Aromatic Organic Small-Molecule Material with (020) Crystal Plane Activation for Wide-Temperature and 68000 Cycle Aqueous Calcium-Ion Batteries

Calcium-ion batteries are an emerging energy storage device owing to the low redox potential of Ca 2+ /Ca and the naturally abundant reserves of the Ca element. However, the high charge density and large radius of Ca 2+ lead to a low calcium storage capacity or unsatisfactory cycling performance for most electrode materials. Herein, we report the organic crystal 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) as an anode material for aqueous calcium-ion batteries (ACIBs) in a water-in-salt electrolyte. PTCDI delivers a high discharge capacity of 131.8 mAh g –1, excellent rate performance (86.2 mAh g –1 @10000 mA g –1 ), and an ultralong life of 68000 cycles (over 470 days) with a high capacity retention of 72.7%. The calcium storage mechanism of PTCDI is shown to be an enolization reaction by in situ attenuated total reflectance Fourier-transform infrared and ex situ X-ray photoelectron spectroscopy. The activation mechanism of PTCDI microribbon splitting along the (020) crystal plane is studied by in situ X-ray diffraction, 3D tomography reconstruction technologies, and ex situ transmission electron microscopy. In addition, the Ca 2+ storage sites and diffusion pathways of PTCDI are studied by density functional theory calculations. Finally, by matching a high-voltage Prussian blue analogue cathode, the assembled aqueous calcium-ion full cells exhibit excellent wide-temperature operating capability (−20 to +50 °C) and an ultralong life of 30000 cycles. Further, an aqueous calcium-ion pouch cell is constructed and exhibits a long lifetime of over 500 cycles.