Understanding the diffusion mechanisms of Li ions through host materials and the resulting phase evolution of intercalated phases is of paramount importance for designing electrode materials of rechargeable batteries. The formation of lithiation gradients and discrete domains during intercalation leads to the development of strain within the host material and is responsible for the observed capacities of most cathode materials being well below theoretically predicted values. Such mesoscale heterogeneity has also been implicated in the loss of capacity upon cycling. Due to their inherent complexity, the analysis of such heterogeneity is rather complex and precise understanding of the evolution of metal sites remains underexplored. In this work, we use phase-pure, single-crystalline V 2 O 5 nanowires with dimensions of 183 ± 50 nm and lengths spanning tens of microns as a model cathode material and demonstrate that V K-edge Xray absorption near-edge structure can be used as an effective probe of the local valence and geometry of vanadium sites upon lithiation. We demonstrate that a highly lithiated phase is nucleated and grows at the expense of a homogeneous low-lithiumcontent α-phase without mediation of a solid-solution with intermediate lithium content. Density functional theory calculations allow for assignment of the pre-edge feature to dipolar transitions that are particularly sensitive to the V 3d−O 2p hybridization of the vanadyl bond and the local geometry of the distorted [VO 5 ] square pyramid. The quantitative analysis of multiple vanadium sites and their evolution as a function of Li-ion content provides insight into the mechanism of phase evolution and the nature of lithiation gradients. The phase coexistence and segregation is further observed in scanning transmission X-ray microscopy images of individual lithiated V 2 O 5 nanowires. The mechanisms and the dynamics of nucleation and growth unraveled here are of great importance for the design and discovery of Li-ion cathode materials.
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 2 O 5 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 2 O 5 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 2 O 5 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 2 O 5 in comparison to the thermodynamically stable α-V 2 O 5 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 2 O 5 . 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 2 O 5 . 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.
Design rules for X-ray phosphors are much less established as compared to their optically stimulated counterparts owing to the absence of a detailed understanding of sensitization mechanisms, activation pathways and recombination channels upon high-energy excitation. Here, we demonstrate a pronounced modulation of the X-ray excited photoluminescence of Tb(3+) centers upon excitation in proximity to the giant resonance of the host Gd(3+) ions in solid-solution Gd1-xTbxOCl nanocrystals prepared by a non-hydrolytic cross-coupling method. The strong suppression of X-ray excited optical luminescence at the giant resonance suggests a change in mechanism from multiple exciton generation to single thermal exciton formation and Auger decay processes. The solid-solution Gd1-xTbxOCl nanocrystals are further topotactically transformed with retention of a nine-coordinated cation environment to solid-solution Gd1-xTbxF3 nanocrystals upon solvothermal treatment with XeF2. The metastable hexagonal phase of GdF3 can be stabilized at room temperature through this topotactic approach and is transformed subsequently to the orthorhombic phase. The fluoride nanocrystals indicate an analogous but blue-shifted modulation of the X-ray excited optical luminescence of the Tb(3+) centers upon X-ray excitation near the giant resonance of the host Gd(3+) ions.
Presented herein is an investigation of a promising ternary metal sulfide catalyst that is capable of electrochemically converting CO2 to liquid and gas fuels such as methanol and hydrogen.
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