Synergistic Copper and Carbon Nanotube Coatings Enable High-Performance Na2+2xFe2-x (SO4)3 Cathode Material for Sodium-Ion Batteries.

Abstract

Sodium iron sulfate (Na2+2xFe2-x(SO4)3, NFS) is a promising cathode material for sodium-ion batteries (SIBs) for large-scale energy storage systems due to its low cost and long life. However, NFS acts as a near-insulator and cannot reversibly store sodium ions effectively. To address this issue, this work develops a straightforward solid-state process to realize a simultaneous copper and carbon co-coating on Na2.5Fe1.75(SO4)3 (NFS) cathode material, NFS@C@Cu, via one-step calcination under the synergistic influence of CuSO4 and Carbon Nanotubes. Among all samples, NFS@C@Cu-0.07, containing 0.89 wt.% Cu, delivers the highest discharge capacity of 103.1 mA h g-1 at 0.05 C, approaching its theoretical capacity and representing a significant improvement over solo carbon-coated NFS (94.8 mA h g-1). Furthermore, it exhibits excellent stability, retaining 87.2% of initial capacity after 10 000 cycles at 50 C, along with commendable rate performance. This study demonstrates that in addition to carbon coating, constructing copper coating on NFS particles can significantly enhance the electronic conductivity and reduce internal cell resistance, thereby improving overall electrochemical performance. This work provides a promising avenue for the design of high-performance polyanion-type cathode materials for sodium-ion batteries.

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