Tai-Chou Lee

Electrochemical performance of Na/NaFePO4 sodium-ion batteries with ionic liquid electrolytes

Rechargeable Na/NaFePO4 cells with a sodium bis(trifluoromethanesulfonyl)imide (NaTFSI)-incorporated butylmethylpyrrolidinium (BMP)–TFSI ionic liquid (IL) electrolyte are demonstrated with an operation voltage of ∼3 V. High-performance NaFePO4 cathode powder with an olivine crystal structure is prepared by chemical delithiation of LiFePO4 powder followed by electrochemical sodiation of FePO4. This IL electrolyte shows high thermal stability (>400 °C) and non-flammability, and is thus ideal for high-safety applications. The effects of NaTFSI concentration (0.1–1.0 M) on cell performance at 25 °C and 50 °C are studied. At 50 °C, an optimal capacity of 125 mA h g−1 (at 0.05 C) is found for NaFePO4 in a 0.5 M NaTFSI-incorporated IL electrolyte; moreover, 65% of this capacity can be retained when the charge–discharge rate increases to 1 C. This ratio (reflecting the rate capability) is higher than that found in a traditional organic electrolyte. With a 1 M NaTFSI-incorporated IL electrolyte, a 13% cell capacity loss after 100 charge–discharge cycles is measured at 50 °C, compared to the 38% observed in an organic electrolyte under the same conditions.

In Situ Growth of Hollow Gold–Silver Nanoshells within Porous Silica Offers Tunable Plasmonic Extinctions and Enhanced Colloidal Stability

Porous silica-coated hollow gold-silver nanoshells were successfully synthesized utilizing a procedure where the porous silica shell was produced prior to the transformation of the metallic core, providing enhanced control over the structure/composition of the bimetallic hollow core. By varying the reaction time and the precise amount of gold salt solution added to a porous silica-coated silver-core template solution, composite nanoparticles were tailored to reveal a readily tunable surface plasmon resonance that could be centered across the visible and near-IR spectral regions (∼445-800 nm). Characterization by X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, scanning electron microscopy, and transmission electron microscopy revealed that the synthetic methodology afforded particles having uniform composition, size, and shape. The optical properties were evaluated by absorption/extinction spectroscopy. The stability of colloidal solutions of our composite nanoparticles as a function of pH was also investigated, revealing that the nanoshells remain intact over a wide range of conditions (i.e., pH 2-10). The facile tunability, enhanced stability, and relatively small diameter of these composite particles (∼110 nm) makes them promising candidates for use in tumor ablation or as photothermal drug-delivery agents.

Ionic liquid electrolytes for high-voltage rechargeable Li/LiNi0.5Mn1.5O4 cells

A high-voltage LiNi0.5Mn1.5O4 cathode material with a cubic spinel structure is synthesized using a citric-acid-assisted sol–gel process. Butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI)-based ionic liquids (ILs) with various kinds of Li salts, namely LiTFSI, LiPF6, and their mixtures, are used as electrolytes for Li/LiNi0.5Mn1.5O4 cells. The IL electrolytes show high thermal stability (>400 °C) and non-flammability, and are thus ideal for high-safety applications. At 25 °C, LiTFSI is more suitable than LiPF6 as an IL electrolyte in terms of cell capacity, rate capability, and cyclic stability. The IL electrolytes clearly outperform the conventional organic electrolytes at 50 °C, since the latter decomposes at high voltage and corrodes both the Al current collector and LiNi0.5Mn1.5O4, degrading the electrode performance. At such an elevated temperature, using LiPF6 to partially substitute LiTFSI in the IL electrolyte can effectively suppress Al pitting corrosion and thus improves the cell performance. In the 0.4 M LiTFSI/0.6 M LiPF6 mixed-salt IL electrolyte, an LiNi0.5Mn1.5O4 discharge capacity of 115 mA h g−1 (at 0.1 C) is obtained at 50 °C with a high cell voltage of ∼4.7 V.

Plasmonically Enhanced Photocatalytic Hydrogen Production from Water: The Critical Role of Tunable Surface Plasmon Resonance from Gold–Silver Nanoshells

Gold-silver nanoshells (GS-NSs) having a tunable surface plasmon resonance (SPR) were employed to facilitate charge separation of photoexcited carriers in the photocalytic production of hydrogen from water. Zinc indium sulfide (ZnIn2S4; ZIS), a visible-light-active photocatalyst, where the band gap varies with the [Zn]/[In] ratio, was used as a model ZIS system (E(g) = 2.25 eV) to investigate the mechanisms of plasmonic enhancement associated with the nanoshells. Three types of GS-NS cores with intense absorptions centered roughly at 500, 700, and 900 nm were used as seeds for preparing GS-NS@ZIS core-shell structures via a microwave-assisted hydrothermal reaction, yielding core-shell particles with composite diameters of ∼200 nm. Notably, an interlayer of dielectric silica (SiO2) between the GS-NSs and the ZIS photocatalyst provided another parameter to enhance the production of hydrogen and to distinguish the charge-transfer mechanisms. In particular, the direct transfer of hot electrons from the GS-NSs to the ZIS photocatalyst was blocked by this layer. Of the 10 particle samples examined in this study, the greatest hydrogen gas evolution rate was observed for GS-NSs having a SiO2 interlayer thickness of ∼17 nm and an SPR absorption centered at ∼700 nm, yielding a rate 2.6 times higher than that of the ZIS without GS-NSs. The apparent quantum efficiencies for these core-shell particles were recorded and compared to the absorption spectra. Analyses of the charge-transfer mechanisms were evaluated and are discussed based on the experimental findings.

Relationship of cathode pore-size distribution and rated capacity in Li/MnO2 cells

Abstract An improved MnO 2 cathode has been evaluated. It was found that the high porosity and the pore-size distribution (macropores with a diameter > 2000 A representing >25% of the total pore volume) of the electrode are two determinant factors in performing a high (80%) utilization of the Li/MnO 2 at high discharge rate.

Electrochemical properties of an AgInS2 photoanode prepared using ultrasonic-assisted chemical bath deposition

This study focuses on preparing a AgInS2 film electrode and studying its electrochemical properties. The AgInS2 film after 400 °C thermal treatment had the orthorhombic structure and a direct energy band gap of 1.98 eV. The thickness of AgInS2 film used in this study was 758.9 ± 40.9 nm. In order to understand the photoelectrochemical properties, electrochemical impedances of the AgInS2 photoanode in response to a light intensity of 75 mW cm−2 were scrutinized. It was found that homogeneous AgInS2 films were obtained with increasing coatings. In addition, these dense films can effectively suppress the dark current. Charge transfer resistance and space charge capacitance can be retrieved from impedance spectra by fitting the experimental data to the models. In fact, Randles model fitted the data better than other complicated models. Under illumination, the space charge capacitance and charge transfer resistance are strongly correlated to the onset of the photo-enhanced current density, suggesting a direct carrier transfer to the electrolyte from the valence band of the semiconductor photoanode, rather than from the surface states.

Eco-Efficient Synthesis of Highly Porous CoCO3 Anodes from Supercritical CO2 for Li+ and Na+ Storage

An eco-efficient synthetic route for the preparation of high-performance carbonate anodes for Li+ and Na+ batteries is developed. With supercritical CO2 (scCO2 ) as the precursor, which has gas-like diffusivity, extremely low viscosity, and near-zero surface tension, CoCO3 particles are uniformly formed and tightly connected on graphene nanosheets (GNSs). This synthesis can be conducted at 50 °C, which is considerably lower than the temperature required for conventional preparation methods, minimizing energy consumption. The obtained CoCO3 particles (ca. 20 nm in diameter), which have a unique interpenetrating porous structure, can increase the number of electroactive sites, promote electrolyte accessibility, shorten ion diffusion length, and readily accommodate the strain generated upon charging/discharging. With a reversible capacity of 1105 mAh g-1 , the proposed CoCO3 /GNS anode shows an excellent rate capability, as it can deliver 745 mAh g-1 in 7.5 min. More than 98 % of the initial capacity is retained after 200 cycles. These properties are clearly superior to those of previously reported CoCO3 -based electrodes for Li+ storage, indicating the merit of our scCO2 -based synthesis, which is facile, green, and can be easily scaled up for mass production.

An electrochemical investigation of the temperature dependence of inorganic electrolytes in rechargeable lithium batteries

Abstract The temperature dependence on the electrolyte stability of LiAlCl 4 and LiGaCl 4 in sulfur dioxide on a platinum electrode was studied by cyclic voltammetry and conductivity methods, whereas the temperature dependence on the electrolyte stability and electrode film conductivity on a lithium electrode was studied by the a.c. impedance technique. LiGaCl 4 with a better conductivity behavior is more stable than LiAlCl 4 . For both electrolytes below 25 °C, the conductivity increases with increasing temperature, but above 25 °C it decreases with increasing temperature. The implication of an ionic transport mechanism change is discussed.

Silver-Free Gold Nanocages with Near-Infrared Extinctions

This article reports the preparation of silver-free Au nanocages from cubic palladium templates. Pd nanocubes were subjected to galvanic replacement with Au3+ to produce Pd@Au nanocages having tunable dimensions (i.e., edge length, gold layer thickness, and hollow pore size), which allowed selectable positioning of the optical extinction maxima from the visible to the near infrared. These new nanocages circumvent the problems associated with previous Ag-derived gold alloy nanocages, which suffer from the toxicity of residual silver and the possible fragmentation of such alloyed nanostructures, thereby limiting their potential applications. In contrast, the present materials represent stable, nontoxic, tunable, and hollow plasmonic nanostructures.

A rechargeable Li/LiχCoO2 cell incorporating a LiCF3SO3NMP electrolyte

Abstract Preliminary cell performance of a rechargeable Li/Li χ CoO 2 cell at room temperature and 80 °C with a LiCF 3 SO 3 NMP electrolyte has been evaluated. The results indicate that the cells using N -methyl-2-pyrrolidinone (NMP) solution at room temperature can deliver energy densities of 100–110 W h/kg at 1 mA/cm 2 discharge rate over 100 cycles while the cells at 80 °C or those using propylene carbonate (PC) solution can only last 40 cycles. Cathode preparation is discussed in terms of reaction time, number of firings, particle size, and type of binder. This is the first report to be published on the use of a LiCF 3 SO 3 NMP electrolyte in rechargeable lithium cells.