Method of manufacturing plastic bonded battery plates having controlled porosity

A method of manufacturing plastic bonded electrodes and battery plates, having a preselected porosity. The electrodes and battery plates are adapted for use in electro-chemical power sources. A loose active mass is mixed with a plastic material and a plasticizing and/or pore forming agent. The mixture is homogenized and thereafter rolled onto a metallic current collector so that a plastic skeleton is formed thereon or therein. The active mass then reacts with an electrolyte so as to act as a two-phase electrode.

BACKGROUND OF THE INVENTION 
The invention concerns a manufacturing method of plastic bonded electrodes 
for electrochemical power sources, such as batteries, wherein the plates 
have a controlled porosity. The manufacturing method includes a rolling 
step. 
The rolling method of the invention is suitable for the manufacture of 
porous electrodes of batteries, especially for cadmium electrodes for 
alkaline batteries. 
In the known methods for manufacturing such electrodes and plates, the 
active mass for the electrode is either enclosed in a perforated steel 
pocket or it is deposited in pores of a porous nickel body. In the first 
case the starting products for the manufacturing method is a prepressed 
longish rectangular briquette which is prepared by pressing the active 
mass; such briquettes are subsequently enclosed in perforated steel 
pockets that are thereafter coupled by welding to assume the form of 
electrododes of a desired shape. In the second case a porous nickel body, 
prepared by sintering a nickel powder, is several times impregnated by 
salt solutions of the electrochemically active metals which are 
subsequently precipitated in the form of hydroxides or oxides. 
Further methods for preparing electrodes of the aforedescribed types are 
known from the literature. For example, in the method described in U.S. 
Pat. No. 3,009,980 (Nov. 21, 1961) a layer of a pasty active mass is 
deposited on the current collector by dipping; the liquid is removed by 
drying and the resulting semi-finished product is further treated by 
rolling. 
A high labor consumption and the difficulty in attaining an optimum porous 
structure due to the use of a pasty active mass constitute important 
disadvantages of this known method. 
A further known method (described in German Patent No. 1,187,698) for 
preparing plastic bonded electrodes, uses an electrochemically active 
material and two different resins. One of the resins serves as a pore 
forming agent which is removed by dissolving it after the electrode has 
been formed. 
The main disadvantages of the last described known manufacturing method are 
the large labor consumption which is required and the necessity of 
extracting one resin from the electrode body. 
The afore-described manufacturing methods, as described in the literature 
of the state of the art, are therefore not used in the mass production of 
electrodes of the aforedescribed type. 
SUMMARY OF THE INVENTION 
It is a general object of this invention to provide a manufacturing method 
for plastic bonded electrodes of the aforedescribed type wherein the 
disadvantages of the manufacturing methods of the state of the art are 
eliminated or at least minimized. In the manufacturing method of this 
invention a loose mixture of an active mass, plastics, a plasticizing 
agent or also a pore forming agent is homogenized. The resulting material 
is then rolled on a collector. 
Under the term "pore forming agent" a substance is to be understood which, 
during the electrode manufacturing process, fills up the space of future 
pores and which is to be removed by evaporation, sublimation, dissolving, 
a chemical reaction, or in any other suitable way, after the electrode 
body has been prepared. Such pore forming agent may be either a solid or 
liquid substance such as, for example, oxalic acid, potassium chloride, 
ammonium chloride, ammonium oxalate, ethanol, benzene, toluene, or the 
like. 
The term "plasticizing agent" implies a liquid substance which is designed 
for wetting the surface of plastic particles and, simultaneously, for 
reducing the friction of the plastic or other particles of the active 
mass. Consequently, a non-coherent active mass is capable of flowing or 
creeping if exposed to a pressure during the rolling process. 
As wetting agents, all the organic solvents which are capable to wet 
polytetrafluoroethylene, such as, for instance, ethanol, petroleum, 
cyclohexanone, etc., are suitable to this purpose. Preferably, it is 
possible to combine the functions of a plasticizing agent and of a pore 
forming agent by using a liquid plasticizing agent which, on the one hand, 
improves the creep of the mixture to be processed in the rolling process 
and which, on the other hand, assumes the function of a pore forming 
agent. The added liquid acts as a pore forming agent; the amount of added 
liquid controls the porosity of the rolled material over broad limits and 
this liquid may be removed by a simple evaporation step at the end of 
rolling operation. The transfer of the specially prepared mass by rolling 
onto the current collector is carried out, for instance, so that one of 
the edges of the collector remains exposed and uncovered by the active 
layer and thus serves for making an electric connection. 
The method of the invention is based on the knowledge that a loose mass 
exhibits a high flowability or ability to creep, which is required for the 
deposition at room temperature. This creeping or flowing action is 
obtained from a preparation of a mixture of pulverized electro-chemically 
active materials and materials suitable to be deposited by rolling 
procedure. The materials are wetted by a liquid plasticizing agent so that 
the mixture keeps its loose condition without, however, losing its 
capability of flowing when exposed to a pressure during the rolling 
process. 
The plasticity of the mixture and consequently the rolling process is 
positively influenced by the presence of particles of bonding material 
exhibiting a suitable shape and size. The creep of the material is 
desirable especially for the preparation of an electrode having an active 
mass rolled onto the current collector which normally would prevent the 
creep of a pulverized material. By utilizing the afore-mentioned 
properties of materials an electrode having a uniform porosity and 
thickness can be prepared. Thus a favorable porous structure of an 
electrode made of an active material with a broad spectrum of grain size 
is obtained by the aforementioned method where the material is reinforced 
by the dispersed plastics so that it forms a solid skeleton of the 
electrode. 
The manufacturing method for electrodes according to the invention, uses 
furthermore the recognized property that by addition of various amounts of 
liquid agents the final porosity of the electrode may be regulated, so 
that after the transferring the material by rolling, the liquid agent is 
removed in a simple way (for instance by evaporation). The thus obtained 
electrode functions in an alkaline electrolyte as a two-phase electrode 
where at the same time the contact area of both phases, namely the liquid 
electrolyte and the solid electrochemically active material, may be 
optimized by means of the controlled porosity. 
It is possible to form an optimum porous structure by adding to the initial 
mixture a further pore forming agent whose function consists in that 
during the preparation of the electrode it occupies the volume of the 
future pores. The pore forming agent is removed for instance by 
evaporation, dissolving, thermal decomposition, chemical or 
electrochemical reaction in the cell during the manufacture or the putting 
into operation of electrodes. 
The rolling of the electrodes may be also performed between heated rollers; 
or it is possible to perform the rolling in two stages: in the first stage 
the rolling is carried out between unheated rollers, and in the second 
stage the rolling is carried out between heated rollers. A melting down of 
a part of the plastic, a more stable bonding with the active mass, and the 
sintering together of the particles is effected by heating the mass to a 
suitable predetermined temperature. 
The advantage obtained by the method for manufacturing electrodes of this 
invention resides in the considerable saving of labor consumption and the 
optimalized porous structure obtained by the method. The electrodes made 
in accordance with the method of this invention have a practically time 
constant capacity; may be made with a reduced amount of the active mass; 
the active mass is consumed with greater efficiency; the electrodes can 
absorb a higher current load. These circumstances are conditioned by an 
increase of the contact area of the porous electrode with a liquid 
electrolyte. 
The electrodes according to the invention are therefore especially suitable 
for electrochemical power sources having a large specific energy. 
In order that the method of the invention may be better understood and 
carried into practice, some preferred embodiments thereof will now be 
described with reference to the accompanying drawings which, however, are 
not intended to limit in any way the invention scope.

DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 illustrates the sequence of the various steps of the method of the 
invention. 
FIG. 2 illustrates schematically an electrode or battery plate 4, which 
comprises a current collector grid 2 made of a Fe based stretch metal. The 
grid 2 is covered with an active mass 3 shown mirror-reversed. A tab 1 is 
welded to the grid 2. 
The advantages of the manufacturing method will be best shown in practical 
examples of using the invention; however, the invention is in no way 
limited to the examples. 
EXAMPLE NO. 1 
4.5 parts of a pulverized active cadmium mass is thoroughly mixed in a 
disintegrator with 0.5 parts of pulverized polytetrafluoro ethylene of a 
suitable type; then one part of the ethyl alcohol is added during constant 
stirring. The resulting mixture is freely deposited on the current 
collector made of an iron stretch metal whose thickness and size of the 
mesh is regulated by the desired thickness of the electrode. The mixture 
deposited on the collector passes between two rotating parallel rollers 
adjusted to the desired thickness of the electrode. The resulting strip is 
exposed to the air and following the evaporation of the liquid agent it is 
cut to the desired size for the electrode. A contact tab is welded or 
bonded to the current collector 2 following the removal of the active mass 
from one edge of the stretch metal. 
EXAMPLE NO. 2 
The preparation of the mixture and the rolling process is carried out 
similarly as in the example 1. The prepared strip is dried at an increased 
temperature so that at the same time the vapors of the liquid agent are 
recycled by being reverse condensed, exhausted and used anew. The 
electrode strip is further processed as in example No. 1. 
EXAMPLE NO. 3 
The preparation of the mixture is performed as in example No. 1. The 
subsequent rolling is performed so that one of the edges of the current 
collector is exposed, that is left without the active mass during the 
rolling operation. The subsequent processing will be performed as in the 
examples Nos. 1 or 2. 
EXAMPLE NO. 4 
A sheet is made of the mixture prepared according to example No. 1 between 
the rollers of a conventional rolling machine. This sheet is thereafter 
deposited on one or both sides of the current collector by being rolled 
in. The resulting electrode strip is further processed as in the examples 
Nos. 1 to 3. 
EXAMPLE NO. 5 
A mixture consisting of 4 parts of pulverized active cadmium mass and of 
one part of pulverized polytetrafluoro ethylene of a suitable type are 
prepared and there is added one part of ethyl alcohol. The mixture is 
freely deposited on the current collector for instance an iron stretch 
metal and is rolled onto the collector between two rollers that are heated 
to a temperature of 300.degree. C. 
EXAMPLE NO. 6 
The mixture is prepared as in example No. 1 by using 4 parts of pulverized 
active cadmium mass and one part of pulverized polyethylene. The further 
processing is the same as in example No. 5 and the temperature of the 
rollers is kept within the temperature range of 100.degree. to 170.degree. 
C. according to the type of polyethylene used. 
An electrode having a thickness 1.2 mm., prepared according to examples 1 
to 3, retains during the cycling, after 1000 charge-discharge cycles, 
still 60% of the theoretical capacity (1 g. of the active mass corresponds 
to 0.3 Ah). The slope of the linear part of the current-voltage 
characteristic corresponds to 1 Ohm.cm.sup.2. 
Although the invention has been illustrated and described with reference to 
several preferred embodiments of the method of the invention, it is to be 
expressly understood that it is in no way limited to the disclosure of 
such preferred embodiments, but is capable of numerous modifications 
within the scope of the appended claims.