Constructing ion-pore structures in hard carbon via in situ polymerization to promote sodium ion diffusion to achieve rapid sodium ion storage

Abstract

Sodium-ion batteries play an important role in large-scale energy storage applications, and hard carbon materials show promising commercial applications as anodes for these batteries. Many novel hard carbon preparation methods, like pre-treatment and high-temperature carbonization, have been reported. However, hard carbon still faces challenges such as an unclear sodium storage mechanism, poor low-temperature performance, and safety issues. In SIBs, achieving high ion diffusion efficiency under high-rate and low-temperature conditions is key to high performance. However, developing SIBs with high diffusion efficiency remains a challenge. The current research focuses on increasing the plateau capacity by engineering closed nanopores, adjusting the aperture of the nanopores, and increasing the slope capacity by engineering defects in the carbon lattice. This study demonstrates a top-down strategy to improve the pore structure of electrode materials by adding ethanol during the fabrication of resorcinol–formaldehyde resin (RF) precursors to accelerate the diffusion of Na⁺ in resin-based hard carbon. Adding ethanol creates vapors during the pyrolysis reaction, forming pores in the cross-linked phenolic resin matrix. These pores provide an additional diffusion path for Na⁺. In addition, adding ethanol slowed down the generation of intermediates, which affected the particle size and structure of the microspheres. This modulation contributes to forming a more homogeneous pore structure, further optimizing the diffusion path of Na⁺. The hard carbon prepared with 5 mL ethanol (HC-5) exhibited a higher ion diffusion rate in SIBs.