摘要/Abstract

可充电镁电池成本低、安全性好，是非常有前景的下一代二次电池，其关键之一是开发高性能的正极材料.本工作首次利用分级结构氮掺杂碳纳米笼（hNCNC）作为可充电镁电池的正极材料，展现出高放电比容量（71 mAh·g^-1@100 mA·g^-1）、优异的倍率性能（60 mAh·g^-1@2000 mA·g^-1）和长循环稳定性（1000圈容量保留率83%@1000 mA·g^-1）.hNCNC呈现电容行为主导的储镁机制，理论研究表明镁离子主要吸附在微孔边缘的碳原子、吡啶氮或吡咯氮等活性位点上.其优异储镁性能可归因于：（1） hNCNC的大比表面积（1590 m²·g^-1）、丰富微孔缺陷和高吡啶/吡咯氮含量（4.49 at.%）有效提升了储镁容量；（2） hNCNC的高导电性、多级孔道结构及N掺杂导致的高浸润性有利于电荷传输，降低了电池的等效串联电阻，从而改善了倍率性能；（3） hNCNC的稳定碳骨架结构及其表面吸附储镁机制使其具有优异的长循环稳定性.
关键词: 可充电镁电池, 正极材料, 氮掺杂碳纳米笼, 电容行为, 高倍率长寿命性能
Rechargeable magnesium batteries (rMBs) are promising next-generation secondary batteries owing to the low-cost, high safety and dendrite-free property of Mg metal. The key of rMBs technology is to develop high-performance cathode materials. Usually, the intercalation-type cathodes such as Mo₆S₈, MoS₂ and Ti₃C₂T_x suffer from the inferior rate performance owing to the sluggish Mg²⁺ ions solid-diffusion kinetics, and the conversion-type cathodes such as S and CuS are beset with the poor cycling stability owing to the pulverization and loss of active species. Recently, sp² carbon materials exhibited considerable magnesium storage performance through an interfacial charge storage/release. Ideal carbon-based cathodes for rMBs should possess the features of high specific surface area and abundant active sites for magnesium storage, high conductivity and porous structure for facilitating charge transfer, as well as high mechanical stability. Herein, we employed the hierarchical nitrogen-doped carbon nanocages (hNCNC) featuring large surface area, abundant surface defects, coexisting micro-meso-macropores and high conductivity as the rMBs cathode for the first time, which exhibited high discharge capacity of 71 mAh·g^-1 at 100 mA·g^-1, excellent rate performance (60 mAh·g^-1 at 2000 mA·g^-1) and ultra-high cycling stability (83% capacity retention after 1000 cycles at 1000 mA·g^-1). The capacitive magnesium storage mechanism is predominant in the charging-discharging process. Theoretical studies reveal that magnesium ions are adsorbed on the carbon, pyridinic-nitrogen or pyrrolic-nitrogen atoms at the edge of micropores. The excellent magnesium storage performance of hNCNC is attributed to the following reasons:(i) the hNCNC with large surface area (1590 m²·g^-1), abundant micropore defects and high content of pyridinic and pyrrolic nitrogen (4.49 at.%) provides sufficient active sites for magnesium storage, resulting in the high discharge capacity; (ii) the coexisting micro-meso-macropores structure, good conductivity and improved wettability via N-doping facilitate the charge transfer kinetics, and decrease the equivalent series resistance of rMBs, thereby leading to the improved rate capability; (iii) the robust scaffold of hNCNC and the capacitive-dominated magnesium storage mechanism ensure the high cycling stability. This study demonstrates the high-rate and durable performance of hNCNC in rMBs, and suggests a promising strategy to improve the rMBs performance by increasing edges and suitable dopants of nanocarbons.
Key words: rechargeable magnesium battery, cathode material, nitrogen-doped carbon nanocage, capacitive-dominated mechanism, high-rate long-life performance


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