A N, S co-doped carbon produced from soybean pods for use as the anode in sodium-ion batteries

Hard carbon (HC) is a promising anode material for sodium-ion batteries (SIBs) because of its large interlayer spacing and structural stability. However, it has a low initial Coulombic efficiency (ICE) and poor reversible specific capacity which limit its use. We report the use of biomass soybean pods as the precursor of a nitrogen and sulfur co-doped hard carbon with abundant mesopores, fabricated by a simple, low-cost and sustainable approach. When used as the anode for sodium-ion batteries, the relatively high nitrogen doping (~17%, mass fraction), ultra-low specific surface area (9.6 m2 g−1), and the large amount of pyridinic-N coordination combine to give an excellent electrochemical performance. The material has an outstanding electrochemical performance with a specific capacity of 425.0 mAh g−1, and an ICE of 83.7% at a current density of 0.1 A g−1. The capacity retention rate is as high as 99.5% after 110 cycles within a wide current range of 0.05-1 A g−1. An assembled full cell has an excellent cycling stability with 97.3% capacity retention after 100 cycles at a current density of 0.2 A g−1, and the energy density can reach as high as 141.2 Wh kg−1 at 1 A g−1.