Electrochemical Sodiation Mechanism in Magnetite Nanoparticle-Based Anodes: Understanding of Nanoionics-Based Sodium...

Electrochemical ion storage behaviors of Fe₃O₄ 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 Fe₃O₄ 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 Fe₃O₄ (50 nm average crystallite size) and 100 mA h/g─about a tenth of the theoretical prediction for complete conversion─for Fe₃O₄ nanoparticles (8.7 nm average particle size), respectively. A fundamental and mechanistic study of material evolution as functions of DoD and SoC shows that Fe₃O₄ 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 Fe₃O₄ anode in sodium-ion batteries is nonviable. A density functional theory investigation points to the impracticality of the intercalation of Na⁺ ions into Fe₃O₄ 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 Fe₃O₄ composite electrodes in a NaPF₆-based electrolyte system.