Min Wan, Wuxing Zhang*, Yunhui Huang*

Introduction

In recent years, sodium ion batteries (SIBs) have received increasing research attentions as alternative for lithium ion batteries in the application of large-scale electric energy storage (EES) ^1-4. SIBs have the advantages of high sodium abundance and low cost, and plenty of materials are investigated for sodium ion storage, such as layered oxides and tunneled oxides ^5-10. Except these, Prussian blue and its derivatives (PBAs) with the merits of high specific capacity, facile synthesis and low cost, are also identified as promising electrode materials for SIBs ^11-17.

PBAs usually have a face-centered cubic structure: A_xM₁[M₂(CN)₆]_y□_1-y•nH₂O (0<x<2, 0<y<1), in which A represents alkali cations, M transition metal ions, and □the vacancies of [M₂(CN)₆] occupied by coordinated H₂O¹⁸.The 3D open framework of PBAs can provide large interstitial sites for fast Na⁺ insertion/extraction. Among these PBAs, sodium iron hexacyanoferrate (A=Na, M₁=M₂=Fe, Fe-HCF) has two electrochemically active sites, which leads to a high theoretical specific capacity of 170 mAh g^{-1 19}. However, Fe-HCF always suffers from low coulombic efficiency and poor cycling stability due to the lattice vacancies in the crystal structure and the side reactions between Fe-HCF and electrolyte ^20-22.

In order to solve these drawbacks of Fe-HCF, it is desirable to synthesize high-quality Fe-HCF with few lattice vacancies via a single iron source method. We propose a facile co-precipitation method to synthesize core-shell structured Fe-HCF@Ni-HCF with outstanding electrochemical performance as cathode material for SIBs. The high-quality Fe-HCF@Ni-HCF integrates the advantages of both Fe-HCF and Ni-HCF, which exhibits high specific capacity, excellent cycling stability and promoted rate performance. It retains 78 % capacity at 200 mA g^-1 after 800 cycles and delivers an average coulombic efficiency of approximately 100 % during cycling. The result further proves that Fe-HCF is a promising cathode material in the practical application of SIBs.

Reference

1. Armand, M.; Tarascon, J. M. Nature 2008, 451, (7179), 652-657.
2. Dunn, B.; Kamath, H.; Tarascon, J.-M. Science 2011, 334, (6058), 928-935.
3. Pan, H.; Hu, Y.-S.; Chen, L. Energ Environ Sci 2013, 6, (8), 2338.
4. Choi, J. W.; Aurbach, D. Nature Reviews Materials 2016, 1, (4), 16013.
5. Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-González, J.; Rojo, T. Energ Environ Sci 2012, 5, (3), 5884.
6. Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S. Nat Mater 2012, 11, (6), 512-7.
7. Cao, Y.; Xiao, L.; Wang, W.; Choi, D.; Nie, Z.; Yu, J.; Saraf, L. V.; Yang, Z.; Liu, J. Adv Mater 2011, 23, (28), 3155-60.
8. Park, Y. U.; Seo, D. H.; Kwon, H. S.; Kim, B.; Kim, J.; Kim, H.; Kim, I.; Yoo, H. I.; Kang, K. J Am Chem Soc 2013, 135, (37), 13870-8.
9. Saravanan, K.; Mason, C. W.; Rudola, A.; Wong, K. H.; Balaya, P. Advanced Energy Materials 2013, 3, (4), 444-450.
10. Yuan, D.; He, W.; Pei, F.; Wu, F.; Wu, Y.; Qian, J.; Cao, Y.; Ai, X.; Yang, H. Journal of Materials Chemistry A 2013, 1, (12), 3895.
11. Wessells, C. D.; Huggins, R. A.; Cui, Y. Nat Commun 2011, 2.
12. Lu, Y.; Wang, L.; Cheng, J.; Goodenough, J. B. Chemical Communications 2012, 48, (52), 6544-6546.
13. Nie, P.; Shen, L.; Luo, H.; Ding, B.; Xu, G.; Wang, J.; Zhang, X. Journal of Materials Chemistry A 2014, 2, (16), 5852-5857.
14. Pasta, M.; Wessells, C. D.; Liu, N.; Nelson, J.; McDowell, M. T.; Huggins, R. A.; Toney, M. F.; Cui, Y. Nat Commun 2014, 5.
15. Wessells, C. D.; Peddada, S. V.; Huggins, R. A.; Cui, Y. Nano Lett 2011, 11, (12), 5421-5425.
16. Wessells, C. D.; McDowell, M. T.; Peddada, S. V.; Pasta, M.; Huggins, R. A.; Cui, Y. Acs Nano 2012, 6, (2), 1688-1694.
17. Shen, L.; Wang, Z.; Chen, L. Chemistry-a European Journal 2014, 20, (39), 12559-12562.
18. Fang, C.; Huang, Y.; Zhang, W.; Han, J.; Deng, Z.; Cao, Y.; Yang, H. Advanced Energy Materials 2015, n/a-n/a.
19. Wu, X.; Luo, Y.; Sun, M.; Qian, J.; Cao, Y.; Ai, X.; Yang, H. Nano Energy 2015, 13, 117-123.
20. Asakura, D.; Li, C. H.; Mizuno, Y.; Okubo, M.; Zhou, H. S.; Talham, D. R. Journal of the American Chemical Society 2013, 135, (7), 2793-2799.
21. Yang, D.; Xu, J.; Liao, X.-Z.; Wang, H.; He, Y.-S.; Ma, Z.-F. Chemical Communications 2015, 51, (38), 8181-8184.
22. Tang, Y.; Zhang, W.; Xue, L.; Ding, X.; Wang, T.; Liu, X.; Liu, J.; Li, X.; Huang, Y. J. Mater. Chem. A 2016, 4, (16), 6036-6041.