Koichi Kibe

Understanding Volume Change in Lithium-Ion Cells during Charging and Discharging Using In Situ Measurements

Because structural change in lithium cobalt oxide (LiCoO 2 ) cathode is primarily responsible for the performance degradation of lithium-ion cells in simulated satellite operation, it is important to investigate the operating-condition effect on cell-volume change. In this work, we used in situ strain-gauge measurement to probe the total volume change during charging and discharging of five 50 Ah-class lithium-ion cells with graphite anodes and LiCoO 2 cathodes. Some interesting phenomena concerning the correlation of the taper voltage with the strain change at the end of the charge were found in the strain trend curve. To explain these phenomena, we examined the strain change of a commercial 0.65 Ah-class lithium-ion polymer cell with the same electrodes as a function of taper voltage by using in situ load-cell measurement and were able to deduce that the cell-volume change during charging correlated to the structure transition of the LiCoO 2 cathode from the initial hexagonal phase (H1) to a new hexagonal phase (H2) at a taper voltage near 4.00 V. We conclude that the taper voltage should be maintained below 4.00 V to maximize the cycle life of lithium-ion cells with graphite anodes and LiCoO 2 cathodes during practical satellite operation.

High-Concentration Trimethyl Phosphate-Based Nonflammable Electrolytes with Improved Charge–Discharge Performance of a Graphite Anode for Lithium-Ion Cells

We developed trimethyl phosphate (TMP)-based nonflammable electrolytes with a high TMP content exceeding 70% to increase the safety of lithium-ion cells with a graphite anode. TMP exhibits good oxidation stability and poor reduction stability at the graphite anode; therefore, we focused our efforts on suppressing TMP reduction decomposition at the graphite anode during charging. We selected a graphite material, named STG, with a surface partly coated by amorphous carbon particles to improve the TMP reduction stability. A new ternary mixed additive, 2 wt % vinylene carbonate +8 wt % vinyl ethylene carbonate +2 wt % cyclo hexane, was developed to exert a synergistic effect to improve the charge-discharge performance of the STG anode in TMP-based electrolytes. We further found that a high-concentration lithium bisperfluoroethylsulfonyl imide [LiN(SO 2 C 2 F 5 ) 2 ], of 2 mol dm - 3 was effective for suppressing TMP decomposition at the STG anode surface. Consequently, we were able to realize excellent cycling performance of an STG anode with over 70% TMP nonflammable electrolytes by applying the above approaches. This is the first report of such excellent performance of a graphite anode with high-content TMP-based nonflammable electrolytes.

Paper-Thin InGaP/ GaAs Solar Cells

A paper-thin, lightweight InGaP/GaAs solar cell with high efficiency and flexibility has been developed. A high-efficiency thin-film cell can be obtained for cell fabrication both before and after removing the substrate. Introducing a tunnel junction as the contact layer between the cell and metal film improves cell characteristics (Fill Factor (FF) and open-circuit voltage (Voc)). A highly doped n-type layer is necessary for good ohmic contact at the metal film interface. High radiation resistance of a thin-film cell was confirmed for a GaAs cell with a one-micron base layer. The thin-film cell was laminated for better handling. The laminated cell efficiency was about 22%. Anti-reflective coating is necessary on the laminate film to improve cell efficiency. A prototype unit panel using the laminated cells was developed for space application. An output power per weight of over 1W/g is possible for the unit panel. However, development of a bypass diode with thin-film structure is currently a problem, and reliability tests need to be performed for the unit panel

Development status of “Space Solar Sheet”

The prototype solar sheet using the paper-like thin film InGaP/GaAs solar cells and thin bypass diodes we newly developed was fabricated. The output power of the prototype solar sheet is 11.3W (Voc=11.88V, Isc=1.12A, F.F.=0.854), and the weight is about 16.7g. So output power per weight is about 0.68W/g. The preliminary reliability tests (thermal shock test, humidity test, high temperature vacuum test) are carried out in the solar sheet. Good reliability of the solar sheet has been confirmed in these tests.

A Feasibility Study of Commercial Laminated Lithium-Ion Polymer Cells for Space Applications Endurance Testing for Space Environment

Lithium-ion polymer cells are expected to provide power storage in microsatellites due to their high energy density, high voltage, and high flexibility in configuration. Our previous work demonstrated the excellent life performance of polymer electrolyte (PE)-type lithium-ion polymer cells in a vacuum. In this work, we determine whether this type of cell cycles normally in a space environment. We conducted endurance testing for γ-ray radiation and vibration of the PE cells, simulating a microsatellite launch. The γ-ray radiation testing revealed that these cells have excellent resistance to g-ray exposure in simulated low-Earth-orbit (LEO) and geostationary-Earth-orbit (GEO) environments. Vibration testing in an ultrahigh vacuum (10 - 6 Pa) demonstrated that the cells could endure a microsatellite launch when fastened only with aluminum tape. During this testing, we did not detect any gas components associated with cell solvents. The promising results led us to conclude that PE cells can store power well for an LEO or GEO microsatellite.

Analysis of Radiation Response and Recovery Characteristics of Amorphous Silicon Solar Cells

The radiation response and recovery characteristics of amorphous silicon (a-Si) thin-film solar cells were investigated. The solar cells studied were a-Si/a-Si dual-junction cell and a-Si/a-SiGe cell. The cells were irradiated with monoenergetic protons from 0.05 to 10 MeV to several fluences. For the cells, the displacement damage effects could model the proton-induced degradation much better than the ionizing radiation effects could. Thus, the changes in photovoltaic parameters could be plotted by a single characteristic curve against the displacement damage dose (Dd) even if the proton energy was low enough to create local damage in the active layers. The results indicated that the proton-induced degradations of the a-Si solar cell correlated with the displacement damage effects. In addition, we have reported the recovery characteristics by thermal annealing and light illumination. The electrical outputs of the cells significantly recovered at 70 and 130 degC but were not restored by light illumination

Analysis of Anomalous Degradation of Cu(In,Ga)Se2 Thin-Film Solar Cells Irradiated with Protons

We analyzed the radiation response of Cu(In,Ga)Se2 (CIGS) solar cells to high-fluence protons. An in situ measurement system was constructed to measure the electrical performance of the CIGS solar cells immediately after proton irradiation. Using this system, abrupt degradation of Isc in the cells irradiated with 0.38 MeV protons at room temperature was observed, which was caused by a decrease in the effective acceptor density of the CIGS absorbing layer according to its capacitance–voltage characteristics. Admittance spectra of proton irradiated CIGS solar cells indicate that the proton-induced defect might be an InCu antisite defect. This defect is a donor-like defect. Therefore, it compensates the acceptor density. The reduction in the density caused by majority carrier trapping by the defect caused an increase in the resistivity of the CIGS absorbing layer. Consequently, a type-conversion of the absorbing layer in the cells occurred because of high-fluence protons.

Investigation of Non-Ohmic Properties for Thin Film InGaP/GaAs Solar Cells

An efficient, flexible and lightweight thin-film InGaP/GaAs solar cell has been developed. We discovered a reduction in open-circuit voltage (Voc) of the thin-film InGaP/GaAs cell due to the non-ohmic characteristics in the interface between the metal film and p-type contact layer on the rear of the cell. Additionally, the non-ohmic characteristics influenced the temperature coefficient of Voc. Cells with 2.34 V and 2.08 V Voc were evaluated in this investigation. The difference in Voc was caused by a difference in carrier concentration of the layer which contacts thin metal film on the rear of the cell. These results indicate that the reduction in Voc of the thin-film cell is due to an opposite voltage generated in the interface between the contact layer and the metal film. The characteristics were improved by introducing a tunnel junction into the interface. Good ohmic characteristics in the interface between the rear of the thin-film cell and the metal film is required for improving efficiency

Current Injection Effects on the Electrical Performance of 3J Solar Cells Irradiated with Low and High Energy Protons

To compare high and low energy proton irradiation effects on the recovery behavior of triple-junction (3J) solar cells, 3J solar cells designed for space applications were irradiated with protons at 50 keV and 10 MeV, and their electrical performance was measured in situ under AM0 illumination. The electrical performance of the solar cells decreases with increasing proton fluence. For 50 keV-proton irradiation, the remaining factors of short circuit current (Jsc), open circuit voltage and maximum power become 81, 81 and 56 % respectively at a fluence of 1.2times1012/cm2. For 10 MeV-proton irradiation, these values are 83, 69 and 47 % at a fluence of 3times10 13/cm2. After proton irradiation, current injection into solar cells was performed at current densities between 0.03 and 0.25 A/cm2. The values of Jsc increase with increasing injected charge, and no significant difference in the increase behavior of Jsc is observed between 3J solar cells irradiated with 50 keV- and those irradiated with 10 MeV-protons. The obtained result suggests that the origin of defects annealed by current injection in 3J solar cells irradiated with protons at 50 keV is the same as that in 3J solar cells irradiated with protons at 10 MeV

Study on Optimum Structure of AlInGaP Top Cell for Triple-Junction Space Solar Cell

The purpose of this study is to improve the performance of triple-junction solar cells. The AlInGaP single-junction (SJ) cell was studied because it has greater radiation resistance than the conventional InGaP top cell. AlInGaP SJ cells with varied layer thicknesses and carrier concentrations of the base layer were prepared, and structural dependencies were investigated in order to determine an optimum structure for the AlInGaP top cell. The preferred cell for the 10-year mission on geostationary Earth orbit would have a base layer thickness of 1250 nm and a base layer carrier concentration of 3.0times1016 cm-3 or lower. This paper also compared radiation resistance of the AlInGaP SJ cell with that of the InGaP SJ cell. Even though InGaP is known to have excellent radiation resistance, the AlInGaP SJ cell exhibited better resistance than the InGaP SJ cell. It was demonstrated that AlInGaP is a superior radiation-resistant material for advanced 3J top cell