Flash graphene induced low-temperature synthesis of a single-crystalline Na2.72Fe1.64(SO4)3 cathode for boosted sodium storage

Abstract

Sodium iron sulfate (NFS) compounds are used as low-cost cathodes for sodium-ion batteries (SIBs), but their performance is far from satisfactory due to the poor phase purity and polycrystalline textures that are caused by the low sintering temperature. Herein, novel and highly stable Na_2.72Fe_1.64(SO₄)₃ single crystals are developed via a flash graphene assisted ball-milling process followed by low-temperature sintering. The highly conductive flash graphene facilitates the formation of large crystal nuclei during the early stages of NFS structure evolution in ball-milling. Furthermore, the subsequent sintering process is significantly enhanced by the well-established conductive network formed by flash graphene. The as-formed homogeneous single-crystal structure results in prominently enhanced electrochemical performance. Remarkably, an ultra-long cycling life of 15 000 cycles at 15C with a capacity retention of 80.7% is achieved. Moreover, a reversible capacity of 110.5 mA h g⁻¹ and much improved rate capability (81 mA h g⁻¹ at 15C) are also demonstrated. Ex situ XRD manifests that the single-crystal NFS only generates a minimal volume fluctuation of 1.8% during Na⁺ insertion/extraction, reflecting superior structure stability. This study provides new avenues to develop advanced polyanionic cathode materials for SIBs.