Phillip E. Stallworth

Achieving electrochemical capacitor functionality from nanoscale LiMn2O4 coatings on 3-D carbon nanoarchitectures

Conformal nanoscale coatings of Na+-birnessite manganese oxide (MnOx) produced via redox reaction between aqueous permanganate (NaMnO4·H2O) and the carbon surfaces of fiber-paper-supported carbon nanofoams are converted to LiMn2O4 spinel through topotactic exchange of Na+ for Li+ in the as-deposited lamellar birnessite, followed by mild thermal treatments to complete the transformation to LiMn2O4. The evolution of the birnessite-to-spinel conversion is verified with X-ray diffraction, solid-state nuclear magnetic resonance, X-ray absorption spectroscopy, electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. The mild conditions used to convert birnessite to spinel ensure that the conformal nanoscale nature of the oxide coating is retained throughout the macroscopically thick (170 μm) carbon nanofoam substrate during the conversion process. The architecture of the LiMn2O4–carbon nanofoam facilitates rapid ion/electron transport, enabling the LiMn2O4 to insert and extract Li+ from aqueous electrolytes at scan rates as high as 25 mV s−1, and with a relaxation time of 37 s as derived from electrochemical impedance. This architectural expression of nanoscale LiMn2O4 delivers full theoretical capacity (148 mA h g−1) at 2 mV s−1.

Lithium-7 nuclear magnetic resonance and Ti K-edge X-ray absorption spectroscopic investigation of electrochemical lithium insertion in Li4/3+xTi5/3O4

Abstract The spinel compound Li4/3+xTi5/3O4 is known to undergo reversible lithium intercalation up to x=1 with almost no change in lattice parameters, hence its designation as a “zero strain” intercalation compound. Structural changes that accompany electrochemical Li intercalation into this compound were studied by both 7 Li nuclear magnetic resonance (NMR) and Ti K-edge X-ray absorption fine structure (XAFS). The NMR results demonstrate that Li occupancies do not follow a simple distribution between two possible sites, one tetrahedral and one octahedral. The presence of at least one additional octahedral site is suggested. Line width measurements show that the Li+ ions do not return to their original distribution after cycling. XAFS results indicate the presence of modest static disorder in Tiue5f8O and Tiue5f8Ti distances above x=0.5. Both methods thus reveal subtle structural details previously unobserved by X-ray diffraction (XRD).