Lithium battery charge: 10s Lithium breakthrough could charge batteries in 10 seconds
A new version of lithium battery technology caneither provide a higher storage density than current batteries, orcan charge and discharge as fast as a supercapacitor, emptying itsentire charge in under 10 seconds.
It's getting difficult to overstate theimportance of battery technology. Compact, high-capacity batteriesare an essential part of portable electronics already, but improvedbatteries are likely to play a key role in the auto industry, andmay eventually appear throughout the electric grid, smoothing overinterruptions in renewable power sources. Unfortunately, batterytechnology often involves a series of tradeoffs among factors likecapacity, charging time, and usable cycles. Today's issue ofNature reports on a new version of lithium batterytechnology that may just be a game-changer.
The new work involves well-understood technology,relying on lithium ions as charge carriers within the battery. Butthe lithium resides in a material that was designed specifically toallow it to move through the battery quickly, which means chargescan be shifted in and out of storage much more rapidly than intraditional formulations of lithium batteries. The net result is abattery that, given the proper electrodes, can perform a completedischarge in under 10 seconds—the sort of performance previouslyconfined to the realm of supercapacitors.
This appears to be one of those cases whereapplications badly lagged theory. Since lithium ions are theprimary charge carriers in most batteries, the rates of chargingand discharging the batteries wind up proportional to the speed atwhich lithium ions can move within the battery material. Real-worldbattery experience would suggest that lithium moves fairly slowlythrough most types of batteries, but theoretical calculationssuggested that there was no real reason that should be thecase—lithium should be able to move quite briskly.
A number of recent papers suggested that, in atleast one lithium battery class (based on LiFePO4), theproblem wasn't the speed at which lithium moved—instead, it couldonly enter and exit crystals of this salt at specific locations.This, in turn, indicated that figuring a way to speed up thisprocess would increase the overall performance of thebattery.
To accomplish this, the authors developed aprocess that created a disorganized lithium phosphate coating onthe surfaces of LiFePO4 crystals. By tweaking the ratioof iron to phosphorous in the starting mix and heating the materialto 600°C under argon for ten hours, the authors created a materialthat has a glass-like coating that's less than 5nm thick, whichcovers the surface of pellets that are approximately 50nm across.That outer coating has very high lithium mobility, which allowscharge to rapidly move into and out of storage in theLiFePO4 of the core of these pellets. In short, becauselithium can move quickly through this outer coating, it can rapidlylocate and enter the appropriate space on the LiFePO4crystals.
The results are pretty astonishing. At lowdischarge rates, a cell prepared from this material dischargescompletely to its theoretical limit (~166mAh/g). As the authors putit, "Capacity retention of the material is superior." Running itthrough 50 charge/discharge cycles revealed no significant changein the total capacity of the battery.
But the truly surprising features of the cellcame when the authors tweaked the cathode to allow higher currentsto be run into the cell. Increasing the rate by a factor of 100dropped the total capacity down to about 110mAh/g, but increasedthe power rate by two orders of magnitude (that's a hundred-foldincrease) compared to traditional lithium batteries. Amazingly,under these conditions, the charge capacity of the battery actuallyincreased as it underwent more charge/discharge cycles. Doublingthe charge transport from there cut the capacity in half, but againdoubled the power rate. At this top rate, the entire battery woulddischarge in as little as nine seconds. That sort of performancehad previously only been achieved using supercapacitors.
At this point, the authors calculate, the primarylimiting factor is no longer storing lithium in the battery;instead, getting the lithium in contact with an electrode is whatslows things down. The electrodes also become a problem becausethey need to occupy more of the volume of the battery in order tomaintain this rate of charge, which lowers the charge density.That's a major contributor to the halving of the battery's capacitymentioned in the previous paragraph.
A more significant problem is that thesebatteries may wind up facing an electric grid that was never meantto deal with them. A 1Wh cell phone battery could charge in 10seconds, but would pull a hefty 360W in the process. A batterythat's sufficient to run an electric vehicle could be fully chargedin five minutes—which would make electric vehicles incrediblypractical—but doing so would pull 180kW, which is mostcertainly not practical.