The present invention relates to the field of the manufacturing of graphene, and in particular to a method of producing graphene by exfoliation of graphite. Further, the present invention relates to a graphene electrode for use in lithium-ion batteries.

Graphene is a two-dimensional carbon allotrope, wherein single layers of sp2- bonded carbon atoms are arranged in a hexagonal lattice. Due to its extraordinary electrical, thermal, and physical properties, graphene has attracted tremendous interest. The most common techniques available for the production of graphene are chemical vapour deposition, unzipping of carbon nanotube, epitaxial growth on SiC, and methods based on the exfoliation of graphite as graphene may be described as individual layers of graphite. However, for most applications the major bottleneck is the requirement of bulk quantities of graphene. Hence, the main obstacle for a wide commercial availability of high quality graphene is represented by a lack of reproducible, low cost, environmental friendly and large scale methods for its production.

Exfoliation techniques mainly include micromechanical exfoliation of graphite, liquid phase exfoliation of graphite, or chemical exfoliation of graphite oxide. Exfoliation methods separate the individual layers of graphite by weakening the interlayer bonding force of graphite. Methods using expandable graphite use a step of heating the expandable graphite to convert the liquid or solid phase intercalant into gas phase. Gas phase produce a high increase of the intercalant volume. The volume variation generates a pressure which forces the adjacent graphene layers to separate. After dispersing the expanded graphite in a solvent, for example by sonication, exfoliation generates few-layered graphene flakes. Ionic liquids have emerged as promising solvents to aid sonication-based graphite exfoliation. Document EP 2 518 103 A2 describes a method of manufacturing a graphene dispersion by exfoliating graphite in an ionic liquid. However, the method does not allow for a large scale production of high quality graphene.

Generally, known methods useful for bulk production do not yield high quality graphene while methods for high quality graphene production are not viable to produce bulk quantities of material. Therefore, it is an object of the present invention to provide a method that allows for a scalable method for the synthesis of high quality graphene.

The problem is solved by a method of producing graphene, the method comprising the steps of:

a) providing expandable graphite and subjecting the expandable graphite to thermal

treatment,

b) dispersing the expanded graphite obtained from step a) in an ionic liquid, and

c) exfoliating the expanded graphite into graphene in the ionic liquid,

wherein for exfoliating the dispersion of expanded graphite in the ionic liquid obtained from step b) is exposed to microwave irradiation.

It has been surprisingly found that the method enables to provide a high yield method for bulk quantity production of high quality graphene. Exfoliating expanded graphite in an ionic liquid using microwave irradiation provides for a thorough exfoliation. This allows to omit a centrifugation step to separate the not-exfoliated material and provides higher yields compared to standard liquid phase exfoliation methods. Exfoliating expanded graphite in an ionic liquid using microwave irradiation allows to obtain high quality graphene with very high yields. The process yield in terms of carbon is about 100%. This high yield further is achieved since no graphite oxidation leading to C0 ₂ and other compounds formation is performed.

Advantageously, the graphene obtained by the method of the invention possesses a high quality in terms of low defects and number of layers. Further, the method is highly

reproducible. The use of low-cost expendable graphite, the elimination of the cost and time consuming centrifugation step, and the efficient energy transfer granted by the microwave treatment provide an overall low cost process. Additionally, the method is environmentally friendly as no toxic and volatile solvents are used. The use of a microwave reactor provides for an appropriate instrument setup to make large-scale production of high-quality graphene available. The method of the invention hence enables a reproducible, scalable, low cost and environmental friendly method for the synthesis of high quality few-layer graphene. This is hugely advantageous, as this allows for a commercial use of graphene, for example allowing the realization of lithium-ion batteries or supercapacitors performing at temperatures as low as -40°C due to graphene anodes.

The term "graphene" refers to a single-atom-thick sheet hexagonally arranged, sp2-bonded carbon atoms that is not an integral part of a carbon material. The term "graphene layer" refers to single-atom-thick sheet hexagonally arranged, sp2 -bonded carbon atoms occurring within a carbon material structure, i.e. graphite. As used herein, the term "graphene" particularly refers to particles comprising a thickness ranging from lnm to 50 nm. The term "few-layer graphene" refers to a 2D, sheet-like material, either as a free-standing flake or substrate-bound coating, comprising a small number, particularly between 3 and 10, of stacked layer of graphene of extended lateral dimension which have been separated from pristine 3D carbon material, i.e. graphite. For example, few-layer graphene can be manufactured by exfoliation from graphite. The term "high-quality graphene" refers to particles of a low number of layers such as single-, double- and few-layer graphene and with a small presence of intrinsic and extrinsic defects such as presence of oxygen-containing groups and deformations of the graphene network.

Graphite has a layered crystal structure of sheets of carbon atoms that have an interlayer distance of 0.335 nm. As used herein, the term "expandable graphite" refers to pretreated graphite in which the layered crystal graphite structure is intercalated with small molecules such as sulphur or nitrogen compounds. The layered, planar structure of graphite allows that atoms or small molecules can intercalate between the carbon layers. Expandable graphite appears as a dry material with a minimal acidity since the intercalant is sealed within its carbon lattice. During this process so-called expandable graphite is produced. Expandable graphite is commercially available, or can be manufactured for example by acid treatment of graphite flake in nitric and sulphuric acid. The expandable graphite still retains the interlayer distance of natural flake graphite, and is chemically stable under air condition and can be easily stored. Graphite particularly flake graphite can be treated with acid such as sulphuric acid, nitric acid, or acetic acid to intercalate into the crystal layers of the graphite. The introduction of acid into the graphite layers can be supported by treatment with oxidants like ¾(¾, HNO ₃ or KM11O4 or by electrochemical treatment. The graphite can have a purity of about 95% up to 99.5%. After the reaction, the expandable graphite can be neutralized, washed, and dried.

While the carbon atoms are tightly bound to each other within a layer, the layers themselves can be expanded and separated. When expandable graphite is exposed to heat treatment an expansion of the intercalated graphite layers is induced. Expandable graphite can expand to more than several hundred times than the original volume. As used herein, the term

"expanded graphite" refers to expandable graphite after such heat treatment. Generally, expanded graphite, differently from expandable graphite, shows an increased interlayer spacing and higher carbon content both due to the heat treatment procedure. A separation of the graphene layers can be effected when the interlayer π-π interactions are sufficiently weakened. The separation of the graphene layers from bulk graphite is denoted "exfoliation". The exfoliation mechanism is thought to be caused by molecular intercalation and/or solvent diffusion between the graphene layers of graphite. Key factors responsible for a successful exfoliation of graphite are the solvent in which the exfoliation takes place and the driving force used to effect the separation of the layers.

The method of the invention enables a thorough exfoliation and provides a high-yield method for bulk quantity production of high quality graphene. This is particularly based on step c) of the method, wherein the expanded graphite is exfoliated into graphene in an ionic liquid using microwave irradiation. The step of exposing the expanded graphite to microwave irradiation as used herein refers to the applying microwave radiation to the ionic liquid dispersed expanded graphite to effect heating. Suitable microwave irradiation may have wavelengths in the range of 1 m to 1 mm. The step of exposing the dispersed, particularly sonicated, expanded graphite to microwave irradiation provides for a thermal exfoliation.

Advantageously, microwave heating can effect a rapid heating of the expanded graphite, which saves time and energy.

Without being bound to a specific theory, it is assumed that exfoliating the expanded graphite into graphene in an ionic liquid using microwave irradiation takes advantage of the high heating efficiency of the ionic liquid in a microwave field. Advantageously, the absolute temperature used for the exfoliation may be low. In embodiments, the exfoliation under microwave irradiation in step c) is accomplished in a temperature range of > 100 °C to < 500 °C, preferably in a temperature range of > 150 °C to < 200 °C, more preferred in a temperature range of > 160 °C to < 170 °C. It was found that the high heating efficiency of the ionic liquid in a microwave field lead to ionic oscillations which generated heat energy that permits a further exfoliation and size reduction of the sonicated expanded graphite to graphene. As a result, already a temperature in a range of > 150 °C to < 200 °C, or even > 160 °C to < 170 °C may be sufficient for exfoliation.

Further, the microwave irradiation allows for a short heating period. In embodiments, the exfoliation in step c) is accomplished under microwave irradiation in a time range of > 10 s to < 600 s, particularly in a time range of > 30 s to < 180 s, preferably in a time range of > 60 s to < 120 s, more preferred in a time range of > 80 s to < 90 s and/or in a power range of > 10 W to < 1500 W, preferably in a power range of > 500 W to < 900 W, more preferred in a power range of > 700 W to < 850 W. The power range may be selected depending on the microwave reactor. For example, when working at a power in a range of > 700 W to < 850 W, microwave irradiation in a time range of > 80 s to < 90 s may be sufficient. By heating using microwave irradiation, the exfoliation of sonicated expanded graphite can occur in less than three or two minutes. For example, thorough exfoliating may be achieved at a temperature range of > 160 °C to < 170 °C, holding the temperature for only 80 to 90 seconds.

Advantageously, the exfoliation under microwave irradiation may be performed in air. No additional process steps to ensure the absence of oxygen, or cost intensive facilities to achieve vacuum are necessary. Advantageously, to operate in air can provide cost saving of the entire process since no vacuum or inert atmosphere is requested.

Preferably, the exfoliation is performed under stirring. Stirring advantageously supports heat transfer and avoids heat peaks in the dispersion. Stirring may be performed in a range of > 50 rpm to < 1500 rpm, preferably in a range of > 200 rpm to < 1000 rpm, more preferred in a range of > 400 rpm to < 600 rpm (revolutions per minute).

The method of the invention provides a very efficient exfoliation of graphite. SEM images showed that the entire or at least nearly the entire graphite has been exfoliated to graphene during the multistep synthesis. Hence, it is not necessary to separate not-exfoliated material from the graphene. This allows omitting a centrifugation step usually used in standard liquid phase exfoliation methods. As a result, the yield is higher with respect to standard liquid phase exfoliation methods. In embodiments, in a step d) the graphene is separated from the ionic liquid by vacuum filtration and by rinsing with water. Advantageously, the filtration is used only to separate the graphene from the ionic liquid. It is not necessary to separate from not-exfoliated material. For example, the graphene may be separated from the ionic liquid through vacuum filtration, be washed with ultrapure water and dried overnight in oven at 80°C under atmospheric conditions.

In step b) of the method the expanded graphite obtained from step a) is dispersed in an ionic liquid. As used herein, the term "ionic liquid" refers to a salt in the liquid state, particularly a salt whose melting point is below 100 °C. Ionic liquids usually comprise anions and cations. In a preferred embodiment, the ionic liquid used in the method is an imidazolium-based ionic liquid. The cation of the imidazolium-based ionic liquid is an imidazolium cation.

In step b) of the method, π-π interactions between the ionic liquid and the graphene layers are established. In this step, the ionic liquid acts as dispersing agent for the expanded graphite particles. Advantageously, the imidazolium-based ionic liquid showed good effects as dispersing agent. Particularly, imidazolium shows good effects in promoting a fragmentation of the expanded graphite into thin graphite microplatelets, and also in stabilizing the microplatelets in the ionic liquid by avoiding a re-stack of the expanded graphitic layers. In an embodiment, the imidazolium cation is selected from the compound according to the R5

formula (1): (1)

wherein:

R ¹ , R ² , R ³ , R ⁴ , R ⁵ is selected, the same or each independently of the other, from the group comprising hydrogen, halogen, Ci-C ₂ o-alkyl, C ₂ -C ₂ o-alkenyl, C ₂ -C ₂ o-alkynyl and/or C ₆ -C ₁₀ - aryl, which may contain a heteroatom selected from O, N, Si and/or S, and/or may optionally be substituted with CI, Br, F, I, OH, NH ₂ and/or SH.

The term "alkyl" according to the invention is to be understood as meaning straight-chain or branched alkyl groups. The term "Ci-C ₂ o-alkyl" as used herein refers to straight-chain or branched alkyl groups having 1 to 20 carbon atoms. Preferred are Ci-Cio-alkyl groups, particularly Ci-Cg-alkyl groups selected from the group comprising methyl, ethyl and the isomers of propyl, butyl, pentyl, hexyl, heptyl or octyl, such as, for example, isopropyl, isobutyl, tert.-butyl, sec. -butyl and/or isopentyl. The term "alkenyl" according to the invention is to be understood as meaning straight-chain or branched alkyl groups having at least one or several double bonds. The term "C ₂ -C ₂ o-alkenyl" as used herein refers to straight-chain or branched alkenyl groups having 2 to 20 carbon atoms and at least one or several double bonds. Preferred are C ₂ -Cio-alkenyl groups, particularly C ₂ -C ₈ -alkenyl groups, preferably selected from the group comprising propenyl, allyl, butenyl, pentenyl and/or hexenyl. The term "alkynyl" according to the invention is to be understood as meaning straight-chain or branched alkyl groups having at least one or several carbon-carbon triple bonds. The term "C ₂ -C ₂ o-alkynyl" as used herein refers to straight-chain or branched alkynyl groups having 2 to 20 carbon atoms and at least one or several triple bonds. Preferred are C ₂ -Cio-alkynyl groups, particularly C ₂ -C8-alkynyl groups, preferably selected from the group comprising propynyl, butynyl, pentynyl and/or propargyl. The term "C6-Cio-aryl" according to the invention is to be understood as meaning aromatic groups having 6 to 10 carbon atoms.

Preferably, the term "C6-Cio-aryl" refers to carbocycles. Preferred C ₆ -Cio-aryl is selected from the group comprising benzyl, and phenyl.

In embodiments, R ¹ , R ² , R ³ , R ⁴ , R ⁵ is selected, the same or each independently of the other, from the group comprising hydrogen, Ci-C ₂ o-alkyl, C ₂ -C ₂ o-alkenyl, C ₂ -C ₂ o-alkynyl and/or C ₆ - Cio-aryl, which may contain a heteroatom selected from O, N, Si and/or S, and/or may optionally be substituted with OH, NH ₂ and/or SH. In these embodiments, the ionic liquid does not contain halogens and is environmental friendly.

Preferred is an imidazolium-based ionic liquid with a donor number > 0 kcal mol ^"1 , wherein the donor number is defined as a quantification of the tendency to donate electron pairs to acceptors. In preferred embodiments, the imidazolium cation is selected from the group of

1,3-dialkyl imidazolium cations, wherein R ² , R ⁴ and R ⁵ is hydrogen, and R ¹ and R ³ is Ci-Cio- alkyl, preferably Ci-Cs-alkyl. Preferred are l-Ci_8-alkyl-3-methylimidazolium cations, preferably selected from the group comprising l-ethyl-3-methylimidazolium and l-butyl-3- methylimidazolium. l-Ci_8-alkyl-3-methylimidazolium cations showed particularly favorable effects in promoting a fragmentation of the expanded graphite and in stabilizing the microplatelets in the ionic liquid.

Preferably, the anion of the ionic liquid is a halide free anion. The use of halide free anions in respect to halide containing anions avoids the formation of environmental dangerous compounds. In embodiments, the anion of the ionic liquid is a halide free anion selected from the group comprising [CH ₃ C0 ₂ ]-, [HS0 ₄ ] ^" , [C0 ₃ ] ₂ ^" , [HC0 ₃ ] ^" , [N0 ₂ ] ^" , [N0 ₃ ] ^" , [S0 ₄ ] ₂ ^" , Ρ0 ₄ ] ₃ ^" , [HP0 ₄ ] ₂ ^~ , [H ₂ P0 ₄ ] ^~ , [HS0 ₃ ] ^~ , [Ci-5-alkyl OS0 ₃ ] ^~ such as [CH ₃ OS0 ₃ ] ^" and [C ₂ H ₅ OS0 ₃ ] ^" and/or [Ci_5-alkylOCH ₂ C0 ₂ ] ^" . Preferably, the anion is selected from the group comprising [CH ₃ C0 ₂ ] ^~ , [Ci_ ₅ -alkyl OS0 ₃ ] ^~ such as [CH ₃ OS0 ₃ ] ^" and [C ₂ H ₅ OS0 ₃ ] ^~ and/or [Ci_ ₅ -alkylOCH ₂ C0 ₂ ] ^" such as [CH ₃ OCH ₂ C0 ₂ ] ^~ (MeOAc ). In most preferred embodiments, the anion is acetate.

Particularly 1,3-dialkyl imidazolium acetate is environmental friendly and can be recycled for further use.

The expanded graphite obtained from step a) may be added to the ionic liquid such as 1-ethyl- 3-methylimidazolium acetate (EMIMAc), or the ionic liquid may be added to the expanded graphite. With respect to stabilizing the microplatelets in the ionic liquid by avoiding a re- stack of the e panded graphitic layers, it is preferred that the dispersion contains an amount of ionic liquid that exceeds the weight of the expanded graphite. In embodiments, in step b) a dispersion comprising the expanded graphite in a range of > 0.2 wt.-% to < 10 wt.-%, preferably in a range of > 0.5 wt.-% to < 5 wt.-%, more preferably in a range of > 0.8 wt.-% to < 1 wt.-%, based on the total weight of the dispersion, is prepared. Such dispersion showed particular favorable stability.

The expanded graphite may be dispersed in the ionic liquid by stirring and/or sonication. Preferably, sonication can be used as driving force to overcome the π-π interactions between the adjacent graphene layers and to speed or enhance the dispersion of the expanded graphite in the ionic liquid. In embodiments, in step b) dispersing the expanded graphite is

accomplished by sonication. Advantageously, sonication supports a fragmentation of the expanded graphene layers of the expanded graphite particles, and thin graphite microplatelets are obtainable. Parameters affecting the sonication are sonication time and sonication power. Preferably, sonically dispersing the expanded graphite is accomplished in a time range of > 1 h to < 300 h, preferably in a time range of > 30 h s to < 150 h, more preferred in a time range of > 60 h to < 70 h. Also preferred, the sonication is accomplished using an instrument power range of > 10 W to < 1000 W, preferably in a power range of > 50 W to < 500 W, more preferred in a power range of > 90 W to < 100 W. Such time and power ranges for the sonication can show good results. The dispersion may be submitted to sonication under continuous stirring. The stirring serves to keep the dispersion homogeneous. For example, the dispersion may be submitted to 70 hours of sonication under 600 rpm continuous stirring. For sonication the dispersion can be treated with waves >20 kHz. Step b) may be performed in air. Also the thermal treatment of the expandable graphite in step a) may be performed in air. Advantageously, all the steps of the method may proceed in air. This allows to omit process steps to ensure the absence of oxygen, or cost intensive facilities to achieve vacuum and provides for a low cost method. The thermal treatment of the expandable graphite in step a) the may be accomplished by heating the expandable graphite in any type of heating source. The heating of expandable graphite induces the expansion of the graphite particles. In embodiments, the thermal treatment of the expandable graphite in step a) is accomplished by heating the expandable graphite in a microwave reactor. Microwave offers faster and more homogeneous heating of the reaction vessels and is therefore superior to most conventional heating treatments. Further, microwave reactors offer an environmental friendly and reproducible synthesis of expanded graphite with high reaction yields. Advantageously, only the intercalated species converted to gas phase to produce the expanded graphite are lost upon heating. The temperature and time ranges for the heating may vary. Preferably, the extendable graphite may be heated to a temperature in the range of > 100 °C to < 1500 °C, preferably in a range of > 200 °C to < 500 °C, more preferred in a range of > 230 °C to < 250 °C. The expandable graphite may be heated for a time in the range of > 10 s to < 600 s, particularly in a time range of > 30 s to < 180 s, preferably in a time range of > 60 s to < 120 s, more preferred in a time range of > 80 s to < 90 s, and/or in a power range of > 10 W to < 1500 W, preferably in a power range of > 500 W to < 900 W, more preferred in a power range of > 700 W to < 850 W. Such ranges assure the expansion of the graphenc layers in the expandable graphite. The thermal treatment of the expandable graphite may be performed with or without stirring. For example expandable graphite may be rapidly heated in air at a microwave power of 850W. When a temperature of 250°C is reached, the instrument may modulate the microwave power holding the temperature for 90 seconds, without stirring, before cooling down to 30°C.

In summary, the high yield of the new microwave irradiation step in ionic liquid environment, the elimination of the cost and time consuming centrifugation step, provide valuable advantages. After the steps a), b) and c) the graphene may separated from the ionic liquid by vacuum filtration and rinsed with water. The yield of the method can be as high as or nearly 100% in terms of carbon. Another aspect of the invention refers to graphene prepared by the method according to the invention. Advantageously, the method provides to obtain few-layer graphene. The number of layers n can be in a range of 1 < n < 30, preferably in the range of 2 < n < 15, more preferred in the range of 3 < n < 10. In preferred embodiments, the final material can be classified a as few-layer graphene having a number of layers n a range of 3 < n < 10.

In embodiments, the graphene can have a particle thickness in a range of > 1 nm to < 50 nm, preferably in the range of > 2 nm to < 25 nm, more preferred in the range of > 3 nm to < 10 nm, and/or a particle lateral size in a range of > 10 nm to < 5 μιη, preferably in the range of > 50 nm to < 3 μιη, more preferred in the range of > 100 nm to < 2 μιη. Graphene of these dimensions fulfils the main requests for graphene commercialization and can be used for several applications.

The graphene produced using the method of the invention has desirable properties, particularly a high electrical conductivity. The graphene produced using the method of the invention can be employed as active material to manufacture electrodes for electrochemical energy storage devices such as lithium-ion batteries, lithium-ion capacitors and

supercapacitors. Another aspect of the invention refers to the use of graphene prepared by the method according to the invention as electrode material for electrochemical energy storage devices. Particularly, the graphene prepared by the method according to the invention is usable as active material for electrodes used in lithium-ion batteries, lithium-ion capacitors and supercapacitors.

Surprisingly, it was found that electrodes manufactured from graphene prepared by the method of the invention showed an exceptional low temperature performance, as low as -40 °C, when used as electrode active material. This allows the realisation of lithium-ion batteries, lithium-ion capacitors and supercapacitors with an operative temperature range significantly extended with respect to state-of-the-art devices, which is considered an essential aspect for the invention.

Another aspect of the invention hence refers to electrode material for electrochemical energy storage devices prepared by the method according to the invention. Yet another aspect of the invention refers to an electrode comprising graphene prepared by the method according to the invention as electrode material. The electrode can particularly be an electrode for lithium-ion batteries, lithium-ion capacitors and supercapacitors. The few-layer graphene obtained by the method of the invention can have a number of layers n in a range of 1 < n < 30, preferably in the range of 2 < n < 15, more preferred in the range of 3 < n < 10. Further, the graphene can have a particle thickness in a range of > 1 nm to < 50 nm, preferably in the range of > 2 nm to < 25 nm, more preferred in the range of > 3 nm to < 10 nm, and/or a particle lateral size in a range of > 10 nm to < 5 μιη, preferably in the range of > 50 nm to < 3 μιη, more preferred in the range of > 100 nm to < 2 μιη.

Another aspect of the invention refers to an electrochemical energy storage device, comprising an electrode as described above, particularly a lithium-ion battery, a lithium-ion capacitor or a supercapacitor. A lithium-ion battery for example can comprise an anode manufactured from active material prepared by the method as described, an electrode of a cathodic material, and an electrolyte.

Unless otherwise defined, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

The examples which follow serve to illustrate the invention in more detail but do not constitute a limitation thereof.

In the figures show:

Figure 1 shows a SEM micrograph of a scale of 1000 x in Figure 1 a) and 10000 x in Figure

1 b) of expandable graphite particles.

Figure 2 shows a SEM micrograph of a scale of 1000 x in Figure 2 a) and 10000 x in Figure

2 b) of the expanded graphite particles as obtained after heat treatment at 250°C in a microwave reactor.

Figure 3 shows a SEM micrograph of a scale of 1000 x in Figure 3 a) and 10000 x in Figure

3 b) of the expanded graphite after dispersion in l-ethyl-3-methylimidazolium acetate by sonication.

Figure 4 shows a SEM micrograph of a scale of 1000 x in Figure 4 a) and 10000 x in Figure

4 b) of obtained graphene after microwave irradiation in ionic liquid according to an embodiment of the method.

Figure 5 shows RAMAN spectroscopy of the expandable graphite (ES 100), the expanded graphite (MW 100), the expanded graphite after sonication (TG 100), and the graphene (GRAL). Figure 5b) shows a band deconvolution of the 2D band of the graphene (GRAL) of Figure 5a). Figure 6 shows a powder X-ray diffraction (XRD) pattern of the expandable graphite (ES 100), the expanded graphite (MW 100), the expanded graphite after sonication (TG 100), and the graphene (GRAL).

Figure 7 shows the specific discharge capacity of graphene (GRAL) and graphite (SLP30) versus temperature at a specific current of 20 mA g ^" and 50 mA g ^" in Figure 7a) and Figure 7b), respectively.

Figure 8 shows the galvanostatic charge-discharge cycles at different temperatures of graphene (GRAL) and graphite (SLP30) using a specific current of 20 mA g ^"1 in Figures 8a) and 8b) or 50 mA g ^"1 in Figures 8c) and 8d), respectively.

Example 1

Preparation of graphene

Step a) Thermal treatment of expandable graphite

0.13 g of expandable graphite (Graphit Kropfmuhl, ES 100 CIO grade) were transferred to a 30 mL-borosilicate glass reaction vial. The vial was rapidly heated to 250°C in air using an Anton Paar Mono wave 300 microwave reactor to induce a rapid expansion of the graphite particles. The instrument started to irradiate the borosilicate glass reaction vial at maximum microwave power of 850W. When the temperature of 250°C was reached, the instrument modulated the microwave power holding the temperature for 90 seconds, without stirring, before cooling down to 30°C. This procedure was repeated for 15 times.

High resolution scanning electron microscope (SEM) analysis was carried out on a ZEISS Auriga® microscope on untreated and heat treated graphite particles. As shown in Figure 1, the expandable graphite before heat treatment showed a compact particle structure with several tens of micrometers thickness. The expanded graphite after heat treatment is shown in Figure 2 exhibits an expansion of the intercalated graphite layers induced by the heat treatment. The expanded graphite shows an accordion-like morphology wherein the graphene layers appear expanded and partially separated as clearly visible in the higher magnification micrograph of Figure 2b).

Step b) Preparing a dispersion of expanded graphite in ionic liquid

165 g of l-ethyl-3-methylimidazolium acetate (EMIMAc) (Ionic Liquid Technologies GmbH, product code IL-0189) were added to 1.8 g of the expanded graphite obtained from step a) to prepare a dispersion of ca. 1 %.-wt. The dispersion was then submitted to 70 hours of sonication using a Sartorius LABSONIC® M ultrasonic homogenizer operating at 100 W under continuous stirring in the 300 rpm - 600 rpm range under air condition at ambient temperature (20 ± 2°C).

Characterization of the morphology as well as particle size and shape of the dispersed expanded graphite after sonication were carried out by SEM analysis on the ZEISS Auriga® microscope. A 20 mL sample of the sonicated expanded graphite was taken from the dispersion using a vacuum filtration apparatus equipped with Fluoropore™ PTFE filter membranes with a pore size of 0.2 μιη and a diameter of 47 mm, than washed three times with 300 mL of ultrapure water (Milli-Q® Water) each and dried overnight in an oven (WTP Binder) at 80°C under atmospheric conditions.

As shown in Figure 3, the expanded graphite after dispersion by sonication showed a fragmentation of the expanded graphene layers previously observed in Figure 2 obtaining thin graphite microplatelets. The thickness of the thin graphite microplatelets was in a range of 10 nm to 30 nm, and the particle size was in a range of 0.5 μιη to 5 μιη.

Step c) Exfoliation using microwave irradiation 20 mL of the dispersion of thin graphite microplatelets in the ionic liquid obtained from step b) were transferred to a 30 mL-borosilicate glass reaction vial. The vial was then exposed to microwave irradiation using the Anton Paar Mono wave 300 microwave reactor as used in step a). The instrument started to irradiate the borosilicate glass reaction vial at maximum microwave power of 850W. When a temperature of 170°C was reached, the instrument modulated the microwave power holding the temperature for 90 seconds, under 600 rpm stirring condition, before cooling down to 30°C. This procedure was repeated until total utilization of the volume of the dispersion of thin graphite microplatelets in the ionic liquid. Step d) Filtration

The cooled graphene dispersions were first collected in a 250 mL beacker and then separated from the ionic liquid using a vacuum filtration apparatus equipped with Fluoropore™ PTFE filter membranes with a pore size of 0.2 μιη and a diameter of 47 mm. After filtration the graphene was washed three times with 300 mL of ultrapure water (Milli-Q® Water) each and dried overnight in an oven (WTP Binder) at 80°C under atmospheric conditions.

The dried graphene was subjected to SEM and RAMAN spectroscopy, XRD characterization and surface area measurements by BET and elemental analysis for characterisation of the obtained material.

The SEM analysis of the obtained graphene as shown in Figure 4 showed an exfoliation of the thin graphite microplatelets. As observable in the low magnification SEM micrograph 4b), particles size was reduced and the thickness lowered. The thickness of the obtained graphene particles was in a range of 3 nm to 8 nm, and the particle size was in a range of 0.1 μιη to 2 μιη.

The BET specific surface area of the starting expandable graphite and the manufactured graphene was determined using nitrogen adsorption (Brunauer-Emmett-Teller method) using a Micrometrics ASAP 2020 (Accelerated Surface Area and Porosimetry Analyzer,

Micromeritics) at liquid nitrogen temperature. Further, elemental analysis of the starting expandable graphite and the manufactured graphene was determined using a vario EL III Element Analyzer. The Table 1 summarizes the results of BET and CFIN-elemental analysis.

Table 1

The elemental analysis shows that the carbon content, based on the total weight, of the graphene increased significantly compared to the carbon content of the expandable graphite. Further, the BET surface area analysis shows that the specific surface area during the graphene production also increased significantly. The increase in the BET surface area is related to the further exfoliation and size reduction of the particles during the synthesis process. RAMAN spectroscopy of probes of the expandable graphite (ES 100), the expanded graphite (MW 100) obtained from step a), the expanded graphite after sonication (TG 100) in step b), and the graphene (GRAL) obtained after microwave irradiation in step c) and filtration was performed using a Bruker SENTERRA dispersive Raman microscope (20X magnification) with a green semiconductor-laser (532 nm, 20 mW) and is shown in Figures 5a and 5b. The Figure 5a shows the D band, the G band, and the 2D band. The D band at - 1350 cm ^"1 is related to disordered structures such as edges and defects, and D band intensity is considered proportional to the level of defects. The significantly decreased peak intensity from expandable graphite to graphene indicates the low presence of defects in the graphene. The G band at ~ 1580 cm ^"1 is related to the crystalline structure and the G band intensity is proportional to the number of graphene layers. As can be taken from the Figure 5a, the G peak narrowed from expandable graphite to graphene. This indicates that the graphene has a higher crystalline structure. Moreover, the peak intensity decreased from expandable graphite to graphene. This indicates that the graphene has a lower number of layers with respect to the starting material of expandable graphite and to the synthesis intermediates of expanded graphite and thin graphite microplatelets after sonication. The 2D band at ~ 2700 cm ^"1 is related to the number of graphene layers.

The Figure 5b shows a band deconvolution of the 2D band of graphene. The deconvolution of the 2D band of graphene confirms the presence of few-layer, mono-layer and bi-layer graphene in the probe. Single layer graphene has a single symmetric 2D peak at 2659 cm ^"1 relating to one scattering process, bilayer graphene has a peak at 2678 cm ^"1 , and few-layer (3 < n < 10) graphene a peak at 2718 cm ^"1 . The presence of different types of graphene causes an overlap of different bands. It can be taken from the Figure 5b that the obtained graphene mostly comprises few-layer graphene particles.

The preservation of the crystal structure of the graphene was confirmed by X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer equipped with Cu-Κα radiation (λ = 0.154 nm). As shown in Figure 6, the observed diffraction peaks in the XRD pattern of the expandable graphite (ES 100), the expanded graphite (MW 100), the thin graphite

microparticles (TG 100) obtained by sonication of the expanded graphite, and the graphene (GRAL) could be clearly determined as belonging to the single graphite crystal peak (2Θ = 26.50). The height of the peaks as shown in Figure 6 is related to the XRD equipment setting and to the possible particle orientation during the sample preparation. The peak position of the graphene (GRAL) is similar to the single graphite crystal peak. This confirmed that the graphene possessed a crystalline structure. The characterisation confirms that the graphene as obtained by the method of the invention can be classified as a high quality few layer (3 < n < 10) crystalline graphene. The procedure was repeated in a 10 mL-borosilicate vial using the experimental conditions as described above and yielded reproducible high quality few layer graphene.

Example 2

Electrochemical characterization Electrode preparation

Electrodes comprising either graphene obtained from Example 1 or graphite (TIMCAL SLP30 grade) for comparison as the active material were prepared by casting a slurry of 85 wt.% active material, 10 wt.% sodium carboxymethyl cellulose (CMC, Walocel® CRT 2000PA, Dow Wolff Cellulosics, Germany) and 5 wt.% TIMCAL Super C65 using ultrapure water (Milli-Q® Water) as the solvent onto dendritic copper foils (Carl Schlenk AG) and dried in an oven at 80°C in air. Subsequently, electrodes having a diameter of 12 mm were punched. The electrode active material mass loading ranged between 0.6 and 0.9 mg cm ^" . Electrolyte solution

Ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC), all battery grade, were purchased at UBE Corporation. LiPF ₆ powder (battery grade, >99.99% trace metals basis) was purchased at Sigma-Aldritch®.

Electrolyte solutions were prepared by mixing the solvents in their respective weight ratios and dissolving the lithium salt powder in an appropriate amount to yield a 1M solution.

Electrochemical testing Electrochemical tests were performed using Swagelok T-cells assembled in an argon filled glovebox (MBraun). Graphene and graphite (TIMCAL SLP30 grade) electrodes were used as working electrodes and lithium metal foil (battery grade, Rockwood Lithium) as counter and reference electrodes. 1M LiPF ₆ in EC:DEC:DMC 1 : 1 : 1 (weight) electrolytic solution was employed as the electrolyte and polypropylene disks (Freudenberg FS 2226) with 12 mm and 7 mm of diameter was employed as separators.

The electrochemical tests (galvanostatic charge-discharge cycles), for both graphene and graphite (TIMCAL SLP30 grade), were performed within the potential range from 0.005 V and 3 V vs Li7Li using a VSP 4 Channels Potentiostat/Galvanostat (Bio-Logic, France) using two different specific currents (20 mA g ^"1 and 50 niA g ^"1 ). The tests were conducted in a climatic chamber (BINDER MK-53-E2) at 20°C, 0°C, -20°C, -30°C and -40°C.

Sequences of charge-discharge cycles had been set as follows:

The Figures 7a and 7b show the specific discharge capacity of graphene (GRAL) and graphite (SLP30) versus temperature at a specific current of 20 mA g ^"1 and 50 mA g ^"1 , respectively. The values of specific capacities on the "Specific Discharge Capacity vs Temperature" graphs of Figure 8 refer to the last cycle of each sequence. As can be taken from Figures 7a and 7b at low temperatures (< 0°C), the graphene (GRAL) based anode electrodes outperformed conventional graphite (SLP30) electrodes as presently used in commercially available lithium-ion batteries while granting similar performance at temperature above 0°C. This allows the realisation of lithium-ion batteries, lithium-ion capacitor s and supercapacitors with an operative temperature range significantly extended with respect to state-of-the-art devices.

The Figure 8 shows the galvanostatic charge-discharge cycles at different temperatures of graphene (GRAL) and graphite (SLP30) using specific current of 20 mA g ^"1 (Figure 8a, Figure 8b) and of 50 mA g ^"1 (Figure 8c, Figure 8d), respectively. Graphene showed a high coulombic efficiency, approaching that of SLP30 graphite ~ 100%, and, at potentials < 0.5 V, both in charge and in discharge, the characteristic staging behavior of graphite (SLP30). However, as testified by the voltage profiles recorded at temperatures below 0°C, the lithium insertion kinetics are much improved with respect to graphite (SLP30).

In summary, the high yield of the microwave irradiation step in ionic liquid environment, the elimination of the cost and time consuming centrifugation step, are considered as essential aspect for the method of the invention. The exceptional low temperature performance of the resulting graphene material when used in lithium-ion anodes further is considered an essential aspect for the invention.