Carbon Nanotubes for Rechargeable Na/Cl₂ Batteries

Rechargeable Na/Cl₂ batteries were developed for the first time using multiwalled carbon nanotube (MWCNT) positive electrodes in SOCl₂-based electrolytes. At room temperature, these batteries delivered high cycling specific capacities up to 3500 mA h g^-1 (normalized to CNT mass) with ∼3.9 V discharge voltage at up to 2 C rates over >140 cycles. In situ Raman spectroscopy experiments combined with real-time optical microscopy imaging revealed reversible formation and reduction of SCl₂ and S₂Cl₂ species during battery operation, responsible for the additional plateaus to the main Cl^-/Cl₂ redox reactions. Cryo-TEM revealed NaCl nanocrystals inside the hollow inner space of CNTs through battery cycling, suggesting Cl^-/Cl₂ redox reactions reaching hollow CNTs likely through defects and open ends on MWCNTs. High-resolution electron energy loss spectroscopy (EELS) mapping revealed Cl uniformly distributed along CNTs in the charged state, suggesting CNTs as a novel carbon material to host Cl^-/Cl₂ redox and store chlorine for reversible conversion between NaCl and Cl₂ and battery rechargeability.