Facile synthesis of Al-stabilized lithium garnets by a solution-combustion technique for all solid-state batteries

Garnet-type solid electrolytes with cubic modification (c-LLZO, Li7La3Zr2O12) are considered to be one of the most promising candidates for SSLBs with desirable properties such as high ionic conductivity (about 1 mS cm-1) at room temperature, a wide electrochemical operational window, and good stability against reduction by Li metal. The synthesis and processing of garnets through conventional wet-chemical, solid-state reaction and nitrate-combustion approaches often requires one or more of the following processing conditions (energy intensive milling steps, multiple and long periods of calcination) to attain a conductive cubic phase making synthesis time intensive. Herein, we report a facile fuel-assisted solution combustion method using carbohydrazide-nitrate mixtures to synthesize cubic-Li6.28Al0.24La3Zr2O12 (Al-LLZO); compared to other nitrate-combustion approaches, utilizing a nitrogen containing fuel source (CH6N4O) offers drastic reduction in the synthesis duration at relatively low temperatures. Selection of the right fuel to oxidizer ratio and annealing conditions is found to be critical for attaining phase purity and particle growth size of LLZO powders. Cubic phase Al-LLZO with a particle size of up to ∼200 nm was attained at temperatures as low as 800 °C upon calcining the as-combusted powders for 4 h. The green pellets attained high relative densities of 90-92% and ionic conductivities up to 0.45 mS cm-1 at low sintering conditions of 1100 °C for 6 h compared to longer sintering duration (∼10-24 h) for LLZO prepared with a common solid-state reaction or wet chemical methods using conventional pressure-less sintering methods. Sintered pellets exhibited a low activation energy of 0.29 eV likely due to the low grain boundary resistance. Synthesizing sub-micron sized Al-LLZO powders through low-cost facile synthesis approaches is of great importance in the fabrication of composite electrolytes and catholytes.