Tuning the properties of ceramic solid electrolytes for lithium batteries

The potential of all-solid-state batteries with lithium metal anodes to be safer, more energy dense and better performing energy storage devices than current lithium ion batteries has led to a major drive in developing solid electrolytes that combine high ionic conductivity with suitable interfacial and mechanical properties. This work analyses the electrochemical and mechanical properties of two ceramic solid electrolytes, Li1.4Al0.4Ge1.6(PO4)3 (LAGP) and Li6.5La3Zr1.5Ta0.5O12 (LLZTO), and their interfacial behaviour with a lithium metal anode. A novel synthetic route to a 3D hybrid solid electrolyte with tuneable properties and a new approach to diagnose cell failure in all-solid-state batteries are presented. A hybrid electrolyte with 3D bicontinuous ordered ceramic and polymer microchannels was successfully synthesised using 3D printing. While the ceramic solid electrolyte channels enable high ionic conduction pathways, the presence of non-conductive polymer channels provides the hybrid with structural stability and resilience to fracture. Full control of the hybrid’s parameters such as the choice of ceramic electrolyte, polymer, microarchitecture and ceramic-to-polymer ratio was demonstrated. The LAGP-epoxy hybrid electrolyte with a gyroid microarchitecture achieved the highest ionic conductivity of 1.6 x 10-4 S·cm-1, which is only lowered by the volume of non-conductive polymer present and the channel tortuosity. The favourable combination of the epoxy polymer’s fracture strength and the ‘easy-to-fill’ gyroid channels, improved the hybrid’s electrochemical and mechanical performance compared to a LAGP pellet. The variation of the ceramic electrolyte and the microarchitecture influence the hybrid’s electrochemical properties, while the polymer volume fraction provides a suitable 3D backbone to increase the hybrid’s mechanical stability and cycle life. Despite the sintering challenges of LLZTO, a possible synthetic route to generate LLZTO-epoxy hybrid electrolytes was also demonstrated. Additionally, inhibiting lithium dendrite growth in LLZTO and other solid electrolytes is another vital challenge. Hence, this work also presents a new method to diagnose dendrite growth by combining galvanostatic cycling with ex-situ magnetic resonance imaging. This approach allowed the identification of lithium dendrites in short-circuit cells and monitoring of their location in 3D space at different stages of cycling.