Understanding the Electrochemical Formation and Decomposition of Li2O2 and LiOH with Operando X-ray Diffraction

The lithium air, or Li-O2, 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 Li2O2 and LiOH discharge products, held at least partially responsible for the limited reversibility and poor rate performance, is hard to measure operando under realistic electrochemical conditions. This study uses Rietveld refinement of operando X-ray diffraction data during a complete discharge-charge cycle to reveal the detailed structural evolution of Li2O2 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 Li2O2 and LiOH and show how the average crystallite shape evolves during discharge and charge. Li2O2 is shown to form via a nucleation and growth mechanism, whereas the decomposition appears to start at the smallest Li2O2 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, H2O depletion is suggested to result in substoichiometric Li(OH)1-x , which appears to be preferentially decomposed during charging. Operando 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.