A Boost for Battery Life and Capacity
A new chemical trick for making nanostructured materials could help increase the range and reliability of electric cars and lead to better batteries that could help stabilize the power grid.
Researchers at the Pacific Northwest National Laboratory (PNNL) in Richland, WA, have developed the technique, which can turn a potential electrode material that cannot normally store electricity into one that stores more energy than similar battery materials already on the market.
In work published in the journal Nano Letters, the PNNL researchers show that paraffin wax and oleic acid encourages the growth of platelike nanostructures of lithium-manganese phosphate. These “nanoplates” are small and thin, allowing electrons and ions (atoms or molecules with a positive or negative charge) to move in and out of them easily. This turns the material–which ordinarily doesn’t work as a battery material because of its very poor conductivity–into one that stores large amounts of electricity.
When the researchers measured the performance of the material, they discovered that it could store 10 percent more energy than the theoretical maximum energy capacity of a comparable commercial electrode material–lithium-iron phosphate, which is used in power tools and some hybrid and electric vehicles.
The approach could open the door to using a wide range of candidate battery materials that are now limited by their ability to conduct electricity and lithium ions. Research in the area has reached the point at which most of the battery materials left to be studied have bad conductivity, says Daiwon Choi, an energy materials researcher at PNNL. The new method provides a simple way to increase their conductivity. He says the method could also be compatible with conventional battery-manufacturing techniques.
Both lithium-iron phosphate and lithium-manganese phosphate are attractive at battery electrodes because they have a stable atomic structure. This crystalline structure–called olivine–is far more stable than the crystal structure of electrode materials used in laptop and cell-phone batteries. As a result, olivine materials can last much longer than the three years that cell-phone battery materials typically last. Some manufacturers claim that lithium-iron phosphate batteries could last for over 30,000 complete charge and discharge cycles without losing much of their capacity to store energy–enough for the battery to last 50 years, Choi says.
In theory, lithium-manganese phosphate could last for a similar number of cycles, because it has a similar crystalline structure. But it has the added advantage of potentially being able to store 20 percent more energy than lithium-iron phosphate, since it operates at a higher voltage. However, it has been particularly hard to modify lithium-manganese phosphate to overcome the fact that it’s an electrical insulator.
Previous attempts have required processing precursor materials in a liquid solution before creating solid battery materials–a process that’s too expensive for commercial production. The new method developed at PNNL eliminates this separate liquid-processing step, simplifying the process and making it compatible with existing manufacturing techniques.
To prepare the material, the researchers mix chemical precursors with paraffin wax and oleic acid. The wax and acid work together to cause the precursor materials to form crystals of a well-controlled size and shape without clumping up. The wax liquefies at the high temperatures used to process the material and acts as a solvent that replaces the separate liquid processing step used in earlier research.
So far, the material can only be charged at low rates (although it delivers power fast enough for many applications). Choi says one of the next steps is to develop a better process for coating the nanoplates with carbon, which should improve conductivity.
Although lithium-manganese phosphate is attractive because it stores more energy than lithium-iron phosphate, both take up a relatively large amount of volume compared to other types of electrodes for lithium ion batteries. Jeff Dahn, professor of physics and chemistry at Dalhousie University, says this could ultimately make them more attractive for stationary applications–such as storing power on the electricity grid to help smooth out variability from renewable sources–than for electric vehicles.
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