Superior sodium–lithium intercalation and depressed moisture sensitivity of a hierarchical sandwich-type nanostructure for a graphene–sulfate composite: a case study on Na2Fe(SO4)2·2H2O

Abstract

Sulfate-based (SO₄²⁻) polyanionic materials with low cost and ionic-conduction break fresh ground for “rocking-chair” systems. But the high moisture sensitivity and limited conductivity led to their poor crystal stability and inferior alkaline-ion intercalation chemistry. Here we report the design of graphene-based sandwich-type nanoarchitecture for sulfate. The three-dimensional graphene-based network not only provides continuous electron/ion pathways for fast intercalation kinetics, but also effectively protects the crystal structure from deterioration to depress moisture sensitivity. As a case study, the hydrated sulfate (Na₂Fe(SO₄)₂·2H₂O)–graphene composite with a sandwich-type structure is prepared by a facile low-temperature synthesis. The depressed moisture sensitivity of hierarchical graphene–Na₂Fe(SO₄)₂·2H₂O compared to the pristine one is demonstrated by comparing their hydration process, and moreover, a shell–core hydration mechanism is disclosed. The hierarchical composite exhibits improved electronic conductivity and better sodium–lithium insertion capability than the pristine one. It delivers a reversible capacity of 72 and 69 mA h g⁻¹ with redox potentials of 3.415/3.234 V (vs. Na⁺/Na) and 3.579/3.483 V (vs. Li⁺/Li) in sodium and lithium intercalation systems, respectively. Moreover, it also exhibits superior high rate capabilities and good cycling stability, which delivers 81% (for sodium) and 70% (for lithium) of the capacity at a 5 C rate. Therefore, the hierarchical sandwich-type architecture is favorable to realizing superior electrochemical performance for the sulfate, which presents a significant step forward in the development of low-cost large-scale batteries.