Aging of Tesla's 18650 Lithium-Ion Cells: Correlating Solid-Electrolyte-Interphase Evolution with Fading in Capacity and Power

Abstract

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. For years to come, Li-ion batteries are considered as one of the most attractive energy storage devices for electric vehicles. They benefit from a high specific energy density combined with a good cycle life. In order to further enhance conventional lithium-ion technology it is necessary to develop new materials and to improve existing battery concepts including, e.g., also the ways how cells are fabricated on an industrial scale. In particular, the latter includes aging studies on commercially available batteries. Such studies are essential for predicting the practical lifetime of the batteries and for assisting in identifying the main failure mechanisms that may, for example, involve lithium plating, passivating surface films properties, co-intercalation, dissolution or electrochemical re-plating of metal ions.In particular, for automotive applications, where long cycle and calendar life is indispensable, accurate knowledge about the origins of battery aging is a major point of interest since it helps predict the operational lifetime of the systems. Understanding and identifying the main failure mechanisms, induced by certain operating conditions, would be a significant step forward in terms of reliability and lifetime costs of electric vehicles. Due to the fact that battery aging is a very complex matter, which is sensitively influenced by many factors such as temperature, storage and operating conditions as well as the types of active materials or electrolytes used, the variety of studies that focused on different aging phenomena is immense. Studies range from detailed material and component tests, e.g., on electrolytes, anodes and cathodes used, to investigations on fully commercial systems. As early as 1979 Peled 1,2 introduced the idea of the f...

Durability and Reliability of EV Batteries Under Electric Utility Grid Operations: Impact of Frequency Regulation Usage on Cell Degradation

“…More details on these cylindrical 18650-size graphite/LiNi x Co 1-x-y Al y O 2 (NCA) cells, as well as the batch cell-to-cell variations analysis, can be found in previous works [25,26,36,37]. These cells are similar with the type of cells used in some EVs, such as the Tesla Model S [39,40]. The twelve cells chosen were within the outlier boundaries of the cell-to-cell variations distribution [36].…”

Comparison of Hydrogen Powertrains with the Battery Powered Electric Vehicle and Investigation of Small-Scale Local Hydrogen Production Using Renewable Energy

“…The former creates another problem for electric vehicles; since a complete discharge will decrease the batteries' life, not all the stored energy can be used. An analysis of Tesla's lithium-ion cells has shown that after 500 cycles at 40 • C, roughly 12% of the capacity faded, which increases even more at 60 • C to 22% [71]. With a total of 1000 cycles and a range of 550 km per cycle, at an average speed of 70 km/h (same speed used for the combustion engine), this would result in roughly 7900 operating hours or 550,000 kilometres.…”

Protection Devices in Commercial 18650 Lithium-Ion Batteries

“…Today, 18650 Li-ion batteries are widely used in e-cigarettes [3], power banks [4], electric vehicles [5], and even deep space CubeSats [6]. The global 18650 Li-ion batteries market reached USD 6.03 billion in 2019 and is expanding at a compound annual growth rate of 1.7% to reach USD 6.69 billion by the end of 2024 [7].…”