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

While manganese oxide (MnO2) 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 MnO2 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 MnO2. Also, theoretical calculations of phonon energy associated with the models of interlayer cation-incorporated MnO2 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 MnO2 electrode are correlated quantitatively with the amount of charge stored in the MnO2 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 MnO2 electrode.