The identification of energy storage systems alternatives to lithium-based technologies is now seen as a valid step for the development of economically sustainable secondary batteries.^[1] Among these, sodium-based batteries appear as very promising candidates due to the low cost and high abundance of sodium. In addition, they hold promises for accelerated development thanks to the knowledge gained in the lithium-ion battery field.^[2-4]

Herein, we present recent advances in positive electrode materials for sodium-ion batteries, giving an overview of the promising properties of this kind of technology.

Sodium-based layered oxides represent an attractive class of compounds for application in sodium-ion batteries. Among them, it has been widely reported that the P-type materials offer better performance in terms of electrochemical behavior respect to the their O3, and among the P-type structures the superior performance of P2-type respect to P3 ones has been demonstrated.^[4-6]

Low cost and non-toxic element-based oxides, such as Na_xMnO₂, have attracted large attention in view of their environmentally friendly characteristics and high specific capacity. However, due to the presence of Jahn-Teller active Mn³⁺, the P2-type structure undergoes several phase transition during the (de)-sodiation process. Moreover, Mn-based oxides show a relatively low operating voltage based on the redox couple Mn^3+/4+, affecting the practical implementation of these materials. Partial substitution of manganese with magnesium, titanium or iron may increase the operating voltage and improve the structural stability.^[7]

Following the pioneering work of Komaba’s group on P2-Na_xMn_0.5Fe_0.5O₂ that demonstrated the activity of the Fe^3+/4+ redox couple in sodium cells, layered oxides Na_x(Mn-Fe)O₂ based on low-cost and non-toxic Mn and Fe have received considerable interest.^[8]However, poor capacity retention upon cycling represents the major issue to be addressed.

We will show that, nickel substitution is a successful strategy to address this issue. Herein we report on the study of different P2-(Ni-Fe-Mn)O₂ oxides which show improved electrochemical response in terms of capacity retention and operating average voltage.^[6,9]

The results presented, deal with a comprehensive evaluation of critical parameter affecting structural and electrochemical properties of the oxide compounds. The influence of the transition metal doping elements into the oxide compounds has been investigated, pointing out exceptional differences within the structural properties, including phase transition upon cycling and electrochemical behavior. ^[6,10]

The P2-(Ni-Fe-Mn)O₂ systems reveal promising performance, however, the structural transition at high voltage remains unexplained to date. Since the Na-(Ni-Mn)O₂ system is considered a promising candidate due to the low cost of manganese and the high redox potential of the Ni^2+/4+couple, herein we also report recent studies performed on the substitution of Fe with unexplored elements characterized by low atomic mass and low cost.

The suitability of the developed materials for sodium battery application has been finally demonstrated by cycling tests revealing the excellent electrochemical performance of lab-scale full Na-ion cells.

[1] D. Larcher, J.-M. Tarascon, Nat. Chem., 7, 19 (2015) ;

[2] V. Palomares, M. Casas-Cabanas, E. Castillo-Martınez, Man H. Han, T. Rojo, Energy Environ. Sci., 6, 2312 (2013);

[3] S.-W. Kim, D.-H. Seo, X. Ma, G. Ceder, K. Kang, Adv. Energy Mater., 2, 710 (2012);

[4] K. Kubota, N. Yabuuchi, H. Yoshida, M. Dahbi, S. Komaba, MRS Bulletin, 39,416 (2014)

[5] C. Delmas, C. Fouassier, P. Hagenmuller, Physica, 99, 81 (1980);

[6] I. Hasa, D. Buchholz, S. Passerini, J. Hassoun, ACS Appl. Mater. Interfaces, 7, 5206 (2015); 

[7] R.J. Clèment, P.G. Bruce, C.P. Grey, J. Electrochem. Soc., 162, A2589 (2015);

[8] N. Yabuuchi, M. Kajiyama, J. Iwatate, H. Nishikawa, S. Hitomi, R. Okuyama, R. Usui, Y. Yamada, S. Komaba, Nat. Mat., 11, 512 (2012);

[9] I. Hasa, D. Buchholz, S. Passerini, B. Scrosati, J. Hassoun, Adv. Energy Mater. 2014, doi:10.1002/aenm.201400083;

[10] E. Talaie, V. Duffort, H.L. Smith, B. Fultz, L.F. Nazar, Energy Environ. Sci., 8, 2512 (2015);