On the impact of the negative electrode in Graphite/LFP cells during first and second life aging experiments

In order to reuse batteries, aging models must describe cell aging in both first and second life to predict the remaining useful life. However, most of the aging studies on lithium-ion batteries are focused first life experiments from 100 % to 70 % of State of Health. The lack of data is a barrier to understand the aging and its impact on long-term use. This study aims to highlight the discrepancy of aging between cells in the same test condition and the mismatch between experimental results and aging models in a long-term aging study. Experimental tests are conducted on cylindrical 18650 cells in different cases of use: electric vehicle use, fast charge, reduce voltage window, partial or complete depth of discharge. Experiments were conducted over a two-year period to study cells aging from a pristine state to the end-of-life. The aging of the negative electrode is estimated and have a strong impact on the cell aging in long term use, regardless of the test case. In high C-rate cycling or with a complete depth of discharge, the impact of the negative electrode on the cell aging is significantly higher. An empirical method is presented to take into account the electrode aging in order to exhibit the impact of the electrode on the cell. The impact of this finding in tested on existing model and then discussed. • The aging of 18650 Graphite/LFP cells is studied in 7 test cases over 2 years: some cells reached 30 % of state of health. • The effect of use in electric vehicles, fast charge and depth of discharged are discussed. • The impact of the negative electrode aging on the full cell is highlighted.