Mitigating Interfacial Capacity Fading in Vanadium Pentoxide by Sacrificial Vanadium Sulfide Encapsulation for Rechargeable Mg-Ion Batteries.

Rechargeable Mg-ion Batteries (RMB) containing a Mg metal anode offer the promise of higher specific volumetric capacity, energy density, safety, and economic viability than lithium-ion battery technology, but their realization is challenging. The limited availability of suitable inorganic cathodes compatible with electrolytes relevant to Mg metal anode restricts the development of RMBs. Despite the promising capability of some oxides to reversibly intercalate Mg+2 ions at high potential, its lack of stability in chloride-containing ethereal electrolytes, relevant to Mg metal anode hinders the realization of a full practical RMB. Here the successful in situ encapsulation of monodispersed spherical V2O5 (≈200 nm) is demonstrated by a thin layer of VS2 (≈12 nm) through a facile surface reduction route. The VS2 layer protects the surface of V2O5 particles in RMB electrolyte solution (MgCl2 + MgTFSI in DME). Both V2O5 and V2O5@VS2 particles demonstrate high initial discharge capacity. However, only the V2O5@VS2 material demonstrates superior rate performance, Coulombic efficiency (100%), and stability (138 mA h g-1 discharge capacity after 100 cycles), signifying the ability of the thin VS2 layer to protect the V2O5 cathode and facilitate the Mg+2 ion intercalation/deintercalation into V2O5.

High-energy all-solid-state lithium batteries enabled by Co-free LiNiO2 cathodes with robust outside-in structures.

A critical current challenge in the development of all-solid-state lithium batteries (ASSLBs) is reducing the cost of fabrication without compromising the performance. Here we report a sulfide ASSLB based on a high-energy, Co-free LiNiO2 cathode with a robust outside-in structure. This promising cathode is enabled by the high-pressure O2 synthesis and subsequent atomic layer deposition of a unique ultrathin LixAlyZnzOδ protective layer comprising a LixAlyZnzOδ surface coating region and an Al and Zn near-surface doping region. This high-quality artificial interphase enhances the structural stability and interfacial dynamics of the cathode as it mitigates the contact loss and continuous side reactions at the cathode/solid electrolyte interface. As a result, our ASSLBs exhibit a high areal capacity (4.65 mAh cm-2), a high specific cathode capacity (203 mAh g-1), superior cycling stability (92% capacity retention after 200 cycles) and a good rate capability (93 mAh g-1 at 2C). This work also offers mechanistic insights into how to break through the limitation of using expensive cathodes (for example, Co-based) and coatings (for example, Nb-, Ta-, La- or Zr-based) while still achieving a high-energy ASSLB performance.

High-Entropy Co-Free O3-Type Layered Oxyfluoride: A Promising Air-Stable Cathode for Sodium-Ion Batteries.

Sodium-ion batteries have recently emerged as a promising alternative to lithium-based batteries, driven by an ever-growing demand for electricity storage systems. The present workproposes a cobalt-free high-capacity cathode for sodium-ion batteries, synthesized using a high-entropy approach. The high-entropy approach entails mixing more than five elements in a single phase; hence, obtaining the desired properties is a challenge since this involves the interplay between different elements. Here, instead of oxide, oxyfluoride is chosen to suppress oxygen loss during long-term cycling. Supplement to this, lithium is introduced in the composition to obtain high configurational entropy and sodium vacant sites, thus stabilizing the crystal structure, accelerating the kinetics of intercalation/deintercalation, and improving the air stability of the material. With the optimization of the cathode composition, a reversible capacity of 109 mAh g-1 (2-4 V) and 144 mAh g-1 (2-4.3 V) is observed in the first few cycles, along with a significant improvement in stability during prolonged cycling. Furthermore, in situ and ex situ diffraction studies during charging/discharging reveal that the high-entropy strategy successfully suppresses the complex phase transition. The impressive outcomes of the present work strongly motivate the pursuit of the high-entropy approach to develop efficient cathodes for sodium-ion batteries.

Interface Engineering of an RGO/MoS2/Pd 2D Heterostructure for Electrocatalytic Overall Water Splitting in Alkaline Medium.

To achieve sustainable production of H2 at ambient temperature, highly active and stable electrocatalysts are the key to water splitting technology commercialization for hydrogen and oxygen production to replace Pt and IrO2 catalysts. Herein, a modified interface of palladium (Pd) and reduced graphene oxide (RGO)-supported molybdenum disulfide (MoS2) prepared by the solvothermal followed by chemical reduction method is established, in which abundant interfaces are formed. The phase structure, composition, chemical coupling, and morphology of the two-dimensional nanostructures are established by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy, respectively. A structural phase transformation in MoS2 is observed from trigonal (2H) to octahedral (1T) by virtue of Pd addition, which is well established from XRD, Raman, and XPS studies. For oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), the RGO/MoS2/Pd (RMoS2Pd) catalyst exhibits extremely low overpotential (245 mV for OER and 86 mV for HER) to achieve benchmark current density, with small values of Tafel slope (42 mV dec-1 for OER and 35.9 mV dec-1 for HER) and charge transfer resistance. The quantitative study shows the hydrogen production rate of RMoS2Pd of 335 µmol h-1 with excellent stability in alkaline medium, which is superior to MoS2, RMoS2, and MoS2Pd. The improved performance of RMoS2Pd is attributed to the combined synergetic effect of 1T MoS2, sulfur vacancy, and conducting RGO sheet, which efficiently accelerate the overall electrochemical water splitting.

Visible-Light-Mediated Electrocatalytic Activity in Reduced Graphene Oxide-Supported Bismuth Ferrite.

Reduced graphene oxide (RGO)-supported bismuth ferrite (BiFeO3) (RGO-BFO) nanocomposite is synthesized via a two-step chemical route for photoelectrochemical (PEC) water splitting and photocatalytic dye degradation. The detailed structural analysis, chemical coupling, and morphology of BFO- and RGO-supported BFO are established through X-ray diffraction, Raman and X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy studies. The modified band structure in RGO-BFO is obtained from the UV-vis spectroscopy study and supported by density functional theory (DFT). The photocatalytic degradation of Rhodamine B dye achieved under 120 min visible-light illumination is 94% by the RGO-BFO composite with a degradation rate of 1.86 × 10-2 min-1, which is 3.8 times faster than the BFO nanoparticles. The chemical oxygen demand (COD) study further confirmed the mineralization of an organic dye in presence of the RGO-BFO catalyst. The RGO-BFO composite shows excellent PEC performance toward water splitting, with a photocurrent density of 10.2 mA·cm-2, a solar-to-hydrogen conversion efficiency of 3.3%, and a hole injection efficiency of 98% at 1 V (vs Ag/AgCl). The enhanced catalytic activity of RGO-BFO is explained on the basis of the modified band structure and chemical coupling between BFO and RGO, leading to the fast charge transport through the interfacial layers, hindering the recombination of the photogenerated electron-hole pair and ensuring the availability of free charge carriers to assist the catalytic activity.

AC Conduction and Time-Temperature Superposition Scaling in a Reduced Graphene Oxide-Zinc Sulfide Nanocomposite.

We report, herein, the results of an in depth study and concomitant analysis of the AC conduction [σ'(ω): f=20 Hz to 2 MHz] mechanism in a reduced graphene oxide-zinc sulfide (RGO-ZnS) composite. The magnitude of the real part of the complex impedance decreases with increase in both frequency and temperature, whereas the imaginary part shows an asymptotic maximum that shifts to higher frequencies with increasing temperature. On the other hand, the conductivity isotherm reveals a frequency-independent conductivity at lower frequencies subsequent to a dispersive conductivity at higher frequencies, which follows a power law [σ'(ω)∝ω(s) ] within a temperature range of 297 to 393 K. Temperature-independent frequency exponent 's' indicates the occurrence of phonon-assisted simple quantum tunnelling of electrons between the defects present in RGO. Finally, this sample follows the "time-temperature superposition principle", as confirmed from the universal scaling of conductivity isotherms. These outcomes not only pave the way for increasing our elemental understanding of the transport mechanism in the RGO system, but will also motivate the investigation of the transport mechanism in other order-disorder systems.