Key Role for Nonlinear Phenomena of Li-O2 Batteries in Discharge Properties

Abstract

Li-O2 batteries have attracted much attention as one of the candidates of future energy devices because of their ultimate energy density. This specific property of Li-O2 batteries is derived from the discharging reactions, which consist of the dissolution of Li metal of anode and oxygen reduction reaction (ORR) on the cathode.1-3 However, many issues to apply them for practical use, such as the limited capacity and low energy efficiency, have remained undissolved. In typical Li-O2 batteries, carbon materials are applied as cathodes on which the discharge product, Li2O2, forms and deposits. Due to the inherently low electron conductivity of Li2O2, deposition on the cathode prevents continuous discharge, and causes the cell voltage to drop sharply during constant current discharging. This nonlinear phenomenon, which is called “Sudden cell death”, is known to restrict the discharge capacity of Li-O2 batteries.Herein, we present that the negative differential resistance (NDR), which appears in the ORR of Li-O2 batteries, plays a key role in the discharge properties.4-6 The existence of NDR in the potential region of discharge implies that LiO2, which is an intermediate of the discharge reaction, impedes the ORR and its coverage varies with the cell voltage. Since LiO2 is desorbed from the cathode at higher cell voltage than NDR region, ORR proceeds via “the solution pathway” where Li2O2 forms in the electrolyte is promoted. Meanwhile, the lower cell voltage leads “the surface pathway” in which the adsorbed LiO2 is further reduced to Li2O2 on the cathode surface. Therefore, these results rationally revealed that there is a transition of pathways from the solution to the surface in the galvanostatic discharging process, and this transition causes “Sudden cell death”. The cell voltage where the transition region of reaction pathways appears shifts with cathode materials and electrolytes, therefore, the design of appropriate condition of usage is important to induce the maximum performance of Li-O2 batteries.References1) A. C. Luntz, B. D. McCloskey, Chem. Rev., 114, 11721-11750 (2014).2) C. Shu, J. Wang, J. Long, H. K. Liu, S. X. Dou, Adv. Mater., 31, e1804587 (2019).3) W. J. Kwak, Rosy, D. Sharon, C. Xia, H. Kim, L. R. Johnson, P. G. Bruce, L. F. Nazar, Y. K. Sun, A. A. Frimer, M. Noked, S. A. Freunberger, D. Aurbach, Chem. Rev., 120, 6626-6683 (2020).4) Y. Hase, Y. Komori, T. Kusumoto, T. Harada, J. Seki, T. Shiga, K. Kamiya, S. Nakanishi, Nat. Commun., 10, 596 (2019).5) K. Nishioka, K. Morimoto, T. Kusumoto, T. Harada, Y. Hase, K. Kamiya, S. Nakanishi, Chem. Lett., 48, 562-565 (2019).6) Y. Hase, K. Nishioka, Y. Komori, T. Kusumoto, J. Seki, K. Kamiya, S. Nakanishi, J. Phys. Chem. Lett., 11, 7657-7663 (2020).