Reaction Heterogeneity in LiNi_0.8Co_0.15Al_0.05O₂ Induced by Surface Layer

Through operando synchrotron powder X-ray diffraction (XRD) analysis of layered transition metal oxide electrodes of composition LiNi_0.8Co_0.15Al_0.05O₂ (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 Li₂CO₃ forms on the electrode particle surface during exposure to air through reaction with atmospheric CO₂. 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 Li₂CO₃ 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_0.33V₂O₅ Nanowires with CdSe Quantum Dots: Toward Nanoscale Heterostructures with Tunable Interfacial Energetic Offsets for Charge Transfer

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_<i>x</i>V₂O₅ 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_<i>x</i>V₂O₅ 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_<i>x</i>V₂O₅ 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_<i>x</i>V₂O₅ 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_<i>x</i>V₂O₅ nanowires

Programming Interfacial Energetic Offsets and Charge Transfer in β‑Pb_0.33V₂O₅/Quantum-Dot Heterostructures: Tuning Valence-Band Edges to Overlap with Midgap States

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_0.33V₂O₅ heterostructures by both linker-assisted assembly and surface precipitation and contrasted these materials with CdSe/β-Pb_0.33V₂O₅ heterostructures prepared by the same methods. Increased valence-band (VB) edge onsets in X-ray photoelectron spectra for CdS/β-Pb_0.33V₂O₅ heterostructures relative to CdSe/β-Pb_0.33V₂O₅ 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_0.33V₂O₅. 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_0.33V₂O₅ NWs, along with electron transfer into the conduction band of the β-Pb_0.33V₂O₅ 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 LiMnPO₄ upon Chemical Delithiation?

Olivine MnPO₄ is the delithiated phase of the lithium-ion-battery cathode (positive electrode) material LiMnPO₄, which is formed at the end of charge. This phase is metastable under ambient conditions and can only be produced by delithiation of LiMnPO₄. We have revealed the manganese dissolution phenomenon during chemical delithiation of LiMnPO₄, which causes amorphization of olivine MnPO₄. The properties of crystalline MnPO₄ obtained from carbon-coated LiMnPO₄ and of the amorphous product resulting from delithiation of pure LiMnPO₄ were studied and compared. The phosphorus-rich amorphous phases in the latter are considered to be MnHP₂O₇ and MnH₂P₂O₇ from NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The thermal stability of MnPO₄ is significantly higher under high vacuum than at ambient condition, which is shown to be related to surface water removal

Electrode Reaction Mechanism of Ag₂VO₂PO₄ Cathode

The high capacity of primary lithium-ion cathode Ag₂VO₂PO₄ 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 Ag₂VO₂PO₄ 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⁺ 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 V₂O₅

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-V₂O₅ 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 V₂O₅ 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-V₂O₅ particles wherein lithiation-induced phase transformations induce phase segregation). Barriers to Li-ion as well as polaron diffusion are substantially diminished for metastable ζ-V₂O₅ in comparison to the thermodynamically stable α-V₂O₅ 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-V₂O₅. The 1D ζ-phase thus facilitates a continuous lithiation pathway that is markedly different from the successive intercalation-induced phase transitions observed in α-V₂O₅. 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 V₂O₅

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-V₂O₅ 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 V₂O₅ 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-V₂O₅ particles wherein lithiation-induced phase transformations induce phase segregation). Barriers to Li-ion as well as polaron diffusion are substantially diminished for metastable ζ-V₂O₅ in comparison to the thermodynamically stable α-V₂O₅ 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-V₂O₅. The 1D ζ-phase thus facilitates a continuous lithiation pathway that is markedly different from the successive intercalation-induced phase transitions observed in α-V₂O₅. 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_0.8Co_0.15Al_0.05O₂ Electrodes Due to Electrochemical and Thermal Stress

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 Li₂CO₃, 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_0.8Co_0.15Al_0.05O₂ (NCA) particle electrodes using a LiPF₆ ethylene carbonate:dimethyl carbonate (1:1 volume ratio) electrolyte. Although initial surface metal reduction does correlate with surface Li₂CO₃ and LiF, these species are found to decompose upon charging and are absent above 4.25 V. While there is trace LiPF₆ 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_0.8Co_0.15Al_0.05O₂ Electrodes Due to Electrochemical and Thermal Stress

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 Li₂CO₃, 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_0.8Co_0.15Al_0.05O₂ (NCA) particle electrodes using a LiPF₆ ethylene carbonate:dimethyl carbonate (1:1 volume ratio) electrolyte. Although initial surface metal reduction does correlate with surface Li₂CO₃ and LiF, these species are found to decompose upon charging and are absent above 4.25 V. While there is trace LiPF₆ 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 H₂ Production over Highly Dispersed Ruthenia on Rutile TiO₂ Nanorods

The immobilization of miniscule quantities of RuO₂ (∼0.1%) onto one-dimensional (1D) TiO₂ nanorods (NRs) allows H₂ evolution from water under visible light irradiation. Rod-like rutile TiO₂ structures, exposing preferentially (110) surfaces, are shown to be critical for the deposition of RuO₂ 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 RuO₂(110) grown as 1D nanowires on rutile TiO₂(110), which occurs only at extremely low loads of RuO₂, 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