ABSTRACT
Solid polymer electrolytes (SPEs) have emerged as promising candidates for sodium-based batteries due to their cost-effectiveness and excellent flexibility. However, achieving high ionic conductivity and desirable mechanical properties in SPEs remains a challenge. In this study, we investigated an AB diblock copolymer, PS-PEA(BuImTFSI), as a potential SPE for sodium batteries. We explored binary and ternary electrolyte systems by combining the polymer with salt and [C3mpyr][FSI] ionic liquid (IL) and analyzed their thermal and electrochemical properties. Differential scanning calorimetry revealed phase separation in the polymer systems. The addition of salt exhibited a plasticizing effect localized to the polyionic liquid (PIL) phase, resulting in an increased ionic conductivity in the binary electrolytes. Introducing the IL further enhanced the plasticizing effect, elevating the ionic conductivity in the ternary system. Spectroscopic analysis, for the first time, revealed that the incorporation of NaFSI and IL influences the conformation of TFSI- and weakens the interaction between TFSI- and the polymer. This establishes correlations between anions and Na+, ultimately enhancing the diffusivity of Na ions. The electrochemical properties of an optimized SPE in Na/Na symmetrical cells were investigated, showcasing stable Na plating/stripping at high current densities up to 0.7 mA cm-2, maintaining its integrity at 70 °C. Furthermore, we evaluated the performance of a Na|NaFePO4 cell cycled at different rates (C/10 and C/5) and temperatures (50 and 70 °C), revealing remarkable high-capacity retention and Coulombic efficiency. This study highlights the potential of solvent-free diblock copolymer electrolytes for high-performance sodium-based energy storage systems, contributing to advanced electrolyte materials.
ABSTRACT
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.
ABSTRACT
Zwitterionic materials can exhibit unique characteristics and are highly tunable by variation to the covalently bound cationic and anionic moieties. Despite the breadth of properties and potential uses reported to date, for electrolyte applications they have thus far primarily been used as additives or for making polymer gels. However, zwitterions offer intriguing promise as electrolyte matrix materials that are non-volatile and charged but non-migrating. Here we report a family of zwitterions that exhibit molecular disorder and plasticity, which allows their use as a solid-state conductive matrix. We have characterized the thermal, morphological and structural properties of these materials using techniques including differential scanning calorimetry, scanning electron microscopy, solid-state NMR and X-ray crystallography. We report the physical and transport properties of zwitterions combined with lithium salts and a lithium-functionalized polymer to form solid or high-salt-content liquid electrolytes. We demonstrate that the zwitterion-based electrolytes can allow high target ion transport and support stable lithium metal cell cycling. The ability to use disordered zwitterionic materials as electrolyte matrices for high target ion conduction, coupled with an extensive scope for varying the chemical and physical properties, has important implications for the future design of non-volatile materials that bridge the choice between traditional molecular and ionic solvent systems.
Subject(s)
Electric Power Supplies , Lithium , Electric Conductivity , Electrolytes/chemistry , Lithium/chemistry , Solvents/chemistryABSTRACT
Sodium batteries have emerged as a promising alternative for large-scale energy storage applications due to the low cost and high abundance of sodium. Sodium batteries require safe, high-voltage, and cost-effective electrolytes and cathode materials for their practical applications to be realized. In the present study, Na metal cells with a mixed-phase electrolyte comprising a high concentration of Na salt in an organic ionic plastic crystal (OIPC), namely, triisobutylmethylphosphonium bis(fluorosulfonyl)imide, are investigated-coupled with either a sodium vanadium phosphate-carbon composite (NVP/C) or a sodium iron pyrophosphate (NFpP) cathode. The performance of the Na/NVP/C and Na/NFpP cells are evaluated using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic cycling at 60 °C and room temperature. The results reported herein indicate the performance improvement in terms of cycling stability, with high Coulombic efficiency at 60 °C granted by the OIPC and ionic liquid mixtures, compared to a conventional organic solvent electrolyte.
