A metal-free polypyrrole graphite secondary battery with an anion shuttle mechanism

A novel secondary battery with a porous, doped polypyrrole layer (PPy) (nominal thickness 100-300 mu m) as the negative electrode and a pristine composite 80 wt.% natural graphite/20 wt.% polypropylene (CPP) as the positive electrode is described in detail. The current collectors are fabricated of 24 wt,% carbon black/76 wt.% polypropylene. Thus, an entirely metal-free battery is fabricated. The optimum electrolyte is 0.2 M LiClO4 in propylene carbonate, containing 20 mM water. The theoretical energy density is about 80 Wh kg(-1) at a thermodynamic cell voltage of 1.7 V. Practical values of 20-30 Wh kg(-1) are feasible for bipolar systems. 6 cm(2) and 20 cm(2) cells are galvanostatically cycled at 0.5 mA cm(-2). The cells are readily rechargeable, and an anion shuttle mechanism is operative. The Ah efficiency is more than 90%, The very positive potentials, which are attained on the positive electrode during the charge process (at about 2 V versus standard hydrogen electrode) lead to some losses due to the irreversible oxidation of the solvent molecules. Thus, redoping of PPy is not a quantitative process on discharge. As a consequence, negative capacity is lost in an early stage. Solvent/electrolyte systems with an improved anodic stability and eventually periodical full oxidation of the PPy may give a solution to this problem. The active mass utilization of the porous PPy layer is thoroughly optimized.