The study also provides the first direct experimental evidence to support a particular model of the electrochemical reaction. The results, published August 4, 2014, in Nature Communications, could provide help battery makers to optimize materials for faster-charging batteries with higher capacity.
"Our work was focused on developing a method to track structural and electrochemical changes at the nanoscale as the battery material was charging," said Brookhaven physicist Jun Wang, who led the research. The group was interested in chemically mapping what happens in lithium iron phosphate - a material commonly used in the cathode, or positive electrode, of electrical vehicle batteries - as the battery charged. "We wanted to catch and monitor the phase transformation that takes place in the cathode as lithium ions move from the cathode to the anode," she said.
Getting as many lithium ions as possible to move from cathode to anode through this process, known as delithiation, is the key to recharging the battery to its fullest capacity so it will be able to provide power for the longest possible period of time. Understanding the subtle details of why that doesn't always happen could ultimately lead to ways to improve battery performance, enabling electric vehicles to travel farther before needing to be recharged.
In operando 2D chemical mapping of multi particle lithium iron phosphate cathode during fast charging (top to bottom). The called-out close-up frame shows that as the sample charges, some regions become completely delithiated (green) while others remain completely lithiated (red). This inhomogeneity results in a lower overall battery capacity than can be attained with slower charging, where delithiation occurs more evenly throughout the electrode.
Many previous methods used to analyze such battery materials have produced data that average out effects over the entire electrode. These methods lack the spatial resolution needed for chemical mapping or nanoscale imaging, and are likely to overlook possible small-scale effects and local differences within the sample, Wang