Bipolar Redox Behavior of Polyaniline Cathodes in Rechargeable Lithium Batteries

Polyaniline is a promising organic cathode for rechargeable batteries due to its high theoretical capacity (295 mAh g−1), robust polymer structure, and low cost. Conventional studies mostly regard it as a p-type material and overlook the influence of various polyaniline forms on electrochemical behavior. Herein, we propose that polyaniline is inherently bipolar and comprehensively investigate this across three typical forms: the leucoemeraldine base (LB), emeraldine base (EB), and pernigraniline base (PB). Thorough structural characterization revealed that LB, EB, and PB possess intrinsic N−/−NH− moiety ratios of 0:1, 1:2, and 1:1, respectively, challenging the conventional understanding of EB and PB structures. Systematic electrochemical tests in p-type (2.0−4.2 V), n-type (0.8−3.0 V), and bipolar modes (0.8−4.2 V) confirmed a proportional relationship between p-type/n-type capacities and the −NH−/N− proportions. Among the three, EB exhibited the best bipolar performance, delivering an apparent reversible capacity of 360 mAh g−1 with 97% retention over 200 cycles, corresponding to nearly full utilization of theoretical capacity after subtracting the KB contribution. Moreover, we revealed a gradual shift in EB from p-type to n-type reactions during cycling, resulting from electrochemical deprotonation at high potentials. These findings are essential for future development of polyaniline-based materials and batteries.