U.S. patent application number 12/460423 was filed with the patent office on 2010-06-03 for method and apparatus for maintaining cell wall integrity during thermal runaway using an outer layer of intumescent material.
This patent application is currently assigned to Tesla Motors, Inc.. Invention is credited to Weston Arthur Hermann, Vineet Haresh Mehta, Alex Prilutsky.
Application Number | 20100136385 12/460423 |
Document ID | / |
Family ID | 42223102 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100136385 |
Kind Code |
A1 |
Hermann; Weston Arthur ; et
al. |
June 3, 2010 |
Method and apparatus for maintaining cell wall integrity during
thermal runaway using an outer layer of intumescent material
Abstract
A method and apparatus is provided in which a layer of an
intumescent material surrounds the casing of a battery, the layer
helping to prevent the formation of perforations in the battery
casing during a thermal runaway event and, if a perforation is
formed, inhibiting the flow of hot, pressurized gas from within the
battery. A sleeve, surrounding the cell, may be used to contain the
intumescent material during the thermal event.
Inventors: |
Hermann; Weston Arthur;
(Palo Alto, CA) ; Mehta; Vineet Haresh; (Mountain
View, CA) ; Prilutsky; Alex; (San Mateo, CA) |
Correspondence
Address: |
Telsa Motors, Inc.
c/o Patent Law Office of David G. Beck, P.O. Box 1146
Mill Valley
CA
94942
US
|
Assignee: |
Tesla Motors, Inc.
San Carlos
CA
|
Family ID: |
42223102 |
Appl. No.: |
12/460423 |
Filed: |
July 17, 2009 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12504712 |
Jul 17, 2009 |
|
|
|
12460423 |
|
|
|
|
Current U.S.
Class: |
429/50 ;
429/185 |
Current CPC
Class: |
H01M 2200/00 20130101;
H01M 10/6556 20150401; H01M 50/1245 20210101; Y02E 60/10 20130101;
H01M 10/613 20150401; H01M 50/124 20210101; H01M 10/625 20150401;
H01M 10/643 20150401; H01M 50/116 20210101; H01M 10/653
20150401 |
Class at
Publication: |
429/50 ;
429/185 |
International
Class: |
H01M 2/08 20060101
H01M002/08; H01M 10/50 20060101 H01M010/50 |
Claims
1. A battery assembly, comprising: a battery, said battery
comprising: a cell case having an outer surface, a first end and a
second end, wherein said first end is closed by a cell case bottom,
and wherein said second end is comprised of a central open portion;
an electrode assembly contained within said cell case, wherein a
first electrode of said electrode assembly is electrically
connected to said cell case; and a cap assembly mounted to said
cell case, said cap assembly closing said central open portion of
said second end, wherein said cap assembly further comprises a
battery terminal electrically isolated from said cell case and
electrically connected to a second electrode of said electrode
assembly; and a layer of an intumescent material surrounding said
outer surface of said cell case.
2. The battery assembly of claim 1, wherein said battery has an
18650 form-factor.
3. The battery assembly of claim 1, wherein said intumescent
material is selected from the group of intumescent materials
consisting of graphite-based intumescent materials, thermoplastic
elastomers, ceramic-based intumescent materials,
vermiculite/mineral fiber based intumescent materials, and ammonium
polyphosphate based intumescent materials.
4. The battery assembly of claim 1, wherein said intumescent
material has a start expansion temperature in the range of
100.degree. C. to 300.degree. C.
5. The battery assembly of claim 1, wherein said intumescent
material has a start expansion temperature in the range of
200.degree. C. to 300.degree. C.
6. The battery assembly of claim 1, wherein said intumescent
material is biologically inert.
7. The battery assembly of claim 1, further comprising a sleeve,
wherein said sleeve surrounds said intumescent material, and
wherein said sleeve is spaced apart from said intumescent
material.
8. The battery assembly of claim 7, wherein said sleeve is
fabricated from a material with a yield strength of at least 75
MPa.
9. The battery assembly of claim 7, wherein said sleeve is
fabricated from a material with a yield strength of at least 150
MPa.
10. The battery assembly of claim 7, wherein said sleeve is
fabricated from a material with a yield strength of at least 250
MPa.
11. The battery assembly of claim 7, further comprising at least
one spacer ring, wherein said at least one spacer ring separates an
interior surface of said sleeve from an exterior surface of said
intumescent material prior to the thermal runaway event.
12. The battery assembly of claim 1, further comprising a second
layer of said intumescent material, said second layer covering said
cell case bottom, wherein said second layer inhibits the escape of
hot, pressurized gas from within said battery through a perforation
formed in said cell case bottom during said thermal runaway
event.
13. The battery assembly of claim 12, wherein said second layer of
said intumescent material is integral with said layer of said
intumescent material surrounding said outer surface of said cell
case.
14. A method of preventing the formation of a perforation in an
outer surface of a battery case during a thermal runaway event, the
method comprising the step of covering said outer surface of said
battery case in a layer of an intumescent material, wherein said
covering step is performed prior to said thermal runaway event, and
wherein said intumescent material expands during said thermal
runaway event.
15. The method of claim 14, further comprising the step of
surrounding said layer of said intumescent material with a sleeve,
said surrounding step further comprising the step of separating
said layer of said intumescent from an interior surface of said
sleeve by a preset distance.
16. The method of claim 14, wherein said covering step further
comprises the step of coating said outer surface of said battery
case with said intumescent material to form said layer.
17. The method of claim 14, wherein said covering step further
comprises the steps of forming a strip of said intumescent material
and wrapping said strip of said intumescent material around said
outer surface of said battery case.
18. The method of claim 17, further comprising the step of bonding
said strip of said intumescent material in place.
19. The method of claim 14, wherein said covering step further
comprises the step of covering an exterior bottom surface of said
battery case in said layer of said intumescent material.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/504,712, filed Jul. 17, 2009, the
disclosure of which is incorporated herein by reference for any and
all purposes.
FIELD OF THE INVENTION
[0002] The present invention relates generally to batteries, and
more particularly, to a means for maintaining cell wall integrity
during thermal runaway.
BACKGROUND OF THE INVENTION
[0003] Batteries can be broadly classified into primary and
secondary batteries. Primary batteries, also referred to as
disposable batteries, are intended to be used until depleted, after
which they are simply replaced with one or more new batteries.
Secondary batteries, more commonly referred to as rechargeable
batteries, are capable of being repeatedly recharged and reused,
therefore offering economic, environmental and ease-of-use benefits
compared to a disposable battery.
[0004] Although rechargeable batteries offer a number of advantages
over disposable batteries, this type of battery is not without its
drawbacks. In general, most of the disadvantages associated with
rechargeable batteries are due to the battery chemistries employed,
as these chemistries tend to be less stable than those used in
primary cells. Due to these relatively unstable chemistries,
secondary cells often require special handling during fabrication.
Additionally, secondary cells such as lithium-ion cells tend to be
more prone to thermal runaway than primary cells, thermal runaway
occurring when the internal reaction rate increases to the point
that more heat is being generated than can be withdrawn, leading to
a further increase in both reaction rate and heat generation.
Eventually the amount of generated heat is great enough to lead to
the combustion of the battery as well as materials in proximity to
the battery. Thermal runaway may be initiated by a short circuit
within the cell, improper cell use, physical abuse, manufacturing
defects, or exposure of the cell to extreme external
temperatures.
[0005] Thermal runaway is of major concern since a single incident
can lead to significant property damage and, in some circumstances,
bodily harm or loss of life. When a battery undergoes thermal
runaway, it typically emits a large quantity of smoke, jets of
flaming liquid electrolyte, and sufficient heat to lead to the
combustion and destruction of materials in close proximity to the
cell. If the cell undergoing thermal runaway is surrounded by one
or more additional cells as is typical in a battery pack, then a
single thermal runaway event can quickly lead to the thermal
runaway of multiple cells which, in turn, can lead to much more
extensive collateral damage. Regardless of whether a single cell or
multiple cells are undergoing this phenomenon, if the initial fire
is not extinguished immediately, subsequent fires may be caused
that dramatically expand the degree of property damage. For
example, the thermal runaway of a battery within an unattended
laptop will likely result in not only the destruction of the
laptop, but also at least partial destruction of its surroundings,
e.g., home, office, car, laboratory, etc. If the laptop is on-board
an aircraft, for example within the cargo hold or a luggage
compartment, the ensuing smoke and fire may lead to an emergency
landing or, under more dire conditions, a crash landing. Similarly,
the thermal runaway of one or more batteries within the battery
pack of a hybrid or electric vehicle may destroy not only the car,
but may lead to a car wreck if the car is being driven or the
destruction of its surroundings if the car is parked.
[0006] One approach to overcoming this problem is by reducing the
risk of thermal runaway. For example, to prevent batteries from
being shorted out during storage and/or handling, precautions can
be taken to ensure that batteries are properly stored, for example
by insulating the battery terminals and using specifically designed
battery storage containers. Another approach to overcoming the
thermal runaway problem is to develop new cell chemistries and/or
modify existing cell chemistries. For example, research is
currently underway to develop composite cathodes that are more
tolerant of high charging potentials. Research is also underway to
develop electrolyte additives that form more stable passivation
layers on the electrodes. Although this research may lead to
improved cell chemistries and cell designs, currently this research
is only expected to reduce, not eliminate, the possibility of
thermal runaway. Accordingly, what is needed is a means for
maintaining cell integrity during a thermal runaway event, thereby
minimizing damage to adjacent cells and materials. The present
invention provides such a means.
SUMMARY OF THE INVENTION
[0007] The present invention provides a method and apparatus for
inhibiting the formation of perforations in the battery casing
during a thermal runaway event.
[0008] In at least one embodiment of the invention, a battery
assembly is provided comprised of a battery and a layer of an
intumescent material surrounding an outer surface of the battery
cell case. The intumescent material may be comprised of
graphite-based intumescent materials, thermoplastic elastomers,
ceramic-based intumescent materials, vermiculite/mineral fiber
based intumescent materials, and ammonium polyphosphate based
intumescent materials. Preferably the intumescent material
comprising the layer has a start expansion temperature in the range
of 100.degree. C. to 300.degree. C., and more preferably in the
range of 200.degree. C. to 300.degree. C. Preferably the
intumescent material is biologically inert. The battery assembly
may further comprise a sleeve that surrounds the intumescent layer
and is spaced apart from the intumescent layer. Preferably the
sleeve is comprised of a material with a yield strength of at least
75 MPa; and/or a yield strength of at least 150 MPa; and/or a yield
strength of at least 250 MPa. The battery assembly may further
comprise at least one spacer ring that separates the interior
surface of the sleeve with the exterior surface of the intumescent
material. The battery assembly may further comprise a second layer
of the intumescent material, which may be integral with the layer
of intumescent material surrounding the outer surface of the cell
case, wherein the second layer covers the cell case bottom.
[0009] In at least one embodiment of the invention, a method of
preventing the formation of a perforation in the outer surface of a
battery case during a thermal runaway event is provided, the method
comprising the step of covering the outer surface of the battery
case in a layer of an intumescent material, wherein the covering
step is performed prior to the thermal runaway event. The method
may further comprise the step of surrounding the layer of
intumescent material with a sleeve, wherein the sleeve is separated
from the intumescent layer by a preset distance. The covering step
may comprise the step of coating the outer surface of the cell case
with the intumescent material. The covering step may further
comprise the step of forming a strip of the intumescent material
and wrapping the strip around the outer surface of the battery
case, which may be bonded in place.
[0010] A further understanding of the nature and advantages of the
present invention may be realized by reference to the remaining
portions of the specification and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a simplified cross-sectional illustration of a
cell utilizing the 18650 form-factor;
[0012] FIG. 2 illustrates the cell shown in FIG. 1, modified to
increase failure resistance in accordance with the prior art
approach;
[0013] FIG. 3 illustrates a preferred embodiment of the invention
utilizing a high strength sleeve;
[0014] FIG. 4 illustrates an alternate embodiment utilizing
multiple high strength sleeves;
[0015] FIG. 5 illustrates an alternate embodiment utilizing an
inner, thermally insulating layer and an outer, high strength
layer;
[0016] FIG. 6 illustrates a modification of the configuration shown
in FIG. 5, the modified configuration including multiple layers
that alternate between thermally insulating material and high
strength material;
[0017] FIG. 7 illustrates an alternate embodiment utilizing a layer
of thermally insulating material interposed between the electrode
assembly and the inner surface of the cell casing;
[0018] FIG. 8 illustrates an alternate embodiment utilizing a high
heat capacity layer;
[0019] FIG. 9 illustrates an alternate embodiment utilizing a
coating of an intumescent material applied to the outer surface of
the battery casing;
[0020] FIG. 10 illustrates a modification of the configuration
shown in FIG. 9, in which an outer containment sleeve is positioned
around said intumescent coated cell;
[0021] FIG. 11 illustrates a modification of the configuration
shown in FIG. 3, the modified configuration including a multi-piece
design that covers the end surface of the cell; and
[0022] FIG. 12 illustrates a modification of the configuration
shown in FIG. 3, the modified configuration including a single
piece design that covers the end surface of the cell.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0023] In the following text, the terms "battery", "cell", and
"battery cell" may be used interchangeably and may refer to any of
a variety of different cell chemistries and configurations
including, but not limited to, lithium ion (e.g., lithium iron
phosphate, lithium cobalt oxide, other lithium metal oxides, etc.),
lithium ion polymer, nickel metal hydride, nickel cadmium, nickel
hydrogen, nickel zinc, silver zinc, or other battery
type/configuration. The term "battery pack" as used herein refers
to multiple individual batteries contained within a single piece or
multi-piece housing, the individual batteries electrically
interconnected to achieve the desired voltage and capacity for a
particular application. It should be understood that identical
element symbols used on multiple figures refer to the same
component, or components of equal functionality. Additionally, the
accompanying figures are only meant to illustrate, not limit, the
scope of the invention and should not be considered to be to
scale.
[0024] FIG. 1 is a simplified cross-sectional view of a
conventional battery 100, for example a lithium ion battery
utilizing the 18650 form-factor. Battery 100 includes a cylindrical
case 101, an electrode assembly 103, and a cap assembly 105. Case
101 is typically made of a metal, such as nickel-plated steel, that
has been selected such that it will not react with the battery
materials, e.g., the electrolyte, electrode assembly, etc.
Typically cell casing 101 is fabricated in such a way that the
bottom surface 102 is integrated into the case, resulting in a
seamless lower cell casing. The open end of cell case 101 is sealed
by a cap assembly 105, assembly 105 including a battery terminal
107, e.g., the positive terminal, and an insulator 109, insulator
109 preventing terminal 107 from making electrical contact with
case 101. Although not shown, a typical cap assembly will also
include an internal positive temperature coefficient (PTC) current
limiting device, a current interrupt device (CID), and a venting
mechanism, the venting mechanism designed to rupture at high
pressures and provide a pathway for cell contents to escape.
Additionally, cap assembly 105 may contain other seals and elements
depending upon the selected design/configuration.
[0025] Electrode assembly 103 is comprised of an anode sheet, a
cathode sheet and an interposed separator, wound together in a
spiral pattern often referred to as a `jellyroll`. An anode
electrode tab 111 connects the anode electrode of the wound
electrode assembly to the negative terminal while a cathode tab 113
connects the cathode electrode of the wound electrode assembly to
the positive terminal. In the illustrated embodiment, the negative
terminal is case 101 and the positive terminal is terminal 107. In
most configurations, battery 100 also includes a pair of insulators
115/117. Case 101 includes a crimped portion 119 that is designed
to help hold the internal elements, e.g., seals, electrode
assembly, etc., in place.
[0026] In a conventional cell, such as the cell shown in FIG. 1, a
variety of different abusive operating/charging conditions and/or
manufacturing defects may cause the cell to enter into thermal
runaway, where the amount of internally generated heat is greater
than that which can be effectively withdrawn. As a result, a large
amount of thermal energy is rapidly released, heating the entire
cell up to a temperature of 900.degree. C. or more and causing the
formation of localized hot spots where the temperature may exceed
1500.degree. C. Accompanying this energy release is the release of
gas, causing the gas pressure within the cell to increase.
[0027] To combat the effects of thermal runaway, a conventional
cell will typically include a venting element within the cap
assembly. The purpose of the venting element is to release, in a
somewhat controlled fashion, the gas generated during the thermal
runaway event, thereby preventing the internal gas pressure of the
cell from exceeding its predetermined operating range.
[0028] While the venting element of a cell may prevent excessive
internal pressure, this element may have little effect on the
thermal aspects of a thermal runaway event. For example, if a local
hot spot occurs in cell 100 at a location 121, the thermal energy
released at this spot may be sufficient to heat the adjacent area
123 of the single layer casing wall 101 to above its melting point.
Even if the temperature of area 123 is not increased beyond its
melting point, the temperature of area 123 in concert with the
increased internal cell pressure may quickly lead to the casing
wall being perforated at this location. Once perforated, the
elevated internal cell pressure will cause additional hot gas to be
directed to this location, further compromising the cell at this
and adjoining locations.
[0029] It should be noted that when a cell undergoes thermal
runaway and vents in a controlled fashion using the intended
venting element, the cell wall may still perforate due to the size
of the vent, the material characteristics of the cell wall, and the
flow of hot gas traveling along the cell wall as it rushes towards
the ruptured vent. Once the cell wall is compromised, i.e.,
perforated, collateral damage can quickly escalate, due both to the
unpredictable location of such a hot spot and due to the
unpredictable manner in which such cell wall perforations grow and
affect neighboring cells. For example, if the cell is one of a
large array of cells comprising a battery pack, the jet of hot gas
escaping the cell perforation may heat the adjacent cell to above
its critical temperature, causing the adjacent cell to enter into
thermal runaway. Accordingly, it will be appreciated that the
perforation of the wall of one cell during thermal runaway can
initiate a cascading reaction that can spread throughout the
battery pack. Furthermore, even if the jet of hot gas escaping the
cell perforation from the first cell does not initiate thermal
runaway in the adjacent cell, it may still affect the health of the
adjacent cell, for example by weakening the adjacent cell wall,
thereby making the adjacent cell more susceptible to future
failure.
[0030] As previously noted, cell perforations are due to localized,
transient hot spots where hot, pressurized gas from a concentrated
thermal event is flowing near the inner surface of the cell.
Whether or not a cell transient hot spot perforates the cell wall
or simply dissipates and leaves the cell casing intact depends on a
number of factors. These factors can be divided into two groups;
those that are based on the characteristics of the thermal event
and those that are based on the physical qualities of the cell
casing. Factors within the first group include the size and
temperature of the hot spot as well as the duration of the thermal
event and the amount of gas generated by the event. Factors within
the second group include the wall thickness as well as the casing's
yield strength as a function of temperature, heat capacity and
thermal conductivity.
[0031] FIG. 2 illustrates the conventional approach to improving
the failure resistance of a cell, where failure is defined as a
thermally induced wall perforation. As shown, in cell 200 the
thickness of casing 201 has been significantly increased, thereby
improving the cell's failure resistance at the expense of cell
weight. Assuming that cell mass is not an issue, which it is not
for many consumer applications where only a few cells are used, the
conventional approach to preventing wall perforations during
thermal runaway is quite effective. Unfortunately, for those
applications in which the battery pack may include hundreds or even
thousands of cells, for example the battery pack of an electric
vehicle, the added mass of this approach is very unattractive since
performance is directly tied to mass. For instance, if the
conventional approach only adds 4 grams per cell, for a battery
pack with 10,000 cells, this increase adds up to 40 kg.
Accordingly, for these applications the conventional approach to
improving cell failure resistance is unacceptable.
[0032] In addition to recognizing the weight constraints placed on
the batteries within a large battery pack and the factors that
contribute to the initiation and growth of wall perforations during
thermal runaway, the present inventors also recognize that once a
cell enters into thermal runaway, it is no longer viable.
Accordingly, at this point the primary purpose of the cell casing
is to control the direction and pathway for the hot, escaping gas
generated by the thermal runaway event. In recognition of these
design parameters, the intent of the present invention is to
minimize, if not altogether eliminate, the escape of hot,
pressurized gas from the sides of a cell where the escaping gas can
adversely affect neighboring cells. Rather than allow the hot gas
to escape through wall perforations, the present invention forces
this gas to exit the cell from either end surface, or in some
embodiments, from only one of the cell end surfaces.
[0033] FIG. 3 illustrates one embodiment of the invention. In this
embodiment, a conventional cell such as that shown in FIG. 1 is
modified by sliding a sleeve 301 lengthwise over the outer casing
wall 101. Typically sleeve 301 is added after fabrication of the
cell has been completed, but prior to the inclusion of the cell
within the cell's intended application, e.g., a battery pack.
Sleeve 301 may be added by the cell manufacturer or added by
another party. Preferably there is minimal clearance between the
inner surface of sleeve 301 and the outer surface of cell casing
101. In at least one embodiment, sleeve 301 is press-fit, also
commonly referred to as a force fit, onto case 101, for example
using a hydraulic press-fit system.
[0034] The use of a separate sleeve offers a number of advantages
over the conventional approach of simply increasing the wall
thickness of casing 101. First and foremost, the goals of
significantly decreasing the risk of hot, pressurized gas escaping
through the cell wall and redirecting this escaping gas to the cell
ends are both achieved while adding much less weight to the cell
than would be required to achieve the same performance using the
conventional, wall-thickening approach. Second, as sleeve 301 is
preferably added after completion of cell processing, this approach
can be used with virtually any manufacturer's cell since it does
not affect cell manufacturing. Third, while the material selected
for casing 101 must be non-reactive with the cell contents (e.g.,
electrolyte and electrode assembly), no such material constraints
are placed on sleeve 301. Accordingly the material used for sleeve
301 can be selected based on its ability to minimize or eliminate
the escape of hot, pressurized gas from the cell sidewalls.
[0035] During thermal runaway, when a transient hot spot (e.g.,
spot 121 in FIG. 1) develops, sleeve 301 performs several
functions. Initially, it draws off heat from the cell case 101 at
the location adjacent to the transient hot spot (e.g., location 123
of case 101 in FIG. 1), thereby slowing down the perforation of the
wall. If a perforation in casing 101 starts, then sleeve 301
prevents the immediate and rapid growth of the perforation that
would otherwise occur due to the pressurized hot gas being directed
through the perforation. Additionally, due to the thermal contact
resistance between case 101 and sleeve 301, and given the limited
duration of the thermal event, sleeve 301 is more resistive to
perforation than case 101. The failure resistance of sleeve 301 is
further enhanced by fabricating the sleeve from a material that
exhibits high yield strength at high temperature. Preferably the
yield strength of sleeve 301 is at least 250 MPa at room
temperature, more preferably at least 250 MPa at a temperature of
1000.degree. C., still more preferably at least 500 MPa at room
temperature, and yet still more preferably at least 500 MPa at a
temperature of 1000.degree. C.
[0036] Given the thermal contact resistance between the case and
the sleeve, and given that the sleeve is preferably constructed of
a material with the desired material characteristics, typically
only casing 101 will be perforated during thermal runaway, leaving
outer sleeve 301 intact. As a result, adjacent cells are not
subjected to a stream of high temperature, pressurized gas.
Additionally, in this situation, and given the minimal clearance
between the outer surface of case 101 and the inner surface of
sleeve 301, sleeve 301 prevents the rapid growth of the case wall
perforation, thereby minimizing the amount of gas that escapes
through the case wall perforation. This effect is aided by the cell
case dimensions expanding slightly during thermal runaway due to
the increased internal pressure, thereby further improving the
contact between the outer cell surface and the interior surface of
the sleeve. Accordingly little, if any, hot gas escapes through the
case wall perforation and that which does escape is redirected by
sleeve 301 to either cell end where its effects on neighboring
cells are minimized.
[0037] It will be appreciated that there are a variety of materials
suitable for use in constructing sleeve 301. Exemplary materials
include various engineering and high strength structural steels,
plated steel, stainless steel, titanium and titanium alloys, and
nickel alloys. The preferred technique or techniques for
fabricating sleeve 301 depend on the selected material as well as
the shape of the cell for which the sleeve is to be used. For
example, if the cell has the 18650 form-factor, sleeve 301 is
cylindrically shaped. Sleeve 301 may be fabricated using any of a
variety of techniques including, but not limited to, (i) drawing
the sleeve, (ii) wrapping a strip or sheet of the desired material
around the cell and welding or mechanically coupling the edges
together, (iii) bending and welding a strip or sheet of the desired
material into the desired shape, or (iv) bending a strip or sheet
of the desired material into the desired shape and coupling the two
edges together using mechanical interconnects. Sleeve 301 may be
bonded into place.
[0038] FIG. 4 illustrates an alternate embodiment of the invention.
In this embodiment, a conventional cell such as that shown in FIG.
1 is modified by wrapping the cell with a plurality of layers
401-403. Preferably three layers are used, as shown, although it
will be appreciated that either a fewer number or a greater number
of layers can be used in this embodiment. Preferably all of the
layers are comprised of a material exhibiting high yield strength
at high temperatures, more specifically a yield strength of at
least 250 MPa at room temperature, still more preferably at least
250 MPa at a temperature of 1000.degree. C., still more preferably
at least 500 MPa at room temperature, and yet still more preferably
at least 500 MPa at a temperature of 1000.degree. C. The plurality
of layers may be comprised of the same material, or of one or more
different materials. Exemplary materials include various
engineering and high strength structural steels, plated steel,
stainless steel, titanium and titanium alloys, and nickel
alloys.
[0039] The inventors have found that due to the thermal contact
resistance between layers 401-403, in addition to the thermal
contact resistance between casing 101 and the innermost layer 401,
the use of multiple layers is more effective than a single layer
sleeve as shown in FIG. 3. Following this approach, and even using
the same material, the additive thickness of layers 401-403 can be
less than the thickness of sleeve 301 while achieving the same
level of perforation resistance. For example, in one test the
inventors found that three 25 micron thick layers of stainless
steel were just as effective as a single, 100 micron thick layer of
stainless steel. Accordingly, further mass reductions can be
achieved by encasing the cell in multiple layers as opposed to a
single layer sleeve.
[0040] As in the previous embodiment, there is minimal clearance
between the layers as well as between the innermost layer and the
outer surface of cell casing 101. The layers of this embodiment can
be fabricated as individual sleeves of gradually increasing inside
diameter or, as preferred, fabricated from a single strip or sheet
that is wrapped multiple times around the outside of the cell case.
The layers, and/or the final, outermost edge of the outermost
layer, can be bonded, welded, or held in place with mechanical
interconnects.
[0041] As in the prior embodiment, the layers of the present
embodiment control the growth of any cell wall perforations while
preventing hot, pressurized gas from being directed at adjacent
cells.
[0042] In an alternate preferred embodiment of the invention, a
pair of layers (FIG. 5), or multiple pairs of layers (FIG. 6),
surround the battery case (e.g., cell case 101 in FIG. 1). Although
the embodiment shown in FIG. 6 only includes two sets of layers, it
will be appreciated that additional layer pairs can be used with
the invention.
[0043] The innermost layer of each layer pair, e.g., layer 501 in
FIG. 5 and each layer 601 in FIG. 6, is comprised of a thermal
insulator, preferably a light weight thermal insulator. The
outermost layer of each layer pair, e.g., layer 503 in FIG. 5 and
each layer 603 in FIG. 6, is comprised of a high yield strength
material. Preferably the high yield strength material used in layer
503/603 has a yield strength of at least 250 MPa at room
temperature, more preferably at least 250 MPa at a temperature of
1000.degree. C., still more preferably at least 500 MPa at room
temperature, and yet still more preferably at least 500 MPa at a
temperature of 1000.degree. C. Exemplary high yield strength
materials include, but are not limited to, various engineering and
high strength structural steels, plated steel, stainless steel,
titanium and titanium alloys, and nickel alloys.
[0044] In one configuration, the material used for the thermal
insulator layers, e.g., layer 501 in FIG. 5 and each layer 601 in
FIG. 6, is capable of withstanding a relatively high temperature,
for example, capable of withstanding temperatures of more than
500.degree. C. continuously and/or withstanding temperatures of
more than 1000.degree. C. for a period of at least 10 seconds
and/or withstanding temperatures of more than 1400.degree. C. for a
period of at least 1 second. It will be appreciated that there are
numerous materials and composite materials that can be used to
fabricate this layer. Exemplary materials include, but are not
limited to, fiberglass, mineral wool, silica/silica fibers,
alumina, Kevlar.RTM., Nomex.RTM., calcium-silicate or
calcium-magnesium-silicate fibers, some ceramics, etc.
[0045] In an alternate configuration, the material used for the
thermal insulator layers, e.g., layer 501 in FIG. 5 and each layer
601 in FIG. 6, is an insulator with a relatively low melting
temperature, for example, with a melting temperature of less than a
500.degree. C., more preferably less than 350.degree. C., and still
more preferably less than 250.degree. C. Exemplary materials
include, but are not limited to, plastics, more specifically a
polymer such as polyethylene or polypropylene.
[0046] During a thermal runaway event, when a transient hot spot is
formed (e.g., spot 121 of FIG. 1), the thermally insulating layer
(e.g., layer 501) significantly reduces the temperature of the high
yield strength layer (e.g., layer 503) adjacent to the hot spot,
thereby drastically reducing the risk of a perforation forming in
the outermost, high yield strength layer. The use of two or more
layer pairs as shown in FIG. 6 further reduces this risk by
requiring that the hot, pressurized gas from the transient hot spot
ablate two or more thermally insulating layers and two or more high
yield strength layers. It will be appreciated that by lowering the
temperature near the transient hot spot, the high yield strength
layer can be thinner than would otherwise be required, resulting in
further weight gains. In this embodiment, one of the primary
purposes of the high yield strength layer (e.g., layer 501, layer
601) is to provide a strong support layer for the thermal
insulator, thereby preventing its rapid ablation by the
pressurized, hot gas.
[0047] The thermally insulating layer (e.g., layer 501, layers 601)
can be fabricated in a variety of ways, depending upon the material
or materials used to fabricate the layer. For example, this layer
can be formed by bonding or otherwise attaching a thin layer of
suitable thermal insulator to the cell case, or to the underlying
high yield strength layer in the configuration utilizing multiple
layer pairs. This thermally insulating layer may be preformed using
a weaving, machining, or other technique. Alternately, this layer
can be fabricated by rolling, dipping or spraying the cell, or the
underlying high yield strength layer, with the thermally insulating
material. Alternately, this layer can be deposited on the inner
surface of the high strength layer (e.g., layer 503, layers 603)
prior to the assembly of the high strength layer onto the cell. As
previously described relative to layers 301 and 401-403, high
strength layer 503 can be fabricated as a sleeve or as a strip or
sheet of suitable material that is bent to shape and held together
by welding, bonding or mechanical interconnects.
[0048] In a modification of the embodiment shown in FIG. 5, a layer
of thermally insulating material is interposed between the inside
wall surface of case 100 and the exterior surface of the electrode
assembly (FIG. 7). As in the prior embodiments shown in FIGS. 5 and
6, the thermally insulating layer 701 shown in FIG. 7 minimizes the
transfer of thermal energy from the transient hot spot to the next
layer which, in this configuration, is outer cell casing 101. Given
the relatively short duration of the thermal event, the delay
provided by layer 701 is sufficient to prevent the pressurized hot
gas accompanying the transient hot spot from perforating casing
101. In this embodiment, case 101 provides the strength and support
necessary to ensure that the region of layer 701 adjacent to the
hot spot is not rapidly ablated away, thereby eliminating its
usefulness.
[0049] Although the use of a single, light weight, thermally
insulating layer 701 as shown in FIG. 7 can dramatically improve
the cell's resistance to wall perforations during thermal runaway,
the inclusion of such a layer is not without its drawbacks.
Primarily, the material or materials selected for use in layer 701
must be non-reactive with the electrolyte and electrode assembly.
Additionally, the inclusion of layer 701 requires modifying the
cell fabrication and assembly process.
[0050] As in the previous embodiment, preferably thermally
insulating layer 701 is fabricated from a thermal insulator that is
capable of withstanding temperatures of more than 500.degree. C.
continuously and/or withstanding temperatures of more than
1000.degree. C. for a period of at least 10 seconds and/or
withstanding temperatures of more than 1400.degree. C. for a period
of at least 1 second. Exemplary materials include, but are not
limited to, fiberglass, mineral wool, silica/silica fibers,
alumina, Kevlar.RTM., Nomex.RTM., calcium-silicate or
calcium-magnesium-silicate fibers, some ceramics, etc.
[0051] Preferably layer 701 is formed by depositing or otherwise
coating the inside surface of case 101 with the selected thermal
insulator, this deposition/coating step being performed prior to
assembling the electrode assembly within the cell casing.
Alternately, the exterior surface of the electrode assembly may be
deposited or otherwise coated with the selected thermal insulator,
this deposition/coating step being performed prior to assembling
the electrode assembly within the cell casing. Alternately, a
sheet/strip of the selected thermal insulator can be formed, after
which the sheet/strip is wrapped around the outside surface of the
electrode assembly before assembly within the cell casing.
Alternately, a sleeve of the selected thermal insulator can be
formed, with or without a bottom surface, and inserted within the
cell case prior to final assembly.
[0052] In a variation of the embodiment described above, layer 701
is not formed from a thermal insulator, but formed from the same
separator, or a similar separator, as that used during the
fabrication of the electrode assembly (i.e., assembly 103). By
using the same separator, fabrication is simplified as the winding
process that is used to create the electrode jellyroll can be used
to create the additional separator layers, simply by lengthening
the separator material relative to the lengths of the anode and
cathode materials. Typically the separator is fabricated from a
thermoplastic, such as polyethylene, polypropylene or some
combination of the two. In a slight modification of this
embodiment, in addition to wrapping extra layers of separator
around the jellyroll, layers of one or more metals are also wrapped
around the jellyroll. The metal or metals within the extra layers
may be the same metals used to form the cathode and/or anode of the
electrode assembly, as long as these additional metal layers are
not part of the active electrode assembly.
[0053] During a thermal runaway event, the additional layers of
separator, or the additional layers of the separator plus metal
layer(s), absorb some of the energy released during the thermal
runaway event, thereby inhibiting the formation of perforations in
the cell case wall. These extra separator layers, with or without
the additional non-active metal layer(s), also help to prevent
short circuits from occurring between the electrode assembly and
the cell case.
[0054] In another variation of the embodiment described above,
layer 701 is not formed from a thermal insulator or from a
separator, but formed from at least one layer of metal. The at
least one layer of metal is non-reactive with the electrode
assembly, including the electrolyte, and is not an active element
of the electrode assembly. An exemplary metal for at least some
cell chemistries is steel, or a steel alloy. Preferably this layer
or layers of metal (e.g., layer 701) is wound around the electrode
assembly prior to inserting the assembly into cell casing 101.
[0055] FIG. 8 illustrates an alternate preferred embodiment of the
invention utilizing the addition of a layer 801 surrounding the
outer surface of the cell case, layer 801 comprised of a material
with a latent heat of fusion of at least 200 kJ/kg, more preferably
with a latent heat of fusion of at least 300 kJ/kg, and still more
preferably with a latent heat of fusion of at least 350 kJ/kg.
Preferably the material has a melting point between 300.degree. C.
and 1500.degree. C., and more preferably between 500.degree. C. and
1200.degree. C. Although a variety of materials can be used for
layer 801, preferably layer 801 is fabricated from aluminum or an
aluminum alloy.
[0056] During thermal runaway, when a transient hot spot (e.g.,
spot 121 in FIG. 1) develops, layer 801 rapidly draws off the heat
from the cell case 101 at the location adjacent to the transient
hot spot. By rapidly drawing off the heat, the temperature in this
region of the case (e.g., location 123 of case 101 in FIG. 1) never
reaches its melting point or becomes so weak that the internal cell
pressure can perforate the cell casing at this location. It will be
appreciated that although a portion of layer 801 adjacent to the
transient hot spot may melt during this process, given the
relatively short duration of the event, and given that layer 801 is
not repeatedly subjected to thermal runaway conditions, the loss of
this portion of layer 801 via melting does not compromise the
performance of this embodiment.
[0057] As with the embodiment illustrated in FIG. 3, layer 801 may
be drawn (e.g., as a sleeve), fabricated by bending and welding a
strip or sheet into the desired shape, or fabricated by bending a
strip or sheet into the desired shape and coupling the two edges
together using mechanical interconnects. Layer 801 may be bonded
into place, welded into place, press-fit into place, or otherwise
fit to cell case 101. As in the embodiments discussed above and
illustrated in FIGS. 3-6, there is minimal clearance between layer
801 and the outer surface of cell casing 101.
[0058] FIG. 9 illustrates an alternate preferred embodiment of the
invention in which the outer surface of the cell case is coated
with a layer 901 of an intumescent material. During thermal
runaway, the exterior surface of the battery heats up, this heating
process typically initiating at one or more transient hot spots
(e.g., spot 121 in FIG. 1). Intumescent layer 901 begins to expand
as soon as the material's start expansion temperature (SET) is
reached. Accordingly, layer 901 typically will begin to expand at a
location near the transient hot spots, and then will continue to
expand as the entire battery heats up past the SET temperature.
After expansion, the intumescent material of layer 901 hardens.
Intumescent layer 901 helps to prevent the formation of a
perforation in the battery case near the transient hot spots and,
if one is formed, helps to contain the escaping hot, pressurized
gas.
[0059] FIG. 10 illustrates a slight modification of the embodiment
shown in FIG. 9. In the modified configuration, an outer sleeve
1001 surrounds the intumescent covered cell. Preferably sleeve 1001
has a yield strength of at least 75 MPa, more preferably at least
150 MPa, and yet still more preferably at least 250 MPa. The
interior surface of sleeve 1001 is spaced apart from the
intumescent covered cell by a spacing 1003, thus insuring that
intumescent layer 901 has room to expand. At the same time, sleeve
1001 helps to prevent layer 901, after expansion, from being
ablated away by the hot, pressurized gas escaping from within the
cell. Sleeve 1003 may be held in place using various means, such as
with one or more spacer rings 1005. Alternately, the battery pack
mounting substrate may be configured to maintain the position of
sleeve 1003 relative to the intumescent covered battery.
[0060] Intumescent layer 901 can be fabricated from any of a
variety of intumescent materials, for example, graphite-based
intumescent material (e.g., expandable graphite in a polymeric
binder), thermoplastic elastomers, ceramic-based intumescent
material, vermiculite/mineral fiber based intumescent material, and
ammonium polyphosphate based intumescent material. Preferably the
selected intumescent material has a SET temperature in the range of
100.degree. C. to 300.degree. C., and more preferably in the range
of 200.degree. C. to 300.degree. C. Preferably the intumescent
material selected for layer 901 is biologically inert, thus
insuring that if the intumescent covered cell is used in an
application with limited airflow, the layer's activation will be a
non-toxic event.
[0061] Preferably layer 901 is formed by depositing or otherwise
coating the outer surface of case 101 with the intumescent
material. Alternately, a sheet/strip of the intumescent material
can be pre-formed, after which the sheet/strip is wrapped around
the outside surface of the cell casing. In this configuration, the
sheet/strip of the intumescent material is preferably bonded in
place.
[0062] Although the embodiments described above successfully
mitigate the effects of cell wall perforations, at least some of
these embodiments may allow gas escaping from the cell to travel
between the cell wall and the sleeve(s), exiting from either end of
the cell. While the inventors have found that this is acceptable
for most applications, they envision that for some applications it
may be preferable to limit the escaping gas to a single cell end,
and preferably the same end that includes the venting element.
Accordingly, for such applications, one end of the sleeve(s) is
closed, using either a multi-piece or a single piece design. FIG.
11 illustrates a multi-piece design based on the embodiment shown
in FIG. 3 that includes both a sleeve layer 1101 and a bottom layer
1103. FIG. 12 illustrates design based on the embodiment shown in
FIG. 3 in which layer 1201 covers both the sidewall and the bottom
surface of case 101. It will be appreciated that such a design
configuration can be applied to the other embodiments described
above. For example, the bottom cell surface may covered with a
layer pair, the layer pair comprised of a thermally insulating
material and a high yield strength material; alternately, the
bottom cell surface may be covered by a high heat capacity
material; alternately, a thermally insulating material may be
interposed between the bottom of the electrode assembly and the
inner surface of the bottom of the cell casing; alternately, the
bottom cell surface may be covered by an intumescent material
layer.
[0063] Although the preferred embodiment of the invention is
utilized with a cell using the 18650 form-factor, it will be
appreciated that the invention can be used with other cell designs,
shapes and configurations.
[0064] As will be understood by those familiar with the art, the
present invention may be embodied in other specific forms without
departing from the spirit or essential characteristics thereof.
Accordingly, the disclosures and descriptions herein are intended
to be illustrative, but not limiting, of the scope of the invention
which is set forth in the following claims.
* * * * *