Professor of Chemical Engineering
1997 our research group moved into the D-level of Tan Hall and established
a unique facility to develop spectroscopic techniques that we could
use to study materials chemistry and processing. With a specific emphasis
on solid state NMR and magnetic resonance imaging, a major component
of our research plan is the construction and application of new hardware
(such as NMR probes) to solve complex materials science problems in
the areas of catalysis, electrochemistry, and semiconductor and polymer
processing. At present the D-level lab houses five NMR or MRI spectrometers,
most of which are intricately woven into home-built apparatuses.
Using NMR we can directly observe lithium in the bulk of a battery electrode and gain a unique insight into the local atomic and electronic environment of the lithium ion. By studying the changes in this local environment during electrochemical cycling of the material, we can explore the critical connection between the atomic-scale structure of the electrode and the resulting electrochemical performance. Our recent research has ranged from fundamental solid-state chemistry to applied electrochemistry and focuses on both novel and well-studied materials.
One type of electrode material we have explored is the lithium-manganese-oxide spinel, LiMn2O4, system. Spinels are simply groups of minerals that are essentially oxides of magnesium, ferrous iron, zinc, or manganese. It is well known that the LiMn2O4 system can withstand many more charge-discharge cycles before failure when Cr, Al, or other metal ions substitute for some of the manganese in the spinel crystal. The mechanism of failure and the role of metal substitution are still subjects of debate. We have used NMR and other techniques to study the evolution of the atomic-scale structure of spinel materials upon charge-discharge cycling and after failure. Our results suggest that the dominant mode of failure is through manganese dispersion via a lithium-for-manganese ion exchange process. We have furthermore demonstrated that substitution of the manganese promotes covalence in the Li-O-Mn bond, producing a more robust material that can withstand the rigors of long-term electrochemical cycling.
Left: Crystal structure of the spinel containing lithium, manganese, and oxygen in addition to vacant sites. Right: The local atomic structure spectrum for LiFePO4, a novel electrode material.