DUAL FIBER ELECTRODE MATS FOR BATTERIES AND APPLICATIONS OF SAME

Abstract

A dual fiber mat for making an electrode includes first nanofibers and second nanofibers. The first fibers contain particles for electrochemical reaction and a binder. The second fibers contain particles for electron conduction and a binder. For a Li-ion battery anode, the first fibers include a polymer binder composed of an electron conducting polyfluorene derivative polymer (PFM or PEFM) or PVDF or PAA and silicon nanoparticles or silicon nanorods embedded in the binder. For a Li-ion battery cathode, the first fibers include a binder composed of an electron conducting polymer (PFM or PEFM) or PAA or PVDF and LiCoO2 or LiFePO4 or Li2MnO3 particles embedded in the binder. The second nanofibers include a PFM or PEFM binder or non-conductive polymer binder and electrically conductive nanoparticles embedded in the binder. The dual fiber mat has a thickness in a range of about 50-1000 m.

Claims

1. A multiple fiber mat for making an electrode, comprising: a first type of nanofibers comprising an electrically conductive nanoparticles embedded in a polymer binder; and one or more types of nanofibers comprising one or more electrochemically active nanoparticles with one or more polymer binders, where the one or more types of nanofibers and the first type of nanofiber are distinguishable in terms of particle/polymer compositions.

2. The multiple fiber mat of claim 1, wherein the multiple fiber mat has a thickness of about 5-1000 m.

3. The multiple fiber mat of claim 1, wherein the multiple fiber mat is a dual fiber mat composed of two different types of fibers, and the dual fiber mat comprises: the first type of type of nanofibers, comprising a polyfluorene derivative polymer (PFM or PEFM) and silicon nanoparticles embedded in the PFM or PEFM; and a second type of nanofibers, comprising a non-conductive polymer binder and electrically conductive nanoparticles embedded in the non-conductive polymer binder.

4. The dual fiber mat of claim 3, wherein the dual fiber mat has a thickness in a range of about 5-1000 m.

5. The dual fiber mat of claim 3, wherein the electrically conductive nanoparticles comprises at least one of carbon nanoparticles and copper nanoparticles.

6. The dual fiber mat of claim 3, wherein the non-conductive polymer binder comprises at least one of polyacrylic acid (PAA), carboxy methyl cellulose, and polyvinylidene fluoride (PVDF).

7. The dual fiber mat of claim 3, wherein the first type of nanofibers and the second type of nanofibers are distributed evenly in the fiber mat, such that the second type of nanofibers form fiber-fiber contact with the first type of nanofibers, and provide numerous node points and pathways for electrons to pass to/from the silicon nanoparticles or the PFM to a metal plate of the electrode.

8. The dual fiber mat of claim 3, wherein the first type of nanofibers have an average diameter of less than about 1 m.

9. The dual fiber mat of claim 3, comprising about 50-80% of the first type of nanofibers and 20-50% of the second type of nanofibers.

10. The dual fiber mat of claim 3, wherein the second type of nanofibers comprises about 30-80% of the electrically conductive nanoparticles.

11. A dual fiber mat, comprising: a first type of fibers having a first polymer and a first particle material; and a second type of fibers having a second polymer and a second particle material.

12. The dual fiber mat of claim 11, wherein the first particle material comprises first nanoparticles or first nanorods.

13. The dual fiber mat of claim 12, wherein the first particle material comprises silicon nanoparticles or silicon nanorods or TiO.sub.2 nanoparticles.

14. The dual fiber mat of claim 12, wherein the first particle material comprises LiCoO.sub.2, LiFeO.sub.2, sulfur-loaded carbon particles, or Li.sub.2MnO.sub.3, and other spinel and olivine structured materials

15. The dual fiber mat of claim 11, wherein each of the first polymer and the second polymer comprises a polyfluorene derivative polymer (PFM or PEFM).

16. The dual fiber mat of claim 11, wherein the second particle material comprises electrically conductive particles.

17. The dual fiber mat of claim 16, wherein the second particle material comprises carbon nanoparticles or copper nanoparticles.

18. The dual fiber mat of claim 11, wherein the first and second polymer has a formula of: ##STR00007## where each of X and Y is selected from H, OH, COOH, and a halide.

19. The dual fiber mat of claim 18, wherein the second polymer comprises at least one of PAA and PVDF.

20. The dual fiber mat of claim 11, wherein the first type of fibers and the second type of fibers are distributed evenly in the fiber mat, such that the second type of fibers form fiber-fiber contact with the first type of fibers, and provide numerous node points and pathways for electrons to pass to/from the first nanoparticles or the first polymer to a metal plate of the electrode.

21. The dual fiber mat of claim 11, wherein a thickness of the dual fiber mat is about 50-1000 m, and a diameter of the first type of fiber is less than 1 m.

22. The dual fiber mat of claim 11, wherein the dual fiber mat comprises about 50-80% of the first type of fibers and about 20-50% of the second type of fibers, and the second type of fibers comprises about 30-80% of the second particles.

23. A multiple fiber mat electrode comprising a multiple fiber mat, wherein the multiple finer mat has two or more types of fibers, containing different particles and/or polymer binders.

24. The multiple fiber mat electrode of claim 23, wherein the electrode is used in an electrochemical device or process.

25. The multiple fiber mat electrode of claim 24, wherein the electrochemical device comprises at least one of a battery, a fuel cell, a water electrolyzer, an electrochemical reactor and a sensor.

26. A method of manufacturing a dual fiber mat for an electrode, comprising: providing a first solution having a first polymer and a first particle material, and a second solution having a second polymer and a second particle material; and co-spinning the first solution and the second solution to respectively form first fibers and second type of fibers, so as to form the dual fiber mat.

27. The method of claim 26, wherein the first particle material comprises silicon nanoparticles or silicon nanorods or TiO.sub.2 nanoparticles, and the first polymer comprises PFM or PEFM or PAA or PVDF.

28. The method of claim 26, wherein the first particle material comprises LiCoO.sub.2, LiFeO.sub.2, sulfur-loaded carbon particles, Li.sub.2MnO.sub.3, or another spinel or olivine material and the first polymer comprises PFM or PEFM or PAA or PVDF.

29. The method of claim 26, wherein the second particle material comprises electrically conductive particles.

30. The method of claim 29, wherein the electrically conductive particles comprises carbon nanoparticles or copper nanoparticles.

31. The method of claim 26, wherein the second polymer comprises polyfluorene derivative polymer (PFM or PEFM), PAA, or PVDF.

32. The method of claim 26, wherein the dual fiber mat has a thickness of about 50-1000 m.

33. The method of claim 26, wherein a diameter of the first type of fiber is less than 1 m.

34. The method of claim 26, wherein the dual fiber mat comprises about 50-80% of the first type of fibers and about 20-50% of the second type of fibers, and the second type of fiber comprises about 30-80% of the second particles.

35. The method of claim 26, wherein the first type of fibers are silicon/PFM or silicon/PEFM fibers, and the second type of fibers are carbon/PVDF fibers.

36. The method of claim 26, wherein the first type of fibers are silicon/PAA fibers, and the second type of fibers are carbon/PAA fibers.

37. The method of claim 26, further comprising providing a third, fourth, or fifth solution having a third, fourth, or fifth particle material and a third, fourth, or fifth polymer, wherein the third, fourth, or fifth solution is co-spun with the first solution and the second solution to form third, fourth, or fifth fibers.

38. The method of claim 37, where there are three different fibers, the third particle material comprises at least one of silicon nanoparticles, silicon nanorods, carbon nanoparticles, and copper nanoparticles, and the third polymer comprises at least one of PFM, PEFM, PAA, and PVDF.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The accompanying drawings illustrate one or more embodiments of the present invention and, together with the written description, serve to explain the principles of the invention. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like elements of an embodiment.

[0054] FIG. 1A is a schematic view of a dual fiber mat according to one embodiment of the present invention, where first type of fibers and second type of fibers are randomly distributed in the dual fiber mat.

[0055] FIG. 1B shows the first type of fiber in FIG. 1A.

[0056] FIG. 1C shows the second type of fiber of FIG. 1A.

[0057] FIG. 2A is a schematic view of a dual fiber mat according to one embodiment of the present invention, where first type of fibers and second type of fibers are distributed in the dual fiber mat with a pattern.

[0058] FIG. 2B shows the first type of fiber in FIG. 2A.

[0059] FIG. 2C shows the second type of fiber of FIG. 2A.

[0060] FIG. 3 shows a method of manufacturing a dual fiber mat according to one embodiment of the present invention.

[0061] FIG. 4 shows a scanning electron microscope (SEM) image of a compacted/welded electrospun dual fiber mat anode containing Si/PAA and C/PAA fibers in a weight ratio of 65/35 according to one embodiment of the present invention.

[0062] FIG. 5 shows Li-ion battery half-cell charge-discharge curves for the first and second cycles with a Li metal cathode and an electrospun dual fiber mat anode containing 50 wt % Si/PAA fibers and 50 wt. % C/PAA fibers, where the Si/PAA fibers contain 65 wt % Si particles and the C/PAA fibers contain 65 wt. % C particles according to one embodiment of the present invention.

[0063] FIG. 6 shows repeat unit structure of PEFM binder containing (P) polyfluorene with octyl side chains, (E) fluorine with trietyleneoxide monomethylether side chains, (F) fluorenone and (M) methyl benzoate ester subunits.

[0064] FIG. 7 shows structure of poly(9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic ester), that is PFM.

DETAILED DESCRIPTION OF THE INVENTION

[0065] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout.

[0066] The terms used in this specification generally have their ordinary meanings in the art, within the context of the invention, and in the specific context where each term is used. Certain terms that are used to describe the invention are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description of the invention. For convenience, certain terms may be highlighted, for example using italics and/or quotation marks. The use of highlighting and/or capital letters has no influence on the scope and meaning of a term; the scope and meaning of a term are the same, in the same context, whether or not it is highlighted and/or in capital letters. It will be appreciated that the same thing can be said in more than one way. Consequently, alternative language and synonyms may be used for any one or more of the terms discussed herein, nor is any special significance to be placed upon whether or not a term is elaborated or discussed herein. Synonyms for certain terms are provided. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification, including examples of any terms discussed herein, is illustrative only and in no way limits the scope and meaning of the invention or of any exemplified term. Likewise, the invention is not limited to various embodiments given in this specification.

[0067] It will be understood that when an element is referred to as being on another element, it can be directly on the other element or intervening elements may be present there between. In contrast, when an element is referred to as being directly on another element, there are no intervening elements present. As used herein, the term and/or includes any and all combinations of one or more of the associated listed items.

[0068] It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below can be termed a second element, component, region, layer or section without departing from the teachings of the present invention.

[0069] It will be understood that when an element is referred to as being on, attached to, connected to, coupled with, contacting, etc., another element, it can be directly on, attached to, connected to, coupled with or contacting the other element or intervening elements may also be present. In contrast, when an element is referred to as being, for example, directly on, directly attached to, directly connected to, directly coupled with or directly contacting another element, there are no intervening elements present. It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed adjacent to another feature may have portions that overlap or underlie the adjacent feature.

[0070] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms a, an, and the are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms comprises and/or comprising, or includes and/or including or has and/or having when used in this specification specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.

[0071] Furthermore, relative terms, such as lower or bottom and upper or top, may be used herein to describe one element's relationship to another element as illustrated in the figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation shown in the figures. For example, if the device in one of the figures is turned over, elements described as being on the lower side of other elements would then be oriented on the upper sides of the other elements. The exemplary term lower can, therefore, encompass both an orientation of lower and upper, depending on the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as below or beneath other elements would then be oriented above the other elements. The exemplary terms below or beneath can, therefore, encompass both an orientation of above and below.

[0072] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0073] As used herein, around, about, substantially or approximately shall generally mean within 20 percent, preferably within 10 percent, and more preferably within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the terms around, about, substantially or approximately can be inferred if not expressly stated.

[0074] As used herein, the terms comprise or comprising, include or including, carry or carrying, has/have or having, contain or containing, involve or involving and the like are to be understood to be open-ended, i.e., to mean including but not limited to.

[0075] As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the invention.

[0076] In one aspect, the present invention is related to a multiple fiber mat for manufacturing a battery electrode. The multiple fiber mat is composed of two or more different type of fibers. Each type of the fibers contains particles and polymer. The different types of fibers have different particles and/or polymer binders. In another aspect, the present invention relates to a process for making the multiple fiber mat. By preparing the fiber mat this way, the present invention allows for a thick battery electrode. For example, this invention allows for a thick Lithium (Li) battery electrode (high areal capacities) with a controlled void volume (for electrolyte penetration and high volumetric capacities) and short Li+ ion transport pathways in the radial fiber direction (for fast charge/discharge rates).

[0077] In certain embodiments, the multiple fiber mat of the invention can be applied to other battery systems, e.g., metal batteries including Na or magnesium batteries. In certain embodiments, the multiple fiber mat of the present invention may be used in high energy density metal-air batteries, e.g., Li-air batteries, and redox flow batteries.

[0078] FIG. 1A is a schematic view of a dual fiber mat (one type of multiple fiber mat) according to one embodiment of the present invention, where first type of fibers and second type of fibers are randomly distributed in the fiber mat. As shown in FIG. 1A, a dual fiber mat electrode 100 is formed from first type of fibers 110 and second type of fibers 130. The first type of fibers 110 and the second type of fibers 130 are randomly distributed in the fiber mat 100. In certain embodiments, the weight percentage of the first type of fibers 110 in the dual fiber mat 100 is about 50%-80, and the weight percentage of the second type of fibers 130 in the dual fiber mat 100 is about 20%-50%.

[0079] Referring to FIG. 1B, the first type of fibers 110 includes a first polymer 112 and first particles 114 attached to or embedded in the first polymer 112. In certain embodiments, the first polymer 112 is an electrically conductive binder, such as a polyfluorene derivative polymer (PFM or PEFM with/without a suitable carrier polymer), and the first particles 114 may be silicon (Si) nanoparticles or Si nanorods.

[0080] Referring to FIG. 1C, the second type of fibers 130 includes a second polymer 132 and second particles 134 attached to the second polymer 132. In certain embodiments, the second polymer 132 may be an electrically conductive binder, such as PFM, or a non-conductive polymer binder, such as PAA or PVDF; the second particles 134 may be electrically conductive particles, such as carbon or Cu nanoparticles.

[0081] In certain embodiments, the second polymer has the formula of:

##STR00002##

where each of X and Y is selected from the group consisting of H, OH, COOH, and halide such as F. In one example, X is H, Y is COOH, and the second polymer is PAA. In another example, both X and Y are F, and the second polymer is PVDF.

[0082] In certain embodiments, the first type of fibers 110 and the second type of fibers 130 may be co-spun to form the dual fiber mat 100. In the dual fiber mat 100, the second type of fibers 130 make fiber-fiber contact with the first type of fibers 110, thus providing numerous node points and pathways for electrons to pass to/from Si surface and or the PFM polymer to a metal plate current collector at the back of the electrode.

[0083] FIG. 2A is a schematic view of a fiber mat according to one embodiment of the present invention, where first type of fibers and second type of fibers are distributed in the dual fiber mat with a pattern. As shown in FIG. 2A, a dual fiber mat 200 is formed from first type of fibers 210 and second type of fibers 230. The first type of fibers 210 and the second type of fibers 230 are distributed in the fiber mat 200 with a pattern. In certain embodiments, the weight percentage of the first type of fibers 210 in the fiber mat 100 is about 50%-80, and the weight percentage of the second type of fibers 230 in the fiber mat 200 is about 20%-50%.

[0084] Referring to FIG. 2B, the first type of fibers 210 includes a first polymer 212 and first particles 214 attached to the first polymer 212. In certain embodiments, the first polymer 212 is an electrically conductive binder, such as PFM, and the first particles 214 may be Si nanoparticles or Si nanorods.

[0085] Referring to FIG. 2C, the second type of fibers 230 includes a second polymer 232 and second particles 234 attached to the second polymer 232. In certain embodiments, the second polymer 232 may be an electrically conductive binder, such as PFM, or a non-conductive polymer binder, such as PAA or PVDF; the second particles 234 may be electrically conductive particles, such as carbon or Cu particles.

[0086] In certain embodiments, the first fibers 210 and the second type of fibers 230 may be co-spun to form the dual fiber mat 200. In the dual fiber mat 200, the second type of fibers 230 make fiber-fiber contact with the first type of fibers 210, thus providing numerous node points and pathways for electrons to pass to/from Si surface and or the PFM polymer to a metal plate current collector at the back of the electrode.

[0087] In certain embodiments, the first type of fibers 210 and the second type of fibers 230 are distributed in the fiber mat 200 with patterns. For example, in one pattern, the first type of fibers 210 are substantially disposed in a vertical direction, while the second type of fibers 230 are disposed in a horizontal direction that is substantially perpendicular to the vertical direction. In certain embodiments, the first type of fibers 210 and the second type of fibers 230 are evenly distributed in the fiber mat 200. In certain embodiments, the first type of fibers 210 and the second type of fibers 230 may be in the form of layers, and the layers of the first type of fibers 210 and the layers of the second type of fibers 230 are alternatively disposed.

[0088] In certain embodiments, the first polymer 212 has the formula of:

##STR00003##

where each of X and Y is selected from H, OH, COOH, and a halide such as F, and n is a positive integer. In certain embodiments, the first polymer 212 includes at least one of PAA, or PVDF.

[0089] In certain embodiments, the first polymer 212 has the formula of:

##STR00004##

In certain embodiments, the first polymer 212 includes a poly(ethylene oxide) (PEO) or polyvinyl alcohol.

[0090] In certain embodiments, the second polymer 214 has the formula of:

##STR00005##

where each of X and Y is selected from H, OH, COOH, and a halide such as F, and n is a positive integer. In certain embodiments, the second polymer 232 includes at least one of PAA, or PVDF.

[0091] In certain embodiments, the second polymer 232 has the formula of:

##STR00006##

In certain embodiments, the second polymer 232 includes PEO or polyvinyl alcohol.

[0092] In certain embodiments, the first type of fibers 210 is different from the second type of fibers 230.

[0093] In certain embodiments, in addition to the first polymer 212 and the first particle material 214, the first solution may further includes another polymer, such as an unchanged polymer such as polyphenlysulfone, and/or an ionically conductive polymer such as perfluorosulfonic acid polymer. In certain embodiments, the first solution may further include a catalyst, such as a platinum (Pt) catalyst.

[0094] In certain embodiments, in addition to the second polymer 232 and the second particle material 234, the second solution may further includes another polymer, such as an unchanged polymer such as polyphenlysulfone, and/or an ionically conductive polymer such as perfluorosulfonic acid polymer. In certain embodiments, the second solution may further include a catalyst, such as a Pt catalyst.

[0095] In certain embodiments, the dual fiber mat may further include third fibers having a third particle material and a third polymer. The third particle material may include at least one of silicon nanoparticles, silicon nanorods, carbon nanoparticles, and copper nanoparticls, and the third polymer may include at least one of PFM, PEFM, PAA, and PVDF.

[0096] In certain embodiments, the first type of fibers 210, the second type of fibers 230, and the third fibers are different from each other.

[0097] In certain aspects, the present invention relates to a method of manufacturing a dual fiber mat. In certain embodiments, as shown in FIG. 3, the method 300 includes the following steps. At operation 302, a first solution is provided. The first solution may include a first particle material and a first polymer as described above. The first particle material may be silicon nanoparticles or silicon nanorods. The first polymer may be PFM or PEFM.

[0098] At operation 304, a second solution is provided. The second solution may include a second particle material and a second polymer as described above. The second particle material may include electrically conductive particles, such as carbon nanoparticles or copper nanoparticles. The second polymer may include PFM, PEFM or a non-conductive polymer binder PAA or PVDF.

[0099] Once the first solution and the second solution are prepared, at operation 306, the first solution and the second solution are used to perform co-spinning to form a dual fiber mat. Co-spinning of the first and second solutions may be performed with a variety of electrospinning apparatuses or devices. For example, each of the first solution and the second solution is respectively filled into one of two syringes. Each of the syringes has a needle. A target is positioned with a predetermined distance to the needles. The needles are respectively connected to a power supply, and the target is grounded. During co-spinning, an electrical potential is applied to each of the needles for drawing out the corresponding solutions in the syringes toward the target. The flow rates for each solution and the electrical potentials applied to each of the needles may be controlled separately and differently such that the electrospinning for both the first and second solutions may be performed simultaneously to achieve co-spinning. As the drawn out solutions travel through the air, at least a portion of the solvent evaporates, resulting in the first type of fibers and the second type of fibers. The first type of fibers and the second type of fibers may be then collected by a rotating cylinder of the target. Thus, the first type of fibers and the second type of fibers may be obtained and organized to form the dual fiber mat for further processing.

[0100] It should be particularly noted that, unless otherwise stated in the present invention, the steps of the method may be arranged in a different sequential order, and are thus not limited to the sequential order as shown in FIG. 3. For example, the first solution is prepared first, and then the second solution is prepared; or the second solution is prepared first, and then the first solution is prepared; or the first solution and the second solution are prepared at the same time.

[0101] In certain embodiments, the method further includes a step of further processing the dual fiber mat. The further processing may include mechanical compaction and interfiber welding of intersecting fibers.

[0102] In certain aspects, the manufactured dual or multiple fiber mat is used to make an anode or an cathode in an electrochemical device.

[0103] These and other aspects of the present invention are further described in the following section. Without intending to limit the scope of the invention, further exemplary implementations of the present invention according to the embodiments of the present invention are given below. Note that titles or subtitles may be used in the examples for the convenience of a reader, which in no way should limit the scope of the invention. Moreover, certain theories are proposed and disclosed herein; however, in no way should they, whether they are right or wrong, limit the scope of the invention so long as the invention is practiced according to the invention without regard for any particular theory or scheme of action.

EXAMPLE 1

Si Anode for Li-Ion Batteries

[0104] In this example, the present invention provides a Si particle anode for Li-ion batteries. Si nanoparticles or nanorods are embedded in an electrically conductive binder, such as a polyfluorene derivative polymer (PFM). Although this binder is electrically conductive, the conductivity may not be sufficiently high for use in a thick anode (with high energy density). The max thickness of a Si anode with PFM may be only about 5-10 microns (m), whereas anodes may be needed with a thickness of about 50-1000 m.

[0105] In certain embodiments, nanofiber anodes have been electronspun with Si nanoparticles and Si nanorods and PFM polymer.

[0106] In certain embodiments, nanofiber Si/PFM anodes has been tested in Li-ion battery coin cells, in terms of energy capacity, cycle life, and charge/discharge rates.

[0107] In one embodiment, PFM electrically conductive polymer is provided by L. Gao, Lawrence Berkeley National Lab. In one embodiment, Si nanorods are provided by Professor Sreeram Vaddiraju from Texas A&M.

EXAMPLE 2

Dual Fiber Mats

[0108] In this example, partially to make an anode with a thickness of about 50-1000 m, two different fibers are co-spun to form dual fiber mats for an electrode. The electrode may be anode or cathode.

[0109] The first type of fibers may be Si/PFM fibers that contain Si nanoparticles or nanorods embedded in PFM or other electrically conductive binder. The second type of fibers contains electrically conductive particles, such as carbon or Cu particles. In certain embodiments, those particles are nanoparticles. The second type of fibers further contains a polymer. The polymer may be PFM, PEFM,or a non-conductive polymer binder, such as PAA or PVDF.

[0110] The second type of fiber mat will make fiber-fiber contact with the Si-containing fibers, thus providing numerous node points and pathways for electrons to pass to/from the Si surface and or the PFM polymer to a metal plate current collector at the back of the electrode. This second co-spun fiber provides electric conductivity for a thick nanofiber mat electrode with high areal and volumetric energy densities.

[0111] In certain embodiments, the use of a Si/PFM fiber for lithium ion intercalation/de-intercalation with a fiber diameter <1 m will ensure high Li+ transport rates into and out of the porous fiber mat and fast charge/discharge reaction times.

[0112] In certain embodiments, dual fiber electrospinning has been used to prepare membranes and electrodes for fuel cells.

EXAMPLE 3

Dual Fiber Mats with High Concentration of Si Particles

[0113] In certain embodiments, a dual fiber mat is provided. The dual fiber mat is manufactured from co-electrospinning of two different fibers. The first type of fibers contains Si nanoparticles or nanorods embedded in PFM or another electrically conductive binder. The second type of fibers contains electrically conductive particles, such as carbon or Cu particles embedded in PFM, PEFM or a non-conductive polymer binder, such as PAA or PVDF. By co-spining the first type of fibers and the second type of fibers, dual fiber mats are manufactured.

[0114] In one example, the first type of fibers is Si/PFM fibers and the second type of fibers is carbon/PVDF fibers. The dual fiber mats contains about 50-80% of the first type of fibers of Si/PFM, and about 2050% of the second type of fibers of carbon/PVDF (or some other conductive particle in a polymer binder). The second type of fibers has a very high loading (30-80%) of carbon nanoparticles to ensure a high electrical conductivity. A percolation threshold of electrically conductive nanofibers (i.e., about 30 vol %) may not be needed here, so the system offers important advantages, as compared to adding conducting carbon particles directly to the Si/PFM fibers to boost the electrical conductivity of these fibers.

EXAMPLE 4

[0115] A dual fiber electrospinning setup consists of two different electrospinning inks. One of the inks is prepared by mixing Si nanoparticles (50-70 nm diameter) with poly(acrylic acid), abbreviated as PAA (450 kDa molecular weight) in a solvent mixture of isopropanol, butanol and methanol, while the second type of fiber ink is prepared by mixing conductive carbon black powder (Vulcan XC-72R) with PAA in 1-propanol as the solvent. Both inks have a total solids content of 15 wt. %, with the weight ratio of the Si:PAA and C:PAA in each of the respective inks is 65:35. The inks were electrospun at room temperature and 20% relative humidity using separate single needle spinnerets at the following conditions: (i) a flow rate of 0.75 mL/hr for both inks, (ii) 8 kV bias voltage for both inks, and (iii) 8 cm spinneret-to-collector distance for both inks. After electrospinning, multiple dual fiber Si/PAA-C/PAA mats were stacked to obtain a Si areal loading of 1.08 mg/cm.sup.2. The stack was then compacted on a hydraulic press at a pressure of 90 MPa, and interfiber contacts were welded by exposing the compacted mat to methanol vapor at room temperature for 1 hour. Since the inks were spun at the same flow rate (i.e., the mat contains 50% Si/PAA fibers and 50% C/PAA fibers) with the same ink composition, the Si:C weight ratio was 1:1 in the final fabricated mat. These fiber mats were then tested as the working electrode (anode) in CR2032 Li-ion battery half cell using a Li metal counter/reference electrode (cathode), a Celgard 2500 separator, and an electrolyte containing 1.2 M LiPF.sub.6 in 3/7 EC/DEC with 30 wt. % FEC additive. FIG. 4 shows an SEM image of the compacted and welded electrospun Si/PAA+C/PAA dual fiber mat. The two fiber types are indistinguishable.

[0116] FIG. 5 shows a representative charge/discharge curve of the first two cycles for the dual fiber mat anode at a charge/discharge rate of 0.1 C. This electrospun dual fiber mat electrode contained 50% Si/PAA fibers and 50% C/PAA fibers (a 1:1 weight ratio of Si:C, where the overall electrode weight ratio of Si:C:PAA was 32.5:32.5:65. During the first cycle, the coulombic efficiency was 66% due to formation of a solid-electrolyte interphase (SEI) layer on the anode surface. The coulombic efficiency rose to 87% during the second cycle. At the end of the second cycle, the gravimetric capacity of the anode was 1300 mAh/g.sub.electrode, (g.sub.electrode includes the weight of si, C, and PAA), corresponding to a Si gravimetric capacity of 3597 mAh/g.sub.Si, thereby indicating excellent Si material utilization.

[0117] This example shows that one can have an electrochemically active (but non-electrically conductive) particles in one fiber (non-conducting Si particles) and have a second type of fiber type in an electrode mat that conducts electrons (the fibers containing C and PAA). The excellent Si utilization (3598 mAh/g.sub.Si) means that electrons were passing through the C/PAA fiber and entering/exiting the Si/PAA fiber where they were participating in Li.sup.+ intercalation de-intercalation reactions.

[0118] In summary, certain embodiments of the present invention, among other things, have the following advantages:

[0119] 1. Electrospinning Si with an electrically conductive binder at a Si particle content >60%.

[0120] 2. The dual fiber electrode concept with:

[0121] (i) One fiber for lithiation/de-lithiation of Si, where Si nanoparticles are embedded in an electrically conductive and chemically stable polymer (such as PFM or PEFM) or embedded in a non-conductive and chemical stable polymer (such as PAA or PVDF) and

[0122] (ii) the second type of fiber distributed uniformly or non-uniformly throughout the electrodes and composed of electrically conductive particles (e.g., carbon or Cu) in an inert polymer binder (e.g., poly(acrylic acid), carboxymethyl cellulose, or PVDF) or in a conductive PFM or PEFM binder, where the particle content is sufficiently high to provide good electron conduction (good electrical conductivity; much better than the electrically conductive polymer binder alone).

[0123] 3. Electrospinning fibers with Si nanorods.

[0124] 4. The technology is applicable to both anodes and cathodes in a Li-ion battery. For the latter case, electrically conductive particles are intermixed with fibers containing lithium cobalt oxide or lithium iron phosphate, for example, and PVDF.

[0125] 5. The technology can be used in batteries other than a Li-ion battery.

[0126] 6. The technology can be used in a proton exchange membrane or alkaline fuel cell or in a water electrolyzer.

[0127] The foregoing description of the exemplary embodiments of the present invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.

[0128] The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to activate others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.