The electrochemistry of Fe₃O₄/Polypyrrole Composite electrodes in lithium-ion cells: The role of polypyrrole in capacity retention

Two series of magnetite (Fe₃O₄) composite electrodes, one group with and one group without added carbon, containing varying quantities of polypyrrole (PPy), and a non-conductive polyvinylidene difluoride (PVDF) binder were constructed and then analyzed using electrochemical and spectroscopic techniques. Galvanostatic cycling and alternating current (AC) impedance measurements were used in tandem to measure delivered capacity, capacity retention, and the related impedance at various stages of discharge and charge. Further, the reversibility of Fe₃O₄ to iron metal (Fe0) conversion observed during discharge was quantitatively assessed exsitu 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, attributed to decreased reaction with the electrolyte in the presence of PPy. This study illustrated the beneficial role of PPy addition to Fe₃O₄ based electrodes was not strongly related to improved electrical conductivity, but rather to improved ion transport related to the formation of a more favorable surface electrolyte interphase (SEI).