image: (a) Conventional rechargeable Na metal battery with a potential-dependence redox reaction for solid-electrolyte interphase (SEI) formation on Na metal anode through FSI− anion decomposition. (b) Rechargeable Na-Cl2 battery with a spontaneous chemical reaction between the FSI− anion and AlCl3 in the chloroaluminate electrolyte to form AlF3 at the carbon cathode, which further facilitates the oxidation of NaCl as an efficient Lewis-acidic catalyst.
Credit: ©Science China Press
Rechargeable sodium-chlorine (Na-Cl2) batteries have emerged as a promising option for next-generation energy storage due to their high energy density and low cost. A key component of this battery system is the chloroaluminate electrolyte, typically composed of aluminum chloride (AlCl3) and thionyl chloride (SOCl2) mixed with fluorinated additives such as sodium bis(fluorosulfonyl)imide (NaFSI). These additives were long believed to stabilize the Na metal anode by forming a F-rich protective layer, an idea borrowed from their behavior in conventional Li and Na metal batteries.
However, this assumption remains unverified. The strong Lewis acidity of AlCl3 and the high reactivity of SOCl2 could instead cause parasitic reactions with FSI− anions in unexpected ways, leading to a different underlying mechanism from the conventional alkali metal batteries.
A research team led by Prof. Hao Sun from Shanghai Jiao Tong University has now clarified this mechanism through detailed analysis of the electrolyte, anode, cathode over battery cycling. The team discovered that, rather than forming a protective film on the anode, the FSI− anions react rapidly with chloroaluminate species in the electrolyte. This reaction triggers a Cl−F exchange, producing AlF3 on the cathode. As a strong Lewis acid, AlF3 plays a critical catalytic role by accelerating the oxidation of NaCl to Cl2 during charging. The process significantly enhances the overall performance of rechargeable Na-Cl2 batteries.
Based on this discovery, the team engineered a polymerized ionic liquid catalyst with FSI− anions and integrated it into the cathode, which delivered a record current density of 30,000 mA g−1 and a cycle life of 300 cycles, outperforming state-of-the-art Na-Cl2 and Li-Cl2 batteries. This work has clarified the long-held misunderstanding on the role of fluorinated additives in rechargeable Na-Cl2 batter system, and reveals a paradigm shift i.e., transforming conventional anode-protective additives into efficient cathode catalysts. These findings can inspire the innovation in high-energy-density, high-rate battery systems, including sulfur- and oxygen-based battery chemistries.
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See the article:
Unveiling cathode catalysis of fluorinated electrolyte additives for high-performance Na-Cl2 batteries
https://doi.org/10.1093/nsr/nwaf333
Journal
National Science Review