We developed trimethyl phosphate ͑TMP͒-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 ͓LiN͑SO 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.

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 ͑H1͒ to a new hexagonal phase ͑H2͒ 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. In a spacecraft, the battery system is one of the most massive onboard components.1,2 Improvement in the energy density of the onboard battery system can help realize a lightweight power storage device, and hence contribute to lower launch costs and enable missions that have critical weight and/or volume margins. The specific advantages of lithium-ion technology offer the possibility of huge reductions in battery mass. It has been reported that over 20 spacecraft with onboard lithium-ion batteries have been launched in recent years.3-9 These spacecraft, including satellites, Mars rovers, and space vehicles, demonstrate the normal operation of onboard lithium-ion batteries in a space environment.A lithium-ion battery in a spacecraft generally consists of many lithium-ion cells connected in series and parallel to meet the power requirements of the bus and the mission. These cells cycle under various operating conditions and environments, such as ultrahigh vacuum states, radiation, long cycle-life requirements, and short charge and discharge intervals limited strictly by the spacecraft orbit.10 Typically, a spacecraft in low Earth orbit ͑LEO, within 1000 km of the Earth͒ periodically experiences about 60 min of sunshine and 30 min of eclipse. This requires that the onboard rechargeable cells store power derived from solar cells over short intervals of 60 min, and that they generate enough power to meet the electrical demands of the bus and the mission at a very short interval of 30 min. Additionally, the onboard rechargeable cells must operate without interruption for more than 30000 cycles to meet the general LEO mission life requirement of 5 years. To facilitate the application of lithium-ion cells in a spacecraft, these cells must be cycled under moderate conditions, such a...

We tested the effect of two new additives, cyclo hexane ͑CH͒ and 1-methyl-2-pyrrolidinone ͑NMP͒, on the cycling performance of a carbon-coated artificial graphite ͑AG͒ anode on a lithium-ion cell to investigate suppression of irreversible capacity loss of the graphite anode during the first-cycle charge. Both CH and NMP additives effectively increased the coulombic efficiency of the graphite anode during the first cycle. We attribute this phenomenon to the dissolution of the poly ͑vinylidene fluoride͒ ͑PVdF͒ binder of the AG anode due to CH or NMP addition, which improved the PVdF elasticity and reduced the contact area between the AG particles and the electrolyte. Consequently, adding CH or NMP reduced the loss of lithium ions in the first-cycle charge. Cycle-performance testing of the Li/AG half-cell indicated that we could achieve maximum discharge capacity and coulombic efficiency by applying an additive amount ranging from 2 to 5% for both CH and NMP. The cycling performance testing of the LiCoO 2 half-cell suggested that these two additives also have good oxidation stability and are therefore worth applying in lithium-ion cells with graphite anodes.

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 ͑PE͒-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 ␥-ray radiation and vibration of the PE cells, simulating a microsatellite launch. The ␥-ray radiation testing revealed that these cells have excellent resistance to ␥-ray exposure in simulated low-Earth-orbit ͑LEO͒ and geostationary-Earth-orbit ͑GEO͒ environments. Vibration testing in an ultrahigh vacuum ͑10 −6 Pa͒ 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.Rapid progress in commercial and consumer microelectronics has catalyzed the use of microsatellites for both civil and military missions. 1-3 The limited payload mass, volume, and power of a microsatellite necessitate small, lightweight, and inexpensive onboard components. This is particularly true for the rechargeable battery system, which is generally one of the most massive onboard components. 4,5 In 2000, we began a feasibility study of commercial laminated lithium-ion polymer cells for space applications at the Japan Aerospace Exploration Agency ͑JAXA, formerly the National Space Development Agency of Japan͒. 6 Our goal was to determine whether these cells can be used to store power for a microsatellite. This current paper provides a review of the latest results of endurance testing in a space environment.Laminated lithium-ion cells may be generally classified as liquidtype lithium-ion cells or lithium-ion polymer cells, depending on the electrolyte state. A lithium-ion polymer cell contains polymer support material that forms gel electrolytes by incorporating organic solvents. This advanced lithium-based battery chemistry is of interest for the following reasons: ͑i͒ high cell voltage of 3.6 V, compared with 1.3 V for conventional alkaline chemistries; ͑ii͒ high specific energy and energy density; ͑iii͒ high flexibility in configuration; ͑iv͒ improved coulombic and energy efficiencies comparable to those of conventional systems, along with low self-discharge rates; and ͑v͒ a wide range of operating temperatures centered on the ambient temperature. Consequently, the battery can achieve a desired system voltage with fewer cells and, hence, reduced system complexity and lower manufacturing, assembly, and launch costs. In addition, the above advantages allow for design approaches for power storage systems of satellites that differ from established space-engineering techniques. For example, aluminum tape may be used as a simple means to affix the cells. This renders battery stacki...

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