Capacitive electrical energy storage system

Abstract

Capacitive Electrical Energy Storage System The invention relates to a capacitive electrical energy storage system comprising: a liquid electrolyte; a positive electrode comprising: a first substrate made of conductive material; a first two-dimensional or three-dimensional conductive nanostructure extending over the substrate; and an insulator comprising at least one layer of dielectric material conforming to the shape of the first conductive nanostructure; and a negative electrode comprising: a second substrate made of conductive material; a second two-dimensional or three-dimensional conductive nanostructure extending over the substrate; Figure for the abstract: Fig. 1

Description

Title of the invention: Capacitive electrical energy storage system Technical field

[0001] The invention relates to capacitive electrical energy storage systems and in particular, to hybrid capacitive electrical energy storage systems between supercapacitor and electrolytic capacitor.

[0002] BACKGROUND

[0003] Energy storage is essential, on the one hand for nomadic or autonomous electronic products to free themselves from the constraint of the electricity distribution network. They incorporate energy storage and management systems and sometimes energy conversion. On the other hand, many electronic devices cannot withstand micro-power cuts from the network and must therefore be supported. The electrochemical storage systems that meet these needs are classified according to their mass energy density (in Wh / kg) or volume energy density (Wh / L) and their power density (W / kg or W / L). These different technologies are based on different charge storage mechanisms, each of which has its intrinsic limits in terms of charge and discharge kinetics (power density) and maximum charge and discharge capacity (energy density).Existing technologies cannot provide both high storage capacity (autonomy) and high power (responsiveness).

[0004] Being able to provide both powers similar to those of electrolytic capacitors which are used in most electronic circuits at present and capacities of the order of those of supercapacitors would make it possible to improve the compactness of systems, particularly in power electronics and in particular in embedded or autonomous systems.

[0005] Several electrolyte capacitive storage systems exist and are currently commercially available. Examples include aluminum (Al) and tantalum (Ta) electrolytic capacitors, as well as supercapacitors. Al capacitors have structured electrodes and electronic insulating material layer thicknesses typically on the scales of 108 to 106 μm, whereas supercapacitors have structured electrodes typically on the scales of 10 10 to 108 μm and do not have an electronic insulating material layer.

[0006] The capacitance of a capacitor is defined by the following formula:

[0007] C=^

[0008] Where e0 is the dielectric permittivity of the vacuum, er the relative permittivity of the material insulator, S the surface area of ​​the electrode and e the thickness of the insulator.

[0009] In an Al electrolytic capacitor, the positive electrode is composed of an etched Al sheet in order to increase its surface area. This sheet is then anodized in order to generate a layer of alumina (A12O3) on the surface which acts as an insulator. By controlling the parameters during this anodization phase, the thickness of Al2O3 can be increased and in the same way the voltage of the capacitor, which can then reach several hundred Volts. The negative electrode is an Al sheet with a thin native oxide. The capacity of an electrolytic capacitor is limited by the surface area of ​​the etched Al sheet while its voltage is limited by the anodization process.

[0010] Ta electrolytic capacitors use a pressed and sintered powder of metallic Ta for the positive electrode. The specific surface area of ​​Ta electrodes is typically 1 to 10 m2 / g (it is noted that the density of Ta is high, of the order of 16.5 g / m3). As with their Al counterparts, a layer of tantalum oxide (Ta2O5), which acts as an insulator, is obtained by anodizing the metallic electrode. The thickness of this layer is from ten to a few hundred nanometers, for a voltage of a few Volts to 200 V typically. Ta capacitors have a larger volume capacity than Al capacitors because their insulating layer is generally thinner and the permittivity of Ta2O5 can reach 27 compared to 8 to 10 for Al2O3. They have capacities of a few pF to several tens of mF.They are widely used in surface mount device (SMD) formats because they are more resistant to the soldering process than Al capacitors. Today, a 100 pF / 6.3 V tantalum electrolytic capacitor has a typical volume of 1 mm3.

[0011] Supercapacitors are capacitive storage devices with very high capacity (from a few hundred mF to a few thousand Farads) and more limited power. Like all families of capacitors, their operation is based on the separation of electrical charges. This is the result of electrostatic interactions between the ionic charges of the electrolyte and the electrical charges of the electrode. Their high capacity comes from their microporous electrodes, most often based on nanostructured carbons with very large specific surfaces (1500 to 2500 m2 / g). In supercapacitors, there is no solid insulating layer with high breakdown voltage, which explains the low voltage of these devices, typically 2.5 to 3V.More precisely, the molecules of the solvents composing the electrolyte (with dielectric constants 2 to 4 times higher than those of solid metal oxides) which surround the solvated ions, ensure the separation of charges with the electrode. The thickness of this layer of molecules is of the order of a nanometer. This phenomenon, as well as the large surface area of ​​the . electrodes explain the high capacity of these devices. They therefore have energy densities up to 10,000 times higher than other types of capacitors.

[0012] SUMMARY

[0013] In order to overcome the drawbacks of capacitive electrical energy storage systems, the invention proposes an asymmetric capacitive electrical energy storage system with nanostructured electrodes. In particular, the present invention proposes capacitive electrical energy storage systems comprising a nanostructured positive electrode thus generating a very large surface conductive architecture, covered by a nanometric layer of an insulating material.

[0014] To this end, the invention has as its first object a capacitive electrical energy storage system comprising: - a liquid electrolyte; - a positive electrode comprising: • a first substrate made of conductive material; • a first two-dimensional or three-dimensional conductive nanostructure extending over the substrate; and • an insulator comprising at least one layer of dielectric material and conforming to the shape of the first conductive nanostructure; and - a negative electrode comprising: • a second substrate made of conductive material; • a second two-dimensional or three-dimensional conductive nanostructure extending over the substrate;

[0015] Compared to state-of-the-art electrolytic capacitors using nanostructured symmetrical electrodes covered with insulation, the use of an asymmetrical approach, with a negative electrode not covered with insulation, ensures the voltage rise of the device without causing electrochemical degradation at the negative electrode / electrolyte interface. The potential of the negative electrode in fact remains close to its initial stability value (open circuit potential) whatever the state of charge of the device.

[0016] The conductive material substrate, used as a conductive support for the first and second nanostructures, makes it possible to ensure the dual role of current collector and source of the insulator repair element. Self-repair is a characteristic that no other technology based on nanostructured electrodes currently offers and which makes it possible to ensure better stability in storage and in use. This stability is comparable to that of electrolytic capacitors, which are also designed to be able to benefit from a means of repairing / regenerating the insulating metal oxide layer because it is not stable over time during storage over extended periods. The insulator used in the present invention is similar to that of electrolytic capacitor technologies.

[0017] In one embodiment, the first substrate and / or the second substrate is made of aluminum, tantalum or niobium.

[0018] These materials make it possible to obtain a passivation layer allowing self-repair of the insulation during use of the system.

[0019] In one embodiment, the first conductive nanostructure and / or the second conductive nanostructure comprises a set of structures extending parallel to each other and perpendicular to the surface of the substrate. The structures may be juxtaposed without contact with each other, may be in point contact or even be entangled.

[0020] In one embodiment, the set of structures is in at least one material selected from the group consisting of carbon nanotubes, graphene, silicon nanowires, zinc oxide nanowires, aluminum-doped zinc oxide nanowires.

[0021] Conductive nanostructures allow the increase in surface area and thus the increase in capacity and thus the compactness at identical capacity.

[0022] In one embodiment, the insulator comprises at least two faces arranged opposite one another and spaced apart.

[0023] Advantageously, the space between the structures makes it possible to increase the capacity of the storage system.

[0024] In one embodiment, at least one layer of dielectric material of the insulator is alumina.

[0025] Advantageously, alumina (A12O3) has a high dielectric strength, allowing high voltages despite a low layer thickness.

[0026] In one embodiment, a total thickness of the at least one layer of dielectric material of the insulator is between 5 and 20 nm.

[0027] The low thickness of the layer thus makes it possible to leave a space between the structures of the first nanostructure, even when the structures are dense.

[0028] The second subject of the invention is a method of manufacturing the storage system according to the first subject, the method comprising the following steps:

[0029] a. the manufacture of the positive electrode comprising the following sub-steps: • supply of the first substrate, • growth or deposition of the first two-dimensional or three-dimensional conductive nanostructure on the first substrate; and • depositing the insulator over the entire surface of the two-dimensional or three-dimensional conductive nanostructure so as to match the shape of the nanostructure and the surface of the first substrate devoid of nanostructure; and b. manufacturing the negative electrode comprising the following sub-steps: • providing the second substrate; and • growth or deposition of the second two-dimensional or three-dimensional conductive nanostructure on the second arranged substrate.

[0030] In one embodiment, the growth of the first and / or second two-dimensional or three-dimensional conductive nanostructure is carried out by chemical vapor deposition.

[0031] Chemical vapor deposition makes it possible to obtain nanostructures comprising confined and narrow structures and thus a high density of structures. This makes it possible to increase the surface area of ​​the electrode.

[0032] In one embodiment, the deposition of the insulator is carried out by atomic layer deposition (ALD).

[0033] Atomic layer deposition allows the deposit of thin, uniform layers that conform to the shape of the nanostructure and leave a space between structures of the nanostructure. Brief description of the drawings

[0034] The invention will be better understood and other advantages will appear on reading the description which follows, given without limitation and thanks to the figures among which:

[0035] [Fig-1] [Fig.l] illustrates an example of a capacitive energy storage system electric according to the invention;

[0036] [Fig.2a] [Fig.2a] illustrates an example of a positive electrode according to the invention;

[0037] [Fig.2b] [Fig.2b] illustrates an example of a positive electrode according to the invention; and

[0038] [Fig.3] [Fig.3] illustrates an example of a method of manufacturing the storage system according to the invention. DETAILED DESCRIPTION

[0039] The invention relates to a capacitive electrical energy storage system 100. [Fig.l] illustrates an example of a capacitive electrical energy storage system.

[0040] As illustrated in [Fig.l], the capacitive electrical energy storage system 100) comprises a liquid electrolyte Ey, a positive electrode E+ and a negative electrode E-. The positive electrode E+ comprises a first substrate Subi made of conductive material, a first two-dimensional or three-dimensional conductive nanostructure Nanol extending over the substrate Subi and an insulator Iso comprising at least one layer of dielectric material and conforming to the shape of the first conductive nanostructure Nanol. The negative electrode E- comprises a second substrate Sub2 made of conductive material and a second two-dimensional or three-dimensional conductive nanostructure Nano2 extending over the substrate Sub2.

[0041] In one example, the first substrate Subi and / or the second substrate Sub2 is made of a conductive material comprising aluminum, tantalum or niobium. Advantageously, the first substrate Subi and / or the second substrate Sub2 is made of a conductive material capable of spontaneously oxidizing in order to form a passivation layer.

[0042] The two electrodes E+; E- are immersed in an electrolyte consisting of a solvent or mixture of solvents (organic or aqueous) and an ionic salt (organic or inorganic), an electrolyte with formulations similar to those used in the field of state-of-the-art supercapacitors or electrolytic capacitors.

[0043] Figures 2a and 2b illustrate examples of positive electrodes. In these examples, the first conductive nanostructure Nanol may be carbon nanotubes grown on the substrate Subi, and in particular vertically aligned carbon nanotubes. In the example of [Fig.2a], the first conductive nanostructure Nanol comprises a set of structures extending parallel to each other and perpendicular to the surface of the substrate. The first conductive nanostructure Nanol may be in at least one material chosen from the group consisting of carbon nanotubes, silicon nanowires (three-dimensional structure) or graphene (two-dimensional structure).

[0044] In another example, the first conductive nanostructure Nanol may comprise entangled structures, as illustrated in the example of [Fig.2b]. For example, the first conductive nanostructure Nanol may comprise carbon nanotubes that are not parallel and may be in contact with each other.

[0045] As illustrated in Figures 2a and 2b, the insulator Iso matches the shape of the first conductive nanostructure Nanol and the areas of the substrate that are not covered by the nanostructures. The insulator Iso comprises at least one layer of dielectric material. It may for example be formed from a single layer of dielectric material or from at least two layers, at least two of these layers having a different composition. For example, at least one layer of dielectric material of the insulator Iso may be made of alumina (A12O3).

[0046] The Iso insulator may comprise at least two faces arranged opposite each other and spaced apart. Indeed, in order to leave a space between at least two structures of the first Nanol nanostructure (for example between two vertically aligned carbon nanotubes), the layer to be deposited must be sufficiently thin. The space between the different structures may vary. Consequently, certain spaces between the structures may be filled by the Iso insulator layer, when the layer is not sufficiently thin. In another example, all the structures covered with Iso insulator are separated by a space. The Iso insulator may be deposited on the first conductive Nanol nanostructure, for example by atomic layer deposition (ALD). ALD is an advantageous technique for the homogeneous deposition of a dielectric layer on materials with complex geometries. In addition, when the first nanostructure Nanol comprises carbon nanotubes, the space between the nanotubes can be particularly confined. The thickness of the insulating layer Iso is therefore limited to an order of magnitude of a few nanometers to a few tens of nanometers. Thus, ALD makes it possible to deposit a dielectric layer having a small thickness making it possible to match the shape of the nanotubes. In one example, the total thickness of the at least one layer of dielectric material of the insulator is less than 20 nm. Preferably, the thickness is between 5 nm and 20 nm. This thickness range makes it possible to leave a space between nanostructures while obtaining a uniform layer. Moreover, as illustrated in [Fig.2a], when the structures of the first conductive nanostructure Nanol are spaced apart, the insulator Iso 1 is also deposited on the exposed parts of the first substrate Subi. In this case, the substrate Subi is covered either by the structures or by the insulator Iso. Advantageously, the space between the structures makes it possible to increase the capacity of the storage system compared to a planar electrode, i.e. without nanostructure. In particular, without space between them, the surfaces of the positive electrode E+ would be far from the end of the insulator Iso. As indicated above, the formula for the capacity of a capacitor being inversely proportional to the thickness of the insulator, the surfaces do not contribute or contribute negligibly to the capacity of the system. These surfaces would therefore be “lost”.

[0047] It is noted that the conductive nanostructure shown in [Fig.2a] or 2b could also illustrate the second conductive nanostructure of the negative electrode. Thus, the second substrate Sub2 may be made of aluminum, tantalum or niobium. It is noted that the first substrate Subi and the second substrate Sub2 may be made of different or identical materials. Similarly, the first conductive nanostructure Nanol and the second conductive nanostructure Nano2 may be made of different or identical materials. For example, the second conductive nanostructure Nano2 may be made of at least one material selected from the group consisting of carbon nanotubes, graphene, silicon nanowires, zinc oxide (ZnO) nanowires, aluminum (Al) doped zinc oxide (ZnO) nanowires.

[0048] The second conductive nanostructure Nano2 may comprise a set of structures extending parallel to each other and perpendicular to the surface of the substrate Sub2. For example, the second conductive nanostructure Nano2 may comprise vertically aligned and / or entangled carbon nanotubes, grown on the second substrate Sub2, for example an Al foil.

[0049] Advantageously, the use of conductive nanostructures in both electrodes, such as carbon nanotubes, make it possible to obtain higher capacities than those of currently commercialized electrolytic capacitors due to the surface area of ​​these nanostructured electrodes.

[0050] In addition, the layer of dielectric material on the positive electrode E+ makes it possible to obtain high voltages, up to several tens of volts, in the same range as those of electrolytic capacitors, and much higher than those of existing nanostructured technologies. In particular, this is linked to the significant electrochemical stability provided by the covering of the nanostructure in a homogeneous manner by the layer of dielectric and the fact that it is only added to the positive electrode.

[0051] In addition, the present invention allows for better compactness than electrolytic capacitors with equivalent energy density. Indeed, the use of nanostructures increases the surface area and therefore results in an increase in capacity.

[0052] In addition, a spontaneous passivation effect during the first charge-discharge cycles in the event of a local insulation defect between the positive conductive electrode and the electrolyte is enabled by the use of a substrate made of conductive material that can oxidize spontaneously.

[0053] [Fig. 3] is a flowchart illustrating a manufacturing method 200 of the storage system 100 according to the invention. The storage system 100 may be the storage system described above.

[0054] In block 202, the method 200 comprises manufacturing the positive electrode E+. Manufacturing the positive electrode E+ comprises providing the first substrate Subi. For example, the first substrate Subi may be provided on a support. The first substrate Subi may comprise aluminum, tantalum, or niobium.

[0055] In addition, the fabrication of the positive electrode E+ comprises growing or depositing the first two-dimensional or three-dimensional conductive nanostructure Nanol on the first substrate Subi. For example, the first nanostructure Nanol may be carbon nanotubes grown by chemical vapor deposition (CVD) directly on the first substrate Subi. In another example, the carbon nanotubes may be grown on a support and then deposited on the first substrate Subi.

[0056] In addition, the fabrication of the positive electrode E+ comprises the deposition of the insulator Iso over the entire surface of the two-dimensional or three-dimensional conductive nanostructure so as to match the shape of the nanostructure and the surface of the first substrate devoid of nanostructure. The surface of the first substrate devoid of nanostructure represents the areas of the upper face of the first substrate (i.e. the face on which the nanostructure is grown), which are not covered by the nanostructure. For example, the deposition can be done by chemical vapor deposition or by physical vapor deposition (PVD). However, chemical vapor deposition Vapor deposition or physical vapor deposition, where the reactants react in the liquid or vapor phase, can only be used on easily accessible surfaces. Thus, when the nanostructures are close together, these deposition methods are not favored. In another example, the deposition of the insulator can be carried out by atomic layer deposition (ALD). Anodizing, electrochemical deposition and ALD better meet the deposition criteria for complex nanostructures with very high specific surface areas. However, anodizing and electrochemical deposition are limited by the nature of the surface to be anodized or by the resistance of the substrate or the deposited layer. ALD is an advantageous technique for the homogeneous deposition of a dielectric layer on materials with complex geometries, particularly when they are nanostructured.Indeed, ALD allows the deposit of thin and uniform layers that fit the shape of the nanostructures, even when the space between the structures is narrow (e.g., less than 100 nm). For example, ALD allows the deposit of dielectric layers with a total thickness between 5 and 20 nm. In addition, the insulator Iso is deposited so as to envelop the nanostructure and the areas of the substrate that are not covered by nanostructures, as illustrated in Figures 2a and 2b.

[0057] In block 204, the method 200 comprises manufacturing the negative electrode E-. Manufacturing the negative electrode E- comprises providing the second substrate Sub2. For example, the second substrate Sub2 may be provided on a support. The first substrate Subi may comprise aluminum, tantalum, or niobium. In another example, the first substrate Subi may comprise carbon or graphite fabric. The first and second substrates may be the same or made of different materials.

[0058] In addition, the fabrication of the negative electrode E- comprises the growth or deposition of the second two-dimensional or three-dimensional conductive nanostructure on the second substrate. For example, the second nanostructure Nano2 may be carbon nanotubes grown by chemical vapor deposition (CVD) directly on the second substrate Sub2. In another example, the carbon nanotubes may be grown on a support and then deposited on the first substrate Sub2.

[0059] In addition, the manufacturing method 200 may comprise arranging the electrodes E+, E- in the liquid electrolyte. For example, the electrodes E+, E- may be arranged in the electrolyte consisting of a solvent or mixture of solvents (organic or aqueous) and an ionic salt (organic or inorganic).

[0060] Although the invention has been illustrated and described in detail using a preferred embodiment, the invention is not limited to the disclosed examples. Other variations may be deduced by those skilled in the art without departing from the scope of protection of the claimed invention. For example, although carbon nanotubes have been used to illustrate exemplary embodiments, other nanostructures may be used, such as graphene or silicon nanowires. In addition, substrates have been illustrated with aluminum, niobium, or tantalum. However, other conductive materials that can spontaneously oxidize to form a passivation layer may be used.

Claims

Claims

1. Capacitive electrical energy storage system (100) comprising: - a liquid electrolyte (Ey); - a positive electrode (E+) comprising: • a first substrate (Subi) made of conductive material; • a first two-dimensional or three-dimensional conductive nanostructure (Nanol) extending over the substrate; and • an insulator (Iso) comprising at least one layer of dielectric material and conforming to the shape of the first conductive nanostructure; and - a negative electrode (E-) comprising: • a second substrate (Sub2) made of conductive material; • a second two-dimensional or three-dimensional conductive nanostructure (Nano2) extending over the substrate;

2. An electrical energy storage system according to claim 1, wherein the first substrate and / or the second substrate is made of aluminum, tantalum or niobium.

3. An electrical energy storage system according to claim 1 or claim 2, wherein the first conductive nanostructure and / or the second conductive nanostructure comprises a set of structures extending parallel to each other and perpendicular to the surface of the substrate.

4. An energy storage system according to claim 3, wherein the set of structures is in at least one material selected from the group consisting of carbon nanotubes, graphene, silicon nanowires, zinc oxide nanowires, aluminum-doped zinc oxide nanowires.

5. Electrical energy storage system according to one of the preceding claims, in which the insulator comprises at least two faces arranged opposite one another and spaced apart from each other.

6. An electrical energy storage system according to any preceding claim, wherein at least one layer of dielectric material of the insulator is alumina.

7. An electrical energy storage system according to any preceding claim, wherein a total thickness of the at least one layer of dielectric material of the insulator is between 5 and 20 nm.

8. A method of manufacturing (200) the storage system according to any one of claims 1 to 7, the method comprising the following steps: a. manufacturing the positive electrode comprising the following sub-steps: • providing the first substrate, • growing or depositing the first two-dimensional or three-dimensional conductive nanostructure on the first substrate; and • depositing the insulator over the entire surface of the two-dimensional or three-dimensional conductive nanostructure so as to match the shape of the nanostructure and the surface of the first substrate devoid of nanostructure; and b. manufacturing the negative electrode comprising the following sub-steps: • providing the second substrate; and • growing or depositing the second two-dimensional or three-dimensional conductive nanostructure on the second substrate arranged.

9. A method of manufacturing the storage system according to claim 8 wherein the growth of the first and / or second two-dimensional or three-dimensional conductive nanostructure is carried out by chemical vapor deposition.

10. A method of manufacturing the storage system according to claim 8 or claim 9, wherein the deposition of the insulator is carried out by atomic layer deposition (ALD).

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