Layered oxide cathode materials for sodium ion batteries: correlation of electronic structure with electrochemical potential and degradation

与锂离子电池相比，钠基离子电池具有钠的高可用性、低成本和安全性等优势，是未来储能的可能选择。然而，到目前为止，钠离子电池的能量密度比锂离子电池低，这是由于钠离子电池的电压和比电极容量都较低，而且不稳定。离子电池的电压和电极的稳定性与电极的电子结构、电子化学势和碱（脱）插入过程中的变化密切相关。迄今为止，钠离子插入材料的电子结构还没有得到广泛的分析，其对电极电位和降解的影响也没有得到很好的理解。在这个项目中，我们建议分析钠层状氧化物阴极材料的电子结构，并确定其电极电位。我们的目的是将阴极材料的电极电位、本征电压极限和降解现象与其电子结构联系起来。结果与材料的锂类似物进行了比较。目标是更基本地了解电压的来源以及钠基离子电池中的退化现象，以改善其性能。我们将制备薄膜模型碱离子电极，分析其电子结构与碱含量和循环次数的关系，并将结果与结构数据和电化学性能联系起来。此外，我们将制备薄膜电极/固体电解质层堆栈，并确定带对准和绝对电极电位。由于对暴露在液体电解质中的电极的专用分析受到副反应（即固体-电解质间相形成）的阻碍，我们将制造全固态器件模型，允许使用XPS/UPS在特高压条件下对电极进行原位研究。进一步的表征涉及HRTEM/HRSEM和XRD等技术，以及标准的电学和电化学测量。该项目的重点将放在钴酸钠（NaxCoO2）和相关材料的性质上，并与它们的锂类似物进行比较。钴酸盐属于层状氧化物家族，在锂基离子电池中非常成功，在钠基离子电池中也有很好的应用前景。从项目开始时的钴酸钠开始，我们将扩大我们的研究范围，以具有更高稳定性和/或更高潜力的掺杂（例如锂，镍，锰）钴酸钠。

Sodium-based ion batteries are possible candidates for future energy storage with advantages over lithium-ion batteries such as high availability of sodium, low cost and safety. Yet, to date, sodium-ion batteries have a lower energy density than lithium-ion batteries, due to both lower voltage and lower specific electrode capacities, and are not as stable. The voltage of ion batteries and the stability of the electrodes are intimately related to the electrode electronic structure, their electron chemical potential and changes during (de-) insertion of alkali. To date, the electronic structure of sodium-ion insertion materials has not been widely analyzed, and its influence on electrode potential and degradation is not well understood. In this project, we propose to analyze the electronic structure of sodium layered oxide cathode materials and to determine their electrode potentials. We aim at correlating electrode potential, intrinsic voltage limit and degradation phenomena of the cathode materials to their electronic structure. The results are compared to the lithium analogues of the materials. The goal is a more fundamental understanding of the origin of voltage as well as degradation phenomena in sodium-based ion batteries to improve their properties. We will prepare thin film model alkali ion electrodes, analyze their electronic structure as a function of alkali content and number of cycles, and correlate the results with structural data and electrochemical properties. Also, we will prepare thin film electrode/solid electrolyte layer stacks and determine the band alignment and absolute electrode potential. As dedicated analysis of the electrodes exposed to liquid electrolyte is hampered by side reactions (i.e. solid-electrolyte interphase formation), we will manufacture model all-solid-state devices, allowing in-situ investigation of electrodes under UHV conditions with XPS/UPS. Further characterization involves techniques such as HRTEM/HRSEM and XRD, as well as standard electrical and electrochemical measurements. The focus of the project will be on the properties of sodium cobaltate (NaxCoO2) and related materials, also in comparison with their lithium analogues. Cobaltates belong to the family of layered oxides, which are very successful in lithium-based ion batteries and show also promising properties for application in sodium-based ion batteries. Starting with sodium cobaltate at the beginning of the project, we will extent our investigation to doped (e.g. lithium, nickel, manganese) sodium cobaltate with higher stability and/or higher potential.