Stable anode interphase enabled use of protic electrolytes in sodium metal batteries

• Protic solvents tend to dissociate protons and aggressively corrode the anode. • Fine-tuning the N-H bond strength of N-methylacetamide supresses proton dissociation. • Promoting aggregate formation enhances interfacial passivation. Sodium metal batteries based on liquid electrolytes are currently limited to using aprotic solvents, such as carbonate esters and ethers. This as protic solvents fundamentally have proton dissociation due to prevalent hydrogen bonding, leading to undesirable reactivity with the sodium metal anode. Our working hypothesis is that this reactivity can be controlled and reduced by replacing/disrupting the hydrogen bonding with other interactions. We present here the viability by using N-methyl-acetamide as an electrolyte solvent for sodium metal batteries, to which both Na + cations and [FSI] - anions, from the NaFSI electrolyte salt used, can interact to modify the N-H bond strength. Combined with the formation of aggregates by careful composition control, the passivation of sodium metal anodes is effectively improved. Furthermore, distinctly different solid electrolyte interphases are formed, as compared to when using a conventional organic electrolyte, and excellent cycling stability of a full cell using Na 3 V 2 (PO 4 ) 3 as cathode is demonstrated, reaching an average Coulombic efficiency of 99.9%. Overall, we show that protic solvents, given controlled proton activity, offer another route to possibly achieve practical sodium metal batteries.