Structure regulation induced high capacity and ultra-stable cycling of conjugated organic cathodes for Li-ion batteries

Abstract

A polymerization strategy to integrate conjugated structures with rich redox-active units for lithium ion batteries has been rapidly adopted to realize highly efficient polymer cathodes, because polymerization can solve the poor conductivity issue as well as solubility problem, existing in small organic molecules. However, the structural–property correlation of such polymers has not been systematically investigated, and the great impact of the conjugated structures on the overall performance has not been clearly revealed. In this work, we design and synthesize three novel pyrene-4,5,9,10-tetraone (PTO)-based polymers containing different thiophene derivatives as linking units, poly(2,7-thiophene pyrene-4,5,9,10-tetraone) (P(PTO-T1)), poly(2,7-(2,2′-bithiophene)pyrene-4,5,9,10-tetraone) (P(PTO-T2)) and poly(2,7-(thieno[3,2-b]thiophene)pyrene-4,5,9,10-tetraone) (P(PTO-TT)), to tune their electronic structures for high-performance lithium-ion batteries (LIBs). All these three materials deliver high specific capacity and long-cycle stability, which benefits from the high activity of the PTO units, the good conductivity of conjugated frameworks, and the insoluble properties of the polymers. Moreover, polymers with different thiophene moieties show distinct electrochemical performances, among which P(PTO-TT) exhibits the best rate capability (a reversible capacity of 129 mA h g⁻¹ at a high current density of 2 A g⁻¹ and a high capacity retention of 94% after 1200 cycles). Electrochemical and theoretical analyses reveal that an optimized electronic structure of P(PTO-TT) determined by using the thieno[3,2-b]thiophene (TT) linking units is crucial to realize the excellent battery performance. Our work unveils the structure–property correlation of PTO-based polymers as cathode materials, which provides inspiration for rational design of polymer cathodes for high-performance LIBs.