12 research outputs found

    Reaction Heterogeneity in LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> Induced by Surface Layer

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    Through operando synchrotron powder X-ray diffraction (XRD) analysis of layered transition metal oxide electrodes of composition LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> (NCA), we decouple the intrinsic bulk reaction mechanism from surface-induced effects. For identically prepared and cycled electrodes stored in different environments, we demonstrate that the intrinsic bulk reaction for pristine NCA follows solid-solution mechanism, not a two-phase as suggested previously. By combining high resolution powder X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and surface sensitive X-ray photoelectron spectroscopy (XPS), we demonstrate that adventitious Li<sub>2</sub>CO<sub>3</sub> forms on the electrode particle surface during exposure to air through reaction with atmospheric CO<sub>2</sub>. This surface impedes ionic and electronic transport to the underlying electrode, with progressive erosion of this layer during cycling giving rise to different reaction states in particles with an intact versus an eroded Li<sub>2</sub>CO<sub>3</sub> surface-coating. This reaction heterogeneity, with a bimodal distribution of reaction states, has previously been interpreted as a “two-phase” reaction mechanism for NCA, as an activation step that only occurs during the first cycle. Similar surface layers may impact the reaction mechanism observed in other electrode materials using bulk probes such as operando powder XRD

    Integrating β‑Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> Nanowires with CdSe Quantum Dots: Toward Nanoscale Heterostructures with Tunable Interfacial Energetic Offsets for Charge Transfer

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    Achieving directional charge transfer across semiconductor interfaces requires careful consideration of relative band alignments. Here, we demonstrate a promising tunable platform for light harvesting and excited-state charge transfer based on interfacing β-Pb<sub><i>x</i></sub>V<sub>2</sub>O<sub>5</sub> nanowires with CdSe quantum dots. Two distinct routes are developed for assembling the heterostructures: linker-assisted assembly mediated by a bifunctional ligand and successive ionic layer adsorption and reaction (SILAR). In the former case, the thiol end of a molecular linker is found to bind to the quantum dot surfaces, whereas a protonated amine moiety interacts electrostatically with the negatively charged nanowire surfaces. In the alternative SILAR route, the surface coverage of CdSe nanostructures on the β-Pb<sub><i>x</i></sub>V<sub>2</sub>O<sub>5</sub> nanowires is tuned by varying the number of successive precipitation cycles. High-energy valence band X-ray photoelectron spectroscopy measurements indicate that “mid-gap” states of the β-Pb<sub><i>x</i></sub>V<sub>2</sub>O<sub>5</sub> nanowires derived from the stereoactive lone pairs on the intercalated lead cations are closely overlapped in energy with the valence band edges of CdSe quantum dots that are primarily Se 4p in origin. Both the midgap states and the valence-band levels are in principle tunable by variation of cation stoichiometry and particle size, respectively, providing a means to modulate the thermodynamic driving force for charge transfer. Steady-state and time-resolved photoluminescence measurements reveal dynamic quenching of the trap-state emission of CdSe quantum dots upon exposure to β-Pb<sub><i>x</i></sub>V<sub>2</sub>O<sub>5</sub> nanowires. This result is consistent with a mechanism involving the transfer of photogenerated holes from CdSe quantum dots to the midgap states of β-Pb<sub><i>x</i></sub>V<sub>2</sub>O<sub>5</sub> nanowires

    Programming Interfacial Energetic Offsets and Charge Transfer in β‑Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub>/Quantum-Dot Heterostructures: Tuning Valence-Band Edges to Overlap with Midgap States

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    Semiconductor heterostructures for solar energy conversion interface light-harvesting semiconductor nanoparticles with wide-band-gap semiconductors that serve as charge acceptors. In such heterostructures, the kinetics of charge separation depend on the thermodynamic driving force, which is dictated by energetic offsets across the interface. A recently developed promising platform interfaces semiconductor quantum dots (QDs) with ternary vanadium oxides that have characteristic midgap states situated between the valence and conduction bands. In this work, we have prepared CdS/β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> heterostructures by both linker-assisted assembly and surface precipitation and contrasted these materials with CdSe/β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> heterostructures prepared by the same methods. Increased valence-band (VB) edge onsets in X-ray photoelectron spectra for CdS/β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> heterostructures relative to CdSe/β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> heterostructures suggest a positive shift in the VB edge potential and, therefore, an increased driving force for the photoinduced transfer of holes to the midgap state of β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub>. This approach facilitates a ca. 0.40 eV decrease in the thermodynamic barrier for hole injection from the VB edge of QDs suggesting an important design parameter. Transient absorption spectroscopy experiments provide direct evidence of hole transfer from photoexcited CdS QDs to the midgap states of β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> NWs, along with electron transfer into the conduction band of the β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> NWs. Hole transfer is substantially faster and occurs at <1-ps time scales, whereas completion of electron transfer requires 530 ps depending on the nature of the interface. The differentiated time scales of electron and hole transfer, which are furthermore tunable as a function of the mode of attachment of QDs to NWs, provide a vital design tool for designing architectures for solar energy conversion. More generally, the approach developed here suggests that interfacing semiconductor QDs with transition-metal oxide NWs exhibiting intercalative midgap states yields a versatile platform wherein the thermodynamics and kinetics of charge transfer can be systematically modulated to improve the efficiency of charge separation across interfaces

    What Happens to LiMnPO<sub>4</sub> upon Chemical Delithiation?

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    Olivine MnPO<sub>4</sub> is the delithiated phase of the lithium-ion-battery cathode (positive electrode) material LiMnPO<sub>4</sub>, which is formed at the end of charge. This phase is metastable under ambient conditions and can only be produced by delithiation of LiMnPO<sub>4</sub>. We have revealed the manganese dissolution phenomenon during chemical delithiation of LiMnPO<sub>4</sub>, which causes amorphization of olivine MnPO<sub>4</sub>. The properties of crystalline MnPO<sub>4</sub> obtained from carbon-coated LiMnPO<sub>4</sub> and of the amorphous product resulting from delithiation of pure LiMnPO<sub>4</sub> were studied and compared. The phosphorus-rich amorphous phases in the latter are considered to be MnHP<sub>2</sub>O<sub>7</sub> and MnH<sub>2</sub>P<sub>2</sub>O<sub>7</sub> from NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The thermal stability of MnPO<sub>4</sub> is significantly higher under high vacuum than at ambient condition, which is shown to be related to surface water removal

    Electrode Reaction Mechanism of Ag<sub>2</sub>VO<sub>2</sub>PO<sub>4</sub> Cathode

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    The high capacity of primary lithium-ion cathode Ag<sub>2</sub>VO<sub>2</sub>PO<sub>4</sub> is facilitated by both displacement and insertion reaction mechanisms. Whether the Ag extrusion (specifically, Ag reduction with Ag metal displaced from the host crystal) and V reduction are sequential or concurrent remains unclear. A microscopic description of the reaction mechanism is required for developing design rules for new multimechanism cathodes, combining both displacement and insertion reactions. However, the amorphization of Ag<sub>2</sub>VO<sub>2</sub>PO<sub>4</sub> during lithiation makes the investigation of the electrode reaction mechanism difficult with conventional characterization tools. For addressing this issue, a combination of local probes of pair-distribution function and X-ray spectroscopy were used to obtain a description of the discharge reaction. We determine that the initial reaction is dominated by silver extrusion with vanadium playing a supporting role. Once sufficient Ag has been displaced, the residual Ag<sup>+</sup> in the host can no longer stabilize the host structure and V–O environment (i.e., onset of amorphization). After amorphization, silver extrusion continues but the vanadium reduction dominates the reaction. As a result, the crossover from primarily silver reduction displacement to vanadium reduction is facilitated by the amorphization that makes vanadium reduction increasingly more favorable

    Mitigating Cation Diffusion Limitations and Intercalation-Induced Framework Transitions in a 1D Tunnel-Structured Polymorph of V<sub>2</sub>O<sub>5</sub>

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    The design of cathodes for intercalation batteries requires consideration of both atomistic and electronic structure to facilitate redox at specific transition metal sites along with the concomitant diffusion of cations and electrons. Cation intercalation often brings about energy dissipative phase transformations that give rise to substantial intercalation gradients as well as multiscale phase and strain inhomogeneities. The layered α-V<sub>2</sub>O<sub>5</sub> phase is considered to be a classical intercalation host but is plagued by sluggish diffusion kinetics and a series of intercalation-induced phase transitions that require considerable lattice distortion. Here, we demonstrate that a 1D tunnel-structured ζ-phase polymorph of V<sub>2</sub>O<sub>5</sub> provides a stark study in contrast and can reversibly accommodate Li-ions without a large distortion of the structural framework and with substantial mitigation of polaronic confinement. Entirely homogeneous lithiation is evidenced across multiple cathode particles (in contrast to α-V<sub>2</sub>O<sub>5</sub> particles wherein lithiation-induced phase transformations induce phase segregation). Barriers to Li-ion as well as polaron diffusion are substantially diminished for metastable ζ-V<sub>2</sub>O<sub>5</sub> in comparison to the thermodynamically stable α-V<sub>2</sub>O<sub>5</sub> phase. The rigid tunnel framework, relatively small changes in coordination environment of intercalated Li-ions across the diffusion pathways defined by the 1D tunnels, and degeneracy of V 3d states at the bottom of the conduction band reduce electron localization that is a major impediment to charge transport in α-V<sub>2</sub>O<sub>5</sub>. The 1D ζ-phase thus facilitates a continuous lithiation pathway that is markedly different from the successive intercalation-induced phase transitions observed in α-V<sub>2</sub>O<sub>5</sub>. The results here illustrate the importance of electronic structure in mediating charge transport in oxide cathode materials and demonstrates that a metastable polymorph with higher energy bonding motifs that define frustrated coordination environments can serve as an attractive intercalation host

    Mitigating Cation Diffusion Limitations and Intercalation-Induced Framework Transitions in a 1D Tunnel-Structured Polymorph of V<sub>2</sub>O<sub>5</sub>

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    The design of cathodes for intercalation batteries requires consideration of both atomistic and electronic structure to facilitate redox at specific transition metal sites along with the concomitant diffusion of cations and electrons. Cation intercalation often brings about energy dissipative phase transformations that give rise to substantial intercalation gradients as well as multiscale phase and strain inhomogeneities. The layered α-V<sub>2</sub>O<sub>5</sub> phase is considered to be a classical intercalation host but is plagued by sluggish diffusion kinetics and a series of intercalation-induced phase transitions that require considerable lattice distortion. Here, we demonstrate that a 1D tunnel-structured ζ-phase polymorph of V<sub>2</sub>O<sub>5</sub> provides a stark study in contrast and can reversibly accommodate Li-ions without a large distortion of the structural framework and with substantial mitigation of polaronic confinement. Entirely homogeneous lithiation is evidenced across multiple cathode particles (in contrast to α-V<sub>2</sub>O<sub>5</sub> particles wherein lithiation-induced phase transformations induce phase segregation). Barriers to Li-ion as well as polaron diffusion are substantially diminished for metastable ζ-V<sub>2</sub>O<sub>5</sub> in comparison to the thermodynamically stable α-V<sub>2</sub>O<sub>5</sub> phase. The rigid tunnel framework, relatively small changes in coordination environment of intercalated Li-ions across the diffusion pathways defined by the 1D tunnels, and degeneracy of V 3d states at the bottom of the conduction band reduce electron localization that is a major impediment to charge transport in α-V<sub>2</sub>O<sub>5</sub>. The 1D ζ-phase thus facilitates a continuous lithiation pathway that is markedly different from the successive intercalation-induced phase transitions observed in α-V<sub>2</sub>O<sub>5</sub>. The results here illustrate the importance of electronic structure in mediating charge transport in oxide cathode materials and demonstrates that a metastable polymorph with higher energy bonding motifs that define frustrated coordination environments can serve as an attractive intercalation host

    Evolution of the Electrode–Electrolyte Interface of LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> Electrodes Due to Electrochemical and Thermal Stress

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    For layered oxide cathodes, impedance growth and capacity fade related to reactions at the cathode–electrolyte interface (CEI) are particularly prevalent at high voltage and high temperatures. At a minimum, the CEI layer consists of Li<sub>2</sub>CO<sub>3</sub>, LiF, reduced (relative to the bulk) metal-ion species, and salt decomposition species, but conflicting reports exist regarding their progression during (dis)­charging. Utilizing transport measurements in combination with X-ray and nuclear magnetic resonance spectroscopy techniques, we study the evolution of these CEI species as a function of electrochemical and thermal stress for LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> (NCA) particle electrodes using a LiPF<sub>6</sub> ethylene carbonate:dimethyl carbonate (1:1 volume ratio) electrolyte. Although initial surface metal reduction does correlate with surface Li<sub>2</sub>CO<sub>3</sub> and LiF, these species are found to decompose upon charging and are absent above 4.25 V. While there is trace LiPF<sub>6</sub> breakdown at room temperature above 4.25 V, thermal aggravation is found to strongly promote salt breakdown and contributes to surface degradation even at lower voltages (4.1 V). An interesting finding of our work was the partial reformation of LiF upon discharge, which warrants further consideration for understanding CEI stability during cycling

    Evolution of the Electrode–Electrolyte Interface of LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> Electrodes Due to Electrochemical and Thermal Stress

    No full text
    For layered oxide cathodes, impedance growth and capacity fade related to reactions at the cathode–electrolyte interface (CEI) are particularly prevalent at high voltage and high temperatures. At a minimum, the CEI layer consists of Li<sub>2</sub>CO<sub>3</sub>, LiF, reduced (relative to the bulk) metal-ion species, and salt decomposition species, but conflicting reports exist regarding their progression during (dis)­charging. Utilizing transport measurements in combination with X-ray and nuclear magnetic resonance spectroscopy techniques, we study the evolution of these CEI species as a function of electrochemical and thermal stress for LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> (NCA) particle electrodes using a LiPF<sub>6</sub> ethylene carbonate:dimethyl carbonate (1:1 volume ratio) electrolyte. Although initial surface metal reduction does correlate with surface Li<sub>2</sub>CO<sub>3</sub> and LiF, these species are found to decompose upon charging and are absent above 4.25 V. While there is trace LiPF<sub>6</sub> breakdown at room temperature above 4.25 V, thermal aggravation is found to strongly promote salt breakdown and contributes to surface degradation even at lower voltages (4.1 V). An interesting finding of our work was the partial reformation of LiF upon discharge, which warrants further consideration for understanding CEI stability during cycling

    Visible Light-Driven H<sub>2</sub> Production over Highly Dispersed Ruthenia on Rutile TiO<sub>2</sub> Nanorods

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    The immobilization of miniscule quantities of RuO<sub>2</sub> (∼0.1%) onto one-dimensional (1D) TiO<sub>2</sub> nanorods (NRs) allows H<sub>2</sub> evolution from water under visible light irradiation. Rod-like rutile TiO<sub>2</sub> structures, exposing preferentially (110) surfaces, are shown to be critical for the deposition of RuO<sub>2</sub> to enable photocatalytic activity in the visible region. The superior performance is rationalized on the basis of fundamental experimental studies and theoretical calculations, demonstrating that RuO<sub>2</sub>(110) grown as 1D nanowires on rutile TiO<sub>2</sub>(110), which occurs only at extremely low loads of RuO<sub>2</sub>, leads to the formation of a heterointerface that efficiently adsorbs visible light. The surface defects, band gap narrowing, visible photoresponse, and favorable upward band bending at the heterointerface drastically facilitate the transfer and separation of photogenerated charge carriers
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