Dynamics of Defects in 2-D MnO2 During Charge-Discharge Cycling

MnO₂ in its many forms has attractive properties for a wide range of applications from catalysis to charge storage. 2-D MnO₂ nanosheets demonstrate an interesting response to reduction of Mn⁴⁺ to Mn³⁺, where Jahn-Teller distorted Mn³⁺ is displaced out of the plane of the nanosheet forming a “surface Frenkel” defect. We find that introducing this unusual defect results in dramatic improvements in the electrochemical charge storage capacity as well as the catalytic activity for many reactions. Specimens were prepared via exfoliation of nominally defect-free KxMnO2 crystals and by electrodeposition to enable studies of a range of Mn3+ defect content. The talk centers on the application of advanced materials characterization tools for study of complex nanoscale 2-D oxides. In-situ and in-operando studies using high energy X-ray diffraction, X-ray pair distribution functions, EXAFS/XANES and Raman spectroscopy will be discussed. We show that linking electrochemical measurements to local structure characterization results enables us to understand the mechanisms by which charging occurs in supercapacitor electrodes. It is clear that the Mn3+ defects are not altered during electrochemical cycling but that they dramatically alter the electronic band structure, yielding large improvements in charge transfer resistance. Further detailed PDF and Raman studies show that partial nanoscale conversion to tunnel-form MnO2 is a primary degradation mechanism that lowers capacitance compared to the layered MnO2.