Note: Descriptions are shown in the official language in which they were submitted.
CA 02384494 2009-09-09
LITHIUM METAL DISPERSION IN SECONDARY BATTERY ANODES
Field of the Invention
The present invention relates to secondary batteries having high
specific capacities and particularly to anodes for secondary batteries
comprising a host
material such as a carbonaceous material capable of absorbing and desorbing
lithium
in an electrochemical system and lithium metal dispersed in the host material.
Background of the Invention
Lithium and lithium-ion secondary or rechargeable batteries have
recently found use in certain applications such as in cellular phones,
camcorders, and
laptop computers, and even more recently, in larger power applications such as
in
electric vehicles and hybrid electric vehicles. It is preferred in these
applications that
the secondary batteries have the highest specific capacity possible but still
provide
safe operating conditions and good cycleability so that the high specific
capacity is
maintained in subsequent recharging and discharging cycles.
Although there are various constructions for secondary batteries, each
construction includes a positive electrode (or cathode), a negative electrode
(or
anode), a separator that separates the cathode and anode, and an electrolyte
in
electrochemical communication with the cathode and anode. For secondary
lithium
batteries, lithium ions are transferred from the anode to the cathode through
the
electrolyte when the secondary battery is being discharged, i.e., used for its
specific
application. During this process, electrons are collected from the anode and
pass to
the cathode through an external circuit. When the secondary battery is being
charged
or recharged, the lithium ions are transferred from the cathode to the anode
through
the electrolyte.
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Historically, secondary lithium batteries were produced using non-
lithiated compounds having high specific capacities such as TiS2, MoS2, Mn02
and
V205, as the cathode active materials. These cathode active materials were
coupled
with a lithium metal anode. When the secondary battery was discharged, lithium
ions
were transferred from the lithium metal anode to the cathode through the
electrolyte.
Unfortunately, upon cycling, the lithium metal developed dendrites that
ultimately
caused unsafe conditions in the battery. As a result, the production of these
types of
secondary batteries was stopped in the early 1990's in favor of lithium-ion
batteries.
Lithium-ion batteries typically use lithium metal oxides such as
LiCo02 and LiNi02 as cathode active materials coupled with a carbon-based
anode.
In these batteries, the lithium dendrite formation on the anode is avoided
thereby
making the battery safer. However, the lithium, the amount of which determines
the
battery capacity, is totally supplied from the cathode. This limits the choice
of
cathode active materials because the active materials must contain removable
lithium.
Furthermore, the delithiated products corresponding to LiCo02 and LiNi02 that
are
formed during charging (e.g. Li.Co02 and Li.Ni02 where 0.4<x<1.0) and
overcharging (i.e. Li.Co02 and LiõNi02 where x<0.4) are not stable. In
particular,
these delithiated products tend to react with the electrolyte and generate
heat, which
raises safety concerns.'
Summary of the Invention
The present invention is a secondary battery having a high specific
capacity and good cycleability and that operates safely. In accordance with
the
invention, the freshly prepared, secondary battery includes an anode that is
formed of
a host material capable of absorbing and desorbing lithium in an
electrochemical
system and lithium metal dispersed in the host material. Preferably, the
lithium metal
is a finely divided lithium powder and more preferably has a mean particle
size of less
than about 20 microns. The host material comprises one or more materials
selected
from the group consisting of carbonaceous materials, Si, Sn, tin oxides,
composite tin
alloys, transition metal oxides, lithium metal nitrides and lithium metal
oxides.
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Preferably, the host material comprises a carbonaceous material and more
preferably
comprises graphite.
The freshly prepared, secondary batteries of the invention include a
positive electrode including an active material, a negative electrode
comprising a host
material capable of absorbing and desorbing lithium in an electrochemical
system and
lithium metal dispersed in the host material, a separator separating the
positive
electrode and the negative electrode and an electrolyte in communication with
the
positive electrode and the negative electrode. Preferably, the cathode active
material
is a compound that can be lithiated at an electrochemical potential of 2.0 to
5.0 V
versus lithium. For example, the cathode active material can be Mn02, V205 or
MoS2, or a mixture thereof. The lithium metal in the anode is preferably a
finely
divided lithium powder and more preferably has a mean particle size of less
than
about 20 microns. The host material comprises one or more materials selected
from
the group consisting of carbonaceous materials, Si, Sn, tin oxides, composite
tin
alloys, transition metal oxides, lithium metal nitrides and lithium metal
oxides.
Preferably, the host material in the negative electrode comprises a
carbonaceous
material and, more preferably, comprises graphite. The amount of lithium metal
present in the negative electrode is preferably no more than the maximum
amount
sufficient to intercalatek,.alloy with, or be absorbed by the host material in
the
negative electrode. For example, if the host material is carbon, the amount of
lithium
is preferably no more than the amount needed to make LiC6-
The present invention also includes a method of preparing a freshly
prepared anode for a secondary battery that includes the steps of providing a
host
material that is capable of absorbing and desorbing lithium in an
electrochemical
system, dispersing lithium metal in the host material and forming the host
material
and the lithium metal dispersed therein into an anode. The lithium metal and
the host
material is preferably mixed together with a non-aqueous liquid to produce a
slurry
and then applied to a current collector and dried to form the anode.
Alternatively, the
anode can be formed by chemical means by immersing the host material in a
suspension of lithium metal in a non-aqueous liquid, and then formed into an
anode.
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The present invention further includes a method of operating a
secondary battery. First, a freshly prepared, secondary battery is provided
that
includes a positive electrode including an active material, a negative
electrode
comprising a host material capable of absorbing and desorbing lithium in an
electrochemical system and lithium metal dispersed in the host material, a
separator
for separating the positive electrode and the negative electrode, and an
electrolyte in
communication with the positive electrode and the negative electrode. In
particular,
the secondary battery is manufactured with lithium metal dispersed in the host
material of the anode. The freshly assembled battery is in a charged state and
more
preferably is in a fully charged state (with all the removable lithium present
in the
anode of the freshly prepared battery). The freshly prepared secondary battery
is
initially discharged by transmitting lithium ions from the negative electrode
to the
positive electrode through the electrolyte. The secondary battery can then be
charged
or recharged by transmitting lithium ions from the positive electrode to the
negative
electrode through the electrolyte and then discharged again by transmitting
lithium
ions from the negative electrode to the positive electrode through the
electrolyte. The
charging and discharging steps can occur for numerous cycles while maintaining
the
high specific capacities of the cathode active materials and maintaining safe
operating
conditions.
According to an aspect of the present invention, there is provided an
anode for a secondary battery comprising a host material capable of absorbing
and
desorbing lithium in an electrochemical system; and lithium powder, wherein
the
lithium powder is dispersed within the host material, and wherein the amount
of
lithium in said anode is no more than the maximum amount that can be
intercalated
in, alloyed with, or absorbed by the host material. In accordance with an
aspect, the
lithium powder has reduced pyrophoricity as compared to lithium metal, and the
lithium powder comprises lithium metal powder that has been stabilized via
treatment
with CO2. In accordance with another aspect, the lithium powder has a mean
particle
size of less than about 20 microns.
According to another aspect of the present invention, there is provided
a secondary battery comprising: the anode described above, a cathode
comprising an
active material capable of being electrochemically lithiated; a separator for
separating
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the cathode and the anode; and an electrolyte in electrochemical communication
with
the cathode and the anode.
According to another aspect of the present invention, there is provided
a method of preparing an anode for a secondary battery comprising: providing a
host
material that is capable of absorbing and desorbing lithium in an
electrochemical
system; dispersing an amount of lithium powder in the host material, wherein
the
amount is no more than the maximum amount that can be intercalated in, alloyed
with, or absorbed by the host material in the anode; and forming the host
material and
the lithium powder dispersed therein into the anode. In accordance with an
aspect, the
lithium powder has reduced pyrophoricity as compared to lithium metal, and the
lithium powder comprises lithium metal powder that has been stabilized via
treatment
with CO2. In accordance with another aspect, the lithium powder has a mean
particle
size of less than about 20 microns.
These and other features and advantages of the present
invention will become more readily apparent to those skilled in the art upon
consideration of the following detailed description and accompanying drawing,
which
describe both the preferred and alternative embodiments of the present
invention.
Brief Description of the Drawing
Fig. 1 illustrates a simplified secondary battery construction including
a cathode, anode, separator and electrolyte, in accordance with the invention.
Detailed Description of the Preferred Embodiments of the Invention
In the drawings and the following detailed description, preferred
embodiments are described in detail to enable practice of the invention.
Although the
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invention is described with reference to these specific preferred embodiments,
it will
be understood that the invention is not limited to these preferred
embodiments. But to
the contrary, the invention includes numerous alternatives, modifications and
equivalents as will become apparent from consideration of the following
detailed
description and accompanying drawing.
As illustrated in Figure 1, the present invention is a secondary battery
that comprises a positive electrode or cathode 12, a negative electrode or
anode 14,
a separator 16 for separating the positive electrode and the negative
electrode, and an
electrolyte in electrochemical communication with the positive electrode and
the
10 negative electrode. The secondary battery 10 also includes a current
collector 20 that
is in electrical contact with the cathode and a current collector 22 that is
in electrical
contact with the anode. The current collectors 20 and 22 are in electrical
contact with
one another through an external circuit (not shown). The secondary battery 10
can
have any construction known in the art such as a "jelly roll" or stacked
construction.
The cathode 12 is formed of an active material, which is typically
combined with a carbonaceous material and a binder polymer. The active
material
used in the cathode 12 is preferably a material that can be lithiated at a
useful voltage
(e,g. 2.0 to 5.0 V versus lithium). Preferably, non-lithiated materials such
as Mn02,
V205 or MoS2, or mixtiirei thereof;, can be used as the active material, and
more
preferably, Mn02 is used. However, lithiated materials such as LiMn204 that
can be
further lithiated can also be used. The non-lithiated active materials are
preferred
because they generally have higher specific capacities than the lithiated
active
materials in this construction and thus can provide increased power over
secondary
batteries that include lithiated active materials. Furthermore, because the
anode 14
includes lithium as discussed below, it is not necessary that the cathode 12
include a
lithiated material for the secondary battery 10 to operate. The amount of
active
material provided in the cathode 12 is preferably sufficient to accept the
removable
lithium metal present in the anode 14. For example, if Mn02 is the cathode
active
material, then one mole of Mn02 is preferably present in the cathode 12 per
mole of
lithium in the anode 14 to produce LiMn02 in the cathode upon discharge.
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When cathode active materials are used that can be lithiated such as
those described above, the removable lithium that is cycled in the battery is
fully
provided by the anode 14 and the battery is assembled or prepared in a fully
charged
state, as is preferred. Nevertheless, the cathode 12 can also include a minor
amount of
one or more lithiated active materials (e.g. LiCo02 or LiNi02) that do not
further
absorb lithium at a voltage between 2.0V and 5.0V and the battery can still be
provided in a primarily charged state. In this event, the cathode preferably
has less
than 50% (molar) and more preferably less than 10% (molar) of the lithiated
material
(e.g. LiCo02 or LiNi02) as the active material. Because LiCo02 and LiNi02 do
not
further absorb lithium, the presence of these materials in the cathode 12 does
not
reduce the amount of cathode active material needed to accept the removable
lithium
from the anode 14.
= The anode 14 is formed of a host material 24 capable of absorbing and
desorbing lithium in an electrochemical system with lithium metal 26 dispersed
in the
host material. For example, the lithium present in the anode 14 can
intercalate in,
alloy with or be absorbed by the host material when the battery (and
particularly the
anode) is recharged. The host material includes materials capable of absorbing
and
desorbing lithium in an electrochemical system such as carbonaceous materials;
materials containing Si,- Sn, tin oxides or composite tin alloys; transition
metal oxides
such as Co0; lithium metal nitrides such as Li3,CoõN where 0<x<0.5, and
lithium
metal oxides such as Li4Ti5012. The lithium metal 26 is preferably provided in
the
anode 14 as a finely divided lithium powder. In addition, the lithium metal 26
preferably has a mean particle size of less than about 20 microns, more
preferably less
than about 10 microns. The lithium metal can be provided as a pyrophoric
powder or
as a stabilized low pyrophorosity powder, e.g., by treating the lithium metal
powder
with CO2.
The anode 14 is typically capable of reversibly lithiating and
delithiating at an electrochemical potential relative to lithium metal of from
greater
than 0.0 V to less than or equal to 1.5. If the electrochemical potential is
0.0 or less
versus lithium, then the lithium metal will not reenter the anode 14 during
charging.
Alternatively, if the electrochemical potential is greater than 1.5 V versus
lithium then
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the battery voltage will be undesirably low. Preferably, the amount of lithium
metal
26 present in the anode 14 is no more than the maximum amount sufficient to
intercalate in, alloy with, or be absorbed by the host material in the anode
14 when the
battery is recharged. For example, if the host material 24 is carbon, the
amount of
lithium 26 is preferably no more than the amount sufficient to make LiC6. In
other
words, the molar ratio of lithium to carbon in the anode is preferably no more
than
1:6.
In accordance with the invention, the anode 14 can be prepared by
providing a host material that is capable of absorbing and desorbing lithium
in an
electrochemical system, dispersing lithium metal in the host material, and
forming the
host material and the lithium metal dispersed therein into an anode.
Preferably, the
lithium metal and the host material are mixed with a non-aqueous liquid such
as
tetrahydrofuran (THF) and a binder, and formed into a slurry. The slurry is
then used
to form the anode 14, for example, by coating the current collector 22 with
the slurry
and then drying the slurry. The lithium metal can also be provided in the
anode by
immersing the host material in a suspension containing lithium metal in a non-
-
aqueous liquid such a hydrocarbon solvent (e.g. hexane). The lithium metal
used in
the suspension is preferably a finely divided lithium powder as discussed
above. The =
host material can be feinted into the shape of the anode and then dipped into
the
lithium metal suspension or it can be combined with the lithium metal
suspension to
form a slurry and then applied to the current collector and dried to form the
anode.
The non-aqueous liquid used to form the suspension can be removed by drying
the
,. anode (e.g. at an elevated temperature). No matter what method is used,
the lithium
metal is preferably distributed as well as possible into the host material.
Accordingly,
as discussed above, the lithium metal 26 preferably has a mean particle size
of less
than about 20 microns, more preferably less than about 10 microns.
The host material 24 in the anode 14 can include one or more materials
capable of absorbing and desorbing lithium in an electrochemical system such
as
carbonaceous materials; materials containing Si, Sn, tin oxides or composite
tin
alloys; transition metal oxides such as Co0; lithium metal nitrides such as
Li3_CoxIsi
where 0<x<0.5; and lithium metal oxides such as Li4Ti5012. Preferably, as
mentioned
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above, the host material 24 preferably includes graphite. In addition, the
host material
24 preferably includes a small amount of carbon black (e.g. less than 5% by
weight) as
a conducting agent.
As shown in Figure 1, the cathode 12 is separated from the anode 14 by
an electronic insulating separator 16. Typically, the separator 16 is formed
of a
material such as polyethylene, polypropylene, or polyvinylidene fluoride
(PVDF).
The secondary battery 10 further includes an electrolyte that is in
electrochemical communication with the cathode 12 and anode 14. The
electrolyte
can be non-aqueous liquid, gel or solid and preferably comprises a lithium
salt, e.g.,
LiPF6. The electrolyte is provided throughout the battery 10 and particularly
within
the cathode 12, anode 14 and separator 16. Typically, the electrolyte is a
liquid, and
the cathode 12, anode 14 and separator 16 are porous materials that are soaked
in the
electrolyte to provide electrochemical communication between these components.
As mentioned above, the battery 10 includes current collectors 20 and
22, which are used to transmit electrons to an external circuit Preferably,
the current
collector 20 is made of aluminum foil and current collector 22 is made of
copper foil.
The battery 10 of the invention can be prepared by methods known in
the art and preferably has a layer thickness within the following ranges (from
left to
right in Figure 1):
Layer thicknes
Current collector (20) 20-40 pm
Cathode (12) 70-100 pm
Separator (16) 25-35 pm
Anode (14) 70-100 ttm
Current collector (22) 20-40 pm
The battery 10 also includes an electrolyte dispersed throughout the cathode
12, anode
14 and separator 16, and a casing (not shown).
In operation, the freshly prepared secondary battery 10 is initially in a
charged state, more preferably a fully charged state, and is initially
discharged by
transmitting lithium ions from the anode 14 to the cathode 12 through the
electrolyte.
At the same time, electrons are transmitted from the anode 14 to the cathode
12
through the current collector 22, the external circuit, and the current
collector 20. The
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secondary battery 10 can then be charged or recharged by transmitting lithium
ions
from the cathode 12 to the anode 14 through the electrolyte and then
discharged again
as discussed above. The charging and discharging steps can occur for numerous
cycles while maintaining the high specific capacities of the cathode active
materials
and maintaining safe operating conditions.
The secondary battery 10 can be used for various types of applications.
For example, the secondary battery can be used in portable electronics such as
cellular
phones, camcorders, and laptop computers, and in large power applications such
as for
electric vehicles and hybrid electric vehicles.
The present invention provides secondary batteries having a high
specific capacity, safe operating conditions and good cycleability. In
particular,
because lithium metal is provided in the anode, non-lithiated materials can be
used as
the preferred cathode active material in the secondary battery. These non-
lithiated
materials have higher specific capacities than the lithiated materials
presently used in
lithium-ion batteries. Unlike traditional lithium secondary batteries having
non-
lithiated cathode active materials and metallic lithium anodes, it has been
discovered
that secondary batteries produced using non-lithiated cathode active materials
combined with the anodes of the invention operate safely and do not generate
lithium
dendrites upon cycling. Furthermore, the secondary batteries of the present
invention
are safer to operate than lithium-ion batteries, which become unstable when
lithium is
removed from the cathode during charging. In particular, because the cathode
active
material in the secondary batteries of the invention is typically in a fully
charged state
. . when the battery is freshly prepared, it is more stable then the cathode
materials used
in lithium-ion batteries. Moreover, the batteries of the invention can be
charged and
discharged numerous times while maintaining safe operating conditions and the
high
specific capacities of the cathode active materials.
It is understood that upon reading the above description of the present
invention and reviewing the accompanying drawings, one skilled in the art
could
make changes and variations therefrom. These changes and variations are
included in
the spirit and scope of the following appended claims
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