Influence of particle size distribution on the microstructure, density and ionic conductivity of Li_6.25Ga_0.25La₃Zr₂O₁₂ ceramics

Li₇La₃Zr₂O₁₂ (LLZO), especially Ga-doped LLZO, is a promising solid electrolyte for all-solid-state lithium batteries (ASSLB) due to its high ionic conductivity and low electronic conductivity. This study systematically explored the role of ball-milling time–induced powder size distribution in modulating the microstructure and ionic conductivity of Ga-doped LLZO ceramics, challenging the conventional belief that smaller particle size inherently favors sintering. By precisely tuning the secondary ball-milling duration, three representative powder types were prepared: micron-sized softly agglomerated (M0h, 1.09 μm), ultrafine hard-agglomerated (M6h, 0.12 μm), and nanocrystalline hard-agglomerated (M12h, 0.39 μm) powders. Using a pressureless sintering process at 1180 °C, the intrinsic relationship between powder agglomeration state and sintering behavior was elucidated. The M0h powder exhibited high green density (60.6 %) and a friable agglomerate structure, enabling rapid densification to 95.2 % relative density and an ionic conductivity of 5.57 × 10⁻⁴ S·cm⁻¹ within just 0.5 h, with a low grain boundary impedance of 198.7 Ω, significantly superior to the M6h and M12h samples. In contrast, ultrafine hard-agglomerated powder led to a lower initial density (58.6 %) and numerous fine pores after sintering, resulting in a conductivity only 20 % of that of the M0h sample. Despite higher sintering activity, nanocrystalline powder suffered from rapid grain growth and severe lithium volatilization, limiting its ionic conductivity to 4.93 × 10⁻⁴ S·cm⁻¹. In both the M0h and M12h samples, Ga facilitated sintering through the formation of a LiGaO₂ liquid phase. In the M6h sample, lithium loss led to Ga re-dissolution into the solid phase, suppressing liquid-phase formation at grain boundaries and hindering densification. Overall, this study provides new insights into how particle size and agglomeration affect the sintering of LLZO ceramics, offering guidance for optimizing powder properties to develop high-performance solid-state electrolytes.