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.
Highlights
In the mid-20th century, research efforts on ionic conductors had reached an apparent stalemate, when promising materials under investigation were plagued by diminished conductivity for specific ions (O2− for the substituted zirconia) or instability in air as in the case of silver halides [1]
According to recent studies [54,55], particle size distribution for the precursor materials can have an effect on the upper limit for ionic conductivity, making a distinction between materials in the nanoparticle range (
Macro-precursors (5 μm for ZrO2, and 216 μm for SiO2 ). Such claims may not be universal, primarily because (i) the applied ball-milling procedure may be considered anemic, which can be remedied by subsequent jet-milling [55]; and (ii) pre-sintered powders originating from nano- and macro-precursors both displayed the exact same bimodal particle size distribution with peaks at 5 μm and 90 μm [54], albeit at relatively different frequencies, rendering the effect of initial particle size essentially negligible as smaller particles agglomerated into the same size scales during thermal processing
Summary
In the mid-20th century, research efforts on ionic conductors had reached an apparent stalemate, when promising materials under investigation (such as Ca2+ or Y3+ -substituted zirconia and α-AgI) were plagued by diminished conductivity for specific ions (O2− for the substituted zirconia) or instability in air as in the case of silver halides [1]. NASICON materials constitute a class of structurally isomorphous 3D framework compounds of high ionic conductivity, which is comparable to that of liquid electrolytes at elevated temperatures; other key properties include ion storage capacity within their crystal lattice and low thermal expansion [3,4,5,6,7,8,9,10,11,12,13,14,15]. Such properties led to the early employment of NASICON ceramics in a variety of applications, which ranged from ion-exchange membranes to waste management and sensors [16,17,18,19]
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