Investigation of nano-fibrous selenium and its polypyrrole and graphene composite as cathode material for rechargeable Li-batteries

Abstract

Analogous to the well-known Li–S battery, an electrochemical reaction of two Li + ion with one Se atom (2Li+ + Se + 2e− = Li2Se) gives rise to a large specific theoretical capacity of 678 Ah kg−1 at around ∼2.4 V, resulting in 1627 Wh kg−1 of specific energy. Given this much larger energy density than present cathode materials show, selenium nanofiber and its composites with the conducting polymer polypyrrole as well as with graphene have been prepared and electrochemically tested as cathode materials for Li-batteries. At a low current rate of C/120, pure t-Se electrode delivers the theoretical specific capacity, which subsequently drops to 100 Ah kg−1 at C/20. As expected, composite formation with polypyrrole and graphene improves the electrochemical performance compared to trigonal selenium (t-Se) fibers. While polypyrrole coating minimizes the polarization between charging and discharging, graphene wrapping enhances the obtainable capacity at moderate cycling rates. Ex-situ X-ray diffraction of the electrode reveals solid solution behavior in the initial stages of lithiation, finally leading to an electrochemical conversion of selenium to Li2Se.