An eco-friendly citric acid-activated high-performance coal-derived hard carbon anode for sodium-ion batteries

Abstract

Coal is recognized as a cost-effective precursor for anodes in sodium-ion batteries (SIBs) due to its natural abundance, affordability, and tunable structure. However, high-temperature carbonization frequently leads to graphitization of coal, undermining its sodium storage efficiency. In this work, a high-performance hard carbon anode was synthesized by modifying a bituminous coal precursor with citric acid, an environmentally friendly and cost-effective food additive. Citric acid effectively impedes the formation of ordered microcrystalline structures during pyrolysis, leading to increased carbon interlayer spacing. Additionally, its decomposition releases gases that form abundant porous structures in the hard carbon, significantly improving its rate capability. The optimized structure (NHC-3) delivers a high reversible capacity of 315.5 mAh g⁻¹ at 0.02 A g⁻¹ and outstanding rate performance. This work proposes an efficient strategy for enhancing coal-based hard carbon anodes for SIBs in a cost-effective and sustainable manner.