Ionic Conductivity and Dielectric Relaxation of NASICON Superionic Conductors at the Near-Cryogenic Regime

Abstract

With a crystal lattice structure first characterized in the 1970s, NASICON sodium-based superionic conductors have recently found renewed interest as solid electrolytes in sodium-ion and seawater flow batteries due to their exceptional ionic conductivity being on the same scale as liquid electrolytes. Since sodium ions in the crystal lattice move among interstitial positions through site-specific bottlenecks, the overall conductivity is strongly dependent on the NASICON composition. In this work, we report on the synthesis protocols and processing parameters of Na3Zr2Si2PO12 prepared from Na2CO3, SiO2, ZrO2, and NH4H2PO4 precursors by the conventional solid-state reaction (SSR) route. We critically evaluated important observations made in the extended literature on the topic including: (i) the importance of precursor particle size concerning the SSR synthesis, focusing on effective ball-milling protocols; and (ii) the onset of excess zirconia contamination, expanding on the effects of both thermal and pressure processing—the latter often overlooked in the available literature. In approaching the cryogenic regime, the dataset availability concerning ionic conductivity and dielectric permittivity measurements for NASICON was extended, starting from elevated temperatures at 200 °C and reaching into the very low temperature zone at −100 °C.

Characterization and Comparative Study of Energy Efficient Mechanochemically Induced NASICON Sodium Solid Electrolyte Synthesis

“…The hump at 820 °C could be attributed to the conversion of low‐temperature monoclinic ZrO 2 to tetragonal [16] and the onset of NASICON conversion. The hump at 1050 °C might be correlated to crystallization of NASICON [17,18] . Upon investigation of the SS_NCO XRD patterns (Figure 4e) at different processing steps, it was observed that the reflections after the first mixing in the planetary ball mill were mainly from precursor materials predominantly ZrO 2 , implying that 24 h grinding with the mill achieved size reduction and mixing but little or no initial reaction.…”

“…The hump at 1050 °C might be correlated to crystallization of NASICON. [17,18] Upon investigation of the SS_NCO XRD patterns (Figure 4e) at different processing steps, it was observed that the reflections after the first mixing in the planetary ball mill were mainly from precursor materials predominantly ZrO 2 , implying that 24 h grinding with the mill achieved size reduction and mixing but little or no initial reaction. After calcination at 700 °C for 12 h, ZrO 2 and intermediate NaZr 2 (PO 4 ) 3 NASICON reflections were observed.…”

Effect of sintering and annealing on electrochemical and mechanical characteristics of Na3Zr2Si2PO12 solid electrolyte

“…For the pristine Na 3 Zr 2 Si 2 PO 12, the sintered sample's ionic conductivity of 1.2 mS/cm at 60 • C is relatively high. [31][32][33] The total ionic conductivity of NASICON-based solid electrolytes depends on the density of the sample and hence its crystallinity, type of synthesis method, and grain boundaries present. Denser pellets are produced by sintering at higher temperatures and longer soaking time.…”

“…using the sol‐gel technique and spark plasma sintering. For the pristine Na 3 Zr 2 Si 2 PO 12, the sintered sample's ionic conductivity of 1.2 mS/cm at 60°C is relatively high 31–33 …”