U.S. patent application number 13/045563 was filed with the patent office on 2012-09-13 for integral bi-layer separator-electrode construction for lithium-ion batteries.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS LLC. Invention is credited to Xiaosong Huang, Hamid G. Kia.
Application Number | 20120231321 13/045563 |
Document ID | / |
Family ID | 46795855 |
Filed Date | 2012-09-13 |
United States Patent
Application |
20120231321 |
Kind Code |
A1 |
Huang; Xiaosong ; et
al. |
September 13, 2012 |
INTEGRAL BI-LAYER SEPARATOR-ELECTRODE CONSTRUCTION FOR LITHIUM-ION
BATTERIES
Abstract
A porous bi-layer separator composed of a first layer with a
contacting array of non-conducting particles overlaid with a second
layer of a microporous polymer layer, may be fabricated on the
electrode surface of the anode of a lithium-ion battery to form an
integral electrode-separator construction. The bi-layer separator
may prevent development of a direct electronic path between the
anode and cathode of the battery while accommodating electrolyte
solution and enabling passage of lithium ions. Such an integral
separator should be mechanically robust and tolerant of elevated
temperatures. Exemplary bi-layer separators may be fabricated by
sequential deposition of solvent-containing slurries and polymer
solutions with subsequent controlled evaporation of solvent. The
elevated temperature performance of lithium-ion battery cells
incorporating such integral electrode-bi-layer separators was
demonstrated to exceed the performance of similar cells using
commercial and experimental single layer polymer separators.
Inventors: |
Huang; Xiaosong; (Novi,
MI) ; Kia; Hamid G.; (Bloomfield Hills, MI) |
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS
LLC
Detroit
MI
|
Family ID: |
46795855 |
Appl. No.: |
13/045563 |
Filed: |
March 11, 2011 |
Current U.S.
Class: |
429/144 ;
29/623.1; 29/623.5 |
Current CPC
Class: |
Y02E 60/10 20130101;
H01M 2/1686 20130101; Y10T 29/49108 20150115; H01M 10/0525
20130101; Y02P 70/50 20151101; H01M 2220/20 20130101; H01M 2220/30
20130101; H01M 2/1673 20130101; H01M 2/145 20130101; Y10T 29/49115
20150115; H01M 2/1653 20130101; Y02T 10/70 20130101; H01M 2/1646
20130101 |
Class at
Publication: |
429/144 ;
29/623.1; 29/623.5 |
International
Class: |
H01M 2/16 20060101
H01M002/16; H01M 4/26 20060101 H01M004/26 |
Claims
1. A cell for a lithium-ion battery, the cell comprising, as thin
layers, stacked cell components consisting essentially of: an
elemental lithium-containing anode layer having an anode face and
an anode peripheral shape and being carried on an anode current
collector; a lithium ion-accepting cathode layer, having a cathode
face and a cathode peripheral shape and carried on a cathode
current collector, the faces of the anode and cathode facing each
other; a liquid lithium ion electrolyte for transport of lithium
ions between the layer surfaces of the anode and cathode; and a
bi-layer separator, with a peripheral shape, integrally formed on,
and attached to, one of the anode and cathode layers so as to
separate their respective faces, the total thickness of the cell
components being up to about one millimeter; the separator
comprising; a tiered layer of substantially-equiaxed, electrically
non-conductive particles of an average size lying on the layer
surface of one of the anode or cathode, within its peripheral
shape, and providing many non-straight porous paths through the
thickness of the layer of particles, the thickness of the layer
being at least equivalent to twice the average sizes of the
particles; and a layer of a polymer containing pores for
infiltration of the electrolyte through the polymer layer and
through the tiered layer of particles for transport of lithium ions
between the opposing faces of the electrodes; the thickness of the
layer of particles and the polymer layer cooperating to prevent
penetration of the bi-layer by an electrically-conducting body
extending from the anode surface to the cathode surface to form a
short-circuit path in the cell.
2. A lithium-ion battery as recited in claim 1 in which the
ion-transporting polymer layer is formed by phase separation from a
liquid mixture comprising the polymer, a polymer solvent and a
polymer non-solvent.
3. A lithium-ion battery as recited in claim 1 in which the
ion-transporting polymer layer is formed by phase separation from a
liquid mixture comprising polyvinylidene fluoride (PVDF), acetone
and water.
4. A lithium-ion battery as recited in claim 1 in which the
electrically non-conducting particles are oxides or nitrides of one
or more of the group consisting of silicon, aluminum, titanium,
magnesium or calcium.
5. A lithium-ion battery as recited in claim 1 in which the
electrically non-conducting particles are bound to one another and
to the anode or cathode on which they lie.
6. A lithium-ion battery as recited in claim 1 in which the average
particle size ranges from about 0.005 micrometers to 10
micrometers.
7. A lithium-ion battery as recited in claim 1 in which the
thickness of the particle layer ranges from about 5 micrometers to
about 40 micrometers.
8. A lithium-ion battery as recited in claim 1 in which the anode
peripheral shape is greater than the cathode peripheral shape and
the bi-layer peripheral shape is at least as great as the anode
peripheral shape.
9. A lithium-ion cell with a lithium-ion electrolyte, an anode and
a cathode, each of the anode and cathode having a face, and each of
the anode and cathode arranged with their faces in opposition, the
anode face having an integral bi-layer separator; the bi-layer
separator comprising a particle layer in contact with the anode
face and a polymer layer; the particle layer comprising an array of
abutting electrically non-conducting particles with an average
particle size and a plurality of tortuous pores extending through
the layer; the polymer layer having a thickness and opposing sides,
one side being attached to the anode and at least coextensive with,
and overlying the particle layer to maintain the particle layer in
contact with the anode
10. An anode for a lithium-ion battery as recited in claim 9 in
which the electrically non-conducting particles are oxides or
nitrides of one or more of the group consisting of silicon,
aluminum, titanium, magnesium or calcium.
11. An anode for a lithium-ion battery as recited in claim 9 in
which the particles of the particle layer are attached to each
other and the particle layer is attached to the anode surface by a
binder.
12. An anode for a lithium-ion battery as recited in claim 9 in
which the average particle size is less than one-half of the
thickness of the particle layer.
13. An anode for a lithium-ion battery as recited in claim 9 in
which the average particle size ranges from about 0.005 micrometers
to 10 micrometers.
14. An anode for a lithium-ion battery as recited in claim 9 in
which the polymer layer comprises open pores that interconnect the
sides of the layer for accommodation of the lithium-ion electrolyte
and transport of lithium ions, the open-pored polymer layer being
formed by phase separation from a liquid mixture comprising the
polymer, a polymer solvent and a polymer non-solvent.
15. An anode for a lithium-ion battery as recited in claim 9 in
which the polymer of the polymer layer comprises polyvinylidene
fluoride (PVDF) with open pores that interconnect the sides of the
layer for accommodation of the lithium-ion electrolyte and
transport of lithium ions, the open-pored PVDF layer being formed
by phase separation from a liquid mixture comprising PVDF, acetone
and water.
16. An anode for a lithium-ion battery as recited in claim 9 in
which the polymer of the polymer layer is poly(methyl methacrylate)
PMMA formed by evaporation of PMMA-solvent solution to form a
continuous layer of PMMA which, when immersed in the lithium-ion
conducting electrolyte, forms a lithium-ion conducting gel for
transport of lithium ions.
17. A method of making an anode with an integral bi-layer separator
for a lithium-ion battery, the bi-layer separator comprising a
particle layer and a polymer layer and being adapted for transport
of lithium ions to and from the anode while preventing passage of
electrons when immersed in a liquid electrolyte, the method
comprising: spreading a slurry of graphite, carbon black and a
first binder dissolved in a first solvent on a copper current
collector and evaporating the solvent to form an anode with a first
surface in contact with the current collector and a carbonaceous
surface; forming the particle layer of the bi-layer separator by:
spreading a slurry of electrically non-conducting particles in a
solution of a second binder and a second solvent on the
carbonaceous surface of the anode to form a layer of generally
uniform thickness on the anode; partially or completely evaporating
the second solvent to form a particle layer of the bi-layer
separator, the particle layer comprising a plurality of abutting
particles with pores extending throughout the layer, the layer
having two opposing surfaces, an anode surface in contact with the
anode and a particulate surface; then forming the polymer layer of
the bi-layer separator by; applying a generally uniform thickness
of a solution comprising a polymer dissolved in a third solvent to
the particulate surface of the particle layer; evaporating the
third solvent to form the polymer layer overlying, and adhering to,
the particle layer, the polymer layer being adapted for transport
of ions through the polymer when immersed in a liquid
electrolyte.
18. A method of making an anode with an integral bi-layer separator
for a lithium-ion battery as recited in claim 16 in which the
polymer layer is formed by applying a substantially uniform layer
of a polymer solution dissolved in an solution comprising a polymer
solvent and a polymer non-solvent; then selectively evaporating the
polymer solvent at a temperature suitable for forming a
phase-separated non-solvent and polymer mixture and then
evaporating the non-solvent to form a microporous polymer layer in
which pores extend from a first particulate-contacting surface of
the polymer layer to a second opposing surface of the layer are
adapted to be filled with liquid electrolyte for transport of
lithium ions.
19. A method of making an anode with an integral bi-layer separator
for a lithium-ion battery as recited in claim 16 in which the
polymer is polyvinylidene fluoride (PVDF), the polymer solvent is
acetone and the polymer non-solvent is water.
20. A method of making an anode with an integral bi-layer separator
for a lithium-ion battery as recited in claim 16 in which the
polymer layer is formed by applying a substantially uniform layer
of a poly(methyl methacrylate) (PMMA) solution dissolved in an
acetone solvent; then evaporating the acetone to form a PMMA layer
adapted to form a lithium-ion conducting gel when immersed in
liquid electrolyte.
Description
TECHNICAL FIELD
[0001] This invention pertains to bi-layer porous separators
integral with an electrode of a lithium-ion cell to assure
electrical separation between the anode and cathode of the cell.
More specifically, this invention pertains to the sequenced
deposition of conformal, electronically non-conductive, porous
ceramic and polymer coatings on one of the cell electrodes to form
a cooperating electrode-separator construction.
BACKGROUND OF THE INVENTION
[0002] Lithium-ion secondary batteries are common in portable
consumer electronics because of their high energy-to-weight ratios,
lack of memory effect, and slow self-discharge when not in use.
Rechargeable lithium-ion batteries are also being designed and
manufactured for use in automotive applications to provide energy
for electric motors to drive vehicle wheels.
[0003] The basic unit of a lithium-ion battery is an individual
cell which includes an anode, a cathode and a liquid, non-aqueous
electrolyte suitable for carrying and conveying lithium ions.
Lithium-ion batteries of different sizes, shapes and electrical
capabilities may be fabricated by electrically connecting any
suitable number of these cells in parallel, series or a combination
of these to develop a battery of suitable voltage and capacity.
Within an individual cell, the anode on discharge becomes the
cathode on charge, and the cathode on discharge becomes the anode
on charge. From here forward, the electrode that is the anode on
discharge (the negative electrode) will be referred to as the
anode, and, correspondingly, the electrode that is the cathode on
discharge (the positive electrode) will be referred to as the
cathode.
[0004] In the anode, elemental lithium is often stored between the
sheets or layers of a graphite structure forming
lithium-intercalated graphite. During discharge, lithium ions
migrate out of the lithium-graphite while, during charge, the
lithium ions are re-inserted into the graphite. The cathode may be
formed from any lithium based active material that can sufficiently
undergo lithium intercalation and deintercalation. For example, in
various embodiments, cathode may comprise, among others, at least
one of spinel lithium manganese oxide (LiMn.sub.2O.sub.4), lithium
cobalt oxide (LiCoO.sub.2) and nickel-manganese-cobalt oxide
[Li(Ni.sub.xMn.sub.yCo.sub.z)O.sub.2].
[0005] A lithium-ion battery generally operates by reversibly
transporting lithium ions between its negative and positive
electrodes. To prevent physical contact (electron-conducting
contact) between the anode and cathode which would result in an
internal short circuit, a separator, is positioned between the
electrodes. The separator, commonly a polyolefin polymer is
microporous and contains small pores which are filled with
electrolyte to provide pathways for passage of lithium ions from
one electrode to the other. Microporous separator materials in
common use include polyethylene or polypropylene. The microporous
separators may be about twenty-five to about thirty microns thick
and exhibit thirty-five percent or more porosity.
[0006] Each of the negative and positive electrodes is also carried
on or connected to a metallic current collector (typically copper
for the anode and aluminum for the cathode). During battery usage,
lithium is oxidized at the anode to form lithium ions which are
then transported from the anode and received by the cathode,
passing through the ion-conducting electrolyte in the separator
pores to form an internal circuit. The current collectors
associated with the two electrodes are connected by a controllable
and interruptible external circuit which allows an electron current
to pass between the electrodes. The external electron current
serves to electrically balance the internal circuit resulting from
the transport of lithium ions through each cell.
[0007] The battery may then be re-charged by passing a suitable
direct electrical current in the opposite direction between the
electrodes. During recharging, the flow of lithium ions is reversed
and they pass from cathode to anode where they are reduced to
lithium metal and re-intercalated into the graphite.
[0008] In principle, such a discharge-recharge procedure may be
practiced indefinitely. But, under normal operating conditions,
battery life is affected by the degradation of the active materials
(e.g. anode, cathode and electrolyte) and abnormal operation can
induce the formation of lithium dendrites, surface deposits of
lithium on the anode. These dendrites, with continued growth may
penetrate the thin polymer separator and to enable a direct
connection, a short circuit, between anode and cathode. Also, if
any fine metal particles are introduced into the inter electrode
space during manufacture these too may enable a short circuit.
[0009] Penetration of commonly-used polymer microporous separator
materials, such as polyethylene or polypropylene, is more likely at
more elevated cell temperatures. For example at cell temperatures
of greater than 130.degree. C. the separator materials will soften
appreciably and offer reduced resistance to penetration. Even if
penetration of the separator does not occur, any prolonged exposure
to temperatures in excess of 130.degree. C. may result in shrinkage
or even melting of the separator. Clearly, any of these behaviors,
shrinkage, softening or melting, will diminish the separators
ability to provide electrical insulation between the battery anode
and cathode to prevent internal short circuits.
[0010] There is thus a need for a more durable and temperature
tolerant microporous separator for lithium-ion battery cells.
SUMMARY OF THE INVENTION
[0011] This invention provides a method of forming a more
thermally- and mechanically-robust integral combination of a
separator and an electrode for a lithium-ion cell. The improved
separator-electrode construction employs a thin bi-layer separator
portion which is fabricated on, and attached to, a cell electrode
which is itself supported by a coextensive current collector,
usually fabricated of copper foil for the anode and aluminum foil
for the cathode. Because it is common for the anode to be larger
than the cathode, the separator will normally be deposited on the
anode to ensure separation of anode and cathode. However deposition
of a suitable separator on the cathode may also be practiced. In
many embodiments of this invention, the entire current
collector-electrode-separator structure is thin and may range from
less than 100 micrometers to about one millimeter thick.
Individually, the current collectors may be about 20 micrometers
thick, the electrodes may be about 50 micrometers thick and the
bi-layer separator may be about 25 micrometers thick.
[0012] The separator is formed by placing a first layer of
separator material on an electrode surface followed by placement of
a second layer on the first layer such that the electrode and
bi-layer separator form an integral cooperating structure for
assembly in a cell.
[0013] The first layer of the bi-layer separator is a laid-down
fabrication of electrically non-conducting ceramic particles such
as oxides or nitrides. The particles may be generally equi-axed and
substantially uniformly sized and arranged to provide a continuous
pore structure. The pores provide tortuous and non-straight
passages through the layer of ceramic particles. A polymer binder
material may be used to coat the particles, securing them to one
another and to the electrode surface while leaving the porous
passages. The second layer, overlying and at least coextensive with
the ceramic particle layer, is a thin polymer layer or membrane.
The polymer membrane is microporous, and forms interconnected
passages extending through the thickness of the membrane for
filling with a lithium ion-conducting electrolyte and for liquid
transport of lithium ions. In another aspect, a non-porous polymer
layer which forms a Li-ion conducting gel when saturated with
liquid electrolyte may be used.
[0014] The two layers of the bi-layer separator cooperate to
provide superior performance than may be achieved with single layer
separators. The first, particle layer, comprising overlapping hard
ceramic particles resistant to penetration offers improved
resistance to mechanical breaching of the separator, for example by
dendrites, metal fines, or detached electrode particles. The
overlying, and at least coextensive polymer layer, supports and
retains the particle layer by improving the adhesion between the
coating layer and the counter electrode. The flexible and compliant
polymer layer is effective in accommodating volume changes
occurring in the electrode as lithium is inserted into and removed
from the electrode during charge-discharge cycles,.
[0015] The bi-layer separator is formed on an electrode supported
by a metal current collector. The first layer of the bi-layer
separator is fabricated by applying a first layer of ceramic
particles directly on the cell anode. The ceramic particles may be
generally equiaxed and fall within a predetermined size range.
Preferably the maximum particle size is less 50% of the intended
thickness of the ceramic layer. So two or more ceramic particles,
when laid-down on the electrode in tiers will provide
interconnected pores which are filled with liquid electrolyte in
the operating cell. The ceramic particles may be bonded to one
another and to the electrode with a thin coating of a suitable
polymer binder. The polymer binder bonds the particles to one
another and to the electrode surface while retaining porosity for
an electrolyte that fills the pores and contacts the electrode.
[0016] Alternatively the particles may be loose and non-adhering.
Overlying and adhering to the ceramic layer is a polymer layer,
which may be microporous or capable of forming a Li-ion conducting
gel when saturated with electrolyte. The polymer layer may be
secured, at its periphery, to the electrode and also adhere to the
outermost tier of particles to retain the particle layer in
position.
[0017] In service, the integral combination of the electrode and
bi-layer separator cooperates to enhance cell performance. The
layer of ceramic particles is in intimate contact with the
electrode surface and so minimizes the extent of any dendrites
which form and reduce the interfacial resistance. This extends the
useful working life of the cell improves the ability of the battery
to tolerate abuse, and conveys good temperature resistance. The
polymer layer provides additional electronic insulation and serves
to retain the ceramic layer in intimate contact with the electrode
and to suppress spalling or flaking of the ceramic layer. Absent
the polymer layer, spalling or flaking of the ceramic particles may
result from the cyclic growth and shrinkage of the electrode
resulting from insertion and removal of elemental lithium, handling
damage during cell assembly, or in-service thermal or vibratory
stresses. The polymer layer may adhere to any one or more of the
ceramic layer, the electrode or the current collector.
[0018] In an embodiment in which the ceramic particles are bonded
to the electrode and to one another, the particles are incorporated
into a dilute solution of a binder dissolved into a significant
excess of solvent. Generally the binder will be present in an
amount ranging from about 0.2% to 25% by weight of the solvent. The
ceramic particles are added in suitable quantity to form a viscous,
paste-like slurry with the dilute binder solution. Suitable ceramic
particles may be any hard, electrically-insulating compound, often
an oxide or nitride, and may include compounds of silicon,
aluminum, titanium, magnesium or calcium. It is preferred that the
particles have an average particle size of around 1 micrometer but
particles with an average size ranging from about 0.005 micrometers
to about 10 micrometers may be used. The particles may be irregular
in shape but are preferably generally equiaxed so that when stacked
together or compacted they will be in line or point contact with
neighboring particles with pores between adjacent particles.
[0019] This particle-containing layer is primarily tasked with
resisting mechanical penetration. For this reason it is preferred
that the particle size be selected to be less than one-half of the
desired layer thickness. This criterion should ensure that that the
layer is composed of at least two tiers of particles so that
particles in the upper tier may nest in the lower tier and more
completely shield the surface from penetration.
[0020] The particle slurry may be applied to the cell anode in any
convenient manner including a doctor blade, a slot die coater or a
comma bar coater each of which is effective in applying a layer of
generally uniform thickness coextensive with the electrode. The
thickness of the applied slurry may be selected to ensure that the
resulting particle layer thickness ranges from about 5 micrometers
to about 45 micrometers. As deposited, the pores formed between
abutting particles will be filled with the dilute binder solution.
However, evaporation of the solvent will reduce the volume of the
solution, causing it to shrink and leave a series of interconnected
voids or pores behind. A volume fraction of voids of about 30% may
be anticipated. Upon complete evaporation of the solvent the binder
material will be left connecting all the particles and binding them
together. The binder will also be effective in binding the
particles to the graphite-based anode.
[0021] The choice of binder for the ceramic particle layer is
partially informed by the procedure used to deposit the second,
polymer layer of the bi-layer separator because the polymer second
layer of the bi-layer separator is dissolved in a solvent and
applied as a solution. It is preferred that the solvent used to
dissolve the polymer of the second layer not dissolve the particle
layer binder. But, the kinetics of dissolution are slow, while the
coating and drying processes are rapid. So a solvent capable of
dissolving the particle layer binder may be acceptable as may a
solvent which induces swelling in the particle layer binder.
Commonly deposition of the polymer overlayer occurs from a solution
of polymer dissolved in, predominantly, acetone containing modest
quantities of water. Hence it is preferred that the particle binder
material be acetone-insoluble. Suitable acetone-insoluble binder
materials for the ceramic powders may include PAN, polyamide or
polyimide. Suitable solvents for these binders include DMF
(dimethylformamide), DMSO (dimethyl sulfoxide), (THF)
(tetrahydrofuran) and NMP (N-methylpyrrolidone).
[0022] Just as in the case of interparticle contact, there will be
only line or point contact between the deposited ceramic and the
anode surface so that an electrolyte, impregnated into the porosity
of the ceramic layer, may freely contact and convey ions to or from
the anode.
[0023] A solution of polymer, for example polyvinylidene fluoride
(PVDF), dissolved in an acetone-water solvent containing an
appreciable excess of acetone may be applied to the ceramic-binder
layer. Upon evaporating the solvent PVDF will be deposited on the
ceramic layer. But, evaporation of the solvent is selective with
acetone evaporating first and promoting phase separation of the
acetone and water constituents of the solvent as the concentration
of water in the solvent increases. Since the polymer segregates to
the acetone, final evaporation of the acetone produces a
sponge-like, open-pored structure of polymer surrounding the
remaining water. Further evaporation to eliminate the water results
in formation of the desired microporous polymer layer. Again, such
a microporous layer may be impregnated by electrolyte to enable
transfer of lithium ions to and from the anode. The thickness of
the ceramic layer may be about 25 micrometers with a porosity of
about 40%, while the thickness of the polymer layer may be less
than 10 micrometers with a porosity of greater than 40%.
[0024] To obtain good adhesion between the ceramic and polymer
layers, the polymer-solvent solution should partially wet the
binder layer securing the particles so that, after evaporation of
the solvent, the precipitated polymer may bond to the ceramic
binder coating.
[0025] When no binder is used in the first layer, particles can be
sprayed onto the electrode. They can also be deposited on the
electrode by a dispersion coating process as described previously.
If the particles of the powder layer are not attached to one
another and to the electrode by a separate binder, a similar result
may be obtained by coating the unattached powder layer with the
acetone-water-PVDF solution just described. A portion of this
dilute solution when applied to the particle layer may be wicked,
by capillary action into the pores between the particles. Upon
evaporation of the solvent the particles may be bound together and
attached to the electrode by the PVDF. The particles may also be
retained by the porous PVDF overlayer.
[0026] A suitable alternative polymer may be poly(methyl
methacrylate) (PMMA). Paralleling the process just described for
PVDF, a similar process of dissolving PMMA in an acetone-water
solvent solution followed by selective evaporation may be followed
to develop a porous PMMA layer. A non-porous layer of PMMA may also
be applied using acetone alone as a solvent. In this case, the
PMMA, when saturated with liquid electrolyte forms a Li-ion
conducting gel for lithium ion transport within the cell.
[0027] Thus, this invention provides a porous bi-layer separator
membrane with an inner ceramic layer which offers increased
temperature resistance and increased resistance to penetration,
whether by dendrites, metal fines or electrode particles. The
ceramic layer, which may be somewhat brittle and prone to spall
under vibratory loads, is supported by the more compliant, adherent
microporous polymer layer. In turn, the ceramic layer restrains
shrinkage of the polymer layer and will continue to serve as an
electrode separator even if the polymer layer softens or melts.
[0028] The performance of cells fabricated using the bi-layer
separator was compared to that of cells fabricated using a
commercial microporous polypropylene separator. Relative to cells
with the conventional separator, cells with the bi-layer separator
exhibited superior resistance to elevated, about 150.degree. C.,
exposure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is an enlarged, schematic illustration of a single
cell of a lithium-ion battery with bi-layer separator. This view
illustrates the flow of lithium ions in discharge mode without
showing details of the flow through separator structure.
[0030] FIG. 2 is a schematic illustration of a graphite electrode
(anode) on which is deposited a separator comprising a ceramic
layer and an overlying PVDF layer, each shown in partial
cutaway.
[0031] FIG. 3 is a low magnification Scanning Electron Micrograph
of the fracture surface formed by cryogenic fracturing of a cell
anode on which has been deposited a bi-layer, ceramic and
microporous polymer separator to show the anode and separator in
cross-section.
[0032] FIG. 4 is higher magnification cross-sectional Scanning
Electron Micrograph of the separator shown in FIG. 3 to further
illustrate the nature of the ceramic and polymer layers.
[0033] FIG. 5 is a graph illustrating the open circuit voltage as a
function of time for several lithium-ion button cells when
maintained at a skin temperature of 150.degree. C. The cells,
otherwise identical, were fabricated with different separators: a
bi-layer ceramic-polymer separator; a commercial polyolefin
separator; and a PVDF polymer layer only.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0034] The following description of the embodiment(s) is merely
exemplary in nature and is not intended to limit the invention, its
application, or uses.
[0035] Embodiments of this invention are separators, and practices
to fabricate such separators, intended to be positioned between the
facing anode and cathode of a lithium ion battery. Such separators
prevent electronic conduction between the facing electrodes but
enable transport of lithium ions contained in the liquid
non-aqueous electrolyte from anode to cathode as the cell
discharges and from cathode to anode as the cell is charging. In
current practice single-layer separators are commonly used and may
comprise a polyolefin, for example polypropylene or polyethylene.
Such separators are porous and wet by the cell electrolyte and so
incorporate electrolyte for transport of lithium ions through the
separator while preventing electronic conduction. These, single
layer, porous, polyolefin separators, particularly at elevated cell
operating temperatures offer little resistance to penetration, for
example by dendrites. Further, in some polymer separators, porosity
is controlled by stretching the separator film to enlarge the pore
size. When heated, such separators may seek to shrink and revert to
their smaller, unstretched size and in so doing, expose portions of
the opposing electrodes so that they may contact one another and
short out.
[0036] The separators of this invention are bi-layer coatings at
least coextensively applied to, and adherent to, the cell anode,
and similarly intended to accommodate liquid electrolyte and enable
passage of lithium ions while suppressing electronic conduction.
The context of the invention may be best understood by
consideration of a lithium ion battery as shown in FIG. 1.
[0037] The lithium-ion battery 10 shown in FIG. 1 includes an anode
12, a cathode 14, and a bi-layer separator 16 incorporating a
ceramic particle layer 15 and a microporous polymer layer 17. The
bi-layer separator is attached to the surface of anode 12 and
sandwiched between the two electrodes 12, 14. Bi-layer separator 16
is impregnated with electrolyte 19 which fills the pores and
cavities of each of layers 15 and 17 and forms a continuous,
lithium ion-conducting path between anode and cathode and vice
versa. But bi-layer separator 16 functions as an electrical
insulator and so, because it is sandwiched between anode 12 and
cathode 14 prevents physical contact between electrodes 12, 14 to
prevent the occurrence of a short circuit. An anode current
collector 12a and a cathode current collector 14a may be positioned
at or near anode 12 and cathode 14, respectively, to collect and
move free electrons (e) to and from an external circuit 18. An
interruptible external circuit 18 and load (L) 22 connects the
negative electrode 12 (through its current collector 12a) and the
positive electrode 14 (through it current collector 14a).
[0038] The lithium ion battery 10 can include a wide range of other
components that, while not depicted here, are nonetheless known to
skilled artisans. For instance, the lithium ion battery 10 may
include a casing, gaskets, terminal caps, and any other desirable
components or materials that may be situated between or around the
negative electrode 12, the positive electrode 12, and/or the
bi-layer separator 16 for performance related or other practical
purposes. Moreover, the size and shape of the lithium ion battery
10 may vary widely depending on the particular application for
which it is designed. One common, but non-limiting, example, is a
button cell, usually intended for operation of low voltage
hand-held devices which has the form of a thin disk with a diameter
of less than 20 millimeters or so.
[0039] The lithium ion battery 10 can generate a useful electric
current during battery discharge by way of reversible
electrochemical reactions that occur when the external circuit 18
is closed to connect anode 12 and cathode 14. Both the anode and
the cathode may contain intercalated lithium. The chemical
potential difference between cathode 14 and anode 12--approximately
2.5 to 4.2 volts depending on the exact chemical make-up of the
electrodes 12, 14--drives electrons produced by the oxidation of
intercalated lithium at anode 12 through the external circuit 18
toward the cathode 14. Lithium ions, which are also produced at the
anode, are concurrently carried by the electrolyte solution through
the bi-layer polymer separator 16 and towards cathode 14. The
electrons flowing through the external circuit 18 and the lithium
ions migrating across the bi-layer polymer separator 16 in the
electrolyte solution eventually reconcile and form intercalated
lithium at the cathode 14.
[0040] In FIG. 1, elemental lithium intercalated between graphite
planar layers is illustrated as black-filled circles. The lithium
atoms are oxidized to lithium ions (not shown) and are transported
through the liquid electrolyte 19 contained in the cavities (layer
15) and pores (layer 17) of bi-layer separator 16 to cathode 14. At
cathode 14 the lithium ions are reduced to elemental lithium and
are inserted into the crystal structure of cathode 14 composition.
Anions (not shown) are formed in the electrolyte composition and
flow counter to the lithium ions.
[0041] The lithium ion battery 10 can be charged or re-powered at
any time by applying an external power source to the lithium ion
battery 10 to reverse the electrochemical reactions that occur
during battery discharge. The connection of an external power
source to the lithium ion battery 10 compels the otherwise
non-spontaneous oxidation of intercalated lithium at cathode 14 to
produce electrons and lithium ions. The electrons (e), which flow
back towards anode 12 through the external circuit 18, and the
lithium ions, which are carried by the electrolyte across the
bi-layer separator 16 back towards anode 12, reunite at anode 12 to
replenish it with intercalated lithium for consumption during the
next battery discharge cycle.
[0042] In many lithium-ion battery constructions each of the
current collector 12a, anode 12, the separator 16, cathode 14, and
its current collector 14a are prepared as relatively thin layers
(for example, several microns or a millimeter or less in thickness)
and assembled in layers connected in electrical parallel
arrangement to provide a suitable energy package.
[0043] Anode 12 may include any lithium host material that can
sufficiently undergo lithium intercalation and de-intercalation
while functioning as the anode terminal of the lithium ion battery
10. Anode 12 may also include a polymer binder material to
structurally hold the lithium host material together. For example,
in one embodiment, the negative electrode 12 may be formed from
graphite intermingled in at least one of polyvinyldiene fluoride
(PVDF), a nitrile butadiene rubber (NBR), styrene butadiene rubber
(SBR) or carboxymethoxyl cellulose (CMC). Graphite is widely
utilized to form the anode because it exhibits favorable lithium
intercalation and deintercalation characteristics, is relatively
non-reactive, and can store lithium in quantities that produce a
relatively high energy density. The anode current collector 12a may
be formed from copper or any other appropriate electrically
conductive material known to skilled artisans.
[0044] The cathode 14 may be formed from any lithium-based active
material that can sufficiently undergo lithium intercalation and
deintercalation while functioning as the positive terminal of the
lithium ion battery 10. The cathode 14 may also include a polymer
binder material to structurally hold the lithium-based active
material together. One common class of known materials that can be
used to form cathode 14 is layered or spinel lithium transitional
metal oxides. For example, in various embodiments, cathode 14 may
comprise at least one of spinel lithium manganese oxide
(LiMn.sub.2O.sub.4), lithium cobalt oxide (LiCoO.sub.2), a
nickel-manganese-cobalt oxide
[Li(Ni.sub.xMn.sub.yCo.sub.z)O.sub.2], or a lithium iron polyanion
oxide such as lithium iron phosphate (LiFePO.sub.4) or lithium iron
fluorophosphate (Li.sub.2FePO.sub.4F) intermingled in at least one
of polyvinyldiene fluoride (PVDF), an ethylene propylene diene
monomer (EPDM) rubber, or carboxymethoxyl cellulose (CMC). Other
lithium-based active materials may also be utilized besides those
just mentioned. Those alternative materials include, but are not
limited to, lithium nickel oxide (LiNiO.sub.2), lithium aluminum
manganese oxide (Li.sub.xAl.sub.yMn.sub.1-yO.sub.2), and lithium
vanadium oxide (LiV.sub.2O.sub.5), to name but a few. The cathode
current collector 14a may be formed from aluminum or any other
appropriate electrically conductive material known to skilled
artisans.
[0045] Any appropriate electrolyte solution that can conduct
lithium ions between the anode 12 and cathode 14 may be used in
lithium ion battery 10. In one embodiment, the electrolyte solution
may be a non-aqueous liquid electrolyte solution that includes a
lithium salt dissolved in an organic solvent or a mixture of
organic solvents. A non-limiting list of lithium salts that may be
dissolved in an organic solvent to form the non-aqueous liquid
electrolyte solution include LiClO.sub.4, LiAlCl.sub.4, LiI, LiBr,
LiSCN, LiBF.sub.4, LiB(C.sub.6H.sub.5).sub.4 LiAsF.sub.6,
LiCF.sub.3SO.sub.3, LiN(CF.sub.3SO.sub.2).sub.2, LiPF.sub.6, and
mixtures thereof. These and other similar lithium salts may be
dissolved in a variety of organic solvents such as, but not limited
to, cyclic carbonates (ethylene carbonate, propylene carbonate,
butylene carbonate), acyclic carbonates (dimethyl carbonate,
diethyl carbonate, ethylmethylcarbonate), aliphatic carboxylic
esters (methyl formate, methyl acetate, methyl propionate),
.gamma.-lactones (.gamma.-butyrolactone, .gamma.-valerolactone),
chain structure ethers (1,2-dimethoxyethane, 1-2-diethoxyethane,
ethoxymethoxyethane), cyclic ethers (tetrahydrofuran,
2-methyltetrahydrofuran), and mixtures thereof.
[0046] FIG. 2 shows, in partial cutaway, a schematic representation
of a bi-layer separator 32, comprising a ceramic layer 26 and a
microporous polymer layer 30, and representative of one embodiment
of this invention. Ceramic layer 26 consisting of a layered array
of bonded-together, irregularly-shaped particles of similar size
(not shown) is bonded, to the surface 24 of anode 12 facing cathode
14 (not shown). A thin polymer coating (not shown) is used to bond
layer 26 to the anode surface and to bond the particles to one
another within layer 26. Overlying ceramic layer 26 is microporous
polymer layer 30 adhered to ceramic layer 26 at surface 28. For
clarity the layers are shown in partial cutaway but it is intended
that both the ceramic and microporous layers be coextensive with
one another and with anode 12 so that separator 32 may prevent any
direct contact between anode and cathode.
[0047] FIG. 3 shows, in cross-section, a scanning electron
micrograph of a cross-section of a cryogenically-fractured anode
12' with such a bonded bi-layer separator 32' and also showing
anode current collector 12a'. Details of the separator may be seen
at FIG. 4, which shows an enlarged view of the
cryogenically-fractured separator. The polymer layer 30' with
micropores 34 may be clearly distinguished from particles 36 and
the porosity resulting from the interconnected voids 38 between
particles 36 in ceramic layer 26'. It may be noted that the voids
in polymer layer 30' are appreciably smaller than those in ceramic
layer 26'.
[0048] Such a bi-layer separator may be formed by the following
exemplary procedure.
[0049] An anode may be prepared by spreading a slurry of graphite
(90% by weight) with carbon black (6% by weight) and polyvinylidene
fluoride (PVDF) dissolved in N-Methylpyrrolidone (NMP) (remainder)
as a binder on a 20 micrometer thick copper current collector and
drying the deposited slurry at 100.degree. C. for 12 hr.
[0050] A carrier solution for the ceramic coating may be prepared
by dissolving 1 gram of polyacrylonitrile (PAN) in 100 grams of
dimethylformamide (DMF) at 50.degree. C. to form a 1 wt. %
solution. A ceramic slurry may be formed by adding 65 grams of
dried silica powder to this carrier solution and stirring
vigorously to form a uniform dispersion. The silica dispersion may
be used to form the ceramic layer in the bilayer separator
coating.
[0051] A solution for a PVDF coating may be formed dissolving 6
grams of PVDF was in 90 grams of acetone at 50.degree. C., then
adding 4 grams of water to the PVDF solution and stirring at
50.degree. C. to obtain a uniform solution.
[0052] To form the bilayer separator, the silica dispersion may be
uniformly coated on the surface of the graphite anode by any
suitable means such as by extrusion or with a doctor blade. The
thickness of the coating may be adjusted to achieve any desired
thickness in the dried layer. A preferred dried coating thickness
may range from about 5 to 40 micrometers. After partially-drying
the ceramic coating by heating it to 80.degree. C. for 4 min, the
PVDF coating solution may be applied.
[0053] The PVDF coating solution may be applied to the ceramic
coated electrode with a doctor blade or a slot die coater. A
preferred thickness of the PVDF layer is between 5 and 20
micrometers and the applied thickness of the PVDF coating solution
may be adjusted to achieve such thickness after solvent
evaporation.
[0054] Solvent evaporation may result from blowing air across the
surface of PVDF coating solution. Because acetone may be
preferentially evaporated the solvent becomes enriched in water,
resulting, after suitable increase in water concentration to phase
separation into acetone-rich regions containing dissolved PVDF and
water-rich regions containing minimal or no dissolved PVDF.
Continued evaporation will remove the remaining acetone and
precipitate PVDF. Water may be removed by drying the coating at
60.degree. C. for 2 hours to leave an interconnected network of
pores within a PVDF layer.
[0055] The PVDF layer may be effective in further securing the
ceramic particles to the electrode as well as imparting a smoother
coating surface. The PVDF layer is sufficiently effective in
promoting adherence of the ceramic particles to the electrode that
in an aspect the polymer binder may be eliminated and dry ceramic
powders deposited on the binder. The PVDF coating solution may then
be applied to fully envelop the ceramic particles and contact the
electrode surface. Upon solvent evaporation the layer of unbonded
ceramic particles may be fully enclosed by the PVDF polymer
overlayer, attached, at its edges, to the electrode.
[0056] In another embodiment, applicable to both a bonded and
unbonded ceramic layer, poly(methyl methacrylate) (PMMA) may be
employed as the polymer layer. A microporous PMMA layer may be
fabricated using the acetone-water solution approach employed to
deposit PVDF. A non-porous layer of PMMA may be applied by
evaporation of acetone from an acetone-PMMA solution. Such a
non-porous layer of PMMA may also be effective because the PMMA,
when saturated with electrolyte, will form a Li-ion conductive gel
but generally the ionic conductance of such a gel will be less than
the conductance of the microporous polymer layer.
[0057] FIG. 5 shows the superior elevated temperature stability of
a lithium ion cell incorporating a 20 micrometer thick SiO.sub.2/5
micrometer thick microporous PVDF bi-layer separator fabricated as
described. The cell was fabricated as a CR 2325 button cell (23.0
millimeters in diameter.times.2.5 millimeters thick). The cell
employed LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2 (NCM) as the
cathode, graphite as the anode, and 1M LiPF.sub.6 in ethylene
carbonate (EC)/diethyl carbonate (DEC) (1:2 by volume) as the
electrolyte. After charging to 4.3 volts the open circuit cell
voltage was measured as a function of time as the cell skin
temperature was first increased to 150.degree. C. and then
maintained at that temperature using an accelerating rate
calorimeter.
[0058] For comparison, cells constructed using only a 25 micrometer
thick microporous PVDF separator and a commercial polyolefin
separator were also evaluated. The results for these cells are also
shown in FIG. 5.
[0059] As can be seen from FIG. 5, the cell employing the bi-layer
separator (curve 40) initially displayed a modest decline in output
voltage, primarily as the temperature (curve 50) was ramping up to
its set-point of 150.degree. C., and then remained at a
substantially constant output voltage over the test duration.
[0060] The cell with the commercial polyolefin separator (curve
44), like the cell with the bi-layer separator initially shows a
modest decline in output voltage as the temperature increases but
the most obvious feature is the precipitous decline in voltage to
zero output voltage after only a short time at maximum (150.degree.
C.) temperature. The cell with only the PVDF separator (curve 42)
likewise initially tracked the behavior of the bi-layer separator
but then progressively declined to zero output over the total test
time.
[0061] While preferred embodiments of the invention have been
described as illustrations, these illustrations are not intended to
limit the scope of the invention.
* * * * *