A Self-Doped Polyaniline Cathode for Rechargeable Aqueous Zinc Batteries

Abstract

Rechargeable batteries offer promising solutions to the energy shortage issue.[1] In comparison to non-aqueous Li-ion batteries, aqueous rechargeable batteries possess the merits of using low-cost and high-safety water-based electrolytes. Aqueous electrolytes also provide higher ionic conductivities than non-aqueous ones, which promotes better rate performance of the cell.[2] These superiorities make the system suitable for stationary grid-level energy storages. The zinc metal is a convenient anode for aqueous batteries due to its low redox potential (-0.76 V vs. standard hydrogen electrode), high theoretical capacity (5845 mAh cm-3, 819 mAh g-1) and good compatibility with water.[3]-[5] Its abundant earth storage, non-toxicity and stability in air furthermore benefits large-scale productions. Using neutral or slightly acidic solutions to replace the alkaline electrolytes can suppress the dendritic growth of Zn anode.[4] One urgent requirement of zinc battery research is to find a suitable cathode. Benefiting from the long π-electron conjugated systems, conducting polymers hold higher conductivities compared to metal oxides and organic materials.[6] The highly conductive polyaniline (PANI) is a potential cathode, but it tends to deactivate in low acidic electrolytes (i.e. pH > 1) due to the de-protonation of the polymer. Herein, we synthesized a sulfo self-doped PANI electrode by a facile electrochemical copolymerization process. The -SO3 - self-dopant functions as an internal proton reservoir to ensure locally high acidic environment and facilitate the redox process in the weak acidic ZnSO4 electrolyte. In a full zinc cell, the self-doped PANI cathode delivers a high capacity of 180 mAh g-1, excellent rate performance of 70% capacity retention at 50 times current density increase, and long cycle life of over 2000 cycles with coulombic efficiency close to 100%. The detailed charge storage processes were investigated by X-ray photoelectron spectroscopy and X-ray diffraction. They suggest partial proton insertion into self-doped PANI during discharge, which leads to the internal protonation of -N= units and facilitates further reduction. This helps the polymer to maintain high electrochemical activity and stable capacity retention over cycling, in direct contrast to the fast capacity decay with the non-self-doped PANI cathode. Overall, our work utilizes the smart pH adjustment ability of self-dopants to overcome the discrepancy of active conditions between PANI cathode and Zn anode. More importantly, it demonstrates the potential applications of conducting polymers as cathode materials for rechargeable zinc batteries, with the specific energy storage behavior tunable thanks to the rich chemistry of their family.