conducting compound is a combination of: the non-ionic polymer said non-ionic polymer being a thermoplastic, thermoset or elastomer, and the ionic polymer selected from a polycation or a polyanion with their corresponding counterions; the ionic conducting compound forming the two-phase bicontinuous structure.

10. The composite of claim 9, wherein the thermoplastic is selected from the list consisting of poly(vinylene fluoride), poly(vinylidene fluoride-hexafluoropropylene), Polyacrylonitrile, Polyvinyl alcohol and Polyethylene oxide.

11. The composite of claim 9, wherein the thermoset is selected from the list consisting of Phenolics, Epoxies, Aminos Urea-formaldehyde, Polyurethane, Polyesters, Vinyl esters, Polyimides, Silicones, Nitriles.

12. The composite of claim 9, wherein the elastomer is selected from the list consisting of natural rubber, polyisoprene, polychloroprene, polybutadiene, Styrene-butadiene rubber, Acrylonitrile-butadiene rubber, polychloroprene Neoprene, polyester, Polysulphide, polyurethane, silicone.

13. The composite of claim 9, wherein the polycation is selected from the list consisting of poly(diallyldimethylammonium), poly(imidazolium), poly(sulphonium), poly(phosphonium) poly(quaternary amonium) and poly(piridinium).

14. The composite of claim 9, wherein the polyanion is selected from the list consisting of polysaccharides, polysulphates, polyphosphates and polycarboxylates.

15. The composite of claim 1, wherein the ionic conducting compound is a combination of an ionic polymer said ionic polymer selected from a polycation or a polyanion with their corresponding counterions, and an ionic liquid, the ionic conducting compound forming the two-phase bicontinuous structure.

16. The composite of claim 15, wherein the ionic liquid is selected from the list consisting of N-methyl-N-alkylpyrrolidinium bis ((trifluoromethyl)sulfonyl)-imide, N-methyl-N-alkylpyrrolidinium 15 bis(fluorosulfonyl)imide, 1-alkyl-3-methylimidazolium bis ((trifluoromethyl)sulfonyl)-imide and 1-alkyl-3-methylimidazolium bis(fluorosulfonyl)imide.

17. The composite of claim 1, wherein the ionic conducting compound is a combination of the non-ionic polymer said non-ionic polymer being thermoplastic, thermoset or elastomer, and the ionic liquid, the ionic conducting compound forming the two-phase bicontinuous structure.

18. The composite of claim 1, wherein the ionic conducting compound of the composite forms a homogeneous continuous phase and said ionic conducting compound comprises at least the ionic polymer.

19. The composite of claim 1, wherein the ionic conducting compound is forming a homogeneous continuous phase and said ionic conducting compound is a combination of the non-ionic polymer and the ionic liquid.

20. The composite of claim 1, wherein the composite is laminated.

21. A process of obtaining the composite of claim 1, comprising at least one of: performing a solution impregnation method selected from the list consisting of deep coating, spin coating, casting, spray coating, brushing and infusion; or performing a non-solution impregnation method selected from the list consisting of hot pressing and infusion.

22. The process of claim 21, comprising a step of drying after the solution impregnation method.

23. An energy storage device comprising the composite of claim 1.

24. A structural electrochemical capacitor, comprising: at least a pair of the composites of claim 1, the pair of composites linked by an electrical wire forming an electrical circuit, wherein the carbon nanotube fibres are working as electrodes and the ionic conducting compound as electrolyte.

25. The structural electrochemical capacitor of claim 24, wherein the pair of composites and a separator are sandwiched, wherein the separator is selected from a polymeric separator such as cellulosic, porous polypropylene and an inorganic separator such as ceramic membranes and glassy fibre membranes.

26. The structural electrochemical capacitor of claim 24, wherein the structural electrochemical capacitor is laminated.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 Scanning electron micrographs of a CNT fibre filament (b) and its porous structure (a)

(2) FIG. 2. Comparison figure of the cross-sections of a CF composite and a porous CNT fibre composite.

(3) FIG. 3 Electron micrographs of an example of half of a supercapacitor device (for clarity) consisting of an electrode of CNT fibre and continuous non-ionic polymeric porous phase. A) CNT fibre array with polymer, b) interface between the CNT fibre and the polymeric phase and c) top view showing the porous continuous polymer structure.

(4) FIG. 4 a) Schematic diagram of a typical supercapacitor based on liquid electrolyte, b) Schematic diagram of the supercapacitor of the invention

EXAMPLES

Example 1: Characterization of the CNT Fibers and the Laminate Composite of the Invention

(5) FIG. 1 presents an example of the fibre, which is a continuous macrosocopic filament. Because it is comprised of CNTs it has a high porosity (a) and a surface are of around 200 m.sup.2/g, that is, about 1000 times higher than CF. This material is also strong and has proven to be easily integrated in composites to produce very strong and light structures.

(6) FIG. 2 presents a schematic comparison of the cross-sections of a CF composite and a porous CNT fibre composite (present invention), respectively. The much larger specific surface of the CNT fibre results in a better charge storage system. A comparison of standard electrochemical tests of these materials give a specific capacitance of about 0.5-3F/g for CF and around 40F/g for the CNT fibres.

(7) This porosity not only increases overall charge storage performance, but also helps solve one of the main challenges in structural supercapacitors: simultaneous charge and load transfer through the polymer matrix. In a fibre composite structure the matrix that holds the fibres together is normally stiff in order to maximise transfers stress between fibres and therefore composite mechanical performance. In a supercapacitor the matrix requires a high ionic conductivity that enables charge transfer for its operation as a charge storage device. These two properties tend to be mutually exclusive, i.e., the stiffer the matrix the lower its ionic conductivity and vice versa.

(8) FIG. 3 shows electron micrographs of an example of half of a supercapacitor device (for clarity) consisting of an electrode of CNT fibre and continuous non-ionic polymeric porous phase. A) CNT fibre array with polymer, b) interface between the CNT fibre and the polymeric phase and c) top view showing the porous continuous polymer structure.

Example 2: Structural Electrochemical Capacitor by Solution Impregnation Method and Ionic Polymer

(9) A structural electrochemical capacitor comprising two laminate composites composed of CNT fibers, being the active material, impregnated with one ionic conducting compound acting as electrolyte by using a solution impregnating method. The ionic conducting compound is a binary blend of an ionic polymer, in particular a polycation, and a salt that can be combined at different mass ratios. The ionic polymer is poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (pDADMATFSI) and the salt is an ionic liquid named N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide, (PYR.sub.14TFSI). In the present example the ratio of polymer/salt is 40/60 in weight. This ratio was selected because it provided self-standing membranes with an optimum balance between mechanical stability and ionic conductivity (from 10.sup.7 to 10.sup.1 S/cm in the temperature range of 30 C. to 120 C.).

(10) In this example CNT fiber electrodes have circular shape with 1 cm diameter (area 0.7854 cm.sup.2). Impregnation of CNT fibers was performed by using casting impregnation method consisting on adding a diluted acetone solution of ionic conducting compound into CNT fibers electrodes using a micropipette and subsequent drying at 60 C. under vacuum overnight.

(11) The ionic conducting compound solution was easily absorbed by the CNT fiber electrode and hence the porous structure of CNT fiber could be filled with ionic conducting compound. Moreover, an external thin layer of ionic conducting compound layer is formed on top of the CNT fiber electrodes. In this example, this layer will serve both as solid electrolyte and as separator so there is no need to add further physical separator in between two electrodes.

(12) After solidification by drying the solvent, structural supercapacitors were assembled by stacking two electrodes with their surfaces impregnated by ionic compound in contact, as shown in FIG. 4. In this example, structural supercapacitors were pressed by using an electric precision rolling press in order to reduce the internal resistance of the structural supercapacitor.

(13) FIG. 4 shows a schematic view of the structural capacitor of the invention (b) in comparison with a typical supercapacitor construction (a). The structural capacitor of the present invention is comprised of two laminate composites composed of CNT fibers impregnated with one ionic conducting compound. An external thin layer of ionic conducting compound layer is formed on top of the CNT fiber electrodes and is represented as the intermediate layer in between the two faced CNT fiber electrodes in FIG. 4. The CNT fibers act both as active material and current collector whereas the ionic conducting compound is acting as electrolyte.

(14) Electrochemical properties of the exemplary structural electrochemical capacitor were tested by galvanostatic charge-discharge (CD) using a multichannel Bio-Logic VMP3. CD experiments were conducted from 0 to 3.5 V at different current densities: 10, 5, 2 and 1 mAcm.sup.2 and the following parameters were obtained and included in Table 1; specific capacitance of capacitor (C.sub.sc), specific capacitance of individual CNT fiber electrode (C.sub.am), equivalent series resistance (ESR), real specific energy (E.sub.real), maximum specific energy (E.sub.max), specific average power (P.sub.av), maximum specific power (P.sub.max), coulombic efficiency ().

(15) TABLE-US-00001 TABLE 1 Electrochemical properties of structural electrochemical capacitor prepared by solution impregnation method Current density C.sub.sc C.sub.am ESR E.sub.real E.sub.max P.sub.av P.sub.max (mAcm.sup.2) (Fg.sup.1) (Fg.sup.1) (cm.sup.2) (Whkg.sup.1) (Whkg.sup.1) (kWkg.sup.1) (kWkg.sup.1) 1 12 50 81 17 21 3.4 85 97 2 11 45 81 14 19 7 85 96 5 9 37 82 9 16 15 83 91 10 7 30 90 3 13 19 76 85

(16) Maximum specific energy (E.sub.max=0.5C.sub.SCV.sup.2) and maximum specific power (P.sub.max=V.sup.2/4.Math.ESR) are the two key parameters to define the behavior of any electrochemical energy storage device. In this example, structural capacitor shows high values of E.sub.max going from 13 to 21 Whke.sup.1 and P.sub.max going from 76 to 85 kWkg.sup.1. Both parameters are normalized to the mass of active material, that tis, to the mass of the two bare CNT fiber electrodes. Excellent electrochemical performance of this exemplary structural electrochemical capacitor is due to both the high surface area of the CNT fibers that result in high values of capacitance (C.sub.SC) and the wide electrochemical stability window of ionic conducting compound that allow the structural electrochemical capacitor to be charged up to a high operating voltage (V) of 3.5 V.

Example 3: Structural Electrochemical Capacitor by Non-Solution Impregnation Method and Non-Ionic Polymer

(17) A structural electrochemical capacitor comprising two laminate composites composed of CNT fibers, being the active material, impregnated with one ionic conducting compound acting as electrolyte by using a non-solution impregnation method. The ionic conducting compound is a binary blend of a non-ionic polymer and a salt that can be combined at different mass rations. The non-ionic polymer is poly(vinylidene fluoride) (average Mw 534,000,) and the salt is an ionic liquid named N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide, (PYR.sub.14TFSI). In the present example the ratio of polymer/salt is 40/60 in weight. The ratio was chosen due to possibility to obtain a self-standing membrane with an optimum balance between mechanical stability and ionic conductivity (from 10.sup.7 to 10.sup.1 S/cm in the temperature range of 30 C. to 120 C.).

(18) In this example CNT fiber electrodes have circular shape with 1 cm diameter (area 0.7854 cm.sup.2) and the structural supercapacitors were assembled by sandwiching one ionic conducting membrane (1.2 cm diameter) between two CNT fibers electrodes and applying pressure in order to improve CNT fiber impregnation with ionic conducting compound. The ionic conducting membrane was obtained by casting a solution of the ionic conducting mixture in acetone. The membrane serves both as solid electrolyte and as separator, eliminating the need to use an additional physical separator between the two electrodes.

(19) Electrochemical characteristics of the structural supercapacitor are tested by galvanostatic charge-discharge (CD) using a multichannel Bio-Logic VMP3. CD experiments were conducted from 0 to 3.5 V at different current densities: 50, 20, 10, 5, 2 and 1 mAcm.sup.2 and the following parameters were obtained and included in Table 2; specific capacitance of capacitor (C.sub.sc), specific capacitance of individual CNT fiber electrode (C.sub.am), equivalent series resistance (ESR), real specific energy (E.sub.real), maximum specific energy (E.sub.max), specific average power (P.sub.ay), maximum specific power (P.sub.max), coulombic efficiency ().

(20) TABLE-US-00002 TABLE 2 Electrochemical properties of structural electrochemical capacitor prepared by non-solution impregnation method. Current density C.sub.sc C.sub.am ESR E.sub.real E.sub.max P.sub.av P.sub.max (mAcm.sup.2) (Fg.sup.1) (Fg.sup.1) (cm.sup.2) (Whkg.sup.1) (Whkg.sup.1) (kWkg.sup.1) (kWkg.sup.1) 1 10 41 15 17 17 5 401 88 2 10 41 23 16 17 7 258 93 5 9 36 20 14 15 18 300 97 10 8 32 19 11 14 33 308 98 20 7 29 19 7 12 57 325 98 50 5 22 18 2 9 79 328 98

(21) In this example, the structural capacitor shows high values of E.sub.max going from 9 to 17 Whkg.sup.1 and P.sub.max going from 300 to 400 kWkg.sup.1. Both parameters are normalized to the mass of active material, that is, to the mass of the two bare CNT fiber electrodes. Excellent electrochemical performance of this exemplary structural electrochemical capacitor is due to both the high surface area of the CNT fibers that result in high values of capacitance (C.sub.SC) and the wide electrochemical stability window of ionic conducting compound that allow the structural electrochemical capacitor to be charged up to a high operating voltage (V) of 3.5 V. Moreover, the low values of ESR that resulted in very high values of P.sub.max are due to the minimal distance between two CNT fiber electrodes in this particular example.