Influence of alkaline earth metal ions on graphitization of ionically crosslinked alginates

Biomass-derived feedstocks have traditionally been regarded as non-graphitizable carbon precursors due to the inherently low crystallinity of carbonized biomass. Nevertheless, recent advances have highlighted the potential of biomass as a sustainable source for producing graphite, offering a promising alternative to fossil fuel–based carbon materials. Here, we present a strategy for preparing highly crystalline graphitic materials from alginate, a polysaccharide found in algae, by investigating the role of alkaline earth metal ions (i.e. Ca 2+ , Sr 2+ , and Ba 2+ ) during thermal treatment up to 2400 °C. Ionic crosslinking with these ions effectively replaced the original Na + in sodium alginate, leading to the formation of their respective carbides within the carbonized matrix during graphitization. Based on the mechanistic consideration, we verified that the subsequent thermal decomposition of the carbides converted amorphous carbon into graphitic layers, enabling the successful graphitization of non-graphitizable alginate. This mechanism stands in stark contrast to the conventional view that alginate-derived carbons are intrinsically turbostratic. In addition, all graphitic materials derived from alginates crosslinked with alkaline earth metal ions exhibited superior crystalline structures and significantly enhanced electrochemical performance as anode material in lithium-ion batteries. For instance, Sr 2+ -crosslinked alginate produced the most crystalline graphite and demonstrated the highest Li + intercalation capacity of 347.1 mAh/g, establishing alginates crosslinked with alkaline earth metal ions as promising biomass-derived precursors for advanced energy storage applications. • Alkaline earth ions crosslink alginates and enhance their thermal stability. • Embedded metal ions form carbides during the carbonization process. • These carbides decompose, generating graphitic layers in carbonized alginates. • The graphitized alginates reach a peak intercalation capacity of 347 mAh/g. • Metal-assisted conversion upgrades biomass to battery-grade synthetic graphite.