Ionic Conduction Mechanism in Two-Dimensional Layered Compound Na₃Fe(PO₄)₂ with a Glaserite-like Crystal Structure

We report the microscopic mechanism of Na-ion conduction and its relationship with the crystal structure of 2D layered electrode battery material Na3Fe(PO4)2. Our X-ray and neutron diffraction studies reveal that Na3Fe(PO4)2 has a layered crystal structure with alternating Na-ion and transition metal (Fe) oxide layers along the crystallographic c-axis. Using impedance spectroscopy, we investigate the detailed Na-ion conduction in Na3Fe(PO4)2 through the analysis of various interlinked parameters, viz., dc conductivity, ac conductivity, diffusion constant, hopping time of ions, electrical modulus, dielectric constant, etc. Our findings indicate that Na-ion conductions occur through a correlated barrier hopping (CBH) process and are thermally and frequency activated. The temperature-dependent Arrhenius-type ionic conductivity is found to increase ∼ 104 orders of magnitude from ∼ 10–7 to ∼ 10–3 Sm–1 with increasing temperature from 450 to 773 K. The diffusion constant is found to be ∼ 1.24 × 10–13 cm2/s at 553 K, which increases ∼ 103 orders of magnitude and reaches 5 × 10–10 cm2/s at 773 K. Both Na-ion hopping time and conductivity relaxation time have been found to decrease significantly with increasing temperature. Electrical modulus and dielectric study also confirm the CBH process as a microscopic mechanism for Na-ion conduction which remains invariant over the entire measured temperature range. The soft bond valence sum (BVS) analysis of the neutron diffraction pattern reveals that the 2D Na-ion conduction pathways are confined within the Na-ion layers. The relationship between the Na-ion conduction and the underlying crystal structural parameters, such as local environment of conducting Na ions, Na–Na distances, and Na-site occupancies, has been established. Our study, thus, provides valuable insight into the ionic conduction properties and their correlation with the underlying crystal structure, holding immense importance in the field of battery material research.