Metal Sulfide Nanofiber Anodes for High Capacity Sodium Rechargeable Batteries
Authors: Ryu Won-Hee , Taylor André D. 
Published: February 2020

A considerable amount of effort has been devoted to developing innovative and economical energy storage systems, including methods that are optimized for application to large-scale energy storage systems such as smart grids and electric vehicles. Sodium ion batteries have been spotlighted recently owing to lower material costs, the abundance of sodium resources, and analogous electrochemical components with existing Li-ion battery systems. However, unlike Li-ion batteries, current studies on Na-ion batteries have often confronted problems related to the insufficient Na storage capacities of electrode materials, especially anode materials, originating from the larger ionic radius of Na compared with Li (e.g. Li+: 0.76 Å, Na+: 1.02 Å). Therefore, the development of high capacity anode materials that enable facile insertion/extraction of large Na ions is essential for high performance Na-ion batteries.
In recent years, metal sulfides have been considered as functional electrode materials for diverse applications such as electronic devices, energy conversion and storage systems due to their unique electronic properties and structural characteristics, compared to metal oxides. Metal disulfides (MoS2 and WS2) have also shown their possibility as a high capacity anode material (>700 mAh/g) because of (i) the large slab space between their 2-D planes, (ii) their reversible conversion reaction characteristics (MS2 + 4Na+ + 4e- → 2 Na2S + M, M = Mo, W), and (iii) the higher conductivity of Na2S products (metal sulfides) compared to Na2O (metal oxides). Similarly, amorphous metal trisulfides (MS3, M = Mo, W) could be considered as high capacity anode materials due to the structural flexibility of their glassy phase and the excess sulfur as an active component (MS3 + 6Na+ + 6e- → 3Na2S + M). Moreover, the electronic conductivity of MS3 can be improved by excess sulfur in the trisulfide structure. However, Na battery performance of metal trisulfides has not been thoroughly introduced thus far. Moreover, an effective solution is required for the sulfur dissolution issue originating from soluble polysulfide intermediates during the Na2S formation/decomposition process, thereby losing sulfur components and consequently degrading capacity. To address complex concerns, designing hierarchical architecture of the metal sulfide strucutre with a functional coating layer can offer considerable improvement in sodium battery performance. Reducing the size-dimension to the nanoscale and tailoring their morphologies can also afford an increased number of reaction sites and reduced Na ion diffusion.
In this presentation, we report a tailored synthetic strategy used to create heterogeneous metal sulfide (e.g. MoS2, WSx)/oxide core-shell nanofiber materials with hierachical features, and we evaluate them as potential anode materials for high performance Na-ion batteries. The sulfide nanofibers are successfully prepared by electrospinning and subsequent calcination in a reducing atmosphere. Conformal oxide coating is applied to prevent capacity degradation of the metal sulfide anodes originating from sulfur dissolution.

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