Research Background:

Rechargeable sodium-chlorine (Na-Cl₂) batteries emerge as strong candidates for high-performance energy storage technologies due to their advantages of high energy density and low cost. These batteries typically employ a chloraluminate electrolyte composed of aluminium chloride (AlCl₃) and thionyl chloride (SOCl₂). To enhance cycle stability, fluorinated additives such as sodium bis(fluorosulfonyl)imide (NaFSI) are frequently introduced into the electrolyte. It is generally postulated that such additives decompose on the metal surface to form sodium fluoride, constituting a solid electrolyte interphase (SEI) layer that suppresses side reactions between sodium metal and the electrolyte, thereby prolonging battery cycle life. However, this mechanism remains inadequately validated, necessitating clarification of the actual role of fluorinated electrolyte additives in Na-Cl₂ batteries to effectively guide material design and performance enhancement for rechargeable Na-Cl₂ batteries.

In light of this, the research group led by Associate Professor Sun Hao from the School of Chemistry and Chemical Engineering at Shanghai Jiao Tong University, concurrently serving as a resident scientist at the Zhangjiang Advanced Research Institute and the Centre for Frontier Science in Transformative Molecules, conducted systematic investigations. Their findings revealed that fluorinated electrolyte additives exert limited influence on the battery's anode side. Instead, they catalyse the in situ formation of aluminium fluoride (AlF₃) via chemical reactions at the cathode, thereby substantially enhancing the redox reaction kinetics of NaCl/Cl₂. Building upon this mechanism, the team proposed a strategy of directly loading high-performance catalysts onto the cathode, substantially improving the battery's rate capability and cycling performance.

Figure 1. Schematic illustration of the mechanism of FSI⁻ anion in conventional sodium metal batteries and Na-Cl₂ batteries.

The research findings were published in National Science Review under the title ‘Unveiling cathode catalysis of fluorinated electrolyte additives for high-performance Na-Cl₂ batteries’. The first authors are PhD candidates Xu Qiuchen and Tang Shanshan from the Centre for Frontier Science in Transformative Molecules. Corresponding authors are Associate Professor Sun Hao and Dr Zhao Xiaoju. This work received substantial support from the National Natural Science Foundation of China, the Special Fund for Basic Research of Central Universities, the Centre for Frontier Science in Transformative Molecules at Shanghai Jiao Tong University, and the Zhangjiang Advanced Research Institute.

Research content:

Figure 2. Electrochemical performance and interfacial chemistry of fluorinated electrolyte additives at the anode.

Through electrolyte composition analysis, the research team discovered that the FSI⁻ anion in the electrolyte undergoes a spontaneous chemical reaction with AlCl₃, leading to S–F bond cleavage and the generation of AlCl₃F⁻ ions. Na||Al half-cell test results further revealed that the introduction of fluorinated additives did not significantly enhance sodium metal deposition/stripping efficiency. In-depth characterisation of anode interface components via time-of-flight secondary ion mass spectrometry and cryo-transmission electron microscopy revealed that the SEI primarily consisted of sodium chloride, with virtually undetectable signals for sodium fluoride. These findings indicate that the action of fluorinated additives on the anode side is not a primary factor influencing battery performance.

Figure 3. Distribution of AlF₃ on the cathode and catalytic mechanism.

Characterisation of the cycled cathode revealed that AlCl₃F⁻ gradually evolves during cycling, ultimately converting to AlF₃. This compound exhibits an interleaved distribution with NaCl, indicating that AlF₃ catalyses the oxidation reaction of NaCl. Mechanism studies confirm that AlF₃, acting as a strong Lewis acid, effectively promotes electron transfer from NaCl to the carbon substrate. This significantly reduces the reaction energy barrier for NaCl oxidation to Cl₂, substantially enhancing cathode reaction kinetics.

Based on this mechanistic understanding, the research team proposed two battery performance optimisation strategies: firstly, direct incorporation of AlF₃ powder into the cathode achieved high rate performance of up to 25 A g^(−1); Second, a polyion liquid (PIL) containing FSI⁻ anions was designed and synthesised as a cathode catalytic material. The PIL's excellent film-forming properties facilitated uniform AlF₃ distribution, thereby regulating NaCl growth morphology and preventing electrode passivation. This enabled stable battery operation at an ultra-high current density of 30 A g^(−1), with performance far surpassing current state-of-the-art Na-Cl₂ and Li-Cl₂ batteries.

Figure 4. Enhancement of rate capability and cycling performance through direct loading of catalysts onto the cathode.

In summary, this work elucidates the unique mechanism of fluorinated electrolyte additives in Na-Cl₂ batteries. Rather than forming a solid electrolyte interphase at the anode to protect it, these additives undergo spontaneous chemical reactions with the electrolyte to generate AlF₃ catalysts in situ at the cathode. This enhances the kinetics and reversibility of the NaCl/Cl₂ redox reaction. This mechanism provides crucial theoretical support for developing high-power-density, long-lifetime metal-chlorine batteries. It also offers a novel approach to transforming anode protective additives into cathode catalysts, furnishing both theoretical and experimental foundations for advancing high-rate, long-cycle conversion batteries.