High‐Temperature Sintering of Garnet Solid Electrolyte Li₇La₃Zr₂O₁₂: A Comparative Study of Induction Hot Pressing and Spark Plasma Sintering

All-solid-state Li-metal batteries are emerging as a transformative energy storage technology, driven by the demand for higher energy density and enhanced safety in electric vehicles (EVs). Among myriad solid electrolytes, garnet-type Li₇La₃Zr₂O₁₂ (LLZO) has attracted significant attention due to its high room-temperature (RT) Li-ion conductivity (>10^-4 S cm^-1) and chemical stability against metallic Li anodes. Nevertheless, achieving highly dense LLZO remains challenging, as conventional sintering methods, such as pressureless sintering or hot pressing (HP), require prolonged processing times, resulting in Li loss and low densification. Spark plasma sintering (SPS) is considered a superior alternative due to its reputed ability to achieve rapid densification. In this study, induction HP and SPS are systematically compared for densifying Al-doped cubic LLZO powder under various conditions. The findings indicate that both techniques can achieve comparable densification (≈98%) within 5 min, when pressure and temperature are meticulously controlled. Both HP- and SPS-sintered samples show a high RT ionic conductivity exceeding 0.45 mS cm^-1. These results challenge the conventional perception that SPS inherently provides superior densification. The analysis also confirms that densification is predominantly governed by pressure-assisted heating. This work offers insights into optimizing high-temperature sintering of dense oxide solid electrolytes for next-generation solid-state metal batteries.