Structure and Conductivity in LISICON Analogues within the Li4GeO4-Li2MoO4 System.

New solid electrolytes are crucial for the development of all-solid-state lithium batteries with advantages in safety and energy densities over current liquid electrolyte systems. While some of the best solid-state Li+-ion conductors are based on sulfides, their air sensitivity makes them less commercially attractive, and attention is refocusing on air-stable oxide-based systems. Among these, the LISICON-structured systems, such as Li2+2xZn1-xGeO4 and Li3+xV1-xGexO4, have been relatively well studied. However, other systems such as the Li4GeO4-Li2MoO4 system, which also show LISICON-type structures, have been relatively little explored. In this work, the Li4-2xGe1-xMoxO4 solid solution is investigated systematically, including the solid solution limit, structural stability, local structure, and the corresponding electrical behavior. It is found that a γ-LISICON structured solution is formed in the range of 0.1 ≤ x < 0.4, differing in structure from the two end members, Li4GeO4 and Li2MoO4. With increasing Mo content, the ß-phase becomes increasingly more stable than the γ-phase, and at x = 0.5, a pure ß-phase (ß-Li3Ge0.5Mo0.5O4) is readily isolated. The structure of this previously unknown compound is presented, along with details of the defect structure of Li3.6Ge0.8Mo0.2O4 (x = 0.2) based on neutron diffraction data. Two basic types of defects are identified in Li3.6Ge0.8Mo0.2O4 involving interstitial Li+-ions in octahedral sites, with evidence for these coming together to form larger defect clusters. The x = 0.2 composition shows the highest conductivity of the series, with values of 1.11 × 10-7 S cm-1 at room temperature rising to 5.02 × 10-3 S cm-1 at 250 °C.

Local Structure in α-BIMEVOXes (ME = Ge, Sn).

The BIMEVOXes are among the best oxide ion conductors at low and intermediate temperatures. Their high conductivity is associated with local defect structure. In this work, the local structures of two BIMEVOX compositions, Bi2V0.9Ge0.1O5.45 and Bi2V0.95Sn0.05O5.475, are examined using total neutron and X-ray scattering methods, with both compositions exhibiting the ordered α-phase at 25 °C and the disordered γ-phase at 700 °C. While the diffraction data for the α-phase do not allow for the polar (C2) and nonpolar (C2/m) structures to be readily distinguished, measurements of dielectric permittivity suggest the α-phase is weakly ferroelectric in character, consistent with calculations of spontaneous polarization based on a combination of density functional calculations and machine learning methodology. Reverse Monte Carlo (RMC) analysis of total scattering data reveals Ge preferentially adopts tetrahedral geometry at both temperatures, while Sn is found to predominantly adopt octahedral coordination in the α-phase and tetrahedral coordination in the γ-phase. In all cases, V polyhedra are found to consist of tetrahedral, pentacoordinate, and octahedral geometries, as also predicted by the crystallographic analysis and confirmed by 51V solid state NMR spectroscopy. Although similar long-range structures are observed at room temperature, the oxide ion vacancy distributions were found to be quite different between the two studied compositions, with a nonrandom deficiency in vacancy pairs in the second-nearest shell along the ⟨100⟩ tetragonal direction for BIGEVOX10, compared with a long-distance (>8.0 Å) ordering of equatorial vacancies for BISNVOX05. This is attributed to the differences in the preferred coordination geometries of the substituent cations in the two systems. Impedance spectroscopy measurements reveal both compositions show high conductivity in the order of 10-1 S cm-1 at 600 °C.

Defect structure in δ-Bi5PbY2O11.5.

A detailed study of the defect structure in a di-substituted δ-Bi2O3 type phase, δ-Bi5PbY2O11.5, is presented. Using a combination of conventional Rietveld analysis of neutron diffraction data, reverse Monte Carlo (RMC) analysis of total neutron scattering data and ab initio molecular dynamics (MD) simulations, both average and local structures have been characterized. δ-Bi5PbY2O11.5 represents a model system for the highly conducting δ-Bi2O3 type phases, in which there is a higher nominal vacancy concentration than in the unsubstituted parent compound. Uniquely, the methodology developed in this study has afforded the opportunity to study both oxide-ion vacancy ordering as well as specific cation-cation interactions. Oxide-ion vacancies in this system have been found to show a preference for association with Pb2+ cations, with some evidence for clustering of these cations. The system shows a non-random distribution of vacancy pair alignments, with a preference for 〈100〉 ordering, the extent of which shows thermal variation. MD simulations indicate a predominance of oxide-ion jumps in the 〈100〉 direction.