Akira Usami

Theoretical study of application of multiple scattering of light to a dye-sensitized nanocrystalline photoelectrichemical cell

Abstract A new structure of dye-sensitized nanocrystalline photoelectrochemical cell, which permits effective absorption of incident solar energy using thinner dye sensitized film, is proposed based on theoretical examination. The cell effectively confines incident light in the thinner dye-sensitized film by multiple scattering from dispersed TiO 2 particles at the bottom and total reflection between the inserted TiO 2 film and the glass substrate at surface. Under optimal scattering conditions, the backscattered intensity is maximized when the backscattering angle is equal to the critical angle of reflection at the surface. The optical confinement is also effective for long wavelength light.

Imidazolium-Based Room-Temperature Ionic Liquid for Lithium Secondary Batteries Effects of Lithium Salt Concentration

To understand the basic properties of lithium secondary batteries which consist of nonflammable and nonvolatile room-temperature ionic liquid electrolytes, we examined the ionic conductivity, electrolyte/electrode interfacial resistance, and charge-discharge rate characteristics by varying the lithium salt concentration in the room-temperature ionic liquid, lithium salt binary electrolytes. By using a modified imidazolium cation-based room-temperature ionic liquid as an electrolyte, the lithium secondary batteries achieved a stable charge-discharge operation of more than 100 cycles (cathode LiCoO 2 , anode lithium metal, voltage region 3.0-4.2 V, current density 1/8 C). Moreover, we found that an optimal lithium salt concentration exists for obtaining an excellent battery rate performance, which depends on delicate balances in several factors, such as ionic conductivity (viscosity), interfacial resistances at the LiCoO 2 cathode/electrolyte interface, and the lithium metal anode/electrolyte interface.

Reversibility of Lithium Secondary Batteries Using a Room-Temperature Ionic Liquid Mixture and Lithium Metal

Lithium secondary batteries that use a room-temperature ionic liquid containing a lithium salt as an electrolyte are prepared (cathode: , anode: lithium metal). The prepared batteries showed values near the theoretical charge-discharge capacity in the first cycle and excellent reversibility (initial discharge capacity: , 100th discharge capacity: vs , C/8) at room temperature.

Highly reversible lithium metal secondary battery using a room temperature ionic liquid/lithium salt mixture and a surface-coated cathode active material

For the purpose of realizing high-voltage, high-capacity, long-life and safe rechargeable batteries, a lithium secondary battery that uses high-voltage stable ZrO2-coated LiCoO2 cathode powder and a nonvolatile high-safety room temperature ionic liquid was fabricated.

Theoretical simulations of optical confinement in dye-sensitized nanocrystalline solar cells

Abstract The application of light scattering of submicron TiO2 particles to dye-sensitized nanocrystalline photoelectrochemical cells is examined theoretically. Monte Carlo simulations reveal that the increase of absorption path length of photons in nanocrystalline films and optical confinement due to total reflections at solar cell surfaces improve light absorption in the sensitized films remarkably. Contribution of optical confinement to the improvement is much greater than that of the increase of absorption path length. Quantum efficiencies are calculated, considering the recombination of electrons in the nanocrystalline films. The application of light scattering improves the quantum efficiencies remarkably, especially in long wavelength lights. Optical confinement permits utilization of thinner sensitized films. Distributions of light absorption in the scattering films are also discussed. The distributions are not represented as exponential expressions due to light reflections in the vicinities of transparent conductive oxide (TCO) electrodes. Optical confinement decreases the light reflections and improves light absorption next to the TCO electrodes where generated electrons can diffuse without recombinations.

Quaternary Ammonium Room-Temperature Ionic Liquid/Lithium Salt Binary Electrolytes: Electrochemical Study

To determine the properties of the quaternary ammonium cation room-temperature ionic liquid [N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis (trifluoromethylsulfonyl) amide (DEMETFSA)] used in lithium secondary battery electrolytes, the lithium ionic transport properties of electrolytes, the characteristics of the interface of a LiCoO 2 cathode and a metallic lithium anode, and battery performance were widely investigated. A DEMETFSA-LiTFSA binary electrolyte showed high chemical stability with lithium metal electrode and a relatively high lithium cationic transport number (0.13), as determined by electrochemical measurements. The prepared [LiCoO 2 cathode|DEMETFSA-LiTFSA binary electrolyte|lithium metal anode] cell showed sufficient charge/discharge reversibility over 100 cycles (voltage range, 4.2-3.0 V). Moreover, the reversibility of capacities and coulombic efficiencies degraded with increasing upper cutoff voltage owing to cathode/electrolyte interfacial degradation, which were analyzed in detail by impedance measurements.

Improvement of Degradation at Elevated Temperature and at High State-of-Charge Storage by ZrO2 Coating on LiCoO2

A uniform ZrO 2 coating on LiCoO 2 cathode materials for rechargeable lithium batteries was applied by a spray coating technique. The cells showed improved cycle performance and better durability of storing the cell (calendar life) under a high-voltage charging condition (4.2 V-313 K). X-ray diffraction and calorimetric study revealed that no marked change was observed in the bulk properties, such as crystal structure and phase transition, in the cathode during charge and discharge. The suppression of the increase of cathode/electrolyte interfacial impedance was observed by ZrO 2 coating. Thus, the improved electrochemical performance in the higher voltage region (>4.2 V) is ascribed to the stabilization of the interface between the cathode and electrolyte materials.

Rigorous solutions of light scattering of neighboring TiO2 particles in nanocrystalline films

Light scattering of neighboring TiO2 particles in nanocrystalline films penetrated by electrolytes is examined based on rigorous solutions of the T-matrix method. Scattering efficiencies are improved by agglomeration of much larger particles than the nanocrystalline particles. A cluster of relatively small particles among the large particles effectively scatters every incident light because of symmetrical scattering. On the other hand, the improvement of scattering efficiencies with clusters of particles as large as the maximum efficiency is ascribed to significant forward scattering.

Effects of alkyl chain in imidazolium-type room-temperature ionic liquids as lithium secondary battery electrolytes

Lithium secondary batteries that use a room-temperature ionic liquid as an electrolyte were investigated for the purpose of realizing high-safe batteries. For the improvement of stability under charge/discharge operation with electrodes, we focused attention on a series of l-alkyl-3-methyl-imidazolium bis(trifluoromethane sulfonyl)imide. The temperature dependence of ionic conductivity and battery charge-discharge performance were examined by changing the alkyl chain lengths: -methyl/-ethyl/-butyl/-hexyl/-octyl. According to the results, the effects of extending the alkyl chain were confirmed in, for example, the increase in carrier ion number, and the improvement of battery charge-discharge performance characteristics.

A theoretical simulation of light scattering of nanocrystalline films in photoelectrochemical solar cells

A Monte Carlo simulation method of light scattering in nanocrystalline films based on solutions of Maxwells equations is proposed. A nanocrystalline film is assumed to be superposition of randomly distributed nanoparticles and deviation of the nanocrystalline film from the randomness. Since a scattering field of the randomly and densely distributed nanoparticles can be neglected, a scattering field of the nanocrystalline film results in a sum of scattering fields of the deviant parts in the nanocrystalline film. In the method, configurations of the deviant parts are simulated with a random function of a computer language. A simulation converges in small number of the configuration patterns. The simulation theoretically demonstrates that almost all of the incident light to the nanocrystalline films in the photoelectrochemical solar cells penetrate without scattering.