Mapping of Ageing Mechanisms in Cylindrical LiFePO4/Graphite Batteries Cycled Under Deep Discharge Steps

Li-ion batteries are good candidates for electric vehicles and stationary applications due to their high power and energy density. Despite significant improvement over the last decade, lifetime and safety remain major issues for large-scale applications. Thus, a better understanding of factors governing battery lifetime and performance loss is crucial. Various types of Li-ion batteries are commercially available and their characteristics and performance depend strongly on the combination of active materials, cell design and manufacturing process. Therefore, the determination of specific degradation mechanisms and their correlation with cell performances is crucial for working on cell concepts that can meet target lifetimes [1-3].

In this work, cycling performance fade of LiFePO₄-based cylindrical batteries is evaluated under deep charge/discharge steps (100% DOD). The cycling conditions were chosen to represent highly demanding condition of operation, which covers the analysis of ageing phenomena under the largest possible operating DOD. Diagnostic evaluation of the ageing mechanisms included in-situ electrochemical measurements, and ex-situ destructive physico-chemical and electrochemical analyses of cell components. A thorough study on the degradation mechanisms in different areas of the battery is realised in order to establish a geometrical mapping of the cell degradation. This allows identifying areas of inhomogeneous degradation along the jelly-roll of the battery. Degradation mechanisms were evaluated by microstructural (SEM) and crystallographic (XRD) analyses. Elemental analysis of the electrode composition was determined by EDS and ICP techniques. The electrochemical performances of selected areas of the harvested electrodes were determined by galvanostatic testing in half-cell configuration. These results were combined with non-destructive cell electrochemical measurements to provide a global view of the main degradation processes.

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[3] D.P. Abraham, J. Liu, C.H. Chen, Y.E. Hyung, M. Stoll, N. Elsen, S. MacLaren, R. Twesten, R. Haasch, E. Sammann, I. Petrov, K. Amine, and G. Henriksen, Journal of Power Sources, 2003. 119-121: p. 511-516.