It's getting difficult to overstate the importance of battery technology. Compact, high-capacity batteries are an essential part of portable electronics already, but improved batteries are likely to play a key role in the auto industry, and may eventually appear throughout the electric grid, smoothing over interruptions in renewable power sources. Unfortunately, battery technology often involves a series of tradeoffs among factors like capacity, charging time, and usable cycles. Today's issue of Nature reports on a new version of lithium battery technology that may just be a game-changer.
The new work involves well-understood technology, relying on lithium ions as charge carriers within the battery. But the lithium resides in a material that was designed specifically to allow it to move through the battery quickly, which means charges can be shifted in and out of storage much more rapidly than in traditional formulations of lithium batteries. The net result is a battery that, given the proper electrodes, can perform a complete discharge in under 10 seconds—the sort of performance previously confined to the realm of supercapacitors.
This appears to be one of those cases where applications badly lagged theory. Since lithium ions are the primary charge carriers in most batteries, the rates of charging and discharging the batteries wind up proportional to the speed at which lithium ions can move within the battery material. Real-world battery experience would suggest that lithium moves fairly slowly through most types of batteries, but theoretical calculations suggested that there was no real reason that should be the case—lithium should be able to move quite briskly.
A number of recent papers suggested that, in at least one lithium battery class (based on LiFePO4), the problem wasn't the speed at which lithium moved—instead, it could only enter and exit crystals of this salt at specific locations. This, in turn, indicated that figuring a way to speed up this process would increase the overall performance of the battery.
To accomplish this, the authors developed a process that created a disorganized lithium phosphate coating on the surfaces of LiFePO4 crystals. By tweaking the ratio of iron to phosphorous in the starting mix and heating the material to 600°C under argon for ten hours, the authors created a material that has a glass-like coating that's less than 5nm thick, which covers the surface of pellets that are approximately 50nm across. That outer coating has very high lithium mobility, which allows charge to rapidly move into and out of storage in the LiFePO4 of the core of these pellets. In short, because lithium can move quickly through this outer coating, it can rapidly locate and enter the appropriate space on the LiFePO4 crystals.
The results are pretty astonishing. At low discharge rates, a cell prepared from this material discharges completely to its theoretical limit (~166mAh/g). As the authors put it, "Capacity retention of the material is superior." Running it through 50 charge/discharge cycles revealed no significant change in the total capacity of the battery.
But the truly surprising features of the cell came when the authors tweaked the cathode to allow higher currents to be run into the cell. Increasing the rate by a factor of 100 dropped the total capacity down to about 110mAh/g, but increased the power rate by two orders of magnitude (that's a hundred-fold increase) compared to traditional lithium batteries. Amazingly, under these conditions, the charge capacity of the battery actually increased as it underwent more charge/discharge cycles. Doubling the charge transport from there cut the capacity in half, but again doubled the power rate. At this top rate, the entire battery would discharge in as little as nine seconds. That sort of performance had previously only been achieved using supercapacitors.
At this point, the authors calculate, the primary limiting factor is no longer storing lithium in the battery; instead, getting the lithium in contact with an electrode is what slows things down. The electrodes also become a problem because they need to occupy more of the volume of the battery in order to maintain this rate of charge, which lowers the charge density. That's a major contributor to the halving of the battery's capacity mentioned in the previous paragraph.
A more significant problem is that these batteries may wind up facing an electric grid that was never meant to deal with them. A 1Wh cell phone battery could charge in 10 seconds, but would pull a hefty 360W in the process. A battery that's sufficient to run an electric vehicle could be fully charged in five minutes—which would make electric vehicles incredibly practical—but doing so would pull 180kW, which is most certainly not practical.
Nature, 2009. DOI: 10.1038/nature07853
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