P-Type Layered Transition Metal Oxides As Cathode Materials for Sodium-Ion Batteries
Authors: Buchholz Daniel , Chagas Luciana Gomes , Hasa Ivana , Hassoun Jusef , Vaalma Christoph , Passerini Stefano 
Published: February 2020

Currently, sodium-ion batteries face a strong increase of the scientific and commercial interest as they represent a relatively young and unexploited research field and a possible low cost alternative towards the lithium-ion technology as well.
Sodium based raw materials are abundant and easily available, thus resulting in cheaper mining abilities. In addition, the low cost advantage is not only due to the lower price of sodium but rather more interconnected with the feasible use of aluminium as anode current collector since sodium is not alloying with it. The avoidance of copper thus results not only in a cheaper but also in a lighter battery with increased energy density. [1-3]
Concerning positive electrode materials, polyanionic frameworks and layered oxides have been largely investigated and now it is commonly accepted that they are suitable cathode materials for sodium-ion batteries. Generally, superior electrochemical performances compared to other secondary battery systems, except for the lithium-ion technology, have been demonstrated for sodium based systems.[1,4-9]
One major advantage compared to polyanionic compounds surely is the lower molecular weight of layered oxides, finally resulting in improved specific capacities. On the other hand, layered materials often suffer of large volume changes upon the whole (de-)sodiation process leading to a fading electrochemical performance. Consequently, the composition of the transition metal layer in these oxides is crucial to ensure a good electrochemical performance and a stable long term cycling behavior.
Within the class of layered transition metal based oxides, materials with an O- and P-type structure are known, whereas the latter ones exhibit a smaller sodium reservoir but reveal an improved reversibility of the electrochemical redox process at higher potentials, finally leading to an improved electrochemical performance.
In the past we have developed the synthesis of layered NaxMO2(M= transition metal) materials by firing a mixture of a transition metal hydroxide precursor, obtained via co-precipitation method, and sodium hydroxide in a simple two step solid state annealing process in air, followed by a water treatment. [7, 8, 9]
Herein, we report the synthesis and structural characterization of different novel layered oxides of the type NaxMO2 with P-type structure, synthesized at various annealing temperatures.
Furthermore, we demonstrate the influence of structure and the type and ratio of the employed transition metals on the electrochemical performance of these materials.

References

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[5] N. Yabuuchi et al. Nature Mater. 11 (2012) 512
[6] D. Kim et al. Electrochem. Commun. 18 (2012) 66
[7] D. Buchholz et al. Chem. Mater. 25 (2013) 142
[8] D. Buchholz et al. Electrochim. Acta (2013), doi.org/10.1016/j.electacta.2013.02.109
[9] D. Buchholz et al. Tailoring the Electrochemical Performance of P2- Na0.45Ni0.22Co0.11Mn0.66O2 as High Voltage or High Capacity Cathode Material for Sodium-Ion Batteries, 224th ECS Meeting San Francisco, USA, October 2013