Qianqian Li

Atomic-scale observation of lithiation reaction front in nanoscale SnO2 materials.

In the present work, taking advantage of aberration-corrected scanning transmission electron microscopy, we show that the dynamic lithiation process of anode materials can be revealed in an unprecedented resolution. Atomically resolved imaging of the lithiation process in SnO2 nanowires illustrated that the movement, reaction, and generation of b = [1[overline]1[overline]1] mixed dislocations leading the lithiated stripes effectively facilitated lithium-ion insertion into the crystalline interior. The geometric phase analysis and density functional theory simulations indicated that lithium ions initial preference to diffuse along the [001] direction in the {200} planes of SnO2 nanowires introduced the lattice expansion and such dislocation behaviors. At the later stages of lithiation, the Li-induced amorphization of rutile SnO2 and the formation of crystalline Sn and LixSn particles in the Li2O matrix were observed.

Twin boundary-assisted lithium ion transport.

With the increased need for high-rate Li-ion batteries, it has become apparent that new electrode materials with enhanced Li-ion transport should be designed. Interfaces, such as twin boundaries (TBs), offer new opportunities to navigate the ionic transport within nanoscale materials. Here, we demonstrate the effects of TBs on the Li-ion transport properties in single crystalline SnO2 nanowires. It is shown that the TB-assisted lithiation pathways are remarkably different from the previously reported lithiation behavior in SnO2 nanowires without TBs. Our in situ transmission electron microscopy study combined with direct atomic-scale imaging of the initial lithiation stage of the TB-SnO2 nanowires prove that the lithium ions prefer to intercalate in the vicinity of the (101̅) TB, which acts as conduit for lithium-ion diffusion inside the nanowires. The density functional theory modeling shows that it is energetically preferred for lithium ions to accumulate near the TB compared to perfect neighboring lattice area. These findings may lead to the design of new electrode materials that incorporate TBs as efficient lithium pathways, and eventually, the development of next generation rechargeable batteries that surpass the rate performance of the current commercial Li-ion batteries.

Epitaxial TiO2/SnO2 core–shell heterostructure by atomic layer deposition

Taking TiO2/SnO2 core–shell nanowires (NWs) as a model system, we systematically investigate the structure and the morphological evolution of this heterostructure synthesized by atomic layer deposition/epitaxy (ALD/ALE). All characterizations, by X-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction and Raman spectra, reveal that single crystalline rutile TiO2 shells can be epitaxially grown on SnO2 NWs with an atomically sharp interface at low temperature (250 °C). The growth behavior of the TiO2 shells highly depends on the surface orientations and the geometrical shape of the core SnO2 NW cross-section. Atomically smooth surfaces are found for growth on the {110} surface. Rough surfaces develop on {100} surfaces due to (100) − (1 × 3) reconstruction, by introducing steps in the [010] direction as a continuation of {110} facets. Lattice mismatch induces superlattice structures in the TiO2 shell and misfit dislocations along the interface. Conformal epitaxial growth has been observed for SnO2 NW cores with an octagonal cross-section ({100} and {110} surfaces). However, for a rectangular core ({10} and {010} surfaces), the shell also derives an octagonal shape from the epitaxial growth, which was explained by a proposed model based on ALD kinetics. The surface steps and defects induced by the lattice mismatch likely lead to improved photoluminescence (PL) performance for the yellow emission. Compared to the pure SnO2 NWs, the PL spectrum of the core–shell nanostructures exhibits a stronger emission peak, which suggests potential applications in optoelectronics.

Synthesis and stress relaxation of ZnO/Al-doped ZnO core–shell nanowires

Doping nanostructures is an effective method to tune their electrical and photoelectric properties. Taking ZnO nanowires (NWs) as a model system, we demonstrate that atomic layer deposition (ALD) can be adopted for the realization of a doping process by the homo-epitaxial growth of a doped shell on the NW core. The Al-doped ZnO NWs have a layered superlattice structure with dopants mainly occupying the interstitial positions. After annealing, Al(3+) ions diffuse into the ZnO matrix and occupy substitutional locations, which is desirable for dopant activation. The stress accumulated during epitaxial growth is relaxed by the nucleation of dislocations, dislocation dipoles and anti-phase boundaries. We note that the proposed method can be easily adopted for doping different types of nanostructures, and fabricating superlattices and multiple quantum wells on NWs in a controllable way.

Superior flexibility of a wrinkled carbon shell under electrochemical cycling

Nanocarbon composites have been extensively employed in engineering alloy-type anodes in order to improve the poor cyclability caused by the enormous volume changes during lithium (Li+) insertion/extraction. The chemical vapor deposited wrinkled carbon shell (WCS) shows high electrical conductivity, excellent thermal stability and remarkable mechanical robustness, which help in retaining the structural integrity around the tin (Sn) anode core despite ∼250% variation in volume during repetitive lithiation and delithiation. In situ transmission electron microscopy reveals no embrittlement in the lithiated WCS, which fully recovers its original shape after severe mechanical deformation with no obvious structural change. Further analysis indicates that the capacity to accommodate large strains is closely related to the construction of the carbon shell, that is, the stacking of wrinkled few-layer graphenes. Both the pre-existing wrinkles and the few-layer thickness render the carbon shell superior flexibility and good elasticity under bending or expansion of the interior volume. Moreover, the WCS possesses fast lithium ion diffusion channels, which have lower activation barriers (∼0.1 eV) than that on a smooth graphene (∼0.3 eV). The results provide an insight into the improvement in cycle performance that can be achieved through carbon coating of anodes of lithium ion batteries.

Controllable synthesis and in situ TEM study of lithiation mechanism of high performance NaV3O8 cathodes

NaV3O8 nanobelts, nanorods and microrods have been successfully synthesized using a facile, one-step solid-state sintering method. The morphology, crystallinity and purity of NaV3O8 can be easily controlled by the calcination temperature. As a cathode material for Li-ion batteries, NaV3O8 nanorods synthesized at 450 °C show a relatively higher specific discharge capacity of 226 mA h g−1 at 30 mA g−1 and a good cycling performance without considerable capacity loss over 100 cycles at 100 and 300 mA g−1. In situ TEM characterization confirmed that the intercalation/deintercalation of Li+ ions in NaV3O8 is a single-phase reaction process with small lattice change, which can result in obvious cracks and fractures. The SEM characterizations of the electrodes after cycling reveal that the structure destruction is the main reason for the capacity fading of NaV3O8.

Microstructure-dependent conformal atomic layer deposition on 3D nanotopography.

The capability of atomic layer deposition (ALD) to coat conformally complex 3D nanotopography has been examined by depositing amorphous, polycrystalline, and single-crystal TiO(2) films over SnO(2) nanowires (NWs). Structural characterizations reveal a strong correlation between the surface morphology and the microstructures of ALD films. Conformal growth can only be rigorously achieved in amorphous phase with circular sectors developed at sharp asperities. Morphology evolution convincingly demonstrates the principle of ALD, i.e., sequential and self-limiting surface reactions result in smooth and conformal films. Orientation-dependent growth and surface reconstruction generally lead to nonconformal coating in polycrystalline and single-crystal films. Especially, an octagonal single-crystal TiO(2) shell was derived from a rectangular SnO(2) NW core, which was the consequence of both self-limited growth kinetics and surface reconstruction. Models were proposed to explain the conformality of ALD deposition over 3D nanostructures by taking account of the underlying microstructures. Besides the surface morphologies, the microstructures also have significant consequence to the surface electronic states, characterized by the broad band photoluminescence. The comparison study suggests that ALD process is determined by the interplay of both thermodynamic and kinetic factors.

Activated Carbon Modified by CNTs/Ni-Co Oxide as Hybrid Electrode Materials for High Performance Supercapacitors

Hybrid materials of carbon nanotubes (CNTs) coated with nickel-cobalt (Ni-Co) oxide nanoparticles were synthesized using electroless plating. Transmission electron microscopy images showed that Ni-Co oxide nanoparticles were dispersively distributed on the external surface of CNTs. The composites of Ni-Co oxides modified CNTs were used as additives of activated carbon to improve the electrochemical performance of the electrode materials for supercapacitors. The electrochemical properties of the supercapacitors were investigated by galvanostatic charge-discharge, cyclic voltammetry, and alternating current impedance techniques. The maximum specific capacitance reached 215 F g-1, approximately 23% higher than that without addition of the CNT-based composite, while the resistance of the electrode was also reduced by addition of the composite. The results revealed that the supercapacitor had an excellent charge-discharge cycle behavior and electrochemical stability after 1200 continuous cycles. The improved performance of the supercapacitor can be attributed to the modified structure and high electrical conductivity of the electrode materials due to the addition of the hybrid nanocomposite, which is promising for energy storage applications.

Dynamic morphology instability in epitaxial ZnO/AZO (aluminum-doped ZnO) core–shell nanowires

Misfit strain relaxation-induced morphology instability is usually observed in epitaxial heterostructures at high temperatures. In this paper, we report that this morphology instability can occur even at room temperature in epitaxial ZnO/AZO (Al-doped ZnO) core–shell nanowires (NWs). As a result, densely distributed ZnO nanodots (NDs) were self-assembled on the NWs. The growth of NDs was slowed down during aging owing to the gradually reduced misfit strain. The final size and shape of the NDs were highly depended on the shell thickness and the doping ratio. It was proved that the morphology stability could be improved by surface passivation, thinning the shell thickness, or lowering the doping ratio. The results may provide instructive suggestions for the reliable design in strain and surface engineering of nanomaterials.

Nickel-cobalt oxide coated CNTs as additives of activated carbon electrode for high-performance supercapacitors

Nanocomposites of carbon nanotubes (CNTs) coated with nickel-cobalt (Ni-Co) oxide nanoparticles were synthesized using electroless plating. Their morphology and microstructure were investigated by transmission electron microscope (TEM) and X-ray diffraction (XRD), showing that the Ni-Co oxide nanoparticles were uniformly distributed on the outer surface of CNTs and formed nanocrystals. The Ni-Co oxides modified CNTs were added into activated carbon (AC) electrodes of supercapacitors to improve their performance. The electrochemical properties of the supercapacitors were investigated by cyclic voltammetry (CV), alternating current impedance techniques, and charge-discharge curve. A maximum specific capacitance of 215 F·g-1 was achieved, which is ca. 23% higher than that without the addition of the composite, and the electrode resistance is lower than that of other electrodes. The enhanced performance of the supercapacitor is attributed to the modified microstructure and the increase of electrical conductivity of the electrode due to the addition of the nanocomposite. Supercapacitors with such kind of electrode materials have potentials for microelectronic and energy storage devices and systems.