Current collector for electric energy storage

Abstract

Current collector for electrodes for electrical energy accumulators, comprising at least two layers: a substrate (2) made of electrically conductive metallic material and a layer attached to the substrate (3), of non-woven carbon fabric, defining a self-supporting structure to contain an active material (5); said layer defining within it contiguous paths to allow the passage of electrons between said active material and said substrate (2).

Description

[0001] CURRENT COLLECTOR FOR ELECTRIC ENERGY STORAGE

[0002] DESCRIPTION

[0003] FIELD OF THE INVENTION

[0004] The present invention refers to a current collector for electrical energy accumulators, i.e. a current collector intended to be used to realize at least one electrical accumulator, for example a battery or a super-capacitor.

[0005] The concerned current collector finds advantageous use in the electrical or electronic field, in particular in the field of the production and marketing of devices for the accumulation of electrical energy, of any nature and for any application.

[0006] Therefore, the concerned current collector for electrical energy accumulators finds application in the technical field of the production and marketing of devices for the accumulation of electrical energy.

[0007] STATE OF THE ART

[0008] In many technical fields, there has long been a need to have electrical energy storage systems characterized by high specific power, capable of providing large amounts of energy in short intervals of time (and therefore a large demand for deliverable power), for example in the technical field of automotive production, especially electric or hybrid cars. To date, the technical field of production and marketing of electrical energy storage devices offers electric batteries, e.g. lithium ion batteries, which often must be oversized to meet the aforementioned demand for large energy and deliverable power. Oversizing storage devices brings the intrinsic drawback of having to increase the size - and therefore the bulk - of the device itself.

[0009] Usually, an electrical energy storage device comprises at least two electrodes separated from each other by a porous material, known to those skilled in the art as a "separator". An electrode usually comprises at least one current collector, usually made of electrically conductive metallic material and a coating layer comprising an active material, fixed to the collector by means of a binder, usually made of polymeric material. Optionally, a conductive additive, with the function of reducing the electrical resistance of the coating, can be added to the formulation of the coating layer applied to the metallic collector.

[0010] Typically, a current collector is made of a very thin sheet of metallic material, in aluminium (Al) or copper (Cu), with excellent mechanical and electrical properties; the main advantage of these materials is their excellent electrical conductivity, so it is possible to construct electrodes with a large surface area, without the areas furthest from the electrical contacts, between the electrode and the terminals of the device, being affected by the drop in potential due to the distance.

[0011] The main drawback of a known type of metallic collector lies in the fact that the maximum quantity of active material that can be used as a coating is limited. In fact, the increase in active material deposited on the collector results in an increase in the thickness of the coating, affecting the mechanical stability of the electrode, reducing its capacity to extract charge from the active material itself and worsening the maximum power output performance.

[0012] In fact, the ease of electronic exchange with the active material decreases as one moves away, in the direction of the thickness of the coating, from the metal collector. In other words, it is easy to extract or introduce electrons from the layers of active material arranged planarly near the collector while it is more difficult to extract or introduce electrons from the layers of active material deposited further away from the collector.

[0013] This problem is often solved by replacing part of the active material with said conductive additive material — usually carbon-based — designed to improve the electrical conductivity of the active material, in particular in correspondence with the outermost layers and those farthest from the collector. Despite this, the limit of adding a conductive additive material in small percentages can only partially solve the problem.

[0014] Another drawback is that metallic materials are very dense (for example, aluminium has a density of 2.7 g / cm3, while copper has a density of 9.0 g / cm3), and their use as current collectors adds a lot of mass to the electrode, worsening its properties in terms of energy and specific power per unit of weight.

[0015] Carbon-based materials have the advantage of being extremely light, electrically conductive and resistant to corrosion, for this reason they are of great interest for the construction of current collectors.

[0016] The main drawback of the application of current collectors in carbon material is that they do not have electrical conductivity characteristics equal to those of metals and for this reason they cannot be used for the construction of electrodes with a large surface area, since a non-negligible resistance would be introduced, effectively limiting the maximum size of the electrode; furthermore, the non-uniform current distribution determines a worsening of the performance and useful life of the devices. An example of a carbon collector that does not solve the above problems, especially for supercapacitors, is described in the document US5581438A.

[0017] Another example, US10840032, shows a supercapacitor cell having at least one electrode comprising a metallic current collector and a coating layer comprising the active material, which in turn comprises activated carbon, as the active material responsible for the electrical energy storage process, and high-purity carbon nanotubes, as a conductive additive, and is free of polymeric binder. The materials of the coating layer comprising the active material are both porous and conductive to increase the charge storage capacity and decrease the electrode resistance. In general, the carbon nanotube content in the active layer is between 10 and 30% and the purity of the carbon nanotubes is at least 95%. However, this solution is expensive, difficult to implement and has limitations. The electrical conduction properties of the obtained collector depend on the direct contact between the nanotubes mixed with the active material to form, after mixing, a porous coating. However, such contact depends on the percentage of nanotubes in the coating formulation. Furthermore, the use of such a solution involves the exploitation of toxic and carcinogenic solvents, such as 1 -methyl-2-pyrrolidinone, togetherwith complex high- energy treatments of the suspensions to ensure a correct distribution of this specific conductive additive.

[0018] A method to circumvent the problems previously listed and still obtain an increase in conductivity of the active material, can be found in the document US20170062143A1, where the use of current collectors consisting exclusively of porous matrices is exposed, thus eliminating the metal substrate; this solution leads to advantages in terms of specific performance due to a better distribution of the current.

[0019] However, such a solution precludes the possibility of being used in conventional processes due to the poor tensile strength of the sheet and the poor in-plane conductivity. SCOPES THE INVENTION

[0020] The purpose of the present invention is to provide a current collector for electrical energy storage devices that allows to overcome and remedy, at least in part, the drawbacks of the above-mentioned prior art, or at least alternative, compared to the solutions of the prior art.

[0021] A further purpose of the present invention is to provide a current collector for electrical energy storage devices that guarantees high energy, power and useful life values of a finished electrode, while remaining compact and lightweight. Another purpose of the invention is to propose a current collector having more stable properties and greater processability than the solutions proposed in US10840032 and US20170062143A1.

[0022] A further purpose of the present invention is to provide a current collector for electrical energy storage devices that can be produced in a simple, rapid and low-cost way.

[0023] Another purpose of the present invention is to avoid the implementation of nanotubes, as they are difficult to process.

[0024] A further aim of the present invention is to provide a current collector for electrical energy accumulators which presents an alternative and / or improved configuration, both in terms of construction and in terms of functionality, compared to traditional known solutions.

[0025] SUMMARY OF THE INVENTION

[0026] All the objects, either individually or in any combination thereof, and others that will result from the detailed description that follows are achieved, according to the invention, with a current collector for electrical energy accumulators having the characteristics indicated in independent claim 1.

[0027] In particular, the aforementioned objects are achieved with a current collector for electrical energy accumulators, comprising a substrate made of electrically conductive metallic material, on which a layer of non-woven carbon fabric is attached, defining a self- supporting structure capable of hosting an active material, so as to allow a facilitated exchange of electrons between the active material and the substrate.

[0028] Preferably, the layer of non-woven fabric defines a matrix formed by microfibers exclusively made of carbon.

[0029] The microfibers are made of carbon and intersected with each other.

[0030] Preferably, the current collector comprises a bonding layer interposed between said substrate and said carbon nonwoven layer configured to mechanically and electrically bond said substrate to said carbon nonwoven layer.

[0031] Preferably, said bonding layer comprises a conductive carbon or metal component having a polymeric binding matrix.

[0032] Preferably, said carbon nonwoven layer comprises a conductive three-dimensional self- supporting reticular structure comprising a combination of electrically conductive carbon materials.

[0033] Furthermore, the aforementioned objects are achieved by an electrical accumulator comprising at least one electrode containing the current collector of the invention.

[0034] The present invention also provides a method for producing a current collector for electrical energy accumulators according to claim 10.

[0035] Preferably, the method comprises an intermediate step of laying at least one layer of bonding material to mutually attach the nonwoven layer to the substrate.

[0036] The present invention also provides a method for producing an electric accumulator in which two electrodes according to the invention are facing each other with at least one porous separator element, or a solid and / or semi-solid state electrolyte in between.

[0037] The dependent claims describe preferred embodiments of the invention forming an integral part of the present description.

[0038] BRIEF DESCRIPTION OF THE FIGURES

[0039] The present invention is further clarified hereinafter in some of its preferred forms of practical embodiment, reported purely for illustrative and non-limiting purposes, with reference to the attached drawings, in which:

[0040] - Figure 1 shows a schematic sectional view of a current collector according to the present invention;

[0041] - Figure 2 shows a schematic sectional view of an electrode with the current collector of figure 1 comprising a layer of active material;

[0042] - Figure 3 shows a perspective view of a step of the method for producing an electrode according to the invention;

[0043] - Figure 4 shows a comparison graph between the performances of a known type of electrode for supercapacitors (identified as “benchmark” in the attached figure), based on activated carbon, which constitutes the active material responsible for the electrical energy accumulation process in the device, and of a similar electrode for supercapacitors, also based on activated carbon, containing the current collector object of this invention (identified as “AC+UCC” in the attached figure), in terms of retention of the initial capacity as the discharge current of the devices varies;

[0044] - Figure 5 shows a performance comparison graph of a known supercapacitor electrode (identified as “benchmark” in the attached figure), based on activated carbon, which constitutes the active material responsible for the electrical energy storage process in the device, and a similar supercapacitor electrode, also based on activated carbon, containing the current collector of this invention (identified as “AC+UCC” in the attached figure), in terms of specific energy as a function of the specific power output of the devices.

[0045] The same reference numbers and letters in the figures identify the same elements or components or functions.

[0046] It should also be noted that the terms “first”, “second”, “third”, “upper”, “lower” and the like may be used here to distinguish various elements. These terms do not imply a spatial, sequential or hierarchical order for the modified elements unless specifically indicated or inferred from the text.

[0047] The elements and features illustrated in the various preferred embodiments, including the drawings, may be combined with each other without departing from the scope of protection of the present application as described below.

[0048] DETAILED DESCRITPION

[0049] With reference to the attached figures, the reference numeral 1 indicates as a whole a current collector for electrical energy accumulators, the subject of the present invention, in particular intended to be used to realize any type of electrical energy accumulators.

[0050] The term electrical accumulator shall be understood hereinafter as any device capable of storing electrical energy in electrochemical form, such as for example batteries, supercapacitors or the like.

[0051] According to the invention, the current collector 1 for electrical energy accumulators comprises a base 2 made of electrically conductive metallic material.

[0052] The substrate 2 is configured to act as a mechanical support for the layer of non-woven carbon fabric in which it is possible to deposit an active material as well as an electrical connection to external circuits, known per se and not described.

[0053] Furthermore, the substrate 2 is made of electrically conductive materials, i.e. capable of conducting electrons.

[0054] The main purpose of the substrate 2 is to pick up and deposit electrons from and to an active material, described in detail below, arranged in the layer 3.

[0055] The current collector 1 according to the invention comprises at least one layer of carbon non-woven fabric 3, mechanically and electrically connected to said substrate 2.

[0056] Layer 3 defines a three-dimensional reticular structure that contains voids between the mutually intersecting microfibers. The voids define a housing volume for an active material.

[0057] The carbon non-woven fabric layer is a distinct and separate component from the active material, forming the aforementioned self-supporting structure in which to insert the active material.

[0058] Layer 3 has the form of a sheet that can be appropriately shaped to be attached to the substrate.

[0059] The non-woven carbon layer 3 defines, through the intersected microfibers, paths such that, once the active material has been applied, the passage of electrons between the active material and the substrate 2 is facilitated.

[0060] Preferably, the active material is configured to accumulate electrical energy, in particular by faradic electrochemical processes, which involve for example the transfer of electrons (as in the normal batteries), or of the electrostatic type (typical of supercapacitors). The active material can be of the hybrid type, i.e. capable of storing electrical energy with both faradic and electrostatic processes.

[0061] Preferably, the substrate 2 comprises a sheet of conductive material, to be shaped in relation to the shape that is intended to be given to the collector.

[0062] In accordance with a particular variant of the present invention, the substrate 2 is made of metallic material, for example aluminium (Al) or copper (Cu), and preferably equipped with high mechanical properties and electrical conductivity.

[0063] In this way, the substrate 2 can be made with very long sheets, avoiding the risk of high potential drops thanks to the high electrical conductivity.

[0064] The layer 3 is made of carbon-based material in the form of intersected micro fibers.

[0065] Advantageously, the layer 3 is much lighter than an electrically conductive metal layer and resistant to corrosion phenomena.

[0066] Preferably, the current collector 1 comprises a binding layer 4 interposed between the substrate 2 and the layer 3, configured to mechanically and electrically bind the substrate 2 to the layer 3.

[0067] The active material 5 is deposited in the voids present in the non-woven fabric made of intersected micro fibers.

[0068] The carbon micro fibers are in no way carbon nanotubes.

[0069] Preferably, the current collector according to the invention can provide a layer of carbon non-woven fabric for each of the two opposite faces of the substrate 2, creating a sort of sandwich with the substrate 2 at the center.

[0070] Preferably, said binding layer 4 comprises a carbon or metal conductive component and a polymeric binding matrix. The presence of the layer 3, in the form of a self-supporting three-dimensional reticular structure of non-woven fabric, allows to increase the power and energy of the electrical energy accumulators in which the current collector is implemented. In fact, the carbon non-woven fabric layer cooperates with the active material to define paths for the passage of electrons, in order to extract or deposit electric charge from / to the active material, improving the performance of the system.

[0071] The current collector according to the invention ensures an increase in power and a better distribution of the current in the electrode space thanks to a better electrical connection of the active material in all directions and preferably for the entire thickness of the electrode.

[0072] Furthermore, the current collector according to the invention allows an increase in storable energy, thanks to a more efficient use of the active material, by an electrode in which such a current collector is implemented. In the case of a standard electrode, the external layers are very distant from the metal substrate, often not electrically connected to it or undergo non-uniform current densities in space. The presence of the three- dimensional self-supporting reticular structure defined by the carbon non-woven fabric layer allows an improved electrical connection as the current density is uniform in all directions.

[0073] The current collector according to the invention allows to increase the amount of depositable active material, making it possible to build thicker electrodes, thanks to the better mechanical stability of the coating compared to known solutions, to the advantage of the energy accumulation properties of the device, without affecting its power performance and useful life.

[0074] The non-woven fabric used to make the carbon non-woven fabric layer is a material composed of interconnected and intersected carbon micro fibers. Each fiber preferably has a diameter greater than 1 pm and preferably between 5 pm and 50 pm and a length greater than 10 mm and preferably between 15 mm and 80 mm. The fibers of the non-woven fabric are rectilinear and intersected with each other. Preferably, this non-woven fabric layer has a surface density between 4 g / m2 and 30 g / m2. The thickness of this non-woven fabric can range from 20 pm to 300 pm. The empty spaces suitable for housing the active material, defined by this three-dimensional self-supporting reticular structure, can vary from 20 pm to 600 pm.

[0075] The microfibers intersect with each other, and a binder, distinct and separate from the active material 5 and the binding layer 4, dips only the intersection points between the microfibers to lock them together.

[0076] In other words, the fibers are not coated in any way, except at the points of mutual intersection.

[0077] The typical size of a particle of battery active material is about 500 nm when made, for example, of mixed oxides of nickel, cobalt, manganese and lithium (NMC), and between 2 and 10 pm when made of supercapacitor activated carbon, i.e. much smaller than the size of the voids between the micro fibers of layer 3.

[0078] The coating comprising the active material can easily penetrate the voids between the microfibers, impregnating the network structure identified by the non-woven fabric layer 3. Furthermore, the microfibers of a non-woven fabric form a network of rectilinear and randomly arranged microfibers. The proposed solution promotes the conduction of electrons and the mechanical stability of the active material in a greatly improved way compared to solutions known to those skilled in the art, such as the addition of a mixture of carbon nanotubes to the formulation of the coating comprising the active material.

[0079] Furthermore, a substantial difference in electrical conductivity is observed between the solution of the present invention and that shown in US10840032. While in the present invention, the increase in electrical conductivity is already guaranteed by the presence of the non-woven fabric 3, in the mixture shown in US10840032, the increase in electrical conductivity depends on the number of nanotubes, which must be no less than 10-15% by mass of the composition of the mixture of the active material. Below this percentage, an effective electrical connection between the nanotubes is not guaranteed.

[0080] The Applicant has conducted numerous studies on numerous prototypes of the invention described here, the results of which are summarized below.

[0081] The Applicant has constructed two electrical energy accumulators with symmetrical electrodes, in particular super-capacitor cells. A first cell comprises electrodes according to the invention, in which the current collector is constructed according to the methods described in the present invention, with a layer of non-woven carbon fabric 3 and with an aluminium substrate 2, with a density of active material deposited on the current collector per unit area of approximately 4.47 mg / cm2. A second accumulator of a known type, with electrodes made of aluminium sheets, according to methods known to those skilled in the art, with a density of active material deposited on the current collector per unit area of approximately 3.91 mg / cm2.

[0082] The attached figure 4 shows the results for these two cells compared on the same graph, in which the current density per unit mass of active material is present on the abscissa and the percentage retention of capacity is present on the ordinate. Similarly, the attached figure 5 shows the specific energy performance, on the ordinate, for the same specific power, on the abscissa.

[0083] It is evident how the performance of the accumulator equipped with electrodes containing the current collector according to the invention has, in every operating condition, better performance and performance in energy delivered or absorbed, thanks to the ability to accumulate and extract electric charge even from the outermost and most distant layers of the active material with respect to the substrate 2.

[0084] The substrate 2 is made with metal-based materials, for example aluminium, stainless steels, copper. Furthermore, the substrate 2 can be made in other variants, in which the metal material can be combined with polymer matrices, which act as a substrate for metallization processes.

[0085] The nature of the substrate allows the construction of collectors through known industrial processes, e.g. roll-to-roll, offering excellent mechanical resistance and in particular tensile strength, subsequently allowing the deposition of active material with methods such as slot-die coating, blade coating, or other coating methods known to those skilled in the art. Such processes are completely impractical as described in US20170062143A1. Furthermore, the in-plane electrical resistance of the porous matrices shown in US20170062143A1 is higher when compared to a planar non-porous substrate. Finally, if such porous matrices are made of carbonaceous materials, a further complication is added given the poor weldability of these materials.

[0086] In contrast, the present invention provides a metal substrate, weldable by methods known to those skilled in the art, and a layer of carbon non-woven fabric attached to the substrate by a binder layer 4.

[0087] Preferably, the binder layer 4 may be composed of a conductive component and a binder. The conductive component is composed of a carbonaceous material (e.g. graphite). The binder matrix can be composed of polymeric material based on: Polyvinylidene Fluoride (PVDF), Polyvinylidene Fluoride-HexaFluoropropylene (PVDF-HFP), Polyethylene Oxide (PEO), Polyvinyl Alcohol (PVA), Polyvinylpyrrolidone (PVP), Polytetrafluoroethylene (PTFE), Carboxymethylcellulose (CMC), Styrene-Butadiene Rubber (SBR), combinations of the previous ones and their derivatives. The binder layer 4 can be produced either by the wet method, which involves the use of solvents, or by the dry method, which does not involve the use of solvents. The preferable techniques for the deposition of the binder layer 4 on the substrate 2 are for example: dip coating, blade coating or tape casting, spray coating, hot pressing, extrusion or combinations of the previous ones.

[0088] The present invention also provides a method for producing a current collector for electrical energy storage devices comprising:

[0089] - a step of preparing a sheet 11 of electrically conductive metallic material to define the substrate 2;

[0090] - a further step of superimposing a layer of carbon non-woven fabric 12 to define the layer 3, defining a self-supporting structure in which to insert an active material and facilitate the passage of electrons between the substrate 2 and the active material 5;

[0091] - optional step of laying a binding layer 4 on the substrate 2 and / or on the layer 3;

[0092] - a hot rolling or calendering step to mechanically and electrically connect the substrate 2 to the layer 3.

[0093] The present invention also provides a method for producing an electrode for electrical energy storage devices comprising the further step, with respect to the method for producing a current collector, of depositing an active material 5, which may be known in itself, in the layer 3.

[0094] The present invention also provides an electric accumulator comprising at least one electrode equipped with the current collector described above.

[0095] The present invention also provides a manufacturing method for an electric accumulator comprising the creation of a pair of electrodes equipped with at least one current collector as described above, in which an active material 5 is deposited in the layer 3, comprising a further step of arranging the electrodes facing each other with at least one porous separator element or a solid and / or semi-solid state electrolyte interposed. From what has been described, it is clear that the current collector for electrodes for electric energy accumulators, according to the invention, is particularly advantageous because:

[0096] - it is able to overcome, at least in part, the drawbacks of the prior art mentioned above; - it allows to increase the performance of the electric accumulator in which it is implemented;

[0097] - it allows to improve the charging and discharging efficiency of the devices in which it is contained;

[0098] - it can be manufactured on an industrial level with known manufacturing techniques; - represents a solution that does not require changes in the formulation of active material inks, allowing its immediate adoption in an already existing industrial process;

[0099] - presents an alternative and / or improved configuration, both in construction and economic terms, compared to traditional known solutions.

[0100] Implementation variants to the non-limiting example described are possible, without however departing from the scope of protection of the present invention, including all the equivalent embodiments for a technician in the field, to the content of the claims.

[0101] From the description reported above, the technician in the field is able to realize the object of the invention without introducing further construction details.

Claims

CLAIMS1. Current collector for electrodes for electrical energy accumulators, comprising at least two layers:- a substrate (2) made of electrically conductive metallic material;- a layer attached to the substrate (3), of non-woven carbon fabric, defining a self- supporting structure to contain an active material (5); said layer defining within it contiguous paths to allow the passage of electrons between said active material and said substrate (2).

2. Collector according to claim 1, characterized in that said layer (3) defines a three- dimensional conductive reticular structure formed by microfibers made exclusively of carbon.

3. Collector according to claim 1 or 2, wherein said layer is formed by microfibers intersected with each other, and wherein a polymeric binder, distinct and separate from said active material (5) and from a binding layer (4), dips exclusively the intersection points between the microfibers to block them mutually.

4. Collector according to claim 3 wherein the fibres are spatially arranged in a disorderly manner with respect to each other.

5. Collector according to any of the preceding claims 3 or 4, wherein each fibre has a diameter greater than 1 pm and preferably between 5 pm and 50 pm and a length greater than 10 mm and preferably between 15 mm and 80 mm and wherein the fibres are rectilinear and mutually intersecting.

6. Current collector for electrodes for electrical energy accumulators according to any of claims 1 - 5, characterised in that it comprises said binding layer (4) interposed between said substrate (2) and said layer (3) to mechanically and electrically bind said substrate (2) to said layer (3) .

7. Current collector for electrodes for electric energy accumulators according to claim 6, characterized in that said binding layer (4) comprises a carbon or metal-based conductive material and a polymeric binding matrix.

8. Collector according to any of the preceding claims, wherein said layer (3) has a thickness of between 40 pm and 200 pm and a surface density of between 4g / m2 and 30g / m2.

9. Electric accumulator comprising at least one electrode with current collector according to one or more of the preceding claims, wherein said active material (5) is deposited in the layer (3).

10. Method for producing a current collector for electrodes for electric energy accumulators according to any of claims 1 — 8, comprising at least the following operating steps in succession:- preparing at least one sheet (11) of electrically conductive metallic material to define a substrate (2);- superposition of a carbon non-woven fabric (12) defining said layer (3), defining a self- supporting structure to contain an active material (5); said layer (3) defining within it contiguous paths to allow the passage of electrons between the active material (5) and the substrate (2).

11. Method according to claim 10, comprising an intermediate step of laying out said binding layer (4) to connect said layer (3) on said substrate (2) mechanically and electrically.

12. Method for the production of an electric accumulator comprising the creation of a pair of electrodes with at least one current collector according to one or more of the preceding claims 10 or 11, in which an active material (5) is deposited in the layer (3), and a further step of arranging the electrodes facing each other with at least one porous separator element or a solid and / or semi-solid state electrolyte interposed

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