新型氟化磷酸钒氧钠二维复合纳米结构的可控制备及储钠性能研究

Currently, the lack of long-term stable cathode material is the bottleneck to hinder the energy storage and conversion of sodium-ion batteries (SIBs). Herein, this project introduces a novel cathode material, Na1.5VPO4.8F0.7, for SIBs. This new material provides an energy density of ~600 Wh kg-1 and the high potential (~3.8 V vs Na+/Na). The small volume change with two-dimensional Na diffusional pathways leads to low activation barriers for Na diffusion, enabling excellent rate capability. According to the structure and Na diffusional pathways of Na1.5VPO4.8F0.7, the morphology of as-designed 2D nanosheets can benefit the fast Na diffusion and offer more active storing sites. While coating the surface by a thickness of 3~10 nm MOx-y layer that accompanied with oxygen vacancies, has been developed as an effective strategy to improve the electrical conductivity. Comparing with the traditional carbon coating, it can overcome the drawbacks that improve the tap density, power density and energy density. We will combine with the theoretical calculation to study the effect of the coating MOx-y layer structure, form and thickness on the interfacial properties, electrochemical performance and thermal stability of SIBs, and then investigate the transferring theory for sodium ion in cathode material. In site direct resistance, alternating-current impedance and GITT results will be used to obtain the resistance changes of MOx-y coated Na1.5VPO4.8F0.7 during the charging and discharging processes, and further understand the inherent relationship between the oxygen vacancies and the rate controlling step in SIBs. This work is believed to perfect the basic theory of coating technology for cathode materials of SIBs.

当前制约钠离子电池发展的主要因素是缺乏可稳定脱/嵌Na+的长寿命型的电极材料。本项目选择NASICON型Na1.5VPO4.8F0.7为钠离子电池正极材料，平均电压为3.8 V，能量密度高达600 Wh kg-1，在充放电过程中体积变化小，Na+扩散活化能低，传荷动力学快。但室温下其电子导电率偏低导致功率密度性能较差。根据此材料的特点，设计为二维纳米结构，此外，在材料表面包覆一层3~10 nm厚度的含氧缺陷MOx-y层，有效提高其电子导电率，保证高振实密度、能量密度和功率密度。结合理论计算，系统研究包覆层结构、形态及厚度对材料的界面性质、电化学性能及热稳定性能的影响及作用机理。通过交流阻抗、直流阻抗、GITT法研究Na1.5VPO4.8F0.7@MOx-y在充放电过程的阻抗变化，阐明含氧缺陷包覆层与钠离子电池电化学反应速率控制步骤的内在联系，完善包覆技术在钠离子电池正极方面的基础理论。

目前钠离子电池是最有前途替代锂离子电池的储能二次电池。当前制约钠离子电池发展的主要因素是缺乏可稳定脱/嵌Na+的长寿命型的电极材料。本项目选择理想的NASICON型Na1.5VPO4.8F0.7作为钠离子电池正极材料，平均电压为3.8 V，能量密度高（≈600 Wh kg−1），在充放电过程中体积变化很小，Na+在ab平面扩散，传荷动力学快。根据Na1.5VPO4.8F0.7的结构及Na+的传输路径，设计二维及三维微纳结构状的形貌，有利于Na+的快速传输。通过高温下还原性气氛处理材料，在材料表面包覆一层3~5 nm厚度的含氧缺陷V2O5-x、TiO2-x层以及碳层，有效提高Na1.5VPO4.8F0.7的电子导电率，保证高振实密度，进而保证高能量密度和功率密度。通过不同扫速的循环伏安曲线、交流阻抗与直流阻抗结合的方法研究存在含氧缺陷TiO2-x包覆层和碳层的Na1.5VPO4.8F0.7在充放电过程的阻抗变化，阐明含氧缺陷包覆层和碳层与钠离子电池电化学反应速率控制步骤的内在联系，完善包覆技术在钠离子电池正极方面的基础理论。