Alkali metal anodes hold the promise to increase the energy density of current battery technologies significantly. But they are plagued by dendrite-induced internal shorts and solid-electrolyte interphase (SEI) layers that undermine the battery safety and cycle life. The two mutually enhanced phenomena are controlled by both the interfacial chemistry and the ion transport. In this presentation, we will first talk about why and how a naturally selected pore of the separator would be easily and quickly penetrated by Li metal growths driven by an apparent under-limiting current density, which is supposed to trigger only the root-growing whiskers that can be blocked by the porous separator. We will also demonstrate the growth of a non-porous Na ingot with self-modulated shiny-smooth facets, without forming whiskers, mosses, gas bubbles, or disconnected metal particles that usually observed in other in situ or operando studies. The ideal interfacial stability confirmed at the microscopic scale in operando is the key to enable anode-free Na metal full cells with a capacity retention rate of 99.93% per cycle, superior to available anode-free Na and Li cells using liquid electrolytes. Contradictory to the common beliefs established around alkali metal anodes, there is no repeated SEI formation on or within the Na anode, supported by the X-ray photoelectron spectroscopy elemental depth profile analyses, electrochemical impedance spectroscopy diagnosis, and microscopic imaging. The results presented here suggest that our simple operando platform using transparent microcapillary cells is an effective tool for electrolyte stability characterization, which complements existing methods and can help achieve the comprehensive analytical understandings of stable alkali metal anodes.

Analytical understandings of alkali metal electrodeposition toward stable anode-free full cells

ACS Spring Meeting 2021

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