Battery utilizing ceramic membranes

A thin film battery is disclosed based on the use of ceramic membrane technology. The battery includes a pair of conductive collectors on which the materials for the anode and the cathode may be spin coated. The separator is formed of a porous metal oxide ceramic membrane impregnated with electrolyte so that electrical separation is maintained while ion mobility is also maintained. The entire battery can be made less than 10 microns thick while generating a potential in the 1 volt range.

FIELD OF THE INVENTION 
The present invention relates to electrochemical power sources in general 
and relates, in particular, to very thin alkaline dry cell batteries 
formulated using ceramic membranes. 
BACKGROUND OF THE INVENTION 
An alkaline dry cell battery is a storage device for electrical power 
intended to provide electrical power on demand to an electrically powered 
device. A dry cell battery is so called because its electrolyte is in the 
form of a moist paste, which is therefore not capable of being spilled, 
since it is in a semi-solid state. The typical commercially available dry 
cell batteries manufactured today are constructed as a cylinder. The 
cathode is typically a manganese dioxide (MnO.sub.2) powder cathode, with 
additives, formed on the outside of the cylinder of the battery. The 
cathode layer is typically coated onto the interior of a nickel plated 
steel can. The anode, formed of powdered zinc mixed with electrolyte, is 
located centrally in the cylinder of the battery. The centrally located 
cathode surrounded by an alkaline paste containing an electrolyte base, 
such as potassium hydroxide (KOH). Such alkaline dry cells are not 
rechargeable due to the irreversible disintegration of the cathode caused 
by its expansion as electrical power flows from the battery. 
The present invention makes use of the technology of ceramic membranes. 
Ceramic membranes are compositions of matter which consist of a plurality 
of metal oxide particles which are partially fused together to form a 
material which is solid, rigid, stable, but which is also porous. The 
porosity of the ceramic membrane can be controlled by manipulation of 
process conditions during its fabrication so as to create pores in any 
desirable range of pore sizes. Such membranes can also be made over a wide 
range of densities. Typically, porous metal oxide ceramic membranes are 
made by sol-gel processes. In such processes first metal particles are 
formed by a sort of inorganic polymerization/condensation from molecular 
precursors in a solution or suspension. The particles are maintained as 
partially soluble metal oxide particles in suspension by techniques such 
as peptization, aggressive agitation, or other similar means to prevent 
aggregation and resultant precipitation of larger metal oxide particles. 
Such a metal oxide suspension, known as a sol, then has the solvent, 
either water or alcohol, removed from it to create a gelified, semi-solid 
material referred to as a gel. The gel then has further solvent removed 
and then by heating or firing the gel, the particles which make up the gel 
are fused together to form a continuous metal oxide ceramic porous 
membrane material. One class of metal oxide porous ceramic membranes are 
disclosed in U.S. Pat. No. 5,006,248, which describes such materials with 
a uniquely small size range of pores therein. 
SUMMARY OF THE INVENTION 
The present invention is summarized in that a planar dry cell battery is 
constructed utilizing a porous metal oxide ceramic membrane separator 
material, so as to create a battery which is planar and which is formed of 
extraordinarily thin films of material. 
It is an object of the present invention to provide an alkaline dry cell 
battery utilizing a porous ceramic membrane material which is exceedingly 
thin yet, has significant power production, and which thus offers the 
potential for being stacked in compact assemblies to create high voltage 
batteries. 
It is yet another object of the present invention to provide an extremely 
thin film battery which can be readily and efficiently manufactured. 
Yet another object of the present invention is to enable the production of 
thin film batteries which could be manufactured in a variety of 
specialized geometries, to potentially produce self-powered devices. 
Other objects, advantages, and features of the present invention will 
become apparent from the following specification when taken in conjunction 
with the accompanying drawing figure.

DETAILED DESCRIPTION OF THE INVENTION 
The battery of the present invention is constructed so as to be 
extraordinarily thin. As in most batteries, the battery of the present 
invention requires a cathode, an anode, and a separator for the battery. 
Because of the advantages achieved by the use of ceramic membrane 
technology for the construction of these layers, the individual layers, 
and the combination of the layers which make up the battery, can all be 
fabricated in thin films while maintaining the electrical separation and 
the needed surface area to achieve significant electrical power. The 
present invention is illustratively described as an alkaline dry cell type 
battery but it is intended that other classes of battery may also use this 
technology. 
The alkaline manganese dioxide zinc dry cell battery is based on zinc metal 
anode, a manganese dioxide cathode, and a separator which contains an 
absorbed electrolyte capable of charge transfer between the anode and the 
cathode. It is this general approach that is utilized in the illustrative 
battery described here. 
The advantages of a thin cathode for such a battery have previously been 
described by Kordesch in "Primary Batteries--Alkaline Manganese 
Dioxide-Zinc Batteries" in Comprehensive Treatise on Electrochemistry. 
Kordesch reports that manganese dioxide electrodes 0.6 mm thick were shown 
to perform significantly better than the typically used cylindrical 
electrodes which were 3 mm thick. The reason for this is the more 
efficient transport of the reactants (e.g. H.sub.2 O or H+) to the surface 
of the individual MnO.sub.2 particles. Also, he showed that 
rechargeability becomes more efficient in thinner cathodes. In the battery 
described herein, the cathode can be made even thinner, by depositing a 
combination mixture of manganese dioxide and graphite particles onto a 
support consisting of an inert conductive metal foil. The metal foil 
serves as a current collector for the cathode. The use of a mixture of 
manganese dioxide and graphite particles allows for improved conductivity 
within the cathode itself. Deposition of the manganese dioxide and 
graphite particles on the current collector can be achieved by 
spin-coating of the metal surface with a sol-gel derived manganese dioxide 
suspension containing a range of graphite between 7 and 20%, preferably 
around 15%. Using such a spin-coating technique from a sol gel derived 
suspension, it is possible to obtain a cathode layer which is on the order 
of 1 micron thick, coated onto a conductive metal foil backing. The 
cathode does not have to be porous, but does advantageously have a high 
surface area. 
One problem which can occur in rechargeable batteries is the slow 
disintegration of the cathode during cell cycling. This occurs, in part, 
because the cathode expands during discharge. Hence the cathode must be 
well supported in order to have reliable battery design. This cathode 
design makes use of a ceramic material coated onto a metal substrate, 
thereby making the cathode very thin and also adjacent to the flexible 
separator. Yet because of the manner of deposition of the cathode material 
onto the support, a high surface contact area between the cathode and the 
electrolyte is maintained. 
The battery separator in a battery prevents contact between the cathode and 
anode, and thus is essential to the performance of the cell in maintaining 
an electrochemical potential. The battery separator has two principal 
functions. First it must absorb within it the electrolyte, typically an 
electrolytic salt, such as the potassium hydroxide used here. Secondly the 
separator must prevent any penetration of the zinc away from the anode. In 
the battery of the present invention, an aluminium oxide (gamma-AlOOH) 
porous ceramic membrane is used as the battery separator. This material 
achieves both objectives, acting as a good battery separator while also 
introducing into the battery the advantage of a very thin separator layer. 
Such an alumina metal oxide ceramic separator layer is porous and is a 
good insulator. In addition, the pore size range can be selected so as to 
be relatively impermeable to penetration of larger zinc particles. Such a 
membrane can be prepared by spin coating on a finished cathode assembly 
with a layer of a colloidal alumina (gamma-AlOOH) suspension. By spin 
coating such a cathode from a colloidal suspension, it is possible to make 
a porous ceramic aluminium oxide membrane of selectable porosity, and of 
any desired thickness between 1 and 10 microns, or thicker. Such membranes 
may be fired up to temperatures of 500.degree. Centigrade, while retaining 
significant porosity. By manipulating the production of the colloidal sol, 
the size of the pores in the membrane can also be controlled. The porous 
separators may then be impregnated with an electrolyte or salt solution. 
The anode of the thin film battery may be another thin layer, this time 
formed from a mixture mainly of colloidal zinc. The anode may be deposited 
via spin coating from a zinc sol on a cathode-separator assembly in a 
manner similar to preparation of the other layers. Alternatively, a 
ceramic ZnO.sub.2 membrane can be reduced to create a zinc metal membrane 
with a large surface area. If the battery is desired to be rechargeable, 
the anode may also contain a structure such as copper or lead powder onto 
which the zinc can be plated during recharging. 
The thin film battery of the present invention can be constructed by 
building up the appropriate layers from one conductor to the other in 
either order. As described below, the thin film battery is built up by 
successive deposition of layers from the cathode conductor to the anode 
conductor. The process can be begun at either of the two current collector 
or conductive surfaces. 
A schematic of the layers of the thin film battery is shown in the drawing 
FIG. 1. Illustrated in FIG. 1, which is not sealed, is the order of the 
various layers of the thin film battery. The cathode collector layer, a 
metal foil current collecting surface, is designated at 12. The cathode 
itself, consisting of a manganese dioxide derived layer, mixed with 
graphite, is illustrated at 14, and is on the order of 1 micron in 
thickness. Designated at 16 is the aluminium oxide separator layer. The 
zinc anode is indicated at 18, and is approximately equal in thickness to 
the cathode. The anode current collector is indicated at 20, and is again 
a thin metal foil conductive sheet. In other embodiments, the anode 
current conductor 20 may be omitted and electrical contact made directly 
to the zinc metal anode. The overall size of the battery cell, in total, 
can be as little as five to ten microns in total thickness. 
The advantage of this thin film design for a battery over a conventional 
manganese dioxide/zinc battery is that there is a very large interface to 
volume ratio. This should, in theory, increase the potential power surge 
capability of the battery since, there is a greater surface contact 
between the electrodes and the electrolyte. This feature reduces mass 
transfer limitations that might otherwise be present in other geometries. 
It has previously been proposed that the limiting step in the reduction of 
manganese dioxide and an alkaline dry cell is the need for molecular 
diffusion of protons into the crystals of the cathode layer. This design 
which uses a very thin cathode layer, composed of very small particles, 
can theoretically increase the performance of a battery, by eliminating or 
decreasing this factor. In addition, the thin film battery is of a 
strikingly small thickness compared to prior art battery designs, and 
therefore may achieve a higher potential drop per unit length than 
previous designs would allow. For example, since battery cells of this 
design can generate an electrical potential of 1.4 volts with a total 
thickness of about 5 microns, if a series of similar batteries could be 
hooked together in series, and assuming that the aluminium oxide separator 
can effectively limit current leakage, a total voltage of 2800 volts could 
be 1 centimeter thick. 
EXAMPLE 
Two batteries of the design of FIG. 1 were constructed beginning with a 
cathode collector plates of titanium. Two thin titanium plates 
approximately 20.times.20.times.0.5 mm were utilized as the substrate. An 
aqueous suspension containing 30 grams per liter of commercially available 
electrolytic manganese dioxide particles was obtained (Aldrich Chemical). 
The size of the particles in the commercially available suspension was not 
known. The suspension was then applied to the titanium plates using a spin 
coating apparatus (Headway Research PWM101D Spinner, R790 bowl) at 2000 
rpm for 30 seconds or until visibly dry. Four successive coats of the 
manganese dioxide suspension were spun-coated onto the metal surfaces this 
way. Between each coating, the plates were fired at 400.degree. C. for 30 
minutes, to fire the layers of the cathode coating in place. 
Next the battery separators were created for the two batteries. An aqueous 
gamma-alumina (AlOOH) sol was prepared. The alumina sol contained 60 grams 
per liter of alumina particles. The sol was spun onto the finished cathode 
assemblies at 750 rpm for 90 seconds. The coated cathode assemblies were 
fired at 500.degree. C. for 1 hour. The resulting cathode and support 
assemblies were then soaked in 6M KOH over night to impregnate the 
membrane with the potassium hydroxide electrolyte. 
To verify that the cells were capable of generating electricity, a single 
unified zinc foil material was used as both the anode and the anode 
collector. Thin pieces of zinc foil, of approximately 
10.times.10.times.0.1 mm, were cut and positioned directly onto the 
alumina membranes, in the manner illustrated in FIG. 1. Each entire 
assembly was then, in turn, sandwiched tightly between the two insulated 
ends of a C-clamp to ensure intimate contact between all components, 
particularly the zinc foil and the separator. The potential drop between 
the two electrodes was then measured with a digital multi-meter. Both 
batteries demonstrated high, although slightly varying open circuit 
voltages. One of the batteries measured 0.91 volts and the other measured 
1.4 volts. The thickness of the batteries thus created was calculated to 
be between 1 and 5 microns. 
Thus these test batteries, utilizing a planar zinc anode rather than a 
configuration with a higher surface area, demonstrated the feasibility of 
this approach to battery design. It is anticipated that by utilizing a 
spin coated zinc layer, having a higher surface area, that more efficient 
power generation will be achieved. 
It is to be understood that the present invention is not limited to the 
particular embodiments described above, but embraces all such modified 
forms thereof as come within the scope of the following claims.