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

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.