Electrolytes play a central role in cell development, as they significantly influence the behaviour and ageing of lithium-ion batteries. The identification of optimal electrolyte formulations can thus lead to decisive competitive advantages in cell production. Electrolyte decomposition is indicated by gas formation, drying of the cell and/or the formation of cover layers and reactive intermediates. The quantification of electrolyte degradation through classical ageing tests is a time-consuming measurement procedure that only provides initial results after several weeks or months. In this work, a rapid measurement method for determining the calendrical ageing rate of lithium-ion batteries – float-current measurement (FCM) – is therefore used to determine the electrolyte degradation reactions and applied to the optimisation of the formation. The hardware used for the precise measurement of the current was developed at the Institute for Power Electronics and Electrical Drives (ISEA).
In this work, we focus on the ageing behaviour of a high nickel lithium-ion cell. The pouch cell is a 1 Ah lithium nickel cobalt manganese oxide battery (NMC/Graphit-SiOx) which is provided by the Li-FUN Technology Corporation Limited dry without electrolyte. The cells are formed at different temperatures to evaluate the influence of formation on ageing. FCM and electrochemical impedance spectroscopy (EIS) are used to investigate the influence of the formation temperature. To evaluate the ageing bevaviour, the cells are tested with potentiostatic and galvanostatic methods. To gain further information about the influence of the formation process, EIS spectra are recorded frequently and analysed using the distribution of relaxation times as well as an equivalent circuit model.
Our results show that FCM provides a differentiated overview of the stability for different ageing steps in a short time. The ageing behaviour of the investigated cells differs significantly from each other after formation. The degradation of the silicon anode plays a major role in the ageing behaviour of the investigated cells and it mainly influences the ageing of the cells. Limiting these ageing mechanisms can help to improve performance. FCM accelerates the optimisation of the formation process for lithium-ion cells and the lifetime prediction.