Abstract
Formation of the redox active insulators such as Lithium peroxide (Li2O2), Lithium sulfide (Li2S) are the salient features for next generation 'beyond intercalation' batteries like metal-air (O2) and metal-sulfur (Li-S) batteries (1-3). The interest in these batteries arises from high theoretical energies, abundant elements, low cost, and environmental friendliness. The mechanism to deposit redox active insulators (Li2O2 for Li-O2 and Li2S for Li-S) during discharging these batteries governs their rate capability, capacity, and reversibility. The prime obstacles for rate capability, capacity, and reversibility for Li-O2 batteries are the parasitic reactions. Our previous works discovered that formation of highly active singlet oxygen is the main cause of these parasitic reactions (1, 4-6). On charging Li-O2 battery, the practically achieving high reversible capacities face the challenge to decompose large amounts of insulating Li2O2 while suppressing parasitic reactions. These challenges require understanding the detailed Li2O2 formation mechanism and the interplay between chemistry and morphological evolution.
The techniques that we use to decipher the mechanism of the formation of Li2O2 include microscopy and electrochemical generator-collector experiments (hydrodynamic voltammetry and interdigitated electrodes). Previously we illustrated in-situ small and wide-angle X-ray scattering as a novel method to study the morphological evolution in Li-O2 and Li-S batteries (3, 7). The experimental data show that the redox active insulator Li2O2 forms exclusively as particles via solution mediated LiO2 disproportionation during discharging. This contradicts established understanding, stating that the separation between surface adsorbed and solvated LiO2 governs whether Li2O2 grows via a surface mechanism or solution mechanism. We also introduced tools to access complex electrochemical and growth mechanism useful for other systems. This helps us to decipher the mechanism to form and to deposit redox active insulators during charging/discharging of the batteries, and thus to understand the governing factors for capacity, rate capability and reversibility.
References:
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