Electrochemical Na-Insertion/Extraction Properties of Phosphorus Electrodes in Ionic Liquid Electrolytes

The electrochemical Na-insertion/extraction properties of phosphorus as a Na-ion battery anode in ionic liquid electrolytes were investigated by using a thick film without any binder or conductive additive. The ionic liquid with more electrochemically-stable cation structure, 1-((2-methoxyethoxy)methyl)-1-methylpyrrolidinium bis(fluorosulfonyl)amide (Py1MEM-FSA), delivered a high reversible capacity of 310 mA h g−1 at the 100th cycle, whereas the phosphorus electrode in 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide (EMI-FSA) showed a low capacity of only 110 mA h g−1. It was revealed that disintegration of the electrode after cycling was effectively suppressed by applying Py1MEM-FSA instead of an organic electrolyte including propylene carbonate (PC), and that a surface layer induced by the decomposition of EMI-FSA hindered Na-insertion into the active material layer. The performance obtained in Py1MEM-FSA was very superior to that in PC. We applied for the first time a closed-system fire-resistance test to the ionic liquid electrolyte for quantitatively evaluating its non-flammability. The Py1MEM-FSA-based electrolyte exhibited an excellent fire resistance in comparison with the PC-based organic electrolyte, which can be an advantage for realizing a Na-ion battery with a high-energy density and a high safety.