Crystalline core/amorphous shell structured silicon nanowires offer size and structure dependent reversible Na-storage

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Materials for energy and sustainability, 2018, Vol.6 (8), p.3422-3434
• Main Authors: Jangid, Manoj K., Lakhnot, Aniruddha S., Vemulapally, Aditya, Sonia, Farjana J., Sinha, Somika, Dusane, Rajiv O., Mukhopadhyay, Amartya
• Format: Article
• Language: English
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/C7TA10249F

One of the major bottlenecks towards the development of the Na-ion battery system is that graphitic carbon (the commonly used anode material for the Li-ion system) is not suitable for use in Na-ion system. Accordingly, in the pursuit to identify and develop a suitable anode material for the upcoming Na-ion battery system, we report here the feasibility of reversible electrochemical Na-alloying in core/shell-structured Si nanowires having crystalline (c-Si) core and amorphous (a-Si) shell. Vapor–liquid–solid mechanism during nanowire growth allowed systematic variations of the a-Si shell thickness around the c-Si core of constant diameter (∼25 nm; as per the size of Sn catalyst-cum-‘nano-template’ particles). This allowed the development of four different sets of nanowires having overall diameters varying between ∼40 (SiNW-40) and ∼460 nm (SiNW-460); thus providing platforms also for investigating the influences of the dimensional scale and structure of Si ( viz. , amorphous vs. crystalline) towards Na-storage. While negligible reversible Na-capacity could be recorded with the thickest nanowire set, significantly greater Na-capacities could be recorded upon reduction in the overall diameter; leading to a reversible Na-capacity of ∼390 mA h g −1 for the thinnest nanowire set ( i.e. , SiNW-40), which is also the highest reported to-date for ‘stand-alone’ Si-based electrodes. Shortened Na-transport distance through the a-Si shell and increased influence of the more conductive c-Si core towards the lowering of charge transfer resistance, with reduced nanowire thickness, are the causes for such a remarkable dimensional effect. Experimental evidences and analytical computational studies indicate that Na-capacity gets contributed primarily by the ‘bulk’ of the amorphous Si shell, but (interestingly) not by the crystalline Si core.