The feasibility of reversible electrochemical Na-alloying in amorphous silicon (a-Si), along with influences of transport limitations of Na, dimensional aspects of a-Si and usage of few layers graphene (FLG) as interlayer (between a-Si and current collector) on Na-capacities and cyclic stabilities, have been demonstrated here with the use of continuous film electrodes (sans binder/additive). Systematic variations of a-Si film thicknesses have indicated that electrochemical Na-alloying, even though feasible, is 'transport limited', especially for a-Si dimensions beyond ∼100 nm. Nevertheless, decent performances, such as initial reversible Na-capacities of ∼340 mAh g −1 , with ∼120 mAh g −1 retained after 100 cycles, could be achieved upon reduction of film thickness to 50 nm. Analytical computation studies indicate that overall diffusivity of Na in such a-Si electrodes may be of the order of ∼10 −19 m 2 s −1 . In the presence of FLG interlayer (∼7 well-ordered continuous graphene layers), 'transport limitation' related issues got suppressed even for 250 nm thick a-Si film; viz., leading to considerably enhanced Na-capacities and improved cyclic stabilities. Accordingly, reversible Na-capacities recorded with the 250 and 50 nm a-Si films at the end of 100 cycles were ∼120 and ∼240 mAh g −1 ; which are possibly the best reported to-date for Si-based electrode materials. Due to limited lithium reserves in the world as compared to the more widespread and abundant reserves of sodium, there has been recent surge of interests toward the development of Na-ion batteries (SIBs), possibly in lieu of the Li-ion technology.1,2 However, one of the major issues associated with the Na-ion system is that graphitic carbon, the commonly used anode material for Li-ion batteries (LIBs), possesses reversible Na-capacity of just ∼35 mAh g −1 .1, [3][4][5][6][7][8][9] This is an order of magnitude lower compared to the corresponding Li-capacity; 3,10-12 and is caused by the larger size of the Na-ion. Accordingly, metallic anode materials, such as Ge, Sb and Sn, have been investigated for SIBs, but without much success in terms of cyclic stabilities due to dimensional changes (and detrimental stress developments) upon Na-alloying/dealloying. [3][4][5][6] It is a bit surprising that silicon, which possesses possibly the highest theoretical capacity for Li and extensively investigated for LIBs, was hitherto believed to be 'inactive' toward sodiation via electrochemical routes.6-10,13 Kulish et al., 8 via first principles, showed that sodiation of bulk Si is unfavorable, with Na binding energies being 0.6 eV, along with larger diffusion barrier of 1.06 eV. By contrast, the available Na-Si phase diagram 14 indicates that Na-alloying in Si is possible (up to Na 1 Si phase); with more recent calculations predicting that one Si atom can host at most 0.76 Na (i.e., up to Na 0.76 Si; corresponding to specific capacity of ∼725 mAh g −1 ). 11 Such predicted Na-capacity, even though significantly lesser compared to the Li-capacity, would still be co...
