Electrochemical performance and storage mechanism study of conjugate donor–acceptor organic polymers as anode materials of lithium-ion battery

Abstract

The low redox potential of conjugated donor–acceptor (D–A) type polymer due to strong electron interaction between donor units and acceptor units is benefit to increase the power density, which is suitable to serve as the anode materials of lithium-ion battery. In this paper, the D–A type polymers containing phenanthraquinone (PQ) as acceptor unit and thiophene as donor unit was used as anode materials of lithium-ion battery. Three polymers with different content of donor unit are synthesized to study the effects of donor unit content on the electrochemical properties. It is found that the low content of donor unit in the polymer is benefit to the specific capacities, but not benefit to the rate capability of the electrode. The polymers with the donor–acceptor ratio of 1:1, 2:1 and 3:1 display discharge specific capacity of 737 mAh•g−1, 552 mAh•g−1 and 499 mAh•g−1 at the current density of 72 mA•g−1, respectively. The rate capability of polymer with higher donor–acceptor ratio is better due to its lower band gap and better conductivity. The three D–A type molecules all have perfect cycle stability and stable molecular structure. For a situation of C=C group in aromatic rings partly stored lithium ions, one simple method is proposed for the first time to confirm the exactly lithium ion storage sites by using the Natural Bond Orbital (NBO) analysis. It is found that the lithium ions are mainly stored in the PQ units (redox sites A, RSA) and the thiophene units adhered to the PQ unit directly (redox sites B, RSB), while the thiophene units not adhere to the PQ units did not storage Li-ion.