The sodium-ion battery: Study of alternative current conduction mechanisms on the Na3PO4 - Based solid electrolyte

Abstract Sodium-ion batteries have been dominating as a power source in digital cameras, laptop computers, mobile phones and electric/hybrid electric vehicles. This can be explained by their high life cycle and power density which differentiate them from other battery types. Sodium phosphate is considered among the favored solid electrolyte materials for the sodium-ion battery because of its high ionic conductivity. In this study, Na3PO4 was obtained using the classic ceramic method and characterized by X-ray powder diffraction patterns, infrared spectroscopy, differential scanning calorimetry (DSC) and electrical impedance. The Na3PO4 compound crystallized at room temperature in the tetragonal system with a P 4 ‾ 2 1 c space group. The morphology and composition of Na3PO4 were studied by a scanning transmission electron microscope coupled with the energy dispersive X-ray spectroscopy (STEM-EDS). The phase transition at T1 ≈ 603/605 K was confirmed by the differential scanning calorimetry (DSC). Infrared spectroscopy confirmed the presence of the (PO4)3− group and its vibrations. The electrical technique was measured in the 10–106 Hz frequency range and 540–700 K temperature intervals. The frequency dependence of alternative current conductivity was explained using Jonscher law. The alternative current electrical conduction in Na3PO4 was interpreted through several processes, which could be associated with two different models: the overlapping large polaron tunneling (OLPT) model in phase I and the non-overlapping small polaron tunneling (NSPT) model in phase II. The conduction mechanisms of Na3PO4 were explained by Elliott's theory and consequently the Elliott's parameters were calculated.