Influence of relaxation time on the lifetime of commercial lithium-ion cells

• Published in: Journal of power sources 2013-10, Vol.239, p.45-53
• Main Authors: Reichert, M., Andre, D., Rösmann, A., Janssen, P., Bremes, H.-G., Sauer, D.U., Passerini, S., Winter, M.
• Format: Article
• Language: English
• Subjects: Anodes; Applied sciences; Blends; Direct energy conversion and energy accumulation; Electric cells; Electrical engineering. Electrical power engineering; Electrical power engineering; Electrochemical conversion: primary and secondary batteries, fuel cells; Electrochemical impedance spectroscopy; Exact sciences and technology; Graphite; Lifetime; Lithium batteries; Lithium-ion cells; Power sources; Relaxation effect; Relaxation time; Rest; Rest period
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2013.03.053

The influence of rest periods on the lifetime of lithium-ion cells is investigated. The investigations focus on commercially available cells (type 18650) with a lithium–nickel–manganese–cobalt-dioxide (NMC)/lithium–cobalt-dioxide (LCO) blend and lithium–iron–phosphate (LFP) as cathode material and graphite as anode material. The above test cells are subjected to different cycle tests. Important influence factors in addition to the relaxation time are: the current, the temperature and the state of charge (SOC). We were able to detect an increase of capacity (e.g. 0.1% of the nominal cell capacity for LFP test cells) after a rest period of 120 min in our measurements, the so called relaxation effect. Further, the question is answered, whether relaxation affects battery lifetime. This was achieved by cycling cells with and without different rest periods at different temperatures and C-rates with a total of 50 test cells. However, no observable differences between cells with and without rest periods (≤2 h) were found. Moreover, experiments with many small breaks or one larger break showed that the duration of one continuous relaxation time is more important for the aging behavior, than the sum of rest periods. In addition, also impedance spectra were evaluated. •A relaxation effect on cell capacity was detected for rest periods (≥1 h).•The influence of rest periods on the lifetime is insignificant.•A long break is better for the lifetime than many short breaks between each cycle.