Synthesis of Solid-state NASICON Electrolytes for Sodium-ion Batteries

Abstract

This thesis aims to synthesise the sodium super ionic conductor (NASICON) solid-state electrolyte for sodium-ion battery applications using two methodologies: solid-state and sol synthesis. The objective is to develop NASICON with enhanced chemical properties and minimised secondary phases by employing less energy-intensive techniques. This involves transitioning from conventional oven heating to near-infrared (NIR) radiation sintering, achieved through the fabrication of NASICON as a thin film. All samples were characterised by x-ray diffraction (XRD) and scanning electron microscopy (SEM).NASICON was successfully synthesised using the solid-state technique, forming dense pellets (density: 2.80 g/cm3) that are 15 mm in diameter and 2 mm thick (±0.05 mm). The final sintering step was conducted in a tube furnace in an inert environment (argon) at 1180 ºC for 16 hours, with the total oven time for the process being 70 hours and only 1.5% weight fraction secondary phase of ZrO2. Alumina crucibles and a powder bed were used to prevent the pellet fusing to the crucible (Al3+ diffusion) and reduce sodium volatilisation.The sol method also successfully formed a NASICON powder with a low secondary phase content of <2%, aligned with literature values. The powder was prepared in a conventional oven (in air) with reduced sintering times of 3 hours and temperatures of 1000 ºC. Additionally, NASICON was synthesised as a dense 10 µm thin film on a quartz substrate, utilising a sol spray coating technique. The same sintering times were employed as the powder (3 hours) but at lower a lower temperature of 950 ºC. The spray coating technique allowed the film to dry on a hot plate reducing the overall oven heating time from 16 hours (powder) to 3 hours (thin film).NIR radiation was successfully employed to synthesise NASICON as a thin film, a novel technique that has not previously been used in this field. This significantly reduced sintering times to 60 seconds, and overall oven/NIR heating times to 2 hours 2 minutes