Keyu Xie

Materials and Structures for Stretchable Energy Storage and Conversion Devices

Stretchable energy storage and conversion devices (ESCDs) are attracting intensive attention due to their promising and potential applications in realistic consumer products, ranging from portable electronics, bio-integrated devices, space satellites, and electric vehicles to buildings with arbitrarily shaped surfaces. Material synthesis and structural design are core in the development of highly stretchable supercapacitors, batteries, and solar cells for practical applications. This review provides a brief summary of research development on the stretchable ESCDs in the past decade, from structural design strategies to novel materials synthesis. The focuses are on the fundamental insights of mechanical characteristics of materials and structures on the performance of the stretchable ESCDs, as well as challenges for their practical applications. Finally, some of the important directions in the areas of material synthesis and structural design facing the stretchable ESCDs are discussed.

Direct and Seamless Coupling of TiO2 Nanotube Photonic Crystal to Dye-Sensitized Solar Cell: A Single-Step Approach

A TiO(2) nanotube layer with a periodic structure is used as a photonic crystal to greatly enhance light harvesting in TiO(2) nanotube-based dye-sensitized solar cells. Such a tube-on-tube structure fabricated by a single-step approach facilitates good physical contact, easy electrolyte infiltration, and efficient charge transport. An increase of over 50% in power conversion efficiency is obtained in comparison to reference cells without a photonic crystal layer (under similar total thickness and dye loading).

Highly Flexible Graphene/Mn3O4 Nanocomposite Membrane as Advanced Anodes for Li-Ion Batteries

Advanced electrode design is crucial in the rapid development of flexible energy storage devices for emerging flexible electronics. Herein, we report a rational synthesis of graphene/Mn3O4 nanocomposite membranes with excellent mechanical flexibility and Li-ion storage properties. The strong interaction between the large-area graphene nanosheets and long Mn3O4 nanowires not only enables the membrane to endure various mechanical deformations but also produces a strong synergistic effect of enhanced reaction kinetics by providing enlarged electrode/electrolyte contact area and reduced electron/ion transport resistance. The mechanically robust membrane is explored as a freestanding anode for Li-ion batteries, which delivers a high specific capacity of ∼800 mAh g(-1) based on the total electrode mass, along with superior high-rate capability and excellent cycling stability. A flexible full Li-ion battery is fabricated with excellent electrochemical properties and high flexibility, demonstrating its great potential for high-performance flexible energy storage devices.

Design and coupling of multifunctional TiO2 nanotube photonic crystal to nanocrystalline titania layer as semi-transparent photoanode for dye-sensitized solar cell

In order to conquer the challenges in the current technology to couple a dye-sensitized solar cell (DSSC) with a photonic crystal (PC), we propose a novel design of PC-based photoanode, composed of a thick TiO2 nanoparticle absorption layer and a thin TiO2 nanotube photonic crystal (TiO2 NT PC) membrane. In this architecture, the bandgap of TiO2 NT PC can be precisely tailored by modulating the anodization parameters using the current-pulse anodization process. Owing to the selective reflectivity of TiO2 NT PC, the power conversion efficiency (PCE) of the electrodes reveals a strong wavelength dependence. Strategies to enhance the efficiency of the newly designed DSSC and its relation to the selective reflection of the photoanode are discussed and evaluated by experimental and simulated results. Meanwhile, the functionality of TiO2 NT PC, offering both PC and light-scattering effects, has also been clarified. The combined effects of PC and light-scattering yield the maximum enhancement in PCE (39.5%) when the tailored TiO2 NT PC, with the best matching of its reflectance maximum to the dye absorption maximum, is integrated into a DSSC. The work presented here provides new insights into the design and tailoring of a photonic crystal to enhance the PCE of DSSCs for practical applications.

Synthesis of ultralong MnO/C coaxial nanowires as freestanding anodes for high-performance lithium ion batteries

A facile synthesis strategy is reported for the preparation of a freestanding membrane of ultralong MnO/C coaxial nanowires using a novel in situ interfacial polymerization technique. The MnO/C membrane possesses interconnected porous structures with a nanowire diameter of ca. 100 nm and a length of up to hundreds of micrometers. When used as a freestanding anode for lithium ion batteries, the coaxial MnO/C nanocomposites exhibit a high reversible capacity of 832 mA h g−1 at a current density of 100 mA g−1 after 100 cycles, good rate capability and outstanding cycling stability with a specific capacity of 480 mA h g−1 being retained after 600 cycles at a high current density of 1000 mA g−1. The uniform carbon coating formed along the ultralong one-dimensional nanostructure surface is the key-enabling factor that not only improves the electrode reaction kinetics, but also renders excellent cycling performance by accommodating the large volume variation of MnO during charge/discharge processes. The superior electrochemical properties suggest that the facile synthesis strategy can be extended to the fabrication of other freestanding films for potential application in energy storage systems.

Carbon Nanotube–Multilayered Graphene Edge Plane Core–Shell Hybrid Foams for Ultrahigh‐Performance Electromagnetic‐Interference Shielding

Materials with an ultralow density and ultrahigh electromagnetic-interference (EMI)-shielding performance are highly desirable in fields of aerospace, portable electronics, and so on. Theoretical work predicts that 3D carbon nanotube (CNT)/graphene hybrids are one of the most promising lightweight EMI shielding materials, owing to their unique nanostructures and extraordinary electronic properties. Herein, for the first time, a lightweight, flexible, and conductive CNT-multilayered graphene edge plane (MLGEP) core-shell hybrid foam is fabricated using chemical vapor deposition. MLGEPs are seamlessly grown on the CNTs, and the hybrid foam exhibits excellent EMI shielding effectiveness which exceeds 38.4 or 47.5 dB in X-band at 1.6 mm, while the density is merely 0.0058 or 0.0089 g cm-3 , respectively, which far surpasses the best values of reported carbon-based composite materials. The grafted MLGEPs on CNTs can obviously enhance the penetration losses of microwaves in foams, leading to a greatly improved EMI shielding performance. In addition, the CNT-MLGEP hybrids also exhibit a great potential as nano-reinforcements for fabricating high-strength polymer-based composites. The results provide an alternative approach to fully explore the potentials of CNT and graphene, for developing advanced multifunctional materials.

Ferroelectric-Enhanced Polysulfide Trapping for Lithium–Sulfur Battery Improvement

A brand new polysulfide entrapping strategy based on the ferroelectric effect has been demonstrated for the first time. By simply adding the nano-ferroelectrics (BaTiO3 nanoparticles) into the cathode, the heteropolar polysulfides can be anchored within the cathode due to the internal electric field originated from the spontaneous polarization BaTiO3 nanoparticles, and thus significantly improving the cycle stability of Li-S batteries.

Aperiodic TiO2 Nanotube Photonic Crystal: Full-Visible-Spectrum Solar Light Harvesting in Photovoltaic Devices

Bandgap engineering of a photonic crystal is highly desirable for photon management in photonic sensors and devices. Aperiodic photonic crystals (APCs) can provide unprecedented opportunities for much more versatile photon management, due to increased degrees of freedom in the design and the unique properties brought about by the aperiodic structures as compared to their periodic counterparts. However, many efforts still remain on conceptual approaches, practical achievements in APCs are rarely reported due to the difficulties in fabrication. Here, we report a simple but highly controllable current-pulse anodization process to design and fabricate TiO2 nanotube APCs. By coupling an APC into the photoanode of a dye-sensitized solar cell, we demonstrate the concept of using APC to achieve nearly full-visible-spectrum light harvesting, as evidenced by both experimental and simulated results. It is anticipated that this work will lead to more fruitful practical applications of APCs in high-efficiency photovoltaics, sensors and optoelectronic devices.

Enabling effective polysulfide trapping and high sulfur loading via a pyrrole modified graphene foam host for advanced lithium–sulfur batteries

Traditional Li-ion batteries are facing problems due to the intrinsic limitation of their low energy density. As one type of promising Li battery, lithium–sulfur (Li–S) batteries have an advantage of high theoretical energy density (2600 W h kg−1). Nevertheless, the notorious polysulfide shuttle and low sulfur loading are the main obstacles to widespread practical utilization of Li–S batteries. To tackle these issues, we design and construct a pyrrole modified graphene aerogel foam (Py-GF) by a simple hydrothermal and freeze drying method as the sulfur host, where pyrrole provides strong chemical bonding for polysulfide anchoring and graphene aerogel foam serves as a matrix to enhance the conductivity as well as increase the sulfur loading of the cathode simultaneously. The Py-GF@S cathode, with a high sulfur loading of about 6.2 mg cm−2, displays an improved initial specific capacity (1220 mA h g−1 at 0.2C and 985.8 mA h g−1 at 0.5C) and cycle stability (capacity retention of 81% after 100 cycles at 0.5C). We anticipate that the work described here will be helpful to develop Li–S batteries that meet the requirements of practical applications.

Fabrication of a novel TiO2/S composite cathode for high performance lithium–sulfur batteries

Sulfur is an attractive cathode material with a high specific capacity of 1675 mA h g−1, but its rapid capacity decay due to polysulfide dissociation presents a significant technical challenge. Here, we present the fabrication of a TiO2/S composite cathode by encapsulating elemental sulfur into TiO2 nanotube hosts for high performance lithium–sulfur batteries. A high capacity of 913 mA h g−1 has been achieved at a rate of 0.2C in the initial cycle for the TiO2/S composite cathode with a sulfur content of 65 wt% and the reversible capacity remains as high as 851 mA h g−1 after 100 cycles. The improvements of electrochemical performances were attributed to the good dispersion of sulfur in the TiO2 nanotubes and the excellent adsorbing effect on polysulfides of TiO2.