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.
Patently Remarkable
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.
ZPower
www.zpowerbattery.com