Lithium-ion batteries are very popular nowadays, as everyone has one in their cell phones. Although people use batteries of a large variety of technologies and form factors, everyone wants one with high energy density, low weight and volume, superior charge/discharge characteristics, and low cost. The Lithium-ion chemistry is popular, as it tends to meet most of the above desirable characteristics, even though it has several variations.
However, some applications need batteries with unusual construction, form factor, and chemistry, those that the Lithium-ion battery will simply be unable to meet. Consider, for instance, the research under the guidance of Professor Christopher Bettinger, at the Carnegie Mellon University.
The CMU researchers are interested in developing edible, biocompatible batteries. They want to use non-toxic material already present within the body for parts of the battery, such as the acid in the stomach, which they want to use as the electrolyte. For this, the team has developed anodes made of manganese dioxide, already present in the body, and cathodes based on melanin, which the body contains as a pigment. They also claim to have developed other versions of batteries consisting of body-friendly materials.
The researchers are interested in developing body-friendly batteries where the electrodes can dissolve harmlessly within the body after use. Among the batteries the group has developed using different types of soluble cations, most have terminal voltages ranging from 0.5 to 0.7 volts. Although information is still sketchy, one battery was able to deliver about 5 mill watts of power for nearly 20 hours.
However, it is not only humans who need such special batteries. Fish too need batteries to power tracking devices, so scientists can follow their trail and understand how they migrate. For instance, researchers at the Pacific Northwest National Laboratory have developed a battery small enough for injection, but powerful enough to enable tracking movements of salmon. This battery weighs only 70 mg, and has dimensions of 6×3 mm. It is handcrafted with several layers of rolled up material, thereby increasing the internal surface area, and reducing its internal resistance. The scientists have so far handmade over 1000 such rice-grain sized batteries, and implanted nearly 700 of them into fishes for powering tracking devices.
The tiny batteries supply enough power to send a 744-microsecond signal every three seconds for nearly three weeks, or every five seconds lasting over a month. That makes the average energy density of the batteries to be 240 WHr/kg. Compare this with standard silver oxide button microbatteries, which have an energy density of nearly 100 WHr/kg. So far, the scientists have not revealed how they made these measurements and whether the comparison is fair.
Making such small batteries comes with some peculiarly unique issues. One such issue is attaching leads for connection. The microbatteries from the Pacific Northwest have leads internally built in. However, it is not very clear how the researchers from the Carnegie Mellon University connect leads to their edible batteries, especially as the users of such special batteries do not have access to vendors for obtaining standard holders or connectors.