中空核壳型Sn-Co@C复合材料的设计合成及其储钠性能的调控研究

Compared with Sn anode material, Sn-Co alloy possesses higher specific capacity and better cycling stability for sodium storage. However, it still has disadvantages of significant volume expansion after sodiation, low initial coulombic efficiency and slow sodium diffusivity, which hinder its practical applications. In this project, a yolk-shell Sn-Co@C composite is designed and synthesized, together with the performance optimization through regulation and control of the microstructure and morphology, the alloy composition, and the degree of amorphization. Firstly, the volume expansion can be alleviated by optimization of microstructure and morphology. That can prolong cycling life of the material. Subsequently, the crystal structure and composition can be optimized by adjusting the Sn/Co ratio of the alloy, thus achieving the balance of different properties, such as specific capacity, initial coulombic efficiency and diffusion velocity of sodium ions. Additionally, the reduction in energy barrier for the phase transition during sodiation and desodiation can be achieved through amorphization, which may alter the ion storage and transfer properties, consequently further optimizing the electrochemical performance of the material. The study reveals the effects of microstructure, morphology, Sn/Co ratio and amorphization on the electrochemical properties of the yolk-shell Sn-Co@C composite. These all can provide new ideas and theoretical guidance for the development of Sn-based alloy anodes, as well as other alloy-type anode materials.

锡-钴合金负极材料具有较高的储钠比容量，且循环稳定性高于锡负极，但还存在体积膨胀显著、首次效率低以及钠离子传输速率慢等问题，离实际应用仍有距离。本项目拟通过形貌调控、合金成分调控及合金无定型化调控等手段，设计合成一种中空核壳型Sn-Co@C复合材料。通过优化设计材料的微观结构与形貌缓解体积膨胀，提高材料的循环寿命；通过调控材料中的Sn/Co比例优化合金的晶体结构及成分，实现储钠比容量、首次效率及钠离子传输速率等几方面性能的平衡；通过无定型化降低材料在储钠过程中的相变能垒，改变钠离子在材料中的存储和传输特性，进一步优化材料的电化学性能。通过本项目研究，揭示材料的微观结构与形貌、Sn/Co比例及Sn-Co合金的无定型化程度对材料电化学性能的影响规律及作用机理，为锡基合金及其他合金负极材料的开发提供新思路和理论指导。

电动汽车和储能电站的发展对锂离子电池的需求日益增长，而锂离子电池中不可或缺的锂元素在地壳中的储量非常有限。开发钠离子电池是缓解锂资源短缺的有效途径，但目前钠离子电池负极材料的电化学性能仍不尽如人意。锡基负极材料具有较高的能量密度和适中的工作电压，近年来备受关注。然而，充放电过程中的体积膨胀问题和“死钠”现象造成的容量衰减问题在锡基材料中较为突出。本项目针对这些问题，设计了中空核壳型Sn-Co@C复合材料，并实现了这种材料的可控合成。本项目所设计的材料以Sn-Co合金为核，以无定型炭为壳，且在核、壳之间有一定空隙可供核的自由膨胀，能有效解决锡基材料因体积膨胀而带来的粉化、脱落问题。Sn-Co合金在电化学储钠过程中仅锡与钠离子发生反应，钴为惰性且能将反应后的锡分隔成许多微区，即“马赛克”结构。这种结构能缓解“死钠”现象，提高材料的循环稳定性。本项目在实现可控合成的基础上对材料中的Sn/Co比例进行了优化，发现当Sn/Co摩尔比为3:2时复合材料具有优异的电化学性能，此时合金核由SnCo和Sn2Co2.9两相组成。Sn/Co比例优化使得Sn-Co@C复合材料的电化学性能得以进一步提升，优化后的材料在50 mA g-1电流密度下表现出了591 mAh g-1的首次可逆比容量；在100 mA g-1电流密度下循环100圈后的可逆比容量保持在330 mAh g-1。本项目系统地研究了Sn-Co@C复合材料的微观结构形貌和电化学性能之间的关系，阐明了合金成分对电化学性能地影响，对锡基负极材料的微观结构设计及性能优化具有指导意义，对其他相似电池如锂离子电池、钾离子电池等的合金型负极材料设计亦具有借鉴价值。