Feasibility of Reversible Electrochemical Na-Storage and Cyclic Stability of Amorphous Silicon and Silicon-Graphene Film Electrodes

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.