Investigations on the lithium-ion and sodium-ion insertion behavior of amorphous sodium iron carbonophosphate using N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide based ionic liquid electrolyte

In this article, we investigate the electrochemical performance of the amorphous sodium iron carbonophosphate upon transitioning from organic electrolytes (lithium hexafluorophosphate (LiPF₆) –ethylene carbonate (EC): diethyl carbonate (DEC) based) to ionic-liquid (N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide; PMP-FSI) based electrolytes. The amorphous sodium iron carbonophosphate is synthesized using microwave-assisted hydrothermal process to obtain the desired phase and microstructure. The nanoparticle morphology, along with the amorphous structure, results in excellent electrochemical properties when tested as cathode for lithium-ion and sodium-ion batteries with organic and ionic-liquid based electrolytes. It is observed that the room-temperature and high-temperature (∼60 °C) cycling performance is superior when ionic-liquids are used instead of organic electrolytes, resulting in nearly fade-free electrochemical cells. Specific capacity retention of ∼97% is reported after 500 cycles with 3 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/PMP-FSI ionic-liquid at a specific current density of 400 mAg⁻¹ (room temperature), in comparison to ∼70% after 500 cycles for 1 M LiPF₆ in EC: DEC = 1:1. At high temperatures (∼60 °C), ionic-liquids outperform the conventional organic electrolytes in terms of capacity retention and rate capability. Post-mortem analysis of the electrodes indicates that a dense and ionically conductive protective film, composed of anion-reduced moieties, is responsible for the superior high-temperature performance of amorphous sodium iron carbonophosphate with the ionic-liquids.