MIT Researchers Develop Lithium Iron Phosphate Material with Charge/Discharge Rates Comparable to Supercapacitors
Researchers at MIT have developed a lithium iron phosphate electrode material that achieves ultra-high discharge rates comparable to those of supercapacitors, while maintaining the high energy density characteristic of lithium-ion batteries. The new material has a rate capability equivalent to full battery discharge in 10–20 s. A paper on the work led by Gerbrand Ceder, the Richard P. Simmons Professor of Materials Science and Engineering, appeared online 12 March in the journal Nature.
The MIT team realized the fast-charge and discharge capability by creating a glassy lithium phosphate coating on the surface of nanoscale LiFePO4. Glassy lithium phosphates are known to be good, stable Li+ conductors.
Because the material involved is not new, the researchers, led by Gerbrand Ceder, the Richard P. Simmons Professor of Materials Science and Engineering, believe the work could make it into the marketplace within two to three years.
The ability to charge and discharge batteries in a matter of seconds rather than hours may make possible new technological applications and induce lifestyle changes. Such changes may first take place in the use of small devices, where the total amount of energy stored is small. Only 360 W is required to charge a 1 Wh cell phone battery in 10 s (at a 360C charging rate). On the other hand, the rate at which very large batteries such as those planned for plug-in hybrid electric vehicles can be charged is likely to be limited by the available power: 180 kW is needed to charge a 15 kWh battery (a typical size estimated for a plug-in hybrid electric vehicle) in 5 min.
Electrode materials with extremely high rate capability will blur the distinction between supercapacitors and batteries. The power density based on the measured volume of the electrode film, including carbon and binder, is around 65 kW l-1 in the 400C test. Assuming that the cathode film takes up about 40% of the volume of a complete cell, this will give a power density of ~25 kW l-1 at the battery level, which is similar to or higher than the power density in a supercapacitor, yet with a specific energy and energy density one to two orders of magnitude higher. The fact that our material can obtain power densities similar to those of supercapacitors is consistent with there being an exceedingly fast bulk process. For LiFePO4, bulk lithium transport is so fast that the charging is ultimately limited by the surface adsorption and surface transfer, which is also the rate-limiting step in supercapacitors.—Kang and Ceder (2009)
Lithium-ion batteries absorb and release energy via the extraction and insertion of Li+ ions and electrons. The power capability of a lithium battery depends heavily on the rate at which the ions and electrons can move through the electrolyte and electrode structure into the active electrode material.
Much of the work on improving the power rate for lithium-ion batteries has focused on improving electron transport in the bulk or at the surface of the material, or on reducing the path length over which the electron and the Li+ ion have to move by using nano-sized materials, the researchers note.
However, about five years ago, Ceder and colleagues found that computer models of a lithium iron phosphate material predicted that the material’s lithium ions should actually be moving extremely quickly.
Further calculations showed that lithium ions can indeed move very quickly into the material but only through tunnels accessed from the surface. If a lithium ion at the surface is directly in front of a tunnel entrance, it proceeds efficiently into the tunnel. But if the ion isn’t directly in front, it is prevented from reaching the tunnel entrance because it cannot move to access that entrance.
To address that problem, Ceder and Byoungwoo Kang, a graduate student in materials science and engineering, created a new surface structure that allows the lithium ions to move quickly around the outside of the material. When an ion traveling across this material reaches a tunnel, it is instantly diverted into it. Kang is a coauthor of the Nature paper.
Ceder notes that further tests showed that unlike other battery materials, the new material does not degrade as much when repeatedly charged and recharged. This could lead to smaller, lighter batteries, because less material is needed for the same result.
This work was supported by the National Science Foundation through the Materials Research Science and Engineering Centers program and the Batteries for Advanced Transportation Program of the US Department of Energy. It has been licensed by two companies.
Byoungwoo Kang and Gerbrand Ceder (2009) Battery materials for ultrafast charging and discharging. Nature 458, 190-193 doi: 10.1038/nature07853