Investigating an all-organic battery using polyisothianaphthene as a redox-active bipolar electrode material

Polyisothianaphthene has the smallest bandgap among all conjugated polymers and delivers high electrical conductivity. This study uses polyisothianaphthene as an active material to accept both lithium ions and PF6− on its cyclic C–S–C bond and benzene ring during the processes of n-doping and p-doping. This study discovers that lithium polysulfide and lithium sulfide are formed during the first electrochemical reaction; however, the impedance, rate performance, and energy density of polyisothianaphthene cells are not affected by those side products. By contrast, an increment of superior rate (10 C) testing is significantly improved by those new sulfur-based solid electrolyte interphase formations compared with transitional anode materials, such as graphite, silicon, and other conjugate polymers. The surface characteristics of the polyisothianaphthene electrode are investigated through in situ X-ray absorption spectroscopy, in operando Fourier transform infrared spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. Furthermore, the reaction mechanisms of n-doping and p-doping on polyisothianaphthene are discussed. The polyisothianaphthene electrode's acceptance of lithium ions exhibits a specific capacity of 730 mAh g−1 at the second cycle as well as of 106 mAh g−1 when it reacts with PF6−. The battery performance exhibits a capacity of approximately 92 mAh g−1 in the bipolar mode. The low-bandgap–conjugated polyisothianaphthene is shown to have high reversibility in terms of bipolar electrochemical reactions, which indicates that it can be a promising bipolar organic material for use in lithium ion batteries.