Alternative anodes for Na–O2 batteries: the case of the Sn4P3 alloy

Abstract

An alternative anodeto metallic sodium based on the high capacity of Sn₄P₃ alloy is investigated for the first time in Na–O₂ batteries. This alloy is synthesised by an easy and scalable method based on ball milling which is further optimized in order to obtain the most suitable morphology while avoiding agglomeration of the material. First, the stability of Sn₄P₃/C composite within the battery environment is analysed and compared to that of metallic Na. Second, the electrochemical study reveals the formation of sodium superoxide on both the metallic anode and Sn₄P₃ alloy for the first time, which evidences the existence of O₂/O₂⁻ crossover processes. Complementary impedance studies, at various voltages and as a function of cycle number, demonstrate a higher stability of the solid electrolyte interphase (SEI) throughout cycling in addition to presenting lower reactivity and greater stability than the metallic Na system. Regarding the cycling performance, although it is lower than that of metallic Na, it surpasses that exhibited by other alternative anodes under similar conditions. This work therefore demonstrates the viability of a new family of anodes, paving the way for the development of more stable and safer Na–O₂ batteries. Moreover, the importance of oxygen crossover is highlighted which is a common problem regardless of the anode used.