Zhenglong Xu

Electrospun Carbon Nanofibers with in Situ Encapsulated Co3O4 Nanoparticles as Electrodes for High-Performance Supercapacitors

A facile electrospinning method with subsequent heat treatments is employed to prepare carbon nanofibers (CNFs) containing uniformly dispersed Co3O4 nanoparticles as electrodes for supercapacitors. The Co3O4/CNF electrodes with ∼68 wt % active particles deliver a remarkable capacitance of 586 F g(-1) at a current density of 1 A g(-1). When the current density is increased to 50 A g(-1), ∼66% of the original capacitance is retained. The electrodes also present excellent cyclic stability of 74% capacity retention after 2000 cycles at 2 A g(-1). These superior electrochemical properties are attributed to the uniform dispersion of active particles in the CNF matrix, which functions as a conductive support. The onionlike graphitic layers formed around the Co3O4 nanoparticles not only improve the electrical conductivity of the electrode but also prevent the separation of the nanoparticles from the carbon matrix.

Cobalt Carbonate/ and Cobalt Oxide/Graphene Aerogel Composite Anodes for High Performance Li-Ion Batteries

Nanocomposites consisting of ultrafine, cobalt carbonate nanoneedles and 3D porous graphene aerogel (CoCO3/GA) are in situ synthesized based on a one-step hydrothermal route followed by freeze-drying. A further heat treatment produces cobalt oxide nanoparticles embedded in the conductive GA matrix (Co(3)O(4)/GA). Both the composite anodes deliver excellent specific capacities depending on current density employed: the CoCO(3)/GA anode outperforms the Co(3)O(4)/GA anode at low current densities, and vice versa at current densities higher than 500 mA g(-1). Their electrochemical performances are considered among the best of similar composite anodes consisting of CoCO(3) or Co(3)O(4) active particles embedded in a graphene substrate. The stable multistep electrochemical reactions of the carbonate compound with a unique nanoneedle structure contribute to the excellent cyclic stability of the CoCO(3)/GA electrode, whereas the highly conductive networks along with low charge transfer resistance are responsible for the high rate performance of the Co(3)O(4)/GA electrode.

Co3O4/porous electrospun carbon nanofibers as anodes for high performance Li-ion batteries

This paper reports a facile route to synthesize porous carbon nanofibers containing cobalt and cobalt oxide nanoparticles (CoOx/PCNF) as anodes for Li-ion batteries. The Co3O4/PCNF electrode delivers a remarkable capacity of 952 mA h g−1 after 100 cycles and equally excellent rate performance at high current densities. There are several ameliorating mechanisms responsible for the observation: namely (i) high theoretical capacity of Co3O4 particles; (ii) enhanced electronic conductivity and Li ion transfer due to the graphene layers surrounding the Co3O4 particles; (iii) the soft CNF matrix serving as the stress buffer to relieve the volumetric stresses arising from Li ion insertion into the Co3O4 particles. The comparison of electrochemical performance with previous studies based on similar CoO or Co3O4/carbon composite anodes indicates that the above value is among the highest.

Enhanced conversion reaction kinetics in low crystallinity SnO2/CNT anodes for Na-ion batteries

The specific capacities of SnO2 anodes in sodium ion batteries (SIBs) are far below the values expected from theory. Herein, we propose that the kinetically-controlled, reversible ‘conversion reaction’ between Na ions and SnO2 is responsible for Na ion storage in SnO2 anodes where the ion diffusion rate is the limiting factor. This revelation is contrary to the current understanding of the ‘alloying reaction’ as the major reaction process. Aiming to fully utilize the theoretical capacity from the conversion reaction, a composite electrode consisting of carbon nanotubes coated with a mainly amorphous SnO2 phase together with crystalline nanoparticles is synthesized. The SnO2/CNT anodes deliver a superior specific capacity of 630.4 mA h g−1 at 0.1 A g−1 and 324.1 mA h g−1 at a high rate of 1.6 A g−1 due to the enhanced kinetics. The volume expansion of the composite is accommodated by the CNT substrate, giving rise to an excellent 69% capacity retention after 300 cycles. The aforementioned findings give new insight into the fundamental understanding of the electrochemical kinetics of SnO2 electrodes and offer a potential solution to the low capacity and poor cyclic stability of other metal oxide anodes based on conversion reactions.

Three-Dimensional Porous Graphene Aerogel Cathode with High Sulfur Loading and Embedded TiO2 Nanoparticles for Advanced Lithium–Sulfur Batteries

Three-dimensional graphene aerogel/TiO2/sulfur (GA/TiO2/S) composites are synthesized through a facile, one-pot hydrothermal route as the cathode for lithium-sulfur batteries. With a high sulfur content of 75.1 wt %, the conductive, highly porous composite electrode delivers a high discharge capacity of 512 mA h/g after 250 cycles at a current rate of 1 C with a low capacity decay of 0.128% per cycle. The excellent capacities and cyclic stability arise from several unique functional features of the cathode. (i) The conductive graphene aerogel framework ameliorates ion/electron transfer while accommodating the volume expansion induced during discharge, and (ii) TiO2 nanoparticles play an important role in restricting the dissolution of polysulfides by chemical bonds with sulfur.

Heterogeneous, mesoporous NiCo2O4–MnO2/graphene foam for asymmetric supercapacitors with ultrahigh specific energies

A major challenge of state-of-the-art supercapacitors as promising energy storage devices lies in their relatively low capacitances and low specific energies. Herein, we report a strategic assembly of two excellent pseudocapacitive materials. Heterogeneous NiCo2O4–MnO2 arrays consisting of a mesoporous NiCo2O4 nanowire core and a cross-linked MnO2 nanosheet shell are grown on a freestanding graphene foam (GF) for ultrahigh-performance supercapacitors. The electrode exhibits a remarkable gravimetric specific capacitance of 2577 F g−1 at 1 A g−1 and areal capacitance of 5.15 F cm−2 at 2 mA cm−2, as well as exceptional capacitance retention of 94.3% after 5000 cycles. An asymmetric supercapacitor assembled with NiCo2O4–MnO2/GF and CNT/GF composites as the positive and negative electrodes, respectively, delivers a maximum specific energy of 55.1 W h kg−1 at a specific power of 187.5 W kg−1. The core/shell strategy adopted here to deposit two active materials on a 3D conductive matrix offers a new insight into assembling ternary hybrids for high-performance electrodes in real-world applications.

Nanocavity-engineered Si/multi-functional carbon nanofiber composite anodes with exceptional high-rate capacities

A facile and scalable electrospinning method is employed to fabricate in situ N-doped, porous graphitic carbon nanofibers (CNFs) containing Si nanoparticles surrounded by nanocavities as durable high-rate Li-ion anodes. Nanocavities are created within the graphitic carbon spheres by electroless etching of the Si nanoparticles, which function not only as buffer to accommodate the volumetric expansion of Si upon lithiation but also as a conducting network for fast electron/ion transport. The Fe3C catalyst simultaneously formed within the fiber promotes the formation of highly graphitic carbon structures while the nitric acid etchant in situ generates functional CNFs with numerous mesopores and oxygenated functional groups, offering extra reaction sites for Li ions. With the ameliorating structural features acting synergistically, the resultant C–Si/F–CNF electrode delivers an exceptional initial reversible capacity of 1548 mA h g−1 at 0.1 A g−1, and remarkable high-rate capacities of 770 and 580 mA h g−1 at 2.0 and 5.0 A g−1 after 70 cycles with excellent capacity retention.

Carbon-coated mesoporous silicon microsphere anodes with greatly reduced volume expansion

Carbon-coated nanostructured silicon (Si/C) composites are promising anodes for next-generation lithium-ion batteries, but scalable synthesis of such materials with an anti-pulverization capability, high areal capacity and long cycle life still remains a challenge. In this work, mesoporous Si/C microspheres of ∼165 nm diameter are synthesized by magnesiothermic reduction of porous silica followed by chemical vapor deposition of a thin carbon layer. They consist of numerous primary Si nanocrystals of ∼10 nm diameter which are interconnected, surrounded by abundant internal pores and coated with conductive graphitic carbon. These ameliorating structural and functional features offer a unique synergy that contributes to prevention of pulverization of Si/C microspheres with a highly reduced volume expansion of ∼85% upon full lithiation. The Si/C electrodes deliver a reversible capacity of ∼1500 mA h g−1 at 0.1 A g−1, an exceptional long-term stability of ∼90% capacity retention even after 1000 and 2500 cycles at 1.0 and 4.0 A g−1, respectively, and an excellent areal capacity of ∼1.44 mA h cm−2 after 500 cycles.

Controlled synthesis of cobalt carbonate/graphene composites with excellent supercapacitive performance and pseudocapacitive characteristics

Cobalt carbonate hydroxide/graphene aerogel and cobalt carbonate/graphene aerogel (CCH/GA and CC/GA) composites are synthesized as supercapacitor electrodes via a one-pot hydrothermal method. Optimized processing conditions are established by controlling the composite composition and microstructure, and their influence on the capacitance performance of the electrodes is identified. A remarkable specific capacitance of 1134 F g−1 at a current density of 1 A g−1 is obtained for the optimal nanowire-shaped CCH/GA electrode, which is among the highest capacitance values of cobalt compound electrodes with or without nanocarbons reported so far. The electrode also delivers exceptional rate performance and cyclic stability benefiting from the pseudocapacitive characteristics of CC-based active materials and the highly conductive, interconnected 3D-structured GA. The CC/GA electrode presents a high capacity of 731 F g−1 under the same conditions. The ex situ XPS analysis identifies the reversible redox reactions of cobalt cations during charge/discharge cycles as the electrochemical mechanism responsible for the high pseudocapacitive properties of CC-based electrodes.

Carbon nanofibers containing Si nanoparticles and graphene-covered Ni for high performance anodes in Li ion batteries

Freestanding, porous carbon nanofiber (CNF) composites containing Si nanoparticles and graphene-covered Ni particles are synthesized via one-pot electrospinning and thermal treatment. The electrodes made from the Si/Ni/CNF composites deliver a remarkable specific capacity of 1045 mA h g−1 at the 50th cycle at a current density of 100 mA g−1 and an excellent high rate capacity of 600 mA h g−1 at 1 A g−1 after 70 cycles with capacity retention of 81%. These values are among the best for similar electrospun Si-based CNF composite electrodes. Two major ameliorating mechanisms are responsible for the finding. The Si particles are fully encapsulated by the soft CNF matrix which serves as the stress buffer to relieve the volumetric stresses stemming from the intercalation/extraction of Li ions into Si and prevent re-agglomeration of Si particles during charge/discharge cycles. The amorphous carbon and the Ni particles surrounded by the crystallized graphene layers effectively form continuous conductive networks which in turn offer fast ion and electron transport paths, giving rise to high rate performance of the electrodes.