Nucleation and Surface Passivation in Li-Air Batteries

Abstract

Recently, there has been increased evidence that the lithium peroxide in Li-oxygen batteries forms as a result of a continuous nucleation process followed by a growth process [1, 2]. This deposition mechanism is intrinsically heterogeneous and is microscopically different from the traditional approach in which the final product grows uniformly as a thin resistive layer on the surface of the electron conductive material [3, 4]. Similarly to the nucleation and growth of lithium sulfide in Li-S batteries, the deposition of Li2O2 in Li-oxygen batteries starts with the formation of initial Li2O2 seeds and subsequent growth of the Li2O and/or Li2O2 product on the surface of the seeds. This growth can be in the form of direct precipitation of Li2O2 molecules or disproportionation reactions involving LiO2 and Li2O2. In this presentation we introduce a mathematical model that describes the precipitation and growth mechanism in the form of a system of differential equations appropriate for implementation in finite element simulators. In agreement with the classical nucleation theory, the driving force of the nucleation process in our model is the oversaturation of the depositing species. Therefore, the nucleation seeds appear when there is a critical oversaturation condition on the surface of the carbon; the number of nucleation seeds increases in time during the discharge of the battery at a rate which is proportional to the discharge rate of the battery. The model explains quantitatively well the decrease of the specific capacity with the discharge rate and also explains the relatively large Li2O2 grains observed in the cathode of discharged Li-air batteries. [1] S. Lau, L.A. Archer, Nucleation and Growth of Lithium Peroxide in the Li–O2 Battery, Nano Letters, 15 (2015) 5995-6002. [2] Y. Yin, A. Torayev, C. Gaya, Y. Mammeri, A.A. Franco, Linking the Performances of Li–O2 Batteries to Discharge Rate and Electrode and Electrolyte Properties through the Nucleation Mechanism of Li2O2, The Journal of Physical Chemistry C, 121 (2017) 19577-19585. [3] A.C. Luntz, V. Viswanathan, J. Voss, J.B. Varley, J.K. Nørskov, R. Scheffler, A. Speidel, Tunneling and Polaron Charge Transport through Li2O2 in Li–O2 Batteries, The Journal of Physical Chemistry Letters, 4 (2013) 3494-3499. [4] P. Andrei, J.P. Zheng, M. Hendrickson, E.J. Plichta, Some Possible Approaches for Improving the Energy Density of Li-Air Batteries, Journal of the Electrochemical Society, 157 (2010) A1287-A1295.