Codoping triiodide anion in polypyrrole cathode: An effective route to increase the capacity of zinc-ion battery

As one kind of promising energy storage device, zinc-ion batteries have attracted much attention due to their large energy density, environmental friendliness and simple operation. However, the energy density of this kind of device is limited by the capacity of cathode. Conducting polymer such as polypyrrole (PPy) can be used as the cathode material but its capacity is determined by the redox sites of the molecular chains. Here, we use triodide anions to codope PPy to increase its capacity because the oxidized state of PPy and the formed triiodide anions will be discharged together. It is found that the doping of iodide ion can enable Zn//PPy-I battery to obtain a capacity of 145 mAh g −1 based on the PPy, which is increased by 47.0 % compared with pure PPy. Due to the strong electrostatic attraction, the triiodide anion can be fixed into the polymer matrix to avoid the self-discharge caused by triiodide ion shuttle without using expensive ion exchange membrane. Furthermore, the devices exhibit good rate capability and cyclic stability (78.5% of capacity retention after 1000 cycles). This strategy provides a new method to improve the energy density of PPy by introducing additional active doping anions.