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

钠基离子电池是未来能量存储的可能候选者，其具有优于锂离子电池的优点，例如钠的高可用性、低成本和安全性。然而，迄今为止，由于较低的电压和较低的比电极容量，钠离子电池具有比锂离子电池更低的能量密度，并且不稳定。离子电池的电压和电极的稳定性与电极的电子结构、电子化学势以及碱的插入（脱）插过程中的变化密切相关。到目前为止，钠离子插入材料的电子结构尚未得到广泛的分析，其对电极电位和降解的影响也没有得到很好的理解。在本计画中，我们提出分析钠层状氧化物阴极材料的电子结构并测定其电极电位。我们的目的是关联的电极电位，本征电压极限和阴极材料的退化现象，其电子结构。将结果与材料的锂类似物进行比较。其目标是对钠基离子电池中的电压起源以及退化现象有更基本的了解，以改善其性能。我们将制备薄膜模型碱离子电极，分析它们的电子结构作为碱含量和循环次数的函数，并将结果与结构数据和电化学性质相关联。此外，我们将制备薄膜电极/固体电解质层堆叠，并确定能带排列和绝对电极电位。由于暴露于液体电解质的电极的专门分析受到副反应（即固体电解质界面形成）的阻碍，我们将制造模型全固态设备，允许在UHV条件下使用XPS/UPS对电极进行原位研究。进一步的表征涉及HRTEM/HRSEM和XRD等技术，以及标准的电气和电化学测量。该项目的重点将是钴酸钠（NaxCoO 2）和相关材料的性质，并与其锂类似物进行比较。钴酸盐属于层状氧化物家族，其在锂基离子电池中非常成功，并且在钠基离子电池中的应用中也显示出有前途的性质。从项目开始时的钴酸钠开始，我们将研究范围扩大到具有更高稳定性和/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.