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

• Applicants: Dr. René Hausbrand; Dr. Philipp Komissinskiy
• Subject Area: Synthesis and Properties of Functional Materials
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 263335462

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.

• DFG Programme: Research Grants
• Participating Persons: Professor Dr. Lambert Alff; Professor Dr. Wolfram Jaegermann