Oxygen vacancy-mediated amorphous GeOx assisted polysulfide redox kinetics for room-temperature sodium-sulfur batteries.

The practical applications of room-temperature sodium-sulfur (RT Na-S) batteries have been greatly hindered by the natural sluggish reaction kinetics of sulfur and the shuttle effect of sodium polysulfide (NaPSs). Herein, oxygen vacancy (OV)-mediated amorphous GeOx/nitrogen doped carbon (donated as GeOx/NC) composites were well designed as sulfur hosts for RT Na-S batteries. Experimental and density functional theory studies show that the introduction of oxygen vacancies on GeOx/NC can effectively immobilize polysulfides and accelerate the redox kinetics of polysulfides. Meanwhile, the micro-and mesoporous framework, acting as a reactor for storing active S, is conducive to alleviating the expansion of S during the charging/discharging process. Consequently, the S@GeOx/NC cathode affords a reversible capacity of 1017 mA h g-1 at 0.1 A g-1 after 100 cycles, outstanding rate capability of 333 mA h g-1 at 10.0 A g-1 and long lifespan cyclability of 385 mAh g-1 at 1 A g-1 after 1200 cycles. This work furnishes a new way for the rational design of metal oxides with oxygen vacancies and boosts the application for RT Na-S batteries.

Nanostructure Engineering Strategies of Cathode Materials for Room-Temperature Na-S Batteries.

Room-temperature sodium-sulfur (RT Na-S) batteries are considered to be a competitive electrochemical energy storage system, due to their advantages in abundant natural reserves, inexpensive materials, and superb theoretical energy density. Nevertheless, RT Na-S batteries suffer from a series of critical challenges, especially on the S cathode side, including the insulating nature of S and its discharge products, volumetric fluctuation of S species during the (de)sodiation process, shuttle effect of soluble sodium polysulfides, and sluggish conversion kinetics. Recent studies have shown that nanostructural designs of S-based materials can greatly contribute to alleviating the aforementioned issues via their unique physicochemical properties and architectural features. In this review, we review frontier advancements in nanostructure engineering strategies of S-based cathode materials for RT Na-S batteries in the past decade. Our emphasis is focused on delicate and highly efficient design strategies of material nanostructures as well as interactions of component-structure-property at a nanosize level. We also present our prospects toward further functional engineering and applications of nanostructured S-based materials in RT Na-S batteries and point out some potential developmental directions.

Immobilization of mercury in tailings originating from the historical artisanal and small-scale gold mining using sodium polysulfide.

A series of sodium polysulfides (SPSs) with different sulfur indexes was prepared as stabilizers to amend elemental mercury-contaminated artisanal small-scale gold mine (ASGM) tailings in Hubei, China, by controlling the molar ratio of sulfur and sodium sulfides as 1:1, 2:1, 3:1, and 4:1 during the synthesis. XRD, XPS, and laser Raman spectroscopy all suggested that the synthesized SPSs were a mixture of multiple polysulfides, sulfur, sodium sulfides, and sodium thiosulfate. Based on toxicity characteristic leaching procedure test (TCLP), mercury stabilization efficiency of SPSs was evaluated and proved to be more superior than sulfur, sodium sulfide, and also calcium polysulfide, with an optimal stabilization efficiency of 97.16% at SPS/THg = 1:2, SPSs pH = initial pH, and liquid-to-solid ratio = 20:7. A pseudo-second-order kinetic model was able to interpret the stabilization kinetics and demonstrated that mercury stabilization rate increased with the sulfur index in the SPSs, but excess SPSs were potentially to inhibit the precipitation of mercury. Speciation analysis results determined with sequential extraction indicated that the unstable mercury, elemental mercury, and organic-bound mercury fractions decreased respectively by up to 88.6%, 53.5%, and 26.3%. Pearson correlation analysis showed that the mercury stabilization in the mine tailings amended with SPSs mainly occurs from the precipitation of the elemental mercury, and the organic mercury fraction reduction was correlated with the decrease of the unstable mercury.

Facile Stabilization of the Sodium Metal Anode with Additives: Unexpected Key Role of Sodium Polysulfide and Adverse Effect of Sodium Nitrate.

Sodium metal is an attractive anode for next-generation energy storage systems owing to its high specific capacity, low cost, and high abundance. Nevertheless, uncontrolled Na dendrite growth caused by the formation of unstable solid electrolyte interphase (SEI) leads to poor cycling performance and severe safety concerns. Sodium polysulfide (Na2 S6 ) alone is revealed to serve as a positive additive or pre-passivation agent in ether electrolyte to improve the long-term stability and reversibility of the Na anode, while Na2 S6 -NaNO3 as co-additive has an adverse effect, contrary to the prior findings in the lithium anode system. A superior cycling behavior of Na anode is first demonstrated at a current density up to 10 mA cm-2 and a capacity up to 5 mAh cm-2 over 100 cycles. As a proof of concept, a high-capacity Na-S battery was prepared by pre-passivating the Na anode with Na2 S6 . This study gives insights into understanding the differences between Li and Na systems.

Ambient temperature sodium-sulfur batteries.

Ambient- or room-temperature sodium-sulfur batteries (RT Na-S) are gaining much attention as a low-cost option for large-scale electrical energy storage applications. However, their adoption is hampered by severe challenges. This concept paper summarizes first the operating principles, history, recent progress, and challenges of RT Na-S battery technology, and then suggests future directions towards enhancing performance in order for it to be a viable technology.

Highly Reversible Room-Temperature Sulfur/Long-Chain Sodium Polysulfide Batteries.

In a room-temperature sodium-sulfur (RT Na-S) battery, the complicated reduction reaction of the sulfur cathode generally involves two main steps: (i) transformation of elemental sulfur into long-chain soluble sodium polysulfides (Na2Sn 4 ≤ n ≤ 8) and (ii) conversion of the long-chain sodium polysulfides into solid-state short-chain polysulfide Na2S2 or disulfide Na2S. It is found that the slow kinetics of the second step limits the efficiency of discharge and induces irreversible capacity loss during cycling. Accordingly, we present here a RT Na-S cell operated with the sulfur/long-chain sodium polysulfide redox couple to avoid the capacity fade. An advanced cathode structure has been developed by inserting a carbon nanofoam interlayer between the sulfur cathode and the separator to localize the soluble polysulfide species and prevent its migration to the anode. The highly reversible sulfur/long-chain sodium polysulfide cell presented here can provide a stable output energy density of 450 Wh kg(-1) at an extremely low energy cost of ∼$10 kWh(-1) (based on the active material of anode and cathode).