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

摘要

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 andgrowth mechanism, whereas the decomposition appears to start at thesmallest Li2O2 crystallite sizes because oftheir larger exposed surface. In the presence of LiI, platelet LiOHcrystallites are formed by a particle-by-particle nucleation and growthprocess, and at the end of discharge, H2O depletion issuggested to result in substoichiometric Li(OH)1–x, which appears to be preferentially decomposed duringcharging. Operando X-ray diffraction proves the cyclicformation and decomposition of the LiOH crystallites in the presenceof LiI over multiple cycles, and the structural evolution provideskey information for understanding and improving these highly relevantelectrochemical systems.