Achieving over 200 Wh kg^-1 sodium-ion pouch cell by quantitative engineering of hard carbon pores

Energy-dense sodium-ion batteries (SIBs) offer lithium-free, cost-effective solutions for grid-scale energy storage. However, the structural complexity of hard carbon (HC) anodes hinders the establishment of a clear structure-performance relationship, leading to the insufficient performance of current HC when paired with advanced cathodes. In this study, we precisely adjusted the content and size of closed pores in HC using an economical, extensible rosin-assisted pore-promoting strategy, and quantified the effective pore volume for sodium storage through small-angle X-ray scattering experiments. We show that optimizing the closed pore size to increase the effective pore volume is key to enhancing the electrochemical performance of HC. By controlling the size (∼2 nm) of closed pores, we enhance the Na-cluster filled volume fraction of HC, resulting in an extended low-potential plateau (<0.1 V vs. Na⁺/Na) and higher sodium storage capacity. Additionally, we established a positive correlation between the plateau capacity of HC and the effective pore volume. Consequently, the 4.5 Ah pouch-type SIBs assembled with optimized HC here (areal capacity, 2.8 mAh cm^-2) achieved a high energy density of 202 Wh kg^-1, with over 80% capacity retention after 500 cycles at 0.5 C. This research provides a solution for realizing low-cost, advanced SIBs.