Battery technology is advancing fast—just compare the Watt-hours you can stuff into current devices to what was on the market a few decades ago (the Powerbook 140 had a 4200 mAh battery, while the current iPad has nearly three times that capacity). Research is continuing to advance at a rapid pace, which is why it sometimes seems like we cover a new miracle battery every month. We try to make it clear that most of these are still years away from commercialization, and some technologies will just never get there.
It's with that caution in mind that we turn to a battery development that was even picked up by the BBC earlier this week. In a new paper, researchers have reported that they are developing a microscopic battery that's suitable for integrating directly into electronics, just as capacitors now are. Compared to existing capacitors, its performance is impressive. That announcement doesn't mean, however, that these things can be scaled up to work as full-sized batteries that can power gadgets or cars. There are also a few hitches, like rapidly degrading performance, that need to be worked out.
That said, making the batteries is a pretty impressive process. In general, the batteries we buy are made from bulk processes, where materials self-organize into the different parts necessary to get the battery to work. This self-organization can be directed to a greater or lesser degree, but it's still dependent upon the material properties of the battery components.
When thinking about a battery that's small enough to integrate with current electronics, however, those sorts of considerations don't really apply. The electronics are already subject to step-by-step manufacturing processes, and the size of the battery means that bulk processes won't apply. So the researchers behind the new battery focused on structuring its parts on microscopic scales and made them to optimize performance. As a result, the batteries were built with a lot of active material relative to their volume and very short transport distances for the electrons and ions in the battery itself.
To build these microbatteries, the authors started out by laying out a series of tightly packed gold electrodes. They then layered tiny polystyrene pellets on top, which were packed down tightly. With the beads in place, the gold electrodes were used to electrodeposit nickel around the polystyrene beads, which created a metal foam precisely located above the gold electrodes. Once that was in place, the beads were etched away, leaving the metal scaffolding in place.
At this point, a second round of electrodepositing was done to create the anodes and cathodes at alternating electrodes. First, half the electrodes were charged to deposit a nickel-tin mixture at the anode; next, manganese dioxide was deposited at the other half to form the cathode. Lithium ions were then integrated into the resulting battery, and an electrolyte solution was layered on top. The end result was a series of alternating stripes, each about 30 micrometers wide, consisting of metal foams that allowed the rapid transport of ions in and out, and a direct connection to the gold electrodes below to transfer electrons.
Performance-wise, the result was pretty impressive. Like a battery, it stored far more energy than a supercapacitor. But because the ions and electrons could move so quickly, it had the power profile of a supercapacitor (power is the energy transferred per time), meaning that charging and discharging could occur at incredibly high rates. If you scaled this up to the size of a conventional lithium ion battery, the microbattery held about seven times the energy.
Those upsides are the good parts; unfortunately, there are a few downsides. One of them is that the battery loses about five percent of its capacity with each charge/discharge cycle. By 15 cycles, it was down to only two-thirds of its original capacity, and things were even worse at high charge/discharge rates. The authors think that this was a necessary part of the electrodes breaking in, and there may be a longer-term stability once that point is reached. But they didn't actually go on to demonstrate that hypothesis.
Other issues have to do with manufacturing and scaling. The demonstration system had a total volume of 0.03 cubic millimeters, so there's a lot of scaling up to do. And any scaling up will require that the multi-step production process has to happen very efficiently and very cheaply.
This is a nice paper with some interesting ideas, but there's definitely some work to do before the technology could be ready for widespread use, and there are some problems that still need to be overcome before it's worth doing that work.