Electrical Properties and Sinterability for Lithium Germanium Phosphate Li1+xMxGe2−x(PO4)3, M = Al, Cr, Ga, Fe, Sc, and In Systems

The electrical properties and sinterability were studied for Li1+xMxGe2−x(PO4)3, M = Al3+, Cr3+, Ga3+, Fe3+, Sc3+, and In3+ systems. Due to the closer ionic radius of Al3+ and Cr3+ compared to that of Ge4+, those M3+ ions easily substitute the Ge4+ site. Larger cations, such as Ga3+, Fe3+, Sc3+, and In3+, were difficult to substitute the Ge4+ site. The ionic conductivity and sinterability improved with an increase in x for all of the M3+-substituted systems. In particular, an Al3+- or Cr3+-substituted system shows higher conductivity; the maximum conductivity is 2.4 × 10−4 S cm−1 at 298 K for Li1.5Al0.5Ge1.5(PO4)3. The enhancement in the conductivity is attributed to a decrease in the porosity and a lowering of the activation energy in the grain boundaries. The activation energy for Li+ ion conduction of the bulk component was 0.38 eV for Li1+xMxGe2−x(PO4)3 electrolytes, and was almost independent of M3+ substitution.