SIMES Publications

Abstract

We report in situ structural measurements of shock‐compressed single crystal orthoenstatite up to 337 ± 55 GPa on the Hugoniot, obtained by coupling ultrafast X‐ray diffraction to laser‐driven shock compression. Shock compression induces a disordering of the crystalline structure evidenced by the appearance of a diffuse X‐ray diffraction signal at nanosecond timescales at 80 ± 13 GPa on the Hugoniot, well below the equilibrium melting pressure (>170 GPa). The formation of bridgmanite and post‐perovskite have been indirectly reported in microsecond‐scale plate‐impact experiments. Therefore, we interpret the high‐pressure disordered state we observed at nanosecond scale as an intermediate structure from which bridgmanite and post‐perovskite crystallize at longer timescales. This evidence of a disordered structure of MgSiO₃ on the Hugoniot indicates that the degree of polymerization of silicates is a key parameter to constrain the actual thermodynamics of shocks in natural environments.

Abstract

High-valent redox, where over-oxidation of oxygen or transition metals (TMs) drives extensive charge sharing through the formation of short covalent bonds, has historically been avoided in intercalation electrodes because of its association with structural disorder and electrochemical irreversibility. Here, we present a perspective on the origin of these undesirable behaviors and materials design criteria to mitigate them. Drawing parallels between oxygen redox and high-valent TM redox (e.g., Cr^III/VI and V^III/V), we reveal that the defect formation energy landscape is the primary factor controlling the electrochemical reversibility of high-valent redox, as it determines which defects form to accommodate the short covalent bonds as well as the nature of those covalent bonding arrangements. By tuning the defect formation energy landscape, researchers can control the nature of the oxidized species while minimizing structural disorder. These concepts reveal a wide range of previously avoided redox mechanisms as promising candidates for high density energy storage.

Abstract

Replacing the Pb−X octahedral building unit of A^IPbX₃ perovskites (X=halide) with a pair of edge‐sharing Pb−X octahedra affords the expanded perovskite analogs: A^IIPb₂X₆. We report seven members of this new family of materials. In 3D hybrid perovskites, orbitals from the organic molecules do not participate in the band edges. In contrast, the more spacious inorganic sublattice of the expanded analogs accommodates larger pyrazinium‐based cations with low‐lying π* orbitals that form the conduction band, substantially decreasing the band gap of the expanded lattice. The molecular nature of the conduction band allows us to electronically dope the materials by reducing the organic molecules. By synthesizing derivatives with A^II=pyridinium and ammonium, we can isolate the contributions of the pyrazinium‐based orbitals in the band gap transition of A^IIPb₂X₆. The organic‐molecule‐based conduction band and the inorganic‐ion‐based valence band provide an unusual electronic platform with localized states for electrons and more disperse bands for holes upon optical or thermal excitation.

Abstract

Li-ion battery safety issues related to fires and explosions are frequently triggered by Li dendrite growth, which is a predominant reason for separator piercing and cell shorts and is very difficult to detect in the early state. Here, we developed a sensitive detection method based on H₂ gas capture that could detect trace amounts of Li dendrite formation. H₂ gas was generated due to reaction of metallic Li with polymer binders. Even micron-scale Li dendrite (∼2.8 × 10⁻⁴ mg and 50 μm) growth can trigger H₂ capture. Overcharge experiment with a LiFePO₄-graphite battery pack (8.8 kWh) shows that H₂ was captured first among H₂, CO, CO₂, HCL, HF, and SO₂, and the capture time was 639 s earlier than smoke and 769 s earlier than fire. The Li dendrite growth can be completely prevented once H₂ is captured, with neither smoke nor fire observed, which provides an effective method for early safety warning.

Abstract

We report the phase diagram of Nd_1−xSr_xNiO₂ infinite layer thin films grown on SrTiO₃. A superconducting dome spanning 0.125<x<0.25 is found, remarkably similar to cuprates, albeit over a narrower doping window. However, while cuprate superconductivity is bounded by an insulator for underdoping and a metal for overdoping, here we observe weakly insulating behavior on either side of the dome. Furthermore, the normal state Hall coefficient is always small and proximate to a continuous zero crossing in doping and in temperature, in contrast to the ∼1/x dependence observed for cuprates. This suggests the presence of both electronlike and holelike bands, consistent with band structure calculations.

Abstract

It is highly attractive to develop efficient methods to directly extract Li from seawater to secure the supply of Li. However, high concentration of Na in the seawater poses a great challenge in Li extraction. Here, we developed pulsed-rest and pulse-rest-reverse pulse-rest electrochemical intercalation methods with TiO₂-coated FePO₄ electrodes for Li extraction. The method can lower the intercalation overpotential and successfully boost the Li selectivity. Moreover, the pulse-rest-reverse pulse-rest method can also promote electrode crystal structure stability during the co-intercalation of Li and Na and prolong the lifetime of the electrode. We demonstrated 10 cycles of successful and stable Li extraction with 1:1 of Li to Na recovery from authentic seawater, which is equivalent to the selectivity of ∼1.8 × 10⁴. Also, with lake water of higher initial Li/Na ratio of 1.6 × 10⁻³, we achieved Li extraction with more than 50:1 of Li to Na recovery.

Abstract

Understanding the fundamentals of nanoscale heat propagation is crucial for next-generation electronics. For instance, weak van der Waals bonds of layered materials are known to limit their thermal boundary conductance (TBC), presenting a heat dissipation bottleneck. Here, a new nondestructive method is presented to probe heat transport in nanoscale crystalline materials using time-resolved X-ray measurements of photoinduced thermal strain. This technique directly monitors time-dependent temperature changes in the crystal and the subsequent relaxation across buried interfaces by measuring changes in the c-axis lattice spacing after optical excitation. Films of five different layered transition metal dichalcogenides MoX₂ [X = S, Se, and Te] and WX₂ [X = S and Se] as well as graphite and a W-doped alloy of MoTe₂ are investigated. TBC values in the range 10–30 MW m⁻² K⁻¹ are found, on c-plane sapphire substrates at room temperature. In conjunction with molecular dynamics simulations, it is shown that the high thermal resistances are a consequence of weak interfacial van der Waals bonding and low phonon irradiance. This work paves the way for an improved understanding of thermal bottlenecks in emerging 3D heterogeneously integrated technologies.

Abstract

In two-dimensional layered quantum materials, the stacking order of the layers determines both the crystalline symmetry and electronic properties such as the Berry curvature, topology and electron correlation1,2,3,4. Electrical stimuli can influence quasiparticle interactions and the free-energy landscape5,6, making it possible to dynamically modify the stacking order and reveal hidden structures that host different quantum properties. Here, we demonstrate electrically driven stacking transitions that can be applied to design non-volatile memory based on Berry curvature in few-layer WTe₂. The interplay of out-of-plane electric fields and electrostatic doping controls in-plane interlayer sliding and creates multiple polar and centrosymmetric stacking orders. In situ nonlinear Hall transport reveals that such stacking rearrangements result in a layer-parity-selective Berry curvature memory in momentum space, where the sign reversal of the Berry curvature and its dipole only occurs in odd-layer crystals. Our findings open an avenue towards exploring coupling between topology, electron correlations and ferroelectricity in hidden stacking orders and demonstrate a new low-energy-cost, electrically controlled topological memory in the atomically thin limit.

Abstract

Solid-electrolyte-based molten-metal batteries have attracted considerable attention for grid-scale energy storage. Although ZEBRA batteries are considered one of the promising candidates, they still have the potential concern of metal particle growth and ion exchange with the β”-Al₂O₃ electrolyte. Herein, a Li_6.4La₃Zr_1.4Ta_0.6O₁₂ solid-electrolyte-based molten lithium−molybdenum−iron(II) chloride battery (denoted as Li−Mo−FeCl₂) operated at temperature of 250 °C, comprising a mixture of Fe and LiCl cathode materials, a Li anode, a garnet-type Li-ion ceramic electrolyte, and Mo additive, is designed to overcome these obstacles. Different from conventional battery reaction mechanisms, this battery revolutionarily synchronizes the reversible Fe−Mo alloying−dealloying reactions with the delithiation−lithiation processes, meaning that the porous Mo framework derived from Fe−Mo alloy simultaneously suppresses the growth of pure Fe particles. By adopting a Li anode and a Li-ion ceramic electrolyte, the corrosion problem between the cathode and the solid electrolyte is overcome. With similar battery cost ($12 kWh⁻¹), the theoretical energy density of Li−Mo−FeCl₂ battery surpasses that of a Na−FeCl₂ ZEBRA battery over 25%, to 576 Wh kg⁻¹ and 2216 Wh L⁻¹, respectively. Experimental results further prove this cell has excellent cycling performance (472 mAh g_LiCl⁻¹ after 300 cycles, 50 mg active material) and strong tolerance against the overcharge− overdischarge (3−1.6 V) and freezing−thawing (25−250 °C) incidents.

Abstract

The resistance of a conventional insulator diverges as temperature approaches zero. The peculiar low-temperature resistivity saturation in the 4f Kondo insulator (KI) SmB₆ has spurred proposals of a correlation-driven topological Kondo insulator (TKI) with exotic ground states. However, the scarcity of model TKI material families leaves difficulties in disentangling key ingredients from irrelevant details. Here we use angle-resolved photoemission spectroscopy (ARPES) to study FeSb₂, a correlated d-electron KI candidate that also exhibits a low-temperature resistivity saturation. On the (010) surface, we find a rich assemblage of metallic states with two-dimensional dispersion. Measurements of the bulk band structure reveal band renormalization, a large temperature-dependent band shift, and flat spectral features along certain high-symmetry directions, providing spectroscopic evidence for strong correlations. Our observations suggest that exotic insulating states resembling those in SmB₆ and YbB₁₂ may also exist in systems with d instead of f electrons.