Physical and electrochemical properties of zwitterion-based electrolytes with different lithium and sodium salts

Abstract

Metal batteries are promising alternatives to present lithium-ion batteries, recognized for their good energy density and storage capacity, but they still suffer from significant safety and performance challenges. Advancements in safer, more stable electrolytes such as ionic, low volatility electrolytes offer a way to address these challenges. Here we report the first combination of a zwitterionic plastic crystal (ZPC) with different Na salts and compare the properties of these electrolytes with those formed from the first use of LiTFSI with a ZPC, for the development of safer, quasi-solid state and liquid electrolytes for lithium and sodium batteries. Thermal and structural analyses, using techniques such as DSC, EIS, IR spectroscopy and solid state and diffusion NMR, revealed that the conductivity and phase behaviour are highly dependent on the salt type and concentration. 50% NaTFSI–ZPC mixtures showed higher conductivity and transference numbers than equivalent LiTFSI–ZPC mixtures, while the 50% NaFSI–ZPC electrolyte enabled the best Na cycling despite a lower transference number. These findings underscore the potential of ZPCs for the development of efficient electrolytes for next-generation energy storage systems.