Evaluation of electrochemical impedance spectra of metal-air batteries (Li-air/Zn-air) for aqueous, organic and solid electrolytes

Kurzfassung

Zinc-air and Lithium-air batteries have attracted the most attention. Primary Zinc-air batteries are well developed and commonly used as button cells in hearing aids. Besides this, zinc has more advantages, zinc is a cheap metal which is commonly occurring in the environment, production of Zn-Air batteries can be done in any atmosphere which makes it cheap in production and zinc is environmentally benign. Because of all these advantages these Zn-Air batteries are of high interest for stationary energy storage systems. Lithium paired with oxygen theoretically have the highest energy density with 11400 Wh/kg. Due to the challenges introduced by the use of metallic lithium, like water and oxygen free electrolyte on anode side, electrolyte decomposition and Li2O2 precipitation in the cathode, these cells are still in development and not commercially available. The lack of information during operation hinders identification of limiting processes. Electrochemical impedance spectroscopy (EIS) offers a path to measure the reactions and limiting processes, which can be easily identified by EIS and a good understanding of the system can be obtained. Measurements were done for carbon free bifunctional air cathodes, consisting of Ni as conductive agent, Co3O4 as catalyst and PTFE as hydrophobic binder in 7 M KOH in half-cell and Zn-air battery full-cell measurements. Both electrodes were first measured separately, cathodes in a homemade, temperature controlled (25 -65°C in 10°C steps) half-cell with 1 L electrolyte tank and varying feed gases (O2, N2/O2, He/O2, ambient air). Zn-electrodes were tested in a symmetrical setup in commercially available El-Cells. Li-air batteries were tested with a bifunctional air electrodes made of Chromium carbide as conductive media, titanium carbide as catalyst and PTFE as binder. Symmetrical Li-Li setup tests were done in El-Cells with an organic electrolyte consisting of 2 M LiTFSI in DMSO, with and without a Li-conducting membrane at different temperatures and applied currents. Full-cell tests were performed in a homemade cell, sealing the anode compartment from surroundings, and dry O2 feed gas. Separation of electrode testing in symmetrical, or half-cell level allowed identification and characterization of electrode specific processes. Full-cell measurements allowed the investigation of the interactions of each cell component and identification of limiting processes and degradation mechanisms.