Mechanochemical effect on the electrochemical properties of a Na3(VO)2(PO4)2F positive electrode for sodium-ion batteries

Abstract

Sodium vanadium fluorophosphate (NVPF) has shown promising properties as a positive electrode in sodium-ion batteries mainly due to its high operating voltage; however, it has significant electronic and kinetic limitations that must be addressed. In this study, we investigated how a simple and straightforward mechanochemical treatment can be a valuable alternative to overcome these limitations. NVPF electrodes before and after (NVPF-bm) mechanochemical treatment were compared. For NVPF-bm, the contribution from the sodiation/desodiation of Na₁⁺ increased from 23 to 40% as Na₂⁺ decreased from 38 to 48% of the total produced capacity at ∼0.5 C (70 mA g⁻¹); additionally, the discharge capacities were 30% greater than those of NVPF at 0.8 C (100 mA g⁻¹); nevertheless, after 150 cycles, NVPF-bm presented a coulombic efficiency of 96.2% and a capacity retention of 82.6%. The calculated diffusion coefficients for NVPF-bm were 22 × 10⁻¹⁰ and 1.0 × 10⁻¹⁰ cm² s⁻¹, compared to 6.5 × 10⁻¹¹ and 2.5 × 10⁻¹¹ cm² s⁻¹ for NVPF for the sodiation and desodiation processes, respectively. Furthermore, for the charge and discharge processes, NVPF-bm presented a charge transfer resistance three times smaller and diffusion lengths of 2.1 and 0.3 μm, respectively, compared to 8.0 and 13 μm, respectively, for NVPF. These results demonstrate the kinetic enhancements of the electrode as a direct consequence of the mechanochemical treatment. Therefore, this approach impacts not only the synthesis and morphology but also the inherent electrochemical storage capacity of the NVPF.