Carbon Nanotubes for Rechargeable Na/Cl2 Batteries

Abstract

Rechargeable Na/Cl<inf>2</inf> batteries were developed for the first time using multiwalled carbon nanotube (MWCNT) positive electrodes in SOCl<inf>2</inf>-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 SCl<inf>2</inf> and S<inf>2</inf>Cl<inf>2</inf> species during battery operation, responsible for the additional plateaus to the main Cl^-/Cl<inf>2</inf> redox reactions. Cryo-TEM revealed NaCl nanocrystals inside the hollow inner space of CNTs through battery cycling, suggesting Cl^-/Cl<inf>2</inf> 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^-/Cl<inf>2</inf> redox and store chlorine for reversible conversion between NaCl and Cl<inf>2</inf> and battery rechargeability.