Electronic Transport Properties of Carbon-Encapsulated Maricite NaFePO4 as Cathode Material for Sodium-Ion Batteries

Abstract

Electronic transport properties, with detailed dielectric behavior over a wide temperature (123 K – 523 K) and frequency (0.1 Hz – 10 MHz) range of maricite NaFePO₄ (NFP) and carbon-coated NaFePO₄ (NFP@C) as a cathode material for Na-ion batteries are reported. For this, pure phase NFP is synthesized via an ultrasound-assisted sol-gel route, with calcination at 600 °C for 5 h in an N₂ atmosphere. The dielectric studies of NFP revealed very high dielectric constant (ɛ_r′) values (10² < ɛ_r′ < 10³), with non-ideal Debye-like relaxation with frequency. The imaginary part of the dielectric constant (ɛ_r″) demonstrated that conductivity is a major contributor to the overall dielectric loss in the sample. The frequency-dependent ac conductivity (σ_ac) response revealed high conductivity values (10⁻⁴ S cm⁻¹ < σ_ac < 10⁻¹ S cm⁻¹) resulting from long-range hopping of charge carriers for T < 300 K and re-orientational, localized hopping of charge carriers for T > 300 K. Activation energy of 35 meV and 56 meV is found separating this transition range. Further, a preliminary electrochemical performance of these NFP samples is evaluated in a half-cell configuration, revealing a specific capacity of 14 mAh g⁻¹ for NFP and 18.11 mAh g⁻¹ for NFP@C samples.