Regulating the closed pore structure of biomass-derived hard carbons towards enhanced sodium storage

Hard carbon (HC) stands out as a promising anode material for sodium storage due to its unique structure. However, the pore structure regulation of HC still poses a significant challenge. Herein, we propose a combined method of hydrothermal pretreatment, freeze-drying, and high-temperature carbonization to regulate the pore structure of HC. During the hydrothermal processes, the assembly of graphene oxide (GO) with starch facilitates the formation of open pores, which are subsequently transformed into closed pores during high-temperature carbonization. Impressively, a hard carbon with a reversible capacity of 434 mA h g(-1) at 30 mA g(-1) has been prepared through the structure regulation. Compared to the sample without structural regulation, it demonstrates a remarkable increase of 184.21 mA h g(-1), with an enhancement rate of 73.80%. Simultaneously, the skeletal structure of GO serves as a conductive network and framework support, enabling favorable rate performance and excellent cycling stability. Furthermore, the sodium storage mechanism is deduced as "adsorption - intercalation - pore filling". Overall, this study can offer valuable insights into pore structure regulation for anode materials and shed light on sodium storage mechanisms.