Sintering Mechanisms of High-Performance Garnet-type Solid Electrolyte Densified by Spark Plasma Sintering

•Impurity formation mechanisms are studied on SPS of garnet-type solid electrolytes.•Electrolysis produces La2Zr2O7 on anode and amorphous carbon on cathode.•The electrolysis can be suppressed by eliminating Li2CO3 from LLZT powder.•Electromigration causes graphite intrusion on cathode.•SPS-densified LLZT exhibited short circuit at current density of 0.2 mA cm−2.

Garnet-type solid electrolyte Li6.5La3Zr1.5Ta0.5O12 (LLZT) was densified by using a spark plasma sintering (SPS) technique. Formation of impurities in the obtained pellets was studied in detail. It is revealed that impurities are strongly related to the SPS process: electrolysis of LLZT and electromigration of graphite. The electrolysis results in La2Zr2O7 on anode of the SPS process, which is accompanied by reduction of Li2CO3 to amorphous carbon on cathode. The electrolysis on SPS could be successfully suppressed by employing LLZT powder without Li2CO3. When these impurities were removed, pellets obtained by SPS exhibited electrochemical performance comparable with those densified by other methods total ionic conductivity of 6.9 × 10−4 S cm−1 at 298 K and short-circuit prevention up to 100 μA cm−2 on dc polarization. The results confirm great advantage of SPS on manufacturing the dense garnet-type solid electrolytes: a low-temperature (900-1100 °C) and short-sintering-time process (10 min).