Lithium phosphorous oxynitride (LiPON) coated NiFe2O4 anode material with enhanced electrochemical performance for lithium ion batteries

Polycrystalline nickel ferrite (NiFe2O4) thin films were fabricated by pulsed laser deposition (PLD) and coated with nanometer layer of lithium phosphorus oxynitride (LiPON) by radio frequency magnetron sputtering (RF sputtering). Physical characterizations demonstrated that NiFe2O4 thin films were not destroyed by LiPON coating with 50 nm thickness. Discharge capacity of LiPON coated NiFe2O4 thin film after 50 cycles at 5 μA cm−2 was 849 mAh g−1, much higher than the counterpart value of pristine NiFe2O4 thin film (449 mAh g−1), while rate capability was also greatly enhanced. Ex situ TEM investigation confirmed the two-steps reaction mechanism between NiFe2O4 and Li, which includes an irreversible decomposition reaction of NiFe2O4 in the first discharge process and reversible reactions between NiO, Fe2O3 and their nanosized metal particles afterward. The significant improvement on the electrochemical performance was ascribed to the amorphous LiPON layer on the surface, which suppressed the volume expansion of electrode, the formation of delaminated particles and generation of cracks on electrode surface, as proved by ex situ SEM and AFM results. The LiPON coating is an effective approach to enhance the electrochemical performance of conversion materials for lithium ion batteries.