Looking for a breakthrough in stored energy

When it comes to electrical energy storage we seem to be limping along on the back of older, well-worn technology. The French scientist Gaston Plante invented the lead-acid battery in 1859 and lead-acid batteries still remain the mainstay of automotive electrical power today. Nickel-cadmium and nickel-metal-hydride (NiCd and NiMH) rechargeable batteries run most hybrid electric vehicles, but both are mature technologies and future improvements in performance and capacity will be minor at best.

Laptop computers and cellular phones utilize lithium ion cells. Lithium ion batteries have a number of distinct advantages over lead and nickel metal hydride battery technology, but the primary advantage is energy density. Energy density in simple terms is the amount of energy stored relative to the size and weight of the battery.

While it has four times the energy of lead-acid batteries and two to three times the energy of the nickel-cadmium variety, the very nature of lithium-ion electrochemistry allows poorly manufactured, physically damaged or overcharged cells to, shall we say, behave badly. In 2006 there were several well-publicized smoke- and fire-related recalls of Sony-manufactured lithium-ion batteries used in notebook computers sold by Lenovo/IBM, Dell, Apple, and Toshiba.

With battery chemistries powering most mobile applications today having reached an energy density plateau, a little creative alchemy is called for. It’s not hard to see why.

The demand for portable power is skyrocketing as consumer electronics become more sophisticated and users of portable systems want ever-increasing run times and quicker recharges. Research into improving portable power sources seems to be finding two focal points that hold promise. One involves revisiting a couple of previously explored battery chemistries and the second is a new take on a familiar friend—capacitors—but these caps store much more energy than the little components you are used to.

We’ll look at these developments one at a time.

Everything old is new again

A century ago Thomas Edison’s work on the invention of the galvanic battery incorporated nickel-zinc electrochemistry. He explored its development for use in high-power, high-energy demand applications such as powering miner’s safety lamps. While the low-cost, high-energy-density characteristics were promising, Edison’s researchers ultimately settled on alternative electrochemistry that was easier to produce with the technological limitations of the time.

Nickel-zinc and nickel-cadmium batteries are chemically very similar, but voltages differ significantly

Nickel-zinc batteries are chemically very similar to nickel-cadmium. Both use an alkaline electrolyte and a nickel electrode but differ significantly in their voltage (see table ). According to PowerGenix, which has developed a nickel-zinc (NiZn) battery for applications that demand high-discharge-rate capabilities, the nickel-zinc cell delivers more than 0.4 V of additional voltage compared to nickel-cadmium, both at open circuit and under load.

With the additional 0.4 V per cell, 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.

The technology is said to offer compelling performance advantages, most notably a 30% weight and size reduction coupled with higher power and superior low-temperature-discharge behavior. Another prominent feature of the NiZn battery is its low internal resistance, which, the company says, enables the delivery of significantly more power during periods of peak demand than a NiCd or NiMH battery of comparable size.

Both nickel and zinc are nontoxic and recycled easily and inexpensively. The zinc electrode contains no lead, cadmium, or mercury, making it an attractive replacement for lead and cadmium batteries.

Before PowerGenix was able to make available the advantages of the nickel-zinc battery chemistry they needed to solve the technical problems associated with the instability of the zinc electrode in a rechargeable cell. These traditionally have included problems with dendrite formation as well as passivation and shape change of the zinc electrode during cycling. Passivation occurs when an oxide layer is formed on the zinc surface especially during high rate discharge. This may completely insulate the zinc electrode and cause the end of discharge.

The answer can be found in a patented electrolyte formulation that reduces zinc solubility and prevents dendrite shorting problems and a patented electrode composition. The company says its electrode composition avoids cobalt contamination of the zinc electrode and employs a negative cap design that lowers impedance and promotes uniform current density. Cells have been tested to more than 450 deep discharge cycles with no signs of deterioration to the zinc electrode, according to PowerGenix.

The PowerGenix battery is capable of completely recharging in less than two hours and achieves 80% recharge within one hour, a key benefit for cordless power tools and other portable electronics. The company has longer term plans to enter additional markets, including uninterruptible power supplies.

The main pillar of ZPower’s battery strategy is rechargeable silver-zinc batteries that can replace lithium-ion for portable power applications. A technology whose origin traces back 40 years silver-zinc batteries use a water-based chemistry that contains no lithium or flammable solvents or heavy metals. Unlike lithium-ion and lithium-polymer batteries, says ZPower (formerly known as Zinc Matrix Power), silver zinc batteries are free from the problems of thermal runaway, fire, and danger of explosion (and also free from the regulations that limit the size of lithium-containing batteries on airplanes).

In the area of performance, ZPower batteries offer an extremely high ratio of energy to volume (Wh/l) for applications such as notebook computers, cell phones, and consumer electronics. According to the engineering troops at ZPower, silver-zinc is the only rechargeable battery formulation that beats lithium-ion in energy density for consumer applications; ZPower claims its battery formulation offers 30% more energy density than traditional lithium-ion batteries. The company says it can offer a battery at the same size as a lithium-ion battery while offering up to twice the runtime, or a battery with half the footprint of lithium ion and the same runtime. And while lithium-ion technology is reaching a plateau, ZPower says silver-zinc has plenty of headroom to increase both energy density and cycle life.

Inside its battery ZPower employs a separator stack that resists dendrite growth from the zinc anode, while simultaneously resisting degradation from the silver cathode. At the same time it allows ions to move freely between electrodes to minimize the cell’s internal resistance. The zinc anode is a composite polymer electrode that inhibits shape change and dendrite growth while the silver cathode is coated with nano-particles, which are said to enhance conductivity for lower internal resistance.

This summer Zpower’s silver-zinc batteries achieved more than 200 full discharge cycles; the company reports it is ahead of schedule to achieve 300+ cycles at 100% discharge by 2008. Zpower has teamed with Tyco Electronics as a worldwide manufacturing partner to bring silver-zinc battery technology into the marketplace in 2008. The product is initially targeted for notebook computers, cell phones and MP3 players.

Also a bit off the beaten technology track is A123Systems, which replaced the cobalt oxide in lithium-ion cells with nanophosphate, which, as the name implies, is phosphate in particles a few billionths of a meter in size. Compared to other lithium-ion battery chemistries, this battery technology is said to provide all the benefits of Li-ion cells without the fire or explosion hazards, while outperforming normal Li-ion cells in key areas such as total power output and cycle life.

Currently in mass production, the ANR26650M1 cell employs a new type of construction based on a dual plate tubular design that does not use crimp seals and instead opts for an “all laser welded” construction optimized for very low humidity penetration over the life of the battery as well as stronger, thicker dual plate headers.

The technology is currently used in battery packs for DeWalt power tools and is in process of breaking into the hybrid vehicle and full-EV market. General Motors and A123Systems will co-develop cells with nanophosphate battery chemistry for use in GM’s electric drive E-Flex system shown recently in the Chevy Volt concept car. The agreement is expected to expedite the development of the batteries for both electric plug-in vehicles and fuel cell variants.

For average commuters driving 40 miles, GM says a future production version of the Chevy Volt will use zero gasoline and produce zero emissions. GM and A123 also cite these advantages of nanophosphate-based batteries in plug-in electric vehicles:

1) Higher energy density than traditional lithium-ion HEV cells while having one of the highest power to weight ratio of commercially available batteries;

2) Low-impedance growth even at very high charge and discharge rates;

3) Outstanding calendar life;

4) A design that withstands extreme shocks and vibration

5) Excellent performance over a wide temperature range; and

6) An intrinsically safe chemistry (which is especially important in large batteries in automotive use).

It’s a bird, it’s a plane, it’s a capacitor?

Capacitors were first developed in 1745 by a German inventor named Ewald Georg von Kleist. They consist of two plates and a separator. The plates are charged by a power source and, when the power is needed, they can send out their entire charge almost instantaneously. While batteries are good at storing energy, but not good at releasing power, capacitors can release a lot of power at once, but are not as good as batteries when it comes to storing energy.

Batteries are chemical reactors with two electrodes, an anode and a cathode, bathed in an electrolyte solution. Unlike batteries, capacitors store energy in the form of separated electrical charge with the amount of energy that a capacitor can store depending on the insulating material in between the metal surfaces, called a dielectric. While standard electrolytic capacitors are measured in microfarads (one millionth of a farad), high-capacity versions called “supercapacitors” or “ultracapacitors” are beginning to challenge battery technologies in applications demanding quick bursts of power.

Basically the super/ultra cap has two porous plates suspended within an electrolyte, with a voltage applied across the plates. With 10 times the power density of a similar battery and the capacity of being charged and discharged 100 times faster, supercap/ultracap applications include battery power source alternatives, UPS systems as well as part of regenerative power and braking systems for electric and hybrid-electric vehicles. On the negative side ultracapacitors need to be much larger than batteries to hold the same charge.

The amount of energy ultracapacitors can hold is related to the surface area and conductivity of their electrodes. Commercial ultracapacitors can achieve an energy density of 6 Watt-hours per kilogram (Wh/kg), much greater than the energy density of a conventional capacitor (however, this figure is much lower than the energy density reached by lithium-ion batteries, which have about 32 Wh/kg).

A conventional capacitor gets its surface area from plates of a flat conductive material and separates its charged plates with a dielectric material, which can be a plastic or paper film or a ceramic. An ultracapacitor gets its area from a porous carbon electrode material whose surface area can approach 2000 square meters per gram, much greater than can be accomplished using films and plates.

No chemical reactions are involved in the energy storage mechanism, which is highly reversible, allowing the ultra cap to be charged and discharged hundreds of thousands of time. Unlike rechargeable batteries, it does not degrade with each cycle. Also unlike batteries an ultracapacitor can be stored totally discharged. A disposable battery can lose as much as 10 percent of its power a year when it is not used.

The DLR series of supercapacitors from Illinois Capacitor range in capacitance from 4 to 220 farads (F) or more in custom packs, with working voltage ratings from 2 to 2.5 Vdc and operating temperature ranging from -20° to 70°C. The company notes that its supercapacitors can be charged using various methods, including constant current, constant power, constant voltage or by paralleling to an energy source such as a battery or fuel cell. Applications include battery pack alternatives, computer memory backup, UPS systems and emergency or solar-powered lighting.

High capacitance capacitors also can be used to buffer short-term mismatches between the power available and the power required to provide maintenance-free backup power to ensure continued operation or a soft shutdown in the event of power interruptions. Maxwell Technologies, for instance, has just introduced a 75-V Boostcap ultracapacitor module to meet the backup power and power quality requirements of wind turbines and industrial equipment applications. The new module is based on Maxwell’s standard 3,000-F energy cell, and is enclosed in a splash- and dust-resistant enclosure designed to withstand harsh temperature, humidity and vibration conditions.

Maxwell’s ultracapacitor module products range from 15 to 390 V. In transportation applications, they efficiently recapture energy from braking for reuse in hybrid cars and can be used to stabilize automotive power networks and power such subsystems as drive-by-wire steering.

What’s next? Since storage capacity is proportional to the surface area of the electrodes, increasing the surface area will increase the capacitance of the ultracapacitor. Carbon nanotubes have excellent porosity, allowing tiny spaces for the polymer to sit in the tube and act as a dielectric. Scientists at MIT’s Laboratory for Electromagnetic and Electronic Systems (LEES) have been working on nanotube-enhanced ultracapacitors using a matrix of vertically aligned, single-wall carbon nanotubes—each one thirty-thousandth the diameter of a human hairas an electrode. One square centimeter of conductive plate when coated with the nanotubes is said to have a surface area of about 50,000 cm2 .

MIT researchers believe a carbon-nanotube based ultracapacitor would have a power density greater than 100 kW/kg (three orders of magnitude higher than batteries), a lifetime longer than 300,000 cycles and an energy density higher than 60 Wh/kg.

Enable IPC Corp. also is developing power storage devices using advances in thin films and nanotechnology, including ultracapacitors made utilizing common types of carbon sheets with deposited nanoparticles. In July the company announced it had exceeded its target specification of 3,000 farads per liter “by a significant margin for comparably-sized, low-capacitance, lower-priced ultracapacitors, which are used primarily in consumer electronics.” The ultracapacitor technology used originated at the University of Wisconsin (UW) and is being researched under a multi-phased, milestone-based joint development program between Enable IPC and SolRayo LLC – a firm consisting of the UW researchers who invented the technology.

The second of four development phases in the program is said to be close to completion. In this phase, researchers examined and tested various types of carbon and deposition methods. The results are preliminary: real results will not be known until the device is actually packaged in commercially usable designs and full testing is conducted under a variety of conditions.

Murray Slovick