Effects of Cell Reversal on Li-Ion Batteries

Abstract

Cell reversal in lithium ion (Li-ion) batteries is the condition of the anode electrochemical potential rising above that of the cathode, resulting in a negative voltage measured at the cell level. There are two primary reactions that occur at the anode at high potentials which increase cell impedance: oxidation of copper current collector, and oxidization of the carbonate electrolytes to CO2. At the cathode, the reducing potential can lead to the electrodeposition of copper to form dendrites, which pose a shorting risk if they bridge the anode and cathode. Cell reversal can be caused by poorly matched cells, a failure of the battery management electronics, or a defective cell in a pack. Under these conditions, one or several of the cells can go into reversal causing performance decreases or even a dangerous thermal runaway event. This paper examines a pack of commercial 18650 Li-ion cells in simulated geosynchronous orbit (GEO) test under conditions where one or more cells were forced into reversal. Panasonic B cells were matched and assembled into a pack to create a virtual cell. Measurements were collected using matched current shunts, and cells were cycled at a 60% DOD, but with only 90% of removed charge replaced each cycle to simulate a battery management system failure. After failure, cells were non-destructively examined using CT X-ray, and then dissected for failure analysis. The parallel cell pack operated far into reversal, with up to four complete cycles (Fig. 1) completed before all cells shorted. CT X-ray scans of the cells after shorting detectable amounts of copper dendrites in the cell, and demonstrated the first nondestructive test for cell reversal in Li-ion (Fig. 2). Destructive physical analysis of the cells showed extensive copper corrosion at the anode current collector as well as copper dendrites that were found to have fully penetrated the separator in selected areas. These results show that these cells can operate several cycles into deep reversal without going into thermal runaway, despite the observed growth of copper dendrites which pierce the cell separator. This has strong implications on cell safety and battery management. Figure 1