Entropy of Cycled and Aged Li-Ion Batteries for Maritime Conditions

Electrification with Li-ion batteries is a global trend that has been on-going for many years across sectors, driven by a strong decrease in battery prices, increasing energy density and increasing fuel costs. Studies by DNV 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 years for some applications, more recent data states even 2 years for other applications [1].

 It is well-known that the safety of Li-ion batteries is an important issue. E.g. during over-charge of a Lithium-ion battery, the battery may eventually overheat and cause a thermal runaway which can ignite neighboring cells which have not been overcharged. Internal shorting of the electrodes is also reported to create thermal runaway situations. Based on the large amounts of energy in a 1 MWh Li-ion battery, it is absolutely vital that the safety of the battery system is assured. The degradation and ageing of Li-ion batteries will contribute to additional instability and affect the safety performance of the batteries as well. Hence, knowledge and the ability to predict capacity decay and the battery state of health will be vital information to enable safe and long-life operation of marine battery systems.

 In this study commercial Li-ion cells were studied with the entropy spectroscopy technique [2, 3] to establish a State-of-Health indicator for aged cells. The cells tested were Li-ion pouch cells which have been aged for both calendar and cycle life up to 3 years.

 For cycle life, the Li-ion cells were evaluated at various temperatures and charge and discharge rates. The calendar life was scrutinized at various temperatures and at different state of charge. The cells were characterized, based on continuous monitoring of capacity and energy content. In addition, periodic capacity and impedance data was recorded for all cells at the same temperature (25 °C) and discharge rate (1C).

 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 20112011: Rueil-Malmaison, France. p. 12.

2.         Reynier, Y., R. Yazami, and B. Fultz, The entropy and enthalpy of lithium intercalation into graphite. Journal of Power Sources, 2003. 119: p. 850-855.

3.         Viswanathan, V.V., D. Choi, D. Wang, et al., Effect of entropy change of lithium intercalation in cathodes and anodes on Li-ion battery thermal management. Journal of Power Sources, 2010. 195(11): p. 3720-3729.