Structure and Conductivity in LISICON Analogues within the Li(4)GeO(4)–Li(2)MoO(4) System

[Image: see text] 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 Li(2+2x)Zn(1–x)GeO(4) and Li(3+x)V(1–x)Ge(x)O(4), have been relatively well studied. However, other systems such as the Li(4)GeO(4)–Li(2)MoO(4) system, which also show LISICON-type structures, have been relatively little explored. In this work, the Li(4–2x)Ge(1–x)Mo(x)O(4) 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, Li(4)GeO(4) and Li(2)MoO(4). With increasing Mo content, the β-phase becomes increasingly more stable than the γ-phase, and at x = 0.5, a pure β-phase (β-Li(3)Ge(0.5)Mo(0.5)O(4)) is readily isolated. The structure of this previously unknown compound is presented, along with details of the defect structure of Li(3.6)Ge(0.8)Mo(0.2)O(4) (x = 0.2) based on neutron diffraction data. Two basic types of defects are identified in Li(3.6)Ge(0.8)Mo(0.2)O(4) 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.