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.

Probing the electrode/electrolyte interface in the lithium excess layered oxide Li1.2Ni0.2Mn0.6O2.

A detailed surface investigation of the lithium-excess nickel manganese layered oxide Li1.2Ni0.2Mn0.6O2 structure was carried out using X-ray photoelectron spectroscopy (XPS), total electron yield and transmission X-ray absorption spectroscopy (XAS), and electron energy loss spectroscopy (EELS) during the first two electrochemical cycles. All spectroscopy techniques consistently showed the presence of Mn(4+) in the pristine material and a surprising reduction of Mn at the voltage plateau during the first charge. The Mn reduction is accompanied by the oxygen loss revealed using EELS. Upon the first discharge, the Mn at the surface never fully recovers back to Mn(4+). The electrode/electrolyte interface of this compound consists of the reduced Mn at the crystalline defect-spinel inner layer and an oxidized Mn species simultaneously with the presence of a superoxide species in the amorphous outer layer. This proposed model signifies that oxygen vacancy formation and lithium removal result in electrolyte decomposition and superoxide formation, leading to Mn activation/dissolution and surface layer-spinel phase transformation. The results also indicate that the role of oxygen is complex and significant in contributing to the extra capacity of this class of high energy density cathode materials.

Atomic engineering of mixed ferrite and core-shell nanoparticles.

Nanoparticulate ferrites such as manganese zinc ferrite and nickel zinc ferrite hold great promise for advanced applications in power electronics. The use of these materials in current applications requires fine control over the nanoparticle size as well as size distribution to maximize their packing density. While there are several techniques for the synthesis of ferrite nanoparticles, reverse micelle techniques provide the greatest flexibility and control over size, crystallinity, and magnetic properties. Recipes for the synthesis of manganese zinc ferrite, nickel zinc ferrite, and an enhanced ferrite are presented along with analysis of the crystalline and magnetic properties. Comparisons are made on the quality of nanoparticles produced using different surfactant systems. The importance of various reaction conditions is explored with a discussion on the corresponding effects on the magnetic properties, particle morphology, stoichiometry, crystallinity, and phase purity.