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

Abstract

Electrochemical properties of cylindrical (18 650, 17 500) and prismatic (48.3×25.4×7.6 mm) Li-ion cells from different manufacturers including A&T, Panasonic, Polystor, Sanyo, and Sony were studied. Impedance and pulse characteristics 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 (nearly completely discharged) in the temperature regime +35°C to −40°C. The cell ohmic resistance was nearly constant from +35°C to −20°C, but increased by 2–3 times at −40°C. For example, the cylindrical Sony cells showed an average bulk resistance of ∼80 mΩ between 35°C and −20°C and ∼290 mΩ at −40°C for the three OCVs studied. The cell ohmic resistance remained nearly constant with OCV. The NyQuist plot (real vs. imaginary impedance) showed, at high frequencies (2.7–65 kHz), an inductive segment characteristic of a porous electrode and/or a jelly-roll cell design. The NyQuist plots also showed two ill-defined loops, a smaller loop at higher frequencies attributed to the anode electrolyte interface and a larger loop at lower 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 general. Ragone plots, relating energy density and power density or specific power and specific energy, were also constructed to compare the performance characteristics 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 than the rest. For these two cells, the power (W/kg or W/l) didn't reach a plateau in the current range studied. These data should be considered, however, in the context that the A&T and Panasonic cells may be newer (later generation) than the other cells used in this study. However, at higher currents (>2 A) and at lower temperatures, for the Panasonic cells, power reaches a plateau. This behavior is also true for the A&T cells. The cells were pulsed at different temperatures both as a function of OCV and current pulse amplitude. The cell voltage drop is almost linear with pulse current at ambient and slightly subambient temperatures. However, at lower temperatures, the voltage drop is nonlinear with pulse current, suggesting that the contribution 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 drop of 94 mV, 130 mV, and 200 mV, respectively, at room temperature. A similar observation was made for the other Li-ion cells.