Yixian Wang

Metal–Organic Frameworks Mediated Synthesis of One-Dimensional Molybdenum-Based/Carbon Composites for Enhanced Lithium Storage

Molybdenum (Mo)-based compounds with properly engineered nanostructures usually possess improved reversible lithium storage capabilities, which offer great promise to boost the performance of lithium-ion batteries (LIBs). Nevertheless, a lack of efficient and high-yield methods for constructing rational nanostructures has largely restricted the application of these potentially important materials. Herein we demonstrate a metal-organic frameworks (MOFs) mediated strategy to successfully synthesize a series of one-dimensional Mo-based/carbon composites with distinct nanostructures. In this process, starting from well-designed MoO3 nanorods, the crystal control growth is first proposed that a layer of MOFs is achieved to be controllably grown on surfaces of MoO3, forming an obvious core-shell structure, and then the adopted precursor can be in situ transformed into MoO2 or Mo2C which are both well confined in conductive porous carbons through direct carbonization at different temperatures, where the MOFs shell serve as both carbon sources and the reactant to react with MoO3 simultaneously. Benefiting from this design, all optimized products exhibit enhanced electrochemical performances when evaluated as anode materials for LIBs, especially the hollow MoO2/C and core-shell Mo2C/C electrodes, show best reversible capacities up to 810 and 530 mAh g-1 even after 600 cycles at a current density of 1 A g-1, respectively. So this work may broaden the application of MOFs as a kind of coating materials and elucidates the attractive lithium storage performances of molybdenum-based compounds.

A Tunable Molten-Salt Route for Scalable Synthesis of Ultrathin Amorphous Carbon Nanosheets as High-Performance Anode Materials for Lithium-Ion Batteries

Amorphous carbon is regarded as a promising alternative to commercial graphite as the lithium-ion battery anode due to its capability to reversibly store more lithium ions. However, the structural disorder with a large number of defects can lead to low electrical conductivity of the amorphous carbon, thus limiting its application for high power output. Herein, ultrathin amorphous carbon nanosheets were prepared from petroleum asphalt through tuning the carbonization temperature in a molten-salt medium. The amorphous nanostructure with expanded carbon interlayer spacing can provide substantial active sites for lithium storage, while the two-dimensional (2D) morphology can facilitate fast electrical conductivity. As a result, the electrodes deliver a high reversible capacity, outstanding rate capability, and superior cycling performance (579 and 396 mAh g-1 at 2 and 5 A g-1 after 900 cycles). Furthermore, full cells consisting of the carbon anodes coupled with LiMn2O4 cathodes exhibit high specific capacity (608 mAh g-1 at 50 mA g-1) and impressive cycling stability with slow capacity loss (0.16% per cycle at 200 mA g-1). The present study not only paves the way for industrial-scale synthesis of advanced carbon materials for lithium-ion batteries but also deepens the fundamental understanding of the intrinsic mechanism of the molten-salt method.

Substrate-Assisted in Situ Confinement Pyrolysis of Zeolitic Imidazolate Frameworks to Nitrogen-Doped Hierarchical Porous Carbon Nanoframes with Superior Lithium Storage

Porous carbons generated from the direct pyrolysis of metal organic frameworks (MOFs) have shown great potential as a kind of promising electrode material for lithium-ion batteries (LIBs). However, several common drawbacks, such as the inevitable structural damage during carbonization process and intrinsic micropore-dominated feature of MOFs-derived products, largely impede the exposure of active sites and mass transfer, thus usually resulting in inferior electrochemical performances. In this work, an effective and controllable approach was reported to construct nitrogen-doped hierarchical porous carbon nanoframes (N-HPCFs) through in situ pyrolysis transformation of small-sized zeolitic imidazolate frameworks (ZIFs) that are in advance combined with a two-dimensional substrate templated from g-C3N4. Using this strategy, numerous ZIFs derived carbon nanoparticles appear to be well arranged on surfaces of a hollow carbon nanoplatelet produced by the adopted substrate after calcination, and the corresponding final product shows great improvements on both structural stability and pore distribution compared with that from direct pyrolysis of monodispersed ZIFs without the substrate. As expected, when used as the anode material for LIBs, the N-HPCFs electrode exhibits a high lithium storage capacity with good cycle stability (651 mAh g-1 after 400 cycles at the current density of 1 A g-1) and outstanding rate capability. Such enhanced electrochemical performances can be well ascribed to the stable frame structure as well as highly developed transport pathways for ions and electrons. So the synthetic strategy presented in this paper may pave a new way for the further application of ZIFs-based materials.