Sodium-ion batteries (SIBs) are considered as a potential candidate for electrical energy storage system (ESS) because of their advantages such as low-cost system and evenly distributed sodium resources worldwide. However, there is need to explore new electrode materials for the realization of SIBs. Particularly, the anode side of SIBs needs more improvements as commonly investigated anodes in lithium-ion batteries (LIBs) such as graphite and silicon do not intercalate sodium ions.

In the search for high capacity anodes for SIBs, we have investigated electrochemical properties of a nanocomposite based on SnF₂ and acetylene black. The nanocomposite electrode delivers a reversible capacity of 563 mAh g^-1 which is higher than the specific capacity of 323 mAh g^-1 of the micron-sized bare SnF₂ electrode. The sodium insertion/extraction process of the high-performing nanocomposite is revealed by in-situ XRD, ex-situ XAS and TEM techniques. The reaction mechanism revealed using in-situ XRD show the existence of a solid solution of two or more compositions during cycling. Ex-situ XAS reveals the electronic and atomic configurations of SnF₂ at different potential states during dis/charging. The XAS results reveal that the valence change of Sn follows the conversion (SnF₂ + 2Na → Sn + 2NaF) and the alloying (Sn + XNa → SnNa_X) reaction upon sodium insertion into a composite.