Shedding Light on Rechargeable Na/Cl2 Battery

Abstract

Advancing new ideas of rechargeable batteries represents an important path to meeting the ever increasing energy storage needs. Recently we showed rechargeable sodium/chlorine (Na/Cl2) (or lithium/chlorine Li/Cl2) batteries that used a Na (or Li) metal negative electrode, a microporous amorphous carbon nanosphere (aCNS) positive electrode and an electrolyte containing dissolved AlCl3 and fluoride additives in thionyl chloride (SOCl2)1-2. The main battery redox reaction involved conversion between NaCl and Cl2 trapped in the carbon positive electrode, delivering a cyclable capacity of up to 1200 mAh g-1 (based on positive electrode mass) at a ~ 3.5 V discharge voltage1-2. Here, we discovered by X-ray photoelectron spectroscopy (XPS) that upon charging a Na/Cl2 battery, chlorination of carbon in the positive electrode occurred to form C-Cl accompanied by molecular Cl2 infiltrating the porous aCNS, consistent with Cl2 probed by mass spectrometry. Synchrotron X-ray diffraction observed the development of graphitic ordering in the initially amorphous aCNS under battery charging when the carbon matrix was oxidized/chlorinated and infiltrated with Cl2. The C-Cl, Cl2 species and graphitic ordering were reversible upon discharge, accompanied by NaCl formation. The results revealed redox conversion between NaCl and Cl2, reversible graphitic ordering/amorphourization of carbon through battery charge/discharge, and for the first time probed trapped Cl2 in porous carbon by XPS.