Ignacio Andrés Cuevas Zuviría: The complexity of ceramic-based electrolytes for all-solid-state batteries

Abstract

The present energy and mobility transformation, heavily relying on Electric Vehicles (EVs) and renewable energy sources, needs batteries. Lithium-ion batteries are the main candidates for reshaping our transport system. Despite already dominating the energy storage components of the EV market, Li-ion batteries possess safety issues related to their flammable liquid electrolytes. Moreover, they get close to reaching their maximum energy density. Alternative battery technologies, safer and able to store more energy, therefore gather great interest. One prominent example is solid-state batteries, employing ceramic or polymeric electrolytes, and their composites.

This thesis explores in-house processing and characterisation techniques to study the processes in, and improve the performance of, inorganic electrolytes for all solid-state lithium batteries. Inorganic electrolytes are solids with high ionic conductivities, which can enable safe batteries with high power and energy densities. There are, however, many challenges to overcome before they can reach commercialization. Advancements are associated with understanding the properties that control the ionic transport.

One focus of this thesis is treating the electrolyte material Li7La3Zr2O12 (LLZO) with boric acid. Such surface treatment appears to tackle the formation of detrimental Li2CO3, and is therefore explored for both sintered ceramic electrolyte pellets and LLZO powders. Respectively, this strategy is evaluated both by analysing the effect upon sintering, and when implementing the powders in a polymer electrolyte matrix. In contact with the acid, LLZO forms a LiBO2 layer with beneficial effects on conductivity. For LLZO powders, the acid treatment yielded solids with promising grain coalescences upon sintering. When incorporated into polymer electrolyte, the higher ionic conductivity suggests a beneficial role of the LiBO2 layer for the polymer-ceramic contacts.

Another promising inorganic electrolyte is Li1+xAlxTi2-x(PO4)3 (LATP), whose easy processing and high conductivity are shadowed by its instability vs. lithium metal. As a strategy to protect the LATP material, it has here been inserted into different polymer electrolyte matrices. While the composites generally displayed poor synergistic effects between the materials, some promising results were seen for polyesters, not least high transference numbers.

In summary, these results provide a step forward into understanding how a functional all-state battery could be built using ceramic electrolytes, and the importance of tailoring the surfaces – both in ceramic and composite electrolytes.