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- Andersson, Rassmus
(author)
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Discovering new ground in ion transport: Exploring coordination effects in polymer electrolytes : – From method development to battery implementation
- 2024
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Doctoral thesis (other academic/artistic)abstract
- The exponentially increasing demand for portable and stationary energy storage devices is pushing the development of lithium-ion batteries (LIBs). This requires safer and more sustainable electrolytes where solid polymer electrolytes (SPEs) are a viable alternative to the flammable liquid electrolytes used nowadays. However, SPEs are characterized by poor ionic conductivity compared to their liquid equivalents, preventing large-scale implementation. Furthermore, to meet the increasing production rate of batteries, alternative battery chemistries based on more abundant resources than Li are explored. To address these matters, a fundamental understanding of ion transport in SPEs for a range of relevant cations is vital in the development process.In the thesis, the ion transport is explored on a fundamental level for Li+ in addition to cations “beyond Li” such as Na+, K+ and Mg2+ in polyether-, polyester- and polycarbonate-based SPEs, where the core encompasses the connection between the ion coordination strength and the transference number (T+). New methods to investigate these properties have been developed especially targeting these more challenging cations. To study the ion coordination strength, two qualitative and one quantitative methods based on NMR and FTIR, are presented. In addition, eNMR and EIS have been combined to determine T+.Regardless of the cation investigated, the strongest coordination was observed for polyethylene oxide, stemming from its chelating effect on the cations. In contrast, poly(trimethylene carbonate) exhibited the weakest coordination, while poly(ε-caprolactone) fell in between. A direct correlation between the coordination strength and the T+ was also recognized, where strong interactions are accompanied by low T+ and vice versa. Moreover, the divalent Mg2+ displayed particularly interesting transport characteristics, where the [MgTFSI]+ speciation appears to be a large contributor to the net Mg mobility. Lastly, the outcome of incorporating an ion-conducting polymer as the soft segment in polyurethanes is that the transport mechanism of the pure SPE remains. In combination with sustained long-term cycling in lithium metal batteries, the polyurethanes illustrate opportunities for new designs by adjusting the soft segments. Similarly, the properties of poly(1-oxoheptamethylene) can be controlled by tuning its saturation degree, which is crucial for the ion conduction and mechanical properties in lithium metal batteries, since it highly affects the crystallinity and the crosslinking of the systems.In summary, this thesis contributes toward the understanding of ion transport in systems belonging to “next-generation” batteries, where SPEs for lithium-metal batteries as well as for cations “beyond Li” are considered to play an important part.
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