Zhicong Shi

Controllable synthesis of spinel nano-ZnMn2O4via a single source precursor route and its high capacity retention as anode material for lithium ion batteries

Agglomerated pure spinel ZnMn2O4 nanoparticles with flake-shaped structure have been synthesized viacalcination of an agglomerated Zn–Mn citrate complex precursor, which was prepared with high yield by a convenient, environmentally benign and low temperature route. The composition, morphology and thermal decomposition of the Zn–Mn citrate complex were studied by C&H elemental analysis (EA), Fourier transform infrared spectroscopy (FTIR), energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The resulting ZnMn2O4 nanoparticles obtained from the precursor calcination at 700 °C were systematically characterized by XRD, FTIR, N2Adsorption/Desorption, SEM, TEM, HRTEM and selected area electron diffraction (SAED). The results show that the ZnMn2O4 material was agglomerated to form a porous texture in pure phase. The electrochemical properties of the agglomerated ZnMn2O4 material were investigated to determine its reversible capacity, rate and cycling performance as the anode material for lithium ion batteries (LIBs). This ZnMn2O4 material exhibited promising capacity retention of over 200 cycles at varying discharge rates. The electrode also exhibited attractive rate capabilities yielding capacity of 330 mAh g−1 after more than 35 cycles at 600 mA g−1.The ameliorated electrochemical performance can be ascribed to the high crystallinity and porous texture of the ZnMn2O4 material which provided short diffusion paths for lithium ions. Ex situXRD analysis of the electrodes after discharging and charging to the selected voltage was conducted and the possible lithium insertion mechanisms are discussed. This study suggests that the ZnMn2O4 material synthesized via the single source precursor route is a promising anode material for LIBs.

Graphene-encapsulated sulfur (GES) composites with a core–shell structure as superior cathode materials for lithium–sulfur batteries

Relatively uniform sized graphene-encapsulated sulphur (GES) composites with a core (S)–shell (graphene) structure were synthesized in one pot based on a solution-chemical reaction–deposition method. These novel GES particles were characterized by XRD, Raman spectrometry, SEM, TGA, EDS and TEM. The electrochemical tests showed that the present GES composites exhibit high specific capacity, good discharge capacity retention and superior rate capability when they were employed as cathodes in rechargeable Li–S cells. A high sulphur content (83.3 wt%) was obtained in the GES composites. Stable discharge capacities of about 900, 650, 540 and 480 mA h g−1 were achieved at 0.75, 2.0, 3.0 and 6.0 C, respectively. The good electrochemical performance is attributed to the high electrical conductivity of the graphene, the reasonable particle size of sulphur particles, and the core–shell structures that have synergistic effects on facilitating good transport of electrons from the poorly conducting sulphur, preserving fast transport of lithium ions to the encapsulated sulphur particles, and alleviating the polysulfide shuttle phenomenon. The present finding may provide a significant contribution to the enhancement of cathodes for the lithium–sulphur battery technology.

Improving the electrochemical performance of the LiNi0.5Mn1.5O4 spinel by polypyrrole coating as a cathode material for the lithium-ion battery

Conductive polypyrrole (PPy)-coated LiNi0.5Mn1.5O4 (LNMO) composites are applied as cathode materials in Li-ion batteries, and their electrochemical properties are explored at both room and elevated temperature. The morphology, phase evolution, and chemical properties of the as-prepared samples are analyzed by means of X-ray powder diffraction, thermogravimetric analysis, Raman spectroscopy, X-ray photoelectron spectroscopy and scanning and transmission electron microscopy techniques. The composite with 5 wt% polypyrrole coating shows a discharge capacity retention of 92% after 300 cycles and better rate capability than the bare LNMO electrode in the potential range of 3.5–4.9 V vs. Li/Li+ at room temperature. At the elevated temperature, the cycling performance of the electrode made from LNMO–5 wt% PPy is also remarkably stable (∼91% capacity retention after 100 cycles). It is revealed that the polypyrrole coating can suppress the dissolution of manganese in the electrolyte which occurs during cycling. The charge transfer resistance of the composite electrode is much lower than that of the bare LNMO electrode after cycling, indicating that the polypyrrole coating significantly increases the electrical conductivity of the LNMO electrode. Polypyrrole can also work as an effective protective layer to suppress the electrolyte decomposition arising from undesirable reactions between the cathode electrode and electrolyte on the surface of the active material at elevated temperature, leading to high coulombic efficiency.

Graphene-wrapped chromium-MOF(MIL-101)/sulfur composite for performance improvement of high-rate rechargeable Li–S batteries

A graphene/chromium-MOF(MIL-101) composite is investigated to serve as a host for sulfur immobilisation in Li–S batteries. The unique structure with a large specific area and a conductive shell ensures a high dispersion of sulfur in the composite and minimizes the loss of polysulfides to the electrolyte.

Porous LiMn2O4 microspheres as durable high power cathode materials for lithium ion batteries

Porous LiMn2O4 microspheres, which are constructed with nanometer-sized primary particles, have been synthesized by a facile method using porous MnCO3 microspheres as a self-supporting template. The LiMn2O4 microspheres were characterized by XRD, SEM and HR-TEM. The as-synthesized porous LiMn2O4 microspheres exhibit high rate capability and long-term cyclability as cathode materials for lithium ion batteries, with the specific discharge capacity of 119, 107 and 98 mA h g−1 and the corresponding capacity retention of 82, 91 and 80% for up to 500 cycles at 2, 10 and 20 C, respectively. The high rate performance and good cyclability are believed to result from the porous structure, reasonable primary particle size and high crystallinity of the obtained material, which favor fast Li intercalation/deintercalation kinetics by allowing electrolyte insertion through the nanoparticles and high structural stability during the reversible electrochemical process. The high level of Mn4+ concentration on the surface of the sample can alleviate the Jahn–Teller transition, which was triggered normally by the equal amounts of Mn4+/Mn3+ concentration on the surface of the LiMn2O4 cathode material. This good example offering extended cycle life at 20 C rate for the LiMn2O4 microspheres indicates their promising application as cathode materials for high performance LIBs.

Novel Germanium/Polypyrrole Composite for High Power Lithium-ion Batteries

Nano-Germanium/polypyrrole composite has been synthesized by chemical reduction method in aqueous solution. The Ge nanoparticles were directly coated on the surface of the polypyrrole. The morphology and structural properties of samples were determined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Thermogravimetric analysis was carried out to determine the polypyrrole content. The electrochemical properties of the samples have been investigated and their suitability as anode materials for the lithium-ion battery was examined. The discharge capacity of the Ge nanoparticles calculated in the Ge-polypyrrole composite is 1014 mAh g−1 after 50 cycles at 0.2 C rate, which is much higher than that of pristine germanium (439 mAh g−1). The composite also demonstrates high specific discharge capacities at different current rates (1318, 1032, 661, and 460 mAh g−1 at 0.5, 1.0, 2.0, and 4.0 C, respectively). The superior electrochemical performance of Ge-polypyrrole composite could be attributed to the polypyrrole core, which provides an efficient transport pathway for electrons. SEM images of the electrodes have demonstrated that polypyrrole can also act as a conductive binder and alleviate the pulverization of electrode caused by the huge volume changes of the nanosized germanium particles during Li+ intercalation/de-intercalation.

Nitrogen-Doped Carbon-Encapsulated SnO2@Sn Nanoparticles Uniformly Grafted on Three-Dimensional Graphene-like Networks as Anode for High-Performance Lithium-Ion Batteries

A peculiar nanostructure consisting of nitrogen-doped, carbon-encapsulated (N-C) SnO2@Sn nanoparticles grafted on three-dimensional (3D) graphene-like networks (designated as N-C@SnO2@Sn/3D-GNs) has been fabricated via a low-cost and scalable method, namely an in situ hydrolysis of Sn salts and immobilization of SnO2 nanoparticles on the surface of 3D-GNs, followed by an in situ polymerization of dopamine on the surface of the SnO2/3D-GNs, and finally a carbonization. In the composites, three-layer core-shell N-C@SnO2@Sn nanoparticles were uniformly grafted onto the surfaces of 3D-GNs, which promotes highly efficient insertion/extraction of Li(+). In addition, the outermost N-C layer with graphene-like structure of the N-C@SnO2@Sn nanoparticles can effectively buffer the large volume changes, enhance electronic conductivity, and prevent SnO2/Sn aggregation and pulverization during discharge/charge. The middle SnO2 layer can be changed into active Sn and nano-Li2O during discharge, as described by SnO2 + Li(+) → Sn + Li2O, whereas the thus-formed nano-Li2O can provide a facile environment for the alloying process and facilitate good cycling behavior, so as to further improve the cycling performance of the composite. The inner Sn layer with large theoretical capacity can guarantee high lithium storage in the composite. The 3D-GNs, with high electrical conductivity (1.50 × 10(3) S m(-1)), large surface area (1143 m(2) g(-1)), and high mechanical flexibility, tightly pin the core-shell structure of the N-C@SnO2@Sn nanoparticles and thus lead to remarkably enhanced electrical conductivity and structural integrity of the overall electrode. Consequently, this novel hybrid anode exhibits highly stable capacity of up to 901 mAh g(-1), with ∼89.3% capacity retention after 200 cycles at 0.1 A g(-1) and superior high rate performance, as well as a long lifetime of 500 cycles with 84.0% retention at 1.0 A g(-1). Importantly, this unique hybrid design is expected to be extended to other alloy-type anode materials such as silicon, germanium, etc.

Facile low-temperature synthesis of hematite quantum dots anchored on a three-dimensional ultra-porous graphene-like framework as advanced anode materials for asymmetric supercapacitors

A composite consisting of well-dispersed and ultrafine hematite quantum-dots (∼2.7 nm) anchored on a three-dimensional ultra-porous graphene-like framework (denoted as Fe2O3-QDs–3D GF) has been designed by a facile and scalable strategy. In the composite, the ultra-porous 3D GF with high conductivity and high surface area was used as a conductive matrix with surface defective sites for the controllable growth of uniformly dispersed, ultra-small Fe2O3-QDs. The graphene framework can tightly hold a great amount of Fe2O3-QDs, thereby ensuring high utilization of active materials and the required conductivity to individual Fe2O3-QDs. The ultra-small-sized Fe2O3-QDs anchored on the 3D GF can endow the composite with a superior high surface area and enough active sites for electrochemical reactions, thus giving the composite a large specific capacitance. As expected, the as-prepared Fe2O3-QDs–3D GF electrode exhibited a high specific capacitance of 945 F g−1 at 1.0 A g−1 in a three-electrode system in 2.0 mol L−1 KOH aqueous solution. In addition, high-performance asymmetric supercapacitors have been fabricated with Fe2O3-QDs–3D GF as the anode and 3D hierarchical porous graphene (HPG) as the cathode, and they showed a very high energy density of 77.7 W h kg−1 at a power density of 0.40 kW kg−1 and maximum power density of 492.3 kW kg−1, as well as excellent cycling stability.

NaCl multistage-recrystallization-induced formation of 3D micro-structured ribbon-like graphene based films for high performance flexible/transparent supercapacitors

Individual graphene ribbons, which fully exploit the large surface area of graphene sheets, have been fabricated. Constructing these unique structures into efficient macroscopic functional architectures is an important and challenging step towards practical applications. Here, we produce micro-structured interconnected ribbon-like graphene sheets (MRGs), which were induced by the multistage-recrystallization of NaCl templates in a microwave plasma chemical vapor deposition (MPECVD) system. The MRGs along different directions hang in polygonal graphene walls, which connect with each other forming a three dimensional (3D) transparent and self-supporting graphene film (MRG-GF). The MRG-GF with a large surface area, enhanced flexibility and fast ion/electron transport pathways exhibits improved capacitance (4.88 mF cm−2) and super-long cycle life with good cycling stability (capacitance retention was ∼95.5% after 20 000 cycles). Herein, we provide a novel approach for controlled synthesis of graphene ribbons, and graphene ribbon-based functional structures and composites.

Graphene-hollow-cubes with network-faces assembled a 3D micro-structured transparent and free-standing film for high performance supercapacitors

Transparent all-solid-state supercapacitors are advanced power supply devices for multifunctional high-end electronics. However, the areal capacitance is seriously limited due to the low mass loading, as high transmittance usually corresponds to ultra-small electrode thickness, which hinders practical applications. Here, we develop an effective three-dimensional (3D) architecture using plasma-etched micro-structured NaCl as the template. The film is assembled by graphene hollow-cube building units, and the faces of the cubes are graphene network microstructures. The graphene-network-face hollow-cube units (GNHC) fully exploit the large surface area of graphene, ensure the transparency of the film and reduce junction contact resistance between the graphene sheets. As expected, the film exhibits improved areal capacitance (5.48 mF cm−2), high volumetric energy (657.2 μW h cm−3), and power densities (954.3 mW cm−3). Generally, GNHC based films will be promising energy storage materials and very suitable supports or templates for construction of graphene-based composites.