Squaraine organic crystals with strong dipole effect toward stable lithium-organic batteries

Abstract

Although organic redox molecules as electrode materials can achieve a high specific capacity through molecular design engineering, effective strategies to overcome their low cycling stability have not yet been developed. Herein, we establish a novel strategy that utilizes a strong dipole effect to suppress the high solubility of organic molecules and, thus, improve cycling stability. Two organic single crystals of squaraine derivative molecules with different substituents, 2,4-di(piperidin-1-yl)cyclobutane-1,3-dione (SAPD) and 2,4-di(pyrrolidin-1-yl)cyclobutane-1,3-dione (SAPL), were synthesized by facile routes. In particular, the SAPL cathode exhibited a discharge capacity of 371 mA h g–1 at 48 mA g–1 (0.1 C) and a nearly 78% capacity retention after 1000 cycles at 0.24 A g–1 (0.5 C). It also exhibited a capacity of 135 mA h g–1 even at 4.8 A g–1 (10 C) without hybridizing with graphene or carbon nanotubes. We studied the molecular interactions of SAPD and SAPL via detailed density functional theory calculations and revealed their lithium-ion storage mechanisms by using ex-situ technologies. Through comparing their intermolecular interactions, we found the stronger dipole effect can elevate cycling performance better. Hence, our design strategy can serve as a promising path for constructing high-performance lithium-organic batteries with long-term cycling stability.