Qun Guan

Needle-like Co3O4 Anchored on the Graphene with Enhanced Electrochemical Performance for Aqueous Supercapacitors

We synthesized the needle-like cobalt oxide/graphene composites with different mass ratios, which are composed of cobalt oxide (Co3O4 or CoO) needle homogeneously anchored on graphene nanosheets as the template, by a facile hydrothermal method. Without the graphene as the template, the cobalt precursor tends to group into urchin-like spheres formed by many fine needles. When used as electrode materials of aqueous supercapacitor, the composites of the needle-like Co3O4/graphene (the mass ratio of graphene oxide(GO) and Co(NO3)2·6H2O is 1:5) exhibit a high specific capacitance of 157.7 F g(-1) at a current density of 0.1 A g(-1) in 2 mol L(-1) KOH aqueous solution as well as good rate capability. Meanwhile, the capacitance retention keeps about 70% of the initial value after 4000 cycles at a current density of 0.2 A g(-1). The enhancement of excellent electrochemical performances may be attributed to the synergistic effect of graphene and cobalt oxide components in the unique multiscale structure of the composites.

Low Temperature Vacuum Synthesis of Triangular CoO Nanocrystal/Graphene Nanosheets Composites with Enhanced Lithium Storage Capacity

CoO nanocrystal/graphene nanosheets (GNS) composites, consisting of a triangular CoO nanocrystal of 2~20 nm on the surface of GNS, are synthesized by a mild synthetic method. First, cobalt acetate tetrahydrate is recrystallized in the alcohol solution at a low temperature. Then, graphene oxide mixed with cobalt-precursor followed by high vacuum annealing to form the CoO nanocrystal/GNS composites. The CoO nanocrystal/GNS composites exhibit a high reversible capacity of 1481.9 m Ah g−1 after 30 cycles with a high Coulombic efficiency of over 96% when used as anode materials for lithium ion battery. The excellent electrochemical performances may be attributed to the special structure of the composites. The well-dispersed triangular CoO nanocrystal on the substrate of conductive graphene can not only have a shorter diffusion length for lithium ions, better stress accommodation capability during the charge-discharge processes and more accessible active sites for lithium-ion storage and electrolyte wetting, but also possess a good conductive network, which can significantly improve the whole electrochemical performance.

Synthesis of a porous sheet-like V2O5–CNT nanocomposite using an ice-templating ‘bricks-and-mortar’ assembly approach as a high-capacity, long cyclelife cathode material for lithium-ion batteries

Tailoring the nanostructures of electrode materials is an effective way to enhance their electrochemical performance for energy storage. Herein, an ice-templating “bricks-and-mortar” assembly approach is reported to make ribbon-like V2O5 nanoparticles and CNTs integrated into a two-dimensional (2D) porous sheet-like V2O5–CNT nanocomposite. The obtained sheet-like V2O5–CNT nanocomposite possesses unique structural characteristics, including a hierarchical porous structure, 2D morphology, large specific surface area and internal conducting networks, which lead to superior electrochemical performances in terms of long-term cyclability and significantly enhanced rate capability when used as a cathode material for LIBs. The sheet-like V2O5–CNT nanocomposite can charge/discharge at high rates of 5C, 10C and 20C, with discharge capacities of approximately 240 mA h g−1, 180 mA h g−1, and 160 mA h g−1, respectively. It also retains 71% of the initial discharge capacity after 300 cycles at a high rate of 5C, with only 0.097% capacity loss per cycle. The rate capability and cycling performance of the sheet-like V2O5–CNT nanocomposite are significantly better than those of commercial V2O5 and most of the reported V2O5 nanocomposite.

Facile synthesis of graphene supported ultralong TiO2 nanofibers from the commercial titania for high performance lithium-ion batteries

Novel nanocomposites consisting of two-dimensional graphene nanosheets and ultralong TiO2 nanofibers are fabricated via a simple one-pot hydrothermal reaction using commercial TiO2 particles as inorganic precursors. Complex chemical synthesis processes and high cost precursors can be avoided. When used as anodes of lithium ion batteries, the obtained nanocomposites exhibit a superior rate capability and an excellent long-term cycling stability. The nanocomposites maintain a charge capacity of 85 mA h g−1 at 20 C, while the TiO2 nanofibers fail when cycled at 5 C. The nanocomposites also demonstrate an excellent cycling stability with a charge capacity of 92 mA h g−1 after 1000 cycles at 10 C, approximately three times the capacity of the TiO2 nanofibers. The superior electrochemical performance can be attributed to the hybrid structure of the graphene nanosheets and the ultralong TiO2 nanofibers. The graphene nanosheets provide highly electronically conductive pathways and work as protected layers to keep the active material integrated during charging/discharging processes. The ultralong TiO2 nanofibers with high specific surface area have a short ion diffusion distance and provide more accessible sites. By combining the advantages of the graphene nanosheets and TiO2 nanofibers, the nanocomposites exhibit obviously improved electrochemical performances.

Sulfur quantum dots wrapped by conductive polymer shell with internal void spaces for high-performance lithium–sulfur batteries

Lithium–sulfur batteries are promising candidates for the next generation of energy storage systems due to the high specific capacity of the sulfur cathode (1675 mA h g−1) and their low cost. However, the intrinsic insulating properties of sulfur, the dissolution of polysulfides, and the huge volume expansion during cycling still hinder their practical application. We introduced the electroactive polymer poly(N-vinylcarbazole) (PVK) into the lithium–sulfur system as a conductive matrix and sulfur reservoir. Using a facile two-step dissolution–precipitation treatment, novel core–shell sulfur quantum dot/PVK (SQD/PVK) nanocomposites were synthesized, in which a large number of SQDs (about 5 nm in size) with plenty of internal void spaces were encapsulated in the PVK shell. The sulfur core consisted of uniformly dispersed SQDs and large void spaces, which formed effective transportation pathways for both electrons and ions among the SQDs. They also acted as a buffer zone to accommodate the volume expansion during cycling and facilitated wetting of the electrolyte. The conducting PVK shell coated on the surface of the sulfur core can restrain dissolution of the polysulfide and suppress the shuttle effect. Galvanostatic testing showed that this SQD/PVK nanocomposite maintained a specific capacity of 687.7 mA h g−1 after 200 cycles at 0.5 C, corresponding to an 89.7% capacity retention with only 0.05% capacity degradation per cycle. Even after long-term cycling, the electrode could still deliver 488.6 mA h g−1 at a rate of 0.5 C after 600 cycles and 443.9 mA h g−1 at 0.75 C after 500 cycles.

Twisted yarns for fiber-shaped supercapacitors based on wetspun PEDOT:PSS fibers from aqueous coagulation

Recently, fiber-shaped yarn supercapacitors (YSCs) have attracted extensive attention due to their merits of small volume, high flexibility and potential to be woven in textiles for future wearable electronics. PEDOT:PSS possesses properties of high-redox capacitance, high conductivity and high intrinsic flexibility, so PEDOT:PSS yarn electrodes are quite promising in the field of YSCs. However, to the best of our knowledge, twisted yarns based on wetspun PEDOT:PSS fibers for fiber-shaped YSCs have not been reported. Herein, we develop a new coagulation bath with CaCl2 in aqueous solution for preparing meter-long PEDOT:PSS fibers. The PEDOT:PSS fibers with good mechanical properties can be easily woven, sewed, knotted and braided as the YSC electrode. The PEDOT:PSS fiber-based YSCs show a high areal capacitance of 119 mF cm−2 and areal energy density of 4.13 μW h cm−2. Meanwhile, the all-solid-state YSCs are flexible and robust enough to tolerate the long-term and repeated bending without an obvious capacitance drop.

Polymeric cathode materials of electroactive conducting poly(triphenylamine) with optimized structures for potential organic pseudo-capacitors with higher cut-off voltage and energy density

For electrochemical capacitors or supercapacitors, pseudo-capacitors via fast surface reactions are able to store/harvest more electrical energy when compared with electrochemical double layer capacitors (EDLCs) using an ion adsorption route. A combination of pseudo-capacitive materials, including oxides, nitrides and polymers, as well as understanding the charge storage mechanism and the development of advanced nanostructures with the latest generation of nanostructured lithium electrodes has brought the energy density of electrochemical capacitors closer to that of batteries. Electroactive polymeric cathodes with designed structures via electrospinning (without polymeric additives) and surfactant-free precipitation polymerization routes were herein fabricated for the abovementioned goals. The as-prepared polymeric active materials show an electrochemical capacitance of around 200 F g−1 with a higher cut-off voltage up to 4.2 V and an energy density up to 370 Wh kg−1 and power density up to 34 kW kg−1 in an organic electrolyte system.

Multiscale sulfur particles confined in honeycomb-like graphene with the assistance of bio-based adhesive for ultrathin and robust free-standing electrode of Li–S batteries with improved performance

Carbon/sulfur composites are attracting extensive attention due to their improved performances for Li–S batteries. Herein, a hierarchical ultrathin but robust graphene composite membrane, which includes an inherent carbon framework and the embedded sulfur multi-sized particles, is facilely in situ and one-pot synthesized with the assistance of bio-based surfactant/adhesive for flexible cathode without current collectors. It shows better cycling stability and higher specific capacity of 823 mA h g−1 over 100 cycles at 0.5C with a high sulfur content of ∼61 wt% in the electrode. The introduction of biosurfactant and interfacial adhesion enhanced the flexibility and strength of the thinner composite membrane in addition to the increased specific capacity, which may show promising potential for better flexible and wearable electronic devices.

Graphene oxide hydrogel as a restricted-area nanoreactor for synthesis of 3D graphene-supported ultrafine TiO2 nanorod nanocomposites for high-rate lithium-ion battery anodes

Three-dimensional graphene-supported TiO2 nanorod nanocomposites (3D GS-TNR) are prepared using graphene oxide hydrogel as a restricted-area nanoreactor in the hydrothermal process, in which well-distributed TiO2 nanorods with a width of approximately 5 nm and length of 30 nm are conformally embedded in the 3D interconnected graphene network. The 3D graphene oxide not only works as a restricted-area nanoreactor to constrain the size, distribution and morphology of the TiO2; it also work as a highly interconnected conducting network to facilitate electrochemical reactions and maintain good structural integration when the nanocomposites are used as anode materials in lithium-ion batteries. Benefiting from the nanostructure, the 3D GS-TNR nanocomposites show high capacity and excellent long-term cycling capability at high current rates. The 3D GS-TNR composites deliver a high initial charge capacity of 280 mAh g-1 at 0.2 C and maintain a reversible capacity of 115 mAh g-1, with a capacity retention of 83% at 20 C after 1000 cycles. Meanwhile, compared with that of previously reported TiO2-based materials, the 3D GS-TNR nanocomposites show much better performance, including higher capacity, better rate capability and long-term cycling stability.

Gel-type polymer separator with higher thermal stability and effective overcharge protection of 4.2 V for secondary lithium-ion batteries

Overcharge protection by electroactive polymer composite separators is an alternative solution for the alleviation of safety concerns of rechargeable Li or Li-ion batteries. The use of gel-type electrolyte with less free organic solvent may benefit the safety performance as well as provide enhanced thermostability. Herein, a kind of gel-type polymer electrolyte separator, prepared by a facile electroactive conducting polymer solution dip-coating method, for effective overcharge protection of Li-ion batteries with enhanced thermal stability and electrochemical safety due to the introduction of heat-resistant polyfluorene end-capped with polysilsesquioxane, may be beneficial for solution of the Li-ion battery safety problem.