A Study on the Synthesis and Properties of NaSICON Solid Electrolyte

In recent years, there has been a growing interest in developing solid electrolytes due to safety considerations, high energy density, relatively long lifetime, and wide operating range, all of which make them a key to the future of battery technology. However, the availability of cost-effective synthesis routes is a prerequisite to driving progress in the manufacturing and use of such electrolytes. The sodium superionic conductor Na3Zr2Si2PO12 a so-called NaSICON structure, is a promising solid-state sodium electrolyte with high ionic conductivity and good thermal, chemical, and electrochemical stability. However, the reported synthesis methods for this material are time-consuming and energy-intensive. To address this issue, this work aims to develop a simple, time-efficient alternative processing route for the synthesis of solid-state sodium electrolytes. A comparative study was carried out between the developed alternative and commonly reported methods to understand the NaSICON structure's formation mechanism better. The results showed that using precursors with a large surface area together with high-energy milling (HEM) improved the conversion process, and the required processing time and energy was reduced. ... mehrSubsequently, the synthesis conditions of the High Energy Mill (HEM) precursors were determined. Stable cycling of the obtained solid-state electrolyte was observed with Na vs. Na at 1 mA/cm2 and a room temperature conductivity of 1.8 mS/cm was achieved at a reduced total processing time and energy. This marks a new alternative route for the syn-thesis of NaSICON solid-state sodium electrolytes.
Furthermore, to pursue process simplification, the processing was optimized, combined (with-out an intermediate grinding and re-pelletizing step between calcination and sintering) two-step heat treatment were studied. The effect of varying process parameters on microstructural evolution, ionic conductivity, doping, and cycling was investigated. Varying the heat-ing/cooling rate, sintering environment, quenching, and particle size were found to affect the microstructure. The most promising microstructure-property correlation was observed by reducing the chemical potential gradient of the HEM powder by sieving, leading to improved sintering and normal grain growth with good conductivities. The effect of Mg and Mo on the structure, microstructure and conductivity was evaluated, Mg went into a secondary Na3PO4 phase while Mo led to grain growth it has limited solubility in the NaSICON structure, and marginally increased the conductivity.
Finally, the study explores the effects of processing route/composition on the structure and dynamics of Na3Zr2Si2PO12 and Na3.4Zr2Si2.4P0.6O12. High temperature x-ray diffraction (HT-XRD), nuclear magnetic resonance (NMR), and electrochemical impedance spectroscopy (EIS) were employed to investigate the structure and ion dynamics. Two monoclinic NaSICON models with 3Na and 4Na sodium position sub-lattices and a 3Na sodium sublattice rhombo-hedral model were used for the FullProf Rietveld refinement of the high-temperature structure. An attempt was made to determine a correlation between the refinement data and observed motions using NMR. Two migration paths were identified, slow hopping motion observed by NMR was attributed to the slow motion of Na at sites other than the Na3(8f) site in the mono-clinic phase and the difference in lattice potential energy due to the forces governing the distri-bution of P and Si in the lattice, as well as local compositional variation.