Formation of a Lithium-Plated Reference Electrode in Lithium-Ion Batteries: Characterization Using Dynamic Impedance and Simulation Using an Electrochemical Model

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Reference electrodes (REs), enabling one to differentiate the contributions of each electrode, have been widely used in the research of electrochemical systems, such as lithium-ion batteries (LIBs). Metallic lithium or lithium alloy (Li-Sn, Li-Al) is the usual material of choice for REs in LIBs. A typical approach is to form a layer of metallic lithium onto a stable substrate by in-situ plating, termed as lithium-plating method [1][2]. In the present study, a copper-wire based RE is embedded into a 2 Ah laminated LIB, and its formation process, crucial to stability of the RE, is scrutinized. During the formation process, the RE is set as the working electrode, and the positive and negative electrodes are used as the counter electrode successively to obtain a both-side-plated RE. Dynamic electrochemical impedance spectroscopy (DEIS) is used as a characterization tool. Being dynamic means that the impedance measurements are performed while there is a DC current between the RE and the counter electrode [3][4]. Figure 1. (a) shows the evolution of DEIS during the whole formation process. It is revealed that a new semi-circle attributed to the emergence of the solid/electrolyte interphase (SEI) film manifests immediately at the moment the formation starts. Ohmic resistance, SEI film resistance and charge transfer resistance all gradually decrease during the formation process, and arrive at their constant values after 2×104s. In comparison with the potential monitoring, DEIS allows better characterization of the formation process with higher sensitivity and resolution. To understand the above phenomena, a two-dimensional (2D) model, accounting for growth of the RE size and concomitant decrease of the electrolyte porosity [5], is developed. Figure. 1 (b) shows non-uniform current distribution during the plating process with the positive electrode as the counter electrode. As expected, the plating current concentrates on the positive-electrode-facing side. Consequently, a thicker lithium layer is deposited onto this side. Impedance simulation results are compared with the experimental DEIS results. Based on this model, a multistep plating strategy is optimized to improve the stability and durability of RE. Key words Reference electrode; Lithium plating; Electrochemical impedance spectroscopy; Electroplating model; Solid/electrolyte interphase Reference: [1]. D. P. Abraham, S. D. Poppen, A. N. Jansen, J. Liu and D. W. Dees, ELECTROCHIM ACTA, 49, 4763 (2004). [2]. A. N. Jansen, D. W. Dees, D. P. Abraham, K. Amine and G. L. Henriksen, J POWER SOURCES, 174, 373 (2007). [3]. J. Huang, J. Zhang, Z. Li, S. Song and N. Wu, ELECTROCHIM ACTA, 131, 228 (2014). [4]. J. Huang, H. Ge, Z. Li and J. Zhang, ELECTROCHIM ACTA, 176, 311 (2015). [5]. J. Newman and W. Tiedemann, J ELECTROCHEM SOC, 140, 1961 (1993). Figure 1