Fast charging of Li-ion cells: Effect of separator membranes and mapping of “safe lines” to avoid Li plating

An experimental method is introduced that uses microprobe Li/Cu reference electrodes to map “safe” charging regimes to avoid Li metal plating on graphite anodes at rates to 8C. With this method, a Li-ion cell closely approaches the Li plating condition but avoids descending into this regime. This “safe” approach is used to demonstrate effects of the microporous separator on Li plating in cells consisting of a layered nickel-manganese-cobalt oxide cathode and graphite anode. We argue that most of the difference in behaviors for different separators arises from time delayed Li+ ion percolation through the membrane. Our study suggests that explicitly treating kinetics of phase transitions in the lithiated graphite anode is essential for electrochemical models predicting “safe” lines for fast charging cells.