Hard carbons are one of the most promising anode materials for next-generation sodium-ion batteries due to their high reversible capacity, long cycle life, and low cost. The advantage in terms of price of hard carbons can be further improved by using cheaper resources such as biomass waste as precursors. Lignin is one of the richest natural bio-polymer in the earth which can be obtained from woods. As the lignin has three-dimensional amorphous polymeric structure, it is considered as good precursor for producing carbonaceous materials under proper carbonization processes for energy storage devices. In this study, structural properties of lignin-derived hard carbons such as interlayer spacing, degree of disorder and surface defects are controlled. Specifically, lignin-derived hard carbons were synthesized at 1000 °C, 1250 °C, and 1500 °C, and it was confirmed that the structure gradually changed from a disordered structure to ordered structure through X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Hard carbons exhibit sloping regions at high voltage and plateau region at low voltage during the electrochemical processes for sodium ions. As the heat treatment temperature increases, the contribution to the overall reversible capacity of the sloping region decreases and the contribution of the plateau region increases. This trend confirms that it affects reversible capacity, rate-capability, and cycling stability, meaning that an understanding of structural properties and related electrochemical properties is necessary when developing hard carbon as a negative electrode material for sodium ion batteries.

Among the various contenders for next-generation sodium-ion battery anodes, hard carbons stand out for their notable reversible capacity, extended cycle life, and cost-effectiveness. Their economic advantage can be further enhanced by using inexpensive precursors, such as biomass waste. Lignin, one of the most abundant natural biopolymers on Earth, which can be readily obtained from wood, possesses a three-dimensional amorphous polymeric structure, making it a suitable candidate for producing carbonaceous materials through appropriate carbonization processes for energy storage applications. In this work, we synthesized hard carbon using lignin containing CaSO₄ to facilitate partial catalytic graphitization to improve the microstructural features, such as interlayer spacing, degree of disorder, and surface defects. Partial catalytic graphitization enables hard carbon to develop an ordered structure compared with hard carbon carbonized without CaSO₄ as analyzed by X-ray diffraction, Raman spectroscopy, scanning/transmission electron microscopy, and X-ray photoelectron spectroscopy. The CaSO₄-aided partially catalytic graphitized hard carbon (CCG-HC) exhibited improved electrochemical performance, showing a larger portion of the low-voltage plateau—an indicator typically associated with a highly ordered structure—compared to simply carbonized hard carbon (HC). Notably, CCG-HC delivered a reversible capacity of 237 mAh g⁻¹, retained 95.6% of its capacity over 100 cycles at 50 mA g⁻¹, and exhibited 127 mAh g⁻¹ at 1.0 A g⁻¹.