Operando Observation of a Highly Reversible Nonporous Sodium Metal Anode with Shiny-Smooth Surface

Abstract

While alkali metal anodes are considered as the promising alternatives to the ion-hosting materials due to the high energy densities, the nonuniform deposition (dendritic growth) that can lead to internal shorts remains a major obstacle of their practical applications (1,2). Here, according to the operando observation in our special capillary cells, we report a shiny-smooth nonporous sodium metal anode that can be cycled reversibly even at high areal capacities, without the formation of any whiskers, mosses, gas bubbles or disconnected metal particles observed in existing studies (3,4). The plating process follows the lateral surface-growing mechanism, which transforms into tip-growing branch-like dendrites at a critical time when applying over-limiting currents (5). The superior interfacial stability is attributed to the non-accumulative solid electrolyte interphases (SEIs) upon cycles, which is supported by the elemental depth profile analysis. As a result, a reversible high capacity sodium metal anode is achieved in coin cells within a critical penetration-related current density (6 mA cm-2). Our study demonstrates the possibility of plating shiny-smooth reactive alkali metal in liquid electrolytes and provides a reliable option applicable for the next-generation alkali metal batteries. Xu W, Wang J, Ding F, Chen X, Nasybulin E, Zhang Y, et al. Lithium metal anodes for rechargeable batteries. Energy Environ Sci. 2014;7(2):513–37.Zhao C, Lu Y, Yue J, Pan D, Qi Y, Hu YS, et al. Advanced Na metal anodes. J Energy Chem. 2018;27(6):1584–96.Rodriguez R, Loeffler KE, Nathan SS, Sheavly JK, Dolocan A, Heller A, et al. In Situ Optical Imaging of Sodium Electrodeposition: Effects of Fluoroethylene Carbonate. ACS Energy Lett. 2017;2(9):2051–7.Yui Y, Hayashi M, Nakamura J. In situ Microscopic Observation of Sodium Deposition/Dissolution on Sodium Electrode. Sci Rep. 2016;6:1–8.Bai P, Li J, Brushett FR, Bazant MZ. Transition of lithium growth mechanisms in liquid electrolytes. Energy Environ Sci. 2016;9(10):3221–9. Figure 1