U.S. patent application number 10/188339 was filed with the patent office on 2004-08-05 for lithium ion battery electrodes.
This patent application is currently assigned to LITHIUM POWER TECHNOLOGIES, INC.. Invention is credited to Munshi, M. Zafar A..
Application Number | 20040151985 10/188339 |
Document ID | / |
Family ID | 23335606 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040151985 |
Kind Code |
A1 |
Munshi, M. Zafar A. |
August 5, 2004 |
Lithium ion battery electrodes
Abstract
A dimensionally stable, highly resilient, hybrid copolymer
solid-solution electrolyte-retention film for use in a lithium ion
battery in one preferred embodiment has a predominantly amorphous
structure and mechanical strength despite contact with liquid
solvent electrolyte. The film is a thinned (stretched), cast film
of a homogeneous blend of two or more polymers, one of which is
selected for its pronounced solvent retention properties. A very
high surface area inorganic filler dispersed in the blend during
formation thereof serves to increase the porosity of the film and
thereby enhance electrolyte retention. The film is soaked in a
solution of liquid polymer with liquid organic solvent electrolyte
and lithium salt, for absorption thereof. Use of a cross-linked
liquid polymer enhances trapping of molecules of the electrolyte
into pores of the film. The electrolyte film is sandwiched between
flexible active anode and cathode layers to form the lithium ion
battery. Novel methods are provided for forming the electrodes, the
polymer substrate, and other elements of the battery.
Inventors: |
Munshi, M. Zafar A.;
(Missouri City, TX) |
Correspondence
Address: |
DONALD R. GREENE
P. O. BOX 12995
SCOTTSDALE
AZ
85267
US
|
Assignee: |
LITHIUM POWER TECHNOLOGIES,
INC.
Manvel
TX
|
Family ID: |
23335606 |
Appl. No.: |
10/188339 |
Filed: |
July 2, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10188339 |
Jul 2, 2002 |
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09340944 |
Jun 28, 1999 |
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6413676 |
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Current U.S.
Class: |
429/309 ;
429/314; 429/316; 429/317 |
Current CPC
Class: |
H01M 6/168 20130101;
Y10T 29/49108 20150115; H01M 4/131 20130101; Y10T 29/49115
20150115; H01M 4/668 20130101; H01M 4/667 20130101; H01M 4/04
20130101; Y02T 10/70 20130101; H01M 10/0565 20130101; Y02E 60/10
20130101; H01M 4/133 20130101; H01M 4/134 20130101; H01M 50/417
20210101; H01M 50/42 20210101; H01M 6/10 20130101; H01M 6/40
20130101; H01M 10/0585 20130101; H01M 50/497 20210101; H01M 50/426
20210101; Y02P 70/50 20151101; H01M 4/622 20130101; H01M 6/188
20130101; H01M 4/661 20130101; H01M 4/137 20130101; H01M 50/411
20210101; H01M 10/0525 20130101 |
Class at
Publication: |
429/309 ;
429/314; 429/317; 429/316 |
International
Class: |
H01M 010/40 |
Claims
What is claimed is:
1. A solid base polymer material of a polymer electrolyte of a
lithium ion battery, comprising a hybrid copolymer solid-solution
homogeneous blend of at least two polymers, one of said at least
two polymers selected from a polar group having pronounced solvent
retention properties, and the other of said at least two polymers
selected from a second group consisting of polyester (PET),
polypropylene (PP), polyethylene napthalate (PEN), polycarbonate
(PC), polyphenylene sulfide (PPS), and polytetrafluoroethylene
(PTFE), or a combination of two or more polymers from said second
group, said other of said at least two polymers being selected and
having a concentration in said copolymer solid-solution blend
according to at least one desired property of said base polymer
material.
2. The solid base polymer material of claim 1, wherein said one of
said at least two polymers is polyvinylidene fluoride (PVDF).
3. The solid base polymer material of claim 1, wherein said one of
said at least two polymers has a concentration of from 1% to 99%,
with remainder said other of said at least two polymers.
4. The solid base polymer material of claim 1, further including an
acrylate.
5. The solid base polymer material of claim 1, further comprising
at least one of an acrylate, polyethylene oxide (PEO),
polypropylene oxide (PPO), oxymethylene linked PEO; PEO-PPO-PEO
cross-linked with trifunctional urethane;
poly(bis(methoxy-ethoxy-ethoxide))-phosphazene (MEEP);
polyacrylonitrile (PAN); polymethylmethacrylate (PMMA);
polymethyl-acrylonitrile (PMAN); triol-type PEO cross-linked with
difunctional urethane; poly(oligo)
oxyethylene)methacrylate-co-alkali metal methacrylate;
polysiloxanes and their copolymers.
6. The solid base polymer material of claim 1, wherein the ratio of
said at least two polymers in said blend is selected to provide
said at least one desired property of said base polymer.
7. Method of preparing of a solid base polymer material of a
polymer electrolyte of a lithium ion battery, comprising forming a
hybrid copolymer solid-solution homogeneous blend of at least two
polymers; including the steps of selecting a resin of one of said
at least two polymers from a polar group having pronounced solvent
retention properties, selecting a resin of the other of said at
least two polymers from a second group consisting of PET, PP, PEN,
PC, PPS, PTFE, or a combination of two or more polymers from said
second group, and setting the concentration of said other of said
at least two polymers in said copolymer solid-solution blend
according to at least one desired property of said base polymer
material, homogeneously mixing said at least two polymer resins
together to produce a melt-cast hybrid copolymer film, and
biaxially orienting said film in machine direction orientation and
transverse direction orientation by respective stretching of said
film to a desired final thickness.
8. The method of claim 7, wherein the step of biaxially orienting
said film includes stretching said film to a final thickness in a
range from 0.5 to 25 microns.
9. The method of claim 7, wherein said step of selecting a resin of
said one of said at least two polymers comprises selecting PVDF
resin.
10. The method of claim 7, wherein said step of setting the
concentration of selected resin of said other of said at least two
polymers comprises adding an amount by weight of said selected
resin to the mix from 99% to 1%, and the remainder of said mix
being the selected resin of said one of said at least two
polymers.
11. The method of claim 7, wherein said step of mixing to produce a
melt-cast hybrid copolymer film comprises co-extruding said at
least two polymer resins.
12. The method of claim 7, further including the step of adjusting
operating conditions of the co-extrusion, including temperature,
throughput, and die thickness, to control the quality of the
resulting melt-cast film with a die thickness in a range from about
100 microns to about 200 microns prior to biaxial orientation
thereof, and to control the composition of said hybrid copolymer
film.
13. An electrolyte-retaining base polymer material for a lithium
ion battery, comprising a solid-solution polymer thin film cast
from a polymer solution with constituents PP, PVDF and cured
acrylate monomer/oligomer from which a solvent in which said
constituents were dissolved has substantially evaporated; and a
liquid electrolyte solution containing a lithium salt absorbed
within said thin film.
14. Process for manufacturing an electrolyte-retaining base polymer
material for a lithium ion battery, comprising the steps of
dissolving PP, PVDF and acrylate monomer/oligomer in a solvent to
form a polymer solution; casting said polymer solution in a thin
film including evaporating said solvent therefrom to form a
solid-solution polymer thin film; soaking said solid-solution
polymer thin film in liquid electrolyte solution containing lithium
salt, to absorb said electrolyte within said thin film; and curing
said acrylate monomer/oligomer.
15. The process of claim 14, wherein said step of dissolving
comprises adding said PP, PVDF and acrylate monomer/oligomer to a
hydrocarbon to form said polymer solution.
16. The process of claim 14, wherein said step of curing is
performed using one of electron beam and ultraviolet radiation.
17. Method of producing a dimensionally stable,
electrolyte-retention without appreciable swelling, hybrid base
copolymer solid-solution blend film for a lithium ion battery, said
method comprising the steps of homogeneously mixing PVDF and PP to
form a copolymer blend thereof; dispersing very high surface area
inorganic filler consisting of one of fumed silica or alumina in a
concentration range from about 0.1% to about 30% by weight into
said copolymer blend to create porosity therein and to enhance the
mechanical stability of a thin film into which said copolymer blend
with said inorganic filler is cast; and soaking said thin film in a
liquid solvent electrolyte for absorption thereof into said thin
film.
18. The method of claim 17, wherein said step of soaking includes
soaking said thin film in a solvent mixture of at least one of
ethylene carbonate-diethyl carbonate (EC-DEC), EC-dimethyl
carbonate (EC-DMC), PC-EC-DMC or PC-DEC, the solvent mixture
containing a lithium salt, for ionic conduction.
19. The method of claim 17, wherein said step of homogeneously
mixing PVDF and PP comprises co-extrusion thereof, and wherein said
step of dispersing inorganic filler is performed during said
co-extrusion.
20. The method of claim 17, wherein said step of dispersing is
performed using fumed silica in the form of highly pure silica in
amorphous crystalline structure with fine particle size less than
0.05 micron and high specific surface area greater than 100
m.sup.2/g, for substantially even distribution of silica throughout
the cast copolymer blend thin film.
21. The method of claim 17, wherein said step of dispersing is
performed using alumina with high specific surface area greater
than 100 m.sup.2/g and fine particle size less than 0.05 micron,
for substantially even distribution of alumina throughout the cast
copolymer blend thin film.
22. The method of claim 18, wherein said lithium salt is selected
from a group including lithium hexafluorophosphase LiPF.sub.6,
lithium perchlorate LiClO.sub.4, lithium tetrafluoro-borate
LiBF.sub.4, lithium hexafluoroarsenate LiAsF.sub.6, lithium
tetrachloroaluminate LiAlC1.sub.4, lithium trifluoromethane
sulfonate LiCF.sub.3SO.sub.3, lithium bis(trifluoromethane
sulfonyl) imide (lithium imide) LiN(CF.sub.3SO.sub.2).sub.2, and
lithium methide LiC(SO.sub.2CF.sub.3).su- b.3.
23. A dimensionally stable hybrid base copolymer solid-solution
blend film adapted for electrolyte-retention without appreciable
swelling, for a lithium ion battery, said film comprising a cast
homogeneous copolymer blend of PVDF and PP into which is dispersed
very high surface area inorganic filler consisting of at least one
of fumed silica or alumina in a concentration range from about 0.1%
to about 30% by weight, whereby to create porosity in and enhance
mechanical stability of said film; said film having a liquid
solvent electrolyte absorbed therein.
24. A method of fabricating a dimensionally stable, highly
resilient, hybrid base copolymer blend electrolyte film of
predominantly amorphous structure having mechanical strength when
in contact with liquid solvent electrolyte, for a lithium ion
battery, said method comprising the steps of homogeneously mixing
PVDF with one of PP or PEN to form a copolymer blend thereof;
dispersing very high surface area inorganic filler in a
concentration range from about 0.1% to about 30% by weight into
said copolymer blend; and casting and further processing said
copolymer blend with said inorganic filler therein into a thin
film.
25. The method of claim 24, including generating high ionic
conductivity of said hybrid base copolymer blend electrolyte film
at reduced levels of liquid organic solvent by steps of introducing
into said copolymer blend film with said inorganic filler therein a
solution of a liquid polymer with liquid organic solvent
electrolyte and lithium salt, in a concentration of said solution
from about 0.1% to about 40% by weight relative to said copolymer
blend; and immobilizing said liquid organic solvent electrolyte to
allow molecules of said liquid polymer to trap molecules of said
liquid organic solvent electrolyte into pores of said copolymer
blend film.
26. The method of claim 25, wherein said liquid polymer in said
solution is selected to be cross-linkable; and the step of
immobilizing said liquid organic solvent electrolyte is performed
by radiation curing to cross-link said cross-linkable liquid
polymer to achieve said trapping of molecules of said liquid
organic solvent electrolyte into pores of said copolymer blend
film.
27. The method of claim 26, including selecting said cross-linkable
liquid polymer from a group of polymers based on acrylates and
PEO-based materials.
28. The method of claim 26, including selecting an ionizable
polymer as said liquid polymer.
29. The method of claim 25, wherein the step of immobilizing said
liquid organic solvent electrolyte is performed in part by
selecting a non-ionizable polymer as said liquid polymer.
30. The method of claim 25, including selecting as said liquid
polymer a polymer having a hetero atom with a lone pair of
electrons for metal ions of said lithium salt to latch onto and
move during a conduction process from one lone pair site to
another.
31. The method of claim 24, including selecting one of fumed silica
or alumina as said inorganic filler.
32. A dimensionally stable, highly resilient, hybrid base copolymer
solid-solution blend electrolyte film having a predominantly
amorphous structure and mechanical strength despite contact with
liquid solvent electrolyte, for a lithium ion battery, said film
comprising a cast homogeneous copolymer blend of PVDF with one of
PP or PEN, a very high surface area inorganic filler dispersed in
said copolymer blend in a concentration range from about 0.1% to
about 30% by weight, and a solution of liquid polymer with liquid
organic solvent electrolyte and lithium salt in a concentration
range from about 0.1% to about 40% by weight in said film.
33. The hybrid base copolymer solid-solution blend electrolyte film
of claim 32, wherein said inorganic filler consists of one of fumed
silica or alumina.
34. The hybrid base copolymer solid-solution blend electrolyte film
of claim 32, wherein said liquid organic solvent electrolyte is
immobilized, whereby molecules of said liquid polymer trap
molecules of said liquid organic solvent electrolyte into pores of
said film.
35. The hybrid base copolymer solid-solution blend electrolyte film
of claim 32, and an active anode and an active cathode between
which said electrolyte film is sandwiched to form an
electrochemical cell.
36. The hybrid base copolymer solid-solution blend electrolyte film
of claim 35, wherein said active cathode is selected from a group
including oxides, sulfides and selenides.
37. The hybrid base copolymer solid-solution blend electrolyte film
of claim 35, wherein said active cathode is selected from a group
including LiMn.sub.2O.sub.4, Li.sub.xMnO.sub.2, Li.sub.xCoO.sub.2,
Li.sub.xV.sub.2O.sub.5, Li.sub.xV.sub.3O.sub.8,
Li.sub.xV.sub.2S.sub.5, Li.sub.xNbSe.sub.3, Li.sub.xNiO.sub.2,
Li.sub.xNi.sub.yCO.sub.202, Li.sub.xNi.sub.yMn.sub.2O.sub.2,
Li.sub.xCo.sub.yMn.sub.2O.sub.2, or lithium doped electronically
conducting polymers including polypyrrole, polyaniline, and
polyacetylene.
38. The hybrid base copolymer solid-solution blend electrolyte film
of claim 35, wherein said active anode is selected from a group
including tin oxide, ion-insertion polymers, ion-insertion
inorganic electrodes, and carbon insertion electrodes.
39. A thin film lithium ion battery, comprising a resilient
flexible hybrid polymeric electrolyte thin film including a
homogeneous blend of at least two polymers with inorganic filler
dispersed therein to increase surface area and porosity of said
hybrid film, impregnated with a semi-liquid solution of liquid
polymer, organic solvent electrolyte and lithium salt; and a pair
of spaced-apart flexible thin film electrodes, each including a
polymer substrate having an adherent electrically conductive layer
thereon, said hybrid film being tightly sandwiched between said
pair of thin film electrodes.
40. The thin film lithium ion battery of claim 39, wherein said at
least two polymers of said blend in said hybrid film include PVDF
and one of PP or PEN.
41. The thin film lithium ion battery of claim 39, wherein said
inorganic filler consists of one of silica or alumina has a
concentration in a range from about 0.1% to about 30% by weight in
said blend.
42. The thin film lithium ion battery of claim 39, wherein said
solution has a concentration in a range from about 0.1% to about
40% by weight in said hybrid film.
43. The thin film lithium ion battery of claim 39, wherein said
pair of thin film electrodes comprises an anode and a cathode, said
polymer substrate of each said electrode selected from a group of
polymers including PET, PP, PPS, PEN, PVDF and PE, and each said
polymer substrate being metallized.
44. The thin film lithium ion battery of claim 39, wherein said
liquid polymer in said solution is cross-linked.
45. The thin film lithium ion battery of claim 39, wherein said
liquid polymer comprises at least one of an acrylate, polyethylene
oxide (PEO), polypropylene oxide (PPO), oxymethylene linked PEO;
PEO-PPO-PEO cross-linked with trifunctional urethane;
poly(bis(methoxy-ethoxy-ethoxid- e))-phosphazene (MEEP);
polyacrylonitrile (PAN); polymethylmethacrylate (PMMA);
polymethyl-acrylonitrile (PMAN); triol-type PEO cross-linked with
difunctional urethane; poly(oligo)
oxyethylene)methacrylate-co-alkali metal methacrylate;
polysiloxanes and their copolymers.
46. A lithium ion battery electrode, comprising an ultra thin film
metal substrate for at least one of a cathode substrate and an
anode substrate of a lithium ion battery, said ultra thin film
metal substrate having a thickness in a range from about one micron
to about 10 microns.
47. The lithium ion battery electrode of claim 46, wherein said
ultra thin film metal substrate is a cathode substrate comprising
aluminum.
48. The lithium ion battery electrode of claim 46, wherein said
ultra thin film metal substrate is an anode substrate comprising
copper.
49. The lithium ion battery electrode of claim 46, wherein said
ultra thin film metal substrate comprises a metal selected from a
group including copper, aluminum, nickel, zinc, stainless steel, an
alloy thereof, or other electrically conductive alloy.
50. A lithium ion battery electrode, comprising an ultra thin film
metallized polymer substrate including a polymer material selected
from a group of polymers including PET, PP, PPS, PEN, PVDF, and PE,
said polymer substrate having a thickness in a range from about 0.5
micron to about 50 microns.
51. An electrode for lithium ion battery manufacture, comprising a
flexible, ultra thin film metallized polymer substrate constituting
one of a cathode substrate and an anode substrate, said polymer
substrate having a thickness in a range from about 0.5 micron to
about 50 microns and being constructed and adapted for ease of
coating and handling, to avoid kinking and deformation thereof,
during manufacture of lithium ion batteries.
52. A lithium ion battery polymer substrate, comprising a layer of
polymer material, and a low resistance metallization layer having a
conductivity in a range from about 0.01 ohm per square to about 1
ohm per square overlying and adhered to a side of said polymer
material.
53. The lithium ion battery polymer substrate of claim 52, wherein
said layer of polymer material has a non-metallized margin with a
width in the range from about one mm to about three mm.
54. The lithium ion battery polymer substrate of claim 52, wherein
a low resistance metallization layer having a conductivity in said
range overlies and is adhered to each side of said polymer
material.
55. The lithium ion battery polymer substrate of claim 54, wherein
both sides of said layer of polymer material have a non-metallized
margin with a width in the range from about one mm to about three
mm, said non-metallized margin on both sides being present at the
same edge of said layer of polymer material.
56. A method of fabricating a thin film lithium ion rechargeable
battery, comprising the steps of incorporating an ultra thin film
metallized polymer substrate in the battery during fabrication
thereof, wherein the polymer layer in said substrate has a
thickness in a range from about 0.5 micron to about 50 microns, in
conjunction with very thin film battery electrode/electrolyte
structures having thickness less than 5 microns/10 microns,
respectively, wherein the thickness of the metallization layer on
said polymer layer is selected according to desired conductivity
thereof.
57. The method of claim 56, wherein said polymer layer comprises a
polymer material selected from a group including PET, PP, PPS, PEN,
PVDF and PE.
58. The method of claim 57, wherein said metallization for the
anode of said battery is copper.
59. The method of claim 57, wherein said metallization for the
cathode of said battery is aluminum.
60. The method of claim 56, wherein said polymer layer has a
thickness of less than 5 microns.
61. The method of claim 60, wherein said metallization layer has a
thickness of less than about 0.01 micron.
62. The method of claim 56, including a process of coating said
substrate with very thin film active anode material and active
cathode material comprising steps of milling each of said anode and
cathode materials in a separate solvent to reduce the particle size
thereof, injecting said materials directly onto the film substrate
at respective opposite sides thereof, and subsequently drawing said
materials into respective thin films of desired thickness using
wire wound rods or Mayer rods of different wire diameters to
control wet slurry thickness.
63. The method of claim 56, including a process of coating said
substrate with very thin film active anode material and active
cathode material comprising steps of incorporating each of said
materials into its own aerosol mix, spraying atomized aerosol of
each material directly on respective opposite sides of said film
substrate while moving said substrate past the points of aerosol
spray at high speed, and curing the sprayed material either by
drying or radiation.
64. The method of claim 56, including a process of coating said
substrate with very thin film active anode material and active
cathode material by evaporating the respective electrode material
directly onto respective opposite sides of said substrate.
65. A method of coating an ultra thin film metallized polymer
substrate for a thin film lithium ion battery with very thin film
active anode material and active cathode material, said method
comprising the steps of milling each of said anode material and
said cathode material in a separate solvent to reduce the particle
size of the respective material, injecting respective ones of said
materials directly onto said substrate at opposite sides thereof,
and subsequently drawing each said materials at opposite sides of
said substrate into a thin film of desired thickness using wire
wound rods or Mayer rods of different wire diameters to control wet
slurry thickness.
66. A method of coating an ultra thin film metallized polymer
substrate for a thin film lithium ion battery with very thin film
active anode material and active cathode material, said method
comprising the steps of incorporating each of said materials into
its own aerosol mix, spraying atomized aerosol of each material
directly on respective opposite sides of said film substrate while
moving said substrate past the points of aerosol spray at high
speed, and curing the sprayed material either by drying or
radiation.
67. A method of coating an ultra thin film metallized polymer
substrate for a thin film lithium ion battery with very thin film
active anode material and active cathode material, said method
comprising the step of evaporating the respective electrode
material directly onto respective opposite sides of said
substrate.
68. A method of fabricating a thin film lithium ion battery which
comprises laminating anode and cathode elements on respective
opposite sides of a double-metallized polymer substrate, whereby to
yield a highly flexible electrode structure for said battery.
69. The method of claim 68, including providing non-metallized
margins on each of said anode and cathode elements on said opposite
sides of said metallized polymer substrate, and metal spraying
opposite ends of said laminated metallized polymer substrate for
terminations thereto.
70. The method of claim 68, wherein the ratio of substrate
thickness to active electrode thickness is less than about 0.5.
Description
BACKGROUND OF THE INVENTION
[0001] A. Field
[0002] The present invention relates generally to a range of
polymer electrolytes characterized by high ionic conductivity at
room temperature and below, improved stability, and ability to be
formed in very thin film configuration, for use in lithium ion
batteries, and to methods of manufacturing lithium ion batteries
comprising such polymer electrolytes.
[0003] B. Prior Art
[0004] A high energy density rechargeable battery system is
currently a highly sought technology objective because of the
proliferation of power-consuming portable electronics that demand
increasingly greater energy levels, as well as more interest in
practical electric-powered vehicles with significantly improved
range presently unavailable from lead acid batteries. As a result,
lithium rechargeable batteries are the focus of intense
investigation around the world. Table I, below, describes the
available rechargeable lithium systems which are either in
commercial production or under development today. The lithium
solid-state polymer electrolyte battery (system 3 in the Table)
would be the ideal system for such high power-consumption
applications owing to its true flexibility and energy density
together with a capability of very high cycle life. However, in its
present stage of development, this otherwise enviable system is not
viable at temperatures below 60.degree. C.
1TABLE 1 Performance Characteristics of Lithium Rechargeable
Batteries Self- Energy Density Voltage Discharge Cycle System Wh/kg
Wh/liter (V) (%/month) Life Electrolyte 1 Lithium Ion 100-120
260-280 3.6 10-12 500-800 Liquid (Organic) 2 Lithium Ion 100-120
260-280 3.6 <8 2500 Solid-Liq. Polymer 3 Lithium Polymer 250-300
350-400 3.6 <<1 >>1000 Solid (Organic) (Note: In Table
1, Wh/kg is specific energy (gravimetric); Wh/liter is energy
density (volumetric).)
[0005] The lithium ion liquid electrolyte battery (system 1) is
presently the only commercial chemistry described in Table 1. No
generic lithium ion chemistry exists since each manufacturer has
its own chemistry containing different positives, different
negatives, binders, electrolyte and formation process. These are
major factors influencing cycle life and the charge and discharge
profiles. The most common lithium sink (i.e., place where the ion
inserts) negative electrodes in a lithium ion battery are
carbon-type insertion compounds, while layered metal oxides of the
LiMO.sub.2 type (where M=Ni or Co) or spinel lithium manganese
oxides of the LiMn.sub.2O.sub.4 type are currently used as
preferred lithium source positive electrodes. These electrodes are
usually calendared onto metallic current collectors (which are
about 25 to 50 microns thick). The overall process of these
batteries may be written as: 1 C 6 + LiMO 2 discharge charge Li x C
6 + Li 1 - x MO 2 ( 1 )
[0006] As indicated by the above cell reaction, charge and
discharge proceed via intercalation of lithium ions into the carbon
and metal oxide structure, respectively. Cell voltage at full
charge is usually 4.2 volts while cell voltage on discharge is 2.6
volts.
[0007] A microporous polypropylene or polyethylene separator
separates the two electrodes from shorting electrically, and liquid
organic solvents containing a lithium salt as the electrolyte which
is usually absorbed into the separator material and portions of the
electrode provides high ionic conductivity (10.sup.-3 to 10.sup.-2
S/cm) and ease of migration of ions between the electrodes of the
cell. These batteries are commonly used in portable computers,
cellular telephones and camcorders, among other applications. The
specific energy and energy density of the lithium ion battery is
usually about 125 Wh/kg and 260 Wh/l, respectively. The packaged
battery, usually in a hard plastic case, has a much lower energy
density than the individual cell (approximately 20% lower). The
cycle life (i.e., the number of times the battery can be recharged)
of this battery is about 500 to 800 cycles, the self-discharge
(i.e., loss of capacity on standing) per month is about 10%, and
the cost is currently about $1.00 per Watt-hour of energy. These
batteries can be manufactured in near fully automated, high volume
production. Although lithium ion battery technology is being
commercialized very heavily, numerous safety issues have arisen.
For example, cells that are abused under crush test or high
temperature test have been known to explode and ignite.
[0008] To overcome the disadvantages inherent in liquid
electrolytes, and to obtain superior long-tem storage stability, an
interest has arisen in solid polymeric electrolytes in which ion
mobility is achieved through coordination by sites on the polymer
chain of electrolyte ions, thus promoting electrolyte dissolution
and salt dissociation. An all-solid-state battery using an
ionically conductive polymer membrane as the electrolyte would have
several attractive features. It could be produced in virtually any
shape and size, in thin film power cells or thick film energy
cells, by automated fabrication techniques, as well as made
reasonably rugged and leakproof, with low self-discharge, and have
high open-circuit potential using a lithium metal anode. Such
features represent a unique combination of properties and give rise
to the possibility of using them as either secondary or primary
devices across a wide range of applications.
[0009] Polyethylene oxide (PEO), a polymer examined extensively for
the present application, is able to form stable complexes with a
number of salts. Because of its low ionic conductivity of about
10.sup.-9 to 10.sup.-8 S/cm at ambient temperature, batteries using
this material were found to require being operated at a temperature
of 100.degree. C. or higher. A major problem observed with
PEO-based electrolytes at temperatures below 60.degree. C. is their
high crystallinity and associated low ion mobility. The crystalline
structure of many polymers, including PEO, results in a weaker
structure. In recent years, many radically different approaches
have been taken to improve the conductivity of PEO and PEO-based
polymers, which have also led to the proposal of other polymers for
this purpose. These approaches included modification of existing
polymers, synthesis of new polymers, forming composite polymers
with ceramic materials, using plasticizer salts to increase ion
transport and mobility of the cation, using plasticizing solvents
in the polymer again to increase the ionic character of the cation,
and other approaches. Several review articles describe these
approaches in detail, e.g., "Technology Assessment of Lithium
Polymer Electrolyte Secondary Batteries," by M. Z. A. Munshi,
Chapter 19 in Handbook of Solid State Batteries and Capacitors, Ed.
M. Z. A. Munshi (World Scientific Pub. Singapore), 1995, and A.
Hooper, M. Gauthier, and A. Belanger, in: "Electrochemical Science
and Technology of Polymers--2," Ed. R. G. Linford (Elsevier Applied
Science, London), 1987.
[0010] These approaches have not resulted in adequate conductivity
enhancements on the polymer electrolytes desired for battery
operation at room temperature. As a result, another approach has
been taken in which plasticizing solvents or low molecular weight
polymers are added to the polymer electrolyte to increase ionic
conductivity of the PEO-based polymer electrolyte. The purpose of
the latter is to increase the ionic mobility and concentrations of
the charge carriers in the solid polymer electrolyte by enhancing
the dissociation of the lithium salt. It is believed that the
lithium ion is also solvated to the solvent molecule and
participates in enhancing the ionic mobility. Many electrolyte
composites incorporating low molecular weight polymers or liquid
organic solvents have been prepared and have demonstrated high room
temperature conductivity approaching those of the typical
non-aqueous liquid electrolytes.
[0011] For example, Kelly et al, in J. Power Sources, Vol. 14, page
13 (1985) disclosed that adding 20 mole percent of liquid
polyethylene glycol dimethyl ether polymer (PEGDME) to solid PEO
polymer results in an increase in the ionic conductivity of the
final plasticized polymer from 3.times.10.sup.-7 S/cm to 10.sup.-4
S/cm at 40.degree. C. However, it was found that the mechanical
property of this material is very poor. In U.S. Pat. No. 4,654,279
(1987), Bauer et al disclosed the thermal cross-linking of polymers
consisting of epoxies and methacrylates and plasticized with a
solution of LiClO.sub.4 in a 400 MW PEG resulted in a conductivity
of 4.times.10.sup.-4 S/cm at 25.degree. C. The '279 patent
describes a polymeric electrolyte consisting of a two phase
interpenetrating network (IPN) of a mechanically supporting phase
of a continuous network of a cross-linked polymer and an ionically
conducting phase comprising a metal salt and a liquid polymer such
as liquid PEG.
[0012] Many of these low molecular weight polymers have a
relatively low dielectric constant when compared to their liquid
solvent counterpart, and thus limit the number of charge carriers
in the plasticized polymer. In an effort to overcome this
hindrance, high dielectric constant liquid organic solvent such as
ethylene carbonate (EC) and propylene carbonate (PC) have been
incorporated into the host polymer. The purpose was both to
increase the number of charge carriers and further increase the
room temperature conductivity of the polymer. The use of these
organic solvents to plasticize polymers such as poly(vinyl acetal),
poly(acrylonitrile), poly(vinyl acetate), and
hexafluoropropene-vinyliden- e fluoride copolymer (Viton.TM.)
occurred as early as 1975 (see Feuillade and Perche, Journal of
Applied Electrochemistry, Vol. 5, page 63 (1975)). However, the
mechanical properties of these polymers were so poor that they had
to be supported on porous matrices. Later, Armand produced a system
with good room temperature conductivity (10.sup.4 S/cm) and good
mechanical properties by cross-linking the Viton.TM. and
plasticizing with a solution of 1M of LiClO.sub.4 in PC (Proc.
Workshop on Li Non-Aqueous Battery Electrochemistry, The
Electrochemical Soc., Vol. 80-7, page 261 (1980)). Polyvinylidene
fluoride (PVDF) and polyacrylonitrile (PAN) were evaluated in the
early 1980s and have also been doped with a variety of liquid polar
solvent, yielding room temperature conductivities as high as
10.sup.-3 S/cm. Subsequently, PVDF has been the subject of a recent
patent of Bellcore (U.S. Pat. No. 5,296,318).
[0013] The use of PC in an ionically conductive matrix containing
oxygen donor atoms such as PEO complexed with a lithium salt was
first presented by the applicant herein (see paper presented at the
Fall Meeting of the Electrochemical Soc., Oct. 18-23, 1987).
Although room temperature battery performance data was presented,
the polymer electrolyte did not exhibit good mechanical
property.
[0014] In the late 1980s, a series of patents were issued to MHB
Inc., generally relating to the use of liquid organic solvents in
various types of polymeric materials including PEO, materials based
on acrylates, and low MW PEG acrylates. These patents describe
predominantly radiation curing methods for the preparation of an
interpenetrating polymeric network (IPN) containing various types
of polyacrylates and liquid organic solvents. Although electron
beam curing was stated to be the preferred method to polymerize the
IPN, thermal and ultraviolet curing methods were also proposed. It
was thought that containing the PC solution in the matrix of the
polymeric network would therefore yield a high ionic conductivity
comparable to that of PC itself. Indeed, this was demonstrated in
typical polymeric networks yielding conductivities of about
2.times.10.sup.-3 S/cm at room temperature. However, with solvent
contents in the 60% to 80% range, oozing of the liquid was a major
problem. An advantage with using electron beam curing compared to
UV radiation is that the electron beam can penetrate through
metallic components, and hence complete prototype cells can be made
and polymer electrolyte cured in-situ. An advantage with acrylates
is their ability to hold the solvents fairly well, but mechanically
they are very weak. Stronger designs of acrylates have poor solvent
retention capabilities. Acrylates are also good ionic conductors
without solvents.
[0015] An offshoot of the lithium ion liquid electrolyte system is
the lithium ion polymer electrolyte battery (system 2 in Table 1,
above) that has been in development for the past four to five
years. Lithium ion cells utilizing gelled electrolytes offer all
the advantages of lithium ion liquid electrolyte cells and are
becoming widely accepted by many companies not only because they
potentially offer good form factors for a large variety of consumer
electronics devices such as slim notebook computers and cellular
telephones, but because they also offer improved safety over liquid
electrolyte cells. The electrode chemistry is the same, but the
liquid electrolyte (up to 70%) in this case is absorbed in a
polymer membrane instead of the microporous polypropylene
separator. The current technology based on liquid organic solvents
absorbed in polyvinylidene fluoride (PVDF) polymer developed at
Bellcore under U.S. Pat. No. 5,296,318 ensures good interfacial
contact, which leads to relatively low internal cell resistance
and, thus, good rate capability and long cycle life (up to 2500
cycles).
[0016] The current method of fabricating the polymer-solvent
electrolyte involves a complex process in which PVDF is cast from a
plasticizer solution of PVDF and DBP (di-butyl phthalate) to create
some porosity for the liquid organic solvent. The DBP is then
removed using either methanol or di-ethyl ether. The liquid organic
solvent is then added to this polymer. This process is very
expensive and involves hazardous chemicals.
[0017] Ironically, PVDF is non-conducting compared to many of the
above-mentioned polymers, and consequently merely holds the liquid
organic solvents in its structure similar to a sponge holding
water. Because the technology uses an extensive amount of liquid
electrolyte solvent absorbed in a polymer, it is not easy to
manufacture cells at high speed. Automation of this technology may
be very difficult. The gelled electrolyte cells incorporate very
thick electrode/electrolyte structures (50 to 75 microns) onto
metallic current collectors (25-50 microns) that not only add
unnecessary weight and volume to the battery, but result in a lower
cell performance. It is believed that many users incorporate an
expanded gauze made of copper (anode) and aluminum (cathode) to
coat the electrodes, instead of planar copper and aluminum foils.
This adds more weight and volume to the already large percentage of
inactive components of the cell. Present indications regarding this
technology from various sources are that the energy density
(gravimetric and volumetric) are lower than the existing lithium
ion batteries, cycle life is not particularly impressive, and cell
cost runs several dollars per Watt-hour. Like PEO, PVDF is highly
crystalline, thus weakening its strength. On the other hand,
acrylates are amorphous and can hold solvents well because of their
ability to be cured in-situ, thus "trapping" the solvents into the
polymer matrix.
[0018] When this technology emerged, form factors and flexibility
were among its most praised features, but currently it is used to
manufacture only flat prismatic cells which exhibit little
flexibility. Although scientific articles have been published
asserting that such polymer electrolyte batteries can be produced
in very thin film form with flexibility, these batteries tend to
lose performance over time when the cell is oriented because the
solvent is not completely immobilized in the polymer electrolyte.
In a battery standing upright, the liquid solvents travel to the
bottom of the cell, and during charge and discharge the current
along the cell height will differ because of the difference in
conductivity at the bottom and top of the electrode. Such cells
tend not to have very high cycle life, and lose capacity as a
result of poor charging and discharging.
[0019] While the addition of organic plasticizers may offer a
solution to low ionic conductivity in polymer electrolytes, they
necessarily introduce additional deleterious effects on other
electrolyte properties such as stability in contact with the
polymer matrix. Indeed, it is now known through manufacturers and
suppliers of PVDF resins to the battery industry that PVDF is
unstable in the organic solvents presently used in lithium ion
polymer electrolyte batteries, dissolves to an extent, and that the
instability worsens at elevated temperatures. The result is a
breakdown in the PVDF mechanical integrity and loss of separator
property, with the possibility of electrical shorting. Another
problem found with PVDF is that the polymer swells and loses
dimensional stability when it contacts liquid organic solvent.
Consequently, the battery would exhibit poor electrode/electrolyte
interface during thermal cycling and poor mechanical property of
the gel compared to that of the polymer. In addition, polymer
electrolytes based on such designs cannot be manufactured in very
thin film forms to reduce overall resistance and, hence, cell
resistance, since the polymer has insufficient strength to hold the
liquid organic solvents in its matrix. For such a system to be
fully functional it must be based on a thick film concept, which
increases overall cell resistance and reduces energy density
because of a reduction in the active components in the cell.
[0020] Thus, all of the aforementioned prior art techniques which
have been employed in an effort to improve ionic conductivity,
mechanical strength, safety, chemical stability, and cost reduction
by simplifying the synthesis of polymer electrolytes, exhibit one
or more substantial problems.
SUMMARY OF THE INVENTION
[0021] Accordingly, it is a principal object of the present
invention to provide a base polymer material for a polymer
electrolyte that is insoluble in the organic solvents presently
used in lithium ion batteries and is highly stable with
temperature.
[0022] Another object of the invention is to provide a base polymer
material for a polymer electrolyte that exhibits little or no
swelling characteristics when in contact with liquid organic
solvents, compared to PVDF.
[0023] Another object of the invention is to provide a base polymer
material that is predominantly amorphous in nature.
[0024] Still another object is to provide a base polymer material
for a polymer electrolyte that is mechanically stronger than PVDF
when in contact with liquid organic solvents.
[0025] Still another object is to provide a base polymer
electrolyte with ionic conductivity.
[0026] Yet another object of the invention is to provide polymer
electrolyte compositions which are more conductive at lower levels
of organic solvents than prior art polymer electrolyte-solvent
compositions.
[0027] A further object of the invention is to provide polymer
electrolyte compositions in which the solvent is immobilized in the
polymer, to allow lithium ion batteries constructed from such
compositions to be used in any orientation.
[0028] Another object of the invention is to provide polymer
electrolyte compositions that can be manufactured in very thin film
form, provide low resistance and excellent flexibility.
[0029] Still another important object of the invention is to
provide a lithium ion battery with polymer electrolyte compositions
described in the preceding enumerated objects.
[0030] A further object is to provide such lithium ion polymer
electrolyte batteries with ultra-thin current collectors such as
very thin metallic elements or metallized polymer substrates for
improved energy density, power density, higher capacity
utilization, higher cycle life, greater charge-discharge
efficiencies, lower polarization, greater safety, and greater
reliability, and which be produced at high speed, lower cost, and
with improved form factors.
[0031] Another object of the invention is to coat the thin
substrate with very thin active anode and cathode material.
[0032] A related object is to laminate the anode and cathode
elements on both sides of the metallized polymer substrate material
so as to yield a highly flexible electrode.
[0033] The electrolyte of the present invention is preferably a
cationic conductor, is very flexible and somewhat dry, is of low
cost, and in some preferred embodiments of the invention is
constructed in very thin film format. Polymer electrolytes of this
design can be combined with various negative electrodes such as an
alkali metal, alkaline earth metal, transition metal, ion-insertion
polymers, ion-insertion inorganic electrodes, carbon insertion
electrodes, and tin oxide electrodes, among others, and with
various positive electrodes such as ion-insertion polymers, and
ion-insertion inorganic electrodes, to provide batteries and
supercapacitors having high specific energy (Wh/kg) (gravimetric)
and energy density (Wh/l) (volumetric), high cycle life, low
self-discharge, and which provide improved safety.
[0034] One embodiment of a solid base polymer material of a polymer
electrolyte of a lithium ion battery according to the invention is
a thin film that includes a hybrid copolymer solid-solution
homogeneous blend of at least two polymers, one selected from a
polar group having pronounced solvent retention properties, and the
other selected from a second group consisting of polyester (PET),
polypropylene (PP), polyethylene napthalate (PEN), polycarbonate
(PC), polyphenylene sulfide (PPS), and polytetrafluoroethylene
(PTFE), or a combination of two or more thereof. The specific
polymer of the latter group and its concentration in the blend are
selected to tailor at least one desired property of the base
polymer material. In a preferred embodiment, the polymer selected
from the polar group is PVDF. In a two polymer blend, the
concentration of one is in a range from 1% to 99% by weight, and
the remainder being the other. The base polymer material may
include other substances such as an acrylate, polyethylene oxide
(PEO), polypropylene oxide (PPO),
poly(bis(methoxy-ethoxy-ethoxide))-phosphazene (MEEP),
polyacrylonitrile (PAN), polymethylmethacrylate (PMMA),
polymethyl-acrylonitrile (PMAN), etc.
[0035] In a method of preparing such a base polymer material,
resins of the two or more polymer constituents, the one from the
polar group being selected at least in part for its pronounced
solvent retention properties, are blended preferably in a
co-extrusion twin-screw process, to produce a hybrid melt-cast
film. This film is then subjected to stretching by biaxial
orientation in machine direction and transverse direction, to a
desired final thickness, preferably in a range from 0.5 to 25
microns.
[0036] In another embodiment, an electrolyte-retaining base polymer
material for a lithium ion battery is a solid-solution polymer thin
film cast from a solution of PP, PVDF and cured acrylate
monomer/oligomer from which a solvent in which those constituents
were dissolved has substantially evaporated. A liquid or
semi-liquid electrolyte solution containing a lithium salt is
absorbed within the thin film. In a process of manufacture of this
embodiment, the PP, PVDF and acrylate monomer/oligomer are
dissolved in a hydrocarbon solvent to form a polymer solution,
which is then cast in a thin film, in part by evaporation of the
solvent. The film is then soaked in liquid electrolyte solution
containing lithium salt, for absorption of the electrolyte within
the film, and the acrylate monomer/oligomer is cured by subjection
to electron beam or ultraviolet radiation.
[0037] A dimensionally stable embodiment of hybrid base copolymer
solid-solution blend film for a lithium ion battery, the film being
capable of electrolyte retention without appreciable swelling, is
produced by a method in which PVDF and PP are mixed homogeneously
to form a copolymer blend thereof. A very high surface area
inorganic filler--either fumed silica or alumina--is then dispersed
with a concentration in a range from about 0.1% to about 30% by
weight into the copolymer blend to enhance the porosity and
mechanical stability of the thin film into which the copolymer
blend with inorganic filler is cast. Finally, after extrusion of
the resin blend and biaxial orientation, the resultant film is
soaked in a liquid solvent electrolyte for absorption and retention
in the film. Preferably, the film is soaked in a mixture of
ethylenecarbonate-diethyl carbonate (EC-DEC), EC-dimethyl carbonate
(EC-DMC), PC-EC-DMC or PC-DEC, each of the solvent mixtures
containing a lithium salt such as (by way of example) lithium
hexafluorophosphase LiPF.sub.6, lithium perchlorate LiClO.sub.4,
lithium tetrafluoro-borate LiBF.sub.4, lithium hexafluoroarsenate
LiAsF.sub.6, lithium tetrachloroaluminate LiAlCl.sub.4, lithium
trifluoromethane sulfonate LiCF.sub.3SO.sub.3, lithium
bis(trifluoromethane sulfonyl) imide (lithium imide)
LiN(CF.sub.3SO.sub.2).sub.2, or lithium methide
LiC(SO.sub.2CF.sub.3).sub.3 for ionic conduction. Dispersion of the
inorganic filler into the copolymer blend is performed during
co-extrusion of the PVDF and PP.
[0038] Still another embodiment of the invention resides in a
dimensionally stable, highly resilient, hybrid base copolymer blend
electrolyte film of predominantly amorphous structure having
mechanical strength when in contact with liquid solvent
electrolyte, for a lithium ion battery. This embodiment preferably
comprises a copolymer blend of PVDF and either PP or PEN, with the
very high surface area inorganic filler dispersed therein. To
generate high ionic conductivity of the hybrid copolymer
electrolyte film at reduced levels of liquid organic solvent, a
liquid polymer with liquid organic solvent electrolyte and lithium
salt is introduced into the film, and the electrolyte is
immobilized to allow molecules of the liquid polymer to trap
molecules of the electrolyte into pores of the film. Preferably,
the liquid polymer is cross-linkable, such as a polymer based on
acrylates and PEO-based materials, and radiation curing is
performed to cross-link the liquid polymer for trapping of
molecules. Alternatively, some immobilization of the liquid organic
solvent electrolyte may be achieved by using a non-ionizable liquid
polymer.
[0039] Any of these polymer electrolyte films may be used to form
an electrochemical cell, particularly a lithium ion battery, by
tightly sandwiching the film between thin, flexible active anode
and active cathode layers.
[0040] For example, one embodiment of a thin film lithium ion
battery is formed from a resilient flexible hybrid polymeric
electrolyte thin film that includes a homogeneous blend of at least
two polymers with inorganic filler dispersed therein to increase
surface area and porosity of the hybrid film, impregnated with a
semi-liquid or even dry solution of liquid polymer, organic solvent
electrolyte and lithium salt; and a pair of spaced-apart flexible
thin film electrodes, each including a polymer substrate having an
adherent electrically conductive layer thereon, the hybrid film
being tightly sandwiched between the pair of thin film electrodes.
The polymer substrate of each of the anode and cathode is
preferably selected from a group of polymers including PET, PP,
PPS, PEN, PVDF and PE, and each polymer substrate is metallized to
form the conductive layer thereon.
[0041] According to another aspect of the invention, a lithium ion
battery electrode comprises an ultra thin film metal substrate for
at least one of a cathode substrate and an anode substrate of a
lithium ion battery, the ultra thin film metal substrate having a
thickness in a range from about one micron to about 10 microns. The
ultra thin film metallized polymer substrate includes a polymer
material selected from a group of polymers including PET, PP, PPS,
PEN, PVDF, and PE, and has a thickness in a range from about 0.5
micron to about 50 microns, thereby rendering it very flexible for
ease of coating and handling, to avoid kinking and deformation
thereof, during manufacture of lithium ion batteries.
[0042] The lithium ion battery polymer substrate may comprise a
layer of polymer material, and a low resistance metallization layer
having a conductivity in a range from about 0.01 ohm per square to
about 1 ohm per square overlying and adhered to a side of the
polymer material. Preferably, the layer of polymer material has a
non-metallized margin with a width in the range from about one mm
to about three mm. Preferably, also, a low resistance metallization
layer having a conductivity in the aforementioned range overlies
and is adhered to each side of the polymer material, and both sides
of the layer of polymer material have such a non-metallized margin
present at the same edge of the layer of polymer material.
[0043] According to yet another aspect of the invention, a method
of fabricating a thin film lithium ion rechargeable battery
includes incorporating an ultra thin film metallized polymer
substrate in the battery during fabrication thereof, wherein the
polymer layer in the substrate has a thickness in a range from
about 0.5 micron to about 50 microns, in conjunction with very thin
film battery electrode/electrolyte structures having thickness less
than 5 microns/10 microns, respectively, wherein the thickness of
the metallization layer on the polymer layer is selected according
to desired conductivity thereof, e.g., less than about 0.01
micron.
[0044] The invention also provides novel methods of coating an
ultra thin film metallized polymer substrate for a thin film
lithium ion battery with very thin film active anode material and
active cathode material. One method comprises steps of milling each
of the anode material and the cathode material in a separate
solvent to reduce the particle size of the respective material,
injecting respective ones of the materials directly onto the
substrate at opposite sides thereof, and subsequently drawing each
of the materials at opposite sides of the substrate into a thin
film of desired thickness using wire wound rods or Mayer rods of
different wire diameters to control wet slurry thickness.
[0045] Another coating method includes incorporating each of the
materials into its own aerosol mix, spraying atomized aerosol of
each material directly on respective opposite sides of the film
substrate while moving said substrate past the points of aerosol
spray at high speed, and curing the sprayed material either by
drying or radiation. Yet another coating method comprises
evaporating the respective electrode material directly onto
respective opposite sides of the substrate.
[0046] Also according to the invention, a method of fabricating a
thin film lithium ion battery involves laminating anode and cathode
elements on respective opposite sides of a double-metallized
polymer substrate, whereby to yield a highly flexible electrode
structure for the battery. Non-metallized margins are provided on
each of the anode and cathode elements on the opposite sides of the
metallized polymer substrate, and metal is sprayed on opposite ends
of the laminated metallized polymer substrate for terminations
thereto. These techniques enable the provision of a ratio of
substrate thickness to active electrode thickness less than about
0.5.
BRIEF DESCRIPTION OF THE DRAWING
[0047] The above and still further aims, objectives, features,
aspects and attendant advantages of the present invention will
become apparent from the following detailed description of a
preferred embodiment and method of fabrication of a thin film
lithium ion battery in accordance with the invention, constituting
the best mode presently contemplated of practicing the invention,
when taken in conjunction with the accompanying drawing, in
which:
[0048] The sole FIGURE is a perspective and partly exploded view of
the layout of a lithium ion polymer electrolyte battery which is
useful in describing the final structure embodying the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS AND METHODS
[0049] According to a first aspect of the present invention, a
range of base polymer compositions is provided for the membrane of
the electrochemical cell with improved chemical stability in
lithium ion battery solvents and improved chemical stability as a
function of temperature. Polymer materials with high breakdown
voltages or strengths and low dissipation factors, such as those
employed in film capacitors, have been found to be chemically more
stable than other materials with liquid organic solvents.
[0050] PVDF has lower electrical stability and breakdown voltage
than many capacitor-grade polymer materials, including PET, PP,
PEN, PC, PPS, and PTFE. This is true even when PVDF is combined
with hexafluoropropylene (HFP), and despite its good solvent
retention properties, making it unstable to an extent in lithium
ion battery electrolytes.
[0051] Polar polymer PVDF currently used in lithium ion gelled
electrolyte batteries is a partially crystalline linear polymer
with a carbon backbone, in which each monomer
{CH.sub.2--CF.sub.2--} unit has two dipole moments, one associated
with CF.sub.2 and the other with CH.sub.2. It is used in a physical
mixture form with HFP in the ratio of about 9:1 PVDF:HFP, and
solvent cast to make the polymer blend. In the crystalline phase,
PVDF exhibits a variety of molecular conformations and crystal
structures depending on the method of preparation. The extruded or
cast material usually contains 40 and 60% crystalline material in
one or both of the principal crystalline phases, alpha and beta.
The alpha phase predominates in material cast from the melt. This
phase is converted to beta phase by mechanical deformation of the
material at temperatures less than 100.degree. C. PVDF's physical
strength is weakened as a consequence of its crystallinity.
[0052] Although materials based on PET, PP, PEN, PC, PPS and PTFE
are found to have improved chemical stability over PVDF, especially
in common battery electrolyte solvents such as ethylene carbonate,
propylene carbonate, diethyl carbonate, dimethyl carbonate, and so
forth, and in mixtures of these solvents, such materials do not
have the same solvent retention properties as PVDF. The first
aspect of the invention provides new base polymers for polymer
electrolytes such as PET, PP, PEN, PC, PPS, PTFE, and so forth,
with high dielectric strength and lower dissipation factor by
forming a copolymer solid solution of mixed polymers thereof,
either singly or preferably in combination with one another, in
combination with PVDF.
[0053] Each polymer or combination of polymers has its own
particular properties and can be selected to meet specific design
requirements for the battery, such as degree of flexibility,
strength, chemical stability, temperature stability, and processing
difficulties, to name a few. For example, PPS mixed with PVDF in a
concentration of 1 to 99 parts by weight provides a much stronger
tensile strength copolymer than PPS mixed with PET. By way of
further example, a mixture of polyethylene (PE) and PET with PVDF
provides a dimensionally stable material which can be cast in very
thin gauges, compared to PE or PET alone or in mixture without
PVDF.
[0054] The copolymer solid-solution of the present invention, then,
is composed of a first polymer selected from a group consisting of
PET, PP, PEN, PC, PPS, and PTFE, or a combination of two or more
polymers from that group, in mixture with a second polymer selected
from a group consisting of PVDF or other polymers with excellent
solvent retention properties. These first and second polymers are
combined in a ratio ranging from 1 hundredth (1%) to 99 hundredths
(99%) parts by weight of the first polymer and the remainder of the
mixture being of the second polymer, the constituents and the ratio
being selected according to specific design requirements for the
lithium ion battery. For example, the first polymer may be PP in a
proportion of 1 part to 99 parts with the second polymer which may
be PVDF in a proportion of 99 parts to 1 part. PVDF has a melting
point of 171.degree. C., while PP has a melting point of
189.degree. C. The closeness of their melting points ensures good
melt blending and similar rates of cooling of these polymers,
without polymer segregation.
[0055] This two-polymer blend is easily extended to a three or more
polymer blend to tailor the specific properties desired for the
final copolymer solution.
[0056] Other polymer hybrid blends may comprise PVDF, PP, PEN, PPS,
PC, PET, and PTFE. Others are based on high molecular weight groups
such as the different types of acrylates, PEO, PPO, including those
based on random copolymers such as oxymethylene linked PEO; block
copolymers such as PEO-PPO-PEO cross-linked with trifunctional
urethane; comb-branched block copolymers such as MEEP, PAN, PMMA,
PMAN; networks such as triol-type PEO cross-linked with
difunctional urethane; and
poly(oligo)oxyethylene)methacrylate-co-alkali metal methacrylate;
polysiloxanes and their copolymers.
[0057] A typical preparation of a hybrid copolymer solid-solution
blend, using examples of PVDF and PP, includes homogeneously
mixing, by co-extrusion (twin screw), selected homopolymer high
purity PVDF (Solvay) and PP (Exxon or Huntsman) resins to form a
melt-cast film. The resins are fed separately into the extruder via
hoppers, and blended with homogenization to form the resin
melt-cast dielectric film. An extruded method is preferred over the
so-called "blown bubble" method because closer thickness tolerances
can be achieved. Operating conditions of the extrusion process,
including temperature, throughput, die opening and width, etc., may
be adjusted until a good quality melt-cast film is obtained. The
operating conditions will vary for each composition even if the
resin materials are the same. For multi-resin extrusion, the resin
pellets are mixed according to the type of extrusion system
available. For example, for a tertiary component system, resin
copolymers of PVDF and PP could be formed as one component and
either PPS or PC or PET or PEN could be used as the second
component in a twin screw extruder. Alternatively, the entire resin
pellets of the mixture could be melted before extrusion into a
melt-cast.
[0058] Even though PVDF melts at 171.degree. C. and PP melts at
189.degree. C., the resin melt is usually at significantly higher
temperatures, typically at about 250.degree. C. to 300.degree. C.
The resin melt is injected onto a chilling-wheel to cool the resin
and form a solid film. Because of the higher temperature of the
resin melt, adequate heat exists within the polymer during
injection of the melt to allow stretching to a thinner sheet before
ultimately being stretched further and pulled wider to a still
thinner sheet during machine direction orientation (MDO, i.e., in
the direction of the film travel).
[0059] The extrusion temperature and the throughput of the
individual resin will vary depending upon the formulation
composition, and the throughput die opening and width will depend
upon the desired thickness and width of the final film. Typically,
these parameters are varied to achieve a melt-cast resin thickness
in a range from about 100 to 200 microns, with preference for the
thinner end of this range, so as to achieve a final film thickness
of about 4 microns at the end of the processing run, and a width in
a range from about 20 to 30 inches. Hence, if a final film
thickness of about 2 microns is desired, the starting melt-cast
thickness should be in a range from about 50 to 100 microns. The
wide range of variation present in these numbers is because each
polymer will stretch and thin differently, and processing at
various stages must be adjusted to obtain the most optimum film
desired without breaking, wrinkling, or overheating of the film
web, for manufacture at the desired rate. Typically, achievement of
a good quality film means that the film is clean of dust or specks;
has no bubbles, pinholes or other visible inclusions or impurities
within the film; is static-free (to preclude attracting dust
particles); and is wrinkle-free, to mention its principal
attributes.
[0060] The melt-cast film is then bi-axially oriented, first being
pulled along and through several rollers for feeding it under
proper tension into the MDO heated chamber where it is stretched so
as to exit the chamber with a typical film thickness in a range
from about 25 to 50 microns. The MDO chamber has a series of
rollers and tension control system that stretches the film in the
direction of film travel to produce a thinner film with a more
uniform thickness. The film exits the MDO chamber at a speed which
is faster than that at entry, depending upon the film thickness at
the end of the MDO run. For example, a melt-cast film thickness of
100 microns at entry into the MDO chamber, which is stretched
therein to a film thickness of 25 microns, will exit the MDO
chamber at four times its entry speed. Here again, the process
parameters in the MDO chamber are adjusted--to control conditions
such as line speed, film tension, stretching ratios, and so forth,
for optimum film quality and desired final thickness. These
conditions are dependent not simply on the final film thickness,
but primarily on the properties of the resin. The parameters chosen
in the first stage of the film processing, i.e., extrusion and
melt-cast film production, will affect the parameters chosen in the
second stage of the film processing, i.e., the MDO stretching.
[0061] The thinner film exiting the MDO chamber is fed through
additional rollers to maintain proper tension, and then enters the
transverse direction orientation (TDO) chamber where it is
stretched in the transverse direction by the tenter method. This is
a conventional technique in which the film is seized by a
continuous series of mechanical jaws at both ends of the film width
just before the film enters the TDO chamber. As in the case of the
MDO chamber, the TDO chamber is heated. This chamber typically has
a length in a range from about 40 to 100 feet, depending upon the
extent of film production. Width of films being processed through
the chamber may vary from two meters to several meters.
[0062] As the film moves forward in the TDO chamber, the mechanical
jaws move outward and thereby stretch the film to make it thinner
and wider, typically to a final thickness in a range from about 0.5
to 25 microns and a width in a range from about 80 to 400 inches.
At the opposite (i.e., exit) end of the TDO chamber, the jaws are
automatically actuated to release the further thinned and widened
film for winding onto paper or plastic cores. As a result of the
stretching, the speed of the film at exit from the TDO chamber is
considerably faster than the film speed at entry. For example, a
film with 100 micron melt-cast thickness exiting the MDO chamber at
25 microns thick travels at four times the speed at entry, as noted
above, and if it has then been stretched to a final thickness of
2.5 microns, will exit from the TDO chamber with a speed which is
40 times the speed of the original melt-cast film. Here also, the
processing parameters of the TDO chamber will depend upon desired
film thickness as well on production feasibility. Film speed and
stretching ratio in the transverse direction in the TDO chamber
will determine the final film thickness. The dwell time selected
for the film in the TDO is also important, as it controls the
reduction in film thickness without breakage.
[0063] This basic process is believed to be novel to PVDF copolymer
manufacture for lithium ion battery applications.
[0064] According to another embodiment of the invention, polymers
from three groups listed above, e.g., PP, PVDF and acrylate
monomer/oligomer, are dissolved in an appropriate solvent, e.g., a
hydrocarbon, to form a polymer solution. The solvent is evaporated
completely after the polymer solution is cast in a thin film. The
solid-solution thin film polymer is then soaked in a liquid
electrolyte solution containing a lithium salt and then the
acrylate monomer/oligomer is cured using electron beam or
ultraviolet radiation.
[0065] According to yet another embodiment of the invention, a
hybrid base polymer blend is formed wherein the electrolyte film is
dimensionally stable when in contact with liquid solvent
electrolytes and will not swell to any appreciable extent. The
applicants herein have found that dispersing very high surface area
fumed silica or alumina in concentrations of 0.1 to 30 weight
percent to some copolymers blends of PVDF and PP provides improved
mechanical stability when soaked in mixtures of EC-DEC and EC-DMC.
The inorganic fillers can be added during processing of the hybrid
copolymer using the extrusion process. The swelling values observed
for such material blends are less than 8% compared to about 20 to
80% for a wide range of different blends of fluoropolymers,
including poly(chlorotrifluoro-ethylene,
poly(ethylene-chlorotrifluoroethylene), poly(fluorinated
ethylene-propylene), ETFE, PTFE, HFP and PVDF and mixtures of the
fluoropolymers. The addition of the inorganic fillers provides a
simpler method of creating porosity in the polymer structure
compared to the traditional method using plasticizer DBP.
[0066] The fumed silica according to the present invention is
preferably highly pure silica in an amorphous crystalline
structure, with a fine particle size and a very high specific
surface area. The fine particle size is important not only to
maintain the high specific surface area, but also to cause the
silica to be evenly distributed through the polymer electrolyte. A
desirable mean particle size is 0.05 micron or less, and preferably
0.01 micron or less. The surface area in the BET measuring method
is 100 m.sup.2/g or more, and preferably 200 m.sup.2/g. Examples of
some suitable fine particle silicas are: Aerosil 380.TM. from
Nippon Aerosil, Cab-O-Sil.TM. Garde EH-5 from Cabot Corporation,
and Snowtex-O.TM. (constituting a dispersion of silica in water or
alcohol) from Nissan Chemical Industries Ltd. In this case, the
water within the dispersion should be removed before adding liquid
solvent electrolytes.
[0067] In the same manner, very high surface area alumina can be
substituted for the silica, with similar desirable particle size
and surface area, and is available from Degussa Corporation.
[0068] Another embodiment of the invention resides in forming
hybrid base polymer blends in which the film is mechanically
stronger than base PVDF and the final polymer electrolyte is
stronger when in contact with liquid electrolyte. Representative
examples indicate that polymers prepared with PP and PVDF, and
polymers prepared with PEN and PVDF, in varying ratios with high
surface area silica or alumina inorganic filler, have greater
mechanical strength than the homopolymer PVDF. PVDF-solvent polymer
tends to stretch when pulled, whereas the hybrid polymer blends of
the present invention offer greater resilience with less tendency
to stretch and thereby suffer damage. The ability of PVDF-solvent
polymer to stretch suggests that solvent retention in the polymer
is weakened.
[0069] Although the conductivity of a gelled polymer electrolyte
can be increased by the incorporation of conductive liquid organic
solvents, e.g., a lithium salt dissolved in EC-DEC, the integrity
of the liquid electrolyte in the polymer remains the same as that
of PVDF-liquid electrolyte material, to an extent. Another
embodiment of the invention provides a means for generating a high
ionic conductivity of the polymer electrolyte at reduced levels of
liquid organic solvents. This is achieved by introducing into the
base polymer material containing the inorganic filler, a solution
of an ionizable or non-ionizable liquid polymer in a concentration
of from about 0.1 to 40 weight percent (compared to the base
polymer) which contains the liquid organic solvent electrolyte and
lithium salt and cross-linking the liquid polymer using radiation
curing methods.
[0070] Cross-linking a cross-linkable liquid polymer using either
ultraviolet (UV) or electron beam (EB) radiation is the most
preferred method of immobilizing the liquid organic solvent
electrolyte into the base polymer structure, although a
non-ionizable liquid polymer may also reduce the mobility of the
solvent from the polymer. Representative examples of cross-linkable
polymers include those polymers based on acrylates and PEO-based
materials; and those based on non-cross-linkable polymers include
MEEP, polyacrylonitrile, and so forth. Cross-linking is most
preferred because the liquid polymer molecule traps the solvent
molecule during radiation curing into the pores of the base
polymer. The applicant herein has found that no restriction exists
on the type of liquid polymer material used. However, it is
preferable to use a polymer having a hetero atom with a lone pair
of electrons for the metal ions of the salt to latch onto and move
during the conduction process from one lone pair site to
another.
[0071] Preferable polymers include MEEP which demonstrates
excellent ionic conductivities at room temperature (10.sup.-5
S/cm), or more preferably acrylates which have excellent solvent
retention properties and can be cured in-situ trapping the solvents
within the polymer.
[0072] The base hybrid polymer thus formed is no longer
crystalline, but predominantly amorphous in nature.
[0073] The applicant has further found that even the use of a worse
conductive polymer which can be cross-linked, such as PEO, still
works very well in the invention. The liquid polymer material, such
as cross-linkable PEO, is then complexed with a metal salt,
preferably a plasticizer salt of a metal, e.g. lithium
bis(trifluoromethane sulfonyl) imide, LiN(CF.sub.3SO.sub.2).sub.2
or lithium methide, LiC(SO.sub.2CF.sub.3).sub.3 and about 5 to 75%
liquid organic solvents such as EC-DEC or EC-DMC, which yields a
conductivity of about 10.sup.-3 to 3.times.10.sup.-2 S/cm. Polymers
based on various acrylate compositions and lithium imide salt also
yield such conductivity values, and some compositions are half an
order of magnitude or more higher than those based on PEO. A
compromise must be made, however, between conductivity and
mechanical strength. The applicant herein has also found that the
use of the inorganic fillers enhances the ionic conductivity of the
polymer electrolyte by about half an order of magnitude compared to
an absence of filler, in addition to enhancing the mechanical
properties and porosity of the base polymer. The increase in ionic
conduction through the use of an insulator in an ionic conductor is
akin to conductivity enhancement in lithium iodide electrolyte as
described by C. C. Liang, J. Electrochem. Soc., v.120, p. 1289
(1973).
[0074] It will be appreciated that the present invention allows
fabrication of very thin, low resistance, flexible films of this
polymer electrolyte, without loss of mechanical integrity,
conductivity, and mechanical strength. By virtue of introducing the
above-described preferred methods, which produce excellent
mechanical strength and porosity of the base polymer via the
copolymer hybrid design and addition of high surface area alumina
or silica, reduce the swelling properties of the polymer with
liquid solvents, reduce the level of liquid solvents into the
polymer, improve the ionic conductivity of the polymer electrolyte
by introducing plasticizer salts and jonically conductive polymers
into the base polymer, and immobilizing the solvents, gelled
polymer electrolytes as thin as 10 microns or less can be
manufactured simply by selective use of the various components of
the polymer electrolyte. Such polymer electrolytes are not only
thin, but truly flexible; and the thinness of the structure allows
the possibility of lower resistances than are available from liquid
electrolytes absorbed in traditional polypropylene separators.
Traditional separator materials are usually at least 25 microns
thick, and the conductivity of the solvent electrolyte is usually
about 10.sup.-3 to 10.sup.-2 S/cm at room temperature. The design
of gelled polymer electrolytes according to the present invention
suggests that the effective resistance for thinner polymer
electrolyte sections should be at least half that observed in
liquid electrolytes alone. For example, a liquid electrolyte
absorbed in a 25 micron polypropylene separator has an effective
resistance of about 0.25 ohm, while a 10 micron polymer electrolyte
fabricated according to one of the methods of the invention, with
comparable ionic conductivity, has an effective resistance of only
about 0.1 ohm. In addition, the thinness of the polymer electrolyte
allows the film to be highly flexible.
[0075] According to yet another embodiment of the present
invention, an electrochemical cell is provided having improved
performance, in which the cell has a polymer electrolyte layer
fabricated as one of the above-described embodiments, and an anode
and cathode. Each of the anode and the cathode is selected from a
group of materials that provides a very high capacity. The active
cathode may be selected from a wide range of oxides, sulfides and
selenides, or any other group well known in the prior art, e.g.,
LiMn.sub.2O.sub.4, Li.sub.xMnO.sub.2, Li.sub.xCoO.sub.2,
Li.sub.xV.sub.2O.sub.5, Li.sub.xV.sub.3O.sub.8,
Li.sub.xV.sub.2S.sub.5, LiSNbSe.sub.3, Li.sub.xNiO.sub.2,
Li.sub.xNi.sub.yCO.sub.202, Li.sub.xNi.sub.yMn.sub.2O.sub.2,
Li.sub.xCo.sub.yMn.sub.2O.sub.2, lithium doped electronically
conducting polymers such as polypyrrole, polyaniline,
polyacetylene, and so forth. By way of example but not of
limitation, the active anode may be selected from the group
including tin oxide, ion-insertion polymers, ion-insertion
inorganic electrodes, and carbon insertion electrodes.
[0076] Lithium primary battery electrodes are traditionally made by
calendaring the cathode paste onto a nickel or stainless steel
gauze and compacting between heated rollers. In the case of lithium
metal anodes the gauze is used as a substrate material. The
substrate material is typically about 2 to 3 mils thick. The anode
and cathode are typically about 5 to 10 mils thick, with a
microporous polypropylene separator sandwiched between them, and
wound in a jelly-roll manner. Usually, the laminates are very thick
and the electrode length is about two feet in a typical AA size
cell. Rechargeable lithium metal anode batteries were also
constructed in this manner.
[0077] These techniques have changed considerably with the advent
of lithium ion battery construction. The carbon anode is pasted in
relatively thin film form onto a copper foil electrode, and the
lithiated metal oxide cathode is pasted onto an aluminum foil. The
substrate thickness for both anode and cathode is in a range from
about 25 to 35 microns, and the active electrode is about 25
microns thick. Additionally, the length of each electrode in a
typical AA size cell is about twice that of lithium anode cells.
Present electrode/electrolyte component thickness in gelled
electrolyte lithium ion cells is of the order of 50 to 75 microns
each.
[0078] This remains far too thick for optimum electrode utilization
and high rate capability. Metallic current collectors are also
used, not only adding weight but unwanted thickness to the battery.
The thick electrode concept in commercial cells is designed for
maximum capacity, while the thick gelled PVDF electrolyte provides
ease of handling. However, the internal resistance of this battery
is still relatively higher than its liquid electrolyte counterpart,
thus decreasing battery performance. Cells constructed from such a
design cannot be used at high discharge and charge rates. Thick
inactive substrates used in such cell construction effectively
reduce the energy density of the battery. In addition, this design
exposes the cells to risk of high polarization during charge and
discharge, which could lead to breakdown of the liquid solvent
electrolyte and consequently loss of capacity, loss of cycle life
and inadequate safety.
[0079] In yet another of its aspects, the present invention
incorporates ultra thin film metal substrates in thin film lithium
rechargeable batteries, in preferred thickness less than 5 microns
and more preferably less than 2 microns. At present, minimum
thickness available for copper or aluminum foil is about 5 microns.
The design of novel polymer electrolytes of this invention is in
tandem with the design of very thin electrodes/electrolytes
fabricated with low cost, very large surface area, very thin
inactive current collectors. Very thin film (e.g., <<5
microns/<<10 microns) battery electrode/electrolyte
structures designed with very thin metallized polymer films (e.g.,
1 micron) as the substrate material have several significant
advantages. They enable fabrication of low resistance cells that
can operate efficiently at the lower temperatures with
significantly improved materials utilization, provide lower
polarization with attendant greater energy efficiency and safety,
and offer the potential for unprecedented cycle life of several
tens of thousands of cycles from a bulk battery system. The present
invention uses either very thin metallic elements less than 10
microns thick, preferably <0.01 micron, of metallized copper,
onto either PET, PP, PPS, PEN, PVDF or PE for the anode, and
metallized aluminum onto either PET, PP, PPS, PEN, PVDF or PE for
the cathode. It should be understood that the invention is not
limited to copper or aluminum metallization on the polymer
substrate, but may readily be extended to other metallic elements
such as nickel, zinc, stainless steel, alloys of such elements, or
other alloys such as inconel, for example. The polymer substrate
layer ranges in thickness from 0.5 micron to greater than 50
microns, while the thickness of the metallic layer is dependent
upon the conductivity requirement and the desired resistivity of
the metal.
[0080] In a lithium ion battery design, the cathode is coated on an
aluminum metallized polymer. Such material has been found to have
the resistivity (expressed as ohms per square) with thickness of
the deposit as shown in Table 2 below.
2TABLE 2 Resistivity as a Function of Metal Thickness for Aluminum
Resistivity (ohms per square) Metal Thickness (Angstroms) 0.1 3000
0.3 1000 0.37 800 0.5 600 0.75 400 1 300 1.5 200 2 150 2.5 113 3
100
[0081] Resistivity as a function of copper thickness on metallized
polyester is indicated in Table 3 below.
3TABLE 3 Resistivity as a Function of Metal Thickness for Copper
Resistivity (ohms per square) Metal Thickness (Angstroms) 0.1 1500
0.2 750 0.3 600 0.4 300 0.5 200 1 100
[0082] Each polymer substrate electrode material has different
characteristics and thermal and mechanical properties, and each
behaves differently depending upon its use. Ideally, the thickness
of the metal coating should be kept as thin as possible, while
concurrently ensuring that its conductivity is very high.
Preferably, the coating thickness has a conductivity of less than
about 1.0 ohm per square, preferably less than 0.1 ohm per square,
and more preferably about 0.01 ohm per square. This will ensure low
resistance loss during current drain from the metallized substrate.
The metallization may be present on only one side of the polymer
layer or substrate, but is preferably provided on both sides
thereof. Further, the metallization preferably is accomplished to
leave an unmetallized margin or non-metallized margin having a
width in a range from about one millimeter (mm) to about three mm.
Where the metallization is present at both sides of the polymer
substrate, the non-metallization margin is provided at opposite
sides of the polymer material, but on the same edge. When the
substrate is coated with the active material, the coating material
will be applied to the metallized portion and not the margin.
[0083] The invention in another of its aspects also resides in
methods of coating the thin substrate with very thin film active
anode and cathode material. Conventional calendaring of the anode
and cathode from paste or knife-over-roll or doctor blades
techniques will not lead to the desired thickness. Instead the
invention teaches a method whereby the active anode and cathode
materials are milled extensively in a solvent to reduce the
particle size, and then injected directly onto the substrate and
subsequently drawn into thin films of various thickness using wire
wound rods or Mayer rods of different wire diameters. The different
diameter wire controls the wet slurry thickness. This method, which
to the applicant's knowledge has not been used previously in the
battery industry to manufacture electrodes, yields an electrode
material that is extremely uniform and thin. Dry film thickness of
1 micron to greater than 100 microns can be controlled using this
process. The coating may also be made either by spraying atomized
aerosol directly onto a high speed moving substrate and curing
either by drying or radiation (if the material contains radiation
curable elements), or by evaporation of the electrode material
directly onto the substrate.
[0084] Thinner electrode structures also have the advantage that
they undergo significantly less expansion and contraction during
discharge and charge. The polymeric structures within the battery
(i.e., the metallized film substrates, and polymer film
electrolyte) should also accommodate these phenomena. Since the
composite cathode and anode structure must be as thin and smooth as
possible, it is desirable to perform extensive, high speed, wet
attrition milling of the electrode formulation (i.e., active
cathode or anode, carbon, lithium salt, solvents, binder) so as to
obtain thin electrode films of the same form of consistency as
audio and video magnetic tapes. Final particle size of the
composite electrodes is less than 0.1 micron, and preferably less
than 0.05 micron. This will ensure a very thin film and smooth
electrode during the coating process.
[0085] The invention provides very thin film, strong, and yet
flexible and highly conductive polymeric electrolyte and electrode
structures, similar to film capacitor dielectric material that can
be tightly wound in formation of the capacitor.
[0086] Referring to the sole FIGURE of drawing, the three principal
webs, comprising anode 10, hybrid polymer electrolyte film 11 and
cathode 12 are wound as illustrated in the FIGURE, or,
alternatively, may be stacked or laminated, to form lithium ion
polymer battery 15. A tightly wound construction removes air from
between the layers, and allows enhanced and continuous contact
between the layers. The exercise of care is essential to avoid
electrical shorting of the beginning of the turns. The tightly
wound battery-capacitor hybrid is taped at the edge 16, and may
then be strapped in a tray (not shown) which is open on both sides.
This provides access to both ends 17 and 18 of the battery 15 for
schooping or, preferably, metal spraying thereof, first with a high
zinc content solder (harder material) followed by a regular softer
"Babbitt" end spray solder (90% tin:10% zinc). The first end spray
scratches the metallized surface and creates a trough to build a
better electrical and mechanical contact. The tight wind and offset
spacing prevents the zinc from penetrating to the active
components. This combination of end sprays also allows better
contact adhesion with the final termination.
[0087] Subsequently, aluminum leads (not shown) are soldered onto
each of the ends 17 and 18 to form the final termination. The
battery may then be epoxied to maintain pressure on the cell as
well as to protect it further from humidity. This battery-capacitor
hybrid 15 is heated to about 80.degree. C. for a period of from 2
to 5 minutes, to improve the interface. If desired, it may be
heated under vacuum before epoxying, to improve the interface even
further.
[0088] According to a further aspect of the present invention, the
anode and cathode elements are preferably laminated on both sides
of a double-metallized polymer substrate material so as to yield a
highly flexible electrode. By way of contrast, in a typical
state-of-the-art lithium ion battery, a carbon anode is calendared
on both sides of a copper current collector. Rather, in the thin
film, flexible battery 15 of the invention, the carbon 22, for
example, is laminated or coated in a very thin film on both sides
of copper metallized polymer (e.g., PET) material 23 so that no
distortion or damage to the carbon electrode will occur during the
winding operation of the cell components. If even thinner electrode
structures are desired, it is preferable that the electrode
material be evaporated directly onto the metallized polymer. A
similar structure (apart from the specific materials employed,
e.g., aluminum rather than copper for the metallization) may exist
for the cathode. An insulator sheet 25 and a metallized PET current
collector 26 complete the assembly prior to winding.
[0089] In some cases, it is desirable that the coating of the anode
and cathode be carried out with a margin, especially if the tabbing
or current lead can be placed at the end of the jelly-roll
structure. The use of metallized plastic substrates in thickness of
0.7 micron reduces the inactive components of the cell
significantly, compared to state-of-the-art lithium ion polymer
electrolyte battery substrate material, thereby increasing the
energy density of the battery even further. In a typical lithium
metal anode battery, the ratio of substrate thickness to electrode
thickness is about 0.8 to 1. In batteries according to the
invention, if the ratio of substrate thickness to electrode
thickness can be reduced to less than 0.5, when combined with
reduced electrolyte thickness and reduced dead space at the top and
bottom of the jelly roll structure, a significant improvement is
obtained in the energy density, rate capability, reliability, cycle
life, and safety over the prior art device.
[0090] The combined use of advanced thin film battery technology
and manufacturing processes of film capacitors disclosed herein
enables the development of novel power source technologies that can
satisfy many of the requirements which are not presently met by
state-of-the-art batteries.
[0091] It will thus be recognized that polymer electrolytes
fabricated in very thin film form can be used with thin film anode
and cathode electrodes. By designing a battery based on very thin
film active and inactive components, the surface area of the active
plates can be effectively increased to provide the battery with
higher current drain capability, lower resistance, higher energy
content, lower self-discharge, wide operating temperature range,
absence of dendrite formation, higher efficiency, higher capacity
utilization, greater cycle life, and improved reliability and
safety. Such a battery is able to tolerate overcharge, without
emission of any deleterious species or outgassing. Furthermore,
when designed around very thin metallized polymer films (1 micron)
as the substrate material, the energy density may be expected to
improve by at least 25 to 40% over state-of-the-art lithium ion
batteries, with reduced cost. A method of producing cells with the
above attributes is to manufacture very thin film cell components,
and once the individual cell components have been laminated, the
finished cell is heated to about 60-80.degree. C. for about 2
hours. This enhances the electrode/electrolyte interface and allows
better interfacial adhesion and improved cyclability.
[0092] Although certain preferred embodiments and methods have been
disclosed herein, it will be appreciated by those skilled in the
art to which the invention pertains, from a consideration of the
foregoing description, that variations and modifications may be
made without departing from the spirit and scope of the invention.
Accordingly, it is intended that the invention shall be limited
only by the appended claims and the rules and principles of
applicable law.
* * * * *