Guoqing Xin

High-rate lithiation-induced reactivation of mesoporous hollow spheres for long-lived lithium-ion batteries

Mechanical and chemical degradations of high-capacity anodes, resulting from lithiation-induced stress accumulation, volume expansion and pulverization, and unstable solid-electrolyte interface formation, represent major mechanisms of capacity fading, limiting the lifetime of electrodes for lithium-ion batteries. Here we report that the mechanical degradation on cycling can be deliberately controlled to finely tune mesoporous structure of the metal oxide sphere and optimize stable solid-electrolyte interface by high-rate lithiation-induced reactivation. The reactivated Co3O4 hollow sphere exhibits a reversible capacity above its theoretical value (924 mAh g(-1) at 1.12 C), enhanced rate performance and a cycling stability without capacity fading after 7,000 cycles at a high rate of 5.62 C. In contrast to the conventional approach of mitigating mechanical degradation and capacity fading of anodes using nanostructured materials, high-rate lithiation-induced reactivation offers a new perspective in designing high-performance electrodes for long-lived lithium-ion batteries.

Highly thermally conductive and mechanically strong graphene fibers.

A superior mix of big and small Graphene is often described as an unrolled carbon nanotube. However, although nanotubes are known for their exceptional mechanical and conductivity properties, the same is not true of graphene-based fibers. Xin et al. intercalated small fragments of graphene into the gaps formed by larger graphene sheets that had been coiled into fibers. Once annealed, the large sheets provided pathways for conduction, while the smaller fragments helped reinforce the fibers. The result? Superior thermal and electrical conductivity and mechanical strength. Science, this issue p. 1083 Intercalated graphene sheets form compact, ordered fibers with enhanced thermal conductivity and mechanical properties. Graphene, a single layer of carbon atoms bonded in a hexagonal lattice, is the thinnest, strongest, and stiffest known material and an excellent conductor of heat and electricity. However, these superior properties have yet to be realized for graphene-derived macroscopic structures such as graphene fibers. We report the fabrication of graphene fibers with high thermal and electrical conductivity and enhanced mechanical strength. The inner fiber structure consists of large-sized graphene sheets forming a highly ordered arrangement intercalated with small-sized graphene sheets filling the space and microvoids. The graphene fibers exhibit a submicrometer crystallite domain size through high-temperature treatment, achieving an enhanced thermal conductivity up to 1290 watts per meter per kelvin. The tensile strength of the graphene fiber reaches 1080 megapascals.

Large‐Area Freestanding Graphene Paper for Superior Thermal Management

Large-area freestanding graphene papers (GPs) are fabricated by electrospray deposition integrated with a continuous roll-to-roll process. Upon mechanical compaction and thermal annealing, GPs can achieve a thermal conductivity of as high as 1238.3-1434 W m(-1) K(-1) . The super-thermally conductive GPs display an outstanding heat-spread ability and are more efficient in removing hot spots than Cu and Al foils.

Organic–Inorganic Heterointerfaces for Ultrasensitive Detection of Ultraviolet Light

The performance of graphene field-effect transistors is limited by the drastically reduced carrier mobility of graphene on silicon dioxide (SiO2) substrates. Here we demonstrate an ultrasensitive ultraviolet (UV) phototransistor featuring an organic self-assembled monolayer (SAM) sandwiched between an inorganic ZnO quantum dots decorated graphene channel and a conventional SiO2/Si substrate. Remarkably, the room-temperature mobility of the chemical-vapor-deposition grown graphene channel on the SAM is an order-of-magnitude higher than on SiO2, thereby drastically reducing electron transit-time in the channel. The resulting recirculation of electrons (in the graphene channel) within the lifetime of the photogenerated holes (in the ZnO) increases the photoresponsivity and gain of the transistor to ∼10(8) A/W and ∼3 × 10(9), respectively with a UV to visible rejection ratio of ∼10(3). Our UV photodetector device manufacturing is also compatible with current semiconductor processing, and suitable for large volume production.

Advanced phase change composite by thermally annealed defect-free graphene for thermal energy storage

Organic phase change materials (PCMs) have been utilized as latent heat energy storage and release media for effective thermal management. A major challenge exists for organic PCMs in which their low thermal conductivity leads to a slow transient temperature response and reduced heat transfer efficiency. In this work, 2D thermally annealed defect-free graphene sheets (GSs) can be obtained upon high temperature annealing in removing defects and oxygen functional groups. As a result of greatly reduced phonon scattering centers for thermal transport, the incorporation of ultralight weight and defect free graphene applied as nanoscale additives into a phase change composite (PCC) drastically improve thermal conductivity and meanwhile minimize the reduction of heat of fusion. A high thermal conductivity of the defect-free graphene-PCC can be achieved up to 3.55 W/(m K) at a 10 wt % graphene loading. This represents an enhancement of over 600% as compared to pristine graphene-PCC without annealing at a comparable loading, and a 16-fold enhancement than the pure PCM (1-octadecanol). The defect-free graphene-PCC displays rapid temperature response and superior heat transfer capability as compared to the pristine graphene-PCC or pure PCM, enabling transformational thermal energy storage and management.

Cl‐Doped ZnO Nanowire Arrays on 3D Graphene Foam with Highly Efficient Field Emission and Photocatalytic Properties

An environmentally friendly, low-cost, and large-scale method is developed for fabrication of Cl-doped ZnO nanowire arrays (NWAs) on 3D graphene foam (Cl-ZnO NWAs/GF), and investigates its applications as a highly efficient field emitter and photocatalyst. The introduction of Cl-dopant in ZnO increases free electrons in the conduction band of ZnO and also leads to the rough surface of ZnO NWAs, which greatly improves the field emission properties of the Cl-ZnO NWAs/GF. The Cl-ZnO NWAs/GF demonstrates a low turn-on field (≈1.6 V μm(-1)), a high field enhancement factor (≈12844), and excellent field emission stability. Also, the Cl-ZnO NWAs/GF shows high photocatalytic efficiency under UV irradiation, enabling photodegradation of organic dyes such as RhB within ≈75 min, with excellent recyclability. The excellent photocatalytic performance of the Cl-ZnO NWAs/GF originates from the highly efficient charge separation efficiency at the heterointerface of Cl-ZnO and GF, as well as improved electron transport efficiency due to the doping of Cl. These results open up new possibilities of using Cl-ZnO and graphene-based hybrid nanostructures for various functional devices.

Electrospray deposition of a Co3O4 nanoparticles–graphene composite for a binder-free lithium ion battery electrode

A binder-free anode was fabricated from a Co3O4–graphene composite by direct electrospray deposition on current collectors. The highly-interconnected mesoporous structure enables fast ion transport and accommodates the volume expansion of Co3O4 upon Li+ insertion/extraction. The incorporation of highly conductive graphene also improves the charge transport and thus the electrochemical performance.

Microstructure control of macroscopic graphene paper by electrospray deposition and its effect on thermal and electrical conductivities

Macroscopic graphene paper is fabricated by an electrospray deposition approach, and the microstructure can be controlled from highly porous to highly compact geometries by varying deposition parameters including graphene colloid concentration and deposition rate. Free-standing graphene films can be separated from substrates via a simple water exfoliation method in which the surface properties of graphene films and substrates control film exfoliation. Specifically, water exfoliation can be achieved when the contact angle of substrates is 64° or below. Thermal and electrical conductivities of the macroscopic graphene paper upon thermal annealing are measured, enabling the establishment of the process-microstructure-property correlation beneficial for further development and property manipulation of graphene-based materials.

Tunable optical properties and stability of lead free all inorganic perovskites (Cs2SnIxCl6−x)

Organic–inorganic hybrid lead-based perovskites experience significant environmental instability under ambient moist air and are not environmentally benign due to the usage of toxic Pb. Here, we report a new approach to synthesize lead-free all inorganic perovskites (Cs2SnIxCl6−x) using hydriodic acid (HI) demonstrating greatly enhanced environmental stability and tunable optical properties by controlling the I−/Cl− ratios. Single phase perovskites can be achieved with a low iodine or chlorine content, and a phase separation occurs in the binary system with closer iodine and chlorine dopings. UV-vis diffuse reflectance and photoluminescence measurements reveal tunable band gaps of Cs2SnIxCl6−x perovskites from the UV to the infrared region. The mixed halide perovskite with a lower chloride content shows significantly higher photoluminescence intensity. The thermal stability of mixed halide all-inorganic perovskites is continuously improved as the Cl content increases. The synthesis of Sn-based perovskites with tunable optical properties and environmental stability represents one step further toward the realization of the stable lead-free all inorganic perovskites.