Magnetite is a naturally occurring mineral found in the earth’s crust; therefore, it is abundant, environmentally friendly, cheap, and non-toxic. Magnetite is also easy to synthesize and can be prepared through simple, non-hazardous techniques such as through a facile aqueous co-precipitation method. Complete reduction of magnetite involves the transfer of 8 electrons, providing opportunity for an impressive capacity of 925 mAh/g. Notably, the initial reactions are insertion reactions where the lithium ion can insert into the Fe3O4 structure with minimal structural distortion, while upon further reduction, a conversion reaction occurs generating Fe metal and Li2O as products of full discharge.
To further probe the role of the electrically conductive polymer in the mesoscale (bulk) properties of composite electrodes, this study focuses on the Fe3O4 conversion reaction and its relationship to the electrically conductive polymer binder, PPy, including a detailed study of composite electrodes with nanoscale Fe3O4 and PPy. Magnetite, Fe3O4, with 6% and 20% polypyrrole (PPy) was used to prepare composite electrodes with and without added carbon.
Galvanostatic cycling and Electrochemical Impedance Spectroscopy (EIS) measurements were used in tandem to determine delivered capacity, capacity retention, and the related impedance at various stages of discharge and charge. Further, the reversibility of Fe3O4 to iron metal conversion observed during discharge was quantitatively assessed, ex situ, using X-ray Absorption Spectroscopy (XAS). The Fe3O4 composite containing the largest weight fraction of PPy (20 wt%) with added carbon demonstrated reduced irreversible capacity on initial cycles and improved cycling stability over 50 cycles. This study illustrated the beneficial role of PPy addition to Fe3O4 based electrodes partially related to improved electrical conductivity and also to improved ion transport related to the formation of a more favorable surface electrolyte interphase (SEI).