Justin L. Andrews

Lithiation across interconnected V2O5 nanoparticle networks

Electrochemical reactions within Li-ion batteries occur far from equilibrium and are accompanied by considerable heterogeneity. Many electrode materials undergo phase transformations upon insertion of cations. The sequence and propagation of these phase transformations determine energy dissipation and the proportion of actively intercalating materials, which play a vital role in influencing characteristics such as cyclability, degradation, and hysteresis. The heterogeneity within electrode materials stems in large measure from local variations of structure, surface states, and position within the electrode; these factors are poorly understood given limited studies of local structure. Here, we show based on scanning transmission X-ray microscopy studies of Li-ion intercalation within interconnected V2O5 particle networks that interconnects between cathode particles strongly influence the transport of Li-ions and the resulting spatial propagation of phase transformations across the network. Considerable phase heterogeneity is observed across interfaces that are rationalized based on phase field models that suggest that the propagation of Li-rich domains occurs preferentially across a single particle instead of concurrent lithiation and nucleation of Li-rich domains across the entire network. Further phase heterogeneity arises from defects and secondary growth of Li-rich phases at nanowire tips. These findings suggest that mesoscale architectures can potentially be designed with appropriately positioned interconnects to maximize the proportion of actively intercalating regions and to ensure equilibration of local current densities.

Striping modulations and strain gradients within individual particles of a cathode material upon lithiation

The insertion of Li-ions within cathode materials during the discharging of a battery oftentimes brings about one or more structural transformations. The spatiodynamic propagation of phase transformations within a matrix of particles is determined by highly localized intercalation phenomena rather than the global voltage profile. Multiscale inhomogeneities resulting from variations in electrode reactions strongly influence the proportion of actively intercalating electrode materials, define local “hot-spots” wherein the current is greatly amplified during charge/discharge processes, and consequently dictate localized energy dissipation profiles. Multiphasic domains further give rise to localized stress gradients that can induce electrode degradation. However, a clear picture of chemical and stress inhomogeneities remains to be developed for most cathode materials. Here we demonstrate compositional striping modulations between Li-rich and Li-poor domains along the edges of individual nanowires of Li-ion-intercalated V2O5 based on analysis of hyperspectral X-ray microscopy data. Analysis of scanning transmission X-ray microscopy data using singular value decomposition and principal component analysis provides a means to map compositional inhomogenieties across individual nanowires and ensembles of nanowires alike. The compositional maps are further transformed to stress and strain maps, which depict the localization of tensile stress and strain within individual nanowires of LixV2O5. The core–shell and compositional striping modulations manifested here and the resulting strain gradients point to the need to design cathode materials and electrode architectures to mitigate such pronounced local inhomoegeneities in Li-ion intercalation and diffusion.

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.

Systematic Transmission Electron Microscopy Study Investigating Lithium and Magnesium Intercalation in Vanadium Oxide Polymorphs

Magnesium-ion based batteries promise a competitive alternative to conventional lithium-ion battery technology. Batteries combining Mg metal anode with a suitable intercalation-based cathode can offer much higher volumetric energy density, as well as significant cost and safety benefits over lithium ion batteries. Recent first-principles and experimental reports have established that orthorhombic α-V2O5 is a promising intercalation cathode for Mg ion batteries. However, several crucial aspects of the intercalation phenomenon, such as the specific intercalation sites for Mg within α-V2O5 or the formation of different phases upon Mg insertion into α-V2O5 remain unclear. Further systematic characterization of the Mg intercalation behaviour is therefore required.

The electronic structure of ε′-V2O5: an expanded band gap in a double-layered polymorph with increased interlayer separation

Selective elimination of network connectivity has emerged as an effective means of modifying the electronic structure of materials. Given its unique properties and diversity of polymorphs, V2O5 is an outstanding candidate. Recent studies have highlighted the benefit of utilizing metastable materials as cathode materials for multivalent ion batteries. In particular, novel polymorphs accessible from topochemical modification of ternary vanadium oxide bronzes have been identified as particularly interesting intercalation hosts. This is a study of the electronic structure of one such polymorph, e′-V2O5, using soft X-ray spectroscopy measurements and density functional theory calculations. This new double-layered polymorph of V2O5 has an increased interlayer separation that is found to lead to a dramatic increase in the band gap. Furthermore, the distortions brought on by the exfoliation process lead to a complex RIXS spectrum, showing d–d excitations, as well as low-energy charge transfer excitations. The comparison of the measurements and calculations is used to refine the crystal structure of e′-V2O5, which cannot be directly determined from X-ray diffraction data. In addition, distinct aspects of the electronic structure that make such polymorphs useful for correlated electron devices and electrode materials for intercalation batteries are discussed and linked to the crystal structure.

Structure-induced switching of the band gap, charge order and correlation strength in ternary vanadium oxide bronzes

Recently, V2 O5 nanowires have been synthesized as several different polymorphs, and as correlated bronzes with cations intercalated between the layers of edge- and corner- sharing VO6 octahedra. Unlike extended crystals, which tend to be plagued by substantial local variations in stoichiometry, nanowires of correlated bronzes exhibit precise charge ordering, thereby giving rise to pronounced electron correlation effects. These developments have greatly broadened the scope of research, and promise applications in several frontier electronic devices that make use of novel computing vectors. Here a study is presented of δ-Srx V2 O5 , expanded δ-Srx V2 O5 , exfoliated δ-Srx V2 O5 and δ-Kx V2 O5 using a combination of synchrotron soft X-ray spectroscopy and density functional theory calculations. The band gaps of each system are experimentally determined, and their calculated electronic structures are discussed from the perspective of the measured spectra. Band gaps ranging from 0.66 ± 0.20 to 2.32 ± 0.20 eV are found, and linked to the underlying structure of each material. This demonstrates that the band gap of V2 O5 can be tuned across a large portion of the range of greatest interest for device applications. The potential for metal-insulator transitions, tuneable electron correlations and charge ordering in these systems is discussed within the framework of our measurements and calculations, while highlighting the structure-property relationships that underpin them.

Aberration corrected STEM and High Resolution EELS study Investigating Magnesium Intercalation in Vanadium Pentoxide Cathode

Magnesium ion based batteries hold promise as a competitive alternative to conventional Lithium ion battery technology due to several key features. Theoretical volumetric capacity for magnesium metal anode is much higher compared to lithium metal. Furthermore, Mg is more readily available compared to Li, which can potentially lead to cost reduction and switching to Mg offers safety benefits over Li as well. Orthorhombic V2O5 is a well-known intercalation cathode host for Mg-ion batteries owing to its characteristic layered structure and weak vanadium oxygen bonding that facilitates ion intercalation between the layers.