FR3166757A1 - Ready-to-use liquid graphene dispersion for use as a conductive electrode additive - Google Patents

Abstract

The present invention relates to a method for obtaining a liquid graphene dispersion, a liquid graphene dispersion, and its uses, particularly as a conductive electrode additive. In particular, the present invention relates to a liquid graphene dispersion comprising a solvent, a polymer, at least 0.1 g/L of graphene, and having an absolute viscosity, measured at 25°C using a rheometer, of between 5 mPa·s and 10 Pa·s.

Description

Ready-to-use liquid graphene dispersion for use as a conductive electrode additive

The present invention relates to the technical field of conductive electrode additives. In particular, the invention relates to a method for obtaining a liquid graphene dispersion, a liquid graphene dispersion itself, and its uses, preferably as a conductive electrode additive. The invention also relates to an electrode comprising the liquid graphene dispersion according to the invention, as well as a battery comprising such an electrode.

State of the art

The field of energy storage is essential for many sectors, such as portable electronic devices, electric vehicles (EVs) and stationary storage systems for renewable energy.

Among the various energy storage technologies available, electrochemical batteries are particularly important due to their ability to efficiently store and release electrical energy. The operating principle of batteries relies on redox reactions that take place in electrodes, where ions move through an electrolyte between an anode and a cathode during charge and discharge cycles.

Batteries are generally classified according to the materials used for the electrodes and electrolyte, as well as the type of electrochemical reaction. The main types of batteries include:
*Lead-acid batteries: Used mainly in vehicles and for stationary storage, they are characterized by a relatively low cost but have a low energy density and a limited lifespan;
*Nickel-cadmium and nickel-metal hydride batteries: These batteries were used for portable devices and power tools. Their advantage lies in their ability to withstand numerous charge/discharge cycles. However, cadmium is a toxic and polluting element, and the low energy density of these batteries has led to a decrease in their use;
*Lithium-ion batteries: Lithium-ion batteries are currently the dominant technology for electronic devices and electric vehicles. They operate by the movement of lithium ions between the cathode and the anode via a liquid electrolyte. This type of battery is distinguished by its high energy density, low self-discharge rate, and long lifespan. However, these batteries are expensive and have certain safety issues, particularly related to short circuits and overheating. Different variants of lithium-ion batteries exist, depending on the materials used for the cathode ( _LiCoO2 , _LiMn2O4 , _LiFePO4 ), each composition offering a different compromise between energy density, lifespan _, and safety.
*Sodium-ion batteries: More recent, sodium-ion batteries are based on principles similar to those of lithium-ion batteries, but replace lithium with sodium, a much more abundant and less expensive element. Although the energy density of sodium-ion batteries is currently lower than that of lithium-ion batteries, they are of increasing interest for stationary energy storage applications and could reduce dependence on limited lithium resources. Furthermore, they offer the potential for better performance at low temperatures and a lower production cost; and
Solid-state batteries: This emerging technology aims to replace liquid electrolytes with solid electrolytes, improving battery safety and stability by reducing the risk of leaks and fires. Solid-state batteries offer potentially higher energy density, longer lifespans, and better high-temperature performance. They are particularly attractive for electric vehicles, but their development is still limited by manufacturing and cost challenges. However, rapid progress in this area suggests a promising future for this technology, which could outperform lithium-ion batteries in many applications in the long term.

Since energy storage is a major issue for many industrial companies, there are different approaches to improve battery performance to make them as competitive as possible, particularly in terms of energy density, charging speed, lifespan, and safety.

Among these different approaches, we can find the development of new materials for electrodes. The exploration of new materials for cathodes and anodes, such as silicon anodes (which offer a much higher theoretical capacity than graphite) or nickel-rich cathodes, aims to increase the capacity and stability of batteries.

Another approach aims to develop solid electrolytes. Replacing liquid electrolytes with solid electrolytes could improve safety and allow the use of high-capacity electrodes, while eliminating the risk of dendrite formation that can cause short circuits.

Finally, to maximize electrode efficiency, conductive additives are often incorporated to improve electrical conductivity, especially when active materials have low intrinsic conductivity.

Electrode conductive additives, such as carbon black, carbon nanotubes, and carbon fibers, are traditionally used to improve the electronic conductivity of electrodes. Their role is to ensure efficient electron transport within the active materials, while also ensuring a uniform distribution of charges across the electrode.

However, these materials have several limitations. Carbon black, for example, requires relatively high proportions in the electrode to achieve sufficient conductivity levels, which reduces the amount of active material in the battery and thus decreases the overall storage capacity. Furthermore, while carbon nanotubes and fibers have superior conductive properties, they can be costly and present challenges in terms of uniform dispersion within the electrode matrix, leading to suboptimal performance.

In this context, graphene appears as a promising solution. A two-dimensional material composed of a single layer of carbon atoms arranged in hexagonal networks, graphene is a versatile material with a large number of advantageous physical properties such as its electrical and thermal conductivity, mechanical strength, lightness, corrosion resistance, flexibility and transparency.

Despite these advantages, the use of graphene as a conductive electrode additive still presents numerous challenges. One of the main challenges is the difficulty of producing high-quality graphene on a large scale and at a reasonable cost. Furthermore, the homogeneous dispersion of graphene within electrode materials remains a major issue: poor dispersion can lead to the formation of aggregates, thus reducing overall electrochemical performance. In addition, the interactions between graphene and active materials, while promising, are not always optimized, which can limit the expected performance gains.

There is therefore a need to develop low-cost, stable, aggregate-free, ready-to-use graphene-based electrode conductive additives that improve battery performance, preferably in the form of a liquid graphene dispersion.

To meet this need, the invention proposes a liquid dispersion of graphene comprising a solvent, a polymer, at least 0.1 g/L of graphene and having an absolute viscosity measured at 25°C using a rheometer of between 5mPa.s and 10Pa.s.

Advantageously, the dispersion according to the invention is stable, comprises graphene in the form of uniformly distributed sheets and is suitable for use in improving the electrochemical performance of electrodes.

The viscosity of the liquid graphene dispersion makes it possible to obtain a graphene concentration that is particularly interesting for industrial use while guaranteeing the absence of aggregates within the dispersion.

Thus, preferably, the dispersion according to the invention does not include an aggregate.

Advantageously, the liquid graphene dispersion according to the invention does not contain aggregates, in particular aggregates obtained by a phenomenon of re-aggregation of graphene sheets, and this for at least 3 months guaranteeing good reliability, in particular when used as a conductive electrode additive.

Preferably, the dispersion according to the invention comprises graphene in sheet form and at least one sheet has a thickness of less than 10 atomic layers.

The dispersion according to the invention advantageously comprises graphene having a low number of atomic layers, indicative of high quality.

According to one embodiment, the polymer is chosen from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene propylene diene monomer rubber (EPDM), or carboxymethylcellulose (CMC), nitrile butadiene rubber (NBR), styrene butadiene rubber (SBR), lithium polyacrylate (LiPAA), sodium polyacrylate (NaPAA), poly(acrylic acid) (PAA), polyimide, polyamide, sodium alginate, lithium alginate and their combinations.

Advantageously, the polymer according to the invention helps to stabilize the dispersion by contributing to the increase in viscosity.

According to one embodiment, the solvent is a polar aprotic solvent selected from dimethylformamide (DMF), dimethyl sulfoxide (DMSO), propylene carbonate (PC), acetonitrile, tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), 2-methyltetrahydrofuran (Me-THF), cyclopentylmethylether and their combinations.

Advantageously, the solvent is particularly suitable for the solubilization and dispersion of various fillers, polymers and additives and is of sufficient quality for applications in energy storage devices, especially batteries, preferably lithium ion batteries.

According to a particularly preferred embodiment, the dispersion can be characterized in that it comprises 3 vibrational bands characteristic of graphene observed by RAMAN spectrophotometry:
- 1350 cm-1 (peak D);
- 1580 cm-1 (peak G); and
– between 2680 and 2700 cm-1 (peak 2D).

Advantageously, such a RAMAN spectrum demonstrates the presence of unoxidized graphene and therefore its sheet conformation, thus guaranteeing that the dispersion according to the invention makes it possible to take advantage of the advantageous characteristics of graphene in sheet form.

According to another object, the invention relates to a method for obtaining a liquid dispersion of graphene characterized in that it comprises the following steps:
1) Solubilization of graphite carried out under an inert atmosphere forming a graphene solution;
2) Oxidation of the graphene solution obtained in step 1) to obtain an organic dispersion of graphene; and
3) Transfer of the organic graphene dispersion obtained in step 2) into a polymer forming a liquid graphene dispersion.

Advantageously, the transfer into a polymer improves stability and concentrates the liquid dispersion into graphene.

Such a process is particularly useful in the context of the invention, thus providing an alternative to the industrial production of graphene.

Preferably, the polymer of step 3) has an absolute viscosity measured at 25°C under a shear of 10s-1 using a rheometer of between 5mPa.s and 10Pa.s.

Advantageously, such viscosity allows for optimal stability, preventing the phenomenon of graphene sheet re-aggregation for at least 3 months.

According to a preferred embodiment of the invention, step 1) of solubilizing graphite carried out under an inert atmosphere comprises the following steps:
a) Intercalation of at least one alkali metal in graphite leading to a graphite intercalation compound;
b) Chemical exfoliation combined with mechanical exfoliation of the graphite intercalation compound, characterized in that the graphite intercalation compound is mixed with a solvent in a turbulent regime exhibiting:
- a Reynolds number greater than 1000;
- a Froude number less than 1; and
- a shear rate of less than 400s-1, in order to obtain a graphene solution.

Advantageously, the combination of chemical and mechanical exfoliation of a graphite intercalation compound in a turbulent regime characterized by a Reynolds number, a Froude number, and a particular shear rate allows for improved dissolution of the graphite intercalation compound.

More specifically, according to this embodiment, the dissolution of the graphite intercalation compound in the solvent is improved by mechanical exfoliation, preferably obtained through a combination of shear, grinding, and friction. Mechanical exfoliation produces a turbulent regime characterized by a Reynolds number of the system that compares inertial effects to viscous effects, and by a Froude number that compares inertial effects to the effects of gravity. In other words, the Reynolds number, the Froude number, and the shear rate characterize the mechanical exfoliation of step b) for all systems. Therefore, the Froude number, Reynolds number, and shear rate characterize the mechanical exfoliation of step b) necessary to achieve improved dissolution of the graphite intercalation compound in a solvent.

Preferably, the graphite intercalation compound is in the form of a binary compound with the formula KC8.

Chemical exfoliation, on the other hand, is preferably carried out by exposing the graphite intercalation compound to a polar aprotic solvent. Preferably, said polar aprotic solvent has a dielectric constant between 5 and 200.

Preferably, chemical exfoliation is carried out with a compound ratio of graphite intercalation / aprotic polar solvent between 1 and 50g/L.

Advantageously, the ratio of the graphite intercalation compound to the aprotic polar solvent associated with mechanical exfoliation allows for the solubilization of good quality graphene. Preferably, at least one sheet, and even more preferably each sheet of graphene obtained according to the invention, has a sheet thickness of less than 10 atomic layers, preferably less than 5 atomic layers.

The invention also relates to the use of the liquid graphene dispersion according to the invention as an additive, preferably as an electrode conductive additive.

Preferably, the graphene dispersion according to the invention can be used as a conductive electrode additive to improve at least one property of an energy storage device selected from storage capacity, cycling capacity, SEI (solid electrolyte interphase) irreversibility, accessible power, specific capacity, temperature resistance, reliability or combinations thereof.

Finally, the invention also aims to:
*an electrode for an energy storage device comprising a liquid graphene dispersion according to the invention; and
*an energy storage device comprising an electrode according to the invention.

Other features and advantages will become apparent from the detailed description of the invention and the examples which are purely illustrative and in no way limiting of the scope of the invention.

Brief description of the Figures

FIG. 1 There FIG. 1 is a graphical representation of the UV-visible absorption spectrum of different liquid graphene dispersions according to the invention over time (T0 to J+90).

FIG. 2 There FIG. 2 is a graphical representation of the evolution of absorbance over time of different liquid dispersions of graphene according to the invention as a function of the graphene concentration.

FIG. 3 There FIG. 3 is a graphical representation of a RAMAN spectrum of a liquid dispersion of graphene according to the invention preserved for 45 days.

FIG. 4 There FIG. 4 is a graphical representation of normalized discharge curves for a reference electrode comprising super P C65 at different cycling regimes.

FIG. 5 There FIG. 5 is a graphical representation of the comparison of the power handling of the electrodes during discharges with 10% additive (super PC65 or graphene (dispersion according to the invention)).

FIG. 6 There FIG. 6 is a graphical representation of the comparison of the power handling of the electrodes with different proportions of graphene (dispersion according to the invention).

FIG. 7 There FIG. 7 is a graphical representation of the comparison of the power handling of the electrodes with 4% graphene (dispersion according to the invention) and different proportions of PVDF for electrodes having 45% porosity.

FIG. 8 There FIG. 8 is a graphical representation of the comparison of the power handling of electrodes comprising different contents of PVDF and graphene (dispersion according to the invention).

Detailed description of the invention

Definition s

For the purposes of this invention, "electrode conductive additive" means a substance introduced into the composition of an electrode to improve its electrical conductivity. This additive facilitates the transfer of electrons within the electrode, thereby optimizing the electrochemical performance of any energy storage device.

For the purposes of this invention, "aggregate" means an agglomerated structure formed from several sheets of graphene in the 3 directions of space, the size of which is large compared to the dimensions of the graphene sheet.

For the purposes of this invention, "inert atmosphere" means a gas or mixture of gases that does not promote the re-oxidation of reduced graphene sheets into neutral graphene sheets. The process according to the invention can therefore be carried out under an argon or nitrogen atmosphere.

For the purposes of this invention, a "graphite intercalation compound" is defined as a compound comprising at least two individual negatively or positively charged graphene planes intercalated by positive or negative counterions. Graphite alkali salts are a specific type of graphite intercalation compound in which the graphene planes are negatively charged and the counterions are alkali ions. They can be formed by intercalating at least one alkali metal into graphite.

For the purposes of this invention, an "atomic layer" is defined as a layer composed of a single atom in one spatial direction. A graphene sheet consists of at least one atomic layer.

For the purposes of this invention, "liquid dispersion of graphene" means a dispersion of graphene having an absolute viscosity between 5mPa.s and 10Pa.s, preferably between 100mPa.s and 10Pa.s.

For the purposes of this invention, "energy storage devices" are defined as systems capable of accumulating energy in electrochemical, electrostatic, or chemical form and releasing it as electrical energy as needed. This includes, but is not limited to, batteries, supercapacitors, and fuel cells. These devices are designed to provide a sustainable and flexible power source for a variety of applications, ranging from electronic devices to electric vehicles.

For the purposes of this invention, an "electrode" is a conductive component of an energy storage device that allows the flow of electrons to or from an external circuit. The electrode is generally in contact with an active material that participates in electrochemical reactions, thereby facilitating the storage or release of energy within the device.

For the purposes of this invention, "carrier gas" means a main gas in which another gas or liquid, reactive or not, will be diluted and introduced into the medium.

The term "solid electrolyte interface" or "SEI," also referred to as "solid electrolyte interphase" in the context of this invention, refers to a thin layer that forms at the interface between the liquid electrolyte and the electrode during the first charging cycle of a battery, particularly in lithium-ion batteries. This layer consists of electrolyte decomposition products and materials from the electrode. The SEI acts as a barrier, preventing further electrolyte decomposition and thus protecting the electrode from undesirable reactions. Although the SEI is a solid layer, it must be sufficiently conductive to allow the passage of ions (e.g., lithium ions) between the electrolyte and the electrode. A stable SEI helps extend battery life by minimizing the consumption of active material and reducing degradation cycles. The formation and composition of the SEI are crucial aspects for the development of higher-performing and longer-lasting batteries.

The term "Froude number" in the context of this invention refers to the ratio of kinetic energy to gravitational potential energy. The Froude number is defined as follows:

Fr = n².d / g

n, d corresponds to the rotation speed (revolutions/sec) and the diameter of the stirring element (m)

g: acceleration due to gravity (m/s²)

For the purposes of this invention, "Reynolds number" means the ratio of inertial forces to viscous forces. The Reynolds number is defined as follows:
The calculation is performed using the parameters extracted from the process as follows:

Re = n.d².ρ / µ

n, d corresponds to the rotation speed (revolutions/sec) and the diameter of the stirring element (m)

ρ, µ correspond to the density (kg/m3) and the dynamic viscosity of the fluid (Pa.s)

For the purposes of this invention, "PVDF or one of its derivatives" means any polymer obtained from polyvinylidene fluoride (PVDF), or modified by substitution, copolymerization or chemical functionalization, including in particular copolymers of PVDF with other fluorinated monomers, PVDF latexes, as well as grafted or functionalized polymers providing additional properties.

For the purposes of this invention, "turbulent regime" means a flow regime characterized by chaotic changes in pressure and flow velocity.

For the purposes of the invention, "without aprotic polar solvent" means that the liquid dispersion of graphene contains less than 0.1% of aprotic polar solvent.

For the purposes of this invention, "RAMAN spectrophotometry" means a non-destructive vibrational spectroscopy method that allows the determination of the molecular composition and external structure of a material.

For the purposes of this invention, "stable" means that the liquid graphene dispersion does not contain aggregates. Therefore, destabilizing phenomena such as creaming or sedimentation do not occur over time, particularly for at least 3 months.

For the purposes of this invention, "absolute viscosity" refers to the resistance of an incompressible fluid to laminar flow. It is measured experimentally using a rheometer.

For the purposes of this invention, "stable viscosity" means that the viscosity exhibits a limited variation over time, defined by a fluctuation that remains within half a decade on a logarithmic scale. More precisely, this means that the viscosity can vary by a maximum factor of 3.16, i.e., an increase or decrease of between 30% and 50%, without exceeding these limits over a period of at least three months. For example, a viscosity that changes from 25-30 Pa·s to 30-35 Pa·s over the course of a month remains within the stability criteria for this range of variation.

Liquid dispersion of graphene

The present invention therefore relates to a liquid dispersion of graphene comprising a solvent, a polymer, at least 0.1 g/L of graphene and having an absolute viscosity measured at 25°C using a rheometer of between 5mPa.s and 10Pa.s.

Advantageously, such a dispersion is stable, graphene-concentrated, homogeneous, and simple to use.

According to one embodiment, the dispersion according to the invention comprises at least 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 10 g/L of graphene.

Preferably, the dispersion according to the invention comprises between 0.3 and 50g/L, in particular between 0.5 and 50g/L, preferably between 1 and 50g/L, more preferably between 1 and 20g/L, even more preferably between 1 and 10g/L of graphene.

According to one embodiment, graphene represents between 1 and 10% by weight relative to the total weight of the dispersion.

According to one embodiment, graphene represents between 1 and 5% by weight, in particular between 1 and 4% by weight, preferably between 1 and 3% by weight relative to the total weight of the dispersion.

Graphene is present in the form of sheets in the liquid graphene dispersion.

Preferably, the graphene sheets present in the liquid graphene dispersion have a thickness of less than 10 atomic layers, and even more preferably less than 5 atomic layers. In one embodiment, at least one graphene sheet present in the liquid graphene dispersion has a thickness of less than 10 atomic layers, and even more preferably less than 5 atomic layers. Preferably, each sheet present in the liquid graphene dispersion has a thickness of less than 10 atomic layers, and even more preferably less than 5 atomic layers.

In one embodiment, at least one graphene sheet has fewer than four atomic layers, preferably fewer than three atomic layers, and fewer than two atomic layers. In another embodiment, at least one graphene sheet is single-layered, preferably at least two, and in yet another embodiment, all graphene sheets are single-layered.

According to one embodiment, the graphene sheets present in the liquid dispersion exhibit:
- an average thickness of between 1 and 8 nm, in particular between 1 and 5 nm, preferably between 2 and 5 nm, more preferably between 2 and 3 nm and/or
- an average lateral size of at least 200nm, in particular at least 300nm, preferably at least 500nm, more preferably at least 1µm.

Advantageously, the liquid dispersion of graphene according to the invention presents sheets of graphene of good quality.

In the context of the invention, the average thickness can be measured by atomic force microscopy (AFM), by RAMAN spectroscopy, in particular from the full width at half maximum (FWHM) of the 2D band, as well as by transmission electron microscopy (TEM).

Average thickness is a parameter directly related to the number of atomic layers in graphene, making it a good indicator of graphene quality. For example, graphene with an average thickness of 15 nm comprises several dozen atomic layers. At this thickness, graphene loses most of the unique properties of monolayer graphene and becomes more like graphite. Its conductivity may be lower due to the increased interlayer resistance, and its specific surface area is also reduced, limiting its effectiveness in applications where maximum interaction with the environment is desired (such as additives in electrodes or composite materials). In terms of dispersion, thicker graphene may have a greater tendency to agglomerate, which can negatively impact its overall performance.

Preferably, the graphene sheets present in the liquid dispersion have an average thickness of less than 10nm, in particular less than 8nm, preferably less than 5nm, more preferably less than 3nm, even more preferably less than 2nm.

Lateral size also influences the conductivity of graphene as well as its ability to disperse in solutions or composites. Graphene with a large lateral size offers a longer conductive path and reduces the number of junctions between the sheets, which can improve conductivity. However, excessively large lateral sizes can impair the material's ability to disperse uniformly in certain matrices, potentially limiting its applications in composites or electrodes.

According to one embodiment, the graphene sheets present in the liquid dispersion have an average lateral size between 300nm and 5µm, in particular between 300nm and 2µm, preferably between 300nm and 2µm, more preferably between 500nm and 800nm.

According to a particular embodiment, the graphene sheets present in the liquid dispersion have an average lateral size of between 1 and 2µm.

In the context of the invention, the average lateral size of the graphene sheets is measured by transmission electron microscopy or by atom shape microscopy.

The liquid graphene dispersion according to the invention is stable, preferably for a period of at least 3 months. Thus, for at least 3 months, the liquid graphene dispersion according to the invention exhibits no aggregation, in particular no aggregation resulting from the reaggregation of graphene sheets. In the context of the invention, it is possible to verify the stability of the liquid dispersion over time by monitoring the viscosity. Indeed, if the viscosity increases significantly, that is, a viscosity fluctuation exceeding half a decade on a logarithmic scale over time, this indicates that graphene reaggregation is occurring.

Thus, according to a preferred embodiment of the invention, the liquid dispersion of graphene exhibits a stable viscosity, preferably for a period of at least 3 months.

Preferably, the liquid dispersion of graphene exhibits an absolute viscosity measured at 25°C under the effect of a shear of 10s-1 using a rheometer between 100mPa.s and 10Pa.s.

Advantageously, the particular viscosity of the liquid graphene dispersion according to the invention allows it to guarantee a particularly high stability and graphene concentration.

Preferably, the dispersion according to the invention is ready for use. Indeed, it requires no preparation step and can be used as is, particularly for uses as an additive, preferably as an electrode conductive additive.

According to one embodiment, the liquid graphene dispersion according to the invention comprises a polymer representing between 2 and 10% by weight, in particular between 2 and 5% by weight relative to the total weight of the dispersion.

Preferably, the dispersion according to the invention comprises a polymer selected from Polyvinylidene Fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene propylene diene monomer rubber (EPDM), or carboxymethylcellulose (CMC), nitrile butadiene rubber (NBR), styrene butadiene rubber (SBR), lithium polyacrylate (LiPAA), sodium polyacrylate (NaPAA), poly(acrylic acid) (PAA), polyimide, polyamide, sodium alginate, lithium alginate and combinations thereof.

According to a particularly preferred embodiment, the dispersion according to the invention comprises Polyvinylidene Fluoride (PVDF).

According to one variant, the dispersion according to the invention comprises Polyvinylidene Fluoride (PVDF) or one of its derivatives.

According to one embodiment, the liquid graphene dispersion according to the invention comprises a solvent representing between 80 and 97% by weight relative to the total weight of the dispersion.

According to one embodiment, the dispersion according to the invention comprises a polar aprotic solvent.

According to a preferred embodiment, the dispersion according to the invention comprises a polar aprotic solvent having a dielectric constant between 5 and 200.

According to a preferred embodiment, the dispersion according to the invention comprises a polar aprotic solvent selected from dimethylformamide (DMF), dimethyl sulfoxide (DMSO), propylene carbonate (PC), acetonitrile, tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), 2-methyltetrahydrofuran (Me-THF), cyclopentylmethylether and their combinations.

Preferably, the dispersion according to the invention comprises N-methyl-2-pyrrolidone (NMP).

According to one embodiment, the dispersion according to the invention comprises N-methyl-2-pyrrolidone (NMP) and polyvinylidene fluoride (PVDF).

Depending on the nature of the polymer and the solvent used, a person skilled in the art is able to select a suitable method to verify the absence of aggregates, in particular by optical microscopy, by correlation between microscopy and observation of the Tyndall effect, by measurement of conductivity, by monitoring of viscosity or by spectrophotometry.

According to a preferred embodiment of the invention, the liquid dispersion of graphene exhibits an absorption spectrum in UV-visible spectroscopy comprising a peak at 269 nm.

Absorbance can be measured using a UV spectrophotometer. UV spectroscopy is one of the techniques used according to the invention to ensure the absence of aggregates.

Advantageously, the presence of a peak at 269 nm is a parameter describing the presence of graphene. It is thus an indicator of the quality of the dispersion, demonstrating that the solution contains graphene and not graphite or oxidized graphene. Therefore, the presence of a peak at 269 nm is an indirect indicator of the stability of the liquid graphene dispersion.

According to another embodiment, the liquid graphene dispersion according to the invention has a UV-visible absorption spectrum that does not include a peak at 230nm.

When the liquid graphene dispersion includes a peak at 230nm in its UV-visible absorption spectrum, this indicates the presence of graphene oxide, an undesirable element at this stage of the process.

The liquid dispersion of graphene according to the invention can be characterized using 3 vibrational bands characteristic of graphene observed by RAMAN spectrophotometry, namely:
- 1350 cm-1 (pic D);
- 1580 cm-1 (peak G); and
- between 2680 and 2700 cm-1 (peak 2D).

These three peaks are characteristic of a graphitic signature and demonstrate the presence of non-functionalized and non-oxidized graphene.

Thus, according to a particularly preferred embodiment, the liquid dispersion of graphene comprises 3 vibrational bands characteristic of graphene observed by RAMAN spectrophotometry:
- 1350 cm-1 (peak D);
- 1580 cm-1 (peak G); and
- between 2680 and 2700 cm-1 (peak 2D).

The presence of peak D indicates disorder or a defect (sp3) in the graphene sheets. Peak G indicates the presence of graphene (sp2 vibration in the plane). Finally, peak 2D indicates the number of layers. When peak 2D is narrow, symmetrical, and intense, it indicates a single sheet. When peak 2D is broad, has a shoulder, and is of low intensity, it indicates graphite.

Thus, the intensity of the vibrational bands of the liquid graphene dispersion can be used to define the quality of said dispersion.

According to one embodiment, the liquid dispersion of graphene comprises:
- a peak intensity D / peak intensity G ratio of less than 1.5; and
- a 2D peak intensity / G intensity ratio greater than 1.

According to another embodiment, the liquid dispersion of graphene comprises:
- a peak D with a full width at half maximum (FWHM) of less than 33 cm-1; and/or
- a 2D peak with a full width at half maximum less than 55cm-1, preferably less than 50cm-1.

Advantageously, the intensity and full width at half maximum of the vibrational bands allow us to demonstrate the quality of the graphene present in the dispersion.

UV spectroscopy and RAMAN spectrophotometry are parameters that can be measured over time to show the stability and/or quality of the liquid graphene dispersion according to the invention.

According to a preferred embodiment of the invention, the liquid dispersion of graphene can be obtained by the process for obtaining a liquid dispersion described or by any other embodiment.

Thus, liquid graphene dispersion can be obtained by implementing the following steps:
1) Solubilization of graphite carried out under an inert atmosphere to obtain a graphene solution by implementing the following steps:
a) Intercalation of at least one alkali metal into graphite carried out under an inert atmosphere leading to a graphite intercalation compound; and
b) Chemical exfoliation and/or mechanical exfoliation carried out under an inert atmosphere of the graphite intercalation compound, so as to obtain a graphenide solution;
2) Oxidation of the graphene solution obtained in step b) to obtain an organic dispersion of graphene; and
3) Transfer of the organic dispersion of graphene into a polymer forming a liquid dispersion of graphene.

Preferably, liquid graphene dispersion can be obtained by implementing the following steps:
1) Solubilization of graphite carried out under an inert atmosphere to obtain a graphene solution by implementing the following steps:
a) Intercalation of at least one alkali metal into graphite carried out under an inert atmosphere leading to a graphite intercalation compound;
b) Chemical exfoliation and/or mechanical exfoliation carried out under an inert atmosphere of the graphite intercalation compound, so as to obtain a graphenide solution;
b') Removal of aggregates from the graphene solution obtained in step b), preferably by sedimentation and/or centrifugation. More specifically, step b') may include the following steps:
- Sedimentation and rejection of aggregates located in the lower part of the graphene solution and/or
- Centrifugation and rejection of aggregates located in the lower part of the graphene solution.
2) Oxidation of the aggregate-free graphene solution obtained in step b') to obtain an organic graphene dispersion; and
3) Transfer of the organic graphene dispersion into a polymer forming a liquid graphene dispersion; and
4) Evaporation and/or distillation of the aprotic polar solvent.

According to a particularly preferred embodiment, the liquid dispersion of graphene can be obtained by carrying out the following steps:
1) Solubilization of graphite carried out under an inert atmosphere to obtain a graphene solution by implementing the following steps:
a) Intercalation of at least one alkali metal into graphite carried out under an inert atmosphere leading to a graphite intercalation compound;
b) Chemical exfoliation combined with mechanical exfoliation carried out under an inert atmosphere of the graphite intercalation compound, characterized in that the graphite intercalation compound is mixed with a solvent in a turbulent regime exhibiting:
- a Reynolds number greater than 1000;
- a Froude number less than 1; and
- a shear rate of less than 400s-1, in order to obtain a graphene solution;
b') Removal of aggregates from the graphene solution obtained in step b), preferably by sedimentation and/or centrifugation. More specifically, step b') may include the following steps:
- Sedimentation and rejection of aggregates located in the lower part of the graphene solution and/or
- Centrifugation and rejection of aggregates located in the lower part of the graphene solution.
2) Oxidation of the aggregate-free graphene solution obtained in step b') to obtain an organic dispersion of graphene;
3) Transfer of the organic graphene dispersion into a polymer forming a liquid graphene dispersion; and
4) Evaporation and/or distillation of the aprotic polar solvent.

According to a particular embodiment, the dispersion according to the invention comprises:
*between 80 and 97% solvent by weight relative to the total weight of the dispersion;

* between 2 and 10% polymer by weight relative to the total weight of the dispersion; and

* between 1 and 10% graphene by weight relative to the total weight of the dispersion.

According to a particular embodiment, the dispersion according to the invention comprises:
- N-methyl-2-pyrrolidone (NMP); and
- Polyvinylidene fluoride (PVDF).

According to a particular embodiment, the dispersion according to the invention comprises:
- between 2 and 10%, preferably between 3 and 6%, in particular between 3 and 5% of Polyvinylidene Fluoride (PVDF) by weight relative to the total weight of the dispersion;
- between 80 and 97%, preferably between 90 and 95%, in particular between 91 and 94% of N-methyl-2-pyrrolidone (NMP) by weight relative to the total weight of the dispersion; and
- between 1 and 10%, preferably between 2 and 8%, in particular between 3 and 5% of graphene by weight relative to the total weight of the dispersion.

Process obtaining a liquid dispersion graphene

The invention also relates to a method for obtaining a liquid dispersion of graphene.

The process for obtaining a liquid dispersion of graphene according to the invention comprises the following steps:
1) Solubilization of graphite carried out under an inert atmosphere forming a graphene solution;
2) Oxidation of the graphene solution obtained in step 1) forming the organic graphene dispersion; and
3) Transfer of the organic graphene dispersion obtained in step 2) into a polymer forming a liquid graphene dispersion.

Advantageously, the process of obtaining a liquid graphene dispersion makes it possible to obtain a stable, aggregate-free liquid graphene dispersion containing up to 50g/L of graphene.

Preferably, step 1) of graphite solubilization carried out under an inert atmosphere includes the implementation of the following steps:
a) Intercalation of at least one alkali metal into graphite leading to a graphite intercalation compound; and
b) Chemical exfoliation and/or mechanical exfoliation of the graphite intercalation compound.

According to a preferred embodiment, the graphite solubilization process carried out under an inert atmosphere in step 1) comprises the following steps, carried out under an inert atmosphere:
a) Intercalation of at least one alkali metal in graphite leading to a graphite intercalation compound;
b) Chemical exfoliation combined with mechanical exfoliation of the graphite intercalation compound, characterized in that the graphite intercalation compound is mixed with a solvent in a turbulent regime exhibiting:
- a Reynolds number greater than 1000;
- a Froude number less than 1; and
- a shear rate of less than 400s-1,
in order to obtain a graphene solution.

According to another embodiment, the graphite solubilization process carried out under an inert atmosphere in step 1) comprises the following steps:
a) Intercalation of at least one alkali metal in graphite leading to a graphite intercalation compound;
b) Chemical exfoliation of the graphite intercalation compound using a solvent, preferably using an aprotic polar solvent, preferably NMP, to obtain a graphenide solution.

According to another embodiment, the graphite solubilization process carried out under an inert atmosphere in step 1) comprises the following steps:
a) Intercalation of at least one alkali metal by graphite leading to a graphite intercalation compound;
b) Mechanical exfoliation of the graphite intercalation compound characterized in that the graphite intercalation compound is mixed in a turbulent regime exhibiting:
- a Reynolds number greater than 1000;
- a Froude number less than 1; and
- a shear rate of less than 400s-1,
in order to obtain a graphene solution.

Advantageously, the graphite solubilization process according to the invention makes it possible to efficiently solubilize graphite in order to produce a graphene solution comprising a sufficient concentration of good quality graphene for industrial use.

The graphite used in step a) can be natural or synthetic graphite.

Preferably, the graphite of step a) has a carbon content of at least 99%. Preferably, the graphite of step a) has a particle size between 100 µm and 5 mm, in particular between 300 and 800 µm.

According to a preferred embodiment, at least one alkali metal intercalated in graphite in step a) is chosen from potassium, sodium, lithium, rubidium, and cesium. Preferably, at least one alkali metal intercalated in graphite in step a) is potassium.

In another embodiment, the intercalation in step a) is carried out in the presence of an alkali metal salt obtained from an alkali metal. For example, the intercalation can be carried out in the presence of an alkali polyaryl salt of formula A+B-, in which A+ represents a cation of an alkali ion, and B- represents an anion of a polyaromatic compound.

Such alkali polyaryl salts and their manufacturing process are described for example in (C. Stein, J. Poulenard, L. Bonnetain, J. Golé, C.R. Acad. Sci. Paris 260, 4503 (1965); “Synthesis of graphite intercalation compounds”, A. Hérold in Chemical physics of intercalation, A.P. Legrand and S. Flandrois Eds, NATO ASI Series, series B, Vol. 172, pp. 3-45 (1987); F. Béguin and R. Setton New ternary lamellar compounds of graphite, Carbon 13, 293-295 (1975).

According to one embodiment, the polyaromatic compound is chosen from the group comprising naphthalene, benzophenone, fluorenone, benzoquinone and anthraquinone.

In one particular embodiment, the polyaromatic compound is naphthalene.

In another particular embodiment, the alkali polyaryl salt is a potassium polyaryl salt (that is, a salt of formula A+B-, in which A+ represents K+).

Advantageously, the alkali polyaryl salt of formula A+B-, is a potassium salt of naphthalene (Naph- K+).

Thus, after the intercalation of at least one alkali metal into graphite, the graphene sheets constituting the graphite intercalation compound are negatively charged; this is then referred to as graphenide. These negative charges contribute to improving the solubility of the graphene sheets.

Preferably, the graphite intercalation compound of step a) is in the form of a binary compound of formula KC8.

According to a particular embodiment of the invention, the graphite intercalation compound obtained in step a) has an inter-sheet graphene distance of at least 5 Ångström.

The measurement of the inter-sheet distance of graphene can be carried out under an inert atmosphere by methods well known to those skilled in the art, such as X-ray diffraction.

Advantageously, when step b) includes chemical exfoliation, this allows the interlayer Van der Waals interactions of graphite to be replaced by electrostatic interactions. Thus, when exposed to a suitable solvent, the entropy of the counterions of the electrostatic bonds results in a negative free energy of dissolution. In other words, exposing the intercalating compound to a suitable solvent facilitates its solubilization.

According to a preferred embodiment, the chemical exfoliation of step b) is carried out by exposing the graphite intercalation compound to a polar aprotic solvent. Preferably, said polar aprotic solvent has a dielectric constant between 5 and 200.

Preferably, the aprotic polar solvent is chosen from: dimethylformamide (DMF), dimethyl sulfoxide (DMSO), propylene carbonate (PC), acetonitrile, tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), 2-methyltetrahydrofuran (Me-THF), cyclopentylmethyl ether and their combinations.

According to a particularly preferred embodiment, the aprotic polar solvent is NMP.

Preferably, the chemical exfoliation of step b) is carried out with a compound ratio of graphite intercalation / solvent, preferably aprotic polar solvent, of between 1 and 50g/L.

When step b) includes the combination of chemical and mechanical exfoliation, modulation of the ratio of graphite intercalation compound to solvent, preferably aprotic polar solvent, is important to maximize the energy transfer associated with the turbulent regime generated by mechanical exfoliation to the graphite intercalation compound.

Initially, the concentration of graphite intercalating compound is high relative to the solvent volume, preferably aprotic polar. This initial concentration allows for improved exfoliation by combining the shear effect with the friction effect between the graphite intercalating compounds and the grinding medium (paddle mixer, ball mill, magnetic stir bar, agitated attrition or detritifier, or an "Ultra-Turrax").

The combination of the shear/friction/grinding effect can be measured in the mixture comprising the graphite intercalation compound and the solvent, preferably aprotic polar solvent, by measuring the velocity and then by calculating the Reynolds number and Froude number.

Step b) of chemical exfoliation, when combined with mechanical exfoliation, is carried out in a mixture comprising the graphite intercalation compound and a solvent in a turbulent regime having a Reynolds number greater than 1000 and a Froude number less than 1 and a shear rate less than 400s-1.

Unexpectedly, the combination of chemical and mechanical exfoliation of the graphite intercalation compound in a turbulent regime exhibiting a Reynolds number greater than 1000, a Froude number less than 1, and a shear rate less than 400s-1 allows for improved dissolution of the graphite intercalation compound, particularly in the solvent, preferably a polar aprotic solvent.

Preferably, shearing is carried out for 24 to 200 hours.

The shear rate can be measured by torque measurement or evaluated by correlation.

The mechanical exfoliation of step b) can be carried out by any means, in particular any means enabling a Reynolds number greater than 1000, a Froude number less than 1, and a shear rate less than 400s-1 to be obtained.

The Reynolds number, Froude number and shear rate required for mechanical exfoliation can be obtained using a device selected from a paddle mixer, ball mill, magnetic bar, agitated attrition or detritifier or an “Ultra-Turrax”.

Thus, the graphite solubilization process according to the invention makes it possible to obtain graphene sheets with a large lateral size, preferably greater than 300nm, in particular greater than 600nm, and even more preferably greater than 900nm.

The measurement of the lateral size of the sheets according to the invention can be carried out by transmission electron microscopy, by atomic force microscopy or by measurement of light scattering.

Preferably, the graphite solubilization process includes a step b’) of aggregate removal, occurring after step b).

Step b’) of aggregate removal advantageously allows the removal of any aggregates including non-exfoliated graphite intercalation compounds.

Preferably, step b') includes the following steps:
- Sedimentation and rejection of aggregates located in the lower part of the graphene solution and/or
- Centrifugation and rejection of aggregates located in the lower part of the graphene solution.

Advantageously, step b’) of removing aggregates makes it possible to obtain an aggregate-free graphene solution.

The presence of aggregates in the graphene solution can be verified by various methods, including:
-By optical microscopy; A sample of the graphene solution can be taken at different intervals during the centrifugation or sedimentation step to determine when a solution free of aggregates has been obtained. Examination under an optical microscope allows the detection of any aggregates with a minimum size on the order of a micron. In a particular embodiment, the sample of the solution can be analyzed under an optical microscope with a magnification of 50x to 100x.
- By correlation between microscopy and observation of the Tyndall effect; the passage of a laser through the graphene solution makes it possible to ensure that it contains particles in solution at a scale small enough that they are not visible under microscopy.
- By correlation between microscopy and Raman spectroscopy; the observation of a specific wavelength of carbon, preferably at 1064nm, makes it possible to ensure the presence of carbon not visible in microscopy.
- Measurement of conductivity; Since the graphene sheets are charged in the graphene solution, it is possible to measure the conductivity of the graphene solution through the contribution of the graphene and the counter-ions to it.

According to one embodiment, step 1) of solubilizing graphite under an inert atmosphere comprises the following steps:
a) Intercalation of at least one alkali metal in graphite leading to a graphite intercalation compound;
b) Chemical exfoliation and/or mechanical exfoliation of the graphite intercalation compound, so as to obtain a graphenide solution
b') Removal of aggregates from the graphene solution obtained in step b), preferably by sedimentation and/or centrifugation.

Preferably, step 1) of graphite solubilization carried out under an inert atmosphere comprises the following steps:
a) Intercalation of at least one alkali metal in graphite leading to a graphite intercalation compound;
b) Chemical exfoliation combined with mechanical exfoliation of the graphite intercalation compound, characterized in that the graphite intercalation compound is mixed with a solvent in a turbulent regime exhibiting:
- a Reynolds number greater than 1000;
- a Froude number less than 1; and
- a shear rate of less than 400s-1, in order to obtain a graphene solution;
b') Removal of aggregates from the graphene solution obtained in step b), preferably by sedimentation and/or centrifugation

The oxidation of the graphene solution from step 2) can be carried out using a gas mixture comprising oxygen and at least one carrier gas. Preferably, the carrier gas is chosen from among the neutral gases, in particular nitrogen or argon.

Preferably, the gas mixture comprises a mixture of oxygen and nitrogen.

Advantageously, the gas composition and humidity are controlled to oxidize the graphene solution.

Oxidation of the graphene solution neutralizes the negative charges of the graphene sheets, making them dispersible in aqueous solvents.

Preferably, the oxidation of the graphene solution in step 2) is carried out using a gas mixture comprising between 70 and 85% nitrogen and 30 to 15% oxygen.

Advantageously, the gas mixture ensures stability and repeatability of the process without depending on the quality of the ambient air.

According to one variant, the invention relates to a process for obtaining a liquid dispersion of graphene comprising an oxidation step in synthetic air.

Step 3) of transferring the organic graphene dispersion is preferably carried out in a viscous matrix having an absolute viscosity measured at 25°C using a rheometer between 5mPa.s and 10Pa.s, preferably between 100mPa.s and 10Pa.s.

Advantageously, the viscous matrix prevents the phenomenon of reaggregation of the graphene sheets.

According to one embodiment, the polymer of step a) is chosen from Polyvinylidene Fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene propylene diene monomer rubber (EPDM), or carboxymethylcellulose (CMC), nitrile butadiene rubber (NBR), styrene butadiene rubber (SBR), lithium polyacrylate (LiPAA), sodium polyacrylate (NaPAA), poly(acrylic acid) (PAA), polyimide, polyamide, sodium alginate, lithium alginate and combinations thereof.

Preferably, the transfer of the organic graphene dispersion into a polymer does not include a degassing step.

In a particular embodiment of the invention, the process for obtaining a liquid dispersion of graphene includes an additional step 4) of evaporation and/or distillation of the aprotic polar solvent.

The liquid graphene dispersion obtained in step 4) includes a portion of aprotic polar solvent. To remove the remaining proportion of aprotic polar solvent, step 4) includes at least one evaporation and/or distillation.

Preferably, step 4) should include at least:
- evaporation over a period of between 6 and 10 days, preferably 7 days; and/or
- a distillation at an evaporation rate of between 100mL/h and 2L/h, preferably 500mL/h.

Advantageously, step 4) allows obtaining a liquid dispersion of graphene without aprotic polar solvent.

According to one embodiment, the process for obtaining a liquid dispersion of graphene comprises the following steps:
1) Solubilization of graphite carried out under an inert atmosphere to obtain a graphene solution by implementing the following steps:
a) Intercalation of at least one alkali metal into graphite carried out under an inert atmosphere leading to a graphite intercalation compound;
b) Chemical exfoliation or mechanical exfoliation carried out under an inert atmosphere of the graphite intercalation compound, so as to obtain a graphenide solution:
b') Removal of aggregates from the graphene solution obtained in step b), preferably by sedimentation and/or centrifugation. More specifically, step b') may include the following steps:
- Sedimentation and rejection of aggregates located in the lower part of the graphene solution and/or
- Centrifugation and rejection of aggregates located in the lower part of the graphene solution.
2) Oxidation of the aggregate-free graphene solution obtained in step b') to obtain an organic dispersion of graphene;
3) Transfer of the organic graphene dispersion into a polymer forming a liquid graphene dispersion; and
4) Evaporation and/or distillation of the aprotic polar solvent.

According to one embodiment, the liquid graphene dispersion obtained at the end of step 3) and/or 4) has an absolute viscosity measured at 25°C using a rheometer of between 5mPa.s and 10Pa.s, preferably between 100mPa.s and 10Pa.s.

According to a preferred embodiment, the process for obtaining a liquid dispersion of graphene comprises the following steps:
1) Solubilization of graphite carried out under an inert atmosphere to obtain a graphene solution by implementing the following steps:
a) Intercalation of at least one alkali metal by graphite carried out under an inert atmosphere leading to a graphite intercalation compound;
b) Chemical exfoliation combined with mechanical exfoliation carried out under an inert atmosphere of the graphite intercalation compound, characterized in that the graphite intercalation compound is mixed with a solvent in a turbulent regime exhibiting:
- a Reynolds number greater than 1000;
- a Froude number less than 1; and
- a shear rate of less than 400s-1, in order to obtain a graphene solution;
b') Removal of aggregates from the graphene solution obtained in step b), preferably by sedimentation and/or centrifugation. More specifically, step b') may include the following steps:
- Sedimentation and rejection of aggregates located in the lower part of the graphene solution and/or
- Centrifugation and rejection of aggregates located in the lower part of the graphene solution.
2) Oxidation of the aggregate-free graphene solution obtained in step b') to obtain an organic dispersion of graphene;
3) Transfer of the organic graphene dispersion into a polymer forming a liquid graphene dispersion; and
4) Evaporation and/or distillation of the aprotic polar solvent

According to one variant, when the process for obtaining a liquid dispersion of graphene includes step 4), said process may include an optional step 5) of recycling the aprotic polar solvent.

Step 5) of recycling the aprotic polar solvent may include the following steps:
- Recovery of vapors from the solvent evaporated or distilled during step 4);
- Purification by membrane separation or extractive distillation;
- possibly reuse in the process by injection of the recycled aprotic polar solvent in step b).

Advantageously, step 5) reduces solvent losses and lowers production costs of the liquid graphene dispersion.

According to another embodiment of the invention, the process for obtaining a liquid dispersion of graphene comprises the following steps:
1) Solubilization of graphite carried out under an inert atmosphere to obtain a graphene solution by implementing the following steps:
a) Intercalation of at least one alkali metal by graphite carried out under an inert atmosphere leading to a graphite intercalation compound;
b) Chemical exfoliation combined with mechanical exfoliation carried out under an inert atmosphere of the graphite intercalation compound in a viscous matrix characterized in that the graphite intercalation compound is mixed with a solvent in a turbulent regime exhibiting:
- a Reynolds number greater than 1000;
- a Froude number less than 1; and
- a shear rate of less than 400s-1, in order to obtain a graphene solution;
b') Removal of aggregates from the graphene solution obtained in step b), preferably by sedimentation and/or centrifugation. More specifically, step b') may include the following steps:
- Sedimentation and rejection of aggregates located in the lower part of the graphene solution and/or
- Centrifugation and rejection of aggregates located in the lower part of the graphene solution.
2) Oxidation of the aggregate-free graphene solution obtained in step b') to obtain an organic dispersion of graphene.

Thus, according to one variant, step b) of chemical and/or mechanical exfoliation is carried out on a compound intercalating graphite in a viscous matrix. Step 3) of the process is then carried out simultaneously with step b).

Use s

According to another aspect, the invention relates to the use of graphene dispersion according to any of the previously described embodiments as an additive, preferably as an electrode conductive additive.

Advantageously, the dispersion according to the invention is particularly well-suited to such an application. Indeed, its high concentration of high-quality graphene, its stability, its lack of aggregation, and its homogeneity make it a particularly suitable conductive electrode additive.

Preferably, the graphene dispersion according to the invention can be used as a conductive electrode additive to improve at least one property of an energy storage device selected from storage capacity, cycling capacity, SEI (solid electrolyte interphase) irreversibility, accessible power, specific capacity, temperature resistance, reliability or combinations thereof.

Preferably, the energy storage device is chosen from a supercapacitor, a fuel cell or a battery.

When the said energy storage device is a battery, the battery can be chosen from a lithium ion battery, a lithium polymer battery, a lithium air battery or a sodium ion battery.

Advantageously, the dispersion according to the invention is particularly suited for use as an electrode conductive additive for batteries, in particular lithium ion batteries, lithium polymer batteries, lithium air batteries or sodium ion batteries.

Advantageously, the solvent and polymer of the dispersion according to the invention are suitable for use as a conductive electrode additive. For example, the polymer, such as NMP, ensures mechanical resistance, particularly through increased viscosity, to cycling (charging/discharging) when used as a conductive electrode additive.

According to one embodiment, the dispersion according to the invention can be used as a conductive electrode additive for an energy storage device, in particular a battery, preferably a lithium ion battery.

According to a particular embodiment, the invention relates to the use, as a conductive electrode additive, of a dispersion according to the invention comprising:
* a solvent selected from dimethylformamide (DMF), dimethyl sulfoxide (DMSO), propylene carbonate (PC), acetonitrile, tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), 2-methyltetrahydrofuran (Me-THF), cyclopentylmethyl ether and their combinations;
*a polymer selected from Polyvinylidene Fluoride (PVDF), Polytetrafluoroethylene (PTFE), Ethylene Propylene Diene Monomer Rubber (EPDM), or Carboxymethylcellulose (CMC), Nitrile Butadiene Rubber (NBR), Styrene Butadiene Rubber (SBR), Lithium Polyacrylate (Li-PAA), Sodium Polyacrylate (NaPAA), Poly(acrylic acid) (PAA), Polyimide, Polyamide, Sodium Alginate, Lithium Alginate and combinations thereof.

According to a particular embodiment, the invention relates to the use as a conductive electrode additive of a dispersion according to the invention comprising N-methyl-2-pyrrolidone (NMP), and polyvinylidene fluoride (PVDF).

According to a particular embodiment, the invention relates to the use, as a conductive electrode additive, of a dispersion according to the invention comprising
- between 2 and 10%, preferably between 3 and 6%, in particular between 3 and 5% of Polyvinylidene Fluoride (PVDF) by weight relative to the total weight of the dispersion;
- between 80 and 97%, preferably between 90 and 95%, in particular between 91 and 94% of N-methyl-2-pyrrolidone (NMP) by weight relative to the total weight of the dispersion; and
- between 1 and 10%, preferably between 2 and 8%, in particular between 3 and 5% graphene by weight relative to the total weight of the dispersion
as a conductive electrode additive.

Electrode and battery

The invention also relates to an electrode for an energy storage device comprising the dispersion according to the invention.

Advantageously, the electrode according to the invention benefits from improved properties thanks to the presence of the dispersion according to the invention. Indeed, the dispersion according to the invention makes it possible, in particular, to improve at least one property of the electrode chosen from among electrical conductivity, cyclic stability, homogeneity of current distribution, accessible power, specific capacitance, temperature resistance, and combinations thereof.

According to one embodiment, the electrode comprises between 1 and 10% by mass of dispersion according to the invention relative to the total mass of the electrode.

According to a particular embodiment, the electrode comprises 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% by mass of dispersion according to the invention relative to the total mass of the electrode.

According to one embodiment, the electrode according to the invention comprises:
* between 1 and 10% by mass of dispersion according to the invention relative to the total mass of the electrode;
*between 1 and 10% by mass of polymer, preferably PVDF, relative to the total mass of the electrode; and
*between 80 and 98% by mass of active material, preferably graphite or any material suitable for forming an electrode for an energy storage device, relative to the total mass of the electrode.

According to a preferred embodiment, the electrode comprises between 1 and 10% by mass of PVDF relative to the total mass of the electrode, in particular between 1 and 5% by mass of PVDF relative to the total mass of the electrode.

Advantageously, the presence of PVDF within the electrode ensures the mechanical stability of the graphene sheets on the electrode. However, an excessive concentration of PVDF, particularly above 10%, is counterproductive to the electrode's performance.

The electrode according to the invention is adapted for an energy storage device chosen from a supercapacitor, a battery or a fuel cell.

According to one embodiment, the invention relates to an electrode for a battery, preferably a lithium ion battery comprising the dispersion according to the invention.

According to a particular embodiment, the electrode according to the invention can be obtained by carrying out the following steps:
*Grinding of an active material to obtain a powder
*Mixing of the powder with the graphene dispersion according to the invention
*Optionally, solvent and/or polymer can be added to adapt the viscosity to the electrode deposition method.
*Pouring of the resulting mixture; and
*oven drying to obtain the electrode according to the invention.

Finally, the invention also relates to an energy storage device comprising an electrode according to the invention.

According to one embodiment, the energy storage device is chosen from a battery, a fuel cell or a supercapacitor.

Preferably, the storage device according to the invention is a battery.

When the energy storage device is a battery, said battery is chosen from a lithium ion battery, a lithium polymer battery, a lithium air battery or a sodium ion battery.

Advantageously, the energy storage device according to the invention has improved properties, including improved electrical conductivity, increased energy density, and optimized high-speed charge/discharge performance.

Examples

Example 1: Measurement of the stability of a dispersion according to the invention

For the realization of this example, a graphene dispersion according to the invention comprising NMP and PVDF was used.

Different amounts of graphene were added to a liquid dispersion according to the invention comprising 5% PVDF by weight relative to the total weight of the dispersion in NMP in order to evaluate the stability and quality of the graphene dispersion over time.

First, the absorbance of the dispersions was measured by diluting the graphene dispersion in NMP until the absorbance was less than one. The absorbance profile shows that the NMP solvent absorbs in the UV range from 200 to 300 nm. This masks the graphene absorbance peak at 269 nm. Since the solvent is transparent in the visible range (300–800 nm), the signal from the graphene can be tracked over time ( FIG. 1 ).

The evolution of absorbance ( FIG. 2 Over time, the absorbance value shows a decrease of approximately 10-15% within the first fifteen days, with stabilization occurring within 70-80% of the initial value. This is consistent with visual observations where a slight sediment is visible in the samples after some time.

Thus, even with a concentration as high as 5%, the graphene dispersion is stable over a period of 45 days, with only a fraction of the dispersed material settling during this period.

Raman spectroscopy is performed on dried graphene droplets deposited on a substrate obtained from the PVDF-NMP dispersion. FIG. 3 is prepared from a formulation stored for 45 days. The analysis is performed on three areas with a minimum of fifty acquisitions for each area. The average spectrum described in the FIG. 3 shows the typical vibration bands of graphene: the D band at 1350cm-1, the G band at 1580cm-1 and the 2D band at 2690cm-1.

Peak fitting yields a full width at half maximum (FWHM) of 34.1 cm⁻¹ for peak D, 45.0 cm⁻¹ for peak 2D, and intensity ratios of 0.8 for peak D/G and 0.6 for peak 2D/G. These values reflect the high quality of the graphene spectrum.

Example 2:

The electrodes used in this example were manufactured in a laboratory, which means they do not have the same quality characteristics as industrially produced electrodes.

The performance of the two types of electrodes was evaluated by adding two additives, namely either the liquid dispersion of graphene according to the invention, or Super P C65, a reference carbon material.

All electrodes were made on 1 g, in order to have homogeneous solutions and inks.

Electrode fabrication protocol

The electrodes in this example were made in the following way:
Weigh the graphite and then grind it in a mortar.
*Take the necessary amount of graphene solution to obtain the desired proportions, and redisperse with ultra-turrax for a few minutes;
*Add NMP, if necessary, to get the right viscosity then mix with ultra-turrax until a homogeneous ink is obtained;
*Filling of the ink, then evaporation of the NMP in an 80°C oven;
*Cutting of the electrodes, and vacuum drying for 1 night.

The active material (graphite) loadings are between 3 and 3.5 mg/cm2.

The initial porosity of the electrodes is 75%, it was reduced to 45% by calendering.

A – Electrochemical study:

To perform electrochemical analyses, the method used is galvanostatic battery cycling. Galvanostatic cycling consists of passing a constant current through an electrochemical cell during charge and discharge cycles.

The different objectives are to measure the power handling of the batteries and to evaluate the lifespan of the cells.

The protocol used is as follows:
*A constant negative current is applied to discharge the cell down to a potential of 20 mV;
*A constant potential of 20 mV is applied for 30 minutes to determine the discharge of the cell, while avoiding the deposition of lithium metal.
*A constant positive current is applied to charge the cell up to a potential of 1.2 V.
These cycling steps are repeated at different cycling regimes.

The chosen current is calculated based on the mass of the electrode, the chosen cycling regime and the maximum capacitance of the material, which is 0.08mA in the case of these electrodes.

The first load curve is described at the FIG. 4 .

Several phenomena are observed that occur during the first reduction of graphite.

The plateaus formed at different potentials are associated with the different stages of lithium insertion. At a potential of 0.8 V, a first plateau is observed, which corresponds to the formation of the SEI.

Indeed, during the reduction process, the electrolyte shrinks and forms a passivation layer on the surface of the graphite electrode. This SEI (Selective Electrolyte Implant) forms a protective layer that prevents electrode corrosion and electrolyte degradation. This layer forms only during the first few cycles (primarily during the first discharge) and is crucial for proper battery function.

B: Comparison of SEI formation between different electrodes

For a graphite + SuperP C65 electrode, approximately 20% of the capacity of the first discharge is consumed for the formation of the SEI.

For a graphite + dispersion electrode, this percentage is reduced to approximately 14%.

This means that, in a battery using graphene, fewer electrons are used to form the SEI.

Cycling parameter

The cycling protocol for each battery was as follows to evaluate the power handling of the different electrodes tested.

Different current values were imposed corresponding to the cycling regime; a C/20 regime indicates that the discharge or charge takes place in 20 hours.

*2 cycles at C/20;
*4 cycles at C/10;
*4 cycles at C/5;
*4 cycles at C/2;
*4 cycles at C;
*4 cycles at 2C;
*4 cycles at C/10

Stopping potentials: 20 mV vs Li+/Li during discharge and 1.2 V during charge

During discharge, at the end of the galvanostatic mode, the 20 mV potential is maintained for 30 minutes (potentiostatic mode); this protocol is often noted as CCCV (constant current constant voltage).

For this example, the Solid Electrolyte Interphase (SEI) was evaluated by comparing the first charge at C/20 to the first discharge at C/20. During the first discharge, the graphite lithiashes, and the SEI forms through the reduction of the electrolyte at low potential. In contrast, during the first charge, only the lithium intercalated within the graphite structure is extracted. The difference in capacitance between these two stages therefore corresponds to that associated with SEI formation. Although SEI continues to form slightly during subsequent cycles, this method remains relevant for comparison.

The last four cycles at C/10 are useful for determining if the battery has degraded during rapid charge/discharge cycles by simply comparing the capacity value at C/10 before and after the power test. Two batteries were tested for each electrode tested.

Only the discharge phase of the galvanostatic cycling will be studied. Indeed, this provides information on the specific electrochemical reactions that occur, as well as on the maximum capacity obtained as a function of the operating regime.

The different discharge curves for the electrode including the super P C65 are presented at the FIG. 4 .

C. Evaluation of power handling

Power is defined by the speed at which a battery can release energy. To quantify this performance, a cycling protocol is applied, which increases the charge and discharge frequency, thus accelerating the kinetics of electrochemical reactions.

To compare power performance in this example, the electrode capacitances are compared. Capacities are measured at a given cycling rate and expressed as C/X, where X represents the number of hours. These capacities are then expressed as a percentage of the capacity measured at a low operating speed, specifically during the second cycle at a rate of C/20.

The results of the power handling assessment are presented to the FIG. 5 .

The results show that electrodes containing 10% by mass of graphene (dispersion according to the invention) and 10% by mass of PVDF exhibit power performance comparable to that of reference electrodes containing 10% by mass of Super P C65 and 10% by mass of PVDF. Although the performance of the graphene-based electrodes (dispersion according to the invention) is slightly lower than C/5, both types of electrodes display a capacitance reduction similar to a 2C regime, due to strong polarization. These results are promising, as they demonstrate competitive behavior under demanding conditions.
[0266] It is also noteworthy that, for both the reference electrodes and the graphene-based electrodes, the performance measured at C/10 after the power tests remains equivalent to that observed before these tests. This suggests that the tested batteries do not undergo significant degradation after rapid charge and discharge cycles, which confirms the robustness of these systems.

In addition, the power handling of different electrodes containing different graphene contents was tested, namely 4% and 6% by mass of graphene (dispersion according to the invention) relative to the total mass of the electrode.

There FIG. 6 This figure presents a comparison of power performance as a function of the proportion of graphene added. Generally, it is expected that increasing the proportion of the additive would improve power performance. However, the data shown in the figure reveals the opposite effect. Indeed, the power performance at 4% graphene by mass relative to the total electrode mass is higher than that at 6% graphene by mass relative to the total electrode mass, which itself is higher than that at 10% graphene by mass relative to the total electrode mass.

Finally, power handling measurements of the electrodes were carried out by varying the PVDF content, which acts as a binding polymer.

The electrode studied was composed of 92% graphite by mass relative to the total electrode mass, 4% PVDF by mass relative to the total electrode mass, and 4% graphene by mass (dispersion according to the invention) relative to the total electrode mass. Despite a low proportion of PVDF, the electrode exhibited excellent mechanical properties. The power performance of the sample containing 4% PVDF by mass relative to the total electrode mass was compared to that of a sample containing 10% PVDF by mass relative to the total electrode mass, both samples having in common the addition of 4% graphene by mass (dispersion according to the invention) relative to the total electrode mass.

The results presented at the FIG. 7 These results demonstrate that the power handling capacity is significantly higher for electrodes containing 4% by mass of graphene (dispersion according to the invention) relative to the total electrode mass and 4% by mass of PVDF relative to the total electrode mass, compared to those with 10% by mass of PVDF relative to the total electrode mass and 4% by mass of graphene (dispersion according to the invention) relative to the total electrode mass. This confirms that 4% by mass of PVDF relative to the total electrode mass is sufficient to fulfill its role as a binder, allowing the graphene to be less enclosed and, consequently, to perform its function as an electronic conductor more effectively.

The electrode exhibiting the best power handling is the one with 4% by mass of graphene (dispersion according to the invention) relative to the total mass of the electrode and 4% by mass of PVDF relative to the total mass of the electrode.

The results of the power handling of the different electrodes used in this example have been compiled in the FIG. 8 .

Claims (32)

A liquid dispersion of graphene, comprising a solvent, a polymer, at least 0.1 g/L of graphene and having an absolute viscosity measured at 25°C under the effect of a 10 s-1 shear using a rheometer of between 5 mPa.s and 10 Pa.s, characterized in that it is obtained by a process comprising the implementation of the following steps:
1) Solubilization of graphite carried out under an inert atmosphere forming a graphene solution;
2) Oxidation of the graphene solution obtained in step 1) to obtain an organic dispersion of graphene; and
3) Transfer of the organic graphene dispersion obtained in step 2) into a polymer forming a liquid graphene dispersion. Dispersion according to the preceding claim, characterized in that the graphene is in the form of sheets and in that at least one sheet has a thickness of less than 10 atomic layers. Dispersion according to the preceding claim, characterized in that at least one sheet has a sheet thickness of less than 5 atomic layers. Dispersion according to any one of the preceding claims, characterized in that graphene represents between 1 and 10% by weight relative to the total weight of the dispersion. Dispersion according to one of the preceding claims, characterized in that it does not comprise any aggregate. Dispersion according to one of the preceding claims, characterized in that it is ready for use. Dispersion according to any one of the preceding claims, characterized in that the graphene sheets exhibit:
- an average thickness between 1 and 8 nm and/or;
- an average lateral size of at least 200nm. Dispersion according to one of the preceding claims, characterized in that the polymer represents between 2 and 10% by weight relative to the total weight of the dispersion. Dispersion according to any one of the preceding claims, characterized in that the polymer is selected from Polyvinylidene Fluoride (PVDF), Polytetrafluoroethylene (PTFE), Ethylene Propylene Diene Monomer Rubber (EPDM), or Carboxymethylcellulose (CMC), Nitrile Butadiene Rubber (NBR), Styrene Butadiene Rubber (SBR), Lithium Polyacrylate (LiPAA), Sodium Polyacrylate (NaPAA), Poly(acrylic acid) (PAA), Polyimide, Polyamide, Sodium Alginate, Lithium Alginate and combinations thereof. Dispersion according to one of the preceding claims, characterized in that the solvent represents between 80 and 97% by weight relative to the total weight of the dispersion. Dispersion according to any one of the preceding claims, characterized in that the solvent is a polar aprotic solvent. Dispersion according to the preceding claim, characterized in that the polar aprotic solvent has a dielectric constant between 5 and 200. Dispersion according to any one of claims 11 or 12, characterized in that the polar aprotic solvent is selected from dimethylformamide (DMF), dimethyl sulfoxide (DMSO), propylene carbonate (PC), acetonitrile, tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), 2-methyltetrahydrofuran (Me-THF), cyclopentylmethylether and combinations thereof. Dispersion according to one of the preceding claims, characterized in that the dispersion exhibits an absorption spectrum in UV-visible spectroscopy with a peak at 269nm. Dispersion according to any one of the preceding claims, characterized in that it comprises 3 vibrational bands characteristic of graphene observed by RAMAN spectrophotometry:
- 1350 cm-1 (pic D);
- 1580 cm-1 (peak G); and
- between 2680 and 2700 cm-1 (peak 2D). Dispersion according to the preceding claim, characterized in that it presents:
- a peak intensity D / peak intensity G ratio of less than 1.5; and
- a 2D peak intensity / G intensity ratio greater than 1. Dispersion according to claim 15 or 16, characterized in that it comprises:
- a peak D with a full width at half maximum (FWHM) of less than 33 cm-1; and/or
- a 2D peak with a full width at half maximum (FWHM) of less than 55cm-1. Dispersion according to any one of the preceding claims, characterized in that it comprises:
*between 80 and 97% solvent by weight relative to the total weight of the dispersion;
* between 2 and 10% polymer by weight relative to the total weight of the dispersion; and
* between 1 and 10% graphene by weight relative to the total weight of the dispersion. Dispersion according to any one of the preceding claims, characterized in that step 1) of solubilization carried out under an inert atmosphere of the graphite of the process comprises the following steps:
a) Intercalation of at least one alkali metal into graphite leading to a graphite intercalation compound; and
b) Chemical exfoliation combined with mechanical exfoliation of the graphite intercalation compound, characterized in that the graphite intercalation compound is mixed with a solvent in a turbulent regime exhibiting:
- a Reynolds number greater than 1000;
- a Froude number less than 1; and
- a shear rate of less than 400s-1, in order to obtain a graphene solution. A method for obtaining a liquid dispersion of graphene according to any one of the preceding claims, characterized in that it comprises the following steps:
1) Solubilization of graphite carried out under an inert atmosphere forming a graphene solution;
2) Oxidation of the graphene solution obtained in step 1) to obtain an organic dispersion of graphene; and
3) Transfer of the organic graphene dispersion obtained in step 2) into a polymer forming a liquid graphene dispersion. A process according to the preceding claim, characterized in that the liquid dispersion from step 3) has an absolute viscosity measured at 25°C using a rheometer of between 5mPa.s and 10Pa.s. A process according to any one of claims 20 or 21, characterized in that the polymer of step 3) is selected from Polyvinylidene Fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-propylene-diene monomer rubber (EPDM), or carboxymethylcellulose (CMC), nitrile-butadiene rubber (NBR), styrene-butadiene rubber (SBR), lithium polyacrylate (Li-PAA), sodium polyacrylate (NaPAA), poly(acrylic acid) (PAA), polyimide, polyamide, sodium alginate, lithium alginate and combinations thereof. A process according to any one of claims 20 to 22, characterized in that the solubilization step carried out under an inert atmosphere of graphite comprises the following steps:
a) Intercalation of at least one alkali metal into graphite leading to a graphite intercalation compound; and
b) Chemical exfoliation combined with mechanical exfoliation of the graphite intercalation compound, characterized in that the graphite intercalation compound is mixed with a solvent in a turbulent regime exhibiting:
- a Reynolds number greater than 1000;
- a Froude number less than 1; and
- a shear rate of less than 400s-1, in order to obtain a graphene solution. A process according to the preceding claim, characterized in that the chemical exfoliation of step b) is carried out by exposing the graphite intercalation compound to a polar aprotic solvent, said polar aprotic solvent having a dielectric constant between 5 and 200. A process according to the preceding claim, characterized in that the aprotic polar solvent is selected from dimethylformamide (DMF), dimethyl sulfoxide (DMSO), propylene carbonate (PC), acetonitrile, tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), 2-methyltetrahydrofuran (Me-THF), cyclopentylmethylether and their combinations. A process according to any one of claims 23 to 25, characterized in that the graphite intercalation compound is in the form of a binary compound of formula KC8. A process according to any one of claims 23 to 26, characterized in that the chemical exfoliation is carried out with a compound ratio of graphite intercalation / aprotic polar solvent of between 1 and 50g/L. Use of a liquid graphene dispersion according to any one of claims 1 to 19 as an electrode conductive additive. Electrode of an energy storage device, comprising a liquid dispersion of graphene according to any one of claims 1 to 19. Electrode according to the preceding claim, characterized in that the energy storage device is chosen from a supercapacitor, a battery or a fuel cell. Electrode according to claim 29 or 30, characterized in that it comprises:
* between 1 and 10% by mass of dispersion according to the invention relative to the total mass of the electrode;
*between 1 and 10% by mass of polymer, preferably PVDF, relative to the total mass of the electrode; and
*between 80 and 98% by mass of active material, preferably graphite or any material suitable for forming an electrode for an energy storage device, relative to the total mass of the electrode Battery comprising an electrode according to any one of claims 29 to 31.