An Explanation of the Ageing Mechanism of Li-Ion Batteries by Metallographic and Material Analysis

Li-ion batteries are a key technology for both electro-mobility and stationary energy storage systems. In order to be able to represent and improve their service life in these applications, a better understanding of the processes which lead to the degradation of the individual cells is essential. The work presented in this article focuses on the comparative post mortem analysis of type 18650 commercially available cells containing the state of the art active materials (Cathode: LiMn2O4 (LMO) and Li(Ni1/3Mn1/3Co1/3)O-2 (NMC), Anode: Graphite). These cells were subjected to various different ageing procedures. Amongst other effects, the cells investigated revealed signs of crack formation in the LMO- and NMC-particles, a loss in the mechanical integrity of the cathode active mass and plastic deformation of cell structure together with pronounced delamination between the active mass layers, the separator and the current collector.