Na Metal Batteries: Interface Design from Liquid to Solid Systems
Authors: Zhang Shumin , Zhao Yang , Sun Xueliang 
Published: May 2020

With a high demand on advanced energy storage systems, sodium metal batteries (SMBs) are believed to be an emerging and competitive candidate due to the rich resource, low cost, and suitable redox potential of Na metal.1 However, the SEI layer formed in Na carbonate electrolytes is relatively unstable, which leads to nonuniform ionic flux during repetitive Na stripping/plating process as well as low columbic efficiency (CE). In addition to that, the poor SEI layer may result in the growth of mossy or dendritic Na, causing serious safety issues. 2 All-solid-state Na metal batteries (ASSSMBs) equipped with solid-state electrolytes (SSEs) are much reliable for the elimination of safety hazards that liquid electrolytes associate with. In the state-of-the-art SSEs, sulfide-based SSEs exhibit high ionic conductivity, moderate synthetic conditions, low grain boundary resistance, and malleable properties3, thus they are more promising to be actually employed in the next-generation high-performance ASSSMBs. However, these sulfide SSEs can thermodynamically react with metal anode to form a mixed ionic and electronic conducting interface, continuously depleting both the SSE and Na anode.4 Meanwhile, the resultant large polarization can lead to the growth of interfacial resistance and deterioration of battery performance. Similarly with that in liquid system, uneven Na+ deposition also occurr at the sulfide SSE/Na anode interface, causing the formation of dendrites and short circuits of the batteries.4 As such, the electrolyte/Na anode incompatibility is the priority to be solved for realizing high-performance SMBs. Even though some effective approaches have been reported to address this issue, the developments of SMBs is still in its infancy.
Herein, we demonstrate the inorganic−organic coating via advanced molecular layer deposition (MLD) 5, 6 as a protective layer for metallic Na anode in SMBs. By protecting Na anode with controllable alucone layer, the dendrites and mossy Na formation are effectively suppressed and the lifetime of liquid-state battery has been significantly improved.7 Moreover, we extend this alucone film in ASSSMBs to stabilize the active Na anode/electrolyte interface, limiting the decomposition of the sulfide based electrolyte (Na3SbS4 and Na3PS4) and Na dendrite growth. Protected by this film, full battery performance is remarkably improved and Na-Na symmetric cells are stabilized nearly 500 hours at room temperature. The modified interface is further characterized by X-ray photoelectron spectroscopy (XPS) depth profiling, which provides spatially resolved evidence of the synergistic effect between the dendrite-suppressed sodiated alucone and the insulating unsodiated alucone. Such coupled layers reinforce the protection of the Na metal/electrolyte interface.8 Our works identify alucone as an effective and bi-functional coating material as its derivatives can stabilize the metal/electrolyte interface, paving the way for rapid development and wide utilization of SMBs.

References:

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Figure 1