Sensitization of Perovskite Strontium Stannate SrSnO 3 towards Visible-Light Absorption by Doping

Perovskite strontium stannate SrSnO 3 is a promising photocatalyst. However, its band gap is too large for efficient solar energy conversion. In order to sensitize SrSnO 3 toward visible-light activities, the effects of doping with various selected cations and anions are investigated by using hybrid density functional calculations. Results show that doping can result in dopant level to conduction band transitions which lie lower in energy compared to the original band gap transition. Therefore, it is expected that doping SrSnO 3 can induce visible-light absorption

Elucidating lithium-ion and proton dynamics in anti-perovskite solid electrolytes

All-solid-state Li-ion batteries are currently attracting considerable research attention as they present a viable opportunity for increased energy density and safety when compared to conventional liquid electrolyte-based devices. The Li-rich anti-perovskite Li3−xOHxCl has generated recent interest as a potential solid electrolyte material, but its lithium and proton transport capabilities as a function of composition are not fully characterised. In this work, we apply a combination of ab initio molecular dynamics and 1H, 2H and 7Li solid-state NMR spectroscopy to study the mobility of lithium ions and protons in Li3−xOHxCl. Our calculations predict a strongly exothermic hydration enthalpy for Li3OCl, which explains the ease with which this material absorbs moisture and the difficulty in synthesising moisture-free samples. We show that the activation energy for Li-ion conduction increases with increasing proton content. The atomistic simulations indicate fast Li-ion diffusion but rule out the contribution of long-range proton diffusion. These findings are supported by variable-temperature solid-state NMR experiments, which indicate localised proton motion and long-range Li-ion mobility that are intimately connected. Our findings confirm that Li3−xOHxCl is a promising solid electrolyte material for all-solid-state Li-ion batteries

High voltage structural evolution and enhanced Na-ion diffusion in P2-Na2/3Ni1/3-xMgxMn2/3O2 (0 < x < 0.2) cathodes from diffraction, electrochemical and ab initio studies

We have presented a detailed investigation of the effects of Mg substitution on the structure, electrochemical performance and Na-ion diffusion in high voltage P2-type Na2/3Ni1/3-xMgxMn2/3O2 (0 <x< 0.20) cathode materials for Na-ion batteries. Structural analysis using neutron diffraction showed that Mg2+ substitutes random Ni2+ on the 2b sites from ordered [(Ni2+/Mn4+)O6] honeycomb units along the ab-plane, leading to an AB-type structure that can be indexed using the P63 space group. Within the sodium layers, high Mg-substituting levels (i.e. x = 0.2) caused a disruption in the typical Na zig-zag ordering observed in the undoped material, leading to a more disordered Na distribution in the layers. Load curves of the x = 0.1, 0.2 materials show smooth electrochemistry, indicative of a solid-solution process. Furthermore, DFT calculations showed an increase on Na-ion diffusivity on the Mg-substituted samples. Enhanced cycling stability was also observed in these materials; structural analysis using high-resolution in-operando synchrotron X-ray diffraction show that such an improved electrochemical performance is caused by the suppression of the O2 phase and switch to the formation of an OP4 phase. Ab-initio studies support our experimental evidence showing that the OP4 phase (cf. O2) is the most thermodynamically stable phase at high voltages for Mg-substituted compounds. Finally, we have provided evidence using diffraction for the x = 1/2 and x = 1/3 intermediate Na+-vacancy ordered phases in P2-Na 2/3Ni1/3Mn2/3O2

Simulations for new battery materials

Using first-principles density functional calculations, LiNiO2-related cathode materials are studied. It is found that in contrast to previous studies, the hole state in Li doped NiO shows predominately Ni character and is accompanied by a local Jahn-Teller distortion. We show that this is consistent with experiments. A new potential ground state LiNiO2 cell is found in which charge disproportionation Ni3+Ni2++Ni4+ occurs. However another cell in which the Jahn-Teller distortions of Ni3+ octahedral are in a zigzag ordering, is close in energy. Therefore we suggest that in real LiNiO2 samples, the two phases coexist. This explains the absence of long range ordering in LiNiO2. Rock-salt LiMO2 compounds crystallise in three different structures depending on the cation ordering. We show that this cannot be explained by the size effect and propose that the exchange interaction between M ions is responsible for the ordering. Both size difference between Li and M and the exchange interaction between nearest-neighbouring M ions favour the layered structure, whereas the exchange interaction between second-nearest-neighbouring M ions destabilises the layered structure. The defect formation energies are low in LiNiO2, consistent with the difficulty to synthesise truly stoichiometric LiNiO2. The tendency for Ni to be present in the Li layers can be explained by super-exchange interactions. Therefore with Co substitution for Ni, the nonmagnetic Co ions screen these interactions and destabilise the presence of Ni in the Li layer. The same effect is found with Al substitution from our calculations. We also show why substitution of Ni by Mn increases the concentration of the interlayer mixing defects worse compared to LiNiO2. In addition, a correlation between the oxygen charge and the defect formation of oxygen vacancy is found. It appears that the lower the effective oxygen charge, the smaller the defect formation energy

The nature of the hole states in Li doped NiO

We have performed density functional calculations on Li0.125Ni0.875O using both the HSE06 hybrid functional and the DFT+U method. Contrary to previous calculations, both methods show that the system is better described with the hole localized on the nickel ion (which is thus formally Ni3+) rather than in the oxygen valence band. We discuss the experimental results in the light of this finding and show that it is consistent with the available data

Sensitization of Perovskite Strontium Stannate SrSnO 3

Perovskite strontium stannate SrSnO3 is a promising photocatalyst. However, its band gap is too large for efficient solar energy conversion. In order to sensitize SrSnO3 toward visible-light activities, the effects of doping with various selected cations and anions are investigated by using hybrid density functional calculations. Results show that doping can result in dopant level to conduction band transitions which lie lower in energy compared to the original band gap transition. Therefore, it is expected that doping SrSnO3 can induce visible-light absorption

Lithium Extraction Mechanism in Li-Rich Li₂MnO₃ Involving Oxygen Hole Formation and Dimerization

Lithium-rich oxide electrodes with layered structures have attracted considerable interest because they can deliver high energy densities for lithium-ion batteries. However, there is significant debate regarding their redox chemistry. It is apparent that the mechanism of lithium extraction from lithium-rich Li₂MnO₃ is not fully understood, especially in relation to the observed O₂ evolution and structural transformation. Here, delithiation and kinetic processes in Li₂MnO₃ are investigated using <i>ab initio</i> simulation techniques employing high level hybrid functionals as they reproduce accurately the electronic structure of oxygen hole states. We show that Li extraction is charge-compensated by oxidation of the oxide anion, so that the overall delithiation reaction involves lattice oxygen loss. Localized holes on oxygen (O^–) are formed as the first step but are not stable leading to oxygen dimerization (with O–O ∼ 1.3 Å) and eventually to the formation of molecular O₂. Oxygen dimerization facilitates Mn migration onto octahedral sites in the vacated lithium layers. The results suggest that reversible oxygen redox without major structural changes is only possible if the localized oxygen holes are stabilized and oxygen dimerization suppressed. Such an understanding is important for the future optimization of new lithium-rich cathode materials for high energy density batteries