Realizing an aqueous sodium-ion battery with a super-high discharge voltage based on a novel FeSe2@rGO anode

Abstract

Aqueous sodium-ion batteries (ASIBs) promise particularly increased operational safety and lower manufacturing cost than the current state-of-the-art organic electrolyte-based lithium-ion batteries. However, the output voltages of reported ASIBs are still restricted to under 1.5 V due to the generally high redox potentials of currently reported anode materials. In this work, we innovatively selected FeSe₂ as the anode material for an ASIB to obtain a high output voltage. The electrochemistry of FeSe₂ is demonstrated to be based on the reversible conversion reaction between FeSe₂ and Na_xFe₂Se₄/Na₂Fe, while the inferior cycling stability is proved to be caused by the dissolution of the Na₂Fe intermediate. By encapsulation of reduced graphene oxide, the capacity and cycling stability of FeSe₂ have been improved, giving rise to a high capacity of 100 mA h g⁻¹ and capacity retention of 72.2% after 100 cycles. Together with an elaborate aqueous/acetonitrile hybrid electrolyte and the Na₃V₂(PO₄)₂F₃ cathode with a high redox potential, the as-assembled FeSe₂@rGO//Na₃V₂(PO₄)₂F₃ ASIB presents an excellent electrochemical performance in terms of a super high output voltage (∼1.7 V), high energy density (53.4 W h kg⁻¹), and outstanding rate performance (78.2% retention rate at 5 A g⁻¹).