Weifeng Huang

Self-Assembled Alluaudite Na2Fe3-xMnx(PO4)(3) Micro/Nanocompounds for Sodium-Ion Battery Electrodes: A New Insight into Their Electronic and Geometric Structure

A series of alluaudite Na2 Fe3-x Mnx (PO4 )3 microcompounds, which self-assembled from primary nanorods, were prepared successfully through a solvothermal method. As a promising candidate cathode for sodium-ion batteries, it is necessary to obtain a deeper understanding of the relationship between the structure and physicochemical properties of these materials. The local electronic and geometric environments were systematically investigated, for the first time, by using a combination of soft/hard X-ray absorption, IR, and Mössbauer spectroscopy. The results show that the electrochemical performance is not only associated with morphology, but also with the electronic and crystalline structure. With the introduction of manganese into the lattice, the long-range order maintains the isostructural framework and the lattice parameters expand as expected. However, for short-range order, PO4 tetrahedra and MO6 octahedra (M=Fe and Mn) become more severely distorted as a function of Mn concentration. Meanwhile, larger MnO6 octahedra will compress the space of FeO6 octahedra, which will result in stronger core/electron-electron interactions for Fe, as characterized by hard/soft X-ray absorption spectra. These slight changes in the electronic and local structures lead to different electrochemical performances with changes to the manganese content. Moreover, other physicochemical properties, such as magnetic behavior, are also confirmed to be correlated with these different electron interactions and local geometric environments.

Detailed investigation of Na2.24FePO4CO3 as a cathode material for Na-ion batteries

Na-ion batteries are gaining an increased recognition as the next generation low cost energy storage devices. Here, we present a characterization of Na3FePO4CO3 nanoplates as a novel cathode material for sodium ion batteries. First-principles calculations reveal that there are two paths for Na ion migration along b and c axis. In-situ and ex-situ Fe K-edge X-ray absorption near edge structure (XANES) point out that in Na3FePO4CO3 both Fe2+/Fe3+ and Fe3+/Fe4+ redox couples are electrochemically active, suggesting also the existence of a two-electron intercalation reaction. Ex-situ X-ray powder diffraction data demonstrates that the crystalline structure of Na3FePO4CO3 remains stable during the charging/discharging process within the range 2.0–4.55 V.

A novel CoN electrocatalyst with high activity and stability toward oxygen reduction reaction

Carbon-supported CoN (CoN/C) nanoparticles have been synthesized by heating at reflux in the solution of o-xylene and subsequent thermal annealing under a NH3 reducing atmosphere. The as-prepared CoN/C composite exhibited high oxygen reduction reaction (ORR) activity and excellent stability as a new efficient non-precious metal electrocatalyst.

A New Route Toward Improved Sodium Ion Batteries: A Multifunctional Fluffy Na0.67FePO4/CNT Nanocactus

To improve the performance of energy storage systems, the rational design of new electrode configurations is a strategic initiative. Here, we present a novel monodisperse fluffy alluaudite Na0.67FePO4, prepared by a modified solvothermal method, as promising electrode for sodium ion battery. This porous Na0.67FePO4 with nanocactus-like morphology is composed by nanorods within an open three-dimensional structure. This unique nanocactus-based morphology offers three important advantages when used as electrode for sodium ion battery: (i) provides an open frame structure for a large Na+ ions transport; (ii) reduces the sodium ion and electron transport path by ≈20 nm; (iii) offers a large surface area for a more efficient interface between the electrode and the electrolyte. The electrochemical investigation revealed that this fluffy Na0.67FePO4 nanocactus exhibits the high discharge capacity of 138 mAh g(-1). Moreover, a battery with a Na0.67FePO4/CNT hybrid electrode delivered a discharge capacity as high as ≈143 mAh g(-1), coupled to an excellent stable cyclability (no obvious capacity fading over 50 cycles at a current rate of 5 mA g(-1)). This enhanced mechanism was studied by means of absorption measurements and ex situ XAFS characterizations. Results of the characterization of the Na0.67FePO4 suggests that the outstanding performance can be associated with the unique fluffy nanocactus morphology.

Decoupling the Lattice Distortion and Charge Doping Effects on the Phase Transition Behavior of VO2 by Titanium (Ti4+) Doping

The mechanism for regulating the critical temperature (TC) of metal-insulator transition (MIT) in ions-doped VO2 systems is still a matter of debate, in particular, the unclear roles of lattice distortion and charge doping effects. To rule out the charge doping effect on the regulation of TC, we investigated Ti4+-doped VO2 (TixV1-xO2) system. It was observed that the TC of TixV1-xO2 samples first slightly decreased and then increased with increasing Ti concentration. X-ray absorption fine structure (XAFS) spectroscopy was used to explore the electronic states and local lattice structures around both Ti and V atoms in TixV1-xO2 samples. Our results revealed the local structure evolution from the initial anatase to the rutile-like structure around the Ti dopants. Furthermore, the host monoclinic VO2 lattice, specifically, the VO6 octahedra would be subtly distorted by Ti doping. The distortion of VO6 octahedra and the variation of TC showed almost the similar trend, confirming the direct effect of local structural perturbations on the phase transition behavior. By comparing other ion-doping systems, we point out that the charge doping is more effective than the lattice distortion in modulating the MIT behavior of VO2 materials.

Fabrication of graphene-encapsulated Na3V2(PO4)3 as high-performance cathode materials for sodium-ion batteries

Na3V2(PO4)3 (NVP) has been in the spotlight as a potential candidate of next generation batteries to overcome the limitation of lithium resources on Earth. Here a thin-layer graphene-encapsulated NVP composite (NVP/G) was synthesized by self-assembly of surface modified NVP and graphene oxide and then followed by reduction to compensate for the intrinsic low electronic conductivity of NVP and strengthen its structure stability. The as-synthesized hybrid composite as a cathode for sodium-ion batteries (SIBs) exhibits excellent high specific capacity and superior rate performance with discharge capacities of 115.2 mA h g−1 at 0.2 C and 70.1 mA h g−1 at 30 C, It also shows an excellent cycling stability with about 86.0% capacity retention at 5 C after 300 cycles. Ex situ X-ray absorption spectroscopy (XAS) characterization confirmes the local geometrical environment around vanadium is highly conserved during the sodiation/desodiation process, associated with an electrochemically active V3+/V4+ redox couple. Hence the as-prepared hybrid composite can be considered as a promising cathode material for high-rate SIBs, thanks to the effect interface interaction between NVP nanoparticle sand graphene films.

Durability Enhancement of Intermetallics Electrocatalysts via N-anchor Effect for Fuel Cells

Insufficient durability and catalytic activity of oxygen reduction reaction (ORR) electrocatalyst are key issues that have to be solved for the practical application of low temperature fuel cell. This paper introduces a new catalyst design strategy using N-anchor to promote the corrosion resistance of electrocatalyst. The as-synthesized N-Pt3Fe1/C shows a high electrocatalytic activity and a superior durability towards ORR. The kinetic current density of N-Pt3Fe1/C as normalized by ECSA is still as high as 0.145 mA cm−2 and only 7% loss after 20000 potential cycles from 0.6 to 1.2 V (vs. NHE) in O2-bubbling perchloric acid solution, whereas Pt3Fe1/C shows 49% loss under the same tests. The N-anchor approach offers novel opportunities for the development of ORR catalyst with excellent electrochemical properties.

Low-Temperature Fabrication of Au–Co Cluster Mixed Nanohybrids With High Magnetic Moment of Co

In recent years, the synergistic effects of Au-based hybrids have generated enormous scientific interest. The hybrids of Au and Co are expected to exhibit attractive properties. In this paper, we report the successful fabrication of the nanohybrids between bulk-immiscible Au and Co with chain-like structures via a mild solution method. Elemental mapping, XRD and EXAFS data reveal that the as-prepared AuCo nanohybrids might be of cluster mixed configuration. A sequential redox and imperfection-promoted aggregation/diffusion process is proposed to elucidate the formation mechanism of the nanohybrids. The as-prepared products exhibit a temperature-independent saturation magnetization with the magnetic moment of Co as high as ~2.95 μ(B) for each Co atom at 300 K, much higher than the bulk value (~1.7 μ(B) for each Co atom) and approaching the theoretical value of an atomic Co (~3.0 μ(B) for each Co atom).

Formation of graphene-encapsulated CoS2 hybrid composites with hierarchical structures for high-performance lithium-ion batteries

Transition metal sulfides (TMSs) are considered as the most promising alternative anode materials for advanced lithium-ion batteries (LIBs). Here, we report a hierarchically structured CoS2 nanosphere/graphene (CoS2/G) composite, fabricated by a simple hydrothermal method. This composite, assembled with CoS2 nanoparticles uniformly distributed on the graphene, exhibits excellent electrochemical performance. In particular, the CoS2/G electrode material delivers a high rate capability of around 398 mA h g−1 at a current density of 3500 mA g−1. Moreover, a discharge capacity of about 400 mA h g−1 can be obtained after 1000 cycles at a current density of 500 mA g−1. X-ray absorption spectroscopy is used to characterize the sample for the first time, and the results demonstrate that CoS2/G is reduced to metallic Co and Li2S when discharged to 0.01 V. In subsequent charge–discharge processes, the metallic Co cannot be fully oxidized to CoS2, which is the main cause of capacity loss for the CoS2 electrode.

Three-dimensional hollow spheres of the tetragonal-spinel MgMn2O4 cathode for high-performance magnesium ion batteries

We report a simple and template-free method for preparing uniform tetragonal-spinel MgMn2O4 (T-MgMn2O4) hollow spheres, which self-assembled from small sized nanocrystals. The obtained T-MgMn2O4 hollow spheres with a micro/nanostructure exhibit excellent rate performance and long cycle life as the cathode for magnesium ion batteries.