All Organic Sodium‐Ion Batteries with Na4C8H2O6

Abstract Developing organic compounds with multifunctional groups to be used as electrode materials for rechargeable sodium‐ion batteries is very important. The organic tetrasodium salt of 2,5‐dihydroxyterephthalic acid (Na 4 DHTPA; Na 4 C 8 H 2 O 6 ), which was prepared through a green one‐pot method, was investigated at potential windows of 1.6–2.8 V as the positive electrode or 0.1–1.8 V as the negative electrode (vs. Na + /Na), each delivering compatible and stable capacities of ca. 180 mAh g −1 with excellent cycling. A combination of electrochemical, spectroscopic and computational studies revealed that reversible uptake/removal of two Na + ions is associated with the enolate groups at 1.6–2.8 V (Na 2 C 8 H 2 O 6 /Na 4 C 8 H 2 O 6 ) and the carboxylate groups at 0.1–1.8 V (Na 4 C 8 H 2 O 6 /Na 6 C 8 H 2 O 6 ). The use of Na 4 C 8 H 2 O 6 as the initial active materials for both electrodes provided the first example of all‐organic rocking‐chair SIBs with an average operation voltage of 1.8 V and a practical energy density of about 65 Wh kg −1 .