Compared to lithium, sodium is a highly abundant and cheap element. Although it has obvious weakness on gravimetric energy density due to the higher weight (M(Li)= 6.941 g/mol vs. M(Na)= 22.989 g/mol) (Hollemann et al.) and redox potential (the standard redox potential of sodium is 0.3 V above that of lithium), this cost benefit still makes it attractive to employ sodium ion batteries for large scale applications (Ellis und Nazar 2012). Such an application is for instance stationary energy storage for fluctuating renewable energy sources (e.g. solar energy or wind energy) (Barpanda et al. 2014) (Palomares et al. 2012). To keep the costs low, it is also necessary not to employ expensive and toxic transition metals, such as cobalt.

Layered transition metal oxides, having the formula AMO₂ (A- alkali metal, M- transition metals, O-oxygen), have already gained a lot of success in lithium-ion technology. There material are commonly  crystallize in a O3-type layered structure, such as LiCoO₂ and LiNiO₂, which means that Li⁺ ions are located between sheets of transition metal oxides and are octahedrally coordinated by oxygen anions. Sodium based materials, which structure in the similar order, may also be the proper cathode candidate for sodium ion batteries. Several layered sodium oxides with different compositions have been synthesized. A promising one is the compound Na_1.0Ni_0.5Mn_0.5O₂, offering a high theoretical capacity of around 240 mAh/g when being desodiated completely. Practically, it can deliver a reversible capacity of around 185 mAh/g in the voltage range of 2.2V – 4.5V against Na/Na⁺ (Komaba et al. 2012). Starting from the initial O3 layered structure, many phase transitions are reported to take place during desodiation in the charge process and sodiation process, leading to a complex voltage profile. However, the final structure of the compound strongly depends on the sodium content as well as on the transition metal ratio. Beside O3 layered type structures, P2 and P3 layered type structures are reported as well for varied molar ratios between Na, Mn and Ni.

In this work, Na_xNi_0.5Mn_0.5O₂ cathode material was synthesized with varying sodium content, ranging from 0.5≤ x ≤1.2. Half cells were cycled to examine the influence of sodium content on structural changes and electrochemical performance. The electrodes and active materials were examined by x-ray diffraction and scanning electron microscopy. Since best electrochemical performance as well as highest phase purity was found for a sodium content of x=0.9, this material was chosen for further investigations. In detail, in situ x-ray diffraction was conducted to observe structural changes during charging / discharging, the influence of different cut off potentials on cycling stability were tested and corresponding electrode surfaces were examined via scanning electron microscopy and x-ray photoelectron spectroscopy (XPS).

Literature

Barpanda, Prabeer; Oyama, Gosuke; Nishimura, Shin-ichi; Chung, Sai-Cheong; Yamada, Atsuo (2014): A 3.8-V earth-abundant sodium battery electrode. In: Nat Comms 5. DOI: 10.1038/ncomms5358.

Ellis, Brian L.; Nazar, Linda F. (2012): Sodium and sodium-ion energy storage batteries. In: Current Opinion in Solid State and Materials Science 16 (4), S. 168–177. DOI: 10.1016/j.cossms.2012.04.002.

Hollemann, A. F.; Wiberg, E.; Wiberg, N.: Lehrbuch der anorganischen Chemie. In: de Gruyter, zuletzt geprüft am 11.11.2014.

Komaba, Shinichi; Yabuuchi, Naoaki; Nakayama, Tetsuri; Ogata, Atsushi; Ishikawa, Toru; Nakai, Izumi (2012): Study on the Reversible Electrode Reaction of Na 1– x Ni 0.5 Mn 0.5 O 2 for a Rechargeable Sodium-Ion Battery. In: Inorg. Chem. 51 (11), S. 6211–6220. DOI: 10.1021/ic300357d.

Palomares, Verónica; Serras, Paula; Villaluenga, Irune; Hueso, Karina B.; Carretero-González, Javier; Rojo, Teófilo (2012): Na-ion batteries, recent advances and present challenges to become low cost energy storage systems. In: Energy Environ. Sci. 5 (3), S. 5884. DOI: 10.1039/c2ee02781j.