Understanding the Relation Between Intrinsic Parameters of Substituents and Physical-Chemical Properties of NVP

Abstract

NASICON-type Na₃V₂(PO₄)₃ (NVP) is regarded as an intriguing cathode material choice for sodium ion batteries (SIBs) due to its cycling stability and relatively high capacity. However, its voltage and electronic conductivity still need to be improved for larger-scale fast-charging applications (e.g. electric vehicles and mobile phones). In this work, we investigate the influence of Vanadium (V) substitution by other environmentally friendly, cheap, and/or high-valent transition metal (TM) elements on the electrochemical performance of NVP. Density functional theory calculation was used to study the volume change, voltage, conductivity, and redox mechanism during charge/discharge of different compositions. It is found that a substitution of 50% of V by Mn, Mo or W ions resulting in Na₃VMn(PO₄)₃ (NVMnP), Na₃VMo(PO₄)₃ (NVMoP), and Na₃VW(PO₄)₃ (NVWP) significantly alters the cathode materials’ physical and chemical properties, notably decreasing the band gap. In particular, NVMnP has lesser than 1 eV theoretical band gap and provides a higher voltage, while NVWP a much lower voltage in comparison to NVP. This means that NVMnP and NVWP can be promising cathode and anode materials respectively. This work also establishes a relation between fundamental properties of substituents (i.e. ionization energy and ionic size) and the overall performance of NVP.