Title: Nanostructured battery electrodes

Technical field

The present invention relates to electrodes, and more particularly, to nanostructured battery electrodes.

Background

Lithium-ion batteries have several advantages such as high energy to weight ratios, lack of memory effect, and a slow charge loss when not in use. Traditionally, lithium cobalt oxide is used as the cathode and carbon or graphite is used as the anode in a lithium ion battery. However, the commercial electrodes have approximately 50% active material by volume, with a significant portion of the electrode volume occupied by liquid electrolyte, void space and additives. This results from the use of slurries of microspheres with active material in the liquid electrolyte. The lithium ions diffuse through the liquid electrolyte into the slurry and travel a short distance into the microspheres of active material. The binder, a kind of additives, holds the microspheres together so that the electrode does not fall apart. Conductivity aides, another kind of additives, are added to provide electron paths from one microsphere to another. The reason why a solid or almost solid structure could not be used is that there would not be sufficient ionic pathway to allow the ions to rapidly diffuse into the solid structure. The ionic conductivity inside the solid structure during the lithiation/delithiation process is far too low, which hinders the commercial utilization.

The document US10403894 has disclosed an anode made with lithium metal coated carbon nanotubes, because such material is expected to store much more energy than the traditional carbon anode. In US10403894, the carbon nanotubes are only coated with a thin layer of lithium metal, the space between the nanotubes is left empty, which results in a large porous volume inside the electrode as illustrated in the figures IB, 3A.

It is also possible to coat the carbon nanotubes with silicon and sulfur, which stores also more energy than traditional material. Since these materials, like silicon and sulfur, expand during the lithiation/delithiation process, it is necessary to leave also large porous volume inside the electrode so that the expansion will not destroy the electrode

Inside the above-mentioned kinds of electrodes, the porous volume is filled with liquid electrolyte. The coating process seals the opening of the nanotubes, especially the top opening of the nanotubes of the electrode, which prevents the access of ions from electrolyte into the interior of the channel of the nanotubes. Consequently, these electrodes do not rely on ion transport inside the interior channel of the nanotubes. There are two ways for lithium ion diffusion down the height of the electrode on the exterior of the nanotubes: firstly, the lithium ions diffusing through the liquid electrolyte; secondly, the lithium ions diffusing on the exterior surface of the infiltration coating.

This method has a few drawbacks. Additional electrolyte increases the cost and battery mass. In addition, the liquid electrolyte used in this kind of battery requires packaging that wastes volume and occupies a greater ratio of the battery volume as the size of the battery shrinks. Finally, the carbon nanotubes are grown on a copper (Cu) substrate, which is used as a current collector. The shape and the footprint of the electrode are limited by the size and shape of the Cu metal foil, which cannot be easily mass produced for this kind of battery.

Advantages of the invention The present disclosure has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by electrodes currently available. Compared to the state of the art, the present invention has the following advantages:

- it uses the conventional electrode materials, which have been proven safe in commercial lithium ion battery cells for decades;

- the conventional electrode materials expand less than 20% during lithiation, which allows long cycle life;

- the electrode is essentially a solid structure, so that the overall capacity per volume is increased and the risk of leakage is avoided, which facilitates the storage and transportation;

- it enables a high volume fraction of active material in the electrode, which leads to a high energy storage per volume;

- it bases on the high ionic conductivity of the fast ion diffusion channels. The distance that ions need to travel into the active material is on the nanoscale, which allows for high charge/discharge rate;

- it can be mass produced at microbattery sizes.

The present invention concerns an electrode comprising a 3D nanostructure in the form of a matrix of channels extending continuously throughout the total internal volume of the electrode. The space around the 3D nanostructure inside the total volume of the electrode is infiltrated with a low- expanding (<20% during full lithiation) electrochemically active material. The 3D nanostructure of matrix of channels serves as electron and/or ion channels for the flows of electrons and/or ions. The low-expanding electrochemically active material includes carbon and metal oxides, including layered oxides, spinal oxides and polyanion oxides, especially lithium cobalt oxides, and lithium aluminum oxide. These materials are ceramics among the five basic material categories (metal, polymer, ceramic, semiconductor, and composite). It is necessary to point out that the ionic lithium and lithium bonded into a ceramic are of fundamentally different nature than lithium as a metal, since the chemical structures are different. The lithium as metal has metallic bonds where the electrons are non-localized. Neither ionic lithium nor lithium bonded into a ceramic possess metallic bonds and are consequently not metals.

The present invention focuses on the nanostructure with fast ion diffusion channels for the flows of the ions. Meanwhile, certain embodiments, which can be adapted for transmitting the flows of electrons, will also be discussed.

Different from the state of the art, in which ion diffusion in the electrode is realized on the exterior of the nanotubes coated with active material, the present invention proposes the use of an interconnected matrix of rapid ion diffusion channels extending throughout the whole volume of the electrode.

The porous volume inside the electrode around the 3D nanostructure is fully infiltrated with a low-expanding electrochemically active material to achieve a porosity as low as possible. The full infiltration, which makes the electrode becoming a non-porous structure, closes up the two continuous pathways of the lithium ion diffusion on the exterior of the ion channels. The lithium ion diffusion on the interior of the ion channels is fast.

Brief summary of the invention

The invention concerns a nanostructure electrode comprising,

- a three-dimensional (3D) nanostructure being in the form of a matrix of channels extending continuously throughout the total internal volume of the electrode, - the space around the 3D nanostructure inside the total internal volume of the electrode being infiltrated with at least one kind of low-expanding electrochemically active material with a lithiation volume expansion less than 20%.

The matrix of channels of the 3D nanostructure is a void space in the form of interconnected pores filled with at least one high ionic conductivity material with an ionic conductivity of each channel being larger than 10 ^L -4 S/cm (10 ^L —2 S/m) so that the matrix channels of the 3D nanostructure serve as fast ion diffusion channels for flows of the ions, and the at least one kind of low-expanding electrochemically active material serves as a storage source for the ions.

The invention concerns a plurality of variations of the electrode, which include the following modes of realization either alone or in combination.

- the fast ion diffusion channels are filled with a liquid electrolyte as the high ionic conductivity material.

- the fast ion diffusion channels are filled with a solid electrolyte as the high ionic conductivity material.

- the fast ion diffusion channels are filled with alternative layers of at least one high surface ionic conductivity material separated by layers of void space.

- the fast ion diffusion channels are filled with superimposed alternative layers of heterogeneous materials to realize the said at least one high ionic conductivity material.

- the at least one kind of low-expanding electrochemically active material occupies at least 50% of the total internal volume of the electrode.

- the median aspect ratio of length/diameter of the channels is greater than 100:1, with 10 micrometer length at the least and 100 nanometer diameter at the most. - the channels have open accesses at least one external surface of the electrode.

- the low-expanding electrochemically active material is a lithium metal oxide.

- the low-expanding electrochemically active material is carbon.

The present invention concerns also a battery comprising:

- a cathode,

- an electrolyte in contact with the cathode; and

- an anode in contact with the electrolyte;

- the cathode and/or the anode is an above-described electrode.

The present invention concerns also several variations of the battery, which has the following features either alone or in combination: the battery is a lithium ion battery.

- the electrolyte is an inorganic solid electrolyte, a polymer electrolyte, or a gel polymer electrolyte.

The present invention concerns a method to fabricate an electrode comprising:

- generation of a three-dimensional (3D) nanostructure, e.g. nanotubes, nanorods, aerogels, being in the form of a matrix of channels extending continuous throughout the total internal volume of the electrode,

- infiltration of the space around the 3D nanostructure inside the total internal volume of the electrode with at least one kind of low-expanding electrochemically active material with a lithiation volume expansion coefficient of less than 20%,

- removing the three-dimensional (3D) nanostructure to leave a void space in the form of interconnected pores, filling the void space in the form of interconnected pores filled with at least one material with a high ionic conductivity of each channel being larger than 10 ^L -4 S/cm so that the matrix channels of the 3D nanostructure serve as fast ion diffusion channels for flows of the ions.

The present invention concerns the variations of the method:

- the interconnected pores are electrical conductively coated

- the electrical conductively coated interconnected pores are filled with liquid/solid electrolyte.

Brief description of the drawings

FIG.l is a cross-section diagram of the infiltration process of the nanostructure;

FIG.2 is a cross-section diagram of the infiltrated nanostructure at different infiltration percentage;

FIG.3 shows the different pathways in different kinds of electrodes;

FIG.4 is a cross-section diagram of an infiltrated nanostructure with tube form pores fulfilled with electrolyte ;

FIG.5 is a cross-section diagram of an electrode;

FIG.6 is a cross-section diagram of a battery;

FIG.7 is a cross-section diagram of an embodiment of ionic channels inside the electrode;

FIG.8 is a cross-section diagram of another embodiment of ionic channels inside the electrode;

FIG.9 is a diagram to illustrate the battery dimension;

FIG.10 is a diagram to illustrate the passage of ionic flows from cathode to anode.

Detailed description of the preferred embodiment

It is to be understood that both the foregoing general description and the following detailed description are illustrative and explanatory and are not restrictive. The section headings used herein are for organizational purposes and are not to be construed as limiting the subject matter described.

Nanostructure

The document US10403894 describes the growth and structural characterization of graphene-carbon hybrid nanostructure grown directly on a metal foil of Cu. This is a possible choice of the nanostructure. The document US20110183206 describes the growth and structural characterization of vertically aligned carbon nanotubes on the substrate of a stainless-steel foil, which is another candidate of the nanostructure. These methods generate the nanostructure in the form of a three-dimensional (3D) matrix of nanotubes or nanorods. In the above-mentioned methods, the nanostructure grows directly on the metal foil substrate, which is conductive and is used directly as the current collector.

The present invention uses a 3D nanostructure in the form of a matrix of channels extending continuously throughout the total internal volume of the electrode. The 3D nanostructure could be formed from a VACNT array, or any other forms of nanotubes or nanorods, or even an aerogel. As an illustrative example without any limitation effect, a method to generate the 3D nanostructure is give hereafter. It is possible to use the photolithography to pattern the footprint of a nanostructure onto a silicon wafer in order that the fabricated nanostructure can be detached/separated from the substrate. This method allows to realize a very small and unusual shaped footprint. Footprint means the 2D area occupied by a 3D object. For example, a skyscraper has a very small footprint compared to a two story building even if they have the same volume. Any other kind of substrate for example quartz is also allowed to be used for growing the VACNT. The geometry of the nanostructure fabricated on the silicon wafer substrate can be controlled to realize any desired three-dimensional shape. By controlling the pattern of the catalyst using photolithography, any size and shape of the nanostructure footprint can be obtained in a two-dimensional plane. A target height of the nanostructure from nanometer scale up to centimeter scale can be achieved by controlling the upwards extrusion growth on the catalyst.

This possibility opens two important aspects for the application:

1.For the batteries as components on PCB boards, it is necessary that the footprints of the components take up as little board space as possible.

2.For the wearable devices or micro robotics, any irregular footprint, or even any 3D shape can be fabricated according to the available space within the device to maximize space usage, knowing till now there are only small-size batteries with square or rectangular footprint .

As illustrated in figure 1, the wavy shape columns represent a small part of the nanotube matrix (11), for example a matrix of vertically aligned carbon nanotubes. Each nanotube continues in both upper and lower directions and repeats the pattern until reaches a desired length, which can be as small as 1 micrometer and as large as several centimeters. Thus, the size of the electrode can vary between several micrometers to several centimeters, and the shape can be designed as completely irregular, which is defined by the pattern of the catalyst. The diameter of the nanotubes and the distance between the nanotubes are in the order of tens of nanometers. The adjacent nanotubes touch each other in some places (12), like the branches of the trees across the branches from nearby trees. The resulted three-dimensional (3D) matrix of nanotubes is like a forest with nanoporous scaffold, which can generally present more than 95% empty space (13) in its volume. In certain embodiment where the CNTs (11) take up between 1-5% of the volume, it is possible to get an empty space (13) occupying 95%, or even 99% of the volume.

An infiltration process is carried out to fill the empty space between the VACNT with an electrochemically active material, e.g. graphite or LiCo0 ₂ (14). The arrows (15) in the figure 1 indicate the infiltration direction. As the infiltration process continues, the surface of the nanotubes is coated with more and more electrochemically active material (14). The coated layer becomes thicker and thicker so that the coatings merge together to fill in the empty space between the nanotubes.

Figure 2 illustrates the development of the infiltration thickness from zero infiltration to full infiltration. The figure 2A shows cross-sectional diagram of a bare VACNT without infiltration. The figure 2B shows cross-sectional diagram of the VACNT with a conformal coating of infiltration material. For the infiltration process, some deposition methods coat conformally while others coat with small spheres that grow in size as the infiltration continues. The difference depends on the nucleation and island growth mechanism that dominate during the infiltration process. The figure 2C shows cross-sectional diagram of the VACNT with a coating of spheres. Both 2B and 2C are the examples of partial infiltration. The figure 2D shows cross-sectional diagram of the VACNT fully infiltrated.

A full infiltration means that the empty space inside the electrode around the 3D nanostructure is fully infiltrated with a low-expanding electrochemically active material to limit the porosity as low as possible. For example, the CNTs can take up 1-5% of the volume according to the best estimation The lowest porosity that has been achieved is around 5%. With the situation that the CNTs take 5% of the volume, the infiltration can reach 90% of the total volume.

Inside the nanostructure, the nanotubes are interwoven throughout the volume. It is like the trees with branches across each other. When the full infiltration is applied, the infiltrated material locks all the places where the nanotubes touch each other and fulfills the whole empty space between the nanotubes, thus creating a fully rigid nanostructure.

A fully infiltrated nanostructure is basically a solid brick that can vary from several micrometers to several millimeters in the thickness. The full infiltration makes the nanostructure rigid enough to be manipulated physically. For example, the fully infiltrated nanostructure can be picked up with tweezers, moved from one substrate to another, and epoxied onto a destined surface.

The coating method for the infiltration can be either atomic layer deposition (ALD), melt infiltration, electrodeposition, electroless deposition, infusion with nanoparticles or chemical vapor deposition. The full infiltration process is carried out with a low-expanding electrochemically active material, like a metal oxide, especially a lithium metal oxide, or carbon.

In the case of full infiltration, the electrochemically active material chosen for the infiltration expands a small amount during charging or discharging so that the expansion of the active material does not cause the damage of the electrode structure. One example of such low-expansion active material is LiCoCk, which only expands by about 1%. For the next-generation lithium storage materials like silicon and sulfur, they are both hindered by issues related to volume expansion and parasitic reactions with the electrolyte. The full infiltration process does not work for these materials, because the expansion can lead to the deformation of the electrode. The figure 3 illustrates the diffusion paths of the lithium ion in different kinds of electrodes. Generally, lithium ions can diffuse several microns into graphite and hundreds of nanometers into lithium metal oxides. It is generally considered that the shorter the diffusion distance into the active material, the faster the battery can charge and discharger. The figure 3A shows the traditional electrode, which is composed of electrochemically active material in the form of chunks (31) surrounded by the liquid electrolyte (32). The lithium ions diffuse fast in the liquid electrolyte (32) and diffuse slowly into the electrochemically active material (31). The figure 3B shows the partially infiltrated VACNT as an electrode. The VACNT is used as the 3D nanostructure (33), which is partially infiltrated with active material (34). The empty space between the partially infiltrated VACNT is filled with liquid electrolyte (35). The lithium ions diffuse fast down in the liquid electrolyte (35) and diffuse slowly into the active material (34). The figure 3C shows the fully infiltrated VACNT as an electrode. The VACNT is a 3D nanostructure being in the form of a matrix of channels extending continuously through the total internal volume of the electrode. The space around the 3D nanostructure inside the total internal volume of the electrode is fully infiltrated with active material (37).

The VACNT can transport electrons with its good conductivity. For the transportation of the ions, it is necessary to build special ionic channels. For that reason, the VACNT is removed to leave a void space in the form of interconnected pores filled with at least one high ionic conductivity material so that an ionic conductivity of each channel is larger than 10 ^L - 4 S/cm (10 ^L —2 S/m). With the matrix channels of the 3D nanostructure serving as fast ion diffusion channels, the flows of lithium ions travel fast inside the ion diffusion channels (36), instead of diffusing at the exterior of the channels (37). In addition, the ions experience a slow diffusion from the interior ionic channels into the active material surrounded the ionic channels. Due to the nanoscale spacing between channels, the electrodes can charge faster than conventional battery electrodes. Another clear advantage is that the fast ion diffusion only takes place inside the ionic channels, which takes orders of magnitude less volume than the other methods. In addition, the electrochemically active material as a storage material increases significantly its volume, which augments significantly the storage capacity and the volumetric capacity of the electrode. Since the ionic channels extends in the total internal volume of the electrode, the ionic channels provides short distance for ions to travel into the active material, which enables fast battery charge and discharge.

For the electrodes as thick as 1mm, the continuous pathways through the whole length enable an areal capacity up to 20 times greater than current technology.

Thin conductive layers can be added to the carbon nanotubes before infiltration process in order to enhance electrical conductivity for electrons. The thin conductive layers could also make the nanostructure mechanically more robust, which would be particularly useful if wet processing is used to infiltrate the electrochemically active material.

The 3D nanostructure can be made using nanotubes/nanorods with other kinds of materials, so far, they provide a 3D matrix of channels extending in the total internal volume of the electrode. The 3D nanostructure can also be made using an aerogel.

In addition, the full infiltration is not obligatory. The infiltration can be realized to a level that up to 20% of the internal volume could be left as void space for the expansion of the active material during lithiation. Throughout the total internal volume of the electrode, the 3D nanostructure becomes the dominant pathway for the electrons and/or ions flows, and that any other pathway for electrons and/or ions flows is secondary, or is negligible. Often, this infiltration level can also provide enough rigidity of the structure for physical treatment.

For the embodiment of electrode with fast ionic channels, the nanotubes inside the infiltrated nanostructure are burned out. For example, in an electrode comprising a nanostructure of vertically aligned carbon nanotubes (VACNT) partially infiltrated with carbon or fully infiltrated with lithium metal oxide, the VACNT can be burned out using oxygen or water at high temperature (>500 C for O2, >700 C for H2O). After the burning out process, the carbon nanotubes as the scaffold of the nanostructure is removed, leaving the infiltrated lithium metal oxide inside the nanostructure to support the structure.

The figure 4 shows the cross-section diagram of an infiltrated nanostructure (41). The VACNT is removed leaving tube form pores (40). The tube form pores (40) are coated with an electrically conductive layers (43) and/or an artificial solid electrolyte interphase (SEI) for creating transport pathways for the electrons and/or ions. The electrically conductive layer increases electrical or thermal conductivity The artificial SEI prevents the channels clogging with SEI during the lithiation process. The artificial SEI could be a thin coating of AI2O3 or HfO that could be applied with ALD before filling the pores with the high ionic conductivity material. The interior void space inside the tube form pores (40) can be filled with a high ionic conductivity material (44), e.g. electrolyte, to further facilitate the lithium ion diffusion.

A concrete example to fabricate an electrode with an infiltrated nanostructure of carbon using the burning-out process is explained hereafter. Firstly, applying a thin layer coating of silica, silicon or silicon dioxide to the VACNT by CVD or ALD. Secondly, the VACNT is burned out by annealing in air at 500°C, thus leaving an interlocked structure of silica or silicon oxide scaffold as inner channels. Thirdly, the silica scaffold is full infiltrated with carbon, fourthly, the silica or silicon oxide is removed by etching with an HF or KOH, which leaves a fully infiltrated carbon electrode with interior channels in the form of nanoscale tube form pores. Finally, using ALD to coat the interior channel walls with an artificial SEI (AI2O3 or HfO). Then filling the remaining pore volume with a high ionic conductivity material such as a a liquid electrolyte.

The figure 5 illustrates the cross-section diagram of an electrode (51). The electrode comprises a 3D nanostructure (52) of fast ion diffusion channels coated with electrochemically active material (53), which occupies more than 80% of the total internal volume of the electrode. In order that the lithium ions can diffuse along the interior of the channels inside the electrode, it is necessary to have the open access (54) of the channels at the surface of the electrode. One possibility is to avoid the sealing of the open access of the channels at the electrode surface by physical / chemical treatment. Another possibility is to do a normal infiltration process, which seals the open access of the channels at the electrode surface. Then using an etching method to remove the top layers of the electrochemically active material on the nanostructure and to expose the inner channels, which provide a rapid pathway for the lithium ions. The sealed top layers can be removed either by dry etch, wet etch, or by mechanically polishing. It is also possible to remove material from the sides to allow ions to pass into the electrode from the sides. In addition, a thin layer of conductive material is deposited on the surface opposite to the non-sealed surface of the nanostructure to form a current collector (55). The thickness of this conductive layer is more than ten times thinner than the metal foils currently used as substrate and current collector.

For the case in which the electrochemically active material is carbon, it is not necessary to remove the top layers, since the lithium ion transport through the sealed top layers is not as slow for graphitic carbon. And this carbon electrode can be used directly for the battery without metal layer deposition, since graphite is conductive. However, removing the top layers of carbon would increase the charge/discharge speed of the ions.

A infiltration of around 80% of the internal volume can increase the volumetric capacity by >70%. The thinner metal current collector, or even a current collector based on graphite without metal, further increases the volumetric and areal capacity.

The figure 6 illustrate an example of a battery (61) composed of the electrodes made on the fully infiltrated nanostructure for electron flows. By the way, regarding a battery composed of the electrodes for ion flows, the nanostructure of VACNT is simply replaced by the fast ion channels. For the cathode (62), the nanostructure is made from a matrix of vertically aligned carbon nanotubes (63). This nanostructure is fully infiltrated with metal oxide (64), e.g. lithium metal oxide. An aluminum thin film (65) is deposited on the surface of the nanostructure to form a current collector. The opposite side (66) of the nanostructure is etched to expose the inner pores.

For the anode (67), the nanostructure is also made from a matrix of vertically aligned carbon nanotubes (68). This nanostructure is fully infiltrated with graphite carbon (69). A copper thin film (70) is deposited on the surface of the nanostructure to form a current collector. The opposite side (71) of the nanostructure is etched to expose the inner pores. Another possible arrangement is for the sides that are perpendicular to the electrical contact to be etched to expose the pores and for the anode and cathode to sit next to each other.

The non-sealed side of each electrode is put into contact with a layer of solid electrolyte or a separator and liquid electrolyte (72). Thus, the battery is realized in a form of sandwich, with the solid electrolyte inserted between the cathode and the anode, both of which are made of fully infiltrated nanostructure. The solid electrolyte can be realized, but not limited to, gel-polymer, polymer, inorganic solid electrolyte or a porous separator. The electrodes can be separated by any effective form of electrolyte.

In one embodiment, the carbon infiltrated electrode is annealed at a temperature >1000°C in a vacuum or in an inert atmosphere to eliminate trapping of lithium ions and increase coulombic efficiency. The carbon infiltration has dangling bonds and defects that can trap lithium ions. The annealing process can heal these defects and make a more graphitic structure to avoid the trapping.

As illustrated in the figure 6, the major volume of the battery is occupied by the storage material of the electrodes, which largely enhance the storage capacity. The electrolyte layer is relatively much thinner compared to the electrodes.

The advantage of this kind of battery is high energy storage per volume because of the use of the electrodes with up to 90% active storage material by volume and little volume is taken up by the current collectors and the contribution of the separator is greatly reduced. Additionally, the battery can be scaled down to small sizes without sacrificing volumetric capacity.

Usually, a battery gets smaller, it loses out on capacity. With the present invention, it is possible to have a footprint as small as 25 micrometer2 ^m ² )and a total size smaller than 10 micrometer3 (mih ³ ) without a significant decrease in volumetric capacity. In the present invention, we are not using a material with improved storage capacity but increasing the amount of storage material per volume of the electrode. The unique aspect of the geometry is that the 3D nanostructure provides a continuous path for electrons and ions throughout the volume of a thick electrode and the spacing between channels is nanoscale, which dramatically reduces the diffusion distance into the active material.

The vertically aligned nanotubes allow us to do very thick electrodes with small footprint, while still having significant capacity, which could be valuable for fitting the battery into electronic devices. For example, the solid state lithium ion batteries with capacities <lmAh, and dimension < lmm ³ are good candidates for the application including, but not limiting to, medical devices, micro sensors, active RFID tags, wearable electronics, and micro robotics.

Solid-state micro battery can be packaged separately as PCB components. More advantageously, Solid-state micro battery can be packaged to be integrated into other electronic components like, medical devices, micro sensors, active RFID tags, wearable electronics, and micro robotics.

Another advantage is the rapid charge and discharge times, because on one hand the ion channels provide rapid ion transport passages, and on other hand the spacing between channels is in the nanoscale, which shortens the transport distance outside of the ion channels.

The figure 7 illustrates an example of ionic channels inside the electrode. The void space of the 3D nanostructure in the form of interconnected pores is filled with one high ionic conductivity material, which presents a high surface per volume.

As explained in the examples before, the fully infiltrated VACNT can be burned out to leave a void space 71 of 3D nanostructure in the form of interconnected pores. In fact, before the burning process, the fully infiltrated VACNT can be coated at its interior surfaces by alternative layers of sacrificial material and structural material. By removing the sacrificial material and the VACNT, the remaining structural material, which presents layers of void space in between, has a high surface per volume. If this structural material itself has a high surface ionic conductivity, the ions can flow on the surface of the porous structural material layers, which serve as fast ion diffusion channels. It is also possible to coat the structural material with a high surface ionic conductivity material if the structural material does not have a high surface ionic conductivity.

It is also possible to keep the VACNT, and only to remove the sacrificial material. For example, the fully infiltrated VACNTs can be coated with alternative layers of a sacrificial material of silicon and structural material of graphite, with the thickness of each layer being around l-5nm. Then, the sacrificial layers of silicon can be selectively etched with KOH and leaving the VACNT with graphite layers separated by porous layers. Since the graphite has a large surface per volume and has a high ionic conductivity, which is measured by its high surface diffusion coefficient and high maximum surface lithium concentration, the structural graphite layers serve as fast ion diffusion channels for the flow of ions. While, the VACNTs serve as fast electron conduction channels. The graphite is given as an illustrative but not limitative example. Beside the graphite, other material with a high surface ionic conductivity can also be used, e.g. graphene.

The figure 8 illustrate another example of ionic channels inside the electrode. The void space of the 3D nanostructure in the form of interconnected pores is filled with two alternative layers of materials 81 and 82. Both of these two materials are low ionic conductivity material, which do not conduct ion well. However, at the interface of the two low ionic conductivity materials, the ionic conductivity is high due the interface effect.

Thus, we can do a conclusion of the different variations of the electrode. A functional electrode must have transport pathways/channels for electrons and ions, which can be either separated or combined together.

For the electron conduction, it is necessary to have electron conduction channels inside the electrode, if the active material is electrically insulting during any part of the charge/discharge cycle. The electron conduction channels are either realized by the supporting 3D nanostructure during the growing process, e.g. VACNT, when it is electronic conductive, or realized by an electronic conductive coating, when the supporting 3D nanostructure during the growing process is not conductive.

For the ions transport, one possibility is to create channels with high ionic conductivity that are evenly distributed throughout the nanostructure. One possibility is to burn or etch out the supporting 3D nanostructure, e.g. VACNT. Then the void space is filled with electrolyte that has a high ionic conductivity. Another possibility is to introduce alternative layers of different materials to realize the double aim of large surface per volume and high surface ionic conductivity. If the alternative layers are composed of sacrificial layers and structural layers, the sacrificial layers can be selectively removed to increase the surface per volume, and the structural layer is chosen with a material of a high surface ionic conductivity. If the alternative layers are composed of different structural layers, which remain after the coating, the interface of the different layers could for certain material combinations present a high interfacial ionic conductivity pathway. In either case, when using alternating layers, the goal is to create a heterogenous structure that in the aggregate behaves as if the channels are filled with a homogenous high ionic conductivity material.

The following paragraphs present the basic models that define the relationship between the energy storage and the charge/discharge speed of the electrodes.

A battery contains three basic components: an anode, a cathode and a separator. The volume of each of the three component is illustrated in the figure 9. Since all the three components have the same cross-sectional surface area, the different volume of each component is simply expressed by the width of each component. w _a is the width of the anode. w _c is the width of the cathode, and w _g is the width of the separation between the two electrodes: anode and cathode. w _a and w _c are related by the volumetric capacity of the anode (C _a ) and the cathode (C _c ) by the equation below:

The overall capacity of the battery is the inverse of the sum

1 of the capacity normalized volumes (-) of the anode and cathode multiplied by an adjustment factor that accounts for the gap volume as seen in the equation below:

From the equations, it is clear that to fully understand the capacities of the anode and cathode, it is necessary to understand how the volume of each electrode is determined. The volume of each electrode is composed of four main categories:

1.Ion Conduction/diffusion Channels: each electrode contains a 3D nanostructure in the form of a matrix of channels extending continuously throughout the total internal volume of the electrode. The matrix of channels of the 3D nanostructure is a void space in the form of interconnected pores filled with at least one high ionic conductivity material presenting an ionic conductivity of each channel being larger than 10 ^L -4 S/cm so that the matrix channels of the 3D nanostructure serve as fast ion conduction/diffusion channels for flows of the ions. The channels in the network are separated by <250 nm, for VACNT, the separation distance is usually around 50-100 nm. The network also penetrates the entire volume of the electrode providing access to all the active material. The volume taken up by the nanostructure can vary between 1-30%, preferably between 10-15%. Vic represents the fraction/ratio of electrode volume occupied by this category.

2.Active Material: the space around the 3D nanostructure inside the total internal volume of the electrode is infiltrated with at least one kind of active material that stores ions. The design of the electrode is to maximize the amount of this material which increases the volumetric energy density. The volume of the active material is defined by the volume of the other 3 categories and can vary between 50-94%, preferably between 70-80%. V _AMC represents the fraction/ratio of electrode volume occupied by this category.

3.Mandatory Void Space: The different methods of infiltrating a 3D nanostructure are not perfect. They leave some amount of void space. For a uniform nanostructure (like a VACNT forest), the volume is around 5%. V _MVS represents the fraction of electrode volume occupied by this category.

4.Void Space for Expansion: The active materials expand when taking up ions. It is prudent to leave void space for this expansion so that the electrodes do not distort. The amount of expansion varies with active material. For the example of Li-ion battery, during a lithiation-delithiation circle, the material as Si, Ge, Sn can expand by more than 200%, the graphite expands by around 10%, but LiCoCb only expands by about 1%. (see the link https://www.sciencedirect .com/science/article/pii/S136970 2114004118)

Generally, a low-expanding active material with a lithiation volume expansion less than 20% is used for the infiltration process in the present invention. Thus, the volume can vary between 1-20%. V _E vs represents the fraction/ratio of electrode volume occupied by this category.

For example, the capacity of a cathode is given by the following equations:

^A _MC (1 V _MVS V _IC V _{EV c} ) where C _cm is the volumetric capacity of the active material of the cathode. Putting it together gives the following equation that gives the capacity of the electrode:

One of the ways this present invention gains capacity by increasing the values of V _Ama and V _AMC In a conventional powder- based electrode, more than half of the volume can be the space between particles where the electrolyte allows for rapid ion transport. By using a precisely designed nanostructure for ion conduction, the volume fraction of active material can be increased to 70%-80% while also increasing charge/discharge speed as discussed in the following paragraphs. The figure 10 illustrates the path of travel of an ion during charging. The ion begins in the active material of the cathode, and it takes three steps to reach the anode.

1.The ion diffuses through the active material of the cathode to reach the ion conduction channels of the cathode.

2.The ion transports along the ion conduction channels of the cathode till reaching the separator.

3.The ion transports across the separator.

Ions in the separator take exactly the three steps in reverse 3,2,1 to reach the active material of the anode. It turns out step 2 is the key step in limiting the charge/discharge speed in most embodiments of this invention. Step 1 is extremely fast because of the small spacing (~50 nm) between ion conduction channels. Step 3 is faster than the step 2 because the separator has a shorter(smaller) width than the electrodes and has a greater (larger) portion of its volume filled with high ionic conductivity materials.

Since the step 2 is the key limiting step, the charge/discharge speed of the battery can be approximated as equivalent to the speed of the step 2. Charge rates for batteries are usually measured in C rate where 1C would be the current needed to charge the battery in 1 hour and IOC would be the current needed to charge the battery in 0.1 hours. For step 2 we can calculate the diffusion limited C rate. The equation below gives a maximum current density (jii _m ) limited by diffusion:

(see C. Heubner, M. Schneider, and A. Michaelis, "Diffusion- Limited C-Rate:A Fundamental Principle Quantifying the Intrinsic Limits of Li-Ion Batteries," Adv. Energy Mater., vol. 10, no. 2, p. 1902523, Jan. 2020). Here, z is the charge number of the carrier (1 for Li ⁺ ), F is the electron charge, c is the ion concentration, w is the width of the electrode, and D is the effective diffusion coefficient which is defined as below

Here, Do is the diffusivity of the material, and e and t are the porosity and tortuosity respectively. The most difficult of these numbers is the tortuosity calculation for the channels. The diffusion limited C rate (DLC) is given by the following equation:

2zFDc

DLC w ² C where C is the volumetric capacity of the electrode. To be clear, a higher DLC means a shorter charge time. The units for DLC are usually hr ¹ .

The following paragraphs give some typical value to calculate the diffusion limited C rate (DLC).

The capacity and charge rate of the battery created can be calculated using the equations above. First, consider the capacity using the equation given above:

Each variable must be considered in turn. The easiest is the volumetric capacities of the anode and cathode materials, C _am and C _cm respectively. For the sake of this calculation, we approximate both as 800 mAh/cm ³ . Next consider the widths of the anode, cathode, and separator, w _a , w _c , and w _g respectively. If w _a is about 20 pm then w _c is about 18 pm. Since the ion conduction channels, mandatory void space, and material capacity are the same in the case of the anode and cathode in this case, the difference between w _a and w _c is due to the void space for expansion. In this example, w _g is 10 pm.

Finally, consider the volume fractions of active material (VAMa and VAM _C ) . These are calculated using :

V _AMC (1 V _MVS V _IC V _{EV c} )

Where VMVS IS 0.05 and Vic is 0.15. Finally, we consider void space for expansion (V _E vs _a and V _E vs _c ) . For a graphitic anode, the expansion is 10.4% and for a LiCoCh cathode it is ~1%. The volume fractions must be adjusted down from these because the active material only takes up part of the electrode. For this calculation, V _E vs _a is approximated as 0.01 and V _EV s _c is approximated as 0.08. This makes VAMa 0.72 and VAM _C 0.79. These numbers lead to an approximate capacity (C _totai ) of 239 mAh/cm ³ or considering an average of 3.8V, 930 mWh/cm ³ .

Now, consider the DLC. Combining the above equation for DLC and D, we get the following :

The following table gives a list of values for the various variables in this equation for our specific example.

These numbers give an approximate DLC of 9.76 hr ¹ . This corresponds to a charge time of ~6 min. This represents a dramatic decrease in charge time over the state of the art. This is only possible because the ion channels provide access to every part of the active material such that the Li only needs to diffuse a nanoscale distance the solid active material. In conventional electrodes, microscale particles are used. Li diffusing microns into these materials requires long times limiting the charge speed.