Probing the Charge Storage Mechanism of a Pseudocapacitive MnO2 Electrode Using in Operando Raman Spectroscopy

While manganese oxide (MnO 2 ) has been extensively studied as an electrode material for pseudocapacitors, a clear understanding of its charge storage mechanism is still lacking. Here we report our findings in probing the structural changes of a thin-film model MnO 2 electrode during cycling using in operando Raman spectroscopy. The spectral features (e.g., band position, intensity, and width) are correlated quantitatively with the size (Li +, Na +, and K + ) of cations in different electrolytes and with the degree of discharge to gain better understanding of the cation-incorporation mechanism into the interlayers of pseudocapacitive MnO 2 . Also, theoretical calculations of phonon energy associated with the models of interlayer cation-incorporated MnO 2 agree with the experimental observations of cation-size effect on the positions of Raman bands. Furthermore, the cation-size effects on spectral features at different potentials of MnO 2 electrode are correlated quantitatively with the amount of charge stored in the MnO 2 electrode. The understanding of the structural changes associated with charge storage gained from Raman spectroscopy provides valuable insights into the cation-size effects on the electrochemical performances of the MnO 2 electrode.