Localized Average Voltage Distribution on the Surface of NMC622 Electrode Measured Using Integrated Al-Voltage Probes during the Cell Cycling at Different C-Rates

Abstract

Studying localized voltage distribution of electrode surfaces in Li-ion batteries (LIBs) during cell cycling at constant current enables better understanding of the cell degradation phenomenon and prediction of the state of the health. In many studies, individual electrode potentials are analyzed during cell cycling at different C-rates using a reference electrode integrated into the cell, studying the electrochemical reaction mechanism of single electrodes [1,2]. However, for long length electrodes in pouch cells, nonuniform voltage distributions are developed on the electrode surfaces due to inhomogeneous lithiation and de-lithiation during cell cycling. This voltage distribution can cause a variety of issues including loss of energy, accelerated cell degradation, and overcharge or over-discharge of the cell at specific points on the electrodes. For example, recent literature shows that the region of the electrode closest to the current collector tab in pouch cells has the highest state of charge [3].Usually, pouch cells are constructed with different dimensions, depending on the application. However, there is a lack of information on the local voltages at electrode surfaces during charge and discharge at different C-rates. To understand the degradation mechanisms of active electrode materials in the pouch cell application, it is therefore important to spatially quantify the voltage distribution on the electrode surfaces at different C-rates.In this study, two pairs of thin film Al voltage probes were placed carefully onto the surface of NMC622 rectangular cathode to monitor the voltage distribution between equidistant points parallel and perpendicular to the tab direction along the electrode. Following this, a pouch cell was constructed with the cathode (2 mAhcm-2) sandwiched between two graphite-based anodes with an areal charge density of 2.4 mAhcm-2. The surface electrode potentials were measured individually against a standard reference Li/Li+ electrode in-operando during the formation cycle as well as during charge and discharge at rates of C/10, C/5, and C/3.The results show that the electrochemical potential close to the tab is always higher than that further away. A potential gradient exists perpendicular to the tab direction, which increases and decreases continuously during charging and discharging respectively. The potential difference between two lateral points parallel to the tab direction is significantly lower than the difference between two points perpendicular to the tab direction. This voltage distribution across the electrode usually manifests due to the inhomogeneous lithium diffusion along the plane of the electrode. This can be influenced by the electronic and ionic conductivity of the electrode along its plane. Therefore, in addition, Electrochemical Impedance Spectroscopy (EIS) measurements were performed, both perpendicular and parallel to the tab directions at different states of charge (SOC) of the cell. The EIS measurements confirm that the resistance perpendicular to the tab directions is higher than that parallel to the tab direction. Therefore, our results show that it is highly recommended to place voltage and temperature sensors closer to the tabs to monitor the state of health of the cell.[1] Sols L and Engineering A 1995 Current and Potential Distribution in Parallel Electrodes 244 236–44[2] Raccichini R, Amores M and Hinds G 2019 Critical review of the use of reference electrodes in li-ion batteries: A diagnostic perspective Batteries 5 1–24[3] Liu J, Kunz M, Chen K, Tamura N and Richardson T J 2010 Visualization of charge distribution in a lithium battery electrode J. Phys. Chem. Lett. 1 2120–3