Patent Number 06979513...
“Battery Including Carbon Foam Current Collectors”
Inventors: Kelley & Votoupal
Assignee: Firefly Energy Corporation
by Donald Georgi
The words “foam current collectors” immediately stimulated interest because of BD’s prior coverage of the Alvin Snaper patent regarding foam plates for batteries. ( See BD issue 67, pp. 9-12.) The fundamental concept of replacing lead plate current collectors is the same in this patent, but the Sanper patent identified the material as metal structures, whereas this patent identifies the replacement plates as some form of carbon. With either concept, active materials are deposited on the surface of the foam, reducing total weight and possibly offering additional advantages of weight reduction, corrosion resistance and higher power density.
Many details of the assignee, Firefly Energy Corp. has been covered in other releases and publications. Suffice it to say that there is sufficient credibility in the concept to have the government provide a $2.5 million contract with Firefly to assist development of the concept. The progression of the technology is far from secure as fundamental manufacturing methods must be married with design requirements for electrochemical performance, safety and reliability. The road is long and the distance traveled to date is short.
The scope of the patent is to first produce a flat plate, Lead-acid chemistry battery taking advantage of the corrosion and weight reduction of the carbon foam electrode which replaces the lead material. Wording in the patent leaves open the application to spiral wrap and other chemistries, although the claims focus on acid electrolytes and lead-based construction. (Graphic from the patent.) +
While this patent background discusses conditions and problems of both Lead-acid and nickel-based batteries, the claims focus on lead-based active materials and acidic electrolytes. This would lead one to think that the commercial applications will be dedicated to the field of Lead-acid batteries. There may be other prior art with foams used both as current collectors and electrodes in other chemistries to make the Lead-acid focus simpler to patent.
“A battery having current collector constructed of carbon foam. The carbon foam includes a network of pores into which a chemically active material is disposed to create either a positive or negative plate for the battery. The carbon foam resists corrosion and exhibits a large amount of surface area. The invention includes a method for making the disclosed carbon foam current collector used in the battery.”
This is a close up representation of the porous structure of the foam current collectors (large oval shaped bodies) with active materials and electrolyte surrounding the collector. Foam structure increases the surface area of the active material by a factor of up to 2,000 times. This would greatly increase the current density leading to much greater current delivery. The patent discusses details of pore size and open porosity . (Graphic from the patent.) +
A conventional Lead-acid battery uses pure lead or a doped lead substrate as a current collector upon which the active (lead-oxide) materials are placed. It is the surface materials on both the positive and negative plates which carry out the electrochemical reactions with the sulfuric acid-based electrolyte, to produce electricity or to be recharged. The underlying lead plates transfer electrons to or from the external battery terminals. Since the chemical reaction is carried out at the active material surface, the underlying lead is not essential for the electrochemical reaction. Lead is already present in the active material and lead is cheap, so conventional wisdom has followed lead as the underlying current collecting part of the electrode.
If one thinks of the underlying collector as a framework rather than as an electrochemical partner, the opportunity for alternative elements or compounds to provide this function become possible. Such is the approach in both the Snaper (metal foam current collector) and this carbon foam current collector patent.
Why change the material and construction of the collectors? The first advantage is that the underlying lead of the positive plate does come in contact with sulfuric acid of the electrolyte and experiences the anodic potential, progressively corroding the plate to lead dioxides which increase resistance and cause the plate to expand and eventually fracture. In the carbon plate, there is no chemical reaction to cause resistance increase and swelling.
A 10 X photograph of the carbon foam materials reveals the highly irregular surface on which the active material will be deposited. The patent describes a range of sources for the foam from commercially available materials to those prepared from carbon, graphite and carbonized or graphitized wood. (Graphic from the patent.) +
The concept of using a solid carbon replacement has been patented (# 5,512,390) specifically with a graphite form of carbon, but the graphite carbon may be too brittle to withstand shock. Construction details have limited surface area slowing charging and discharging.
Moving to the carbon foam version of the carbon electrode eliminates the corrosion, and simultaneously offers significantly greater surface areas upon which to deposit the electrode active material. The greater electrochemical surface area allows the current density to increase, providing higher power density and charge acceptance.
Since the patent makes note of the brittle nature of graphite, it could be assumed that the carbon foam does not exhibit the graphite brittleness.
Carbon Foam differences
Just as carbon electrodes of Lithium-ion batteries have different forms, the carbon structures of this patent are broadly defined,. starting with carbon foams which may be partly amorphous. Another graphite foam called “PocoFoam,” manufactured by Poco Graphics, Inc. can be used. Graphite foam is considered to form in layered structures which may have higher conductivity than carbon foam.
Other foams produced from organic materials are mentioned as possible collectors, defining a wide range of source materials to implement the carbon foam structure.
Some detail of the construction of a metal current collector tab to the base current collector is mentioned along with a cutting method from preformed sheets of carbon foams.
The description of the current collector tab (21) terminal construction is shown with the entire plate in 2A and up close in 2B. The tab could be an extension of the foam, coated with a highly conductive electrochemically compatible material such as lead oxide to reduce internal resistance. (Graphic is from the patent.) +
Flat or Spiral
Most of the material would lead the reader to think that flat plate technology was the defining structure. But the description mentions a pliable foam which might leave the door open for a spiral wrapped version.
Chemistries other than Lead-acid
The description is rich in the application of the carbon foam to Lead-acid chemistries as well as zinc and nickel chemistries, but as the description progresses, the reader is left with a focus on Lead-acid improvements which could be applied to automotive/hybrid applications.
While there are 23 claims, the claims can be broken down into two groups. The first group is led by claim 1 which integrates the foam material into a positive plate and talks about an acidic electrolyte only. Claims 2-12 expand on details of the positive plate such as porosity, resistivity, and materials such as carbon form graphitized wood. Again, claim 7 talks about a lead oxide paste, but no nickel/alkaline structures are mentioned.
Claim 13 identifies the foam negative plate and claims 14-23 use the same form of expanding on porosity etc. applied to that plate. Claim 18 mentions the lead oxide, but again no mention of nickel/alkaline applications are made.
It will definitely be a matter for those much more skilled in patent law to decide on how much nickel/alkaline application this patent has.
Show us the data
Patents are good and this one may have a real step-up contribution to Lead-acid chemistry. But at the same time, the benefits may be highly speculative. In the detailed description near the end, there is a group of sentences which repeatedly use the term “may” with statements about carbon-based foam batteries. They may offer greater specific power - they may provide reduced charging times - they many be suitable for short charge time applications - they may capture braking energy - they may provide pliable plates - they may be less susceptible to damage and they may perform well in vehicular applications.
These and many more things may be a part of the future, but the future will only accept the fundamental concept of carbon foam current collectors in Lead-acid batteries when the data and field experience and economics proves them true. BD