Understanding Lithium Inventory Loss and Sudden Performance Fade in Cylindrical Cells during Cycling with Deep-Discharge Steps

The cycling performance fade of LFP-based Li-ion cylindrical batteries is evaluated under maximum cycling voltage amplitude. Diagnostic evaluation of the aging mechanisms included in situ electrochemical measurements and ex situ destructive physicochemical and electrochemical analyses of cell components. SEM, EDS, XRD, and electrochemical measurements of harvested electrodes confirmed that the primary cell performance degradation modes are loss of active lithium inventory (LLI) and loss of active material (LAM) related to graphite electrode. Aging phenomena were associated with the progressive decomposition of the electrolyte. Cell capacity loss was concluded to be dominated by SEI layer growth, which also led to a sharp power loss together with localized lithium plating on the negative electrode surface upon prolonged cycling. The graphite surface was polymerized and inactivated in localized central parts of the jelly-roll, leading to large cavities as a result of metallic lithium and electrolyte reactions. No degradation of the structure or performance of the LFP positive electrode was detected. In this paper, aging processes are examined in the overall context of cell performance fade during accelerated cycling operation.