Excellent lithium storage performance of bipyridinium-based polymerized ionic liquids as anode in lithium-ion batteries

Abstract

For organic electrode materials, slow Li+ diffusion kinetics is a main obstacle to the realization of high lithium storage capacity through a super-lithiation reaction. Here, bipyridinium-based polymerized ionic liquids (PILs) have been synthesized by the ionothermal trimerization of 1,1-dicyanomethyl bipyridinium bromide ILs in molten ZnCl2 matrix, and are used as an anode material for the first time. In which, the ionic chains are beneficial for fast Li+ diffusion, as well as the p-π resonance structure between methylene and aromatic ring helps the electron conduction. Electrochemical analysis shows PILs-400 can deliver 1201.5 mAh g−1 at 0.1 A g−1 and almost no capacity loss with the capacity of 873.6 mAh g−1 over 500 cycles at 2 A g−1. Theoretical calculation reveals that all functional groups of triazine, methylene, and bipyridinium have taken part in the redox reaction, and this reaction is associated with a three-step insertion process of 19 Li+ for each repetitive unit in forming Li19C14N5, along with the reversible formation of N-Li and C-Li bonds.