Electrification with Li-ion batteries is a global trend across many sectors, driven by a strong decrease in battery prices and increasing energy density. Recent studies by DNV GL of the electrification of ships (e.g. Fellowship) have shown a large potential of reduction in energy consumption and emissions of CO₂, NO_x and particulate matters through electrification [1]. In addition, cost calculations have shown the possibilities for return of investment through electrification within 3-5 years for some applications [1].

 Knowledge and the ability to predict capacity decay and the battery state of health will be vital information to enable long, safe and profitable operation of marine battery systems. Based on the large amounts of energy in a 1 MWh Li-ion battery pack, it is absolutely imperative that also the safety of the battery system is assured [2].

 At Nordic maritime conditions the sea water temperature is normally in the range of 5-15 °C (winter/summer). Hence, sea water can be used as a source of active cooling of on-ship battery packs. For sea-bed installations the battery packs will normally be operated at the actual surrounding temperatures. These temperatures can induce Li-plating above certain charge rates.

 In our work we utilize several complementary characterization techniques for investigating the aging mechanisms relevant for maritime use for different Li-ion battery cell chemistries. An example is 30 Ah high-energy commercial NMC-graphite pouch cells cycled at 5°C at 1.5 C charge and discharge rates, which are realistic conditions for some specific maritime applications. After only 19 cycles it was observed that the capacity dropped to 77% of its original capacity. Li-plating is suspected as the cause of this accelerated capacity decay. Several in-situ techniques were applied to characterize and diagnose the observed ageing including:

• Entropy spectroscopy (ES) is a battery’s entropy spectrum with respect to state of charge [3]. The entropy spectrum is specific to a Li-ion battery’s anode and cathode chemistries. The changes in entropy spectra can be used as indicators of a battery’s state-of-health [3, 4] and as a diagnostic tool. This method has been extended at IFE to record entropy spectra for cells up to 50 Ah size, and to evaluate new electrode chemistries, such as Si-containing anode materials.
•  Incremental capacity analysis (ICA) [5] involves measuring the incremental change in capacity as a function of voltage during charge and discharge, also known as dQ/dV analysis. Dubarry et al. has shown that incremental capacity analysis during aging gives information (qualitative and quantitative) on the degradation mechanisms involved [5-7].       

EA and ICA for the NMC-graphite cells are presented in the figure below. One cell was subject to low temperature high-rate cycling at 5°C, the other at 45 °C. The characterization was done during C/10 cycling at 25 °C for both.  As can be seen in the figure (top), a distinct shift in the entropy peak at around 13 J/molK from 3.65 V to 3.85 V is found for the cell cycled at 5°C. This shift indicates a loss in active Lithium, shifting the cycling window of the cell corresponding to a shift of about 20% SoC. Similarly, we observe a shift of the dQ/dV peaks (bottom), both in position and intensity. As a comparison, the dQ/dV peaks for the cell cycled at 45 °C shows no significant shift and only a slight decrease in intensity corresponding to the overall drop in the cells' capacity to 90% after 420 cycles.

 In our presentation we will correlate the ageing effects observed by the in-situ electrochemical analysis methods discussed with ex-situ material characterization techniques after opening the cycled pouch cells. 

 References

1. Vartdal, B.-J. and C. Chryssakis, Potential Benefits of Hybrid Powertrain Systems for Various Ship Types, in International scientific conference on hybrid and electric vehicles, RHEVE 2011. 2011: Rueil-Malmaison, France. p. 12.
2. Corvus Energy, Corvus Energy hosts the most important lithium ion battery safety event of the year: Thermal Runaway Mitigation Testing, S. Puchalski, Editor. 2015, Corvus Energy: www.corvus-energy.com.
3. Viswanathan, V.V., et al., Journal of Power Sources, 2010. 195(11): p. 3720-3729.
4. Reynier, Y., R. Yazami, and B. Fultz, Journal of Power Sources, 2003. 119: p. 850-855.
5. Dubarry, M., et al., Electrochemical and Solid State Letters, 2006. 9(10): p. A454-A457.
6. Dubarry, M. and B.Y. Liaw, Journal of Power Sources, 2009. 194(1): p. 541-549.
7. Dubarry, M., et al., Journal of Power Sources, 2011. 196(7): p. 3420-3425.