CN108123099B

CN108123099B - Hybrid energy storage device including support wire

Info

Publication number: CN108123099B
Application number: CN201710944389.9A
Authority: CN
Inventors: R·A·罗杰斯基; S·克兰科维斯基; 李钧
Original assignee: Kansas State University; Catalyst Power Technologies Inc
Current assignee: Kansas State University; Catalyst Power Technologies Inc
Priority date: 2012-07-03
Filing date: 2013-07-03
Publication date: 2021-06-08
Anticipated expiration: 2033-07-03
Also published as: KR20200122424A; WO2014008433A1; KR20200122426A; JP7045032B2; GB2518110B; CN112349880A; JP2021022575A; CN104662726A; KR20220066203A; CN112349878A; CN112349879A; KR102339237B1; CN112349877A; KR102294208B1; GB201500515D0; KR102339235B1; JP6272851B2; GB2518110A; JP2022091795A; JP2015528985A

Abstract

A novel hybrid lithium ion anode material based on a Si shell coaxially coated on Carbon Nanofibers (CNF). The unique cup-stacked graphite microstructure makes CNF an effective Li⁺An embedding medium. Highly reversible Li⁺Insertion and extraction are observed at high power rates. More importantly, the highly conductive and mechanically stable CNF core optionally supports a coaxially coated amorphous Si shell with much higher theoretical specific capacity by forming a fully lithiated alloy. The addition of surface effect dominant sites in close proximity to the embedding medium results in a hybrid device that includes the advantages of both batteries and capacitors.

Description

Hybrid energy storage device including support wire

The present application is a divisional application of patent application hybrid energy storage device containing support wires of invention application No. 201380035121.3 filed on 7/3/2013.

Cross Reference to Related Applications

The application is as follows:

continuation of the U.S. non-provisional patent application 13/779,409 filed on 27.2.2013;

partial continuation of U.S. non-provisional patent application serial No. 13/725,969 filed on 21/12/2012;

and claims the benefit and priority of the following U.S. provisional patent applications:

61/667,876 submitted on 7/3/2012,

61/677,317 submitted on 30/7/2012,

61/806,819 submitted on 29 th month 3 of 2013, and

61/752,437 filed on 14/1/2013.

The present application also relates to 13/779,472, 13/779,522, and 13/779,571 U.S. non-provisional patent applications, all filed on 26/3/2013.

The disclosures of all of the above-mentioned provisional and non-provisional patent applications are hereby incorporated by reference.

The invention was accomplished with the funding of a project entitled CMMI-1100830 and EPSCoR Award EPS-09038 awarded by the national science Foundation. The united states government may have certain rights in the invention.

Technical Field

The present invention is in the field of energy storage devices including, but not limited to, batteries, capacitors, and fuel cells.

Background

Rechargeable lithium ion batteries are key electrical energy storage devices for power supply in portable electronic devices, power tools, and future electric vehicles. Improvements in specific energy capacity, charge/discharge rates and cycle life are critical to their broader application.

In current commercial lithium ion batteries, graphite or other carbonaceous materials are formed by forming fully embedded LiC₆The compound was used as an anode with a theoretical capacity limit of 372 mAh/g. Conversely, silicon is formed by forming a fully lithiated metallic Li_4.4Si has a much higher theoretical specific capacity of 4,200 mAh/g. However, the large volume expansion of lithiated silicon, up to-300% -400%, causes significant structural stresses that previously inevitably led to cracking and mechanical failure, which significantly limited the lifetime of prior art silicon anodes.

Disclosure of Invention

In some embodiments, an energy storage device includes a hybrid core-shell NW (nanowire) configuration in a high performance lithium ion anode by incorporating a Vertically Aligned Carbon Nanofiber (VACNF) array that is coaxially coated with an amorphous silicon layer. The vertically aligned CNFs comprise multi-walled carbon nanotubes (MWCNTs) that are selectively grown on copper substrates using a direct current biased plasma chemical vapor deposition (PECVD) process. The Carbon Nanofibers (CNF) grown by this method can have a unique internal morphology that distinguishes them from the hollow structure of common MWCNTs and conventional solid carbon nanofibers. One of the distinguishing features is that these CNFs optionally consist of a series of bamboo-like nodes that traverse a main hollow central channel. This microstructure may be attributed to the stacking of the tapered graphite cups as discussed further elsewhere herein. At larger length scales, these PECVD-grown CNFs are typically uniformly aligned perpendicular to the substrate surface and well separated from each other. They may be free of any or minimal entanglement and thus form a brush-like structure known as a VACNF array. The diameter of the individual CNFs may be selected to provide the desired mechanical strength to make the VACNF array robust and able to maintain its integrity through silicon deposition and wet electrochemical testing.

Various embodiments of the present invention include several types of support filaments other than VACNF. These support filaments may include, for example, nanowires, carbon sheets, or other structures described herein. Other embodiments do not include any support wires and instead use an adhesive.

Various embodiments of the present invention include an energy storage system comprising an electrically conductive substrate; a plurality of vertically aligned carbon nanofibers grown on a substrate, the carbon nanofibers comprising a plurality of multi-walled carbon nanotubes; and an electrolyte comprising one or more charge carriers.

Various embodiments of the present invention include an energy storage system comprising an electrically conductive substrate; a plurality of vertically aligned carbon nanofibers grown on a substrate; and an intercalation material layer disposed on the plurality of vertically aligned carbon nanofibers and configured to have a lithium ion storage capacity of between about 1,500 and 4,000mAh per gram of intercalation material.

Various embodiments of the present invention include an energy storage system comprising an electrically conductive substrate; a plurality of vertically aligned carbon nanofibers grown on a substrate; and an intercalation material layer disposed on the plurality of vertically aligned carbon nanofibers and configured such that ion storage capacities of the intercalation material at charging rates of 1C and 3C are about the same.

Various embodiments of the present invention include a method of producing an energy storage device, the method comprising providing a substrate; growing carbon nanofibers on a substrate, the carbon nanofibers having a stacked-cone (stacked-cone) structure; and applying an intercalation material to the carbon nanofibers, the intercalation material configured for intercalation of charge carriers.

Various embodiments of the present invention include an energy storage system comprising: an electrolyte comprising one or more charge carriers; a conductive substrate; a plurality of vertically aligned support filaments attached to a substrate; an embedding material disposed on each of the support filaments and configured to reversibly adsorb members of charge carriers within a body of the embedding material; and a binder disposed on the intercalation material and including a plurality of nanoparticles, each nanoparticle configured to provide surface effect dominant sites configured to adsorb members of the charge carriers through interaction of induced current on a surface of the nanoparticle.

Various embodiments of the present invention include an energy storage system comprising: an electrolyte comprising one or more charge carriers; a conductive substrate; a plurality of support wires attached to a substrate; an intercalation material disposed on each of the support filaments and configured to reversibly adsorb members of charge carriers in a body of the intercalation material; and a binder disposed on the intercalation material and including a plurality of surface effect dominant sites configured to facilitate intercalation of the charge carriers into the intercalation material.

Various embodiments of the present invention include an energy storage system comprising: an electrolyte comprising one or more charge carriers; a conductive substrate; an intercalation material configured to reversibly adsorb members of charge carriers in a majority of the intercalation material; and a binder disposed on the embedding material and including nanoparticles, each nanoparticle configured to provide surface effect dominant sites configured to supply electrons to the members of the charge carriers through interaction of induced current on the surface of the nanoparticle.

Various embodiments of the present invention include an energy storage system comprising: a cathode; and an anode separated from the cathode by an electrolyte comprising one or more charge carriers, the anode comprising an intercalation material configured to intercalate and donate electrons to the charge carriers at a first reaction potential, and a plurality of nanoparticles comprising surface effect dominant sites configured to donate electrons to the charge carriers at a second reaction potential, an absolute difference between the first and second reaction potentials being less than 2.4V.

Various embodiments of the present invention include a system comprising: means for establishing an electrical potential gradient at an anode of a charge storage device, the anode comprising an electrolyte, a plurality of surface effect dominant sites, an intercalation material, and a substrate; means for receiving charge carriers of an electrolyte at one of the surface effect dominant sites; means for receiving electrons at the charge carriers from one of the surface effect dominant sites; and means for receiving charge carriers at the embedding material.

Various embodiments of the present invention include a method of producing an energy storage device, the method comprising: providing a conductive substrate; growing a support wire on a substrate; applying an embedding material to the supporting nanofibers, the embedding material configured for the embedding of charge carriers; and applying a plurality of surface effect dominant sites in close proximity to the embedding material.

Various embodiments of the present invention include a method of producing an anode, the method comprising: providing a conductive substrate; mixing an adhesive material, surface effect dominant sites configured to accept electrons from charge carriers at a first reaction potential and an intercalation material configured to accept charge carriers or electrons from charge carriers at a second reaction potential; and applying the adhesive material, the surface effect dominant sites, and the embedding material to the substrate.

Various embodiments of the present invention include a method of producing an energy storage device, the method comprising: providing a conductive substrate; providing a support wire; applying an embedding material to the support filaments, the embedding material configured for embedding of charge carriers; and surface effect dominant sites are added to the support filaments.

Various embodiments of the present invention include a method of charging a charge storage device, the method comprising: establishing an electrical potential between a cathode and an anode of a charge storage device, the charge storage device comprising an electrolyte; receiving first charge carriers of an electrolyte at surface effect dominant sites of an anode; transferring electrons of the anode to a first charge carrier; receiving second charge carriers of an electrolyte at the intercalation material of the anode; and electrons are transferred from the intercalation material to the second charge carrier.

Various embodiments of the present invention include a method of charging a charge storage device, the method comprising: establishing an electrical potential gradient at an anode of a charge storage device, the anode comprising an electrolyte, a plurality of nanoparticles having surface effect dominant sites, an intercalation material, and a substrate; receiving a first charge carrier of an electrolyte at one of the surface effect dominant sites; transferring an electron from one of the surface effect dominant sites to a first charge carrier; receiving second charge carriers at the intercalation material of the anode; and electrons are transferred from the intercalation material to the second charge carrier.

Various embodiments of the present invention include an energy storage system comprising: a conductive substrate; a carbon nanofiber or other support filament connected to the conductive substrate, the carbon nanofiber comprising a plurality of exposed nanoscale edges along a length of the carbon nanofiber; and an embedding material configured to form a shell over at least a portion of the carbon nanofibers.

Various embodiments of the present invention include an energy storage system, such as a battery or an electrode, comprising: a conductive substrate; a carbon nanofiber or other support filament attached to the conductive substrate, the carbon nanofiber comprising a plurality of cup-shaped structures along the length of the carbon nanofiber; and an embedding material configured to form a shell over at least a portion of the carbon nanofibers.

Various embodiments of the present invention include an energy storage system comprising: a conductive substrate; carbon nanofibers or other supporting filaments connected to the conductive substrate; and an intercalation material configured to form a shell on at least a portion of the carbon nanofibers, the intercalation material being disposed in feather-like structures along the length of the carbon nanofibers.

Various embodiments of the present invention include an energy storage system comprising: a conductive substrate; a carbon nanofiber attached to the conductive substrate; and an embedding material configured to form a shell over at least a portion of the carbon nanofibers, the embedding material configured such that expansion of the embedding material does not cause delamination of the embedding material from the carbon nanofibers.

Various embodiments of the present invention include a method of producing an energy storage device, the method comprising providing an electrically conductive substrate; adding carbon nanofibers on a conductive substrate, the carbon nanofibers each comprising a plurality of exposed nanoscale edges along a length of the carbon nanofiber; and applying an intercalation material to the carbon nanofibers, the intercalation material configured for intercalation of charge carriers.

Various embodiments of the present invention include a method of producing an energy storage device, the method comprising providing an electrically conductive substrate; adding carbon nanofibers on a conductive substrate, the carbon nanofibers each comprising a plurality of cup-shaped structures along a length of the carbon nanofiber; and applying an intercalation material to the carbon nanofibers, the intercalation material configured for intercalation of charge carriers.

Various embodiments of the present invention include a method of producing an energy storage device, the method comprising providing an electrically conductive substrate; adding carbon nanofibers on the conductive substrate; and applying an intercalation material to the carbon nanofibers, the intercalation material configured for intercalation of charge carriers, and the intercalation material being disposed in feather-like structures along the length of the carbon nanofibers.

Drawings

Fig. 1A and 1B illustrate a CNF array comprising a plurality of CNFs grown on a substrate, according to various embodiments of the invention.

Fig. 2A-2C illustrate a plurality of vertically aligned CNFs in different states, according to various embodiments of the present invention.

Fig. 3A-3C illustrate details of CNFs according to various embodiments of the invention.

Fig. 4 illustrates a schematic diagram of a cone-stack structure of a CNF according to various embodiments of the present invention.

Fig. 5A-5C illustrate electrochemical characteristics of CNFs-3 μm long according to various embodiments of the present invention.

Fig. 6A-6C illustrate scanning electron microscope images of 3 μm long CNFs according to various embodiments of the present invention.

Fig. 7A-7C illustrate results obtained using CNFs comprising a silicon layer as an anode of a lithium ion battery, according to various embodiments of the present invention.

Fig. 8 illustrates how the capacity of a CNF array varies with charge rate according to various embodiments of the present invention.

Fig. 9 illustrates a raman spectrum of a CNF array according to various embodiments of the present invention.

FIGS. 10A-10C show Li during 15 charge-discharge cycles according to various embodiments of the invention⁺Variations in insertion-extraction capacity and coulombic efficiency.

Fig. 11A-11C show scanning electron microscope images of newly prepared CNF arrays according to various embodiments of the present invention.

Fig. 11D shows a cross-section of a nanofiber/silicon composite comprising more than one CNF.

Fig. 12 illustrates an array of carbon nanofibers comprising fibers of 10 μm length according to various embodiments of the present invention.

Fig. 13 illustrates a method of producing CNF arrays and/or CNFs according to various embodiments of the invention.

Fig. 14A illustrates a CNF including a power enhancing material according to various embodiments of the present invention.

Fig. 14B illustrates details of the power enhancing material illustrated in fig. 14A, according to various embodiments of the invention.

Fig. 14C illustrates an alternative detail of the power enhancing material illustrated in fig. 14A, according to various embodiments of the invention.

Fig. 15 illustrates an electrode surface including a power enhancing material and unaligned CNFs coated by an intercalation material, according to various embodiments of the invention.

Fig. 16 illustrates an electrode surface including power enhancing material, unaligned CNFs, and free intercalation material, according to various embodiments of the invention.

Fig. 17 illustrates an electrode surface including an intercalation material and a power enhancing material without CNFs, according to various embodiments of the invention.

Fig. 18 illustrates an electrode surface including surface effect dominant sites arranged in close proximity to CNFs, according to various embodiments of the invention.

Fig. 19 and 20 illustrate electrode surfaces including surface effect dominant sites disposed in close proximity to free intercalation material, according to various embodiments of the invention.

Fig. 21 illustrates a method of assembling an electrode surface according to various embodiments of the invention.

Fig. 22 illustrates a method of operating a charge storage device according to various embodiments of the invention.

Detailed Description

Fig. 1A and 1B illustrate a CNF array 100 according to various embodiments of the present invention, the CNF array 100 including a plurality of CNFs 110 grown on a conductive substrate 105. In fig. 1A, the CNF array 100 is shown in a lithium extraction (discharge) state, and in fig. 1B, the CNF array 100 is shown in a lithium insertion (charge) state. The CNFs 110 in these and other embodiments discussed herein are optionally vertically aligned. CNF110 is grown on copper substrate 105 using a dc-biased plasma chemical vapor deposition (PECVD) process. As discussed above, CNF110 grown by this method can have a unique morphology comprising stacks of conical graphite structures similar to cups or cones or helices. This creates a very fine structure that promotes lithium intercalation. Such a structure is referred to elsewhere herein as a "cone stack" structure. On a larger length scale, these CNFs 110 are typically aligned uniformly perpendicular to the substrate surface and well separated from each other. The diameter of the individual CNFs can be selected to provide the desired mechanical strength so that the CNF array 100 is robust and able to maintain its integrity through silicon deposition and wet electrochemical cycling. A seed layer may optionally be used to grow CNF110 on substrate 105. In use, CNF array 100 is placed in contact with electrolyte 125, which electrolyte 125 may be a solid or a liquid, or a combination of a solid and a liquid and further comprises one or more charge carriers, such as lithium ions. The CNFs 110 are configured such that some of the electrolyte 125 is disposed between the CNFs 110 and/or is able to reach the substrate 105 through gaps between the CNFs 110.

The diameter of the individual CNFs 110 illustrated in fig. 1A and 1B is nominally between 100 and 200nm, but diameters between 75 and 300nm or other ranges are possible. The CNF110 may optionally be tapered along its length. CNF110 produced using the techniques discussed herein has excellent conductivity along the axis (σ ═ 2.5x10⁵S/m) and forms a robust ohmic contact with the substrate 105. The open spaces between CNFs 110 enable silicon layer 115 to be deposited onto each CNF to form a tapered coaxial housing, mostly at the tip 120 of CNF 110. This design enables the entire silicon layer 115 to be electrically connected through CNF110 and remain sufficiently active during charge-discharge cycles. The expansion that occurs upon alloying of lithium with silicon layer 115 can be easily accommodated in the radial direction, e.g., perpendicular to the long dimension of CNF 110. The charge and discharge capacity and cycle stability of the non-silicon coated CNF110 and the silicon coated CNF110 can be compared. The addition of silicon layer 115 provides a C/2 rate of up to 3938mAh/g_SiOf (2) is significant⁺Embedding (charging) capacity and retention of 1944mAh/g after 110 cycles_Si. This charge/discharge rate and corresponding capacity is significantly higher than previous configurations using silicon nanowires or mixed Si-C nanostructures. Fig. 1A and 1B are perspective views.

In various embodiments, nominal silicon thicknesses of from 0.01 up to 0.5, 1.0, 1.5, 2.5, 3.0, 4.0, 10, 20, 25 μm (or more) can be deposited on CNFs 110 that are 3 μm long to form CNF arrays 100 such as those illustrated in fig. 1A and 1B. Likewise, in various embodiments, nominal silicon thicknesses of from 0.01 up to 0.5, 1.0, 1.5, 2.5, 3.0, 4.0, 10, 20, 25 μm (or more) can be deposited on CNFs 110 that are 10 μm long to form CNF array 100. In some embodiments, the nominal thickness of the silicon is between 0.01 μm and the average distance between CNFs 110.

Using CNF array 100, lithium ion storage was obtained at C/2 rates up to-4,000 mAh/g specific mass capacity. At the same power rate, this capacity is significantly higher than those obtained with silicon nanowires alone or other silicon nanostructured carbon mixtures. The improved performance is attributed to the efficient charge collection and short Li by CNF110 in this mixture structure⁺Path length and fully activated silicon shell. Good cycling stability was demonstrated over 110 cycles. In various embodiments, the CNF array 100 has a lithium ion storage capacity of about 750, 1500, 2000, 2500, 3000, 3500, or 4000mAh per gram of silicon, or any range between these values. As used herein, the term "nominal thickness" (e.g., of silicon) is the amount of silicon that produces a flat layer of silicon of that thickness on the substrate 105. For example, a nominal thickness of 1.0 μm of silicon is the amount of silicon that would result in a 1.0 μm thick silicon layer if silicon were deposited directly on the substrate 105. The nominal thickness is reported as it can be easily measured by weight using methods known in the art. A nominal thickness of 1.0 μm will result in a smaller thickness of silicon layer 115 on CNF110 because the silicon is distributed over a larger area of the CNF110 surface.

Fig. 2A-2C illustrate a CNF array 100 having an average fiber length of about 3 μm according to various embodiments of the invention. FIGS. 2A-2C are Scanning Electron Microscope (SEM) images. Fig. 2A shows a plurality of vertically aligned CNFs 110 without silicon layer 115. Fig. 2B shows a plurality of vertically aligned CNFs 110 including silicon layer 115. Fig. 2C shows a plurality of vertically arranged CNFs 110 in a taken-out (discharged) state after undergoing 100 lithium charge-discharge cycles. The CNFs 110 are firmly attached to the copper substrate 105 and are substantially uniformly vertically aligned and randomly distributed on the substrate surface. The sample used in this study had a 1.11x10 sample size⁹CNF/cm²Average areal density (from SEM image top view)Calculated), corresponding to an average nearest neighbor distance of-330 nm. The average length of CNF110 in FIGS. 2A-2C is-3.0 μm and>90% of the CNFs had lengths in the range of 2.5 to 3.5 μm. The diameter extends from-80 nm to 240nm with an average value of-147 nm. An inverted tear-drop shaped nickel catalyst at the tip 120 is present at the tip of each CNF110, covering the hollow channel at the center of the CNF, which promotes CNF110 tip growth during the PECVD process. The size of the nickel catalyst nanoparticles defines the diameter of each CNF 110. Longer CNFs 110 of up to 10 μm have also been used in some studies that will be discussed in the later section.

In various embodiments, the average nearest neighbor distance can vary between 200-450nm, 275-385nm, 300-360nm, or similar distances. Furthermore, the average length of the CNF110 can be between about 2-20, 20-40, 40-60, 60-80, 80-100, 100-. Such as 1 mm long standard carbon nanofibers, are known in the art. In various embodiments, the average diameter can vary between about 50-125, 100-200, 125-175 (nm), or other ranges.

Amorphous silicon layer 115 is deposited on CNF array 100 by magnetron sputtering. The open structure of the brush-like CNF array 100 makes it possible for silicon to reach deep down into the array and create conformal structures between the CNFs 110. As a result, a thick silicon coating was formed at the CNF tip, followed by a gradually thinning coaxial silicon shell around the lower part of the CNF, presenting an interesting tapered core-shell structure similar to a cotton swab. The amount of silicon deposition is characterized by the nominal thickness of the silicon film on the plane using a Quartz Crystal Microbalance (QCM) during sputtering. Li⁺The insertion/extraction capacity is normalized to the total silicon mass resulting from the nominal thickness. At a nominal thickness of 0.50 μm, the silicon-coated CNFs 110 separated well from each other, forming an open core-shell CNF array structure (shown in fig. 2B). This structure allows the electrolyte to freely reach the entire surface of silicon layer 115. In the illustrated embodiment, the average tip diameter is-457 nm, compared to the average diameter of CNF110 of-147 nm prior to application of silicon layer 115. The average radial silicon thickness at the tip 120 is estimated to be-155 nm. This is significantly less than the nominal silicon thickness of 0.50 μmMuch because most of the silicon is spread along the full length of the CNF. In alternative embodiments, other radial silicon thicknesses in the 10-1000, 20-500, 50-250, 100-200 (nm) ranges or different ranges are seen. As discussed elsewhere herein, the cone stack of CNFs 110 provides additional fine structure for silicon layer 115. The stacked cone structure may alternatively be the result of a spiral growth mode that produces a stacked cone structure when viewed in cross-section.

The perspective electron microscope (TEM) images in fig. 3A-3C illustrate further structural details of the silicon coated CNF 110. Silicon layer 115 of-390 nm silicon is produced directly on top of the tip 120 of CNF110 of-210 nm diameter. The largest portion of the cotton-wool-like silicon layer 115, which is-430 nm in diameter, occurs near the extreme end of the tip 120. The coaxial silicon layer 115 around CNF110 showed a feathery texture with adjusted contrast, distinct from the uniform silicon deposit on the tip (see fig. 3A). This is likely the result of the cone-stack microstructure of the PECVD-grown CNF 110. This is distinguished from the literature that such CNFs 110 include a non-uniform cupping-like graphite structure along the central axis of the CNF 110. The use of this variation in the diameter of CNF110 was previously disclosed in commonly owned U.S. patent application serial number 12/904,113 filed on 13/10/2010.

It can be clearly seen in fig. 3B that the cone-stack structure along the length of each CNF consists of one to five, five to fifteen or more than ten cup-shaped graphite layers, as indicated by the dashed lines. At the edges of each cone stack, the side edges of some of the graphite layers are exposed. At these exposed edges, lithium may be able to penetrate between the graphite layers. At the molecular level, the cup-shaped structure comprises a cone of graphene and/or graphite sheets with which lithium can interact. The cup rim is a nanoscale rim and may have the properties of a graphene rim, while properties such as between graphene sheets may be found between graphite layers. The rim of the cup provides carbon nanofibers that appear to be vertically aligned through which lithium ions can move. The new microstructure of VACNF results in a stack of exposed graphite edges along the length of the CNF sidewall, e.g., cup edge. These nanoscale edges are similar to those typically found on graphene/graphite sheets and ribbons. These exposed cup edges result in varying silicon nucleation rates, thus producing a tailored silicon shell texture. These exposed edges also form a good interface between the VACNF core and the Si shell to facilitate fast electron transfer in the hybrid structure. Two different structures of the Si shell can be controlled by changing the growth process of VACNF. The region of VACNF that includes the cup structure results in a feather-like Si shell, while the region of VACNF that does not include the cup structure has a similar Si structure as observed at the tip of VACNF. The VACNF is optionally configured to have one or more regions along the length of the VACNF with no cup stacking structures at the tip. In alternative embodiments, the carbon nanofibers further comprise a cup stack structure having exposed graphite/graphene or graphite edges but not vertically aligned and/or even directly attached to the substrate. Although the use of a "cup-stacked" graphite microstructure is discussed elsewhere herein, other methods of producing exposed nanoscale edges of graphite sheets include opening carbon nanotubes using acid. The exposed nanoscale edges produced in this manner are expected to also provide the advantages of controlling and/or immobilizing the Si shell and may be included in some embodiments. For example, the edges of graphitic nanoscale bands can be used to influence the growth of Si shells around these bands.

As used herein, the term "nanofiber" is meant to include structures having at least two nanometer-scale (less than one micron) dimensions. These include, for example, wires, tubes, and ribbons, where the thickness and width are on the nanometer scale but the length may or may not be on the nanometer scale. The term nanofiber is meant to exclude graphene sheets, where the thickness may be on the order of nanometers but both the length and width are on the order of nanometers. In various embodiments, the support filaments discussed herein are nanofibers.

The resolution and contrast of fig. 3B and 3C are limited due to the fact that the electron beam needs to penetrate several hundred nanometers thick CNF or Si-CNF mixtures, but this structural feature is consistent with high resolution TEM studies in the literature using smaller CNFs. This unique structure creates broken graphite edge clusters along the CNF sidewalls that cause varying nucleation rates during silicon deposition and thus adjust the silicon layer 115 density on the CNF110 sidewalls. The adjusted density resulted from FIG. 3A (100 nm)²) Block 310 indicates an ultra-high surface area silicon structure. The feathered silicon structure of silicon layer 115 provides an excellent lithium ion interface that results in a very high lithium capacity and also transfers electrons quickly to CNF 110. The dark area at the tip 120 in fig. 3A is the nickel catalyst for CNF growth. Other catalysts may also be used.

Fig. 3B and 3C are images recorded before (3B) and after (3C) the lithium insertion/extraction cycle. The sample in fig. 3C is in a delithiated (discharged) state when it is removed from the electrochemical cell. The dashed line in fig. 3B is the visual guide of the cone-stacked graphite layers within CNF 110. The long dashed line in fig. 3C indicates the sidewall surface of the CNF 110.

As discussed elsewhere herein, the cone-stack structure of CNF110 is very different from commonly used Carbon Nanotubes (CNTs) or graphite. The stacked cone structure results in improved Li relative to standard carbon nanotubes or nanowires⁺Embedded even though silicon layer 115 is not added. For example, the cone-stacked graphite structure of CNF110 allows Li⁺Embedded into the graphite layers (not just at the ends) through the side walls of the CNF 110. Li passing through the wall of each CNF110⁺The transfer path is very short (with D-290nm in some embodiments), completely different from the long path from the open end in a commonly used seamless Carbon Nanotube (CNT). Fig. 4 illustrates a schematic diagram of a cone-stack structure of the CNF 110. In this particular embodiment, the average of the parameters is: radius r of CNF_CNF74nm CNF wall thickness t_w-50nm, graphite taper angle θ 10 °, and graphite taper length D t_w/sin θ＝290nm。

FIGS. 5A-5C graphically illustrate the electrochemical properties of CNF110 that is-3 μm long. This characteristic illustrates the phenomenon described with respect to fig. 4. FIG. 5A shows relative Li/Li at 0.1, 0.5, and 1.0mV/s scan rates⁺Cyclic Voltammogram (CV) from 1.5V to 0.001V for the reference electrode. A lithium disk was used as the counter electrode. Data were taken from the second cycle and normalized to the exposed geometric surface area. FIG. 5B shows electrostatic charge-discharge curves at C/0.5, C1, C/2 power rates, corresponding to 647, 323, and 162mA/g (normalized to estimated carbon mass) or 71.0, 35.5, and 17.8 μ A/cm, respectively²Current density (normalized to geometric surface area). Fig. 5C shows the insertion and extraction capacity (to the left vertical axis) and coulombic efficiency (to the right vertical axis) versus cycle number at a C/1 charge-discharge rate. (C/1 discharge rate 1 hour, C/2 discharge rate 120 minutes, 2C/0.5 30 minutes, etc.).

Newly assembled half-cells typically exhibit comparisons to Li/Li⁺The Open Circuit Potential (OCP) of the reference electrode, uncoated CNF110 anode, was-2.50 to 3.00V. CV measured between 0.001V and 1.50V shows Li when the electrode potential is 1.20V or less⁺The embedding begins. The first cycle from OCP to 0.001V involves the formation of the necessary protective layer, i.e., the Solid Electrolyte Interphase (SEI), by the decomposition of solvents, salts and impurities, and thus presents a large cathodic current. The subsequent CV showed a smaller but more stable current. When the electrode potential extends to a negative value, with Li⁺The intercalation-related cathodic current rose slowly until a sharp cathodic peak at 0.18V appeared. The reversal of positive values after the electrode potential reached the lower limit of 0.001V, lithium extraction was observed over the entire range up to 1.50V as indicated by the continuous anode current and the broad peak at 1.06V.

CV feature and segment embedding into graphite and Li of CNF array 100⁺The CV characteristics of those CNF arrays 100 that diffuse slowly into the hollow channels of CNTs are slightly different. Lithium ion intercalation into CNF110 is likely due to the unique structure of CNF110, passing through intercalation between graphitic layers from the sidewalls. The TEM image in FIG. 3C shows that the graphite in the cone stack within CNF110 is stacked in Li⁺Is slightly destroyed during the insertion-extraction cycle, probably due to Li⁺Large volume changes occur upon embedding. Some debris and nanoparticles that are white objects are observed inside CNF110 as well as on the external surface. These show penetration into the interior of the CNF through the side walls.

The electrostatic charge-discharge curve in FIG. 5B shows that Li increases as the power rate increases from C/2 to C/0.5(C/0.5 is also referred to as "2C")⁺The storage capacity is reduced. For easier comparison of ratios (especially for those above C/1), we use the fractional notation C/0.5 herein instead of the more commonly used one in the literature“2C”。Li⁺Embedding and extraction Capacity normalization to estimated quality of CNF110 (1.1 × 10)⁴g/cm²) The mass of the CNF110 is calculated from the hollow vertically arranged CNF structure and the following average parameters: length (3.0 μm), density (1.1X 10)⁹CNF per cm²) An outer diameter (147nm) and a hollow inner diameter (49nm, outer diameter-1/3). The density of the solid graphite wall of CNF110 is assumed to be equal to graphite (2.2 g/cm)³) The same is true. At normal C/2 rate, the embedding capacity is 430mA h g^-1And the take-out capacity was 390mA hr g^-1The two are slightly higher than the theoretical value of graphite 372mA h g^-1This may be due to SEI formation and Li⁺Irreversibly embedded in a hollow compartment within CNF 110. At all power rates, it was found that the extraction capacity was greater than 90% of the intercalation value, and that both intercalation capacity and extraction capacity decreased by-9% when the power rate was increased from C/2 to C/1, and decreased by-20% when the power rate was increased from C/1 to C/0.5, comparable to graphite electrodes.

After 20 cycles at C/1 rate through charge-discharge cycling, the embedded capacity was found to be from 410mA h g^-1Slightly reduced to 370mA h g^-1And the extraction capacity is kept at 375mA h g^-1And 355mA h g^-1In the meantime. The total coulombic efficiency (i.e., the ratio of the take-out capacity to the built-in capacity) was-94% except for the formation of SEI on the CNF110 surface in the first two cycles. SEI films are known to readily form on carbonaceous anodes during initial cycling, which allows lithium ions to diffuse, but are electrically insulating, resulting in increased series resistance. TEM images (fig. 3C) and SEM images (fig. 6A) show that an uneven thin film was deposited on the CNF110 surface during charge-discharge cycles. In some embodiments, the SEI acts as a sheath to increase the mechanical strength of the CNF110, preventing them from collapsing into clumps by the cohesive capillary forces of the solvents as observed in studies using other polymer coatings.

Fig. 6A-6C illustrate CNF110 scanning electron microscope images that are 3 μm long according to various embodiments of the present invention. Fig. 6A shows CNF110 in a delithiated (discharged) state after an insertion/extraction cycle. Fig. 6B shows CNF110 including silicon layer 115 in a delithiated state after 100 cycles. Fig. 6C shows CNF110 including silicon layer 115 in a lithiated state after 100 cycles. These images are in a perspective view at 45 degrees.

Fig. 7A-7C illustrate results obtained using CNF110 including silicon layer 115 as the anode of a lithium ion battery. These results were obtained using a nominal silicon thickness of 0.50 μm. FIG. 7A shows s at 0.10, 0.50 and 1.0mV^-1At scanning rate relative to Li/Li⁺Cyclic voltammogram between 1.5V and 0.05V. The measurements were taken after the samples had undergone 150 charge-discharge cycles and data for the second cycle at each scan rate is shown. FIG. 7B shows the electrostatic charge-discharge curves of the samples at C/0.5, C/1 and C/2 power rates, 120 cycles. All curves were taken from the second cycle at each ratio. Fig. 7C shows the insertion and extraction capacity (to the left vertical axis) and coulombic efficiency (to the right vertical axis) of two CNF arrays 100 (used as electrodes) as a function of the number of charge-discharge cycles. The first CNF array 100 is first adjusted with one cycle at C/10 rate, one cycle at C/5 rate, and two cycles at C/2 rate. The remainder of the 96 cycles were then tested at the C/2 insertion rate and C/5 removal rate. The solid and hollow squares represent the insertion and extraction volumes, respectively. The second electrode was first conditioned in two cycles at rates of C/10, C/5, C/2, C/1, C/0.5, C/0.2, respectively. The next 88 cycles were then tested at the C/1 rate. The coulombic efficiencies of the two electrodes are expressed in solid diamonds (first electrode) and open diamonds (second electrode), with a majority of the overlap of 99%.

The CV in fig. 7A presents characteristics very similar to those of silicon nanowires. Li in comparison to uncoated CNF array 110⁺Embedded cathode wave and Li⁺Both of the extracted anodic waves are shifted to lower values (below-0.5 and 0.7V, respectively). After application of silicon layer 115, the peak current density increases by a factor of 10 to 30 and is proportional to the scan rate. Apparently, alloy-formed Li⁺Intercalation into silicon is much faster than that into uncoated CNF, which is limited by Li between graphitic layers⁺Slow diffusion of (2). In previous studies of pure silicon nanowires, no cathodic peak at-0.28V was observed. The three anodic peaks representing the transfer of lithium silicon alloy into amorphous silicon are similar to those using silicon nanowires, although shifted 100 to 200mV to lower potentials.

The electrostatic charge-discharge curve for a CNF array comprising silicon layer 115 shown in fig. 7B includes two notable features: (1) even after 120 cycles at a C/2 rate, -3000 mA h (g) was obtained_Si)^-1High Li⁺Insertion (charging) and extraction (discharging) capacities; and (2) Li at C/2, C/1, C/0.5 power rates⁺The capacities are almost the same. In other words, the capacity of CNF array 100 operating as an electrode does not decrease when the charge rate increases from C/2 to C/1 and C/0.5. With respect to these charging rates, the capacity is in various embodiments nearly independent of the charging rate. Total Li for CNF array 100 including silicon layer 115⁺The storage capacity is about 10 times higher than CNF array 100 lacking silicon layer 115. This occurs even if the low potential limit of the charging cycle is increased from 0.001V to 0.050V. Thus, Li⁺The amount of embedding into the CNF core appears to be negligible. The specific capacity is calculated by dividing only the silicon mass, which is determined from the measured nominal thickness and 2.33g cm^-3The bulk density of (3) is calculated. This method is chosen as a suitable metric for comparing the specific capacity of the silicon layer 115 with the theoretical value of bulk silicon. For a 3.0 μm long CNF110 deposited with a 0.456 μm nominal thickness of silicon layer 115, the actual mass density of silicon layer 115 is-1.06 x10^-4g cm^-2Mass density of the CNF110 (-1.1X 10)^-4g cm^-2). The corresponding coulombic efficiency in fig. 7B is greater than 99% at all three power rates, which is much higher than the coulombic efficiency of CNF110 without silicon layer 115.

Fig. 8 illustrates how the capacity of CNF array 100 varies with charge rate according to various embodiments of the present invention. Data for a number of cycles are shown. Fig. 8 shows the average specific discharge capacity for a set of cycles using the same current ratio as a function of the charge rate (C-ratio) required to reach the total capacity (C/h, e.g., total capacity/hour) in a set hour. The vertical lines are centered at C/4, 1C, 3C and 8C. CNF array 100 is first adjusted symmetrically in two cycles at rates C/8, C/4, C/2, C/1, C/0.8, C/0.4, and C/0.16, respectively, and then tested at a C/1 symmetry ratio for the next 88 cycles. The above steps are repeated from 101 cycles to 200 cycles. Starting at 201 cycles, the electrodes were cycled symmetrically with five cycles under each of C/4, C/3, C/2, C/1, C/0.75, C/0.66, C/0.50, C/0.33, C/0.25, C/0.20, and C/0.15, and then tested at the C/1 symmetry ratio for the next 45 cycles. The above steps are repeated from 301 cycles to 400 cycles and from 401 cycles to 500 cycles. When the C ratio was changed by 32 times, the change in capacity was small (< 16%). After 100 cycles, the electrode showed increased capacity when the C ratio was changed from 3C to 8C. Thus, faster charging rates result in improved capacity. At both high and lower ratios (C/4 and 8C), high capacity (>2,700mAh/g) was obtained. As the C ratio increases, the capacity at ratios above 3C increases. The decrease in specific capacity with cycle number is due to known correctable factors.

Both CV and charge-discharge measurements indicate Li⁺Embedding into silicon layer 115 is rapid and highly reversible, which is a desirable feature for high performance lithium ion battery anodes. This was further demonstrated with two long cycle tests on two identical samples under different test conditions (see fig. 7C): (1) slow asymmetry test at C/2 rate for embedding and C/5 rate for extraction; and (2) fast symmetry testing at the C/1 rate for both embedding and extraction. The two data sets showed long cycles in addition to the initial conditioning cycle (4 cycles of the former and 12 cycles of the latter at different low ratios)>A coulomb efficiency of 98%. In the slow asymmetry test, the embedded capacity dropped only 8.3% from 3643mA h g at cycle 5^-1To 3341mA h g at cycle 100^-1. Even at the C/1 charge-discharge rate, the insertion capacity decreased only 11%, from 3096mA h g at cycle 13^-1To 2752mA hr g at 100 th cycle^-1. Li between these two data sets⁺The difference in capacity is mainly attributable to the initial conditioning parameters and small sample-to-sample variations. This is due to the similar values of the insert-extract capacities during the first few conditioning cycles at C/10 and C/5 rates in FIG. 7CNote that. The faster ratios (C/0.5 for

cycles

9 and 10 and C/0.2 for cycles 11 and 12 of sample # 2) were found to be detrimental and cause irreversible decreases in capacity. However, the electrode becomes stable after longer cycles. As shown in FIG. 7B, the charge-discharge curves measured with sample #1 after 120 cycles were almost the same at the C/2, C/1 and C/0.5 rates. This is for a four times charging rate variation.

At 3000 to 3650mA h g^-1The specific capacity of silicon layer 115 within the range is consistent with the highest value of amorphous silicon anodes outlined in the literature. Notably, the entire silicon shell in CNF array 110 is paired with Li⁺Intercalation is active and retains almost 90% of its capacity over 120 cycles, to our knowledge, except for flat ultrathin films (<50nm) silicon film, which has never been achieved before. The specific capacities disclosed herein are significantly higher at similar power rates than reported using other nanostructured silicon materials, including-2500 mA hr g of silicon nanowires at C/2 rates^-1And-2200 mA hr g at C/1 rate^-1And-800 mA h g at C/1 rate with randomly oriented carbon nanofiber-silicon core-shell nanowires^-1. Clearly, the coaxial core-shell nanowire structure on well-separated CNFs 110, such as included in various embodiments of the present invention, provides enhanced charge-discharge rates, almost complete Li of silicon, relative to the prior art⁺Storage capacity and long cycle life.

As shown in FIG. 7C, the abnormally high embedding capacity (-4500mA hr g)^-1) Always observed in the initial cycle, which is 20-30% higher compared to the later cycle. In contrast, the take-off value is relatively stable throughout the cycle. The extraordinary intercalation capacity can be attributed to a combination of three irreversible reactions: (1) forming a thin SEI (surface electrode interphase) layer (of several tens of nanometers); (2) lithium with SiO present on the surface of the silicon_xReaction (SiO)_x+2xLi→Si+xLi₂O); and (3) will have a higher theoretical capacity (-4200mA h g^-1) Conversion of the starting crystalline silicon coating to a lower capacity<3800mA h g^-1) Amorphous silicon of (2). TEM image (FIG. 3C) and SEM image (FIG. 6B) showIt is shown that after charge-discharge cycling, an uneven SEI can be deposited on the surface of silicon layer 115. Such an elastic SEI film can help to fix silicon layer 115 on the surface of CNF110 when CNF array 110 is subjected to a bulky expansion-contraction cycle that occurs during charge-discharge cycles. The significant difference between the SEM images in fig. 6B and 6C indicates a large expansion of the silicon layer 115 in the lithiated (charged) state relative to the non-lithiated state. Note that the generation of SEI during the initial charge-discharge cycle causes the difference seen in silicon layer 115 between fig. 3A and 3B (although some swelling may be due to oxidation of lithium by air when the electrochemical cell is disassembled for imaging). In fig. 3B, silicon interacts with the electrolyte to produce an SEI that fills the gaps between the feather-like structures. The interaction may include mixing, chemical reaction, charge coupling, encapsulation, and/or the like. Thus, silicon layer 115 appears relatively uniform in fig. 3B. However, silicon layer 115 now comprises interleaved silicon layers (feathered structures) and SEI layers. Each of these intersecting layers may be on the order of tens of nanometers. The SEI layer may be an ion permeable material that is the product of the interaction between the electrolyte and the silicon layer 115 (or other electrode material).

The crystalline and amorphous structure of the silica shell is visualized by raman spectroscopy. As shown in fig. 9, original CNF array 100 comprising silicon layer 115 shows a thickness of 350 to 550cm corresponding to amorphous silicon^-1Multiple broad bands overlapping in range and corresponding to nano-crystal silicon at 480cm^-1Higher sharp bands at the point. After the charge-discharge test, the sharp peak disappeared while the broad band was merged to 470cm^-1A single peak at (a). Bare CNF110 does not show any features in this range. The crystalline silicon peak shifts down-40 cm from the peak measured with a single crystal silicon (100) wafer^-1And moves down-20 to 30cm from other microcrystalline silicon materials^-1. This shift is likely due to much smaller crystal size and large disorder. The initial silicon layer 115 is likely to consist of nanocrystals embedded in an amorphous matrix associated with the feathered TEM image in fig. 3A. After the initial cycling, the silicon nanocrystals were converted to amorphous silicon, consistent with TEM images after cycling tests (see fig. 3B and 3C). However, it is not limited toSilicon layer 115 apparently does not slide along the length of the CNF compared to the large longitudinal expansion of pure silicon nanowires (up to 100%). This indicates that in some embodiments, the expansion of silicon relative to the carbon nanofibers is primarily radial rather than longitudinal. In some embodiments, expansion occurs between feather-like structures of Si. For example, the expansion of one feather may be in the direction of the nearest neighboring feather above and below, thereby filling the gaps between the feathers. In either case, the expansion that occurs in this manner results in a significant reduction in delamination of the silicon relative to the prior art. Thus, silicon layer 115 adheres strongly to CNF110 over 120 cycles. In Li⁺The volume change of the silicon shell during embedding is controlled by the radial expansion while the CNF-silicon interface remains intact.

Various embodiments of the present invention include CNFs 110 having different lengths and silicon shell thicknesses. One factor that may be controlled when generating the CNFs 110 is the open space between each CNF110, e.g., the average distance between CNFs 110 within the CNF array 100. This space allows silicon layer 115 to expand radially when charged, and thus provides stability in some embodiments. Because the optimal electrode structure depends on both the length of CNF110 and the thickness of silicon layer 115, it is sometimes desirable to use longer CNFs 110 and thicker silicon layers 115 in order to obtain higher total Li⁺A storage capacity. Longer CNFs 110 are associated with greater storage capacity. FIGS. 10A-10C show Li in 15 charge-discharge cycles using three 10 μm long CNF110 samples with silicon layers 115 deposited with nominal thicknesses of 0.50, 1.5, and 4.0 μm, respectively⁺Variations in insertion-extraction capacity and coulombic efficiency. After adjustment at the C/10 rate for the first cycle and at the C/5 rate for the second cycle, the asymmetric ratio (C/2 for insertion and C/5 for withdrawal) was used in the subsequent cycle similar to the measurement of sample #1 in FIG. 7C. This scheme provides nearly 100% coulombic efficiency and minimal drop off during cycling. During sputtering, the nominal thickness is measured in situ using a quartz crystal microbalance.

Up to 3597mA h g was obtained with silicon layers 115 of 0.50 and 1.5 μm thickness, respectively^-1And 3416 mA h g^-1Which is very similar to the specific capacity of a 0.50 μm thick silicon layer 115 used on a 3.0 μm long CNF110 (see fig. 7C). The capacity remained almost unchanged over 15 cycles. However, an electrode with a nominal silicon thickness of 4.0 μm showed only 2221mA hr g^-1Significantly lower specific capacity. This indicates that silicon layers 115 come into contact with each other from adjacent CNFs 110 due to expansion, limiting their further expansion and limiting lithium diffusion between CNFs 110. As a result, only a small part of the silicon coating is active in lithium intercalation. The cycling stability is correspondingly worse than for the sample with the thinner silicon layer 115.

The same amount of Si (500nm nominal thickness) on a CNF array 110 comprising 10 μm long CNFs 110 gives Li with 3 μm long CNFs 110⁺Storage capacity (3643mA h g)^-1See FIG. 7C) almost the same amount of Li⁺Storage capacity (3597mA h g^-1See fig. 6A), although the carbon mass is more than 3 times. This is strong evidence that the contribution of CNF110 is in the calculation of Li⁺Is negligible in storage. Very little Li⁺Ion intercalation into CNF110 in silicon coated samples is likely, which contributes to structural stability over multiple charge-discharge cycles.

Li in three samples well correlated with the structure of three samples⁺The change in stored specific capacity is shown by the SEM pictures illustrated in fig. 11A-11C. FIGS. 11A-11C show scanning electron microscope images of a newly prepared CNF array 100 (on a-10 μm long CNF 110). Silicon layer 115 was grown using nominal silicon thicknesses of (a) 0.50 μm, (b)1.5 μm, and (c)4.0 μm, measured in situ during deposition using a quartz crystal microbalance. All images are 45 perspective. At 0.50 μm nominal silicon thickness, the average tip diameter on a 10 μm long CNF was found to be-388 nm, much smaller than the average diameter of-457 nm on a 3.0 μm long CNF 110. Silicon layer 115 is thinner but more uniformly spread along 10 μm long CNF 110.

It should be noted that it takes 120 minutes to grow 10 μm CNF110, which is about six times as long as growing 3 μm CNF 110. Some nickel catalysts pass NH during long PECVD processes₃Is slowly etched, resulting in a nickel nanoparticle sizeContinues to decrease and results in a tapered tip 120 (as shown in fig. 12). The length variation of CNFs 110 also increases with long CNFs 110. These factors together reduce the shielding effect of the tip 120. As a result, CNFs 110 coated with silicon layer 115 are well separated from each other even at a nominal silicon thickness of 1.5 μm. The SEM image of 1.5 μm silicon on 10 μm CNF array 100 (fig. 11B) is very similar to the SEM image of 0.50 μm silicon on 3.0 μm CNF array 110 (fig. 2B). But when the nominal silicon thickness is increased to 4.0 μm, silicon layers 115 significantly merge with each other and fill most of the space between CNFs 110 (see fig. 10C). This reduces the free space required to accommodate the volumetric expansion of silicon layer 115. As a result, Li⁺The storage specific capacity is obviously reduced.

Fig. 11A and 11B each include approximately the same number of CNFs 110, however, with substantially fewer visible tips 120 in fig. 11B. This is because silicon layer 115 is capable of forming a nanofiber/silicon composite that includes a single CNF110 (a cross-section of which is shown in fig. 1A). Alternatively, silicon layer 115 can form a nanofiber/silicon composite that includes two, three, or more CNFs 110 under a single silicon cap. This occurs when two or more CNFs 110 come together during the silicon layer 115 deposition process. A nanofiber/silicon composite is a structure that includes a continuous silicon layer 115 encapsulating one or more CNFs 110. A cross-section of a nanofiber/silicon composite comprising two CNFs 110 is illustrated in fig. 11D. In various embodiments, at least 1%, 5%, or 10% of the nanofiber/silicon composites comprise more than one CNF 110.

In various embodiments, examples of CNF array 100 with nominal silicon thicknesses of 0.50 and 1.5 μm have 3208 + -343 and 3212 + -234 mA h g, respectively^-1Comparable mass-specific capacity. Samples with a nominal silicon thickness of 4.0 μm produced 2072. + -. 298mA hr g^-1Much lower capacity. Thinner silicon coatings are fully activated and provide the maximum lithium insertion capacity that amorphous silicon can deliver. On the other hand, the area-specific capacity varies with the silicon thickness from 0.373. + -. 0.040mA h cm with a silicon thickness of 0.50 μm^-2Proportionally increased to 1.12 + -0.08 mA h cm of silicon thickness of 1.5 μm^-2But dropped from the linear curve to produce 1.93 + -0.28 mA at 4.0 μm nominal silicon thicknessh cm^-2. At a nominal silicon thickness of 4.0 μm, only a small fraction of the additional silicon in the thick silicon coating is actively involved in lithium storage. The thickness of 4.0 μm is larger than the average distance between CNFs 110. The electrochemical results are consistent with the structure shown in the SEM image in fig. 11C, which shows that the spaces between CNFs 110 are substantially filled.

In various embodiments of the invention, the structure of the CNF array 100 comprises a silicon layer of about 200 to 300nm radial thickness on a CNF110, the CNF110 having a length of about 30-40, 40-75, 75-125 microns (or greater or combinations thereof) and a diameter of about-50 nm. In some embodiments, these CNF arrays 100 are grown on conductive foil having a thickness in the range of-10 microns, -10-20 microns, -10-50 microns, or greater. In various embodiments, silicon (corresponding to a nominal thickness of 1.5 μm on a plane) is deposited on CNFs 100 that are 10 μm long to form CNF arrays 100. This is done while maintaining an open vertical-type core-shell nanowire structure with individual CNFs 110 well separated from each other, so that lithium ions can penetrate the CNF array 100 between the CNFs 110. This unique hybrid configuration allows silicon layer 115 to be Li on⁺Free to expand/contract in the radial direction during insertion and extraction. Even at a C/1 rate, with 3000 to 3650mA h g^-1High performance lithium storage of mass-to-specific capacity. This capacity matches the maximum expected from amorphous silicon of similar quality, indicating that the silicon layer 115 is fully activated. This 3D nanostructured configuration enables efficient electrical connection of large amounts of silicon material while maintaining short Li⁺An insert-extract path. As a result, a high capacity approaching the theoretical limit over 120 charge-discharge cycles is possible. When the ratio is increased by a factor of 20 from C/10 to C/0.5 (or 2C), the change in capacity is small. The significantly improved charge and power rates and unusually high capacity at cycling stability make this new structure an alternative anode material for high performance lithium ion batteries. The same core-shell concept can be achieved by using TiO₂、LiCoO₂、LiNiO₂、LiMn₂O₄、LiFePO₄、Li₂O、Li₂O₂Or the like is applied to the cathode instead of the silicon shellA material.

Fig. 13 illustrates a method of producing the CNF array 100 and/or CNF110 disclosed herein. In a provide substrate step 1310, a substrate 105 is provided. Substrate 105 is optionally suitable for CNF110 growth. The substrate 105 may comprise a variety of materials, such as copper. The substrate 105 may alternatively be a conductive foil having a thickness as described elsewhere herein. In an optional provide nucleation sites step 1320, nucleation sites for growth of CNF110 are provided on substrate 105. A variety of nucleating materials, such as nickel particles, are known in the art. The nucleation sites may optionally be provided in a density such that an average distance between CNFs 110 is produced, such as the densities taught elsewhere herein. Providing nucleation sites step 1320 is optional in embodiments where nucleation is not required for growth of CNF110 or similar structures, or where adhesive is used to attach CNF110 to substrate 105 after being grown elsewhere.

In the grow CNF step 1330, CNF110 is grown on substrate 105, or in some embodiments, is isolated from substrate 105. CNF110 may optionally be grown to produce stacked cone structures as taught elsewhere herein to produce structures with exposed graphite edges along their length, or to produce similar variable structures. CNF110 can be grown to any length as taught elsewhere herein. Growth may alternatively be accomplished using PECVD methods such as those taught or referenced in "A high-performance lithium-ion battery based on the core-shell chromatography structure of silicon-coated vertically aligned carbon n-fibers" Klankowski et al J.Mater.chem.A., 2013,1, 1055.

In apply silicon layer step 1340, an embedding material, such as silicon layer 115, is applied to the grown CNF 110. In some embodiments, applying Si layer step 1340 occurs before CNF110 is attached to substrate 105. The applied material may have any nominal thickness as taught elsewhere herein so as to produce a silicon layer 115 thickness of tens or hundreds of nanometers. In some embodiments, applying the Si layer step 1340 includes growing an embedding material in the exposed edge-dependent structure along the length of the CNF 110. For example, when CNF110 includes the cup-like structures discussed herein, applying Si layer step 1340 includes growing feather-like structures as illustrated in the figures, such as fig. 3A.

In an optional apply PEM step 1345, a Power Enhancing Material (PEM) is added to CNF array 100 or CNF 110. In some embodiments, applying PEM step 1345 occurs before CNF110 is attached to substrate 105. The PEM typically includes an adhesive and surface effect dominant sites, as discussed in more detail elsewhere herein. In an optional conditioning step 1350, the CNF array 100 produced using steps 1310-.

Fig. 14A illustrates a CNF110 attached to a substrate 105 and a tie layer 1410, wherein the CNF110 includes a power enhancing material 1420, according to various embodiments of the invention. The power enhancement material 1420 is applied as a layer over the embedding material, for example over the silicon layer 115. Fig. 14B illustrates details of the power enhancing material 1420 illustrated in fig. 14A, in accordance with various embodiments of the present invention. Power enhancement material 1420 includes surface effect dominant sites 1430 and optional adhesive 1440. Silicon layer 115 is but one example of an embedding material. While silicon layer 115 is used as an example herein, it is understood that other types of embedding materials can be substituted for or combined with silicon. Such alternative or additional intercalation materials include Ag, Al, Bi, C, Se, Sb, Sn and Zn. The CNF110 illustrated in fig. 14A is typically one of many CNFs 110 within the CNF array 100.

In some embodiments, the surface effect dominant sites 1430 include a surface of a nanoparticle configured to adsorb charge carriers in faradaic interactions, e.g., so as to undergo a redox reaction with the charge carriers. Because, typically, for these nanoparticles, the interaction of induced current between the charge carriers and the nanoparticle surface dominates the interaction of the bulk induced current, they are referred to as "surface effect dominated". Thus, the charge carriers are more likely to react at the surface than the bulk of the nanoparticles. For example, lithium ions will be more likely to adsorb onto the nanoparticle surface rather than being absorbed into the bulk of the nanoparticle. These nanoparticles are sometimes referred to as surface redox particles. The interaction of the induced currents results in pseudocapacitors (pseudocapacitors) that are capable of storing a large amount of loosely bound charge and thus provide significant power density. Under the pseudocapacitance, electrons are exchanged (e.g., supplied). In this case between the charge carriers to the nanoparticles. When some of the electrical potential results in some intercalation of charge carriers into the nanoparticles, this does not constitute a host of interaction at the surface effect dominant sites 1430 and can degrade some types of nanoparticles. The interaction of the induced current is an interaction that transfers (e.g., is supplied with) electric charges due to an electrochemical interaction.

The nanoparticles including the surface effect dominant sites 1430 can be composed of a transition metal oxide, such as TiO₂、Va₂O₅、MnO、MnO₂NiO, tantalum oxide, ruthenium oxide, rubidium oxide, tin oxide, cobalt oxide, nickel oxide, copper oxide, iron oxide, and/or the like. They may also be composed of metal nitrides, carbon, activated carbon, graphene, graphite, titanates (Li)₄Ti₅O₁₂) Crystalline silicon, tin, germanium, metal hydrides, iron phosphate, polyaniline, mesophase carbon, and/or the like. It is to be appreciated that mixtures of the above and/or other materials having desired induced current properties can be included in the surface effect dominant sites 1430. In various embodiments, these nanoparticles can be less than 1, 2, 3, 5, 8, 13, 21, or 34 nanometers in diameter. The lower limit of the nanoparticle size is a function of the size of the molecules that make up the material. The nanoparticles include at least some molecules. The smaller size provides a larger surface to volume ratio of possible adsorption sites. However, particles comprising only one pair of molecules have reduced stability. The nanoparticles may optionally be multilayered. For example, they can be included in the transition metals Co, Ni, Mn, Ta, Ru, Rb, Ti, Sn, V₂O₂TiO on FeO, Cu or Fe cores₂A layer (or any other nanomaterial discussed herein) or a graphene/graphite layer on a core of some other material. In some embodiments, the different core materials affect the reaction potential of the surface material. Amount of surface effect dominant sites 1430Optionally selected according to the desired power and energy density. For example, a greater power density may be achieved by having a greater number of surface effect dominant sites 1430 per mass of embedding material, or a greater energy density may be achieved by having a greater amount of embedding material per number of surface effect dominant sites 1430. An advantage of some embodiments of the present invention is that both historically high energy and power densities can be achieved simultaneously.

By adsorbing charge carriers on the surface of the nanoparticles, the charge carriers are able to provide a power density such as previously obtained with capacitors alone. This is because the release of charge does not depend on the diffusion of charge carriers but on the intercalation material. In addition, by placing the surface effect dominant sites 1430 in close proximity to the intercalation material, charge carriers can move from the intercalation material to the surface effect dominant sites 1430 (or directly to the electrolyte). This results in an energy density equal to or greater than that of conventional batteries. Both the energy density of the battery and the power density of the capacitor are obtained in the same device. It should be noted that during discharge, charge carriers embedded within the material are able to move to the surface effect dominant sites 1430 and thus recharge these sites.

In some embodiments, the surface effect dominant sites 1430 are disposed on larger particles. For example, the particle size may be greater than 1,10, 25, 100 or 250 microns (but substantially less than 1 mm). Activated carbon, graphite, and graphene are materials that can be included in particles of these sizes. For example, activated carbon can be included in the power enhancing material 1320 when the activated carbon has a pore size with surface effect dominant sites 1430 that is similar to the nanoparticle diameter of the teachings above. For the purposes of this disclosure, nanoparticles are particles having an average diameter of less than 1 μm.

Optional adhesive 1440 is configured to hold surface effect dominant sites 1430 in proximity to the embedded material. In some embodiments, the distribution of surface effect dominant sites 1430 is uniform throughout adhesive 1440. For example, nanoparticles including surface effect dominant sites 1430 may be mixed with adhesive 1440 prior to application of adhesive 1440 to an embedding material to produce a relatively uniform distribution. Alternatively, the nanoparticles may be applied to the surface of the embedding material prior to application of the binder 1440. This can result in a greater concentration of surface effect dominant sites 1430 (within adhesive 1440) closest to the embedding material than in areas of adhesive 1440 away from the embedding material. Adhesive 1440 is optional in embodiments where surface effect dominant sites 1430 or associated nanoparticles are attached directly to the embedding material, e.g., to silicon layer 115.

The adhesive 1440 is permeable (e.g., porous) to charge carriers of the electrolyte. Examples of suitable materials for binder 1440 include polyvinylidene fluoride (PVDF), styrene butadiene rubber, poly (acrylic acid) (PAA), carboxymethyl cellulose (CMC), and/or the like. Other adhesives that meet permeability requirements may be used. Adhesive 1440 may optionally include a material that increases its electrical conductivity. For example, the binder 1440 may include conductive polymers, graphite, graphene, metal nanoparticles, carbon nanotubes, carbon nanofibers, metal nanowires, super-P (conductive carbon black), and/or the like. The material is preferably in a concentration high enough to make the adhesive 1440 conductive, e.g., at the percolation threshold.

The addition of surface effect dominant sites 1430 in close proximity to the embedding material (e.g., silicon layer 115) does not necessarily require the use of vertically aligned CNFs 110 or any supporting wires. For example, fig. 15 illustrates an electrode surface including a power enhancing material 1320 and unaligned CNFs 110 coated by an intercalation material, according to various embodiments of the invention. In these embodiments, CNF110 is not directly attached to substrate 110, but is held in close proximity to substrate 110 by adhesive 1440. In some embodiments, CNF110, including a cup-like structure such as that illustrated in fig. 3B, is used in a non-connected configuration such as that illustrated in fig. 15. In these embodiments, the cup-shaped structure still helps prevent delamination of the silicon from the underlying CNF 110. While CNF110 is used herein as an example of a supporting filament, it is to be understood that other types of supporting filaments discussed herein in any example can be used in addition to or in place of the carbon nanofibers of CNF 110.

The embodiment illustrated by fig. 15 can be produced, for example, by first growing unattached CNFs 110. These are then coated with silicon layer 115 (or some other embedding material) so that the embedding material is substantially in contact with CNF110 as a coating layer. The coated CNF110 is then mixed with surface effect dominant sites 1430 and adhesive 1440. Finally, the resulting mixture is disposed on a substrate 105.

Fig. 16 illustrates an electrode surface including power enhancing material 1320, unaligned CNFs 110, and free embedding material 1610, according to various embodiments of the invention. In these embodiments, the embedding material 1610 need not be disposed as a coating around the CNF 110. The embedding material 1610 is free, meaning that the embedding material 1610 is not confined to the CNF110 surface, yet it is held close to the substrate 105 by the adhesive 1440.

The embodiment illustrated in fig. 16 can be produced, for example, by co-mixing (in any order) adhesive 1440, surface effect dominant sites 1430, embedment material 1610, and CNF 110. The mixture is then applied to a substrate 105. In these embodiments, CNF110 may or may not be attached to substrate 105 by means other than adhesive 1440. The embedding material 1610 may and/or may not contact the CNF110 or the substrate 105. Likewise, surface effect dominant sites 1430 may optionally be in contact with substrate 105, CNF110, and/or embedded material 1610. The embedding material 1610 may optionally include particles, suspensions, clusters, and/or droplets of embedding material having a size of at least 0.1, 0.6, 1, 1.5, 2, 3, 5, 7, 9, 10, 13, 15, 18, 21, or 29 μm, or any range therebetween. In alternative embodiments, other sizes are possible.

Fig. 17 illustrates an electrode surface including adhesive 1440, surface effect dominant sites 1430, and embedding material 1610 without support filaments according to various embodiments of the invention. In these embodiments, surface effect dominant sites 1430 and embedded material 1610 are held in proximity to substrate 105 by adhesive 1440.

Fig. 18 illustrates an electrode surface similar to that illustrated in fig. 15. However, in the embodiment illustrated by fig. 18, surface effect dominant sites 1430 are concentrated in close proximity to the embedment material 1610. For example, in some embodiments, at least 2%, 10%, 25%, 50%, 75%, or 85% of the surface effect dominant sites 1430 are on the particles in contact with the embedment material 1610. The increased concentration of surface effect dominant sites 1430 closest to the embedding material 1610 can be obtained using methods described elsewhere herein. This results in a greater concentration of surface effect dominant sites 1430 at the surface of embedded material 1610 relative to other volumes within adhesive 1440.

Fig. 14C, 19, and 20 illustrate electrode surfaces similar to those illustrated in fig. 14B, 16, and 17, respectively. However, in the embodiment illustrated by these figures, surface effect dominant sites 1430 are disposed in close proximity to the free embedding material, according to various embodiments of the invention. As in the embodiment illustrated by fig. 18, in some embodiments, at least 2%, 10%, 25%, 50%, 75%, or 85% of surface effect dominant sites 1430 are in contact with embedment material 1610. In some embodiments, a higher concentration of nanoparticles including surface effect dominant sites 1430 is disposed within the surface of the embedding material 1610 at 5 nanometers than between these surfaces at 10 and 15 nanometers. The increased concentration of surface effect dominant sites 1430 closest to the embedment material 1610 can be obtained by selecting an appropriate zeta potential of the nanoparticles and the embedment material 1610 in solution such that the nanoparticles form an electrostatic bilayer at the surface of the embedment material 1610. The zeta potential is the potential of the interfacial bilayer at the location of the surface relative to a point in the bulk liquid remote from the surface. The zeta potential can alternatively be higher than 25mV (absolute). In other embodiments, the nanoparticles are applied to the surface of the embedding material 1610 prior to application of the adhesive 1440.

The embedding material 1610 as illustrated in fig. 16-20 can include any single or combination of materials discussed herein (including or not including silicon) relative to the silicon layer 115. Likewise, CNF110 as illustrated in fig. 16-20 can include any single one or combination of the various types of fibers discussed herein (including or excluding carbon nanofibers). For example, the CNFs 110 may include branched fibers, multi-walled fibers, wires, aerogels, graphite, carbon, graphene, boron nitride nanotubes, and the like. The number of surface effect dominant sites 1430 and CNFs 110 shown in these and other figures herein is for illustration purposes only. For example, in practice the number of surface effect dominant sites 1430 can be greater. Likewise, the amount and size of embedding material 1610 and silicon layer 115 are for illustration purposes. Alternative embodiments may include greater or lesser amounts and greater or lesser dimensions. Likewise, the depth of the PEM 1420 and the length of the CNF110 can be different than the depth and length shown in the figures.

In various embodiments, the amount of nanoparticles that include surface effect dominant sites 1430 may be selected so as to result in at least 0.1, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 2, 3, 5, 10, 25, 50, or 100 (or any range therebetween) times (as measured in the discharge state) the monolayer of nanoparticles on the surface of the embedding material 1610 or silicon layer 115. As used herein, a 0.1 monolayer means 10% and a 10 times monolayer is a 10 monolayer. In various embodiments, the amount of nanoparticles that include surface effect dominant sites 1430 can be selected to result in at least 1,5, 10, 20, 50, 100, 250, or 500 nanolayers (or any combination therebetween) of nanoparticles on the surface of the embedded material 1610 (as measured in the discharge state). Other densities of coverage, as measured in a single layer or depth, are possible. When the coverage of nanoparticles (including surface effect dominant sites 1430) approaches a monolayer of 1.0 layers, the nanoparticles are able to form a layer between the embedding material 1610 and the charge carriers of the electrolyte that move throughout the adhesive 1440. For example, in some embodiments, the electrolyte includes lithium as a charge carrier. Lithium is able to move throughout the adhesive 1440 and undergo a faradaic reaction with the surface effect dominant sites 1430, where electrons are supplied to the lithium from one of the surface effect dominant sites 1430. The electrons are transferred (e.g., supplied) from the substrate 105 to the nanoparticles via the embedding material 1610. Because the nanoparticles form a barrier, only a limited amount of charge carriers reach the embedding material 1610 at this stage of the charging process. Charging is controlled by the reaction at the surface effect dominant site 1430. In some embodiments, charging can be rapid because it is not necessary that the charge carriers be embedded in the embedding material 1610 before the faradaic reaction with the charge carriers occurs. The presence of the surface effect dominant sites 1430 greatly increases the surface area over which the reaction of the induced current initially prior to embedding can occur. Surface effect dominant sites 1430 facilitate the embedding of charge carriers into embedding material 1610. The charge carriers can be embedded in a form as received at the surface effect dominant sites 1430 or in an alternative form such as a metal oxide. If intercalated in a metal oxide, the oxygen of the oxide may be recycled back to the surface effect dominant sites 1430 after intercalation.

In some embodiments, because the nanoparticles form an incomplete barrier, some charge carriers still reach the embedding material 1610 during the charging phase (e.g., the initial phase of charging an energy storage device including an electrode as discussed herein). Because some embodiments of the embedding material 1610, such as silicon, expand when charge carrier embedding occurs, the surface area of the embedding material 1610 is also increased. This reduces the surface coverage of the nanoparticles on the surface of the embedding material 1610 and reduces the effectiveness of the nanoparticles to form a barrier to charge carriers. Thus, a greater number of charge carriers per unit time can reach the embedding material 1610 as charging proceeds. This optionally continues until charging is controlled by the reaction within the embedding material 1610. The reduction in surface coverage may also increase the average fraction of surface effect dominant sites 1430 on each nanoparticle exposed to the electrolyte. The phrase "surface coverage" as used herein is used to represent the density of a substance on a surface and may be measured as the number of monolayers (or fraction thereof), as thickness, or as concentration, etc.

In some embodiments, energy storage at the surface effect dominant sites 1430 occurs at an electrical potential at which surface reactions of the induced current occur but charge carrier intercalation into the nanoparticles including the surface effect dominant sites 1430 does not occur. This prevents degradation of the nanoparticles due to repeated intercalation and de-intercalation of charge carriers and allows for longer cycle life. Under the same electrode, it may be desirable to store energy within the embedding material 1610 by reactions of induced current that occur at higher potentials, which may optionally include potentials that cause charge carriers to embed into nanoparticles having surface effect dominant sites 1430. This can occur in some embodiments of the invention because there is a potential drop between the substrate 105 and the electrolyte 125.

In a specific example, where lithium is the charge carrier, the surface effect dominant sites 1430 are on the TiO₂On the nanoparticles and the embedding material 1610 is mainly silicon. In other embodiments, it will be appreciated that the particular voltage depends on the chemicals included in the surface effect dominant sites 1430 and the embedding material 1610, the reactions that occur during charging, and the like. In various embodiments, the difference in electrical potential between the surface effect dominant sites 1430 and the substrate 105 is at least 0.001, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0, 1.3, 1.7, 2.0, 2.2, or 2.4V, or any range therebetween. The term "potential" as used herein is used to refer to the absolute value of the electrostatic potential (e.g., | x |).

Fig. 21 illustrates a method of assembling an electrode surface according to various embodiments of the invention. The assembled electrode surface may be used, for example, as an anode in a battery, capacitor, or hybrid device. The method illustrated in fig. 21 may alternatively be used to produce various electrodes discussed elsewhere herein.

In provide substrate step 2110, a conductive substrate is provided. The provide substrate step 2110 is similar to the provide substrate step 1310. In provide substrate step 2110, a substrate 105 is provided that is optionally suitable for growth of CNF110 or other support filaments. As discussed herein, the substrate 105 may include a variety of materials, such as Cu, Au, Sn, and the like. The substrate 105 may optionally include nucleation sites as discussed elsewhere herein.

In an optional provide CNF step 2120, CNF110 (or any other supporting filament discussed herein) is provided. In embodiments where electrodes without supporting filaments are produced (such as those illustrated by fig. 17 and 20), the provide CNF step 2120 is optional. In some embodiments, CNF110 is provided by growing CNF110 on substrate 105. In some embodiments, the CNF110 is provided by adding the CNF110 to a mixture that is subsequently applied to the substrate 105. In some embodiments, CNF110 is produced separately from substrate 105 and subsequently attached to substrate 105.

In provide embedding material step 2130, embedding material 1610 is provided. In some embodiments, embedding material 1610 is first applied to CNF 110. In various embodiments, the embedding material 1610 is applied as a colloidal suspension in a solvent using vapor deposition, as a paste, or the like.

In provide Surface Effect Dominant Sites (SEDS) step 2140, surface effect dominant sites 1430 are provided. As discussed elsewhere herein, the surface effect dominant sites 1430 may be disposed on nanoparticles or larger structures, such as graphite, graphene, or activated carbon. The surface effect dominant sites 1430 can be provided as using sputter deposition, using electroplating, using a suspension evaporated in adhesive 1140 or solvent, as a spray, or the like. In some embodiments, the zeta potential of the embedment material 1610 is selected such that the surface effect dominant sites 1430 are concentrated at the surface of the embedment material 1610.

In application step 2150, embedment material 1610, surface effect dominant sites 1430, and optionally CNF110 are applied to substrate 105. These materials can be applied in a variety of sequences and combinations. For example, embedding material 1610 can be applied to CNF110 (perhaps already attached to substrate 105), and then surface effect dominant sites 1430 can be applied over embedding material 1610. Alternatively, the free CNF110, embedding material 1610 may be mixed first, and then surface effect dominant sites 1430 and adhesive 1140 added separately or in combination. Based on the teachings herein one of ordinary skill in the art will appreciate that in various embodiments, these components can be mixed or added in any order or combination. Additionally, the components can be mixed before or after being applied to the substrate 105. Step 2110-2150 can be performed in any order. The applying step 2150 is optionally followed by a conditioning step 1350.

In some embodimentsFig. 21 illustrates a method that includes mixing embedding material 1610 and surface effect dominant sites 1430 in suspension in a solvent with a sufficient amount of dispersion. The dispersion is optionally applied to CNF 110. The solvent of the dispersion then evaporates from the mixture, resulting in a powder or coating on the CNF 110. The binder 1440 can be added to the suspension before or after application to the CNF 110. In some embodiments, the application of surface effect dominant sites 1430 occurs at the final stage of the deposition of the embedding material 1610 by altering the material sputtered onto the substrate 105. In these embodiments, for example, TiO₂Can be added to the sputtering mixture after substantially all of the embedding material 1610 is deposited. This produces TiO₂As surface effect dominant sites 1430 on top of the embedding material 1610.

Fig. 22 illustrates a method of operating a charge storage device according to various embodiments of the invention. The method may be used, for example, when charging a charge storage device. In some embodiments, the method includes attaching a charging device to both an anode and a cathode of the charge storage device via a wire. The charge storage device places an electrical potential at the anode and the cathode, resulting in a potential gradient between them. The potential gradient drives electrons into the anode. The steps illustrated in fig. 22 may optionally occur simultaneously, e.g., they can occur simultaneously or at overlapping times relative to each other.

In establish potential step 2210, a potential is established at the charge storage device. The potential may be between an anode and a cathode of the charge device. Such a potential will result in a potential gradient between the substrate 105 and the electrolyte 125 within the charge storage device. This potential gradient can produce a difference in potential between the locations of surface effect dominant sites 1430 and embedded material 1610. In various embodiments, the potential difference value is at least 0.001, 0.1, 0.3, 0.4, 0.5, 0.8, 1.0, 1.3, 1.7, 2.0, or 2.4V, or any range therebetween.

In receive lithium step 2220, charge carriers for which lithium is only one possible example are received at one of the surface effect dominant sites 1430. The charge carriers are optionally received throughout the adhesive 1440.

In a transfer electrons step 2230, electrons are transferred (e.g., supplied) from the surface effect dominant sites 1430 to the charge carriers received in the receive lithium step 2220. The transfer may include sharing of electrons between the surface effect dominant sites 1430 and the charge carriers. The electrons are transferred in a reaction that induces current and are typically conducted from the substrate 105. This transfer occurs when the charge carriers are at the surface of the surface effect dominant sites 1430 and at the potential of that location. The reaction potential for electron transfer depends, for example, on the reaction potential of the charge carriers and the reaction potential of the surface effect dominant sites 1430. The reaction potential can depend on both the surface effect dominant sites 1430 and the nearby embedding material 1610. As used herein, the term "reaction potential" is used to refer to the potential at which a reaction occurs at a significant rate. The reaction potential of the reaction can be illustrated by, for example, the peak in the cyclic voltammogram. In another example, the reaction Li occurs in an electrochemical cell⁺+e^-→ Li or 2Li⁺+MO+2e^-→Li₂The desired potential for O + M (where M is any of the transition metals discussed herein) is the reaction potential for these reactions. The reaction potential can be highly dependent on the environment in which the reaction takes place. For example, the second reaction described above may be on TiO with a diameter in the range of 2-10nm₂The reaction potential is lower in the presence of nanoparticles. Likewise, the reaction potential can be influenced by the energy required for intercalation or by the close proximity of surface effect dominant sites 1430 and the intercalation material 1610.

In the intercalation step 2240, charge carriers, of which lithium is only one possible example, are intercalated within the intercalation material 1610. This step may include the transfer of charge carriers into the interior of the body of the embedding material 1610. The charge carriers can be received at the intercalation material 1610 as the same chemistry received at the surface effect dominant sites 1430 in the receive lithium step 2220, or alternatively as a chemistry generated at the surface effect dominant sites 1430. For example, charge carriers can be present at the embedding material 1610 as chemicals received at the surface effect dominant sites 1430Oxides of matter (e.g. Li)₂O, etc.) is received.

In a transfer electrons step 2250, electrons are transferred from the intercalation material 1610 to the charge carriers of the intercalation lithium step 2240. The electrons are transferred in a reaction that induces current and are typically conducted from the substrate 105. This transfer occurs when the charge carriers are within the embedding material 1610 and at the potential of that location. The reaction potential for electron transfer may depend on the reaction potential of the charge carriers and the reaction potential of the intercalation material 1610. The potential of the conductive strip can be affected by both the embedding material 1610 and the nearby surface effect dominant sites 1430. The surface-dominated sites 1430 can facilitate the transfer of lithium from the electrolyte 125 to the intercalation material 1610. As discussed elsewhere herein, this movement may be via, for example, Li₂The intermediate oxide of O occurs. The work function for this electron transfer can be different from the work function for the electron transfer in the transfer electrons step 2230. For example, in various embodiments, the work function is at least 0.001, 0.1, 0.3, 0.4, 0.5, 0.8, 1.0, 1.3, 1.7, 2.0, or 2.4V, or any combination therebetween. In some embodiments, lithium is thermodynamically favored to be intercalated into the intercalation material 1610 than into the host of the nanoparticle including the surface effect dominant sites 1430. However, the presence of surface effect dominant sites 1430 can facilitate the embedding of charge carriers into the embedding material 1610.

If the charge carriers are converted to oxides in the transfer electrons step 2230, then in some embodiments, the transfer electrons step 2250 includes transferring oxygen from the intercalation material 1610 back to the surface-effect dominant sites 1430. This oxygen, which is received at the intercalation material 1610 as an oxide of the charge carriers, is released from the charge carriers during intercalation. After the oxygen is transferred back to the surface effect dominant sites 1430, the oxygen can then be used for further occurrences of the transfer electrons step 2230, i.e., the oxygen is recycled.

Although the description of fig. 22 above assumes that the charge carriers received in the receive lithium step 2220 and the charge carriers of the intercalate lithium step 2240 are two different separate charge carriers (which may be of the same type), in various embodiments, step 22 is performed in various embodiments20. 2230 and 2240 can be carried out by the same individual charge carriers. For example, in some embodiments, receiving lithium step 2220 includes receiving charge carriers at one of the surface effect dominant sites 1430. Then, the transfer electrons step 2230 includes a reaction of the charge carriers with surface effect dominant sites 1430 to produce an intermediate compound. In some embodiments, the reaction includes 2Li⁺+_MO+2e^-→Li₂O + M (where M is any of the transition metals discussed herein and Li₂O is the resulting intermediate compound). In the lithium intercalation step 2240, an intermediate compound (e.g., Li)₂O) is embedded in the embedding material 1610, or one (or both) of Li in the intermediate compound is from Li₂O of O is transferred to the intercalation material (e.g. Li)_xSi) is used. This transfer may result in the regeneration of an MO, which is split in the transfer electron step 2230. It should be noted that in this example, the same individual Li atom is included in each of steps 2220-2230 and 2240. The transfer electrons step 2250 is not required in these embodiments of the method illustrated by fig. 22. In some embodiments, including, for example, Li₂It is possible that both the reaction sequence of the intermediates of O and the reaction sequence not including the intermediates occur during a single charge cycle.

Various embodiments are particularly illustrated and/or described herein. However, it will be appreciated that modifications and variations are covered by the above teachings and within the purview of the appended claims without departing from their spirit and intended scope. For example, while the examples discussed herein focus on CNFs having a cone-stack structure, the teachings may be applied to other materials having similar or alternative structures. Likewise, while copper substrates and lithium charge carriers are discussed herein, other substrates and charge carriers will be apparent to one of ordinary skill in the art. Silicon layer 115 may alternatively be formed of an embedded material in addition to or as an alternative to silicon. For example, tin, germanium, carbon, graphite, graphene, silicon, other materials discussed herein, or combinations thereof can be used as the intercalation material. In addition, aerogels, nanowires, TiO₂(titanium oxide), MetalWires, carbon wires, or boron nitride nanofibers can be used in place of the carbon nanofibers discussed herein. The relative concentrations of adhesive 1440, surface effect dominant sites 1430, embedment material 1610, and CNF110, as well as other components in the figure, can vary significantly from the relative concentrations shown.

The electrodes taught herein may be included in a wide variety of energy storage devices, including capacitors, batteries, and hybrids thereof. These energy storage devices can be used, for example, in lighting systems, portable electronic devices, load balancing devices, communication devices, backup power sources, vehicles, and computing devices. The concepts taught herein can be applied to cathodes as well as anodes in many cases.

Details of VACNF growth and silicon deposition, microscopy and spectroscopy characteristics, and electrochemical cell assembly and charge-discharge testing are known in U.S. provisional application 61/667,876 filed on 3/7/2012.

The embodiments discussed herein are illustrative of the invention. Various modifications and adaptations of the described methods and/or specific structures may become apparent to those skilled in the art when such embodiments of the invention are described with reference to the illustrations. All such modifications, adaptations, or variations that rely upon the teachings of the present invention and through which these teachings have advanced the art are considered to be within the spirit and scope of the present invention. The description and drawings are, accordingly, not to be taken in a limiting sense, and it is to be understood that the invention is not in any way limited to the illustrated embodiments.

Claims

1. An energy storage system comprising:

a conductive substrate;

a carbon nanofiber attached to the conductive substrate, the carbon nanofiber comprising a plurality of nanoscale edges exposed along a length of the carbon nanofiber; and

an embedding material configured to form a shell over at least a portion of the carbon nanofibers;

wherein each exposed nanoscale edge is configured to control growth of the intercalation material, and expansion of the intercalation material is primarily in a radial direction of the carbon nanofiber, the each exposed nanoscale edge comprising a plurality of graphene edges;

wherein the respective exposed nanoscale edges are part of a cup-shaped structure.

2. The system of claim 1, wherein the respective exposed nanoscale edges comprise edges of a plurality of graphite sheets.

3. The system of claim 1 or 2, wherein the respective exposed nanoscale edges are configured to provide a moving path for charge carriers to an interior of the carbon nanofiber.

4. The system of claim 1, wherein each of the cup-shaped structures comprises a wall having a plurality of graphite sheets.

5. The system of claim 1 or 2, wherein the intercalation material is arranged in a pinnate structure along the length of the carbon nanofiber.

6. The system of claim 1 or 2, further comprising nanoparticles disposed between the intercalation material and the electrolyte, wherein the amount of nanoparticles is at least 1.1 times the amount of a monolayer of nanoparticles.

7. The system of claim 1 or 2, further comprising an electrolyte in contact with the intercalation material and including charge carriers.

8. The system of claim 7, wherein the charge carrier comprises lithium and the intercalation material comprises silicon.

9. The system of claim 1 or 2, wherein the carbon nanofiber is one of a plurality of vertically aligned carbon nanofibers attached to the conductive substrate.

10. The system of claim 1 or 2, wherein the conductive substrate, the carbon nanofibers, and the intercalation material are configured to act as an anode.

11. The system of claim 1 or 2, further comprising a plurality of nanoparticles coupled to the intercalation material, each of the nanoparticles configured to provide surface effect dominant sites configured to adsorb charge carriers by faradaic interactions on the nanoparticle surface.