Large-scale electrical energy storage systems are one of the core technologies in renewable energies and smart grid, among which sodium-ion batteries show great promise due to the abundant sodium resources. Searching for suitable electrode materials to satisfy the long-term stability requirement is an important step to realize the practical sodium-ion batteries. Recently, many layered Na_xMO₂ (M: 3d transition metals) oxides have been proposed as positive electrode materials for sodium-ion batteries. Layered metal oxides have attracted widespread attention as cathodes for sodium-ion batteries because of easy synthesis, highly reversible Na deintercalation/intercalation and high energy density. However, the practical applications have been hindered by two major challenges. Unlike LiMO₂, almost all the Na_xMO₂ are not stable against moisture (either they can be oxidized by water/oxygen/carbon dioxide or water molecular can be intercalated into alkali metal layer). This will not only increase the substantial cost for material storage, transportation and battery production but also lead to the inconsistency of the battery performance. Therefore, it is very important to explore air-stable layered oxides in the viewpoint of practical applications. Furthermore, although significant progress has been made in new layered metal oxides with abundant elements such as Fe and Mn to form Na_xFe_1-yMn_yO₂, their Na storage performance is not satisfactory. It is necessary to incorporate toxic Ni or Co into transition metal layer to achieve better performance. However, as noted above, Ni or Co is widely used in lithium-ion batteries, particularly, with the growing market of electric vehicles, the cost of Ni or Co will definitely increase. Therefore, this is not a good choice for sodium-ion batteries and it is essential to explore Ni/Co-free layered oxides with superior performance.

Here we report a new Co/Ni-free layered oxide, O3-Na_0.9[Cu_0.22Fe_0.30Mn_0.48]O₂, showing unexpectedly superior stability against moisture and excellent electrochemical cycling stability. A prototype sodium-ion battery using this cathode and hard carbon anode is demonstrated to have an energy density of 210 Wh/kg and superior rate capability and negligible capacity fade. These desirable characteristics meet the requirements for large-scale electrical energy storage, thus paving the way of developing low-cost and high-energy sodium-ion batteries for practical applications. On the basis of full cell system, we have developed the first 2 Ah practical soft package sodium-ion battery for room-temperature energy storage application in IOP-CAS. We will present the results in this talk.

Acknowledgement.  This work was supported by funding from the NSFC (51222210, 11234013) and the One Hundred Talent Project of the Chinese Academy of Sciences.

References

(1)   Mu, L. Q.; Xu, S.Y.; Li, Y.M.; Hu, Y. -S.; Li, H.;Chen, L.Q.; Huang X.J. Adv. Mater. DOI: 10.1002/adma.201502449

(2)   Pan, H. L.; Hu, Y. S.; Chen, L. Q. Energy Environ. Sci. 2013, 6, 2338-2360.

(3)   Xu, S.Y.; Wu, X.Y.; Li, Y.M.; Hu, Y. -S.; Chen, L.Q. Chin. Phys. B 2014, 23,118202.

(4)   Mu, L. Q.; Hu, Y. -S.; Chen, L.Q. Chin. Phys. B 2015, 3, 038203.

(5)   Hu, Y. -S.; et al. Several Chinese patents have been filed.

(6)   Li, Y. M.; Xu, S. Y.; Wu, X. Y.; Yu, J. Z.;  Wang, Y. S.; Hu, Y.-S.;  Li, H.; Chen, L. Q.; Huang, X. J. J. Mater. Chem. A 2015, 3, 71-77.