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Chem Soc Rev Review Article Fig. 33 (a) First galvanostatic charge and discharge profiles of the Sn4P3 electrode (top). The ex situ HR-TEM bright-field images, enlarged HR-TEM images and corresponding FFT patters at each point. (Reproduced from ref. 460 with permission, Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.) (b) Long cycling performances of yolk–shell Sn4P3@C nanospheres at 1.5C. (Reproduced with permission from ref. 463, Copyright 2015 The Royal Society of Chemistry.) (c) Self-healing effect of the conversion reaction for the Sn4P3 anode. (Reproduced with permission from ref. 465, Copyright 2016 American Chemical Society.) View Article Online with Na per single atom produce a high specific capacity during the alloying–dealloying reactions.99 Metals (Sn, Bi), metalloids (Si, Ge, As, Sb) and polyatomic nonmetal compounds (P) in group 14 (Fig. 34) or 15 elements (Fig. 37) in the periodic table have been widely studied as potential anode materials for SIBs. However, depending on the host materials and electrochemical sodiation levels, the large Na+ ion causes huge volume changes during the alloying–dealloying reaction. This repetitive volume changes under the constraints imposed by the battery packaging give rise to complex mechanical stresses in active particles, ultimately leading to their fracture or pulverization.240 To date, through various experimental works focusing on electrochemical and mechanical responses of these alloys to Na interactions, the desirable architectures and/or enhanced electrode designs have been investigated. In this section, we specifically discussed group 14 and group 15 elements with a focus on their Na alloy reaction mechanisms and summarize various strategies for high performance sodium anode materials. 3.3.1 Alloying compounds in group 14 3.3.1.1. Silicon. In recent years, Si-based anode materials were intensively studied for LIBs due to their abundance in Earth’s crust and their high specific capacity through electrochemical alloying reactions with Li.467,468 Theoretically, silicon can uptake 4.4 Li+ ions per Si atom and deliver a high specific capacity of 4000 mA h g1. Morito et al. firstly demonstrated the phase diagram between Na and Si, which indicated the fully sodiated form of Na–Si.469 However, based on a single-atom diffusion model, Morito et al. deduced that bulk Si is not a promising anode material for Na batteries because Si can only uptake 1Na per Si atom and exhibit poor Na diffusion kinetics469 (Fig. 34a). Through the computational calculation, desirable electrode design to facilitate Na+ intercalation and migration into Si was suggested such as structural modification and control of the activation barrier.470–476 Structurally modified Si was predicted to demonstrate better electrochemical perfor- mance such as in amorphous Si due to more favorable binding between Na and Si. Reasonable activation barriers for Na+ diffusion were also predicted, where 0.4 eV was required for Na+ migration in amorphous Si.457 Based on theoretical works, Xu et al. experimentally proved reversible electrochemical Na+ ion uptake in Si for the first time475 (Fig. 35a and b). Xu et al. prepared nanoparticles containing both amorphous and Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3579 Open Access Article. Published on 28 March 2017. Downloaded on 7/1/2019 3:41:21 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.PDF Image | Sodium-ion batteries present and future
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