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A stunning array of battery types provides portable power for a sea of applications —from traction motors in interurban buses to flea-power transmitters in wireless mesh networks that harvest microscopic amounts of energy from small photovoltaic cells or piezo beams (Fig. 1). Despite a history that traces back at least to 1800, engineers continue to be presented with new chemistries and novel ways of exploiting the possibilities of Galvani potentials between different materials (see “Battery Basics,” ED 20733).

THIN-FILM BATTERIES
Some of the most interesting developments involve thin-film technologies. Their energy and power density, relative to their size, make them attractive in energy harvesting. Particularly significant for fast charging is their very low equivalent series resistance (ESR). Also, they don’t self-discharge, so they may remain ready for use for a decade or longer.

Coupled with supercapacitors, these batteries round out the energy-storage picture for many mesh-network applications. It’s even possible to integrate a photovoltaic cell with a thin-film battery to make a self-charging battery.

According to one thin-film maker, Oak Ridge Micro Energy, John Bates and a team of researchers at the Oak Ridge National Laboratory (ORNL) developed the first thin-film rechargeable lithium batteries. As the name suggests, thin-film batteries are fabricated by deposition directly onto chips or chip packages.

Unlike conventional batteries, thin-film batteries offer bendability (when fabricated on thin plastics); they can be shaped into whatever form-factor required by an application. They also scale nicely in terms of size. (Oak Ridge Micro characterizes that as a constant $/cm².)

Furthermore, thin-film batteries exhibit extreme temperature tolerance. Operational tests have been conducted between –20°C and 140°C. In assembly, they’re unaffected by heating to 280°C, which means they can handle automated solder-reflow.

During fabrication, different layers are deposited by sputtering or evaporation. In Oak Ridge’s batteries, the stack from current collector to anode is less than 5 µm thick. Depending on substrate and package, total battery thickness can be anywhere from 0.35 to 0.62 mm. Figure 2 illustrates the discharge charge characteristics of thin-film lithium-ion (Li-ion) batteries. (Voltage starts at 4.0 V because Li-ion cells have lower operating voltages than those of batteries with lithium anodes for comparable current densities.)

Flexible thin-film batteries have current limitations. To achieve high current densities, it’s necessary to heat-treat the cathode at temperatures of 700°C or even higher. This tends to discourage the use of flexible polymer substrates for cathode films in certain applications.

Oak Ridge has examples of discharge curves measured for a lithium battery fabricated on a 0.005-in. thick polyimide sheet where the cathode anneal temperature was never allowed to rise above 400°C. In that case, the internal resistance of the battery was roughly 60 times higher than a lithium battery, made on a rigid ceramic substrate with a cathode of comparable thickness, that was annealed above 700°C.

THIN-FILM BATTERY MAKERS
Oak Ridge’s lithium thin-film batteries aren’t the only game in town. Front Edge Technology’s flexible NanoEnergy battery is a miniature power source designed for highly space-limited micro devices such as smart cards, portable sensors, and RFID tags. NanoEnergy batteries can be made as thin as 0.002 in., including packaging.

These batteries use the lithium-phosphorous-oxynitride (LiPON) ceramic electrolyte, developed by Oak Ridge National Laboratory. The cathode material is lithium cobalt oxide (LiCoO₂), and the anode is lithium. They contain no liquid or environmentally hazardous material. (Front Edge says that the small amount of lithium metal in the battery won’t cause fire even if the hermetic seal is broken.)

A 0.25-mAh battery can be charged to 70% of rated capacity in two minutes and to full capacity in four minutes. Any battery can be discharged at rates of more than 10C (and more than 20C in pulsed discharge), and the company says they’re good for more than 1000 charge/discharge cycles at 100% depth discharge. Selfdischarge is less than 5% per year.

Physically, the NanoEnergy batteries can be customized to fit specific size requirements. A 20- by 25- by 0.3-mm battery has a 0.1-mAh capacity. Stretch that to 42 by 25 mm and fatten it to 0.4 mm, and you have 0.5-mAh capacity.

Charging is simple—just apply a constant 4.2 V. You can’t overcharge a NanoEnergy battery. When charged at 4.2 V and discharged at 1 mA to 3.0 V, the battery loses less than 10% capacity over 1000 charge/discharge cycles. The charging time required to obtain 95% of the rated capacity is four minutes at the first cycle and increases to six minutes at the thousandth cycle.

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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” at www.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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While current lithium battery makers are challenged to provide a performance improvement of 7% or so, Deuber said that by leveraging manufacturing efficiencies, ZPower’s silver-zincs might achieve something closer to 10% per annum for several years. The ability to recycle exhausted batteries hinges on the technology used recover silver from used X-ray film. Deuber says that element of the plan is now in place, though batteries won’t need to be recycled for at least a year after the first laptops are sold.

Looking at applications from handheld devices to electric vehicles, PowerGenix’s nickel-zinc batteries are chemically very similar to nickel cadmium (NiCd). But they lack the environmental problems of cadmium while offering a 0.4-V boost that brings the open-circuit full-charge voltage to 1.74 V. Both use an alkaline electrolyte and a nickel electrode. The discharge reaction is:

H₂O + Zn + 2NiOOH = ZnO +2Ni(OH)₂

With the additional 0.4-V per cell advantage over NiCd, an inherent value of the nickel-zinc cell lies in the reduced cell count required for a multicell battery. For higher-voltage applications, the advantages associated with fewer cells are quickly apparent through a smaller footprint, lighter weight, and lower impedance battery.

Using zinc presented some challenges to PowerGenix, though. The company took a patented electrolyte formulation that reduces zinc solubility to eliminate dendrite shorting problems. Dendrites are famously the cause of “memory effect” that plagued NiCd batteries. PowerGenix also developed advances in both the positive and negative electrode composition that eliminate the need for heavy metal elements.

The batteries, fabricated in AAA, AA, and D sizes, fit into packs for heavier-duty applications. This lets the company take advantage of the current alkaline battery supply chain and have the batteries manufactured on existing NiMH lines.

Last year, PowerGenix showed a rechargeable D-cell battery pack for hybrid electric vehicles (HEVs). The company explained that the display model, installed in a Prius, could deliver 30% more power and increased energy density than the similarly sized NiMH battery packs that come standard in the car. The company also said that nickel zinc can be easily integrated into existing hybrid vehicle designs at about one-half the cost per watt-hour.

Mature Battery Types
Ubiquitous alkaline and carbon-zinc cells produce approximately the same electromotive force (EMF) of 1.5 V. For NiCd and NiMH cells, the EMF is 1.2 V. Higher electrochemical potential changes give lithium cells EMFs of 3 V or more.

NiCd cells have an anode made of cadmium hydroxide and a cathode of nickel hydroxide, immersed in an alkaline electrolyte comprising potassium, sodium, and lithium hydroxides. The cells are rechargeable and deliver a voltage of 1.2 V during discharge. NiCd is essentially dead these days. The European Union banned cadmium for most uses in 2004, though Saft in France continues to make them, presumably for military purposes. (The company uses cadmium from recycled NiCd batteries.)

While NiCd batteries have been around since the 19th century, NiMH batteries were first developed around 1980. Nickel-metalhydride anodes are made of a metal alloy capable of absorbing and desorbing hydrogen. Their nickel-hydroxide cathodes are immersed in an alkaline electrolyte solution of potassium, sodium, and lithium hydroxides. NiMH cells are rechargeable and deliver a voltage of 1.2 V. They have very similar properties to NiCd units and share the same manufacturing processes and most components. They also have excellent energy density by volume (up to 140 Wh/L).

Lead-Acids Old and New
Lead-acids that you top-up with distilled water are called flooded batteries. “Low-maintenance” lead-acid batteries, also known as valve-regulated lead-acid (VRLA) or recombinant batteries, use less acid than flooded batteries and offer better power density and cranking (short-duration power delivery) performance. “Recombinant” implies that at high recharge currents, some of the oxygen generated at the positive plates recombines with hydrogen from the negative plates, making it unnecessary to add water manually.

Despite the lack of battery caps, they aren’t really “sealed.” Although they won’t spill if tipped, there’s a safety valve that vents hydrogen if necessary. There are two kinds of VRLAs: gel batteries and absorbent glass mat (AGM) batteries.

Gel batteries use an electrolyte in which the sulfuric acid is mixed with silica, producing a gel. Alternatively, in AGM VRLA batteries, a glass fiber mat soaks up the electrolyte. Instead of an array of parallel plates, some AGM batteries are built like capacitors, with long, thin cells, wound into spirals.

Firefly Energy has developed a carbon foam-based lead-acid battery with an energy density of 30% to 40% more than the battery’s original 38-Wh/kg density.