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Since the state-of-the-art Li-ion batteries are close to reaching their theoretical limit in energy density, it becomes crucial to develop next-generation batteries that enable better safety, higher energy density, and longer lifetime. One such next-generation technology is solid-state batteries, employing solid-state electrolytes. Both polymer and inorganic electrolytes are well-explored in this context. While polymers are flexible and easily processable, their ionic conductivities are generally insufficient. Inorganic ceramics can be good ionic conductors, but display interfacial issues. Therefore, combining polymeric and ceramic material in composites polymer electrolytes (CPEs) can – in principle – be beneficial to merge the advantages of both categories. However, it is still unclear how to best construct such systems, and how the ions are actually transported in them. 

This thesis explores ionic transport in CPEs, both with ion-conducting (“active”) and non-ion-conducting (“passive”) ceramic fillers. The focus is on the amorphous polymer material poly(trimethylene carbonate) (PTMC), the active ceramic filler Li₇La₃Zr₂O₁₂ (LLZO), and the passive ceramic fillers LiAlO₂ (LAO) and NaAlO₂ (NAO). The ionic transport mechanism in PTMC:LLZO CPEs is determined to be dependent on two main factors: particle loading and surface chemistry. An increase in ionic conductivity up to 30 wt% of Li₇La₃Zr₂O₁₂ is seen due to formation of additional transport pathways along the polymer-ceramic interfaces, while higher loadings affect the ionic conductivity negatively. While this can partly be explained by particle agglomeration, the presence of Li₂CO₃ on the Li₇La₃Zr₂O₁₂ surface also contributes to retard the ionic movement along the interfaces. Therefore, boric acid treatment is explored as a strategy to enable a Li₂CO₃-free surface of Li₇La₃Zr₂O₁₂ particles, which renders improved ionic transport and battery performance. Boron-treated Li₇La₃Zr₂O₁₂ shows formation of LiBO₂, which yields a negative zeta-potential, indicative of interactions between the ceramic particles and Li⁺ ions. That the surface chemistry – rather than the bulk – of the ceramic filler ultimately controls the overall transport, opens the door towards employment of passive fillers. It is shown that LiAlO₂  particles can increase the ionic conductivity by one order of magnitude and the Li⁺ transference number to almost 1, effectively rendering the LiAlO₂-based CPE a single-ion conductor. These enhanced ionic transport properties can be explained by the ability of LiAlO₂  particles to promote better ion-ion separation through the attraction of negatively charged TFSI anions to the surface. This renders considerably improved battery performance, enabling cycling in Li||NMC cells. Similar effects are also seen for the analogous Na-ion battery system. 

Thereby, considering that the bulk conductivity of active fillers does not contribute to the overall ionic conduction in CPEs, and that passive fillers such as LiAlO₂  can greatly enhance the ionic transport because of its surface chemistry enabling greater ion-ion separation and favorable transport pathways, this thesis provides guidelines for future design of solid-state conductors for Li- and Na-batteries.