Comparison of the physicochemical and electrochemical behaviour of mixed anion phosphonium based OIPCs electrolytes for sodium batteries

The physicochemical properties of the phosphonium-based organic ionic plastic crystal (OIPC), triisobutylmethylphosphonium bis(fluorosulfonyl)amide (P1i444FSI) in mixtures with sodium salts consisting of different anion structures, sodium bis(fluorosulfonyl)amide (NaFSI), sodium bis(trifluromethanesulfonyl)amide (NaNTf2) and sodium hexafluorophosphate (NaPF6) were investigated. The phase behaviour, ionic conductivity and electrochemical performance at a sodium concentration of 20 mol% for each anion system were compared. 20 mol% P1i444FSI/NaPF6 displays complicated phase behaviour with additional phase transitions and a higher melting temperature compared to pure P1i444FSI, indicating the formation of a new compound which is different from pure P1i444FSI or NaPF6. The system consisting of 20 mol% NaNTf2 exists as a supercooled liquid across the whole temperature range with a glass transition at − 73 °C. At temperatures corresponding to their liquid state, the ionic conductivity values for both 20 mol% NaFSI and NaNTf2 systems are substantially higher than the NaPF6 system and are approximately similar. Na symmetrical cell cycling at room temperature and 50 °C for these two systems at current densities of 0.1 and 0.25 mA cm− 2 exhibited stable and reversible sodium stripping and plating behaviour with very low polarisation potentials. In contrast, for cells based on the 20 mol% NaPF6 electrolyte, an extended time or higher current density is required in order to form a stable SEI layer before stable cell polarisation behaviour is reached.