Investigating the low-temperature impedance increase of lithium-ion cells

Low-temperature performance loss is a significant barrier to commercialization of lithium-ion cells in hybrid electric vehicles. Increased impedance, especially at temperatures below 0 degrees C, reduces the cell pulse power performance required for cold engine starts, quick acceleration, or regenerative braking. Here we detail electrochemical impedance spectroscopy data on binder-and carbon-free layered-oxide and spinel-oxide electrodes, obtained over the + 30 to -30 degrees C temperature range, in coin cells containing a lithium-preloaded Li4/3Ti5/3O4 composite (LTOc) counter electrode and a LiPF6-bearing ethylene carbonate/ethyl methyl carbonate electrolyte. For all electrodes studied, the impedance increased with decreasing cell temperature; the increases observed in the midfrequency arc dwarfed the increases in ohmic resistance and diffusional impedance. Our data suggest that the movement of lithium ions across the electrochemical interface on the active material may have been increasingly hindered at lower temperatures, especially below 0 degrees C. Low-temperature performance may be improved by modifying the electrolyte-active material interface (for example, through electrolyte composition changes). Increasing surface area of active particles (for example, through nanoparticle use) can lower the initial electrode impedance and lead to lower cell impedances at -30 degrees C. (c) 2007 The Electrochemical Society.