Cell gelling agent

A cell having an anode comprised of an anode metal gelled with a hydrolyzed polyacrylonitrile gelling agent.

This invention relates to gelled anodes for electrochemical cells and 
particularly to alkaline cells having zinc anodes. 
In order to maintain homogeneity within the anode of alkaline cells whereby 
anodic materials such as amalgamated zinc powders are kept uniformly 
dispersed for maximum electrochemical activity, such anodes are kept in a 
gelled state. Common gelling materials utilized in commercial cells 
include carboxymethylcellulose (CMC), starch graft copolymers such as 
Waterlock A-221 from Grain Processing Corporation, and polyacrylic acid 
such as Carbopol from B. F. Goodrich Co. Other gelling materials include 
cross-linked polyacrylamides such as described in U.S. Pat. No. 3,884,721, 
and cross linked CMC as described in U.S. Pat. No. 4,435,488. 
The efficacy of the gelling materials in electrochemical cells such as 
alkaline cells is determined by studies of cell performance factors, 
gassing, long term storage, ease of handling during manufacturing and of 
course compatability with the cell components. The aforementioned prior 
art gelling materials having exhibited varying degrees of utility with 
respect to the enumerated criteria. Thus, for example, while both the 
starch graft copolymers and the polyacrylic acid provide improved 
characteristics relative to the previously used CMC, improvements remain 
to be made in extending the storage like of anodes made with the starch 
graft copolymer and the handling of anodes made with the polyacrylic acid. 
It is an object of the present invention to provide a novel gelling agent 
for use in making gelled anodes with improved storage life, stability and 
handling. This and other objectives, features and advantages of the 
present invention will become more evident from the following discussion. 
Generally, the present invention comprises a method of making gelled anodes 
for alkaline cells with the use of hydrolyzed polyacrylonitrile 
(particularly alkali hydrolyzed) as a gelling agent, the anodes so formed 
and the cells containing such anodes. 
Polyacrylonitrile having the repeating units: 
##STR1## 
when hydrolyzed with alkali materials at elevated temperatures such as 
with treatment of concentrated KOH at 80.degree. C. forms a polymeric 
structure: 
##STR2## 
with acrylate, amide, and nitrile groups with some degree of cross linking 
as shown. The hydrolyzed polyacrylonitrile is comprised of various short 
and long chain branches with various degrees of cross linking and 
solubilities generally related to the hydrolysis parameters. An example of 
a commercially available alkali hydrolyzed polyacrylonitrile is Waterlock 
A-400 from Grain Processing Corporation. 
Materials such as the aforementioned Waterlock A-221 are comprised of 
starch backbones having polyacrylonitrile grafted thereto which grafted 
materials are thereafter hydrolyzed. Such materials have been considered 
to be highly absorbent with high gel strengths. It has however been 
discovered that elimination of the starch backbone and the use of the 
hydrolyzed polyacrylonitrile without such starch backbone in 
electrochemical cells provides an unexpected advantage of improved 
performance after periods of storage. 
The gelled anodes of the present invention are made in accordance with the 
prior art practice of either pregelling the anode in the form of a slurry 
and thereafter dispensing the gel into the cells or forming the gel in 
situ. In the former instance, the hydrolyzed polyacrylonitrile is admixed 
with an active anode material such as powdered zinc and a controlled 
amount of the cell electrolyte which is generally an alkaline 30-40% KOH 
aqueous solution. In the in situ processing, the anodic material and the 
hydrolyzed polyacrylonitrile are mixed and dispensed into the cell 
container in the dry state and then activated into a gel by the presence 
of the cell electrolyte. Lubricants and additives such as glycerine, 
polyhydric alcohols, mineral oil and the like to facilitate handling and 
processing, may be additionally added to the anode mixture. 
The amount of hydrolyzed polyacrylonitrile utilized in the gelled anode may 
range from 0.6 to 1.5% with a preferred range of 0.8 to 1.2% and a 
preferred amount of about 0.9%. This compares favorably with other gelling 
agents which require substantially greater amounts of gelling agents 
usually in the area of about 2-3% by weight. 
The hydrolyzed polyacrylonitrile may be utilized either alone as the sole 
gelling agent or in admixture with other gelling agents such as starch 
graft copolymers, CMC or polyacrylic acid with varying degrees of 
effectiveness. 
In the present invention, the anode is a gelled mixture of the electrolyte 
solution and a metal in a particulate or porous form. The metal useful in 
the anode of the present invention can be any metal generally used in 
cells having an aqueous electrolyte. Such metals can include aluminum, 
cadmium, calcium, copper, indium, iron, lead, magnesium, manganese, 
mercury, nickel, tin, zinc and other metals well known in the art, used 
either alone or in alloys, amalgamations and admixtures. The anode metal 
can be used in the cell as a powder, as granules or in any other 
particulate form. 
In the preferred cell, the anode metal comprises powdered amalgamated zinc. 
Powdered metals provide the largest exposure of anode surface area to the 
electrolyte. Further, the finer the anode metal powder, the greater the 
ability of the gel to retain the particles uniformly throughout the gel, 
which acts to maintain the exposure of the anode metal to the electrolyte. 
The particle size of the preferred anode metal powder is of the order of 
from 0.03 to 0.9 millimeter in diameter. The most preferred size of powder 
to be used depends on many factors and it can be readily determined by one 
skilled in the art. 
The electrolyte solutions which can be gelled by the agents of the present 
invention, include all aqueous electrolyte solutions usable in 
electrochemical cells. In the preferred embodiment of the present 
invention alkaline electrolyte solutions are employed. These include, but 
are not limited to, hydroxides of alkali and alkaline earth metals. Sodium 
and/or potassium hydroxide are the most commonly used alkaline 
electrolytes. 
The hydrolyzed polyacrylonitrile gelling agent of the present invention can 
be used with all cathodes heretofore useful in aqueous electrochemical 
cells. These cathodes include, but are not limited to metal oxides, such 
as cadmium oxide and hydroxide, mercuric oxide, lead oxide, manganese 
dioxide, nickel oxide and hydroxide, silver oxide and air. 
In order to more fully illustrate the efficacy of the present invention the 
following comparative examples are presented. It is understood, however, 
that such examples are illustrative in nature and any enumeration of 
detail therein should not be construed as limitations on the present 
invention. Unless otherwise indicated, all parts are parts by weight.

EXAMPLE 1 
Amalgamated zinc (6.5% Hg), 35% KOH solution, and hydrolyzed 
polyacrylonitrile gelling agent (Waterlock A-400) are admixed and formed 
into a slurry, with the relative ratios being 1228:754:18 (about 0.9% 
gelling agent). About 4.75 grams of the the slurry mixture are dispensed 
into each of five alkaline AA size cells as the anode thereof (Cell nos. 
1-5). The cells are filled with 1.0 gram of the 35% KOH solution as 
electrolyte and an MnO.sub.2 cathode with the cells being anode limited. 
The cells are stored for different periods of time and under different 
temperature conditions and are then discharged with a continuous load of 
3.9 ohms with the results being given in the Table below. 
EXAMPLE 2 
(PRIOR ART) 
Five cells (6-10) are made as in Example 1 but with a starch graft 
copolymer (Water-Lock 221) as the gelling agent present in about 1% by 
weight. The cells are stored under the same conditions and are then 
discharged with the same continuous load with the results being given in 
the Table below. 
TABLE 
______________________________________ 
Storage 
Cell Condi- OCV Hours to % Zn 
# tion volts 1.0 v 0.8 v 0.65 v 
Utilization 
______________________________________ 
1 Fresh 1.564 3.32 5.04 5.20 62.0 
2 130.degree. F., 
1.567 3.24 4.69 4.76 57.1 
1 wk 
3 130.degree. F., 
1.564 3.21 4.59 4.65 55.8 
2 wks 
4 130.degree. F., 
1.558 2.97 4.33 4.36 52.2 
4 wks 
5 160.degree. F., 
1.563 2.86 4.33 4.37 51.8 
1 wk 
6(PA) 
Fresh 1.562 3.46 5.01 5.15 62.0 
7(PA) 
130.degree. F., 
1.555 3.28 4.51 4.55 55.3 
1 wk 
8(PA) 
130.degree. F., 
1.545 3.03 4.26 4.29 51.8 
2 wks 
9(PA) 
130.degree. F., 
1.539 2.94 4.11 4.13 50.0 
4 wks 
10(PA) 
160.degree. F., 
1.543 2.78 4.12 4.15 49.1 
1 wk 
______________________________________ 
In view of the above examples it is evident that the hydrolyzed 
polyacrylonitrile gelling agent of the present invention provides a 
significant increase of cell capacity in cells which are discharged after 
high temperature storage when compared to cells containing starch graft 
copolymer gelling agents. 
The hydrolyzed polyacrylonitrile gelling agent of the present invention 
provides similar discharge characteristics when compared to the prior art 
polyacrylic acid gelling agents. However, the hydrolyzed polyacrylonitrile 
provides several physical advantages which make it more suitable for 
manufacturing processes. The hydrolyzed polyacrylonitrile is not adhesive 
in nature and will not detrimentally stick to machinery. Additionally, it 
does not foam as does the polyacrylic acid and therefore provides a more 
stable higher density gel. 
It is understood that the above examples were presented for illustrative 
purposes and that changes in cell components and relative ratios of 
components may be made without departing from the scope of the present 
invention as defined in the following claims.