ETH Zurich team develops new low-temp synthesis route for high-conductivity garnet structures for solid-state Li-ion batteries
Ga-doped Li7La3Zr2O12 (Ga-LLZO) garnet structures are promising electrolytes for all-solid state Li-ion-batteries. LLZO not only has a high ionic conductivity of 10-4 S cm-1, which greatly surpasses that of all the other garnets, it also has excellent stability even in molten Li. Unlike many other solid electrolytes, LLZO does not suffer from conductivity degradation upon exposure to humid atmospheres. (Earlier post.)
However, the synthesis of garnet-type fast Li-ion conductors depends upon conventional sol–gel and solid state syntheses and sintering that are usually done at temperatures above 1050 ˚C. This process results in micron-sized particles and potential Li-loss, which are unfavorable for further processing and electrode–electrolyte assembly. Now, a team at ETH Zurich has developed a novel low-temperature synthesis-processing route to stabilize the cubic phase of LLZO, while keeping the nanocrystallites at ~200–300 nm. Their paper is published in the RSC journal Journal of Materials Chemistry A.
Solid inorganic electrolytes … can enable the use of high capacity electrode materials (e.g. sulphur, manganese and vanadate based cathodes ), which are otherwise not very stable and safe to be used in liquid organic electrolytes for the current Li-ion battery technology. The use of metallic Li anodes, so further boosting practical energy densities, could also be possible with ceramic electrolytes by preventing the damage of dendritic Li growth. Solid inorganic electrolytes also provide additional advantages such as better thermal and chemical stabilities and the elimination of liquids and separators in electrochemical cells.
… Among the solid Li-ionic electrolytes, Li7La3Zr2O12 (LLZO) garnets and doped variants revealed the highest Li-ion conductivities in the range of ~10-4 S cm-1 at room temperature … However, only the cubic phase exhibits superior Li-ion conductivity close to the classic liquid Li-ion electrolytes; its stabilization over the lower conducting tetragonal phase and the management of Li-losses relative to sintering conditions remain challenging. Though initial studies suggested a high temperature stabilization of the high Li-ion conducting cubic phase, recent reports showed that the stabilization of the cubic phase mostly depends on the formation of disordered and partially occupied Li-sites and vacancies that cannot be obtained without a stabilizing agent; vice versa either full or empty Li sites exist for the low-conductive tetragonal phase. Currently, there is much debate surrounding the best stabilization strategy for the desired cubic LLZO phase.
… In this work, we report a novel low temperature synthesis- processing route for cubic Li6.4Ga0.2La3Zr2O12 through which the synthesis-sintering temperature is decreased by ~200 ˚C compared to the state-of-art, viz. nano-particles of the compounds are obtained at a temperature as low as 600 ˚C by a modified sol–gel combustion method utilizing mainly nitrate precursors.—Afyon et al.
The ETH Zurich team also demonstrated in demonstrate in model experiments that the tetragonal to cubic LLZO phase transformation can occur at a very low temperature of 100 ˚C by post-synthetic Ga-doping of originally tetragonal nano-powders. Only low thermal activation is required to incorporate Ga3+ into the Li-garnet structures.
The sintered pellets of Li6.4Ga0.2La3Zr2O12 delivered high bulk Li-ion conductivities in the range of ~4.0 10-4 S cm-1 at 20 ˚C.
The team also developed and presented a basic model of the chemical and microstructural evolution from the nano-particle formation to dense sintered pellets of submicron grain size—which can be used for all-solid state battery electrolytes.
The new synthesis route and the utilization of nano-c-Li6.4Ga0.2La3Zr2O12 could open new pathways in terms of a simplified solid electrolyte–electrode assembly and the prevention of Li-loss during synthesis and processing due to the strongly lowered temperatures and high sintering activity.—Afyon et al.
Semih Afyon, Frank Krumeich and Jennifer L. M. Rupp (2015) “A shortcut to garnet-type fast Li-ion conductors for all-solid state batteries” J. Mater. Chem. A doi: 10.1039/C5TA03239C