Nanoconfined Chlorine Redox Chemistry in Multi-Walled Carbon Nanotubes: A Breakthrough for Rechargeable Na/Cl2 Batteries
Rechargeable sodium-chlorine (Na/Cl2) batteries derived from thionyl chloride (SOCl2) primary systems offer high theoretical energy density and wide-temperature operation, but their reversibility is constrained by chlorine shuttle and unstable sodium-metal interfaces. Dai et al. demonstrate a Na/Cl2 battery using multi-walled carbon nanotubes (MWCNTs) as the cathode host, a sodium-metal anode, and a SOCl2-based electrolyte containing AlCl3, potassium bis(fluorosulfonyl)imide (KFSI), and NaCl additives. The cell delivers an initial discharge capacity of 5400 mAh g−1 (carbon mass basis), a reversible capacity of 3500 mAh g−1, and a discharge plateau near 3.9 V at room temperature, sustaining over 140 cycles at rates up to 2 C with near 100% Coulombic efficiency. The dual-function KFSI additive suppresses sodium dendrites via electrostatic shielding from the lower K+/K redox potential and forms a NaF/KF-rich solid-electrolyte interphase. In situ Raman spectroscopy reveals reversible SCl2 and S2Cl2 formation at the end of charge, contributing an additional ~3.9 V plateau, while the main Cl−/Cl2 redox plateau remains at ~3.55 V. Cryogenic transmission electron microscopy shows NaCl nanocrystals deposited within the hollow cores of MWCNTs, and electron energy loss spectroscopy confirms uniform chlorine distribution on nanotube surfaces in the charged state. A high defect density (Raman D/G ratio = 1.01) and large pore volume (2.48 cm3 g−1) are identified as critical enablers of superior battery performance.