WO1993008123A1

WO1993008123A1 - Flat micrometric graphite, their preparation method and use

Info

Publication number: WO1993008123A1
Application number: PCT/EP1992/002317
Authority: WO
Inventors: Guy Furdin; Jean-François MARECHE; Albert Herold
Original assignee: Centre National De La Recherche Scientifique
Priority date: 1991-10-15
Filing date: 1992-10-08
Publication date: 1993-04-29
Also published as: FR2682370A1; FR2682370B1

Abstract

Micrometric graphite particles, their preparation method and use are described. This graphite material is characterized in that it essentially consists of single crystal particles with a particle-form factor of 50-1000, which have, when deposited in a layer a few millimetres thick on a plane support, a mosaic structure in which the angular carbon plane misalignment is 15, as determined by X-ray diffraction. The flat micrometric graphite is useful as an electrode material in rechargeable or non-rechargeable batteries.

Description

The present invention relates to micron particles of graphite, a process for their preparation, and their applications.

Different powdery carbonaceous materials, graphite or not, are known. They are industrially prepared by processes using different technologies.

Carbon blacks are obtained by cracking hydrocarbons in the vapor phase. They are in the form of nanometric powder, the particles of which are agglomerated or not. Some of these materials can be graphitized by a subsequent heat treatment.

Natural graphites are obtained after washing the ore and purifying the graphite.

Artificial graphites are obtained by a controlled treatment of precursors (pyrolysis of coal pitch or petroleum), followed by graphitation. The resulting products are then either dry ground in ball mills, or wet ground in ball mills. Each product is identified by its particle size. We can thus distinguish large flakes (diameter between 0.2 and 2 mm), small flakes (diameter less than 0.35 mm), fine particles (diameter less than 112 μm), very fine particles (smaller diameter at 20 μm).

For many applications of the lubrication type, it is sought to respect the sheet structure, that is to say an ultra-thin crystalline structure. However, conventional grinding techniques break the grains in all directions.

An object of the present invention is to provide a graphite material in the form of flat micron graphite particles. The invention also relates to a method for obtaining the flat micron particles from graphitizable or partially graphitized carbons.

PLACEMENT Finally, the invention relates to applications of said material.

The graphitized material according to the present invention is characterized in that it consists essentially of monocrystalline particles having a form factor of between 50 and 1000 and which, after deposition on a flat support in a layer of a few millimeters in thickness have a mosaicite , determined by X-ray diffraction, of the order of + 15 ° angle. Preferably, the form factor of the particles is greater than 100.

The form factor represents the ratio of the diameter Φ of the particles to their thickness d.

The graphite material of the present invention is obtained by a two-step process. During a first step, an expanded material is prepared from a graphite or graphitisable precursor material. In a second step, the expanded material is subjected to grinding. The grinding results from the alternating action of an ultrasonic probe and a rotary mill with shearing and cavitation effect. The alternation of these two types of grinding is particularly effective for fragmenting pre-ground expanded graphite particles into unitary crystallites. In fact, the combination of ultrasound and the rotary grinder makes it possible to obtain fine particles having a very high form factor, of the order of 100 to 200. The duration of the grinding fixes the maximum dimension of the particles.

As an example of a rotary shear mill, mention may be made of the Ultra Turrax mills, from the company Janke and Kunkel.

Preferably, the expanded material is subjected to a pre-grinding which has the essential purpose of wetting the expanded graphite with an appropriate liquid. Any liquid wetting graphite can be used. By way of example, mention may be made of benzene, toluene or cyclohexane, benzene and toluene being particularly effective. Preferably, mixtures containing from 1 to 60 g / l are used of expanded graphite. The mixture of expanded graphite and. wetting liquid is pre-ground, for example by a helical turbine. This pre-grinding phase allows homogenization of the mixture and results in significant fragmentation of the expanded graphite flakes.

Drying after grinding, which aims to remove the wetting liquid, is useful to prevent particles from agglomerating. The wetting liquid must be removed to avoid recompaction of the flat micron graphite particles. This removal can advantageously be carried out by lyophilization, preferably when benzene or cyclohexane is used as a wetting liquid.

As a precursor material, any graphitized or graphitizable material can be used in which an element can be inserted. By way of example, mention may be made of natural graphite from Madagascar or natural graphite from Brazil. Madagascar graphite is in the form of quasi-onocrystalline particles having an average diameter of 0.5 mm and an average thickness of a few tenths of mm. It contains less than 0.5% ash. Brazilian graphite is found in the form of quasi-monocrystalline particles with an average diameter of 40 μm and a thickness of a few tens of μm to a few hundred μm. Mention may also be made of graphite or graphitisable pitch cokes.

The precursor material can be expanded by a process consisting in interposing a reagent in the precursor material, then subjecting the insertion compound obtained, optionally washed with water and dried, to a sudden rise in temperature (thermal flash). The rapid departure of the intercalate is accompanied by an irreversible one-dimensional expansion (or exfoliation) of each graphite particle. As reagent, it is possible to use, for example, ferric chloride-ammonia mixtures or sulfuric acid. However, reagents having an exothermic decomposition, such as those described in patent application FR 91.97516, are particularly advantageous. Among

MPLA EMENT these reagents having an exothermic decomposition, mention may be made of perchloric acid, used alone or as a mixture with nitric acid, or even nitromethane. The apparent density of the expanded graphite thus obtained is very low (a few grams per d 3 against 2 kg per dm3 for the precursor material). Its specific area is of the order of a few tens of m∑ / g, while it is of the order of 1 m2 / 9 for the precursor material.

The present invention is described in more detail in the following examples, given by way of illustration but not limitation, and which relate to the preparation of the graphitized material of the present invention, from different graphitized or graphitizable materials.

The particle size distribution histogram was established by counting and determining the particle size using a device using light scattering, for example of the Malvern type manufactured by Instrumat which implements the scattering of a laser beam from particles suspended in a carrier liquid. The histogram is spread between 0 and 100 μm. It depends on the grinding time in the first four hours and on the wetting power of the liquid used. It gives the main dimension of the particles.

The morphology of the product obtained is determined by examination with a scanning electron microscope. The particles are deposited on a previously polished brass support. The accelerating voltage of the incident electron beam is 21 KV. The thickness of the particles is such that the scratches of the support are visible by transparency, as are the copper nodules in the brass matrix. This phenomenon, linked to the existence of backscattered electrons, highlights particles whose thickness is less than 0.1 μm. This examination under the electron microscope gives results point by point.

The specific area of the flat micron graphite obtained was evaluated by using the krypton adsorption isotherms carried out at 77. Qualitative analysis of isothermal shows that the product is characterized by a surface homogeneity identical to that encountered with natural graphite. The quantitative use of isotherms makes it possible to determine the specific area of flat micron graphite. Given the sheet morphology of the flat micron graphite particles, the covering power is deduced therefrom.

The crystallinity of the material is determined by electron diffraction. The particles are deposited by filtration on a front nucleopore membrane. to be collected on an observation grid. The small thickness of the particles makes it possible to achieve the diffraction conditions giving the maximum resolution and resulting in an onopunctuated diffraction plate comprising the reflections [hko] of graphite.

The depositability of flat micron graphite particles was assessed on a layer of particles deposited by decantation on a thin beryllium plate. The X-ray diffractometric recording, performed in transmission, shows the hko reflections, but also the 002 reflection, whose presence and intensity allow a good appreciation of the state of disorientation of the particles.

EXAMPLE 1

A natural graphite, UF4, supplied by Le Carbone Lorraine and presenting a particle size distribution histogram compatible with an average diameter of 6.4 μ was used. 1 g of this material was brought into contact with 4 ml of a mixture composed of 70% commercial perchloric acid and fuming nitric acid (50/50 volume) for 2 hours. After reaction, this heterogeneous medium was filtered under vacuum. The cake recovered after filtration was placed in the bottom of a reactor previously heated to red. This first step led to the expansion of the particles of the starting material. The expanded particles were then subjected to pre-grinding for approximately 1/4 h using of a helical turbine at 2000 rpm. Then they were subjected to grinding for 3 hours, said grinding consisting of alternating sequences of 10 min of grinding using a rotary Ultra-Turrax mill and sequences of 5 min of ultrasound. For this grinding step, the particles were wetted with cyclohexane.

The product obtained has a distribution histogram compatible with an average diameter of 6.2 mμ. The observation of the particles in scanning electron microscope shows ^a morphological state very different from that of the material before ^"grinding. After treatment, .the particles are transparent under 1'impact of the electron beam. The diameter / thickness ratio was therefore greatly increased after grinding.

EXAMPLE 2

The procedure of ^r example 1 was repeated with a non-graphitized pitch coke obtained by carbonization

1100 ° C of a coal pitch. This non graphitized pitch coke presented a histogram compatible with an average diameter of 50 mμ.

The product obtained has a distribution histogram compatible with an average diameter of 28 mμ.

EXAMPLE 3

The procedure of Example 1 was reproduced with a graphite pitch coke, obtained by a heat treatment at 2500 ° C. of the non graphite pitch coke used in Example 2. This graphite pitch coke had a compatible histogram with an average diameter of 50 mμ.

The product obtained has a distribution histogram compatible with an average diameter of 12 mμ.

These results show that the process of the present invention is effective for cokes, and more particularly for graphite cokes, therefore easier to insert. EXAMPLE 4

An insertion compound was prepared from Madagascar graphite and sulfuric acid.

The insertion compound obtained was washed with water and dried at 110 "C. A sudden rise in temperature up to 1000" C caused the graphite to expand.

Various mixtures containing from 1 to 60 g of expanded graphite in 1 1 of benzene were subjected to pre-grinding for approximately 1/4 h using a helical turbine at 2000 rpm. Then, each mixture was subjected alternately to an ultrasonic probe (20KHz) and to a grinder of the Ultra Turrax type (20,000 rpm). Each sequence lasted 15 min, including 5 min of ultrasound and 10 min of Ultra Turrax. The reactor was thermostatically controlled to avoid evaporation of the carrier liquid.

The benzene was removed by solidification of the mixture at 0 ° C. and dynamic pumping of the vapors with trapping of these at 180 "C. After this last treatment, the flat micron graphite is obtained in the dry state. The distribution of the The size of the particles subjected to stirring in a carrier liquid was determined using a Malvern type device Figure 1 gives the evolution of the particle size as a function of the grinding time for a mixture at 1 g / 1 It is noted that all the particles have a diameter of less than 30 μm.

The particles obtained have a thickness of less than 0.1 μm, a specific area of the order of 20 m2 / α ;. and therefore a covering power of lOm∑ / çj-

The electron diffraction diagram of the flat micron graphite obtained indicates an almost perfect monocrystallinity of the material.

A layer of a few tenths of a mm thick, which results from the superposition of several thousand elementary particles, is characterized by a disorientation of the carbon planes of + 15 ° angle. This result reflects the excellent ability of the material to deposit flat. EXAMPLE 5 An insertion compound was prepared from graphite from Brazil and a mixture of nitric acid - perchloric acid - water, according to the procedure described in patent application FR 91.97516.

The insertion compound obtained was washed with water and dried at 110 ° C. A sudden rise in temperature to 600 "C caused the graphite to expand.

Various mixtures containing from 1 to 60 g of expanded graphite in 1 1 of benzene were subjected to pre-grinding for approximately 1/4 h using a helical turbine at 2000 rpm. Then, each mixture was subjected alternately to an ultrasonic probe (20 kHz) and to an Ultra Turrax type mill (2000 rpm). Each sequence lasted 15 min, ^• including 5 min of ultrasound and 10 min of Ultra Turrax.

The benzene was removed by solidification of the mixture at 0 ° C and dynamic pumping of the vapors with trapping of them at 180 "C. After this last treatment, the flat micron graphite is obtained in the dry state.

The size distribution of the particles subjected to stirring in a carrier liquid was determined using a Malvern type device. Figures 2 and 3 respectively give the evolution of the particle size as a function of the grinding time for a mixture of 1 g / 1 and 10 g / 1 of graphite. Note that all the particles have a diameter of less than 30 μm and that 33% of the particles have a diameter of between 4 and 14 μm

FIG. 4a and FIG. 4b respectively represent the particle size distribution histogram of the product obtained from a 10 g / 1 mixture ground for 5 h and for 11 h.

The particles obtained have a thickness of less than 0.05 μm and a specific area of the order of 40 τs .- / q, and consequently a covering power of 20m2 / g-

The electron diffraction diagram of the flat micron graphite obtained indicates an almost perfect monocrystallinity of the material. GMP flat micron graphite of the ^• present invention can be advantageously used as an electrode material in batteries, rechargeable or not. For this application, the flat micron graphite is used in the form of a mixture with Mn0 ₂ or with any other material whose electrochemical and / or morphological properties are similar to those of nθ ₂ , for example V ₂ O ₅ . Indeed, a percolation phenomenon occurs in GMP-Mn0 ₂ mixtures, even for GMP contents of the order of 1%. The following example 6 illustrates this property.

EXAMPLE 6 A mixture of GMP and MnO ₂ is homogenized after having been wetted with cyclohexane. Homogenization is carried out using an ultrasonic probe with a power of 500 watts in a volume of 100 cm ³ with a frequency of 20 KHz, for 10 min. The cyclohexane is then extracted from the mixture by lyophillization. The recovered powder is pelletized under a pressure of 1000 kg / cm ² . The electrical conductivity is measured on the patch obtained by the Van Der Pauw method. According to this method, on the pad mentioned above, silver lacquer contacts are made as fine as possible at the four vertices A, B, C and D of a rectangle. First, a weak current is circulated, of the order of μA, or even nA, from A to B, noting the value of the voltage appearing between C and D for different intensities of the current. The curve representing the voltage as a function of the intensity of the current is a straight line having a slope RI. Then, repeat the operation by passing a current from D to A and note the voltage appearing between points B and C. The line representing the variation of the voltage between B and C as a function of the intensity of current a an R2 slope. The resistivity p of the layer formed by the patch is given by the formula

in which e represents the thickness of the layer, RI and

R2 the resistances obtained by the voltage and current measurements and f (Rl / R2) represents the Van Der function

Pauw which is tabulated.

The results of the measurements are collated in the following table.

BOARD

These results show that the percolation phenomenon is achieved in these two-phase media with a GMP content as low as 1%.

In conventional batteries, particles of graphitized thermal black are spheroids and the percolation threshold is only obtained with black percentages close to 30%. All of the properties of the flat micron graphite particles of the present invention allow their use for many applications. Thus, the particles of the present invention can be used in conventional tribology. They can also be used as an insertion compound with an insert suitable for tribology applications under vacuum or at elevated temperature. They can also be used as such, mixed with pitch, to promote the graphitization of the pitch by a heat treatment.

Claims

1. Graphite material characterized in that it consists essentially of monocrystalline particles having a form factor of between 50 and 1000 and having, after deposition in a layer of a few millimeters thick on a flat support, a mosaicism determined by diffraction of the X-rays, of the order of + 15 ° angle.

2. Graphite material according to claim 1, characterized in that the form factor is greater than

100.

3. Method for preparing a graphite material according to any one of claims 1 or 2, characterized in that it consists in preparing an expanded graphite from a graphite or graphitizable precursor material, then in grinding the graphite expanded obtained, said grinding being carried out by the alternating action of an ultrasonic probe and a rotary shear mill.

4. Method according to claim 3, characterized in that the precursor material is expanded by inserting an interlayer in said material, then by subjecting the insertion compound obtained to a sudden rise in temperature.

5. Method according to claim 3, characterized in that the expanded graphite is pre-ground in a wetting liquid.

6. Method according to claim 3, characterized in that the wetting liquid is removed after grinding.

7. Method according to claim 6, characterized in that the wetting liquid is removed by lyophilization.

8. Application of the graphitized material according to any one of claims 1 to 2, to the preparation of electrode materials.

9. Application of the graphitized material according to any one of claims 1 to 2, in the graphitization of pitch.

REPLACEMENT SHEET