Solid state electrolytes (SSEs) have a wider electrochemical and thermal stability window than either liquid or polymer electrolytes, making them a desirable component of high voltage lithium ion batteries. The superior thermal stability of SSEs reduces the need for added coolant in a multi-cell battery allowing for more efficient stacking. The SSE also provides structural stability, lending itself to smaller and inherently safer batteries.

Recently LISICON sulfates, perovskites and garnets have been investigated because they exhibit a highly mobile sub-lattice of lithium. Among these the lithium garnet Li₇La₃Zr₂O₁₂ is a suitable candidates for use as a SSE as it is stable against lithium metal. Room temperature ionic conductivities as high as 1.3 S/cm have been reported for the cubic phase of this material, however the thermodynamically stable tetragonal phase has an ionic conductivity orders of magnitude bellow the cubic phase. Higher conductivity values and stabilization of the cubic phase may be obtained by chemical substitution. The strategy adopted in this study is the substitution of Li for Al, Ga, In, and Sc; La for Ca, Sr, Ba and the lanthanide series; and Zr for Hf and Ta. The lithium occupation and local environment will be studied in these structures to give insight into the phase transition from the cubic to the tetragonal phase. Preliminary PXRD results have confirmed the cubic phase can be stabilized by substitution of 25% of Zr for Ta.

The structure of these garnets will be characterized by NMR and diffraction techniques. To avoid Li volatilization and the formation of undesirable reaction products, the garnets will be consolidated by spark plasma sintering. The ionic conductivities will be determined by pulsed field gradient NMR and impedance spectroscopy. Half cells will be assembled by sintering a layered composition of the electrolyte and active material. The half cells will be cycled against lithium and the electrolyte/electrode interface will be characterized by microscopy techniques. Compatible electrode materials will be determined in this way and an entire solid state cell will be assembled by spark plasma sintering. These cells will be characterized by impedance spectroscopy and an analysis of their charge discharge behavior will be performed.