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.

No abstract

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...

No abstract

No abstract