High Precision Coulometry and Calorimetry Measurements of Side Reactions for Si/NCA and G/NMC811 Cells
Authors: Streck Luiza , Roth Thomas , Noel Andreas , Zilberman Ilya , Jossen Andreas 
Published: February 2024

Side reactions taking place at both anode and cathode can be investigated via different methods. Common methods include the high precision coulometry [1-3] and the isothermal calorimetry [4, 5]. In this study, pure silicon anode (70 wt%) Si/NCA cells and pure graphite G/NMC811 cells are investigated via the methods mentioned above. These measurements include voltage holds and charge and discharge endpoint slippage analysis, which is described in [6]. Glazier et al. [4] already showed that after sufficient time, the heat flow during a voltage hold is equivalent to the side reactions taking place. The aim of this study is to investigate the dependency of side reaction currents and parasitic heat on different voltages and temperatures. In addition, their behaviour during the transient phase caused via relaxation processes and edge effects [7] comparing silicon and graphite materials.
The cells voltage was held at different state-of-charge (SOC) conditions, low, medium, and high as well as at different temperatures. At the beginning and at the end of the experiment, check-ups were performed to compare the capacity loss and to analyse the endpoint slippage. During the measurements, the heat flow and the leakage current of the cells were measured simultaneously.
The preliminary results show that for the low and for the high SOCs, a strong opposing transient phase is present at the beginning of the voltage hold. The strong transient phase is expected for silicon-based materials due to their high relaxation characteristics, meaning also higher leakage currents for this material. Additionally, it was found that the different pre-histories of the cells influence the amount of leakage current measured. Meaning that the actual side reaction current is only achieved after edge effects and relaxation processes cease, which can take a long time for pure Si anodes, up to 1000 hours at 25°C. Moreover, the transient phase of the thermal and the electrical signals was correlated for the different materials.
The study of side reactions for lithium ion cells is investigated in the project “ExZellTUM III”, funded by the German Federal Ministry of Education and Research (BMBF) under grant number 03XP0255.

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