Anode-free sodium metal batteries are emerging as a compelling alternative to lithium-ion batteries, leveraging the abundance of sodium and eliminating the reliance on critical metals such as Li, Co, and Ni. Unlike sodium-ion batteries, which require a host structure like hard carbon, zero-excess anodes utilize metallic sodium on the anode, enabling a significantly higher theoretical capacity of 1166 mAh g^-1.[1] Despite their potential, these batteries face challenges due to the high reactivity of sodium, including uneven deposition, dendrite formation, electrolyte consumption, and the accumulation of isolated sodium, all of which demand further investigation.[1,2]

In this study, we investigate sodium plating and stripping on carbon-coated aluminum foil, focusing on three electrolytes: 1 M NaPF₆ in diglyme (2G), a locally high concentrated electrolyte based on sodium bis(fluorosulfonyl)imide:monoglyme:1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (NaFSI:1G:TTE) in the molar ratio of 1:1.5:1.5, and carbonate-based  1 M NaPF₆ in ethylene carbonate/propylene carbonate (EC/PC). We studied the sodium morphologies by utilizing an operando optical microscope. To gain insights into electrolyte properties, including the deposition, we investigated the coordination shells towards Na⁺, ionic conductivity, and viscosity, which influence deposition behavior. Furthermore, we assessed the cycling performance through galvanostatic cycling of AlǁNa cells, and the evolution of the interphase resistances was studied by impedance spectroscopy.

It has been shown that ion mobility, coordination towards Na⁺, and cathodic electrolyte stability play a pivotal role regarding the sodium plating morphology. In the case of the glyme-based electrolyte, both high ion mobility and electrochemical stability lead to uniform crystal growth, as seen in the ToC. Conversely, the locally high-concentrated electrolyte exhibits lower ion mobility and poorer stability compared to diglyme-based, resulting in root-like growth. While the carbonate-based offers the highest Na-ion mobility, its low Coulombic efficiency shows low stability towards sodium metal and the formation of mossy-like sodium structures with thick SEI layers, and electrically isolated sodium. Based on these observations, the mechanisms behind cell failure become clearer, highlighting the critical factors for developing optimized electrolytes for future anode-free sodium metal batteries.