Unlocking the role of electrolyte concentration for Na-O2 batteries

Abstract

Na-O2 batteries have attracted great interest in recent years mainly due to their high energy density, in theory having prospects to outperform the commercialized lithium-ion batteries. In the quest for optimization, a recently explored approach is to use highly concentrated electrolytes (HCEs). The knowledge of molecular level of solvation as function of electrolyte concentration and its impact on Na-O2 battery performance is, however, still very limited. In this work, experimental and computational methods are used to characterize the cation solvation and when the emergence of anions into the cation first solvation shell occurs, which affects the de-solvation process and formation of discharge products. Furthermore, the solid electrolyte interphase (SEI) formed using HCEs demonstrates presence of anion fragments, with poorer protection of the Na metal anode. Moreover, the use of HCEs is also linked to lowered capacity, possibly due to a decrease in the size of the cubic-shaped discharge products as the electrolyte concentration increases, causing clogging of the pores of the air cathode. Thus, increasing the electrolyte salt concentration seems to have a detrimental effect on the cyclability of Na-O2 batteries. Instead, electrolytes with a lower than conventional salt concentration show the best performance, which highlights the importance of carefully tuning the cation solvation alongside overall physico-chemical properties to enhance battery performance.