Yudai Huang

Nano- LiFePO4 ∕ MWCNT Cathode Materials Prepared by Room-Temperature Solid-State Reaction and Microwave Heating

The precursors of LiFePO 4 were prepared by room-temperature solid-state reaction using CH 3 COOLi, NH 4 H 2 PO 4 , FeC 2 O 4 ·2H 2 O as raw materials. Citric acid was added to reduce the particle size of LiFePO 4 , and multiwalled carbon nanotubes (MWCNTs) were adopted to improve the electrical conductivity. The LiFePO 4 samples were synthesized for a few minutes by microwave heating. X-ray diffraction and transmission electron microscope were used to characterize their structures and morphologies. The results showed that all samples had the same olivine phase structure. When both citric acid and MWCNTs were added, nanoparticles of LiFePO 4 homogeneously coated on the MWCNTs. An electrochemical test showed that LiFePO 4 , with both citric acid and MWCNTs addition, demonstrated an excellent electrochemical capacity of 145 mAh/g at C/2 and stable cycle ability. Citric acid expansion reduced the particle size, thus reducing the length of diffusion or improving the reversibility of the lithium ion intercalation/deintercalation. MWCNTs functioned as a lead to connect the LiFePO 4 particles and enabled active materials to transport lithium ions and electrons at fast rates.

Super high-rate, long cycle life of europium-modified, carbon-coated, hierarchical mesoporous lithium-titanate anode materials for lithium ion batteries

Europium-modified, carbon-coated, hierarchical mesoporous Li4Ti5O12 microspheres were prepared via the co-precipitation method. X-ray diffraction (XRD) and Raman analyses revealed that europium ions were doped into 16d Li+/Ti4+ sites of Li4Ti5O12. Microscopic observations reveal that primary nanoparticles of Li4−x/2Ti5−x/2EuxO12@C (x = 0.004) are assembled into hierarchical mesoporous microspheres, with an average particle size of about 473.4 nm and a uniform particle size distribution. X-ray photoelectron spectroscopy demonstrated that partial Ti4+ is reduced to Ti3+ induced by carbon coating and double-valence state of europium (Eu2+/Eu3+) doping into the Li4Ti5O12. The samples exhibit excellent electrochemical properties including fast lithium storage performance, outstanding cycle stability and high rate capability. The highest initial discharge capacity of Li4−x/2Ti5−x/2EuxO12@C (x = 0.004) reached 198.7 mA h g−1 and the discharge capacity still maintained 173.4 mA h g−1 at 5C after 1000 cycles. Even cycled at 100C, the discharge capacity of Li4−x/2Ti5−x/2EuxO12@C (x = 0.004) maintained 92.1 mA h g−1. The excellent electrochemical performance can be attributed to the hierarchical mesoporous structure combined with modified strategies including europium doping and carbon coating, which not only improved the lithium-ion diffusion coefficient, but also increased the electronic conductivity. Moreover, the electrical conductivity between the Li4Ti5O12 particles was enhanced by carbon coating and the bulk electronic conductivity of Li4Ti5O12 was also improved by the presence of Ti3+.

Decoration of Silica Nanoparticles on Polypropylene Separator for Lithium–Sulfur Batteries

A SiO2 nanoparticle decorated polypropylene (PP) separator (PP-SiO2) has been prepared by simply immersing the PP separator in the hydrolysis solution of tetraethyl orthosilicate (TEOS) with the assistance of Tween-80. After decoration, the thermal stability and the electrolyte wettability of the PP-SiO2 separator are obviously improved. When the PP-SiO2 separator is used for lithium-sulfur (Li-S) batteries, the cyclic stability and rate capability of the batteries are greatly enhanced. The capacity retention ratio of the Li-S battery configured with the PP-SiO2 separator is 64% after 200 cycles at 0.2 C, which is much higher than that configured with the PP separator (45%). Moreover, the rate capacity of the Li-S batteries using the PP-SiO2 separator reaches 956.3, 691.5, 621, and 567.6 mAh g-1 at the current density of 0.2, 0.5, 1, and 2 C, respectively. The reason could be ascribed to that the polar silica coating not only alleviates the shuttle effect but also facilitates Li-ion migration.

Simple in situ synthesis of carbon-supported and nanosheet-assembled vanadium oxide for ultra-high rate anode and cathode materials of lithium ion batteries

A simple and efficient precipitation method has been developed for the in situ synthesis of a two-dimensional vanadium oxide@carbon nanosheet (2D V2O5@C NS). The crystalline structure, morphology and electrochemical performance of the as-prepared material were characterized systematically. The results demonstrate that the thickness of nanosheet is about 50 nm, and a thin C shell is successfully coated in situ on the surface of the V2O5 NS core. Benefiting from the intrinsic increased conductivity of the 2D V2O5@C NS and its robust NS structure, when the as-synthesized material is used as an anode material, it exhibits large reversible discharge capacity (860 mA h g−1 at 0.5 A g−1), good cycling performance (a high capacity of 802 mA h g−1 at 1.0 A g−1 after 200 cycles) and an ultra-high rate capability (reversible capabilities of 705 mA h g−1 at 2.0 A g−1, and 554 mA h g−1 at 3.0 A g−1). As a cathode material, the material also shows superior rate performance (reversible capabilities of 189, 166, 147, 139, 132, and 126 mA h g−1 at 0.1, 0.2, 0.5, 0.8, 1.0, and 1.2 A g−1, respectively). This work demonstrates a novel method for preparing vanadium-based NS material for high-performance lithium ion batteries.

Self-assembled sandwich-like vanadium oxide/graphene mesoporous composite as high-capacity anode material for lithium ion batteries

Sandwich-like V2O5/graphene mesoporous composite has been synthesized by a facile solvothermal approach. The crystalline structure, morphology, and electrochemical performance of the as-prepared materials have been investigated in detail. The results demonstrate that the 30-50 nm V2O5 particles are homogeneously anchored on conducting graphene sheets, which allow the V2O5 nanoparticles to be wired up to a current collector through the underlying conducting graphene layers. As an anode material for lithium ion batteries, the composite exhibits a high reversible capacity of 1006 mAh g(-1) at a current density of 0.5 A g(-1) after 300 cycles. It also exhibits excellent rate performance with a discharge capacity of 500 mAh g(-1) at the current density of 3.0 A g(-1), which is superior to the performance of the vanadium-based materials reported previously. The electrochemical properties demonstrate that the sandwich-like V2O5/graphene mesoporous composite could be a promising candidate material for high-capacity anode in lithium ion batteries.

CdSe Ring- and Tribulus-Shaped Nanocrystals: Controlled Synthesis, Growth Mechanism, and Photoluminescence Properties.

With air-stable and generic reagents, CdSe nanocrystals with tunable morphologies were prepared by controlling the temperature in the solution reaction route. Thereinto, the lower reaction temperature facilitates the anisotropic growth of crystals to obtain high-yield CdSe ring- and tribulus-shaped nanocrystals with many branches on their surfaces. The photoluminescence properties are sensitive to the nature of particle and its surface. The products synthesized at room temperature, whose surfaces have many branches, show higher blue shift and narrower emission linewidths (FWHM) of photoluminescence than that of samples prepared at higher temperature, whose surfaces have no branches. Microstructural studies revealed that the products formed through self-assembly of primary crystallites. Nanorings formed through the nonlinear attachment of primary crystallites, and the branches on the surfaces grew by linear attachment at room temperature. And the structure of tribulus-shaped nanoparticle was realized via two steps of aggregation, i.e., random and linear oriented aggregation. Along with the elevation of temperature, the branches on nanocrystal surfaces shortened gradually because of the weakened linear attachment.

Two-dimensional dysprosium-modified bamboo-slip-like lithium titanate with high-rate capability and long cycle life for lithium-ion batteries

Two-dimensional dysprosium-modified bamboo-slip-like Li4Ti5O12 (2D Dy-B-LTO) has been synthesized by a one-pot hydrothermal method. The structure and morphology of the as-prepared materials were analyzed by X-ray diffraction (XRD) and electron microscopy. The results show that dysprosium is doped into both 8a and 16d sites of Li4Ti5O12, which is determined by Rietveld analysis of XRD data. Dysprosium-modified Li4Ti5O12 shows bamboo-slip-like nanosheet morphology and the possible formation mechanism is proposed. The as-prepared samples exhibit superior high-rate capability and excellent cycle performance. The initial discharge capacity of Li4−x/3Ti5−2x/3DyxO12 (x = 0.02) is 181.8 mA h g−1 at 20C; surprisingly, even when cycled at 100C, the discharge capacity still retains 140.9 mA h g−1 after 1000 cycles. The improved electrochemical performance could be attributed to bamboo-slip-like nanosheets in conjunction with dysprosium doping, which can offer more ion and electron transporting channels and increase the amount of Ti3+/Ti4+ mixing as charge compensation.

High-Capacity, High-Cycling Cathode Material Synthesized by Low-Temperature Solid-State Coordination Method for Lithium Rechargeable Batteries

Cathode materials of the Li 1+x Mn 2 O 4-v F v family were successfully synthesized by a low-heating-temperature solid-state coordination method using lithium acetate, manganese acetate, lithium fluoride, citric acid, and polyethylene glycol (PEG) 400 as raw materials. X-ray diffractometry (XRD) proved that the Li 1+x Mn 2 O 4-y F y powders were well-crystallized pure phase. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the Li 1+x Mn 2 O 4-y F y powders consisted of small and uniformly sized particles, with the diameter of the particles about 30-50 nm. Galvanostatic cycling results suggested that the Li 1.05 Mn 2 O 3.95 F 0.05 gave an initial capacity of 128 mAh/g and had a retained capacity of 112 mAh/g at the 100th cycle within the potential range from 3.0 to 4.35 V and of 129 and 106 mAh/g between 3.0 and 4.8 V, respectively, which indicated that the sample possesses excellent electrochemical properties. The introduction of Li and F in LiMn 2 O 4 apparently increases the capacity and significantly decreases the rate of capacity degradation during charge-discharge cycling.

In Situ Chelating Synthesis of Hierarchical LiNi1/3Co1/3Mn1/3O2 Polyhedron Assemblies with Ultralong Cycle Life for Li‐Ion Batteries

Layered lithium transition-metal oxides, with large capacity and high discharge platform, are promising cathode materials for Li-ion batteries. However, their high-rate cycling stability still remains a large challenge. Herein, hierarchical LiNi1/3 Co1/3 Mn1/3 O2 polyhedron assemblies are obtained through in situ chelation of transition metal ions (Ni2+ , Co2+ , and Mn2+ ) with amide groups uniformly distributed along the backbone of modified polyacrylonitrile chains to achieve intimate mixing at the atomic level. The assemblies exhibit outstanding electrochemical performances: superior rate capability, high volumetric energy density, and especially ultralong high-rate cyclability, due to the superiority of unique hierarchical structures. The polyhedrons with exposed active crystal facets provide more channels for Li+ diffusion, and meso/macropores serve as access shortcuts for fast migration of electrolytes, Li+ and electrons. The strategy proposed in this work can be extended to fabricate other mixed transition metal-based materials for advanced batteries.

Improved rate capability and cycling stability of bicontinuous hierarchical mesoporous LiFePO4/C microbelts for lithium-ion batteries

Bicontinuous hierarchical mesoporous LiFePO4/C (BHM-LFP/C) microbelts have been synthesized using a simple dual-solvent electrospinning method for the first time. The use of a dual-solvent is beneficial for the electrospinning synthesis, which increases the salt solubility, as well as decreases the surface tension to improve the spinnability. The sample exhibits a high reversible capacity (153 mA h g−1 at 0.5C), and achieves an excellent high rate cycling performance. The enhanced electrochemical performance can be attributed to the unique microbelts providing bicontinuous electron/ion pathways, and the thoroughly mesoporous structure facilitating electrolyte penetration, especially for hierarchical architectures, which have three-dimensional diffusion channels for Li+. This method can be extended to fabricate other electrospun multi-element oxide cathode materials for advanced lithium-ion batteries.