Hengcai Wu

Super-aligned carbon nanotube/graphene hybrid materials as a framework for sulfur cathodes in high performance lithium sulfur batteries

We report the use of super-aligned carbon nanotube/graphene (CNT/G) hybrid materials as a 3D conducting framework for sulfur accommodation. The CNT network acts as a skeleton to form a self-sustained cathode that is binder-free, highly conductive, and flexible. Graphene with a 2D sheet structure extends in an additional dimension to provide improved restriction for sulfur/polysulfides. Moreover, the CNT/G hybrid framework enables better dispersion of sulfur and allows each sulfur particle to closely attach to the conductive components, which greatly enhance the electronic conductivity and thereby approach the full potential of the active materials. With an optimized CNT/G ratio in the framework, the S–CNT/G nanocomposite exerts improved mechanical performance and favorable electrochemical characteristics compared to the S–CNT composite. Based on its superior structure, the S–CNT/G nanocomposite achieves a high discharge capacity of 1048 mA h g−1 at 1 C with a capacity fade as low as 0.041% per cycle over 1000 charge–discharge cycles. Excellent high-rate and long-term cycling performances are also revealed. These results demonstrate the great potential of the S–CNT/G nanocomposite as a flexible and binder-free cathode for Li–S batteries.

Entrapping electrode materials within ultrathin carbon nanotube network for flexible thin film lithium ion batteries

A novel design of a flexible thin film electrode for lithium ion batteries is reported. We employ ordered carbon nanotube (CNT) film, directly pulled from aligned CNT arrays, as a flexible skeleton. The functional electrode material is introduced by a one-step spray-painting approach. The electrode is self-sustained as a result of the strong interactions among CNTs. In such an electrode configuration, the CNT network acts as micro electron pathways and its excellent mechanical properties also ensure flexibility. The electrodes fabricated in this way are electrochemically and mechanically superior in comparison with those prepared by the traditional slurry cast method. A full battery that contains a LiFePO4 cathode and a Li4Ti5O12 anode exhibits a high areal capacity over 200 μA h cm−2, a stable output voltage of 1.82 V, excellent reversibility, high flexibility, and light polarization in both flat and bent conditions. As a result, we suggest such electrodes hold great promise for thin film lithium ion batteries to satisfy energy storage demand in revolutionary portable electronics.

Self‐assembly of 3D Carbon Nanotube Sponges: A Simple and Controllable Way to Build Macroscopic and Ultralight Porous Architectures

Macroscopic and 3D superaligned CNT (SACNT) sponges are fabricated through a simple, low-cost, controllable, and scalable self-assembly method without using organic binder. Sponges with specific shapes and densities can be achieved. SACNT sponges are ultralight (1-50 mg cm-3 ), highly porous (97.5%-99.9%) with honeycomb-like hierarchical structure, and highly conductive. Using SACNT sponges as templates, various materials with honeycomb-like structure can be obtained for wide applications.

Cross-stacked carbon nanotube film as an additional built-in current collector and adsorption layer for high-performance lithium sulfur batteries.

Cross-stacked carbon nanotube (CNT) film is proposed as an additional built-in current collector and adsorption layer in sulfur cathodes for advanced lithium sulfur (Li-S) batteries. On one hand, the CNT film with high conductivity, microstructural rough surface, high flexibility and mechanical durability retains stable and direct electronic contact with the sulfur cathode materials, therefore decreasing internal resistivity and suppressing polarization of the cathode. On the other hand, the highly porous structure and the high surface area of the CNT film provide abundant adsorption points to support and confine sulfur cathode materials, alleviate their aggregation and promote high sulfur utilization. Moreover, the lightweight and compact structure of the CNT film adds no extra weight or volume to the sulfur cathode, benefitting the improvement of energy densities. Based on these characteristics, the sulfur cathode with a 100-layer cross-stacked CNT film presents excellent rate performances with capacities of 986, 922 and 874 mAh g(-1) at cycling rates of 0.2C, 0.5C and 1C for sulfur loading of 60 wt%, corresponding to an improvement of 52%, 109% and 146% compared to that without a CNT film. Promising cycling performances are also demonstrated, offering great potential for scaled-up production of sulfur cathodes for Li-S batteries.

Multifunctional Interlayer Based on Molybdenum Diphosphide Catalyst and Carbon Nanotube Film for Lithium–Sulfur Batteries

A multifunctional interlayer, composed of molybdenum diphosphide (MoP2 ) nanoparticles and a carbon nanotube (CNT) film, is introduced into a lithium-sulfur (Li-S) battery system to suppress polysulfide migration. Molybdenum diphosphide acts as the catalyst and can capture polysulfides and improve the polysulfide conversion activity during the discharge/charge processes. The CNT film acts as a conductive skeleton to support the MoP2 nanoparticles and to ensure their uniform distribution. The CNT film physically hinders polysulfide migration, acts as a current collector, and provides abundant electron pathways. The Li-S battery containing the multifunctional MoP2 /CNT interlayer exhibits excellent electrochemical performance. It delivers a reversible specific capacity of 905 mA h g-1 over 100 cycles at 0.2 C, with a capacity decay of 0.152% per cycle. These results suggest the introduction of the multifunctional CNT/MoP2 interlayer as an effective and practical method for producing high-performance Li-S batteries.

Sandwich-structured cathodes with cross-stacked carbon nanotube films as conductive layers for high-performance lithium-ion batteries

A simple and feasible strategy of using cross-stacked super-aligned carbon nanotube (SACNT) films as conductive layers to prepare sandwich-structured LiCoO2 cathodes for high-performance lithium-ion batteries (LIBs) is reported. Owing to the super-aligned feature, the SACNTs are fully dispersed and form a homogeneous and efficient conductive network in the electrodes. Meanwhile, the sandwiched electrode structure, consisting of a repeating and alternating stack of LiCoO2 layers and SACNT films, ensures that each layer of active materials can adhere to the SACNT conductive layers, realizing sufficient electron transfer throughout the electrodes regardless of the thickness of the electrodes. With the introduction of three separate SACNT conductive layers, significant improvements on the conductivity as well as the cell performance are achieved. The sandwich-structured LiCoO2–2 wt% Super P–SACNT cathodes possess an impressive rate capability (109.6 mA h g−1 at 10C and 1668% improvement compared with that without SACNT films), showing the best rate performances reported so far for commercial micro-sized LiCoO2 particles. The easy fabrication procedure, compatible method for commercialization, low cost, and outstanding electrochemical performances of the sandwich-structured electrode demonstrate its great potential for the large-scale production of high-performance electrodes for LIBs.

Self-Expansion Construction of Ultralight Carbon Nanotube Aerogels with a 3D and Hierarchical Cellular Structure

A novel and simple strategy is developed to construct ultralight and 3D pure carbon nanotube (CNT) aerogels by the spontaneous expansion of superaligned CNT films soaked in a piranha (mixed H2 SO4 and H2 O2 ) solution, followed by cryodesiccation. The macroscopic CNT aerogels have an extremely low apparent density (0.12 mg cm-3 ), ultrahigh porosity (99.95%), high specific surface area (298 m2 g-1 ), and a hierarchical cellular structure with giant and ultrathin CNT sheets as cell walls. The pure CNT aerogels show high adsorption abilities for various kinds of solvents, and have great potential in widespread applications such as energy storage, catalysis, and bioengineering.

Synergistic effect of manganese oxide nanoparticles and graphene nanosheets in composite anodes for lithium ion batteries

A graphene-Mn3O4-graphene (GMG) sandwich structure with homogeneous anchoring of Mn3O4 nanoparticles among flexible and conductive graphene nanosheets (GSs) is achieved through dispersion of the GSs in Mn(NO3)2 solution and subsequent calcination. Mn3O4 nanoparticles are 50 ~ 200 nm clusters consisting of 10 ~ 20 nm primary particles, and serve as spacers to prevent the re-stacking of the GSs. GSs provide a highly conductive network among Mn3O4 nanoparticles for efficient electron transfer and buffer any volume change during cycling. Due to the strong synergistic effect between Mn3O4 and GSs, the capacity contributions from GSs and Mn3O4 in GMG are much larger than capacities of pure GSs and Mn3O4. Consequently, the GMG composite electrodes show excellent electrochemical properties for lithium ion battery applications, demonstrating a large reversible capacity of 750 mAh g−1 at 0.1 C based on the mass of GMG with no capacity fading after 100 cycles, and high rate abilities of 500 mAh g−1 at 5 C and 380 mAh g−1 at 10 C.

Ultrastretchable carbon nanotube composite electrodes for flexible lithium-ion batteries

Ultra-stretchable carbon nanotube (CNT) composite electrodes for lithium-ion batteries are fabricated by coating CNT films and active material powders on biaxially pre-strained polydimethylsiloxane (PDMS) substrates. The wrinkled structures that form during the pre-straining and release process extend along the strain axis to protect the CNT composite structures from fracture. The CNT composites demonstrate excellent stability and high durability with resistance increase of less than 12% after 2000 cycles at 150% strain. Both CNT/Li4Ti5O12 (LTO) anodes and CNT/Li(Ni1/3Co1/3Mn1/3)O2 (NCM) cathodes maintain excellent electrochemical properties at cyclic 150% strain in different axes. The full lithium-ion battery consisting of the stretchable CNT/LTO anode and CNT/NCM cathode is able to withstand 150% strain in different axes without large decreases in performance. Stretchable batteries fabricated by the scalable, highly efficient, and low-cost biaxial pre-strain process with excellent durability and electrochemical properties will have potential applications in flexible devices.

TiO2-Nanocoated Black Phosphorus Electrodes with Improved Electrochemical Performance

Black phosphorus (BP) is a promising electrode material with high energy density for lithium-ion batteries. However, volumetric expansion of BP upon lithiation leads to rapid capacity fading of the electrode. Herein, BP composite electrodes are prepared by mixing microsized BP particles with carbon nanotubes and KetjenBlack as dual conducting agents, which facilitate the construction of stable and conductive networks in the electrodes. An ultrathin TiO2 nanocoating is deposited on the surface of the BP composite electrode by electron-beam evaporation. The TiO2 nanocoating acts as a protective layer to prevent the BP particles from directly contacting the electrolyte by forming a Li xTi yO z passivation coating on the electrode surface. The Li xTi yO z passivation layer suppresses propagation of the formed irreversible solid electrolyte interlayer on the BP particles, resulting in an improved Coulombic efficiency of the BP electrode. Moreover, the Li xTi yO z passivation layer facilitates lithium-ion diffusion and electron transfer and thus superior cycling and rate performance of the BP electrode are achieved.