Regulating Microstructure in Lignite-Derived Hard Carbon via Altering the Thermal Transformation Pathway for High-Plateau Sodium-Ion Batteries

Abstract Carbonization, a thermally driven transformation process, serves as an indispensable step for converting coal precursor into hard carbon. However, the traditional one-step high-temperature carbonization process ignores the discrepancies in coal pyrolysis behavior across different temperature stages, rendering it a nonequilibrium thermal transformation pathway. Herein, based on the pyrolysis characteristics of lignite, a two-step carbonization strategy was adopted to synthesize tailored lignite-derived hard carbons. Compared with one-step carbonization, this two-step carbonization strategy establishes a relative-equilibrium thermal transformation pathway. Experimental results and ex situ characterizations confirm that precarbonization induces the moderate production of condensed aromatic ring structures and free radical fragments in intermediates via depolymerization and polycondensation reactions. After precarbonization treatment, these condensed aromatic ring structures and free radical fragments not only facilitate the formation of microcrystallines with suitable sizes but also inhibit excessive graphitization during the postcarbonization stage, endowing the hard carbon with abundant pseudographitic carbon and more closed pores. Thus, the resulting lignite-derived hard carbons achieve an enhanced reversible capacity of 261.9 mAh g–1, an improved plateau region of 169.7 mAh g–1, and excellent rate performance. This work provides a design principle for regulating the microstructure of lignite-derived hard carbons by modulating the intrinsic carbonization process, thereby providing a promising route for fabricating high-performance anode materials for sodium-ion batteries.