Capacity Enhancement and Discharge Mechanisms of Room‐Temperature Sodium–Sulfur Batteries

Abstract

A strategy for capacity and cyclability enhancement of roomtemperature sodium-sulfur (Na-S) batteries is reported by inserting a nanostructured, carbon-based interlayer between the sulfur cathode and the separator. The interlayer localizes the soluble polysulfide species and prevents its migration to the sodium anode. Electrochemical and spectroscopic characterizations along with thermodynamic analyses indicate that the charge/discharge of the Na-S cell involves complicated transition processes through a series of long-chain (Na 2 S n , 4 n 8) and short-chain (Na 2 S n , 1 n < 4) sodium-polysulfide intermediates. The results obtained in this work show that the cell can provide a remarkable capacity of 400 Ah kg À1 and an energy density of 720 Wh kg À1 (based on the sulfur cathode) after 20 cycles.Energy storage is one of the key aspects of global sustainability and societal welfare. Among the various available energystorage technologies, the Li-ion battery has conquered the portable electronic market for over 20 years. However, the limited capacity of traditional lithium-insertion-compound cathodes makes it difficult to meet the high energy-density demands of stationary electricity storage and next-generation electric vehicles. In this regard, lithium-sulfur (Li-S) batteries have recently received great attention as the most promising power sources for electric vehicles and grid storage, owing to the high theoretical energy density of 2600 Wh kg À1 , based on the light weight of elemental sulfur and the two-electron charge-transfer reactions.[1]However, because of the limited lithium resources on a global scale and its high cost, Na-based compounds have recently made a comeback in energy-storage technologies.[2] The use of Na instead of Li in rocking-chair-type batteries, with Na intercalation chemistry, has made interesting progress in recent years.[3] However, similar to the traditional Li-ion cathodes, capacities of the Na-insertion cathodes are too low to meet the high energy-density demands of future large-scale energy-storage systems. Na-based battery technologies, with high-capacity sulfur cathodes and high operating temperatures, have been under development for over 40 years.[4] However, the high operating temperatures and the use of b-alumina solid electrolytes have led to cost and safety concerns.To address these issues, the development of room-temperature (RT) Na-S batteries, analogous to the Li-S batteries, have been reported since 2006.[5] A few research groups have focused on the operation of Na-S batteries at ambient temperatures by utilizing Na metal as the anode, sulfur-carbon composite materials as the cathode, and electrolytes based on organic solvents or polymers.[5] A more recent study showed high utilization of sulfur as the active material in an advanced S/CNT@MPC cathode (CNT@MPC = microporous-carbon-coated carbon nanotubes).[5i] However, issues existing in the RT Na-S batteries are even more severe than those in the Li-S batteries, in terms of low utilization of active material, high capa...

Na2S–Carbon Nanotube Fabric Electrodes for Room‐Temperature Sodium–Sulfur Batteries

“…The diffusion of sodium ions would be low due to the highly localized and viscous sodium polysulfides formed around unreacted Na 2 S. This kinetic barrier accounts for the long charge time in the first charge. The first discharge, as well as the subsequent discharge, shows two voltage plateaus, which are similar to those of conventional sulfur–carbon composite electrodes4k (Figure S2 in the Supporting Information). The subsequent charge shows a complete two‐plateau charging process unlike the first charge, indicating the discharged product is much more active than the pristine Na 2 S within the MWCNT electrode.…”

“…However, operation of a Na–S battery at room temperature faces much more challenges than Li–S batteries, especially in terms of utilization of the sulfur active material and capacity fade during cycling. Recently, quite a few advances have been made towards improving the overall performances of the RT Na–S batteries, such as advanced sulfur–carbon cathode nanomaterials,4g, i unique cell configuration structures,4h, k, l smart cycling strategies,4j etc. In all of these previous studies, the RT Na–S cells were fabricated in a fully charged state with sulfur as the starting cathode material.…”

Differences in Electrochemistry between Fibrous SPAN and Fibrous S/C Cathodes Relevant to Cycle Stability and Capacity

“…Table S3 summarizes the different poly(sulfide)s formed during discharge together with the corresponding voltage at which they are formed. [35][36][37][38] Upon discharge, S/ACF in 1M LiTFSI in DME/DOL as electrolyte shows a maximum in the range of 2.3-2.4 V, which correspond to long-chain poly(sulfide)s, i.e. Li 2 S 8 and Li 2 S 4 , respectively.…”

Rational Design of Binders for Stable Li‐S and Na‐S Batteries

“…In a typical discharge process, the electrons in the external circuit are captured by sulfur cathode through a two-step reduction reaction with Li + /Na + : a higher reduction process corresponding to conversion of cyclo-S 8 to high-order polysulfides (M 2 S x , 4 ≤ x ≤ 8, M = Li, Na), and a lower reduction process involving the conversion of M 2 S x to lower-order species, which are assumed to ultimately convert to solid M 2 S ( Figure 1A). [22] The critical challenges arising from a similar cathodic reaction in Li-S and Na-S systems can be concluded into the following aspects ( Figure 1D). For Li-S batteries, the formation of a series of soluble polysulfides (Li 2 S x , 4 ≤ x ≤ 8) at the first step (≈2.3 V) and solid Li 2 S 2 /Li 2 S at the second step (≈2.1 V) achieves a theoretical capacity of 418 and 1254 mAh g −1 , respectively.…”