WO2023275733A1

WO2023275733A1 - Carbon/active material hybride electrodes for lithium ion batteries

Info

Publication number: WO2023275733A1
Application number: PCT/IB2022/055981
Authority: WO
Inventors: Pierfrancesco ATANASIO; Francesca Anna SCARAMUZZO; Andrea Capasso; Marco Rossi; Mauro Pasquali
Original assignee: Nanoshare 4.0 Srl
Priority date: 2021-06-29
Filing date: 2022-06-28
Publication date: 2023-01-05
Also published as: IT202100017024A1

Abstract

The present invention relates to the procedure for preparing a negative electrode for lithium-ion batteries wherein the electrode is composed of a plurality of layers of carbonaceous material and active material supported on a substrate that acts as a current collector. The invention also relates to the production of nanostructured electrodes for other devices suitable for the production and storage of electricity, such as supercapacitors.

Description

CARBON/ACTIVE MATERIAL HYBRIDE ELECTRODES FOR LITHIUM ION BATTERIES

Technical field The present invention relates to the procedure for manufacturing a negative electrode for batteries wherein the electrode is composed of a scaffolding consisting of a plurality of layers of carbonaceous material and active material supported on a collecting substrate of the type normally used for electrodes.

The invention also relates to the development of the method for preparing multilayer electrodes for lithium-ion batteries specifically and, more generally, for the production of multilayer electrodes that can also be used in other fields of energy production and storage. Therefore, the composition of the electrodes as such, the procedures for making the individual component materials and the methodologies for manufacturing the complete electrodes are included. More in detail, the present invention relates to the production of hybrid carbon/active material electrodes for lithium ion accumulators and their production with liquid phase deposition and chemical vapor deposition (CVD) techniques. The electrodes are made of a hybrid multilayer carbon/active material. The invention is generally placed in the context of the research for new anodic materials for the production of electrochemical devices for energy storage, in particular lithium-ion accumulators. The materials object of the invention are carbon/active material composites: the carbon component consists of nanostructured forms of graphite obtained through CVD processes (plasma activated CVD - Plasma Enhanced - and hot wire - or HFPECVD). The electrochemically active material, on the other hand, is deposited on the electrodes through liquid phases, but can also be deposited through electrochemical techniques or CVD processes. Background

Lithium-ion batteries are lightweight and provide a high energy density, making them very promising for use as a power source in all-electric and hybrid vehicles. In lithium-ion batteries, charges and discharges are based on the reversible processes of intercalation and de-intercalation of Li⁺ ions in the positive and negative electrodes of the battery.

Examples of electrochemically active materials used as anodes in lithium ion cells, in addition to graphite (C), are: silicon (Si), tin (Sn), and germanium (Ge). The conjugation of two components, a graphitic carbonaceous support and an active species such as Si, Ge or Sn, allows to overcome the limitations of both materials. Indeed, graphite, which intercalates lithium ions between its crystalline planes, offers a limited theoretical capacitance (372 mAh/g), while the active materials already mentioned, which form alloys with lithium, provide theoretical capacities up to an order larger (3590 mAh/g for Li-isSU alloy). It should be remembered, however, that compared to the high theoretical capacitance and the adequate performance as active substances of negative electrodes, these materials have reduced electronic conductivity and considerable volumetric variations during the charge and discharge cycles of the accumulator (up to 300%). For instance, the volume of the active substance of the negative electrode when it is subjected to reduction, at the moment of charging, increases considerably. Conversely, during discharge the active material shrinks and the volume decreases. As a result, the morphology of the active substance layer of the negative electrode collapses and the structure changes from crystalline to amorphous, whereby the electrical conduction paths between materials (such as between the active substance of the negative electrode and the collector) are interrupted, resulting in a reduction in battery capacitance. Therefore, following the charge and discharge cycles, the volumetric variation leads to the fragmentation of the electrode, which involves the loss of electrical contact between the particles of active material, with a consequent decrease in the performance of the electrode itself. Furthermore, the crumbling of the active material causes a continuous renewal of the solid layer that is created in the electrode/solution interphase called SEI (Solid Electrolyte Interface), with consequent alteration of the electrode kinetics and excessive electrolyte consumption. The consequence of these phenomena is the progressive loss of capacitance during the operating cycles.

The considerable volumetric variation is due to the alligation/dealligation process of lithium during the charge and discharge cycles of the battery, and is a typical phenomenon during alloys formation in which a metal with a very small atomic radius, such as Li, goes to insert itself in large quantities into a predefined crystalline structure, as in the case of the most common active materials mentioned.

In view of the technical problems highlighted above, the aim was to obtain hybrid electrodes (carbon/active material) capable of withstanding the volumetric variations the active material undergoes by means of a rigid and conductive structure (graphite), which can also have an active role in the electrochemical process. The carbonaceous structure, therefore, has the double role of containing the volume variations of the active material and of intercalating lithium ions. Among the active materials capable of alloying and reversibly releasing lithium, silicon is the most studied also because it represents a widely available and inexpensive material.

The solutions envisaged so far in the literature, while reaching high capacitance values, do not solve the problem of the progressive decline in performance as the number of charge and discharge cycles increases. The construction techniques proposed so far, both for economic and technical- practical reasons, are not suitable for scale-up production at an industrial level.

Among the most common forms of carbon nanostructures are carbon nanowalls (CNWs), defined as graphitic structures having the basal planes arranged mostly perpendicularly (90° +/- 20°) with respect to the plane of the substrate growth, interconnected in a morphology similar to "petals" arranged to form a three-dimensional matrix. This matrix welcomes other materials in the form of nanoparticles or other structures small enough to spread or grow inside the receptacles that are created between adjacent nanowalls. CNWs have a width at the base between 10 nm and 1 pm, a width at the tip, or crest, between 0.3 nm and 10 nm, and a vertical development between 100 nm and 100 pm. The size of the receptacles between adjacent nanowalls is between 30 nm and 30 pm.

US patent 8,551 ,657 B2, for instance, describes the manufacture of an anode for secondary lithium-ion batteries, consisting of carbon nanowalls which support the active material; in the case of the cited patent the exemplified active material is tin and the nanowalls deposition takes place in a CVD reactor fed by a mixture containing precursor, inert gas with carrier function and hydrogen. The composite material used as anode obtainable with the procedure described in US 8,551 ,657 B2 consists of a single nanowalls deposit on whose surface a layer of active material directly exposed to the electrolytic solution is applied. Consequently, during the charge and discharge cycles of the battery, the active material undergoes the usual large volumetric variations, without any kind of protection by the carbonaceous matrix. The resulting battery, therefore, loses capacitance as the number of cycles increases, has poor efficiency and short life.

The use of the carbon nanowalls as an electrode material (active principle of the negative electrode) is known, and is described both in patents (for instance the aforementioned US patent 8,551 ,657 B2, the Japanese patent application No. 2008-239369, JP- A-2008-239369, and Japanese Patent Application No. 2008-24570, JP-A-2008-24570), and in other types of publications (e.g. N. Kitada, “Application of Carbon Nanowalls to Negative Electrode Material for Lithium Ion Secondary Battery Abstracts of Exchange Meeting of Monodukuri Gijutsu 2008, Kanagwa Industrial Technology Center, 13 Nov 2008).

The nanowalls production is generally performed through low pressure chemical vapor deposition (or CVD) techniques with plasma, as shown in US patent application 2007/0184190 A1.

CVD techniques are widely studied and applied for the manufacture of carbonaceous based materials because, in addition to nanowalls, they also allow to obtain other types of carbon nanostructures such as graphene, activated carbon, nanotubes and nanowires. Gas phase precursors contain a carbon source (usually methane CFU, acetylene C2H2, ethylene C2H4, ethanol C2H5OH or fluorinated compounds such as CF4 or C2F6) and may contain hydrogen (H2) and other gases. The precursors are transported to the deposition zone by a flow of inert gas (helium (Fie), argon (Ar), nitrogen (N2)). For growth, a support substrate is used which can be metallic, glassy or ceramic. The substrate can be coated with a metal (such as Fe, Co and Ni) in the form of films or particles that act as a catalyst for the formation of specific nanostructures.

The growth of the nanostructures occurs in the CVD chamber as a result of chemical reactions that require a high temperature of the growth substrate and an independent energy source for gas activation. This energy source can be of a thermal nature (FHot Wire CVD, FIFCVD), or derive from a plasma (Plasma Enhanced, PE) generated by electrical discharges, which can be excited in direct current (DC), alternating current (AC), radio frequency (RFPECVD) or microwaves (MWPECVD).

The deposition of carbon nanostructures occurs when a gaseous precursor is activated and dissociated near the hot wire and by the chemical plasma conditions that occur in the volume affected by the electrical discharges. Carbon-based radicals that form under certain plasma and temperature conditions give rise to the deposition of nanostructures. As regards the nanowalls already used for lithium-ion batteries, in the literature (e.g. patent applications US2007/184190 A1 and US2015/0037515 A1) syntheses are described which generally involve the use of gaesous hydrogen in the feeding mixture. The use of hydrogen involves plant engineering and safety problems in terms of process scalability due to the high reactivity of the species in contact with atmospheric oxygen, with consequent risks associated with its storage and handling. Eliminating or replacing the hydrogen in the mixture brings greater simpleness in the instrumentation and scale-up of the process and less risks for operators and plants (AIP Conference Proceedings 1873, 02006, 2017).

Two paths can be followed for the manufacture of electrodes based on CVD- deposited carbon nanostructures. The first consists in synthesizing the nanostructures ex situ and then transferring them with chemical cold spreading processes on the electrode. The second consists in directly synthesizing the carbon nanostructures in situ on a substrate which will then become an integral part of the electrode.

Ex situ methods are necessary when the electrode materials and geometries are not compatible with the CVD growth conditions: the material produced must be separated from the growth substrate producing a powder, suspended in a solvent in the presence of binders and additives and spread on the power rail. This procedure represents a further step in terms of process and requires the use of organic solvents. Furthermore, in lithium-ion batteries, the electrode must be structurally cohesive and self-supporting, and the materials firmly anchored to conductive supports that guarantee the passage of electrons and structural cohesion: ex situ methods, which according to what has been described do not allow to obtain monolithic materials, do not lead to systems that fully meet these characteristics. To be used in in situ procedures, the support whereon the growth of the nanostructures takes place should have the following characteristics: a) it should resist the thermochemical conditions of growth and not lose the mechanical and chemical characteristics (softening, formation of carbides) in the interval 600-1000°C and in the presence of carbon radicals; b) it should be electrically conductive and able to survive the chemical environment and the mechanical stress of manufacturing and use of the lithium ion accumulator.

As regards the deposition system of the active material, liquid phase depositions such as suspensions in ethanol, propanol, supercritical CO2 and other solvents are described in the literature. In the case of US patent 8,551 ,657 B2, the active material (Sn) is dispersed using carbon dioxide in supercritical conditions. This system guarantees excellent performance in the solvent removal phase, as carbon dioxide can be quickly and effectively returned to the gaseous phase. However, the use of carbon dioxide in supercritical conditions requires special precautions and dedicated instrumentation, which makes it impractical in view of the industrial scale-up of the overall process. In addition to liquid phase deposition techniques, other methods are described in the literature that allow effective deposition of the active material. In particular, carbon nanostructures/active material composite anodes were obtained by carrying out electrodepositions (for instance of Ge, Materials Letters 144, 50-53, 2015, 10.1016/j.matlet.2015.01.022) or through multiple steps of CVD (ACS Nano 4, 2233-2241 , 2010, 10.1021 /nn901632g).

The electrochemical performances of the anodes obtained up to now with these methods are encouraging but not yet fully satisfactory. By way of example, in US patent 8,551 ,657 B2 a hybrid anode for secondary lithium ion batteries CNW/active material is described wherein the deposition of nanowalls takes place in a CVD reactor powered by a mixture containing precursor, carrier and hydrogen, while the typical active material is tin. The electrode obtainable with the described procedure consists of a single nanowalls deposit on whose surface a layer of active material directly exposed to the electrolytic solution is applied. Consequently, during the charge and discharge cycles of the battery, the active material undergoes the usual great volumetric variations, without any kind of protection by the carbonaceous matrix. The resulting battery, therefore, loses capacitance as the number of cycles increases, has poor efficiency and short life.

As described in detail in the following paragraphs, the present invention aims to overcome the limitations of the existing devices illustrated up to now, describing hybrid electrodes for lithium-ion accumulators based on nanostructured carbonaceous material/active material able to withstand the volumetric variations to which the active material is subjected under operating conditions and their manufacture with economic and easily scalable methods. As for the synthesis of the nanostructured carbonaceous material, the invention is part of the in situ methods, with the carbonaceous nanostructures that grow directly on a substrate that will act as a current collector or electrode for the final device. The deposition of electrochemically active materials is carried out on the same substrate by liquid phase techniques, CVD techniques or electrochemical depositions.

If not specifically excluded in the detailed description that follows, what is described in this chapter is to be considered as an integral part of the detailed description.

Summary of the invention

The present invention provides a solution to the technical problem represented by the pulverization of the active material and the structural collapse of the electrode also for those active materials which show a significant volume variation during the electrode charging and discharging processes.

In fact, from the following description and the attached figures it is evident that the carbon nanostructures according to the invention do not form a layer per se, but are separated by spaces and voids when deposited on the surface of a collector.

It is also described that the active material in a non-aggregated form is pretreated, so that after said pretreatment its particles/structures have dimensions smaller than the distances (w) between the carbon nanostructures deposited by the CVD process and/or smaller than the elements of the carbon nanostructures, or, according to a preferred embodiment, of the walls of the nanowalls.

Furthermore, the active material, in the form of a suspension, is infiltrated within the carbon nanostructures and does not form a layer perse, in the usual meaning of the term.

Therefore, it is an object of the present invention a negative electrode for a lithium ion battery comprising a current collector which supports a scaffolding consisting of one or more alternating layers of nanostructured carbonaceous material which in turn supports, traps and confines nanostructures of active material. The current collector and the active material do not undergo alterations at the working temperature of CVD techniques.

In the electrode of the invention, each layer of nanostructured carbonaceous material supports the active material by embedding it in order to contain the volume variations following the charging and discharging cycles. In this way, the overall volume of the entire scaffolding remains almost constant.

This electrode organization limits the structural collapse of the active material as it is embedded in the carbonaceous structure. In addition, excellent electrical conductivity is guaranteed between the carbonaceous walls that embed the nanoparticles of active material, even if this should undergo heavy structural deformation phenomena during the cycles. The result is a low resistance to charge transfer and high efficiency in current accumulation with consequent excellent battery performance both in terms of absolute capacitance and electrode stability; all this translates into an increase in the number of cycles and therefore in the life of the battery.

The negative electrode of the invention, i.e. the manufacture of a scaffolding consisting of alternating layers of nanostructured carbonaceous material in which the active material is embedded, can be made with n layers wherein n is equal to or greater than 1 supported on a collector/support.

The process for manufacturing this layered scaffolding includes the following steps:

• Preparing the active material in the form of disaggregated particles, by means of liquid phase techniques, electrochemical techniques or CVD processes;

• Applying alternating layers of active material and carbonaceous material regardless of which is the first layer deposited on the collector/support, taking care that in the case of applying only two layers, the second and last layer (that is, the one directly exposed to the electrolytic solution) is not the layer of active material, as in this case the active material would have no protection and would be subject to structural deformations related to volume changes during cycling which induce a high loss of capacitance with few cycles. The electrode would therefore have unacceptable application characteristics, especially in terms of the number of charges/discharges of the accumulator whereon it is installed.

More specifically, the method for obtaining the electrode comprises the following fundamental steps:

(i). Pretreatment of the active material in a non-aggregated form; so that the resulting structures have dimensions smaller than the distances (w) between the carbon nanostructures deposited with the CVD technique (w between 30 and 300 nm), preferred dimensions <200 nm, particularly preferred 20-100 nm; the structures of active material will be disaggregated, dried and suspended in a volatile solvent such as ethanol for subsequent application in the form of a suspension in said volatile solvent. This step is necessary when preparation methods such as, for instance, liquid phase techniques are used. Examples of pretreatments of the active material are also described in Electrochimica Acta 176, 1182-1190, 2015,

10.1016/j.electacta.2015.07.171 ; Part. Part. Syst. Charact. 34, 1700141 , 2017, 10.1002/ppsc.201700141 ;

(ii). Deposition of the disaggregated structures of active material obtained in step (i) on a substrate. The suspension of the active material structures prepared in step (i) is applied on the starting substrate according to traditional dip coating, spray, brush methods, even if more sophisticated techniques can also be used, such as that using supercritical fluids. In the event that different preparation techniques are used (such as, for instance, electrochemical techniques or CVD processes), the structures of active material can be obtained directly on the electrode. (iii). Deposition and growth of carbon nanostructures (0D or 1 D type, also in the form of arrays, such as for instance nanofibers, nano-rods, nanowalls, nanotubes) on the substrate by CVD or similar techniques (a technique that does not use hydrogen in the gaseous feed mixture is preferred). The treatments of deposition of active material [step (ii)] and growth of carbon nanostructures through CVD technique [step (iii)] can be carried out independently and alternately between them, i.e. it is possible to carry out step (ii) and then in succession step (iii) or vice versa, taking care however that the last layer is made of carbonaceous material. In this way, the active material will substantially not result in direct contact with the electrolytic solution used during the operation of a battery which comprises the negative electrode of the invention. The electrolytic solutions that can be used are all those that can be used for common lithium-ion devices. Typically, these solutions involve the use of organic solvents alone or in a mixture such as dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, tetrahydrofuran, diethyl ether, g-butyrolactone. The electrolyte, typically composed of lithium salts such as LiPF6, LiCIC , L1BF4, LiAsF6, is dissolved in these solvents.

The optimal number of layers depends on the Si load per surface unit that is desired, since this is directly proportional to the number of layers and for each step a silicon quantity between 0.05 and 0.3 mg/cm² is deposited.

In this way, the active material is embedded (trapped and confined) in a multilayer structure based on carbon nanostructures, that is, in a structure consisting of successive layers obtained from the repeated execution of steps (ii) and (iii). The active material, therefore, is not all on the surface, directly exposed to the electrolytic solution, and is not subject to the huge volumetric variations the metal undergoes during the lithiation/delithiation that occurs during the discharge and charge cycles of the battery, thus resolving the problem of loss of capacitance with cycles. Consequently, the alternation of deposited layers of carbon nanostructures and active material prevents a layer of active material from structurally collapsing even when it shows a significant change in volume during charging and discharging.

Another object of the present invention relates to a negative electrode for a lithium battery, which includes at least one layer of active material and at least one layer of carbon nanostructures applied on the current collector that supports the electrode material, so that the layer of active material is not directly in contact with the electrolytic solution used. Furthermore, since the active material on the negative electrode is supported on the carbon nanostructures which have high electrical conductivity, the electrical conductivity between the active material of the negative electrode and the current collector through the carbon nanostructures will always be ensured. Therefore, the electrode will have a high conductivity, improving both the performance in terms of capacitance and cycling of the battery even at high currents. Furthermore, the morphology and dimensions of the active material and carbonaceous material, both nanostructured, result in excellent packing of the active material in the carbonaceous material layers, whereby the energy density of the negative electrode increases.

As regards the morphological and structural characteristics of the negative electrode described above and object of the present invention, the active material in the form of nanostructured nanoparticles can be silicon, tin, germanium, zinc, aluminum, magnesium, indium, cadmium, lead, bismuth, antimony or an alloy of these metals with lithium.

Another object of the present invention relates to a negative electrode for a lithium battery wherein the current collecting supports a plurality of layers of active material and carbon nanostructures.

Furthermore, the subject of the present invention is the use of the electrode already described in energy storage devices such as batteries and supercapacitors, as well as the devices made with said electrode. This energy storage device therefore comprises a negative electrode as described herein and a positive electrode, separated from the electrolytic solution. This device has at least one of the following advantages: the volume retention ratio after the charge and discharge cycles is high; the internal resistance of the battery is low; battery life, in terms of number of cycles, is good.

An object of the invention is therefore to improve the performance of lithium batteries, which are used as a source of energy to power any type of electronic device; in general, from electric mobility to avionics devices, UAVs (Unmanned Aerial Vehicles), remotely driven aircraft (i.e. drones) or unmanned aerial vehicles and electric vehicles, but also in storage systems coupled to renewable energy sources and in so-called consumable applications, such as personal computers, portable terminals, telephone devices or working tools, portable survival systems, biomedical devices, data recording and backup tools, security systems, sensor and environmental networks.

Further objects, scopes and advantages will become evident from the detailed description of the invention. Brief description of the Figures.

The features, advantages, and technical and industrial significance of the present invention will be illustrated in the detailed description of exemplary embodiments of the invention. These exemplary embodiments of the invention will be described with reference to the attached figures which are not to be considered limitative of the scope of the invention and wherein:

Figure 1 shows the flow diagram of the production process of the hybrid nano composite material;

Figure 2 shows the SEM image of a typical carbonaceous support used as a substrate for the manufacture of electrodes; Figure 3 shows the SEM image of graphitic carbon nanostructures (nanowalls in this micrograph) grown on the carbonaceous support of Figure 2 in an FIFPECVD reactor;

Figure 4 shows the SEM image of the carbon nanostructures (nanowalls in this micrograph) of Figure 3 coated with active material (silicon nanoparticles in this micrograph) following a liquid phase deposition process;

Figure 5 shows the SEM image of multilayer material, obtained according to the invention: (i) by growing the carbon nanostructures according to Figure 3 on the carbon paper according to Figure 2, (ii) then carrying out a liquid deposition of the active material according to Figure 4 and finally (iii) coating the active material with an additional layer of carbon nanostructures (nanowalls in this micrograph);

Figure 6 shows the schematic representation of the different materials both of the known art (6A) and those that can be progressively obtained through the methodology described in the present invention (6B-6E). In the representation, Fig. 6A (for comparison) represents an electrode composed of a support whereon first the carbonaceous material and then the active material are deposited; Fig. 6B represents an electrode composed of a support whereon first the active material is deposited and then the carbonaceous material; Fig. 6C represents the electrode of Figure 6B, on top of which additional active material is deposited; Fig. 6D represents the electrode of Figure 6C on the top of which additional carbonaceous material is deposited; Fig. 6E represents the electrode of Figure 6A on top of which additional carbonaceous material is deposited. As can be seen from the following detailed description of the invention, the number of depositions and the order of alternation of active material and carbonaceous material are not limited to what is exemplified in Figures BA BE, since the deposition process of the invention can be repeated an arbitrary number of times, starting from the deposition of the active material or alternatively from the deposition of the carbonaceous material.

It should be noted that the figure does not reflect the real proportions between the dimensions or the morphologies of the various elements, which are evidently represented schematically by way of example.

Figure 7 represents a graph correlating the performance in terms of absolute capacitance shown in mAh over the first 55 cycles of multiple electrodes. Figure 8 represents the performance in terms of absolute capacitance expressed in mAh of the electrode (c) already shown in Figure 7 but for 145 cycles and shows how the multilayer material maintains its operativity even for long periods. Detailed description

In the context of the present invention, the following definitions are given.

The term "carbonaceous material" means a material composed of carbwhereon is an electron conductor and which is capable of intercalating lithium ions in its structure. With the words “carbon nanostructure or nanostructures” or “graphitic nanostructure(s)” we mean particular carbonaceous materials composed of distinguishable elements having at least a dimension less than 300 nm. Said distinguishable elements consist of sp² hybridized carbon atoms organized in the form of a hexagonal mesh lattice. The lattice can be a single layer or a multilayer one. Carbon nanostructures or graphitic nanostructures can be oriented in any direction with respect to the support whereon they are deposited, and in any direction with respect to each other.

The term “0D nanostructure” refers to a nanostructure in which all three spatial dimensions are less than 300 nm and are comparable to each other. Typical examples of 0D nanostructures are spherical shaped nanoparticles, cubic shaped nanoparticles or quantum dots.

The term "1 D nanostructure" refers to a nanostructure in which one of the three spatial dimensions is much larger (at least one order of magnitude) than the other two and can largely exceed the nanometer scale. The other two dimensions are less than 300 nm. Examples of 1 D nanostructures are nanotubes, nano-rods or nanowires.

The term “2D nanostructure” refers to a nanostructure in which two of the three spatial dimensions are much larger (at least one order of magnitude) than the third and can largely exceed the nanoscale. The remaining dimension is less than 300 nm.

The term "nanostructures in an array form" means a set of 0D or 1 D nanostructures homogeneous in chemical structure, morphology and dimensions distributed in an orderly manner according to a 2D type arrangement defined as above or 3D (in case the three spatial dimensions are comparable to each other and greater than those of the 0D nanostructures as defined above). A typical example of nanostructures in an array form are nanostructures obtained on a substrate by electrochemistry. The term "nanowalls" or CNW (Carbon Nano Walls) refers to a particular type of carbon nanostructure, known to the skilled in the art, described above, which in this invention is used as an example of all graphitic nanostructures, both for its active role in the intercalation of lithium ions and as an element to embed, support, receive and confine the active material as defined below. Within the scope of the present invention, the CNWs are arranged in a generically, although not strictly, two-dimensional way and rise from the surface of a substrate (such as, for instance, the collector defined below) locally in a direction generally perpendicular to that of the surface of the substrate itself. In directions locally parallel to the substrate surface, the nanowalls take on random directions.

With the words "active material(s)" or "anodic active material(s)", we mean materials, typically in the form of nanometric particles, based on metals or semimetals capable of reversibly alloying or intercalating lithium ions, which in this invention are used for their role as electrochemically active components in lithium ion accumulators. In this invention, in addition to being capable of alloying with lithium, the active material is resistant to the operating conditions of CVD. Examples of suitable materials are those based on tin (Sn), silicon (Si), germanium (Ge), zinc (Zn), aluminum (Al), magnesium (Mg), indium (In), cadmium (Cd), lead (Pb), bismuth (Bi) and antimony (Sb), their compounds and their alloys (comprising lithium alloys with these metallic elements).

The term "lithium alloy" as used herein is meant to refer to a substance that is capable of forming an alloy with lithium and is capable of reversibly including and releasing lithium at room temperature. Said materials and alloys can be used alone or in mixture in the form of films, 0D or 1 D nanostructures (for instance nanoparticles, nano-rods, nanowires, nanotubes) with a size between 10 nm and 500 nm, preferably between 20 nm and 200 nm. Examples of preparation of said materials in the form of nanoparticles are described in "Synthesis and Optical Characterization of Silicon Nanoparticles" (Journal of Nanoscience and Nanotechnology, Volume 13, Number 1 , January 2013, pp. 384-387 (4)) or in "Silicon nanowire growth by electron beam evaporation: Kinetic and energetic contributions to the growth morphology" (Journal of Crystal Growth Volume 300, Issue 2, 15 March 2007, Pages 288-293, 10.1016/j.jcrysgro.2006.11.329). The term "substrate" or "support" or "collecting substrate" or more simply "collector" means a current conducting material used as a support for both positive and negative active electrode materials, known to the expert in the art. The current collecting substrate has the function of bringing the electrons from the electrochemically active material to the outside or vice versa. The set of substrate plus carbonaceous material plus active material constitutes the electrode (cathode or anode). As a collecting substrate for electrodes, for the uses according to the invention, metal nets or thin sheets of copper, nickel, tungsten, aluminum, conductive or semiconductive alloys, carbonaceous, graphitic materials, such as pyrolytic carbon, graphite, carbon foam, are used in the form of fiber, woven or non-woven or in other porous form with open porosity, such as, for instance, porous carbon, carbon cloth, hard carbon, carbon nanotubes; in the form of powders, sheets, puffed materials or foams, thin graphite sheets (carbon paper), or other materials of similar performance, and their combinations. The term "hybrid nano-composite material" or simply "hybrid material" means the set of the scaffoldings of the layers obtained with the process of the invention and applied to a substrate to make the negative electrode of the invention. The set of these layers consists of the alternation of layers of active material and layers of graphitic nanostructures 0D, 1 D or in the form of arrays as defined above. The alternation of layers composing the nano-composite hybrid material on the substrate is made so that the active material is not directly exposed to the solution. In the hybrid electrode (or negative electrode) as already described, the carbon nanostructures can be contained in the active substance layer of the negative electrode in an amount from 0.1% by volume to 70% by volume, in particular from 1 % by volume to 50% by volume, based on the full volume of the active substance layer of the negative electrode. In the negative electrode as described below for a lithium battery, the carbon nanostructures can have a wall thickness from 1 nm to 100 nm, in particular from 3 nm to 50 nm. In the negative electrode as described below, the distance between adjacent carbon nanostructures is such as to create porosities with an average diameter between 10 nm and 10,000 nm, in particular between 50 nm and 500 nm.

As is evident from Fig. 6A-6E, the word "layer" means an overlap of carbon nanostructures that define nanoreceptacles or nanorecesses and nanostructures of active material that at least partially fill said nanorecesses in such a way that the so-called "layer" of carbon nanostructures that defines nanorecesses is interpenetrated with said "layer" of active material. In fact, the "layer" of carbon nanostructures defines a sort of scaffolding within which recesses/receptacles are housed at least in part the nanostructures of active material and the nanostructured carbonaceous material supports, traps and confines said active material nanostructures which penetrate directly in the interstices formed by the carbonaceous material nanostructures so as to form interpenetrated layers; it is therefore the same alternation of full/empty that allows the anode to withstand the volumetric variations.

As will be better clarified below, the active material nanostructures and the carbonaceous ones are superimposed in succession with respect to each other.

The active material will therefore be of such dimensions and morphologies as to be able to be incorporated and received in whole or in part in the nanorecesses and on the surface of the carbonaceous nanostructures (such as nanowalls), the carbonaceous nanostructures forming a high porosity "layer", with a high vacuum content inside them and allowing to obtain as a result "layers" of active material interpenetrated in the "layers" of carbonaceous material.

With the words "electrode" or "hybrid electrode" or "negative electrode" we mean the set of the current connector whereon the hybrid nano-composite material, defined above, has been supported. The term electrode therefore means the set of all materials, comprising the active material, which has the task of intercalating and de-intercalating the lithium ions.

The terms "energy storage device" or "battery" or "electrochemical device" or "device" mean an electrochemical system consisting of a cell or several cells connected in series or in parallel (battery). The cell consists of two electrodes, respectively an anode and a cathode, interfaced and separated by means of a separator soaked in the electrolytic solution. The separator consists of a cellulosic membrane or a porous polymeric film that physically keeps the two electrodes separate but allows the passage of ions.

The present invention relates to the manufacture, by means of a combination of processes (for instance CVD and liquid phase deposition or several CVD processes in sequence), of hybrid electrodes in nanometric form consisting of a plurality of layers of carbon nanostructures and active material, where carbon nanostructures are of the type 0D, 1 D, or in the form of arrays, such as for instance nanowires, nanotubes, nanowalls or other particle forms of known nanometric dimensions. The plurality of said layers forms a hybrid material, applied on the substrate, manufactured in such a way that, at least for one layer, the active material is not directly exposed to the electrolytic solution. In other words, at least one layer of active material is embedded in carbon nanostructures or has a layer of carbon nanostructures on it.

The invention also relates to the use of hybrid materials in the field of lithium- ion energy storage devices, with the aim of overcoming the limitation of the loss of performance due to the excessive fragmentation of the active material during the operation cycles (Li alligation/de-alligation), thanks to the coating of the active material with a protective and electroconductive layer of highly graphitic carbon, such as nanowalls.

In the present invention, carbonaceous nanostructures have, in reality, multiple functions: they act as a mechanical support for the deposition of the electrochemically active material in the form of a nanostructure, as a covering and protection of the same active material, as an electronic conductor and as an ionic conductor of Li⁺ (diffusion of the lithium ion between the graphite layers).

In the exemplary embodiment of the invention shown in figures 3-5 the carbon nanostructures were deposited by conventional CVD instrumentation already described in "Plasma enhanced hot wire CVD growth of thick carbon nanowalls layers" (AIP Conference Proceedings 1873, 02006, 2017), preferably in a hot wire deposition system (hot wire CVD, hot wire chemical vapor deposition, HFCVD), or in a hot wire deposition system with plasma application (plasma activated hot wire CVD, hot wire plasma enhanced chemical vapor deposition, HFPECVD). The wire is kept at a temperature above 2000°C. A gaseous precursor flow based on chemical compounds containing carbon is inserted into the deposition apparatus, but in the absence of an external hydrogen flow, only the carbonaceous precursor and the carrier gas are fed. The deposition of the active material which, as mentioned, can be silicon but also other metals and semimetals capable of forming an alloy with lithium, can take place on the substrate before or after the deposition of the carbon nanostructures (for instance the nanowalls), at outside the CVD reactor through various methods such as liquid phase deposition methods. In this case the deposition of the electrochemically active material is carried out starting from a liquid suspension of the material itself. It is also possible to use other techniques such as electrochemical or CVD techniques: in this case the active material can be grown directly on the substrate. By having an adequate CVD reactor (for instance with two chambers), it will thus be possible to carry out controlled depositions and growths of carbon nanostructures and structures of active material without ever taking the sample outside, until the end of the process.

The treatments of "deposition of carbon nanostructures" and "deposition of active material" can be repeated in an arbitrary sequence and for an indefinite number of times in order to:

- increase the load of active material in the electrode;

- always maintain intimate contact between the active material and the carbon;

- encapsulate the nanoparticles of the active material in a continuous nanostructured carbon matrix with a highly graphitic structure such as, for instance, the nanowalls;

- allow the manufacture of electrodes containing electrochemically active materials different from each other.

In this way, at most a single layer of active material is directly exposed to the electrolytic solution. Consequently, during the charge and discharge cycles of the battery (lithiation/delithiation) the active material:

- is less prone to pulverization, - it is not dispersed and therefore does not lose electronic contact with the electrode,

- limits the production of SEI (Solid Electrolyte Interface) by reducing the consumption of electrolyte thus solving the problem of loss of capacitance with working cycles.

The process for making the electrodes according to the invention comprises the following basic steps:

(i) Pretreating the active material to obtain it in a non-aggregated form, necessary when using preparation methods such as, for instance, liquid phase techniques. Silicon is particularly preferred as the active material;

• Supplying the active material in the form of structures of suitable size. This size is not specifically limited, as long as it is less than the interspace between the elements of the carbonaceous nanostructures (such as, for instance, the walls of the nanowalls). In an implementation of the invention, 0D nanostructures were used as defined above with the particularly preferred morphology of particles with a diameter of around 100 nm, preferably between 20 and 200 nm. In the case of silicon particles they will have a weight percentage equal to 2% based on silica;

• Initially subjecting the structures of active material to a surface cleaning treatment. For instance, in the case of silicon, a solution of hydrofluoric acid in a hydroalcoholic mixture can be used. The ratio of the mass of treated structures (in grams) to the required volume of solution (in milliliters) can typically be 1 :20. The mixture is left under magnetic stirring, at atmospheric pressure and at room temperature generally for about 30 minutes; then, the solid is filtered and washed in ethanol; • Drying the powder obtained in the previous step, for instance at 70- 90°C under vacuum for 2-5 hours, and storing in an inert atmosphere;

• Dispersing the dried structures thus obtained in ethanol or another solvent capable of creating a stable suspension. The solvent chosen should have good volatility characteristics so that it is easy to remove it from the substrate during the deposition phase. The ratio of the mass of structures (in grams) to the volume of solvent (in milliliters) can be 0.1 :5

• Sonicating the suspension of active material to eliminate any aggregates and centrifuging it, recovering the sedimented structures; the sonication and centrifugation cycles are repeated up to ten times, recovering the accumulated supernatants and the dispersed precipitate. The suspension is finally centrifuged one last time, typically at 5300 rpm.

• Suspending the resulting solid deposit again. The suspension can have a concentration between 0.1 and 10 grams of active material per liter of solvent and should be stable, i.e. it should be a suspension in which the structures remain in disaggregated form for the time necessary for the preparation of the electrodes. In one of the exemplary embodiments of the invention, suspensions of silicon particles with a diameter of around 100 nm were prepared in ethanol, which were found to be stable for up to six months.

The pre-treatment is within the knowledge of the expert in the field and is functional to obtaining a stable suspension of the structures of active material, as indicated above. For this purpose, other conventional techniques can also be used to obtain active material in a non-aggregated form (Electrochimica Acta 176, 1182-1190, 2015, 10.1016/j.electacta.2015.07.171 ; Part. Part. Syst. Charact. 34, 1700141 , 2017, 10.1002/ppsc.201700141 ). (ii) Depositing or growing active material in the form of non-aggregated structures on a substrate by liquid phase techniques, electrochemical techniques or growth by CVD (Materials Letters 144, 50-53, 2015; ACS Nano 4, 2233-2241 , 2010). In the event that non-aggregated structures have been obtained as described in step (i), it will be necessary:

• Applying the suspension on the substrate according to conventional methods such as dip, spray, brush coating or the like, always starting from active material concentrations of 0.1-10 g/l of solvent in the suspension to obtain a surface density of the coating after drying of 0.05-3 mg/cm² of geometric surface. However, these values depend on the characteristics of the support and are not to be considered binding. Usable substrates are indicated above and are used in the form of flat articles of the desired dimensions;

A dip coating application involves immersing the substrate in the suspension, typically for times from 10 minutes to 2 hours at room temperature and atmospheric pressure, with a minimum suspension volume ratio used per unit of geometric surface area of the substrate greater than 0,5 ml/cm², then extracting the substrate and leaving it to dry in an inert atmosphere (N2 or Ar). The material thus produced can be stored in an inert atmosphere or directly subjected to the growth of carbonaceous nanostructures.

(iii) Depositing and growing the carbonaceous nanostructures (e.g. nanowalls) on a substrate with CVD techniques

• The substrate is inserted into a special housing or "sample holder" placed inside the deposition chamber with the surface whereon the growth will take place facing a heater or "wire" (in refractory conductive material) and one or more electrodes or "grids" (in refractory conductive material). Once all the elements are inside the reaction chamber, the reactor is closed and brought to working conditions. In the case of the HFPECVD device, the operating conditions may be: system is evacuated and then brought to an absolute pressure of 25 mbar under the flow of a carrier gas, such as helium, the substrate heated to a temperature between 500°C and 1000°C, and the wire heated up to a temperature between 2000 and 2500°C.

• Upon reaching the stability conditions, the mixture, preferably Fte-free, containing a carbonaceous precursor selected from: CPU, C2H2, C2H4, ethanol or fluorinated compounds such as CF4 or C2F6, and a carrier selected from Fie, Ar or N2 is introduced. For instance, the feed can be a mixture containing helium and methane (Fie and CPU) with a percentage of methane between 1% and 15%, under a flow of Fie (10- 10000 SCCM or Standard Cubic Centimeters per Minute). Finally, the deposition apparatus is activated by applying suitable electrical potentials, in the case of FIFPECVD, to the wires and other electrodes. The growth process can take place in traditional equipment already described in the scientific literature and in other patent documents (e.g. US2007/184190 A1 and US2015/0037515 A1 ). In an exemplary embodiment of the invention, the deposition technique presents a novelty with respect to the state of the art since in the feed mixture only a carbonaceous precursor and a carrier gas are used, in the absence of hydrogen (as described in "Plasma enhanced hot wire CVD growth of thick carbon nanowalls layers", AIP conference proceedings 1873, 02006, 2017). The duration of the process is between 10 min-100 h depending on the desired thickness of the carbonaceous nanostructure layer. At the end of the process, the plasma producing power supply is interrupted, the temperatures of the wire and substrate are brought back to values close to the ambient temperature under the flow of the inert carrier and the chamber is brought back to atmospheric pressure. The sample can be extracted from the system already as electrode material ready for electrochemical applications, or subjected to further deposition of active material and new growth of carbonaceous nanostructures.

• Preferably the distance between single nanostructures, for instance the distance between the surfaces of the opposite walls of two nanowalls, varies from about 10 nm to 10000 nm, generally preferably from about 50 nm to 1000 nm. Too small a gap below the above range can occasionally cause a reduction in energy density because the volume proportion of the carbonaceous nanostructures in the electrode is large.

The deposition treatments of active material [step (ii)] and growth of carbonaceous nanostructures (such as nanowalls) using the CVD technique [step (iii)] can be carried out independently and alternately one another, i.e. first step (ii) and then in succession step (iii) or vice versa, in any case an alternation of steps (ii) and (iii) is preferred, repeated at least once so that, in operation (during the operation of the negative electrode), at least one of the layers of active material deposited is not exposed to the electrolytic solution. The number n of deposition cycles (where each cycle is composed of a deposition step of structures of active material, step (ii), and a growth process of carbon nanostructures, step (iii), carried out independently and alternately one another) on the substrate can be increased thus obtaining a multilayer electrode material (preferably but not limitedly n > 6, preferably in applications for fast charging devices n < 12 and in applications for power devices n > 20). The surface density of active material obtained for a single deposition is 0.05- 3 mg/cm² of geometric surface.

In this way the active material (for instance silicon) is trapped and confined in a multilayer material based on carbonaceous nanostructures, furthermore the active material is not all on the surface, directly exposed to the electrolytic solution. Consequently, the electrode as a whole is not damaged by the huge volumetric variations of the active material during the lithiation/delithiation processes that occur during the discharge and charge cycles, thus solving the problem of loss of capacitance with the cycles.

Characteristics and advantages of the electrodes object of the invention With the described procedure, the hybrid material produced is composed of active material of such dimensions and morphologies as to be able to be incorporated and received in the receptacles and on the surface of the carbonaceous nanostructures (such as for instance nanowalls). The carbonaceous nanostructures form a high porosity layer, with a high vacuum content inside them. Under the conditions used in one of the implementations of this invention, a growth of 1 hour of carbonaceous nanostructures in the form of nanowalls produces a portion of void equal to 10-120 times the average volume of the nanostructures of active material in the form of silicon nanoparticles. In the implementation of the invention wherein carbonaceous nanostructures in the form of nanowalls are used, these have morphological and dimensional characteristics comparable, for the single layer, to the characteristics described in US 8,551 ,657 B2. In fact, as clearly evident from figures 3-5, there is a monolayer of nanowalls within which the nanoparticles of active material exposed to the electrolytic solution are inserted.

In the negative electrode of the invention these morphological and dimensional characteristics are applicable to the different layers of carbon nanostructures deposited, and overall provide unexpectedly improved performances, especially in terms of capacitance and number of life cycles, as indicated below.

The electrode thus obtained can operate for a high number of cycles while maintaining excellent retention capacitance, without undergoing breakages that affect its operation. The proposed electrode quickly reaches a coulombic efficiency (ratio between the capacitance recorded in charge and the capacitance recorded in discharge in the same cycle) of 99% after the tenth cycle, starting from an initial efficiency of 85%.

The electrochemical performances obtained by the electrodes of the invention were compared with those of a conventional electrode of the type shown in figure 6A. The electrodes used in one of the implementations of the invention consist of a substrate of carbon paper whereon layers of silicon nanoparticles and CNW have been deposited in succession according to the steps (ii) and (iii) illustrated above in the order and in the number described below. The performances of these electrodes have been compared in the long term and are shown in figure 7. In particular, the electrode of the known art (of the type illustrated in Fig. 6A and indicated with (a) in figure 7) is compared with the electrodes (b), (c) and (d) prepared as follows:

- electrode (a) (circles in Fig. 7), electrode of the known art morphologically similar to the electrode proposed in US patent 8,551 ,657 B2 (distribution of the layers illustrated in Fig. 6A).

- electrode (b) (losenges in Fig. 7 and distribution of the layers illustrated in Fig. 6E) obtained by deposition in succession of: nanostructured carbonaceous material + active material + nanostructured carbonaceous material;

- electrode (c) (triangles in Fig. 7) obtained by deposition in succession of: nanostructured carbonaceous material + 4 times (active material + nanostructured carbonaceous material)

- electrode (d) (squares in Fig. 7) obtained by deposition in succession of: nanostructured carbonaceous material + 6 times (active material + nanostructured carbonaceous material)

Practically, in the electrodes (b), (c) and (d) the external layer is composed of carbonaceous nanostructures (in the present case CNW) and the active material (in the present case the silicon particles) is completely encapsulated, as experimentally demonstrated through SEM analysis, a typical example of which is shown in figure 5.

The graph in figure 7 shows the absolute capacitance values (mAh), detected by the multichannel potentiostat/galvanostat (model VMP, serial n° 0051 , Perkin Elmer Instruments), which records the measurement for each cycle, for the four electrodes. These values were obtained by imposing a discharge current of 0.1 mA and a charge current equal to 1/10 of the discharge current in the potential range 0.04-1 2V (vs Li⁺/Li, whose standard potential reference E° is E° Li⁺/Li = -3.040 V) at room temperature. From the comparison of the results obtained, it is evident that the electrode (a) obtained as described in US patent 8,551 ,657 B2 has a limited stability, as its capacitance quickly reaches 0 after just 20 cycles. The electrodes (b), (c) and (d), on the contrary, remain active for at least 45 cycles, clearly showing the improvement given to the system by the presence of at least one layer of carbonaceous nanostructures that covers and protects at least one layer of active material. The experimentally obtained results show that already from the 20^th cycle the electrodes of the invention exhibit capacitance values unexpectedly higher than those of the known art. For instance, the electrode (a) shows a value above 0.2 mAh compared to a value less than 0.1 mAh of the electrode according to the known art. It is also evident that at the 25^th cycle, where the capacitance of the electrode of the known art is practically zero, the values remain well above 0.1 mAh for the electrodes of the invention. Specifically, for 1 layer [electrode (b)] and for 4 layers [electrode (c)] the capacitance values are respectively 0.15 and 0.34 mAh; always at the 25^th cycle, also the electrodes with 6 [electrode (d)] 8 and 12 layers have very high capacitance values, respectively equal to 0.82, 1.30 and 2.27 mAh.

Figure 8 shows the trend of the electrode capacitance (c) up to 145 cycles. As evident from the graph, after an initial loss of capacitance, the electrode is around 0.18 mAh. Since the number of charge and discharge cycles of the electrode is closely related to the overall life of the device, it is evident that the methodology proposed in the present invention allows greater stability of the electrodes obtained and therefore a significant improvement in their application.

From the point of view of the absolute capacitance, for the first 15 cycles the electrode (b) shows values comparable with those of the electrode (a) prepared according to the methodology described in US patent 8,551 ,657 B2, and then maintains a significantly higher capacitance in the subsequent cycles Therefore, already adding a single outer layer of carbonaceous material causes an unexpected improvement in the life of the electrode prepared according to the invention compared to the electrode of the known art.

By increasing the number of deposition steps and, consequently, the quantity of active material, it is natural to expect an increase in the absolute capacitance values. In fact, as evident from the values shown in figure 8, already with the electrode (c) there is a certain increase, but the electrode (d), obtained by carrying out in addition 6 deposition cycles (active material + nanostructured carbonaceous material), to maintain its operation up to over 145 electrode charge/discharge cycles, looks much better. The obtained capacitance values can be further increased with subsequent depositions. Electrode (d) has been selected since it allows to demonstrate that, in steady state, a capacitance of around 0.6 mAh is obtained, a value that the comparison electrode (a) is able to maintain only in the very first cycles, before encountering a decline in performance. Other electrodes object of the invention offer results comparable to those observed in figures 7 and 8 although not explicitly shown. For instance, an electrode like the one shown in figure 6B, characterized by a support whereon the active material is deposited first and then the carbonaceous material, shows similar results in terms of capacitance and average life to the electrode of figure 6E (electrode (b) of figure 7).

In conclusion, from the comparison of the values shown in figure 7 for the proposed electrodes, the applicative advantages in terms of performance due to the presence of several layers of active material, of which at least one covered by a layer of carbonaceous nanostructures, are clear, which constitutes a form of encapsulation and protection of the active material. The growth of the carbon nanostructures can take place as a first process directly on the starting support as it is, or on substrates whereon the electrochemically active material has already been deposited. The overall process can comprise an arbitrary number of successive deposition steps of the active material (step ii) and growth of carbonaceous nanostructures (step iii). The alternating repetition of these two steps allows to obtain a multilayer carbonaceous matrix material with an interconnected graphitic structure in which structures of active material are soaked. Figures 6 illustrate some schematic examples of the different multi-layer structures that can be generated during the subsequent process steps, both starting from a deposition of active material carried out according to step (ii) (Figures 6B, 6C and 6D) and starting from the deposition of carbonaceous nanostructures carried out according to step (iii) (Figures 6A and 6E). As previously indicated in the description of the invention, the process can be repeated an arbitrary number of times until the desired number of layers is obtained. The deposition of the active material can take place through liquid phase techniques starting from the suspension of particles by soaking the substrate inside the suspension itself, by spraying the suspension itself on the substrate, by "drop casting" of the suspension on the substrate or by "spin coating" of the suspension on the substrate. Furthermore, the deposition of the active material can also take place through other techniques, such as electrochemical deposition techniques from electrolytic solutions or chemical vapor deposition (CVD). In an implementation of this invention, illustrated below, the deposition took place from a liquid phase by soaking the support in a suspension of silicon in ethanol obtained by modifying the methodology described by X. Zhou et al. (Zhou, X., Cao, AM., Wan, LJ. Et al. Nano Res. (2012) 5: 845. https://doi.org/10.1007/s12274-012-0268-4). In the same implementation, the growth of carbonaceous nanostructures (in this case CNW) takes place inside a HFPECVD reactor, already described and used in AIP Conference Proceedings 1873, 02006, 2017. The reactor consists of a sealed chamber, equipped with a suitable cooling system and evacuated by a special system of vacuum pumps. Inside the chamber there is a heating element, or "hot wire", maintained at a temperature above 2000°C and special systems for thermal control of the substrates in the deposition area. In the chamber there are also electrode systems to which a potential difference suitable for the formation of electric discharges on the wire itself is applied. The growth rate of carbonaceous nanostructures, intended as an increase in the vertical development or thickness of the deposit, is between 50 and 10000 nm/h.

Figure 4 shows a SEM image of a support coated with carbonaceous nanostructures (in this case nanowalls), whereon particles of active material (in this case silicon nanoparticles) have been deposited. Since the size of the deposited particles is smaller than that of the receptacles available between the carbonaceous nanostructures and on their surface, it is possible to generate an anodic material where the electrochemically active material is well dispersed and distributed inside the pores and on the surface of the carbonaceous matrix itself. It is known that for a material such as silicon, for instance, the change in the volume of the active material during the alligation and de-alligation cycles reaches up to 300% (McDowell M. T., Lee S. W., Nix W. D., Cui Y., Adv. Mater. 2013, 25, 4966-4985 DOI: 10.1002/adma.201301795). By varying the growth time of the carbonaceous nanostructures, it is possible to modulate the ratio between the volume occupied by the nanostructures and the empty spaces and that between silicon and carbon, in such a way as to obtain a layer capable of containing and withstanding the volumetric variation of silicon or other active material without crushing the electrode. In the event that the carbonaceous nanostructures are CNW, a growth of 1 h before and after the deposition step of the active phase produces a portion of vacuum equal to 10-120 times the average volume of the nanoparticles used in the implementation of this invention. This empty volume can completely accommodate the volumetric variation to which the active material is subjected. The possibility of constructing a hybrid material consisting of the overlapping of layers of active material and carbon nanostructures envisaged by the present invention is an undoubted advantage, because the active material (Figure 4) shows a high dispersion in the carbon matrix.

The encapsulation by carbonaceous nanostructures improves the containment of the active material and avoids crushing during volumetric expansions, whose effects would shorten the life of the electrode. For instance, the CNWs used in an exemplary embodiment of the invention have a full/empty ratio of the order of 50% or more, as defined in "Design-Considerations regarding Silicon/Graphite and Tin/Graphite Composite Electrodes for Lithium-Ion Batteries", (Otero, M., Fleim, C., Leiva E. P. M., Wagner, N., Friedrich A., Sci. Rep., 2018, 8, 15851 , DOI: 10.1038/s41598-018-33405-y) so they are capable to accommodate volumetric variations of 300% of the fraction represented by the active material during the charge and discharge cycles of the accumulator. The resulting electrode, therefore, has a very long operating life. The electrode of the invention, having at least one layer of active material not directly exposed to the electrolytic solution, is not in itself subject to fragmentation due to huge volumetric variations that the exposed material undergoes during the lithiation/delithiation that occurs during the discharge and charge cycles of the battery. In addition, the active material is homogeneously distributed along the surface of the carbonaceous nanostructures, ensuring a good compromise between the high mass ratio of the active material/carbon, the large surface area of the active material and the efficient displacement of the active material so that the carbonaceous matrix can contain its volumetric variations at best.

Finally, the high degree of graphitization of the carbonaceous nanostructures used ensures a synergistic contribution to the electrochemical performance of the material due to the intercalation processes of lithium ions along the hexagonal planes of the sp² carbon. All these factors make it possible to obtain an electrode capable of mechanically resisting many successive cycles, solving the problem of decreased performance and the reduced capacitance retention of these hybrid materials as shown in figure 8.

The present invention is also advantageous since it allows to obtain a high physical contact between the two components, which ensures a good electronic conductivity of the electrode, thus freeing from the need for an additional conductor which acts as a support for the electrode material and as a current collector. The invention, in fact, allows to directly produce a monolithic nanostructured electrode composed of active material dispersed within a self-supporting electro-conductive structure ("free-standing"). The material produced can be used directly inside a device and does not require further chemical or mechanical treatments. Current conduction is guaranteed by the high graphitization of the carbon nanostructures and by the conductive nature of the starting supports used which allows the passage of electrons and lithium ions. A further advantage of the invention is the simpleness with which the deposition of the active material can be carried out. In the event that a liquid phase deposition is chosen, in fact, the suspension of the structures of active material can be easily produced, manipulated and stored. The preparation of the suspension requires a few simple cyclical steps and the use of cheap solvents, which are not harmful to the operator and which can be used in conditions of ambient temperature and/or atmospheric pressure without particular safety precautions (for instance ethanol). The stable suspension thus described can be stored for a long time, thus avoiding having to produce it at each deposition, it allows to easily soak the substrate whereon to deposit the material, maintaining high dispersion and homogeneity during the process, and the spraying of the solution itself on the substrate. Liquid phase deposition methods based on supercritical CO2, on the other hand, require particular instrumentation in order to be produced and used. In the event of choosing the growth of active material through CVD, by having adequate equipment it is possible to carry out the entire electrode preparation process directly inside the chamber.

The possibility of carrying out successive depositions, i.e. alternating the deposition of the active material with the growth in CVD of the nanostructured carbon, allows to control the quantity of active material that is added cumulatively in the different layers of the composite material. This factor allows to easily control the desired load, which is well dispersed and encapsulated.

The devices of the invention minimize the formation of SEI (Solid Electrolyte Interface) since, by limiting the fragmentation of the material, the continuous exposure of the new surface to the action of the electrolytic solution, leading to the formation of new layers of SEI, is limited.

All the advantages described so far involve, as shown in Figures 7 and 8, the achievement of higher and more stable electrochemical performance in terms of absolute capacitance. The electrodes of the invention solve the problem of loss of capacitance as the cycles increase, thus allowing the construction of devices that last longer than the single charge and discharge cycle and can have a longer overall life in terms of number of operating cycles.

The ability to build devices with a longer overall life decreases the need for new electrodes and electronic components for the devices. This results in fewer devices in circulation and a lower impact of the processes associated with the disposal of exhausted devices. In a world where global challenges require greater environmental awareness, and the possibility of following a green approach acquires more and more added value, this invention brings the advantage of reducing waste materials and emissions.

The lithium batteries and the electrochemical devices that can be produced with the electrodes object of the present invention, can be used in electric mobility, both in avionics devices, UAVs, unmanned aerial vehicles and electric vehicles, but also in storage systems coupled to renewable energy sources and in the so-called consumable applications (telephone devices, portable terminals, computers, working tools), in portable survival systems, in biomedical devices, in data recording and backup tools, in security systems, sensor and environmental networks. The following examples are provided to illustrate the invention and are not to be considered limitative of its scope.

EXAMPLE 1

The active material used in this implementation consists of commercial silicon nanoparticles (Sigma Aldrich Silicon nanopowder, < 100 nm particle size (TEM), > 98% trace metals basis, CAS Number 7440-21-3), with a diameter between 20 and 200 nm and a weight percentage equal to 2% in silica. The nanoparticles are initially subjected to a surface cleaning treatment in a 20% by weight solution of hydrofluoric acid in a 1 :1 v/v ethanohwater mixture. The ratio of the mass of treated nanoparticles (in grams) to the required volume of solution (in milliliters) is 1 :20. The mixture is left under magnetic stirring, at atmospheric pressure and at room temperature for 30 minutes. The solid is filtered and washed with ethanol. The powder obtained is dried at 70-90°C under vacuum for 2-5 hours and finally stored in an inert atmosphere inside a glove box in an Ar atmosphere.

The dried nanoparticles are dispersed in ethanol; the ratio between the mass of nanoparticles (in grams) and the volume of ethanol (in milliliters) is 1 :40. The suspension is sonicated and centrifuged at a speed of 4200 rpm to eliminate any aggregates. The residual sedimented nanoparticles are resuspended in ethanol, sonicated and centrifuged. The sonication and centrifugation cycles are repeated 5-9 times. The supernatants are accumulated and the precipitate dispersed. The solution is finally centrifuged one last time at 5300 rpm. The nanoparticles are dried by keeping them at 70- 90°C under vacuum for 2-5 hours.

To prepare the suspension to be used to deposit the active material, the dried nanoparticles are redispersed in ethanol and sonicated. The suspension thus obtained and used for the deposition has a concentration between 0.1 and 4 grams of silicon nanoparticles per liter of solvent. The substrate used in our implementation is a carbon paper sheet cut to the desired size and 0.28 mm thick. The substrate is soaked in the suspension for 10 minutes - 2 hours at room temperature and atmospheric pressure, with a minimum ratio of suspension volume used per geometric surface unit of the substrate greater than 0.5 ml/cm². At the end of the process the substrate is removed from the suspension and left to dry in an inert atmosphere (N2 or Ar). The material thus produced can be stored in an inert atmosphere or directly subjected to the growth of carbonaceous nanostructures. Through this deposition procedure it is possible to achieve a silicon load per unit of geometric surface between 0.05 and 0.3 mg/cm². In this implementation, the carbonaceous nanostructures are nanowalls obtained via HFPECVD. This treatment can be carried out both on substrates already subjected to the deposition of silicon and on virgin substrates. The substrate is inserted on the sample holder and introduced into the deposition chamber with the surface whereon the growth will take place facing the grid and the wire. Once all the elements are inside the reaction chamber, the HFPECVD reactor is hermetically closed. The system is evacuated and then brought to an absolute pressure of 25 mbar under carrier flow; in the present implementation the carrier gas is helium. Subsequently the substrate is heated up to a temperature of 650°C, while the wire is brought to 2200°C. When the temperatures are stable, the mixture containing the carbonaceous precursor is introduced. In the present implementation the mixture contains 3.0% ChU and He under a flow of 120 SCCM. Finally, a direct current electrical discharge involves the wire as a cathode (i.e. it plays the role of a negative electrode and emits electrons) and generates the plasma inside the chamber. The anode consists of the grid and/or the substrate itself. In the present example the applied voltage (to the grid and/or to the substrate) is 30V for a plasma current of 2A. From this moment on the growth of carbon nanostructures on the substrate surface begins. Growth occurs both when the plasma develops between the grid and the wire, and when it develops between the substrate and the wire, and when it develops on both. The duration of the process is between 10 min-100 h depending on the desired thickness of the nanowalls layer. At the end of the process, the power supply to the plasma source is interrupted, and the temperature of the wire and substrate is brought back to room temperature and atmospheric pressure under the flow of the inert carrier. In case the carbon nanostructures have been obtained on virgin substrates, the sample must undergo at least one liquid phase deposition and at least one new growth of alternating carbonaceous nanostructures before it can be used. In the event that the carbon nanostructures have been obtained on substrates already subjected to the deposition of silicon, the sample can already be used as an electrode material for electrochemical applications, but it can also be subjected to at least one further deposition from liquid phase and possibly also at least one new growth of alternating carbonaceous nanostructures.

Claims

1. A process for producing a negative electrode which is capable of releasing lithium ions during the discharge of a battery, said electrode comprising:

- one or more layers of nanostructured carbonaceous material, said one or more layers of nanostructured carbonaceous material defining nanorecesses and said carbonaceous material being selected from 1 D type and 0D type nanostructures, and combinations thereof; and

- one or more layers of a nanostructured active material capable of forming an alloy with lithium, the nanostructures of said one or more layers of active material filling the nanorecesses of the nanostructured carbonaceous material, so that said layer of nanostructured carbonaceous material supports and traps said active material, said layers being supported on a collecting support, said process comprising the basic steps of:

• Preparing the active material in a non-aggregated form; so that the resulting nanostructured active material has dimensions smaller than the distances (w) between the nanostructures of carbonaceous material obtained with the CVD technique according to the next step;

• Depositing the carbonaceous material through vapor deposition (CVD) processes

• Applying alternating layers of the active material and the carbonaceous material regardless of which is the first layer deposited on the collecting support, taking care that in the case of applying only two layers, the one made of active material is in contact with the support and the one made of carbonaceous material is placed on top of the layer of active material.

2. The process according to claim 1 wherein the number of deposited layers of active material and the number of layers of carbonaceous material are each greater than 1 , preferably greater than 4, more preferably between 6 and 12, more preferably greater than 20.

3. The process according to any one of claims 1-2 wherein the carbonaceous material is constituted or comprises carbon nanostructures selected from: 1 D type nanostructures, also in the form of arrays, such as nanofibers, nano-rods, nanowalls, nanowires, nanotubes; preferably nanowalls; 0D nanostructures, such as spherical-shaped or cubic-shaped nanoparticles and combinations thereof; preferably said nanostructures having a minimum size between 0.3 nm and 1 pm, more preferably between 10 nm and 200 nm, the distance between adjacent nanostructures being between 30 nm and 30 pm.

4. The process according to any one of claims 1-3 wherein the active material consists of or comprises: silicon, tin, germanium, zinc, aluminum, magnesium, indium, cadmium, lead, bismuth, antimony or an alloy of these metals with lithium.

5. The process according to any one of claims 1 -4 wherein the active material is prepared in the form of a liquid suspension.

6. The process according to any one of claims 1-5 wherein the one or more layers of nanostructured carbonaceous material supports and traps all or part of said active material.

7. A negative electrode capable of releasing lithium ions during the discharge of a battery, said electrode comprising one or more layers of nanostructured carbonaceous material, said one or more layers of nanostructured carbonaceous material defining nano-recesses, and one or more layers of a nano-structured active material whose nanostructures fill the nano-recesses and wherein the one or more layers of nano-structured carbonaceous material that define the nano-recesses supports and traps said active material, said layers being deposited on a collecting support and being alternated with each other, taking care that in the presence of only two layers, the one made of active material is in contact with the support and the one made of carbonaceous material is placed above the layer of active material.

8. The negative electrode according to the preceding claim wherein both the number of deposited layers of active material and the number of layers of carbonaceous material are greater than 1 , preferably greater than 4, more preferably between 6 and 12, more preferably greater than 20.

9. The negative electrode according to the previous claim which maintains a capacitance greater than 0.1 mAh at the 25^th charge and discharge cycle, by imposing a discharge current of 0.1 mA and a charging current equal to 1/10 of the discharge current in the potential range 0.04-1.2V vs Li⁺/Li at room temperature.

10. The negative electrode according to any one of claims 7-9 wherein nanowalls are used as carbonaceous material and as active material a chemical species capable of forming an alloy with lithium is used.

11. The negative electrode according to any one of claims 7-10 wherein the support is selected from: metal nets or thin sheets of a metal chosen from: copper, nickel, tungsten, aluminum, conductive or semiconductor alloys; carbonaceous materials, such as graphite materials, pyrolytic carbon, graphite, carbon foam, in the form of fiber, woven or non-woven or in other porous form with open porosity, such as, for instance, porous carbon, carbon cloth, hard carbon, carbon nanotubes; in the form of powders, sheets, puffed materials or foams or thin sheets of graphite, carbon paper, and combinations thereof.

12. The negative electrode according to any one of claims 7-11 wherein the one or more layers of nanostructured carbonaceous material supports and traps all or part of said active material.

13. An energy storage device comprising the negative electrode according to any one of claims 7-12, said device preferably being a battery or a supercapacitor.

14. An article comprising the energy storage device or the battery or the supercapacitor according to the preceding claim.

15. The article according to the preceding claim which finds use and application in: electric mobility; avionics devices; UAV (Unmanned Aerial Vehicle), such as remotely driven aircrafts such as drones or unmanned aircraft; electric vehicles; storage systems coupled to renewable energy sources; personal computer; portable terminals; telephone devices; working tools; portable survival systems; biomedical devices; data recording and backup tools; security systems; sensor and environmental networks.