Electrochemical cell

This invention relates to an electrochemical cell having spirally wound electrodes and an electrolyte which enhances plating of the anode metal during voltage reversal. The safety of such cells is improved by concentrating the current, during voltage reversal, between an outer segment of the anode and a metal sheet connected to the cathode whereby anode metal plates only onto the metal sheet. Thus, the hazardous condition of plating anode metal onto the cathode is avoided.

This invention relates to an electrochemical cell having a spirally wound 
electrode stack and an electrolyte which enhances plating of the anode 
metal during voltage reversal. The safety of such cells is improved by 
concentrating the current, during voltage reversal, between a segment of 
the anode and a metal sheet connected to the cathode whereby anode metal 
plates primarily onto the metal sheet. Thus, the hazardous condition of 
plating anode metal onto the cathode is avoided. 
Consumers often inadvertently place fresh cells in series with partially 
discharged cells in battery operated devices. Voltage reversal of the 
partially discharged cells occurs when these cells have exhausted their 
capacity but continue to be forced discharged by the fresh cells. High 
energy density electrochemical cells, such as lithium cells, have become 
widely available for consumer use during the past decade. During voltage 
reversal of lithium cells a deposit of lithium can form on the cathode. 
Occasionally the deposit can grow to sufficient size to bridge the gap 
between the electrodes and short circuit the cell. 
The morphology of the lithium deposit is highly dependent on the components 
of the electrolyte, i.e. the electrolyte salt and solvents. Salts commonly 
used in primary lithium cells include LiCF.sub.3 SO.sub.3, LiAsF.sub.6, 
LiBF.sub.4, LiPF.sub.6, and LiClO.sub.4. Each of these salts has a 
different effect on the morphology of plated lithium. It is also true that 
the solvents used in the electrolyte will effect the morphology and, in 
fact, a reactive solvent present in sufficient amount will have a leveling 
effect and mask the differences among the salts. However, commonly used 
electrolyte formulations employ solvent combinations which are not highly 
reactive, e.g. propylene carbonate and dimethoxyethane in a 1/1 volume 
ratio. Thus, for a given non-reactive solvent formulation, it has been 
found that, of the above listed salts, LiClO.sub.4 enhances the lithium 
plating process such that a coherent, metallic deposit is formed on the 
cathode, which deposit is more plate-like than dendritic. The result of 
having a plate-like deposit is to create intimate contact between the 
plated lithium and the cathode. If a short circuit occurs between the 
anode and the cathode during voltage reversal, the intimate mixture of 
lithium on the cathode is heated and this heating can cause the mixture to 
react violently. In addition to LiClO.sub.4, LiAsF.sub.6 and LiPF.sub.6 
are also salts which enhance the formation of a coherent, metallic lithium 
deposit. 
Various designs have been used by manufacturers to protect a cell from 
hazardous conditions during voltage reversal. U.S. Pat. Nos. 4,385,101, 
4,482,615, and 4,622,277 disclose a variety of approaches to improve the 
safety of spirally wound lithium cells during voltage reversal. While 
these patents disclose effective expedients for safety when dendritic 
deposits are formed, they are not as effective for providing safety when 
plate-like deposits form.

The present invention relates to spirally wound lithium cells wherein the 
electrolyte comprises a salt, such as LiClO.sub.4, which, when used, 
results in the formation of plate-like deposits of lithium during voltage 
reversal. Generally, such cells comprise an anode, a cathode, and a 
separator spirally wound together such that said separator is between said 
anode and cathode. According to the present invention an anode tab is 
located on a section of the anode which is not fully utilized during 
discharge. An electrically conductive member is located across from the 
face of said anode section which does not bear said tab. Said member is 
electrically coupled to the cathode, such as by physical contact, and is 
insulated from said anode section by said separator. During voltage 
reversal anode metal is preferentially plated onto said sheet. 
With specific reference to the FIGS., FIG. 1 shows a cross section through 
a spirally wound electrode stack 10 wherein the cathode 12 is longer than 
the anode 14. The relative lengths of the anode 14 and cathode 12 are such 
that, when these electrodes are spirally wound together having separator 
16 located therebetween, only the outermost segment of anode 14 
(designated 18 and extending from D to E) forms a part of the outer 
circumference of the electrode stack, as shown in FIG. 1. The remainder of 
the circumference of the electrode stack is formed by an outermost segment 
of cathode 14 (designated 19), whereby the outermost segment of anode is 
shorter than the outermost segment of cathode. A metal tab 20 is attached 
to the inner surface of anode segment 18. This tab functions as the 
electrical contact between the anode and the negative terminal of the 
cell. In accordance with the present invention, an electrically conductive 
member 22 is located along the circumference of the spirally wound 
electrode stack such that the member is in mechanical and electrical 
contact with cathode outer segment 19. It is important that a section of 
separator 16 is located between segment 18 of the anode and member 22, as 
shown, to prevent a short circuit therebetween. An insulating means 24, 
discussed more fully below, is located between the inner surface of anode 
segment 18 and the adjacent cathode surface. The function of means 24 is 
to provide an ion impermeable or high resistance barrier between anode 
segment 18 and the adjacent cathode so that the anode metal cannot plate 
onto that section of cathode. As discussed more fully below, during forced 
discharge or voltage reversal, a deposit of plate-like lithium 
preferentially forms on the surface of member 22 facing anode segment 18. 
Should a short circuit occur because such plate-like deposit contacts 
anode segment 18, the short circuit safely shunts the forced discharge 
current through the cell without causing large negative voltages in the 
reversed cell. 
In order to ensure the effectiveness of the present invention it is 
preferred that anode tab 20 and member 22 are associated with a segment of 
the anode which is not fully utilized during discharge. The "extra" amount 
of anode material is needed in order to effectively create a short circuit 
between member 22 and the anode segment opposite thereto. While extra 
anode material is required, it is preferred that the outer segment 18 not 
exceed about 10% of the total anode length. Anode material in excess of 
this amount is not needed to create a short circuit and thus would occupy 
space in the cell which could otherwise be occupied by cathode material 
which would be utilized during discharge. 
Referring again to FIG. 1, the anode 14 can be described as comprising four 
consecutive regions (A-B, B-C, C-D, D-E) which differ from each other by 
the rate at which they are consumed during discharge. The rate at which 
these regions are discharged is related to the amount of cathode material 
which "sandwiches" these regions of anode. The discharge rate in turn 
determines the amount of anode material which is utilized. Beginning at 
the inner end, A, of the anode 14, and moving outwardly along the anode 
length, the first region is defined by the length A-B. This length of 
anode has a segment of cathode across from its inner surface which cathode 
segment does not have any anode juxtaposed with its other side. Thus, the 
cathode material in this segment is discharged only by the anode segment 
A-B. The rate at which anode segment A-B is consumed is greater than the 
rate of discharge in the next anode length B-C because anode length B-C is 
sandwiched on both sides by a cathode segment which itself has anode on 
both sides. Thus, the anode length B-C discharges less cathode material 
per unit length than the length A-B and as a result length B-C is consumed 
at a lower rate during discharge. 
The next anode length moving outwardly along the anode spiral is C-D. This 
length has similar discharge characteristics as length A-B since this 
length has an adjacent cathode segment which does not have anode on both 
sides. Thus, anode length C-D is discharged at a higher rate than length 
B-C and will be consumed before length B-C during discharge. The fourth 
anode length is D-E (also designated as anode segment 18). This length is 
discharged at the lowest rate of the four regions because D-E has cathode 
across from only the inner surface. Therefore, anode length D-E is 
consumed at the slowest rate during discharge. 
The four anode regions will be consumed in the following order during 
discharge. Regions A-B and C-D are consumed first because they are 
discharged at the highest rate. Region B-C is discharged at an 
intermediate rate and is consumed to a lesser degree than A-B or C-D. 
Region D-E is consumed the least of all four regions because it is 
discharged at the slowest rate. 
The location of anode tab 20 can be on either segment B-C or D-E, which 
sections have sufficient lithium remaining at the end of discharge to 
create a short circuit. However, with the specific design shown in FIG. 1 
it is preferred to locate tab 20 on section D-E because this section has 
the greater amount of lithium remaining at the end of discharge. The end 
of discharge is reached as the cell is forced discharged when section C-D 
is virtually consumed and section B-C becomes electrically disconnected 
from region D-E where the tab is located. At this point section D-E cannot 
sustain a positive voltage at a high current density and voltage reversal 
occurs. 
The present invention operates in conjunction with the above described 
phenomena as follows with reference to FIG. 1. In a preferred embodiment a 
conductive member 22, comprising a metal foil is located along the 
circumference of the spirally wound electrode stack such that the metal 
foil is in contact with the cathode. The metal foil 22 also extends over 
the entire outer surface of anode segment 18 and slightly beyond the outer 
edge thereof. It is necessary that the metal foil 22 be held at the 
cathode potential in order that lithium will plate thereto during voltage 
reversal should such occur. Should such a cell be force discharged beyond 
its capacity the only lithium connected to tab 20 would, for the reasons 
set forth above, be segment 18. If the cell is forced into voltage 
reversal lithium will begin to form plate-like deposits on that portion of 
metal foil 22 which is across from anode segment 18. As discussed below, 
insulating means 24 prevents lithium from plating in the opposite 
direction onto the cathode such that lithium can only plate onto the metal 
foil 22. If this deposit grows to sufficient thickness to contact anode 
segment 18 a short circuit occurs. This short circuit safely shunts the 
forced discharge current through the cell without causing any hazardous 
conditions which could otherwise occur. 
In a preferred embodiment insulating means 24 is a piece of ion impermeable 
tape having a polyester backing and an acrylate adhesive. The adhesive can 
be omitted whereby the polyester film is held in place by pressure between 
the electrodes. However, the use of an adhesive simplifies the manufacture 
by holding the film in place until the electrode stack is fully wound. The 
dimensions of the tape should be such so as to cover a substantial portion 
of surface of anode segment 18 which faces the cathode. Unexpectedly, it 
has been discovered that means 24 does not need to cover the entire inner 
surface of anode segment 18, however means 24 should cover at least about 
66%, and preferably at least about 80% of the inner surface of said anode 
segment. Thus, when the cell is driven into voltage reversal the lithium 
cannot plate through the ion impermeable tape to the cathode. It is 
thereby ensured that lithium will plate onto the metal foil 22. In another 
embodiment means 24 is a piece of material, such as a non-woven 
polypropylene mat having a greater weight density per unit length than the 
separator 16. The greater weight density per unit length will insure that 
means 24 is a more resistive to plate lithium through than the separator 
16. The greater resistance will ensure that lithium will preferentially 
plate through the lower resistance pathway to the metal foil 22. 
According to the present invention, conductive member 22 and insulating 
means 24 are essential even when insulating means 24 covers the entire 
inner surface of anode segment 18. In this latter situation, one might 
expect that lithium would be totally blocked from plating to the cathode 
and that conductive member 22 would not be necessary. However, plating of 
lithium onto the cathode would still occur because current passes through 
the lithium at the edge of the tape. When metal foil 22 is present, it 
acts as a counter electrode and lithium preferably plates thereto. 
With the spirally wound cell design shown in FIG. 1, it is desirable to 
include a cathode edge protector 26. Edge protector 26 is placed between 
the outermost edge of cathode 12 and the lithium anode 14 lying 
immediately behind the cathode edge in order to protect against sharp 
points on the edge of the cathode from piercing through separator 16 and 
causing a short to anode 14. In order for the present invention to operate 
properly, protector 26 should be made from a highly porous material so 
that plating to metal foil 22 is not hampered. Suitable materials include 
non-woven fabrics made from polyolefins such as polypropylene or 
polyethylene. Polypropylene is the preferred material. 
The features and advantages of the present invention are demonstrated in 
the following examples. 
COMATIVE EXAMPLE A 
Four 2/3A size lithium/manganese dioxide cells are built having a lithium 
foil anode, a manganese dioxide cathode, and a microporous polypropylene 
separator spirally wound together with the separator between the anode and 
cathode. The lithium anode is 8.7 inches long, 0.9 inch wide, and 0.007 
inch thick. The manganese dioxide cathode is 9.3 inches long, 1 inch wide, 
and 0.015 inch thick. The anode and cathode are spirally wound together 
with a 1 mil microporous polypropylene separator therebetween such that 
about 0.6 inch of anode lies along the outer circumference of the spirally 
wound electrode stack. A metal anode tab comprising nickel is located on 
the inner surface of this outer segment of anode. A piece of adhesive tape 
comprising a Mylar backing and an acrylate adhesive and about 0.4 inches 
long is applied over the metal anode tab onto the lithium surface. The 
cells are filled with an electrolyte comprising 0.65 molar LiClO.sub.4 in 
a mixture of propylene carbonate and dioxolane. Each cell has an open 
circuit voltage of about 3.2V and has a capacity of about 1.4 A-Hr to a 2 
volt cutoff under a 100 ohm load. 
One of the cells is discharged by about 40% of its original capacity. This 
cell is then connected in series with the other three undischarged cells. 
This arrangement simulates the situation where a consumer connects a 
partially discharged cell with fresh cells. A 6 ohm resistor is used to 
discharge the four cells connected in series. FIG. 2 shows the voltage and 
temperature characteristics of the partially discharged cell. The figure 
shows that within the first hour the voltage of the partially discharged 
cell is driven below zero volts. Lithium plates onto the cathode as long 
as the cell voltage remains below zero. After about one hour the cell 
temperature reaches a peak and begins to fall because the current being 
supplied by the three "driver" cells begins to fall. Slightly after two 
hours a short circuit occurs, said short being caused by plated lithium 
which makes contact between the anode and cathode. The short is evidenced 
by the voltage of the cell falling to zero. A current surge results from 
the short and causes tremendous heating. The cell temperature goes off 
scale in the FIG. but is measured to be about 442.degree. C. This 
temperature is the result of hazardous reactions between the chemicals in 
the cell causing thermal runaway thereof. 
EXAMPLE 1 
Three 2/3A size lithium/manganese dioxide cells are built identically to 
the above described cells. 
A fourth cell is built identically except that, in accordance with the 
present invention, a piece if aluminum foil 1 inch wide and 1 mil thick is 
wrapped around the circumference of the spirally wound electrodes. The 
aluminum foil is held apart from contacting the outer anode segment by the 
separator. A wrapping of separator holds the foil in place prior to 
insertion into the cell can. This cell is discharged to 50% of its 
original capacity and is then connected in series with the three 
undischarged cells. 
The four cells are then discharged through a 6 ohm resistor. FIG. 3 shows 
the temperature and voltage characteristics of the cell made in accordance 
with the present invention. The cell temperature rises to about 95.degree. 
C. but the cell voltage is not driven to large negative values as in the 
previous example. Rather, the voltage does not go below about -1 volt. The 
current is safely shunted through the cell by the plate-like lithium 
deposit on the metal foil as described above. 
While the above example described the metal foil as extending along the 
entire circumference of the spirally wound electrode stack, the length of 
the metal foil could be less. The minimum length is that length which 
would both cover the outer anode segment and contact a sufficient length 
of cathode to ensure good electrical contact between the foil and the 
cathode. It is preferred that the metal foil extend from between about 50% 
to 100% of the circumference of the spirally wound electrodes. 
The metal foil is preferably aluminum, but it can be comprised of metals 
other than aluminum. The only requirement of the metal is that it be 
compatible with the cell environment. Other suitable metals include 
titanium, tantalum, niobium, stainless steel, nickel, and those metals 
which can alloy with lithium such as aluminum. 
The metal foil should be sufficiently thick so that it can be handled 
easily. However, it should not be so thick as to occupy space which could 
otherwise be occupied by active materials. It is preferred the foil 
thickness be between about 0.5 and 5 mils. 
In addition to using a metal foil, it could be desirable to use a laminate 
of a metal foil and an adhesive tape, as shown in FIG. 4. In such case, 
the laminate should have an adhesive tape layer 28 which extends beyond 
the metal foil layer 22 so that the adhesive can be used to hold the foil 
in place. The foil thickness can be very thin (i.e. less than 0.5 mil) if 
a laminate is used because the tape backing provides the needed mechanical 
strength. Both the tape backing and the adhesive must, of course, be 
compatible with the cell environment. 
The above examples described the use of an insulating means comprised of an 
adhesive tape having a Mylar backing and an acrylate adhesive. Other 
suitable backings include polyester, vinyl, cellophane, ultra high 
molecular weight polyethylene, and ultra high molecular weight 
polypropylene. Other suitable adhesives include silicone and rubber based 
adhesives. 
An alternative to the design described in the examples would be to have the 
anode tab and metal foil arrangement located inside the electrode stack 
and associated with the section of the anode corresponding to section B-C 
in FIG. 2. The actual location of the anode tab and the metal foil will 
depend on the particular configuration of the spirally wound electrodes. 
For a given configuration, the location of the anode tab should be on a 
section of the anode which remains at the end of discharge. The metal 
foil, electrically coupled to the cathode, is then placed opposite to said 
section of anode in the manner described above. 
In the most commonly used spirally wound cell designs the cell can 
functions as the negative terminal, i.e. "can negative" and the cell cover 
functions as the positive terminal. Connection of the electrodes to the 
terminals is achieved by a variety of methods well known in the art. With 
the "can negative" design the outer surface of the cathode and the metal 
foil must be covered by a layer or layers of separator to prevent short 
circuit to the cell can which is connected to the anode. However, if a 
cell design had the cell can as the positive terminal, i.e. "can 
positive", and the cell top as the negative terminal it would not be 
necessary to interpose separator between the cathode and the cell can. In 
this embodiment the cell can would function as the metal foil in the "can 
negative" design and a separate piece of metal foil would not be 
necessary. During voltage reversal lithium would plate from the outer 
segment of the anode to the cell can and the same result would be achieved 
as with the metal foil in the "can negative" design. 
As previously noted, the present invention is most effective in cells 
having electrolytes comprising salts which give rise to a plate-like 
deposit of lithium. While the above description has referred specifically 
to electrolytes containing LiClO.sub.4, other salts which give plate-like 
deposits such as LiAsF.sub.6 and LiPF.sub.6 are useful in accordance with 
the present invention. 
The specific example described the presently disclosed invention as used in 
a lithium/manganese dioxide cell. However, the invention is broadly useful 
in conjunction with any solid cathode. Classes of suitable cathodes 
include metal oxides, carbon fluorides, metal sulfides, transition metal 
polysulfides, metal halides such as CFx, V.sub.2 O.sub.5, WO.sub.3, 
MoO.sub.3, MoS.sub.2, lead oxides, cobalt oxides, copper oxides, CuS, 
CuS.sub.2, iron sulfides, NiS, Ag.sub.2 CrO.sub.4, Ag.sub.3 PO.sub.4, 
TiS.sub.2, and mixtures thereof. The present invention could also be used 
in cells having anodes other than lithium. Suitable anode materials 
include alkali and alkaline earth metals such as lithium, sodium, 
potassium, calcium, magnesium, aluminum and alloys thereof. 
The previous examples are intended to be illustrative of the presently 
disclosed invention. It is to be understood that deviations can be made 
but still remain within the scope of the presently disclosed invention.