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

Abstract

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 ₄GeO ₄–Li ₂MoO 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 ₄GeO ₄ and Li ₂MoO 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 ₃Ge _0.5Mo _0.5O ₄) is readily isolated. The structure of this previously unknown compound is presented, along with details of the defect structure of Li _3.6Ge _0.8Mo _0.2O ₄ ( x = 0.2) based on neutron diffraction data. Two basic types of defects are identified in Li _3.6Ge _0.8Mo _0.2O 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.

Abstract

A γ-LISICON-structured solution is formed in the Li _{4−2 x} Ge _{1− x} Mo _x O ₄ system in the range of 0.1 ≤ x < 0.4. Two basic types of defects are identified in the γ-phase involving interstitial Li ⁺-ions in octahedral sites, with evidence for these coming together to form larger defect clusters. With increasing Mo content, the β-phase becomes increasingly more stable than the γ-phase, and at x = 0.5, a previously unknown β-phase (β-Li ₃Ge _0.5Mo _0.5O ₄) is readily isolated.