The implementation of safe high voltage Li-ion batteries, capable of powering demanding technologies such as electric vehicles, is precluded by the current use of organic polymer electrolytes which have been implicated as the cause of fires in operating battery cells. A safer alternative to these are solid state garnet electrolytes, which display impressive ionic conductivites and can, in fact, act as heat sinks. However, the Li-ion diffusion properties in this particular class of materials are very poorly understood due to grain boundary effects in bulk measurements and existing problems with reliable synthetic routes. We have successfully prepared two phase pure Li-stuffed garnets which display great promise as solid electrolytes and we now request the beamtime at the EMU beamline necessary to determine the Li-ion diffusion characteristics of these materials using muon spectroscopy.