Interface and Interlayer Dynamics in 2-D MnO2 via In-Situ Characterization

Supercapacitor electrodes can be prepared from MnO2 nanosheets exfoliated from highly crystalline powder particles and subsequently flocculated to form porous solid electrodes. Restacking of the 2-D nanosheets in packets of ~5 layers affords high surface area electrodes with highly flexible interlayer spaces for electrochemical activity. The talk centers on the application in-situ and in-operando studies using high energy X-ray diffraction, X-ray pair distribution functions, EXAFS/XANES and Raman spectroscopy. We show that linking electrochemical measurements to local structure characterization results enables us to understand the mechanisms by which charging occurs in supercapacitor electrodes. The data show that the re-stacked 2-D nanosheets go through a phase transition during charge/discharge wherein the interlayer chemistry alters the interlayer stacking in a subtle but detectable manner. The presence vs. absence of interlayer water and electrochemically active ions including Na, K or Rb define the interlayer stacking. Using a combined approach that tracks the Mn oxidation state, the local structures (via PDF and Raman) and charge/discharge electron exchange forms a clear picture of the physical mechanisms at play, including mechanisms of capacity fade.