Understanding the Electrochemical Formation and Decomposition of LiO and LiOH with X-ray Diffraction.

The lithium air, or Li-O, battery system is a promising electrochemical energy storage system because of its very high theoretical specific energy, as required by automotive applications. Fundamental research has resulted in much progress in mitigating detrimental (electro)chemical processes; however, the detailed structural evolution of the crystalline LiO and LiOH discharge products, held at least partially responsible for the limited reversibility and poor rate performance, is hard to measure under realistic electrochemical conditions. This study uses Rietveld refinement of X-ray diffraction data during a complete discharge-charge cycle to reveal the detailed structural evolution of LiO and LiOH crystallites in 1,2-dimethoxyethane (DME) and DME/LiI electrolytes, respectively. The anisotropic broadened reflections confirm and quantify the platelet crystallite shape of LiO and LiOH and show how the average crystallite shape evolves during discharge and charge. LiO is shown to form via a nucleation and growth mechanism, whereas the decomposition appears to start at the smallest LiO crystallite sizes because of their larger exposed surface. In the presence of LiI, platelet LiOH crystallites are formed by a particle-by-particle nucleation and growth process, and at the end of discharge, HO depletion is suggested to result in substoichiometric Li(OH) , which appears to be preferentially decomposed during charging. X-ray diffraction proves the cyclic formation and decomposition of the LiOH crystallites in the presence of LiI over multiple cycles, and the structural evolution provides key information for understanding and improving these highly relevant electrochemical systems.