Germanium Germanium as a donor dopant in garnet electrolytes

Cubic Li7La3Zr2O12 (LLZO) garnet electrolytes continue to be viewed as an enabler of all-solid-state lithium battery technologies, or as protective membranes for next-generation lithium battery systems. Supervalent do pants at the lithium sublattice are commonly used to stabilise the conductive cubic phase, through the creation of lithium vacancies. The use of germanium (Ge4+) as a higher valent dopant substituting for Li+ was studied here and shown to stabilise the cubic LLZO phase through substitution of x = 0.10 mol of Ge (at the tetrahedral 24d Li sites of the space group la-3d, based on the neutron powder diffraction result). This substitution preference follows that of Al3+ (having a similar ionic radius and reported to reside at 24d sites), but with a lower critical concentration for cubic phase stabilisation, in agreement with charge neutrality arguments to obtain the required Li content (ca. 6.4-6.6 per formula unit) for optimum conductivity. The x = 0.10 composition gave the highest bulk Li ion conductivity of 2.8 x 10(-4) S cm(-1) at 25 degrees C, on the order of reported values for Al-doped LLZO. Surface chemical analysis using time of flight secondary ion mass spectrometry showed Ge homogeneously distributed within the grains as well as some Li-, O- and Ge- enrichment along the grain boundaries. Cyclic voltammetry of the cells containing Ge-doped LLZO showed a redox stability up to +5V.