Don't write off lithium-ion (Li-ion) batteries just yet, but their competition is gaining ground. Formerly known as Zinc Matrix Power Inc., ZPower appeared at last month's Intel Developer Forum (IDF) in San Francisco to discuss an early, unnamed adopter of its technology as well as reveal more about its plans to get around the high cost of silver.
It all hinges on a plan to get companies that recover silver from used X-ray film to start recovering silver from exhausted batteries. Laptop users would have to return worn-out batteries to the store for credit on new packs, just as many people return soda bottles for deposit today.
ZPower's technology is based on what's commonly called the silver-oxide battery, but with various twists in the manufacture, particularly of the zinc anode. Silver-oxide batteries have been used since at least World War II in radiosonde balloons and since the 1960s in satellites.
The zinc is the anode, and the silver forms the cathode. (On the electromotive series, zinc is listed at -0.762 V and silver at +0.800 V.) In past implementations, the electrolyte has typically been sodium hydroxide or potassium hydroxide. The drawback to earlier silver-oxide batteries has been their limited ability to be recharged, due to corrosion of the zinc.
ZPower's secret sauce isn't that secret, as much of it is in the company's patents: "Upon replating, this zinc diffusion leads to the well-known phenomena of electrode shape change and the presence of zinc dendrites within the batteries. This shape change includes an agglomeration of the zinc towards the center of the battery while simultaneously depleting zinc from the edges," says U.S. patent 6,582,851.
"Dendrites can readily be formed due to the zinc concentration gradients within the battery. Their tree-like structures have as their most undesirable effect the rupture of the separator membranes leading to battery shorting," it continues.
More succinctly, says ZPower CEO Ross Dueber, "The zinc essentially turns to sludge." ZPower's solution is a proprietary aqueous gel that holds zinc particles in suspension and reduces the corrosion problem, permitting a greater number of recharge cycles.
"The present invention provides a material that surrounds the zinc in a three-dimensional lattice matrix which induces the zinc to re-plate in the same mesh size as it was originally assembled. Second, the material has been designed to be mechanically stable despite zinc cycling," says the patent.
"The anode paste of the invention remains electrically interconnected during the entire charge cycle. Finally, the anode has high ionic transport, excellent accommodation to zinc density changes and, optionally, high hydrogen transport," the patent explains.
Dueber says the zinc-in-a-gel concept led to the original "Zinc- Matrix"company name. But relating that technology description to laptop batteries proved to be a challenge for investors, so the company became ZPower. The ZPower technology includes two other key proprietary elements. First, its separator stack resists dendrite growth while simultaneously resisting degradation from the silver cathode and minimizing internal resistance. Second, its nanoparticle silver cathode lowers internal resistance.
Moving Toward Production
At IDF, the company showed a prototype laptop battery pack developed for that undisclosed OEM with six cells in the same form factor as conventional Li-Ion cells. Each cell is rated for 1.4 to 1.6 V, and ZPower claims an energy density greater than 500 W/L. The company also showed an MP3 player powered by a single cell.
The accompanying table, abbreviated from one that ZPower disclosed at the forum, compares the characteristics of the company's prototype cells with conventional Li-ions, both in the standard 10- by 34- by 50-mm form factor used in laptop battery packs. ZPower's charge capacity is higher, but at a lower open-circuit voltage. Also, ZPower's energy density is roughly 20% higher, but charging time is longer and the total number of cycles is less.
In that regard, Dueber notes that discharge depth greatly influences cycle life. If users consistently run the batteries completely down, they can expect fewer cycles. He also says the ZPower batteries are designed with some charging headroom. They can't be "overcharged," but occasionally, they can be charged to a higher amp-hour value than baseline to sustain a laptop for a cross-country plane trip.
Perhaps most importantly, according to the company, silveroxide batteries don't have a failure mode that involves bursting into inextinguishable flames.