Local degradation and differential voltage analysis of aged lithium-ion pouch cells

Knowledge of the underlying degradation mechanisms in a lithium-ion cell is crucial for the development process and control of automotive lithium-ion battery packs. In this paper, a local post-mortem degradation analysis is performed on two differently cycled large-format pouch cells to improve the understanding of the underlying degradation mechanisms. For each cell, anode and cathode sheets are extracted and electrode coins are cut out from the sheets in an equally spaced, five-by-five pattern to assemble three-electrode test cells. Subsequently, the local capacities and differential voltage curves of the test cells are determined showing an inhomogeneous loss of lithium inventory for one cell and a capacity degradation of the cathode for the other cell. In a parallel connection of the test cells, the influence of the degradation mechanisms on the pouch cell's differential voltage curve is shown and an approach is validated for calculating the resulting differential voltage curve by superimposing the single measured curves. In addition, the local discharge resistance is evaluated, revealing for both aged cells that the measured resistance increase at pouch cell level is the result of an increased resistance of the cathode and a decreased anode resistance.