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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