Iron-Based Mixed Phosphate Na₄Fe₃(PO₄)₂P₂O₇ Thin Films for Sodium-Ion Microbatteries

Iron-based polyanionic materials can be exploited to realize low cost, durable, and safe cathodes for both bulk and thin film sodium-ion batteries. Herein, we report pulsed laser deposited mixed phosphate Na₄Fe₃(PO₄)₂P₂O₇ as a positive electrode for thin film sodium-ion microbatteries. The bulk material and thin films of Na₄Fe₃(PO₄)₂P₂O₇ are employed by solution combustion synthesis (SCS) and the pulsed laser deposition (PLD) technique, respectively. Phase purity and the nature of the crystallinity of the thin films were confirmed by grazing incidence X-ray diffraction and transmission electron microscopy. Identification of surface roughness and morphology was obtained from atomic force microscopy and scanning electron microscopy, respectively. Emerging electrochemical properties were observed from charge-discharge profiles of the thin films, which are well comparable to bulk material properties. The Na₄Fe₃(PO₄)₂P₂O₇ thin film electrodes delivered a highly reversible Na⁺ storage capacity of ∼120 mAh g^-1 with an excellent stability of over 500 cycles. Electrochemical analysis results revealed that the thickness of the film affects the storage capacity.