We have developed a new laboratory X-ray diffraction technique to probe
meso-scale inhomogeneities in battery cathode materials. Conventional
diffraction uses an X-ray beam that covers most of the cathode
surface, giving average properties over all locations. In contrast,
our technique uses a laboratory diffractometer with a sub-millimeter
beam to extract location-specific structure information. Up until now,
this has only been reported with synchrotron radiation. We have used
this technique to uncover inhomgeneities related to a variety of
factors, such as preparation technique, the size and position of the
anode, and the charge rate of the cell. For example, in one
experiment, sections of an LiMn₂O₄ cathode were found to lag behind
the global state in areas of poor contact with the current
collector. This effect was not seen at slow rates (C/10), however,
suggesting that alternate electrical pathways are still available,
though kinetically less viable. This technique can also be used to map
an electrode's cross-section to find effects occuring along the
direction of charge flow.