Na3V2(PO4)2F3/C钠离子电池无机/有机杂化SEI膜原位构筑及性能调控

Sodium ion battery is regarded as an excellent candidate for large-scale electrochemical energy storage technology because of the Na abundance, low cost, relatively high energy density. However, the problems of inferior storage performance and severe self-discharge, which are rooted in the side reactions between electrode and electrolyte, restrain its practical application. An innovative idea, in-situ forming an inorganic-organic hybrid SEI film with high Na+ ionic conductivity and low electron conductivity on the electrode surface, is proposed here in this project to stabilize the interphase between electrode and electrolyte. By utilizing Na3V2(PO4)3 as an inner reference electrode, Na-ion full cells with various electrochemical windows could be designed and built, which contribute to clarify the mechanism of carbonate-based electrolytes decomposition and shuttling effect of their decomposition products. With the help of film-forming additives containing functional group (phosphates, sultone, and borates), the interfacial stability between electrode and electrolyte can be enhanced, which is beneficial to improve the storage and cycling performance of Na3V2(PO4)2F3/C battery. In combination of dV/dQ and dQ/dV methods, interfacial reaction rates and polarization information of the cathode and anode could be quantitatively analyzed, and the capacity fading mechanism of Na3V2(PO4)2F3/C battery during storage and cycling process can be revealed. On the basis of these research results, the internal relationship of interfacial composition, interfacial stability, and electrochemical performance will be established. The fundamental study throughout this project provide more logical proofs to understand the theory of manipulating the interfacial properties in the Na-ion battery, and offer more effective research methods for boosting its practical application.

钠离子电池具有资源丰富、成本低廉、能量密度较高等突出优势，在大规模电化学储能领域极具发展潜力。但是，源于电极/电解液界面副反应的存储性能差、自放电大的问题制约了其实际应用。本项目创新性提出原位构筑高钠离子电导、低电子电导的无机/有机杂化SEI膜思路，以期实现电极/电解液界面的高稳定性。以Na3V2(PO4)3为内参比电极，构建具有不同电化学窗口的钠离子电池电解液研究体系，揭示碳酸酯基电解液分解机理及分解产物穿梭机制；利用含特征官能团(磷酸基、磺内酯基、硼酸基)添加剂的定向/协同作用，提高界面稳定性，改善Na3V2(PO4)2F3/C存储及循环性能；通过dV/dQ和dQ/dV法定量分析正负极材料界面反应速率和极化情况，原位无损分析电池存储和循环过程中容量衰减机制，建立界面成分、界面稳定性和电化学性能之间的内在联系。通过本研究，为钠离子电池的界面调控提供理论依据，为推动其实际应用提供有效方法。

针对氟磷酸钒钠/硬碳（NVPF/HC）钠离子电池自放电明显、存储性能差等关键问题，本项目开展了NVPF制备及改性， NVPF/HC钠离子电池电解液组分设计、界面调控及其作用机理方面的研究。主要研究结论如下：.通过碳热还原法，以葡萄糖为碳源制备了碳包覆层紧凑、均一的NVPF材料，经水洗后材料的纯度及电化学性能进一步提升，水洗后的NVPF材料在10 C倍率下材料的首次放电容量为107 mAh g-1，循环1000次后容量保持率为83%。.通过原位电化学紫外-可见光光谱和原位循环伏安测试方法，系统地研究了碳酸酯类、醚类有机溶剂的电化学窗口与硬碳负极的兼容性。线性碳酸酯易在硬碳表面分解，而环状碳酸酯可在硬碳表面形成较为稳定的SEI膜，阻止电解液进一步分解，提高NVPF/HC钠离子电池的首次库伦效率和循环性能。相较于碳酸酯溶剂，采用醚类电解液的硬碳负极倍率性能更好。0.1 C时， EC/DMC与DME两种电解液的充电比容量接近，但DME体系在10 C时比容量为200.71 mAh g-1，而EC/DMC体系仅有1.38 mAh g-1，醚类电解液较快的去溶剂化速度、更高的离子电导率及形成的低阻抗SEI膜是其倍率性能优异的内在原因。.系统研究了LiODFB、NaODFB、VC、1,3-PS、PES、DTD和SN添加剂在NVPF/HC钠离子电池正负极界面的定向/协同作用机理。其中，VC可以在硬碳表面形成稳定的SEI膜，但过量VC易在NVPF正极被氧化分解；1,3-PS可以分别在正负极表面形成富含硫酸盐的SEI膜组分；NaODFB则可形成富含NaF的SEI膜；综合而言，NaODFB、VC、1,3-PS和SN存在协同作用，可显著抑制碳酸酯类电解液在高温下的分解，大幅度提升NVPF/HC钠离子电池的高温自放电和循环性能。NVPF/HC钠离子电池在55度、0.1 C下循环60次后的容量保持率高达99%。本项目对钠离子电池电解液的组分设计与优化进行了广泛和深入的研究，对后续钠离子电池科研工作具有重要的借鉴意义。