Unveiling cathode catalysis of fluorinated electrolyte additives for high-performance Na-Cl2 batteries

Rechargeable sodium-chlorine (Na-Cl2) batteries offer a promising solution for next-generation energy storage, due to their high electrochemical performance and reliance on abundant, cost-effective materials. The electrolyte typically comprises a mixture of aluminum chloride (AlCl3) and thionyl chloride (SOCl2), with the addition of sodium bis(fluorosulfonyl)imide (NaFSI) and sodium trifluoromethanesulfonimide (NaTFSI) as F-containing additives. These additives have been considered to form a fluorinated solid-electrolyte interphase layer on the Na metal anode to enhance cycling stability, a mechanism analogous to those observed in conventional alkali metal batteries. Here we reveal a previously unrecognized spontaneous reaction between these additives and AlCl3 in the electrolyte, producing AlF3 on the cathode. This enables facilitated NaCl/Cl2 oxidation due to the strong Lewis acidity of AlF3, and suppresses the parasitic reactions. These findings not only correct the mechanism misunderstanding of fluorinated additives in rechargeable Na-Cl2 batteries but, in a broader context, open a new avenue for turning conventional anode protective additives into efficient cathode catalyst for high-rate and long-life energy storage solutions.