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Lithium concentration and diffusion:  Lithium-ion batteries are an attractive power source for a variety of mobile power applications (laptops, automobiles, cell phones) due to their relatively large energy and power density.  The adoption of Li-ion batteries in automotive applications has been limited for number reasons: recharging times are long in order to prevent thermal run-away, for instance from penetration of the separator; effective battery-pack lifetimes are limited by the number of charge/discharge cycles due to irreversible changes to electrodes;fully-electric automobile ranges are still limited by capacity of the battery pack in comparison with the internal combustion engine;all of which are impacted by the lithium diffusion and migration within the battery.  Similar to water management in fuel cells, the in situ measurement of the lithium distribution is both critical and challenging due to the metallic casing and probing a light element.  In this sense, neutron imaging can provide unique information due to the very large neutron total scattering cross-section (σT) of 6Li.  With the recent advent of high resolution (10 μm) imaging detectors, there is sufficient discrimination between anode, separator, and cathode.  By utilizing the isotopic sensitivity of neutrons – both 6Li and 1H have a large σT while both 7Li and 2H have a relatively small σT – it possible to study lithium migration or changes in the electrolyte distribution.  In situ high resolution neutron radiography measurement of lithium migration is performed using prismatic (planar) cells, as this yields a well defined geometry.  Before cycling the battery, a reference image is acquired; as the battery charges and discharges, changes in the neutron intensity can be directly correlated with changes in the lithium distribution.  Using this method, initial experiments conducted in December, 2009 successfully visualized the depletion of lithium from the anode and plating of lithium in the activated carbon cathode.  Future experiments will focus on the solid-electrolyte interface, and dendrite formation after multiple charge/discharge sequences. 

Providing critical in situ, non-destructive measurements of the lithium migration will enable battery developers to advance designs for automotive applications.  In particular, these measurements can yield insight into: lithium dendrite formation, which can penetrate the separator during charging and limits the charging time; how optimize intercalation of lithium into cathodes; and reducing the effects of solid-electrolyte interface on battery performance.

Daniel Hussey