A shortcut to garnet-type fast Li-ion conductors for all-solid state batteries†
Ga-doped Li7La3Zr2O12 garnet structures are among the most promising electrolytes for all-solid state Li-ion-batteries. The synthesis and processing of garnet-type fast Li-ion conductors depend on conventional sol–gel and solid state syntheses and sintering that are usually done at temperatures above 1050 °C to reach the high Li-ion conducting cubic phase. This process results in micron-sized particles and potential Li-loss, which are unfavorable for further processing and electrode–electrolyte assembly. Here, we tackle this problem and report a novel low temperature synthesis-processing route to stabilize the cubic phase of Li7La3Zr2O12, while keeping the nanocrystallites at ∼200–300 nm. Li7La3Zr2O12 phases are obtained at temperatures as low as 600 °C by a modified sol–gel combustion method utilizing mainly nitrate precursors, and the sintering temperature is lowered by ∼200 °C compared to the state-of-art. Through a new model experiment, we also shed light on the conditions influencing the tetragonal to cubic phase transformation via homogeneous Ga-diffusion and incorporation occurring at a surprisingly low temperature of ∼100 °C for a post-annealing step. The sintered pellets of the newly obtained Li6.4Ga0.2La3Zr2O12 deliver high bulk Li-ion conductivities in the range of ∼4.0 × 10−4 S cm−1 at 20 °C, and a wide thermal operation window is accessible through its characteristic activation energy of ∼0.32 eV. We report that there is an optimum in sintering-processing conditions for the cubic c-Li6.4Ga0.2La3Zr2O12 solid state electrolytes and their Li-ionic conductivity and the (Raman) near order characteristics that can be tracked through changes in Li–O vibrational modes. Based on this alternative route, low-temperature synthesized powders can be sintered to relatively dense pellets at around only 950 °C. At higher sintering temperatures (e.g. 1100 °C), Li-losses progress as confirmed by structural studies and a reduction of both ceramic pellet density and ionic conductivity, as well as distortions in the Li-sublattice, are found. Through this work, an alternative low temperature processing route for Ga-doped Li7La3Zr2O12 garnet type electrolytes for all-solid state batteries is suggested. The new synthesis method and the use of c-Li6.4Ga0.2La3Zr2O12 nanoparticles could open pathways in terms of preventing Li-loss during the process and advancing future solid electrolyte–electrode assembly options for all-solid state Li-ion batteries.