Scott Misture1 Peng Gao1 Robert Koch1 Madeleine Flint1

1, Alfred University, Alfred, New York, United States

Investigation of charge storage mechanisms is of great importance in developing new electrochemical energy storage materials. Here we study the intercalation mechanisms, electrochemical strains and cyclic stability of defective 2-D nanosheet δ-MnO2 electrodes using a combination of high energy X-ray scattering and X-ray spectroscopy. MnO2 nanosheets have a peculiar Mn3+ defect, one that is displaced from the plane of the nanosheet to form a “surface Frenkel” defect. The quantity of sodium ions intercalated correlates with the surface Mn3+ Frenkel defect density as determined by gravimetric and X-ray pair distribution function (PDF) analysis. Electrochemical strain manifests as contractions of the Mn-O bond lengths with increasing charge state, and the data shows that the in-plane Mn-Mn coordination also increases while the surface displaced Mn concentration decreases. This result indicates that the surface coordinated Mn3+ may be extracted from and re-inserted into the plane of the nanosheet during charge/discharge cycling. EXAFS studies of K and Rb intercalation show that there is significant disorder in the local environments around these cations, but do not definitively identify the intercalation sites.