Highly resistive cell separator for bi-polar battery

Material is incorporated in each of the cell separator membranes of an electrochemical cell having a layered construction wherein a plurality of commonly connected metal anode layers and a plurality of commonly connected cathode layers are arranged in a closely packed, stacked sandwich construction, separated by a plurality of thin insulating separator membranes which gives each cell separator a known resistance value which can be measured thereby allowing total cell resistance to be readily tested.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed generally to non-aqueous electrochemical 
cells or batteries having a plurality of sandwiched electrode layers and, 
more particularly, to an improved cell separator construction which allows 
reliable testing with respect to locating high or low resistance shorts 
which may develop in the cells during cell construction. 
2. Description of the Related Art 
Bipolar, multi-electrode layer cells of cylindrical construction are 
typical electrochemical cells of the class of interest in the present 
invention. They include a plurality of stacked battery components exposed 
to an electrolyte solution within the elongated housing of the cell which 
may be of metal or of a polymeric or plastic or prismatic material and is 
normally cylindrical in shape. The battery components are arranged in a 
concentric sandwich construction within the housing in which a plurality 
of metal anode structure layers and a plurality of carbon or other cathode 
electrode structures are arranged alternately with the electrode 
structures spaced by a plurality of separator membranes. The plurality of 
metal anode structures, each of which may be a lithium grid pressed into a 
nickel foil backing, generally employed within the cell may be arranged to 
be joined in parallel and in direct physical contact with the internal 
wall of the housing to form one pole of the bipolar cell. The plurality 
cathode structures, which may be carbon electrodes pressed into nickel 
grid cathode collector plates, are likewise joined to a second lead which 
is conducted outside the cell through an insulated connection to form a 
pin terminal on the top of the battery. A plurality of anode and cathode 
layers are normally pressed together in stacked fashion to form an 
extremely thin, high energy density composite cell. Because of the dense 
nature of the o cell construction and the relatively large area between 
the plurality of sets of oppositely charged cell plates, it is important 
that the integrity of the separators be consistent throughout to prevent 
internal shorts from robbing needed battery power. 
In this regard, it has heretofore been somewhat difficult to ascertain the 
integrity of typical bipolar battery construction in the form of locating 
high or low resistance shorts which might occur between layers of the 
stacked electrodes. The present method involves the monitoring of known or 
presumed capacitance values between cells to determine if imperfections 
exist. The primary problem with this method, however, is that cell 
capacitance tends to vary over a wide range due to the variation in 
distance between anode and cathode in the stacked cells due to variation 
in the effective cell compression. This, of course, makes the method 
conductive fibers which may be unreliable. Another method is to attach a 
small lead wire to each collector plate to measure internal cell 
resistance. This approach is not always possible due to space limitations 
within the stacked construction together with increased intercell leakage 
due to a plurality of exposed lead wires. Thus, there remains a definite 
need to provide a simple yet reliable method of monitoring the quality 
control in multi-layer, stacked, bipolar battery construction. 
SUMMARY OF THE INVENTION 
By means of the present invention, problems associated with monitoring the 
quality control in the construction of bipolar multi-layer batteries with 
respect to determining the presence of high or low resistance shorts is 
solved by the provision of a simple yet reliable method and apparatus for 
checking cell resistance. This is accomplished by making the cell 
separators slightly conductive but having very high resistance values, on 
the order of megohms. Whereas, variations in the high resistance can 
readily be measured to monitor the integrity of cell construction, the 
very high resistance values utilized add but a very small amount of load 
current per cell, i.e., in the order of microamps, thereby detracting very 
little from cell performance. 
The invention may be accomplished using any of several embodiments to 
produce cell separators of known high resistance. In one embodiment, 
conductive fibers, which may be carbon fibers, are admixed in a 
ceramic-type thin membrane separator medium capable of compression into 
sandwiched construction. In another embodiment, a normally non-conducting 
separator is provided with a conductive spot of measured resistance. This 
method allows the cell to have a measured resistance as opposed to an 
infinite resistance as applied, for example, to a dry reserve cell. During 
construction of the cell, the total resistance can be measured simply 
between the top cell collector plate and the bottom cell lead. This value 
will be equal to the addition of resistances in the stack. If a short is 
present during construction, it can easily be determined by comparing 
measured resistance to the calculated resistance.

DETAILED DESCRIPTION 
The accompanying figures are deemed to be illustrative of typical 
embodiments of the invention and the cross-sectional elevational view of 
FIG. 1 shows a typical construction which can be used with active or 
reserve cells. The cell or battery is depicted generally by 10 and 
includes a plurality of individual stacked parallel connected subcells or 
cell portions as at 11 compressed within a housing having side walls 12 
and a bottom member 13 spaced from the bottom cell by a separator 15. The 
individual layers making up the subcells are better illustrated in FIGS. 
2A and 2B. Thus, each includes an anode member 20, which is normally a 
grid structure of active alkali metal such as lithium, and which is, in 
turn, pressed into a nickel collector member 22, which may be a thin foil, 
or the like. The anode/collector combination is separated from a carbon or 
other cathode member 24 by a separator membrane, typically of a ceramic or 
paper material, shown at 26. The cathode, in turn, is carried on a metal 
grid cathode collector which is typically a nickel grid mesh indicated by 
28. 
The thin membrane ceramic or paper separator 26, in FIG. 2A, is provided 
with an amount of fibrous conductive materials such as carbon fibers shown 
as at 30 which impart a slight conductance across the separator membrane 
which, in turn, is translated into a known separator resistance value. In 
the embodiment of FIG. 2B, the conductance or, conversely, the resistance 
value of the separator is determined from a spot or small defined area of 
known conductance such as that depicted at 32. 
FIG. 3 denotes a typical equivalent circuit for measuring the resistance 
across the plurality of separator membranes through the multi-layer cell 
construction. Thus, the total measurable resistance through the total cell 
can be measured from A to N very accurately in a well-known manner and, in 
this way, even small shorts within the system can be measured and, if 
necessary, pinpointed by measuring the resistance across each of the cell 
segment configurations or separators. 
The invention has been described herein in considerable detail in order to 
comply with the Patent Statutes and to provide those skilled in the art 
with the information needed to apply the novel principles and to construct 
and use such specialized components as are required. However, it to be 
understood that the invention can be carried out using specifically 
different materials, equipment and devices and that various modifications 
can be accomplished without departing from the scope of the invention 
itself.