Kostiantyn V. Kravchyk

Monodisperse Antimony Nanocrystals for High-Rate Li-ion and Na-ion Battery Anodes: Nano versus Bulk

We report colloidal synthesis of antimony (Sb) nanocrystals with mean size tunable in the 10-20 nm range and with narrow size distributions of 7-11%. In comparison to microcrystalline Sb, 10 and 20 nm Sb nanocrystals exhibit enhanced rate-capability and higher cycling stability as anode materials in rechargeable Li-ion and Na-ion batteries. All three particle sizes of Sb possess high and similar Li-ion and Na-ion charge storage capacities of 580-640 mAh g(-1) at moderate charging/discharging current densities of 0.5-1C (1C-rate is 660 mA g(-1)). At all C-rates (0.5-20C, e.g. current densities of 0.33-13.2 Ag(1-)), capacities of 20 nm Sb particles are systematically better than for both 10 nm and bulk Sb. At 20C-rates, retention of charge storage capacities by 10 and 20 nm Sb nanocrystals can reach 78-85% of the low-rate value, indicating that rate capability of Sb nanostructures can be comparable to the best Li-ion intercalation anodes and is so far unprecedented for Na-ion storage.

Monodisperse and Inorganically Capped Sn and Sn/SnO2 Nanocrystals for High-Performance Li-Ion Battery Anodes

We report a facile synthesis of highly monodisperse colloidal Sn and Sn/SnO2 nanocrystals with mean sizes tunable over the range 9-23 nm and size distributions below 10%. For testing the utility of Sn/SnO2 nanocrystals as an active anode material in Li-ion batteries, a simple ligand-exchange procedure using inorganic capping ligands was applied to facilitate electronic connectivity within the components of the nanocrystalline electrode. Electrochemical measurements demonstrated that 10 nm Sn/SnO2 nanocrystals enable high Li insertion/removal cycling stability, in striking contrast to commercial 100-150 nm powders of Sn and SnO2. In particular, reversible Li-storage capacities above 700 mA h g(-1) were obtained after 100 cycles of deep charging (0.005-2 V) at a relatively high current of 1000 mA h g(-1).

Unraveling the Core–Shell Structure of Ligand-Capped Sn/SnOx Nanoparticles by Surface-Enhanced Nuclear Magnetic Resonance, Mössbauer, and X-ray Absorption Spectroscopies

A particularly difficult challenge in the chemistry of nanomaterials is the detailed structural and chemical analysis of multicomponent nano-objects. This is especially true for the determination of spatially resolved information. In this study, we demonstrate that dynamic nuclear polarization surface-enhanced solid-state NMR spectroscopy (DNP-SENS), which provides selective and enhanced NMR signal collection from the (near) surface regions of a sample, can be used to resolve the core-shell structure of a nanoparticle. Li-ion anode materials, monodisperse 10-20 nm large tin nanoparticles covered with a ∼3 nm thick layer of native oxides, were used in this case study. DNP-SENS selectively enhanced the weak 119Sn NMR signal of the amorphous surface SnO2 layer. Mössbauer and X-ray absorption spectroscopies identified a subsurface SnO phase and quantified the atomic fractions of both oxides. Finally, temperature-dependent X-ray diffraction measurements were used to probe the metallic β-Sn core and indicated that even after 8 months of storage at 255 K there are no signs of conversion of the metallic β-Sn core into a brittle semiconducting α-phase, a phase transition which normally occurs in bulk tin at 286 K (13 °C). Taken together, these results indicate that Sn/SnOx nanoparticles have core/shell1/shell2 structure of Sn/SnO/SnO2 phases. The study suggests that DNP-SENS experiments can be carried on many types of uniform colloidal nanomaterials containing NMR-active nuclei, in the presence of either hydrophilic (ion-capped surfaces) or hydrophobic (capping ligands with long hydrocarbon chains) surface functionalities.

Colloidal Tin–Germanium Nanorods and Their Li-Ion Storage Properties

We report a facile colloidal synthesis of tin-germanium (Sn-Ge) heterostructures in the form of nanorods with a small aspect ratio of 1.5-3 and a length smaller than 50 nm. In the two-step synthesis, presynthesized Sn nanoparticles act as a low-melting-point catalyst for decomposing the Ge precursor, bis[bis(trimethylsilyl)amido]Ge(II), and for crystallization of Ge via solution-liquid-solid growth mechanism. Creation of such Sn-Ge nanoheterodimers can serve as a well-controlled method of mixing these nearly immiscible chemical elements for the purpose of obtaining Sn-Ge nanocomposite electrodes for high-energy density Li-ion batteries. Comparable mass content of Sn and Ge leads to synergistic effects in electrochemical performance: high charge storage capacity above 1000 mAh g(-1) at a relatively high current density of 1 A g(-1) is due to high theoretical capacity of Ge, while high rate capability is presumably caused by the enhancement of electronic transport by metallic Sn. At a current density of 4 A g(-1), Sn-Ge nanocomposite electrodes retain up to 80% of the capacity obtained at a lower current density of 0.2 A g(-1). Temporally separated lithiation of both elements, Sn and Ge, at different electrochemical potentials is proposed as a main factor for the overall improvement of the cycling stability.

Zeolite-Templated Carbon as an Ordered Microporous Electrode for Aluminum Batteries

High surface area porous carbon frameworks exhibit potential advantages over crystalline graphite as an electrochemical energy storage material owing to the possibility of faster ion transport and up to double the ion capacity, assuming a surface-based mechanism of storage. When detrimental surface-related effects such as irreversible capacity loss due to interphase formation (known as solid-electrolyte interphase, SEI) can be mitigated or altogether avoided, the greatest advantage can be achieved by maximizing the gravimetric and volumetric surface area and by tailoring the porosity to accommodate the relevant ion species. We investigate this concept by employing zeolite-templated carbon (ZTC) as the cathode in an aluminum battery based on a chloroaluminate ionic liquid electrolyte. Its ultrahigh surface area and dense, conductive network of homogeneous channels (12 Å in width) render ZTC suitable for the fast, dense storage of AlCl4- ions (6 Å in ionic diameter). With aluminum as the anode, full cells were prepared which simultaneously exhibited both high specific energy (up to 64 Wh kg-1, 30 Wh L-1) and specific power (up to 290 W kg-1, 93 W L-1), highly stable cycling performance, and complete reversibility within the potential range of 0.01-2.20 V.

Monodisperse colloidal gallium nanoparticles: synthesis, low temperature crystallization, surface plasmon resonance and Li-ion storage.

We report a facile colloidal synthesis of gallium (Ga) nanoparticles with the mean size tunable in the range of 12–46 nm and with excellent size distribution as small as 7–8%. When stored under ambient conditions, Ga nanoparticles remain stable for months due to the formation of native and passivating Ga-oxide layer (2–3 nm). The mechanism of Ga nanoparticles formation is elucidated using nuclear magnetic resonance spectroscopy and with molecular dynamics simulations. Size-dependent crystallization and melting of Ga nanoparticles in the temperature range of 98–298 K are studied with X-ray powder diffraction, specific heat measurements, transmission electron microscopy, and X-ray absorption spectroscopy. The results point to delta (δ)-Ga polymorph as a single low-temperature phase, while phase transition is characterized by the large hysteresis and by the large undercooling of crystallization and melting points down to 140–145 and 240–250 K, respectively. We have observed size-tunable plasmon resonance in the ultraviolet and visible spectral regions. We also report stable operation of Ga nanoparticles as anode material for Li-ion batteries with storage capacities of 600 mAh g–1, 50% higher than those achieved for bulk Ga under identical testing conditions.

Evaluation of Metal Phosphide Nanocrystals as Anode Materials for Na-ion Batteries.

Sodium-ion batteries (SIBs) are potential low-cost alternatives to lithium-ion batteries (LIBs) because of the much greater natural abundance of sodium salts. However, developing high-performance electrode materials for SIBs is a challenging task, especially due to the ∼50% larger ionic radius of the Na(+) ion compared to Li(+), leading to vastly different electrochemical behavior. Metal phosphides such as FeP, CoP, NiP(2), and CuP(2) remain unexplored as electrode materials for SIBs, despite their high theoretical charge storage capacities of 900-1300 mAh g(-1). Here we report on the synthesis of metal phosphide nanocrystals (NCs) and discuss their electrochemical properties as anode materials for SIBs, as well as for LIBs. We also compare the electrochemical characteristics of phosphides with their corresponding sulfides, using the environmentally benign iron compounds, FeP and FeS(2), as a case study. We show that despite the appealing initial charge storage capacities of up to 1200 mAh g(-1), enabled by effective nanosizing of the active electrode materials, further work toward optimization of the electrode/electrolyte pair is needed to improve the electrochemical performance upon cycling.

Polypyrenes as High-Performance Cathode Materials for Aluminum Batteries

The pressing need for low-cost and large-scale stationary storage of electricity has led to a new wave of research on novel batteries made entirely of components that have high natural abundances and are easy to manufacture. One example of such an anode-electrolyte-cathode architecture comprises metallic aluminum, AlCl3 :[EMIm]Cl (1-ethyl-3-methylimidazolium chloride) ionic liquid and graphite. Various forms of synthetic and natural graphite cathodes have been tested in recent years in this context. Here, a new type of compelling cathode based on inexpensive pyrene polymers is demonstrated. During charging, the condensed aromatic rings of these polymers are oxidized, which is accompanied by the uptake of aluminum tetrachloride anions (AlCl4- ) from the chloroaluminate ionic liquid. Discharge is the fast inverse process of reduction and the release of AlCl4- . The electrochemical properties of the polypyrenes can be fine-tuned by the appropriate chemical derivatization. This process is showcased here by poly(nitropyrene-co-pyrene), which has a storage capacity of 100 mAh g-1 , higher than the neat polypyrene (70 mAh g-1 ) or crystalline pyrene (20 mAh g-1 ), at a high discharge voltage (≈1.7 V), energy efficiency (≈86%), and cyclic stability (at least 1000 cycles).

Nanocrystalline FeF3 and MF2 (M = Fe, Co, and Mn) from metal trifluoroacetates and their Li(Na)-ion storage properties

With demands placed on batteries constantly increasing, new positive electrode materials with higher energy density, satisfactory power density, and long-term cycling capabilities, composed of highly abundant elements with low-cost, are desired. One such low-cost cathodic material is iron(III) fluoride – FeF3. Its theoretical capacity for single-electron reduction, accompanied by the insertion of Li+ ions, is 237 mA h g−1. Herein we present a new synthesis for nanocrystalline FeF3 using inexpensive iron trifluoroacetate as a molecular single-source precursor. We also report an adaptation of this simple chemistry to several transition metal difluorides (M = Fe, Co, and Mn). With FeF3, a high capacity of 220 mA h g−1 was attained at a moderate current density of 100 mA g−1 (∼0.5 C). In addition to high capacity, we see the evidence for high rate-capability. Capacities of up to 155 mA h g−1 were observed with 1-minute (10 A g−1) charge–discharge ramps, and at least 88% of this capacity was retained after 100 cycles. When tested as a sodium cathode, FeF3 exhibits capacities of up to 160 mA h g−1 at a current rate of 200 mA h g−1.

Popcorn-Shaped FexO (Wüstite) Nanoparticles from a Single-Source Precursor: Colloidal Synthesis and Magnetic Properties

Colloidal nanoparticles (NPs) with myriads of compositions and morphologies have been synthesized and characterized in recent years. For wüstite FexO, however, obtaining phase-pure NPs with homogeneous morphologies have remained challenging. Herein, we report the colloidal synthesis of phase-pure FexO (x ≈ 0.94) popcorn-shaped NPs by decomposition of a single-source precursor, [Fe3(μ3-O)(CF3COO)(μ-CF3COO)6(H2O)2]·CF3COOH. The popcorn shape and multigrain structure had been reconstructed using high-angle annular dark-field scanning transmission electron micrograph (HAADF-STEM) tomography. This morphology offers a large surface area and internal channels and prevents further agglomeration and thermal tumbling of the subparticles. [Fe3(μ3-O)(CF3COO)(μ-CF3COO)6(H2O)2]·CF3COOH behaves as an antiferromagnetic triangle whose magnetic frustration is mitigated by the low symmetry of the complex. The popcorn-shaped FexO NPs show the typical wüstite antiferromagnetic transition at approximately 200 K, but behave very differently to their bulk counterpart below 200 K. The magnetization curves show a clear, unsymmetrical hysteresis, which arises from a combined effect of the superparamagnetic behavior and exchange bias.