Analysis of the age-, current- and temperature-dependent expansion of cylindrical NCM<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg" display="inline" id="d1e1539"><mml:mo>|</mml:mo></mml:math> Graphite Li-ion battery cells using strain gauges
Jessica Hemmerling, Alexander Fill, Kai Peter Birke
Abstract
This study investigates cylindrical Li-ion cells’ age-, current- and temperature-dependent expansion with Lithium–Nickel–Cobalt–Manganese–Oxide cathodes and graphite anodes. The radius change of the rigid outer casing of the battery cells measured with strain gauges at suitable locations around the circumference of the cells allows conclusions about the mechanisms occurring in the electrodes. The lithiation of graphite, accompanied by a volume expansion of the electrode of up to 12%, leads to an externally measurable increase in the radius, as the collected data show. This expansion is not only SoC-dependent but is also influenced by temperature, current, and aging. The analysis of differential voltage and incremental capacity reveal the areas of structural transformations where phase transitions occur. Incremental radius change analysis shows that these phase transformations have mechanical causes that become more evident with aging, as both reversible and irreversible expansion become greater as the cells age. Higher ambient temperatures result in higher reversible radius change and faster volumetric aging, i.e. irreversible expansion. Also, higher currents provide faster aging but show only a small to no effect on irreversible expansion. However, since expansion is related to many operating variables of Li-ion cells, it can be used as a parameter for determining the condition and for determining safety-relevant parameters. • Analysis of the influence of operating parameters (current and temperature) on the expansion of cylindrical battery cells. • Investigation of aging with regard to the expansion of cylindrical battery cells. • Linking various effects in the expansion curve with phase transformations using differential capacity analysis and differential stress analysis.