Impact of Metal Oxide Nanoparticles on SEI and on Sodium Deposition - Dissolution

Sodium metal batteries (SMBs) are emerging as a next-generation rechargeable technology due to sodium's abundance, cost-effectiveness, and high theoretical specific capacity. However, challenges such as uneven sodium plating, dendrite formation, and low coulombic efficiency (CE) limit their performance. This study explores the impact of various additives to carbonate based electrolyte, including metal oxide nanoparticles (Al2O3, TiO2) and vinylene carbonate (VC), on the electrochemical behavior of SMBs, focusing on sodium deposition morphology, solid electrolyte interphase (SEI) composition, and capacity losses. The VC additive contributed to a thicker SEI, characterized by an inorganic-rich inner layer and polymer-rich outer layer, resulting in the highest CE across current rates. In contrast, Al2O3 samples demonstrated intermediate performance, with NaOH on the SEI surface and reduced polymer content compared to VC. Additionally, we show that capacity losses from isolated "dead" sodium represent the largest contributor to total capacity losses, with cells containing VC and 1% Al2O3 exhibiting the lowest total capacity losses. Moreover, the study demonstrates the uneven distribution of sodium deposition on the aluminum CC, especially its tendency to favor regions with lower fluorine concentration in the SEI. The findings from this study are important for optimizing SMB design, stability, and cycling efficiency.