Li-oxide garnets

A flurry of recent research has shown that Li-oxide garnets with the general composition "Li7 La3 Zr 2 O12" (LLZO) are excellent fast ion conductors that could be used in solid-state batteries. Work has shown that the cubic high temperature phase has better ion conductivity than the low temperature tetragonal phase. And, here, it has been demonstrated that small amounts of Al, substituting at the Li positions in the garnet structure, stabilizes the cubic garnet structure at ambient conditions (Geiger et al. 2011). Though much research on "Li7 La3 Zr 2 O12" garnet has now been made in the past several years, there remain a number of important uncertainties. This is especially true with regard to the role of substitutional cations (e.g. Al, Ga) and their effect on ionic conductivity. The distribution and occupancies of Li cations over different crystallographic sites in the garnet structure are also uncertain. Moreover, phase stability and the precise stoichiometry of LLZO having additional substituting cations are not understood. To better understand these issues, we propose to investigate Ga- and Fe3+- and Fe2+-containing "Li7 La3 Zr 2 O12" garnets in a systematic and detailed manner. Three solid solution series of Ga, Fe3+- and Fe2+-containing "Li7 La3 Zr 2 O12" garnets will be synthesized. Their chemical composition will be determined, including the Li contents by electron microprobe and ICP measurements and their crystal structures investigated by X-ray and neutron diffraction, the latter done at low temperatures. The results will give crystal chemical information on the site distribution and occupancies of the Ga, Fe and Li ions. Mössbauer spectroscopic measurements will be carried out in order to obtain site-partitioning behavior of Fe in LLZO, something that has not been done to date. NMR MAS spectra of Li and Ga will also be made to determine their site distribution and to obtain local crystal chemical information. The possible incorporation of H+ for Li will be investigated by IR and Raman spectroscopy. Ion conductivity on a variety of samples will be carefully measured by impedance spectroscopy in order to determine which solid solution garnets show the best conductivity behavior. Our goal is to ultimately understand the relationships between garnet composition and crystal chemistry in relation to ion conductivity behavior in LLZO.

Rechargeable Li-ion batteries are essential power sources for a large variety of portable electronic devices. Most Li-ion batteries now in use have liquid and polymer- supported electrolytes and they can have a number of unwanted, if not dangerous, properties such as dendrite formation, leakage and flammability. Thus, there is great current interest in finding and developing new solid-state fast Li-ion conductors that are thermally and chemically stable and that have high energy densities. Recent research along these lines has shown that certain Li-oxide garnets have the necessary high-ionic conductivities, as well as good chemical and physical properties, to be considered as potential electrolytes in solid- state batteries.In the present project the cubic Li-oxide garnet, Li7La3Zr2O12, named LLZO, doped with Ga (LLZO:Ga) and Fe3+ (LLZO:Fe) could be successfully synthesized. LLZO:Ga had a better crystallization behavior than LLZO:Fe and LLZO:Al. The ionic conductivities of LLZO:Ga and LLZO:Fe are among the highest ion conductivities observed for solid state electrolytes. From this point of view, LLZO:Ga and LLZO:Fe are well qualified to be used as solid state electrolytes in all solid state batteries, which are very important for energy storage.