The development of solid polymer electrolytes (SPEs) for lithium batteries has become a topic of intense research interest towards improving the energy density and reliability of lithium batteries. In particular, the preparation of solid-state lithium metal batteries has attracted increased attention because of their potential for maximizing the potential energy density by using solid lithium metal as the anode material. By acting as both the electrode separator and the Li+ conducting medium, SPEs can also offer higher Li+ transference, increased chemical stability, higher operating temperatures, and diminish the potential for dendrite formation which can lead to battery failure. Due to their relatively high ionic conductivity (~10^-5 S/cm), PEG and PEO based electrolytes have been the most widely studied polymer materials for solid polymer electrolyte applications. However, these current state-of-the-art SPEs suffer from a limited operating voltage potential window, poor mechanical properties, and low thermal stability. Phosphonium ionenes offer an interesting alternative to PEG due to their increased mechanical and thermal properties, and initial characterization has shown that these unique polyelectrolytes have competitive ionic conductivity. This research focuses on the structure-property relationships of PEG-segmented phosphonium ionene block copolymers that combine the favorable Li+ conductivity of PEG along with the increased mechanical and thermal properties found in phosphonium ionenes. Prepared by step-growth polymerization, we are able to synthesize PEG-segmented phosphonium ionenes to molecular weights up to 25 kg/mol where the molecular weight of each incorporated PEG segment is 1 kg/mol. Thermal characterization using differential scanning calorimetry shows that these materials retain the characteristic crystallinity of PEG while also exhibiting increased glass transition temperatures typical of phosphonium ionenes. While studies on Li+ transference are ongoing, initial results indicate that the ionic conductivity in symmetrical stainless steel coin cells is similar to that found in typical PEG-based SPEs.

Investigating the structure-property relationships of PEG-segmented phosphonium ionenes for lithium battery applications

ACS Spring 2022

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