To understand the structural and compositional factors controlling lithium transport in sulfides, we explored the Li5AlS4–Li4GeS4 phase field for new materials. Both parent compounds are defined structurally by a hexagonal close packed sulfide lattice, where distinct arrangements of tetrahedral metal sites give Li5AlS4 a layered structure and Li4GeS4 a three-dimensional structure related to γ-Li3PO4. The combination of the two distinct structural motifs is expected to lead to new structural chemistry. We identified the new crystalline phase Li4.4Al0.4Ge0.6S4, and investigated the structure and Li+ ion dynamics of the family of structurally related materials Li4.4 M 0.4 M′0.6S4 (M = Al3+, Ga3+ and M′ = Ge4+, Sn4+). We used neutron diffraction to solve the full structures of the Al-homologues, which adopt a layered close-packed structure with a new arrangement of tetrahedral (M/M′) sites and a novel combination of ordered and disordered lithium vacancies. AC impedance spectroscopy revealed lithium conductivities in the range of 3(2) × 10–6 to 4.3(3) × 10–5 S cm–1 at room temperature with activation energies between 0.43(1) and 0.38(1) eV. Electrochemical performance was tested in a plating and stripping experiment against Li metal electrodes and showed good stability of the Li4.4Al0.4Ge0.6S4 phase over 200 h. A combination of variable temperature 7Li solid state nuclear magnetic resonance spectroscopy and ab initio molecular dynamics calculations on selected phases showed that two-dimensional diffusion with a low energy barrier of 0.17 eV is responsible for long-range lithium transport, with diffusion pathways mediated by the disordered vacancies while the ordered vacancies do not contribute to the conductivity. This new structural family of sulfide Li+ ion conductors offers insight into the role of disordered vacancies on Li+ ion conductivity mechanisms in hexagonally close packed sulfides that can inform future materials design.
The structure and Li + ion dynamics of a new class of ABO 3 perovskite with Li on both the A-and B-sites are described. La 3 Li 3 W 2 O 12 is synthesized by solid state reaction at 900 °C and shown by powder X-ray diffraction to adopt the structure of a monoclinic double perovskite (A 2 )BB′O 6 , (La 1.5 Li 0.5 )WLiO 6 , with rock salt order of W 6+ and Li + on the B-site. High resolution powder neutron diffraction locates A-site Li in a distorted tetrahedron displaced from the conventional perovskite A-site, which differs considerably from the sites occupied by Li in the well studied La 2/3−x Li 3x TiO 3 family. This is confirmed by the observation of a lower coordinated Li + ion in the 6 Li magic angle spinning nuclear magnetic resonance (NMR) spectra, in addition to the B-site LiO 6 , and supported computationally by density functional theory (DFT), which also suggests local order of A-site La 3+ and Li + . DFT shows that the vacancies necessary for transport can arise from Frenkel or La excess defects, with an energetic cost of ∼0.4 eV/vacancy in both cases. Ab initio molecular dynamics establishes that the Li + ion dynamics occur by a pathway involving a series of multiple localized Li hops between two neighboring A-sites with an overall energy barrier of ∼0.25 eV, with additional possible pathways involving Li exchange between the A-and B-sites. A similar activation energy for Li + ion mobility (∼0.3 eV) was obtained from variable temperature 6 Li and 7 Li line narrowing and relaxometry NMR experiments, suggesting that the barrier to Li hopping between sites in La 3 Li 3 W 2 O 12 is comparable to the best oxide Li + ion conductors. AC impedance-derived conductivities confirm that Li + ions are mobile but that the long-range Li + diffusion has a higher barrier (∼0.5 eV) which may be associated with blocking of transport by A-site La 3+ ions.
This study investigates the effect of 1 mmol dm(-3) concentrations of a selection of small cationic molecules on the performance of a fuel cell grade oxygen reduction reaction (ORR) catalyst (Johnson Matthey HiSPEC 3000, 20 mass% Pt/C) in aqueous KOH (1 mol dm(-3)). The cationic molecules studied include quaternary ammonium (including those based on bicyclic systems) and imidazolium types as well as a phosphonium example: these serve as fully solubilised models for the commonly encountered head-groups in alkaline anion-exchange membranes (AAEM) and anion-exchange ionomers (AEI) that are being developed for application in alkaline polymer electrolyte fuel cells (APEFCs), batteries and electrolysers. Both cyclic and hydrodynamic linear sweep rotating disk electrode voltammetry techniques were used. The resulting voltammograms and subsequently derived data (e.g. apparent electrochemical active surface areas, Tafel plots, and number of [reduction] electrons transferred per O2) were compared. The results show that the imidazolium examples produced the highest level of interference towards the ORR on the Pt/C catalyst under the experimental conditions used.
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