Investigation of Homogeneity and Reversibility of Deposited Lithium on the Graphite Anode Surfaces

Summary:

Lithium-ion batteries (LiBs) are considered to be one of the most important storage devices for electric energy, especially with regard to automotive applications. To ensure safety, investigations on aging processes, especially lithium plating, are of great interest. A process accelerating cell aging that occurs especially during charging at low temperatures, high C-rates or high SOCs is lithium plating. Li-plating is the deposition of metallic lithium on the anode surface. The state-of-the-art LiB uses graphite as anode material. Due to kinetic limitations at low temperature or high C-rates the potential can locally drop below 0 V Li/Li+ which thermodynamically favours lithium plating. Furthermore, high SOCs and especially manufacturing defects can affect the formation of lithium plating. The deposited lithium can be reversible or irreversible. The irreversible plated lithium results in the loss of active lithium which causes capacity fading. Ideally lithium is plated homogeneous as mossy lithium. However, dendritic lithium deposition is also possible which can penetrate the separator and cause a short circuit. In the worst case this can lead to a thermal runaway. To better understand these safety risks investigations of lithium plating behaviour in the presence of manufacturing defects are of great interest. The experimental approach is described on this poster.
To obtain deeper inside into the lithium plating behavior intentionally installed defects are used in this study to represent possible errors occurring during cell manufacturing. The inserted defects are a separator fold, inserting a non-conductive object and detaching active material. Separator fold is mimicked by inserting two extra layers of separator in the middle of the cell. As a non-conductive object that could possibly be inserted into a cell while manufacturing commonly used PP-tape is inserted. The defect of detached active material is represented by dot and line defects. Hereby the active material is detached during folding (line defect) or scratching (dot defect). In order to be able to investigate the influences of the built-in defects, a quantitative method with high detection strength is required. For this purpose, an already existing GC-BID method, which detects the resulting hydrogen from the reaction of metallic lithium with water, was shortened and its applicability was proven. In addition, lithium deposition was simulated with a Cu││NMC cell with different charge capacities of 0.5, 1.0, 1.5 and 2.0 mAh. The GC-BID method was already used to validate in situ NMR measurements.
Schwieters et al.[1] showed the influence of external pressure on the lithium and transition metal distribution and the suitability of LA-ICP-MS for this purpose. Therefore, LA-ICP-MS will be used to investigate the influence of the above-mentioned defects on the behaviour of lithium plating and to determine its homogeneity.
[1] T. Schwieters, M. Evertz et al., J. of Power Sources, 2018, 380, 194-201