Solid-state single-ion conducting polymer electrolytes have drawn considerable interest for secondary lithium batteries due to their potential for high electrochemical stability and safety, but applications are limited by their low ionic conductivities. Specifically, poly(ethylene oxide) (PEO) based electrolytes have the highest reported Li⁺ conductivities for these materials; however, their potential is limited due to the ion transport mechanism being coupled to segmental relaxations of the cation solvating polymer chain. To investigate the potential of single-ion conducting polymer electrolytes lacking polar matrices, we synthesized three para-polyphenylene-based, side-chain polymer electrolytes with various pendent anion chemistries (–SO₃ ⁻, –PSI⁻, and –TFSI⁻) with differing binding affinities to Li⁺. Compared with the previously reported lithium poly(4-styrenesulfonyl(trifluoromethylsulfonyl)imide) (LiPSTFSI), the side-chain polymers showed at least 3 orders of magnitude higher conductivity with the same –TFSI⁻ anion (6.7 × 10⁻⁶ S cm⁻¹ compared with 1.2 × 10⁻¹⁰ S cm⁻¹ at 150 °C). We found that the side-chain electrolyte showed a dielectric relaxation dominated transport mechanism through use of dielectric spectroscopy analysis. The conductivity is highly dependent on the charge delocalization and size of the pendent anion, which provides a pathway forward for the engineering of polymeric ion conductors for electrochemical applications.

Magnesium-ion-conducting solid polymer electrolytes have been studied for rechargeable Mg metal batteries, one of the beyond-Li-ion systems. In this paper, magnesium polymer electrolytes with magnesium bis(trifluoromethane)sulfonimide (Mg(TFSI) ₂ ) salt in poly( ε -caprolactone- co -trimethylene carbonate) (PCL-PTMC) were investigated and compared with the poly(ethylene oxide) (PEO) analogs. Both thermal properties and vibrational spectroscopy indicated that the total ion conduction in the PEO electrolytes was dominated by the anion conduction due to strong polymer coordination with fully dissociated Mg ²⁺ . On the other hand, in PCL-PTMC electrolytes, there is relatively weaker polymer–cation coordination and increased anion–cation coordination. Sporadic Mg- and F-rich particles were observed on the Cu electrodes after polarization tests in Cu|Mg cells with PCL-PTMC electrolyte, suggesting that Mg was conducted in the ion complex form (Mg _x TFSI _y ) to the copper working electrode to be reduced which resulted in anion decomposition. However, the Mg metal deposition/stripping was not favorable with either Mg(TFSI) ₂ in PCL-PTMC or Mg(TFSI) ₂ in PEO, which inhibited quantitative analysis of magnesium conduction. A remaining challenge is thus to accurately assess transport numbers in these systems.

In this study, the amphiphilic salt lithium trifluoromethanesulfonylimide octadecane (C18LiTFSI) was used as a basis to investigate the effects of anion density and cation coordination sites within blended electrolytes with strong ionic aggregation. C18LiTFSI was previously reported as a single-component, ion-condensed electrolyte with a wide layered liquid crystalline phase regime. Three additive molecules with varyingly sized polar sulfonyl groups attached to an octodecane-tail were synthesized and mixed with C18LiTFSI. The thermal properties, morphology, and ionic conductivity of the blended electrolytes were characterized. It was found that the blended electrolytes exhibited layered liquid crystalline morphology over a narrower temperature range than the pure salt, and the ionic conductivity of the blended liquid crystalline electrolytes were generally lower than that of the pure salt. Surprising, the additives were found to have the greatest effect on the bulk ionic conductivity of the semicrystalline phase of the electrolytes. Addition of minor fractions of methylsulfonyloctadecane to C18LiTFSI resulted in increases in conductivity of over two orders of magnitude at room temperature, while addition of ethylsulfonyloctadecane or isopropylsulfonyloctadecane with the larger head group resulted in decreased ionic conductivity over the entire composition space and temperature range investigated.