All Organic Sodium‐Ion Batteries with Na₄C₈H₂O₆

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 (Na4DHTPA; Na4C8H2O6), 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 (Na2C8H2O6/Na4C8H2O6) and the carboxylate groups at 0.1-1.8 V (Na4C8H2O6/Na6C8H2O6). The use of Na4C8H2O6 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).