Abstract
Increasing the ionic conductivity has for decades been an overriding goal in the development of solid polymer electrolytes. According to fundamental theories on ion transport mechanisms in polymers, the ionic conductivity is strongly correlated to free volume and segmental mobility of the polymer for the conventional transport processes. Therefore, incorporating plasticizing side chains onto the main chain of the polymer host often appears as a clear-cut strategy to improve the ionic conductivity of the system through lowering of the glass transition temperature (Tg). This intended correlation between Tg and ionic conductivity is, however, not consistently observed in practice. The aim of this study is therefore to elucidate this interplay between segmental mobility and polymer structure in polymer electrolyte systems comprising plasticizing side chains. To this end, we utilize the synthetic versatility of the ion-conductive poly(trimethylene carbonate) (PTMC) platform. Two types of host polymers with side chains added to a PTMC backbone are employed, and the resulting electrolytes are investigated together with the side chain-free analogue both by experiment and with molecular dynamics (MD) simulations. The results show that while added side chains do indeed lead to a lower Tg, the total ionic conductivity is highest in the host matrix without side chains. It was seen in the MD simulations that while side chains promote ionic mobility associated with the polymer chain, the more efficient interchain hopping transport mechanism occurs with a higher probability in the system without side chains. This is connected to a significantly higher solvation site diversity for the Li+ ions in the side-chain-free system, providing better conduction paths. These results strongly indicate that the side chains in fact restrict the mobility of the Li+ ions in the polymer hosts.
Highlights
Solid polymer electrolytes (SPEs) are considered as potential candidates in the realization of all-solid-state batteries
We turn our interest toward materials with plasticizing side groups and have considered three carbonate-based host polymers: Poly(trimethylene carbonate) (PTMC), Poly(2-butyl-2-ethyltrimethylene carbonate) (PBEC), and PHEC (Figure 1)
While PTMC has a backbone without side chains, PBEC contains relatively short noncoordinating side chains, and PHEC is characterized by a longer side chain that includes a potentially ion-coordinating ether oxygen
Summary
Solid polymer electrolytes (SPEs) are considered as potential candidates in the realization of all-solid-state batteries. The strong dependence on polymer segmental motion makes the polymer flexibility decisive for ion mobility, and a low Tg, a high degree of free volume, and a large volume of amorphous domains are necessary for fast ion transport.[2,3] Second, a decoupled motion can be distinguished, where the ions undergo a hopping motion between fixed sites again either intra- or interchain similar to the conduction in a ceramic material.[4] Here, the connectivity of good ionic transport paths is important for ionic mobility, and thereby that the polymer matrix can provide available sites for ions to jump into, while polymer flexibility is less crucial This renders possibilities to reach “superionic” conductivities, which are not restricted by the Walden rule.[5,6] Naturally, there are intermediate cases between these two extremes
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