Understanding the Reaction Processes in Li-Ion Batteries By Voltage Relaxation Analysis
Authors: Min Yu-Jeong , Shin Heon-Cheol 
Published: August 2024

The open circuit voltage (OCV) transient observed after current interruption is a very useful electrochemical signal providing crucial information about a Li-ion battery (LIB), including its state of charge and health, and kinetics of internal processes. Generally, OCV transient is divided into immediate and long-term relaxations, which are probably related to current-dependent kinetic processes and changes in the instantaneous thermodynamic state, respectively. Previous studies have attempted to analyze the pulse and relaxation voltages of LIBs under Li concentration gradients in both the electrolyte and electrode, with much success. Nevertheless, they have been limited in understanding what is really happening inside the cell in detail. This was mainly due to the fact that the experiments were performed on full cells with overlapping anode and cathode signals, which hindered the interpretation of detailed internal reactions, and for the same reason, only unsystematic phenomenological interpretations were possible. So it is necessary to separate relaxation signals from the anode and cathode for more accurate analysis. Even in a half-cell, where the signal contribution of the Li counter electrode (CE) to the overall cell signal is believed to be very small, the degree of potential relaxation attributable to the CE might be non-negligible due to the concentration change of active ions around the electrode. Therefore, in order to fully understand the electrochemical characteristics of the working electrode (WE), a precise analysis of the voltage relaxation of a single electrode is required.
In this study, various test cells, including three-electrode half-cells and full cell, and a voltage-controllable symmetrical cell, were fabricated to analyze the pulse and relaxation process in order to further understand the reaction kinetics of the battery. In particular, we aimed to distinguish the detailed processes and to understand the overvoltage evolution during battery operation. For this purpose, Ni-rich layered oxide and graphite were used as the cathode and anode, respectively. Li metal was adopted as the CE and RE in both the half-cell and three-electrode cells. In the two- and three-electrode cathode half-cell experiments, it was confirmed that the overpotential contribution of Li CE in half-cells was one-third to half of total overpotential in specific operating conditions. Considering the relatively fast redox reaction rate of Li, this result strongly implies that a concentration overpotential due to the CE can play a critical role in OCV relaxation. In other words, a typical test cell structure where the opposing areas of WE and CE are similar, total cell overpotential is greatly affected by CE. Supporting this argument, a cell designed with a large CE area and abundant electrolyte showed the very small contribution of CE. In experiments with two- and three-electrode anode half-cells, the contribution of Li CE was similar to that of the cathode half-cell. Next, to investigate the change in the effect of Li CE on the overall overpotential as the cell degrades under repeated cycles, the OCV relaxation with cycles was analyzed for the test cells used above. The analysis revealed an obvious cycle dependence of the long-term OCV relaxation signal, which was confirmed to be largely an effect of Li CE. To investigate the unexpected voltage behavior of Li, Li symmetric cells were configured to obtain voltage curves over time with cycling. From the analysis results, it is proposed that the voltage curve variation is mainly due to different local reaction resistances and highly tortuous surface morphology of the Li surface.
In conclusion, the overpotential of Li CE had a large impact on total overpotential in the commonly used two-electrode half-cells, which may cause errors in voltage curve interpretation. Under these circumstances, we further investigated whether the systematic and thorough analysis of the OCV relaxation could reliably separate the cathode and anode overpotential signals in the full cell, as well as the detailed reaction signals at each electrode. To this end, we performed an overpotential analysis using two- and three-electrode cells, as well as a voltage-controllable symmetrical cell, cross-validating the results with those obtained in the half-cell experiments above and electrochemical impedance spectroscopy. In this presentation, we will show how to derive the overpotential contribution of the cathode and anode, and the detailed reaction processes from the short- and long-term OCV relaxation curves obtained from a full-cell. Furthermore, we will discuss the reliability of the separated signals and possible future work to improve their accuracy.