Lithium metal has the highest volumetric and gravimetric energy density of all negative-electrode materials when used as an electrode material in a lithium rechargeable battery. However, the formation of lithium dendrites and/or 'moss' on the metal electrode surface can lead to short circuits following several electrochemical charge-discharge cycles, particularly at high rates, rendering this class of batteries potentially unsafe and unusable owing to the risk of fire and explosion. Many recent investigations have focused on the development of methods to prevent moss/dendrite formation. In parallel, it is important to quantify Li-moss formation, to identify the conditions under which it forms. Although optical and electron microscopy can visually monitor the morphology of the lithium-electrode surface and hence the moss formation, such methods are not well suited for quantitative studies. Here we report the use of in situ NMR spectroscopy, to provide time-resolved, quantitative information about the nature of the metallic lithium deposited on lithium-metal electrodes.

Monoclinic α-NaMnO 2 is re-investigated electrochemically as a positive electrode material for sodium ion batteries. About 0.85 Na can be deintercalated from NaMnO 2 and 0.8 Na be intercalated back during potentiostatical intermittent charge and discharge. Galvanostatical cycling between 2.0 V and 3.8 V gives 185 mAh/g discharge capacity for the first cycle at C/10 rate and 132 mAh/g remains after 20 cycles. Charge and discharge curves are significantly different indicating more hysteresis than is typical for lithium intercalation compounds. We also explain the remarkable difference between layered LiMnO 2 and NaMnO 2 upon alkali removal.

Van der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2 heterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices.

Despite prolonged scientific efforts to unravel the hydration structures of ions in water, many open questions remain, in particular concerning the existences and structures of ion clusters in 1∶1 strong electrolyte aqueous solutions. A combined ultrafast 2D IR and pump/probe study through vibrational energy transfers directly observes ion clustering in aqueous solutions of LiSCN, NaSCN, KSCN and CsSCN. In a near saturated KSCN aqueous solution (water/KSCN molar ratio ¼ 2.4∕1), 95% of the anions form ion clusters. Diluting the solution results in fewer, smaller, and tighter clusters. Cations have significant effects on cluster formation. A small cation results in smaller and fewer clusters. The vibrational energy transfer method holds promise for studying a wide variety of other fast short-range molecular interactions.T he solution properties of ions in water are relevant to a wide range of systems, including electrochemistry, biological environments, and atmospheric aerosols (1, 2). For more than 100 yr, tremendous scientific efforts have been devoted to unravel the hydration structures of ions in water (1-11). However, many fundamental questions remain open, in particular concerning the existence, concentration, and structure of ion clusters in 1∶1 strong electrolyte aqueous solutions. Whether strong 1∶1 electrolytes (especially salts of Na þ and K þ ) form ion pairs or clusters in water has been considered a key issue for understanding many important problems, e.g., the excess ionic activity in 1∶1 electrolytes (12), ion dependent conformational and binding equilibria of nucleic acids (13), the concentration difference between Na þ and K þ in living cells, protein denaturation by salts (14, 15), and ion concentration dependent properties of ion channels (16).The properties of aqueous solutions of 1∶1 strong electrolytes deviate from the ideal dilute solution at extremely low concentrations (<10 −5 M). The deviations were generally believed to be caused by the attraction between ions of opposite charge and the repulsion of ions of the same charge, leading to the development of the Debye-Hückel theory (17, 18). However, this theory begins to fail at a very low concentration (∼10 −3 M), as the assumptions upon which the theory was based become invalid. The formation of ion pairs containing two ions of opposite charge has been proposed to be primarily responsible for this failure (1, 2). Recently, calculations from molecular dynamics (MD) simulations, suggested that, clusters with more than one ion of the same charge which are traditionally viewed as unlikely, could be a major factor contributing to the nonideality of solutions at medium or high concentrations (12,19). However, these predicted ion clusters cannot be investigated by the usual tools for probing molecular structures and particle sizes in liquids, e.g., X-ray or neutron diffraction (20), or the dynamic light scattering (19,21), because the contribution of ion-ion correlations to the total scattering pattern is too small compared to the contributions ...

The cooperative Jahn-Teller effect (CJTE) refers to the correlation of distortions arising from individual Jahn-Teller centres in complex compounds. The effect usually induces strong coupling between the static or dynamic charge, orbital and magnetic ordering, which has been related to many important phenomena such as colossal magnetoresistance and superconductivity. Here we report a Na5/8MnO2 superstructure with a pronounced static CJTE that is coupled to an unusual Na vacancy ordering. We visualize this coupled distortion and Na ordering down to the atomic scale. The Mn planes are periodically distorted by a charge modulation on the Mn stripes, which in turn drives an unusually large displacement of some Na ions through long-ranged Na-O-Mn(3+)-O-Na interactions into a highly distorted octahedral site. At lower temperatures, magnetic order appears, in which Mn atomic stripes with different magnetic couplings are interwoven with each other. Our work demonstrates the strong interaction between alkali ordering, displacement, and electronic and magnetic structure, and underlines the important role that structural details play in determining electronic behaviour.

The design of efficient and stable photocatalysts for robust CO 2 reduction without sacrifice reagent or extra photosensitizer is still challenging. Herein, a single-atom catalyst of isolated single atom cobalt incorporated into Bi 3 O 4 Br atomic layers is successfully prepared. The cobalt single atoms in the Bi 3 O 4 Br favors the charge transition, carrier separation, CO 2 adsorption and activation. It can lower the CO 2 activation energy barrier through stabilizing the COOH* intermediates and tune the rate-limiting step from the formation of adsorbed intermediate COOH* to be CO* desorption. Taking advantage of cobalt single atoms and two-dimensional ultrathin Bi 3 O 4 Br atomic layers, the optimized catalyst can perform light-driven CO 2 reduction with a selective CO formation rate of 107.1 µmol g −1 h −1 , roughly 4 and 32 times higher than that of atomic layer Bi 3 O 4 Br and bulk Bi 3 O 4 Br, respectively.

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Carbonaceous materials have great potential for applications as anodes of alkali-metal ion batteries, such as Na-ion batteries and K-ion batteries (NIB and KIBs). We conduct an in situ study of the electrochemically driven sodiation and potassiation of individual carbon nanofibers (CNFs) by transmission electron microscopy (TEM). The CNFs are hollow and consist of a bilayer wall with an outer layer of disordered-carbon (d-C) enclosing an inner layer of crystalline-carbon (c-C). The d-C exhibits about three times volume expansion of the c-C after full sodiation or potassiation, thus suggesting a much higher storage capacity of Na or K ions in d-C than c-C. For the bilayer CNF-based electrode, a steady sodium capacity of 245 mAh/g is measured with a Coulombic efficiency approaching 98% after a few initial cycles. The in situ TEM experiments also reveal the mechanical degradation of CNFs through formation of longitudinal cracks near the c-C/d-C interface during sodiation and potassiation. Geometrical changes of the tube are explained by a chemomechanical model using the anisotropic sodiation/potassiation strains in c-C and d-C. Our results provide mechanistic insights into the electrochemical reaction, microstructure evolution and mechanical degradation of carbon-based anodes during sodiation and potassiation, shedding light onto the development of carbon-based electrodes for NIBs and KIBs.