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.

An advanced cathode for Na-ion batteries with high rate and excellent structural stability.

Layered P2-Na(x)[Ni(1/3)Mn(2/3)]O(2) (0 < x < 2/3) is investigated as a cathode material for Na-ion batteries. A combination of first principles computation, electrochemical and synchrotron characterizations is conducted to elucidate the working mechanism for the improved electrochemical properties. The reversible phase transformation from P2 to O2 is observed. New configurations of Na-ions and vacancy are found at x = 1/3 and 1/2, which correspond to the intermediate phases upon the electrochemical cycling process. The mobility of Na-ions is investigated using the galvanostatic intermittent titration technique (GITT) and the Na diffusion barriers are calculated by the Nudged Elastic Band (NEB) method. Both techniques prove that the mobility of Na-ions is faster than Li-ions in the O3 structure within the 1/3 < x < 2/3 concentration region. Excellent cycling properties and high rate capability can be obtained by limiting the oxygen framework shift during P2-O2 phase transformation, suggesting that this material can be a strong candidate as a sustainable low-cost Na-ion battery cathode.