U.S. patent application number 13/470392 was filed with the patent office on 2012-09-06 for electrolyte-absorbing, non-permeable sealing materials.
This patent application is currently assigned to EVEREADY BATTERY COMPANY, INC.. Invention is credited to Mark A. Schubert.
Application Number | 20120225349 13/470392 |
Document ID | / |
Family ID | 39763024 |
Filed Date | 2012-09-06 |
United States Patent
Application |
20120225349 |
Kind Code |
A1 |
Schubert; Mark A. |
September 6, 2012 |
Electrolyte-absorbing, Non-permeable Sealing Materials
Abstract
An engineered thermoplastic sealing member for LiFeS.sub.2 and
other nonaqueous cells is disclosed. The optimal material displays
a propensity to absorb at least 10 weight percent of an ether-based
electrolyte while, at the same time, displaying a vapor
transmission rate of less than 500 ((g.times.mil)/(100
in.sup.2.times.days).
Inventors: |
Schubert; Mark A.; (Medina,
OH) |
Assignee: |
EVEREADY BATTERY COMPANY,
INC.
St. Louis
MO
|
Family ID: |
39763024 |
Appl. No.: |
13/470392 |
Filed: |
May 14, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12125142 |
May 22, 2008 |
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13470392 |
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10682223 |
Oct 9, 2003 |
7923137 |
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12125142 |
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10856189 |
May 28, 2004 |
7670715 |
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10682223 |
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Current U.S.
Class: |
429/185 |
Current CPC
Class: |
C09K 3/1012 20130101;
C09K 3/10 20130101; H01M 10/0569 20130101; C09K 3/1009 20130101;
Y02E 60/10 20130101; H01M 10/052 20130101; C09K 2200/0667 20130101;
H01M 2/08 20130101; H01M 10/0568 20130101; H01M 4/581 20130101;
C09K 2200/0655 20130101; C09K 2200/0617 20130101 |
Class at
Publication: |
429/185 |
International
Class: |
H01M 2/08 20060101
H01M002/08; H01M 10/0569 20100101 H01M010/0569; H01M 4/64 20060101
H01M004/64; H01M 4/13 20100101 H01M004/13; H01M 4/58 20100101
H01M004/58; H01M 10/0568 20100101 H01M010/0568; H01M 4/38 20060101
H01M004/38 |
Claims
1. A lithium iron disulfide battery comprising: a jellyroll
electrode assembly comprising an anode consisting of lithium or a
lithium alloy, a cathode including iron disulfide coated onto a
current collector and a separator disposed between the anode and
the cathode; an electrolyte consisting essentially of one or more
organic solvents and at least one solute dissolved therein, said
one or more organic solvents including at least 50 volume percent
of one or more ethers; a housing; a closure assembly affixed over
an opening in the housing; and at least one sealing member disposed
between the housing and the closure assembly, said sealing member
made from an engineered thermoplastic comprising PPA; wherein the
engineered thermoplastic has an absorption at 71.degree. C. greater
than or equal to 10 weight percent and a vapor transmission rate of
less than 500 (g.times.mil)/(100 in.sup.2.times.day), said
absorption and vapor transmission rate both relative to the
electrolyte.
2. The battery of claim 1 wherein the one or more ethers are
selected from the group consisting of: 1,2-dimethoxyethane,
1,2-diethoxyethane, di(methoxyethyl)ether, triglyme, tetraglyme,
diethyl ether, 1,3-dioxolane, tetrahydrofuran, 2-methyl
tetrahydrofuran, 3-methyl-2-oxazolidinone and combinations
thereof.
3. The battery of claim 2 wherein the electrolyte consists of at
least 90 volume percent of the one or more ethers.
4. The battery of claim 1 wherein the electrolyte consists of at
least 90 volume percent of the one or more ethers.
5. The battery of claim 1 wherein the at least one solute selected
from the group consisting of: lithium bromide, lithium perchlorate,
lithium hexafluorophosphate, potassium hexafluorophosphate, lithium
hexafluoroarsenate, lithium trifluoromethanesulfonate, lithium
iodide and combinations thereof
6. The battery of claim 2 wherein the at least one solute selected
from the group consisting of: lithium bromide, lithium perchlorate,
lithium hexafluorophosphate, potassium hexafluorophosphate, lithium
hexafluoroarsenate, lithium trifluoromethanesulfonate, lithium
iodide and combinations thereof.
7. The battery of claim 1 wherein the engineered thermoplastic
consists essentially of PPA.
8. The battery of claim 1 wherein the engineered thermoplastic
consists essentially of PPA having between 5 and 40 weight percent
of an impact modifier.
9. The sealing member of claim 8 wherein the engineered
thermoplastic is semi-crystalline.
10. The sealing member of claim 1 wherein the engineered
thermoplastic is semi-crystalline.
11. The sealing member of claim 7 wherein the engineered
thermoplastic is semi-crystalline.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/125,142, filed on May 22, 2008, which is a
continuation-in-part of U.S. patent application Ser. No. 10/682,223
filed on Oct. 9, 2003, now U.S. Pat. No. 7,923,137 issued on Apr.
12, 2011 and U.S. patent application Ser. No. 10/856,189 filed on
May 28, 2004, now U.S. Pat. No. 7,670,715, issued on Mar. 2, 2010,
both entitled "Nonaqueous Cell with Improved Thermoplastic Sealing
Member".
BACKGROUND OF INVENTION
[0002] This invention relates to the field of engineered
thermoplastic sealing members for electrochemical cells and, more
particularly, to a novel engineered thermoplastic sealing member
with an affinity for absorbing electrolyte but possesses low vapor
transmission (i.e., permeation) of that electrolyte through the
sealing member. As used herein, the term "engineered thermoplastic"
contemplates the type of materials described in Volume 2 of the
Engineered Materials Handbook, first published by ASM International
in 1988, and specifically includes all thermoplastics, with or
without fillers or reinforcers (such as
acrylonitrile-butadiene-sytrene, acetal, acrylic, fluorocarbon,
nylon, phenoxy, polybutylene, polyaryl ether, polycarbonate,
chlorinated polyethers, polyether sulfone, polyphenylene oxide,
polysulfone, polyimide, rigid polyvinyl chloride, polyphenylene
sulfide, thermoplastic urethane elastomers and other similar
reinforced plastics), that have sufficient mechanical, chemical and
thermal properties necessary to withstand exposure to non-aqueous
organic solvents at temperature extremes potentially as great as
-55.degree. C. to 85.degree. C. over a period of time that may be
as long as 10-15 years and all without degradation that would
comprise the material's ability to act as a hermetic seal for a
battery housing.
[0003] The ability to withstand extreme temperature conditions,
including thermal cycling and thermal shock between high and low
temperatures, is becoming more important for nonaqueous cells,
particularly in consumer-sized lithium batteries (e.g.,
International Electrotechnical Commision sizes FR6 or FR03).
Specifically, transportation regulations limit total weight loss of
such batteries to fractional percentages of the battery's original
weight after being subjected to certain tests/conditions. However,
such weight loss is typically, if not entirely, attributed to vapor
transmission of the volatile nonaqueous electrolytes through and/or
around the sealing mechanism(s) utilized by that housing. Not
surprisingly, the more volatile the electrolyte, the greater the
need for effective sealing.
[0004] In response, a wide variety of cell designs have been
developed for effectively sealing nonaqueous cells. The design
depends, at least in part, on the size of the cell, the type of
electrode and electrolyte materials used in the cell and the power
requirements of the devices to be powered by the cell. Because the
cathode/electrolyte materials are so reactive, lithium cells with
large liquid cathode (e.g., lithium-sulfur dioxide (Li/SO.sub.2)
and lithium-thionyl chloride (Li/SOCl.sub.2)) often have housings
in which metal components are hermetically welded, and glass seals
are used to seal metal components that must be electrically
insulated and to seal small apertures in the housings. These types
of housings tend to be expensive due to the materials and the
manufacturing processes and equipment required. As such, these
solutions have little bearing on consumer battery systems, where
cost and ease of manufacture are two of the most prominent
motivations in cell design.
[0005] Other, less complex means are used to seal cells that
utilize solid electrodes, including consumer-sized lithium
batteries, with the more recent trend being toward engineered
plastics in place of less refined or more common materials in order
to improve sealing performance. Because of its relatively low cost
and ease of manufacture, thermoplastic seal members are often used
to effect a seal between rigid components of the cell housing
(i.e., the container and closure). For example, a thermoplastic
gasket or grommet can be compressed between the inside top edge of
the cell container (e.g., a steel can) and the periphery of the
cover which forms the closure over the open top of the can.
Hermetic sealing of the container is important both to keep the
active materials and electrolyte within the cell housing, as well
as to keep water out.
[0006] Thermoplastic seals may also be used to effect a hermetic
seal of the safety vent aperture(s) in the cell housing. Here, a
conforming thermoplastic seal plugs the vent hole. Alternatively,
the plug may be a rigid material, such as a glass or metal ball
blocking the opening of a cylindrical thermoplastic bushing which
lines the interior of the vent hole. In either case, the
thermoplastic seal functions as a pressure relief vent for the
cell. These arrangements are also desirable because electrolyte may
be dispensed into the cell through the vent hole after the cover
has already been sealed to the battery housing. Other designs which
utilize a thermoplastic member to form a compressive seal between
rigid parts of a rupturable vent are also possible.
[0007] FIG. 1 shows an example of a cylindrical lithium battery 10
that is common to lithium-iron disulfide cells and/or other lithium
cell types (e.g., lithium-manganese dioxide, etc.). Notably, the
cell design shown in FIG. 1 is also applicable to certain
embodiments of the invention described herein. This design has two
thermoplastic seal members--gasket 16, which in combination with
top cover 40 encloses in the open end of can 12, and bushing 34,
which in combination with vent ball 32 encloses vent aperture 30.
Both thermoplastic seal members provide a compressive seal. Since
the can 12 and cover 40 are electrically connected to opposite
electrodes within the cell, gasket 16 must also provide electrical
insulation between the top cover 40 and the can 12. The bushing 34
and vent ball 32 comprise a pressure relief vent for the cell. When
the internal cell pressure exceeds a predetermined abnormally high
level, the vent ball 32 (and sometimes the bushing 34) are forced
out of the vent aperture 30, leaving an opening through which
pressure is released. Cells sealed with both a gasket between the
can and cover and a pressure relief vent comprising a bushing and
vent plug disposed in an aperture in the cell cover are disclosed
in U.S. Pat. No. 4,329,405 (issued May 11, 1982), U.S. Pat. No.
4,437,231 (issued Mar. 20, 1984), U.S. Pat. No. 4,529,673 (issued
Jul. 16, 1985), 4,592,970 (issued Jun. 3, 1986), U.S. Pat. No.
4,927,720 (issued May 22, 1990) and U.S. Pat. No. 4,931,368 (issued
Jun. 5, 1990) and U.S. Pat. No. 5,015,542 (issued May 14, 1991),
the entire disclosures of which are incorporated herein.
[0008] For any cell type, the seal member material must be such
that a suitable seal is maintained for an acceptable period of time
and under the temperature conditions that the cell is expected to
withstand during transportation, storage and use. Common
characteristics of a good seal member include stability of the
material in the internal cell and external environments,
impermeability to the liquids and gases that are to be sealed
within or outside the cell, and the formation and maintenance of a
complete seal path (i.e., with no voids or gaps) at each seal
interface. As noted above, consumer non-aqueous batteries are often
expected to deliver service at temperatures as low as 0.degree. C.,
and sometimes even as low as -40.degree. C. or -55.degree. C.
Additionally, these cells often experience high temperatures
exceeding 40.degree. C. or, more often than not, 71.degree. C. and
higher. Thus, the seal must maintain its physical shape and
integrity, while remaining non-reactive with the organic solvents,
solutes and electrochemically active materials (e.g., iron
disulfide, lithium, etc.), across that entire range of
temperatures.
[0009] Compressibility of the seal is significant in order to allow
for slight variations in manufacturing and/or component tolerances.
As a result, the material used in a seal should have sufficient
compressibility that can be maintained for a prolonged period of
time. However, some thermoplastic materials under compressive
stress tend to flow to relieve that stress. This is referred to as
stress relaxation or "cold flow" of the material. Furthermore,
thermoplastic materials tend to exhibit even greater stress
relaxation at higher temperatures, thereby reducing the time that
sufficient compression can be maintained.
[0010] Temperature also affects the compression of thermoplastic
materials in another way. Different materials will expand and
contract by different amounts in response to increases and
decreases, respectively, in ambient temperature. Therefore,
thermoplastics used to form compressive seals between more rigid
components (e.g., a metal can and a metal cover) should expand and
contract at close to the same rate as the rigid materials in order
to maintain sufficient gasket compression over the greatest
temperature range possible. If the thermoplastic is used as a
compressive seal and it has a diverging coefficients of thermal
expansion as compared to the rigid components in contact with the
seal, unwanted gaps may form between the seal and rigid members,
resulting in leaking and potential weight loss.
[0011] Thermoplastic materials available for use in nonaqueous cell
seal members are more limited than for aqueous cells. Specifically,
the active materials in nonaqueous cells are very reactive with
water, so the seal must prevent water from permeating into the cell
(as compared to aqueous cells, where water transport may be less
restricted). A seal for a nonaqueous cell must also have a low
vapor transmission rate for the electrolyte solvents used in the
cell itself. Since the vapor transmission rate of thermoplastic
material is generally dependent in part upon the vapor pressure of
the solvent, low vapor transmission rates are generally more
difficult to achieve for nonaqueous electrolytes containing ethers
and/or other organic solvents with low boiling points common to
such cell systems. Finally, and most significantly, any
thermoplastic must not dissolve or degrade when exposed to the
organic solvents commonly used in nonaqueous cells, thereby
limiting the number of available material combinations.
[0012] Although the prior art generically teaches that a seal
member's ability withstand temperature fluctuations can be improved
by using engineered thermoplastic materials that maintain
dimensional stability and do not crack under extreme temperature
conditions, the problem of reducing electrolyte permeation and the
rate of transmission of through the gasket is not addressed. This
problem, as well as the electrolyte's propensity to absorb into the
sealing material, is generally greater at higher temperatures and
with more volatile organic solvents with lower boiling points.
[0013] Notably, as used throughout this specification, "absorb" and
"absorption" refer to a material's propensity to enter into and be
held by the sealing material (much as sponge would absorb water).
In contrast, "permeation" is the process by which the electrolyte
is absorbed into the material and then diffuses through that
material such that it is released on the opposite (i.e.,
non-sealed) side. Numerous references teach that absorption and/or
permeation are unwanted characteristics in a sealing material. For
example, U.S. Pat. No. 4,333,995 discusses the disadvantages of
electrolyte absorption because it causes deterioration of gasket
elasticity which, in turn, leads to electrolyte leakage. Similarly,
U.S. Pat. No. 5,462,820 recommends polypropylene as a non-aqueous
battery gasket material because it does not swell or dissolve in
organic solvents, with the swelling presumably an undesirable trait
caused by electrolyte absorption.
[0014] Polypropylene is commonly used a material for lithium cell
(e.g., Li/MnO.sub.2 and Li/FeS.sub.2) gaskets. Gaskets have been
made with other thermoplastic materials for the purpose of
improving the ability of the cell to withstand higher temperatures
than with polypropylene.
[0015] Sano et al., in U.S. Pat. No. 5,624,771, disclose the use of
polyphenylene sulfide ("PPS"), rather than PP, as a gasket material
for a lithium cell to improve resistance of the cell to high
temperatures. PPS was used to reduce gasket deformation due to cold
flow under the high compressive load conditions the gasket was
subjected to in the cell. However, a blown asphalt sealant was
required and glass filler and elastomer content both had to be
maintained at less than 10 wt. % of the material in order to
prevent leakage of electrolyte through the resulting gaskets.
[0016] In U.S. Pat. No. 5,656,392, PPS and
tetrafluoride-perfluoroalkyl vinylether copolymer (PFA) are
identified as suitable for making a gasket for a cell that is
useable at high temperatures. Here again, the addition of a glass
fiber filler to the resin (to extend the stability of the gasket
configuration) small amounts of PE and/or PP (to extend the
temperature range that can be tolerated by the cells on a cyclic
thermal shock test) are taught. But as above, gaskets containing
more than 10 weight percent glass fiber were undesirable because
cells made with such highly filled thermoplastic materials leaked
on a temperature cycling test. The addition of more than 10 weight
percent of PE and/or PP was also undesirable because of cell
leakage and a continuously usable temperature of less than
150.degree. C. for the gasket. The use of an asphalt sealant is
also preferred/required.
[0017] In U.S. Pat. No. 6,025,091 Kondo et al. disclose a cell with
a metal can sealed with a metal terminal cap and a gasket
comprising polybutylene terephthalate ("PBT"). The gasket material
can be PBT alone, PBT mixed with another polymer or PBT reinforced
with inorganic materials such as glass fibers, glass beads and
certain organic compounds. Kondo et al. disclose that the invention
solves the problems of creeping and cracking of the gasket material
when the cell is exposed to high temperature. The preferred cell
type was a secondary cell, either with an alkaline or nonaqueous
electrolyte (e.g., a lithium ion cell). A particularly preferred
electrolyte contained LiCF.sub.3SO.sub.3, LiClO.sub.4, LiBF.sub.4
and/or LiPF.sub.6 dissolved in a mixed solvent comprising propylene
carbonate or ethylene carbonate and 1,2-dimethoxyethane and/or
diethyl carbonate and 1,2-dimethoxyethane and/or diethyl
carbonate.
[0018] In the mid-1980's Union Carbide Corp. also manufactured a
1/3 N size Li/MnO.sub.2 cell (Type No. 2L76) with a gasket made
from PBT (GAFITE.RTM. from GAF Chemicals). These cells had a spiral
wound electrode design and contained an electrolyte with comprising
a mixture of lithium perchlorate and lithium
trifluoromethanesulfonate salts in a solvent containing 50 volume
percent each of propylene carbonate and 1,2-dimethoxyethane.
[0019] When a pressure relief vent for the cell is incorporated
into the seal member, the characteristics of the thermoplastic seal
member that affect the operation of the pressure relief vent must
also be considered when selecting a suitable thermoplastic resin.
Ethylene-tetrafluoroethylene copolymer ("ETFE") is commonly used
for vent bushings in consumer Li/FeS.sub.2 cells with pressure
relief vent designs similar to that in FIG. 1. When the internal
cell pressure reaches a predetermined level, the vent ball or the
vent ball and the vent bushing are forced outward to create an
opening in the cell. When tested on a thermal shock test, ETFE-only
bushings can sometimes undergo sufficient stress relaxation to
cause a partial or complete loss of compression between the vent
ball and cover or cause activation of the pressure relief vent
undesirably low internal cell pressures.
[0020] Another approach to minimizing weight loss during thermal
cycling is through the selection of electrolyte. Both U.S. Pat. No.
5,624,771 and U.S. Pat. No. 5,656,392 teach that high boiling point
solvents, such as y-butyrolactone (boiling point 202.degree. C.)
and propylene carbonate (boiling point 241.degree. C.), can be used
as electrolyte solvents to achieve the desired high temperature
cell performance, either alone or in combination with the gasket
materials mentioned above. Notably, these solvents can maintain
practical low temperature (-20.degree. C.) cell operation in a
Li/(CF).sub.n coin cell, and because of their low-volatility, they
are at a lesser risk of volatilizing and/or absorbing and
permeating through the sealing material (obviously, all solvents
contemplated herein will neither absorb nor permeate through the
metal container/housing used in nonaqueous-type batteries).
However, electrolytes containing a large amount of low boiling
point solvents such as these do not perform as well on high power
discharge in lithium batteries, which can be a significant
disadvantage for high power discharge applications.
SUMMARY OF INVENTION
[0021] In view of the advantages described in the Parent
Applications, a further examination of the interaction between the
selection of electrolyte, the selection of an engineered
thermoplastic sealing material and the rate of absorption and
permeation thereof was undertaken. It was discovered that impact
modified polyphenylene sulfide ("PPS") and polyphthalamide ("PPA")
disclosed in the Parent Applications absorb ether-based
electrolytes at high temperatures while at the same time remaining
resistant to permeation of that electrolyte at high temperatures.
In essence, these materials behave like a wine cork, in that they
absorb liquid and swell or expand to create a more effective seal,
but at the same time they do not allow the liquid to easily
permeate through the material. The beneficial effect of the
swelling is that it closes off any gaps between the seal and the
housing, while the low permeability of the materials safeguard
against electrolyte egress (and any ambient fluid ingress, e.g.,
atmospheric moisture) through the seal material itself.
Significantly, the material must display an acceptable level of
performance in both regards (i.e., high absorption and low
permeation) in order to function as an acceptable sealing material
according to this invention. To the extent these properties are not
inherent to common thermoplastics, selection of an appropriate
engineered thermoplastic is one of the key aspects of this
invention.
[0022] In order to better demonstrate this so-called "wine cork
effect", tests for the equilibrium absorption, nominally at
70.degree. C., and the vapor transmission rate, nominally at
75.degree. C., were developed for specific polymer/liquid
combinations. Acceptable parameters for each trait were then
defined, with relatively high absorption and low
transmission/permeation necessary to impart the desired effect and
characteristics for the sealing material. This work was
specifically performed on the engineered thermoplastic materials
described in the Parent Applications, although it is believed the
new characteristics defined herein are equally applicable to other
engineered thermoplastics that meet the criteria set forth
below.
[0023] As a result, an electrochemical cell, a sealing member for
such cell and/or a method for making such cells are all
contemplated. In each case, the sealing member is made from an
engineered thermoplastic resin having an absorption at 71.degree.
C. greater than or equal to 10 weight percent and a vapor
transmission rate of less than 500 (g.times.mil)/(100
in.sup.2.times.day), said absorption and vapor transmission rate
both relative to the electrolyte consisting essentially of one or
more organic solvents and at least one solute dissolved therein,
said one or more organic solvents including at least 50 volume
percent of one or more ethers.
[0024] The sealing member has a tubular, circular shape or is
otherwise hollow and cylindrical. Optimally, the sealing member is
injection molded and may be cold-crystallized to increase its
crystallinity. When incorporated in a cell, the sealing member is
disposed between the container/housing and the closure assembly,
which may be in the form of a top cover or an assembly including a
venting mechanism. Impact modifiers, thermal-stabilizing fillers
and judicious selection of resins such PPS (and more ideally PPS
with at least 10 weight percent of a thermal stabilizing filler)
and PPA (and more ideally PPA with between about 5 to 40 weight
percent impact modifiers) can all be selected to achieve the
desired levels of absorption and vapor transmission.
[0025] The electrolyte is composed from a majority, by volume, of
ethers although other organic solvents are contemplated. The
electrolyte must have at least one solute dissolved therein.
Preferably, the electrolyte includes 90 to 98 volume percent of
ethers, such as DIOX, DME, THF and the like.
[0026] The preferred cell and battery according to the invention
has a lithium-based anode and iron disulfide in the cathode. The
cell can be constructed in a spirally wound configuration.
BRIEF DESCRIPTION OF DRAWINGS
[0027] In the drawings:
[0028] FIG. 1 is a cross-sectional view of a cylindrical
electrochemical battery cell, with one thermoplastic seal member
between the can and cover and another thermoplastic seal member
between the cover and vent ball;
[0029] FIG. 2 is a cross-sectional view of a test membrane for a
vapor transmission rate test; and
[0030] FIG. 3 is a graph showing creep strain as a function of time
for three resin types at 85.degree. C., with an initial applied
stress of 10,000 kPa.
[0031] FIGS. 4A and 4B are exemplary plots of the absorption of a
DIOX-DME-LiI based electrolyte solution by PPA and PP materials,
respectively speaking, at various temperatures.
[0032] FIG. 5 is an exemplary plot of dynamic absorptions of PP and
PPA materials in a DIOX-DME solution normalized to their
equilibrium uptake, M., versus the square root of time.
[0033] FIGS. 6A and 6B show the first and second heat cycles of a
DSC thermogram for a PPS engineered thermoplastic having glass
fiber and impact modifier.
DETAILED DESCRIPTION OF INVENTION
[0034] Unless otherwise specified, the following definitions,
methods and criteria apply throughout this disclosure: [0035] All
disclosed characteristics and ranges are as determined at room
temperature (20-25.degree. C.), and boiling points are at one
atmosphere pressure; [0036] Aperture means an opening in a material
that extends from an area within one surface to an area within an
adjacent surface of the material; an open end of a container such
as a can or a tube is not an aperture; [0037] Coefficient of
thermal expansion is determined in the flow direction between
50.degree. C. and 90.degree. C. according to ASTM E831 and
expressed in cm/cm/degree Celsius; [0038] Creep strain rate is
determined by Dynamic Mechanical Analysis using a Tritec 2000 DMA
from Triton Technologies, Ltd., UK, at a test temperature of
85.degree. C.; resin is compression molded to form a 0.25 mm thick
film and cut to a width of 2.3 mm; the initial gauge length is 2.0
mm, a constant tensile force of 6 N is applied to give a constant
tensile stress of 10,000 kPa; [0039] Crimp release pressure means
the internal cell pressure at which the cell housing deforms
sufficiently to break the container/seal member/cell cover seal and
release pressure from the cell; [0040] DIOX refers to a
dioxolane-based solvent, typically 1,3-dioxolane unless otherwise
specified but also including substituted variants of 1,3-dioxolane
where appropriate; [0041] DME refers to a dimethoxyethane-based
solvent, typically 1,2-dimethoxyethane unless otherwise specified
but also including diglyme, triglyme and the like where
appropriate; [0042] Electrolyte means a solution containing one or
more solutes dissolved within one or more liquid, organic solvents;
[0043] Equilibrium Absorption is the percent weight gain that
occurs upon submersion into electrolyte, at a given temperature,
when the weight gain becomes constant versus time; [0044] Heat
deflection temperature is determined at 18.56 kg/cm.sup.2 (264
pounds per square inch) according to ASTM D648 and expressed in
degrees C.; [0045] Impact modifiermeans a polymer modifier added
primarily to alter the physical and mechanical properties of a
thermoplastic material and functioning by absorbing impact energy
and dissipating it in a nondestructive fashion; elastomers can be
used as impact modifiers, including but not limited to natural
rubbers, acrylics and styrenic elastomers, chlorinated
polyethylene, EVA copolymers, ethylene-propylene copolymers and
terpolymers, polybutadiene and polyisoprene; [0046] Mold shrinkage
is determined on a 1/8 inch (3.175 mm) thick specimen according to
ASTM D955 and expressed in (inches/inch).times.10.sup.-3
[(mm/mm).times.10.sup.-3]; [0047] Thermal-stabilizing filler is a
material which, when added to a base resin, will decrease the
resin's coefficient of thermal expansion by at least 20 percent and
increase the heat deflection temperature by at least 20.degree. C.;
[0048] Toughness is determined using a notched Izod impact test
according to ASTM D256; [0049] Vapor Transmission Rate ("VTR") is a
quantitative measure of a material's permeability at a specified
temperature and pressure, expressed as weight loss in grams per
unit time and per 100 in.sup.2 of surface area, multiplied by the
path length for permeation in mils ( 1/1000 of an inch). The units
are expressed as (g.times.mil)/(days.times.100 in.sup.2); [0050]
Venting means the opening of the pressure relief vent of a cell;
[0051] Vent pressure means the internal cell pressure at which the
pressure relief vent opens to release pressure from the cell;
[0052] The invention will be better understood with reference to
FIG. 1, which shows an FR6 type cylindrical battery cell having a
housing sealed by two thermoplastic seal members (a gasket and a
vent bushing). Cell 10 has a housing that includes a can 12 with a
closed bottom and an open top end that is closed with a cell cover
14 and a gasket 16. The can 12 has a bead or reduced diameter step
near the top end to support the gasket 16 and cover 14. The gasket
16 is compressed between the can 12 and the cover 14 to seal an
anode 18, a cathode 20 and electrolyte within the cell 10. The
anode 18, cathode 20 and a separator 26 are spirally wound together
into an electrode assembly. The cathode 20 has a metal current
collector 22, which extends from the top end of the electrode
assembly and is connected to the inner surface of the cover 14 with
a contact spring 24. The anode 18 is electrically connected to the
inner surface of the can 12 by a metal tab (not shown). An
insulating cone 46 is located around the peripheral portion of the
top of the electrode assembly to prevent the cathode current
collector 22 from making contact with the can 12, and contact
between the bottom edge of the cathode 20 and the bottom of the can
12 is prevented by the inward-folded extension of the separator 26
and an electrically insulating bottom disc 44 positioned in the
bottom of the can 12. Cell 10 has a separate positive terminal
cover 40, which is held in place by the inwardly crimped top edge
of the can 12 and the gasket 16. The can 12 serves as the negative
contact terminal. Disposed between the peripheral flange of the
terminal cover 40 and the cell cover 14 is a positive temperature
coefficient (PTC) device 42 that substantially limits the flow of
current under abusive electrical conditions. Cell 10 also includes
a pressure relief vent. The cell cover 14 has an aperture
comprising an inward projecting central vent well 28 with a vent
hole 30 in the bottom of the well 28. The aperture is sealed by a
vent ball 32 and a thin-walled thermoplastic bushing 34, which is
compressed between the vertical wall of the vent well 28 and the
periphery of the vent ball 32. When the cell internal pressure
exceeds a predetermined level, the vent ball 32, or both the ball
32 and bushing 34, are forced out of the aperture to release
pressurized gases from the cell 10.
[0053] Alternatively, a cell design which omits the ball vent in
favor of a foil vent, as contemplated in U.S. Patent Publication
No. 2005/0244706 (which is incorporated herein). Here, only a
gasket is used, and the vent well, vent hole, bushing and vent ball
are replaced by an impermeable foil or vapor barrier. All other
parts of the cell remain the same, as do the construction and
design principles discussed herein. The gasket of the present
invention has equal applicability to the foil vent and ball vent
designs. In the preferred embodiments of the invention and
regardless of whether the member is a gasket or a bushing, the
sealing member is tubular (i.e., a cylinder with an open, axial
center) with an essentially circular cross sectional shape for ease
of manufacture by injection molding.
[0054] The materials used for cell components depend in part on the
cell type, including the electrochemistry. For lithium and lithium
ion cells, there are many similarities in suitable materials.
Ultimately, to the extent that the invention is most ideally suited
to consumer-sized lithium batteries, particular emphasis should be
placed on the availability, cost and ease of manufacture/handling
these materials possess.
[0055] The gasket comprises an engineered thermoplastic material
that is resistant to cold flow at high temperatures (e.g.,
75.degree. C. and above), chemically stable (resistant to
degradation, e.g., by dissolution or chemical reaction) when
exposed to the internal environment of the cell and resistant to
the transmission of air gases into and electrolyte vapors from the
cell. Gaskets are preferably made from engineered thermoplastic
resins that absorb electrolyte materials but resist permeation of
the electrolyte therethrough.
[0056] The preferred polymeric sealing materials have diffusion
kinetics and solubility that are conversely related, as expressed
by the equation P =D X S, where permeability, P, is equal to the
product of the diffusion, D, and solubility, S, coefficients.
Notably, vapor transmission rate ("VTR") is another
experimentally-derived metric indicative of the permeability of any
given engineered thermoplastic.
[0057] Dynamic absorption experiments are also used to determine
the diffusion coefficient for a material. The technique is based on
classical diffusion theory of Fick's Law. Fick's Law states that
the amount of a given material (Q) passing normally through a plane
is proportional to the concentration gradient, according to the
following equation, where C is the concentration of the diffusing
material, x is the distance in the direction normal to the plane,
and D is the diffusion coefficient:
Q = - D .differential. C .differential. x ##EQU00001##
[0058] When D is assumed to be independent of concentration and
when the flux is assumed to be one dimensional (approximated as a
sheet), the mass absorbed at a given time (M.sub.t), normalized to
the equilibrium uptake (M.infin.), can be expressed as follows,
where l is the distance measured outwards from the center of the
sheet:
M t / M .infin. = 2 Dt .pi. l 2 ##EQU00002##
[0059] Note that this equation for uptake equilibrium is
semi-infinite and only applies at times near zero. Therefore, a
plot of M.sub.t/M.sub..infin. versus t.sup.0.5 should generate a
linear relationship at times before the equilibrium uptake has been
reached. From the slope of this line (m), one can calculate the
diffusion coefficient, which can be expressed as follows:
D = .pi. ( ml 2 ) 2 ##EQU00003##
[0060] To measure this dynamic absorption experimentally, gaskets
are submerged in an electrolyte, preferably containing an
electrolyte with an all-ether solvent blend of DIOX and DME and a
solute, such as LiI and/or lithium triflate. These gaskets remain
submerged for predetermined times and at predetermined temperatures
before being removed from the electrolyte filled vial, at which
time excess electrolyte droplets are removed from the surface of
the gaskets and then the gaskets are weighed. The weight gain is
representative of the absorbed amount of electrolyte, and the
equilibrium absorption is then reported as a percent weight gain
for that particular material.
[0061] By way of example rather than limitation, FIGS. 4A and 4B
show the dynamic absorption measurements of a 0.75 molal lithium
iodide salt dissolved in 65:35:0.2 solvent blend of DIOX:DME:
3,5-dimethylisoxazole ("DMI") into PPA and PP materials,
respectively. As shown in these figures, the amount of electrolyte
absorption in PPA is significantly higher than PP, as the
equilibrium absorption reaches 70% weight gain for PPA but only 10%
for PP.
[0062] Another difference illustrated in FIGS. 4A and 4B is the
rate at which PP and PPA are absorbed. FIG. 4A shows that at
60.degree. C., PPA requires over 100 days to reach its equilibrium
absorption. In contrast, FIG. 4B shows that PP requires only
minutes to reach its equilibrium absorption. In fact, PP absorbs
electrolyte so quickly that reliable measurements could not be
obtained at 71.degree. C.
[0063] The absorption rate differences are further analyzed with
reference to FIG. 5, which plots the dynamic absorption data
according to the M.sub.t/M.sub..infin. relationship described
above. In particular, the dynamic absorption data for PP at 45 and
60.degree. C. is plotted against the same data for PPA. When shown
in this manner, one can appreciate the large difference in
absorption rate between PP and PPA. To the extent this plot is
linear versus radical time, one can use this plot to calculate the
diffusion coefficient, D, for the electrolyte being studied because
the slope of the line, m, as contemplated in the foregoing
equations.
[0064] Permeability can also be quantitatively measured via a
standardized experimental regime and expressed as a Vapor
Transmission Rate ("VTR"). In practice, a series of identical vials
are filled with the electrolyte of interest and then fitted with
individual membranes made from the various engineered thermoplastic
materials of interest. The thickness of each membrane is
measured/known, and each membrane is then affixed to its vial via a
sealant and metal ring crimped around the edges in order to insure
a hermetic seal. The metal ring includes an aperture of known
diameter/surface area so as to leave a known surface area of the
membrane exposed to the electrolyte vapors on one side and a
controlled atmosphere on the opposite side. All of the vials are
then stored under identical conditions, typically 71.degree. C. and
ambient pressure, and monitored for relative weight loss over a set
period of time. Finally, based on these parameters, comparative VTR
rates can be developed for the electrolyte/engineered thermoplastic
combination.
[0065] Through judicious selection of engineered thermoplastic
materials and control of the conditions under which the material(s)
are molded or otherwise formed, even more control over the
resultant sealing member's properties (either a gasket or a
bushing) can be attained. By way of example rather than limitation,
the properties of semi-crystalline engineered thermoplastics, such
as PPS, depend on its level of crystallinity, which is dictated by
the conditions of the molding process, especially the mold
temperature. Often, high mold temperatures (e.g., >135.degree.
C. in the case of PPS) are needed to reach maximum crystallinity of
such thermoplastics, but such high temperatures make injection
molding of such parts impractical, if not impossible. In contrast,
if the mold temperature occurs below the glass transition
temperature (e.g., 85.degree. C. in the case of PPS), the injection
molding process for the part is effective but the plastic itself
will have an amorphous structure that leads to unwanted softening
of the material when the resultant part is exposed to high
temperatures (e.g., >100.degree. C. for a period of 5 hours)
because the molten polymer is quenched so rapidly that the polymer
chains do not have time to arrange into their preferred crystalline
lattice. Therefore, to arrive at a sealing member with the desired
levels crystallinity and processability, engineered thermoplastic
parts can be injection molded and then annealed in order to create
a semi-crystalline part. In doing so, the annealing temperature
should nonetheless be minimized to lessen any potential thermal
oxidative embrittlement of the bushing. This and similar processes
are also known as "cold-crystallization," and such cold
crystallized engineered thermoplastics and techniques can be used
to achieve the desired levels of performance in terms of absorption
and permeation, as described above.
[0066] The annealing and semi-crystallinity of an engineered
thermoplastic can be determined through any number of well known
analytical techniques. For example, a comparative study of two
separate heating cycles in a differential scanning calorimetry
("DSC") thermograms on PPS material, as seen in FIGS. 6A and 6B, is
possible. FIG. 6A shows the first heating cycle of a DSC thermogram
for a PPS part, including glass fiber and impact modifier, without
any annealing. Note the exotherm at 117.degree. C. and the
endotherm at 279.degree. C. that are present in FIG. 6A. The
exotherm at 117.degree. C. represents the annealing exotherm, where
the polymer chains arrange into a crystalline form and release the
latent heat of fusion associated with the liquid-solid phase
change. Once the PPS reaches 117.degree. C. the annealing process
occurs quickly in light of the heating rate for this particular DSC
thermogram (i.e., 10.degree. C/min). The endotherm at 279.degree.
C. is the melting of the PPS crystalline domain. FIG. 6B shows the
second heating cycle of the DSC thermogram of the same sample after
it has been allowed to cool. Here, no annealing exotherm is found
because the slow (10.degree. C./min) rate of cooling after the
first cycle allowed sufficient time for full crystallization to
take place. Therefore, no crystallinity is left for annealing and
the annealing exotherm is absent from FIG. 6B. X-Ray diffraction
can also be used to confirm the amorphous state of such engineered
thermoplastics.
[0067] In light of the foregoing, electrolyte absorption and VTR
rates have been developed for the preferred ether electrolyte and
materials which represent the currently known consumer
battery-grade engineered thermoplastics. The data for each is set
forth in Table 1 below.
TABLE-US-00001 TABLE 1 Absorption at 71.degree. C. and VTR at
75.degree. C. for 65:35:0.2 DIOX:DME:DMI electrolyte having 0.75 m
LiI Absorption Vapor Transmission Rate Material (wt. %) (g*mil/100
in.sup.2*days) PP 8 700 PBT 6 370 ETFE 5 300 ETFE + 25 wt. % glass
fibers 3 100 PPS + <10 wt. % impact modifier + 30 wt. % glass
fibers 7 80 PPS + .gtoreq.10 wt. % impact modifier 10 100 PPS +
.gtoreq.10 wt. % impact modifier + 35 wt. % glass fibers 11 360 PPS
+ .gtoreq.10 wt. % impact modifier 13 210 PPA + .gtoreq.5 wt. %
impact modifier 70 70
[0068] Based upon subsequent experiments with actual full cell
constructions, an engineered thermoplastic material must absorb at
least 10 wt.% of an ether-based electrolyte (i.e., a solvent blend
that is >50% ethers) while having a VTR of less than 500 in
order to obtain the desired "wine cork" effect. More preferably,
the absorption should be at least 12 wt. % and/or the VTR less than
300. Also, while a preferred combination has been identified (i.e.,
electrolyte containing at least one solute dissolved in a solvent
blend consisting of at least 50% ethers), it is believed that
similar experiments, conducted on the same materials but different
specific electrolyte blends, would nevertheless yield the same
benefits.
[0069] Without intending to be confined by any particular theory
regarding why high absorption and low VTR is desirable in
engineered thermoplastic sealing members according to this
invention, it is believed that the excess "free" volume inherent in
certain engineered thermoplastic material will provide more
molecular scale porosity for passage of electrolyte vapors through
the material, provided those vapors have the appropriate generic
electrolyte solvent characteristics (to the extent the solute does
not volatize, it is believed the solute choice has little to no
affect on absorption or permeation). Mobility of the polymer chains
may also play a role, insofar as the Activate State Theory applies.
That is, the free volume involved in a given diffusional jump of a
penetrant molecule must be supplied by a momentary fluctuation in
the polymer segmental position due to local thermal fluctuations.
Therefore, the ability of the polymer to provide segmental motion
(polymer backbone movement of two to four monomer units, e.g.
rotations around chain backbone bonds, torsional oscillations,
etc.) will dramatically impact its ability to allow permeation of
low molecular weight compounds.
[0070] The temperature at which absorption occurs is also
important. A flexible or rubbery material will behave differently
than one that is essentially in a rigid or glassy state. These two
different states can be differentiated by their glass transition
temperatures (Tg). The glassy state of a polymer occurs below its
Tg, which immobilizes segmental motion. In the rubbery state (i.e.,
above Tg), local molecular motions are significantly enhanced.
Therefore, more free volume and the rubbery state work together to
provide a much higher level of diffusion for electrolyte into the
material. The molding conditions for engineered thermoplastics can
be manipulated accordingly to optimize this phenomenon.
[0071] Furthermore, the role of the electrolyte composition and its
propensity to absorb but not permeate a specific material cannot be
overlooked. The absorption and permeation tendencies of an
electrolyte composition will vary depending on the engineered
thermoplastic, primarily based on the solvent's boiling point and
interactions with the other components in the electrolyte blend.
However, from a battery performance perspective, the electrolyte
composition must still consist of a solvent blend and solute that
enables good high rate performance while avoiding unwanted
reactions with the cell components. Given the number of engineered
thermoplastics known to be compatible with the compatible organic
solvents/solutes (as disclosed herein), and further considering the
various types and amounts engineered thermoplastic additives
(impact modifiers and thermal stabilizing fillers) along with
potential further alterations by post-molding processes such as
cold crystallization, there are potentially hundreds, if not
thousands, of permutations to consider when selecting
electrolyte/engineered thermoplastic pairings according to the
invention, and the synergistic effects of any particular
electrolyte and plastic combination must be verified
experimentally.
[0072] Two preferred resins to make gaskets possessing the
aforementioned properties are polyphenylene sulfide and
polyphthalamide and combinations thereof as base resins. The base
resin and can be blended with modifiers to provide the desired
properties of absorption and vapor transmission. Small amounts of
other polymers, reinforcing inorganic fillers and/or organic
compounds may also be added to the base resin of the gasket. A
preferred base resin displaying the desired characteristics of
absorption and vapor transmission is polyphthalamide. In one
embodiment, polyphthalamide can be used alone. In another
embodiment an impact modifier is added to the polyphthalamide. For
example, from about 5 to 40 weight percent of an impact modifier
can be added to the resin. An example of a suitable polyphthalamide
resin is RTP 4000 from RTP Company, Winona, Minn., USA, another
preferred material is available as AMODEL.RTM. ET 1001 L from
Solvay Advanced Polymers, LLC, Alpharetta, Ga., USA.
[0073] Another preferred base resin is polyphenylene sulfide to
which up to 40 weight percent of an impact modifier is added, along
with optional amounts of thermal stabilizing fillers, preferably in
the range of 20 to 40 weight percent when present. Appropriate
materials may be available as FORTRON.RTM. SKX 382 from Ticona-US,
Summit, N.J., USA, and/or as XTEL.RTM. XE 5300, XTEL.RTM. XE 3035
and XTEL.RTM. XE3200 from Chevron Phillips Chemical Company LLC,
The Woodlands, Tex., USA.
[0074] To maintain the desired compression of the gasket between
the container and cover, it is generally desirable to use gasket
materials with relatively low coefficients of thermal expansion to
minimize the effects of temperature. When the CTE is too high
excessive overstress (resulting in excessive cold flow) can occur
at high temperatures, and excessive contraction can occur at low
temperatures. It is also preferable for the CTE's of the container,
cell cover and gasket to be relatively close to one another so that
dimensions of their interface surfaces will change by about the
same amount in response to temperature changes, thereby minimizing
the effects on gasket compression over a broad temperature range.
The importance of the CTE's of the gasket, container and cell cover
materials can be reduced by using a cell cover design like that
shown in FIG. 1, where the cover has a generally vertical wall that
has some radial spring characteristics.
[0075] Heat deflection temperature (HDT) is a measure of a resin's
tendency to soften when subjected to heat. The higher the HDT, the
more rigid the material remains when heated. Preferably the resin
used to make the gasket has an HDT of at least 50, preferably at
least 75 and more preferably at least 100.degree. C. at a pressure
of 18.56 kg/cm.sup.2.
[0076] Creep strain rate is another measure of the material's
tendency to soften when subjected to heat. The lower the creep
strain rate, the more rigid the material remains when heated. When
the creep strain rate is too high the material can flow
excessively, resulting in a loss of compression of the gasket
between the container and cell cover. Ideally the average creep
strain rate of the resin is zero. An average creep strain rate of
no greater than 0.01 percent/min. between 100 and 200 minutes at
85.degree. C. with a constant applied force of 6 N is preferred.
More preferably the average creep strain rate is no more than about
0.007 percent/min., and most preferably it is no more than about
0.004 percent/min. Most preferably the average creep strain rate is
no more than 0.002 percent/min.
[0077] The gasket will also be resistant to the forces applied
during and after cell manufacturing, when the gasket is initially
compressed, to prevent damage, such as cracks through which
electrolyte can leak. Impact modifiers can be included in the resin
to increase the impact resistance of the material.
[0078] To further improve the seal at the interfaces between the
gasket and the cell container and the cell cover, the gasket can be
coated with a suitable sealant material. A polymeric material such
as EPDM can be used in embodiments with an organic electrolyte
solvent.
[0079] The vapor transmission rates of water and the electrolyte
solvent should also be low to minimize the entry of water into the
cell and loss of electrolyte from the cell. Water in the cell can
react with the active materials, and the internal resistance of the
cell can increase to an undesirable level if too much electrolyte
solvent is lost.
[0080] The vent bushing is a thermoplastic material that is
resistant to cold flow at high temperatures (e.g., 75.degree. C.
and above). This can be achieved by including more than 10 weight
percent, preferably at least 15 percent, thermal-stabilizing filler
in the thermoplastic material. Preferably no more than 40, more
preferably no more than 35, weight percent thermal-stabilizing
filler is added. The base resin of the thermoplastic material is
one that is compatible with the cell ingredients (anode, cathode
and electrolyte). The resin can be formulated to provide the
desired sealing, venting and processing characteristics. The resin
is modified by adding a thermal-stabilizing filler to provide a
vent bushing with the desired sealing and venting characteristics
at high temperatures.
[0081] The wall of the vent bushing between the vent ball and the
vent well in the cover can be designed to be thin (e.g., 0.006 to
0.015 inch as manufactured) and compressed by about 25 to 40
percent when the bushing and ball are inserted into the cover.
[0082] Suitable polymeric resins for the vent bushing include
ethylene-tetrafluoroethylene, polybutylene terephthlate,
polyphenylene sulfide, polyphthalamide,
ethylene-chlorotrifluoroethylene, chlorotrifluoroethylene,
perfluoroalkoxyalkane, fluorinated perfluoroethylene polypropylene
and polyetherether ketone. Ethylene-tetrafluoroethylene copolymer
(ETFE), polyphenylene sulfide (PPS), polybutylene terephthalate
(PBT) and polyphthalamide (PPA) are preferred, especially for use
in a cell with an electrolyte solvent containing a large percentage
of highly volatile (high vapor pressure, low boiling point) ether
compounds.
[0083] A suitable thermal-stabilizing filler is one which, when
added to the thermoplastic resin, decreases the CTE of the resin by
at least 20 percent and increases the HDT of the resin by at least
20.degree. C. Such fillers may be inorganic materials, such as
glass, clay, feldspar, graphite, mica, silica, talc and
vermiculite, or they may be organic materials such as carbons. It
may be advantageous for the filler particles to have a high average
aspect ratio, such as fibers, whiskers, flakes and platelets.
[0084] Glass can be used as a thermal-stabilizing filler. A
preferred type of glass is E-glass. The lengths of the glass fibers
will affect the material properties to some extent, particularly
the thermal and mechanical properties, more so than the thermal
expansion. The fiber length can vary depending on the base resin
use. For example, with PBT as the base resin, shorter fibers seem
to work well, while with other base resins, longer fibers may be
better. The glass fiber length can be controlled in any suitable
manner. In general, milling produces shorter fibers than
chopping.
[0085] In addition to thermal-stabilizing fillers, an impact
modifier may also be added to the vent bushing material to increase
its resiliency for manufacturing and/or operation. As with the
gasket materials described above impact modifiers can also be used
in some cases. Preferred materials displaying the desired
absorption and vapor transmission rate characteristics are sold as
XTEL.RTM. XE 5030 or XTEL.RTM. XE 3035 by Chevron Phillips Chemical
Company LLC, The Woodlands, Tex., USA.
[0086] The vent bushing can be manufactured using any suitable
process. Injection molding is an example. Because the length of the
glass fibers in the thermoplastic material can be reduced during
injection molding of the vent bushings, the possible effects on the
vent bushing characteristics should be considered before using
reground scrap from molding. The molding parameters used should be
those that provide a smooth surface on the molded bushings (e.g.,
Society of the Plastics Industry Standard Surface Finish D3 or
better). Molding parameters will vary with the type of material
being molded, and the suppliers of such resins typically provide
guidance on appropriate injection molding parameters. For the
preferred PPS resins, the preferred changes to standard molding
conditions are a mold temperature of about 38.degree. C., followed
by cold crystallization of the part at about 130.degree. C. for
approximately 60 minutes in the event a preferred form of the
semi-crystalline material is desired.
[0087] The mixture of base resin and filler used to make the vent
bushing preferably has a heat deflection temperature (HDT) of at
least 90.degree. C. (preferably at least 150.degree. C. and more
preferably at least 190.degree. C.) and a coefficient of thermal
expansion (CTE) between 50 and 90.degree. C. of no greater than
7.0.times.10.sup.-5 (preferably no greater than 5.0.times.10.sup.-5
and more preferably no greater than 3.0.times.10.sup.-5)
cm/cm/.degree. C.
[0088] To maintain the desired compression of the bushing between
the cover and vent ball, it is generally desirable to use materials
for the vent bushing that have low coefficients of thermal
expansion to minimize the effects of temperature. When the CTE is
greater than 5.0.times.10.sup.-5 cm/cm/.degree. C., excessive
overstress (resulting in excessive cold flow) can occur at high
temperatures and excessive contraction can occur at low
temperatures. Both of these undesirable conditions can result in
insufficient compression in the vent bushing to provide a good seal
against the cell cover and the vent ball, leading to loss of
electrolyte from the cell, water ingress into the cell and opening
of the pressure relief vent under normal storage and use
conditions.
[0089] It is also preferable for the CTE's of the cell cover, vent
ball and vent bushing to be close to one another so that dimensions
of the cover, ball and bushing interface surfaces will change by
about the same amount in response to temperature changes, thereby
minimizing the effects on bushing compression over a broad
temperature range.
[0090] The heat deflection temperature is a measure of the
material's tendency to soften when subjected to heat. The higher
the temperature, the more rigid the material remains when exposed
to heat. When the HDT is too low the material can flow excessively
at high temperatures, resulting in a loss of compression of the
vent bushing between the cell cover and the vent ball.
[0091] Notably, while the discussion above is bifurcated into ideal
materials for gaskets and vent bushings, these characteristics and
engineering principles used in selecting a particular engineered
thermoplastic are equally applicable to any and all sealing members
utilized in non-aqueous cell designs. As an example, if a closure
design is used that requires a higher HDT or a modified CTE, the
material selection (including addition/removal of glass fibers or
impact modifiers) for a gasket required in that design can be
altered according to the teachings identified above as pertinent to
the vent bushings. The same will hold true for virtually any
engineered thermoplastic part that may be utilized.
[0092] The cell container is often a metal can with an integral
closed bottom, though a metal tube that is initially open at both
ends may also be used instead of a can. The can is generally steel,
plated with nickel on at least the outside to protect the outside
of the can from corrosion. The type of plating can be varied to
provide varying degrees of corrosion resistance or to provide the
desired appearance. The type of steel will depend in part on the
manner in which the container is fonned. For drawn cans the steel
can be a diffusion annealed, low carbon, aluminum killed, SAE 1006
or equivalent steel, with a grain size of ASTM 9 to 11 and equiaxed
to slightly elongated grain shape. Other steels, such as stainless
steels, can be used to meet special needs. For example, when the
can is in electrical contact with the cathode, a stainless steel
may be used for improved resistance to corrosion by the cathode and
electrolyte.
[0093] The cell cover is typically metal. Nickel plated steel may
be used, but a stainless steel is often desirable, especially when
the cover is in electrical contact with the cathode. The complexity
of the cover shape will also be a factor in material selection. The
cell cover may have a simple shape, such as a thick, flat disk, or
it may have a more complex shape, such as the cover shown in FIG.
1. When the cover has a complex shape like that in FIG. 1, a type
304 soft annealed stainless steel with ASTM 8-9 grain size may be
used, to provide the desired corrosion resistance and ease of metal
forming. Formed covers may also be plated, with nickel for
example.
[0094] The terminal cover should have good resistance to corrosion
by water in the ambient environment, good electrical conductivity
and, when visible on consumer batteries, an attractive appearance.
Terminal covers are often made from nickel plated cold rolled steel
or steel that is nickel plated after the covers are formed. Where
terminals are located over pressure relief vents, the terminal
covers generally have one or more holes to facilitate cell
venting.
[0095] The vent ball can be made from any suitable material that is
stable in contact with the cell contents and provides the desired
cell sealing and venting characteristic. Glasses or metals, such as
stainless steel, can be used. The vent ball should be highly
spherical and have a smooth surface finish with no imperfections,
such as gouges, scratches or holes visible under 10 times
magnification. The desired sphericity and surface finish depend in
part on the ball diameter. For example, in one embodiment of a
Li/FeS.sub.2 cell, for balls about 0.090 inch (2.286 mm) in
diameter the preferred maximum sphericity is 0.0001 inch (0.00254
mm) and the preferred surface finish is 3 microinches (0.0762
.mu.m) RMS maximum. For balls about 0.063 inch (1.600 mm) in
diameter, the preferred maximum sphericity is 0.000025 inch
(0.000635 mm), and the preferred maximum surface finish is 2
microinches (0.0508 .mu.m) RMS.
[0096] In one embodiment of an FR6 Li/FeS.sub.2 cell according to
FIG. 1, the upstanding side wall of the gasket is 0.0205 inch
(0.521 mm) thick as manufactured. The diameters of the cell cover,
gasket and crimped can are such that the gasket is compressed by
about 30 percent of its original thickness to provide a good seal.
The gasket is preferably coated with a sealant such as ethylene
propylene diene terpolymer (EPDM), but other suitable sealant
materials can be used. The initial vent bushing wall thickness is
0.0115 inch (0.292 mm). It is compressed by about 30 to 35 percent
of its original thickness in the sealed cell. A sealant could be
used between the vent bushing and the cell cover or between the
vent bushing and the vent ball, or a sealant could be applied over
the cover, bushing and ball to improve the seal, but preferably no
sealant is used in order to avoid adversely affecting cell venting
or the vent pressure.
[0097] An anode for a lithium cell contains lithium metal, often in
the form of a sheet or foil. The composition of the lithium can
vary, though the purity is always high. The lithium can be alloyed
with other metals, such as aluminum, to provide the desired cell
electrical performance. When the anode is a solid piece of lithium,
a separate current collector within the anode is generally not
used, since the lithium metal has a very high electrical
conductivity. However, a separate current collector can be used to
provide electrical contact to more of the remaining lithium toward
the end of cell discharge. Copper is often used because of its
conductivity, but other conductive metals can be used as long as
they are stable inside the cell.
[0098] An anode for a lithium ion cell includes one or more
lithium-intercalable materials (capable of insertion and
deinsertion of lithium ions into their crystalline structure).
Examples of suitable materials include, but are not limited to
carbons (e.g., graphitic, mesophase and/or amorphous carbons),
transition metal oxides (e.g., those of nickel, cobalt and/or
manganese), transition metal sulfides (e.g., those of iron,
molybdenum, copper and titanium) and amorphous metal oxides (e.g.,
those containing silicon and/or tin). These materials are generally
particulate materials that are formed into the desired shape.
Conductive materials such as metal, graphite and carbon black
powders may be added to improve electrical conductivity. Binders
may be used to hold the particulate materials together, especially
in cells larger than button size. Small amounts of various
additives may also be used to enhance processing and cell
performance. The anode generally includes a current collector;
copper is a common choice. The current collector may be a thin
metal foil sheet, a metal screen, an expanded metal or one or more
wires. The anode mixture (active material and other ingredients)
can be combined with the current collector in any suitable manner.
Coating and embedding are examples.
[0099] Because lithium and lithium alloy metals are typically
highly conductive, a separate current collector within the anode is
often unnecessary in lithium and lithium alloy anodes. When an
anode current collector is required, as is often the case in
lithium ion cells, the current collector can be made from a copper
or copper alloy metal.
[0100] A cathode for a lithium cell contains one or more active
materials, usually in particulate form. Any suitable active cathode
material may be used. Examples include FeS.sub.2, MnO.sub.2,
CF.sub.x and (CF).sub.n.
[0101] A cathode for a lithium ion cell contains one or more
lithium-intercalated or lithium-intercalable active materials,
usually in particulate form. Any suitable active
lithium-intercalated or lithium-intercalable material may be used,
alone or in combination with others. Examples include metal oxides
(e.g., those of vanadium and tungsten), lithiated transition metal
oxides (e.g., those including nickel, cobalt and/or manganese),
lithiated metal sulfides (e.g., those of iron, molybdenum, copper
and titanium) and lithiated carbons.
[0102] In addition to the active material, a cathode for a lithium
or lithium ion cell often contains one or more conductive materials
such as metal, graphite and carbon black powders. A binder may be
used to hold the particulate materials together, especially for
cells larger than button size. Small amounts of various additives
may also be used to enhance processing and cell performance.
[0103] A cathode current collector may be required. Aluminum,
copper and related alloys are all commonly used materials.
Typically, the thickness of the collector is selected to provide
the minimal tensile strength required for coating and manufacturing
operations while still maximizing the amount of coating and
electrochemically active materials that can be loaded on to the
collector.
[0104] To provide good high power discharge performance it is
desirable that the separator have the characteristics (pores with a
smallest dimension of at least 0.005 .mu.m and a largest dimension
of no more than 5 .mu.m across, a porosity in the range of 30 to 70
percent, an area specific resistance of from 2 to 15 ohm-cm.sup.2
and a tortuosity less than 2.5) disclosed in U.S. Pat. No.
5,290,414, issued Mar. 1, 1994, and hereby incorporated by
reference.
[0105] Suitable separator materials should also be strong enough to
withstand cell manufacturing processes as well as pressure that may
be exerted on the separator during cell discharge without tears,
splits, holes or other gaps developing that could result in an
internal short circuit. To minimize the total separator volume in
the cell, the separator should be as thin as possible, preferably
less than 25 .mu.m thick, and more preferably no more than 22 .mu.m
thick, such as 20 .mu.m or 16 .mu.m. A high tensile stress is
desirable, preferably at least 800, more preferably at least 1000
kilograms of force per square centimeter (kgf/cm.sup.2). For an FR6
type cell the preferred tensile stress is at least 1500
kgf/cm.sup.2 in the machine direction and at least 1200
kgf/cm.sup.2 in the transverse direction, and for a FR03 type cell
the preferred tensile strengths in the machine and transverse
directions are 1300 and 1000 kgf/cm.sup.2, respectively. Preferably
the average dielectric breakdown voltage will be at least 2000
volts, more preferably at least 2200 volts and most preferably at
least 2400 volts. The preferred maximum effective pore size is from
0.08 .mu.m to 0.40 .mu.m, more preferably no greater than 0.20
.mu.m. Preferably the BET specific surface area will be no greater
than 40 m.sup.2/g, more preferably at least 15 m.sup.2/g and most
preferably at least 25 m.sup.2/g. Preferably the area specific
resistance is no greater than 4.3 ohm-cm.sup.2, more preferably no
greater than 4.0 ohm-cm.sup.2, and most preferably no greater than
3.5 ohm-cm.sup.2. These properties are described in greater detail
in U.S. Patent Publication No. 2005/0112462, which is hereby
incorporated by reference.
[0106] Separator membranes for use in lithium batteries are often
made of polypropylene, polyethylene or ultrahigh molecular weight
polyethylene, with polyethylene being preferred. The separator can
be a single layer of biaxially oriented microporous membrane, or
two or more layers can be laminated together to provide the desired
tensile strengths in orthogonal directions. A single layer is
preferred to minimize the cost. Suitable single layer biaxially
oriented polyethylene microporous separator is available from Tonen
Chemical Corp., available from EXXON Mobile Chemical Co.,
Macedonia, N.Y., USA. Setela F20DHI grade separator has a 20 .mu.m
nominal thickness, and Setela 16MMS grade has a 16 .mu.m nominal
thickness. Suitable separators with similar properties are also
available from Entek Membranes in Lebanon, Oreg., USA.
[0107] Electrolytes for lithium and lithium ion cells are
nonaqueous electrolytes. In other words, they contain water only in
very small quantities (e.g., no more than about 500 parts per
million by weight, depending on the electrolyte salt being used) as
a contaminant. Suitable nonaqueous electrolytes contain one or more
electrolyte salts dissolved in an organic solvent. Any suitable
salt may be used, depending on the anode and cathode active
materials and the desired cell performance. Examples include
lithium bromide, lithium perchlorate, lithium hexafluorophosphate,
potassium hexafluorophosphate, lithium hexafluoroarsenate, lithium
trifluoromethanesulfonate and lithium iodide. Suitable organic
solvents include one or more of the following: dimethyl carbonate,
diethyl carbonate, methylethyl carbonate, ethylene carbonate,
propylene carbonate, 1,2-butylene carbonate, 2,3-butylene
carbonate, methyl formate, .gamma.-butyrolactone, sulfolane,
acetonitrile, 3,5-dimethylisoxazole, n,n-dimethyl formamide and
ethers. The salt/solvent combination will provide sufficient
electrolytic and electrical conductivity to meet the cell discharge
requirements over the desired temperature range.
[0108] While the electrical conductivity is relatively high
compared to some other common solvents, ethers are often desirable
because of their generally low viscosity, good wetting capability,
good low temperature discharge performance and good high rate
discharge performance. This is particularly true in Li/FeS.sub.2
cells because the ethers are more stable than with MnO.sub.2
cathodes, so higher ether levels can be used. The
electrolyte-cathode compatibility, coupled with the absorption and
permeation characteristics of ethers with the engineered
thermoplastics contemplated herein, also causes the Li/FeS.sub.2
chemistry to derive the most benefit from this invention. Suitable
ethers include, but are not limited to acyclic ethers such as
1,2-dimethoxyethane, 1,2-diethoxyethane, di(methoxyethyl)ether,
triglyme, tetraglyme and diethyl ether; and cyclic ethers such as
1,3-dioxolane, tetrahydrofuran, 2-methyl tetrahydrofuran and
3-methyl-2-oxazolidinone. For the purposes of absorption and
permeation in engineered thermoplastics according to this
invention, the preferred ethers include DIOX and DME, provided in
volume percentages ranging from 10:90 to 90:10, and more preferably
between 25:75 and 65:35, DIOX:DME. Nominal amounts (e.g., <2
volume percent, and more preferably <0.2 volume percent) of an
optional third solvent, such as DMI (or any of the other organic
solvents identified herein), are also provided in a preferred
electrolyte. The preferred solutes are lithium iodide, lithium
triflate and combinations thereof, with between 0.75 and 1.25 molal
of lithium iodide being preferred. Other known lithium solutes are
also appropriate.
[0109] Specific anode, cathode and electrolyte compositions and
amounts can be adjusted to provide the desired cell manufacturing,
performance and storage characteristics. Preferably, the amounts of
electrochemically active materials will be balanced as taught in
U.S. Pat. Nos. 6,849,360 and 7,157,185, incorporated herein, so as
to maximize the performance of the cell.
[0110] The invention is particularly useful for cells having
electrolyte solvents with a very high level (e.g., a total of at
least 80 volume percent) of ethers with very low boiling points
(e.g., no greater than 90.degree. C. at sea level). The advantage
is even greater when the volume percent of ethers in the solvent is
at least 90 percent, and even more so with at least 98 volume
percent ethers in the solvent.
[0111] The cell can be closed and sealed using any suitable
process. Such processes may include, but are not limited to,
crimping, redrawing, colleting and combinations thereof. For
example, for the cell in FIG. 1, a bead is formed in the can after
the electrodes and insulator cone are inserted, and the gasket and
cover assembly (including the cell cover, contact spring and vent
bushing) are placed in the open end of the can. The cell is
supported at the bead while the gasket and cover assembly are
pushed downward against the bead. The diameter of the top of the
can above the bead is reduced with a segmented collet to hold the
gasket and cover assembly in place in the cell. After electrolyte
is dispensed into the cell through the apertures in the vent
bushing and cover, a vent ball is inserted into the bushing to seal
the aperture in the cell cover. A PTC device and a terminal cover
are placed onto the cell over the cell cover, and the top edge of
the can is bent inward with a crimping die to hold retain the
gasket, cover assembly, PTC device and terminal cover and complete
the sealing of the open end of the can by the gasket.
[0112] The above description is particularly relevant to FR6 type
cylindrical Li/FeS.sub.2 cells with nonaqueous electrolytes and to
pressure relief vents comprising an engineered thermoplastic
gasket, bushing and vent ball. However, the invention may also be
adapted to other types of cells, such as non-cylindrical (e.g.,
prismatic) cells, cells with other active materials, cells with
other electrolyte solvents (e.g., water) and cells with other
pressure relief vent designs, including foil vent-type designs. For
example, the aperture and pressure relief vent can be located in a
cell cover or the container. The aperture can be defined by a
uniform opening, such a straight cylindrical opening, or it may be
nonuniform, with a reduced diameter opening in one section, such as
the aperture in the cell cover in FIG. 1. The seal member sealing
the aperture in the housing can be an engineered thermoplastic
plug, or it can be a bushing into which a plug is inserted. The
plug can be of any suitable solid shape, including but not limited
to, a sphere, an ellipsoid, an ovoid and a cylinder. Cells
according to the invention preferably have spiral wound electrode
assemblies such as that shown in FIG. 1, another electrode
configuration, such as folded strips, stacked flat plates, bobbins
and the like.
[0113] The invention and its features and advantages are further
illustrated in the following examples.
EXAMPLE 1
[0114] Gaskets for FR6 type cells similar to the gasket shown in
FIG. 1 were injection molded from polypropylene homopolymer
(PROFAX.RTM. 6524), polybutylene terephthlate (VALOX.RTM. 310),
ethylene tetrafluoroethylene copolymer (TEFZEL.RTM. 2185),
polyphenylene sulfide with 15 weight percent impact modifier
(FORTRON.RTM. SKX 382) and polyphthalamide with 10-30 weight
percent impact modifier (AMODEL.RTM. ET 1001 L).
[0115] FR6 type cells were also made according to FIG. 1 and the
above description. The cells had the following features
(quantitative values are design averages): [0116] can
material--diffusion annealed, low carbon, aluminum killed, SAE 1006
steel; ASTM 9 to 11 grain size, equiaxed to slightly elongated
shape; nickel plated; about 0.010 inch (0.254 mm) thick, to provide
a 0.0095 inch (0.241 mm) thick can wall; [0117] can CTE about
1.25.times.10.sup.-5 cm/cm/.degree. C.; [0118] cell cover
material--0.013 inch (0.330 mm) thick type 304 soft annealed
stainless steel; ASTM 8-9 grain size; post-plated with nickel;
[0119] cell cover CTE--1.72.times.10.sup.-5 cm/cm/.degree. C.;
[0120] cell cover vent well inside diameter--0.105 inch (2.67 mm);
[0121] gasket material--varies per example but using the materials
identified above; [0122] gasket wall thickness--0.0205 inch (0.521
mm); [0123] gasket sealant coating material--EPDM with 56% ethylene
and 9% diene; [0124] gasket compression--about 32 percent of the
initial gasket wall thickness; [0125] vent ball material (if
present)--440C stainless steel (per ASTM A276); [0126] vent ball
surface finish (if present)--3 microinches (0.0762 .mu.m) RMS max;
[0127] vent ball sphericity (if present)--0.0001 inch (0.00254 mm)
max; [0128] vent ball CTE (if present)--1.02.times.10.sup.-5
percent per degree C.; [0129] vent ball diameter (if
present)--0.090 inch (2.29 mm); [0130] electrolyte
composition--9.14 w t % LiI solute in a solvent blend of 63.05 wt %
1,3-dioxolane, 27.63 wt % 1,2-dimethoxyethane and 0.18 wt %
3,5-dimethylisoxazole; [0131] electrolyte quantity--1.6 g; [0132]
cell internal void volume--10 percent; [0133] vent bushing material
(if present)--ETFE with no filler; [0134] vent bushing wall
thickness (if present)--0.0115 inch (0.292 mm); and [0135] vent
bushing compression (if present)--about 32 percent of the bushing
wall thickness.
[0136] Samples of both undischarged and fully discharged FR6 cells
having the PP gasket were tested on a thermal shock test. The fully
discharge cells were prepared by continuously discharging at 200 mA
to a discharge voltage of 0.5 volt. In the thermal shock test,
cells were stored for 6 hours at 75.degree. C., followed by storage
for 6 hours at -40.degree. C.; this was repeated 10 times, with no
more than 30 minutes between the test temperature extremes. After
temperature cycling the cells were stored for 24 hours at room
temperature. Each cell tested was weighed before and after testing
to determine the total weight loss, including weight loss around
and through the vent bushing as well as weight loss around and
through the gasket. Each cell was also examined to determine if the
cell had vented during the test. Sixteen percent of the
undischarged cells and 58 percent of the fully discharged cells
vented during the test. Of the cells that did not vent, the average
weight loss during the test attributed to the vent bushing was
about 23.7 mg for the undischarged cells and about 1.7 mg for the
fully discharged cells.
EXAMPLE 2
[0137] FR6 type cells were made according to FIG. 1 and the above
description, except that the cell covers (14) did not have vent
holes (30), so vent bushings and vent balls were not used. These
cells were then tested to determine the amount of weight loss
during storage at reduced pressure followed by thermal shock. Some
cells with each gasket type were tested in an upright orientation
(as shown in FIG. 1), and others were inverted.
[0138] Cells were first stored for about 6 hours at room
temperature and a pressure of about 11.6 kPa; weight loss was not
significant. In the thermal shock portion of the test, cells were
stored for 6 hours at 75.degree. C., followed by storage for 6
hours at -40.degree. C.; this was repeated 10 times, with no more
than 30 minutes between the test temperature extremes; after
temperature cycling the cells were stored for 24 hours at room
temperature. For three of the gasket types (PP, PPS and PPA), three
lots of cells, made at different times, were tested; for the other
two gasket types (PBT and ETFE), only one lot of each was made and
tested. The change in mass was determined for each cell. The
average weight loss results are summarized in Table 2 for each cell
lot. The average weight loss was better for cells with PBT, ETFE,
PPS and PPA gaskets than for cells with PP gaskets, with cells made
with PBT and PPA gaskets being the best overall. There was no
substantial difference in average weight loss due to cell
orientation during the test.
TABLE-US-00002 TABLE 2 Gasket Type PP PBT ETFE PPS PPA Average
0.0081 -- -- 0.0099 0.0000 Weight 0.0079 -- 0.0032 0.0104 0.0012
Loss (g) 0.0090 0.0010 -- 0.0059 0.0010
EXAMPLE 3
[0139] Cells from Example 2 were stored for 3 weeks at 85.degree.
C. and then weighed to determine the amount of additional weight
loss after storage at 85.degree. C. Some cells were stored in an
upright orientation (as shown in FIG. 1), and others were inverted.
The average weight losses are shown for each lot of cells with PP,
ETFE and PPA gaskets in Table 3. The average additional weight loss
was significantly less for cells with ETFE, PPS and PPA gaskets
than for cells with PP gaskets. Some of the cells with each gasket
type were autopsied and examined. The PBT gaskets had cracks on the
surfaces that had been exposed to the electrolyte in the cells,
indicating degradation of the material.
TABLE-US-00003 TABLE 3 Gasket Type PP ETFE PPS PPA Average 0.0700
-- 0.0328 0.0018 Weight 0.0748 0.0093 0.0368 0.0005 Loss (g)
EXAMPLE 4
[0140] Gaskets made with different grades of PBT were submerged in
various solutions at 70.degree. C. and examined periodically to
determine the source of the cracking observed in Example 3. The
results are summarized in Table 4; "fail" indicates cracking after
7 days or less, and "pass" indicates no cracking after 60 days.
Gaskets with all PBT grades tested failed when tested in the
electrolyte used in the cells in Example 1. Gaskets did not fail
when tested in solutions that did not contain both lithium and
iodide ions in a nonaqueous solvent.
TABLE-US-00004 TABLE 4 Solute Type and Solvent Concentration
Components and Gasket (moles/liter solvent) Volume Ratio Material
Results Li I DIOX:DME:DMI VALOX .RTM. fail 0.75 65:35:0.2 310 Li I
DIOX:DME:DMI CELANEX .RTM. fail 0.75 65:35:0.2 1600A Li I
DIOX:DME:DMI VALOX .RTM. fail 0.75 65:35:0.2 HR326 Li I DIOX:DME
CELANEX .RTM. fail 0.75 65:35 1600A none DIOX:DME:DMI VALOX .RTM.
pass 65:35:0.2 310 none DIOX:DME:DMI CELANEX .RTM. pass 65:35:0.2
1600A LiCF.sub.3SO.sub.3 DIOX:DME:DMI CELANEX .RTM. pass 1.0
65:35:0.2 1600A KI DIOX:DME:DMI CELANEX .RTM. pass saturated
65:35:0.2 1600A NaI DIOX:DME:DMI CELANEX .RTM. pass 0.75 65:35:0.2
1600A LiI distilled water CELANEX .RTM. pass 0.75 1600A
EXAMPLE 5
[0141] Tables 5 shows properties of materials used in Example 1.
Table 5 shows typical CTE, HDT and toughness characteristics for
the grades of PP, PBT, PPS and PPA shown, where available.
TABLE-US-00005 TABLE 5 CTE HDT at 18.56 Material Material
(cm/cm/.degree. C.) .times. kg/cm.sup.2 Toughness Type Grade
10.sup.-5 (.degree. C.) (Joules/m) PP PRO-FAX .RTM. -- -- 37 6524
PBT VALOX .RTM. 8.1 54 54 310 ETFE TEFZEL .RTM. 12.6 74 (no break)
HT2185 PPS FORTRON .RTM. 8.4 82 507 SKX 382 PPA AMODEL .RTM. 7.5
120 960 ET 1001 L
EXAMPLE 6
[0142] Test samples made from the PP, PPS and PPA resins used in
Example 1 were tested at 85.degree. C. to determine the tensile
creep strain rate of those resins. The testing was done using a
Tritec 2000 DMA (Triton Technologies, Ltd., UK). The test samples
were made by compression molding the virgin resin to form a 0.25 mm
thick film and then cutting individual samples 2.3 mm wide. An
initial gauge length of 2.0 mm was used, and a constant tensile
force of 6 N (tensile stress of 10,000 kPa) was applied. The
results are plotted in the graph in FIG. 3, which shows the percent
creep stain as a function of time. After application of the initial
tensile stress, a flat line indicates a creep strain rate of zero
(i.e., no material flow). The average creep strain rate for a given
time interval (e.g., between 100 and 200 minutes) is calculated by
subtracting the creep strain at 100 min. from the creep strain at
200 min. and dividing the difference by 100 min. The creep strain
values at 100 and 200 minutes and the average creep strain rate are
shown in Table 7. The average creep strain rates of the PPS and PPA
materials were substantially better than that of PP, with PPA being
the best.
TABLE-US-00006 TABLE 6 Creep Creep Ave. Creep Material Material
Strain at Strain at Strain Rate Type Grade 100 min. (%) 200 min.
(%) (%/min.) .times. 10.sup.-3 PP PRO-FAX .RTM. 41.7 43.2 15 6524
PPS FORTRON .RTM. 2.9 3.2 3 SKX 382 PPA AMODEL .RTM. 7.4 7.4 0 ET
1001 L
EXAMPLE 7
[0143] Other thermoplastic materials were considered as possible
substitutes for ETFE to make vent bushings for the FR6 cells in
Example 1.
[0144] Table 7 shows CTE, HDT and mold shrinkage characteristics
provided by suppliers of a number of thermoplastic materials. For
the materials in Table 7, the CTE and HDT values for the glass
filled resins are generally more suitable than those for unfilled
resins for use in making seal members. The electrolyte transmission
rates through unfilled ETFE and PBT are similar, and adding 15-25
weight percent glass filler to these resins can substantially
reduce the electrolyte vapor transmission rate at high storage
temperatures. Other material properties can also affect the vapor
transmission rate, as evident in comparing the results for
VALOX.RTM. DR51 and LNP WF1004M.
TABLE-US-00007 TABLE 7 Mold Shrinkage Glass Filler Thermoplastic
CTE HDT at (in./in. .times. 10.sup.-3) Base length Material (%
.times. 10.sup.-5 264 psi (flow (transverse Resin (wt. %) (.mu.m)
Grade per .degree. C.) (.degree. C.) direction) direction) EFTE 0
-- TEFZEL .RTM. 9.3 74 12 28 HT2185 EFTE 16 73.sup..sctn. LNP 107
FP1004M EFTE 25 290.sup..dagger. TEFZEL .RTM. 1.7 210 10 18 HT2004
PBT 0 -- VALOX .RTM. 14 54 19 20 310 PBT 0 -- VALOX .RTM. 7.9 121
12 14 365 PBT 15 548.sup..dagger. VALOX .RTM. 2.2 191 6 11 DR51 PPS
40 RYTON .RTM. 1.5 260 R-4-230NA .sup..sctn.milled fibers
.sup..dagger.chopped fibers
[0145] Table 8 shows the vapor transmission rates of water and the
desired organic electrolyte (9.14 wt % LiI solute in a solvent
blend of 63.05 wt % 1,3-dioxolane, 27.63 wt % 1,2-dimethoxyethane
and 0.18 wt % 3,5-dimethylisoxazole) through a number of
thermoplastic materials at different temperatures. The vapor
transmission rates were determined using the following method,
adapted from ASTM E96-80 (Standard Test Method for Water Vapor
Transmission of Materials): [0146] 1. mold a thermoplastic test
membrane according to the membrane 100 in FIG. 2, where the height,
outside diameter and inside diameter at wall 101 are suitable for
providing a seal between the bottle and seal in steps 2 and 5
below, the membrane thickness between wall 101 and hub 103 is 0.020
inch (0.508 mm) and the test surface area (step 9 is the surface
area of the membrane between wall 101 and hub 103 [for the serum
bottle and seal described in the examples in steps 2 and 5 below, a
suitable test membrane has a wall outside diameter of 0.770 inch
(19.56 mm), a wall inside diameter of 0.564 inch (14.33 mm), a hub
diameter of 0.127 inch (3.23 mm), a hub length of 0.075 inch (1.91
mm) below the lower test surface and a test surface area of 0.237
in..sup.2 (1.529 cm.sup.2)]; [0147] 2. put about 8 ml of liquid
(water or electrolyte) into a 15 ml bottle (e.g., Wheaton Serum
Bottle, 25 mm diameter.times.54 mm high, Cat. No. 06-406D); [0148]
3. apply sealant (e.g., G.E. Silicone II for testing at up to
60.degree. C.; vacuum grease for testing at up to 75.degree. C.) to
the lip of the bottle; [0149] 4. place the test membrane over the
top of the bottle; [0150] 5. place a seal with a 5/8 inch (15.88
mm) diameter center hole (e.g., Wheaton Aluminum Seal Cat. No.
060405-15) over the test membrane and crimp the seal tightly onto
the bottle; [0151] 6. weigh the sealed bottle; [0152] 7. store the
bottle at the desired test temperature and reweigh (at room
temperature) at regular intervals (e.g., monthly for 6 months at
room temperature; daily for 2 weeks at 60.degree. C. and 75.degree.
C.); [0153] 8. determine the total weight loss (use a negative
value to indicate a weight gain) over the test period; [0154] 9.
calculate the vapor transmission rate in g0.001 in./day100
in..sup.2 (g0.0254 mm/day0.65416 cm.sup.2) using the average total
weight loss from step 8 (excluding any individual samples that are
extremely high due to loss of seal) and the formula [(ave. weight
loss in grams/day)(membrane thickness in inches/1000)(100)/(test
surface area of membrane)], where day=24 hours; and [0155] 10.
perform steps 2-9 on an empty bottle, and correct the calculated
vapor transmission rate for the test liquid by subtracting the
result from step 9 for the empty bottle from the result from step 9
for the bottle containing the test liquid.
TABLE-US-00008 [0155] TABLE 8 Vapor Transmission Rate Glass Filler
(g 0.0254 mm/day 0.65416 cm.sup.2) avg. Thermoplastic Water
Electrolyte Base length Material room room Resin (wt. %) (.mu.m)
Grade temp. 60.degree. C. 75.degree. C. temp. 60.degree. C.
75.degree. C. PP 0 -- PRO-FAX .RTM. 0.2 7 18 8 437 1394 6524 EFTE 0
-- TEFZEL .RTM. 0.6 7 20 6 140 314 HT2185 EFTE 25 290 TEFZEL .RTM.
0.7 4 13 5 48 173 HT2004 PBT 0 -- VALOX .RTM. 1 11 35 4 129 372 310
PBT 15 548 VALOX .RTM. 1 11 27 7 52 155 DR51 PBT 16 LNP 0.7 10 28 5
115 312 WF1004M
[0156] The PP material had the lowest water vapor transmission rate
at room temperature, but its electrolyte vapor transmission rate at
60.degree. C. and 75.degree. C. was much higher than any of the
others. The electrolyte vapor transmission rates for the PPS and
PPA materials were substantially lower than those of PBT and
ETFE.
EXAMPLE 8
[0157] Vent bushings were injection molded from TEFZEL.RTM. 2185,
TEFZEL.RTM. HT2004, VALOX.RTM. DR51, RYTON.RTM. PRO9-60 and
RYTON.RTM. R-4-230NA. The TEFZEL.RTM. resins were obtained from E.
I. duPont de Nemours & Co. (Wilmington, Del., USA), the
VALOX.RTM. materials were obtained from G.E. Plastics, General
Electric Company (Pittsfield, Mass., USA), the RYTON.RTM. materials
were obtained from Chevron Phillips Chemical Company, LP (Houston,
Tex., USA) and the other materials were custom blended by LNP
Engineering Plastics (Exton, Pa., USA). The filled thermoplastic
materials were filled with glass fibers. The TEFZEL.RTM. HT2185
material contained 75 weight percent regrind. The other materials
were 100 percent virgin, with no regrind. The bushings made from
RYTON.RTM. PRO9-60 and R-4-230NA were not acceptable for use in
cells. The RYTON.RTM. PRO9-60 would not properly fill the mold
during molding and the bushings molded from the RYTON.RTM.
R-4-230NA had weak weld lines, indicating that either modification
of the resins to improve molding or changes in molding parameters
would be necessary in order to produce suitable bushings.
EXAMPLE 9
[0158] Vent bushings from Example 4 made with TEFZEL.RTM. 2185,
TEFZEL.RTM. HT2004 and VALOX.RTM. DR51 were used to make FR6 cells
that were otherwise like the FR6 cells in Example 1.
[0159] Undischarged samples of the FR6 cells were tested on the
thermal shock test described in Example 1. The at the cell cover
apertures (i.e., through and around the vent bushings are
summarized in Table 9.
[0160] Those lots with vent bushings made from glass-filled ETFE
and PBT had lower average weight losses than lots with bushings
made with the unfilled resins. Lot Dl performed the best, with only
0.5 mg of weight loss during the thermal cycling test.
TABLE-US-00009 TABLE 9 Bushing Ave. Weight Material Bushing Loss
Lot Type Material Grade (mg) A1 Unfilled TEFZEL .RTM. 38.5 ETFE
2185 A2 Unfilled TEFZEL .RTM. 15.6 ETFE 2185 B1 ETFE with TEFZEL
.RTM. 5.5 25% Glass HT2004 B2 ETFE with TEFZEL .RTM. 4.9 25% Glass
HT2004 C2 Unfilled VALOX .RTM. 1127.6 PBT 365 D1 PBT with VALOX
.RTM. 0.5 15% Glass DR51 D2 PBT with VALOX .RTM. 7.2 15% Glass
DR51
[0161] Samples of the FR6 cells were also tested to determine the
average vent pressures--at room temperature, at 75.degree. C., and
at room temperature following the thermal shock test. The results
are summarized in Table 10.
TABLE-US-00010 TABLE 10 Vent Pressure [psi (kg/cm.sup.2)] Bushing
Bushing At At Room Temp. Material Material Room At after Lot Type
Grade Temp. 75.degree. C. Thermal Shock A Unfilled TEFZEL .RTM. 846
596 199 ETFE 2185 (59.5) (41.9) (14.0) B ETFE with TEFZEL .RTM. 955
775 315 25% Glass HT2004 (67.1) (54.5) (22.1) C Unfilled VALOX
.RTM. 1175 757 462 PBT 365 (82.7) (53.2) (32.5) D PBT with VALOX
.RTM. 1170 926 1299 15% Glass DR51 (82.3) (65.1) (91.3)
[0162] To prevent cell venting under normal operating conditions,
FR6 cells made as described in Examples should have minimum vent
pressures above 100 psi (7.0 kg/cm.sup.2) at room temperature and
above 135 psi (9.5 kg/cm.sup.2) at 75.degree. C. With both ETFE and
PBT as the base resin, the addition of glass filler did not result
in a substantially lower vent pressure at room temperature, and it
increased the average vent pressure at 75.degree. C. and at room
temperature following the thermal shock test to provide greater
assurance that cells would not vent during storage and normal
use.
[0163] Although the present invention has been described in
considerable detail with reference to certain preferred versions
thereof, other versions are possible. Therefore, the spirit and
scope of the appended claims should not be limited to the
description of the preferred versions contained herein.
[0164] Each feature disclosed in this specification (including the
accompanying claims, abstract, and drawings) is one example only of
a generic series of equivalent or similar features, and each of the
features disclosed may be replaced by alternative features serving
the same, equivalent or similar purpose, unless expressly stated
otherwise.
* * * * *