There is no Moore's Law for batteries, but that doesn't mean you can't apply the same advances that have yielded increasingly powerful computer chips to the problem of powering those chips. This month, Marc Madou, a microelectromechanical systems expert at the University of California, Irvine, and his collaborators plan to demonstrate powerful chip-scale batteries made using techniques borrowed largely from semiconductor manufacturing. The inventors say the resulting microbatteries, capable of delivering an intense burst of electricity or a steady flow of relatively low current, would be suitable for many applications, including sensors, cellphones, hearing aids, and cardiac defibrillators. "You could use it for all types of tiny electronic gadgets," Madou says.
Batteries generate electricity through a chemical reaction in which an electrolyte ferries ions between a pair of electrodes: the positively charged anode and the negatively charged cathode. The amount of current generated depends on how much of the chemical reaction takes place--which depends on both the surface area of the electrodes and the volume of the battery's electrolyte. The more surface area there is, the more current can be generated within the same volume. That presents a fundamental challenge for small batteries, which have neither expansive electrodes nor large volumes of electrolyte.
Microbattery designers typically try to work around limitations on current by interleaving two stacks of thin, platelike electrodes. That increases surface area and makes intense bursts of current possible. But because of the low volume of the electrolyte, the microbatteries still can't store much energy.
Madou and collaborator Chunlei Wang say they have improved current flow even further and made room for more electrolyte by designing a unique electrode made out of carbon that looks like a bed of blunted nails [see photo, " Pillars of Power"]. The nails in this case are 400 micrometers tall, with 20-µm gaps in between. Each alternating row comprises anodes, followed by a row of cathodes. Because the electrolyte can get in between the prongs, there is a huge increase in surface area. And putting the anodes and cathodes in close proximity means the ions in the electrolyte do not have to travel far. It is the microscale equivalent of raising the speed limit on a highway and widening it to 10 lanes. According to the researchers, a battery using such an electrode can generate 78 percent more power than a stacked-plate microbattery of the same volume.
To make the carbon posts for the electrodes, the team used high-aspect-ratio photolithography techniques that allowed them to build tiny towers of plastic. They then heated the plastic spikes to about 540 C until only the plastic's carbon remained. Next Madou and his team attached leads to each point on the "bed of nails," yielding a never-before-seen capability: control over groups of electrodes that can be turned on or off separately, allowing a single battery to produce a wide range of voltages and currents.
The team plans to present a completed carbon-pillar electrode battery--whose energy storage they estimate will be roughly 360 kiloamperes per cubic meter--this month at the 206th meeting of the Electrochemical Society in Honolulu. Compared with the smallest commercial batteries such as those used in pacemakers, the 1-by-1-by-0.3-millimeter microbattery they're developing should deliver three to five times as much energy per unit volume.