Enzuo Liu

Carbon-Encapsulated Fe3O4 Nanoparticles as a High-Rate Lithium Ion Battery Anode Material

A facile and scalable in situ synthesis strategy is developed to fabricate carbon-encapsulated Fe3O4 nanoparticles homogeneously embedded in two-dimensional (2D) porous graphitic carbon nanosheets (Fe3O4@C@PGC nanosheets) as a durable high-rate lithium ion battery anode material. With assistance of the surface of NaCl particles, 2D Fe@C@PGC nanosheets can be in situ synthesized by using the Fe(NO3)3·9H2O and C6H12O6 as the metal and carbon precursor, respectively. After annealing under air, the Fe@C@PGC nanosheets can be converted to Fe3O4@C@PGC nanosheets, in which Fe3O4 nanoparticles (∼18.2 nm) coated with conformal and thin onion-like carbon shells are homogeneously embedded in 2D high-conducting carbon nanosheets with a thickness of less than 30 nm. In the constructed architecture, the thin carbon shells can avoid the direct exposure of encapsulated Fe3O4 to the electrolyte and preserve the structural and interfacial stabilization of Fe3O4 nanoparticles. Meanwhile, the flexible and conductive PGC nanosheets can accommodate the mechanical stress induced by the volume change of embedded Fe3O4@C nanoparticles as well as inhibit the aggregation of Fe3O4 nanoparticles and thus maintain the structural and electrical integrity of the Fe3O4@C@PGC electrode during the lithiation/delithiation processes. As a result, this Fe3O4@C@PGC electrode exhibits superhigh rate capability (858, 587, and 311 mAh/g at 5, 10, and 20 C, respectively, 1 C = 1 A/g) and extremely excellent cycling performance at high rates (only 3.47% capacity loss after 350 cycles at a high rate of 10 C), which is the best one ever reported for an Fe3O4-based electrode including various nanostructured Fe3O4 anode materials, composite electrodes, etc.

Graphene Networks Anchored with Sn@Graphene as Lithium Ion Battery Anode

A facile and scalable in situ chemical vapor deposition (CVD) technique using metal precursors as a catalyst and a three-dimensional (3D) self-assembly of NaCl particles as a template is developed for one-step fabrication of 3D porous graphene networks anchored with Sn nanoparticles (5-30 nm) encapsulated with graphene shells of about 1 nm (Sn@G-PGNWs) as a superior lithium ion battery anode. In the constructed architecture, the CVD-synthesized graphene shells with excellent elasticity can effectively not only avoid the direct exposure of encapsulated Sn to the electrolyte and preserve the structural and interfacial stabilization of Sn nanoparticles but also suppress the aggregation of Sn nanoparticles and buffer the volume expansion, while the interconnected 3D porous graphene networks with high electrical conductivity, large surface area, and high mechanical flexibility tightly pin the core-shell structure of Sn@G and thus lead to remarkably enhanced electrical conductivity and structural integrity of the overall electrode. As a consequence, this 3D hybrid anode exhibits very high rate performance (1022 mAh/g at 0.2 C, 865 mAh/g at 0.5 C, 780 mAh/g at 1 C, 652 mAh/g at 2 C, 459 mAh/g at 5 C, and 270 mAh/g at 10 C, 1 C = 1 A/g) and extremely long cycling stability even at high rates (a high capacity of 682 mAh/g is achieved at 2 A/g and is maintained approximately 96.3% after 1000 cycles). As far as we know, this is the best rate capacity and longest cycle life ever reported for a Sn-based lithium ion battery anode.

2D Space-Confined Synthesis of Few-Layer MoS2 Anchored on Carbon Nanosheet for Lithium-Ion Battery Anode

A facile and scalable 2D spatial confinement strategy is developed for in situ synthesizing highly crystalline MoS2 nanosheets with few layers (≤5 layers) anchored on 3D porous carbon nanosheet networks (3D FL-MoS2@PCNNs) as lithium-ion battery anode. During the synthesis, 3D self-assembly of cubic NaCl particles is adopted to not only serve as a template to direct the growth of 3D porous carbon nanosheet networks, but also create a 2D-confined space to achieve the construction of few-layer MoS2 nanosheets robustly lain on the surface of carbon nanosheet walls. In the resulting 3D architecture, the intimate contact between the surfaces of MoS2 and carbon nanosheets can effectively avoid the aggregation and restacking of MoS2 as well as remarkably enhance the structural integrity of the electrode, while the conductive matrix of 3D porous carbon nanosheet networks can ensure fast transport of both electrons and ions in the whole electrode. As a result, this unique 3D architecture manifests an outstanding long-life cycling capability at high rates, namely, a specific capacity as large as 709 mAh g(-1) is delivered at 2 A g(-1) and maintains ∼95.2% even after 520 deep charge/discharge cycles. Apart from promising lithium-ion battery anode, this 3D FL-MoS2@PCNN composite also has immense potential for applications in other areas such as supercapacitor, catalysis, and sensors.

Porous graphitic carbon nanosheets as a high-rate anode material for lithium-ion batteries.

Two-dimensional (2D) porous graphitic carbon nanosheets (PGC nanosheets) as a high-rate anode material for lithium storage were synthesized by an easy, low-cost, green, and scalable strategy that involves the preparation of the PGC nanosheets with Fe and Fe3O4 nanoparticles embedded (indicated with (Fe&Fe3O4)@PGC nanosheets) using glucose as the carbon precursor, iron nitrate as the metal precursor, and a surface of sodium chloride as the template followed by the subsequent elimination of the Fe and Fe3O4 nanoparticles from the (Fe&Fe3O4)@PGC nanosheets by acid dissolution. The unique 2D integrative features and porous graphitic characteristic of the carbon nanosheets with high porosity, high electronic conductivity, and outstanding mechanical flexibility and stability are very favorable for the fast and steady transfer of electrons and ions. As a consequence, a very high reversible capacity of up to 722 mAh/g at a current density of 100 mA/g after 100 cycles, a high rate capability (535, 380, 200, and 115 mAh/g at 1, 10, 20, and 30 C, respectively, 1 C = 372 mA/g), and a superior cycling performance at an ultrahigh rate (112 mAh/g at 30 C after 570 charge-discharge cycles) are achieved by using these nanosheets as a lithium-ion-battery anode material.

Covalent functionalization of graphene by azobenzene with molecular hydrogen bonds for long-term solar thermal storage

Reduced graphene oxide-azobenzene (RGO-AZO) hybrids were prepared via covalent functionalization for long-term solar thermal storage. Thermal barrier (ΔEa) of cis to tran reversion and thermal storage (ΔH) were improved by molecular hydrogen bonds (H-bonds) through ortho- or para-substitution of AZO. Intramolecular H-bonds thermally stabilized cis-ortho-AZO on RGO with a long-term half-life of 5400 h (ΔEa = 1.2 eV), which was much longer than that of RGO-para-AZO (116 h). RGO-para-AZO with one intermolecular H-bond showed a high density of thermal storage up to 269.8 kJ kg−1 compared with RGO-ortho-AZO (149.6 kJ kg−1) with multiple intra- and intermolecular H-bonds of AZO according to relaxed stable structures. Thermal storage in experiment was the same order magnitude to theoretical data based on ΔH calculated by density functional theory and packing density. Photoactive RGO-AZO hybrid can be developed for high-performance solar thermal storage by optimizing molecular H-bonds.

Fabrication of in-situ grown graphene reinforced Cu matrix composites.

Graphene/Cu composites were fabricated through a graphene in-situ grown approach, which involved ball-milling of Cu powders with PMMA as solid carbon source, in-situ growth of graphene on flaky Cu powders and vacuum hot-press sintering. SEM and TEM characterization results indicated that graphene in-situ grown on Cu powders guaranteed a homogeneous dispersion and a good combination between graphene and Cu matrix, as well as the intact structure of graphene, which was beneficial to its strengthening effect. The yield strength of 244 MPa and tensile strength of 274 MPa were achieved in the composite with 0.95 wt.% graphene, which were separately 177% and 27.4% enhancement over pure Cu. Strengthening effect of in-situ grown graphene in the matrix was contributed to load transfer and dislocation strengthening.

A high energy density azobenzene/graphene hybrid: a nano-templated platform for solar thermal storage

Effective conversion of light into heat is an emerging field showing great potential for large-scale applications, markedly driven by novel molecules and structures. Unfortunately, until now, it is still hindered by a low storage capacity and short-time storage. A nano-template for covalently attaching new azobenzene chromophores on graphene as solar thermal fuels is presented here, in which the intermolecular hydrogen bond and proximity-induced interaction, resulting from a high functionalization density and inter-planar bundling interaction, remarkably improve both the storage capacity and lifetime. This nanoscopic template exhibits a high energy density up to 112 W h kg−1 and long-term storage with a half-life of more than one month (33 days), which are also confirmed by the calculations using density functional theory, simultaneously maintaining an excellent cycling stability tuned by visible light for 50 cycles. Our work develops a promising class of solar thermal fuels with high energy density, which outperform previous nano-materials and are comparable to commercial soft-packing Li-ion batteries.

Three-Dimensional Network of N-Doped Carbon Ultrathin Nanosheets with Closely Packed Mesopores: Controllable Synthesis and Application in Electrochemical Energy Storage

A flexible one-pot strategy for fabricating a 3D network of nitrogen-doped (N-doped) carbon ultrathin nanosheets with closely packed mesopores (N-MCN) via an in situ template method is reported in this research. The self-assembly soluble salts (NaCl and Na2SiO3) serve as hierarchical templates and support the formation of a 3D glucose-urea complex. The organic complex is heat-treated to obtain a 3D N-doped carbon network constructed by mesoporous nanosheets. Especially, both the mesoporous structure and doping content can be easily tuned by adjusting the ratio of raw materials. The large specific surface area and closely packed mesopores facilitate the lithium ion intercalation/deintercalation accordingly. Besides, the nitrogen content improves the lithium storage ability and capacitive properties. Due to the synergistic effect of hierarchical structure and heteroatom composition, the 3D N-MCN shows excellent characteristics as the electrode of a lithium ion battery and supercapacitor, such as ultrahigh reversible storage capacity (1222 mAh g(-1) at 0.1 A g(-1)), stable long cycle performance at high current density (600 cycles at 2 A g(-1)), and high capacitive properties (225 F g(-1) at 1 A g(-1) and 163 F g(-1) at 50 A g(-1)).

Salt-template-assisted synthesis of robust 3D honeycomb-like structured MoS2 and its application as a lithium-ion battery anode

Constructing a 3D porous architecture from 2D MoS2 nano-building blocks has been considered as a promising approach to prevent restacking and thus achieve superior properties in energy conversion and storage, catalysis, sensors, and so on. In this work, a novel salt (NaCl) template-assisted solid-phase synthesis strategy is developed to fabricate a new type of MoS2 with a robust 3D honeycomb-like structure without the necessity to use a solution reaction. As a demonstration of structural advantages, the 3D honeycomb-structured MoS2 with a highly crystalline architecture and intimate interfacial bonding between adjacent MoS2 walls is utilized as a lithium-ion battery anode, exhibiting an unprecedented initial coulombic efficiency (∼93.5%), large specific capacity, high rate capability and excellent cycle stability.

Synthesis of uniform and superparamagnetic Fe3O4 nanocrystals embedded in a porous carbon matrix for a superior lithium ion battery anode

A facile and scalable strategy for the synthesis of discrete, homogeneous and small (mostly 5–15 nm) Fe3O4 nanocrystals embedded in a partially graphitized porous carbon matrix was developed, which involved the simple mixing of a metal precursor (Fe(NO3)3·9H2O), a carbon precursor (C6H8O7), and a dispersant (NaCl) in an aqueous solution followed by calcination at 600 °C for 2 h under Ar. As the anode materials for lithium-ion batteries, the Fe3O4/carbon composite with 55.24 wt% Fe3O4 exhibited superior electrochemical performances, such as high reversible lithium storage capacity (834 mA h g−1 at 1 C after 60 cycles, 1 C = 924 mA g−1), high Coulombic efficiency (∼100%), excellent cycling stability, and superior rate capability (588 mA h g−1 at 5 C and 382 mA h g−1 at 10 C). These excellent electrochemical performances could be attributed to the robust porous carbon matrix with a partially graphitized structure for embedding a mass of small Fe3O4 nanocrystals, which not only provided excellent electronic conductivity, short transportation length for both lithium ions and electrons, and enough elastic buffer space to accommodate volume changes upon lithium insertion/extraction, but also could effectively avoid agglomeration of the Fe3O4 nanocrystals and maintain the structural integrity of the electrode during the charge–discharge process. It is believed that the Fe3O4/carbon composite synthesized by the current method is a promising anode material for high energy and power density lithium-ion batteries.