ABSTRACT
Magnetic resonance imaging provides a noninvasive method for in situ monitoring of electrochemical processes involved in charge/discharge cycling of batteries. Determining how the electrochemical processes become irreversible, ultimately resulting in degraded battery performance, will aid in developing new battery materials and designing better batteries. Here we introduce the use of an alternative in situ diagnostic tool to monitor the electrochemical processes. Utilizing a very large field-gradient in the fringe field of a magnet, stray-field-imaging (STRAFI) technique significantly improves the image resolution. These STRAFI images enable the real time monitoring of the electrodes at a micron level. It is demonstrated by two prototype half-cells, graphiteâ¥Li and LiFePO4â¥Li, that the high-resolution (7)Li STRAFI profiles allow one to visualize in situ Li-ions transfer between the electrodes during charge/discharge cyclings as well as the formation and changes of irreversible microstructures of the Li components, and particularly reveal a non-uniform Li-ion distribution in the graphite.
Subject(s)
Electric Power Supplies , Energy Transfer , Equipment Failure Analysis/methods , Lithium/chemistry , Magnetic Resonance Imaging/methods , Magnetometry/methods , IonsABSTRACT
Stray field imaging (STRAFI) has provided an alternative imaging method to study solid materials that are typically difficult to obtain using conventional MRI methods. For small volume samples, image resolution is a challenge since extremely strong gradients are required to examine narrow slices. Here we present a STRAFI probe for imaging materials with quadrupolar nuclei. Experiments were performed on a 19.6 T magnet which has a fringe field gradient strength of 72 T/m, nearly 50 times stronger than commercial microimagers. We demonstrate the ability to acquire (7)Li 1D profiles of liquid and solid state lithium phantoms with clearly resolved features in the micrometer scale and as a practical example a Li ion battery electrode material is also examined.
ABSTRACT
Octaethyl porphyrin (OEP) and its Ni and Zn derivatives are considered as model compounds in biochemical, photophysical, and fossil fuel chemistry. They have thus been investigated by high-resolution solid-state (13)C NMR using powders, but peak assignment has been difficult because of large line widths. Arguing that a significant cause of broadening might be the anisotropic bulk magnetic susceptibility, we utilized single crystals in our (13)C cross-polarization magic angle spinning (CP-MAS) measurements and observed a nearly 2-fold line narrowing. This enhanced resolution enabled us to assign chemical shifts to each carbon for all the three compounds. The new assignments are now in agreement with X-ray structural data and allowed us to probe the motional dynamics of the methyl and methylene carbons of the OEP side chains. It is apparent that the use of single crystals in (13)C CP-MAS measurements has a significantly wider impact than previously thought.
