Energy and power characteristics of Li-ion cells

摘要

At Sandia National Laboratories we are evaluating the energy and power characteristics of commercially available Li-ion cells. Cells of several different sizes ( 20 Ah, 1.1 Ah, 0.750 Ah and 0.5 Ah) and geometries (cylindrical and prismatic) from several manufacturers were studied. The cells were pulse discharged at increasing currents (50 mA to 1000 mA) over a range of temperatures (+35°C to -40°C) and at different states of charge (4.1 V, open circuit voltage, fully charged, 3.6 V OCV partiallydischarged and 3.1 V OCV nearly discharged) and the voltage drop was recorded. The voltage drop was small at ambient and near ambient temperatures indicating that the total cell internal impedance was small under these conditions. However, at -40°C thevoltage drop was significant due to an increase in the cell internal impedance. At a given temperature, the voltage drop increases with decreasing state-of-charge (SOC) or OCV. The cell impedance and other electrochemical properties as a function oftemperature and SOC were also measured. The Ragone data indicate that the specific power and specific energy of Li-ion cells of different sizes are comparable and therefore scaling-up to 20 Ah does not affect either the energy or the power.