Abhishek Parija

Mapping polaronic states and lithiation gradients in individual V2O5 nanowires

The rapid insertion and extraction of Li ions from a cathode material is imperative for the functioning of a Li-ion battery. In many cathode materials such as LiCoO2, lithiation proceeds through solid-solution formation, whereas in other materials such as LiFePO4 lithiation/delithiation is accompanied by a phase transition between Li-rich and Li-poor phases. We demonstrate using scanning transmission X-ray microscopy (STXM) that in individual nanowires of layered V2O5, lithiation gradients observed on Li-ion intercalation arise from electron localization and local structural polarization. Electrons localized on the V2O5 framework couple to local structural distortions, giving rise to small polarons that serves as a bottleneck for further Li-ion insertion. The stabilization of this polaron impedes equilibration of charge density across the nanowire and gives rise to distinctive domains. The enhancement in charge/discharge rates for this material on nanostructuring can be attributed to circumventing challenges with charge transport from polaron formation.

Mapping Catalytically Relevant Edge Electronic States of MoS2

Molybdenum disulfide (MoS2) is a semiconducting transition metal dichalcogenide that is known to be a catalyst for both the hydrogen evolution reaction (HER) as well as for hydro-desulfurization (HDS) of sulfur-rich hydrocarbon fuels. Specifically, the edges of MoS2 nanostructures are known to be far more catalytically active as compared to unmodified basal planes. However, in the absence of the precise details of the geometric and electronic structure of the active catalytic sites, a rational means of modulating edge reactivity remain to be developed. Here we demonstrate using first-principles calculations, X-ray absorption spectroscopy, as well as scanning transmission X-ray microscopy (STXM) imaging that edge corrugations yield distinctive spectroscopic signatures corresponding to increased localization of hybrid Mo 4d states. Independent spectroscopic signatures of such edge states are identified at both the S L2,3 and S K-edges with distinctive spatial localization of such states observed in S L2,3-edge STXM imaging. The presence of such low-energy hybrid states at the edge of the conduction band is seen to correlate with substantially enhanced electrocatalytic activity in terms of a lower Tafel slope and higher exchange current density. These results elucidate the nature of the edge electronic structure and provide a clear framework for its rational manipulation to enhance catalytic activity.

Roadblocks in Cation Diffusion Pathways: Implications of Phase Boundaries for Li-ion Diffusivity in an Intercalation Cathode Material

Increasing intercalation of Li-ions brings about distortive structural transformations in several canonical intercalation hosts. Such phase transformations require the energy dissipative creation and motion of dislocations at the interface between the parent lattice and the nucleated Li-rich phase. Phase inhomogeneities within particles and across electrodes give rise to pronounced stress gradients, which can result in capacity fading. How such transformations alter Li-ion diffusivities remains much less explored. In this article, we use layered V2O5 as an intercalation host and examine the structural origins of the evolution of Li-ion diffusivities with phase progression upon electrochemical lithiation. Galvanostatic intermittent titration measurements show a greater than 4 orders of magnitude alteration of Li-ion diffusivity in V2O5 as a function of the extent of lithiation. Pronounced dips in Li-ion diffusivities are correlated with the presence of phase mixtures as determined by Raman spectroscopy and X-ray diffraction, whereas monophasic regimes correspond to the highest Li-ion diffusivity values measured within this range. First-principles density functional theory calculations confirm that the variations in Li-ion diffusivity do not stem from intrinsic differences in diffusion pathways across the different lithiated V2O5 phases, which despite differences in the local coordination environments of Li-ions show comparable migration barriers. Scanning transmission X-ray microscopy measurements indicate the stabilization of distinct domains reflecting the phase coexistence of multiple lithiated phases within individual actively intercalating particles. The results thus provide fundamental insight into the considerable ion transport penalties incurred as a result of phase boundaries formed within actively intercalating particles. The combination of electrochemical studies with ensemble structural characterization and single-particle X-ray imaging of phase boundaries demonstrates the profound impact of interfacial phenomena on macroscopic electrode properties.

Fabrication and Electrochemical Performance of Structured Mesoscale Open Shell V2O5 Networks

Crystalline vanadium pentoxide (V2O5) has attracted significant interest as a potential cathode material for energy storage applications due to its high theoretical capacity. Unfortunately, the material suffers from low conductivity as well as slow lithium ion diffusion, both of which affect how fast the electrode can be charged/discharged and how many times it can be cycled. Colloidal crystal templating (CCT) provides a simple approach to create well-organized 3-D nanostructures of materials, resulting in a significant increase in surface area that can lead to marked improvements in electrochemical performance. Here, a single layer of open shell V2O5 architectures ca. 1 μm in height with ca. 100 nm wall thickness was fabricated using CCT, and the electrochemical properties of these assemblies were evaluated. A decrease in polarization effects, resulting from the higher surface area mesostructured features, was found to produce significantly enhanced electrochemical performance. The discharge capacity of an unpatterned thin film of V2O5 (∼8.1 μAh/cm2) was found to increase to ∼10.2 μAh/cm2 when the material was patterned by CCT, affording enhanced charge storage capabilities as well as a decrease in the irreversible degradation during charge-discharge cycling. This work demonstrates the importance of creating mesoscale electrode surfaces for improving the performance of energy storage devices and provides fundamental understanding of the means to improve device performance.