Decay Processes and Life Predictions for Lithium Ion Satellite Cells

We have pursued the following steps in the life qualification of lithium ion cells for GEO satellites: • Establish a parametric test data base for flight cells as a function of use and storage. • Use careful DPA of the cells to determine the fundamental changes occurring within the cell. • Determine performance projections for the cell based on the observed internal changes and the measured rates of performance change. This process has been most seriously pursued for the qualification of lithium ion technology for the 15-year GEO mission and with electrochemical cells manufactured by SAFT. Empirically, we find (employing parallel real cycling and storage testing followed by additive decay summation) that the cells demonstrate approximately a 10% loss in energy storage capacity taken through the full mission life. The validity of the approach is based on the decay process elucidation which results for the DPA. Cell DPA conclusively shows that all wear out process in the SAFT cell when operated in the controlled GEO environment arise in the positive electrode. Positive electrode coulombic storage capacity loss appears to be a consequence of structural changes which occur as a result of cycling and possibly destructive reactions between the active material and electrolyte environment during both cycling and storage. Impedance growth is also isolated to the positive electrode. The growth appears to be entirely diffusional in nature. Statistical analysis of test data show that there is a “film-like” t 1/2 component during periods of storage and a component linear with time (or cycles) during cyclic use. DPA evidence appears to support that finding as an amorphous surface film develops on storage. The thickness of this film may actually be reduced by cyclic operation and we presume that the continued impedance growth is due to changes in the mobility of lithium within the crystal structure. The DPA and life test correlation have allowed us to validate the empirical extrapolation of cell performance to end of life. Combined with the fact that we see relatively small changes (~10%) over mission life this provides the confidence required for flight use.