Decay of Open Circuit Voltage during Storage of Lithium Ion Batteries

Summary:

Calendaric ageing of lithium-ion battery (LIB) is relevant to most applications of LIB. Classical testing methods, which involve long-term storage with regular check-up (CU) are slow and cost intensive. New methods have been presented in the past, such as float current measurement [1] or the high precision coulometry (HPC) [2]. With these methods, the calendaric ageing rate can be determined faster than with long-term storing tests. However, both methods have drawbacks. The float current measurement method lack precision and the equipment for HPC is very costly. As an alternative, we present the galvanic separated, high precision measurement of the open circuit voltage (GS-OCV). The galvanic separation ensures that the battery is not altered during the testing, while the high precision voltage measurement allows detecting smallest changes in the electrode potentials. A further advantage of the GS-OCV is that the equipment can be produced for a relatively competitive price.
Initial investigations of the mostly decreasing open circuit voltage (OCV) aimed at determining its time dependency.For this end, the GS-OCV acquired voltage data at a rate of one measurement every 2 hours over the course of 4 months. Subject of the testing were a set of NMC/G-Si batteries. The experiments were conducted at different temperatures (25°C,30°C, 35°C,40°C, 45°C and 55°C) und storage voltages (4.05 V, 3.85V, 3.71V, 3.5V and 3.48V), to investigated temperature and SOC dependency of the OCV decay. The OCV decay was associated with the self-discharge by Zilberman et al. [3]. A linear model was fitted to the data with the aim to extract the slope of the voltage decay its intercept. The evaluation of the test results allowed calculation of the reaction entropy of the intercalation [4-6] and of the activation energy of the self-discharge [7].

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