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Organic ionic plastic crystals (OIPCs) appear as promising materials to replace traditional liquid electrolytes, especially for use in solid state batteries. However, OIPCs show low conductive properties relative to liquid electrolytes, which presents an obstacle for their widespread applications. Recent studies revealed very high ion mobility in solid phases of OIPCs, yet the ionic conductivity is significantly (~100 times) suppressed because of strong ion-ion correlations. To understand the origin of the ion-ion correlations in OIPCs, we employed broadband dielectric spectroscopy, light scattering and NMR diffusion measurements in liquid and solid phases of Hexafluorophosphate - Diethyl(methyl)(isobutyl)phosphonium [PF6][P1,2,2,4]. The results confirmed significant decrease in conductivity of solid phases of this OIPC through ion-ion correlations. Surprisingly, these ionic correlations suppress charge displacement on rather long time scales comparable to the time of ion diffusion on the ~1.5 nm length scale. We ascribe the observed phenomena to momentum conservation in motion of mobile anions and emphasize that microscopic understanding of these correlations might enable design of OIPCs with strongly enhanced ionic conductivity. 

Sodium metal batteries are an emerging technology that shows promise in terms of materials availability with respect to lithium batteries. Solid electrolytes are needed to tackle the safety issues related to sodium metal. In this work, a simple method to prepare a mechanically robust and efficient soft solid electrolyte for sodium batteries is demonstrated. A task-specific iongel electrolyte was prepared by combining in a simple process the excellent performance of sodium metal electrodes of an ionic liquid electrolyte and the mechanical properties of polymers. The iongel was synthesized by fast (<1 min) UV photopolymerization of poly(ethylene glycol) diacrylate (PEGDA) in the presence of a saturated 42%mol solution of sodium bis(fluorosulfonyl)imide (NaFSI) in trimethyl iso-butyl phosphonium bis(fluorosulfonyl)imide (P111i4FSI). The resulting soft solid electrolytes showed high ionic conductivity at room temperature (≥10−3 S cm−1) and tunable storage modulus (104–107 Pa). Iongel with the best ionic conductivity and good mechanical properties (Iongel10) showed excellent battery performance: Na/iongel/NaFePO4 full cells delivered a high specific capacity of 140 mAh g−1 at 0.1 C and 120 mAh g−1 at 1 C with good capacity retention after 30 cycles. 

Hexamethylguanidinium bis(fluorosulfonyl)imide ([HMG][FSI]) has recently been shown to be a promising solid state organic ionic plastic crystal with potential application in advanced alkali metal batteries. This study provides a detailed exploration of the structural and dynamic behavior of [HMG][FSI] mixtures with the sodium salt NaFSI across the whole composition range from 0 to 100 mol%. All mixtures are solids at room temperature. A combination of differential scanning calorimetry (DSC), synchrotron X-ray diffraction (SXRD) and multinuclear solid state NMR spectroscopy is employed to identify a partial phase diagram. The 25 mol% NaFSI/75 mol% [HMG][FSI] composition presents as the eutectic composition with the eutectic transition temperature at 44 °C. Both DSC and SXRD strongly support the formation of a new compound near 50 mol% NaFSI. Interestingly, the 53 mol% NaFSI [HMG][FSI] composition was consistently found to display features of a pure compound whereas the 50 mol% materials always showed a second phase. Many of the compositions examined showed unusual metastable behaviour. Moreover, the ion dynamics as determined by NMR, indicate that the Na + and FSI − anions are signifcantly more mobile than the HMG cation in the liquid state (including the metastable state) for these materials. 

A critical challenge for next-generation lithium-based batteries lies in development of electrolytes that enable thermal safety along with use of high-energy-density electrodes. We describe molecular ionic composite (MIC) electrolytes based on an aligned liquid crystalline polymer combined with ionic liquids and concentrated Li salt. This high strength (200 MPa) and non-flammable solid electrolyte possesses outstanding Li+ conductivity (1 mS·cm-1 at 25 °C) and electrochemical stability (5.6 V vs Li|Li+) while suppressing dendrite growth and exhibiting low interfacial resistance (32 Ω·cm2) and overpotentials (≤ 120 mV @ 1 mA·cm-2) during Li symmetric cell cycling. A heterogeneous salt doping process modifies a locally ordered polymer-ion assembly to incorporate an inter-grain network filled with defective LiFSI & LiBF4 nanocrystals, strongly enhancing Li+ conduction. This modular material fabrication platform shows promise for safe and high-energy-density energy storage and conversion applications, incorporating the fast transport of ceramic-like conductors with the superior flexibility of polymer electrolytes.