All Organic Sodium‐Ion Batteries with Na 4 C 8 H 2 O 6

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 .