On the Reversibility and Fragility of Sodium Metal Electrodes

Abstract

Metallic sodium is receiving renewed interest as a battery anode material because the metal is earthabundant, inexpensive, and offers a high specific storage capacity (1166 mAh/g at-2.71 V vs the This article is protected by copyright. All rights reserved. 2 standard hydrogen potential). Unlike metallic lithium, the case for Na as the anode in rechargeable batteries has already been demonstrated on a commercial scale in high-temperature Na||S and Na||NiCl 2 secondary batteries, which increases interest. We investigate the reversibility of room temperature sodium anodes in galvanostatic plating/stripping reactions using in-situ optical visualization and galvanostatic polarization measurements. It is discovered that electronic disconnection of mossy metallic Na deposits (-orphaning‖) is a dominant source of anode irreversibility in liquid electrolytes. The disconnection is shown by means of direct visualization studies to be triggered by a root-breakage process during the stripping cycle. As a further step towards electrode designs that are able to accommodate the fragile Na deposits, electrodeposition of Na is demonstrated in non-planar electrode architectures which provide continuous and morphology agnostic access to the metal at all stages of electrochemical cycling. On this basis, we report nonplanar Na electrodes which exhibit exceptionally high levels of reversibility (Coulombic Efficiency > 99.6% for 1mAh/cm 2 Na throughput) in room-temperature, liquid electrolytes.

Sodium Cyclopentadienide as a New Type of Electrolyte for Sodium Batteries

“…In comparison, Figure 3 b shows the typical potential response of a cell with NaPF 6 electrolyte (in EC : DMC 1 : 1) under the same CC conditions. This type of potential curve was observed not only during our studies with NaPF 6 and NaClO 4 (supporting information and [11] ), but also in previous studies on lithium metal anodes using different electrolytes [19–21] …”

“…Its decay is due to the lower barrier for the stripping and plating processes on already existing structures (pits and dendrites, respectively). Overall, such potential curves are typical for mossy dendritic growth [20,21] . The absence of this peak in the potential response of the NaCp cell indicates that there is no such energy benefit stripping from pits and plating onto existing dendrites/electrodeposits.…”

“…This type of potential curve was observed not only during our studies with NaPF 6 and NaClO 4 (supporting information and [11] ), but also in previous studies on lithium metal anodes using different electrolytes. [19][20][21] Figure 1. Arrhenius plot of the ionic conductivity in a temperature range from À 30°C to 50°C (continuous line to guide the eye).…”

“…Overall, such potential curves are typical for mossy dendritic growth. [20,21] The absence of this peak in the potential response of the NaCp cell indicates that there is no such energy benefit stripping from pits and plating onto existing dendrites/electrodeposits. Thereby, NaCp could prevent the ongoing growth of these structures and enable a more homogenous sodium deposition even after continuous cycling.…”

Solid Electrolyte Interphases on Sodium Metal Anodes

“…Unlike Li metal anodes, which have been reported to fail due to the uncontrolled growth of dendrites and consequent internal short circuit, Na metal anodes typically fail because of their electronic disconnection from the underlying metal ( Figure 3d), as well as the depletion of the electrolyte, which tends to be erroneously concluded in the absence of the in operando optical visualization setup. [53] Summarily, although SEI research has been extensively conducted for many years, there are still several challenges in reference to the accurate design and construction of a stable SEI, and more vigorous attention needs to be invested toward understanding the fundamentals of SEIs.…”