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

Abstract

Polyanionic phosphates have the potential to act as low-cost cathodes and stable framework materials for Na ion batteries. The mixed phosphates Na 4 M 3 (PO 4 ) 2 P 2 O 7 (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.

A Low‐Cost and Environmentally Friendly Mixed Polyanionic Cathode for Sodium‐Ion Storage

“…Further, the migration path related to active Na ions (point a to point e in Figure b), and the corresponding energies were simulated (Figure c). From the results, we estimate that the sodium diffusion barriers of NFOS are about 0.5 eV, at the same level as the previously reported cathodes …”

Exploring the Ni redox activity in polyanionic compounds as conceivable high potential cathodes for Na rechargeable batteries

“…27 A combination of different polyanionic groups could contribute to outstanding ion diffusion and voltage profile in Na-ion batteries as well as structure stability during the Na (de-)insertion process. Interestingly, a theoretical prediction reported by Islam and colleagues 28 has pointed out that doping the Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) with Ni can lead to a significant increase in the cell voltage.…”

MgFeSiO4 as a potential cathode material for magnesium batteries: ion diffusion rates and voltage trends

“…The Pedone model 55 was used for the MD simulations, which was employed successfully in recent studies on Na-ion conducting battery materials. 50,56 Three initial congurations were investigated and the results of the three structures were averaged. Simulation runs were carried out using the NVT ensemble and a time step of 2 fs for long runs of 6 ns, at temperatures in the range 300-1500 K. Preequilibrium runs of 4 ps with NVE and NPT ensembles were rst used for stable congurations.…”

“…Computer simulations have been used to investigate a range of materials for lithium and sodium batteries. 40,42,[56][57][58][59] Density functional theory (DFT) calculations were performed using a plane wave basis set implemented in the VASP code.…”