Diffusion vent for a rechargeable metal-air cell

The present invention provides a system for venting gas from within a case housing a metal-air cell having a cathode and an anode with a separator positioned therebetween. The cell has at least one gas vent on the anode side of the separator that extends from the interior surface of the case to the exterior surface. Each gas vent has a gas vent cap thereon on the interior surface of the case. A gas permeable, hydrophobic membrane is positioned between each gas vent and gas vent cap. The gas vent, with the gas vent cap positioned thereon, prevents the anode from blocking the venting of gas through the membrane.

TECHNICAL FIELD 
This invention relates generally to metal-air cells and, more particularly, 
to vent systems for the exhaust of gasses generated within a metal-air 
cell. 
BACKGROUND OF THE INVENTION 
During the operation of an electrochemical cell, such as a metal-air cell, 
gasses are released during the electrochemical reaction. These gasses must 
be vented from the interior of the case housing of the cell or cell 
operation and efficiency may be compromised. It is also desirable to 
prevent the passage of liquids into or out of the cell and to prevent the 
intrusion of contaminates while the internal gasses are being vented from 
the cell. 
Metal-air cells include an air-permeable cathode and a metallic anode 
separated by an aqueous electrolyte. During the operation of a cell such 
as a zinc-air cell, oxygen from ambient air is converted at the cathode to 
hydroxide ions, zinc is oxidized at the anode and reacts with these 
hydroxide ions such that water and electrons are released to provide 
electrical energy. Various gasses are released within the cell structure 
during this electrochemical reaction causing the internal pressure in the 
cell case to increase with continued use. Because the cathode is usually 
not capable of supporting high hydrostatic pressures (typically less than 
2 psi), the gasses generated within the cell case must be vented at low 
pressures to protect the cathode. 
While venting internal gasses is possible through mechanical devices that 
can open and close to the atmosphere, these devices must reseal each time 
after venting. The hermeticity of the case may be sacrificed by the 
opening and closing a mechanical seal. The control of electrolyte leakage 
and equilibrium vapor pressure also may be difficult depending upon the 
size of the opening as well as the length of time in which the mechanical 
seal is open. Environmental contaminants, such as carbon dioxide, also may 
enter through the opening. Further, the relative humidity of the ambient 
air that enters the cell through the opening is also of concern. If the 
relative humidity of the ambient air is too high, then the battery may 
fail due to a condition called flooding. However, if the relative humidity 
of the ambient air is too low, then the battery may fail due to drying 
out. 
Known methods for venting gasses generated from within a cell include that 
described in commonly owned U.S. Pat. No. 5,362,577, issued Nov. 8, 1994, 
entitled "Diffusion Vent for a Rechargeable Metal-Air Cell," disclosing a 
vent system providing at least one gas exit hole that is sufficiently 
small to prevent electrolyte leakage and also to prevent the intake of 
excess carbon dioxide or excess water vapor from the atmosphere. The 
disclosure of U.S. Pat. No. 5,362,577 is incorporated herein by reference. 
Generally, this invention also discloses the use of combinations of gas 
permeable, hydrophobic membranes, such as polypropylene, and diffuser 
materials, such as polyethylene, to cover the gas exit hole. A recess also 
may be provided within the case such that the gas exit hole communicates 
between the atmosphere and the recess. A gas collection area is defined by 
the recess formed in the case wall or by the gas diffuser membrane 
attached to the case. 
In a preferred embodiment of the invention described in U.S. Pat. No. 
5,362,577, the case has at least one recess defined on its interior 
surface. The recess extends towards the exterior of the case so as to 
define a gas collection area, with at least one gas exit hole 
communicating with the atmosphere. The gas exit hole has a smaller 
cross-sectional area than the cross-sectional area of the recess. The gas 
exit hole is covered with a gas permeable, hydrophobic membrane. A gas 
diffuser is retained or attached within the recess. A second gas 
permeable, hydrophobic membrane is then attached to the interior surface 
of the case and covers the gas diffuser and first gas permeable, 
hydrophobic membrane. The gas diffuser both supports the inner membrane 
when under pressure and laterally diffuses the gas between the membranes. 
The membranes are ultrasonically welded to the case. 
While the system of U.S. Pat. No. 5,362,577 provides superior venting of 
exhaust gas from a battery case while maintaining a hermetic seal, what is 
needed is a simplified venting system. This simplified system would 
provide venting for the case while eliminating the need for some of the 
materials and construction techniques currently used. It is also desirable 
for a vent system to prevent the ingress of gasses, such as oxygen, that 
may corrode the anode and otherwise impede the operation of the cell. 
Further, because the anode tends to expand during discharge, it is 
desirable to provide structural support for the cell to ensure that the 
anode does not block the vent. These additional goals must be accomplished 
while maintaining an adequate venting system and insuring the hermeticity 
of the cell. 
SUMMARY OF THE INVENTION 
Generally described, the present invention provides a system for venting 
gas from within a case housing a metal-air cell having a cathode and an 
anode with a separator positioned therebetween. The cell has at least one 
gas vent on the anode side of the separator that extends from the interior 
surface of the case to the exterior surface. Each gas vent has a gas vent 
cap thereon on the interior surface of the case. A gas permeable, 
hydrophobic membrane is positioned between each gas vent and gas vent cap. 
The gas vent, with the gas vent cap positioned thereon, prevents the anode 
from blocking the venting of gas through the membrane. 
Specific embodiments include the use of Teflon as a gas membrane and 
polypropylene for the case, the gas vent, and the gas vent cap. The Teflon 
membrane is bonded to the polypropylene by pinching the membrane between 
the gas vent and the gas vent cap and ultrasonically welding the vent and 
the cap together. While Teflon and polypropylene are generally 
incompatible for joining, a type of adhesion between the materials is 
achieved. The use of Teflon as the membrane eliminates the need for the 
porous gas diffuser sheet as used in U.S. Pat. No. 5,362,577. The gas vent 
and the gas vent cap are dome shaped with several channels therein to 
permit the drainage of electrolyte. These chapels permit self draining 
when the cell is tilted or rotated. Further, the use of this dome shape 
provides mechanical support for the anode and anode collector to prevent 
the cell case from swelling and to prevent the anode from blocking the 
vents. 
The present invention further adds a valve for pressure differential relief 
and to prevent the intrusion of gasses. Left unprotected, oxygen from the 
atmosphere can intrude into a diffusion vent and cause corrosion of the 
zinc anode. The invention solves this problem through the use of a burp 
valve or a flapper valve that seals out oxygen and other gasses but allows 
hydrogen to escape when internal pressure is created. The valve can be 
adjusted to vary the force or internal pressure required to open the 
valve. 
It is thus an object of the present invention to provide a vent system that 
exhausts gasses generated within a battery case. 
It is another object of the present invention to provide a vent system that 
exhausts gasses generated from within a battery case while maintaining the 
hermetic seal of the case. 
It is a further object of the present invention to provide a vent system 
that exhausts gasses generated from within a battery case while preventing 
excess water loss or gain within the case. 
It is a still further object of the present invention to provide a vent 
system that exhausts gasses generated from within a battery case while 
minimizing gas intake. 
It is a still further object of the present invention to provide a gas vent 
for a battery cell with self draining channels therein for orientation 
independence of cell. 
It is a still further object of the present invention to provide for the 
use of Teflon as a gas permeable, hydrophobic membrane within a 
polypropylene battery cell case. 
It is a still further object of the present invention to provide a gas vent 
in a battery cell case that provides structural support to a cell. 
It is a still further object of the present invention to provide an 
adequate gas collection area near the gas vent for efficient diffusion 
through the membrane. 
It is a still further object of the present invention to provide a pressure 
relief valve for a battery cell. 
It is a still further object of the invention to provide a venting system 
for a battery cell that prevents the ingress of oxygen and other gasses 
into the cell. 
Other objects, features, and advantages of the present invention will 
become apparent upon review of the following detailed description of the 
preferred embodiments of the invention, when taken in conjunction with the 
drawing and the claims.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawings, in which like numerals represent like parts 
throughout the several views, FIGS. 1-8 show the preferred embodiment of 
one or more vent systems 15 positioned in a cell case 20 of a metal-air 
cell 10. The vent systems 15 provide for exhausting excess gasses 
generated within the cell case 20 during operation of the cell 10 to 
prevent the build-up of pressure therein. Each vent system 15 includes a 
gas vent 30, a gas vent cap 50, and a gas membrane 65. 
FIG. 1 is a side cross-sectional view of a metal-air cell 10 showing the 
preferred embodiment of the vent system 15 in the cell 10. One or more 
vent systems 15 are formed in a top wall 18 of the cell case 20. The vent 
systems 15 are located over and provide support for an anode 115 and an 
anode screen 116. The anode screen 116 also may be covered by an 
electrolyte reservoir pad (not shown) to keep the anode screen 116 moist. 
In a bottom wall 19, the cell case 20 includes a plurality of openings 100. 
An air cathode 105 is disposed within the cell case 20 such that the 
openings 100 expose the cathode 105 to the atmosphere. A cathode support 
110 secures the cathode 105 in position as well as containing and 
supporting the anode 115, an absorbent separator material 120, and an 
electrolyte 125. The electrolyte 125 partially fills the cell case 20 
thereby defining a liquid volume within the cell case 20. The remaining 
non-solid volume contains a gas, such as hydrogen, that is generated when 
the cell 10 is recharged. 
FIG. 2 is a plan view of an interior surface 21 of the top wall 18 of the 
cell case 20 on the anode 115 side of the cell 10. The interior surface 21 
of the cell case 20 contains a plurality of gas vents 30 arranged at 
spaced intervals. The gas vents 30 are molded into the wall 18 of the cell 
case 20. The gas vents 30 are positioned to ensure that at least one gas 
vent 30 may communicate with the gas volume within the cell case 20. The 
cell case 20 may be oriented through at least a 90 degree vertical 
rotation from the horizontal position and at least one vent 30 will be 
above the maximum level of the electrolyte 125 of the cell case 20. 
Preferably, the vents 30 are aligned in a diagonal array as shown in FIG. 
2, but also may be positioned near the edges or corners of the cell case 
20. It should be appreciated that the number of vents 30 and the position 
of the vents 30 may vary depending upon the amount of electrolyte 125 used 
within the cell case 20 and depending upon the shape and contents of the 
cell case 20. Also, the number of vents 30 may vary according to the 
cross-sectional area of the vents 15. 
The interior surface 21 of the cell case wall 18 also contains a plurality 
of support posts or pegs 25 arranged at uniformly spaced intervals, as is 
shown in FIGS. 2 and 3. The pegs 25 provide structural support for the 
anode 115 and the anode screen 116 while providing an area for gasses 
generated within the cell 10 to collect. The pegs 25 are approximately 
0.120 inches in height from the interior surface 21 of the cell case 20. 
The cell case 20 itself is generally made from polypropylene. The cell case 
20 of this embodiment is approximately 4.86 inches in length and 
approximately 2.56 inches in width. One or more cells 10 may be stacked 
together to form a battery pack (not shown). 
The gas vents 30 are placed approximately 1.9 inches away from each other 
along the length of the cell case 20 and approximately 0.8 inches away 
from each other along the width of the cell case 20, along a diagonal line 
as shown. The gas vents are dome-like in shape. A central dome 36 is 
surrounded by an annular plateau 37. The central dome 36 is encircled by a 
series of ribs 31 of approximately 0.012 inches in height, positioned on 
the annular plateau 37. The ribs 31 are best shown in FIGS. 2, 5, and 6. 
The dome-like shape of the gas vent 30 is shown in FIG. 3, which is a side 
cross-sectional view of the cell case 20, and in FIG. 6 which is a 
perspective view of the gas vent 30. The gas vent 30 is approximately 0.10 
inches in height from the interior surface 21 of the cell case 20. The 
dome-like shape of the gas vent 30 also creates a recess 35 on the 
exterior surface 22 of the cell case 20. The recess 35 is approximately 
0.41 inches in diameter. 
A gas exit hole 40 is located in the center of the gas vent 30. The gas 
exit hole 40 is approximately 0.045 inches in diameter. The gas exit hole 
40 extends from the recess 35 on the exterior surface 22 to the interior 
surface 21 of the cell case 20. On the interior surface 21 of the cell 
case 20, the gas exit hole 40 extends into a gas collection area 46 and a 
central depression 47. The gas collection area 46 is formed as a series of 
gas channels 45 extending outward in spoke-like fashion from the central 
depression 47 to an outer circular channel 48. The channels are defined by 
a group of roughly triangular plateaus 49 surrounding the central 
depression 47 with the gas exit hole 40 positioned in its center. The 
channels 45 are approximately 0.025 inches wide. FIG. 4 shows a top plan 
view of the gas exit hole 40 surrounded by the gas collection area 46 and 
the plateaus 49. 
FIG. 7 is a top plan view of the gas vent cap 50 with several tabs 55 
extending therefrom. The cap 50 includes an annular ring 51 with, in this 
embodiment, four tabs 55 protruding upwards and away from the interior 
surface 21 of the cell case 20. The tabs 55 are evenly spaced on the ring 
51 and define between them four radial cap channels 56. The ring 51 
further defines a central opening 60 in its interior that is surrounded by 
the tabs 55. As is shown in FIG. 8, the cap 50 is designed to fit matingly 
with the vent 30 such that the central dome 36 defining the gas collection 
area 46 and the plateaus 49 are positioned in the central opening 60. When 
the cap 50 is installed on the vent 30, the plateaus 49 are slightly 
elevated above the radial cap channels 56. This slight elevation of the 
plateaus 49 over the cap channels 56 ensures that electrolyte 125 drains 
away from the vent system 15. The cap 55 is constructed from 
polypropylene. 
FIG. 8 is a side cross-sectional view of the top cell wall 18 of cell case 
20 showing the gas vent 30 with the cap 50 and the gas membrane 65 in 
place, taken along line 8--8 of FIG. 7. The gas membrane 65 is positioned 
between the gas vent 30 and the gas vent cap 50 such that the gas exit 
hole 40 and the channels 45 of the gas collection area 46 are completely 
sealed. The gas membrane 65 is made from gas permeable 
polytetrafluouroethylene ("PTFE" or "Teflon"). While Teflon and 
polypropylene are generally not compatible for joining, a type of adhesion 
is achieved between the materials by the use of ultrasonic welding and a 
pinch backup. The membrane 65 is stretched over the dome-shaped gas vent 
30 and is supported by the plateaus 49 of the gas collection area 46. The 
membrane 65 is pinched between the ribs 31 of the gas vent 30 and the cap 
50. The cap 50 is then ultrasonically welded to the gas vent 30 along an 
outer annular rib 32 to ensure a hermetic seal. Other welding techniques 
or adhesives may also be used. When the cap 50 and the gas membrane 65 are 
in place over the gas vent 30, the height of the vent system 15 from the 
interior surface 21 of the cell case 20 is approximately 0.120 inches, or 
the same height as the pegs 25. 
The gas membrane 65 is hydrophobic and gas permeable. The membrane 65 
allows gas to be vented from the cell 10 but is substantially impermeable 
to water. The membrane 65 therefore prevents liquid loss or intrusion from 
or into the cell. The properties of Teflon membranes are well known to 
those skilled in the art. 
FIGS. 9-10 show an alternative embodiment of the present invention. This 
alternative embodiment is similar to that described above, but has 
distinctly shaped tabs 55 positioned on the ring 56 of the gas cap 50 and 
has an increased number of gas channels 45 positioned in the gas 
collection area 46 of the gas vent 30. As is shown in FIG. 9, this 
embodiment has eight cantilevered tabs 200 for increased structural 
support for the anode 115. These cantilevered tabs 200 extend the width of 
the ring 205 and cantilever over the central opening 210. FIG. 9 also 
shows an alternative embodiment of the gas vent 30 with eight gas channels 
215 and plateaus 220. Further, the gas channels 215 extend outward through 
two sets of circular outer channels 225. This embodiment of the gas vent 
30 provides for a more diffuse gas collection area 230. This embodiment of 
the gas vent 30 is also shown in plan view in FIG. 2. While the number of 
the plateaus 220 and the tabs 200 are even in this alternative embodiment, 
there is no requirement that they be equal. The number of tabs 200 is 
determined by the amount of structural support needed for the anode 115. 
FIG. 11 is a plan view of the exterior surface 22 of the cell case 20. A 
pressure relief valve or "burp valve" 70 is mounted over each gas exit 
hole 40 and recess 35. The burp valve 70 prevents oxygen and other gasses 
from entering the cell 10 via the vent systems 15 while still allowing 
hydrogen gas to escape. The burp valve 70 uses a circular piece of thin, 
flexible plastic material that deforms under pressure but resiliently 
returns to its original shape and position in the absence of such 
pressure. A thin slit 71 is positioned in the center of the burp valve 70. 
The periphery 72 of the burp valve 70 is secured to the exterior surface 
22 of the cell case 20 such that the burp valve 70 completely surrounds 
the recess 35. The periphery 72 of the burp valve 70 is secured to the 
cell case 20 by means of ultrasonic welding, adhesives, or other bonding 
means. Other embodiments of the burp valve 70 include the use of a flapper 
valve (not shown) having a strip of the thin flexible plastic material 
that is secured on one end to the cell case 20. 
In use, hydrogen gas generated at the anode 115 of the cell 10 collects 
within the interior surface 21 of the cell case 20 in the area between the 
pegs 25 and the vent systems 15. The gas membrane 65 in each vent system 
15 permits the hydrogen gas to pass into the channels 45 of the gas 
collection area 46 and to escape through the gas exit hole 40. The gas 
membrane 65 allows hydrogen gas to be exhausted but substantially prevents 
water vapor from exiting or entering the cell 10 such that hermeticity of 
the cell 10 is maintained. The vent systems 15 are positioned such that at 
least one vent system 15 is free to communicate with the gas volume. 
The gas vent 30 in each vent system 15 is dome-like in shape to provide 
additional structural strength to the cell case 20 and to provide the gas 
collection area 46. The gas collection area 46 is defined by a series of 
plateaus 49 encircling the gas exit hole 40. The plateaus 49 ensure that 
the gas membrane 65 is raised over the gas exit hole 40 and the gas 
collection area 46 to provide a sufficient surface area for the diffusion 
of gasses through the membrane 65. The gas cap 50 with the tabs thereon 
also provides structural support to the cell case 20. 
This structural support and the gas collection area provided by the gas 
vent system 15 are desired because the anode 115 tends to expand during 
discharge of the cell 10. This expansion may block the gas exit hole 40 
and can create a "wet blanket" effect that prevents the vent system 15 
from operating. The pegs 25 and the gas vent systems 15 provide structural 
support to the cell case 20 to resist the expansion of the anode 115. More 
tabs 55 can be added to the gas cap 50 depending upon the amount of 
support needed to keep the anode 115 away from the membrane 65. 
Efficiency of the cell 10 is also increased by ensuring proper draining of 
electrolyte 125 away from the vent system 15. Each of the gas caps 50 has 
a series of radial draining channels 56 to ensure that electrolyte 125 
drains away from the vent 30 when the cell 10 is moved or rotated, rather 
than collecting on the vent 30 and preventing the exhaust of hydrogen. 
Each vent system 15 may also have a pressure release valve or burp valve 70 
on the exterior surface 22 of the cell casing 20. The burp valve 70 
provides for the release of internal pressure and prevents the intrusion 
of gasses from the atmosphere. In its undeformed position, the burp valve 
70 seals the vent system 15 to prevent gasses from entering the cell 10. 
When hydrogen is generated by the cell 10, the internal pressure causes 
the thin strip of flexible material to expand such that the slit 71 opens 
to the atmosphere and allows the hydrogen to escape. Upon the release of 
the hydrogen, the pressure in the cell 10 returns to normal and the burp 
valve 70 reseals over the gas exit hole 40. Similarly, in the flapper 
valve embodiment, the escaping hydrogen gas forces the unattached end of 
the flapper valve to extend away from the exterior surface 22 of the cell 
case 20, thereby permitting the gas to escape. Upon the release of the 
hydrogen, the pressure in the cell 10 returns to normal and the flapper 
valve reseals over the gas exit hole 40. 
The burp valve 70 likewise prevents the ingress of gasses from the 
atmosphere. This feature is particularly helpful if a group of cells 10 is 
stacked into a battery pack (not shown). Oxygen and other gasses can 
corrode the anode 115 and compromise the operation and efficiency of the 
cell 10 if they are allowed to intrude into the cell casing 20. Further, 
the byproducts of the corrosion process also can clog the membrane 65. 
The ingress of gasses is a concern particularly during recharge of the cell 
10. The zinc anode 115 has a lesser volume then the zinc and zinc oxide 
present during discharge of the cell 10. This reduced volume causes 
somewhat of a vacuum effect in the interior of the cell case 20. Ambient 
air and carbon dioxide tend to enter the cell case 20 and cause discharge 
of the anode 115. More pressure is required to permit the ingress of 
gasses, however, if the surface area of the burp valve 70 on the side of 
the recess 35 is smaller than the surface area facing the atmosphere. In 
this design, the burp valve 70 therefore effectively prevents the ingress 
of gasses into the cell 10. Further, the size of the recess 35 can be 
changed so as to vary the amount of pressure required to penetrate the 
burp valve 70. 
In sum, the vent system 15 of the present invention has the novel ability 
to maintain the hermetic seal of the cell case 20 so that contaminates do 
not enter the cell 10 and so that electrolyte 125 is retained within the 
cell 10 during the exhaustion of excess hydrogen gas. The invention also 
provides structural support to the cell to resist expansion of the anode 
115. The invention further permits the release of internal pressure while 
preventing the intrusion of oxygen and other gasses into the cell case 20 
such that corrosion or undesired discharge of the zinc anode 115 is 
eliminated or greatly delayed. The invention as a whole provides three 
stages of relief: (1) liquid/gas separation, (2) pressure differential 
relief, (3) prevention of gas intrusion. These advantages are obtained 
through the use of fewer materials and simplified construction techniques 
as compared to the prior art. These advantages are accomplished in the 
preferred embodiment through the novel joining of Teflon and polypropylene 
by ultrasonic welding with a pinch back up to achieve a hermetic seal. 
The foregoing relates only to the preferred embodiments of the present 
invention, and many changes may be made therein without departing from the 
scope of the invention as defined by the following claims.