Microstructure regulation of lignite-based hard carbons and their sodium storage properties

The diversified clean and high-value utilization of coal plays an important role in promoting the low-carbon sustainable development of China's coal industry, and the materialization of coal is one of the important ways to enhance the level of its clean and efficient utilization. In this study, lignite-based hard carbons (LHC) were prepared by high-temperature carbonization (1000−1600 ℃) method using Huating lignite as precursor, taking full advantage of lignite's aromatic ring structure, well-developed primary pores and abundant surface active groups. The formation mechanism and evolution behavior of graphite-like microcrystals and defect structures including amorphous carbon, nanopores, and surface functional groups in LHC was explored, while the influence of carbonization temperature on the microstructure of LHC was further revealed. The electrochemical performance of different LHC as anode for sodium ion batteries (SIBs) was also tested by galvanostatic charge/discharge, galvanostatic intermittent titration and cyclic voltammetry, the influence mechanism of microstructure on the electrochemical sodium storage performance of anode materials was explored, and the electrochemical sodium storage mechanism of LHC was eventually elucidated. The results show that graphite-like microcrystalline structure and defective structures such as amorphous carbon, nanopores, and oxygen/nitrogen-containing functional groups of LHC can be effectively regulated by adjusting the carbonization temperature. When the carbonization temperature was 1400 ℃, the LHC-1400 was rich in graphite-like microcrystals with reasonable layer spacing (0.371 nm), and also had appropriate amorphous carbon and nanopores with a specific surface area of 4.92 m²/g, and oxygen/nitrogen-containing functional groups including C—O, C=O, O—C=O, pyridinic-N, pyrrolic-N, graphitic-N. The LHC-1400 as anode for SIBs can deliver a high reversible capacity of 275 mAh/g, and has a reversible capacity of 111 mAh/g at a current density of 0.2 A/g with a capacity retention rate 96% after 200 cycles, which exhibits good rate performance and cycle stability. The excellent sodium storage performance of LHC is closely related to the functions and roles of their different microstructures. The graphite-like microcrystals with reasonable layer spacing in LHC can provide a transport channel for the rapid intercalation and de-intercalation of Na⁺ to provide capacity with intercalated sodium storage; the defect structures such as amorphous carbon, open nanopores, and oxygen/nitrogen-containing in LHC can provide sufficient active sites for Na⁺ storage to contribute capacity by adsorption, and a small amount of closed pores in LHC can provide sufficient space for the storage of Na⁺ and supply sodium storage capacity by pore filling. The “adsorption-insertion-filling” sodium storage methods in LHC are synergistic with each other to realize its efficient electrochemical energy storage.