Abstract
Sodium ion batteries, as an important alternative to lithium ion batteries, and a new kind of energy powering device, have attracted intensive attention because of the high natural occurrences of sodium on the earth and also its similar chemical nature to lithium.[1] Sodium ion has larger radius than lithium ion, nevertheless, which would immediately affect Na+ ion insertion into active materials, resulting in a poor battery performance.[2] Therefore, it has become an urgent task to find novel materials with outstanding sodium storage properties. The novel application of BiOI, as a new anode material for sodium ion batteries, is reported here for the first time. BiOI microspheres were directly synthesized by a simple solution method, which is promising for large-scale industrial production. The Na-storage mechanism was explored in details by ex-situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), which are powerful tools for investigating electrochemical reaction mechanisms. Our results showed that the layered BiOI underwent irreversible structure changes in charge-discharge processes. The final discharged product was detected to be a sodium bismuth alloy compound, while the charged product was not BiOI instead of the Bi metal. It acts as the active material taking part in the following electrochemical reactions, especifically the alloying-dealloying reactions. Furthermore, we also found a facile way through coating BiOI with rGO to improve the cycling performance of this novel anode material. The detailed understanding and the modifying methods might shed lights on investigating and applying other kinds of materials toward a broader range of both energy and other areas. [1] C. Chen, Y. Wen, X. Hu, X. Ji, M. Yan, L. Mai, P. Hu, B. Shan, Y. Huang, Na(+) intercalation pseudocapacitance in graphene-coupled titanium oxide enabling ultra-fast sodium storage and long-term cycling, Nature Communications, 6 (2015) 6929. [2] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Sodium-Ion Batteries, Advanced Functional Materials, 23 (2013) 947-958. Figure 1
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