Mengqiang Wu

Effective thermal transport properties in multiphase biological systems containing carbon nanomaterials

Here we report computational results from an off-lattice Monte Carlo investigation of the effective thermal transport properties in multiphase biological systems containing carbon nanomaterials. A three-phase system that consists of a cell, healthy tissue and carbon nanotubes (CNTs) was built in silico for this study. The CNTs were embedded in both the cell and the healthy tissue. The effective thermal conductivity (Keff) of such biological systems can be predicted by taking into account the dispersion of the CNTs and the interfacial thermal resistances (ITRs) between any pair of components. We quantitatively investigated the effects of the distribution (CNTs at different locations in the system), concentration (0.01–0.1 vol%), and morphology (diameter of 2–10 nm, length of 200–800 nm) of the CNTs on the Keff of the biological systems. Additionally, we studied the effects of the ITRs between any pair of components (0.05–76.5 × 10−8 m2 K W−1) on the Keff of the biological systems. The results showed that greater enhancement of the Keff values of the biological systems can be achieved by using longer CNTs in higher concentration, and reducing the ITRs between the CNTs and their surroundings. Finally, CNTs embedded on the cell membrane have a stronger effect than being dispersed within the cell or in the tissue surrounding the cell.

A Facile Approach to Tune the Electrical and Thermal Properties of Graphene Aerogels by Including Bulk MoS2

Graphene aerogels (GAs) have attracted extensive interest in diverse fields, owing to their ultrahigh surface area, low density and decent electrical conductivity. However, the undesirable thermal conductivity of GAs may limit their applications in energy storage devices. Here, we report a facile hydrothermal method to modulate both the electrical and thermal properties of GAs by including bulk molybdenum disulfide (MoS2). It was found that MoS2 can help to reduce the size of graphene sheets and improve their dispersion, leading to the uniform porous micro-structure of GAs. The electrical measurement showed that the electrical conductivity of GAs could be decreased by 87% by adding 0.132 Vol % of MoS2. On the contrary, the thermal conductivity of GAs could be increased by ~51% by including 0.2 vol % of MoS2. The quantitative investigation demonstrated that the effective medium theories (EMTs) could be applied to predict the thermal conductivity of composite GAs. Our findings indicated that the electrical and thermal properties of GAs can be tuned for the applications in various fields.

Investigation of the electrochemical performance of polyvinylidene fluoride-derived LiFePO4/C composite nanospheres

The wide application of LiFePO4 (LFP) in high-power lithium-ion batteries is limited due to its two main drawbacks: poor electronic conductivity and low lithium-ion diffusivity, which can be greatly improved through a combination of reducing the LFP crystallites to nanoscale and introducing a conductive carbon coating layer. It is well accepted that the choice of carbon precursors has a significant impact on the ultimate lithium storage property of the LiFePO4/carbon (LFP/C) composite. In this work, LFP/C core–shell composite nanospheres using polyvinylidene fluoride (PVDF) as carbon source (LFP/C-PVDF) were prepared and the electrochemical performances in lithium half cells were investigated. The electrochemical properties of LFP/C composite derived from glucose (LFP/C-GLU) and the bare LFP without carbon coating were also investigated for comparison. It was found that LFP/C-PVDF displayed a higher capacity, better rate capability and smaller polarization than its LFP/C-GLU and LFP counterparts, which could be ascribed to lower surface and charge-transfer impedances, and an enhanced lithium-ion diffusivity, as revealed by electrochemical impedance spectroscopy analysis. Our study demonstrates that PVDF is a facile and potential carbon precursor for LiFePO4 in high-performance lithium-ion battery application.

Enhanced Electrochemical and Thermal Transport Properties of Graphene/MoS2 Heterostructures for Energy Storage: Insights from Multiscale Modeling

Graphene has been combined with molybdenum disulfide (MoS2) to ameliorate the poor cycling stability and rate performance of MoS2 in lithium ion batteries, yet the underlying mechanisms remain less explored. Here, we develop multiscale modeling to investigate the enhanced electrochemical and thermal transport properties of graphene/MoS2 heterostructures (GM-Hs) with a complex morphology. The calculated electronic structures demonstrate the greatly improved electrical conductivity of GM-Hs compared to MoS2. Increasing the graphene layers in GM-Hs not only improves the electrical conductivity but also stabilizes the intercalated Li atoms in GM-Hs. It is also found that GM-Hs with three graphene layers could achieve and maintain a high thermal conductivity of 85.5 W/(m·K) at a large temperature range (100-500 K), nearly 6 times that of pure MoS2 [∼15 W/(m·K)], which may accelerate the heat conduction from electrodes to the ambient. Our quantitative findings may shed light on the enhanced battery performances of various graphene/transition-metal chalcogenide composites in energy storage devices.

Variation of carbon coatings on the electrochemical performance of LiFePO4 cathodes for lithium ionic batteries

The applications of LiFePO4 (LFP) in high-power lithium ion batteries (LIBs) are limited due to its two major drawbacks: poor electronic conductivity and low lithium ion diffusivity, which could be greatly improved chiefly by reducing the size of LFP crystallites to nanoscale and introducing a conductive carbon-coating layer. In this study, asphalt-derived and glucose-derived carbon proved to be soft carbon-coating (SCC) and hard carbon-coating (HCC), respectively. Asphalt and glucose were therefore used as carbon precursors to prepare varied carbon-coated LFP nanoparticles. The electrochemical properties of the LFP/carbon composites were studied using cyclic voltammetry, electrochemical impedance spectroscopy and charge/discharge cycling. The effects of variation of carbon coatings on the electrochemical performance of LiFePO4 cathodes was investigated in detail, and it was found that LFP/SCC showed a superior performance in capacity and rate capability than that of LFP/HCC. It was therefore concluded that soft carbon coating on LFP exhibits better electrochemical performance than hard carbon coating, demonstrating that asphalt could be used as a cheap and efficient carbon source material of LiFePO4 cathodes for LIBs.

Graphene enhanced silicon/carbon composite as anode for high performance lithium-ion batteries

Silicon-based anode materials for lithium ion batteries (LIBs) have become a hot research topic due to their remarkably high theoretical capacity (4200 mA h g−1). However, the large volume change (>300%) of Si electrodes during the lithium ion insertion/extraction process leads to a rapid decay of the reversible capacity. In our report, carbon/graphene double-layer coated-silicon composite (Si/carbon/graphene, Si/C/G) is prepared via a facile hydrothermal process. It is demonstrated that the Si/C/G composite displayed an exceedingly ameliorated electrochemical performance in both cycling stability and rate capability. The specific capacity of the Si/C/G electrode is maintained at 2469 mA h g−1 after 50 cycles under 0.2 A g−1, and above 1500 mA h g−1 after 300 cycles at 2 A g−1. More notably, even at an ultrahigh rate of 32 A g−1, the specific capacity could still reach 471 mA h g−1. Hence the presented simple approach enables massive fabrication of the Si/C/G composite as a promising anode material for high performance LIBs.

Graphene coated Co3V2O8 micro-pencils for enhanced-performance in lithium ion batteries

Transition metal vanadates have attracted much attention for high capacity anodes of lithium ion batteries (LIBs). However, they have obvious drawbacks (short cycle-lives and low rate performance) because of the intrinsically low electronic conductivity and serious volume variation during Li-ion desorption and insertion. In particular, pure Co3V2O8 micro-pencils (pCVO MPs) have a stable and regular crystal structure, large tap density and uniform grain size, but, unfortunately, they have not exhibited expected electrochemical performance. Herein, we report the successful preparation of reduced graphene oxide coated Co3V2O8 micro-pencils (rGO@CVO MPs) through a facile approach combining hydrothermal synthesis with thermal reduction. When tested as anodes for LIBs, rGO@CVO MPs exhibit superior electrochemical performance compared to that of pure Co3V2O8 micro-pencils (pCVO MPs). The anodes of rGO@CVO MPs show a high reversible capacity of 760 mA h g−1 over 200 cycles at 200 mA g−1, and 500 mA h g−1 can remain after 500 cycles at 1000 mA g−1, with an increase in 200 mA h g−1 in contrast to the pCVO MPs. It is consequently demonstrated that the composite material (rGO@CVO MPs) is a promising anode material for LIBs.

Uniform Co3V2O8 microspheres via controllable assembly for high-performance lithium-ion battery anodes

As recently explored lithium-ion battery (LIB) anode materials, transition metal vanadates have attracted much attention due to their excellent electrochemical performance. In this work, we successfully synthesized solid Co3V2O8 microspheres (sCVO MSs) via a facile and controllable hydrothermal reaction, and the magical formation mechanism was also revealed by the different morphologies of sCVO MSs at different reaction times. When tested as LIB anode materials, sCVO MSs showed superior reversible capacities (∼780 mA h g−1 after 250 cycles at 1000 mA g−1 and as high as 550 mA h g−1 even after 400 cycles at 5000 mA g−1) and excellent rate performance (a capacity of ∼500 mA h g−1 at a current density of 2000 mA g−1). It is accordingly demonstrated that uniform sCVO MSs have great potential to be used as high-performance LIB anodes.