Understanding improved electrochemical properties of NiO-doped NiF2-C composite conversion materials by X-ray absorption spectroscopy and pair distribution function analysis.

The conversion reactions of pure NiF2 and the NiO-doped NiF2-C composite (NiO-NiF2-C) were investigated using X-ray absorption spectroscopy (XAS) and pair distribution function (PDF) analysis. The enhanced electronic conductivity of NiO-NiF2-C is associated with a significant improvement in the reversibility of the conversion reaction compared to pure NiF2. Different evolutions of the size distributions of the Ni nanoparticles formed during discharge were observed. While a bimodal nanoparticle size distribution was maintained for NiO-NiF2-C following the 1st and 2nd discharge, for pure NiF2 only smaller nanoparticles (∼14 Å) remained following the 2nd discharge. We postulate that the solid electrolyte interphase formed upon the 1st discharge at large overpotential retards the growth of metallic Ni leading to formation of smaller Ni particles during the 2nd discharge. In contrast, the NiO doping and the carbon layer covering the NiO-NiF2-C possibly facilitate the conversion process on the surface preserving the reaction kinetics upon the 2nd discharge. Based on the electronic conductivity and surface properties, the resulting size of the Ni nanoparticles is associated with the conversion kinetics and consequently the cyclability.