Materials development toward higher-voltage energy storage devices

JSPS奖学金的目标是确定用于钠离子电池的新的高电位正极材料。为此，我们研究了钠基杂多阴离子SO4-PO3F固溶体Na2Fe(SO4)2-x(PO3F)x.2H2O的电化学性质。到目前为止，PO3F单元还没有作为正极材料的聚阴离子进行研究，我们感兴趣的是它的诱导效应及其对电化学性能的影响。恒电流循环显示PO3F多阴离子的氧化还原电位升高。这将为一类新的高电位正极材料开辟道路。然而，该官能团的一个缺点是其热稳定性较低。因此，我们试图寻找更多的合成路线，以确保该化合物的制备方法简单且可重复性。第二个项目包括含有Ti4和CO2的钠基层状氧化物的电化学和结构表征，其在Na(去)插层过程中表现出很大的滞后，表明CO2在氧化时发生了自旋转变。我们正在使用多角度的方法，并辅之以表征方法，如非原位软、硬X射线吸收、磁性测量和恒电流间歇滴定技术(GITT)实验，以了解潜在的氧化还原机理。此外，我们还进行了密度泛函理论计算，从理论上补充了实验工作。各份出版物正在筹备中。

The goal of this JSPS fellowship is to identify new high-potential cathode materials for sodium-ion batteries. To this end, we studied the electrochemical properties of a Na-based heteropolyanionic SO4-PO3F solid-solution Na2Fe(SO4)2-x(PO3F)x.2H2O. The PO3F unit has not been studied as a polyanion for cathode materials so far and we were interested in its inductive effect and its impact on electrochemical performances. Galvanostatic cycling revealed elevated redox potentials for the PO3F polyanion. This would open the pathway to a new class of high-potential cathode materials. However, one drawback of this functional group is its low thermal stability. Therefore, we try to find additional synthesis routes to ensure an easy and reproducible preparation method of this compound.A second project consists of the electrochemical and structural characterization of a Na-based layered oxide containing Ti4+ and Co2+, which displays a large hysteresis during Na (de)intercalation suggesting a spin transition of Co2+ upon oxidation. We are using a multi-angle approach with complementing characterization methods such as ex situ soft and hard X-ray absorption, magnetic measurements and galvanostatic intermittent titration technique (GITT) experiments to understand the underlying redox mechanism. Furthermore, we performed Density Functional Theory calculations to complement the experimental work with a theoretical viewpoint. The respective publication is under preparation.