Nonaqueous Polymer Combustion Synthesis of Cubic Li7La3Zr2O12 Nanopowders

Garnet-type lithium lanthanum zirconate (Li7La3Zr2O12, LLZO) shows great promise as a solid electrolyte for future solid-state lithium batteries as it possesses a uniquely beneficial combination of high ionic conductivity, electrochemical stability against metallic lithium, and generally low reactivity in ambient conditions. Conventionally synthesized by using solid-state reactions, LLZO powders have also been prepared by using variations of sol-gel or combustion synthesis with sacrificial organic templates or polymers containing metal nitrate precursors. Herein, a novel nonaqueous polymer (NAP) method using metalorganic precursors and poly(vinylpyrrolidone) is demonstrated to easily form LLZO nanopowders. Compared to similar techniques using aqueous solutions with metal nitrates, the NAP method confers greater control over synthesis conditions. Undoped cubic phase LLZO is obtained after calcination at 700-800 °C between 0 and 4 h, and the NAP process is easily extended to Ta-doped LLZO. To elucidate the general formation mechanism of nanosized LLZO in the NAP combustion synthesis, scanning transmission electron microscopy is used to perform energy dispersive X-ray and electron energy loss spectral imaging. The results show that in situ formation of a carbonaceous foam during combustion physically segregates pockets of reagents and is responsible for maintaining the small particle size of the as-synthesized material during combustion and crystallization. The room temperature ionic conductivity of nanosized Ta-doped LLZO synthesized by using the NAP method was studied under various sintering conditions, with ionic conductivities between 0.24 and 0.67 mS cm-1, activation energies between 0.34 and 0.42 eV, and relative densities in excess of 90% obtained by sintering at 1100 °C for between 6 and 15 h.