Electrolyte-Electrode Interfaces in Sodium Metal Batteries

Licentiate thesis, 2025

Sodium metal batteries (SMBs) can achieve high energy densities owning to metal anodes with high specific capacity and low redox potential, and in addition sodium is abundant. Yet, the inherent high chemical and electrochemical reactivity of sodium metal has (so far) limited electrolytes to be created from aprotic solvents (carbonate esters, ethers) because protic solvents dissociate protons via hydrogen bonds and react aggressively. The high reactivity also makes it difficult to correlate electrolyte chemistry with the solid electrolyte interphase (SEI) formation and dissolution, hindering the design of high‑performant SMBs.
Herein, we introduce a protic electrolyte based on the N-methylacetamide (NMA) solvent and the NaFSI salt. The coordination of Na⁺ and [FSI]^- to NMA disrupts the hydrogen bonding and strengthens the N-H bond. Optimizing the salt concentration promotes aggregate formation that enhances the SEI stability. We further applied distribution of relaxation times (DRT) analysis to separate the SEI stability and decomposition from other overlapping processes, using more conventional carbonate electrolytes, and could semi-quantify the SEI loss by comparing post-stripping resistances.
Compared to standard organic electrolytes, the optimized NMA formulation produces a chemically distinct, more robust SEI, while the SEI fracture, detachment, and dissolution shows up in conventional carbonate electrolytes when the underlying metal is stripped.