Yue-Feng Xu

A composite material of uniformly dispersed sulfur on reduced graphene oxide: Aqueous one-pot synthesis, characterization and excellent performance as the cathode in rechargeable lithium-sulfur batteries

AbstractSulfur-reduced graphene oxide composite (SGC) materials with uniformly dispersed sulfur on reduced graphene oxide sheets have been prepared by a simple aqueous one-pot synthesis method, in which the formation of the composite is achieved through the simultaneous oxidation of sulfide and reduction of graphene oxide. The synthesis process has been tracked ex situ by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy, which both confirm that the majority of graphene oxide has been reduced during the synthesis reaction. The sulfur contents in the SGC, determined by thermogravimetry and elementary analysis, have been adjusted in the range from 20.9 to 72.5 wt.%. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images reveal that most of the sulfur is uniformly dispersed on the reduced graphene oxide sheets, for which no sulfur in particulate form could be observed. The SGC materials have been tested as the cathode of rechargeable lithium-sulfur (Li-S) batteries, and demonstrated a high reversible capacity and good cycleability. The SGC-63.6%S can deliver a reversible capacity as high as 804 mA·h/g after 80 cycles of charge/discharge at a current density of 312 mA/g (ca. 0.186 C), and 440 mA·h/g after 500 cycles at 1250 mA/g (ca. 0.75 C).

Facile synthesis of hierarchical micro/nanostructured MnO material and its excellent lithium storage property and high performance as anode in a MnO/LiNi0.5Mn1.5O4-δ lithium ion battery

Hierarchical micro/nanostructured MnO material is synthesized from a precursor of MnCO3 with olive shape that is obtained through a facile one-pot hydrothermal procedure. The hierarchical micro/nanostructured MnO is served as anode of lithium ion battery together with a cathode of spinel LiNi0.5Mn1.5O(4-δ) material, which is synthesized also from the precursor of MnCO3 with olive shape through a different calcination process. The structures and compositions of the as-prepared materials are characterized by TGA, XRD, BET, SEM, and TEM. Electrochemical tests of the MnO materials demonstrate that it exhibit excellent lithium storage property. The MnO material in a MnO/Li half cell can deliver a reversible capacity of 782.8 mAh g(-1) after 200 cycles at a rate of 0.13 C, and a stable discharge capacity of 350 mAh g(-1) at a high rate of 2.08 C. Based on the outstanding electrochemical property of the MnO material and the LiNi0.5Mn1.5O(4-δ) as well, the MnO/LiNi0.5Mn1.5O(4-δ) full cell has demonstrated a high discharge specific energy ca. 350 Wh kg(-1) after 30 cycles at 0.1 C with an average high working voltage at 3.5 V and a long cycle stability. It can release a discharge specific energy of 227 Wh kg(-1) after 300 cycles at a higher rate of 0.5 C. Even at a much higher rate of 20 C, the MnO/LiNi0.5Mn1.5O(4-δ) full cell can still deliver a discharge specific energy of 145.5 Wh kg(-1). The excellent lithium storage property of the MnO material and its high performance as anode in the MnO/LiNi0.5Mn1.5O(4-δ) lithium ion battery is mainly attributed to its hierarchical micro/nanostructure, which could buffer the volume change and shorten the diffusion length of Li(+) during the charge/discharge processes.

Porous Graphitic Carbon Loading Ultra High Sulfur as High-Performance Cathode of Rechargeable Lithium-Sulfur Batteries

Porous graphitic carbon of high specific surface area of 1416 m(2) g(-1) and high pore volume of 1.11 cm(3) g(-1) is prepared by using commercial CaCO3 nanoparticles as template and sucrose as carbon source followed by 1200 °C high-temperature calcination. Sulfur/porous graphitic carbon composites with ultra high sulfur loading of 88.9 wt % (88.9%S/PC) and lower sulfur loading of 60.8 wt % (60.8%S/PC) are both synthesized by a simple melt-diffusion strategy, and served as cathode of rechargeable lithium-sulfur batteries. In comparison with the 60.8%S/PC, the 88.9%S/PC exhibits higher overall discharge capacity of 649.4 mAh g(-1)(S-C), higher capacity retention of 84.6% and better coulombic efficiency of 97.4% after 50 cycles at a rate of 0.1C, which benefits from its remarkable specific capacity with such a high sulfur loading. Moreover, by using BP2000 to replace the conventional acetylene black conductive agent, the 88.9% S/PC can further improve its overall discharge capacity and high rate property. At a high rate of 4C, it can still deliver an overall discharge capacity of 387.2 mAh g(-1)(S-C). The porous structure, high specific surface area, high pore volume and high electronic conductivity that is originated from increased graphitization of the porous graphitic carbon can provide stable electronic and ionic transfer channel for sulfur/porous graphitic carbon composite with ultra high sulfur loading, and are ascribed to the excellent electrochemical performance of the 88.9%S/PC.

Hierarchical Mn2O3 Hollow Microspheres as Anode Material of Lithium Ion Battery and Its Conversion Reaction Mechanism Investigated by XANES

Hierarchical Mn2O3 hollow microspheres of diameter about 6-10 μm were synthesized by solvent-thermal method. When serving as anode materials of LIBs, the hierarchical Mn2O3 hollow microspheres could deliver a reversible capacity of 580 mAh g(-1) at 500 mA g(-1) after 140 cycles, and a specific capacity of 422 mAh g(-1) at a current density as high as 1600 mA g(-1), demonstrating a good rate capability. Ex situ X-ray absorption near edge structure (XANES) spectrum reveals that, for the first time, the pristine Mn2O3 was reduced to metallic Mn when it discharged to 0.01 V, and oxidized to MnO as it charged to 3 V in the first cycle. Furthermore, the XANES data demonstrated also that the average valence of Mn in the sample at charged state has decreased slowly with cycling number, which signifies an incomplete lithiation process and interprets the capacity loss of the Mn2O3 during cycling.

Tuning the structure and property of nanostructured cathode materials of lithium ion and lithium sulfur batteries

A great deal of progresses has been made in the pursuit of high energy and high power battery devices in the past decades on account of the growing demand for clean and sustainable energy. The tremendous challenges in increasing the specific energy and power density of batteries lie mainly in the cathode materials. Lots of attempts have been made to improve the reversible capacity and rate capability of cathode materials for lithium ion batteries in the past few years. On the one hand, the rate capability of the cathode materials depends strongly on their surface structures, which determine the kinetics of lithium ion transportation and intercalation. Through tuning the surface structure of cathode materials, the density of channels for fast Li+ diffusion can be increased, and therefore enhance greatly the surface/interfacial transportation kinetics of lithium ions. On the other hand, the reversible capacity of the cathode materials of lithium ion batteries is in direct proportion to the number of electrons transferred, thus exploration of high capacity cathode materials beyond intercalation, such as sulfur cathodes, has been conducted extensively. The utilization efficiency of sulfur active materials, the reaction kinetics and trapping of soluble polysulfides depend also on the structure of sulfur cathodes. This review outlines recent developments in structure-tuning of the most appealing cathode materials, including layered lithium metal oxides, olivine structured LiFePO4, spinel LiMn2O4 and LiNi0.5Mn1.5O4, as well as sulfur cathodes. The structure-dependent properties of the cathode materials are summarized, mainly focusing on electrochemical performance, which can provide an in-depth understanding and rational design of high performance cathode materials. Further direction and perspectives of research in the present field are also addressed.

Modification of epoxy resin through the self-assembly of a surfactant-like multi-element flame retardant

In order to develop a multi-element, synergistic, flame-retardant system, the combination of DOPO, POM and POSS was achieved using the classical Kabachnik–Fields reaction. The as-designed POSS-bisDOPO was characterized by FT-IR, 1H NMR, 13C NMR, 31P NMR, 2D NMR, and MS. POSS-bisDOPO was introduced into epoxy resins to obtain flame-retardant materials. The LOI value can reach 34.5% when the content of POSS-bisDOPO is 20 wt%. The TGA results showed that the char yield was significantly improved in cured POSS-bisDOPO/EP. The ATR-FTIR results, optical images and SEM analyses indicated that the residual char had a compact and coherent appearance in the inner layer, while the outer structure was intumescent and multi-porous. Therefore, by isolating heat and oxygen more efficiently, the char played an important role in improving the thermal stability and flame retardancy of cured POSS-bisDOPO/EP. The three-point bending test results showed that the mechanical strength of POSS-bisDOPO/EP was higher than those of pure EP and POSS–NH2/EP due to the outstanding reinforcement effect of the unique nanostructure of POSS-bisDOPO assembled in the EP matrix. These data indicated that POSS-bisDOPO not only obviously enhances the flame retardancy, but also improves the mechanical properties of epoxy resins.

Superiority of the bi-phasic mixture of a tin-based alloy nanocomposite as the anode for lithium ion batteries

The nanostructured mixture of a Sn2Fe and Sn2Co alloy composite with uniform cubic shaped particles has been synthesized by a reduction-thermal diffusion alloying reaction. The textural properties of the as-prepared samples were characterized by field-emission scanning electron microscopy, transmission electron microscopy and powder X-ray diffraction. Compared with the Sn2Fe alloy, the alloy composite exhibits better reversibility and cycle performance. At a charge/discharge current density of 50 mA g−1, a reversible capacity of 510 mA h g−1 can be maintained after 50 cycles, and its capacity in the 50th cycle was retained at ca. 85% of that in the second cycle. When the current density is increased to 1000 mA g−1, a reversible capacity of 443 mA h g−1 can be still obtained. The ab initio simulation results indicate that Sn2Fe and Sn2Co have similar crystal structures, demonstrating that the two kinds of alloys can be mixed uniformly by the thermal diffusion alloying reaction. The superior electrochemical performance can be attributed to the homogeneously dispersed inactive metallic material (Fe and Co) nanostructure, which partly accommodates the volume change and also retains the integrity of the active material and matrix, resulting in good cycle performance of the composite electrode.

Origin of Structural Evolution in Capacity Degradation for Overcharged NMC622 via Operando Coupled Investigation

The nickel-rich layered oxide materials have been selected as promising cathode materials for the next generation lithium ion batteries because of their large capacity and comparably high operating voltage. However, at high voltage (beyond 4.30 V vs Li/Li+), the members of this family are all suffering from a rapid capacity decay, which was commonly concerned with crystal lattice distortion and related cation disordering. In this work, the quasi-spherical Ni-rich layered LiNi0.6Co0.2Mn0.2O2 (QS-NMC622) material was successfully synthesized through the carbonate co-precipitation method. A coupled measurement, which is a combination of potentiostatic intermittent titration technique (PITT) and in situ X-ray diffraction (XRD), was deployed to simultaneously capture the structural changes and lithium ion diffusion coefficient of QS-NMC622 material during the first cycle. With help of in situ XRD patterns and high-resolution transmission electron microscope (HR-TEM) images, a defective spinel framework of Fd3̅m space group was detected along with a rapid decreasing lattice-parameter c and lattice distortion at deep delithiated state, which causes poor kinetics related to lithium ion mobility. The new-born framework seems to transform and remain as full spinel structure in the parent phase to the end of charge/discharge with high voltage, which could deteriorate both the surface and body structure stability during the subsequent cycles. This established coupled in situ measurement could be applied to simultaneously investigate the structure transformation and kinetics of cathode materials during charge/discharge.