Intercalation Pseudocapacitance of Cation-Exchanged Molybdenum-Based Polyoxometalate for the Fast and Stable Zinc-Ion Storage.

Recently, intercalation pseudocapacitance has received significant interest as an abnormal charge storage mechanism owing to the battery-like intercalation energy storage into the bulk electrodes and the fast charge storage kinetics of electrochemical capacitors. However, intercalation pseudocapacitance of molybdenum-based polyoxometalates (POMs) for high-performance Zn ion battery (ZIB) cathodes is yet to be exploited. Herein, we demonstrate the fast and reversible intercalation pseudocapacitance of vanadium-substituted Keggin-type molybdenum-based POMs (XPMoV), where H of HPMoV is replaced by X cations (X = Li, Na, K, or Rb). This cation exchange allows cation-exchanged XPMoV to exhibit the morphological evolution into an anisotropic rodlike structure and to achieve a pillar effect on the improved chemical and structural integrity. Despite the micron-size rod morphology and the contracted lattice of (100) plane, the intercalation pseudocapacitance kinetics of XPMoV was dominated by the fast surface-confined electrochemistry and became highly reversible after the 1st cycle activation process by co-intercalation of Li+ and Zn2+ ions. Therefore, the ZIB with the KPMoV cathode delivered a high rate capability of 74.0 mAh g-1 at 20,000 mA g-1 and 87% capacity retention over 2000 cycles at 1000 mA g-1, far exceeding HPMoV and other Mo-based cathodes. This study paves the way to design the fast and reversible intercalation pseudocapacitance of POMs and the cation exchange chemistry into the improved (electro)chemical and structural integrity.

Layered Double Hydroxide Quantum Dots for Use in a Bifunctional Separator of Lithium-Sulfur Batteries.

Functional separators, which are chemically modified and coated with nanostructured materials, are considered an effective and economical approach to suppressing the shuttle effect of lithium polysulfide (LiPS) and promoting the conversion kinetics of sulfur cathodes. Herein, we report cobalt-aluminum-layered double hydroxide quantum dots (LDH-QDs) deposited with nitrogen-doped graphene (NG) as a bifunctional separator for lithium-sulfur batteries (LSBs). The mesoporous LDH-QDs/NG hybrids possess abundant active sites of Co2+ and hydroxide groups, which result in capturing LiPSs through strong chemical interactions and accelerating the redox kinetics of the conversion reaction, as confirmed through X-ray photoelectron spectroscopy, adsorption tests, Li2S nucleation tests, and electrokinetic analyses of the LiPS conversion. The resulting LDH-QDs/NG hybrid-coated polypropylene (LDH-QDs/NG/PP) separator, with an average thickness of ∼17 µm, has a high ionic conductivity of 2.67 mS cm-1. Consequently, the LSB cells with the LDH-QDs/NG/PP separator can deliver a high discharge capacity of 1227.48 mAh g-1 at 0.1C along with a low capacity decay rate of 0.041% per cycle over 1200 cycles at 1.0C.

Extremely Foldable and Highly Porous Reduced Graphene Oxide Films for Shape-Adaptive Triboelectric Nanogenerators.

Here, a thin and foldable porous reduced graphene oxide (rGO) fabricated by a solvent casting method (SC-rGO) is introduced. The SC-rGO is superior to aluminum as a positive triboelectric material in triboelectric nanogenerators (TENGs), significantly enhancing TENG output performance. The film shows extremely foldable features, where it could be folded by 1/16 size. The electrical properties and device performance of SC-rGO are optimized varying thicknesses from 5 to 30 µm. A 30 µm thick TENG with a non-annealed SC-rGO film (STENG) shows the highest output of about 255 µW cm-2 due to its high carrier concentration, low work function, and high surface area. After annealing, STENG performance is optimized with a 10 µm thick SC-rGO because their work functions decreases, while the corresponding carrier concentrations decrease according to the thickness of the SC-rGO films. The SC-rGO films are highly durable and stable, where their output and conductivity show negligible changes after 100 000 cycles of mechanical deformation. A large SC-rGO with a size of 13 × 3 cm2 is fabricated and is attached inside a person's arm to demonstrate the shape-adaptive characteristics. Consequently, 170 V is obtained and it turns on 19 green light emitting diodes by simply touching the STENG.

Interfacially Polymerized Polyamide Interlayer onto Ozonated Carbon Nanotube Networks for Improved Stability of Sulfur Cathodes.

Lithium-sulfur (Li-S) batteries are considered promising energy-storage devices owing to the high specific capacity and low cost of the S cathode. However, they suffer from capacity decay and poor coulombic efficiency arising from the dissolution of long-chain polysulfides and their shuttling. A facile and scalable method was developed to directly coat a thin (≈57.3 nm) and porous polyamide (PA) interlayer onto a S cathode by interfacial polymerization. This PA interlayer prevented the shuttling of polysulfides by creating a physical barrier and, through chemical interactions between the amide functionalities of PA and the polysulfides, allowing access to the S electrode by the Li ions. The resulting PA-coated cathode exhibited approximately 64.2 % capacity retention over 1000 cycles at 1 C with only 0.0358 % decay per cycle and a moderate capacity of 1008 mAh g-1 at 0.1 C.

Surface-Modified Sulfur Nanorods Immobilized on Radially Assembled Open-Porous Graphene Microspheres for Lithium-Sulfur Batteries.

The assembly of two-dimensional conductive nanomaterials into hierarchical complex architectures precisely controlling internal open porosity and orientation, external morphology, composition, and interaction is expected to provide promising hosts for high-capacity sulfur cathodes. Herein, we demonstrate rod-like nanosulfur (nS) deposited onto radially oriented open-porous microspherical reduced graphene oxide (rGO) architectures for improved rate and cyclic capabilities of lithium-sulfur (Li-S) batteries. The combined chemistry of a spray-frozen assembly and ozonation drives the formation of a radially oriented open-porous structure and an overall microspherical morphology as well as uniform distribution and high loading of rod-like nS. Moreover, an optimum composition and strong bonding of the rGO/nS hybrid enables the optimization of redox kinetics for high sulfur utilization and high-rate capacities. The resulting rGO/nS hybrid provides a specific capacity and first-cycle Coulombic efficiency of 1269.1 mAh g-1 and 98.5%, respectively, which are much greater than those of ice-templated and physically mixed rGO/nS hybrids and radially oriented open-porous rGO/bulk sulfur with the same hybrid composition. A 4C capacity of 510.3 mAhg-1 and capacity decay of 0.08% per cycle over 500 cycles (70.9% of the initial capacity over 300 cycles) also support the synergistic effect of the rod-like nS strongly interacting with the radially oriented open-porous rGO microspheres.