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Abstract Kagome materials have become solid grounds to study the interplay among geometry, topology, correlation, and magnetism. Recently, niobium halide semiconductors Nb 3 X 8 ( X  = Cl, Br, I) have been predicted to be two-dimensional magnets and these materials are also interesting for their breathing kagome geometry. However, experimental electronic structure studies of these promising materials are still lacking. Here, we report the spectroscopic evidence of flat and weakly dispersing bands in breathing-kagome semiconductor Nb 3 I 8 around 500 meV binding energy, which is well supported by our first-principles calculations. These bands originate from the breathing kagome lattice of niobium atoms and have niobium d -orbital character. They are found to be sensitive to the polarization of the incident photon beam. Our study provides insight into the electronic structure and flat band topology in an exfoliable kagome semiconductor, thereby providing an important platform to understand the interaction of geometry and electron correlations in two-dimensional materials.

Hydrated vanadium pentoxide (VOH) can deliver a gravimetric capacity as high as 400 mA h g −1 owing to the variable valence states of the V cation from 5+ to 3+ in an aqueous zinc ion battery. The incorporation of divalent transition metal cations has been demonstrated to overcome the structural instability, sluggish kinetics, fast capacity degradation, and serious polarization. The current study reveals that the catalytic effects of transition metal cations are probably the key to the significantly improved electrochemical properties and battery performance because of the higher covalent character of 55% in the Cu–O bond in comparison with 32% in the Mg–O bond in the respective samples. Cu( ii ) pre-inserted VOH (CuVOH) possesses a significantly enhanced intercalation storage capacity, an increased discharge voltage, great transport properties, and reduced polarization, while both VOH and Mg( ii ) pre-inserted VOH (MgVOH) demonstrate similar electrochemical properties and performances, indicating that the incorporation of Mg cations has little or no impact. For example, CuVOH has a redox voltage gap of 0.02 V, much smaller than 0.25 V for VOH and 0.27 V for MgVOH. CuVOH shows an enhanced exchange current density of 0.23 A g −1 , compared to 0.20 Amore »g −1 for VOH and 0.19 A g −1 for MgVOH. CuVOH delivers a zinc ion storage capacity of 379 mA h g −1 , higher than 349 mA h g −1 for MgVOH and 337 mA h g −1 for VOH at 0.5 A g −1 . CuVOH shows an energy efficiency of 72%, superior to 53% for VOH and 55% for MgVOH. All of the results suggest that pre-inserted Cu( ii ) cations played a critical role in catalyzing the zinc ion intercalation reaction, while the Mg( ii ) cations did not exert a detectable catalytic effect.« less

Hydrated vanadates are promising layered cathodes for aqueous zinc-ion batteries owing to their specific capacity as high as 400 mA h g −1 ; however, the structural instability causes serious cycling degradation through repeated intercalation/deintercalation reactions. This study reveals the chemically inserted Mn( ii ) cations act as structural pillars, expand the interplanar spacing, connect the adjacent layers and partially reduce pentavalent vanadium cations to tetravalent. The expanded interplanar spacing to 12.9 Å reduces electrostatic interactions, and transition metal cations collectively promote and catalyze fast and more zinc ion intercalation at higher discharge current densities with much enhanced reversibility and cycling stability. Manganese expanded hydrated vanadate (MnVO) delivers a specific capacity of 415 mA h g −1 at a current density of 50 mA g −1 and 260 mA h g −1 at 4 A g −1 with a capacity retention of 92% over 2000 cycles. The energy efficiency increases from 41% for hydrated vanadium pentoxide (VOH) to 70% for MnVO at 4 A g −1 and the open circuit voltage remains at 85% of the cutoff voltage in the MnVO battery on the shelf after 50 days. Expanded hydrated vanadate with other transition metal cations for high-performance aqueous zinc-ionmore »batteries is also obtained, suggesting it is a general strategy for exploiting high-performance cathodes for multi-valent ion batteries.« less

Li-ion diffusion and lithiation kinetics in MnO/C nanocomposites were systematically investigated by monitoring the change in the charge transfer resistance and the ion diffusion coefficient, and the kinetically predominant process at various charge/discharge states. Crystal field analysis and density functional theory (DFT) calculations were introduced to reveal the relationship between the electronic structure of the phase compositions, the displayed electrochemical potential and its profile. The split 3d orbitals in the Mn ion determine the ordering of the electron migration and energy difference, leading to the different potential profiles in the lithiated/delithiated process. The phase compositions strongly affect the intrinsic properties of the MnO/C nanocomposites, increasing the ion diffusion coefficient from ∼10 −15 to 10 −11 cm 2 s −1 when the electrode progressed from the fully charged to fully discharged state, while both the surface redox reaction and the solid-state diffusion could be the limiting process depending on the lithiation/delithiation states. In addition, the MnO/C anode delivers an energy efficiency of 90% in a Li-ion hybrid capacitor, suggesting a promising and competitive application in the future.

Lithium (Li) metal anodes have regained intensive interest in recent years due to the ever-increasing demand for next-generation high energy battery technologies. Li metal, unfortunately, suffers from poor cycling stability and low efficiency as well as from the formation of dangerous Li dendrites, raising safety concerns. Utilizing solid-state electrolytes (SSEs) to prevent Li dendrite growth provides a promising approach to tackle the challenge. However, recent studies indicate that Li dendrites easily form at high current densities, which calls for full investigation of the fundamental mechanisms of Li dendrite formation within SSEs. Herein, the origin and evolution of Li dendrite growth through SSEs have been studied and compared by using Li 6.1 Ga 0.3 La 3 Zr 2 O 12 (LLZO) and NASICON-type Li 2 O–Al 2 O 3 –P 2 O 5 –TiO 2 –GeO 2 (LATP) pellets as the separators. We discover that a solid electrolyte interphase (SEI)-like interfacial layer between Li and SSE plays a critical role in alleviating the growth of dendritic Li, providing new insights into the interface between SSE and Li metal to enable future all solid-state batteries.