PERFORMANCES OF SAFT LITHIUM-ION CELLS IN LEO CYCLING

Based on good LEO lifetime tests results, Saft proposes currently VES16 Lithium-ion for 12 years mission in Low Earth Orbit (LEO) up to 30%DoD or more within a temperature range from +10°C to +30°C. The first flight experimentation started at the end of 2015, confirming its ability to withstand the environmental stresses induced by a space launch and the in-flight constraints generated by a LEO mission. The upcoming launches scheduled from 2016 will significantly increase the flight heritage of this cell. From the beginning of VES16 Lithium-ion cell development, a wide lifetime tests program has been implemented in order to evaluate the different degradation modes due to the conditions applied in LEO cycling, as presented in Table 1. This program has allowed assessing the influence of different parameters such as EOCV (End Of Charge Voltage), DoD (Depth of Discharge), charge current and temperature. New MP176065 XTD Lithium-ion cell, developed & qualified by Saft, with an operating range extended from -40°C to +85°C, has already been selected for EXOMARS Rover mission. Promising results in accelerated LEO cycling conditions at 30%DOD has been observed and compared to VES16 lithium-ion cell results. Manufactured on a mass production industrial line, this cell is well suited for low cost missions (eg. Mega Constellations...). The article will thus present the whole LEO cycling results available for the two cells, and will provide afterwards the correlation status of Saft Li-ion Model (SLIM) with all the experimental data acquired.


VES16 LITHIUM-ION CELLS LEO LIFE-TIME TESTS RESULTS
This chapter focuses on the analysis of the life-test results performed in accelerated (A) or in real time (RT) mode according to VES16 lithium-ion cells LEO lifetests program described in Tab.These LEO Life-Test results have permitted to assess the influence of the cycling parameters such as DOD/charge current, EOCV and temperature on the evolution of the cell electrical characteristics such as EODV, Capacity, Internal Resistance and Energy, as presented in the following paragraphs.The evolution of these cell electrical characteristics is measured through a check-up sequence, involving a residual discharge and a reference test at 4.1Volts @20°C, performed at 500 cycles, then every 1000 cycles.This reference test is based on charge up to 4.1V, then a discharge down to 2.7V under C/2 discharge rate @20°C with pulses 1.5C-30seconds at 20& 60%DOD.It allows also demonstrating the correlation of the LEO life-test results with the SLIM V3 ageing prediction model.

EOC Voltage Influence
The evaluation of the EOCV impact on the VES16 cell behaviour is performed from the results obtained in LEO cycling at 20%DOD -@20°C.
That is done on QM2/QM3 cells in the EOC Voltage range from 3.9 to 4.05Volts, then on FM3 cells in the EOC Voltage range from 4.05 to 4.1Volts.
After respectively 25000 cycles on QM3 cells & 27000 cycles on QM2 cells in LEO cycling at 20%DOD, no significant effect of EOCV between 3.9 & 4.05Volts on EOD Voltage variation slope and IR0%DOD increase, as presented in Figure 1 and 2.

EOCV & Cycle Number
We can just note that capacity loss is reduced by 4% (i.e.6% after extrapolation at 60000 cycles) when EOC Voltage is limited to 3.9Volts, refer to Fig. 3.However, the available capacity is reduced by 9% due to the limitation of SOC (|15%) between EOCV = 4.05 and 3.9Volts.

Figure 3. Available capacity vs. EOCV & Cycle Number
After 15500 cycles in LEO cycling at 20%DOD on FM3 cells, no significant effect of EOCV between 4.05 & 4.1Volts has been observed on EOD Voltage variation slope, as presented in Figure 4.

DOD & Charge Current Influence
The evaluation of DOD & charge current impact on the VES16 cell behaviour is performed from the results obtained in LEO cycling at EOCV=4.05Volts -@20°C.
That is done on QM2/QM3 cells within DOD range from 20 to 30%, then on FM cells within DOD range from 20% to 50%.
In LEO cycling conditions, DOD & charge current change together.That comes from the fact that the charge duration is limited and requires to compensate DOD increase by a higher charge current, as presented in After respectively 27000 cycles on QM2 and 25000 cycles on QM3 cells in LEO cycling at EOCV=4.05Volts @20°C, a very limited EOD voltage evolution slope increase is observed from -1.5mV to -1.6mV per 1000 cycles between 20% & 30%DoD, as presented in Fig. 9.     2.3mV per 1000 cycles between 20°C & 30°C, as presented in Fig. 17.

Figure 17. EOD Voltage evolution vs. Temp. & Cycle Number
When the temperature increases from 20°C to 30°C, higher capacity loss is observed, |4% after respectively 20000 cycles on QM2 cells & 25000cycles on QM3 cells, and |7% expected at 60000 cycles, as presented in Fig. 18.

Figure 18. Available capacity vs. Temp. & Cycle Number
No visible effect of DOD & charge current increase on QM2/QM3 cells internal resistance, as presented in Fig. 19.

Figure 19. Internal Resistance vs. Temp. & Cycle Number
After 15000 cycles on FM3 cells in LEO cycling at EOCV=4.05Volts -20%DOD, the same EOD voltage evolution slope is observed at 10°C & 20°C, about -2.4mV per 1000 cycles, as presented in Fig. 20.

Figure 20. EOD Voltage evolution vs. Temp. & Cycle Number
No significant capacity/energy losses are also measured within the temperature range from 10°C to 20°C, as presented in Fig. 21 and 22.As shown in Fig. 30, the EOD voltages measured or extrapolated are much higher than the predicted ones, around 15mV after 25000 cycles on QM3, 25 mV after 27000 cycles on QM2, and 50mV at 50000 cycles.
As presented in Fig. 31, the available capacity calculated by SLIM V3 fits at less 6% with the worst case 30%DoD LEO life-tests data after 50000 cycles.That can be explained by the very good ability of the negative electrode design to sustain LEO cycling conditions with larger charge current up to 2.57Amps and lower temperature down to 0°C.Up to 50000 cycles, the error between the available capacity/EODV predicted by SLIM V3 ageing model and the ones extrapolated from the LEO life-tests data at 30%DOD are respectively less than 50mV & 6%.

MP176065 XTD LITHIUM-ION CELLS LEO LIFE-TIME TESTS RESULTS
New MP176065 XTD Lithium-ion cell presents promising results in accelerated LEO cycling conditions at 30%DOD, comparable to VES16 lithium-ion cell ones.We can even observe a lower capacity loss of |4% after 6000 cycles, as shown in Fig. 32.

Figure 10 .Figure 11 .
Figure 10.Available capacity vs. DOD & Cycle Number No visible effect of DOD & charge current increase on the QM2/QM3 cells internal resistance, as presented in Fig.11.

Figure 25 .
Figure 25.EOD Voltage evolution vs. Temp.& Cycle NumberNo significant capacity/energy losses are also measured within the temperature range from +5°C to +20°C, as presented in Fig.26and 27.

Table 1 :
VES16 Li-ion cells On-Going LEO Life-Tests Program

Table 2 :
Charge conditions vs. DoD