Design of a full-ionic coordination microenvironment electrolyte for high-temperature/voltage sodium-metal batteries

Abstract

Traditional liquid electrolytes suffer from poor safety, inadequate thermal stability and complex organic composition, which hinders the development of high specific energy and long-cycle-life sodium-metal batteries (SMBs). In this work, an ionic salt electrolyte (ISE) with a full ionic coordination microenvironment has been prepared from an N-methylpropylpyrrolidinium bisfluorosulfonylimide ([C₃mpyr][FSI]) ionic liquid and NaFSI with high stability/conductivity without any organic solvents/additives, which exhibits good flame-retardant properties, excellent long-term cycling stability, and outstanding high-temperature cycling and high-voltage performance. The ISE constructed with full ionic composition increases the transference number of Na⁺, which is conducive to reducing the concentration polarization on the electrode surface and improving the cycling stability of SMBs. The electrostatic shielding effect of [C₃mpyr]⁺ and the sodium repellent activity of the alkyl chain can regulate the uniform deposition/stripping of Na⁺, reducing the side reactions between the electrolyte and electrodes. In addition, this stable, fully ionically coordinated microenvironment promotes the formation of a stable, inorganic-rich SEI during cycling. Therefore, this ionic salt electrolyte enabled 900 stable cycles of layered oxide cathodes in SMBs with a high initial efficiency of 98% and exhibited excellent cycling performance at 80 °C or 4.3 V.