Scientists led by the University of Texas at Austin (UTA) claim to have solved a long-standing mystery for lithium-ion battery researchers, explaining why certain electrode materials have been shown to exhibit higher storage capacities than should have been physically possible.
In collaboration with the Qingdao University & Shandong University in China, as well as the Massachusetts Institute of Technology and Canada’s University of Waterloo, the group was able to demonstrate several compounds based on transition metal oxides that exhibit up to three times the storage capacity of today’s commercially available batteries, and to explain a phenomenon that enables these materials to have a capacity beyond what should be their theoretical limit.
“For nearly two decades, the research community has been perplexed by these materials' anomalously high capacities beyond their theoretical limits,” said UTA associate professor Guihua Yu. “This work demonstrates the very first experimental evidence to show the extra charge is stored physically inside these materials via space charge storage mechanism.”
The mechanism is described in the paper Extra storage capacity in transition metal lithium-ion batteries revealed by in situ magnetometry, published in Nature Materials. The group demonstrated that additional energy is stored at the surface of the metal oxide, thanks to metallic nanoparticles formed during discharge of the battery. These nanoparticles showed strong surface capacitance and the ability to store large numbers of electrons.
The phenomenon was shown to be the dominant source of extra capacity in iron oxide electrodes, and also to exist in cobalt and nickel oxides, as well as iron fluoride and nitrides. While understanding of these material’s behavior within a battery is still limited, the group is convinced that its findings represent the overcoming of a significant challenge to further development.
To observe the mechanism in action, they employed a technique called in situ magnetometry. This is commonly used in physics to study charge storage at very small scales, and relies on measuring variations in magnetism to quantify charge capacity. “The most significant results were obtained from a technique commonly used by physicists but very rarely in the battery community,” Yu said. “This is a perfect showcase of a beautiful marriage of physics and electrochemistry.”