Self-discharge or side reactions on Li-Ion batteries have been extensively investigated in the literature [1,2]. The amount of side reactions depends mainly on the materials utilized and on the test conditions. High temperature and high voltage can increase self-discharge currents [3]. Due to the rise of Na-Ion batteries in the market, more effort is needed to understand the side reactions, which can help to improve the Na-Ion system. This study applies voltage hold and high-precision coulometry to investigate the side reactions of Na-Ion cells.
In this regard, a high-resolution test device from Battery Dynamics with a coulombic efficiency (CE) precision of < 35 ppm was utilized due to its enhanced voltage and current resolution, which is necessary for this precise characterization. The experiments were performed on commercial cylindrical 18650 cells with 12 Na-Ion cells compared to 12 Li-Ion cells (six LFP/G and six NMC811/G cells). Voltage holds of 300h were performed at 25°C, 40°C, and 55°C for six states of charge (SOCs), including 10%, 40%, 50%, 70%, 90%, and 100% SOC. Analyzing the leakage current of a voltage hold provides valuable information about the amount of side reactions taking place. In addition, between the voltage holds capacity check-ups were performed to investigate the end-point slippage of the charge/discharge phases, to examine which electrode is responsible for most of the degradation, and to analyze the CE values for the different test conditions, investigating the irreversible losses taking place. [1] P. Keil et al., “Calendar Aging of Lithium-Ion Batteries I. Impact of the Graphite Anode on Capacity Fade.” J. Electrochem. Soc., 163, A1872-A1880 (2016). [2] T. Roth et al., “Relaxation Effects in Self-Discharge Measurements of Lithium-Ion Batteries.” J. Electrochem. Soc., 170, 020502 (2023) [3] L. Streck et al., “A comparison of voltage hold and voltage decay methods for side reactions characterization.” J. Electrochem. Soc., 170, 40520 (2023).