The hydrous ruthenium oxide has been formed by a sol-gel process. The precursor was obtained by mixing aqueous solutions of RuC13 9 xH20 and alkalis. The hydrous ruthenium oxide powder was obtained by annealing the precursor at low temperatures. The crystalline structure and the electrochemical properties of the powder have been studied as a function of the annealing temperature. At lower annealing temperatures the powder is in an amorphous phase with a high specific capacitance. Specific capacitance as high as 720 F/g was measured for the powder formed at 150~ When the annealing temperature exceeded 175~ the crystalline phase was formed, and the specific capacitance dropped rapidly. The surface area of the powder and the resistivity of the pellet made from these powders have also been studied. The specific surface area and the resistivity decreased as the annealing temperature increased. A capacitor was made with electrodes comprised of hydrous ruthenium oxide and HzSO4 electrolyte. The energy density of 96 J/g (or 26.7 Wh/kg), based on electrode material only, was measured for the cell using hydrous ruthenium oxide electrodes. It was also found that hydrous ruthenium oxide is stable in H2SO4 electrolyte.

Among the various energy-storage systems, lithium-ion capacitors (LICs) are receiving intensive attention due to their high energy density, high power density, long lifetime, and good stability. As a hybrid of lithium-ion batteries and supercapacitors, LICs are composed of a battery-type electrode and a capacitor-type electrode and can potentially combine the advantages of the high energy density of batteries and the large power density of capacitors. Here, the working principle of LICs is discussed, and the recent advances in LIC electrode materials, particularly activated carbon and lithium titanate, as well as in electrolyte development are reviewed. The charge-storage mechanisms for intercalative pseudocapacitive behavior, battery behavior, and conventional pseudocapacitive behavior are classified and compared. Finally, the prospects and challenges associated with LICs are discussed. The overall aim is to provide deep insights into the LIC field for continuing research and development of second-generation energy-storage technologies.

No abstract

A model for predication of the gravimetric and volumetric energy densities of Li-air batteries using aqueous electrolytes is developed. The theoretical gravimetric/volumetric capacities and energy densities are calculated based on the minimum weight of the electrolyte and volume of air electrode needed for completion of the electrochemical reaction with Li metal as an anode electrode. It was determined that both theoretical gravimetric/volumetric capacities and energy densities are dependent on the porosity of the air electrode. For instance, at a porosity of 70%, the maximum theoretical cell capacities are 435 mAh/g and 509 mAh/cm 3 in basic electrolyte, and 378 mAh/g and 452 mAh/cm 3 in acidic electrolyte. The maximum theoretical cell energy densities are 1300 Wh/kg and 1520 Wh/L in basic electrolyte, and 1400 Wh/kg and 1680 Wh/L in acidic electrolyte. The significant deduction of cell capacity from specific capacity of Li metal is due to the bulky weight requirement from the electrolyte and air electrode materials. In contrast, the Li-air battery using a nonaqueous electrolyte does not consume electrolyte during the discharge process and has high cell energy density. For Li-air batteries using both aqueous and nonaqueous electrolytes, the weight increases by 8-13% and the volume decreases by 8-20% after the cell is fully discharged.

The formula describing the energy density of asymmetric cells, which consists of a battery-type electrode ͑such as lithium intercalated compound͒ and an electrochemical capacitor-type electrode ͑such as activated carbon͒, was derived. From the formula, the optimal mass ͑or volume͒ ratio of battery electrode to capacitor electrodes and electrolyte can be obtained for achieving the maximum theoretical gravimetric ͑or volumetric͒ energy density. The voltage swing of the cell during charge and discharge cycles was also described. Relationships between the energy density, ion concentration of the electrolyte, specific capacity of battery electrode, specific capacitance of capacitor electrode, and maximum operational voltage were also given. Three specific asymmetric systems, including carbon/LiPF 6 ethylene carbonate:dimethyl carbonate (EC:DMC)/Li x Ti 5 O 12 , carbon/LiPF 6 EC:DMC/WO 2 , and Ni͑OH͒ 2 /KOH H 2 O/carbon were evaluated for their maximum theoretical energy density and swing voltage. It was found that for asymmetric cells using nonaqueous electrolyte, the maximum energy density ͑about 30 Wh/kg͒ was limited mainly by the electrolyte due to the low ion concentration; however, for asymmetric cells using aqueous electrolytes, the maximum energy density ͑about 40 Wh/kg͒ was limited mainly by the capacitor electrode. The maximum operational voltage always plays an important role in the maximum energy density.

A physics-based model is proposed for the simulation of Li-air batteries. The model is carefully calibrated against published data and is used to simulate standard Li-air batteries with nonaqueous (organic) electrolyte. It is shown that the specific capacity is mainly limited by the oxygen diffusion length which is a function of the oxygen diffusivity in the electrolyte and the discharge current density. Various approaches to increase the specific capacity of the cathode electrode and the energy density of Li-air batteries are discussed. It is shown that, in order to increase the specific capacity and energy density, it is more efficient to use a nonuniform catalyst that enhances the reaction rate only at the separator-cathode interface than a catalyst uniformly distributed. Using uniformly distributed catalysts will enhance the current and power density of the cell but will not increase significantly the specific capacity and energy density. It is also shown that the specific capacity and energy density can be increased by suppressing the reaction rate at the oxygen-entrance interface in order to delay the pinch-off of the conduction channel in this region. Other possibilities to enhance the energy density such as using solvents with high oxygen solubility and diffusivity, and partly wetted electrodes are discussed.

No abstract

Electrochemical capacitors can be divided into two types depending on whether the salt concentration in the electrolyte changes during charging and discharging. In the first type of capacitor, such as double-layer capacitors, the salt concentration in the electrolyte reduces during the charging of the capacitor. The maximum energy density of this type of capacitor will depend not only on the specific capacitance and the operating voltage, but also on the salt concentration of the electrolyte. In this paper, a formula describing the dependence of energy density on specific capacitance, operating voltage, and salt concentration is given based on the optimized weight (or volume) ratio of the electrode material and the electrolyte. It shows that f or electrochemical capacitors using nonaqueous electrolytes, the maximum energy density of the capacitor will be limited mainly by the low salt concentrations of the electrolyte. The relationship between the energy density and the mass density of the electrode is also given. The optimum mass density of the electrode can be obtained based on the value of the theoretical energy density for capacitors with different electrolytes. In the second type of capacitor, such as pseudocapacitors with metal oxide electrodes, the salt concentration in the electrolyte remains constant during charging and discharging. The maximum energy density of this type of capacitor will be limited mainly by specific capacitance and operating voltage.