Non-Blinking Luminescence from Charged Single Graphene Quantum Dots.

Photoluminescence blinking behavior from single quantum dots under steady illumination is an important but controversial topic. Its occurrence has impeded the use of single quantum dots in bioimaging. Different mechanisms have been proposed to account for it, although controversial, the most important of which is the non-radiative Auger recombination mechanism whereby photocharging of quantum dots can lead to the blinking phenomenon. Here, the singly charged trion, which maintains photon emission, including radiative recombination and non-radiative Auger recombination, leads to fluorescence non-blinking which is observed in photocharged single graphene quantum dots (GQDs). This phenomenon can be explained in terms of different energy levels in the GQDs, caused by various oxygen-containing functional groups in the single GQDs. The suppressed blinking is due to the filling of trap sites owing to a Coulomb blockade. These results provide a profound understanding of the special optical properties of GQDs, affording a reference for further in-depth research.

Design principles for heterointerfacial alloying kinetics at metallic anodes in rechargeable batteries.

How surface chemistry influences reactions occurring thereupon has been a long-standing question of broad scientific and technological interest. Here, we consider the relation between the surface chemistry at interfaces and the reversibility of electrochemical transformations at rechargeable battery electrodes. Using Zn as a model system, we report that a moderate strength of chemical interaction between the deposit and the substrate-neither too weak nor too strong-enables highest reversibility and stability of the plating/stripping redox processes. Focused ion beam and electron microscopy were used to directly probe the morphology, chemistry, and crystallography of heterointerfaces of distinct natures. Analogous to the empirical Sabatier principle for chemical heterogeneous catalysis, our findings arise from competing interfacial processes. Using full batteries with stringent negative electrode-to-positive electrode capacity (N:P) ratios, we show that such knowledge provides a powerful tool for designing key materials in highly reversible battery systems based on Earth-abundant, low-cost metals such as Zn and Na.

Designing interphases for practical aqueous zinc flow batteries with high power density and high areal capacity.

Aqueous zinc flow batteries (AZFBs) with high power density and high areal capacity are attractive, both in terms of cost and safety. A number of fundamental challenges associated with out-of-plane growth and undesirable side reactions on the anode side, as well as sluggish reaction kinetics and active material loss on the cathode side, limit practical deployment of these batteries. We investigated artificial interphases created using a simple electrospray methodology as a strategy for addressing each of these challenges. The effectiveness of the electrospray interphases in full cell zinc-iodine flow batteries was evaluated and reported; it is possible to simultaneously achieve high power density [115 milliwatts per square centimeter (mW/cm2)] and high areal capacity [25 milliampere hour per square centimeter (mA·hour/cm2)]. Last, we extended it to aqueous zinc-bromine and zinc-vanadium flow batteries of contemporary interest. It is again found that high power density (255 and 260 mW/cm2, respectively) and high areal capacity (20 mA·hour/cm2) can be simultaneously achieved in AZFBs.

Highly Reversible Sodium Metal Battery Anodes via Alloying Heterointerfaces.

As a promising pathway toward low-cost, long-duration energy storage, rechargeable sodium batteries are of increasing interest. Batteries that incorporate metallic sodium as anode promise a high theoretical specific capacity of 1166 mAh g-1 , and low reduction potential of -2.71 V. The high reactivity and poor electrochemical reversibility of sodium anodes render sodium metal anode (SMA) cells among the most challenging for practical implementation. Here, the failure mechanisms of Na anodes are investigated and the authors report that loss of morphological control is not the fundamental cause of failure. Rather, it is the inherently poor anchoring/root structure of electrodeposited Na to the electrode substrate that leads to poor reversibility and cell failure. Poorly anchored Na deposits are prone to break away from the current collector, producing orphaning and poor anode utilization. Thin metallic coatings in a range of chemistries are proposed and evaluated as SMA substrates. Based on thermodynamic and ion transport considerations, such substrates undergo reversible alloying reactions with Na and are hypothesized to promote good root growth-regardless of the morphology. Among the various options, Au stands out for its ability to support long Na anode lifetime and high reversibility (Coulombic Efficiency > 98%), for coating thicknesses in the range of 10-1000 nm. As a first step toward evaluating practical utility of the anodes, their performance in Na||SPAN cells with N:P ratio close to 1:1 is evaluated.

Production of fast-charge Zn-based aqueous batteries via interfacial adsorption of ion-oligomer complexes.

Aqueous zinc batteries are attracting interest because of their potential for cost-effective and safe electricity storage. However, metallic zinc exhibits only moderate reversibility in aqueous electrolytes. To circumvent this issue, we study aqueous Zn batteries able to form nanometric interphases at the Zn metal/liquid electrolyte interface, composed of an ion-oligomer complex. In Zn||Zn symmetric cell studies, we report highly reversible cycling at high current densities and capacities (e.g., 160 mA cm-2; 2.6 mAh cm-2). By means of quartz-crystal microbalance, nuclear magnetic resonance, and voltammetry measurements we show that the interphase film exists in a dynamic equilibrium with oligomers dissolved in the electrolyte. The interphase strategy is applied to aqueous Zn||I2 and Zn||MnO2 cells that are charged/discharged for 12,000 cycles and 1000 cycles, respectively, at a current density of 160 mA cm-2 and capacity of approximately 0.85 mAh cm-2. Finally, we demonstrate that Zn||I2-carbon pouch cells (9 cm2 area) cycle stably and deliver a specific energy of 151 Wh/kg (based on the total mass of active materials in the electrode) at a charge current density of 56 mA cm-2.

Textured Electrodes: Manipulating Built-In Crystallographic Heterogeneity of Metal Electrodes via Severe Plastic Deformation.

Control of crystallography of metal electrodeposit films has recently emerged as a key to achieving long operating lifetimes in next-generation batteries. It is reported that the large crystallographic heterogeneity, e.g., broad orientational distribution, that appears characteristic of commercial metal foils, results in rough morphology upon plating/stripping. On this basis, an accumulative roll bonding (ARB) methodology-a severe plastic deformation process-is developed. Zn metal is used as a first example to interrogate the concept. It is demonstrated that the ARB process is highly effective in achieving uniform crystallographic control on macroscopic materials. After the ARB process, the Zn grains exhibit a strong (002) texture (i.e., [002]Zn //ND). The texture transitions from a classical bipolar pattern to a nonclassical unipolar pattern under large nominal strain eliminate the orientational heterogeneity of the foil. The strongly (002)-textured Zn remarkably improves the plating/stripping performance by nearly two orders of magnitude under practical conditions. The performance improvements are readily scaled to achieve pouch-type full batteries that deliver exceptional reversibility. The ARB process can, in principle, be applied to any metal chemistry to achieve similar crystallographic uniformity, provided the appropriate temperature and accumulated strains are employed. This concept is evaluated using commercial Li and Na foils, which, unlike Zn (HCP), are BCC crystals. The simple process for creating strong textures in both hexagonal and cubic metals and illustrating the critical role such built-in crystallography plays underscores opportunities for developing highly reversible thin metal anodes (e.g., hexagonal Zn, Mg, and cubic Li, Na, Ca, Al).

On the crystallography and reversibility of lithium electrodeposits at ultrahigh capacity.

Lithium metal is a promising anode for energy-dense batteries but is hindered by poor reversibility caused by continuous chemical and electrochemical degradation. Here we find that by increasing the Li plating capacity to high values (e.g., 10-50 mAh cm-2), Li deposits undergo a morphological transition to produce dense structures, composed of large grains with dominantly (110)Li crystallographic facets. The resultant Li metal electrodes manifest fast kinetics for lithium stripping/plating processes with higher exchange current density, but simultaneously exhibit elevated electrochemical stability towards the electrolyte. Detailed analysis of these findings reveal that parasitic electrochemical reactions are the major reason for poor Li reversibility, and that the degradation rate from parasitic electroreduction of electrolyte components is about an order of magnitude faster than from chemical reactions. The high-capacity Li electrodes provide a straightforward strategy for interrogating the solid electrolyte interphase (SEI) on Li -with unprecedented, high signal to noise. We find that an inorganic rich SEI is formed and is primarily concentrated around the edges of lithium particles. Our findings provide straightforward, but powerful approaches for enhancing the reversibility of Li and for fundamental studies of the interphases formed in liquid and solid-state electrolytes using readily accessible analytical tools.

Stabilizing Zinc Electrodeposition in a Battery Anode by Controlling Crystal Growth.

Reversible electrodeposition of metals at liquid-solid interfaces is a requirement for long cycle life in rechargeable batteries that utilize metals as anodes. The process has been studied extensively from the perspective of the electrochemical transformations that impact reversibility, however, the fundamental challenges associated with maintaining morphological control when a intrinsically crystalline solid metal phase emerges from an electrolyte solution have been less studied, but provide important opportunities for progress. A crystal growth stabilization method to reshape the initial growth and orientation of crystalline metal electrodeposits is proposed here. The method takes advantage of polymer-salt complexes (PEG-Zn2+ -aX- ) (a = 1,2,3) formed spontaneously in aqueous electrolytes containing zinc (Zn2+ ) and halide (X- ) ions to regulate electro-crystallization of Zn. It is shown that when X = Iodine (I), the complexes facilitate electrodeposition of Zn in a hexagonal closest packed morphology with preferential orientation of the (002) plane parallel to the electrode surface. This facilitates exceptional morphological control of Zn electrodeposition at planar substrates and leads to high anode reversibility and unprecedented cycle life. Preliminary studies of the practical benefits of the approach are demonstrated in Zn-I2 full battery cells, designed in both coin cell and single-flow battery cell configurations.

The early-stage growth and reversibility of Li electrodeposition in Br-rich electrolytes.

The physiochemical nature of reactive metal electrodeposits during the early stages of electrodeposition is rarely studied but known to play an important role in determining the electrochemical stability and reversibility of electrochemical cells that utilize reactive metals as anodes. We investigated the early-stage growth dynamics and reversibility of electrodeposited lithium in liquid electrolytes infused with brominated additives. On the basis of equilibrium theories, we hypothesize that by regulating the surface energetics and surface ion/adatom transport characteristics of the interphases formed on Li, Br-rich electrolytes alter the morphology of early-stage Li electrodeposits; enabling late-stage control of growth and high electrode reversibility. A combination of scanning electron microscopy (SEM), image analysis, X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and contact angle goniometry are employed to evaluate this hypothesis by examining the physical-chemical features of the material phases formed on Li. We report that it is possible to achieve fine control of the early-stage Li electrodeposit morphology through tuning of surface energetic and ion diffusion properties of interphases formed on Li. This control is shown further to translate to better control of Li electrodeposit morphology and high electrochemical reversibility during deep cycling of the Li metal anode. Our results show that understanding and eliminating morphological and chemical instabilities in the initial stages of Li electroplating via deliberately modifying energetics of the solid electrolyte interphase (SEI) is a feasible approach in realization of deeply cyclable reactive metal batteries.

Spontaneous and field-induced crystallographic reorientation of metal electrodeposits at battery anodes.

The propensity of metal anodes of contemporary interest (e.g., Li, Al, Na, and Zn) to form non-planar, dendritic morphologies during battery charging is a fundamental barrier to achievement of full reversibility. We experimentally investigate the origins of dendritic electrodeposition of Zn, Cu, and Li in a three-electrode electrochemical cell bounded at one end by a rotating disc electrode. We find that the classical picture of ion depletion-induced growth of dendrites is valid in dilute electrolytes but is essentially irrelevant in the concentrated (≥1 M) electrolytes typically used in rechargeable batteries. Using Zn as an example, we find that ion depletion at the mass transport limit may be overcome by spontaneous reorientation of Zn crystallites from orientations parallel to the electrode surface to dominantly homeotropic orientations, which appear to facilitate contact with cations outside the depletion layer. This chemotaxis-like process causes obvious texturing and increases the porosity of metal electrodeposits.

Proton Intercalation/De-Intercalation Dynamics in Vanadium Oxides for Aqueous Aluminum Electrochemical Cells.

Understanding cation (H+ , Li+ , Na+ , Al3+ , etc.) intercalation/de-intercalation chemistry in transition metal compounds is crucial for the design of cathode materials in aqueous electrochemical cells. Here we report that orthorhombic vanadium oxides (V2 O5 ) supports highly reversible proton intercalation/de-intercalation reactions in aqueous media, enabling aluminum electrochemical cells with extended cycle life. Empirical analyses using vibrational and x-ray spectroscopy are complemented with theoretical analysis of the electrostatic potential to establish how and why protons intercalate in V2 O5 in aqueous media. We show further that cathode coatings composed of cation selective membranes provide a straightforward method for enhancing cathode reversibility by preventing anion cross-over in aqueous electrolytes. Our work sheds light on the design of cation transport requirements for high-energy reversible cathodes in aqueous electrochemical cells.

Reversible epitaxial electrodeposition of metals in battery anodes.

The propensity of metals to form irregular and nonplanar electrodeposits at liquid-solid interfaces has emerged as a fundamental barrier to high-energy, rechargeable batteries that use metal anodes. We report an epitaxial mechanism to regulate nucleation, growth, and reversibility of metal anodes. The crystallographic, surface texturing, and electrochemical criteria for reversible epitaxial electrodeposition of metals are defined and their effectiveness demonstrated by using zinc (Zn), a safe, low-cost, and energy-dense battery anode material. Graphene, with a low lattice mismatch for Zn, is shown to be effective in driving deposition of Zn with a locked crystallographic orientation relation. The resultant epitaxial Zn anodes achieve exceptional reversibility over thousands of cycles at moderate and high rates. Reversible electrochemical epitaxy of metals provides a general pathway toward energy-dense batteries with high reversibility.

Reversible Electrochemical Lithium-Ion Insertion into the Rhenium Cluster Chalcogenide-Halide Re6Se8Cl2.

The cluster-based material Re6Se8Cl2 is a two-dimensional ternary material with cluster-cluster bonding across the a and b axes capable of multiple electron transfer accompanied by ion insertion across the c axis. The Li/Re6Se8Cl2 system showed reversible electron transfer from 1 to 3 electron equivalents (ee) at high current densities (88 mA/g). Upon cycling to 4 ee, there was evidence of capacity degradation over 50 cycles associated with the formation of an organic solid-electrolyte interface (between 1.45 and 1 V vs Li/Li+). This investigation highlights the ability of cluster-based materials with two-dimensional cluster bonding to be used in applications such as energy storage, showing structural stability and high rate capability.

Magnesium-ion battery-relevant electrochemistry of MgMn2O4: crystallite size effects and the notable role of electrolyte water content.

MgMn2O4 nanoparticles with crystallite sizes of 11 (MMO-1) and 31 nm (MMO-2) were synthesized and their magnesium-ion battery-relevant electrochemistry was investigated. MMO-1 delivered an initial capacity of 220 mA h g-1 (678 mW h g-1). Electrolyte water content had a profound effect on cycle retention.

Synthesis and photocatalytic activity of single-crystalline hollow rh-In2O3 nanocrystals.

We report here for the first time the hollow, metastable, single-crystal, rhombohedral In(2)O(3) (rh-In(2)O(3)) nanocrystals synthesized by annealing solvothermally prepared InOOH solid nanocrystals under ambient pressure at 400 °C, through a mechanism of the Kirkendall effect, in which pore formation is attributed to the difference in diffusion rates of anions (OH(-) and O(2-)) in a diffusion couple. The InOOH solid nanocrystals were prepared via a controlled hydrolysis solvothermal route by using In(NO(3))(3)·4.5H(2)O as a starting material and glycerol-ethanol as a mixed solvent. The glycerol-ethanol mixed solvent plays a key role on the formation of the intermediate InOOH, thus the final product of rh-In(2)O(3). The as-synthesized In(2)O(3) nanocrystals present excellent photocatalytic degradation of rhodamine B (RhB) and methylene blue (MB) dyes, which present ∼92% degradation of RhB or MB after 4 or 3 h reaction in the presence of the as-synthesized In(2)O(3) nanocrystals, respectively.

Improved performances of ß-Ni(OH)2@reduced-graphene-oxide in Ni-MH and Li-ion batteries.

Incorporation of reduced graphene oxide into ß-Ni(OH)(2) presents high performances with specific discharge capacity of 283 mA hg(-1) after 50 cycles in Ni-MH batteries, and 507 mA hg(-1) after 30 cycles in Li ion batteries.

MgCO3·3H2O and MgO complex nanostructures: controllable biomimetic fabrication and physical chemical properties.

In this paper, we report a method of biomimetic synthesis of MgCO(3)·3H(2)O and MgO Viburnum opulus-like complex nanostructures with superhydrophobicity and adsorption properties. The MgCO(3)·3H(2)O complex nanostructures can be obtained by changing experimental parameters, including concentrations of reactants (dextran and MgCl(2)), molar ratios of reactants, and reaction time. The phase structure of as-synthesized samples was characterized by X-ray diffraction (XRD). The morphology and structure are studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared (FT-IR) spectroscopy. The MgCO(3)·3H(2)O complex nanostructures exhibited superhydrophobicity, due to their unique superstructures, and was proved by the contact angle (CA) measurement. We also show that a simple calcination of these unusually shaped MgCO(3)·3H(2)O results in spontaneous formation of MgO complex nanostructures while the unique shape can be maintained, and the as-synthesized MgO nanostructures show excellent adsorption property. These unique structures and properties will open up a wide range of potential applications in material and environmental protection.