非金属掺杂碳包覆电极材料用于高性能可充钠电池正极的研究

Undoubtedly, rechargeable lithium ion battery is the most promising energy storage system, and the most dominant cathode materials of today's rechargeable Li-ion batteries are lithium transition metal oxides, such as LiCoO2, LiNiO2, and LiMn2O4, etc., the limited ores resources of these materials make them expensive. In the future, if the real electric vehicles (EVs) or hybrid EVs (HEVs) generation comes, it will put considerable strain on resources and hence cost effectiveness, therefore will limit its practical application. Sodium-based battery seems to be a good choice when consideration of above-mentioned issues, since it has been attracted considerable attentions in past several years. The most studied cathode materials for rechargeable sodium battery are mainly those of materials containing corrosive fluorine or sulfur, for instance, MF3 (M=Fe, V, Mn, Co and Ti), Cu2S, NaV1-xCrxPO4F, Na2FePO4F, of which are relatively less environmentally friendly. Moreover, fluorides and sulfate are basically unstable in moisture atmosphere, which easily leads to decomposition, thus inevitably induce some unexpected reactions during the cell operation.It seems very necessary and desirable to develop new kind of electrode materials for SIBs application with good electrochemical performance. In this project, non-metal doped carbon-coating will be firstly applied to modify the cathode materials of sodium-ion batteries, with the aim to improve their electrochemical performance. It is expective that that different non-metal element with various electron shell structure, for instance, N (2s22p3), B (2s22p1),S (3s23p4), will bring about different dopant effects to the carbon-coating layer, therefore, the effect mechanisms will also different. Various characterizations and electrochemical tests will be used to investigate the effect mechanism, and finally to prepare the cathode material of SIBs with the improved electrochemial performance.

钠离子电池是一类新型规模储电备选化学电源之一，对其关键材料的研发是走向实际应用的关键。本申请针对钠电池正极材料导电性差、以及由此导致的结构不稳定、可逆容量小、循环稳定性差等问题，通过碳（2s22p2）包覆以及非金属[氮(2s22p3)、硼(2s22p1)、硫(3s23p4)]掺杂碳包覆设计制备具备可逆钠离子嵌入/脱嵌的正极材料；通过调控碳包覆层的电子结构以及缺陷密度，对材料的电子导电性进行改性；通过电化学测试手段对其在钠离子可逆嵌入/脱嵌过程中的电化学行为，包括电化学嵌/脱钠特征，电子传导、钠离子扩散机理，循环寿命等进行研究;通过分析掺杂前后材料的电化学行为，揭示非金属掺杂碳包覆层对材料改性的本质原因；探索非金属掺杂碳包覆钠电池正极材料的可控制备技术，通过优化非金属掺杂种类、掺杂密度、掺杂深度等调整材料表面特性，最终实现高性能钠离子电池正极材料制备技术突破。

项目的主要研究目的是针对钠离子电池正极材料（主要聚阴离子类）导电性能不佳的特点，采用非金属元素（N， B， S等）掺杂碳包覆的技术对正极材料进行改性研究。首先通过溶胶凝胶法结合瞬间冷冻干燥的处理手段制备出碳包覆均匀的、具有多孔结构的Na3V2(PO4)3这种多孔结构的形貌能够显著的增加材料的比表面积，使电解液充分地与Na3V2(PO4)3接触，增加Na3V2(PO4)3电极中活性位点的数量，使其表现出了更为优异的电化学性能。其次，通过原位制备方法合成了n型氮掺杂碳包覆修饰的Na3V2(PO4)3电极材料，适量氮掺杂碳包覆表现出更为优异的电化学性能。氮掺入碳包覆层后会产生三种掺杂构型：吡啶氮、吡咯氮和石墨氮。吡啶氮和吡咯氮不仅能够提高碳包覆层的电子电导率，而且还能够打破碳包覆层中原有的碳碳骨架结构，使之产生大量的缺陷，而石墨氮仅仅是取代部分的碳原子，不会对碳碳骨架结构产生破坏，因此不能产生缺陷。当石墨氮比例最少时，也就是吡啶氮和吡咯氮最多时，氮掺杂碳包覆的Na3V2(PO4)3 （NVP-C-N142）电化学性能最优。然后，首次制备了新型硼掺杂碳包覆技术对钠离子电池电极材料Na3V2(PO4)3的修饰，并显示出了极为有效的改性效果。在不同硼掺杂量的样品中，均出现四种掺杂类型：BC4，BC3，BC2O和BCO2。不同硼掺杂类型对于碳包覆的Na3V2(PO4)3的改性机理是各不相同的。与BC4和BC3相比，BC2O和BCO2中由于O原子的引入，能够明显的打破碳层的碳碳骨架结构，使碳层产生大量的缺陷，有利于提高Na+的扩散速度，进而改善Na3V2(PO4)3的电化学性能。 因此，BC2O和BCO2含量最多的NVP-C-B0.38%样品表现出了最为优异的电化学性能。最后，我们通过双纳米碳（碳包覆和CNTs）协同修饰Na3V2(PO4)3正极材料使其电化学性能进一步提升。氮掺杂不仅能够提高碳包覆层的电子电导率，还能改善Na+通过碳包覆层的扩散速度；而碳纳米管能够显著改善电子在多个Na3V2(PO4)3颗粒间的传输速度。因此，双纳米碳协同修饰Na3V2(PO4)3正极材料表选出了极为优异的电化学性能，特别是倍率性能和循环稳定性。以上由基金支持的相关成果已发表在Advanced Energy Materials，Journal of Materials Chemistry A上。