褐煤基硬炭微观结构调控及其储钠特性

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

煤炭的多元化清洁高值利用对促进我国煤炭工业低碳可持续发展具有重要作用,而煤的材料化是提升煤炭清洁高效利用水平的重要途径之一。充分利用褐煤含芳环结构、原生孔隙发达、表面活性基团丰富等特点,通过高温炭化(1000~1600℃)处理华亭褐煤制备出褐煤基硬炭,探究褐煤基硬炭中类石墨微晶、无定形碳与纳米孔及表面官能团等缺陷结构在高温炭化过程中的形成机制与演变规律,揭示炭化温度对煤基硬炭微观结构的影响,并通过恒电流充放电、恒电流间歇滴定及循环伏安测试等研究不同褐煤基硬炭用作钠离子电池负极材料的电化学性能,探索微观结构对负极材料电化学储钠性能的影响机制及褐煤基硬炭的电化学储钠机理。研究表明,通过调节炭化温度可实现对褐煤基硬炭中类石墨微晶、无定形碳、纳米孔、含氧/含氮官能团等缺陷结构的调控。当炭化温度为1400℃时,所制褐煤基硬炭LHC-1400富含合理层间距(0.371 nm)的类石墨微晶,兼有适宜含量的无定形碳和纳米孔等缺陷结构,其比表面积为4.92 m^(2)/g,且含有C—O、C=O、O—C=O及吡啶氮、吡咯氮、石墨氮等含氧/含氮官能团。该硬炭用作钠离子电池负极材料的可逆容量达275 mAh/g,且在0.2 A/g电流密度下可逆容量为111 mAh/g,经200次循环后容量保持率仍可达96%,展现出良好的倍率性能和循环稳定性。褐煤基硬炭优异的储钠性能与其不同微观结构所发挥的功能与作用密切相关。硬炭中合理层间距的类石墨微晶可为Na^(+)的快速嵌入/脱出提供传输通道,以插层储钠来提供容量;硬炭中的无定形碳、开放的纳米孔和含氧/含氮等缺陷结构可为Na^(+)存储提供足够的活性位点,以吸附储钠来贡献容量;而硬炭中少量封闭孔则可为Na^(+)的存储提供足够的空间,以填充储钠来提供容量。褐煤基硬炭中“吸附-插层-填充”3种储钠方式相互协同,最终实现其高效的电化学储能。

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 aro-matic 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 influ-ence 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 carboniza-tion 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 m^(2)/g,and oxygen/ni-trogen-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 deintercalation 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.