Additive manufacture of sodium beta alumina solid state battery electrolyte

Energy has been one of the hottest topic in the last two decades and therefore the energy storage devices have attracted significant attention. Na ion battery is considered as an alternative to Li ion battery for next generation energy storage devices. β/β’’ alumina solid electrolyte (BASE) for solid-state Na ion battery has been widely investigated since last century, however the fabrication of BASE with complex 3D-structure is still unfulfilled. In this project water-based BASE ink formulations were developed for material extrusion additive manufacturing technique. The fabrication of BASE with complex 3D-structure was achieved which massively extended the potential of BASE for future applications.

The fabrication of BASE via additive manufacturing was investigated from the powder to electrolyte. The BASE powder was synthesised by a solid-state method with MgO dopant as β’’phase stabiliser. The calcination temperature for the BASE powder synthesis was investigated and 1250 ^oC was finally used. The particle size of the BASE powder was reduced to d₅₀=1.12 μm by planetary ball-milling for extrusion-based 3D-printing afterwards.

To develop BASE ink formulation, the additives were investigated firstly. The binder + plasticiser system was investigated and optimised by rheology analysis, after which an aqueous gel consisting of methylcellulose 15cP of 10wt% as binder and PEG 300 of 15wt% was used for BASE ink formulation. A markable achievement of this project is the finding of citric acid as a unique dispersant for BASE water-based ink. The optimisation of the dispersant was conducted by zeta potential measurement and settling test after which 10wt% citric acid according to the BASE amount was used for BASE ink formulation. After the investigation of the additives, the BASE ink formulation was optimised by changing the solid loading to obtain a BASE ink with printable rheology. Finally, a BASE ink with solid loading of 67wt% was optimised for the extrusion-based 3D-printing afterwards.

The relation between the printing speed and printing pressure for the optimised BASE ink was investigated by 3D-Bio-plotter, an extrusion-based 3D-printer. By using the optimised printing speed and printing pressure, the fabrication of the BASE with porous structure and curved design was achieved, which gives infinitive compatibility for the future applications.

The sintering of the BASE was optimised in terms of the ionic conductivity. The ionic conductivity of the BASE was characterised by electrochemical impedance spectroscopy (EIS). The effect of the densification, microstructure, crystal phase and structure resulted by different sintering conditions on the ionic conductivity of the BASE was investigated. The ionic conductivity of the BASE was found to be affected by the densification, microstructure, crystal phase and structure comprehensively and it was very difficult to determine their effect independently.

Overall, the highest ionic conductivity of 0.081 S·cm^-1 at 350 ^oC of 3D-printed BASE sample was obtained by sintering at 1600 ^oC for 5 minutes, which is comparable with the highest ionic conductivity of 0.119 S·cm^-1 at 350 ^oC of conventional pellet BASE sample obtained by sintering at 1600 ^oC for 10 minutes. Thus, the additive manufacture was successfully applied on BASE fabrication at the end of this project, which established a foundation of complex 3D-structure fabrication of the BASE.