Adsorption and Reactivity of Na⁺ Salt and Room-Temperature Ionic Liquid at Na Metal Surface

Sodium-ion batteries are cost-effective and sustainable alternatives to lithium-ion batteries. Recently, metal anode batteries have gained prominence over metal-ion systems due to their higher theoretical capacities. However, sodium metal batteries employing conventional organic electrolytes face persistent challenges, including dendrite formation and unstable solid-electrolyte interphase (SEI) layers. Thus, the choice of electrolyte plays a critical role in improving battery stability and performance. Ionic liquids (ILs) offer a promising pathway to improved performance owing to their high thermal and electrochemical stability and excellent ionic conductivity. In this work, we employ density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations to gain atomistic insights into SEI formation and stability by investigating the interactions of the IL containing N,N-methylpropylpyrrolidinium (PYR₁₃ ⁺) cation, bis-(fluorosulfonyl)-amide (FSI^-) and bis-(trifluoromethylsulfonyl)-amide (TFSI^-) anions (PYR₁₃FSI and PYR₁₃TFSI), along with sodium salts NaFSI and NaTFSI, with the Na(110) metallic surface. Detailed geometric and electronic structure analyses capture the anode/electrolyte interfacial chemistry, encompassing both IL and salt interactions and their decomposition pathways to model the early stages of SEI formation. Our results show that FSI^- interacts more strongly than TFSI^- and undergoes spontaneous dissociation upon structural relaxation, whereas PYR₁₃ ⁺ and TFSI^- remain intact. AIMD trajectories at 298 K over 5-10 ps reveal the formation of decomposition products, primarily NaF, which is known to contribute to SEI stabilization. Compared to FSI^-, TFSI^- exhibits delayed decomposition, and PYR₁₃ ⁺ cation remains intact throughout. Overall, these findings unravel the distinct interfacial behaviors of FSI^- and TFSI^- and highlight the critical role of anion chemistry in governing SEI formation and stability in sodium metal batteries.