Using a Reference Electrode inside Li-Ion Cell As an Operando Sensor to Detect Aging Mechanisms

Abstract

Lithium-ion batteries (LIBs) are a key technology in the massive deployment of EVs. Besides enhancing the capabilities and performances of the batteries, we want to accelerate their charging rate to increase their driving range. However, special attention must be paid to safety and thus to understanding the aging mechanisms inside the cells. Evaluating the origin of capacity decay that is linked to loss of cyclable lithium (LLI: Loss of Lithium Inventory) or active materials (LAM: Loss of Active Material) can be obtained by two different methodologies but both have certain limitations: Non-invasive [1], of which one of the most popular is Incremental Capacity Analysis (ICA). ICA operates by slipping or contracting the electrochemical profiles of the electrodes to fit the experimental curve of the cell obtained at low current. This method can provide contradictory information if not well used [2].Post Mortem Analysis (PMA) [3], LLI and LAM are evaluated after sampling of disks of electrodes retrieved by dismantling the cell at discharged state and re-assembled in half coin-cells versus lithium. However, coin cells are assembled by introducing fresh electrolyte and separator that can be not representative of the real state of impregnation of the electrodes and separator in the cell after cycling. Moreover, sampling must be performed on different areas where heterogeneities are observed (gas bubble, lithium deposition) to be representative of the global behavior of the electrode considering their respective surface [4,5]. Embedded sensors inside the batteries are a major asset to monitor the internal parameters and prevent failures. The implementation of a reference electrode [6], as an electrochemical sensor, provides interesting results in terms of operando degradation detection since it allows the potential of the positive and negative electrodes to be monitored individually. The objective of our study was to assess the efficiency of the reference electrode, as a sensor, to quantify the aging mechanisms occurring inside the cell and detect lithium plating onset. The reference electrode could be used to modulate the current and reduce the overpotential of the negative electrode avoiding lithium metal being plated. Such sensor implementation is in the scope of several European projects including the INSTABAT project [7]. We will try to demonstrate in the presentation that the reference electrode can be an additional tool for lithium-ion cell diagnosis and cell management. For that, we will base our demonstration on cycling tests performed on instrumented NMC/Graphite pouch cells. The cycling protocol is either formulated as a worldwide harmonized light vehicle test cycle (WLTC) for discharging and 1C for charging and or at CC-CV/CC in charge/discharge and at different temperatures. A correlation with post-mortem analyses will give answer about the consistency of the response in potential given by the reference electrode and for example if it is able to detect the formation of localized metallic lithium.Figure 1 : (left) Electrode balancing during a cell checkup allowing precise monitoring of the cell degradation using individual electrode potentials. (right) Understanding the conditions of the appearance of lithium plating when increasing the charging rate.