Chemically Preintercalated Bilayered K_<i>x</i>V₂O₅·<i>n</i>H₂O Nanobelts as a High-Performing Cathode Material for K‑Ion Batteries

Tailoring the structure of the electrode material through chemical insertion of charge-carrying ions emerged as an efficient approach leading to enhanced performance of energy storage devices. Here, we for the first time report the effect of chemically preintercalated K⁺ ions on electrochemical charge storage properties of bilayered vanadium oxide (δ-V₂O₅) as a cathode in nonaqueous K-ion batteries, a low-cost alternative to Li-ion batteries, which is attractive for large-scale energy storage. δ-K_0.42V₂O₅·0.25H₂O with expanded interlayer spacing of 9.65 Å exhibited record high initial discharge capacity of 268 mAh·g^–1 at a current rate of C/50 and 226 mAh·g^–1 at a current rate of C/15. K-preintercalated bilayered vanadium oxide showed capacity retention of 74% after 50 cycles at a constant current of C/15 and 58% capacity retention when the current rate was increased from C/15 to 1C. Analysis of the mechanism of charge storage revealed that diffusion-controlled intercalation dominates over nonfaradaic capacitive contribution. High electrochemical performance of δ-K_0.42V₂O₅·0.25H₂O is attributed to the facilitated diffusion of electrochemically cycled K⁺ ions through well-defined intercalation sites, formed by chemically preintercalated K⁺ ions

Morphological Instability in Topologically Complex, Three-Dimensional Electrocatalytic Nanostructures

Advances in electrocatalyst functionality have resulted from the evolution of complex nanostructured materials with increasing degrees of compositional and morphological complexity. Focused almost entirely on pushing the boundaries of intrinsic activity, electrocatalytic material development often overlooks stability. Operating in parallel to the typical mechanisms of electrochemical material degradation, three-dimensional nanomaterials are susceptible to an additional degradation process known as coarsening. Driven by the reduction of surface free energy, surface diffusion evolves the nanoporous morphology toward a solid spherical particle. Here, using nanoporous NiPt alloy nanoparticles (np-NiPt/C) as a representative three-dimensional electrocatalytic material, we demonstrate that coarsening is the dominant mechanism of degradation as observed during accelerated durability testing (ADT). The upper potential limit (UPL) of the ADT protocol is found to have a significant impact on coarsening, with the rate roughly scaling with the UPL. Here we demonstrate the viability of a methodology to limit the coarsening process by decoration of the surface with a foreign metal impurity, Ir, possessing a surface diffusivity lower than that of the catalytic species. Ir, present in a low coverage with negligible impact on the intrinsic activity, dramatically slows morphology evolution. This strategy is shown to result in significant improvements in the electrochemically active surface area and transition-metal alloying component retention up to a UPL of 1.1 V versus the reversible hydrogen electrode. This proof-of-concept result demonstrates the utility of this strategy for improving the balance between activity and stability for three-dimensional electrocatalytic nanomaterials with potential application to a broad range of nanoscale geometries and compositions

Thickness-Dependent Crossover from Charge- to Strain-Mediated Magnetoelectric Coupling in Ferromagnetic/Piezoelectric Oxide Heterostructures

Magnetoelectric oxide heterostructures are proposed active layers for spintronic memory and logic devices, where information is conveyed through spin transport in the solid state. Incomplete theories of the coupling between local strain, charge, and magnetic order have limited their deployment into new information and communication technologies. In this study, we report direct, local measurements of strain- and charge-mediated magnetization changes in the La_0.7Sr_0.3MnO₃/PbZr_0.2Ti_0.8O₃ system using spatially resolved characterization techniques in both real and reciprocal space. Polarized neutron reflectometry reveals a graded magnetization that results from both local structural distortions and interfacial screening of bound surface charge from the adjacent ferroelectric. Density functional theory calculations support the experimental observation that strain locally suppresses the magnetization through a change in the Mn-e_g orbital polarization. We suggest that this local coupling and magnetization suppression may be tuned by controlling the manganite and ferroelectric layer thicknesses, with direct implications for device applications