Conductive fibrous materials

Abstract

There is provided a conductive fibrous material comprising a plurality of carbonaceous fibers, wherein each carbonaceous fiber is fused to at least one other fiber. The carbonaceous fibers may be fused at fiber-to-fiber contact points by a polymer. The process of making the conductive fibrous material comprises mixing a phenolic polymer with a second polymer to form a polymer solution, preparing phenolic fibers having nano- or micro-scale diameters by electrospinning the polymer solution, and subsequent carbonization of the obtained phenolic fibers, thereby generating carbonaceous fibers, wherein each carbonaceous fiber is fused to at least one other fiber. The conductive fibrous material may be useful in electrode materials for energy storage devices.

Claims

1. A conductive fibrous material comprising a plurality of carbonaceous fibers, wherein the carbonaceous fibers have a diameter of about 0.1 m to about 15 m and each carbonaceous fiber is fused to at least one other fiber, wherein the carbonaceous fibers are fused at fiber-to-fiber contact points by a low-melting point polymer selected from the group consisting of homo-polyether, co-polyether, co-polyether-polyester, and polymer blends thereof, wherein the weight ratio between a carbonaceous fiber precursor to polymer is from 99:1 to 80:20, and wherein the carbonaceous fibers are electrospun.

2. The material according to claim 1, wherein a major portion of the carbonaceous fibers are fused at multiple fiber-to-fiber contact points.

3. The material according to claim 2, wherein the polymer has a melting point of below 200 C.

4. The material according to claim 2, wherein the polymer has a molecular weight from about 100,000 to 1,000,000.

5. The material according to claim 1, wherein the polymer is selected from the group consisting of polyethylene oxide, polypropylene oxide, and poloxamer 407, preferably wherein the homo- or co-polyether is selected from the group consisting of homo-(polyalkylene oxide), co-(polyalkylene oxide), and poloxamer.

6. The material according to claim 1, wherein the carbonaceous fiber precursor is a phenolic fiber, preferably wherein the carbonaceous fiber precursor is derived from lignin, and preferably wherein the lignin is selected from the group consisting of: organosolv lignin, softwood kraft lignin, hardwood kraft lignin and lignosulfonate.

7. The material according to claim 1, wherein said material is doped with nitrogen, preferably wherein the content of nitrogen is in the range of 0.1% to 0%.

8. The material according to claim 1, wherein the carbonaceous fibers have nano- or micro-scale diameters.

9. A process for forming a conductive fibrous material according to claim 1, comprising: (a) mixing a phenolic polymer with a second polymer to form a polymer solution, wherein the weight ratio of phenolic polymer to second polymer is selected as being from about 80 to 99 weight percent of phenolic polymer to about 1 to 20 weight percent of second polymer; (b) preparing phenolic fibers having nano- or micro-scale diameters by electrospinning the polymer solution of step a); (c) carbonizing the phenolic fibers obtained through step b), thereby generating carbonaceous fibers, wherein each carbonaceous fiber is fused to at least one other fiber, wherein the second polymer is a low melting point polymer selected from the group consisting of homo-polyether, co-polyether, co-polyether-polyester, and polymer blends thereof.

10. The process according to claim 9, further comprising stabilizing the phenolic fibers prior to carbonization, preferably wherein the stabilizing comprises heat treating and oxidizing the phenolic fibers.

11. The process according to claim 9, wherein the carbonizing comprises heat treating the phenolic fibers in an inert atmosphere.

12. The process according to claim 9, comprising the step of selecting the phenolic polymer as being derived from lignin, preferably wherein the lignin is selected from the group consisting of: organosolv lignin, softwood kraft lignin, hardwood kraft lignin and lignosulfonate.

13. The process according to claim 9, comprising the step of selecting the second polymer as having a melting point below 200 C.

14. The process according to claim 9, wherein the second polymer has a molecular weight from about 100,000 to 1,000,000.

15. The process according to claim 9, wherein the second polymer is selected from the group consisting of polyethylene oxide, polypropylene oxide, and poloxamer 407, preferably wherein the homo- or co-polyether is selected from the group consisting of homo-(polyalkylene oxide), co-(polyalkylene oxide), and poloxamer.

16. The process according to claim 9, wherein the polymer solution of step a) further comprises a polar solvent, preferably wherein the polar solvent is selected from the group consisting of acetone, acetonitrile, N,N-dimethylformamide (DMF), tetrahydrofuran (THF), ethyl acetate (EtOAc), formamide, dimethyl sulfoxide (DMSO), acetamide water, ethanol, and methanol.

17. The process according to claim 9, further comprising the step of doping the carbonaceous fibers with nitrogen.

18. An electronic device comprising the conductive fibrous material of claim 1, a fibrous mat comprising the material according to claim 1, or an electrode material for energy storage devices comprising the material of claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

(2) FIGS. 1a and 1b show the Scanning Electron Microscope (SEM) images of lignin-derived electrospun nanofibers prior to stabilization and carbonization.

(3) FIG. 1a shows the SEM image of lignin-derived electrospun nanofibers wherein the weight ratio of lignin:PEO is 97:3. Diameters of the fibers are around 1 m.

(4) FIG. 1b shows the SEM image of lignin-derived electrospun nanofibers wherein the weight ratio of lignin:PEO is 90:10. Diameters of the fibers are around 1 m.

(5) FIGS. 2a and 2b show the SEM images of lignin-derived electrospun nanofibers after being subjected to stabilization and carbonization.

(6) FIG. 2a shows the SEM image of lignin-derived carbonaceous nanofibers wherein the weight ratio of lignin:PEO is 97:3. Diameters of the fibers are around 500 nm.

(7) FIG. 2b shows the SEM image of lignin-derived carbonaceous nanofibers wherein the weight ratio of lignin:PEO is 90:10. Diameters of the fibers are around 500 nm.

(8) FIG. 3 depicts the disclosed conductive fibrous material comprising fused carbonaceous fibers in 2032 coin cells with lithium foil as the counter electrode.

(9) FIG. 4a shows the survey X-ray photoelectron spectra (XPS) of nitrogen-doped carbonaceous fibers.

(10) FIG. 4b shows the C1s XPS of nitrogen-doped carbonaceous fibers.

(11) FIG. 4c shows the N1s XPS of nitrogen-doped carbonaceous fibers.

(12) FIG. 5 shows the Raman spectra of lignin-derived carbonaceous fibers.

(13) FIG. 6 shows the Electrochemical Impedance Spectroscopy (EIS) of lignin-derived non-fused electrospun carbonaceous fibers, fused electrospun carbonaceous fibers and N-doped fused electrospun carbonaceous fibers.

(14) FIG. 7 shows the cyclic voltammogram of N-doped fused electrospun carbonaceous fibers. The voltage window and scanning rate are 3.0-0.0 V and 0.01 mV s.sup.1, respectively.

(15) FIG. 8 shows the initial discharge/charge curves of carbon samples at a current rate of 30 mA g.sup.1.

(16) FIG. 9a shows the specific capacitances of lignin-derived carbonaceous fibers, lignin-derived nitrogen-doped carbonaceous fibers, PAN-derived carbonaceous fibers and graphite at various current densities.

(17) FIG. 9b shows the specific capacitances of lignin-derived carbonaceous fibers and lignin-derived nitrogen-doped carbonaceous fibers at various current densities.

(18) FIG. 10 shows the cycling stability at a current density of 372 mA g.sup.1 (1C) of nitrogen-doped carbonaceous fibers.

(19) FIG. 11 shows a proposed structure of lignin.

(20) FIG. 12 shows a schematic diagram of the electrospinning process.

EXAMPLES

(21) Non-limiting examples of the invention and a comparative example will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

(22) Materials

(23) Lithium foil and 1M LiPF.sub.6 in ethylene carbonate/dimethyl carbonate (1:1 v/v ratio) were used as the working electrode, counter electrode and electrolyte, respectively. All were assembled into 2032 button cells in an argon-filled glove box with moisture and oxygen levels of less than 1 ppm. The galvanostatic charge/discharge tests were performed using a NEWARE battery tester at different current densities with, a cutoff voltage window of 0.005-3.0 V. The cyclic voltammetry test was performed on an electrochemical workstation (PGSTAT302, Autolab) within a voltage window of 3.0-0 V and at a scan rate of 0.01 mV s.sup.1. The electrochemical impedance spectroscopy (EIS) study was conducted using the same electrochemical workstation in a frequency range of 106 to 10.sup.2 Hz and at an a.c. amplitude of 5 mV.

(24) The morphologies of the nanofibers were observed under a field-emission scanning electron microscope (FESEM, JEOL JSM 6700) at an accelerating voltage of 5 kV. X-ray photoelectron spectra (XPS) were obtained on a VG ESCA LAB-220i XL X-ray photoelectron spectrometer with an exciting source of AI. Elemental analysis was performed on elemental analyze (EA, flash 1112 series) and raman spetra was recorded on micro raman system.

Example 1: General Synthesis

(25) Conductive Fibrous Material Comprising Fused Carbonaceous Fibers

(26) The disclosed free-standing N-doped fused carbonaceous nanofibers may be fabricated via electrospinning solutions of lignin and polyethylene oxide (PEO) in dimethylformamide (DMF) followed by thermal stabilization and subsequent carbonization with urea.

(27) The weight ratio of lignin to PEO can be tuned from 99:1 to 80:20 via varying the concentration of the polymer solution.

(28) FIGS. 1a and 1b show SEM images of the as-spun nanofibers with diameters around 1 m prior to stabilization and carbonization. The as-spun fibers were stabilized in air by heating from room temperature to 200 C. at 1 C./min and subsequently carbonized under argon at different temperatures from 500 to 1000 C. It was observed that high proportions of PEO could result in fusing together of adjacent carbonaceous fibers (as shown in FIG. 2), which could be ascribed to the strong melting behavior of PEO during heat treatment. FIGS. 2a. and 2b show SEM images of the carbonaceous fibers after stabilization and carbonization with diameters of around 0.5 m. As the polymer and lignin-derived fibers decompose during the carbonization process, the diameters of the resulting carbonaceous fibers are smaller with respect to their diameters prior to carbonization.

(29) N-Doped Conductive Fibrous Material

(30) To further improve electrochemical performance, the disclosed carbonaceous fiber mats were vacuum impregnated with urea aqueous solution, which were then subjected to high temperature treatment under argon to realize high levels of nitrogen doping. The content of nitrogen can be tuned by varying the concentration of the urea solution and could reach as high as 12.6% as characterized by elemental analysis. The resulting carbonaceous fiber mats remain free-standing and tough enough to be punched to electrode materials.

Example 2: Preparation of Fused Electrospun Carbonaceous Fiber Mats from Lignin

(31) 436 mg Alcell lignin and 48 mg polyethylene oxide (Mw 600K) were dispersed into 2 mL N,N-dimethylformamide (DMF) under magnetic stirring and the suspension was heated at 60 C. for 6 h. After cooling down to room temperature naturally under continuous stirring, the solution was placed in a 1 mL plastic syringe fitted with a flap tip 22 G needle and was electrospun using a horizontal electrospinning setup. Typically, electrospinning was performed at 6.5-7.0 kV with a feeding rate of 1 mL/h and the needle tip-to-plate substrate distance was 10 cm. The nanofibers were collected on aluminum foil and dried at 70 C. under vacuum overnight. The dried nanofibers were thermostabilized in a tube furnace under atmospheric environment. The temperature was ramped from 25 C. to 200 C. at 1 C. min.sup.1 and kept at 200 C. for 2 h. The stabilized fibers were then heated from 200 C. to 900 C. at 10 C. min.sup.1 under a flow of argon (150 cm.sup.3 STP/min) and carbonized at 900 C. for 2 h.

Example 3: N-Doping of the Carbonaceous Fiber Mats

(32) 20 mg lignin-derived carbonaceous fiber mats (carbonized at 500 C. for 2 h) were immersed in 20 mL 10% urea aqueous solution and then dried under vacuum for several hours. The resulting mixture was heated from 25 C. to 900 C. at 5 C. min.sup.1 and kept at 900 C. for 2 h. After cooling down to room temperature, the obtained N-doped carbonaceous fiber mats were washed with deionized water and dried at 60 C. under vacuum overnight.

Example 4: X-Ray Photoelectron Spectroscopy (XPS) Analysis

(33) X-ray photoelectron spectroscopy (XPS) analysis was conducted to study the elemental composition and chemical states of the elements that exist in the N-doped fused electrospun carbonaceous fibers. FIG. 4a displays the survey spectrum (0-1000 eV) of the sample, which includes C1s, N1s and O1s without any other impurities. The C1s spectrum ranging from 283 to 291 eV is shown in FIG. 4b, which after peak fitting could be divided into three obvious peaks successfully. The binding energy at 284.6 eV may be attributed to graphitic carbon, which constitutes almost half of the carbon material. The peak centered at 285.4 eV corresponds to CC/CH and the peak at 288.1 eV could be assignable to small amounts of NCN group. The N1s spectrum in FIG. 4c can be fitted into three obvious peaks locating at 398.4 eV, 399.5 eV, and 401.0 eV, which indicate proportions of pyridinic nitrogen, pyrrolic nitrogen and graphitic nitrogen, respectively. The peak intensity ratio of D-band (1350 cm.sup.1, amorphous carbon) to G-band (1580 cm.sup.1 graphitic carbon) in the Raman spectra (FIG. 5) further demonstrates the proportions of graphitic structure in the lignin-derived carbon fibers.

Example 5: Electrochemical Performance Testing and Structure Characterization

(34) The electrochemical properties of the lignin-derived carbonaceous fibrous mats as anode material for LIBs are evaluated through assembling 2032 coin cells with lithium foil as the counter electrode. The electrochemical impedance spectroscopy (EIS) analysis is shown in FIG. 6, which shows the electrical resistance of the carbon samples. The fused carbonaceous fiber mats exhibit very low interfacial resistance as well as charge transfer resistance, much lower than that of the non-fused ones. The conductivity may be further improved by doping nitrogen into the carbonaceous fibers. Cyclic voltammograms are performed to study the reaction mechanism of the N-doped carbonaceous fibers (as shown in FIG. 7). The intercalation of lithium can be observed below 1.2 V and the extraction of lithium from carbon occurred at low potential with a broad peak extending to 1.5 V. These results are typical for carbon materials as anode in lithium-ion batteries. The initial charge-discharge curves at a current density of 30 mA g.sup.1 are depicted in FIG. 8. Compared to the lignin-derived pure carbonaceous fibers which have a good capacity above 400 mA g.sup.1, the carbonaceous fibers with nitrogen doping exhibit much higher capacity at 576 mA g.sup.1.

Comparative Example 1

(35) The specific capacitances of the presently disclosed lignin-derived carbonaceous fibers and presently disclosed lignin-derived N-doped carbonaceous fibers were compared with PAN-derived carbonaceous fibers and graphite at various current rates.

(36) FIG. 9 shows that the lignin-derived fused carbonaceous fibers without nitrogen doping exhibit similar performance as the PAN-derived carbonaceous fibers. However, the rate capacities of the nitrogen-doped carbonaceous fibers far exceeded those of the PAN-derived carbonaceous fibers and even at a very high current density of 2 A g.sup.1, a satisfying capacity value of 183 mA h g.sup.1 was still achieved with the N-doped carbonaceous fibers. Moreover, the N-doped carbonaceous fibers also showed good cyclic stability at a current rate of 1 C (FIG. 10).

APPLICATIONS

(37) The disclosed conductive fibrous material and the process of making the same may be used in an electrode for a lithium-ion battery in a variety of applications including, electronic devices, such as computers and various hand-held devices, motor vehicles, power tools, photographic equipment, and telecommunication devices.

(38) The disclosed conductive fibrous material may be fabricated from low-cost, abundant and renewable feedstock which advantageously leads to reduced costs, easy accessability and processability.

(39) The disclosed conductive fibrous material may comprise carbonaceous fibers that are fused to each other which advantageously leads to high conductivity, specific surface area and enhanced electrochemical performances when used as an electrode material in lithium-ion batteries.

(40) The disclosed conductive fibrous material may further be doped with nitrogen which advantageously leads to further improvised specific capacities and electrical conductivity when used in an electrode material for lithium-ion batteries.

(41) The disclosed conductive fibrous material may be made by a simple and straightforward fabrication process which can be conducted on a large-scale.

(42) It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.