Sodium Ion Diffusion and Voltage Trends in Phosphates Na4M3(PO4)2P2O7 (M = Fe, Mn, Co, Ni) for Possible High-Rate Cathodes

Polyanionic phosphates have the potential to act as low-cost cathodes and stable framework materials for Na ion batteries. The mixed phosphates Na4M3(PO4)(2)P2O7 (M = Fe, Mn, Co, Ni) are a fascinating new class of materials recently reported to be attractive Na ion cathodes which display low-volume changes upon cycling, indicative of long-lifetime operation. Key issues surrounding intrinsic defects, Na ion migration mechanisms, and voltage trends have been investigated through a combination of atomistic energy minimization, molecular dynamics (MD), and density functional theory simulations. For all compositions, the most energetically favorable defect is calculated to be the Na/M antisite pair. MD simulations suggest Na+ diffusion extends across a 3D network of migration pathways with an activation barrier of 0.20-0.24 eV, and diffusion coefficients (D-Na) of 10(-10)-10(-11) cm(2) s(-1) at 325 K, suggesting good rate capability. The voltage trends indicate that doping the Fe-based cathode with Ni can significantly increase the voltage, and hence the energy density.