CATHODE FOR A CELL OF A LITHIUM-ION BATTERY, ITS
MANUFACTURING PROCESS AND THE BATTERY INCORPORATING IT
5 The present invention relates to a cathode that is
usable in a cell of a lithium-ion battery, to a process
for manufacturing this cathode, and to a lithium-ion
battery having one or more cells incorporating this
cathode.
10 There are two main types of lithium storage
battery: lithium-metal batteries, where the negative
electrode is made of lithium metal (which material
causes problems with safety when in the presence of a
liquid electrolyte), and lithium-ion batteries, where
15 the lithium remains in the ionic state.
Lithium-ion batteries consist of at least two
conductive Coulombic electrodes of different
polarities, the negative electrode or anode (generally
made of graphite) and the positive electrode or cathode
20 (generally made of an oxide of a transition metal, such
as an oxide of vanadium or cobalt, or made of a
lithiated iron phosphate such as, for example,
described in documents US-B1-6 514 640 or WO-A1-
2011/092283), between which electrodes a separator is
25 located, which separator consists of an electrical
insulator imbibed with an aprotic electrolyte based on
Li+ cations ensuring the ionic conductivity. The
electrolytes used in these lithium-ion batteries
usually consist of a lithium salt, for example of
30 formula LiPF6, LiAsF6, LiCF3SO3 or LiClO4, which is
dissolved in a mixture of non-aqueous solvents such as
acetonitrile, tetrahydrofuran, or more often a
carbonate, for example of ethylene or propylene.
The active material of the cathode of a lithium35
ion battery allows reversible insertion/removal of
lithium into/from this cathode, and the higher the mass
fraction of this active material in the cathode, the
higher its capacity. The cathode must also contain an
electrically conductive compound, such as carbon black
40 and, in order to provide it with sufficient mechanical
cohesion, a polymer binder. A lithium-ion battery is
thus based on the reversible exchange of lithium ions
between the anode and the cathode during the charging
and discharging of the battery, and, for a very low
5 weight, by virtue of the physical properties of
lithium, such a battery has a high energy density.
The cathodes of lithium-ion batteries are most
often manufactured using a process comprising, in
succession, a step of dissolving or dispersing the
10 various ingredients of the cathode in a solvent, a step
of spreading the obtained solution or dispersion on a
metallic current collector, and then lastly a step of
evaporating this solvent. Many types of polymer binders
can be used, among which mention may firstly be made of
15 PVDF (polyvinylidene fluoride), which is more easily
compatible with a cathode operating at a high operating
voltage (more than 4 V) because of the presence of
fluorine, but also, for example, polyacrylonitriles
(PAN) with polybutylacrylate latexes.
20 Processes for manufacturing the cathodes of
lithium-ion batteries that use an organic solvent have
many drawbacks with respect to the environment and
safety. In particular, it is, in this case, necessary
to evaporate large amounts of such solvents, which are
25 toxic or inflammable.
As for processes that use an aqueous solvent to
manufacture these cathodes, their major drawback is
that the cathode must be very thoroughly dried before
they can be used, traces of water being known to limit
30 the useful lifetime of lithium storage batteries.
It is therefore highly desirable, for lithium-ion
batteries, to prepare cathodes that are manufactured
without using solvents. It is in this context that
processes for manufacturing cathodes for lithium-ion
35 batteries using melt processing techniques (for example
extrusion) have been described in the literature.
Unfortunately, these melt processes cause major
difficulties in the case of lithium-ion batteries,
which, as is known, require a mass fraction of active
40 material in the polymer mixture of the cathode of at
least 90% for the latter to have sufficient capacity in
the lithium-ion battery. However, at such contents of
active material the viscosity of the cathode polymer
mixture becomes very high, and leads to a risk of the
5 mixture over-heating or losing its mechanical cohesion
once it is in use.
Document US-B2-6 939 383 describes the extrusion
of a polymer composition comprising a poly(ethylene
oxide)-poly(propylene oxide)-poly(glycidyl ether)
10 copolymer for a ionically conducting polymer for
solventless implementation of a cathode for a lithiumpolymer
battery. However, the mass fraction of active
material in the single cathode polymer composition
manufactured in this document is only 64.5%.
15 Document US-A-5 749 927 discloses a process for
the continuous preparation of lithium-polymer batteries
by extrusion, which process comprises compounding the
active material with an electrical conductor and a
solid electrolyte composition comprising a polymer, a
20 lithium salt and a propylene carbonate/ethylene
carbonate mixture greatly in excess over this polymer.
In this document, the mass fraction of active material
present in the cathode polymer composition is also
below 70%.
25 Thus, a major drawback of these known melt
processes for manufacturing cathodes for lithium
storage batteries is that the mass fractions of active
material in the cathode polymer composition are
insufficient to obtain high-performance cathodes
30 specifically for lithium-ion batteries.
One aim of the present invention is therefore to
provide a process, for manufacturing a cathode, that
overcomes all of the aforementioned drawbacks, and this
aim is achieved since the Applicant has discovered,
35 surprisingly, that if an active material and additives
comprising a crosslinked elastomer matrix, an
electrically conductive filler and a non-volatile (i.e.
having a boiling point above 150oC at an atmospheric
pressure of 1.013×105 Pa) organic compound, are hot
40 compounded, without solvent evaporation, then a cathode
polymer composition is obtained that is usable in a
lithium-ion battery comprising an electrolyte based on
a lithium salt and on a non-aqueous solvent, with a
fraction of this active material in the composition
5 clearly higher than those obtained previously by melt
processing, and advantageously greater than or equal to
90%, the one or more organic compounds advantageously
being used as a solvent for the electrolyte.
A cathode according to the invention, usable in a
10 cell of a lithium-ion battery comprising an electrolyte
based on a lithium salt and on a non-aqueous solvent,
is thus based on a polymer composition, obtained by
melt processing and without solvent evaporation, that
is the product of a hot compounding reaction between an
15 active material and additives comprising a polymer
binder and an electrically conductive filler, and the
cathode is such that the binder is based on at least
one crosslinked elastomer and such that these additives
furthermore comprise at least one non-volatile organic
20 compound usable in this electrolyte solvent, the
composition comprising the active material in a mass
fraction advantageously greater than or equal to 90%.
It will be noted that this very high mass fraction
of active material in the cathode according to the
25 invention ensures that the or each cell obtained is a
high-performance cell and therefore that the lithiumion
battery incorporating them is a high-performance
battery.
It will also be noted that the uniform
30 distribution of said at least one crosslinked elastomer
in the composition ensures the mechanical strength of
the cathode.
Advantageously, said active material may comprise
at least one lithiated polyanionic compound or complex
35 having an operating voltage below 4 V and preferably
carbon coated, such as a lithiated metal M phosphate of
formula LiMPO4 (also called phosph-olivine), such as a
carbon-coated lithium-iron phosphate having the formula
C-LiFePO4.
Specifically, it will be noted that the active
material used in the composition of the present
invention may consist of elementary carbon-coated
particles or agglomerates of elementary particles
5 comprising a carbon coating or deposit.
Preferably, said at least one elastomer is a
peroxide-crosslinked diene elastomer and, even more
preferably, a hydrogenated nitrile rubber (HNBR). Also
preferably, said at least one elastomer may be present
10 in said composition in a mass fraction between 1% and
5%.
Advantageously, said at least one non-volatile
organic compound may comprise a carbonate, preferably a
carbonate of at least one olefin such as ethylene,
15 which is preferably used in the electrolyte
composition.
It will be noted that using such a carbonate, such
as an ethylene carbonate, advantageously allows:
- the filler content in the composition to be
20 increased;
- the inherent risks with respect to the toxicity
of volatile organic compounds (VOC), used in
conventional processes for manufacturing cathodes, to
be avoided because this carbonate is a product that is
25 solid at room temperature and much less hazardous to
handle; and
- the cathode polymer composition to be used
without evaporating the carbonate beforehand, and the
incorporation of the electrolyte into the cathode to be
30 made easier because this carbonate is one of the main
constituents of electrolytes used at the present time
in lithium-ion batteries.
Also advantageously, said at least one organic
compound may be present in said composition in a mass
35 fraction between 0.1% and 5%.
It will be noted that the invention allows the
salts required for operation of the cathode to be
incorporated during its manufacture.
According to another feature of the invention,
40 said additives may furthermore comprise a crosslinking
system that is present in the composition in a mass
fraction between 0.05% and 0.20%, and that preferably
comprises an organic peroxide and a crosslinking coagent
in the case where said at least one elastomer is
5 a diene elastomer such as a hydrogenated nitrile rubber
(HNBR).
According to another feature of the invention,
said electrically conductive filler may be chosen from
the group consisting of carbon black, graphite,
10 expanded graphite, carbon fibres, carbon nanotubes,
graphene and their mixtures, and is present in the
composition in a mass fraction between 1% and 5%.
A process, according to the invention, for
manufacturing a cathode such as defined above, is
15 characterized in that it comprises:
a) melt compounding, without solvent evaporation,
in an internal mixer or an extruder, of said active
material and said additives comprising said binder and
said organic compound in the solid state in order to
20 obtain said composition in the crosslinkable state,
this active material preferably comprising at least one
lithiated polyanionic compound or complex such as a
carbon-coated lithiated iron phosphate of formula CLiFePO4;
and
25 b) crosslinking and optionally hot forming this
composition, in order to obtain said crosslinked
composition.
According to another feature of the invention,
step a) may be carried out by compounding said binder
30 into a powder premix of the other ingredients of the
composition, for example at a temperature between 60oC
and 80oC in an internal mixer.
According to another feature of the invention,
step b) may be carried out by hot pressing the
35 crosslinkable composition.
Advantageously, the process of the invention may
then comprise a step c) of rolling said crosslinked
composition so as to deposit it on a metallic current
collector with which said cathode is equipped.
A lithium-ion battery according to the invention
comprises at least one cell comprising an anode, for
example a graphite-based anode, a cathode such as
defined above, and an electrolyte based on a lithium
5 salt and on a non-aqueous solvent.
According to another advantageous feature of the
invention said electrolyte solvent may comprise said at
least one non-volatile organic compound of the cathode.
According to another aspect of the invention, said
10 cathode comprises a metallic current collector making
contact with at least one film made of said polymer
composition.
Other features, advantages and details of the
present invention will become clear from reading the
15 following non-limiting description of an exemplary
embodiment of the invention, given by way of
illustration.
Example 1:
20
A cathode polymer composition was prepared in an
Haacke internal mixer at 70oC, the composition
comprising the following constituents expressed in mass
fraction (%):
25
HNBR binder (“Therban 4307”) 2.68
Carbon black 2.68
Ethylene carbonate 0.54
Active material C-LiFePO4 93.97
Crosslinking system:
Dicumyl peroxide 0.08
Triallyl cyanurate (TAC) 0.05
The various compounds were introduced into this
internal mixer in succession, starting with the
hydrogenated nitrile rubber by way of a crosslinkable
30 diene elastomer (HNBR binder), and then a premix in
powder form of the other ingredients above. After this
compounding, and hot pressing at 170oC for 10 minutes
simultaneously allowing the binder to be crosslinked, a
1 mm-thick electrode able to form a cathode inside a
cell of a lithium-ion battery, after deposition on a
current collector with which this cathode is equipped,
5 was obtained directly.
It will be noted that the very high mass fraction
(higher than 93%) of active material in this cathode
ensures that the or each cell obtained is a highperformance
cell and therefore that the lithium-ion
10 battery incorporating them is a high-performance
battery.
“Control” example, not according to the invention:
15 A “control” composition having the same
formulation (i.e. the same amounts of the same
ingredients) as that of Example 1 was prepared by
dispersing/dissolving said ingredients in a MIBK
(methyl isobutyl ketone) solvent – i.e. using a process
20 that did not conform to the melt process of the
invention – this “control” composition being deposited
by coating on a current collector.
It has be observed that the “control” cathode
obtained by dispersing/dissolving had intrinsic
25 physical properties that were very different to those
of the cathode of Example 1, especially in terms of
morphology (scanning electron microscope “SEM”
micrographs), bulk density and electrical conductivity,
as may be seen in Table 1 below.
30 In particular, it may be noted that the bulk
density of the cathode obtained by melt processing
(Example 1, without solvent) is clearly higher than 1 –
lying between 1.5 and 2 – i.e. more than two times
higher than the bulk density of the “control” cathode
35 obtained with solvent.
Table 1:
Cathode of
Example 1
“Control”
cathode
Bulk density 1.854 0.777
Electrical
conductivity (S/cm)

CLAIMS
1. Cathode usable in a cell of a lithium-ion
battery comprising an electrolyte based on a lithium
5 salt and on a non-aqueous electrolyte solvent, the
cathode being based on a polymer composition, obtained
by melt processing and without solvent evaporation,
that is the product of a hot compounding reaction
between an active material and additives comprising a
10 polymer binder and an electrically conductive filler,
characterized in that said binder is based on at least
one crosslinked elastomer and in that said additives
furthermore comprise at least one non-volatile organic
compound usable in said electrolyte solvent, the
15 composition comprising said active material in a mass
fraction greater than or equal to 90%.
2. Cathode according to Claim 1, characterized in
that said active material comprises at least one
20 lithiated polyanionic compound or complex having an
operating voltage below 4 V and preferably carbon
coated, such as a lithiated metal M phosphate of
formula LiMPO4, where M is for example an iron atom.
25 3. Cathode according to Claim 1 or 2,
characterized in that said at least one elastomer is a
peroxide-crosslinked diene elastomer, preferably a
hydrogenated nitrile rubber (HNBR).
30 4. Cathode according to one of the preceding
claims, characterized in that said at least one
elastomer is present in said composition in a mass
fraction between 1% and 5%.
35 5. Cathode according to one of the preceding
claims, characterized in that said at least one organic
compound comprises a carbonate, preferably a carbonate
of at least one olefin such as ethylene.
6. Cathode according to one of the preceding
claims, characterized in that said at least one organic
compound is present in said composition in a mass
fraction between 0.1% and 5%.
5
7. Cathode according to one of the preceding
claims, characterized in that said additives
furthermore comprise a crosslinking system that is
present in said composition in a mass fraction between
10 0.05% and 0.20%, and that preferably comprises an
organic peroxide and a crosslinking co-agent in the
case where said at least one elastomer is a diene
elastomer such as a hydrogenated nitrile rubber (HNBR).
15 8. Cathode according to one of the preceding
claims, characterized in that said electrically
conductive filler is chosen from the group consisting
of carbon black, graphite, expanded graphite, carbon
fibres, carbon nanotubes, graphene and their mixtures,
20 and is present in said composition in a mass fraction
between 1% and 5%.
9. Process for manufacturing a cathode according
to one of the preceding claims, characterized in that
25 it comprises:
a) melt compounding, without solvent evaporation,
in an internal mixer or an extruder, of said active
material and said additives comprising said binder and
said organic compound in the solid state in order to
30 obtain said composition in the crosslinkable state,
this active material preferably comprising at least one
lithiated polyanionic compound or complex such as a
carbon-coated lithiated iron phosphate of formula CLiFePO4;
and
35 b) crosslinking and optionally hot forming this
composition, in order to obtain said crosslinked
composition.
10. Manufacturing process according to Claim 9,
40 characterized in that step a) is carried out by
compounding said binder into a powder premix of the
other ingredients of the composition, for example at a
temperature between 60oC and 80oC in an internal mixer.
5 11. Manufacturing process according to Claim 9 or
10, characterized in that step b) is carried out by hot
pressing said crosslinkable composition.
12. Manufacturing process according to one of
10 Claims 9 to 11, characterized in that it then comprises
a step c) of rolling said crosslinked composition so as
to deposit it on a metallic current collector with
which said cathode is equipped.
15 13. Lithium-ion battery comprising at least one
cell comprising an anode, for example a graphite-based
anode, a cathode and an electrolyte based on a lithium
salt and on a non-aqueous solvent, characterized in
that said cathode is as defined in one of Claims 1 to
20 8.
14. Lithium-ion battery according to Claim 13,
characterized in that said electrolyte solvent
comprises said at least one non-volatile organic
25 compound of the cathode.
15. Lithium-ion battery according to Claim 13 or
14, characterized in that said cathode comprises a
metallic current collector making contact with at least
30 one film made of said polymer composition.