Electrochemical Behavior of Surface-Fluorinated Cathode Materials for Lithium Ion Battery

Abstract Electrochemical properties and thermal stability of carbon-coated LiFePO 4 cathode are improved by surface fluorination using NF 3 gas at 298 K. The LiFePO 4 fluorinated at 6.67 kPa-NF 3 showed that the discharge capacity was about 10% higher than that of untreated LiFePO 4 . Electrochemical impedance spectra indicates the lowest resistance ( R ct  = 35.55 Ω) and largest exchange current density ( i 0  = 0.72 mA). However, in the case of LiFePO 4 fluorinated at 50.66 kPa-NF 3 , the electrochemical properties are negatively affected by the formation of resistive fluoride films. Regarding the thermal stability, the decomposition temperature (495 K) of LiFePO 4 fluorinated at 6.67 kPa-NF 3 was 14 K higher than that of untreated LiFePO 4 . Surface fluorination using F 2 gas improves the electrochemical properties and thermal stability of LiFePO 4 as a promising cathode of high power lithium-ion cells for HEVs. The LiNi 0.5 Mn 1.5 O 4 is used as a 5 V class cathode for lithium secondary battery. Because of its high redox potential near 5 V versus Li/Li + , it is attracting much attention as high energy density cathode material. The effect of surface fluorination on the electrochemical properties and Mn dissolution from LiNi 0.5 Mn 1.5 O 4 cathode has been investigated. Fluorinated LiNi 0.5 Mn 1.5 O 4 particles were prepared at 298 and 373 K by 6.67 kPa-F 2 for 1 h. X-ray diffraction and scanning electron microscopy indicated that the surface fluorination with F 2 gas did not affect the crystal structure and particle morphology of LiNi 0.5 Mn 1.5 O 4 . However, X-ray photoelectron spectroscopy data proved the existence of fluorinated surface layers at the surface of LiNi 0.5 Mn 1.5 O 4 . The discharge capacity and cycle stability of LiNi 0.5 Mn 1.5 O 4 fluorinated at 298 K was higher than that of untreated sample. The Mn dissolution of LiNi 0.5 Mn 1.5 O 4 is also restrained by the surface fluorination. Thus the surface fluorination improves the electrochemical properties of LiFePO 4 and LiNi 0.5 Mn 1.5 O 4 as the promising cathode active materials for lithium-ion cells.