Chunyan Ding

NiCo2O4 nanosheet supported hierarchical core–shell arrays for high-performance supercapacitors

Two types of homogeneous NiCo2O4 nanosheet@NiCo2O4 nanorod and heterogeneous NiCo2O4 nanosheet@NiO nanoflake hierarchical core–shell arrays are synthesized via facile solution methods in combination with a simple thermal treatment. In both cases, the NiCo2O4 nanosheets serve as the core backbone for anchoring the shell materials. The two as-prepared hierarchical nanoarrays are evaluated as supercapacitor electrodes and demonstrate excellent electrochemical performance with high specific capacitance (1925 and 2210 F g−1 for NiCo2O4@NiCo2O4 and NiCo2O4@NiO at 0.5 A g−1, respectively), good rate capability, and superior cycling stability. The superior capacitive performance is mainly due to the unique hierarchical core–shell architecture with faster ion/electron transfer, improved reactivity, and enhanced structural stability. Our work can allow for the fabrication of various NiCo2O4 nanosheet supported hierarchical nanostructures for applications in energy storage, catalysis, and sensing.

Hierarchically constructed NiCo2S4@Ni(1-x)Co x (OH)2 core/shell nanoarrays and their application in energy storage.

We report a new type of core-shell heterostructure consisting of a rod-like NiCo2S4 (NCS) core and an urchin-like Ni(1-x)Co x (OH)2 (NCOH) shell via a simple hydrothermal route coupled with a facile electrodeposition. NCS nanorod arrays (NRAs) can not only act as excellent electrochemically active materials by themselves, but they can also serve as hierarchical porous scaffolds capable of fast electron conduction and ion diffusion for loading a large amount of additional active materials. Moreover, it is observed that the urchin-like NCOH nanosheets coating could bind the inner NCS nanorods together and thereby reinforce the whole structure mechanically. Meanwhile, more effective pathways for electrons are available in the NCS@NCOH hybrids than an individual NCS nanorod. Benefiting from both structural and compositional features, the NCS@NCOH electrode exhibits greatly improved electrochemical performance with high capacity (3.54 C cm(-2) at 1 mA cm(-2)) and excellent cycling stability (78% capacity retention after 4000 cycles). Moreover, a battery-type device is also fabricated by using NCS@NCOH as a positive electrode and activated carbon (AC) as a negative electrode, displaying high capacity (2.51 C cm(-2) at 2 mA cm(-2)) and good durability (88.8% capacity retention after 4000 cycles) as well.

General synthesis of graphene-supported bicomponent metal monoxides as alternative high-performance Li-ion anodes to binary spinel oxides

Engineering two transition metals into an integrated spinel oxide anode provides great opportunity towards high-performance lithium-ion batteries (LIBs). Spinels with high-valence transition metal oxides (TMOs) however tend to exhibit low initial coulombic efficiency (ICE) due to the irreversible Li2O generated during the first discharge process. Herein, we report a simple and general strategy to synthesize elaborate graphene framework (GF) supported low-valence bicomponent transition metal monoxide anodes (e.g., ZnO–MnO microcubes, ZnO–CoO polyhedra, NiO–CoO nanowires, and (FeO)0.333(MnO)0.667 microspheres, etc.), which can efficiently address the low ICE issue. As a proof of concept demonstration, we show that the ZnO–MnO/GF is indeed more advantageous as an LIB anode over the spinel ZnMn2O4/GF counterpart as well as many other ZnMn2O4-based anodes. Benefiting from the enhanced reversibility of Li+ uptake/extraction and graphene hybridization, the ZnO–MnO/GF electrode exhibits significantly improved ICEs at various current densities, superior rate capability (286 mA h g−1 even at a high current density of 6 A g−1; ∼2.9 min charging/discharging), and extended cycling life (1123 mA h g−1 after 300 cycles) with respect to ZnMn2O4/GF. Such improvements have also been observed for the ZnO–CoO/GF electrode and other analogues. This versatile electrode design could advance our understanding and control of complex TMO-based anodes to gain high ICE and capacity.

MOF-derived Zn–Mn mixed oxides@carbon hollow disks with robust hierarchical structure for high-performance lithium-ion batteries

Hollow metal oxides and carbon hybrids with hierarchical and robust nanoarchitecture hold great potential as high-performance electrode materials. Herein, a relatively unexplored hollow and hierarchical metal–organic framework (MOF) assembled by parallel stacked triangular sub-MOFs were successfully synthesized via a facile co-precipitation method. The hollow MOFs were then converted to binary metal oxides@carbon composites, exemplified herein as Zn–Mn mixed oxides@carbon (ZnxMnO@C) hybrids. The obtained ZnxMnO@C inherits the unique hollow hexagonal nanodisks (HHNDs) structure of the MOF precursor, and each triangular plate-like subunit consists of a continuous carbon matrix embedded uniformly within the ultrafine ZnxMnO nanoparticles. When evaluated as an anode material for lithium ion batteries, the ZnxMnO@C HHNDs exhibited high specific capacity (1050 mA h g−1 at 0.1 A g−1 after 200 cycles) and remarkable cycling performance up to 1000 cycles. It is believed that besides the protection of the carbon matrix, the unique hierarchically hollow structure with parallel stacked subunits endows the ZnxMnO@C hybrid with additional capability to withstand lithiation/delithiation strain. Moreover, kinetics-analysis based on cyclic voltammograms (CVs) reveals that the high lithium storage capacity is primarily attributed to fast kinetics originating from pseudocapacitive contribution. This also accounts for the good rate capabilities of ZnxMnO@C HHNDs (713 and 330 mA h g−1 at 1 and 10 A g−1, respectively). Furthermore, full cells with Zn0.5MnO@C anodes and LiMn2O4 cathodes are assembled and show good cycling stability over 120 cycles. This study demonstrates a new hollow structure of MOFs and its usefulness in developing robust and hierarchical metal oxide/carbon composites for electrochemical storage applications.

Template-free fabrication of graphene-wrapped mesoporous ZnMn2O4 nanorings as anode materials for lithium-ion batteries

We rationally designed a facile two-step approach to synthesize ZnMn2O4@G composite anode material for lithium-ion batteries (LIBs), involving a template-free fabrication of ZnMn2O4 nanorings and subsequent coating of graphene sheets. Notably, it is the first time that ring-like ZnMn2O4 nanostructure is reported. Moreover, our system has been demonstrated to be quite powerful in producing ZnMn2O4 nanorings regardless of the types of Zn and Mn-containing metal salts reactants. The well-known inside-out Ostwald ripening process is tentatively proposed to clarify the formation mechanism of the hollow nanorings. When evaluated as anode material for LIBs, the resulting ZnMn2O4@G hybrid displays significantly improved lithium-storage performance with high specific capacity, good rate capability, and excellent cyclability. After 500 cycles, the ZnMn2O4@G hybrid can still deliver a reversible capacity of 958 mAh g-1 at a current density of 200 mA g-1, much higher than the theoretical capacity of 784 mAh g-1 for pure ZnMn2O4. The outstanding electrochemical performance should be reasonably ascribed to the synergistic interaction between hollow and porous ZnMn2O4 nanorings and the three-dimensional interconnected graphene sheets.

A general strategy toward graphitized carbon coating on iron oxides as advanced anodes for lithium-ion batteries

Integration of carbon materials with benign iron oxides is blazing a trail in constructing high-performance anodes for lithium-ion batteries (LIBs). In this paper, a unique general, simple, and controllable strategy is developed toward in situ uniform coating of iron oxide nanostructures with graphitized carbon (GrC) layers. The basic synthetic procedure only involves a simple dip-coating process for the loading of Ni-containing seeds and a subsequent Ni-catalyzed chemical vapor deposition (CVD) process for the growth of GrC layers. More importantly, the CVD treatment is conducted at a quite low temperature (450 °C) and with extremely facile liquid carbon sources consisting of ethylene glycol (EG) and ethanol (EA). The GrC content of the resulting hybrids can be controllably regulated by altering the amount of carbon sources. The electrochemical results reveal remarkable performance enhancements of iron oxide@GrC hybrids compared with pristine iron oxides in terms of high specific capacity, excellent rate and cycling performance. This can be attributed to the network-like GrC coating, which can improve not only the electronic conductivity but also the structural integrity of iron oxides. Moreover, the lithium storage performance of samples with different GrC contents is measured, manifesting that optimized electrochemical property can be achieved with appropriate carbon content. Additionally, the superiority of GrC coating is demonstrated by the advanced performance of iron oxide@GrC compared with its corresponding counterpart, i.e., iron oxides with amorphous carbon (AmC) coating. All these results indicate the as-proposed protocol of GrC coating may pave the way for iron oxides to be promising anodes for LIBs.