曽根 理嗣

Since battery performance is the main factor affecting on-orbit satellite operation, it is very important to understand the degradation mechanism of battery performance and operating conditions. In particular, a lithium (Li) ion cell used as a satellite power source may be subjected to high charge and discharge rates, and the cell volume change during the charge-discharge cycle might cause the mechanical degradation of the electrode. In this work, we used two in situ methods, load cell measurement and X-ray observation, to investigate the cell volume change of a commercial Li-ion polymer cell with charging and discharging An excess cell volume change that was delayed with respect to the change induced by the state of charge was observed at high charge and discharge rates. We attributed this phenomenon to the slow diffusion of Li ions in the electrode active materials and slow structural change in the outer region of the electrode particle. Furthermore, we deduced that the outer region of the electrode particle, rather than the inner region, was mainly responsible for the excess cell volume change. © 2004 The Electrochemical Society. All rights reserved.

Polymer Electrolyte Fuel Cell (PEFC) systems are being developed at JAXA for applications to transfer vehicles for shot-term missions and larger spacecraft in the future. For space applications in a closed environment, we are developing a system in which the fuel is perfectly consumed and the oxygen is recycled. We prepared a six-cell-stack fuel cell with a gas-water separator and demonstrated its performance without external humidification. The effective surface area of the Pt catalyst layer on the MEA was 162 cm2. The performance of the fuel cell was checked for 1,000 hours while it nominally generated 60 A. We also demonstrated the fuel cell performance through the simulated Space Shuttle operations. The PEFC performed stably during these operations, providing its applicability for future space missions.

<p>We performed a cycle test with 25% and 40% depth of discharge (DoD) using 30 Ah prismatic Lithium-Ion secondary cells designed for ground applications. For 25% DoD, we observed adequate cycle performance exceeding 18,000 cycles. For 40% DoD, the end of discharge voltage (EoDV) decreased to 3.0 volt after 10,000 charge-discharge cycles, which enabled us to disassemble the cell and check the reason of performance degradation. Through the analysis, we found that the reduced cell performance mainly caused by capacity loss in the negative electrode, especially in the middle of the cell stack. The positive electrode also degraded slightly, and the impedance of the positive electrode increased. The discharge capacity during the 40% DoD test might not be restored by simulated low Earth orbit cycling because of these degradations.</p>

<p>For a thin film lithium-ion secondary battery, it is important to use a polymer or gel electrolyte with high ionic conductivity and good mechanical strength. In this work, two types of poly(vinylidene fluoride hexafluopropylene) (PVdF-HFP) were compared as preparing gel electrolyte for lithium-ion secondary battery. The PVdF-HFP copolymers polymerized by the emulsion method (PVdF-HFP_e) and the suspension method (PVdF-HFP_s), were applied in this study. The gel electrolyte was prepared by casting the THF solution of mixture of PVdF-HFP, ethylene carbonate (EC), propylene carbonate (PC) and LiClO₄ salt. All of the gel electrolytes exhibited a typical arrhenius behavior in temperature dependence of ionic conductivity, and gave a high ionic conductivity of 1-3 mS cm⁻¹ at ambient temperature when adding the 1.75 ml plasticizer to 1 g PVdF-HFP. Especially, the PVdF-HFP_s gel showed a higher mechanical strength over 4 M Pa. With the cyclic voltammetry, the PVdF-HFP_s gel electrolyte showed anodic stability up to above 4.0 V on Ni, Al, and stainless steel (SUS), and cathodic stability down to Li deposition potential on Cu, Ni and SUS. It was also suggested that the anode (MCMB) or cathode (LiCoO₂) active material could work in the PVdF-HFP_s gel electrolyte.</p>

© 1998, American Institute of Aeronautics and Astronautics, Inc. Using 50 Ah cells designed for H-2 orbiting plane-X (denoted as HOPE-X), we compared CC-CV charge controlling method with CC-multistep way. This study revealed that both methods realized almost the same capacity of the cells. To know the further influence of the constant voltage (CV) level where constant current (CC) is switched to constant voltage (CV), commercial cells with different negative electrodes were charged at different CV level. This measurement revealed that the capacity of cells increases proportionally to the level of the constant charge voltage. All these measurements were performed at various temperature. The higher capacity of the cells were obtained with increasing temperature.

The proton conductivity of Nafion 117 was measured under various conditions of humidity and temperature using a four-electrode ac impedance method. The conductivity of this membrane without heat-treatment was ca. 7.8 × 10-2 S cm-1 at ambient temperature and 100% relative humidity; it varied strongly with the humidity and heat-treatment of the membrane. After heat-treatment, the membrane showed a slight dependence of conductivity on temperature. From 21 to 45°C, its conductivity at a given relative humidity decreased with increasing temperature, while from 45 to 80°C it increased with temperature.

Electrochromism of Prussian blue (PB) involving proton insertion/extraction was investigated in an aqueous HCl solution using a Ta2O5·nH2O/PB laminate, formed by spin-coating a peroxo-polytantalate solution on an electrodeposited PB film. Reversible and stable electrochromic performance was observed with more than 10000 repeated coloring and bleaching cycles, demonstrating that the film of Ta2O5·nH2O serves as a proton-conductive protecting layer to prevent PB from dissolving in a strong acid solution. Insertion behavior of proton into the PB framework is compared with those for K+, Rb+ and Cs+ by means of a cyclic voltammogram and equilibrium potential measurements. The equilibrium potentials measured as a function of the degree of intercalation have shown that the maximum accommodation of proton is 3.0 (per Fe3+4[Fe11(CN)6]3 unit), which is in agreement with K+ and Rb+. However, the potential-composition curve observed with the PB-proton system is quite different from others not only in the potential range but also in shape, featuring by a conspicuous inflection at the middle region of the intercalation level. The intercalation behaviors are also discussed using a lattice statistical model.

Two kinds of solid state ECD prototypes were fabricated by combining electrochromic films and a Ta2O5·nH2O proton conducting film. Electrochromic characteristics of these devices were comparable to those using liquid electrolyte. We also demonstrated reversible EC characteristics of Prussian blue with insertion and extraction of proton, using a Ta2O5·nH2O film. © 1994.

Peroxo-polytantalic acid (Ta-IPA) was obtained by a direct reaction of Ta(OC2H5) with H2O2. Using its water-based solution, amorphous thin films with excellent homogeneity could easily be fabricated on ITO-glass substrates by a conventional spin-coating technique. Electrical conductivity of this sample was measured with an ac impedance method, revealing that both temperature and humidity dependences of ionic conductivity (ca. 4.0×10-1 S cm-1 at 20°C) were quite small. Furthermore, all solid state ECD prototype was fabricated by combining this film electrolyte with electrochromic film (hydrous tungsten oxide film derived from peroxo-polytungstic acid). Electrochromic characteristics of this all solid state device were favorable. © 1993.