Enhanced Cyclability of Lithium–Sulfur Batteries by a Polymer Acid-Doped Polypyrrole Mixed Ionic–Electronic Conductor

A mixed ionic–electronic conductor (MIEC) of polypyrrole (PPy) synthesized with poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAAMPSA), which is water-dispersible and is in the form of nanoparticles intertwined by the PAAMPSA, is explored as an additive in sulfur cathodes for rechargeable lithium–sulfur (Li–S) batteries. A S-MIEC composite containing a sulfur content of 75 wt % was synthesized by an in situ deposition of sulfur with MIEC. The sulfur retains an orthorhombic phase randomly mixed with MIEC nanoparticles, exhibiting a lower thermal decomposition temperature than the pristine sulfur. Cathodes containing the S-MIEC composite were prepared and evaluated in half cells by cyclic voltammetry and galvanostatic cycling. The S-MIEC composite cathode shows excellent electrochemical stability as the MIEC facilitates ion and electron transfer and capture intermediate polysulfides within the electrodes. The MIEC in the composite electrodes forms a porous, 3D heterostructure providing good electrochemical contact upon cycling as indicated by scanning electron microscopy and electrochemical impedance spectroscopy. The sulfur in the S-MIEC composite retains a capacity of >600 mA h g –1 at low rates and 500 mA h g –1 at 1 C after 50 cycles.