Multiscale Computational Model for Particle Size Evolution during Coprecipitation of Li-Ion Battery Cathode Precursors.

Next generation lithium ion batteries require higher energy and power density, which can be achieved by tailoring the cathode particle morphology, such as particle size, size distribution, and internal porosity. All these morphological features are determined during the cathode synthesis process, which consists of two steps, (i) coprecipitation and (ii) calcination. Transition metal hydroxide precursors are synthesized during the coprecipitation process, whereas their oxidation and lithiation occur during calcination. The size and size distribution of crystalline primary and aggregated secondary particles and their internal porosity are determined during coprecipitation. Operating conditions of the chemical reactor, such as solution pH, ammonia concentration, and stirring speed control the final morphological features. Here, a multiscale computational model has been developed to capture the nucleation and growth of crystalline primary particles and their aggregation into secondary transition metal hydroxide precursor particles. The simulations indicate that increasing solution pH and decreasing ammonia concentration lead to smaller sizes of the secondary particles. A phase map has been developed that can help identify the synthesis conditions needed for a specified particle size and size distribution.

Rotating Ring-Disk Electrode Method for the Detection of Solution Phase Superoxide as a Reaction Intermediate of Oxygen Reduction in Neutral Aqueous Solutions.

A method is herein described that allows for solution phase superoxide generated via the reduction of dioxygen in neutral aqueous solutions at a rotating disk electrode to be oxidized at a concentric Au ring electrode bearing a covalently linked monolayer of 3-mercapto-1-propanol, a modified surface that blocks the oxidation of solution phase of hydrogen peroxide. Experiments were performed in which the potential of a glassy carbon disk electrode was linearly scanned in the oxygen reduction region and the ring voltage was poised at a value at which superoxide oxidation ensued yielded bell-shaped ring currents. This behavior is consistent with changes in the relative rates constant for the processes involved in the mechanism of oxygen reduction on this carbonaceous material induced by the applied potential which so far had remained undetected using other techniques.

Quantitative aspects of ohmic microscopy.

The potential difference between two microreference electrodes, Δφ(sol), immersed in an aqueous sulfuric acid solution was monitored while performing conventional cyclic voltammetric experiments with a Pt disk electrode embedded in an insulating surface in an axisymmetric cell configuration. The resulting Δφ(sol) vs E curves, where E is the potential applied to the Pt disk electrode were remarkably similar to the voltammograms regardless of the position of the microreference probes. Most importantly, the actual values of Δφ(sol) were in very good agreement with those predicted by the primary current distribution using Newman's formalism (Newman, J. J. Electrochem. Soc. 1966, 113, 501-502). These findings afford a solid basis for the development of ohmic microscopy as a quantitative tool for obtaining spatially resolved images of electrodes displaying nonhomogenous surfaces.

Adsorption of Cd2+ on carboxyl-terminated superparamagnetic iron oxide nanoparticles.

The affinity of Cd(2+) toward carboxyl-terminated species covalently bound to monodisperse superparamagnetic iron oxide nanoparticles, Fe(3)O(4)(np)-COOH, was investigated in situ in aqueous electrolytes using rotating disk electrode techniques. Strong evidence that the presence of dispersed Fe(3)O(4)(np)-COOH does not affect the diffusion limiting currents was obtained using negatively and positively charged redox active species in buffered aqueous media (pH = 7) devoid of Cd(2+). This finding made it possible to determine the concentration of unbound Cd(2+) in solutions containing dispersed Fe(3)O(4)(np)-COOH, 8 and 17 nm in diameter, directly from the Levich equation. The results obtained yielded Cd(2+) adsorption efficiencies of ~20 µg of Cd/mg of Fe(3)O(4)(np)-COOH, which are among the highest reported in the literature employing ex situ methods. Desorption of Cd(2+) from Fe(3)O(4)(np)-COOH, as monitored by the same forced convection method, could be accomplished by lowering the pH, a process found to be highly reversible.

A facile route to hollow nanospheres of mesoporous silica with tunable size.

Hollow mesoporous silica nanospheres (HMSNs) with tunable sizes of both sphere diameter (around 100 nm) and shell thickness have been successfully fabricated.