MobilNachhaltigTC Plattling MoMo
M. Uitz, Michael Sternad, S. Breuer, C. Täubert, T. Traußnig, V. Hennige, I. Hanzu, M. Wilkening
Aging of Tesla's 18650 Lithium-Ion Cells: Correlating Solid-Electrolyte-Interphase Evolution with Fading in Capacity and Power
Journal of The Electrochemical Society, vol. 164, no. 14
The long-term performance of commercial lithium-ion batteries used in today's electric vehicles is of utmost importance for automotive requirements. Here, we use Tesla's 18650 cells manufactured by Panasonic to elucidate the origins of capacity fading and impedance increase during both calendar and cycle aging. Full cell testing is systematically carried out at three different temperatures (25°C, 40°C, 60°C). The cells are galvanostatically cycled at different C-rates (0.33 C – 1 C) and calendar aging is monitored at 4 different state-of-charges (SOC). Operation at high temperatures turns out to have the largest effect on both the capacity and direct current (DC) impedance. As an example, after 500 cycles at 25°C and 40°C capacity fading is approximately 12%, while at 60°C the fading reaches 22%. Our DC impedance measurements reveal the same trend. Post mortem analysis indicate that aging is strongly related to changes of the solid electrolyte interphase (SEI). Hence, the changes in performance are correlated with the change in composition (and thickness) of the SEI formed. In particular, we quantitatively measure the formation of electrically insulating LiF and find a correlation between overall DC impedance of the cells and lithium fluoride of the SEI.
MobilNachhaltigTC Plattling MoMo
W. Schmidt, P. Bottke, Michael Sternad, P. Gollob, V. Hennige, M. Wilkening
Small Change—Great Effect: Steep Increase of Li Ion Dynamics in Li 4 Ti 5 O 12 at the Early Stages of Chemical Li Insertion
Chemistry of Materials, vol. 27, no. 5, pp. 1740-1750
Lithium titanate (LTO) is one of the most promising anode materials for large-scale stationary electrochemical storage of energy produced from renewable sources. Besides many other aspects, such as negligible formation of passivation layers and no volume expansion during lithiation, the success of LTO is mainly based on its ability to easily accommodate and release Li ions in a fully reversible way. This feature is tightly connected with Li self-diffusion. As yet, little information is available about microscopic Li diffusion properties and elementary steps of Li hopping at low intercalation levels, i.e., at values of x being significantly smaller than 1. Here, we used 7Li spin-locking NMR relaxometry to probe absolute hopping rates of LTO (homogeneous) solid solutions in quasi-thermodynamic equilibrium. As a result, the largest increase of Li diffusivity is observed when small amounts of Li are inserted. Strong Coulomb repulsions caused by the simultaneous occupation of neighboring 8a and 16c sites serve as an explanation for the enhanced Li diffusivity found. At even larger values of x, Li mobility slows down but is still much faster than in the host material with x = 0. Our results experimentally corroborate the outcome of recently published calculations on the DFT level focusing on both dynamic and structural aspects. The findings favor the formation of LTO solid solutions upon chemical lithiation; the steep increase in Li diffusivity found might also help with understanding the flat insertion potential observed.