Identification of Li ion battery cell aging mechanisms by half-cell and full-cell open-circuit-voltage characteristic analysis

For a good understanding of the aging and its prediction for Li ion cells, an identification of aging mechanisms, such as loss of mobile Li or loss of active material, is essential. The results of this identification might be used as input parameter or as validation for a Li ion cell aging model. In this publication, the changes of the open-circuit-voltage (OCV) characteristic are analyzed carefully to draw conclusions regarding the aging mechanisms that reduce cell capacity. For this, a model which calculates the OCV characteristic of the automotive scale full-cell (FC) based on the anode and cathode half-cell (HC) OCV characteristics is suggested. The resulting changes of the FC OCV due to anodic and cathodic side reactions as well as loss of active material (LAM) are simulated. The analysis of these simulation results shows that each aging mechanism effects the OCV characteristic in a unique way, like a fingerprint, which can help to identify the aging mechanisms if unknown. Finally, the reverse mode operation, in which the model is used to identify the aging mechanisms based on the changes in the OCV, is validated on FC level. For this, a cell is disassembled after significant aging and the storage capacities of anode and cathode are measured on HC level. Even though inhomogeneities must be expected for the large automotive scale FC with more than 30 Ah capacity, loss of mobile Li inventory (LLI) and LAM can be identified and quantified.