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.

Effect of Stereochemically Active Electron Lone Pairs on Magnetic Ordering in Trivanadates.

Stereoactive electron lone pairs derived from filled 5/6s2 states of p-block cations are an intriguing electronic and geometric structure motif that have been exploited for diverse applications such as thermoelectrics, thermochromics, photocatalysis, and nonlinear optics. Layered trivanadates are dynamic intercalation hosts, where the insertion of cations can be used to tune electron correlation, charge localization, and magnetic ordering. However, the interaction of 5/6s2 stereoactive electron lone pairs with layered trivanadates remains unexplored. In this study, we contrast s- and p-block trivanadates and map off-centering in the coordination environment and reduction in symmetry arising from the stereochemical activity of lone pair cations to the emergence of filled antibonding lone-pair 6s2-O 2p hybridized states. The former is studied by high-resolution single-crystal X-ray diffraction studies of TlV3O8 and isostructural RbV3O8 to probe distinct differences in Tl and Rb coordination environments and the resulting modulation of V-V interactions in V3O8 slabs. The latter has been probed by variable-energy hard X-ray photoelectron spectroscopy (HAXPES) measurements, which manifest orbital-specific contributions from bonding and antibonding interactions of stereoactive Tl 6s2 electron lone pairs in TlV3O8. The spectroscopic assignment of valence band states to stereoactive lone pairs is further corroborated by first-principles electronic structure calculations, crystal orbital Hamilton population analyses, and electron localization function maps. The presence of the Tl 6s2 electron lone pair in TlV3O8 brings about the off-centering of Tl+ cations, which leads to anisotropy in Tl-O bonds. The off-centering of Tl ions weakens V-O bonds in one direction, which subsequently strengthens directional V-V coupling. Magnetic measurements reveal ferromagnetic signatures for both RbV3O8 and TlV3O8. However, the differences in V···V interactions significantly affect the energy balance of the superexchange interactions, resulting in an ordering temperature of 140 K for TlV3O8 as compared to 125 K for RbV3O8. The results demonstrate the distinctive effects of stereochemically active lone pairs in modifying electronic structure near the Fermi level and for mediating superexchange interactions.

Linker-Assisted Assembly of Ligand-Bridged CdS/MoS2 Heterostructures: Tunable Light-Harvesting Properties and Ligand-Dependent Control of Charge-Transfer Dynamics and Photocatalytic Hydrogen Evolution.

We used linker-assisted assembly (LAA) to tether CdS quantum dots (QDs) to MoS2 nanosheets via L-cysteine (cys) or mercaptoalkanoic acids (MAAs) of varying lengths, yielding ligand-bridged CdS/MoS2 heterostructures for redox photocatalysis. LAA afforded precise control over the light-harvesting properties of QDs within heterostructures. Photoexcited CdS QDs transferred electrons to molecularly linked MoS2 nanosheets from both band-edge and trap states; the electron-transfer dynamics was tunable with the properties of bridging ligands. Rate constants of electron transfer, estimated from time-correlated single photon counting (TCSPC) measurements, ranged from (9.8 ± 3.8) × 106 s-1 for the extraction of electrons from trap states within heterostructures incorporating the longest MAAs to >5 × 109 s-1 for the extraction of electrons from band-edge or trap states in heterostructures with cys or 3-mercaptopropionic acid (3MPA) linkers. Ultrafast transient absorption measurements revealed that electrons were transferred within 0.5-2 ps or less for CdS-cys-MoS2 and CdS-3MPA-MoS2 heterostructures, corresponding to rate constants ≥5 × 109 s-1. Photoinduced CdS-to-MoS2 electron transfer could be exploited in photocatalytic hydrogen evolution reaction (HER) via the reduction of H+ to H2 in concert with the oxidation of lactic acid. CdS-L-MoS2-functionalized FTO electrodes promoted HER under oxidative conditions wherein H2 was evolved at a Pt counter electrode with Faradaic efficiencies of 90% or higher and under reductive conditions wherein H2 was evolved at the CdS-L-MoS2-heterostructure-functionalized working electrode with Faradaic efficiencies of 25-40%. Dispersed CdS-L-MoS2 heterostructures promoted photocatalytic HER (15.1 µmol h-1) under white-light illumination, whereas free cys-capped CdS QDs produced threefold less H2 and unfunctionalized MoS2 nanosheets produced no measurable H2. Charge separation across the CdS/MoS2 interface is thus pivotal for redox photocatalysis. Our results reveal that LAA affords tunability of the properties of constituent CdS QDs and MoS2 nanosheets and precise, programmable, ligand-dependent control over the assembly, interfacial structure, charge-transfer dynamics, and photocatalytic reactivity of CdS-L-MoS2 heterostructures.

Tunnel-Structured ζ-V2O5 as a Redox-Active Insertion Host for Hybrid Capacitive Deionization.

Much of the earth's water has a salt content that is too high for human consumption or agricultural use. Enhanced oil recovery operations generate massive volumes of produced water waste with a high mineral content that can substantially exacerbate water distress. Current deionization techniques such as reverse osmosis function by removing the water (majority phase) from the salt (minority phase) and are thus exceedingly energy-intensive. Furthermore, these methods are limited in their ability to selectively extract high-value ions from produced water waste and brine streams. Hybrid capacitive deionization holds promise for enabling both desalination and resource recovery. In this work, we demonstrate the construction of a hybrid capacitive deionization cell that makes use of tunnel-structured ζ-V2O5 as a redox-active positive electrode material. By augmenting surface adsorption with Faradaic insertion processes, a 50% improvement in the ion removal capacity for K and Li ions is obtained as compared to a capacitive high-surface-area carbon electrode. The extracted ions are accommodated in surface sites and interstitial sites within the one-dimensional tunnel framework of ζ-V2O5. The kinetics of ion removal depend on the free energy of hydration, which governs the ease of desolvation at the electrode/electrolyte interface. The overall ion removal capacity additionally depends on the solid-state diffusion coefficient. ζ-V2O5 positive electrodes show substantial selectivity for Li+ removal from mixed flow streams and enrichment of the Li-ion concentration from produced water waste derived from the Permian Basin.

Toggling Stereochemical Activity through Interstitial Positioning of Cations between 2D V2O5 Double Layers.

The 5/6s2 lone-pair electrons of p-block cations in their lower oxidation states are a versatile electronic and geometric structure motif that can underpin lattice anharmonicity and often engender electronic and structural instabilities that underpin the function of active elements in nonlinear optics, thermochromics, thermoelectrics, neuromorphic computing, and photocatalysis. In contrast to periodic solids where lone-pair-bearing cations are part of the structural framework, installing lone-pair-bearing cations in the interstitial sites of intercalation hosts provides a means of a systematically modulating electronic structure through the choice of the group and the period of the inserted cation while preserving the overall framework connectivity. The extent of stereochemical activity and the energy positioning of lone-pair-derived mid-gap states depend on the cation identity, stoichiometry, and strength of anion hybridization. V2O5 polymorphs are versatile insertion hosts that can accommodate a broad range of s-, p-, and d-block cations. However, the insertion of lone-pair-bearing cations remains largely underexplored. In this article, we examine the implications of varying the 6s2 cations situated in interlayer sites between condensed [V4O10]n double layers. Systematic modulations of lattice distortions, electronic structure, and magnetic ordering are observed with increasing strength of stereochemical activity from group 12 to group 14 cations. We compare and contrast p-block-layered MxV2O5 (M = Hg, Tl, and Pb) compounds and map the significance of local off-centering arising from the stereochemical activity of lone-pair cations to the emergence of filled antibonding lone-pair 6s2-O 2p-hybridized mid-gap states mediated by second-order Jahn-Teller distortions. Crystallographic studies of cation coordination environments and the resulting modulation of V-V interactions have been used in conjunction with variable-energy hard X-ray photoelectron spectroscopy measurements, first-principles electronic structure calculations, and crystal orbital Hamilton population analyses to decipher the origins of stereochemical activity. Magnetic susceptibility measurements reveal antiferromagnetic signatures for all the three compounds. However, the differences in V-V interactions significantly affect the energy balance of the superexchange interactions, resulting in an ordering temperature of 160 and 260 K for Hg0.5V2O5 and δ-Tl0.5V2O5, respectively, as compared to 7 K for δ-Pb0.5V2O5. In δ-Pb0.5V2O5, the strong stereochemical activity of electron lone pairs and the resulting electrostatic repulsions enforce superlattice ordering, which strongly modifies the electronic localization patterns along the [V4O10] slabs, resulting in disrupted magnetic ordering and an anomalously low ordering temperature. The results demonstrate a versatile strategy for toggling the stereochemical activity of electron lone pairs to modify the electronic structure near the Fermi level and to mediate superexchange interactions.

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.

Effect of crystallite geometries on electrochemical performance of porous intercalation electrodes by multiscale operando investigation.

Lithium-ion batteries are yet to realize their full promise because of challenges in the design and construction of electrode architectures that allow for their entire interior volumes to be reversibly accessible for ion storage. Electrodes constructed from the same material and with the same specifications, which differ only in terms of dimensions and geometries of the constituent particles, can show surprising differences in polarization, stress accumulation and capacity fade. Here, using operando synchrotron X-ray diffraction and energy dispersive X-ray diffraction (EDXRD), we probe the mechanistic origins of the remarkable particle geometry-dependent modification of lithiation-induced phase transformations in V2O5 as a model phase-transforming cathode. A pronounced modulation of phase coexistence regimes is observed as a function of particle geometry. Specifically, a metastable phase is stabilized for nanometre-sized spherical V2O5 particles, to circumvent the formation of large misfit strains. Spatially resolved EDXRD measurements demonstrate that particle geometries strongly modify the tortuosity of the porous cathode architecture. Greater ion-transport limitations in electrode architectures comprising micrometre-sized platelets result in considerable lithiation heterogeneities across the thickness of the electrode. These insights establish particle geometry-dependent modification of metastable phase regimes and electrode tortuosity as key design principles for realizing the promise of intercalation cathodes.

Halide Replacement with Complete Preservation of Crystal Lattice in Mixed-Anion Lanthanide Oxyhalides.

A challenge in anion control in periodic solids is to preserve the crystal lattice while substituting for different anions of widely varying size and hardness. Post-synthetic modification routes that place cations or anions in non-equilibrium configurations are promising; however, such methods remain relatively unexplored for anion placement. Here, we report the synthesis of LaOI nanocrystals by a non-hydrolytic sol-gel condensation reaction and their transformation into LaOBr, LaOCl, and LaOF nanocrystals along hard-soft acid-base principles using post-synthetic metathesis reactions with ammonium halides. Anion displacement proceeds along halide planes, preserving the tetragonal matlockite structure. Energy-variant X-ray excited optical luminesce signatures of alloyed Tb3+ -ions is a sensitive quantum reporter of the preservation of the cation sublattice and hardening of the crystal structure upon anion replacement.

An Atomic View of Cation Diffusion Pathways from Single-Crystal Topochemical Transformations.

The diffusion pathways of Li-ions as they traverse cathode structures in the course of insertion reactions underpin many questions fundamental to the functionality of Li-ion batteries. Much current knowledge derives from computational models or the imaging of lithiation behavior at larger length scales; however, it remains difficult to experimentally image Li-ion diffusion at the atomistic level. Here, by using topochemical Li-ion insertion and extraction to induce single-crystal-to-single-crystal transformations in a tunnel-structured V2 O5 polymorph, coupled with operando powder X-ray diffraction, we leverage single-crystal X-ray diffraction to identify the sequence of lattice interstitial sites preferred by Li-ions to high depths of discharge, and use electron density maps to create a snapshot of ion diffusion in a metastable phase. Our methods enable the atomistic imaging of Li-ions in this cathode material in kinetic states and provide an experimentally validated angstrom-level 3D picture of atomic pathways thus far only conjectured through DFT calculations.