Sodium ion diffusion and voltage trends in phosphates Na4M3(PO4)2P2O7 (M = Fe, Mn, Co, Ni) for possible high-rate cathodes

Stephen M. Wood, Chris Eames, Emma Kendrick, M. Saiful Islam

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Abstract

Polyanionic phosphates have the potential to act as low-cost cathodes and stable framework materials for Na ion batteries. The mixed phosphates Na4M3(PO4)2P2O7 (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 (DNa) of 10–10–10–11 cm2 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.
Original languageEnglish
Pages (from-to)15935-15941
Number of pages7
JournalJournal of Physical Chemistry C
Volume119
Issue number28
Early online date17 Jun 2015
DOIs
Publication statusPublished - 16 Jul 2015

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