Optimisation of the interfaces of solid-state batteries

Abstract

The development of solid-state batteries aims to implement lithium metal in a battery to utilise the total capacity and voltage it applies. A full solid-state battery would also prevent a fire hazard by removing the volatile organics. Solid oxide electrolytes of Li₃ₓLa₂/₃-₃ₓTiO₃ (LLTO) and Li₇La₃Zr₂O₁₂ (LLZO) are promising electrolyte materials that can be used in these applications. However, one of the main limitations is the high interfacial tension at the interface between lithium metal and the solid electrolyte.

In this work, LLTO electrolytes were synthesised using an aqueous system providing a double perovskite structure with a pseudo-cubic structure after sintering the material at 1350°C for 6 hours provided a unit cell parameter of a = 3.86 Å. It was also possible to dope the b-site with aluminium to allow for a cubic phase formation at a lower temperature, 1100°C, at a shorter time of ten minutes.

This thesis describes the use of LLTO and LLZO electrolytes wetting experiments investigating the intrinsic wetting of solid lithium. The results demonstrated that lithium is intrinsically poor at wetting solid electrolyte surfaces with contact angles greater than 90°. The adhesion of lithium to these electrolytes is also low, which explains why pressure is typically implemented to cause lithium to stick to the surface. Using wetting state calculations, most of the interactions between lithium and the solid oxide electrolytes are described as Cassie-Baxter wetting states. By the same calculations, some interactions present capillary filling, providing evidence that electrolyte modification may permit lithium infiltration. This can be done by coating protective sacrificial electrolytes (LLTO) on top of others (LLZO). Another conclusion from the wetting experiments was that altering the surface tension of lithium is one of the easiest and most beneficial ways to allow for the impregnation of the lithium into the pores, improving the contact angle at the micro-scale.

The coating of an LLZO pellet with a sol-precursor of LLTO was successful. It provided some improvement (1x10⁻³ Scm⁻¹) to the ionic conduction when placed in a symmetrical cell with lithium, and the contact angle of lithium to this coated LLZO improved in the microscale. Bulk lithium adhesion and impregnation will require further improvement by either improved coverage of LLTO on the surface of LLZO or by using higher vacuum pumps at a temperature of around 180°C.

The LLTO coating also protects the LLZO surface from absorbing CO₂ to form impurities of La₂Zr₂O₇. LLTO stability with the desired cathode material LiFePO₄ (LFP) is noted and may lead to an implementation of LFP, LLTO and LLZO in a full cell, which would be an exciting step as a proof of concept.