Hierarchical porous hard carbon enables integral solid electrolyte interphase as robust anode for sodium-ion batteries

Hard carbon is the most promising anode for sodium-ion battery applications due to the wide availability and low work voltage. However, it often delivers worse electrochemical performance in ester-based electrolytes. Herein, a hierarchically porous loose sponge-like hard carbon with a highly disordered phase, prepared from the biomass of platanus bark, exhibits superior rate performance with a capacity of 165 mAh·g⁻¹ at a high current of 1 A·g⁻¹, and high retention of 71.5% after 2000 cycles in an ester-based electrolyte. The effect of the hierarchically porous loose sponge-like structure on the formation dynamics of solid electrolyte interphase (SEI), and related properties, was studied via cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT), X-ray photoelectron spectroscope (XPS), Fourier transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS) analysis. These results reveal that the hierarchically porous structure can construct continued connecting channels and accelerate the electrolyte transport, which is beneficial to the reaction kinetics of SEI. Moreover, the mesoporous structure is conducive to good contact between electrolyte and materials and shortens the Na⁺ diffusion path, which in turn facilitates the charge transfer kinetics in the material.