Conversion mechanism of nickel fluoride and NiO-doped nickel fluoride in Li ion batteries

The conversion mechanism of NiF2 and NiO-doped NiF2 during electrochemical cycling was investigated using a combination of structural analysis by ex situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and magnetic analysis by superconducting quantum interference device (SQUID) magnetometry. It was observed that the conversion reactions in both cathode materials were partially reversible; however, they differ in their conversion rate. NiO-doped NiF2 exhibited enhanced electrochemical properties in terms of the conversion potential and reversibility due to the presence of a NiO phase, which has slightly higher electronic conductivity than NiF2. It is suggested that the NiO doping reduced the nucleation sites for Ni nanoparticles, subsequently enhancing the kinetics of the conversion reaction involving the growth of Ni particles formed during lithiation. The ex situ XRD and the magnetic hysteresis data (HC and MS) indicate that the average dimension of the Ni particles formed along with LiF in pristine NiF2 and NiO-doped NiF2 during the 1st lithiation was in the superparamagnetic regime, with 4–5 nm and 8–9 nm particle sizes, respectively. Although the particle size was decreased to the nanoscale, the original NiF2 phase was regenerated by re-lithiation.