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

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-liquidsolid mechanism during nanowire growth allowed systematic variations of the a-Si shell thickness around the c-Si core of constant diameter (similar to 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 similar to 40 (SiNW-40) and similar to 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 similar to 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.