Na ion batteries attract significant research interest since theyprovide potentially high energy density while using low cost andabundant sodium as the active ion.[1–5] Due to the analogy betweenLi and Na ions, different types of materials that have been appliedin Li-ion batteries are also studied for application in Na ion bat-teries and vice versa.[6–9] Si has been extensively investigated sinceit has high theoretical lithiation capacity up to Li4.4Si.[10–12] In gen-eral, the sodiation of Si is anticipated to be different with respectto phase behavior, insertion voltages, and kinetic barriers whencompared to Li ion, for instance, because of the difference inionic radius of Na+ (0.97 Å) and Li+ (0.68 Å).[13,14] From thermalsynthesis of Na Si materials it is known that NaSi is the mostNa rich phase for Na Si binary compounds,[ 15 ] which wouldenable a sizeable capacity of 954 mAh g−1 and be promising forNa-ion battery anodes. [...]