Huayun Xu

Enhanced Lithium Storage Performances of Hierarchical Hollow MoS2 Nanoparticles Assembled from Nanosheets

MoS(2), because of its layered structure and high theoretical capacity, has been regarded as a potential candidate for electrode materials in lithium secondary batteries. But it suffers from the poor cycling stability and low rate capability. Here, hierarchical hollow nanoparticles of MoS(2) nanosheets with an increased interlayer distance are synthesized by a simple solvothermal reaction at a low temperature. The formation of hierarchical hollow nanoparticles is based on the intermediate, K(2)NaMoO(3)F(3), as a self-sacrificed template. These hollow nanoparticles exhibit a reversible capacity of 902 mA h g(-1) at 100 mA g(-1) after 80 cycles, much higher than the solid counterpart. At a current density of 1000 mA g(-1), the reversible capacity of the hierarchical hollow nanoparticles could be still maintained at 780 mAh g(-1). The enhanced lithium storage performances of the hierarchical hollow nanoparticles in reversible capacities, cycling stability and rate performances can be attributed to their hierarchical surface, hollow structure feature and increased layer distance of S-Mo-S. Hierarchical hollow nanoparticles as an ensemble of these features, could be applied to other electrode materials for the superior electrochemical performance.

One-step hydrothermal synthesis of ZnFe2O4 nano-octahedrons as a high capacity anode material for Li-ion batteries

AbstractBinary transition metal oxides are considered as promising anode materials for lithium-ion batteries (LIB), because they can effectively overcome the drawbacks of simple oxides. Here, a one-step hydrothermal method is described for the synthesis of regular ZnFe2O4 octahedrons about 200 nm in size at a low temperature without further annealing being required. The ZnFe2O4 octahedrons were characterized by powder X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The electrochemical performance of the ZnFe2O4 octahedrons was examined in terms of cyclic voltammetry and discharge/charge profiles. The ZnFe2O4 octahedrons exhibit a high capacity of 910 mA·h/g at 60 mA/g between 0.01 and 3.0 V after 80 cycles. They also deliver a reversible specific capacity of 730 mA·h/g even after 300 cycles at 1000 mA/g, a much better performance than those in previous reports. A set of reactions involved in the discharge/charge processes are proposed on the basis of ex situ high-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns of the electrode materials. The insights obtained will be of benefit in the design of future anode materials for lithium ion batteries.

General synthesis of hollow MnO2, Mn3O4 and MnO nanospheres as superior anode materials for lithium ion batteries

The use of manganese oxides as promising candidates for anode materials in lithium ion batteries has attracted a significant amount of attention recently. Here, we develop a general approach to synthesize hollow nanospheres of MnO2, Mn3O4 and MnO, using carbon nanospheres as a template and a reagent. Depending on the calcination temperature, time and atmosphere, hollow nanospheres of MnO2 assembled by randomly dispersed nanosheets, or hollow nanospheres of Mn3O4 and MnO composed of aggregated nanoparticles, are produced. The electrochemical properties of the three hollow nanoparticles are investigated in terms of cycling stability and rate capability. They deliver the specific capacities of 840, 1165 or 1515 mA h g−1 after 60 cycles at 100 mA g−1 for MnO2, Mn3O4 and MnO. Even at 500 mA g−1, the reversible capacities could be still kept at 637, 820, and 1050 mA h g−1 after 150 cycles. The outstanding performances might be related with their hollow structure, porous surface and nanoscale size.

Coaxial MnO/N-doped carbon nanorods for advanced lithium-ion battery anodes

MnO nanorods encapsulated by N-doped carbon are prepared, using polypyrrole-coated MnOOH nanorods as both a template and a precursor. The resulting coaxial nanorods have a one-dimensional shape, nanoscale size and an N-doped carbon coating within one particle, which substantially improves their electrochemical performance. As an anode material for lithium-ion batteries, the coaxial nanorods of MnO/N-doped carbon deliver a specific capacity of 982 mA h g−1 at a current density of 500 mA g−1 after 100 cycles, higher than the values for pure MnO nanostructures and MnO/C nanocomposites. At a current density of 5000 mA g−1, the reversible capacity of the coaxial nanorods could be as high as 372 mA h g−1.

Facile synthesis of hierarchically porous NiO micro-tubes as advanced anode materials for lithium-ion batteries

Hierarchically porous NiO microtubes are synthesized by a high-temperature calcination of Ni(dmg)2 microtubes obtained by a simple precipitation method. The porous NiO microtubes as an anode material for lithium ion batteries exhibit excellent performances, ∼640 mA h g−1 after 200 cycles at 1 A g−1.

Hierarchical core–shell α-Fe2O3@C nanotubes as a high-rate and long-life anode for advanced lithium ion batteries

High-performance anode materials in lithium ion batteries greatly rely on the elaborate control of their size, shape, structure and surface. However, it is difficult to assemble all of the controls within one particle, due to difficulties in their synthesis. Here, hierarchical carbon-coated α-Fe2O3 nanotubes are prepared by a facile hydrothermal reaction between branched MnO2/Fe2O3 nanorods and glucose. The resulting nanotubes realize all these controls in one particle in terms of their nanoscale size, one-dimensional shape, hollow structure, hierarchical surface and carbon coating. Meanwhile, the thickness of the carbon layer could be easily controlled by the ratio between the different reactants. Electrochemical measurements show that the core–shell nanotubes with the thinnest carbon layer give the best cycling and rate performances. They deliver a specific capacity of 1173 mA h g−1 after 100 cycles at a current density of 0.2 A g−1, or 1012 mA h g−1 after 300 cycles at 1 A g−1. Even after 1000 cycles at a current density of 4 A g−1, the specific capacity could be still kept at 482 mA h g−1. The excellent lithium-storage performance could be attributed to the well-designed controls in this nanocomposite and a thin carbon layer, which increases the electron conductivity of the electrode and simultaneously keeps the carbon content lower.

Enhancing the performance of MnO by double carbon modification for advanced lithium-ion battery anodes

Porous hybrid materials with designed micro/nano-sub-structures have been recognized as promising anodes for lithium-ion batteries (LIBs) due to their high capacity and reliable performance. The low electrical conductivity and side-reactions at the interface of electrode/electrolyte prohibit their practical applications. Carbon material modification can effectively enhance the conductivity and mechanical properties, and suppress the direct contact between the electrode and electrolyte, leading to enhanced performance. Herein, unique porous MnO with micro/nano-architectures has been in situ decorated with carbon layers on the surface and by carbon nanotube doping between the particles (denoted as MnO@C/CNTs) by a catalytic chemical vapor deposition (CCVD) treatment. As anodes in LIBs, these MnO@C/CNTs exhibit remarkable cycling performance (1266 mA h g−1 after 300 cycles at 500 mA g−1) and good rate capability (850 mA h g−1 after 100 cycles at 100 mA g−1). The inspiring performance is associated with the carbon modified porous micro/nano-structure features which can buffer the volume expansion and promote the ion/electron transfer at the interface of electrode/electrolyte.

Effect of different carbon sources on the electrochemical properties of rod-like LiMnPO4–C nanocomposites

Olivine structured LiMnPO4 nanorods as an emerging cathode material for lithium ion batteries have recently triggered intensive interest. Herein, LiMnPO4 nanorods have been synthesized via a solvothermal route and then coated with a carbon layer from different carbon sources to improve their electrochemical performance. The LiMnPO4–C nanocomposite obtained from beta-cyclodextrin as the carbon source showed a reversible capacity of 153.4 mA h g−1 at a rate of 0.1 C and maintained it at 120 mA h g−1 after 50 cycles, which is much better than that obtained from ascorbic acid, citric acid, glucose and sucrose. This result can be attributed to the smaller electrode impedance in the nanocomposite obtained from beta-cyclodextrin, based on EIS measurements. Correlated to the molecule structure, it is believed that larger molecules with more oxygenous groups are beneficial to their uniform adsorption on the electrodes and then produce a better electron-conductive carbon layer after calcinations. This result would be very helpful in future work related to carbon coating on other electrodes.

Conductive Polymer-Coated VS4 Submicrospheres As Advanced Electrode Materials in Lithium-Ion Batteries

VS4 as an electrode material in lithium-ion batteries holds intriguing features like high content of sulfur and one-dimensional structure, inspiring the exploration in this field. Herein, VS4 submicrospheres have been synthesized via a simple solvothermal reaction. However, they quickly degrade upon cycling as an anode material in lithium-ion batteries. So, three conductive polymers, polythiophene (PEDOT), polypyrrole (PPY), and polyaniline (PANI), are coated on the surface to improve the electron conductivity, suppress the diffusion of polysulfides, and modify the interface between electrode/electrolyte. PANI is the best in the polymers. It improves the Coulombic efficiency to 86% for the first cycle and keeps the specific capacity at 755 mAh g(-1) after 50 cycles, higher than the cases of naked VS4 (100 mAh g(-1)), VS4@PEDOT (318 mAh g(-1)), and VS4@PPY (448 mAh g(-1)). The good performances could be attributed to the improved charge-transfer kinetics and the strong interaction between PANI and VS4 supported by theoretical simulation. The discharge voltage ∼2.0 V makes them promising cathode materials.

Hierarchical mesoporous octahedral K2Mn1−xCoxFe(CN)6 as a superior cathode material for sodium-ion batteries

A new Prussian blue analogue, hierarchical mesoporous octahedral K2Mn1−xCoxFe(CN)6 (x = 0, 0.05, 0.1 and 0.2), has been successfully synthesized for the first time via a simple magnetically stirred process. K2Mn1−xCoxFe(CN)6 (x = 0, 0.05, 0.1 and 0.2) was selected as a cathode material for sodium ion batteries to increase their lattice parameters, along with the large available tunnels for Na+ diffusion. We also introduce cobalt into the framework to stabilize the crystal structure and improve the electronic conductivity of the KMHFC particles. These composites are characterized via powder X-ray diffractometry, and scanning and transmission electron microscopy. Among the samples, K2Mn0.9Co0.1Fe(CN)6 exhibits the best electrochemical performance as a cathode material for sodium ion batteries, with a discharge capacity of 116 mA h g−1 after 150 cycles at 10 mA g−1. This superior electrochemical property could be ascribed to its unique hierarchical architecture, improved electronic conductivity, and robust structure.