Influence of carbon content on the ionic and electronic conductivities of dense Na3V2(PO4)3/C composites

Sodium vanadium triphosphate (Na₃V₂(PO₄)₃, NVP) is a promising cathode material for Na-ion batteries. Due to its intrinsically low electronic conductivity, it is usually mixed or coated with carbon. However, so far there have been no systematic studies on the ionic and electronic conductivity of carbon-coated NVP particles. In this work, NVP with varying carbon contents are prepared. The powders are sintered as single pellets or sandwiched between a solid electrolyte for measurements in an ion blocking and non-ion blocking configuration. In these two different configurations, two different electrodes are attached and several electrochemical characterization techniques are applied such as impedance spectroscopy, chronoamperometry, and four-point measurements. The NVP/C composites with carbon content >0.1 wt% show a high degree of densification and an amorphous carbon network. The conductivity of NVP in composites with carbon content <0.1 wt% shows dominating ionic conduction with an average value of ∼2 × 10⁻⁶ S cm⁻¹. NVP/C samples with carbon contents >0.1 wt% show a dominance of electronic conduction in the range of 0.01–0.2 mS cm⁻¹ because of the percolated carbon network at the grain boundaries. The ionic conductivity, however, remains almost constant in the same order of magnitude (∼6 × 10⁻⁶ S cm⁻¹).