Xianming Wang

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.

New Additives to Improve the First-Cycle Charge–Discharge Performance of a Graphite Anode for Lithium-Ion Cells

We tested the effect of two new additives, cyclo hexane (CH) and l-methyl-2-pyrrolidinone (NMP), on the cycling performance of a carbon-coated artificial graphite (AG) 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 fluoride) (PVdF) 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.

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.

Cycle-Life Testing of 100-Ah Class Lithium-Ion Battery in a Simulated GEO Operation

In this paper, we review our work on cycle-life testing of a 100-Ah class lithium-ion battery in a simulated GEO operation. The battery consists of ten 100-Ah lithium-ion cells in a series, with a high energy density exceeding 100 Wh/kg at the battery level. We simulate the eclipse period in real-time testing with five depth-of-discharge (DOD) patterns at an ambient temperature of 15oC. We also simulate a sun-shine period in eight-day thermally accelerated full-charge storage at an ambient temperature of 25oC, which in our experience corresponds to full-charge storage of a half year at 0oC. Eighteen eclipse seasons have presently been completed, corresponding to nine years of GEO operation. The battery maintained a high voltage near 3.4 V at the end of the discharge, even when the DOD was set at 70%. The voltage dispersion of ten cells was also sufficiently small in the range of 48 mV. The cell temperature reached a maximum of 29oC and maintained minimal dispersion smaller than 4oC even when the battery was discharged at a high DOD of 70%.