NaFe2PO4(SO4)2: a potential cathode for a Na-ion battery

Abstract

Hexagonal Na₃Zr₂PO₄(SiO₄)₂ is a well-known fast Na⁺ conductor commonly referred to as NASICON. The low cost and wide availability of sodium invites exploration of Na⁺-insertion cathodes with the hexagonal NASICON framework structure. Here we report a low-temperature synthesis of NaFe₂PO₄(SO₄)₂ with the same host framework as NASICON and its performance as the cathode of a Na/Na_1+xFe₂PO₄(SO₄)₂ cell operating on the Fe³⁺/Fe²⁺ redox couple. The cell provides a single-phase reaction having a capacity approaching 100 mA h g⁻¹ at 0.1C after 50 cycles over the voltage range 2 ≤ V ≤ 4 V with a coulomb efficiency approaching 100%. An increase in capacity with cycling is the result of aging of an unoptimized electrode morphology.