Insight into the Multirole of Graphene in Preparation of High Performance Na_2+2xFe_2-x(SO₄)₃ Cathodes

The development of prospective cathode candidates is one of the key issues for the practical application of sodium ion batteries. Although alluaudite Na_2-2xFe_2-x(SO₄)₃ (NFS) cathode combines natural abundance, environmental benign and higher theoretical energy density than conventional polyanionic cathodes, the present available performance is unsatisfying because of limited electronic conductivity. The traditional in situ carbon coating via wet chemistry and thermolysis is infeasible due to the moisture sensitivity and poor thermal stability. In this study, the above issues are simultaneously tackled via a simple but effective method assisted by graphene addition with the merits of high conductivity, hydrophobicity, and stretchable nature. Since the added graphene could affect the growth of NFS particle, the nanoparticle could be wrapped well and form NFS@graphene composite, which could keep from moisture attack. Moreover, the functional groups of the graphene precursor could boost the crystallization. The NFS@graphene could deliver an initial reversible capacity of -106 mAh g^-1 at 25 °C, and a capacity retention of >98% at 0 °C is retained over 700 cycles. The inspiring results would cast insight on the graphene-assisted synthesis and were meaningful for scaling up the Na_2-2xFe_2-x(SO₄)₃ cathodes.