The resultant concentration of electric field lines on faster growing regions of the interface drives the morphological instability loosely termed dendritites 12, 13. ![]() Instead, dendrites on Na (and Li) arise from inhomogeneities in the resistance of the solid–electrolyte interphase (SEI), formed spontaneously on the anode surface when in contact with an electrolyte. Specifically, during battery recharge Na ions deposit in rough, low density and uneven patches on the negative electrode, even at current densities below the limiting current where classical instabilities such as electroconvection that drive rough, dendritic deposition are expected to be unimportant 11, 12. Additionally, recent studies have shown that rechargeable batteries that pair a Na anode with highly energetic O 2-based cathodes are intrinsically more stable during discharge than their Li analogs because the species generated electrochemically in the cathode, the metal superoxide, is more stable when the anode is Na, as opposed to Li 9, 10.Īs with rechargeable batteries comprising Li metal anodes, the Achilles heel of the rechargeable sodium battery is the anode’s susceptibility to failure during the charging process. Metallic sodium has other attractive features as a battery anode, including its relatively high electronegativity and low atomic weight, which combine to give the Na anode a specific capacity (1166 mAh gm −1) that is competitive with Li (3860 mAh gm −1) in many applications 6. The greater natural abundance of sodium and its availability in regions all over the world provide significant cost advantages over Li that have within the last decade helped re-ignite interest in Na-based batteries 6, 7, 8. Although sodium-based batteries pre-date those based on lithium 3, Li has received more recent attention for a variety of reasons, including its greater electronegativity, higher specific energy, low atomic radius 4, 5, and the commercial success of related Li-ion battery technology. Rechargeable batteries based on lithium and sodium metal anodes are of interest for high-energy storage solutions in portable and stationary applications 1, 2. Direct visualization of sodium electrodeposition confirms large improvements in stability of sodium deposition at sodium bromide-rich interphases. These experiments reveal an approximately three-fold reduction in activation energy for ion transport at a sodium bromide interphase. We evaluate this prediction by means of electrochemical measurements and direct visualization studies. In particular, we find that a sodium bromide interphase presents an exceptionally low energy barrier to ion transport, comparable to that of metallic magnesium. ![]() Here we use joint density-functional theoretical analysis to show that the surface diffusion barrier for sodium ion transport is a sensitive function of the chemistry of solid–electrolyte interphase. Chemical instability of liquid electrolytes also leads to premature cell failure as a result of parasitic reactions with the anode. Room-temperature sodium metal batteries are impractical today because morphological instability during recharge drives rough, dendritic electrodeposition. Secondary batteries based on earth-abundant sodium metal anodes are desirable for both stationary and portable electrical energy storage.
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