Impedance, power, energy, and pulse performance characteristics of small commercial Li-ion cells

Electrochemical properties of cylindrical (18650, 17500) and prismatic (48. 3 x 25.4 x 7.6 mm) Li-ion cells from different manufacturers including A&T, Panasonic, Polystor, Sanyo, and Sony were studied. Impedance and pulse cha racteristics of these cells were evaluated for three open circuit voltages (OCVs): 4.1 V (fully charged), 3.6 V (partially discharged), and 3.1 V (nea rly completely discharged) in the temperature regime +35 degrees C to -40 d egrees C. The cell ohmic resistance was nearly constant from +35 degrees C to -20 degrees C, but increased by 2-3 times at - 40 degrees C. For example , the cylindrical Sony cells showed an average bulk resistance of similar t o 80 m Omega between 35 degrees C and -20 degrees C and similar to 290 m Om ega at -40 degrees C for the three OCVs studied. The cell ohmic resistance remained nearly constant with OCV. The NyQuist plot (real vs. imaginary imp edance) showed, at high frequencies (2.7-65 kHz), an inductive segment char acteristic of a porous electrode and/or a jelly-roll cell design. The NyQui st plots also showed two ill-defined loops, a smaller loop at higher freque ncies attributed to the anode electrolyte interface and a larger loop at lo wer frequencies due to the cathode electrolyte interface. A smaller charge transfer resistance (R-ct) at the anode is indicated and the performance of the cell may be improved by reducing the interfacial resistances, in gener al. Ragone plots, relating energy density and power density or specific pow er and specific energy, were also constructed to compare the performance ch aracteristics of these cell types. In the current range studied (20-1000 mA ), the energy/power performance of both A&T and Panasonic cells is better t han the rest. For these two cells, the power (W/kg or W/1) didn't reach a p lateau in the current range studied. These data should be considered, howev er, in the context that the A&T and Panasonic cells may be newer (later gen eration) than the other cells used in this study. However, at higher curren ts (> 2 A) and at lower temperatures, for the Panasonic cells, power reache s a plateau. This behavior is also true for the A&T cells. The cells were p ulsed at different temperatures both as a function of OCV and current pulse amplitude. The cell voltage drop is almost linear with pulse current at am bient and slightly subambient temperatures. However, at lower temperatures, the voltage drop is nonlinear with pulse current, suggesting that the cont ribution of charge transfer resistance to the overall cell impedance under load is nontrivial. In addition, the cell voltage drop for a given current pulse increases with depth of discharge. For example, for a 500 mA pulse at 4.1 V, 3.6 V, and 3.1 V, the Panasonic cells showed an average voltage dro p of 94 mV, 130 mV, and 200 mV, respectively, at room temperature. A simila r observation was made for the other Li-ion cells. (C) 1999 Elsevier Scienc e S.A. All rights reserved.