Uncertainties in entropy due to temperature path dependent voltage hysteresis in Li-ion cells

The determination of reversible heat is often carried out by means of a potentiometric measurement of entropy related effects. In this work, the potentiometric method is more closely investigated regarding the often assumed linear relationship between the open circuit potential (OCP) and temperature and possible associated measurement inaccuracies. Four different, completely equalized lithium-ion cells comprising different chemistries and cell formats are object of this study. Two different temperature profiles with varying frequency and amplitude components were applied and according entropic coefficients were calculated based on the preceding data. The results obtained in this work clearly reveal a non-linear behavior of the voltage to temperature relationship regardless of the cell format, state of charge (SOC), cell chemistry or sign of the derived entropic coefficient. This non-linearity manifests in a certain hysteresis of the OCP depending on the temperature history of the cells. The results suggest to minimize the temperature pulse duration whilst still allowing for a thermal equalization of the cell. Additionally, the authors suggest to minimize the amplitude of the temperature pulse as much as possible, whilst still allowing for an acceptable signal to noise ratio (SNR) for the measured voltage response.