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

Abstract

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...

Recent Progress on the Alloy‐Based Anode for Sodium‐Ion Batteries and Potassium‐Ion Batteries

“…On the other side, amorphous Si can alloy with sodium to absorb 0.76 Na per Si with much lower energy barrier via the theoretical prediction, corresponding to capacity of 725 mAh g −1 accompanied by a volume expansion of 114%. [ 171 ] In addition, Si nanoparticles containing a large ratio of amorphous state [ 168 ] is demonstrated a reversible capacity of 279 mAh g −1 at 10 mA g −1 and a capacity retention of 248 mAh g −1 after 100 cycles at 20 mA g −1 .Si nanoparticles containing a large ratio of amorphous Si can take place reversible Na ion uptake. [ 168 ] So a well‐defined bamboo‐rattle‐like structure [ 167 ] is fabricated that yolk–shell carbon/silicon nanobeads embedded in the carbon nanofibers, which can retain 454.5 and 190 mAh g −1 after 200 and 2000 cycles at 0.05 and 5 A g −1 ( Figure c).…”

“…On the other side, amorphous Si can alloy with sodium to absorb 0.76 Na per Si with much lower energy barrier via the theoretical prediction, corresponding to capacity of 725 mAh g −1 accompanied by a volume expansion of 114%. [171] In addition, Si nanoparticles containing a large ratio of amorphous state [168] is demonstrated a reversible capacity of 279 mAh g −1 at 10 mA g −1 and a capacity retention of 248 mAh g −1 after 100 cycles at 20 mA g −1 .Si [160] Copyright 2018, Wiley-VCH. (d) Reproduced with permission.…”

Si and Ge‐Based Anode Materials for Li‐, Na‐, and K‐Ion Batteries: A Perspective from Structure to Electrochemical Mechanism

“…a‐Si nanomembranes with enhanced Na + transport kinetics is able to store Na through a pseudocapacitive/bulk mechanism . A silicon/graphene and silicon graphite composite alleviates the Na transport limitation issues and improves the electronic conductivity . Ge nanocolumns improve Na kinetics due to their short diffusion length and 430 mAh g −1 are obtained with a capacity retention of 88% after 100 cycles .…”

Stabilizing Tin Anodes in Sodium-Ion Batteries by Alloying with Silicon

“…Reported specific capacities are usually around 250 mAh/g, even for very small (~10 nm diameter) nanoparticles, 26 which points to the slow diffusion of Na in Si. 27,28 Thin films of silicon with few-layer graphene at the interface with the current collector have been shown to increase reversible capacity by almost a factor of 2, suggesting that slow solidstate diffusion is not solely responsible for the low practical capacities. 27 Although Sn promises the second highest specific capacity among Group IV elements, it suffers from large volumetric expansion (~420%) that leads to severe pulverization, which further disrupts the electrode integrity and results in rapid deterioration in cycling capacity.…”

“…27,28 Thin films of silicon with few-layer graphene at the interface with the current collector have been shown to increase reversible capacity by almost a factor of 2, suggesting that slow solidstate diffusion is not solely responsible for the low practical capacities. 27 Although Sn promises the second highest specific capacity among Group IV elements, it suffers from large volumetric expansion (~420%) that leads to severe pulverization, which further disrupts the electrode integrity and results in rapid deterioration in cycling capacity. 29,30 Nanostructuring the Sn to alleviate these problems related to volume expansion effect has been undertaken, but at the expense of the associated Coulombic efficiency.…”