|Ragone plot showing relative position of Axion e3 Supercells|
Axion Power International, the developer of what it calls a lead-acid battery-supercapacitor hybrid energy storage cell, has received a US Patent (No. 7,006,346) for its Positive Electrode of an Electric Double Layer Capacity Technology.
Where conventional lead-acid batteries use lead-based electrodes for both the positive and negative electrodes, Axion e3 Supercells use lead-based positive electrodes and a five-layer assembly consisting of a microporous activated carbon electrode, a corrosion barrier, a current collector, a second corrosion barrier and a second carbon electrode.
These electrode assemblies are then sandwiched together with conventional separators and positive electrodes to make the e3 Supercell, which is filled with an acid electrolyte, sealed and connected in series to the other cells.
In general terms this patent covers a novel method developed by our scientists that is used to fabricate high performance positive electrodes for use in our e3 Supercells. Electrodes based on the new technology are expected to enhance our efforts to develop and produce energy storage devices that offer higher specific energy, higher maximum voltage, and longer cycle life.—Edward Buiel, Axion Chief of Research and Development
|e3 Supercell Single-Cell Assembly|
In the e3 Supercell, the positive (lead) electrode undergoes the same half-reaction as in a conventional lead-acid cell (i.e. lead dioxide reacts with acid and sulfate ions to form lead sulfate and water). But the negative electrode does not undergo a chemical reaction at all.
Instead of having a lead electrode that reacts with sulfate to form lead sulfate, the activated carbon electrode has a very high surface area material that acts to adsorb and desorb protons (H+) ions in solution.
In conventional lead-acid batteries the concentration of acid changes from being very concentrated in the charged state to somewhat diluted in the discharged state as the acid is converted to water.
In contrast, the e3 Supercell stores the protons in the negative electrode in the fully charged state which move to the positive electrode during discharge where they are neutralized to form water. The result is reduced acid concentration swings from the charged to discharged state which reduces grid corrosion on the positive electrode and leads to longer cycle life of the positive electrode.
As a result, the e3 Supercells eliminate premature failures caused by sulfation of the negative electrodes; reduce positive grid corrosion due to the reduction in acid concentration changes during charge and discharge; reduce mass shedding on the positive electrodes; and reduce premature failures caused by dryout as a result of the improved recombination efficiency of oxygen on the negative activated carbon electrode.
Axion claims that e3 Supercell batteries, compared to typical lead acid batteries, use up to 65% less lead; offer longer cycle life in deep discharge applications, significantly higher power rates, significantly faster recharge rates; and can weigh considerably less and require no maintenance.
The cells, however, offer lower energy densities than lead-acid batteries. Axion believes that the e3 Supercell is best suited for applications that require higher power output, rapid charge rates and the ability to withstand a large number of deep discharge cycles.
Although the low energy densities would seem to argue against application in hybrid electric vehicles (HEV), Axion believes that it can engineer e3 Supercell power systems for HEV applications that offer comparable performance for a fraction of the cost.
Axion says that it will begin manufacturing e3 Supercells in 2Q 2006.
|e3 Supercell Comparisons|
|Energy density||Useful life (cycles)||Device cost|