Two-step synthesis of hard carbons anode material from poplar wood with enhanced performance for sodium-ion batteries

Hard carbon (HC) is regarded as the most prospective anode material for sodium-ion batteries. Biomass HC is favored due to the advantages of being inexpensive and easily available. Herein, hydrothermal treatment of poplar lateral branches at 220 °C for 4 h was employed as the first synthesis step for HC precursor with yield of 48.5%. The obtained precursor was subjected to the second step of carbonization under nitrogen atmosphere at 1200 °C, 1400 °C, and 1600 °C for synthesis of HC anode materials. Advantages of two step synthesis were confirmed in terms of inorganic impurities elimination, HC yield, and electrochemical performance. Inorganic impurities reduced from 0.46% in poplar wood to 0.26% in precursor. The HC yield was 34.6% for precursor from hydrothermal treatment of poplar wood, which was much higher than HC yield of 18.5% from direct carbonization of poplar wood. The obtained HC anode materials manifested high capacity, strong rate performance, and long-term stability for sodium-ion batteries as indicated by the capacity of 333 mA h g−1 at 0.1 C and 285 mA h g−1 at 2 C, and capacity retention of 92.9% after 200 cycles at 1 C. This research provides an eco-friendly approach for the high-value utilization of woody biomass as anode material for sodium-ion batteries.