Shedding light on rechargeable Na/Cl₂ battery

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 aluminum chloride and fluoride additives in thionyl chloride [G. Zhu et al., Nature 596, 525-530 (2021) and G. Zhu et al., J. Am. Chem. Soc. 144, 22505-22513 (2022)]. The main battery redox reaction involved conversion between NaCl and Cl₂ trapped in the carbon positive electrode, delivering a cyclable capacity of up to 1,200 mAh g^-1 (based on positive electrode mass) at a ~3.5 V discharge voltage [G. Zhu et al., Nature 596, 525-530 (2021) and G. Zhu et al., J. Am. Chem. Soc. 144, 22505-22513 (2022)]. Here, we identified by X-ray photoelectron spectroscopy (XPS) that upon charging a Na/Cl₂ battery, chlorination of carbon in the positive electrode occurred to form carbon-chlorine (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 probed trapped Cl₂ in porous carbon by XPS.