Volume 51, Issue 12

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: LiMn 2 O 4 (LMO) and Li(Ni 1/3 Mn 1/3 Co 1/3 )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.

Solidification microstructures exhibit a variety of partially very complex morphologies. It is possible to directly correlate the properties of an alloy with its solidification microstructures taking into account length scales and volume fractions of different phases. As the effect of the kinetics during the microstructure formation must be taken into consideration, the phase diagram presents only incomplete information on the solidification structures. Non-equilibrium phases which drastically change the alloy's properties occur in numerous microstructures which, according to the phase diagram, should be single-phase. Owing to the non-equilibrium phases, resulting from the segregation formation, the liquid fraction at a given temperature is generally larger as predicted by the phase diagram and the lever rule. Segregations are best characterized based on phase fractions. Using image analysis technologies, they can usually be measured with good accuracy, just as model calculations which predict statistically validated microstructural parameters nowadays also achieve a very good accuracy. The above will be demonstrated in several examples.