Structure and Conductivity in LISICON Analogues within the Li 4 GeO 4 -Li 2 MoO 4 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 Li 2+2 x 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 4-2 x 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.