A 3.5 V-class organic sodium-ion battery using a croconate cathode

Abstract

Sodium-ion batteries (SIBs) based on organic cathode materials have received considerable attention owing to high theoretical capacities, cost-effectiveness, and raw material abundance. However, high-voltage organic cathodes have been rarely investigated for SIBs mainly due to their electrical insulation and dissolution in electrolytes. In this study, sodium croconate (C5O5Na2) is utilized as a high-voltage cathode material for SIBs, designing an interface using a functionalized multi-walled carbon nanotube (MWCNT). The composite C5O5Na2 cathode with carboxylic-acid-functionalized-(COOH–) MWCNT exhibits a 3.5 V-class high-voltage redox activity with a cycle performance higher than that of non-functionalized MWCNT/C5O5Na2 composite cathodes; the capacity retention ratio is improved from 0 % to 34 % after 30 cycles at 0.1C. Raman, X-ray photoelectron spectroscopy, and solid-state nuclear magnetic resonance reveal the formation of hydrogen bonding between COOH groups on MWCNT and carbonyl groups in C5O5Na2 molecules; leading to suppression of organic molecules and improvement on the cycle performance. A full cell assembled with the C5O5Na2/COOH–MWCNT cathode and a hard carbon anode also shows a high-voltage redox plateau at around 3.5 V and a 1st discharge capacity of 123 mAh g−1 at 1C. These results suggest the feasibility of developing a high-energy-density storage system using completely common-metal components.