To produce high-performance anode materials for lithium/sodium batteries via sustainable strategies is still one of the most essential tasks in battery research.

To produce high-performance anode materials for lithium/sodium batteries via sustainable strategies is still one of the most essential tasks in battery research. A biomass-based carbon–tin oxide composite (BC/SnO 2 ) is prepared through pyrolysis of birch tree waste using phosphoric acid as an activator and its electrochemical performance as a sustainable anode material in lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs) is tested. The physicochemical characterization results proved that SnO 2 has a remarkable impact on BC/SnO 2 porosity, morphology, and physicochemical features. Due to these favorable properties, the BC/SnO 2 anode exhibited far better performance for LIBs and NIBs than bare carbon (BC). Against Li metal, the BC/SnO 2 anode delivered a specific capacity of 319 mA h g −1 while BC delivered only 93.2 mA h g −1 (at 1C) at the end of 120 cycles. The BC/SnO 2 composite showed excellent rate performances at different current densities, exhibiting a capacity of 453 mA h g −1 at the end of 120 cycles. Upon testing against sodium metal, the BC/SnO 2 composite exhibited better cycling stability than BC (233 mA h g −1 compared with 165 mA h g −1 ) at 100 mA g −1 for 120 cycles. A theoretical investigation of the interactions between BC and SnO 2 was performed using the semi-empirical GFN1-xTB method. The stability of the mixed system at high temperatures was confirmed using molecular dynamic simulations. Finally, we analyzed the electronic properties of the BC/SnO 2 composite and drew conclusions about the electrical conductivity. Therefore, our research strategy helps to produce sustainable high-specific capacity anode materials from biomass resources for building cost-effective metal-ion batteries.