Layered sodium transition metal oxides NaxTMO2 are under ongoing investigation as cathode active materials for sodium ion batteries. Among the wide variety of transition metal compositions, sodium contents and – closely linked – different crystal structures [1]-[3], we focus on NaxFe0.1Mn0.6-yNi0.3-zMy+zO2 (M = Mg or Zn) and investigate the effects of Mg/Zn-doping on the electrochemical performance and cycle life.

We synthesised a broad variety of stoichiometric compositions following a straightforward DOE approach, taking the formal oxidation states of the transition metals into account.
The samples underwent a systematic characterization via ICP-OES, XRD and SEM. The structure and active material composition were obtained from XRD and ICP-OES results.

Isovalent substitution of Ni for Mg/Zn led to formation of a P2-structured material, which showed significantly improved capacity retention of over 90% after 100 cycles in electrochemical half-cell cycling experiments. On the other hand, variation of the Mn content allows for mixed P2/O3 and pure O3 structures with increased sodium stoichiometry and high initial discharge capacities (up to 158 mAh/g at C/20).
Generally, this substitution leads to more reversible sodium de-/intercalation by mitigation of structural phase transitions as well as suppression of detrimental sodium-vacancy orderings in the cathode active material.

We will discuss the structural concept of stabilisation and capacity control and compare the effects of Mg and Zn doping with respect to structure, performance and cycle life.
This work contributes to the understanding of the class of layered NaFMNO-cathode active materials and gives guideline to effective doping strategies for sodium ion battery cathode active materials.

ACKNOWLEDMENT: This work was supported by the German Federal Ministry of Education and Research (BMBF) in the project Transition Transfer (03XP0553A).

REFERENCES:
[1] Gonzalo, E., Zarrabeitia, M., Drewett, N. E., López del Amo, Juan Miguel & Rojo, T. Sodium manganese-rich layered oxides: Potential candidates as positive electrode for Sodium-ion batteries. Energy Storage Materials 34, 682–707; 10.1016/j.ensm.2020.10.010 (2021).
[2] Kubota, K., Kumakura, S., Yoda, Y., Kuroki, K. & Komaba, S. Electrochemistry and Solid‐State Chemistry of NaMeO 2 (Me = 3d Transition Metals). Adv. Energy Mater. 8, 1703415; 10.1002/aenm.201703415 (2018).
[3] Pfeiffer, L. F. et al. Layered P2-NaxMn3/4Ni1/4O2 Cathode Materials For Sodium-Ion Batteries: Synthesis, Electrochemistry and Influence of Ambient Storage. Front. Energy Res. 10, 305; 10.3389/fenrg.2022.910842 (2022).