Ambient sodium-ion batteries (SIBs) have attracted great attention for large-scale energy storage system due to the concern of the availability and increasing price of lithium resources.^[1] Among all the reported Na cathode materials up to date, manganese-based sodium layered transition metal (TM) oxides, such as P2-type Na_2/3TM_xMn_1-xO₂ (TM = Ti, V, Cr, Fe, Co, Ni, and mixture of 2 or 3 elements), deliver high reversible capacities and relatively high volumetric density from their compact structural framework.^[2] However, the cycle life of the currently developed P2-Na_2/3TM_xMn_1-xO₂ is far from the demand of practical application. Current research has shown that Mg doping in layered P2-Na_0.67MnO₂ is promising in stabilizing the structure and enhancing the electrochemical properties in terms of cycle life and rate performance, yet further enhancement is still necessary.^[3] In addition, surface modification has been demonstrated to be effective in protecting the integrity of electrode materials during cycling in lithium layered cathode materials and prolonging the cycling stability; however, its influence is not intensively studied in sodium counterparts.^[4] Herein, we synthesized TiO₂ coated and Mg doped P2-type Na_2/3Mn_0.65Ni_0.2Co_0.15O₂ electrode materials and investigated their electrochemical performance and sodium storage mechanism. The preliminary results suggest the cycling stability has been improved by Mg doping. We expect that the sustainable cycling behaviors would be further improved due to the synergistic effects from the composition manipulation of Mg-doping and surface modification of P2-type Na_2/3Mn_0.65Ni_0.2Co_0.15O₂ in SIBs.

Acknowledgement: The study was supported by NSFC (21621091).

References:

[1] a）H. Yu, Y. Ren, D. Xiao, S. Guo, Y. Zhu, Y. Qian, L. Gu, H. Zhou, Angew. Chem. Int. Ed., 2014, 53, 8963 ; b）N. Yabuuchi, M. Kajiyama, J. Iwatate, H. Nishikawa, S. Hitomi, R. Okuyama, R. Usui, Y. Yamada, S. Komaba, Nat. Mater., 2012, 11, 512; c）H. Pan, Y.-S. Hu, L. Chen, Energy Environ. Sci., 2013, 6, 2338.

[2] R. J. Clément, P. G. Bruce, C. P. Grey, J.Electrochem. Soc., 2015, 162, A2589.

[3] a）J. Billaud, G. Singh, A. R. Armstrong, E. Gonzalo, V. Roddatis, M. Armand, T. Rojo, P. G. Bruce, Energy Environ. Sci., 2014, 7, 1387; b）R. J. Clement, J. Billaud, A. Robert Armstrong, G. Singh, T. Rojo, P. G. Bruce, C. P. Grey, Energy Environ. Sci., 2016, 9, 3240.

[4] a）Y. Chen, Y. Zhang, B. Chen, Z. Wang, C. Lu, J. Power Sources, 2014, 256, 20; b）D. J. Comstock, J. W. Elam, J. Phys. Chem. C, 2012, 117, 1677; c）B. Song, M. O. Lai, Z. Liu, H. Liu, L. Lu, J. Mater. Chem.A, 2013, 1, 9954; d）S. Kalluri, M. Yoon, M. Jo, H. K. Liu, S. X. Dou, J. Cho, Z. Guo, Adv. Mater., 2017, n/a; e）Y. Liu, X. Fang, A. Zhang, C. Shen, Q. Liu, H. A. Enaya, C. Zhou, Nano Energy, 2016, 27, 27; f）K. Kaliyappan, J. Liu, B. Xiao, A. Lushington, R. Li, T.-K. Sham, X. Sun, Adv. Funct. Mater., 2017, n/a.