Towards a Stable Layered Vanadium Oxide Cathode for High-Capacity Calcium Batteries.

Calcium-based batteries have promising advantages over multivalent ion batteries. However, the fabrication of highly efficient calcium batteries is limited by the quality of available cathode materials, which motivates the exploration of electrodes that can enable reversible, stable Ca2+ intercalation. Herein, layered vanadium oxide Mgx V2 O5 ·nH2 O is used as a calcium battery cathode, and it exhibits a high capacity of 195.5 mA h g-1 at 20 mA g-1 and an outstanding cycling life (93.6% capacity retention after 2500 cycles at 1 A g-1 ). Combining theoretical analysis and experimental design, a series of layered oxides (Mx V2 O5 ·nH2 O, M = Mg, Ca, Sr) is selected as a model system to identify the Ca storage mechanism. It is found that the hydrated alkaline earth metal ions in the vanadium-based layered oxide interlayers play a critical role as pillared stabilizers to facilitate Ca2+ insertion/extraction. Compared with Ca2+ and Sr2+ , the presence of Mg2+ provides vanadium oxides with a rigid framework that allows for minimized volume fluctuation (a tiny variation of ≈0.15 Å of the interlayer spacing). Such an understanding of the Ca storage mechanism is a key step in the rational design and selection of materials for calcium batteries to achieve a high capacity and long cycle life.

Reconstruction of Supported Metal Nanoparticles in Reaction Conditions.

Metal nanoparticles (NPs) dispersed on a high-surface-area support are normally used as heterogeneous catalysts. Recent in situ experiments have shown that structure reconstruction of the NP occurs in real catalysis. However, the role played by supports in these processes is still unclear. Supports can be very important in real catalysis because of the new active sites at the perimeter interface between nanoparticles and supports. Herein, using a developed multiscale model coupled with in situ spherical aberration-corrected (Cs-corrected) TEM experiments, we show that the interaction between the support and the gas environment greatly changes the contact surface area between the metal and support, which further leads to the critical change in the perimeter interface. The dynamic changes of the interface in reactive environments can thus be predicted and be included in the rational design of supported metal nanocatalysts. In particular, our multiscale model shows quantitative consistency with experimental observations. This work offers possibilities for obtaining atomic-scale structures and insights beyond the experimental limits.

Four-phonon scattering of so-As and improvement of the thermoelectric properties by increasing the buckling height.

In recent years, more and more thermoelectric (TE) materials have been discovered as the research boom of TE materials advances. However, due to the low conversion efficiency, most of the current TE materials cannot meet the commercial demand. The low-dimensional nanomaterials are promising to break the current status quo of low conversion efficiency of TE materials. Here, we predicted a stable two-dimensional TE material, namely so-As, based on density functional theory. The so-As has an ultra-low lattice thermal conductivity,κl= 1.829 W m-1K-1at 300 K, and when the temperature rises to 700 K theκlis only 0.788 W m-1K-1. This might be caused by the strong anharmonic interaction among the so-As phonon and the out-of-plane vibration of the low-frequency acoustic modes. Moreover, the maximumZTvalue of thep-type so-As is 0.18 at room temperature (0.45 at 700 K), while that of then-type can even reach 0.75 at 700 K. In addition, we have also studied the difference between the four- and three-phonon scattering rates. The increase of scattering channels leads to the ultra-lowκl, which is only 3.33 × 10-4W m-1K-1at room temperature, showing an almost adiabatic property. Finally, we adjust the TE properties of so-As by changing the buckling height. With the buckling height is increased by 2%, the scattering rate of so-As is extremely high. WhenTis 700 K, the maximumZTof then-type is 0.94 (p-type can also reach 0.7), which is 25% higher than the pristine one. Our work reveals the impact of buckling height on the TE figure of merit, which provides a direction for future search and regulation of the highZTTE materials.

Cation⊗3π: cooperative interaction of a cation and three benzenes with an anomalous order in binding energy.

Cation-π or cation-π-π interaction between one cation and one or two structures bearing rich π-electrons (such as benzene, aromatic rings, graphene, and carbon nanotubes) plays a ubiquitous role in various areas. Here, we analyzed a new type interaction, cation⊗3π, whereby one cation simultaneously binds with three separate π-electron-rich structures. Surprisingly, we found an anomalous increase in the order of the one-benzene binding strength of the cation⊗3π interaction, with K(+) > Na(+) > Li(+). This was at odds with the conventional ranking of the binding strength which usually increases as the radii of the cations decrease. The key to the present unexpected observations was the cooperative interaction of the cation with the three benzenes and also between the three benzenes, in which a steric-exclusion effect between the three benzenes played an important role. Moreover, the binding energy of cation⊗3π was comparable to cation⊗2π for K(+) and Na(+), showing the particular importance of cation⊗3π interaction in biological systems.

Alkaline earth metal vanadates as sodium-ion battery anodes.

The abundance of sodium resources indicates the potential of sodium-ion batteries as emerging energy storage devices. However, the practical application of sodium-ion batteries is hindered by the limited electrochemical performance of electrode materials, especially at the anode side. Here, we identify alkaline earth metal vanadates as promising anodes for sodium-ion batteries. The prepared calcium vanadate nanowires possess intrinsically high electronic conductivity (> 100 S cm-1), small volume change (< 10%), and a self-preserving effect, which results in a superior cycling and rate performance and an applicable reversible capacity (> 300 mAh g-1), with an average voltage of ∼1.0 V. The specific sodium-storage mechanism, beyond the conventional intercalation or conversion reaction, is demonstrated through in situ and ex situ characterizations and theoretical calculations. This work explores alkaline earth metal vanadates for sodium-ion battery anodes and may open a direction for energy storage.The development of suitable anode materials is essential to advance sodium-ion battery technologies. Here the authors report that alkaline earth metal vanadates are promising candidates due to the favorable electrochemical properties and interesting sodium-storage mechanism.

Critical Dipole Length for the Wetting Transition Due to Collective Water-dipoles Interactions.

The wetting behavior of water on the solid surfaces is fundamental to various physical, chemical and biological processes. Conventionally, the surface with charges or charge dipoles is hydrophilic, whereas the non-polar surface is hydrophobic though some exceptions were recently reported. Using molecular dynamics simulations, we show that there is a critical length of the charge dipoles on the solid surface. The solid surface still exhibited hydrophobic behavior when the dipole length was less than the critical value, indicating that the water molecules on the solid surface seemed not "feel" attractive interactions from the charge dipoles on the solid surface. Those unexpected observations result from the collective interactions between the water molecules and charge dipoles on the solid surface, where the steric exclusion effect between water molecules greatly reduces the water-dipole interactions. Remarkably, the steric exclusion effect is also important for surfaces with charge dipole lengths greater than this critical length.