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

New solid electrolytes are crucial for the developmentof all-solid-statelithium batteries with advantages in safety and energy densities overcurrent liquid electrolyte systems. While some of the best solid-stateLi(+)-ion conductors are based on sulfides, their air sensitivitymakes them less commercially attractive, and attention is refocusingon air-stable oxide-based systems. Among these, the LISICON-structuredsystems, such as Li2+2x Zn1-x GeO4 and Li3+x V1-x Ge x O4, have been relatively well studied. However, othersystems such as the Li4GeO4-Li2MoO4 system, which also show LISICON-type structures,have been relatively little explored. In this work, the Li4-2x Ge1-x Mo x O4 solid solution is investigated systematically,including the solid solution limit, structural stability, local structure,and the corresponding electrical behavior. It is found that a & gamma;-LISICONstructured solution is formed in the range of 0.1 & LE; x < 0.4, differing in structure from the two end members,Li4GeO4 and Li2MoO4. Withincreasing Mo content, the & beta;-phase becomes increasingly morestable than the & gamma;-phase, and at x = 0.5, apure & beta;-phase (& beta;-Li3Ge0.5Mo0.5O4) is readily isolated. The structure of this previouslyunknown compound is presented, along with details of the defect structureof Li3.6Ge0.8Mo0.2O4 (x = 0.2) based on neutron diffraction data. Two basic typesof defects are identified in Li3.6Ge0.8Mo0.2O4 involving interstitial Li+-ionsin octahedral sites, with evidence for these coming together to formlarger defect clusters. The x = 0.2 composition showsthe highest conductivity of the series, with values of 1.11 x10(-7) S cm(-1) at room temperaturerising to 5.02 x 10(-3) S cm(-1) at 250 & DEG;C. A & gamma;-LISICON-structured solution is formed in the Li4-2x Ge1-x Mo x O4 system in therange of 0.1 & LE; x < 0.4. Two basic typesof defects are identified in the & gamma;-phase involving interstitialLi(+)-ions in octahedral sites, with evidence for these comingtogether to form larger defect clusters. With increasing Mo content,the & beta;-phase becomes increasingly more stable than the & gamma;-phase,and at x = 0.5, a previously unknown & beta;-phase(& beta;-Li3Ge0.5Mo0.5O4) is readily isolated.