Ultramicropore Engineering Bridges the Capacity–Kinetics Gap in Hard Carbon for Sodium-Ion Battery

Abstract

Although, hard carbon (HC), is an ideal anode for sodium-ion batteries, its major capacity contribution in the plateau region is often hindered by sluggish kinetics, which limits the use in high-power applications. In this work, lignin is introduced into the cellulose precursor to modify the pyrolysis pathway and results in abundant C═O functional groups, optimized graphite domains, and a tailored pore system rich in both closed pores and ultramicropores. Crucially, ultramicropores play a pivotal role in resolving the trade-off between plateau capacity and kinetics, as they facilitate rapid sodium adsorption, inhibit the decomposition of the electrolyte within the pores, and partially contribute to the capacity of the plateau region. The optimized HC exhibits a high reversible capacity of 353.9 mAh g⁻¹ with an initial coulombic efficiency of 86.3%, excellent rate performance, and stable long-term cycling at room temperature (82.1% retention after 2500th at 1 A g^‒1) and −40°C (80.9% retention after 100th at 37.2 mA g^‒1). Based on the electrochemical performance and in situ characterization, the “adsorption-intercalation-pore filling” mechanism of HC anodes is confirmed, and the role of the ultramicropores in enhancing transport kinetics is demonstrated, which provides novel insights for designing high-power anodes of sodium-ion batteries.