In-Situ Quantification of the Ageing Dynamics in Lithium-Ion Cells up to Failure-Near Conditions

Implementing end-of-life (EOL) lithium-ion batteries from automotive applications in stationary energy storages is of utmost relevance for a sustainable handling of scarce resources. Beneficial from an economic and ecological perspective, such second-life applications urgently require a guarantee for safe operation. Unlike the state of health (SOH), defined by classical performance indicators such as capacity and voltage, the state of safety (SOS) of an aged battery cannot be assessed straightforward. Its determination requires a plethora of cells to be tested which is a particular challenge for new technologies with limited access to EOL batteries. For providing cells with a defined SOH at a reasonable timescale, we herein propose a novel method of greatly accelerating the ageing process of lithium-ion batteries. In a preliminary test series, lithium-ion NMC pouch cells are exposed to incrementally increasing temperatures, current rates and/or states of charge (SOC), until thermal runaway is induced. In this manner, the critical state in proximity to cell failure is spotted for individual and combined stress parameters. Based on this knowledge, cell-specific test parameters for heavily accelerated ageing are developed. In this protocol, electrical abuse conditions are defined by over/under charging and high current rates. Typically, the cells are cycled utilizing a depth of discharge above 100 %. The accelerated aging dynamics under these critical conditions are monitored by systematic capacity, open circuit voltage and electrochemical impedance spectroscopy (EIS) measurements. This enables a comparative assessment of the electrical behaviour, following conventional vs. heavily accelerated ageing. Such knowledge will in turn help to define the threshold to which cyclic ageing can be accelerated without changing the characteristic degradation mechanisms of lithium-ion batteries.