Non-crystalline oligopyrene as a cathode material with a high-voltage plateau for sodium ion batteries

Oligopyrene (OPr, 3-4 pyrene units) is chemically synthesized and used as a high-voltage organic cathode for sodium ion batteries (SIBs). OPr shows anion-dominant transport behaviors during redox-switching in NaClO4 electrolytes, indicating that, when implemented in SIBs, OPr can reversibly incorporate/release perchlorate anions for charge-balance. A composite film, in which OPr maintains a crystalline phase with a layered structure, shows a sloping charge-discharge (C-D) curve (discharge capacity = 42.5 mAh g−1 and average voltage = 2.9 V vs. Na/Na+ at 20 mA g−1), implying a large overpotential due to slow ClO4− diffusion through the crystalline phase. In contrast, a composite film containing amorphous OPr exhibits substantially reduced overpotential with a plateau potential at 3.5 V during discharge. An initial reversible capacity of 121.0 mAh g−1, which is close to one-electron transfer per pyrene unit, is decreased to 95.8 mAh g−1 during the first 10 C-D cycles, but is subsequently stabilized with a decreasing rate of 0.30 mAh g−1 per C-D cycle. The energy density of amorphous OPr (423 Wh kg−1 for the 1st discharge) is so large that it exceeds those of most inorganic-based cathode materials that have been reported thus far.