Lithium-ion batteries (LIBs) offer multiple benefits, e.g., long cycle life, fast charging capabilities that are attractive for energy storage systems in both mobile and stationary applications. However, LIB characteristics and performance degrade overtime [1]. Battery degradation or aging is a dynamic and a permanent process driven mainly by its operation history and storage time. Observed effects include capacity fade and power fade. However, they can occur independently of each other as well and must be assessed accordingly [2]. A frequently used metric for evaluating battery degradation rate and aging levels in LIBs is state of health (SoH), defined as the ratio of available energy storage capacity at certain time to the nominal capacity of the pristine battery. Another metric is based on its internal resistance or capacitance [3]. In general, resistance of the battery increases significantly with its age. Battery capacity begins to fade rapidly as it approaches 80% SoH. Safety of the system becomes questionable as well. 80% SoH has thus been widely accepted as end-of-life (EOL) criterion for large battery packs [4-6].

Safety is crucial in both industrial applications and societal installations [7, 8]. In addition, both second life and recycling are unknown aspects with potential impact to ownership and lifecycle costs of large industrial size batteries; now many products are already in the market and approaching their EOL. LIB aging mechanisms are, however, complicated and largely non-linear. This is because a single degradation process alone is not responsible for the power fade and capacity decrease seen during long-term operation. It, in fact, results from several different processes and their interactions with each other [9]. A deeper understanding of the effects of the operation patterns on the health of the LIBs is therefore required.

Experiments with an intent to distinguish battery degradation mechanisms observed during the second life of lithium cobalt oxide (LCO) batteries from the mechanisms dominant during their first life were thus planned and executed in this study. Primary motive was to answer the question – “are these batteries safe to use beyond the typical EOL criterion of 80% SoH?” Comprehensive cycle life tests using commercial, 930 mAh pouch cells with LCO cathode and graphite anode were conducted in room temperature using the maximum (dis)charge current, specified by the manufacturer. Test matrix had multiple groups. Each group contained at least four batteries for statistical measurements. Different groups were cycled different number of times until their performance degraded by a pre-specified margin.

Interestingly, the measured cycle (first) life for the worst performing cell in the test matrix exceeded the manufacturer’s advertised value (under standard conditions) almost by a factor of two. Moreover, during their first life, parallel samples demonstrated similar performances. However, significant inter-cell variations were noted in the second life cells. Cell thickness also increased by almost 70% due to significant gas generation during this phase. Such findings that lay insight on techno-economic feasibility of second-life applications of these Li-ion batteries are presented herein.

References:

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