Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED LITHIUM ION ELECTROCHEMICAL CELLS
The present invention relates to improvements in the construction of lithium
ion
electrochemical cells, including capacitors, supercapacitors and batteries, by
means of
an improved negative electrode (anode) comprising a mesoporous material that
is active
for lithium insertion.
The mesoporous materials used in the present invention are sometimes referred
to as "nanoporous". However, since the prefix "nano" strictly means 10-9, and
the
pores in such materials may range in size from 10-8 to 10-9 m, it is better to
refer to
them, as we do here, as "mesoporous".
Although, strictly speaking, the term "battery" means an arrangement of two or
more cells, it is used here with its common meaning of a device for storing
and
releasing electrical energy, whether it comprises one or several cells.
The drive towards `convergence' in electronics, i.e. increasing the
functionality
of devices such as mobile phones and personal digital assistants (PDAs) has
increased
the demand for energy and power placed on batteries. At present, we believe
that the
greatest scope for capacity (energy) improvement lies in the development of
the
negative electrode. The majority of commercial lithium ion batteries currently
use
negative electrodes based on carbon. The charge storage capacity of carbon is
typically
in the region of 300 mAh/g.
Alternatives to carbon are materials which are capable of forming alloys with
lithium at low potentials, such as tin, silicon and aluminium. These materials
have
charge storage capacities up to 2000 mAh/g. However, insertion of lithium into
these
materials is accompanied by significant expansion of the structure. This
causes rapid
mechanical breakdown of the material and manifests in cell performance as poor
cycle
life. In addition, expansion of the electrode material during charging can
cause
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expansion of the entire battery, leading to other perfonnance and safety
concerns. As
such, the commercial realisation of these high capacity materials has to date
been
limited and carbon electrodes remain the dominant technology.
In Chemical Communications, 1999, 4, 331-332, J. R. Owen discloses a lithium
ion battery negative electrode consisting of an electrodeposited tin film made
using a
liquid crystal templating route. The paper states: "It would be expected that
extensive
mesoporosity would significantly reduce internal stresses during expansion and
thus
decrease the mechanical degradation of the electrodes". Cycle life was found
to be
poor, however.
We have now surprisingly found that, by engineering higher than normal levels
of porosity into liquid crystal templated materials, these materials are
better able to cope
with expansion on lithiation and so can provide superior cycle life and
reduced overall
particle expansion. Alternatively, higher capacities may be achieved since the
higher
porosity allows a greater degree of lithiation (expansion) before mechanical
breakdown
of the material begins. Particle expansion is reduced since the increased
porosity allows
more of the expansion to be accommodated by the internal mesopores of the
material
rather than outward expansion of the particle.
Porosities typically achieved in liquid crystal templating are in the range of
approximately 13% to 27%. Porosities in excess of 35% would be considered
unusually
high, and the usual porosity is around 23%. However, we have found that
unexpected
benefits may be achieved by using significantly higher porosities.
Thus, the present invention consists in an electrochemical cell comprising a
positive electrode, a negative electrode and a non-aqueous electrolyte, where
the
negative electrode comprises a liquid crystal templated mesoporous material
capable of
forming a lithium insertion alloy, characterised in that the liquid crystal
templated
mesoporous material has a porosity of from 38% to 80%.
The invention is illustrated by the accompanying drawing, in which: Figure 1
compares the cycle life behaviour of two cells; one utilising an anode having
a porosity
of 51 % and made as described in Example 3 and another that used the same
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construction, the only difference being that 39% porous copper-tin material
(made as
described in Example 2, using a liquid crystal template) was used.
The preparation and use of liquid crystalline phases is disclosed in US
Patents
No 6,503,382 and 6,203,925, the disclosures of which are incorporated herein
by
reference.
The electrochemical cell of the present invention may be a capacitor,
supercapacitor or battery. Where it is a battery, this is normally a
secondary, i.e.
rechargeable, battery.
The material capable of forming a lithium insertion alloy may be an element (a
metal or metalloid) or it may be a mixture or alloy of one or more elements
capable of
forming a lithium insertion alloy with one or more elements which cannot form
such an
insertion alloy or a mixture or alloy of two or more elements each capable of
forming a
lithium insertion alloy. Examples of elements that are active for lithium
insertion are
aluminium, silicon, magnesium, tin, bismuth, lead and antimony. Copper is
inactive for
lithium insertion, but alloys of copper with an element, such as tin, which is
active may
themselves be active. Other inactive elements include nickel, cobalt and iron.
There is
an advantage in including these inactive alloying elements in that their
presence
effectively dilutes the active material so that less expansion occurs on
cycling, leading
to further improved cycle life. The preferred active element is tin, and this
is most
preferably used as an alloy with an inactive element, most preferably copper. -
It is a crucial aspect of the present invention that the porosity of the
material
active for lithium insertion should be from 38% to 80%. The porosity herein is
calculated from nitrogen porosimetry (BET) measurements. In general, we have
found
that cycle life improves as porosity increases. However, too high a porosity
will lead to
a reduction in the amount of active material present and so may detract from
cell
performance. Preferably the porosity is in the range from 42% to 75%, more
preferably
from 44% to 70%. Most preferably the porosity is from 50% to 65%.
The material active for lithium insertion is unlikely to have sufficient
mechanical strength on its own to serve as an electrode and, accordingly, it
is preferably
used in the electrochemical cell on a support, which may also function as a
current
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collector. The support material is thus preferably electrically conductive and
preferably
has sufficient mechanical strength to remain intact when formed into a film
which is as
thin as possible. Suitable materials for use as the support include copper,
nickel and
cobalt, of which copper is preferred both for its cost and its electrical
conductivity.
In order to enhance the conductivity of the electrode, the porous material is
preferably mixed with an electrically conductive powder, for example: carbon,
preferably in the form of graphite, amorphous carbon, or acetylene black;
nickel; or
cobalt. If necessary, it may also be mixed with a binder, such as ethylene
propylene
diene monomer (EPDM), styrene butadiene rubber (SBR), carboxy methyl cellulose
(CMC), polyvinyl diene fluoride (PVDF), polytetrafluoroethylene (PTFE),
polyvinyl
acetate or a mixture of any two or more thereof. The porous material,
electrically
conductive powder and optionally the binder may be mixed with an organic
solvent,
such as hexane, water, cyclohexane, heptane, hexane, or N-methylpyrrolidone,
and the
resulting paste applied to the support, after which the organic solvent is
removed by
evaporation, leaving a mixture of the porous material and the electrically
conductive
powder and optionally the binder.
The electrochemical cell also contains a positive electrode. This may be any
material capable of use as a positive electrode in a lithium ion cell.
Examples of such
materials include LiCoO2, LiMnO2, LiNiCoO2, or LiNiAlCoO2. Like the negative
electrode, this is preferably on a support, e.g. of aluminium, copper, tin or
gold,
preferably aluminium.
The electrolyte likewise may be any conventional such material, for example
lithium hexafluorophosphate, lithium tetraborate, lithium perchlorate, or
lithium -
hexafluoroarsenate, in a suitable solvent, e.g. ethylene carbonate, diethylene
carbonate,
dimethyl carbonate, propylene carbonate, or a mixture of any two or more
thereof.
The cell may also contain a conventional separator, for example a microporous
polypropylene or polyethylene membrane, porous glass fibre tissue or a
combination of
polypropylene and polyethylene.
Preparation of the mesoporous material used as the negative electrode in the
cells of the present invention may be by any known liquid crystal templating
method.
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For example, a liquid crystalline mixture is formed and a mesoporous material
is caused
to deposit from it. A variety of methods can be used to effect this
deposition, including
electrodeposition, electroless deposition, or chemical deposition. Of course,
to some
extent, the method of deposition used will depend on the nature of the
material to be
5 deposited.
For example, one method of preparing the mesoporous material comprises
electrodepositing material onto a porous support from a mixture comprising at
least one
source of said material, an organic directing agent and a solvent; by passing
charge
through said mixture until sufficient of said material has been deposited to
form a
mesoporous layer on said porous support; and then removing the organic
directing agent
to produce a mesoporous layer preferably having a substantially regular pore
structure
and uniform pore size within the desired range, e.g. from 2.5 to 50 nm, on
said porous
support.
The nature of the source material used in the mixture will depend on the
nature
of the material to be produced. For example, if the desired material is tin,
then a
compound of tin, e.g. SnBF4, ,SnCH3SO3, SnC14, or SnC12, should be used. If it
is
desired to produce a mixture of two or more elements, e.g. an active and an
inactive
element, then a mixture of the compounds of the respective elements should be
used.
Examples of source materials for inactive elements include CuBF4, CuSO4,
CuC12, or
CoC12.
The organic structure-directing agent is included in the mixture in order to
impart a homogeneous lyotropic liquid crystalline phase to the mixture. The
liquid
crystalline phase is thought to function as a structure-directing medium or
template for
deposition of the mesoporous layer. By controlling the nanostructure of the
lyotropic
liquid crystalline phase, mesoporous material may be synthesised having a
corresponding nanostructure. For example, porous materials formed from normal
topology hexagonal phases will have a system of pores disposed on a hexagonal
lattice,
whereas porous materials formed from normal topology cubic phases will have a
system
of pores disposed in cubic topology. Similarly, porous materials having a
lamellar
nanostructure may be deposited from lamellar phases. Accordingly, by
exploiting the
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rich lyotropic polymorphism exhibited by liquid crystalline phases, liquid
crystal
technology allows precise control over the structure of the porous materials
and enables
the synthesis of well-defined porous materials having a long range spatially
and
orientationally periodic distribution of uniformly sized pores.
Any suitable amphiphilic organic compound or compounds capable of forming a
homogeneous lyotropic liquid crystalline phase may be used as structure-
directing
agent, either low molar mass or polymeric. These may include compounds
sometimes
referred to as organic directing agents. In order to provide the necessary
homogeneous
liquid crystalline phase, the amphiphilic compound will generally be used at
an high
concentration, typically at least about 10% by weight, preferably at least 20%
by
weight, and more preferably at least 30% by weight, based on the total weight
of the
solvent, source material and amphiphilic compound.
Preferably, the organic structure-directing agent comprises an organic
surfactant
compound of the formula RQ wherein R represents a linear or branched alkyl,
aryl,
aralkyl or alkylaryl group having from 6 to about 60 carbon atoms, preferably
from 12
to 18 carbon atoms, and Q represents a group selected from: [O(CH2)m]õOH
wherein m
is an integer from 1 to about 4 and preferably m is 2, and n is an integer
from 2 to about
60, preferably from 4 to 12; nitrogen bonded to at least one group selected
from alkyl
having at least 4 carbon atoms, aryl, aralkyl and alkylaryl; and phosphorus or
sulphur
bonded to at least 2 oxygen atoms. Other suitable structure-directing agents
include
monoglycerides, phospholipids and glycolipids.
Other suitable compounds include surface-active organic compounds of the
formula R1RZQ wherein Rl and R2 represent aryl or alkyl groups having from 6
to about
36 carbon atoms or combinations thereof, and Q represents a group selected
from: -
(OC2H4) õOH, wherein n is an integer from about 2 to about 20; nitrogen bonded
to at
least two groups selected from alkyl having at least 4 carbon atoms, and aryl;
and
phosphorus or sulphur bonded to at least 4 oxygen atoms.
Preferably non-ionic surfactants such as octaethylene glycol monododecyl ether
(C 12E08 , wherein EO represents ethylene oxide) and octaethylene glycol
monohexadecyl ether (C16EO8) or commercial products containing mixtures of
related
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molecules are used as organic structure-directing agents. Other preferred
organic
directing agents include polyoxyalkylene derivatives of propylene glycol, such
as those
sold under the trade mark "Pluronic", and ionic surfactants such as CTAB.
It has been found that the pore size of the porous material can be varied by
altering the hydrocarbon chain length of the surfactant used as structure-
directing agent,
or by supplementing the surfactant by an hydrocarbon additive. For example,
shorter-chain surfactants will tend to direct the formation of smaller-sized
pores
whereas longer-chain surfactants tend to give rise to larger-sized pores. The
addition of
a hydrophobic hydrocarbon additive such as n-heptane, to supplement the
surfactant
used as structure-directing agent, will tend to increase the pore size,
relative to the pore
size achieved by that surfactant in the absence of the additive. Also, the
hydrocarbon
additive may be used to alter the phase structure of the liquid crystalline
phase in order
to control the corresponding regular structure of the porous metal. By a
suitable
combination of these methods, it is possible to control the pore size very
precisely and
over a wide range, extending to much smaller pore sizes (of the order of 1 nm)
than
could be achieved hitherto.
The solvent is included in the mixture in order to dissolve the source
material
and to form a liquid crystalline phase in conjunction with the organic
structure-directing
agent, thereby to provide a medium for deposition of the mesoporous material.
Generally, water will be used as the preferred solvent. However, in certain
cases it may
be desirable or necessary to carry out the deposition in a non-aqueous
environment. In
these circumstances a suitable organic solvent may be used, for example
formamide or
ethylene glycol.
In most cases, the source material will dissolve in the solvent domains of the
liquid crystalline phase, but in certain cases the source material may be such
that it will
dissolve in the hydrophobic domains of the phase.
The mixture may optionally further include a hydrophobic hydrocarbon additive
to modify the pore diameter of the porous metal, as explained more fully
above.
Suitable hydrocarbon additives include n-heptane, n-tetradecane and
mesitylene. The
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hydrocarbon additive may be present in the mixture in a molar ratio to the
structure-
directing agent in the range of 0.1 to 4, preferably 0.5 to 1.
Alternatively, the material of which the mesoporous layer is formed may be
deposited by electroless deposition. The procedure used to fabricate material
by
electroless deposition is essentially the same as that used in chemical
deposition,
described below. The essential difference is that, prior to application of the
liquid
crystal template to a support, the support is sensitised with a metal salt in
order to
promote deposition of the mesoporous material only on the support surface
rather than
throughout the liquid crystal. In summary, the reduction of a metal salt to a
metal is
facilitated by an appropriate reducing agent just as in chemical deposition.
The
presence of the sensitiser confines this deposition to the support surface. A
suitable
sensitiser is tin (II) chloride.
As a further alternative, the material of which the mesoporous material is
formed
may be a metal or other material capable of deposition by reduction or other
chemical
reaction. In this case, the mixture comprises a source material for the metal
or other
element, dissolved in a solvent, and a sufficient amount of an organic
structure-directing
agent to provide an homogeneous lyotropic liquid crystalline phase for the
mixture.
Examples of suitable source materials include compounds of the element which
are
capable of reduction to the element, for example, tin methanesulphonate,
copper
sulphate, SnBF4, SnC14, SnC12, CuBF4, CuC12. The nature of the solvent is not
critical, and is usually aqueo .us.
One or more source materials may be used in the mixture, for reduction to one
or more metals. Thus, by appropriate selection of source material, the
composition of
the porous metal can be controlled as desired. Suitable metals include those
described
above in relation to the electrodeposition method.
A reducing agent is used to reduce the mixture. Suitable ireducing agents
include metals (such as zinc, iron or magnesium), sodium hypophosphite,
dimethyl
borane, hydrogen gas, and hydrazine, preferably sodium hypophosphite or
dimethyl
borane.
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The structure directing agents and solvents used in this embodiment may be any
of those described above in relation to the electrodeposition method.
Typically, the pH of the mixture may be adjusted to a value in the range from
2
to 12. The temperature is generally maintained in the range from 15 to 100 C,
preferably 18 to 80 C, more preferably 20 to 60 C.
The mixture and reducing agent are left to stand for a sufficient period to
precipitate the porous material, typically overnight at room temperature.
Depending on
the nature of the reactants, the mixture may be left for a period of from 15
minutes to 4
weeks, and typically for about 24 hours. Following the reduction, it will
usually be
desirable to treat the porous material to remove the organic material
including the
structure-directing agent, hydrocarbon additive, unreacted source material and
ionic
impurities, for example by solvent extraction or by decomposition in nitrogen
and
combustion in oxygen (calcination). However, for certain applications such
treatment
may not be necessary.
The regular pore structure of the porous metal may for example be cubic,
lamellar, oblique, centred rectangular, body-centred orthorhombic, body-
centred
tetragonal, rhombohedral or hexagonal. Preferably the regular pore structure
is
hexagonal.
The invention is further illustrated by the following non-limiting Examples.
EXAMPLE 1 (COMPARATIVE)
Preparation of 20% Porosity Mesoporous Copper-tin Powder
72 g of BC10-TX surfactant (from Nikkol) was heated until molten. To this was
added a mixture containing 12.0 cm3 of 0.3 M tin(II) methanesulphonate
solution
(aqueous), 12.0 cm 3 of copper(II) sulphate solution (aqueous) and 0.63 g of
sodium
hypophosphite in 24 cm3 of deionised water. The resulting paste was stirred
vigorously
until homogeneous and then allowed to cool to room temperature and allowed to
stand
at room temperature overnight. The surfactant was removed from the resultant
product
via repeated washing in deionised water. The collected powder was dried in
air,
overnight at 60 C.
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EXAMPLE 2
Preparation of 39% Porosity Mesoporous Copper-tin Powder
72 g of BC10-TX surfactant was heated until molten. To this was added a
mixture containing 12.0 cm3 of 0.6 M tin(II) methanesulphonate solution
(aqueous),
5 12.0 cm3 of 0.6 M copper(II) sulphate solution (aqueous) and 0.42 g of
dimethylamine-
borane complex in 24 cm3 of deionised water. The resulting paste was stirred
vigorously until homogeneous and then allowed to cool to room temperature and
allowed to stand at room temperature overnight. The surfactant was removed
from the
resultant product via repeated washing in deionised water. The collected
powder was
10 dried in air, overnight at 60 C and was found to have an average pore size
of 2.5 nrn.
EXAMPLE 3
Preparation ofHijeh Porosity Mesoporous Copper-tin - 51 % Porosity
72 g of BC10-TX surfactant was heated until molten. To this was added a
mixture containing 12.0 cm3 of 1.0 M tin(II) tetrafluoroborate solution
(aqueous), 12.0
cm 3 of copper(II) tetrafluoroborate solution (aqueous), 3.15 g sodium
citrate, 2.23 g
ethylenediaminetetraacetic acid (EDTA) and 2.11 g of sodium hypophosphite in
24 cm3
of deionised water. The resulting paste was stirred vigorously until
homogeneous and
then allowed to cool to room temperature and allowed to stand at room
temperature
overnight. The surfactant was removed from the resultant product via repeated
washing
in deionised water. The collected powder was dried in air, overnight at 60 C
and was
found to have an average pore size of 9-10 nm.
EXAMPLE 4
Preparation of HiQh Porosity Mesoporous Copper-tin Powder - 44% porosity
72 g of BC10-TX surfactant was heated until molten. To this was added a
mixture containing 12.0 cm3 of 1.0 M tin(II) tetrafluoroborate solution
(aqueous), 12.0
cm 3 of copper(II) tetrafluoroborate solution (aqueous), 3.15 g sodium
citrate, 2.23 g
ethylenediaminetetraacetic acid and 2.11 g of sodium hypophosphite in 24 cm3
of
deionised water. The resulting paste was stirred vigorously until homogeneous
and then
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allowed to cool to room temperature and allowed to stand at room temperature
overnight. The surfactant was removed from the resultant product via repeated
washing
in deionised water. The collected powder was dried in air, overnight at 60 C.
Treatment of the powder at 250 C in H2/Ar for 5 hours resulted in a powder
with an
average pore size of 9-10 nm.
EXAMPLE 5
Electrode Fabrication Using SBR/CMC Binder
A lithium ion battery anode based on mesoporous copper-tin fabricated using a
liquid crystal templating route was prepared. The copper-tin material had a
porosity of
51 % as calculated from nitrogen porosimetry (BET) measurements and was
prepared as
described in Example 3. This was done by first mixing the copper-tin material
with
Timcal KS-6 graphite, followed by addition of an aqueous solution of styrene
butadiene
rubber (SBR) and carboxy methyl cellulose (CMC), such that the percentages of
copper-tin, SBR, CMC and graphite in the electrode (after, evaporation of the
water)
were 80 %, 6 %, 4 % and 10% by mass respectively. The resulting paste was then
spread over a 14 m thick copper foil which acted as a current collector, and
the water
was allowed to evaporate leaving a uniform coating of the copper-
tin/SBR/CMC/graphite composite adhered to the copper foil. This composite
electrode
was then calendared to improve adhesion.
EXAMPLE 6
Electrode Fabrication UsinP EPDM Binder
A lithium ion battery anode based on mesoporous copper-tin fabricated using a
liquid crystal templating route was prepared. The copper-tin material had a
porosity of
51 % as calculated from nitrogen porosimetry (BET) measurements and was
prepared as
described in Example 3. This was done by first mixing the copper-tin material
with a
solution consisting of hexane and ethylene propylene diene monomer (EPDM),
followed by addition of Timcal KS-6 graphite, such that the percentages of
copper-tin,
EPDM and graphite in the electrode (after evaporation of the hexane) were 90
%, 5 %
and 5 % by mass respectively. The resulting paste was then spread over a 14 m
thick
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copper foil which acted as a current collector, and the hexane was allowed to
evaporate,
leaving a uniform coating of the copper-tin/EPDM/graphite composite adhered to
the
copper foil.
EXAMPLE 6
Cell Fabrication Using SBR/CMC Bound Electrodes
A lithium ion battery with a footprint area of 1.2 cm2 was fabricated using a
home-made cell housing. The cathode consisted of LiCoO2 supported on aluminium
as
is standard in the industry. The anode consisted of a composite of liquid
crystal
templated mesoporous copper-tin, SBR/CMC and graphite deposited on a copper
foil as
prepared in Example 5, but using the material of Example 4. The separator
consisted of
two layers of Celgard 2400 membrane and contained an electrolyte composed of 1
M
LiPF6 in a mixture of ethylene carbonate and diethylene carbonate (LP30
Selectipur
from Merck). A lithium foil was inserted between the two layers of separator
and acted
as a reference electrode. Once assembled, the cell was cycled at a C/10 rate
with a
depth of discharge of 30 % using a lower voltage limit of 0.005 V vs. Li/Li+
for the
copper-tin composite electrode..
Similar cells were prepared, but using the materials of Example 1(Comparative)
and Example 2 as the anode materials, in place of the material of Example 4.
Figure 1 compares the cycle life behaviour of the three cells; one utilising
an
anode as described in Example 1 (Comparative), one from Example 2 and another
that
used the same construction, but using the 44% porous copper-tin material
prepared as
described in Example 4.
EXAMPLE 8
Cell Fabrication Using EPDM Bound Electrode
A lithium ion battery with a footprint area of 1.2 cm2 was fabricated using a
home-made cell housing. The cathode consisted of LiCoO2 supported on aluminium
as
is standard in the industry. The anode consisted of a composite of liquid
crystal
templated nanoporous copper-tin, EPDM and graphite deposited on a copper foil
as
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prepared in Example 6. The separator consisted of two layers of Celgard 2400
membrane and contained an electrolyte composed of 1 M LiPF6 in a mixture of
ethylene
carbonate and diethylene carbonate (LP30 Selectipur from Merck). A lithium
foil was
inserted between the two layers of separator and acted as a reference
electrode. Once
assembled, the cell was cycled at a C/10 rate with a depth of discharge of 30
% using a
lower voltage limit of 2.5 V.
A similar cell was prepared, but using the material of Example 2 as the anode
materials, in place of the material of Example 3.
Figure 2 compares the cycle life behaviour of the two cells; one utilising an
anode as described in Example 6 and another that used the same construction,
but with a
39 % porous copper-tin material prepared as described in Example 2.
