Bridging sodium storage behavior and microstructure in broad-leaf lignin hard carbon for sodium-ion batteries

Abstract

Hard carbon (HC) has emerged as the most promising anode material for the sodium ion battery because high theoretical capacity and cost-effective properties. However, unsatisfactory specific capacity and initial Coulombic efficiency (ICE) significantly impede its further advancement. Moreover, the correlation between the microstructure and electrochemical performance has been not thoroughly elaborated. Here, a straightforward approach to regulate the closed nanopore and defect structure in the hard carbon matrix by adjusting the pyrolysis temperatures. Higher pyrolysis temperature will promote the growth of a long carbon chain and further fold and shrink generating more closed nanopores, which was beneficial to improve the Na+ plateau capacity. The long-range ordered turbostratic graphite domain structure should be avoided, as it can lead to a reduction in reversible capacity. Through detailed analysis of the hard carbon microstructure evolution and electrochemical performance, the relationship of which has been established. Simultaneously, the optimized hard carbon pyrolyzed at 1500 °C displays a remarkable reversible specific capacity of 338 mAh g-1 at 0.1 C with a high ICE of 87%. Based on the detailed analysis, a microstructure-dependent mechanism ‘adsorption-intercalation-filling’ was proposed for a comprehensive understanding of the sodium ion storage behavior. More significantly, fabricated 18650 cylindrical batteries demonstrate a high reversible capacity of 1175 mAh at 0.1 C with outstanding cycle stability (82.9% after 225 cycles at 0.5 C), outperforming commercial hard carbon counterparts. Our work provides deep insight into the rational design of the hard carbon structure and provides the possibility of building practical SIBs with high performance.