Understanding the electrochemical behaviour of reduced graphene oxide cathodes in all-carbon Na-ion batteries

Abstract

Sodium-ion batteries represent a sustainable and cost-effective solution for grid-scale energy storage. However, the reliance on cathode materials containing scarce transition metals currently limits their wider adoption. Carbonaceous materials present an environmentally sustainable and economically viable alternative. This study investigates the application of reduced graphene oxide as a cathode active material. A detailed analysis of the storage mechanism and its dependence on the morphological and chemical structure revealed that it combines surface capacitance and faradaic reactions. The key factors responsible for high capacity and long cycle life are the open structure of graphene sheets and the presence of functional oxygen and nitrogen groups where Na+ ions are stored in the R–CO + Na⁺ + e⁻ ↔ R–C–O–Na reaction. A good understanding of the mechanism allowed optimisation of cycling conditions in a proof-of-concept all-carbon full cell incorporating reduced graphene oxide and hard carbon as a cathode and an anode, respectively. The system displays good energy density (80 W h kg⁻¹) and remarkable stability over 500 cycles. The gained insights will support the rational design of more efficient carbonaceous electrodes.