Lithium metal deposition is induced by a low temperature charging and causes failure in lithium ion batteries (LIBs). Lithium metal deposition during low temperature charge-discharge cycles in LIBs was investigated by in situ ⁷Li solid state nuclear magnetic resonance (NMR) spectroscopy. Cells composed of LiCoO₂ (LCO) and graphite (MAG) were cycled with a continue current (CC) and pulse current (PC) mode at low temperatures (-5, 0 and 5 ℃) to examine kinetics of Li metal deposition. The growth rate, activation energy driven from Arrhenius low for Li metal deposition are evaluated by collecting cell capacities and NMR peak intensities at selected sampling points (e.g., Fig.1 for 0 ℃). The Li metal deposition started after one cycle and reaches the maximum value which is half of the Li amount contained in LCO after 10-20 cycles with CC mode operation at -5 ℃. While, no Li metal depositions were detected at all in the samples operated with PC modes (e.g., 1/2 duty 1 sec width pulse) even after 360000 cycle at -5 ℃. A good correlation between growth rate (n) of deposited Li metal and capacity fading rate (k) in CC mode operation.

Figure 1. Stacked in situ ⁷Li NMR spectra measured after 1, 10 and 20 cycle charge-discharge at 0 ℃ (experiments were done with two samples for each measurement).