Fabrication and characteristics of a composite cathode of sulfonated polyaniline and Ramsdellite–MnO2 for a new rechargeable lithium polymer battery

The ionic conductivity of a polyacrylonitrile (PAN)-based solid polymer electrolyte is 1.4×10−3 S cm−1, which is sufficient for the electrolyte to be used in a rechargeable lithium polymer battery. The anodic stability of the solid polymer electrolyte is over 4.6 V (vs. Li/Li+). A reduced, highly sulfonated form of polyaniline (SPAn) and Ramsdellite–MnO2 (R-MnO2) are synthesized and used as a cathodic material for a rechargeable lithium polymer battery. Three kinds of cathodes are prepared from SPAn, R-MnO2, and a mixture of SPAn and R-MnO2. The electrochemical properties and diffusion coefficient of lithium ions in each cathode, and the interface between the solid polymer electrolyte and each cathode are investigated by cyclic voltammetry and impedance spectroscopy. The redox processes of the SPAn cathode are two-step reactions. The cathodic and anodic peak currents increase as the cycle number increases. In the redox processes of the R-MnO2 cathode, the cathodic peak current on the second cycle is 62% of that on the first cycle. The Li/R-MnO2 battery has a very high initial discharge capacity, but very poor cycleability. For the composite cathode, the cathodic peak current on the second cycle is 72% of that on the first cycle, i.e., higher than that for the R-MnO2 cathode. The diffusion coefficient of the composite cathode during the discharge process is close to the sum of each variation in the SPAn and R-MnO2 cathodes. The instability of the R-MnO2 cathode at x=0.3 and x=0.2 during the charge process is not observed with the composite cathode. The discharge–charge performance of three types of battery are investigated. The initial discharge capacity of the Li/composite cathode battery is 97.0 m Ah g−1. This battery has higher discharge capacity than the Li/SPAn battery (66.8 m Ah g−1), and better cycleability than the Li/R-MnO2 battery.