Cation reordering instead of phase transitions: Origins and implications of contrasting lithiation mechanisms in 1D ζ- and 2D α-V2O5.

Substantial improvements in cycle life, rate performance, accessible voltage, and reversible capacity are required to realize the promise of Li-ion batteries in full measure. Here, we have examined insertion electrodes of the same composition (V2O5) prepared according to the same electrode specifications and comprising particles with similar dimensions and geometries that differ only in terms of their atomic connectivity and crystal structure, specifically two-dimensional (2D) layered α-V2O5 that crystallizes in an orthorhombic space group and one-dimensional (1D) tunnel-structured ζ-V2O5 crystallized in a monoclinic space group. By using particles of similar dimensions, we have disentangled the role of specific structural motifs and atomistic diffusion pathways in affecting electrochemical performance by mapping the dynamical evolution of lithiation-induced structural modifications using ex situ scanning transmission X-ray microscopy, operando synchrotron X-ray diffraction measurements, and phase-field modeling. We find the operation of sharply divergent mechanisms to accommodate increasing concentrations of Li-ions: a series of distortive phase transformations that result in puckering and expansion of interlayer spacing in layered α-V2O5, as compared with cation reordering along interstitial sites in tunnel-structured ζ-V2O5 By alleviating distortive phase transformations, the ζ-V2O5 cathode shows reduced voltage hysteresis, increased Li-ion diffusivity, alleviation of stress gradients, and improved capacity retention. The findings demonstrate that alternative lithiation mechanisms can be accessed in metastable compounds by dint of their reconfigured atomic connectivity and can unlock substantially improved electrochemical performance not accessible in the thermodynamically stable phase.

Porous Fe-Doped ß-Ni(OH)2 Nanopyramid Array Electrodes for Water Splitting.

We report a highly efficient and stable electrode composed of a porous Fe-doped ß-nickel hydroxide nanopyramid array supported on nickel foam (U-Fe-ß-Ni(OH)2/NF) for overall water splitting. The unique structure is assembled via a self-templated strategy by utilizing the FeNi oxalate (FeNi-C2O4/NF) nanopyramid as the templates, followed by an anion-exchange reaction at room temperature. Due to the intrinsic activity of Fe-doped ß-Ni(OH)2 along with unique porous array structures consisting of two-dimensional (2D) active materials on three-dimensional (3D) conductive substrates, the developed electrode exhibited outstanding electrocatalytic activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in an alkaline medium. The introduced amount of Fe plays a significant role in promoting OER and HER activity compared to the ß-Ni(OH)2 electrode. The optimal electrode (U-Fe-ß-Ni(OH)2/NF-2) generated a current density of 10 mA cm-2 at low overpotentials of 218 mV for the OER and 121 mV for the HER. The electrode also demonstrated considerably stable performance during the continuous water splitting process. Furthermore, we elucidated the promotion mechanisms of the active Fe-doped ß-Ni(OH)2 compound for the OER and HER based on extensive characterization and electrochemical measurements. Hence, this work provides a facile approach to developing low-cost, efficient, and stable hydroxide-based electrodes for bifunctional OER and HER in water splitting.