Anion intercalation into a graphite cathode from various sodium-based electrolyte mixtures for dual-ion battery applications

Electrolyte plays a crucial role for dual-ion batteries by supplying the ions to be intercalated into electrodes. Yet, high operation potentials of such batteries limit the number of suitable electrolytes to be used since they have to be stable upon cycling. Here, sodium-based electrolytes were investigated by applying stability and cycling tests in order to observe their influences on anion intercalation behavior using graphitic cathodes. Intercalation of PF6− anions with organic carbonate solvents revealed relatively better discharge capacities compared to its anionic counterparts and 0.5 M NaPF6/EC:EMC electrolyte was found to be the most promising mixture among them. PC solvent proved detrimental for anion intercalation, but FEC as an electrolyte additive in PC precluded undesired effects of PC and gave rise to reversible cycling. However, the ionic liquids based on TFSI− and FSI− anions, mostly, yielded capacity fading under a charging end potential of 5.0 V vs. Na/Na+, except the 0.7 M NaTFSI/Pip14TFSI electrolyte, showing 98% coulombic efficiency over 200 cycles. During this work, electrochemical tests were conducted and evaluated to comprehend the impacts of electrolytes on the anion intercalation process. EC:EMC-based electrolyte was then used in our further research, where the compositional cathodic investigation was executed and the anion intercalation phenomenon was characterized.