CO₂ activation-induced closed pores in hard carbon derived from bamboo for enhanced sodium-ion storage

Hard carbon (HC) materials prepared from bamboo have shown a promise as anodes of sodium-ion batteries (SIBs), but their capacity needs to be further improved in order to meet commercial requirements. Creating a porous structure in HC particles is an effective route. This work applies low-cost carbon derived from bamboo to enhance its porosity through CO₂ activation at 800 °C. The total amount of CO₂ is fixed and the activation degree can be varied by tuning the amounts of carbon precursor from 1 g to 10 g. After secondary carbonization at 1300 °C, HC materials are obtained as anodes for SIBs. Experimental results demonstrate that the specific surface area of the carbon precursor is substantially enhanced after CO₂ activation and then is dramatically reduced subjected to secondary carbonization. The sample after CO₂ activation delivers a larger closed pore volume than the non-activated one. Excessive activation by CO₂ will reduce the particle size of HC particles. The optimal HC has a high reversible specific capacity of 339.2 mAh g^-1 at 30 mA g^-1 with an initial Coulombic efficiency of 86.3 %, and a capacity retention of 93.6 % at 300 mA g^-1 after 200 cycles. The sodium storage mechanism and the formation process of closed pores are also discussed based on the experimental data. The activation of the carbon precursor by CO₂ offers environmental and economic benefits.