Stable NaTFSI-Based Highly Concentrated Electrolytes for Na-Ion and Na–O₂ Batteries

Sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) and adiponitrile (ADN) have attractive high stability and safety properties for application as electrolytes in Na-ion batteries, but are unusable in modern cells due to significant Al corrosion by NaTFSI, and spontaneous ADN degradation by Na metal. Herein, the electrochemical properties of NaTFSI–ADN electrolytes are investigated as a function of concentration. The ionic conductivity and phase diagram of NaTFSI–ADN is measured, and molecular dynamics (MD) simulations give insight into the solution structure of the electrolyte. The reductive stability of the electrolyte is found to increase drastically with concentration in cyclic voltammetry (CV) experiments, and the parasitic dissolution of Al by TFSI decreases with concentration in linear sweep voltammetry (LSV) and chronoamperometry (CA) tests. In Na-ion cells, the dual effect of reductive stability enhancement and Al corrosion suppression allows the 4.4 M electrolyte to reversibly intercalate high voltage Na3V2(PO4)2F3 (NVPF) cathodes for multiple cycles, while no NVPF intercalation is observed with the standard 1.0 M electrolyte. Na–O2 cells also benefit from the highly concentrated electrolyte, showing significantly longer lifetimes in CV experiments on Na–O2 coin cells. Concentrating NaTFSI–ADN electrolytes offers practical benefits to Na batteries and should be implemented with further stabilizing strategies going forward.