Mechanism Scan in Li-O2 Batteries By Impedance Spectroscopy

Li-O₂ batteries are claimed to be the batteries of the future because they can provide the highest energy density among any other type of batteries since Li⁺ ions and O₂ directly react with each other and, in practice, oxygen is not stored in the battery (1). Despite the great deal of attention in this technology currently there exists a great gap between the theoretically achievable energy density and the provided by Li-O₂ batteries in practice (2). Therefore, to fully exploit the capacity of Li-O₂ batteries, the thermodynamic mechanisms and kinetics that rule and limit their functioning must be understood. In this context, electrochemical impedance spectroscopy (EIS) is a unique technique that allows in-situ monitoring the charge-discharge processes in batteries obtaining kinetic information of the mechanisms developed with different time constant (3). In the present study, the impedance spectra of Li-O₂ batteries have been registered at different voltages of the charge and discharge processes. With the obtained data a novel equivalent circuit model is proposed, where interfacial charge transfer, adsorption and diffusion phenomena have been identified as well as the voltages where oxygen reactions (ORR and OER) appear. For each step is associated a resistance what allows to determine the bottleneck process. Furthermore, different parameters (composition and structure) of the cathode have been methodologically modified and the effect on the mechanism step analyzed. The proposed equivalent circuit model is a relevant tool for the rational design of Li-O₂ batteries.

References:

(1)   N. Imanishi, A. C. Luntz, P. Bruce The Lithium Air Battery: Fundamentals Springer (2013)

(2)   N-S. Choi, Z. Chen, S. A. Freunberger, X. Li, Y-K. Sun, K. Amine, G. Yushin, L. F. Nazar, J. Cho, P. G. Bruce Angew. Chem. Int. Ed. 51, 9994 (2012)

(3)   F. Martinez-Julian, A. Guerrero, M. Haro, J. Bisquert, D. Bresser, E. Paillard, S. Parrerini, G. Garcia-Belmonte J. Phys. Chem. C 118, 6069 (2014)