Victoria A. Nikitina

A molecular dynamics study of the ionic and molecular permeability of alkanethiol monolayers on the gold electrode surface

Ionic and molecular permeability of monolayers of alkanethiols on the Au(111) surface in an aqueous NaF solution is investigated using the molecular dynamics (MD) method. The surface charge-driven penetration of the electrolyte components into the monolayer was found to have a threshold. The mechanism of ion permeation comprises two steps, (1) the formation of a polar channel of water molecules which are drawn into the nonpolar monolayer due to local electric field fluctuations (2) transport of ions through the polar channel to the Au(111) surface. It is concluded that the defect-free monolayers are impermeable for the electrolyte components at electrode potentials related to the stability region of the monolayer. The monolayer of hexadecanethiols (—SC16H33) is less permeable with that of hexanethiols (—SC6H13), this is explained in terms of a closer packing structure for longer hydrocarbon tails.

Lithium Ion Coupled Electron-Transfer Rates in Superconcentrated Electrolytes: Exploring the Bottlenecks for Fast Charge-Transfer Rates with LiMn2O4 Cathode Materials

The charge-transfer kinetics of lithium ion intercalation into LixMn2O4 cathode materials was examined in dilute and concentrated aqueous and carbonate LiTFSI solutions using electrochemical methods. Distinctive trends in ion intercalation rates were observed between water-based and ethylene carbonate/diethyl carbonate solutions. The influence of the solution concentration on the rate of lithium ion transfer in aqueous media can be tentatively attributed to the process associated with Mn dissolution, whereas in carbonate solutions the rate is influenced by the formation of a concentration-dependent solid electrolyte interface (SEI). Some indications of SEI layer formation at electrode surfaces in carbonate solutions after cycling are detected by X-ray photoelectron spectroscopy. The general consequences related to the application of superconcentrated electrolytes for use in advanced energy storage cathodes are outlined and discussed.

Reversible facile Rb+ and K+ ions de/insertion in a KTiOPO4-type RbVPO4F cathode material

In this paper, we report on a novel RbVPO4F fluoride phosphate, which adopts the KTiOPO4 (KTP) type structure and complements the AVPO4F (A = alkali metal) family of positive electrode (cathode) materials for metal-ion batteries. RbVPO4F was synthesized via a freeze-drying assisted solid-state route and characterized via structural, computational and electrochemical methods. RbVPO4F represents the first example of reversible electrochemical Rb+ de/insertion in a crystalline oxypolyanionic framework. The electrochemical measurements on RbVPO4F in a three-electrode cell configuration in RbClO4-saturated propylene carbonate (PC) electrolyte revealed that the material exhibits reversible Rb+ de/insertion within the 0.4–1.3 V vs. Ag+/Ag potential range (∼3.9–4.8 V vs. K+/K) displaying rather high diffusion coefficients of (0.3–1.0) × 10−11 cm2 s−1 comparable to those of K+ in KVPO4F that supports reasonably fast ionic mobility in the KTP structure despite the large ionic radius of the Rb+ ions. The energy barriers of Rb+ ion transport are exceptionally low not exceeding 0.2 eV along the c-axis and correlating well with diffusion coefficients estimated using the DFT+U-NEB methodology and with the experimentally determined transport properties. These results suggest a new paradigm for the development of materials that support many monovalent ion reversible de/insertion processes in a single prototypical structural oxypolyanionic framework.