Search for: All records

The layered transition metal chalcogenides MCrX₂ (M = Ag, Cu; X = S, Se, Te) are of interest for energy storage because chemically Li-substituted analogs were reported as superionic Li⁺ conductors. The coexistence of fast ion transport and reducible transition metal and chalcogen elements suggests that this family may offer multifunctional capability for electrochemical storage. Here, we investigate the electrochemical reduction of AgCrSe₂ and CuCrSe₂ in non-aqueous Li- and Na-ion electrolytes using electrochemical measurements coupled with ex situ characterization (scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy). Both compounds delivered high initial specific capacities (~ 560 mAh/g), corresponding to 6.64 and 5.73 Li⁺/e^- per formula unit for AgCrSe₂ and CuCrSe₂, respectively. We attribute this difference to distinct reduction pathways: 1) Li⁺ intercalation to form LiCrSe₂ and extruded Ag or Cu, 2) conversion of LiCrSe₂ to Li₂Se, and 3) formation of an Ag-Li alloy at the lowest potential, operative only in AgCrSe₂. Consistent with this proposed mechanism, step 3 was absent during reduction of AgCrSe₂ in a Na-ion electrolyte since Ag does not alloy with Na. These results demonstrate the complex reduction chemistry of MCrX₂ in the presence of alkali ions, providing insights into the use of MCrX₂ materials as alkali ion superionic conductors or high capacity electrodes for lithium or sodium-ion type batteries. 

Hydrous transition metal oxides (TMOs) are redox-active materials that confine structural water within their bulk, organized in 1D, 2D, or 3D networks. In an electrochemical cell, hydrous TMOs can interact with electrolyte species not only via their outer surface but also via their hydrous inner surface, which can transport electrolyte species to the interior of the material. Many TMOs operating in an aqueous electrochemical environment transform to hydrous TMOs, which then serve as the electrochemically active phase. This review summarizes the physicochemical properties of hydrous TMOs and recent mechanistic insights into their behavior in electrochemical reactions of interest for energy storage, conversion, and environmental applications. Particular focus is placed on first-principles calculations and operando characterization to obtain an atomistic view of their electrochemical mechanisms. Hydrous TMOs represent an important class of energy and environmental materials in aqueous and nonaqueous environments. Further understanding of their interaction with electrolyte species is likely to yield advancements in electrochemical reactivity and kinetics for energy and environmental applications. 

Materials that undergo ion-insertion coupled electron transfer are important for energy storage, energy conversion, and optoelectronics applications. Cyclic voltammetry is a powerful technique to understand electrochemical kinetics. However, the interpretation of the kinetic behavior of ion insertion electrodes with analytical solutions developed for ion blocking electrodes has led to confusion about their rate-limiting behavior. The purpose of this manuscript is to demonstrate that the cyclic voltammetry response of thin film electrode materials undergoing solid-solution ion insertion without significant Ohmic polarization can be explained by well-established models for finite diffusion. To do this, we utilize an experimental and simulation approach to understand the kinetics of Li⁺insertion-coupled electron transfer into a thin film material (Nb₂O₅). We demonstrate general trends for the peak current vs scan rate behavior, with the latter parameter elevated to an exponent between limiting values of 1 and 0.5, depending on the solid-state diffusion characteristics of the film (diffusion coefficient, film thickness) and the experiment timescale (scan rate). We also show that values < 0.5 are possible depending on the cathodic potential limit. Our results will be useful to fundamentally understand and guide the selection and design of intercalation materials for multiple applications. 

Electrochemical ion insertion into transition metal oxides forms the foundation of several energy technologies. Transition metal oxides can exhibit sluggish ion transport and/or phase-transformation kinetics during ion insertion that can limit their performance at high rates (<10 min). In this study, we investigate the role of structural water in transition metal oxides during Li + insertion using staircase potentiostatic electrochemical impedance spectroscopy (SPEIS) and electrochemical quartz crystal microbalance (EQCM) analysis of WO 3 ·H 2 O and WO 3 thin-film electrodes. Overall, the presence of structural water in WO 3 ·H 2 O improves Li + insertion kinetics compared to WO 3 and leads to a less potential-dependent insertion process. Operando electrogravimetry and 3D Bode impedance analyses of nanostructured films reveal that the presence of structural water promotes charge accommodation without significant co-insertion of solvent, leading to our hypothesis that the electrochemically induced structural transitions of WO 3 hinder the electrode response at faster timescales (<10 min). Designing layered materials with confined fluids that exhibit less structural transitions may lead to more versatile ion-insertion hosts for next-generation electrochemical technologies.