U.S. patent application number 10/204715 was filed with the patent office on 2005-09-22 for surface preparation of natural graphite and the effect of impurities on grinding and the particle distribution.
Invention is credited to Zaghib, Karim.
Application Number | 20050207966 10/204715 |
Document ID | / |
Family ID | 25681570 |
Filed Date | 2005-09-22 |
United States Patent
Application |
20050207966 |
Kind Code |
A1 |
Zaghib, Karim |
September 22, 2005 |
Surface preparation of natural graphite and the effect of
impurities on grinding and the particle distribution
Abstract
The present invention relates to the physical or chemical
specific purification of natural mineral graphite. This
purification is preferably applied to the surface of natural
graphite in order to allow the formation of a passivation film
during the first electrical discharge or the insertion of lithium
in the graphite when the latter is used in a lithium-ion cell. The
grinding to a small size before purification allows the
optimization of the distribution of the particles, resulting in a
more uniform electrode. This grinding is carried out in the
presence of the natural impurities of the graphite that play the
role of a micro-abrasive and result in a hardness of the graphite
that increases its mechanical properties.
Inventors: |
Zaghib, Karim; (US) |
Correspondence
Address: |
BUCHANAN INGERSOLL PC
(INCLUDING BURNS, DOANE, SWECKER & MATHIS)
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Family ID: |
25681570 |
Appl. No.: |
10/204715 |
Filed: |
March 17, 2003 |
PCT Filed: |
February 23, 2001 |
PCT NO: |
PCT/CA01/00233 |
Current U.S.
Class: |
423/448 |
Current CPC
Class: |
C01B 32/215 20170801;
H01M 10/0525 20130101; Y02E 60/10 20130101; H01M 4/583 20130101;
H01M 4/133 20130101 |
Class at
Publication: |
423/448 |
International
Class: |
C01B 031/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 2000 |
CA |
2,299,626 |
Apr 28, 2000 |
CA |
2,307,118 |
Claims
1. A process for the preparation of graphite particles which are
surfaced-purified, said process comprising at least the following
consecutive steps: a) a grinding of the graphite particles, carried
out until said particles attain a size between 1 and 50 .mu.m; b) A
purification of the graphite particles obtained in a) performed for
less than 16 hours, by chemical, mechanical or thermal means, at a
temperature lower than 2,000.degree. C. or by combining these
means, until impurities, as well as sites of corrosion are
substantially eliminated.
2. A process for the purification of the surface of particles of a
natural graphite, said process comprising at least the following
steps: a) a grinding of the natural graphite carried out until said
natural graphite attains a size between 1 and 50 .mu.m; and b) a
purification of the particles obtained in a) carried out until said
natural graphite, as well as sites or corrosion, are substantially
eliminated.
3. A process according to claim 2, wherein the surface of the
particles is purified by chemical or mechanical means for less than
16 hours at a temperature lower than 2,000.degree. C., or by a
combination of these means.
4. A process according to claim 2, characterized in that the
purification of particles as carried out by mechanical means. the
crystallographic parameters L.sub.c and L.sub.a are controlled.
5. A process according to claim 1 or 2, characterized in that the
crystallographic parameters L.sub.c and L.sub.a are controlled.
6. A process according to claim 1 or 2, characterized in that the
graphite is of the StratminGraphite.RTM. type.
7. A process according to claim 1 or 2, characterized in that the
graphite is of the Chinese type.
8. A process according to claim 1 or 2, characterized in that the
graphite is of the Lac Knife type.
9. A process according to claim 1 or 2, characterized in that the
graphite is of the Brazilian type.
10. A process according to any one of claims 1 to 9, characterized
in that steps a) and b) are performed under conditions, which are
adapted to obtain graphite particles having the following
additional properties: an interplanar distance d002 measured by
X-rays, varying between 3.35 .ANG. to 3.38 .ANG.; a specific
surface varying from 0.4 to 55 m.sup.2/g; and a level of purity
varying from 98.5% to 99.99%.
11. A process according to claim 1 or 2, characterized in that the
purification is carried out by chemical means in an acid or in a
basic medium.
12. A process according to claim 11, characterized in that the acid
medium comprises H.sub.2SO.sub.4--HF, H.sub.2SO.sub.4--NH.sub.4F,
H.sub.2SO.sub.4, HNO.sub.3, HCl or their mixtures thereof and the
basic medium comprises a fluoridated derivative.
13. A process according the claim 11, characterized in that the
acid medium comprises HF or a fluoridated derivative generating HF
in said medium.
14. A process according to claim 13, characterized in that the
fluoridated derivative comprises NH.sub.4F, NH.sub.5F.sub.2, XF
wherein X is an alkaline or a rare earth metal or their mixtures
thereof.
15. A process according to claim 12, wherein said acid medium
comprises H.sub.2SO.sub.4 or HCl, as well as a fluoridated
derivative, and additionally sufficient HNO.sub.3 to exfoliate
graphite while inserting itself between the layers of said
graphite.
16. A process according to claim 1 or 4, characterized in that the
mechanical purification is carried out by grinding until the
particles reach a size from 1 .mu.m to 50 .mu.m, followed by the
separation of the impurities by flotation.
17. A process according to claim 11, characterized in that the
purification is carried out by an acid in the presence of at least
two salts generating two acids including HF.
18. A process according to claim 1 or 2, characterized in that said
graphite particles are conditioned in the form of a carbon anode
for a rechargeable electrochemical generator comprising an alkaline
or rare earth metal.
19. A process according to claim 18, characterized in that the
metal is lithium.
20. A process according to claim 18, characterized in that the
anode is prepared by mixing the graphite particles with a binder
and a solvent, and the mixture thereby obtained is then coated onto
a metallic collector.
21. A process according to claim 1, characterized in that the
purification is performed in such a way to eliminate said
impurities and sites of corrosion only on the surface.
22. Graphite particles with a size between about 1 and 50 .mu.m and
where impurities and sites of corrosion have been partially or
totally eliminated from the surface of the particles.
23. A carbon anode based on graphite particles according to claim
22.
24. An anode according to claim 23, characterized in that the
natural graphite particles which are ground and purified have the
following additional properties: an interplanar distance d002
measure by X-rays varying between 3.35 .ANG. and 3.38 .ANG.; a
specific surface varying from 0.4 to 55 m.sup.2/g; and a level of
purity varying from 98.5% to 99.99%.
25. Electrochemical battery comprising an anode according to claim
23.
26. A battery according to claim 25, characterized in that it is a
battery of a lithium-ion type.
Description
FIELD OF INVENTION
[0001] The present invention relates to the physical and chemical
purification specific to the mineral of natural graphite with the
goal of generating a purified graphite which is particularly
advantageous for use in carbon-lithium anodes. This purification is
applied preferentially to the surface of natural graphite to permit
the formation of a passivating layer with the first electrical
discharge or insertion of lithium in the graphite when the graphite
is used in a lithium-ion battery. Grinding to a very small size
before the purification permits the optimisation of the size
distribution of the particles, which gives rise to an electrode
which is much more homogenous. This grinding is done in the
presence of the natural impurities of the graphite which play a
role as a micro abrasive and give a hardness to the graphite which
increases its mechanical properties. It goes without saying that
the purified graphite particles can have many other usages, notably
in the field of car brakes and joints, and the nuclear field.
PRIOR ART
[0002] The negative carbon-lithium electrode has recently caused a
great deal of interest in the industrial but also in the scientific
community. In fact, the use of such an electrode in a rechargeable
battery resolves the crucial problem of lithium metallic electrodes
which are poorly recharged in liquid electrolytes because of the
growth of dendrites once the charged density (C/cm.sup.2) and/or
the current density (mA/cm.sup.2) exceed limiting values for the
good operation of the battery. This major problem has slowed the
arrival of lithium batteries in the classical formats (AA, C, D,
etc.) to the public at large. The first such type of battery was
commercialised at the beginning of the 1990s by Sony Energytech.
This battery is said to be a lithium-ion and consists of a negative
electrode made of carbon-lithium.
[0003] The operating principle of this electrode resides in the
reversible insertion of lithium between layers of carbon. These
layers are characterized by very strong anisotropic connective
carbon-carbon forces within the layers (very strong covalent bonds)
and between layers (very weak Van der Waals forces). Thus, because
lithium is a very weak cation, it may rapidly diffuse between 2 D
layers, forming bonds of the ionic type between layers without
involving irreversible changes of the bonds within the layers. Only
a slight spacing between layers is observed, thus accommodating the
inserted lithium.
[0004] It is well known that the reversibility of electrochemical
insertion of lithium in the carbon is far more favourable than the
Li.sup.+ cation deprived of its sphere of solvation when
transferred from the electrolyte solution towards the interior of
the solid carbon. Thus, the co-insertion of DMSO (dimethyl
sulfoxide) and DME (dimethyl ether) causes a larger spacing between
the layers (>300%) which thus contributes to a larger
disorganisation of the structure of the host. The lithium thus
inserted has, moreover, a more substantial mass and apparent
volume, which reduces its mobility as well as its maximum
concentration inside the layers. In addition, in propylene
carbonate medium, the ternary compound is very unstable, the
solvent reacts to become gaseous propene that can cause a violent
degradation of the battery.
[0005] More recently, it was shown that carbons with an imperfect
crystallinity (turbostratics) can insert lithium in a PC (propylene
carbonate) medium or PC-DME without the co-insertion of the
solvent. The difference in electrochemical behaviour of graphite of
high crystallinity and carbon poorly crystallised such as coke
treated at a temperature less than 1,800.degree. C., may be caused
by an overpressure due to propene generation which is greater than
that with coke. However a first step of discharge results in the
formation of a protective film at the surface of the carbon grains,
produced by the decomposition of the solvent. Once formed, this
film has a sufficient impedance to prevent the electron transfer
necessary for the continued reduction of the solvent. However, it
remains a conductor for Li.sup.+ ions and because of this behaves
as a solid electrolyte. It is also highly probable that this layer
intervenes in the dissolution of the Li.sup.+ ion during its
transfer and/or reduction at the surface of the carbon.
[0006] Electricity consumed during this step cannot be recovered
when the current is inverted. The faraday output of this first
cycle is therefore poor. The reversible capacity measured after the
following cycles is directly related to the nature of the carbon
and the treatment it underwent as well as the nature of the
electrolyte.
[0007] U.S. Pat. No. 5,882,818 deals with graphite of 1-40 .mu.m.
This study is based on the relationship between structure and
electrochemistry. However no information concerning the purity of
graphite or a process to obtain said is taught.
[0008] U.S. Pat. No. 5,756,062 discloses the modification of the
surface of a high purity graphite. The graphite is not however one
obtained directly from the mineral. The chemical modification of
the graphite is performed by fluorine, chlorine or phosphorous
treatment.
[0009] The graphite used conventionally as electrode material in a
lithium-ion battery is generally obtained from two distinct
sources: synthetic graphite or natural thermally highly purified
graphite, preferably treated at temperatures higher than
2,500.degree. C. This type of graphite although of excellent
quality, is however very costly, and this has a direct impact on
the cost of the final product eventually sold in the market.
Moreover, the graphite is only reduced to the powder state after
having been purified or synthesised, this causes certain problems
during the grinding process. In effect, the uniformity of the size
distribution of the particles in the powder is markedly altered,
since pure graphite is very fragile. In fact, it can be said that
the distribution is non-uniform. If a battery is produced directly
with such a non-uniform graphite, it is clear that its lifetime
will be greatly reduced. The alternative is then to filter the
graphite and to collect only those particles having the wanted
size, which involves additional process steps, and an increase in
the cost of the resulting material.
[0010] It would be desirable to develop a purification process of a
low cost graphite having electrochemical properties comparable or
superior to those of the graphite presently being used as a
material for electrodes in lithium-ion batteries. In a preferred
embodiment, the process would result in a graphite powder having a
relatively uniform size distribution of particles, and thus avoid
filtration steps.
SUMMARY OF THE INVENTION
[0011] The present invention relates to a process for the
preparation of graphite particles characterized by at least by the
following consecutive steps:
[0012] a) a grinding of the graphite carried out until a particle
size distribution between 1 and 50 .mu.m is obtained;
[0013] b) a purification of the particles obtained in a) until
impurities as well as sites of corrosion are substantially
eliminated.
[0014] The graphite particles produced by the process of the
invention have a size varying from 1 to 50 .mu.m and are generally
free of impurities and sites of corrosion. Preferably they have the
following properties:
[0015] an interplanar distance d002, measured by X-rays, varying
from 3.35 .ANG. to 3.38 .ANG.;
[0016] a specific surface varying from 0.4 to 55 m.sup.2/g; and
[0017] a level of purity varying from 98.5% to 99.99%.
[0018] The invention also relates to a carbon metal anode,
preferably lithium, based on a natural graphite obtained by the
previously described process. Such an anode is particularly
advantageous in an electrochemical battery such as a lithium-ion
type.
[0019] Finally, in another aspect of the invention includes a
purified graphite comprising particles having an external surface
substantially stripped of impurities and sites of corrosion having
an electric conductivity, used for a number of commercial
applications.
DETAILED DESCRIPTION OF THE INVENTION
[0020] A new method has been developed to produce a purified
graphite in the form of small particles which can be used in an
electrochemical battery, for example, of the lithium-ion type,
while maintaining a relatively uniform particle size distribution.
This type of graphite which can equally be used in other
applications, such as an electric conductor in a cathode
(batteries) or fuel cells, or in the field of cars (breaks and
joints) or in the nuclear field.
[0021] The present invention relates to a purification method,
either chemical or physical, of impurities found on the surface of
the natural graphite, i.e. where the passivating film is formed.
The present method permits the removal of the impurities which can
harm the formation of the passivating film and the cycling of the
carbon-lithium anode. The grinding process is advantageously
conducted before purification, this permits a better control of the
size and the size distribution of the particles, where a more
uniform powder does not require filtration to remove the oversized
and undersized particles.
[0022] The subsequent purification step essentially seeks to remove
impurities from the surface of the graphite particles which
generate an electric conductivity, such as compounds comprising
silica oxide and iron. Those compounds also cause the doping or the
reduction by lithium of the compounds in which they are found.
These phenomena should not be present or at least minimised in the
passivating layer which will be formed at the surface of the
electrode, because they will cause the degradation of the batteries
efficiency, and ultimately a short circuit. On the other hand, the
presence of surface impurities favouring ionic conductivity, such
as calcium fluoride, have no negative impact on the graphite
electrode performance, because they have a strong ionic character
less favourable to electronic conductivity.
[0023] The impurities present in graphite a mineral are generally
the following (in descending order of priority):
Si>Ca>Fe>Mg>S>- ;Al. As mentioned before, these
compounds containing silica must be eliminated, on one hand because
lithium reduces or dopes compounds containing silica (such as
SiO.sub.2, SiO and Si metal), and on the other hand these compounds
of silicium are electronic conductors. This last property is
completely incompatible with the properties of the passivating
film, which represents a key element to a good carbon-lithium anode
which is characterized by a long lifespan.
[0024] To eliminate compounds containing silicium, an acid
treatment is used, for example with H.sub.2SO.sub.4, HNO.sub.3,
HCl, HF or their mixtures thereof A treatment with HF or a
fluoridated derivative allowing the generation of HF in the medium
represents a particularly preferred embodiment. This treatment
equally causes an interaction between the fluoride and the calcium
already present in the mineral, leading to the formation of calcium
fluoride, a compound which is strongly ionic, an electric insulator
as well as a good anionic conductor at high temperatures. Moreover,
the presence of calcium fluoride will not alter the formation of
the passivating layer.
[0025] The present method of purification does not change the size
of the particles produced by the grinding process. There is no
agglomeration of particles, which are free and which may produce an
homogenous mixture with the binder in order to produce an electrode
of good quality (roughness, thickness, porosity, etc.).
[0026] To illustrate the present invention, a graphite mineral from
StratminGraphite (Lac des les--Qubec), of a particle size about 375
.mu.m was used. The particles were first ground until their size
varied between 1 and 50 .mu.m. It should be noted that once the
particle size is less than 1 .mu.m, the graphite loses its
crystallinity and the term intercalation of lithium becomes
doping.
[0027] The grinding process can be done by any method known to the
person skilled in the art. These techniques include jet milling,
air milling, ball milling, etc.
[0028] The purification step by thermal means can be accomplished
by conventional means, i.e., in a furnace and at a sufficiently
high temperature to allow the vaporisation of all impurities
typically between 1,000 and 3,100.degree. C.
[0029] The purification step by chemical means can be accomplished
by different methods using acid compounds containing fluorine,
nitrate, sulphate and chloride or basic compounds such as potassium
or sodium hydroxide, in order to clean the surface of the graphite
and to permit the subsequent formation of a stable passivating film
at the time of the reduction of the electrolyte and during the
first insertion of lithium in the graphite.
[0030] Typically, the purification method by chemical means can
comprise:
[0031] only one step using fluoridated acid, preferably HF or one
fluoridated derivative such as NH.sub.4F, NH.sub.5F.sub.2, etc.
which allow the generation of HF in the reaction mixture;
[0032] only one step using two different compounds, where one is HF
or a fluoridated derivative as described above, and the other may
consists of a non fluoridated acid such as HCl, HNO.sub.3,
H.sub.2SO.sub.4 etc. The use of two different compounds has the
advantage of extracting more impurities that with one single acid
or fluoridated derivative;
[0033] two steps, the first using a non fluoridated acid as defined
above, followed by a second with HF or a fluoridated derivative as
defined above; and
[0034] two steps using a base such as KOH or NaOH followed by HF or
a fluoridated derivative as defined above.
[0035] It will be noted that the addition of HNO.sub.3 to the
purification solvents (H.sub.2SO.sub.4--NH.sub.4F) or
(HCl--NH.sub.4F) allows us to obtain in one step a purified and
exfoliated graphite.
[0036] The purification steps by chemical means can be used
independently or in series. For obvious cost reasons, chemical
purification represents the preferred method of purification.
[0037] During the purification by chemical means, it is very
important to choose concentrations of compounds added to achieve
this end, as well as the operating conditions, in order to avoid
all physical changes or exfoliation of the natural graphite. In
this way, the concentration of HF or of fluoridated derivatives
added for the purification has to be preferably between 10% and 50%
(by weight) and at a process temperature which preferably does not
exceed 250.degree. C. in order to maximise the output. In the case
where another acid is used, the concentration of this acid will
vary preferably between 10 and 30%.
[0038] To carry out the purification by mechanical means, the
graphite is milled by air milling, until the particles attain a
size of approximately 201 .mu.m. The impurities are then separated
by flotation.
DRAWINGS
[0039] The invention will now be illustrated with the aid of the
annexed drawings that are solely given for illustrative purposes
and without limitations, in which
[0040] FIG. 1 is a diagram of the conventional method;
[0041] FIG. 2 is a diagram of the method according to the
invention;
[0042] FIG. 3 illustrates the formation of an anode from graphite
particles obtained according to the method of this invention;
[0043] FIG. 4 illustrates a particle of graphite according to the
invention formed from several crystallites; and
[0044] FIG. 5 illustrates the exfoliation of graphite in a particle
according to the invention.
[0045] In referring to the drawings, it can be seen that according
to the conventional method illustrated in FIG. 1, where
purification proceeds grinding, there still remain many impurities
on the surface. In addition, according to the method of the
invention when the mineral contains calcium, there are no more
impurities found on the surface. All that is found, are particles
of CaF.sub.2 which are produced during the course of purification,
par chemical means precisely.
[0046] FIG. 3 illustrates the formation of an anode with particles
of graphite obtained according to the method of the invention,
which are deposited on a collector. During reduction, there is the
formation of a passivating film, which is at the same time an ionic
inductor, and electronic isolator, which represent ideal conditions
for electrochemistry.
[0047] FIG. 4 shows that the graphite particle according to the
invention is constituted by a group of crystallites. During the
insertion of lithium into the graphite, it is very important that
the Lc is controlled. In this case, very strong acids have been
used for the purification, the Lc becomes very small and graphite
illustrated in FIG. 5 is obtained.
[0048] The following examples are given to illustrate the preferred
embodiments of the invention, and should not be considered as
limiting its scope.
EXAMPLE 1
[0049] A natural graphite having an initial particle size of 375
.mu.m is ground by a process of air milling until the particle size
reaches 10 .mu.m. The size of the main particles obtained (50%
distribution of particles or D50%) is 10.52 .mu.m. The Gaussian
distribution of graphite has only one maximum and no additional
peak. The granulometric distribution was determined with the aide
of a Microtrac.TM. particle analyser built and sold by Leeds &
Northrul. The methanol was used as the carrier fluid. Subsequently,
the ground graphite was leached in an aqueous bath of 30% HF. The
temperature of the mixture is maintained at 90.degree. C., with a
leached time of 180 minutes. The graphite is then filtered, washed
with copious amounts of water, and the powder dried for a period of
24 hours at 120.degree. C.
[0050] The graphite powder obtained is analysed by reversed
diffusion coupled with EDX (Energy Dispersive X-ray). No
exfoliation of the particles was observed. In addition the analysis
by EDX shows that the majority of the impurities remaining are
constituted by calcium. The purity of this sample is 99.6%, as
obtained by the analysis of the impurities found in the ashes after
incineration.
[0051] The graphite is mixed with a binder of polyvinylidene
fluoride (PVDF) (Kruha: KF-1700) and with N-methylpyrolidone in a
mass ratio of 90:10. The mixture is then applied to a collector of
copper by the method of Doctor Blade.TM..
[0052] The graphite electrode thus obtained is dried under vacuum
at 120.degree. C. for 24 hours. The said electrode is placed in a
button size battery of type 2035 (diameter 20 mm, thickness 3,5
mm). A Celgard.TM. separator 2300 soaked in electrolyte 1M
LiPF.sub.6+EC/DMC: 50/50 (ethylene carbonate+dimethylcarbonate) is
used. The metallic lithium is used as a reference and a
counter-electrode.
[0053] The electrochemical tests were conducted at ambient
temperature. The discharge and charge curves were obtained between
0 V and 2.5 V in C/24. The first coulombic output is 85%, which is
superior to commercial graphite used in lithium-ion batteries
(typically 81%). The reversible capacity is 365 mAh/g equivalent at
x=0.98 in Li.sub.xC.sub.6. This obtained value is very close to the
theoretical value for graphite (372 mAh/g). No negative effect is
associated with the presence of residual Ca impurities is
observed.
EXAMPLE 2
[0054] Natural graphite having an initial particle size of 375
.mu.m is ground by a process of air-milling until the particles
attain a size of 10 .mu.m. The graphite is then leached in a mixed
aqueous bath comprising 30% H.sub.2SO.sub.4 and 30% HF. 106.5 ml of
the mixed acid is heated to 90.degree. C., and 30 g of graphite is
then added into the solution. The graphite is leached for 180
minutes in a reactor. The solid is then filtered, washed with
copious amounts of water, and dried at 120.degree. C. for 24 hours.
The size (D50%) is 10.92 .mu.m, and this before and after
purification. The Gaussian distribution for the graphite has only
one single maximum without any peak.
[0055] The analysis of impurities in the graphite by EDX shows
mainly the presence of the elements Ca and F. Analysis of the
impurities of the residual ashes shows the graphite purity to be
99.68%. The electrode preparation and the electrochemical tests are
done using the same procedure described in example 1.
[0056] The coulombic efficiency of the first cycle is 90%. The
irreversible plateau caused by the passivating layer is formed
normally near 800 mV. This means that the elements Ca, F or
CaF.sub.2 have no influence on the formation of the passivating
film. The reversible capacity of graphite is 356 mAh/g, or
equivalent to x=0.96 for the formation Li.sub.xC.sub.6.
EXAMPLE 3
[0057] The natural graphite used in this example is processed in an
identical manner as that described in Example 2, with the exception
that the HF concentration is now 20%. Analysis of the impurities in
the graphite by EDX shows the main presence of the elements Ca and
F. The analysis of the impurities of the residual ashes shows the
graphite to have a purity of 99.75%. The preparation of the
electrode and the electrochemical tests were conducted with the
identical procedures described in example 1.
[0058] The coulombic efficiency of the first cycle is 89%. The
irreversible plateau due to the passivating layer is formed
normally near 800 mV. The reversible capacity of the graphite is
365 mAh/g, equivalent to x=0.98 for the formation
Li.sub.xC.sub.6.
EXAMPLE 4
[0059] The natural graphite used in this example is processed in an
identical manner as that described in Example 2, with the exception
that the concentration of HF is of 10%. The preparation of the
electrode and the electrochemical test are identical to the
procedures described in Example 1.
[0060] The coulombic efficiency of the first cycle is 75%. The
irreversible capacity of 106.7 mAh/g is very high compared to that
of the graphite in Examples 2 and 3, which were leached
respectively in HF 30% and HF 20%. The reversible capacity is 318
mAh/g, equivalent to x=0.85 for the formation of
Li.sub.xC.sub.6.
EXAMPLE 5
[0061] The natural graphite used in this example is processed in an
identical manner as that described in Example 2, with the exception
of the H.sub.2SO.sub.4--HF mixture wherein HF is replaced by
NH.sub.4F, also present in the concentration of 30%.
[0062] The analysis of the impurities in this graphite by EDX shows
the main presence of the elements Ca and F. The analysis of the
impurities of the residual ashes shows the graphite to have a
purity of 99.64%. The preparation of the electrode and the
electrochemical tests are identical to the procedures described in
Example 1.
[0063] The coulombic efficiency of the first cycle is 90%. The
irreversible capacity of the graphite is 44 mAh/g. The reversible
capacity is 352 mAh/g, equivalent to x=0.96 for the formation of
Li.sub.XC.sub.6.
EXAMPLE 6
[0064] The natural graphite used in this example is processed in an
identical manner as described in Example 2 with the exception that
in the H.sub.2SO.sub.4--HF mixture HF is replaced by NH.sub.4F, HF
(NH.sub.5F.sub.2) at a concentration of 30%.
[0065] The analysis of impurities of the graphite by EDX shows the
presence, in majority, of Ca and F. The analysis of the impurities
of the residual ashes shows the graphite to have a purity of
99.57%. The preparation of the electrode and the electrochemical
tests are identical to those described in Example 1.
[0066] The coulombic efficiency of the first cycle is 88%. The
irreversible capacity is 49 mAh/g. The reversible capacity is 346.7
mAh/g, equivalent to x=0.93 for the formation Li.sub.xC.sub.6.
EXAMPLE 7
[0067] A natural graphite having an initial particle size of 375
.mu.m is ground by a process of air-milling until the particles
achieve a size of 10 .mu.m. The graphite is leached in two steps.
Initially, with an aqueous solution of 30% HCl, followed by an
aqueous solution of 30% HF. For each leach step, 106.5 ml of acid
solution is heated to 90.degree. C., and 30 g of graphite are
added. The graphite is leached for 180 minutes in a reactor. The
solid is filtered, washed with copious amounts of water and dried
at 120.degree. C. for 24 hours.
[0068] The size (D50%) is 10.02 .mu.m. The Gaussian distribution of
graphite has a single maximum with no flattening.
[0069] The analysis of impurities in the graphite by EDX shows the
total absence of the elements Si and Ca. The element found in the
impurities, in majority, is sulphur. An analysis of the impurities
found in the residual ashes shows the graphite to have a purity of
99.9%. The preparation of the electrode and the electrochemical
tests performed are identical to the procedures described in
Example 1.
[0070] The coulombic efficiency of the first cycle is 88%. The
irreversible plateau of the passivating layer is normally formed at
800 mV. One can conclude that the presence of sulphur has no
harmful effect on the formation of the passivating film. The
reversible capacity of the graphite is 357 mAh/g which is
equivalent to x=0.96 for the formation of Li.sub.xC.sub.6.
EXAMPLE 8
[0071] The natural graphite used in this example was processed in
the same manner identical to that described in Example 7 but
replacing HCl with HNO.sub.3.
[0072] The size (D50%) of graphite is 10.26 .mu.m. The Gaussian
distribution of the graphite has only one maximum and no additional
peak.
[0073] The analysis of the impurities of the graphite by EDX shows
the presence of the elements Ca and Mg, but the complete absence of
Si. The analysis of impurities found in the residual ashes shows
the graphite purity to be 99.96%. The preparation of the electrode
and the electrochemical tests are identical to the procedures
described in Example 1.
[0074] The coulombic efficiency of the first cycle is 86%. The
irreversible plateau of the passivating layer is formed normally
near 800 mV, which confirms that the elements Ca and Mg have no
influence on the formation of the passivating layer. The reversible
capacity of graphite is 353 mAh/g which is equivalent to x=0.95 for
the formation of Li.sub.xC.sub.6.
EXAMPLE 9
[0075] The natural graphite used for this example is processed in
the same manner as that described in Example 7 with HCl being
replaced by a base notably KOH, at a concentration of 30%.
[0076] The analysis of the impurities of graphite by EDX shows the
presence of the elements Ca and F. The analysis of impurities found
in the residual ashes shows the graphite to have a purity of
99.77%.
[0077] The coulombic efficiency of the first cycle is 88%. The
reversible capacity of graphite is 345 mAh/, which is equivalent to
x=0.93 for the formation of Li.sub.xC.sub.6.
EXAMPLE 10
[0078] A graphite comprising particles of 375 .mu.m is initially
purified in a bath of HF according to the following procedure.
106.5 ml of acid are heated to 90.degree. C., and 30 g of graphite
are added. The graphite is leached for 180 minutes in the reactor.
The solid is then filtered, washed with copious amounts of water
and dried at 120.degree. C. for 24 hours.
[0079] Subsequently, the graphite is ground until the particle size
reaches 10 .mu.m by the procedure previously described. The size
(D50%) is 10.67 .mu.m. The distribution of graphite has two maxima
with a flattening of the level at a size>18 .mu.m. It is well
known that this type of distribution is less favourable to the
optimal functioning of carbon-lithium anodes, and illustrates the
better uniformity of natural graphite ground before its
purification.
[0080] The analysis of impurities of the graphite by EDX shows the
presence of Ca and Mg in large quantity as impurities. An analysis
of the impurities found in the residual ashes shows the graphite to
have a purity of 99.43%. The preparation of the electrode and the
electrochemical tests are identical to the procedures described in
Example 1.
[0081] The coulombic efficiency of the first cycle is 75%. The
value of 110 mAh/g of the irreversible capacity is high due to its
purity (99.43%) and its elevated specific surface (7.08 m.sup.2/g).
The reversible capacity of the graphite is 356 mAh/g, which is
equivalent to x=0.96 in the formation of Li.sub.xC.sub.6.
EXAMPLE 11
[0082] The graphite used in this example is prepared according to
the method of Example 10. The conditions of purification and
grinding are the same as used in Example 7.
[0083] The size (D50%) of the graphite particles is 12.40 .mu.m.
The distribution has two maxima, the first at 10 .mu.m, and the
second at 17.7 .mu.m.
[0084] The analysis of impurities of the graphite by EDX shows the
presence of an intense peak for Si followed by one for Mg and
another for Ca. The analysis of impurities found in the residual
ashes shows the graphite to have a purity of 99.95%. The
preparation of the electrode and the electrochemical tests are
identical to the procedures described in Example 1.
[0085] The coulombic efficiency of the first cycle is 86%. The
irreversible capacity is 59 mAh/g. The reversible capacity is 362
mAh/g, which is equivalent to x=0.97 for the formula
Li.sub.xC.sub.6.
EXAMPLE 12
[0086] The natural graphite used in this example is processed in a
manner identical to that of Example 11 but replacing HCl for
HNO.sub.3.
[0087] The particle size (D50%) of the graphite is 12.11 .mu.m. The
distribution has two maxima, a first at 8 .mu.m and a second at 18
.mu.m.
[0088] The analysis of impurities in the graphite by EDX shows the
presence of Ca. The analysis of the impurities found in the
residual ashes shows the graphite to have a purity of 99.99%. The
preparation of the electrode and the electrochemical tests are
identical to the procedures described in Example 0.1.
[0089] The coulombic efficiency of the first cycle is 88%. The
irreversible capacity is 56 mAh/g. The reversible capacity is 396.9
mAh/g which is equivalent to x=1.066 according to the formulation
Li.sub.xC.sub.6.
[0090] As we can see in Examples 10 through 12, in purifying before
grinding the graphite produced has physical and chemical properties
which are less advantageous.
EXAMPLE 13
[0091] According to the procedure of Example 1, the initial size of
graphite is taken from 375 .mu.m to 10 .mu.m by a process of
grinding. The graphite is then subjected to a thermal treatment at
high temperature (2,800.degree. C.) for 2 hours. The analysis of
the impurities in the graphite by EDX shows the absence of
these.
[0092] The coulombic efficiency of the first cycle is 87%. The
irreversible capacity is 54.7 mAh/g. The reversible capacity is 346
mAh/g, which is equivalent to x=0.93 for the formation of
Li.sub.xC.sub.6.
EXAMPLE 14
[0093] In this example, natural Chinese graphite is used having a
higher Si content but a lower content of Ca than graphite from
StratminGraphite.
[0094] The leaching and the electrochemical preparation of the
graphite are identical to those of Example 2.
[0095] The coulombic efficiency of the first cycle is 88%. The
reversible capacity is 350 mAh/g, which is equivalent to x=0.94 for
the formation of Li.sub.xC.sub.6.
[0096] From this example we note that natural graphite containing
impurities of silicon (silicate) in relatively low concentrations
with respect to Ca and Mg impurities, is favourable to the
preferred embodiments of the invention.
[0097] This purification process allows the control of the
interplanar distance L.sub.c (FIG. 3). In the field of lithium-ion
batteries, the control of this parameter minimises the co-insertion
of solvent and stabilises the passivating film. The fact that in
this purification process, the grinding of particles is done first
and the purification is done second, permits the removal of all new
edges which appear after grinding with oxygen bonds, this fact
reduces the level of oxygen in the product. This process at the
same time eliminates oxidation of graphite and reduces the
irreversible capacity due to the formation of LiO.sub.2.
EXAMPLE 15
[0098] A natural graphite of Brazilian origin (table 1) having an
initial particle size of 323 .mu.m is ground by a process of
air-milling until the particles reach a size of 20 .mu.m. This
powder is processed in a manner identical to that described in
Example 5, with a H.sub.2SO.sub.4--NH.sub.4F mixture at a
concentration of 30%.
[0099] The analysis of the impurities found in the residual ashes
show the graphite to have a purity of 99.99%. The specific surface
is 4.285 m.sup.2/g. The preparation of the electrode and the
electrochemical tests are identical to the procedures described in
Example 1.
[0100] The coulombic efficiency of the first cycle is 91.0%. The
irreversible capacity of the graphite is 37 mAh/g. The reversible
capacity is 353 mAh/g which is equivalent to x=0.95 according to
the formation of Li.sub.XC.sub.6.
EXAMPLE 16
[0101] A natural graphite from Lac Knife (Quebec, Canada) (table 2)
having an initial particle size of 323 .mu.m is ground by an
air-milling process until the particles reach a size of 20 .mu.m
whereas the level of oxygen is 1.9%. The powder is processed in an
identical manner to that described in Example 5, with a
H.sub.2SO.sub.4--NH.sub.4F mixture at a concentration of 30%. After
leaching, the level of oxygen is reduced to 0.18%. It is clear that
the process according to the invention greatly reduces the level of
oxygen.
[0102] The analysis of the impurities found in the residual ashes
shows the graphite to have a purity of 99.24%. The specific surface
is 2.696 m.sup.2/g. The preparation of the electrode and the
electrochemical tests are identical to the procedures described in
Example 1.
[0103] The coulombic efficiency of the first cycle is 91%. The
irreversible capacity of the graphite is 36 mAh/g. The reversible
capacity is 353 mAh/g which is equivalent to x=0.95 according to
the formation Li.sub.XC.sub.6.
EXAMPLE 17
[0104] The natural graphite used in this example is identical to
that used in Example 15. It was processed in an identical manner as
that described in Example 5 however replacing the H.sub.2SO.sub.4
with HCl.
[0105] The analysis of impurities found in the residual ashes shows
the graphite to have a purity of 99.99%. The preparation of the
electrode and the electrochemical tests are identical to the
procedures described in Example 1.
[0106] The coulombic efficiency of the first cycle is 89.4%. The
irreversible capacity of the graphite is 43 mAh/g. The reversible
capacity is 367 mAh/g which is equivalent to x=0.986 according to
the formation Li.sub.XC.sub.6.
EXAMPLE 18
[0107] The natural graphite used in this example is identical to
that used in Example 15 with an average particle size of 10 .mu.m.
The graphite is leached in a bath with an aqueous mixture
comprising 150 ml of HCl (30%) and 1.0 g of CaF.sub.2. The mixture
is heated to 90.degree. C., and 20 g of graphite are added to the
solution. The graphite is leached for 180 minutes in a reactor. The
solid is then filtered, washed with copious amounts of water and
dried at 120.degree. C. for 24 hours.
[0108] The analysis of impurities found in the residual ashes shows
the graphite to have a purity of 99.84%. The preparation of the
electrode and the electrochemical tests are identical to the
procedures described in Example 1.
[0109] The coulombic efficiency of the first cycle is 85%. The
irreversible capacity of the graphite is 64 mAh/g. The reversible
capacity is 366 mAh/g, which is equivalent to x=0.98 according to
the formation Li.sub.XC.sub.6.
EXAMPLE 19
[0110] The natural graphite used in this example is identical to
that used in Example 18. The natural graphite is processed in the
same manner as described in Example 18 but replacing HCl with
H.sub.2SO.sub.4 (30%) and CaF.sub.2 by 1.5 g of LiF.
[0111] The analysis of impurities of the residual ashes shows the
graphite to have a purity of 99.99%. The preparation of the
electrode and the electrochemical tests are identical to the
procedures described in Example 1.
[0112] The coulombic efficiency of the first cycle is 82%. The
irreversible capacity of the graphite is 77.5 mAh/g. The reversible
capacity is 365 mAh/g which is equivalent to x=0.98 according to
the formation Li.sub.XC.sub.6.
1TABLE 1 Brazilian graphite (NATIONAL DE GRAFITE LTDA) CHEMICAL
SUBSTANCE (PPM) Fe Mo Sb As V Cr Cu Ni Pb Co Ca Al Ge Si 103 1.5
<1 <0.5 9.7 6.5 4.2 0.3 <1 0.5 3 528.8 <1 0.3
[0113]
2TABLE 2 Lac Knife graphite (MAZARIN) CHEMICAL SUBSTANCE (PPM) Ba
Be Cd Ce Co Cu Dy Eu La Li Mo Nd Ni Pb Pr 320 <1 <2 59 6 1000
<1 <1 47 73 2400 <25 404 410 <10 Sc Sm V Zn Ga Nb Rb Sn
Sr Ta Te Th U Y Zr 17 20 2600 775 21 23 200 <10 180 <5 <10
<3 64 55 650
[0114] Although the present invention is described by specific
embodiments, it is understood that many variations and
modifications can be added on top of the embodiments, the present
application attempts to cover such modifications, uses or
adaptations of the present invention covering in general, the
principles of the invention including all variations of the present
description which become known or conventional in the field of
activity in which this present invention is found, and which can be
applied to the essential elements mentioned above, and in agreement
with the range of the following claims.
* * * * *