Li-ion Batteries

The potential safety hazards of lithium ion batteries (LIBs), as well as their capacity fade and long charging time, are major challenges that currently prevent the widespread adoption of electric vehicles.  These effects are strongly correlated with lithium plating, a parasitic reaction on graphite anodes that competes with lithium intercalation when LiBs are charged at high rates or under low temperature. Li plating is known for depletion of the lithium inventory, cell shorting, and thermal runaway.

Addressing Li plating problem relies on material engineering and battery operation optimization. The efficacy of these approaches lies in the fundamental understanding of Li plating mechanism. Despite Li ion battery has been invented for 30 years, a systematic understanding on the onset and growth of Li in graphite anode is still lacking. Thermodynamics criterion requires the voltage of graphite to be negative vs. Li metal for Li plating to happen. However, in practice graphite is usually able to tolerate certain degree of negative voltage without plating. Such discrepancy necessitates the incorporation of kinetic factors. Several kinetic mechanisms have been proposed, which include electrolyte transport limitation and solid diffusion limitation. However, neither of them are validated experimentally on graphite electrode.

In this work, we test these hypotheses and elucidate the mechanism of Li plating, by investigating the interplay between Li intercalation and Li plating on a single graphite particle, using in-situ optical microscopy coupled with electrochemical test. The discovery sheds light on directions and guidelines of materials innovation or electrode design for reducing the risk of Li plating and extreme fast charging. 

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