Degradation of LiFePO4 batteries after a real hybrid-bus extended application: Investigation of ageing phenomena and heterogeneity of performance

Li-ion battery rapidly emerged as a key enabling technology towards the widespread adoption of low and zero-emissions vehicles. However, the understanding of long-term degradation still requires consolidation due to the complex interplay of several ageing mechanisms. Furthermore, it is widely reported in the literature that major differences could occur between laboratory-induced and real-world degradation. Hence, this work analyses a batch of 24 LFP cells aged in a hybrid-bus application for up to 8 years. The combined use of a selection of experimental measurements, ex-situ investigations and physical model simulations is the proposed tool to quantify the performance decay, identify the most important degradation mechanism, assess the heterogeneity among cells and relate it with the operating conditions. The characterisation showed significant heterogeneity in residual capacity (from 80% down to 55% with respect to BoL) and impedance, suggesting an uneven module cooling effectiveness. Electrolyte degradation, generating together with Solid-electrolyte interface (SEI) growth a passivation and precipitation layer on the negative electrode surface, is identified as the dominant degradation mechanism, leading to a large loss of both lithium inventory and electrolyte conductivity, as suggested by modelling analyses and confirmed with ex-situ investigations. This work provides novel insights into the state of health of a spent lithium-ion battery from the field as preliminary activity towards battery second life.