A first-principles comparative study of lithium, sodium, and magnesium storage in pure and gallium-doped germanium: Competition between interstitial and substitutional sites

Effects of Se substitution and transition metal doping on the electronic and magnetic properties of a MoSxSe2-x/h-BN heterostructure

The van der Waals heterostructures created by stacking two monolayer semiconductors have been rapidly developed experimentally and exhibit various unique physical properties. In this work, we investigate the effects of Se atom substitution and 3d-TM atom doping on the structural, electronic, and magnetic properties of the MoSe2/h-BN heterostructure, by using first-principles calculations based on density functional theory (DFT). It is found that Se atom substitution could considerably enhance the band gaps of MoSe2/h-BN heterostructures. With an increase in the substitution concentration, the energy band changes from an indirect to a direct band gap when the substitution concentration exceeds a critical value. For 3d-TM atom doping, it is shown that V-, Mn-, Fe-, and Co-doped systems exhibit a half-metallic state and magnetic behavior, while there is no spin polarization in the Ni-doped case. The results provide a theoretical basis for the development of diluted magnetic semiconductors and spin devices based on the MoSxSe2-x/h-BN heterostructure.

Composition- and size-modulated porous bismuth-tin biphase alloys as anodes for advanced magnesium ion batteries

Rechargeable magnesium batteries have huge potential for applications in large scale energy storage systems due to their low cost and abundant sources. Nevertheless, not much attention has been paid to the development of alternative anodes for magnesium ion batteries (MIBs). Herein, we demonstrate a scalable strategy to fabricate bismuth (Bi)-tin (Sn) biphase anodes with a porous (P) structure and controllable compositions/sizes, through the design of triphase precursors with immiscible elements (with a positive enthalpy of mixing between elements) and selective phase corrosion. Here, one phase was selected as the sacrificial component to form three-dimensional porous channels, which differs from the mechanism by which a porous structure is generated in a classical dealloying process. We systematically investigate how the composition and size of P-Bi-Sn anodes affect their Mg storage properties. As an anode for MIBs, the P-Bi3Sn2 electrode exhibits an excellent reversible capacity retention of over 93% for 200 cycles at 1000 mA g-1. Most importantly, the Mg storage mechanism of P-Bi-Sn anodes was unveiled by operando X-ray diffraction.

Lithium Attachment to C60 and Nitrogen- and Boron-Doped C60: A Mechanistic Study

Fullerene-based materials including C60 and doped C60 have previously been proposed as anodes for lithium ion batteries. It was also shown earlier that n- and p-doping of small molecules can substantially increase voltages and specific capacities. Here, we study ab initio the attachment of multiple lithium atoms to C60, nitrogen-doped C60 (n-type), and boron doped C60 (p-type). We relate the observed attachment energies (which determine the voltage) to changes in the electronic structure induced by Li attachment and by doping. We compare results with a GGA (generalized gradient approximation) functional and a hybrid functional and show that while they agree semi-quantitatively with respect to the expected voltages, there are qualitative differences in the electronic structure. We show that, contrary to small molecules, single atom n- and p-doping will not lead to practically useful modulation of the voltage-capacity curve beyond the initial stages of lithiation.

On the Charge State of Titanium in Titanium Dioxide

The oxidation state of titanium in titanium dioxide is commonly assumed to be +4. This assignment is based on the ionic approximation and is used ubiquitously to rationalize phenomena observed with TiO₂. It implies a charge state +4 and that no further oxidation of the metal center is possible. We present a comprehensive electronic structure investigation of Ti ions, TiO₂ molecules, and TiO₂ bulk crystals using different density functional theory and wave function-based approaches, which shows that the charge state of Ti is +3. Specifically, there is evidence of a significant remaining contribution from valence s and d electrons of Ti, including the presence of a nuclear cusp around the Ti core. The charge corresponding to valence s and d states of Ti amounts to 1 e. This suggests the possibility of further oxidation of Ti in TiO₂ compounds and challenges the commonly assumed picture of assigning the oxidation state of Ti in titania to +4.