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.

Phosphate doped hard carbons have shown a significant capacity increase accompanied by an additional plateau in its voltage profile. The work reveals the co-contribution of oxygen atoms in phosphate and carbon atoms for the additional Na-ion storage.