Low volume expansion in S/O diatomically modified hard carbon for stable Na ion storage

Sodium ion batteries (NIBs) show significant potential battery alternatives for lithium ion batteries (LIBs) large-scale energy grid storage. The development of cost-effective anodes for NIBs has become one of the most significant goals in contemporary society, which is characterized by high energy consumption. With merits of high abundance, sustainability and low cost, carbonaceous materials are utilized as highly promising NIB anodes. However, the commonly used carbon materials often suffer from large volume change upon Na⁺ ion intercalation/de-intercalation, leading to unsatisfactory capacity and poor cycling stability of NIBs. This study presents a chemical carbon etching strategy for the synthesis of sulfur (S)/oxygen (O) diatomically modified hard carbon with low volume expansion as a new type of anode for NIBs by the thermolysis of disused sunflower biomass. The S/O diatomically modified on the carbon matrix provides lower adsorption energy for sodium ion adsorption (−3.87 eV), effectively enhancing sodium storage capacity. Structural characterizations reveal that the S/O-modified hard carbon exhibits expanded interlayer spacing (0.392 nm) and large surface area per unit mass (227.7 m² g⁻¹). Density functional theory (DFT) simulations validate that the S/O diatomic modification on carbon promotes sodium cation adsorption and storage, and the volume expansion upon Na ion intercalation is only 2.5 %. Electrochemical measurements demonstrate that the hard carbon electrode provides superior charge–discharge retention (294 mAh g⁻¹ at 0.03 A g⁻¹) and exceptional long-term cycle stability (∼86 % preservation after 20,000 cycles at 5 A g⁻¹). The practical cell was assembled by pairing this anode with a layered oxide cathode, showing highly sustainable capacity (129 mAh g⁻¹) and exceptional long-term cycling stability (86 % after 100 cycles). In addition, the full cells are able to work in a broad operation temperature from −30 to 40 °C (138mAh g⁻¹ at 40 °C and 110mAh g⁻¹ at −30 °C). This synthesis route not only enhances the economic viability of biomass hard carbon materials but also highlights their environmental sustainability, which holds great promise for advancing improvement of high-efficiency NIBs.