Structural Transition of the 2‐Nitropropane Organic Compound at Low Temperature

Structural investigation of the crystallized 2‐nitropropane compound (C₃H₇NO₂) was performed by X‐ray powder diffraction at low temperature. A first crystalline phase, called phase α, is observed below 172 K. This form exhibits a triclinic symmetry with P‐1 space group (a=1.0313(3) nm, b=0.5873(2) nm, c=1.6146(4) nm, α=90.17(2)°, β=92.17(2)° and γ=90.09(2)°), and Z=8). At T_t=172 K, a structural transition is observed which brings to another phase, called phase β (above T_t). This one contains four molecules per unit cell and shows a Pc2₁n symmetry (a=1.0141(3) nm, b=0.5855(2) nm, and c=0.8319(4) nm). In addition to the doubling of the c‐axis, structural networks differ by the different conformations of NO₂ nitro groups and by the orientation of the propyl group in the unit cell. Both crystal structures can be described using infinite zigzag chains of C₃H₇NO₂ molecules showing a regular alternation along the c‐axis. Two orientations of these ribbons, called A and B, are observed. The crystal structures are then built with different distribution of these ribbons within the crystalline network.     

Stable and High-Power Calcium-Ion Batteries Enabled by Calcium Intercalation into Graphite

Calcium-ion batteries (CIBs) are considered to be promising next-generation energy storage systems because of the natural abundance of calcium and the multivalent calcium ions with low redox potential close to that of lithium. However, the practical realization of high-energy and high-power CIBs is elusive owing to the lack of suitable electrodes and the sluggish diffusion of calcium ions in most intercalation hosts. Herein, it is demonstrated that calcium-ion intercalation can be remarkably fast and reversible in natural graphite, constituting the first step toward the realization of high-power calcium electrodes. It is shown that a graphite electrode exhibits an exceptionally high rate capability up to 2 A g⁻¹, delivering ≈75% of the specific capacity at 50 mA g⁻¹ with full calcium intercalation in graphite corresponding to ≈97 mAh g⁻¹. Moreover, the capacity stably maintains over 200 cycles without notable cycle degradation. It is found that the calcium ions are intercalated into graphite galleries with a staging process. The intercalation mechanisms of the “calciated” graphite are elucidated using a suite of techniques including synchrotron in situ X-ray diffraction, nuclear magnetic resonance, and first-principles calculations. The versatile intercalation chemistry of graphite observed here is expected to spur the development of high-power CIBs.     

Lead Halide Post‐Perovskite‐Type Chains for High‐Efficiency White‐Light Emission

Hybrid metal halides containing perovskite layers have recently shown great potential for applications in solar cells and light‐emitting diodes. Such compounds exhibit quantum confinement effects leading to tunable optical and electronic properties. Thus, broadband white‐light emission has been observed from diverse metal halides and, owing to high color rendering index, high thermal stability, and low‐temperature solution processability, these materials have attracted interest for application in solid‐state lighting. However, the reported quantum yields for white photoluminescence (PLQY) remain low (i.e., in the range 0.5–9%) and no approach has shown to successfully increase the intensity of this emission. Here, it is demonstrated that the quantum efficiencies of hybrid metal halides can be greatly enhanced if they contain a polymorph of the [PbX₄]^2? perovskite‐type layers: the [PbX₄]^2? post‐perovskite‐type chains showing a PLQY of 45%. Different piperazines lead to a hybrid lead halide with either perovskite layers or post‐perovskite chains influencing strongly the presence of self‐trapped states for excitons. It is anticipated that this family of hybrid lead halide materials could enhance all the properties requiring the stabilization of trapped excitons.     

Understanding Origin of Voltage Hysteresis in Conversion Reaction for Na Rechargeable Batteries: The Case of Cobalt Oxides

Conversion reaction electrodes offer a high specific capacity in rechargeable batteries by utilizing wider valence states of transition metals than conventional intercalation‐based electrodes and have thus been intensively studied in recent years as potential electrode materials for high‐energy‐density rechargeable batteries. However, several issues related to conversion reactions remain poorly understood, including the polarization or hysteresis during charge/discharge processes. Herein, Co₃O₄ in Na cells is taken as an example to understand the aforementioned properties. The large hysteresis in charge/discharge profiles is revealed to be due to different electrochemical reaction paths associated with respective charge and discharge processes, which is attributed to the mobility gap among inter‐diffusing species in a metal oxide compound during de/sodiation. Furthermore, a Co₃O₄–graphene nanoplatelet hybrid material is demonstrated to be a promising anode for Na rechargeable batteries, delivering a capacity of 756 mAh g^?1 with a good reversibility and an energy density of 96 Wh kg^?1 (based on the total electrode weight) when combined with a recently reported Na₄Fe₃(PO₄)₂(P₂O₇) cathode.     

Using First‐Principles Calculations for the Advancement of Materials for Rechargeable Batteries

Rechargeable batteries have been regarded as leading candidates for energy storage systems to satisfy soaring energy demands and ensure efficient energy use, and intensive efforts have thus been focused on enhancing their energy densities and power capabilities. First‐principles calculations based on quantum mechanics have played an important role in obtaining a fundamental understanding of battery materials, thus providing insights for material design. In this feature article, the theoretical approaches used to determine key battery properties, such as the voltage, phase stability, and ion‐diffusion kinetics, are reviewed. Moreover, the recent contribution of first‐principles calculations to the interpretation of complicated experimental characterization measurements on battery materials, such as those obtained using X‐ray absorption spectroscopy, electron energy‐loss spectroscopy, nuclear magnetic resonance spectroscopy, and transmission electron microscopy, are introduced. Finally, perspectives are provided on the research direction of first‐principles calculations for the development of advanced batteries, including the further development of theories that can accurately describe the dissolved species, amorphous phases, and surface reactions that are integral to the operation of future battery systems beyond Li‐ion batteries.     

Amorphous Cobalt Phyllosilicate with Layered Crystalline Motifs as Water Oxidation Catalyst

The development of a high‐performance oxygen evolution reaction (OER) catalyst is pivotal for the practical realization of a water‐splitting system. Although an extensive search for OER catalysts has been performed in the past decades, cost‐effective catalysts remain elusive. Herein, an amorphous cobalt phyllosilicate (ACP) with layered crystalline motif prepared by a room‐temperature precipitation is introduced as a new OER catalyst; this material exhibits a remarkably low overpotential (η ≈ 367 mV for a current density of 10 mA cm^?2). A structural investigation using X‐ray absorption spectroscopy reveals that the amorphous structure contains layered motifs similar to the structure of CoOOH, which is demonstrated to be responsible for the OER catalysis based on density functional theory calculations. However, the calculations also reveal that the local environment of the active site in the layered crystalline motif in the ACP is significantly modulated by the silicate, leading to a substantial reduction of η of the OER compared with that of CoOOH. This work proposes amorphous phyllosilicates as a new group of efficient OER catalysts and suggests that tuning of the catalytic activity by introducing redox‐inert groups may be a new unexplored avenue for the design of novel high‐performance catalysts.     

Engineering Solid Electrolyte Interphase for Pseudocapacitive Anatase TiO2 Anodes in Sodium‐Ion Batteries

Anatase TiO₂ is considered as one of the promising anodes for sodium‐ion batteries because of its large sodium storage capacities with potentially low cost. However, the precise reaction mechanisms and the interplay between surface properties and electrochemical performance are still not elucidated. Using multimethod analyses, it is herein demonstrated that the TiO₂ electrode undergoes amorphization during the first sodiation and the amorphous phase exhibits pseudocapacitive sodium storage behaviors in subsequent cycles. It is also shown that the pseudocapacitive sodium storage performance is sensitive to the nature of solid electrolyte interphase (SEI) layers. For the first time, it is found that ether‐based electrolytes enable the formation of thin (≈2.5 nm) and robust SEI layers, in contrast to the thick (≈10 nm) and growing SEI from conventional carbonate‐based electrolytes. First principle calculations suggest that the higher lowest unoccupied molecular orbital energies of ether solvents/ion complexes are responsible for the difference. TiO₂ electrodes in ether‐based electrolyte present an impressive capacity of 192 mAh g^?1 at 0.1 A g^?1 after 500 cycles, much higher than that in carbonate‐based electrolyte. This work offers the clarified picture of electrochemical sodiation mechanisms of anatase TiO₂ and guides on strategies about interfacial control for high performance anodes.     

Substrate-induced crystal plastic phase of a discotic liquid crystal

A new phase of a known discotic liquid crystal is observed at the interface with a rigid substrate. The structure of the substrate-induced phase has been characterized by atomic force microscopy, specular X-ray diffraction, and small-angle and wide-angle grazing incidence X-ray diffraction. The substrate-induced phase, which has a thickness of ～30 nm and a tetragonal symmetry, differs notably from the bulk phase. The occurrence of such phase casts a new light on alignment of discotic liquid crystals.