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

Abstract

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.