Yan-Bing He

Low-Temperature Exfoliated Graphenes: Vacuum-Promoted Exfoliation and Electrochemical Energy Storage

A preheated high-temperature environment is believed to be critical for a chemical-exfoliation-based production of graphenes starting from graphite oxide, a belief that is based on not only experimental but also theoretical viewpoints. A novel exfoliation approach is reported in this study, and the exfoliation process is realized at a very low temperature, which is far below the proposed critical exfoliation temperature, by introducing a high vacuum to the exfoliation process. Owing to unique surface chemistry, low-temperature exfoliated graphenes demonstrate an excellent energy storage performance, and the electrochemical capacitance is much higher than that of the high-temperature exfoliated ones. The low-temperature exfoliation approach presents us with a possibility for a mass production of graphenes at low cost and great potentials in energy storage applications of graphene-based materials.

Flexible and planar graphene conductive additives for lithium-ion batteries

Graphene is introduced into a lithium-ion battery (LIB) as a type of novel but powerful planar conductive additive and the flexible graphene-based conducting network is characterized by a novel “plane-to-point” conducting mode with exceptional electron transport properties and unique geometrical nature (a soft and ultrathin planar structure). With a much lower fraction of graphene additives than those of commercial carbon based additives, the graphene-introduced LiFePO4 cathode shows better charge/discharge performance than commercial cases. Graphene also shows a better performance compared to carbon nanotubes, another type of novel conductive additive with similar fractions. These results present us an indication that graphene will possibly find early application as a flexible and planar conductive additive in high performance LIBs, as our further efforts have shown that a graphene-introduced battery is of better performance as compared to real commercial batteries with conventional additives.

Facile synthesis of Li4Ti5O12/C composite with super rate performance

The Li4Ti5O12/C composite with lump morphology and excellent rate performance are synthesized using a facile hydrothermal method followed by a low temperature heat treatment. In the hydrothermal process, the introduction of cetyltrimethylammonium bromide (CTAB) as a surfactant significantly improves the rate performance of Li4Ti5O12/C composite as anode material for lithium ion battery (LIB). The specific capacities of the obtained composite at charge and discharge rates of 0.1, 1, 5, 10 and 20 C are 176, 163, 156, 151 and 136 mA h g−1, respectively, which is apparently larger than those of the Li4Ti5O12/C free from CTAB in the preparation. The Li4Ti5O12/C prepared in presence of CTAB also shows excellent cycling performance at high rate, which is attributed to its larger diffusion coefficient of lithium ion (6.82 × 10−12 cm2 s−1) and smaller charge-transfer resistance (Rct) (19.2 Ω) than those of the composite (1.22 × 10−13 cm2 s−1 and 50.2 Ω) free from CTAB in the preparation. The Li4Ti5O12 particles obtained in presence of CTAB are coated uniformly by a thin carbon layer with a thickness of ∼1 nm, whereas the Li4Ti5O12 particles obtained in absence of CTAB are covered by relatively thick surface layers with a thickness of ∼2.5 nm, which is too thick, blocks the lithium ion diffusion and leads to low ionic conductivity.

Gassing in Li 4 Ti 5 O 12 -based batteries and its remedy

Destructive gas generation with associated swelling has been a major challenge to the large-scale application of lithium ion batteries (LIBs) made from Li4Ti5O12 (LTO) anodes. Here we report root causes of the gassing behavior, and suggest remedy to suppress it. The generated gases mainly contain H2, CO2 and CO, which originate from interfacial reactions between LTO and surrounding alkyl carbonate solvents. The reactions occur at the very thin outermost surface of LTO (111) plane, which result in transformation from (111) to (222) plane and formation of (101) plane of anatase TiO2. A nanoscale carbon coating along with a stable solid electrolyte interface (SEI) film around LTO is seen most effective as a barrier layer in suppressing the interfacial reaction and resulting gassing from the LTO surface. Such an ability to tune the interface nanostructure of electrodes has practical implications in the design of next-generation high power LIBs.

Chemical Dealloying Derived 3D Porous Current Collector for Li Metal Anodes

A 3D porous Cu current collector is fabricated through chemical dealloying from a commerial Cu-Zn alloy tape. The interlinked porous framework naturally integrated can accommodate Li deposition, suppressing dendrite growth and alleviating the huge volume change during cycling. The Li metal anode combined with such a porous Cu collector demonstrates excellent performance and commerial potentials in Li-based secondary batteries.

Exceptional electrochemical performance of freestanding electrospun carbon nanofiber anodes containing ultrafine SnOx particles

SnOx–carbon nanofiber (CNF) composites are synthesized using the electrospinning technique for use as freestanding electrodes in Li-ion batteries. The electrodes made from the composites carbonized at 750 °C (SnOx–CNF-750) with 14.5 wt% SnOx deliver a remarkable capacity of 674 mA h g−1 after 100 cycles when discharged at 0.5 A g−1. This result is considered the highest among those reported in the literature for anodes made from similar electrospun carbon fibers containing SnOx nanoparticles. An increase in carbonization temperature to 950 °C (SnOx–CNF-950) results in a significant reduction of the particle content in the fiber due to aggregation of Sn to form nanoparticles external to the fibers, with concomitant degradation of capacities. The presence of amorphous SnOx particles at the atomic scale embedded in the conductive CNFs is thought to be responsible for the exceptional electrochemical performance of the SnOx–CNF-750 electrodes. These ultrafine particles facilitate the reaction Sn + xLi2O → SnOx + 2xLi+ + 2xe−, making it highly reversible, which is confirmed by the growing peak currents with increasing scan rate indicated by cyclic voltammetry, and the absence of Sn–Sn bonds in the particles revealed by the extended X-ray absorption fine structure spectroscopy (EXAFS). Both the SnOx particle size and content in the fiber play important roles in controlling the rate and cyclic performance of the SnOx–CNF composite electrodes.

Dendrite‐Free, High‐Rate, Long‐Life Lithium Metal Batteries with a 3D Cross‐Linked Network Polymer Electrolyte

A 3D network gel polymer electrolyte (3D-GPE) is designed for lithium metal batteries and prepared by an initiator-free one-pot ring-opening polymerization technique. This 3D-GPE exhibits an unprecedented combination of mechanical strength, ionic conductivity, and more importantly, effective suppression of Li dendrite growth. The produced lithium-based battery presents long life, high rate, and excellent safety.

Multilayer Graphene Enables Higher Efficiency in Improving Thermal Conductivities of Graphene/Epoxy Composites

The effects of number of graphene layers (n) and size of multilayer graphene sheets on thermal conductivities (TCs) of their epoxy composites are investigated. Molecular dynamics simulations show that the in-plane TCs of graphene sheets and the TCs across the graphene/epoxy interface simultaneously increase with increasing n. However, such higher TCs of multilayer graphene sheets will not translate into higher TCs of bulk composites unless they have large lateral sizes to maintain their aspect ratios comparable to the monolayer counterparts. The benefits of using large, multilayer graphene sheets are confirmed by experiments, showing that the composites made from graphite nanoplatelets (n > 10) with over 30 μm in diameter deliver a TC of ∼1.5 W m(-1) K(-1) at only 2.8 vol %, consistently higher than those containing monolayer or few-layer graphene at the same graphene loading. Our findings offer a guideline to use cost-effective multilayer graphene as conductive fillers for various thermal management applications.

Suppressing Self-Discharge and Shuttle Effect of Lithium–Sulfur Batteries with V2O5-Decorated Carbon Nanofiber Interlayer

V2 O5 decorated carbon nanofibers (CNFs) are prepared and used as a multifunctional interlayer for a lithium-sulfur (Li-S) battery. V2 O5 anchored on CNFs can not only suppress the shuttle effect of polysulfide by the strong adsorption and redox reaction, but also work as a high-potential dam to restrain the self-discharge behavior in the battery. As a result, Li-S batteries with a high capacity and long cycling life can be stored and rested for a long time without obvious capacity fading.

A carbon sandwich electrode with graphene filling coated by N-doped porous carbon layers for lithium–sulfur batteries

A sheet-like carbon sandwich, which contains a graphene layer as the conductive filling with N-doped porous carbon layers uniformly coated on both sides, is designed as a novel sulfur reservoir for lithium–sulfur batteries and experimentally obtained by a hydrothermal process of a mixture of graphene oxide, glucose and pyrrole, followed by KOH activation. In the hydrothermal process, graphene oxide is both employed as the precursor for the central graphene filling and a sheet-like template for both-side formation of N-doped porous carbon layers, resulting in an N-doped carbon sandwich structure (N-CS). This carbon sandwich is about 50–70 nm in thickness and has a high specific surface area (∼2677 m2 g−1) and a large pore volume (∼1.8 cm3 g−1), making it a promising high capacity reservoir for sulfur and polysulfide in a lithium–sulfur cell. The sheet-like morphology and the interconnected pore structure of the N-CS, together with a nitrogen content of 2.2%, are transformed to the assembled N-CS/sulfur cell with a high rate performance and excellent cycling stability because of fast ion diffusion and electron transfer. At a 2C rate, the reversible capacity is up to 625 mA h g−1 and remains at 461 mA h g−1 after 200 cycles with only 0.13% capacity fading per cycle. More interestingly, the sheet-like structure helps the N-CS materials form a tightly stacked coating on an electrode sheet, guaranteeing a volumetric capacity as high as 350 mA h cm−3.