Revealing Electrochemical Process of Functional Carbon Dots Stabilized Sodium Metal Anode: Co‐Deposition and Strengthened SEI Films

Abstract Sodium metal batteries with high theoretical energy density and low redox potential possess vast application prospects. However, the inevitable side reactions between Na metal anode and electrolyte, the formation of Na dendrite during cycling, and the uneven electric field distribution at the interface will lead to a decrease in the cycling stability of the battery. Herein, N and S co‐doped carbon dots are employed as electrolyte additives in view of their quantum size, excellent dispersion, and polar surface functional groups. It is found that the quantum‐sized carbon core and sodiophilic surface functional groups of N,S‐CDs enable the homogenous deposition of Na⁺ and the involvement of N,S‐CDs in the formation of solid electrolyte film. The N,S‐CDs co‐deposition guarantees the uniform sodium plating in the initial cycle, avoiding the routine dendrite growth. The strength and stability of SEI are boosted with the assistance of inorganics such as Na₃N and Na₂S, which accelerate the transport of Na⁺ flux and realize rapid charge transfer at the interface, ensuring high reversibility of sodium ion plating/stripping. Na||Cu half‐cells achieve a coulombic efficiency of 99% after 250 cycles under 1.0 mA cm⁻¹, and Na||Na symmetric cells continuously cycle over 1200 h at a current density of 1 mA cm⁻².