Ante-Mortem Analysis and Ageing Testing of SOTA Cylindrical Lithium-Ion Cells

摘要

Cylindrical Lithium-Ion battery cells are among the most popular choices when it comes to traction batteries. They offer high intrinsic safety, high energy, and flexibility in battery system design, all at low cost. Nowadays, these cells are mostly produced in 18650 and 21700 sizes. Some of the highest energy densities of cells currently available on the market can be found in these two formats. This study focuses on the quality of manufacturing, the materials used, and the ageing behavior of five 21700 and three 18650 format cells. They represent the cells with the highest energy densities offered by LG Chem, Panasonic, Samsung, and BMZ within a range from 248.7 Wh/kg up to 267.4 Wh/kg. For each type of cell a destructive ante-mortem analysis was performed to gain insights into the materials used and the quality of the materials and the coatings. Materials were characterized by Energy-dispersive X-ray spectroscopy (EDX) and X-ray crystallography (XRD) and their morphology were examined using a scanning electron microscope. This data serves as a basis for further understanding of the ageing behavior. It was shown that the manufacturers work with very different material compositions for their cells, mainly on the cathode side, but almost all are Nickel rich systems. On the anode side, graphite is the main component and thus still the dominating material. In five cell types the graphite anode is doped with small amounts of silicon. SEM pictures show that two types of cells contain carbon nano-tubes in their material, and one type of cell already had cracks in the cathode before cycling even started. For the ageing tests, six cells per type were cycled at room temperature with different constant current rates and depth of discharge (DOD). Additionally, 2 cells per type were cycled with a downscaled real life profile from two different OEMs. All cells were cycled for half a year or until a remaining capacity of less than 80% of initial capacity was reached - whichever occurred first. Regular checkups at every 75 full equivalent cycles provided full cycle data for comparison of cell types and load profile impacts. It was found that the cells, while having quite similar values in energy density, vary widely in their cycle life capability. Some of the cells show rapid decay at 100% DOD and cannot be cycled until the first checkup, while others achieve a few hundred cycles before reaching less than 80% of initial capacity. Correlation of the data shows that the ageing behavior is more strongly dependent on the manufacturer than on factors like size or active materials used. Therefore, it most likely dependent on manufacturing processes and additives.