Molecular structure design of planar zwitterionic polymer electrode materials for all-organic symmetric batteries.

All-organic symmetric lithium-ion batteries (LIBs) show promising prospects in sustainable energy storage systems, due to their environmental friendliness, structural diversity and low cost. Nevertheless, it remains a great challenge to explore suitable electrode materials and achieve excellent battery performance for all-organic symmetric LIBs. Herein, a squaraine-anthraquinone polymer (PSQ) electrode material was designed through rational molecular engineering. The well-designed extended π-conjugated system, donor-acceptor structure, abundant redox-active sites and rational manipulation of weak inter-/intramolecular interactions endow the PSQ electrode with outstanding electrochemical performance. The capacity of the PSQ cathode can be optimized to 311.5 mA h g by carbon-template polymerization. Impressively, PSQ-based all-organic symmetric LIBs displayed high reversible capacity (170.8 mA h g at 50 mA g), excellent rate performance (64.9% capacity retention at 4000 mA g 50 mA g), ultralong cycle life up to 30 000 cycles at 2000 mA g and 97% capacity retention after 2500 cycles at 500 mA g, which is one of the best comprehensive battery performances among the all-organic LIBs reported thus far.