Closed pore structure engineering from ultra-micropores with the assistance of polypropylene for boosted sodium ion storage

Abstract

Carbonaceous materials with closed pore structures have emerged as promising anodes for sodium-ion batteries. However, the rational design of closed pores and the investigation of their underlying sodium storage mechanisms face significant challenges. Herein, PVDF-derived pyrolytic carbon (PFC) with abundant open ultra-micropores and uniform pore size of 0.55 nm was chosen as the carbon matrix to construct a closed pore model to elucidate the sodium storage process in the closed pores. The transformation of ultra-micropores into closed pores was achieved with the assistance of polypropylene (PP), where the light aromatic compounds derived from PP can be deposited at the open pore entrance of the PFC during the pre-oxidation process, and subsequently pyrolyzed to a carbon layer in the carbonization stage, thus converting the open pores into closed pores. The obtained closed pore enriched carbon (PFCC@PP) showed a noticeable low-potential plateau region in the charge/discharge curve and an improved reversible capacity of 309.3 mA h g⁻¹ with a high initial coulombic efficiency of 87.8%. When coupled with the O3-NaNi_1/3Fe_1/3Mn_1/3O₂ cathode, the assembled Na ion full cell realized a high energy density of 239.2 Wh kg⁻¹ besides the remarkable rate capability. Additionally, theoretical calculations further validated the significant role of closed pores with optimal pore size in efficient Na ion storage. This work not only provides an interesting model for investigating closed-pore structures but also offers new insights into the rational design of carbon materials for advanced energy storage technology.