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In conventional intercalation cathodes, alkali metal ions can move in and out of a layered material with the charge being compensated for by reversible reduction and oxidation of the transition metal ions. If the cathode material used in a lithium-ion or sodium-ion battery is alkali-rich, this can increase the battery's energy density by storing charge on the oxide and the transition metal ions, rather than on the transition metal alone.sup.1-10. There is a high voltage associated with oxidation of O.sup.2- during the first charge, but this is not recovered on discharge, resulting in reduced energy density.sup.11. Displacement of transition metal ions into the alkali metal layers has been proposed to explain the first-cycle voltage loss (hysteresis).sup.9,12-16. By comparing two closely related intercalation cathodes, Na.sub.0.75[Li.sub.0.25Mn.sub.0.75]O.sub.2 and Na.sub.0.6[Li.sub.0.2Mn.sub.0.8]O.sub.2, here we show that the first-cycle voltage hysteresis is determined by the superstructure in the cathode, specifically the local ordering of lithium and transition metal ions in the transition metal layers. The honeycomb superstructure of Na.sub.0.75[Li.sub.0.25Mn.sub.0.75]O.sub.2, present in almost all oxygen-redox compounds, is lost on charging, driven in part by formation of molecular O.sub.2 inside the solid. The O.sub.2 molecules are cleaved on discharge, reforming O.sup.2-, but the manganese ions have migrated within the plane, changing the coordination around O.sup.2- and lowering the voltage on discharge. The ribbon superstructure in Na.sub.0.6[Li.sub.0.2Mn.sub.0.8]O.sub.2 inhibits manganese disorder and hence O.sub.2 formation, suppressing hysteresis and promoting stable electron holes on O.sup.2- that are revealed by X-ray absorption spectroscopy. The results show that voltage hysteresis can be avoided in oxygen-redox cathodes by forming materials with a ribbon superstructure in the transition metal layers that suppresses migration of the transition metal. In oxygen-redox intercalation cathodes, voltage hysteresis can be avoided by forming cathode materials with a 'ribbon' superstructure in the transition metal layers that suppresses transition metal migration., Author(s): Robert A. House [sup.1] , Urmimala Maitra [sup.1] , Miguel A. Pérez-Osorio [sup.1] , Juan G. Lozano [sup.1] [sup.2] , Liyu Jin [sup.1] , James W. Somerville [sup.1] , [...]

TiO2anatase plays a central role in energy and environmentalresearch. A major bottleneck toward developing artificial photosynthesiswith TiO2is that it only absorbs ultraviolet light, owingto its large bandgap of 3.2 eV. If one could reduce the bandgap ofanatase to the visible region, TiO2-based photocatalysiscould become a competitive clean energy source. Here, using scanningtunneling microscopy and spectroscopy in conjunction with densityfunctional theory calculations, we report the discovery of a highlyreactive titanium-terminated anatase surface with a reduced bandgapof less than 2 eV, stretching into the red portion of the solar spectrum.By tuning the surface preparation conditions, we can reversibly switchbetween the standard anatase surface and the newly discovered lowbandgap surface phase. The identification of a TiO2anatasesurface phase with a bandgap in the visible and high chemical reactivityhas important implications for solar energy conversion, photocatalysis,and artificial photosynthesis. [ABSTRACT FROM AUTHOR]

We develop a first-principles computational method for investigating the dielectric screening in extended systems using the self-consistent Sternheimer equation and localized nonorthogonal basis sets. Our approach does not require the explicit calculation of unoccupied electronic states, uses only two-center integrals, and has a theoretical scaling of order 0(N3). We demonstrate this method by comparing our calculations for silicon, germanium, diamond, and LiCl with reference plane-wave calculations. We show that accuracy comparable to that of plane-wave calculations can be achieved via a systematic optimization of the basis set. [ABSTRACT FROM AUTHOR]