1 Introduction Rechargeable Li-ion batteries have been a major part of our everyday lives since their commercialization in the early 1990s. However, current Li-ion technologies suffer from their dependence on scarce and toxic elements, e.g., Ni, Co, and Li [1, 2]. A more sustainable alternative is Na-ion batteries, which have matured significantly in recent years and are becoming a promising substitute for Li-ion technologies [3, 4].