Study on the Electrochemical Reaction Mechanism of NiFe2O4 as a High-Performance Anode for Li-Ion Batteries.

Nickel ferrite (NiFe2O4) has been previously shown to have a promising electrochemical performance for lithium-ion batteries (LIBs) as an anode material. However, associated electrochemical processes, along with structural changes, during conversion reactions are hardly studied. Nanocrystalline NiFe2O4 was synthesized with the aid of a simple citric acid assisted sol-gel method and tested as a negative electrode for LIBs. After 100 cycles at a constant current density of 0.5 A g-1 (about a 0.5 C-rate), the synthesized NiFe2O4 electrode provided a stable reversible capacity of 786 mAh g-1 with a capacity retention greater than 85%. The NiFe2O4 electrode achieved a specific capacity of 365 mAh g-1 when cycled at a current density of 10 A g-1 (about a 10 C-rate). At such a high current density, this is an outstanding capacity for NiFe2O4 nanoparticles as an anode. Ex-situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) were employed at different potential states during the cell operation to elucidate the conversion process of a NiFe2O4 anode and the capacity contribution from either Ni or Fe. Investigation reveals that the lithium extraction reaction does not fully agree with the previously reported one and is found to be a hindered oxidation of metallic nickel to nickel oxide in the applied potential window. Our findings suggest that iron is participating in an electrochemical reaction with full reversibility and forms iron oxide in the fully charged state, while nickel is found to be the cause of partial irreversible capacity where it exists in both metallic nickel and nickel oxide phases.