Remedies for Polysulfide Dissolution in Room-Temperature Sodium-Sulfur Batteries

Rechargeable room-temperature sodium-sulfur (RT-NaS) batteries represent one of the most attractive technologies for future stationary energy storage due to their high energy density and low cost. The S cathodes can react with Na ions via two-electron conversion reactions, thus achieving ultrahigh theoretical capacity (1672 mAh g^-1 ) and specific energy (1273 Wh kg^-1 ). Unfortunately, the sluggish reaction kinetics of the nonconductive S, severe polysulfide dissolution, and the use of metallic Na are causing enormous challenges for the development of RT-NaS batteries. Fatal polysulfide dissolution is highlighted, important studies toward polysulfide immobilization and conversion are presented, and the reported remedies in terms of intact physical confinement, strong chemical interaction, blocking layers, and optimization of electrolytes are summarized. Future research directions toward practical RT-NaS batteries are summarized.

Suppressing Polysulfide Dissolution via Cohesive Forces by Interwoven Carbon Nanofibers for High-Areal-Capacity Lithium-Sulfur Batteries

Nanostructural design renders several breakthroughs for the construction of high-performance materials and devices including energy-storage systems. Although attempts made toward electrode engineering have improved the existing drawbacks, nanoengineering is still hindered by some issues. To achieve practical applications of lithium-sulfur (Li-S) batteries, it is difficult to attain a high areal capacity with stable cycling. Physical encapsulation via nanostructural design not only can resolve the issue of lithium polysulfide dissolution during the electrochemical cycling, but also can offer significant contact resistance, which in turn can decrease the kinetics, particularly at a high sulfur loading. Thus, we demonstrate an electrospun carbon nanofiber (CNF) matrix for a sulfur cathode. This simple design enables a high mass loading of 10.5 mg cm^-2 with a high specific capacity and stable cycling. The CNF-sulfur complex can deliver a high areal capacity of greater than 7 mAh cm^-2, which is related to the excellent electrical conductivity of one-dimensional species. Moreover, we have observed that the reacted sulfur species have adhered well to the junction of the CNF network with specific wetting angles, which are induced by the cohesive force between the narrow gaps in the matrix that trapped the viscous polysulfides during cycling. The results of this study open new avenues for the design of high-areal-capacity Li-S batteries.

Improvement of Cycling Performance of Lithium-Sulfur Batteries by Using Magnesium Oxide as a Functional Additive for Trapping Lithium Polysulfide

Trapping lithium polysulfides formed in the sulfur positive electrode of lithium-sulfur batteries is one of the promising approaches to overcome the issues related to polysulfide dissolution. In this work, we demonstrate that intrinsically hydrophilic magnesium oxide (MgO) nanoparticles having surface hydroxyl groups can be used as effective additives to trap lithium polysulfides in the positive electrode. MgO nanoparticles were uniformly distributed on the surface of the active sulfur, and the addition of MgO into the sulfur electrode resulted in an increase in capacity retention of the lithium-sulfur cell compared to a cell with pristine sulfur electrode. The improvement in cycling stability was attributed to the strong chemical interactions between MgO and lithium polysulfide species, which suppressed the shuttling effect of lithium polysulfides and enhanced the utilization of the sulfur active material. To the best of our knowledge, this report is the first demonstration of MgO as an effective functional additive to trap lithium polysulfides in lithium-sulfur cells.

In Situ EQCM Study Examining Irreversible Changes the Sulfur-Carbon Cathode in Lithium-Sulfur Batteries

In situ EQCM experiments were used to investigate the stability and roughness changes occurring in a sulfur-carbon cathode utilized for a Li-S battery during the charge-discharge process. Results show that the sulfur-carbon cathode gains mass during the first discharge plateau (∼2.4 V) due to the formation of the long chain polysulfides during the discharge (lithiation) process. However, further discharge to below 2.4 V yields an increase in the crystal resistance (Rc) suggesting the sulfur-carbon cathode becomes rougher. During the charge (delithiation) process, the roughness of the sulfur-carbon cathode decreases. Time dependent measurements show that the electrode surface becomes rougher with the deeper discharge, with the change occurring following a step to 1.5 V. The sulfur-carbon cathode exhibits stable Rc and frequency behavior initially, but then becomes rougher in subsequent following cycles.

Surface modification of sulfur electrodes by chemically anchored cross-linked polymer coating for lithium-sulfur batteries

Lithium-sulfur batteries suffer from severe self-discharge due to polysulfide dissolution into electrolytes. In this work, a chemically anchored polymer-coated (CAPC) sulfur electrode was prepared, through chemical bonding by coordinated Cu ions and cross-linking, to improve cyclability for Li/S batteries. This electrode retained specific capacities greater than 665 mAh g(-1) at high current density of 3.35 A g(-1) (2C rate) after 100 cycles with an excellent Coulombic efficiency of 100%.

Ionic liquid enabled FeS2 for high-energy-density lithium-ion batteries

High-energy-density FeS2 cathodes en-abled by a bis(trifluoromethanesulfonyl)imide (TFSI-) anion-based room temperature ionic liquid (RTIL) electrolyte are demonstrated. A TFSI-based ionic liquid (IL) significantly mitigates polysulfide dissolution, and therefore the parasitic redox shuttle mechanism, that plagues sulfur-based electrode chemistries. FeS2 stabilization with a TFSI(-) -based IL results in one of the highest energy density cathodes, 542 W h kg(-1) (normalized to cathode composite mass), reported to date.