Dongniu Wang

Layer by layer assembly of sandwiched graphene/SnO2 nanorod/carbon nanostructures with ultrahigh lithium ion storage properties

Sandwiched structures consisting of carbon coated SnO2 nanorod grafted on graphene have been synthesized based on a seed assisted hydrothermal growth to form graphene supported SnO2 nanorods, followed by a nanocarbon coating. As a potential anode for high power and energy applications, the hierarchical nanostructures exhibit a greatly enhanced synergic effect with an extremely high lithium storage capability of up to 1419 mA h g−1 benefiting from the advanced structural features.

LiFePO4–graphene as a superior cathode material for rechargeable lithium batteries: impact of stacked graphene and unfolded graphene

In this work, we describe the use of unfolded graphene as a three dimensional (3D) conducting network for LiFePO4 nanoparticle growth. Compared with stacked graphene, which has a wrinkled structure, the use of unfolded graphene enables better dispersion of LiFePO4 and restricts the LiFePO4 particle size at the nanoscale. More importantly, it allows each LiFePO4 particle to be attached to the conducting layer, which could greatly enhance the electronic conductivity, thereby realizing the full potential of the active materials. Based on its superior structure, after post-treatment for 12 hours, the LiFePO4–unfolded graphene nanocomposite achieved a discharge capacity of 166.2 mA h g−1 in the 1st cycle, which is 98% of the theoretical capacity (170 mA h g−1). The composite also displayed stable cycling behavior up to 100 cycles, whereas the LiFePO4–stacked graphene composite with a similar carbon content could deliver a discharge capacity of only 77 mA h g−1 in the 1st cycle. X-ray absorption near-edge spectroscopy (XANES) provided spectroscopic understanding of the crystallinity of LiFePO4 and chemical bonding between LiFePO4 and unfolded graphene.

Hierarchically porous LiFePO4/nitrogen-doped carbon nanotubes composite as a cathode for lithium ion batteries

A porous composite of LiFePO4/nitrogen-doped carbon nanotubes (N-CNTs) with hierarchical structure was prepared by a sol–gel method without templates or surfactants. Highly conductive and uniformly dispersed N-CNTs incorporated into three dimensional interlaced porous LiFePO4 can facilitate the electronic and lithium ion diffusion rate. The LiFePO4/N-CNTs composites deliver a reversible discharge capacity of 138 mA h g−1 at a current density of 17 mA g−1 while the LiFePO4/CNTs composites only deliver 113 mA h g−1, demonstrating N-CNTs modified composites can act as a promising cathode for high-performance lithium-ion batteries.

Hierarchical nanostructured core-shell Sn@C nanoparticles embedded in graphene nanosheets: spectroscopic view and their application in lithium ion batteries.

Hierarchical carbon encapsulated tin (Sn@C) embedded graphene nanosheet (GN) composites (Sn@C-GNs) have been successfully fabricated via a simple and scalable one-step chemical vapor deposition (CVD) procedure. The GN supported Sn@C core-shell structures consist of a crystalline tin core, which is thoroughly covered by a carbon shell and more interestingly, extra voids are present between the carbon shell and the tin core. Synchrotron spectroscopy confirms that the metallic tin core is free of oxidation and the existence of charge redistribution transfer from tin to the carbonaceous materials of the shell, facilitating their intimate contact by chemical bonding and resultant lattice variation. The hybrid electrodes of this material exhibit a highly stable and reversible capacity together with an excellent rate capability, which benefits from the improved electrochemical properties of tin provided by the protective carbon matrix, voids and the flexible GN matrices.

Discharge product morphology and increased charge performance of lithium–oxygen batteries with graphene nanosheet electrodes: the effect of sulphur doping

Sulphur-doped graphene was successfully fabricated and its influence on the discharge product formation in lithium–oxygen batteries was demonstrated. The growth and distribution of the discharge products were studied and a mechanism was proposed. This will have significant implication for cathode catalysts and rechargeable battery performance.

In situ self-catalyzed formation of core–shell LiFePO4@CNT nanowires for high rate performance lithium-ion batteries

In situ self-catalyzed core–shell LiFePO4@CNT nanowires can be fabricated by a two-step synthesis, where one-dimensional LiFePO4 nanowires with a diameter of 20–30 nm were encapsulated into CNTs, and 3D conducting networks of CNTs were obtained from in situ carbonization of a polymer. LiFePO4@CNT nanowires deliver a capacity of 160 mA h g−1 at 17 mA g−1, and 65 mA h g−1 at 8500 mA g−1 (50 C, 1.2 minutes for charging and 1.2 minutes for discharging).

N Doping to ZnO Nanorods for Photoelectrochemical Water Splitting under Visible Light: Engineered Impurity Distribution and Terraced Band Structure

Solution-based ZnO nanorod arrays (NRAs) were modified with controlled N doping by an advanced ion implantation method, and were subsequently utilized as photoanodes for photoelectrochemical (PEC) water splitting under visible light irradiation. A gradient distribution of N dopants along the vertical direction of ZnO nanorods was realized. N doped ZnO NRAs displayed a markedly enhanced visible-light-driven PEC photocurrent density of ~160 μA/cm2 at 1.1 V vs. saturated calomel electrode (SCE), which was about 2 orders of magnitude higher than pristine ZnO NRAs. The gradiently distributed N dopants not only extended the optical absorption edges to visible light region, but also introduced terraced band structure. As a consequence, N gradient-doped ZnO NRAs can not only utilize the visible light irradiation but also efficiently drive photo-induced electron and hole transfer via the terraced band structure. The superior potential of ion implantation technique for creating gradient dopants distribution in host semiconductors will provide novel insights into doped photoelectrode materials for solar water splitting.

Soft X-ray XANES studies of various phases related to LiFePO4 based cathode materials

LiFePO4 has been a promising cathode material for rechargeable lithium ion batteries. Different secondary or impurity phases, forming during either synthesis or subsequent redox process under normal operating conditions, can have a significant impact on the performance of the electrode. The exploration of the electronic and chemical structures of impurity phases is crucial to understand such influence. We have embarked on a series of synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy studies for the element speciation in various impurity phase materials relevant to LiFePO4 for Li ion batteries. In the present report, soft-X-ray XANES spectra of Li K-edge, P L2,3-edge, O K-edge and Fe L2,3-edge have been obtained for LiFePO4 in crystalline, disordered and amorphous forms and some possible “impurities”, including LiPO3, Li4P2O7, Li3PO4, Fe3(PO4)2, FePO4, and Fe2O3. The results indicate that each element from different pure reference compounds exhibits unique spectral features in terms of energy position, shape and intensity of the resonances in its XANES. In addition, inverse partial fluorescence yield (IPFY) reveals the surface vs. bulk property of the specimens. Therefore, the spectral data provided here can be used as standards in the future for phase composition analysis.

Atomic layer deposited coatings to significantly stabilize anodes for Li ion batteries: effects of coating thickness and the size of anode particles

A study of the effect of thickness of alumina coating on the electrochemical performances of various sizes of SnO2 electrodes has been systematically studied in this paper. It is found that the different volume changes in various sizes of SnO2 electrodes can be suppressed by optimized thickness of Al2O3 coating layers, with a range from less than 1 nm to up to 3 nm deposited by atomic layer deposition (ALD), which have significant impact on the electrochemical behaviour of the composites. The well-defined and optimized Al2O3 layer could not only relieve mechanical degradation and improve cycling stability, but also form an artificial solid electrolyte interphase (SEI) layer to prevent the chemical reaction between SnO2 and the electrolyte, leading to improved electrochemical performances compared with bare SnO2 electrodes.

Observation of lithiation-induced structural variations in TiO2 nanotube arrays by X-ray absorption fine structure

We report here a study of self-organized TiO2 nanotube arrays both in the amorphous and anatase phases with superior electrochemical performance upon lithiation and delithiation. X-ray absorption fine structure (XAFS) study at the Ti K and L, O K and Li K edges has been conducted to track the behavior. Characteristic features for amorphous and anatase TiO2 are identified. After lithiation, it is found that although no obvious variation of chemical states is apparent at the Ti K and L edges, charge transfer from Ti 3d to O 2p and also partial amorphization of anatase TiO2 are evident from spectral intensities. The Li and O K edge XAFS show the successful intercalation of lithium and reveal the existence of a nearly linear “O–Li–O” arrangement in the lithiated TiO2 nanotube. This study helps in understanding of the lithiation process in nanostructured TiO2 anodes from a spectroscopic viewpoint.