Electrochemical Sodiation Mechanism in Magnetite Nanoparticle-Based Anodes: Understanding of Nanoionics-Based Sodium Ion Storage Behavior of Fe₃O₄

Electrochemical ion storage behaviors of FeO nanoparticles, as a representative transition metal oxide for an environmentally benign and low-cost anode for a sodium-ion battery, are thoroughly investigated through a combination of electrochemical analysis and diagnostics of FeO electrode cells, X-ray-based and spectroscopic analysis of material structure evolution as functions of depth of discharge (DoD) and state of charge (SoC), and first principle modeling. The gravimetric capacity is found to be 50 mA h/g for bulk FeO (50 nm average crystallite size) and 100 mA h/g─about a tenth of the theoretical prediction for complete conversion─for FeO nanoparticles (8.7 nm average particle size), respectively. A fundamental and mechanistic study of material evolution as functions of DoD and SoC shows that FeO does not allow electrochemical incorporation of Na ions into the empty cation positions of the inverse spinel structure, leading to our assertion that electrochemical intercalation of Na ions to conversion of the FeO anode in sodium-ion batteries is nonviable. A density functional theory investigation points to the impracticality of the intercalation of Na ions into FeO and further validates our experimental findings. We propose several possible mechanisms corresponding to the observed low capacity, including formation of solid electrolyte interphases with unfavorable properties and adsorption of Na ions onto surfaces of nanoparticles and/or at heterointerfaces in FeO composite electrodes in a NaPF-based electrolyte system.