Rational design of S, N Co-doped reduced graphene oxides/pyrrhotite Fe₇S₈ as free-standing anodes for large-scale, ultrahigh-rate and long-lifespan Li- and Na-ion batteries

To realize advanced Li- and Na-ion batteries (LIBs and SIBs), a scalable free-standing electrode constructed using S, N co-doped reduced graphene oxides and pyrrhotite Fe₇S₈ (Fe₇S₈@SN-rGO) has been rationally designed and developed via freeze casting. Freeze casting can generate unidirectional open channels inside the electrode, which enables fast ion transport and provides large contact area between electrode and electrolyte. Furthermore, the aligned SN-rGO frameworks provide effective electron transport routes. In terms of structural stability, sandwiched Fe₇S₈ within sturdy SN-rGO walls are protected from volume expansion caused by lithiation and sodiation. Consequently, when used in LIBs, Fe₇S₈@SN-rGO exhibited outstanding rate capability with reversible capacities of 621.1 and 492.1 mA h g⁻¹ at low and high current densities of 0.2 and 4 A g⁻¹, respectively, and excellent cycling stability without drastic capacity decay over 1000 cycles. As an anode for SIBs, the electrode also exhibited a high reversible capacity of 341.3 mA h g⁻¹ at a high current density of 4 A g⁻¹ with a high initial coulombic efficiency of 90.5% and excellent long-term cycling stability for 2000 cycles. The excellent performances of the Fe₇S₈@SN-rGO suggest that the powerful large-scale energy storage systems can be realized via metal colloid/GO-based freeze casting.