Endorsing Na+ storage mechanism in low tortuosity, high plateau capacity hard carbon towards development of high-performance sodium-ion pouch cells

Abstract

Understanding the disordered structure of hard carbon and the sodium-ion storage mechanism is essential for the commercialization of sodium-ion batteries (SIBs). Herein, we have successfully synthesized low tortuosity hard carbon with high plateau capacity at low voltage regions. This study first explores the correlation between the pore structure, sodium-ion diffusion pathway, and half-cell performance. Furthermore, the Na+ storage mechanism has been studied in detail by electrochemical and surface characterizations. The distribution of relaxation times (DRT) was used for a deeper understanding of electrode reactions. Additionally, the correlation between porosity and tortuosity was established with structural evolutions on the synthesized hard carbon. For anode structure, the Na+ pathway was examined by tortuosity experiment and validated by the COMSOL simulation study. The anode electrode performance displayed a low voltage-plateau and high discharge capacity of ∼301 mAh g−1 at 0.1C (∼35 mA g−1 with excellent cyclic stability even at 1C (∼350 mA g−1) and extended up to 1000 cycles with ∼94% capacity retention. Later, the full-cell fabricated with hard carbon as an anode and polyanionic sodium vanadium phosphate (Na3V2(PO4)3) as the cathode exhibited a superior cyclic performance up to 450 cycles at 0.1C. Further, the pouch cell (∼100 mAh designed capacity) was fabricated and exhibited favorable cyclic stability up to 300 cycles and delivered ∼135.2 Wh kg−1 of energy density at the cell level. This research may offer a distinct hard carbon microstructure that will be effective in developing high-performance practical SIBs.