Xufeng Zhou

Graphene modified LiFePO4 cathode materials for high power lithium ion batteries

Graphene-modified LiFePO4 composite has been developed as a Li-ion battery cathode material with excellent high-rate capability and cycling stability. The composite was prepared with LiFePO4 nanoparticles and graphene oxide nanosheets by spray-drying and annealing processes. The LiFePO4 primary nanoparticles embedded in micro-sized spherical secondary particles were wrapped homogeneously and loosely with a graphene 3D network. Such a special nanostructure facilitated electron migration throughout the secondary particles, while the presence of abundant voids between the LiFePO4 nanoparticles and graphene sheets was beneficial for Li+ diffusion. The composite cathode material could deliver a capacity of 70 mAh g−1 at 60C discharge rate and showed a capacity decay rate of <15% when cycled under 10C charging and 20C discharging for 1000 times.

Scalable synthesis of TiO2/graphene nanostructured composite with high-rate performance for lithium ion batteries.

A simple and scalable method is developed to synthesize TiO(2)/graphene nanostructured composites as high-performance anode materials for Li-ion batteries using hydroxyl titanium oxalate (HTO) as the intermediate for TiO(2). With assistance of a surfactant, amorphous HTO can condense as a flower-like nanostructure on graphene oxide (GO) sheets. By calcination, the HTO/GO nanocomposite can be converted to TiO(2)/graphene nanocomposite with well preserved flower-like nanostructure. In the composite, TiO(2) nanoparticles with an ultrasmall size of several nanometers construct the porous flower-like nanostructure which strongly attached onto conductive graphene nanosheets. The TiO(2)/graphene nanocomposite is able to deliver a capacity of 230 mA h g(-1) at 0.1 C (corresponding to a current density of 17 mA g(-1)), and demonstrates superior high-rate charge-discharge capability and cycling stability at charge/discharge rates up to 50 C in a half cell configuration. Full cell measurement using the TiO(2)/graphene as the anode material and spinel LiMnO(2) as the cathode material exhibit good high-rate performance and cycling stability, indicating that the TiO(2)/graphene nanocomposite has a practical application potential in advanced Li-ion batteries.

A scalable, solution-phase processing route to graphene oxide and graphene ultralarge sheets

High yield production of graphene oxide and graphene sheets with an ultralarge size (up to approximately 200 microm) was realized using a modified solution-phase method.

A 3D porous architecture of Si/graphene nanocomposite as high-performance anode materials for Li-ion batteries

A 3D porous architecture of Si/graphene nanocomposite has been rationally designed and constructed through a series of controlled chemical processes. In contrast to random mixture of Si nanoparticles and graphene nanosheets, the porous nanoarchitectured composite has superior electrochemical stability because the Si nanoparticles are firmly riveted on the graphene nanosheets through a thin SiOx layer. The 3D graphene network enhances electrical conductivity, and improves rate performance, demonstrating a superior rate capability over the 2D nanostructure. This 3D porous architecture can deliver a reversible capacity of ∼900 mA h g−1 with very little fading when the charge rates change from 100 mA g−1 to 1 A g−1. Furthermore, the 3D nanoarchitechture of Si/graphene can be cycled at extremely high Li+ extraction rates, such as 5 A g−1 and 10 A g−1, for over than 100 times. Both the highly conductive graphene network and porous architecture are considered to contribute to the remarkable rate capability and cycling stability, thereby pointing to a new synthesis route to improving the electrochemical performances of the Si-based anode materials for advanced Li-ion batteries.

Morphology-controlled solvothermal synthesis of LiFePO4 as a cathode material for lithium-ion batteries

LiFePO4 (LFP) nanoparticles (∼50 nm in size), nanoplates (100 nm thick and 800 nm wide) and microplates (300 nm thick and 3 μm wide) have been selectively synthesized by a solvothermal method in a water–polyethylene glycol (PEG) binary solvent using H3PO4, LiOH•H2O and FeSO4•7H2O as precursors. The morphology and size of the LFP particles were strongly dependent on synthetic parameters such as volume ratio of PEG to water, temperature, concentration, and feeding sequence. The carbon coated nanoparticles and nanoplates could deliver a discharge capacity of >155 mAh g−1 at 0.1C rate (i.e. 17 mA g−1 of current density); in comparison, the carbon coated microplates had a discharge capacity as low as 110 mAh g−1 at 0.1C rate. The Li-ion diffusion coefficients of the carbon coated nanoparticles, nanoplates, and microplates were calculated to be 6.4 × 10−9, 4.2 × 10−9, and 2.2 × 10−9 cm2 s−1, respectively. When the content of conductive Super P carbon (SP) was increased to 30 wt.%, the prepared electrodes could charge–discharge at a rate as high as 20C. Over 1000 cycles at 20C, the nanoparticle electrode could maintain 89% of its initial capacity (126 mAh g−1), the nanoplate electrode showed 79% capacity retention compared to an initial capacity (129 mAh g−1), and the microplate electrode retained 80% of its initial capacity (63.5 mAh g−1).

An Ordered Mesoporous Carbon with Short Pore Length and Its Electrochemical Performances in Supercapacitor Applications

The pores of conventional ordered mesoporous carbons (OMCs) are usually over several micrometers in length, making it difficult for electrolyte access and ion diffusion to the deep pores of the carbon grains when they are used as the electrode of electrochemical double layer capacitors (EDLCs). We have synthesized an ordered mesoporous carbon with a much shorter pore length of 200-300 nm through a hard-template method. The electrochemical properties as an electrode material for EDLC were investigated in an alkaline solution in comparison with the conventional OMC. A maximum capacitance of 14 mu F/cm(2) was obtained for this short pore length OMC (SOMC) in 6 M KOH solution compared with 10 mu F/cm(2) of the conventional OMC. SOMC delivered much better capacity retention than the conventional OMC in lower concentration electrolyte solution. The superior performance of SOMCs was attributed to its having more entrances for electrolyte accessibility and a short pathway for rapid ion diffusion. (c) 2007 The Electrochemical Society.

A Facile One‐Step Solvothermal Synthesis of SnO2/Graphene Nanocomposite and Its Application as an Anode Material for Lithium‐Ion Batteries

Spare capacity: A SnO/graphene nanocomposite is fabricated by a novel solvothermal method (see picture). The nanocomposite exhibits a reversible lithium storage capacity of 838 mAhg in the first cycle and improved cyclability as an anode material for lithium-ion batteries.

Ultrasmall, well-dispersed, hollow siliceous spheres with enhanced endocytosis properties.

The synthesis of ultrasmall, well-dispersed, hollow siliceous spheres (HSSs) by using a block copolymer as the template and tetraethoxysilane as a silica source under acidic conditions is reported. After removing the surfactant core of as-synthesized, spherical, silica-coated block-copolymer micelles, HSSs with a uniform particle size of 24.7 nm, a cavity diameter of 11.7 nm, and a wall thickness of 6.5 nm are obtained. It is shown that by surface functionalization of HSSs with methyl groups during synthesis, HSSs can be further dispersed in solvents such as water or ethanol to form a stable sol. Moreover, the hollow cavities are accessible for further loading of functional components. In addition, it is demonstrated that HSSs possess superior endocytosis properties for HeLa cells compared to those of conventional mesoporous silica nanoparticles. A feasible and designable strategy for synthesizing novel well-dispersed hollow structures with ultrasmall diameters instead of conventional ordered mesostructures is provided. It is expected that HSSs may find broad applications in bionanotechnology, such as drug carriers, cell imaging, and targeted therapy.

synthesis and electrochemical properties of layered lithium transition metal oxides

Layered lithium transition metal oxide cathode materials (Li1.2Ni0.2Mn0.6O2, LiNi1/3Co1/3Mn1/3O2 and LiNi0.5Mn0.5O2), of spherical morphology with primary nanoparticles assembled in secondary microparticles, were generally synthesized through a simple carbonate co-precipitation method. In this method, various carbonates such as Na2CO3, NaHCO3 and (NH4)2CO3 could be employed as the precipitants without careful control of the pH value. Aging treatment on the carbonate slurries at 80 °C could yield spherical microparticles assembled with very fine primary nanoparticles. The carbonate microparticle precursors were calcined at 500 °C and further lithiated at 900 °C to prepare the layered cathode materials. The as-prepared Li1.2Ni0.2Mn0.6O2, LiNi1/3Co1/3Mn1/3O2 and LiNi0.5Mn0.5O2 cathode materials could deliver a capacity of 230, 190 and 153 mAh g−1, respectively, at a charge–discharge current density of 25 mA g−1 in the voltage range of 2.5–4.6 V. When the charge–discharge current density was increased to 250 mA g−1, the Li1.2Ni0.2Mn0.6O2 and LiNi1/3Co1/3Mn1/3O2 showed an initial discharge capacity of 150 and 166 mAh g−1; as for the LiNi0.5Mn0.5O2, the discharge capacity decreased to 67 mAh g−1. After 150 cycles at a current density of 250 mA g−1, both LiNi1/3Co1/3Mn1/3O2 and LiNi0.5Mn0.5O2 showed a capacity decay rate of >25%, while the Li1.2Ni0.2Mn0.6O2 exhibited an excellent cycling performance with almost no capacity decay.

Morphology controlled synthesis and modification of high-performance LiMnPO4 cathode materials for Li-ion batteries

Morphology-controlled monodispersed LiMnPO4 nanocrystals as high-performance cathode materials for Li-ion batteries have been successfully synthesized by a solvothermal method in a mixed solvent of water and polyethylene glycol (PEG). Morphology evolution of LiMnPO4 nanoparticles from a nanorod to a thick nanoplate (∼50 nm in thickness) and to a smaller thin nanoplate (20–30 nm in thickness) is observed by increasing the pH value of the reaction suspension. Electrochemical measurements confirm that the LiMnPO4 thin nanoplates display the best charge–discharge performance, thick nanoplates the intermediate, nanorods the worst, which can be mainly ascribed to the difference in their morphologies and particle sizes in three dimensions. Further modification of LiMnPO4 thin nanoplates with graphene gives rise to an improved electrochemical performance compared with conventional pyrolytic carbon coated ones. The LiMnPO4 thin nanoplate/graphene composites deliver a high capacity of 149 mA h g−1 at 0.1 C, 90 mA h g−1 at 1 C, and even 64 mA h g−1 at 5 C charge–discharge rate, with an excellent cycling stability.