EP4059034A1

EP4059034A1 - Compositions for energy storage devices and methods of use

Info

Publication number: EP4059034A1
Application number: EP20804692.0A
Authority: EP
Inventors: Mohammad AKBARI GARAKANI; Sebastiano BELLANI; Leyla Najafi; Antonio Esau DEL RIO CASTILLO; Vittorio Pellegrini; Francesco Bonaccorso
Original assignee: Fondazione Istituto Italiano di Tecnologia
Current assignee: Fondazione Istituto Italiano di Tecnologia
Priority date: 2019-11-13
Filing date: 2020-11-09
Publication date: 2022-09-21
Also published as: IT201900021096A1; WO2021094897A1

Abstract

Composition for providing electrically conductive covering layers on a substrate of an energy storage device, the composition comprising: a) an electrically conductive material comprising at least one allotrope of carbon, wherein said at least one allotrope of carbon comprises pristine graphene flakes having an atomic content of oxygen equal to or lower than 5%, preferably equal to or lower than 2%, b) a binding agent comprising a polymeric material, c) an environmentally friendly solvent having a boiling point equal to or lower than 150°C.

Description

“Compositions for energy storage devices and methods of use”

Field of the invention

This disclosure relates to electrically conductive compositions. One or more embodiments may regard, for example, electrically conductive compositions suitable to be used in energy storage devices, preferably in electrochemical double layer capacitors.

Background of the invention

Electrochemical double layer capacitors are energy storage devices also known as ultracapacitors or supercapacitors (SCs).

They store energy electrostatically at the interface between an electrode and an electrolyte, exhibiting 10 to 100 times higher energy storage capacity (typically ranging between 1-10 Wh/kg) compared to those of electrolytic capacitors (typically ranging between 0.01-0.3 Wh/kg) thanks to the high-surface area (> 100 m²/g) of their electrodes. Since the energy storage mechanism is based on the intrinsic reversibility of ion adsorption processes, SCs store and release energy in second/sub- second timescales, which means they can complement or even replace batteries (typically operating over minute/hour timescales) for high-power (> 10000 W/kg) applications. These applications include regenerative breaking in electric and hybrid electric vehicles, short-term energy storage in consumer electronics and communication systems, or burst-mode power delivery.

At the same time, SCs can provide superior lifetime (-millions of cycles) compared to that of batteries (several hundred cycles). Indeed, the latter suffer intercalation-alloying induced stresses that can cause material swelling and subsequent electrode failure. In order to extend the application field of SCs, recent research aimed to increase the energy density of SCs (< 10 Wh/kg for commercial SCs), since the latter is still significantly inferior to that of batteries (30-70 Wh/kg for Ni-Cd, 50-120 Wh/kg for Ni-MH batteries, 150-270 Wh/kg for Li-ion batteries).

Generally, electrodes for SCs are made of commercially available nanoporous carbon-structures displaying a balanced micro (pore size between 0.2-2 nm)/mesoporosity (pore size 2-10 nm). In order to increase the energy density of SC electrodes, major strategies handled with tailoring the nanoporous structures of the carbon electrodes, controlling the interaction between carbon, solvent molecules and ions, as well as enlarging the operating voltage window of the electrolytes. Atomistic simulation of electrosorbed ions in nanoconfined environments, and the finding of charge storage in pores smaller than the size of solvated electrolyte ions have paved the way towards the design of nanoporous carbon electrodes with an extraordinary energy density (> 20 Wh/kg for a “full” device, including active material, current collectors, electrolyte, separator, binder and packaging). Despite these progresses, pores with a size of the order of 1 nm still do not significantly contribute yet to the overall capacitance (especially at high-power and low- temperature operation) and both cost-effective formulation of active materials and method of electrode manufacturing are still pursued to establish the above mentioned electrochemical performance in commercial SCs.

Document US 2018/0201740 Al, for example, discloses electrode slurries containing halogenated graphene nanoplatelets. Document US 2014/030590 Al discloses an electrode for an energy storage device comprising a self-supporting layer of a mixture of graphene sheets and spacer particles and/or binder particles. Document US 2017/0170466 Al discloses electrodes of power devices comprising a coating wherein the active material layer includes a composite in particulate form, graphene and a binder.

In this context, the formulation of scalable cost-effective highly performant active materials in form of inks/pastes based on low-boiling point and “green” solvents, the development of methods to process them into SC electrodes compatible with high-throughput roll-to-roll (R2R) coating technologies are pivotal for the realization of advanced SC technologies. In fact, on the one hand, the industrial implementation of “green” solvent-based processing of SC electrodes reduces the human health/environmental impact of the SC production, currently based on hard-to-dispose fluorine (F)-containing binders (e.g., poly(vinylidene difluoride) -PVDF- and polytetrafluoroethylene -PTFE-) and often using hazardous, teratogen, irritating and/or toxic solvents/dispersants (e.g., N-Methyl-2- pyrrolidone -NMP-). Notably, several countries, including USA and European Union (EU), have already limited the use of NMP to a minimum, as assessed by Annex XVII to Reach -Regulation. NMP shall not be used in mixtures at a concentration equal or greater than 0.3%, unless operational conditions ensure exposure of workers below the Derived No-Effect Levels -DNELs- of 4.8 mg/kg per day for dermal exposure and 14.4 mg/m³ for exposure by inhalation. On the other hand, “green” solvent-based SC manufacturing also eliminates the costs associated with the use and recycling of the organic solvents and related binders. For example, NMP is more expensive than water (> 1 US$/kg vs. <0.02 US$/L), the cost of PVDF (8-10 US$/kg) is superior to both alcohol and water/alcohol soluble binder like carboxymethyl cellulose (CMC) (< 5 US$/kg) natural cellulose -NC- (< 2 US$/kg), alginate (~8 US$/kg).

Lastly, the exploitation of advanced active materials beyond commercial activated carbon (ACs) as well as the establishment of scalable active material deposition methods alternative to industrially implemented casting deposition (e.g., doctor blading deposition) could better meet the mechanical/electrochemical stability of F-free and “green” solvent soluble binder-based electrodes without compromising their optimal porosity and wettability. Overall, new technical solutions are needed in order to provide improved ink or paste compositions (comprising active electrically conductive materials and binders/dispersing agents) capable of achieving substrate covering material layers compatible with cost- effective, sustainable, environmentally friendly, high-throughput production methods.

Summary of the invention

The object of this disclosure are compositions for providing electrically conductive covering layers on a substrate of an energy storage device, which are endowed with an optimal trade-off between mechanical stability and electrochemical performance, capable of efficiently operating in a large operating temperature window and attainable through cost-effective, sustainable and environmentally friendly methods.

The above object is achieved thanks to the subject matter recalled specifically in the ensuing claims, which are understood as forming an integral part of this disclosure.

The present disclosure provides a composition for providing electrically conductive covering layers on a substrate of an energy storage device, the composition comprising: a) an electrically conductive material comprising at least one allotrope of carbon, wherein said at least one allotrope of carbon comprises pristine graphene flakes, said pristine graphene flakes having an atomic content of oxygen equal to or lower than 5%, preferably equal to or lower than 2%, b) a binding agent comprising a polymeric material, c) an environmentally friendly solvent having a boiling point equal to or lower than 150°C. In one or more embodiments, the pristine graphene flakes may be present in an amount between 1% and 100% by weight (w/w) of to the electrically conductive material weight.

The pristine graphene flakes are non-oxidized graphene flakes displaying a defect-free flat morphology of their basal planes including structural defects of the sp² structure. Said flakes have an atomic content of oxygen, measured by X-ray photoelectron spectroscopy, equal to or lower than 5%, preferably equal to or lower than 2%. The defect-free flat morphology is assessed by the absence of a linear correlation (R² < 0.5) in the plot of the ratio between the intensity of the D and G peak of their Raman spectra versus the full width half maximum of the G peak (ID/I(G) vs. FWHM(G) plot). The ratio between the intensities of the D and G peaks (I(D)/I(G)) of the pristine flakes Raman spectra is comprised between 0.1 and 2.0, preferably between 0.1 and 1.2.

The environmentally friendly solvent may be selected in the group consisting of water, alcohols, mixtures thereof.

The alcohols may be selected in the group consisting of ethanol (ethyl alcohol), 1-propanol (propan- l-ol or 1-propyl alcohol or n-propanol), 2-propanol (propan-2-ol or isopropyl alcohol or isopropanol), 1 -butanol (n-butanol or n-butyl alcohol), 2-butanol (sec -butanol or 2-butyl alcohol), 2-methyl- 1 -propanol, 2- methyl-2-propanol (tert-butanol or 2-methylpropan-2-ol), 1-pentanol (pentan-l-ol), 2-pentanol (pentan-2-ol or sec-amyl alcohol), 3 -methyl- 1 -butanol (3-methylbutan- l-ol or isoamyl alcohol or isopentyl alcohol), 2,2-dimethyl- 1 -propanol (2,2- dimethylpropan-l-ol or neopentyl alcohol). Preferably, the 2,2-dimethyl- 1- propanol alcohol is used in a mixture with water.

In one or more embodiments, the polymeric material may comprise at least one fluorine-free polymer preferably selected in the group consisting of carboxymethyl cellulose (CMC) and derivatives, preferably sodium carboxymethyl cellulose (Na-CMC), natural cellulose (NC), alginate, polyvinylpyrrolidone (PVP), polyvinyl butyral (PVB).

The binding agent may be contained in the composition in an amount comprised between 1% by weight (w/w) and 30% by weight, preferably between 5% and 10% by weight compared to the total weight of the electrically conductive material plus the binding agent.

In one or more embodiments, the electrically conductive material may further comprise at least one additional allotrope of carbon selected in the group consisting of activated carbon, carbon nanotubes, carbide-derived carbons, carbon black, preferably activated carbon (AC).

The disclosure also provides a method for producing the composition. The method comprises the steps of:

- adding to an environmentally friendly solvent having a boiling point equal to or lower than 150°C, preferably selected in the group consisting of water, alcohols, mixtures thereof: i) an electrically conductive material comprising at least one allotrope of carbon, wherein said at least one allotrope of carbon comprises pristine graphene flakes having an atomic content of oxygen equal to or lower than 5%, preferably equal to or lower than 2%, ii) a binding agent comprising a polymeric material, to obtain a dispersion,

- mixing the dispersion to homogenize the distribution of the electrically conductive material and the binding agent dispersed therein.

In one or more embodiments, the disclosure further provides a method to produce electrically conductive covering layers on a substrate of an energy storage device, wherein the substrate is preferably the surface of a SC electrode.

Brief description of the figures

One or more embodiments will now be described, purely by way of non limiting example, with reference to the annexed figures, in which:

Figure 1 shows in a) the comparison between the Raman spectra of the graphite and the pristine graphene flakes produced by wet jet milling exfoliation, b) the plot of I(D)/I(G) vs. FWHM(G) and c) the statistical analysis of I(D)/I(G).

Figure 2 shows the Ragone plot of representative SCs based on electrodes spray-coated with the compositions according to the present disclosure at room temperature (25°C), in comparison to the Ragone plot of the SC based on spray- coated electrodes using sole ACs as electrically conductive active materials. The Ragone plot of SCs produced by conventional casting deposition (i.e., doctor blading deposition) of an electrode material slurry adopting conventional F- containing binders (i.e., PVDF) and expensive toxic organic solvent (i.e., NMP) are also shown, both for electrode covering layers with graphene flakes and without graphene flakes.

Figure 3 shows the Ragone plots of the SCs based on the spray-coated electrodes obtained according to the present disclosure at temperature of -40°C, 0°C, 50°C and 100°C in comparison to the Ragone plots of the SC based on spray- coated electrodes using sole ACs as electrically conductive active materials at the same operating temperature.

Figure 4 shows the distribution of the I(D)/I(G) ratios for pristine graphene flakes disclosed in the instant application and for halogenated pristine graphene flakes known in the art.

Figure 5 shows a TEM image of representative pristine flakes disclosed in the instant application (left panel) in comparison with an image of fluorinated graphene flakes known in the art (right panel).

Detailed description of the invention

In the following description, numerous specific details are given to provide a thorough understanding of embodiments. The embodiments can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the embodiments.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

The headings provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

The present disclosure relates to the field of compositions suitable to be used in energy storage devices, particularly to be applied on supercapacitor (SC) electrodes.

In one or more embodiments, the present disclosure provides a composition for providing electrically conductive covering layers on a substrate of an energy storage device, the composition comprising: a) an electrically conductive material comprising at least one allotrope of carbon, wherein said at least one allotrope of carbon comprises pristine graphene flakes, said pristine graphene flakes being graphene flakes having an atomic content of oxygen equal to or lower than 5%, preferably equal to or lower than 2%, b) a binding agent comprising a polymeric material, c) an environmentally friendly solvent having a boiling point equal to or lower than 150°C.

The pristine graphene flakes may be produced through graphite exfoliation without exploiting the chemical oxidation- aided graphite exfoliation (e.g., modified Hummer’s methods) followed by thermal reduction (e.g., material annealing at temperature > 700 °C) and/or activation processes (e.g., KOH activation).

Preferably, the pristine graphene flakes are two-dimensional flakes that preserve the crystallinity of the basal planes of the native graphite layers.

The pristine graphene flakes may comprise a single layer or a number of graphene layers of less than 150. More preferably, the two dimensional flakes are single layer graphene flakes or flakes with a number of graphene layer equal to or lower than 5. These flakes are also referred to “single-/few-layer graphene (SLG/FLG)”.

These flakes are non-oxidized flakes. They have an atomic content of oxygen equal to or lower than 5%, preferably lower than 2%, more preferably lower than 1%, as measured by X-ray photoelectron spectroscopy.

These pristine, non-oxidized graphene flakes retain the chemical purity of the pristine graphite, without exhibiting novel functional groups (e.g., oxygen groups).

The atomic content of oxygen of the pristine graphene flakes herein disclosed resembles the one of the starting graphite, commercially available with different chemical purity grades. To achieve flakes with a low oxygen atomic content (equal to or lower than 5%, preferably lower than 2%, more preferably lower than 1%), the starting graphite used to produce the flakes is preferably selected with an atomic content of oxygen measured by x-ray photoelectron spectroscopy lower than 5%, preferably lower than 2%, more preferably lower than 1%.

The pristine graphene flakes are free of defects on their basal plane. This property is assessed by the absence of a linear correlation (R² < 0.5) in the plot of the ratio between the intensity of the D and G Raman peaks against the full width half maximum of G Raman peak, which is extrapolated by the statistical analysis of Raman spectroscopy measurements of samples comprising different flakes. In such condition (i.e., absence of defective basal planes), I(D)/I(G) varies inversely with the crystal size.

In one or more embodiments, the ratio between the intensities of the D and G peaks (I(D)/I(G)) of the pristine flakes Raman spectra is comprised between 0.1 and 2.0, preferably between 0.1 and 1.2. The flakes exhibit superior electrical conductivity compared to graphite/graphene oxides and derivatives, as a result of a defect-free flat morphology of their basal planes. They show a graphite-like long range order of the sp² lattice (contrary to the highly wrinkled defective (reduced) graphene oxides), as well as high surface area (mandatory for the realization of high-capacitance SC electrode) due to their nanometric thickness.

The chemical, morphological and structural properties of the graphene flakes result in electrolyte tribology resembling those of the starting graphite.

Notably, the ultra-low friction/super-lubricating properties of graphene flakes are drastically deteriorated by both chemical functionalization groups (including oxygen functionalities in (reduced) graphene oxides) and defective sp³ domains of defective graphene flakes, in which polar and/or charged moieties adhere via friction-enhancing hydrogen bonds.

The pristine flakes having the features herein disclosed (two dimensional non-oxidised graphene flakes) may be obtained by evaporation and freeze-drying of a dispersion of flakes produced by wet-jet milling exfoliation of graphite, as disclosed in document WO2017/089987A1. The thickness of the flakes is comprised between about 0.3 nm and 40 nm (values measured by atomic force microscopy measurements). The majority of the flakes have thicknesses between 0.3 nm and 2 nm. The statistical distribution of the thickness of the flakes obeys a log-normal distribution with the maximum peaking at value inferior to 1.8 nm. This means that the two-dimensional flakes in the sample are mainly single layer graphene flakes or flakes with a number of graphene layer equal to or lower than 5 (i.e., SLG/FLG). The atomic content of oxygen of the pristine, non-oxidized graphene flakes is lower than 2%.

Figure 1 illustrates the Raman spectroscopy analysis for the pristine graphene flakes. The Raman spectrum of the flakes is compared to that of the starting graphite. The Raman spectrum of the flakes exhibits an increase of the D peak compared to those of native graphite, because of the decrease of the crystal size of the flakes compared to starting graphite. The plot of I(D)/I(G) vs. FWHM(G) does not show a linear correlation (R² < 0.3), which means that the exfoliation process did not induce in-plane defects into the flakes. I(D)/I(G) ranges between 0.1 and 1.2 for the flakes, indicating the presence of flakes with lateral dimension larger than those of the flakes produced by ultrasonication-assisted liquid phase exfoliation, whose I(D)/I(G) can be higher than 1.2. The statistical distribution of the lateral dimension of the flakes (measured by transmission electron microscopy) obeys a log-normal distribution with the maximum peaking at ca. 460 nm, which is higher than those shown by graphene flakes produced by ultrasonication-assisted liquid phase exfoliation (< 350 nm).

When used as active electrically conductive materials for SCs, the pristine graphene flakes boost the mechanical stability of the SC electrodes based on solely AC thanks to the high Young’s module of graphene flakes (~in the order of 1 TPa for single layer graphene flakes).

The graphene flakes enhance the electrochemical performance, in terms of rate capability (i.e., specific energy density at high specific power density), of the AC -based SC electrodes. More specifically, the electrolyte nanotribology on graphene flakes is effective to overcome electrolyte confinement issues in micropores (pore size < 2 nm). In fact, as the electrolyte confinement increases, the translational entropy available to the electrolyte molecules decreases until it is thermodynamically favorable for it to condense to an ordered phase, displaying a solid-like behavior even at temperature in which the bulk is a fluid. Therefore, the fluidity of the electrolyte in a porous film is supposed to be limited by the nanoconfinement effects provided by the irregular surface of the AC particles, while the flat surface of graphene flakes can effectively squeeze out the interfacial region providing fluid-like conditions similar to the solely solvent.

In addition, by exhibiting a long range 2D order of the sp² lattice, the graphene flakes intrinsically acts as “slide” for electrolyte ions, accelerating the electrolyte transport compared to covering layers comprising AC in form of films. The “slide like behavior” of the graphene flakes agrees with the ultra-low nanoscale frictional properties previously reported for FLG films, which behave similarly to super- lubricating highly oriented pyrolytic graphite (HOPG) films (friction coefficient < 0.01).

Noteworthy, previous theoretical and experimental studies demonstrated that super-lubricating properties of graphene-based materials are exhibited only in high- purity graphene, discarding similar behavior for defective graphene including sp³ domains, as well as for graphene oxide films. Concretely for graphene oxide, its surface enriched with oxygen functionalities forms hydrogen bonds enhancing the friction in presence of polar and/or charged moieties. Thereby, the use of pristine, non-oxidized graphene flakes in the composition herein disclosed simultaneously distinctively provides high electrochemically accessible specific surface area (SSA) of the SC electrodes, and ultrafast ions transport within the SC electrodes, boosting the specific energy density of the SCs at high specific power density (i.e., the SC rate capability).

The binding agent of the compositions comprises a polymeric material. The polymeric material may comprise at least one fluorine -free polymer. The at least one fluorine-free polymer has the advantage that it may be better processable in the environmentally friendly solvent.

The at least one fluorine-free polymer may be selected in the group consisting of carboxymethyl cellulose (CMC) and derivatives, preferably sodium carboxymethyl cellulose (Na-CMC), natural cellulose (NC), alginate, polyvinylpyrrolidone (PVP), polyvinyl butyral (PVB). In one or more embodiments, the at least one polymer is sodium carboxymethyl cellulose (Na- CMC).

The binding agent may be contained in the composition in an amount comprised between 1% by weight (w/w) and 30% by weight, preferably between 5% and 10% by weight with respect to the total weight of the electrically conductive material plus the binding agent.

More particularly, said binding agent is capable of inhibit the sedimentation/aggregation of graphitic flakes (especially non-polar graphene flakes) in the selected environmentally friendly solvent and provide physio/electro/chemical properties of the covering layers in form of films, comparable or superior to those obtained by conventional fluorine-containing polymers.

In fact, it has been found that both the physico-chemical stability of the composition and the mechanical stability under mechanical and electrochemical stresses improve with increasing the amount of the at least one polymer in the composition. However, the Inventors of the instant application have observed that an amount higher than 30% by weight with respect to the total amount of the electrically conductive material plus the binding agent results in low electrical conductivity and poor specific energy density (i.e., specific capacitance) once the composition is transformed in covering layers in form of film applied to the substrate.

The compositions herein disclosed allow achieving covering layers, preferably in form of film, endowed with mechanical stability without impeding wettability, ion mobility in the electrode pores and electron conduction. The binding agent comprising a polymeric material preferably comprising at least one fluorine-free polymer may be used for organic electrolytes or ionic liquids that are typically adopted in commercial SCs and high-voltage SCs, respectively, due to their low-cost (< 2 US$/kg) and their surfactant properties impeding active material sedimentation. When the at least one polymer, for example Na-CMC, is added to the aqueous dispersion, hydrophobic interactions between the graphitic nanomaterials and the CMC backbones are established. The carboxylic groups of Na-CMC adsorbed onto the graphite surface dissociate and stabilize the graphene- based inks.

The solvent contained in the composition herein disclosed is an environmentally friendly solvent having a boiling point equal to or lower than 150°C.

The expression “environmentally friendly solvent” refers to a solvent that do not exert adverse effects to the environment and to the health of living organisms.

The environmentally friendly solvent having a boiling point equal to or lower than 150°C may be selected in the group consisting of water, alcohols, mixtures thereof. The alcohols may be selected in the group consisting of ethanol (ethyl alcohol), 1-propanol (propan- l-ol or 1-propyl alcohol or n-propanol), 2- propanol (propan-2-ol or isopropyl alcohol or isopropanol), 1 -butanol (n-butanol or n-butyl alcohol), 2-butanol (sec -butanol or 2-butyl alcohol), 2-methyl- 1 -propanol, 2-methyl-2-propanol (tert-butanol or 2-methylpropan-2-ol), 1-pentanol (pentan-1- ol), 2-pentanol (pentan-2-ol or sec-amyl alcohol), 3 -methyl- 1 -butanol (3- methylbutan-l-ol or isoamyl alcohol or isopentyl alcohol), 2,2-dimethyl- 1- propanol (2,2-dimethylpropan-l-ol or neopentyl alcohol). Preferably, the 2,2- dimethyl- 1 -propanol is used in a mixture with water.

The environmentally friendly solvents may comprise water and ethanol (a biosolvent derived from the processing of agricultural crops).

In one or more embodiments, the solvent may be present in the composition herein disclosed in an amount comprised between 70% and 99.5% by weight (w/w) compared to the total weight of the composition.

For producing compositions that can be deposited by spray coating methods, the amount of solvent is preferably chosen dependently by the solvent composition to result in a composition viscosity <50 mPa s.

In one or more embodiments, the solvent may be present in the composition in an amount comprised between 10% and 70% by weight (w/w) compared to the total weight of the composition. Lower amount of solvent, for example comprised between 10% and 70% by weight (w/w) can be obtained by drying the dispersion at temperature inferior to the temperature degrading chemically the component of the compositions (for example, the binding agents). The products with less than 70% w/w of solid contents compared to the total weight of the composition could result in slurries, which can be deposited by casting method.

In one or more embodiments, the solvent may comprise water in an amount comprised between 10% and 90% by weight with respect to the total amount of the solvent.

In one or more embodiments, when the solvent is a mixture of different solvents, each of them may have a boiling point comprised between 50°C and 150°C.

The boiling point of the solvent used in the composition (equal or lower than 150°C), allow avoiding high-temperature processes (> 100°C) for its removal during and/or after the composition deposition.

Preferably, the solvent suitable to be used in the compositions herein disclosed comprises a mixture of water and ethanol, preferably in a 1: 1 volume ratio (vol/vol).

The use of the solvent herein disclosed, preferably comprising a mixture of water and ethanol, favours the dispersion and stabilization (i.e., impeding material sedimentation) of the electrically conductive material contained in the composition.

In addition, the use of an “environmentally friendly” solvent reduces the environmental and human health impacts of electrode film production.

In agreement with comparative life cycle assessments (LCAs), the transition for conventional organic solvent (e.g., NMP) used for SC fabrication to “environmentally friendly” solvents, preferably water-based solvents, substantially reduces the CO2 equivalent emissions. Furthermore, the use of “green” solvents intrinsically reduce the costs associated to the actions ensuring hazardous organic solvents exposure within the operational condition limits and costs associated to both drying and recovery of the hazardous organic solvents. For example, the equilibrium pressure of NMP is 35-times lower than that of water (in agreement with the lower boiling point of NMP (203°C) compared to that of water (100°C)), potentially requiring relevant drying costs impacting on the overall SC production costs. The recovery of NMP solvent via condensation and/or distillation is also necessary to prevent toxic/irritating solvent release into the environment, thus introducing additional cost compared to “environmentally friendly” solvent processing.

In one or more embodiments, the at least one allotrope of carbon consists of pristine graphene flakes having an atomic content of oxygen equal to or lower than 5%, preferably equal to or lower than 2%.

In one or more embodiments, the composition may comprise as active electrically conductive material at least one further allotrope of carbon selected in the group consisting of activated carbon (AC), carbon nanotubes, carbide-derived carbons, carbon black.

The weight percentage (wt%) of the nano structured allotropes of carbon with respect to the total weight of the electrically conductive active material contained in the compositions herein disclosed can vary, as shown in Table 1.

Table 1

As shown in Table 1, the composition may comprise as electrically conductive material: pristine graphene flakes in an amount comprised between 1% and 100%, activated carbon (AC) in an amount comprised between 0% and 99%, carbon nanotubes in an amount comprised between 0% and 99%, carbide-derived carbons in an amount comprised between 0% and 99%, acetylene black (carbon black) in an amount comprised between 0% and 99%.

For example, commercially available ACs may be used in combination with graphene flakes due to both their high SS A (> 1000 m²/g) and low-cost (~50 US$/kg for SC-grade AC).

In one or more embodiments, activated carbon may be present in the composition in an amount higher than 0% and lower than or equal to 99% by weight (w/w), preferably between 20% and 90% by weight (w/w) compared to the weight of the electrically conductive material.

In one or more embodiments, the electrically conductive material may consist of pristine graphene flakes having an atomic content of oxygen equal to or lower than 5%, preferably equal to or lower than 2%.

In one or more embodiments, the electrically conductive material may consist of pristine graphene flakes having an atomic content of oxygen equal to or lower than 5%, preferably equal to or lower than 2% and activated carbon (AC).

The compositions may be produced by:

The mixing step may be performed by sonicating the dispersion in a bath sonicator, preferably for a time period of at least 30 minutes.

In one or more embodiments, the compositions herein disclosed allows to produce electrically conductive covering layers on a substrate of an energy storage device in form of film.

A method for providing electrically conductive layers of covering material on a substrate of an energy storage device in form of film comprises at least one step of depositing the composition onto the substrate.

In one or more embodiments, the method for providing at least one electrically conductive covering layer on a substrate of an energy storage device comprises the steps of:

- depositing the composition herein disclosed onto said substrate to obtain the covering layer in form of film,

- optionally heating the covering layer in form of film, preferably at a temperature comprised between 40°C and 150°C.

In some embodiments, the method may consist of the step of depositing the composition herein disclosed onto the substrate to obtain the covering layer in form of film.

In one or more embodiments, the depositing step may be carried out by spray coating the composition.

The spray-coating may be carried out by using a mass loading comprised between 0.01 mg/cm² and 1000 mg/cm², preferably between 0.2 mg/cm² and 20 mg/cm², more preferably between 1 mg/cm² and 10 mg/cm². The gas carrier pressure may be comprised between 1 bar and 5 bar, preferably 1 bar. The deposition distance may be comprised between 0.2 cm and 100 cm, preferably between 1 and 10 cm. The aperture of nozzle may be comprised between 0.1 mm and 10 mm, preferably between 1 mm and 2 mm. The temperature of the substrate may be comprised between room temperature (e.g., 20°C or even lower) and 200°C, preferably between 40°C and 150°C, more preferably equal to or lower than 100°C. The line deposition velocity may be higher than 0.01 mm/s, preferably comprised between 0.5 mm/s and 500 mm/s, more preferably between 10 mm/s and 50 mm/s.

In one or more embodiments, the step of heating the covering layer in form of film favors the solvent evaporation. The obtained film is a homogeneous film of electrically conductive layers of covering material. The heating step may be carried out at a temperature comprised between 40°C and 100°C. A temperature of 50°C may be advantageous to obtain homogeneous films of electrically conductive covering material.

The substrate may be a surface of a supercapacitor electrode, preferably a surface of a current collector of a supercapacitor electrode.

The substrate may comprise a material selected in the group consisting of a metal, preferably aluminum (Al), gold (Au), nickel (Ni), stainless steel, a carbon- coated non-electric ally conductive material, preferably plastic, fabric, paper, a graphitic foil, preferably a graphite paper.

Preferably, said substrate may be a flexible substrate. The substrates can be coated by a thin layer (< 2 pm) of electrically conductive nanoparticles (e.g., acetylene black) to promote the adhesion and optimal electrical connection between the covering layer comprising the composition herein disclosed and the substrate.

The covering layer in form of film obtained according to the method disclosed above may have a thickness comprised between 0.5 pm and 500 pm. Notably, thickness higher than 10 mhi, corresponding to electrode material weight > 1 mg/cm², are preferred in commercial-like SCs aiming to large-scale applications (i.e., applications excluding nano-electronics in which low specific energy density is required), since active electrically conductive material weight has to account for about more than 30% of the total mass of the packaged device. Otherwise, the gravimetric electrochemical performance of the latter drastically decreases (by a factor > 10 for active material mass loading of 1 mg cm ²). Preferably, for the preparation of interdigitated electrode for planar SC configurations, a deposition mask is used during spray coating deposition method in order to deposit patterned electrode electrically conductive covering layer in form of film with spatial resolution superior to those achievable by simple spray coating deposition method.

In one or more embodiments, the disclosure provides a supercapacitor electrode comprising an electrically conductive covering layer in form of film obtained by the method above disclosed, comprising the step of depositing the composition object of the instant application.

The final supercapacitors may be obtained by assembling in vertical or planar configuration two identical, preferably spray-coated, electrodes above described. In vertical configuration, a commercial separator is needed to avoid the electrical contact between the electrodes. In planar configurations, the electrode are intrinsically electrically disconnected by the spatial gap between their fingers of the interdigitated electrodes.

In one or more embodiments, when the composition is deposited on a substrate of a supercapacitor configured in a vertical configuration the pristine graphene flakes may be preferably present in an amount comprised between 1% and 30% by weight based to total weight of the electrically conductive material weight. In one or more embodiments, the composition may comprise as electrically conductive material pristine graphene flakes in combination with at least one further allotrope of carbon selected in the group consisting of activated carbon, carbon nanotubes, carbide-derived carbons, carbon black, preferably activated carbon. The at least one further allotrope of carbon, preferably active carbon, may be present in the composition in an amount comprised between 70% and 99% by weight compared to total weight of the electrically conductive material weight. Thus, pristine graphene flakes and activated carbon may be present in the composition in an amount, with respect to the total amount of the electrically conductive material, comprised between 1% and 30% by weight (w/w) and between 70% and 99% by weight (w/w), respectively. The at least one further allotrope of carbon, preferably active carbon, acts as spacers for impeding graphene flakes restacking with a displacement parallel to the current electrode collectors. Therefore, said spacers facilitate the electrolyte access to the electrode surface, boosting the maximum power density of the SCs. Furthermore, the use of spacers favors the full exploitation of the surface area of the graphene flakes, reaching specific surface area of active materials comparable to those of commercially available high-SSA ACs (> 1000 m²/g). Moreover, said at least another allotrope of carbon acts as conductive bridges between graphene flakes, thus creating highly electrically conductive carbon-based nanoporous covering layer in form of films, as required for high power operating SC electrodes.

Consequently, an electrically conductive component, such as those given by acetylene black (carbon black) nanoparticles in commercial -like electrode material formulations, may not be strictly required in the graphene-based electrode material formulation to provide satisfactory low equivalent series resistance -ESR- of the SC electrodes (e.g., < 10 W per 1 cm² of device area).

This allows the SCs electrode covered with the composition herein disclosed to exploit the electrical conductivity of the pristine graphene flakes, improving the specific electrochemical performance of the SC electrodes.

In one or more embodiments, the supercapacitor may be configured in a planar configuration and the pristine graphene flakes may be preferably present in the composition in an amount comprised between 20% and 100% by weight compared to the total weight of the electrically conductive material weight. In one or more embodiments, in planar SC configurations, solely graphene flakes may be used as electrically conductive active material. The in-plane displacement of graphene flakes is parallel to the electrolyte ions movements during the device charging/discharging. Consequently, in absence of graphene flakes restacking, planar SCs based on solely graphene flakes may intrinsically facilitate the flow of the electrolyte ions between the graphene flakes in a short diffusion pathway, maximizing the ion accessibility to the SSA of the substrate covering layer in form of films, thus boosting the rate capability of the SCs (i.e., the maximum power density of the SCs).

The optimized films electrically conductive layers of covering material for both the vertical and planar SC configurations show an optimal balance between the resulting mechanical stability, porosity and wettability of the electrode, otherwise unreachable with commercially available SC active materials, i.e., ACs and typical electrode composition deposition methods (i.e., casting deposition of active material compositions in form of slurry).

As shown in the instant application, symmetric SCs produced by using electrically conductive covering layers comprising the composition herein disclosed display high specific or gravimetric energy density (> 10 Wh/kg) at high specific power density (> 30 kW/kg). For the sake of reference, the SCs composed by electrodes produced by conventional casting deposition, which typically have specific energy density < 10 Wh/kg at specific power density of 15 kW/kg).

In addition, the composition herein disclosed allows improving the rate capability of the SCs compared to commercially available electrode materials. Such rate capability improvement has here been validated in a large operating temperature window (-40/+100°C), as disclosed in the Examples section.

Examples

Example 1. Compositions for supercapacitors

Compositions were formulated by mixing the following components:

- activated carbon (AC) and pristine graphene flakes, alone or in combination, as active electrically conductive material;

- a mixture of water/ethanol (50:50 v/v) as “environmentally friendly” solvent with a boiling point of about 150°C;

- sodium carboxymethyl cellulose (NA-CMC) as polymeric binding agent (capable of mechanically reinforcing the active electrically conductive material, the inhibiting active electrically conductive material sedimentation/aggregation).

Pristine, non-oxidized graphene flakes having an atomic content of oxygen equal to or lower than 5% in form of powder were isolated by drying the single- /few-layer graphene (SLG/FLG) flake dispersion obtained by wet jet milling exfoliation as disclosed in document WO2017/089987A1 and summarized below. The wet jet milling exfoliation process exploits a high pressure (between 180 and 250 MPa) to force the passage of the solvent/graphite mixture through perforated disks, with adjustable hole diameters (0.3-0.1 mm, named nozzle), strongly enhancing the effectiveness of the generated shear forces originating the exfoliation of the graphite.

Experimentally, 200 g of graphite flakes (+100 mesh Sigma Aldrich) were added to 20 L of N-Methyl-2-pyrrolidone (NMP) (> 97%, Sigma Aldrich) and mixed in a container by a mechanical stirrer (Eurostar digital Ika-Werke). The WJM apparatus is equipped with a hydraulic piston supplying a pressure of 250 MPa to push the as-prepared mixture into five sets of different perforated and interconnected disks (i.e., processor). Two jet streams originated at the second disk consisting of two holes with a diameter of 1 mm. Then, the jet streams collided between the second and the third disks, given by a nozzle (i.e., a half-cylinder channel) of diameter 0.3 mm. The shear force generated by the solvent when the sample passed through such a nozzle promoted the graphite exfoliation. Immediately after the processor, the sample was cooled down using a chiller. The processed sample was then collected in another container. The WJM process was repeated three times adjusting the diameter of the nozzle to 0.2 0.15 and 0.1 mm, respectively, to obtain the final SLG/FLG dispersion. The as-obtained SLG/FLG dispersion was dried in a rotary evaporator (Heidolph HEIVAP INDUSTRIAF, FKV Sri, Italy) setting the bath temperature at 80°C and gradually lowering the pressure to 5 mbar, allowing the evaporation of NMP. Subsequently, 1.5 F of dimethyl sulfoxide (DMSO) (Merck KGaA, Germany) was added to the SFG/FFG flakes.

Activated carbon (AC) was purchased by AB-520, MTI Corp. and used as received.

Activated carbon and graphene flakes in different amounts (as shown in detail in Table 3) were mixed obtaining a hybrid composition of the electrically conductive active material.

The binding agent, sodium carboxymethyl cellulose (Na-CMC; 90000 molecular weight), was purchased by Sigma Aldrich and used as received.

The compositions were obtained by dispersing the electrically conductive material (activated carbon, graphene flakes or mixtures comprising activated carbon and graphene flakes) and the polymer sodium carboxymethyl cellulose (Na- CMC) in an aqueous solvent comprising water and ethanol in a volume ratio (v/v) of 1 : 1. The obtained dispersions were ultrasonicated in a bath sonicator (Branson® 5800 cleaner, Branson Ultrasonics) for 30 min before their use to homogenize the distribution of the electrically conductive material and the binding agent.

Table 2 reports the amounts of the composition components produced ad disclosed above. Table 2

* Comparative composition

The compositions “Graphene”, “AC:graphene (50:50)” and “AC:graphene (20:80)” have been preferably used for the realization of SCs in planar configuration.

The composition “AC:graphene (90:10)” has been validated in sandwich-like vertical SC configurations displaying rate capability superior to reference device based on commercially available ACs as active materials. In such vertical configurations, an amount of graphene flakes for example of about 10% by weight with respect to the total amount of the electrically conductive material (“AC:graphene (90:10)”) has been shown to be sufficient to achieve the desired results.

Example 2. Spray coated supercapacitors

The compositions “AC” (comparative composition) and “AC:graphene (90:10)” obtained as disclosed in Example 1 were deposited in form of films onto C-coated Aluminum current collector (En'Safe® 91, Armor) by a manual dual action gravity feed airbrush (FE-180, Fengda) or a custom-built automatized spray coater (Aurel XCEL, Aurel Automation S.p.A.) with: a computer-controlled spray head, movable in the XYZ directions on a heatable stage; a 2 mm orifice diameter nozzle (Nordson EFD) with fluid recirculation, supplied with compressed air. For the case of the manual air brushing deposition, the distance between the spray nozzle and the substrate was set to ~20 cm. For the automatized spray coating deposition, lines were sprayed along the Y-axis while maintaining a Step X of 2 mm and adopting the spray coating parameters shown in Table 3. Table 3

By weighting the current collector before and after the composition deposition, the amount of material comprising the composition deposited by each spray coating sequence is calibrated. The composition mass loadings between 1 and 20 mg cm² were deposited on current collectors by adjusting the amount of sprayed electrode composition. Said mass loadings are comparable to those adopted in commercial-like SCs for large-scale applications), in which active material weight has to account for about more than 30% of the total mass of the packaged device, otherwise the gravimetric performance (i.e., the specific energy density) of the SCs drastically decreases (e.g., by a factor > 10 for active material mass loading of 1 mg cm²).

3. Covering layer characterization

The spray-coated layers in form of films were characterized by Brunauer, Emmett and Teller (BET) surface area and sheet resistance (Rsheet) measurements. Specific surface area measurements of the spray-coated films were carried out in Autosorb-iQ (Quantachrome) by Kr physisorption at temperatures of 77 K. The SSAs were calculated using the multipoint Brunauer, Emmett and Teller (BET) model, (1) considering equally spaced points in the P/P0 range of 0.009 - 0.075. Po is the vapour pressure of Kr at 77 K, corresponding to 2.63 Torr. (2-5). The obtained BET SSAs of spray-coated films are between 40-200 m²/g. The measured BET SSAs for spray-coated “AC” and “AC:graphene (90:10)” films are between 2000-2700 m²/g, depending on the electrode material mass loadings. Such values are superior to those measured for electrode films deposited by conventional casting deposition (i.e., doctor blading deposition) of electrode material (ACs or AC- hybridized graphene flakes) slurries adopting conventional F-containing binders (i.e., PVDF) and expensive and toxic organic solvents (i.e., NMP) (between 1000- 1800 m²/g).

Actually, AC particles act as spacers to avoid restacking of graphene flakes enabling the exploitation of the SSA of graphene flakes beyond their mechanical, electrical and tribological properties.

Contrary, the lower SSA values of films obtained by composition containing graphene as sole active electrically conductive material compared to the films obtained by composition containing graphene and AC are ascribed to the in-plane displacement of the SLG/FLG flakes, which may impede an efficient gas (and ion) transport along the vertical direction of the film. Therefore, the laminar morphology of the films comprising the composition “Graphene” is preferred for the realization of interdigitated electrodes to be used in planar SC configurations. In fact, in such configuration, the in-plane displacement of graphene flakes is parallel to the electrolyte ions movements during the device charging/discharging. This intrinsically facilitates the flow of the electrolyte ions between the graphene flakes in a short diffusion pathway, maximizing the ion accessibility to the SSA of the electrode films.

These observation arose by preliminary experiments carried out by measuring the SSAs of films containing a graphene content higher than that of “AC:graphene (90:10)”, i.e., the “AC:graphene (80:20)” and “AC:graphene (50:50)” films. In particular, the average calculated SSAs of the “AC:graphene (80:20)” and “AC:graphene (50:50)” films are reduced by ~6% and -44% compared to the one of “AC:graphene (90:10)” films (-2600 m²/g) adopting a similar active material mass loading (-1 mg cm ²). Thus, these values confirm the preferred use of the “AC:graphene (90:10)” composition for the development of sandwich-like vertical SC configurations. It is worth to note that similar conclusions have also been obtained by analyzing traditional slurry casted SCs (i.e., SCs based on the electrodes produced through doctor blading deposition of the electrode material slurries adopting conventional F-containing binders, specifically PVDF, and, organic solvents, specifically NMP) adopting the same ratio of AC and graphene for the formulation of active electrically conductive materials. In particular, the calculated SSAs for representative slurry-casted films are -1430 m²/g for AC, -2120 m²/g for “AC:graphene (90:10)”, -1600 m²/g for “AC:graphene (80:20)”, -630 m²/g for “AC:graphene (50:50)” and -40 m²/g for “graphene”. The trend of these values observed by changing the graphene flakes content resemble those observed for spray-coated SCs.

Sheet resistances of the produced films, spray-coated onto glass substrates with a mass loading of -10 mg/cm², were measured by four-probe method (6) by using Jandel RM3000 Test Unit. The “AC:graphene (90: 10)” films exhibit a Rsheet (-40 W sq ¹) lower than that of “AC” films (-80 W sq-1 ). The Rsheet of “AC:graphene (90:10)” is also lower than that of “graphene” film (-70 W sq-1). The AC in the hybrid films act as conductive bridges between SLG/FLG flakes, thus creating highly conductive nanoporous films.

Example 4. Supercapacitor preparation

Supercapacitors were fabricated in a symmetric sandwich-like vertical configuration by stacking two identical electrodes. Films obtained by the deposition of the composition “AC:graphene (90:10)”, as well as the composition “AC” as comparative composition, spray-coated onto C-coated A1 current collector were used as electrodes. The SCs were assembled in both coin cell (CR2032 cases, springs and spacers purchased from MTI Corp.), by using a compact crimper (MSK-PN110-S, MTI Corp.), or pouch cell-configurations, by packaging electrodes in A1 laminated films (MTI Corp.), and contacting electrodes by A1 Tab (MTI Corp.). Conventional organic electrolyte (e.g., 1 M tetraethylammonium tetrafluoroborate -TEABF4- in acetonitrile (ACN) or propylene carbonate (PC), ionic liquid electrolyte (e.g., butyl-trimethylammonium bis(trifluoromethyl sulfonyl)imide -[N 1114] [TFSI]-, methyl-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide -[PYR13] [TFSI]- l-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide-[EMIM][TFSI]-, l-ethyl-3-methylimidazolium tetrafuoroborate -[EMIM][BF4])-, or mixture of organic (g-Butyrolactone, PC and ACN) and ionic liquid electrolytes were used as electrolyte.

Preferably, binary mixture of g-Butyrolactone (50 wt%) and [EMIM][TFSI] (50 wt%) (g- B uty o 1 ac tonc/[ E I M] [ TFS I J ) were used as electrolyte with high voltage window electrochemical stabilities over a wide temperature range (- 50/+100°C). The addition of g-Butyrolactone into [EMIM][TFSI] enhances the ionic conductivity and fluidity of the pure ionic liquid, especially at low temperature(7,8). For instance, at -50°C the ionic conductivity of g- Butyrolactone/[EMIM][TFSI] is still as high as 1.9 mS cm^-1 (~10⁶ mS cm ¹ for pure [EMIM][TFSI]) and its viscosity is 70 mPas. In fact, the g-Butyrolactone addition to the [EMIM][TFSI] suppresses the melting transition of the latter, allowing the SCs to operate at temperature as low as -50°C with voltage window stability resembling those of [EMIM][TFSI], thus larger than that of electrolytes used in commercial -like SCs (typically up to 2.7V at room temperature). A glass fiber membrane (Whatman) was used as electrode separator. The SCs were assembled into Ar-filled glovebox with concentrations of moisture and O2 below 0.1 ppm.

Example 5. Electrochemical characterization

The electrochemical characterization of SCs was carried out with potentiostat/galvanostat (VMP3, Biologic). More specifically, galvanostatic charge/discharge measurements were carried out at different current density, ranging from 0.5 to 50 A g ¹. The specific capacitance (C_g) of the SCs (comprising two electrodes) was determined by using the equation: Cg = (|i|td)/(m_totAV), where |i| is the absolute value of the applied current, td is the discharge time of the CD curve, AV is the voltage cell window and m_tot is the sum of the mass of the two electrodes (excluding the mass of the current collectors). Specific energy density of the SCs was calculated as E = 0.5xC_areaix(AV)². The specific power density of the SCs was calculated by P = E/td. The electrochemical measurements were performed at five different temperatures, i.e., -40°C, 0°C, room temperature (25°C), 50°C and 100°C by placing the SCs inside a temperature-controlled chamber.

Figure 2 shows the Ragone plots obtained at room temperature (25°C) for the SCs based on the electrodes produced according to the disclosed embodiments, adopting an electrode material mass loading of ~5 mg/cm² (excluding the mass of current collectors) and g- B uty ro 1 actonc/[ E I J [TFS I J as electrolyte. The Ragone plots obtained room temperature for the SCs based on the electrodes produced through conventional casting deposition (i.e., doctor blading deposition) of electrode material slurries adopting conventional F-containing binders (i.e., PVDF) and, organic solvents (i.e., NMP) are also shown for comparison. In the figure legend, the SCs are simply named with the name of the electrode film compositions (i.e., “AC” or “AC:graphene (90:10)”). At specific power density above 1500 W/kg, the spray coated “AC:graphene (90:10)” exhibits a specific energy density higher than that of “AC” (+313% at the specific tested power density of 30000 W/kg), as a consequence of the high electron conductivity of the graphene-based electrode films and the ultrafast electron ion transport enabled by graphene flakes. More in detail, the spray coated “AC:graphene (90:10)” exhibits a specific energy density > 10 Wh/kg at specific power density > 30 kW/kg. In addition, spray coating-based manufacturing of SCs enable superior electrochemical performance of the SCs compared to those of slurry casted SCs (e.g., specific energy density more than +50% for specific power density > 7500 W/kg), which show specific energy density < 10 Wh/kg at specific power density of 15 kW/kg.

By increasing the graphene flakes content in the compositions up to 50%, the corresponding SC (“AC:graphene (50:50)”) results in energy density significantly lower than those of the “AC:graphene (90:10)” (i.e., 17.9 Wh/kg vs. 31.6 Wh/kg at the power density of 150 W/kg).

Similar trend was also observed for slurry-casted SCs. Interestingly, these devices have also shown that rate capability of the SCs can improve with increasing the rate of graphene flakes content in the active material (energy density retention for “AC:graphene (50:50)” is >50% at power density of 10000 W/kg relatively to the values measured at 100 W/kg), as a consequence of the optimal tribology properties of the SLG/FLG flakes.

Figure 3 shows the Ragone plots obtained at -40°C, 0°C, 50°C and 100°C for the spray-coated “AC” and “AC:graphene (90:10)” compositions previously measured at room temperature. For all the temperature operation, “AC:graphene (90:10)” improves the SC rate capability of the “AC”, validating the increase of the SC rate capability enabled by the use of graphene flakes as electrode material. Moreover, at -40°C, “AC:graphene (90:10)” exhibits specific energy densities higher than those of “AC” at all the measured specific power density. This indicates that the electrolyte nanotribology on graphene flakes facilitates the access of the electrolyte ions to the surface area of the electrode films at low temperature, in which the translational entropy available to the electrolyte decreases promoting its transformation towards an ordered solid-like phase. Example 6. Halogenated graphene flakes - comparative results

Fluorinated graphene flakes were synthetized according to the disclosure of the prior art document US 2018/0201740 Al. The oxygen and fluorine contents measured through X-ray photoelectron spectroscopy and TEM-coupled energy- dispersive X-ray spectroscopy was approximately equal to 2.0 wt% and 3.9 wt%, respectively.

The structural properties of the fluorinated graphene flakes have been evaluated through Raman spectroscopy measurements, while providing a comparison with the pristine graphene flakes disclosed in the instant application. As shown in Figure 4, the statistical analysis of the Raman spectra of the fluorinated graphene flakes shows that the ratio between the intensities of the D and G peaks (I(D)/I(G)) is different from the ratio I(D)/I(G) of the pristine graphene flakes contained in the composition of the instant application; specifically, the I(D)/I(G) ratios for the fluorinated graphene flakes are markedly higher than 2.0.

In addition, as further evidenced in Figure 5, a transmission electron microscopy (TEM) analysis shows that the pristine graphene flakes disclosed in the instant application have a flat morphology while the fluorinated graphene flakes known in the art show a corrugated/wrinkled surface morphology, which may be also associated to the fluorine-based chemical functionalization.

As previously reported in the literature, the polarity of the functional group of the halogenated graphene flakes, as well as the non-flat morphology, alter the tribological properties expected for pristine graphene, thus increasing the friction of the electrolyte over the flakes.

In contrast the pristine, non-oxidized, graphene flakes showing a ratio I(D)/I(G) of the Raman spectra comprised between 0.1 and 2.0, simultaneously provide high electrochemically accessible specific surface area (SSA) of the SC electrodes and ultrafast ions transport within the SC electrodes, boosting the specific energy density of the SCs at high specific power density (i.e., the SC rate capability).

Naturally, while the principle of the invention remains the same, the details of construction and the embodiments may widely vary with respect to what has been described and illustrated purely by way of example, without departing from the scope of the present invention. References

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2. K. S. Walton, R. Q. Snurr, Applicability of the BET Method for Determining Surface Areas of Microporous Metal-Organic Frameworks. J. Am.

Chem. Soc. 129, 8552-8556 (2007).

3. G. Fagerhmd, Determination of specific surface by the BET method. Materiaux Constr. 6, 239-245 (1973).

4. S. Wang, D. Minami, K. Kaneko, Comparative pore structure analysis of highly porous graphene monoliths treated at different temperatures with adsorption of N<inf>2</inf> at 77.4 K and of Ar at 87.3 K and 77.4 K. Microporous Mesoporous Mater. (2015), doi:10.1016/j.micromeso.2015.01.014.

5. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report) . Pure Appl. Chem. . 87 (2015), p. 1051.

6. S. F. M., Measurement of Sheet Resistivities with the Four-Point Probe. Bell Syst. Tech. J. 37, 711-718 (2018).

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8. L. Dagousset et ah, Electrochemical characterisations and ageing of ionic liquid/r -butyrolactone mixtures as electrolytes for supercapacitor applications over a wide temperature range. J. Power Sources (2017), doi:10.1016/j.jpowsour.2017.05.068.

Claims

1. Composition for providing electrically conductive covering layers on a substrate of an energy storage device, the composition comprising: a) an electrically conductive material comprising at least one allotrope of carbon, wherein said at least one allotrope of carbon comprises pristine graphene flakes, wherein said pristine graphene flakes have an atomic content of oxygen equal to or lower than 5%, preferably equal to or lower than 2%, wherein the ratio between the intensities of the D and G peaks (I(D)/I(G)) of the pristine flakes Raman spectra is comprised between 0.1 and 2.0, preferably between 0.1 and 1.2, b) a binding agent comprising a polymeric material, c) an environmentally friendly solvent having a boiling point equal to or lower than 150°C.

2. Composition according to Claim 1, wherein said pristine graphene flakes are present in an amount between 1% and 100% by weight of the electrically conductive material weight.

3. Composition according to Claim 1 or Claim 2, wherein said environmentally friendly solvent is selected in the group consisting of water, alcohols and mixtures thereof, said alcohols being preferably selected in the group consisting of ethanol, 1- propanol, 2-propanol, 1 -butanol, 2-butanol, 2-methyl- 1 -propanol, 2-methyl-2- propanol, 1-pentanol, 2-pentanol, 3 -methyl- 1 -butanol, 2,2-dimethyl- 1 -propanol, more preferably ethanol.

4. Composition according to any one of the preceding claims, wherein said polymeric material comprises at least one fluorine-free polymer preferably selected in the group consisting of carboxymethyl cellulose (CMC) and derivatives, preferably sodium carboxymethyl cellulose (Na-CMC), natural cellulose (NC), alginate, polyvinylpyrrolidone (PVP), polyvinyl butyral (PVB).

5. Composition according to any one of the preceding claims, wherein the binding agent is contained in the composition in an amount comprised between 1% and 30% by weight (w/w), preferably between 5% and 10% by weight (w/w) compared to the total amount of the electrically conductive material plus the binding agent.

6. Composition according to any one of the preceding claims, wherein said electrically conductive material comprises at least one further allotrope of carbon selected in the group consisting of activated carbon, carbon nanotubes, carbide- derived carbons, carbon black.

7. Composition according to Claim 6, wherein said at least another allotrope of carbon is activated carbon.

8. Composition according to Claim 6 or Claim 7, wherein said activated carbon is present in an amount up to 99% by weight (w/w) compared to the total weight of the electrically conductive material.

9. Method for providing a composition according to anyone of Claims 1 to 8, the method comprising the steps of:

- adding to an environmentally friendly solvent having a boiling point equal to or lower than 150°C, preferably selected in the group consisting of water, alcohols, mixtures thereof: i) an electrically conductive material comprising at least one allotrope of carbon, wherein said at least one allotrope of carbon comprises pristine graphene flakes having an atomic content of oxygen equal to or lower than 5%, preferably equal to or lower than 2%, the ratio between the intensities of the D and G peaks (I(D)/I(G)) of the pristine flakes Raman spectra being comprised between 0.1 and 2.0, preferably between 0.1 and 1.2, ii) a binding agent comprising a polymeric material to obtain a dispersion,

10. Method for providing at least one electrically conductive covering layer on a substrate of an energy storage device, the method comprising the steps of:

- depositing a composition according to any of the preceding Claims 1 to 8 onto said substrate to obtain a covering layer in form of film, optionally heating the covering layer in form of film, preferably at a temperature comprised between 40°C and 100°C.

11. Method according to Claim 10, wherein the depositing step is carried out by spray-coating.

12. Method according to Claim 10 or Claim 11, wherein the substrate is a surface of a supercapacitor electrode, preferably a surface of a current collector of a supercapacitor electrode.

13. Method according to anyone of Claims 10 to 12, wherein the substrate comprises a material selected in the group consisting of a metal, preferably aluminum (Al), gold (Au), nickel (Ni), stainless steel, a carbon-coated non- electrically conductive material, preferably plastic, fabric, paper, a graphitic foil, preferably a graphite paper.

14. Method according to anyone of Claims 10 to 13, wherein the supercapacitor is configured in a vertical configuration and the pristine graphene flakes are present in the composition in an amount comprised between 1% and 30% by weight compared to total weight of the electrically conductive material weight and/or wherein at least one further allotrope of carbon, preferably active carbon, is present in the composition in an amount comprised between 70% and 99% by weight compared to total weight of the electrically conductive material weight.

15. Method according to anyone of Claims 10 to 13, wherein the supercapacitor is configured in a planar configuration and the pristine graphene flakes are present in the composition in an amount comprised between 20% and 100% by weight compared to the total weight of the electrically conductive material weight.