Increasingly Reversible Na/Cl₂ and Li/Cl₂ Batteries

Sodium/chlorine and lithium/chlorine batteries are new types of high-voltage and -capacity batteries reported recently with great potential to be developed into high-energy-density batteries for real-world applications. In this work, we report using cobalt polyphthalocyanine grown on multiwalled carbon nanotubes as the positive electrode in both sodium/chlorine and lithium/chlorine batteries to achieve, for the first time, a stable cycling capacity well exceeding the first discharge capacity in chlorine batteries. We employed various techniques together with theoretical calculations to reveal that cobalt polyphthalocyanine not only facilitates the formation of submicron-sized sodium chloride crystals with excellent electrochemical activity, but also enhances the reversibility of chlorine/chloride redox by forming cobalt-chlorine and sodium-nitrogen bonds during battery cycling. Additionally, cobalt polyphthalocyanine acts as a good storage medium for the chlorine that is formed during battery charging. We also discovered that the metal center can be changed from cobalt to iron and the carbon substrate can be varied from multiwalled carbon nanotubes to graphite for both sodium/chlorine and lithium/chlorine batteries. The chlorine batteries can now operate using only a minimal amount of electrolyte, and we were able to construct a sodium/chlorine battery with ∼150 Wh kg^-1 or ∼325 Wh L^-1 full-cell energy density for the first time.