Effect of Electrode/Electrolyte Coupling on Birnessite (δ-MnO2) Mechanical Response and Degradation.

Abstract

Understanding the deformation of energy storage electrodes at a local scale and its correlation to electrochemical performance is crucial for designing effective electrode architectures. In this work, the effect of electrolyte cation and electrode morphology on birnessite (δ-MnO2) deformation during charge storage in aqueous electrolytes was investigated using a mechanical cyclic voltammetry approach via operando atomic force microscopy (AFM) and molecular dynamics (MD) simulation. In both K2SO4 and Li2SO4 electrolytes, the δ-MnO2 host electrode underwent expansion during cation intercalation, but with different potential dependencies. When intercalating Li+, the δ-MnO2 electrode presents a nonlinear correlation between electrode deformation and electrode height, which is morphologically dependent. These results suggest that the stronger cation-birnessite interaction is the reason for higher local stress heterogeneity when cycling in Li2SO4 electrolyte, which might be the origin of the pronounced electrode degradation in this electrolyte.