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 μm) 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 cm2 and 20 cm2 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.