Structure-activity relationship: Understanding implications of cavity design for potassium-ion storage

Abstract

Structural stability of electrode material is probably the first and foremost requirement for realization of reversible potassium-ion storage in long-lasting battery applications. In this respect, careful cavity design is much needed because it not only establishes interconnected channels for ion transfer and reservoirs for electrolyte to buffer sudden changes in ion concentration, but also contributes a lot to stabilization of electrode structure. In this work, therefore, we tried to provide important insights on cavity design using multiple spherical structures as paradigms. After electrochemical characterization, a quantitative evaluation regime was proposed. This regime was mainly based on geometry and mechanics considerations. As for geometry criterion, surface expansion ratio ζ could be estimated by using r/R = [2−(1 + ζ)3/2]1/3, with r and R denoting yolk and shell radii, respectively. In terms of mechanics criterion, ζ could also be calculated by integrating εe = σe/Y with ζ = [2−(1 − εe)3]2/3 − 1, wherein εe, σe and Y denoted maximum elastic strain, maximum elastic stress and Young’s modulus, respectively. The ζ values deduced from both geometry and mechanics criteria ought to be in good consistency, so as to maintain structural integrity. Accordingly, this evaluation regime could be generalized to establish structure–activity relationship in electrode design and would provide a broad vision in understanding implications of cavity design for potassium-ion storage.