Na₃V(PO₄)₂ cathode material for Na ion batteries: Defects, dopants and Na diffusion

Layered Na3V(PO4)2 has been recently identified as a high rate cathode material for Na ion batteries. We use atomistic simulation based on the classical pair potentials to calculate the most favourable intrinsic defect process, Na migration paths and tetravalent dopant incorporation at V and P sites. The Na-V anti-site defect is the most energetically favourable defect process. The Na Frenkel is the second most favourable intrinsic defect but only higher by 0.19 eV than the anti-site. Two dimensional long range Na ion migration with activation energy of 0.59 eV is observed along the ab plane implying that Na3V(PO4)2 could be a promising cathode material for Na ion batteries. The formation of both Na vacancy and interstitial defects can be simultaneously achieved by substituting Ge on the V site and the P site required for vacancy migration and storage capacity respectively. High exoergic solution energy is calculated for La on the V site suggesting that the formation of Na3(VxLa1−x)(PO4)2 composition should be experimentally possible.