Congxiao Wang

Carbon-coated nano-sized Li4Ti5O12 nanoporous micro-sphere as anode material for high-rate lithium-ion batteries

In this study, we report a facile process for preparing a carbon-coated nanosized Li4Ti5O12 nanoporous micro-sphere (CN-LTO-NMS) by a carbon pre-coating process combined with the spray drying method. The obtained material consists of a micron-size secondary sphere (10–20 μm) accumulated by carbon-coated nanosized primary particles (∼200 nm). The nanosized primary particles and nano-thickness carbon layer uniformly coated over the particles as well as the interconnected nanopores greatly improve its rate capability. As a consequence, the resulting sample delivers a reversible capacity of 160 mAhg−1 at 0.2 C, and shows remarkable rate capability by maintaining 79% of the capacity at 20 C (vs. 0.2 C), as well as excellent cycling stability with a capacity retention of 95% after 1000 cycles at 1 C rate (vs. 0.2 C).

General synthesis of carbon-coated nanostructure Li4Ti5O12 as a high rate electrode material for Li-ion intercalation

A simple approach is proposed to synthesize nanostructured Li4Ti5O12 spinel materials with different morphologies (nanorods, hollow spheres and nanoparticles), in which the TiO2 precursor is first coated with a conductive carbon layer by the chemical vapour decomposition (CVD) method, followed by a solid-state reaction with lithium salt. The Li4Ti5O12 obtained was characterised by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as well as galvanostatic measurements. The results indicate that, by employing the carbon pre-coating process, the carbon-coated nanostructured Li4Ti5O12 can maintain the initial morphologies of the TiO2 precursors and also show significant improvement in the rate capability for lithium-ion intercalation due to both good electronic conductivity and the short lithium-ion diffusion path.

Preparation of three-dimensional ordered mesoporous carbon sphere arrays by a two-step templating route and their application for supercapacitors

A two-step template approach was demonstrated for preparation of the three-dimensional ordered mesoporous carbon sphere arrays. The ordered macroporous silica skeleton was formed from silicon alkoxide precursor templating around polystyrene (latex) spheres, and removal of the polystyrene spheres. These preforms as hard templates were infiltrated with a solution mixture of amphiphilic triblock copolymer PEO-PPO-PEO and soluble resol. By combining evaporation-induced surfactant-templating organic resol self-assembly with thermosetting, carbonization and hydrofluoric acid extraction of silica, the obtained mesoporous carbon has a pore size of 10.4 nm, interconnected window size of about 60 nm, surface area of 601 m2/g and pore volume of 1.70 cm3/g. The electrochemical properties as an electrode material for supercapacitor applications were investigated in nonaqueous electrolyte. They show rectangular-shaped cyclic voltammetry curves over a wide range of scan rates even up to 200 mV/s between 0 and 3 V, and deliver a large capacitance of 14 µF/cm2 (84 F/g), and good cycling stability with capacitance retention of 93% over 5000 cycles.

Improving the electrochemical performance of layered lithium-rich transition-metal oxides by controlling the structural defects

We report the electrochemical properties of layered lithium-rich Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials with various degrees of stacking faults, which are prepared via a facile molten-salt method using a variety of fluxes including KCl, Li2CO3, and LiNO3. The frequency of the stacking faults is highly dependent on the temperature and molten salt type used during the synthesis. A well-crystallized Li1.18Mn0.54Ni0.13Co0.13O2 nanomaterial with a larger amount of stacking faults synthesized at 800 °C for 10 h in an inactive KCl flux delivers a high reversible capacity of ∼310 mA h g−1 at room temperature, while the samples prepared in the chemically active fluxes with a smaller amount of stacking faults show poor electrochemical performance.

A nitrogen-doped ordered mesoporous carbon nanofiber array for supercapacitors

A nitrogen-doped ordered mesoporous carbon nanofiber array was prepared by a combination of a hard-templating and a soft-templating method. The obtained N-enriched carbon material has a high surface area (1030 m2 g−1) and a large pore volume (2.35 cm3 g−1), with uniform mesopores located at 15.0 nm. The material with a nitrogen content of 6.7 wt% shows a high capacitance of 264 F g−1 in 1.0 M H2SO4 electrolyte and a good rate capability with a capacitance retention of 86% at 10 A g−1vs. 0.5 A g−1. It also delivers an excellent cycling stability without capacitance fading over 10000 cycles.

A Comprehensive Study of Effects of Carbon Coating on Li4Ti5O12 Anode Material for Lithium-Ion Batteries

Carbon coating is an important technique used to improve the electrochemical performance of many electrode materials in lithium-ion batteries. In the present work, Li 4 Ti 5 O 12 was selected as the probe material and a series of carbon-coated Li 4 Ti 5 O 12 samples was prepared to discuss the effects of graphitization and thickness of the coated carbon layer on its rate capability for lithium-ion intercalation. The apparent diffusion coefficient (D Li ) and rate capability were studied by cyclic voltammetry experiments using a powder microelectrode. The results show that a higher carbon coating temperature could promote the graphitization of the surface carbon, thus enhancing the electronic conductivity of the electrodes while hindering the lithium-ion diffusion. The thickness of carbon layer is demonstrated to have a major influence on ion diffusion while slightly affecting the electronic conductivity. Considering both the electronic conductivity and ion diffusion in the carbon layer, an optimal condition for carbon coating to achieve a good rate capability is determined to be performed at 800°C with a thickness of ca. 5 nm.

Ordered hierarchical mesoporous/microporous carbon with optimized pore structure for supercapacitors

A series of ordered mesoporous/microporous carbon materials with controllable micropore size was prepared from chlorination of ordered mesoporous titanium carbide/carbon composites obtained by an evaporation induced self-assembly approach. The proportion of micropores can be tuned easily by changing the Ti content in the parent TiC/C composites in order to study the effect of micropore content on rate capability and specific capacitance. Under the optimized condition, an optimal mesoporous/microporous carbon with moderate micropores (52.4% in volume) was obtained. It has a high surface area (1698 m2 g−1) and a large pore volume (1.17 cm3 g−1), with the mesopore size centered at 4.4 nm, and micropore sizes of 0.52, 0.76 and 1.35 nm. It exhibits a high capacitance of 132 F g−1 at the current density of 500 mA g−1 and good rate capability with capacitance retention of 79% at a scan rate of 2000 mV s−1vs. 20 mV s−1 in nonaqueous electrolyte, and also shows a good cycling stability with capacitance retention of 84% over 5000 cycles.

Ordered mesoporous/microporous carbon sphere arrays derived from chlorination of mesoporous TiC/C composite and their application for supercapacitors

Three dimensional ordered mesoporous/microporous carbon microsphere arrays (MMCSA) were derived from chlorination of mesoporous titanium carbide/carbon composites which were prepared by a hard-templating method combined with a soft-templating self-assembly approach. The obtained carbon material has a high surface area (1464.4 m2 g−1) and a large pore volume (1.483 cm3 g−1), with mesopore size at about 8.5 nm, micropore size of 0.69 and 1.51 nm and an interconnected window size of about 60 nm. It exhibits a high capacitance of 134 F g−1 in nonaqueous electrolyte and good cycling stability with capacitance retention of 84% over 5000 cycles. The unique mesoporous structure and sphere array morphology provide a more favorable path for electrolyte penetration and transportation and lower electrode resistance to achieve promising rate capability performance, while the micropores in the mesopore walls greatly enhance the specific capacitance.

Hybrid Aqueous Energy Storage Cells Using Activated Carbon and Lithium-Ion Intercalated Compounds III. Capacity Fading Mechanism of LiCo 1 ∕ 3 Ni 1 ∕ 3 Mn 1 ∕ 3 O 2 at Different pH Electrolyte Solutions

The electrochemical stability of LiCo 1/3 Ni 1/3 Mn 1/3 O 2 in a Li + -containing aqueous electrolyte solution is critically dependent on the pH value of the electrolyte solution. It shows the capacity fading upon cycling in the electrolyte solution below pH 11. The mechanism responsible for the capacity fading has been extensively investigated by means of cyclic voltage, ac impedance, charge/discharge, ex situ X-ray diffraction, and chemical analysis. It was found that LiCo 1/3 Ni 1/3 Mn 1/3 O 2 and its partly charged product is chemically stable in the low pH electrolyte solution, while it is not electrochemically stable during the discharge process in which the proton cointercalation parallel to the lithium ion intercalation occurred, but the intercalated proton cannot be reversibly extracted during the charge process. The intercalated proton limits the intercalation of lithium ion, thus resulting in capacity fading.

Polyimide as anode electrode material for rechargeable sodium batteries

A polyimide synthesized from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) and ethylene diamine (EDA) was evaluated as a new anode material for sodium ion batteries (SIB). The polyimide delivers a discharge specific capacity of 140 mA h g−1 at an average potential of 2 V vs. Na+/Na with an initial coulombic efficiency of 97.6% and exhibits an excellent cycleability with a capacity retention of 90% over 500 cycles. A full SIB with polyimide anode and Na3V2(PO4)3/C cathode and Na4Fe(CN)6/C cathode was proposed.