Application of Single-ion conducting polymer electrolytes (SICPEs)

Polymer electrolytes have been widely studied as a potential candidate for next generation batterie with improved safety and higher energy density. Especially, single-ion conducting polymer electrolytes (SICPEs) have attracted significant attention due to their almost unity lithium-ion transport number, which is believed to help suppress lithium dendrite growth and extend battery cycle life. However, there is still a long way to go before they can be practically applied in batteries, due to their relatively low ionic conductivity at ambient temperature. Therefore, the main goal of this work is to explore various methods that can improve the ionic conductivity of SICPEs at ambient temperature, together with the investigation of the effect of ion transport number on battery performance.

In this dissertation, a SICPE with a “soft” poly(dimethylsiloxane) (PDMS) backbone and poly(ethylene oxide) (PEO) side chains was designed. The obtained SICPE exhibits accelerated segmental dynamics, improved lithium-ion conductivity at ambient temperature (4.7 × 10⁻⁶ S/cm), and good electrochemical stability with the lithium metal electrode.

Then, a detailed investigation of SICPEs copolymerized with different neutral units possessing either flexible or polar structures are presented. The results emphasize the important role of miscibility between plasticizing neutral units and ionic conductive units, while the polarity of sidechains has limited influence on ion dissociation. This work suggests that polyanions with a strongly delocalized charge would be a promising structure to achieve sufficient ionic conductivity.

Lastly, plasticized polymer electrolyte membranes containing either SICPE or dual-ions lithium salt were employed to investigate the transport number’s effect on battery performance by fixing other parameters, such as mechanical robustness and chemical composition. Even with a relatively lower conductivity, the sample with a high lithium-ion transport number demonstrated the best performance in a Li/Li symmetric cell, which is ascribed to the uniform deposition of lithium and the formation of a stable solid electrolyte interface (SEI) layer. A full cell test with the configuration of LiFePO₄ (LFP)/Li is still under investigation. Knowledge gained from the research in this thesis provides a deeper understanding of the mechanism of lithium-ion transport in SICPEs and can guide the design of SICPEs to improve ionic conductivity.