Investigation of Lithium-Ion Diffusion and Irreversible Processes in Dual-Ion Cell

Abstract

In 2012 Placke et al. proposed so called „dual-ion cells“ as a new type of energy storage system. They reported an extraordinary cycling stability with a capacity retention of more than 99% after 500 cycles. Even though the achievable cell voltage in this kind of cells is about 4.6 V, the specific energy is still quite low compared to common lithium-ion batteries. The reason for this is a fundamental difference from conventional lithium-ion batteries, since not only the electrodes, but also the electrolyte acts as active material. During charging, lithium cations are reduced and deposited at a metallic lithium anode, while the anions are intercalated into graphite. During discharging, anions and cations are released back into the electrolyte (see figure 1).[1] Therefore the electrolyte acts as ion source, and the concentration hereby limits the energy density. As the cell voltage of “dual-ion cells” is very high the electrolyte needs to have a wide electrochemical window. For this reason, typically ionic liquid based electrolytes are used for this application. However, the concentration of lithium salts in ionic liquids is strongly limited due to solubility, increasing viscosity and decreasing conductivity.[2] During cycling the concentration underlies changes due to the extraction of lithium salt. This fact gives the possibility to investigate fundamental processes during charging and discharging by measuring the electrolyte resistance. In this work we therefore use electrochemical impedance spectroscopy in 2- and 3-electrode setup to gather information about diffusion processes in the electrolyte and about the reversibility of the processes during charging and discharging. Figure 1: Functionality of a dual-ion cell (derived from [1]). [1] T. Placke, P. Bieker, S. F. Lux, O. Fromm, H.-W. Meyer, S. Passerini, M. Winter, Zeitschrift für Physikalische Chemie, 226, 391 (2012). [2] T. Frömling, M. Kunze, M. Schönhoff, J. Sundermeyer, B. Roling, Journal of Physical Chemistry B, 112, 12985 (2008). Figure 1