[Engineering Feature]
Keep Current With New Battery Technologies
Whether you’re talking micropower reflow-solderable thin-film batteries or lighter, safer batteries for traction motors, groundbreaking advances continue to energize the field of chemically generated electrical power.
Batteries can be stored from –40°C to 80°C without damage, but operating temperature does affect performance. At high temperatures, they can be charged and discharged at a higher rate and with higher capacity. (When operated at very high temperatures up to170°C, capacity drops faster during cycling.) In the cold (down to –40°C), expect reduced charge and discharge rates.
At the 2008 Darnell NanoPower Forum, FrontEdge CEO Simon Nieh described work on integrating photovoltaics with stacks of NanoEnergy cells (Fig. 3). He said that 7-mm diameter, 10-mm thick micro-photovoltaic cell arrays had been successfully stacked with similar-diameter photovoltaic cells. The objective for 2009 was to create a commercial 2.5-mm diameter, 0.16-mmthick stack that would provide 80 mAh per discharge cycle with 400-Wh/L energy density.
The AlwaysReady Smart Battery nanobattery technology from mPhase fulfills a different function than the thin-film batteries discussed so far (Fig. 4). These devices are intended for use as “reserve batteries,” and the chemical reaction that produces energy is never activated until it’s required. A typical application might be in a mission-critical cell phone. Just as the conventional battery was about to give up the ghost, the reserve battery could be activated to provide another 10 minutes of talk time.
The mPhase technology, a “Superhydrophobic NanoStructured Surface” made of nanotubes, normally keeps an electrolyte separate from the battery’s anode and cathode. Upon the application of an electric field, the electrolyte experiences “electrowetting.” That, essentially, is a change in surface tension that permits it to flow through the barrier, producing a voltage across the battery electrodes.
Lithium Battery Types The struggle to continually provide higher capacity and better product safety has led the makers of lithium batteries on a quest for better and better chemistries. For example, lithium-manganesedioxide (Li-MnO2) cells have an anode in metallic lithium and a solid manganese-dioxide cathode immersed in a non-corrosive, non-toxic organic electrolyte. They deliver a voltage of 2.8 V.
In the nearly ubiquitous Li-ion cell, the anode is graphite and the positive cathode is a lithium-bearing metal compound such as lithium cobalt oxide, lithium nickel oxide, lithium aluminum oxide, lithium manganese oxide, or lithium iron phosphate. The non-aqueous electrolyte is a mixture of organic carbonates.
Lithium-thionyl-chloride (Li-SOCl2) cells have a metallic lithium anode and a liquid cathode that consists of a porous carbon current collector filled with thionyl chloride. Their open circuit voltage (OCV) is 3.6 V. Self-discharge is less than 1% per year, and they can achieve a service life of 10 to 20 years. Similarly, lithium-sulfur-dioxide (Li-SO2) cells have a similar metallic lithium anode and a porous carbon current collector filled with a sulfur-dioxide solution. Their OCV is 2.8 V.
As a side note, most of the 400 geostationary satellites in orbit (telecommunications satellites in the main) carried nickel-hydrogen (NiH) batteries until they were superseded by lithium batteries a few years ago. The NiH batteries used gaseous hydrogen acting on a carbon electrode (using a design derived from fuel-cell technology) plus a nickel-hydroxide cathode. The electrolyte was potassium hydroxide, with a zirconium ceramic separator. NiH cells deliver a voltage of 1.2 V.
In November 2005, A123Systems announced a new higherpower, faster-recharging lithium-ion battery system based on doped nanophosphate materials licensed from the Massachusetts Institute of Technology. Output voltage is 3.6 V.
Interesting Alternatives Challenging Li-ion for laptops and cell phones, ZPower’s silver- zinc technology will appear later this year in a laptop from an as-yet unnamed supplier. That’s the word from ZPower CEO Ross Deuber, who also stated that 2010 would see a cell phone with a silver-zinc battery (see “Rechargeable Silver-Zinc Batteries Coming Online” atwww.electronicdesign.com, ED Online 20539).
ZPower’s battery chemistry, sometimes called silver-oxide batteries, updates an old technology with new processes and materials that extend rechargeability performance. The company also has a recycling approach that addresses the cost of silver.
In these batteries, the anode is zinc, and the cathode is silver. Beforehand, the electrolyte has typically been sodium hydroxide or potassium hydroxide. Past recharging limitations have been due to zinc corrosion. When the old batteries were recharged, zinc diffusion during replating made the anode sag and created zinc dendrites. In Deuber’s words, “The zinc essentially turns to sludge.”
According to Deuber, ZPower’s solution is a proprietary gel that holds zinc particles in suspension and reduces the corrosion problem. The ZPower technology also includes a separator stack that resists dendrite growth while simultaneously resisting degradation from the silver cathode and minimizing internal resistance. A nano-particle silver cathode lowers internal resistance.
Compared to conventional Li-ion batteries, ZPower discloses that charge capacity is higher, but at a lower OCV. Energy density is roughly 20% higher, but charging time is longer and the total number of cycles is less, though cycle life is greatly influenced by discharge depth.
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