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/Cl 2 ) (or lithium/chlorine Li/Cl 2 ) batteries that used a Na (or Li) metal negative electrode, a microporous amorphous carbon nanosphere (aCNS) positive electrode and an electrolyte containing dissolved AlCl 3 and fluoride additives in thionyl chloride (SOCl 2 ) 1-2 . The main battery redox reaction involved conversion between NaCl and Cl 2 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 voltage 1-2 . Here, we discovered by X-ray photoelectron spectroscopy (XPS) that upon charging a Na/Cl 2 battery, chlorination of carbon in the positive electrode occurred to form C-Cl accompanied by molecular Cl 2 infiltrating the porous aCNS, consistent with Cl 2 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 Cl 2 . The C-Cl, Cl 2 species and graphitic ordering were reversible upon discharge, accompanied by NaCl formation. The results revealed redox conversion between NaCl and Cl 2 , reversible graphitic ordering/amorphourization of carbon through battery charge/discharge, and for the first time probed trapped Cl 2 in porous carbon by XPS.