Mapping polaronic states and lithiation gradients in individual V2O5 nanowires

Effect of pre-intercalation on Li-ion diffusion mapped by topochemical single-crystal transformation and operando investigation

Limitations in electrochemical performance as well as supply chain challenges have rendered positive electrode materials a critical bottleneck for Li-ion batteries. State-of-the-art Li-ion batteries fall short of accessing theoretical capacities. As such, there is intense interest in the design of strategies that enable the more effective utilization of active intercalation materials. Pre-intercalation with alkali-metal ions has attracted interest as a means of accessing higher reversible capacity and improved rate performance. However, the structural basis for improvements in electrochemical performance remains mostly unexplored. Here we use topochemical single-crystal-to-single-crystal transformations in a tunnel-structured ζ-V2O5 positive electrode to illustrate the effect of pre-intercalation in modifying the host lattice and altering diffusion pathways. Furthermore, operando synchrotron X-ray diffraction is used to map Li-ion site preferences and occupancies as a function of the depth of discharge in pre-intercalated materials. Na- and K-ion intercalation 'props open' the one-dimensional tunnel, reduces electrostatic repulsions between inserted Li ions and entirely modifies diffusion pathways, enabling orders of magnitude higher Li-ion diffusivities and accessing higher capacities. Deciphering the atomistic origins of improved performance in pre-intercalated materials on the basis of single-crystal-to-single-crystal topochemical transformation and operando diffraction studies paves the way to site-selective modification approaches for positive electrode design.

Weakly Polarized Organic Cation-Modified Hydrated Vanadium Oxides for High-Energy Efficiency Aqueous Zinc-Ion Batteries

Vanadium oxides, particularly hydrated forms like V2O5·nH2O (VOH), stand out as promising cathode candidates for aqueous zinc ion batteries due to their adjustable layered structure, unique electronic characteristics, and high theoretical capacities. However, challenges such as vanadium dissolution, sluggish Zn2+ diffusion kinetics, and low operating voltage still hinder their direct application. In this study, we present a novel vanadium oxide ([C6H6N(CH3)3]1.08V8O20·0.06H2O, TMPA-VOH), developed by pre-inserting trimethylphenylammonium (TMPA+) cations into VOH. The incorporation of weakly polarized organic cations capitalizes on both ionic pre-intercalation and molecular pre-intercalation effects, resulting in a phase and morphology transition, an expansion of the interlayer distance, extrusion of weakly bonded interlayer water, and a substantial increase in V4+ content. These modifications synergistically reduce the electrostatic interactions between Zn2+ and the V-O lattice, enhancing structural stability and reaction kinetics during cycling. As a result, TMPA-VOH achieves an elevated open circuit voltage and operation voltage, exhibits a large specific capacity (451 mAh g-1 at 0.1 A g-1) coupled with high energy efficiency (89%), the significantly-reduced battery polarization, and outstanding rate capability and cycling stability. The concept introduced in this study holds great promise for the development of high-performance oxide-based energy storage materials.

Unexpected Two-Dimensional Polarons Induced by Oxygen Vacancies in Layered Structure MoO3-x

Unveiling the effects of oxygen vacancies on the structural stability of layered α-MoO3 is critical for optimizing its physical and chemical properties. Herein, we present experimental evidence regarding the phase stability of α-MoO3 with ∼2% oxygen vacancy concentrations. Interestingly, we report a previously ignored oxygen-deficient orthorhombic MoO3-x phase in space group Cmcm. Further density functional theory calculations reveal a detailed phase transition mechanism from α-MoO3 to MoO3-x. More importantly, we demonstrate that two-dimensional (2D) large polarons must exist to stabilize the MoO3-x crystal structure. 2D large polarons are suspected to exist in numerous quasi-2D systems but have never been found in layered α-MoO3 or other molybdenum oxides. Our work contributes to a basic understanding of the polaronic behavior in MoO3-x and may broaden the application realm of molybdenum oxides.

Controlling Electron Delocalization in Vanadium-Based Hybrid Bronzes through Molecular Templation

We show that the conductivity of hybrid vanadium bronzes-mixed-valence organic-inorganic vanadium oxides-can be tuned over six orders of magnitude through judicious choice of molecular component. By systematically varying the steric profile, charge density, and propensity to hydrogen bond across a series of eight diammonium-based molecules, we engender multiple distinct motifs of V-O connectivity within the two-dimensional vanadium oxide layers of a family of bulk crystalline hybrid materials. A combination of single-crystal and powder X-ray diffraction analysis, variable-temperature electrical transport measurements, and a range of spectroscopic methods, including UV/Visible diffuse reflectance, X-ray photoelectron, and electron paramagnetic resonance are employed to probe how vanadium oxide layer topology correlates with electron localization. Specifically, alkylammonium molecules yield hybrids featuring more corrugated layers that contain V-O tetrahedra as well as a higher ratio of corner-sharing to edge-sharing polyhedra and that exhibit highly localized electronic behavior, while alkyl bipyridinium molecules yield more regular layers with polyhedral edge-sharing that show substantially delocalized electronic behavior. This work allows for the development of design principles based on structure-property relationships and brings the charge transport capabilities of hybrid vanadium bronzes to more technologically relevant levels.

5D Analysis of Capacity Degradation in Battery Electrodes Enabled by Operando CT-XANES

For devices encountering long-term stability challenges, a precise evaluation of degradation is of paramount importance. However, methods for comprehensively elucidating the degradation mechanisms in devices, particularly those undergoing dynamic chemical and mechanical changes during operation, such as batteries, are limited. Here, a method is presented using operando computed tomography combined with X-ray absorption near-edge structure spectroscopy (CT-XANES) that can directly track the evolution of the 3D distribution of the local capacity loss in battery electrodes during (dis)charge cycles, thereby enabling a five-dimensional (the 3D spatial coordinates, time, and chemical state) analysis of the degradation. This paper demonstrates that the method can quantify the spatiotemporal dynamics of the local capacity degradation within an electrode during cycling, which has been truncated by existing bulk techniques, and correlate it with the overall electrode performance degradation. Furthermore, the method demonstrates its capability to uncover the correlation among observed local capacity degradation within electrodes, reaction history during past (dis)charge cycles, and electrode microstructure. The method thus provides critical insights into the identification of degradation factors that are not available through existing methods, and therefore, will contribute to the development of batteries with long-term stability.

A Heterogeneous Single Atom Cobalt Catalyst for Highly Efficient Acceptorless Dehydrogenative Coupling Reactions

A fundamental understanding of metal active sites in single-atom catalysts (SACs) is important and challenging in the development of high-performance catalyst systems. Here, a highly efficient and straightforward molten-salt-assisted approach is reported to create atomically dispersed cobalt atoms supported over vanadium pentoxide layered material, with each cobalt atom coordinated with four neighboring oxygen atoms. The liquid environment and the strong polarizing force of the molten salt at high temperatures potentially favor the weakening of VO bonding and the formation of CoO bonding on the vanadium oxide surface. This cobalt SAC achieves extraordinary catalytic efficiency in acceptorless dehydrogenative coupling of alcohols with amines to give imines, with more than 99% selectivity under almost 100% conversion within 3 h, along with a high turnover frequency (TOF) of 5882 h^-1 , exceeding those of previously reported benchmarking catalysts. Moreover, it delivers excellent recyclability, reaction scalability, and substrate tolerance. Density functional theory (DFT) calculations further confirm that the optimized coordination environment and strong electronic metal-support interaction contribute significantly to the activation of reactants. The findings provide a feasible route to construct SACs at the atomic level for use in organic transformations.

Atomically Strained Metal Sites for Highly Efficient and Selective Photooxidation

Strain engineering is an attractive strategy for improving the intrinsic catalytic performance of heterogeneous catalysts. Manipulating strain on the short-range atomic scale to the local structure of the catalytic sites is still challenging. Herein, we successfully achieved atomic strain modulation on ultrathin layered vanadium oxide nanoribbons by an ingenious intercalation chemistry method. When trace sodium cations were introduced between the V₂O₅ layers (Na⁺-V₂O₅), the V-O bonds were stretched by the atomically strained vanadium sites, redistributing the local charges. The Na⁺-V₂O₅ demonstrated excellent photooxidation performance, which was approximately 12 and 14 times higher than that of pristine V₂O₅ and VO₂, respectively. Complementary spectroscopy analysis and theoretical calculations confirmed that the atomically strained Na⁺-V₂O₅ had a high surficial charge density, improving the activation of oxygen molecules and contributing to the excellent photocatalytic property. This work provides a new approach for the rational design of strain-equipped catalysts for selective photooxidation reactions.

Intercalant-induced V t2g orbital occupation in vanadium oxide cathode toward fast-charging aqueous zinc-ion batteries

Intercalation-type layered oxides have been widely explored as cathode materials for aqueous zinc-ion batteries (ZIBs). Although high-rate capability has been achieved based on the pillar effect of various intercalants for widening interlayer space, an in-depth understanding of atomic orbital variations induced by intercalants is still unknown. Herein, we design an NH4+-intercalated vanadium oxide (NH4+-V2O5) for high-rate ZIBs, together with deeply investigating the role of the intercalant in terms of atomic orbital. Besides extended layer spacing, our X-ray spectroscopies reveal that the insertion of NH4+ could promote electron transition to 3dxy state of V t2g orbital in V2O5, which significantly accelerates the electron transfer and Zn-ion migration, further verified by DFT calculations. As results, the NH4+-V2O5 electrode delivers a high capacity of 430.0 mA h g-1 at 0.1 A g-1, especially excellent rate capability (101.0 mA h g-1 at 200 C), enabling fast charging within 18 s. Moreover, the reversible V t2g orbital and lattice space variation during cycling are found via ex-situ soft X-ray absorption spectrum and in-situ synchrotron radiation X-ray diffraction, respectively. This work provides an insight at orbital level in advanced cathode materials.

Vanadium Oxide: Phase Diagrams, Structures, Synthesis, and Applications

Vanadium oxides with multioxidation states and various crystalline structures offer unique electrical, optical, optoelectronic and magnetic properties, which could be manipulated for various applications. For the past 30 years, significant efforts have been made to study the fundamental science and explore the potential for vanadium oxide materials in ion batteries, water splitting, smart windows, supercapacitors, sensors, and so on. This review focuses on the most recent progress in synthesis methods and applications of some thermodynamically stable and metastable vanadium oxides, including but not limited to V₂O₃, V₃O₅, VO₂, V₃O₇, V₂O₅, V₂O₂, V₆O₁₃, and V₄O₉. We begin with a tutorial on the phase diagram of the V-O system. The second part is a detailed review covering the crystal structure, the synthesis protocols, and the applications of each vanadium oxide, especially in batteries, catalysts, smart windows, and supercapacitors. We conclude with a brief perspective on how material and device improvements can address current deficiencies. This comprehensive review could accelerate the development of novel vanadium oxide structures in related applications.

Chemistry-mechanics-geometry coupling in positive electrode materials: a scale-bridging perspective for mitigating degradation in lithium-ion batteries through materials design

Despite their rapid emergence as the dominant paradigm for electrochemical energy storage, the full promise of lithium-ion batteries is yet to be fully realized, partly because of challenges in adequately resolving common degradation mechanisms. Positive electrodes of Li-ion batteries store ions in interstitial sites based on redox reactions throughout their interior volume. However, variations in the local concentration of inserted Li-ions and inhomogeneous intercalation-induced structural transformations beget substantial stress. Such stress can accumulate and ultimately engender substantial delamination and transgranular/intergranular fracture in typically brittle oxide materials upon continuous electrochemical cycling. This perspective highlights the coupling between electrochemistry, mechanics, and geometry spanning key electrochemical processes: surface reaction, solid-state diffusion, and phase nucleation/transformation in intercalating positive electrodes. In particular, we highlight recent findings on tunable material design parameters that can be used to modulate the kinetics and thermodynamics of intercalation phenomena, spanning the range from atomistic and crystallographic materials design principles (based on alloying, polymorphism, and pre-intercalation) to emergent mesoscale structuring of electrode architectures (through control of crystallite dimensions and geometry, curvature, and external strain). This framework enables intercalation chemistry design principles to be mapped to degradation phenomena based on consideration of mechanics coupling across decades of length scales. Scale-bridging characterization and modeling, along with materials design, holds promise for deciphering mechanistic understanding, modulating multiphysics couplings, and devising actionable strategies to substantially modify intercalation phase diagrams in a manner that unlocks greater useable capacity and enables alleviation of chemo-mechanical degradation mechanisms.

Machine learning accelerates identification of lithiated phases in X-ray images of battery hosts

Santos et al. (2022) propose a machine learning-based approach to identify various lithiated phases across lengthscales in X-ray images of battery particles, thus enabling automatic interpretation of such information in much bigger datasets and creating opportunities to unravel previously inaccessible scientific understanding.

Data science enables X-ray vision

In their recent publication in Patterns, the authors proposed a novel workflow to derive compositional and stress maps for positive electrode materials of Li-ion batteries from hyperspectral X-ray imaging data. They describe their interdisciplinary collaboration, the elements that sustain such collaborations, and their effect on the flourishing of the domain and data science.

Multivariate hyperspectral data analytics across length scales to probe compositional, phase, and strain heterogeneities in electrode materials

The origins of performance degradation in batteries can be traced to atomistic phenomena, accumulated at mesoscale dimensions, and compounded up to the level of electrode architectures. Hyperspectral X-ray spectromicroscopy techniques allow for the mapping of compositional variations, and phase separation across length scales with high spatial and energy resolution. We demonstrate the design of workflows combining singular value decomposition, principal-component analysis, k-means clustering, and linear combination fitting, in conjunction with a curated spectral database, to develop high-accuracy quantitative compositional maps of the effective depth of discharge across individual positive electrode particles and ensembles of particles. Using curated reference spectra, accurate and quantitative mapping of inter- and intraparticle compositional heterogeneities, phase separation, and stress gradients is achieved for a canonical phase-transforming positive electrode material, α-V₂O₅. Phase maps from single-particle measurements are used to reconstruct directional stress profiles showcasing the distinctive insights accessible from a standards-informed application of high-dimensional chemical imaging.

How the Environment Encourages the Natural Formation of Hydrated V2O5

Herein, we report the microscopic and spectroscopic signatures of the hydrated V₂O₅ phase, prepared from the α-V₂O₅ powder, which was kept in deionized water inside an airtight glass container for approximately 2.5 years. The experimental results show an evolution of the V⁴⁺ component in V 2p_3/2 core energy level spectra, and a peak corresponding to σ-OH^- bond appeared in the valence band spectra in the hydrated V₂O₅ powder sample due to the water intercalation. Vanadium metal oxide particles were found to be self-nucleated into micro/nanorods after a long period of exposure to an extremely humid environment. The distinct features in the spectra obtained with high-resolution transmission electron microscopy, micro-Raman scattering, and X-ray photoelectron spectroscopy confirmed the presence of structural water molecules for the first time in the long-aged naturally hydrated V₂O₅ phase.

VO2 as a Highly Efficient Electrocatalyst for the Oxygen Evolution Reaction

Herein, we report high electrocatalytic activity of monoclinic VO₂ (M1 phase) for the oxygen evolution reaction (OER) for the first time. The single-phase VO₂ (M1) nanoparticles are prepared in the form of uniformly covering the surface of individual carbon fibers constituting a carbon fiber paper (CFP). The VO₂ nanoparticles reveal the metal-insulator phase transition at ca. 65 °C (heating) and 62 °C (cooling) with low thermal hysteresis, indicating a high concentration of structural defect which is considered a grain boundary among VO₂ nanoparticles with some particle coalescence. Consequently, the VO₂/CFP shows a high electrocatalytic OER activity with the lowest η₁₀ (350 mV) and Tafel slope (46 mV/dec) values in a 1 M aqueous solution of KOH as compared to those of the vacuum annealed V₂O₅ and the hydrothermally grown VO₂ (M1), α-V₂O₅, and γ′-V₂O₅. The catalytically active site is considered V⁴⁺ components and V^4+/5+ redox couples in VO₂. The oxidation state of V⁴⁺ is revealed to be more favorable to the OER catalysis compared to that of V⁵⁺ in vanadium oxide through comparative studies. Furthermore, the amount of V⁵⁺ component is found to be increased on the surface of VO₂ catalyst during the OER, giving rise to the performance degradation. This work suggests V⁴⁺ and its redox couple as a novel active component for the OER in metal-oxide electrocatalysts.

Selective catalytic oxidation of ammonia to nitric oxide via chemical looping

Selective oxidation of ammonia to nitric oxide over platinum-group metal alloy gauzes is the crucial step for nitric acid production, a century-old yet greenhouse gas and capital intensive process. Therefore, developing alternative ammonia oxidation technologies with low environmental impacts and reduced catalyst cost are of significant importance. Herein, we propose and demonstrate a chemical looping ammonia oxidation catalyst and process to replace the costly noble metal catalysts and to reduce greenhouse gas emission. The proposed process exhibit near complete NH3 conversion and exceptional NO selectivity with negligible N2O production, using nonprecious V2O5 redox catalyst at 650 oC. Operando spectroscopy techniques and density functional theory calculations point towards a modified, temporally separated Mars-van Krevelen mechanism featuring a reversible V5+/V4+ redox cycle. The V = O sites are suggested to be the catalytically active center leading to the formation of the oxidation products. Meanwhile, both V = O and doubly coordinated oxygen participate in the hydrogen transfer process. The outstanding performance originates from the low activation energies for the successive hydrogen abstraction, facile NO formation as well as the easy regeneration of V = O species. Our results highlight a transformational process in extending the chemical looping strategy to producing base chemicals in a sustainable and cost-effective manner.

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

The function of cathode materials is determined by factors transcending decades of length scales, spanning the range from the crystal structure and composition of the compound to the dimensions and morphologies of the particles, their connectivity with other particles and with the conductive matrix, and their spatial location relative to the electrolyte–electrode interface. Mitigating the constraints and degradation mechanisms that limit cathode materials from realizing their full potential requires careful consideration of the electrode structure spanning multiple length scales. In this work, we explore an intriguing concept: For the same exact composition (V₂O₅), can the atomic connectivity be altered to stabilize a metastable polymorph that provides access to an entirely distinctive cation insertion and diffusion mechanism?

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.

Theoretical Design of Transition Metal-Doped TiO2 for the Selective Catalytic Reduction of NO with NH3 by DFT Calculations

Many transition metal oxides supported on TiO2 have been studied for selective catalytic reduction (SCR) of NO with NH3. However, the trade-off exists between the low-temperature activity and N2 selectivity....

Control of crystal size tailors the electrochemical performance of α-V2O5 as a Mg2+ intercalation host

α-V₂O₅ has been extensively explored as a Mg²⁺ intercalation host with potential as a battery cathode, offering high theoretical capacities and potentials vs. Mg²⁺/Mg. However, large voltage hysteresis is observed with Mg insertion and extraction, introducing significant and unacceptable round-trip energy losses with cycling. Conventional interpretations suggest that bulk ion transport of Mg²⁺ within the cathode particles is the major source of this hysteresis. Herein, we demonstrate that nanosizing α-V₂O₅ gives a measurable reduction to voltage hysteresis on the first cycle that substantially raises energy efficiency, indicating that mechanical formatting of the α-V₂O₅ particles contributes to hysteresis. However, no measurable improvement in hysteresis is found in the nanosized α-V₂O₅ in latter cycles despite the much shorter diffusion lengths, suggesting that other factors aside from Mg transport, such as Mg transfer between the electrolyte and electrode, contribute to this hysteresis. This observation is in sharp contrast to the conventional interpretation of Mg electrochemistry. Therefore, this study uncovers critical fundamental underpinning limiting factors in Mg battery electrochemistry, and constitutes a pivotal step towards a high-voltage, high-capacity electrode material suitable for Mg batteries with high energy density.

Electronic and thermodynamic properties of native point defects in V2O5: a first-principles study

The formation of native point defects in semiconductors and their behaviors play a crucial role in material properties. Although the native defects of V2O5 include vacancies, self-interstitials, and antisites, only oxygen vacancies have been extensively explored. In this work, we carried out first-principles calculations to systematically study the properties of possible native defects in V2O5. The electronic structure and the formation energy of each defect were calculated using the DFT+U method. Defect concentrations were estimated using a statistical model with a constraint of charge neutrality. We found that the vanadyl vacancy is a shallow acceptor that could supply holes to the system. However, the intrinsic p-type doping in V2O5 hardly occurred because the vanadyl vacancy could be readily compensated by the more stable donor, i.e., the oxygen vacancy and oxygen interstitial, instead of holes. The oxygen vacancy is the most dominant defect under oxygen-deficient conditions. However, under extreme O-rich conditions, a deep donor of oxygen interstitial becomes the major defect species. The dominant oxygen vacancy under synthesized conditions plays an important role in determining the electronic conductivity of V2O5. It induces the formation of compensating electron polarons. The polarons are trapped at V centers close to the vacancy site with the effective escaping barriers of around 0.6 eV. Such barriers are higher than that of the isolated polaron hopping (0.2 eV). The estimated polaron mobilities obtained from kinetic Monte Carlo simulations confirmed that oxygen vacancies act as polaron-trapping sites, which diminishes the polaron mobility by 4 orders of magnitude. Nevertheless, when the sample is synthesized at elevated temperatures, a number of thermally activated polarons in samples are quite high due to the high concentrations of oxygen vacancies. These polarons can contribute as charge carriers of intrinsic n-type semiconducting V2O5.

Revealing Charge-Transfer Dynamics at Electrified Sulfur Cathodes Using Constrained Density Functional Theory

To understand and control the behavior of electrochemical systems, including batteries and electrocatalysts, we seek molecular-level details of the charge transfer mechanisms at electrified interfaces. Recognizing some key limitations of standard equilibrium electronic structure methods to model materials and their interfaces, we propose applying charge constraints to effectively separate electronic and nuclear degrees of freedom, which are especially beneficial to the study of conversion electrodes, where electronic charge carriers are converted to much slower polarons within a material that is nonmetallic. We demonstrate the need for such an approach within the context of sulfur cathodes and the arrival of Li ions during discharge of a Li-S cell. The requirement that electronic degrees of freedom are arrested is justified by comparison with real-time evolution of the electronic structure. Long-lived metastable configurations provide plenty of time for nuclear dynamics and relaxation in response to the electrification of the interface, a process that would be completely missed without applying charge constraints. This approach will be vital to the study of dynamics at electrified interfaces which may be created deliberately, adding charge to the electrode, or spontaneously, due to finite temperature dynamics in the electrolyte.

Structure and Photoluminescence Properties of Thermally Synthesized V2O5 and Al-Doped V2O5 Nanostructures

Al-free and Al-doped V₂O₅ nanostructures were synthesized by a thermal-chemical vapor deposition (CVD) process on Si(100) at 850 °C under 1.2 × 10⁻¹ Torr via a vapor-solid (V-S) mechanism. X-ray diffraction (XRD), Raman, and high-resolution transmission electron microscopy (HRTEM) confirmed a typical orthorhombic V₂O₅ with the growth direction along [110]-direction of both nanostructures. Metallic Al, rather than Al³⁺-ion, was detected by X-ray photoelectron spectroscopy (XPS), affected the V₂O₅ crystallinity. The photoluminescence intensity of V₂O₅ nanostructure at 1.77 and 1.94 eV decreased with the increasing Al-dopant by about 61.6% and 59.9%, attributing to the metallic Al intercalated between the V₂O₅-layers and/or filled in the oxygen vacancies, which behaved as electron sinks. Thus the Al-doped V₂O₅ nanostructure shows the potential applications in smart windows and the electrodic material in a Li-ion battery.

Recent Progress and Challenges in the Optimization of Electrode Materials for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) have been regarded as one of the promising electrochemical energy storage systems to complement Li-ion batteries owing to the low-cost and high safety characteristics. However, the various challenges including the sluggish solid-state diffusion of highly polarizing Mg²⁺ ions in hosts, and the formation of blocking layers on Mg metal surface have seriously impeded the development of high-performance RMBs. In order to solve these problems toward practical applications of RMBs, a tremendous amount of work on electrodes and electrolytes has been conducted in the last few decades. Creative optimization strategies including the modification of cathodes and anodes such as shielding the charges of divalent Mg²⁺ , expanding the layers of host materials, and optimizing the interface of electrode-electrolyte are raised to promote the technology. In this review, the detailed description of innovative approaches, representative examples, and facing challenges for developing high-performance electrodes are presented. Based on the review of these strategies, guidelines are provided for future research directions on improving the overall battery performance, especially on the electrodes.

Insights into Synergistic Effect of Acid on Morphological Control of Vanadium Oxide: Toward High Lithium Storage

Morphological control is a fundamental challenge of nanomaterial development. Commonly, hierarchical nanostructures cannot be induced by a single driving force, but obtained through balancing multiple driving forces. Here, a feasible strategy is reported based on the synergistic effect of proton and acid anion, leading to the morphological variation of vanadium oxide from nanowire, bundle, to hierarchical nanoflower (HNF). Protons can only induce the formation of nanowire through reducing the pH value ≤ 2. However, acid anions with strong coordination ability, e.g., phosphate radicals, can also participate in morphological regulation at high concentration. Through coordinating with exposed vanadium ions, the enrichment of phosphate radicals at ledge and kink changes the growth directions, giving rise to the advanced structures of bundle and HNF. The lithium ion batteries using HNF as a cathode achieve a 30% improved initial discharge specific capacity of 436.23 mAh g-1 at a current density of 0.1 A g-1, reaching the theoretical maximum value of vanadium oxide based on insertion/desertion of three lithium ions, in addition to strong cyclic stability at 1 A g-1.

Enhanced charge storage of nanometric ζ-V2O5 in Mg electrolytes

V2O5 is of interest as a Mg intercalation electrode material for Mg batteries, both in its thermodynamically stable layered polymorph (α-V2O5) and in its metastable tunnel structure (ζ-V2O5). However, such oxide cathodes typically display poor Mg insertion/removal kinetics, with large voltage hysteresis. Herein, we report the synthesis and evaluation of nanosized (ca. 100 nm) ζ-V2O5 in Mg-ion cells, which displays significantly enhanced electrochemical kinetics compared to microsized ζ-V2O5. This effect results in a significant boost in stable discharge capacity (130 mA h g-1) compared to bulk ζ-V2O5 (70 mA h g-1), with reduced voltage hysteresis (1.0 V compared to 1.4 V). This study reveals significant advancements in the use of ζ-V2O5 for Mg-based energy storage and yields a better understanding of the kinetic limiting factors for reversible magnesiation reactions into such phases.

The split-off terahertz radiating dipoles on thermally reduced α-V2O5 (001) surface

The trapped electron states on a pliable lattice have different localization and physical chemistry characteristics. Here, terahertz time-domain measurements suggest that the formation of vanadyl oxygen defect, in the presence of the surface potential traps and mobile charge carriers, leads to a transient charge distribution that forms terahertz radiating dipoles in V2O5. The emergence of radiating dipoles is evidenced by terahertz responses with a two-valley feature of the thermally reduced α-V2O5 (001) thin films in the temperature range of 300-700 K. The two photoconductance valleys on a several millielectron volts interval are related to two emergent split-off traps, which originate from the VO6 octahedra distortion upon the vanadyl oxygen desorption on the surface. The pliable surface lattices plays a decisive role. So long as the α-V2O5 (001) thin films are covered by a 30 nm-thick Al2O3 capping layer, the distinct two-valley feature disappears completely in the full temperature range. The terahertz radiating dipoles with a fine energy structure is potentially a new measure for charge dynamics on the α-V2O5 (001) surface.

Revealing Nanoscale Chemical Heterogeneities in Polycrystalline Mo-BiVO4 Thin Films

The activity of polycrystalline thin film photoelectrodes is impacted by local variations of the material properties due to the exposure of different crystal facets and the presence of grain/domain boundaries. Here a multi-modal approach is applied to correlate nanoscale heterogeneities in chemical composition and electronic structure with nanoscale morphology in polycrystalline Mo-BiVO₄ . By using scanning transmission X-ray microscopy, the characteristic structure of polycrystalline film is used to disentangle the different X-ray absorption spectra corresponding to grain centers and grain boundaries. Comparing both spectra reveals phase segregation of V₂ O₅ at grain boundaries of Mo-BiVO₄ thin films, which is further supported by X-ray photoelectron spectroscopy and many-body density functional theory calculations. Theoretical calculations also enable to predict the X-ray absorption spectral fingerprint of polarons in Mo-BiVO₄ . After photo-electrochemical operation, the degraded Mo-BiVO₄ films show similar grain center and grain boundary spectra indicating V₂ O₅ dissolution in the course of the reaction. Overall, these findings provide valuable insights into the degradation mechanism and the impact of material heterogeneities on the material performance and stability of polycrystalline photoelectrodes.

Localized Electrons Enhanced Ion Transport for Ultrafast Electrochemical Energy Storage

The rate-determining process for electrochemical energy storage is largely determined by ion transport occurring in the electrode materials. Apart from decreasing the distance of ion diffusion, the enhancement of ionic mobility is crucial for ion transport. Here, a localized electron enhanced ion transport mechanism to promote ion mobility for ultrafast energy storage is proposed. Theoretical calculations and analysis reveal that highly localized electrons can be induced by intrinsic defects, and the migration barrier of ions can be obviously reduced. Consistently, experiment results reveal that this mechanism leads to an enhancement of Li/Na ion diffusivity by two orders of magnitude. At high mass loading of 10 mg cm^-2 and high rate of 10C, a reversible energy storage capacity up to 190 mAh g^-1 is achieved, which is ten times greater than achievable by commercial crystals with comparable dimensions.

Phase stability of intercalated V2O5 battery cathodes elucidated through the Goldschmidt tolerance factor

Orthorhombic V₂O₅ is a promising Mg battery cathode material, and reversible intercalation in the layered α-phase has been claimed experimentally. However, these results, based on electrochemistry and XRD, are controversial. Previous computational studies have predicted high activation barriers (∼1 eV) for ionic migration in α-V₂O₅, although improved Mg²⁺ mobility is expected in the δ-phase. Here, hybrid-exchange density functional theory is used to discuss structure, stability and intercalation in the α and δ phases, beginning with a model system with MV₂O₅ stoichiometry, and varying ionic radius of the M cations. The relative stability of the two phases upon intercalation of M is rationalised through a tolerance factor-like behavioural trend, providing a framework for phase selection using intercalants of different ionic size. This tolerance factor behaviour is due to the presence of ferroelectrically distorted (2 × 2 × 2) perovskite blocks within the α-V₂O₅ structure. The δ-phase is found to undergo a barrierless phase change to α in fully charged (de-intercalated) Mg_xV₂O₅ (x = 0), indicating that stabilisation of δ-Mg_xV₂O₅ is required at low x if the δ phase is to be retained for higher Mg mobility. By employing dispersion interactions to accurately reproduce the interlayer distance, activation barriers for ion migration are found to be higher than reported in previous studies, clarifying questions regarding the extent of Mg intercalation that can be achieved experimentally. Interlayer ions are found to lower activation barriers for Mg²⁺ mobility by up to ∼330 meV in the α phase by expanding the interlayer space. The results address open questions about the electrochemical performance of orthorhombic V₂O₅ as Mg battery cathode material, and indicate atomic level mechanisms that activate ionic mobility in layered V₂O₅.

Transport properties of electron small polarons in a V2O5 cathode of Li-ion batteries: a computational study

Mechanisms and properties of the electron transport at the V₂O₅ cathode of Li-ion batteries were studied by means of first-principles computations.

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.

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.

Revealing reaction mechanisms of nanoconfined Li2S: implications for lithium–sulfur batteries

Using Li₂S as an active material and designing nanostructured cathode hosts are considered as promising strategies to improve the performance of lithium–sulfur (Li–S) batteries.

Thermodynamics and defect chemistry of substitutional and interstitial cation doping in layered α-V2O5

A systematic study of the location and energetics of cation dopants in α-V₂O₅ has been conducted using pair-potential methods, supplemented by first-principles calculations.

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-Sr_x V₂ O₅ , expanded δ-Sr_x V₂ O₅ , exfoliated δ-Sr_x V₂ O₅ and δ-K_x 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.

Tunable Mechanochemistry of Lithium Battery Electrodes

The interplay between mechanical strains and battery electrochemistry, or the tunable mechanochemistry of batteries, remains an emerging research area with limited experimental progress. In this report, we demonstrate how elastic strains applied to vanadium pentoxide (V₂O₅), a widely studied cathode material for Li-ion batteries, can modulate the kinetics and energetics of lithium-ion intercalation. We utilize atomic layer deposition to coat V₂O₅ materials onto the surface of a shapememory superelastic NiTi alloy, which allows electrochemical assessment at a fixed and measurable level of elastic strain imposed on the V₂O₅, with strain state assessed through Raman spectroscopy and X-ray diffraction. Our results indicate modulation of electrochemical intercalation potentials by ∼40 mV and an increase of the diffusion coefficient of lithium ions by up to 2.5-times with elastic prestrains of <2% imposed on the V₂O₅. These results are supported by density functional theory calculations and demonstrate how mechanics of nanomaterials can be used as a precise tool to strain engineer the electrochemical energy storage performance of battery materials.

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.

A skin-integrated transparent and stretchable strain sensor with interactive color-changing electrochromic displays

In this study, we report on the development of a stretchable, transparent, and skin-attachable strain sensor integrated with a flexible electrochromic device as a human skin-inspired interactive color-changing system. The strain sensor consists of a spin-coated conductive nanocomposite film of poly(vinyl alcohol)/multi-walled carbon nanotube/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) on a polydimethylsiloxane substrate. The sensor exhibits excellent performance of high sensitivity, high durability, fast response, and high transparency. An electrochromic device (ECD) made of electrochemically synthesized polyaniline nanofibers and V₂O₅ on an indium-tin-oxide-coated polyethylene terephthalate film experiences a change in color from yellow to dark blue on application of voltage. The strain sensor and ECD are integrated on skin via an Arduino circuit for an interactive color change with the variation of the applied strain, which enables a real-time visual display of body motion. This integrated system demonstrates high potential for use in interactive wearable devices, military applications, and smart robots.

Modeling of phase separation across interconnected electrode particles in lithium-ion batteries

Non-equilibrium lithiation and its relaxation towards equilibrium in a particle network with phase-separating materials.

Rational selection of amorphous or crystalline V2O5 cathode for sodium-ion batteries

Amorphous and crystalline V₂O₅ cathodes in sodium ion batteries express inverse capacity values at low and high current densities.