Degradation Behavior of Graphite-Nickel Cobalt Aluminum Oxide Lithium Ion Cells with Series Connections Including an Overcharged Cell

Cycle life of series-connected lithium-ion battery (LIB) modules in the presence or absence of an overcharged cell is reported, and degradation behavior is characterized by electrochemical impedance spectroscopy (EIS) and differential voltage analysis (DVA). Three-stage capacity decay is solely observed in modules in the presence of an overcharged cell, which is exposed to a state of charge (SoC) of 105% during cycling. The module without overcharged cells or with an overcharged cell at an SoC of >103% exhibits capacity decay as a function of the square root of cycle numbers. EIS and DVA analyses confirmed that the overcharged cell with an SoC of 105% in the module deteriorates more harshly than the other cells and that deterioration occurs especially at the cathode. Cycle dependence of the charge/discharge cutoff voltage of LIBs revealed that the cell capacity of the overcharged cell shrinks with the end point on charge as a fulcrum, thereby decreasing the lower cutoff voltage as cycling progresses. Such a voltage change of an overcharged cell directly affects module capacity, leading to three-stage capacity decay. This study reveals a potential risk in series-connected cells using a wide SoC range and provides critical insights for safe operation without balance circuits.