The Electrochemistry of Fe3O4/Polypyrrole Composite Electrodes in Lithium-Ion Cells: The Role of Polypyrrole in Capacity Retention

Andrea M Bruck,Kenneth J Takeuchi,Cara N Gannett,Esther S Takeuchi,Paul F Smith,David C Bock,Amy C Marschilok

doi:10.1149/2.0361701jes

Abstract

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 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, 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).

Highlights

Li-ion battery (LIB) technology has played a critical role in the widespread adoption of a variety of portable electronic devices
One strategy to increase the capacity of electroactive materials is to use materials capable of multi-electron transfer resulting in high energy density materials.[1]. Towards this end, magnetite, Fe3O4, is a promising nanoscale electroactive material with a high theoretical capacity (926 mAh/g) upon full conversion to Fe metal.[2] over the past several years, the number of published studies of Fe3O4 has been increasing due in part to its high energy density, as well as environmental friendliness and low cost.[3]
An electrically conductive carbon additive and a polymer binder (e.g. polyvinylidene difluoride (PVDF) or polytetrafluoroethylene (PTFE)) are mixed with the electroactive material to form a composite electrode.[3, 4] As mentioned above, future device applications will dictate increases in battery energy density, which may be accomplished by incorporating high capacity electroactive materials into composite electrodes where all the electroactive material is electrochemically accessible and the mass and volume contributions of passive components is minimized

Summary

Introduction

Li-ion battery (LIB) technology has played a critical role in the widespread adoption of a variety of portable electronic devices. An electrically conductive carbon additive (e.g. acetylene black or graphite) and a polymer binder (e.g. polyvinylidene difluoride (PVDF) or polytetrafluoroethylene (PTFE)) are mixed with the electroactive material to form a composite electrode.[3, 4] As mentioned above, future device applications will dictate increases in battery energy density, which may be accomplished by incorporating high capacity electroactive materials into composite electrodes where all the electroactive material is electrochemically accessible and the mass and volume contributions of passive components is minimized. Composite electrodes containing single walled carbon nanotubes have shown good capacity retention for up to hundreds of cycles with nanoscaled Fe3O4 and LixV3O8 electroactive materials with no additional carbon or binder required.[5,6,7]. Our hypothesis was that the dissolved Fe3+ ions may reside in close proximity to the Fe3O4 particles and the PPy generated would be in close proximity to the Fe3O4 particles

Results

Discussion

Conclusion