Versatile Fe2GeS4 for Li/Na–Fe2GeS4 battery cathodes and Li/Na-ion battery anodes

Metal chalcogenides are promising candidates for various energy storage applications. Herein, a ternary Fe2GeS4 compound was synthesized using a simple solid-state method and applied as versatile electrodes for rechargeable Li/Na–Fe2GeS4 battery cathodes and Li/Na-ion battery anodes. In addition, distinctive Li and Na reaction pathways for Fe2GeS4 consisting of insertion and conversion reactions have been demonstrated. For its application as rechargeable Li/Na–Fe2GeS4 battery cathodes, Li‖Fe2GeS4 and Na‖Fe2GeS4 battery systems were electrochemically evaluated by controlling the cut-off voltages within the insertion reaction and they showed high electrochemical performance with high reversible volumetric capacities and cyclability with appropriate cell potentials (Li‖Fe2GeS4: ∼550 mA h cm−3 over 100 cycles with ∼1.6 V; Na‖Fe2GeS4: ∼555 mA h cm−3 over 100 cycles with ∼1.2 V). In addition, for using Fe2GeS4 as an anode for Li-ion batteries (LIBs) and Na-ion batteries (NIBs), graphene-wrapped Fe2GeS4 nanocomposite (Fe2GeS4-rGO) comprising Fe2GeS4 nanocrystallites (∼10–15 nm) embedded on reduced graphene oxide (rGO) was synthesized through a simple two-step synthetic method to improve the electronic/ionic conductivity, structural stability, and mechanical strain relaxation. The electrochemical test results indicated that the Fe2GeS4-rGO exhibited high initial reversible gravimetric capacities (730 mA h g−1 for LIB, 563 mA h g−1 for NIB), high rate capability (520 mA h g−1 at 3C for LIB, 470 mA h g−1 at 2C for NIB), and an excellent cycling performance at a high 1C rate (capacity retention: ∼100% over 500 cycles for LIBs, ∼93% over 200 cycles for NIBs). Therefore, we believe that Fe2GeS4 is a highly applicable compound material for various battery systems.