Nanoconfined Chlorine Redox Chemistry in Multi-Walled Carbon Nanotubes: A Breakthrough for Rechargeable Na/Cl2 Batteries

• • Initial discharge capacity of 5400 mAh g−1 (carbon mass basis) and reversible capacity of 3500 mAh g−1 at room temperature, with a discharge plateau near 3.9 V, exceed prior Na/Cl2 systems using amorphous carbon nanospheres or graphite cathodes; this directly addresses the energy-density bottleneck for stationary storage and electric vehicles. • • Stable cycling for over 140 cycles at rates up to 2 C with near 100% Coulombic efficiency, enabled by the dual-function KFSI additive that forms a NaF/KF-rich SEI and suppresses dendrites via electrostatic shielding from the lower K+/K redox potential; this mitigates sodium-metal anode failure, a persistent barrier to commercialization. • • In situ Raman spectroscopy identifies reversible SCl2 and S2Cl2 species at the end of charge, contributing an additional ~3.9 V plateau, while the main Cl−/Cl2 redox plateau remains at ~3.55 V; this clarifies the reaction mechanism and demonstrates that MWCNTs trap active species, preventing shuttle-induced electrolyte depletion. • • Cryogenic TEM reveals NaCl nanocrystals deposited within the hollow cores of MWCNTs, and EELS confirms uniform chlorine distribution on nanotube surfaces in the charged state; the high defect density (Raman D/G ratio = 1.01) and large pore volume (2.48 cm3 g−1) are critical for accessing internal space and achieving superior performance.