Low-temperature pre-oxidation and high-temperature activation modulate brewer’s spent grains-based hard carbon microcrystalline structure to enhance sodium storage properties

Hard carbon materials are among the most promising materials for sodium-ion batteries. Influenced by sodium storage mechanisms such as intercalation and pore filling, the carbon layer spacing and closed-pore structure significantly affect the electrochemical performance of sodium-ion batteries. However, how to regulate the hard carbon structure through a simple method remains a huge challenge at present. In this work, we prepared hard carbon (OMHC-350–1400) with a large carbon layer spacing and abundant closed-pore structure through a strategy combining low-temperature pre-oxidation and high-temperature carbonization. Thanks to the introduction of a large amount of oxygen elements (from 14.49 % to 22.61 %) by pre-oxidation treatment, the carbon layer spacing of hard carbon widened from 0.366 nm to 0.384 nm, and a large number of closed pores were generated simultaneously. This provides abundant active sites for sodium ion storage. Compared with the hard carbon obtained without pre-oxidation treatment, the discharge capacity of OMHC - 350–1400 at 25 mAh g⁻¹ increased from 263.68 mAh g⁻¹ to 343.52 mAh g⁻¹ , the initial Coulombic efficiency (ICE) increased from 60.3 % to 72.6 %, and it showed a high capacity of 211.44 mAh g⁻¹ in the low-voltage discharge platform, with obvious advantages in electrochemical performance. This work aims to provide some references for the preparation of high-performance bio-based hard carbon. Discharge curves for the second cycles of different structural carbon: MHC-1400,OMHC-1400 • Air peroxidation regulates oxygen-containing functional groups. • Disordered regulation of carbon microcrystals. • An ultra-long discharge plateau of sodium-ion batteries. • Thermogravimetric analysis is performed to determine the temperature of cryogenic pre-oxidation.