Catalytic oxidation of Li ₂ S on the surface of metal sulfides for Li−S batteries

Abstract

Polysulfide binding and trapping to prevent dissolution into the electrolyte by a variety of materials has been well studied in Li−S batteries. Here we discover that some of those materials can play an important role as an activation catalyst to facilitate oxidation of the discharge product, Li 2 S, back to the charge product, sulfur. Combining theoretical calculations and experimental design, we select a series of metal sulfides as a model system to identify the key parameters in determining the energy barrier for Li 2 S oxidation and polysulfide adsorption. We demonstrate that the Li 2 S decomposition energy barrier is associated with the binding between isolated Li ions and the sulfur in sulfides; this is the main reason that sulfide materials can induce lower overpotential compared with commonly used carbon materials. Fundamental understanding of this reaction process is a crucial step toward rational design and screening of materials to achieve high reversible capacity and long cycle life in Li−S batteries. T he ever-increasing demand for energy storage devices with high energy density, low material cost, and long cycle life has driven the development of new battery systems beyond the currently dominant lithium ion batteries (LIBs) (1). Among alternative battery chemistries, lithium−sulfur (Li−S) batteries have attracted remarkable attention due to their high theoretical energy density of 2,600 watt hours per kilogram, 5 times higher than those of state-of-the-art LIBs (2-4). In addition, sulfur, as a byproduct of the petroleum refining process, is naturally abundant, inexpensive, and environmentally friendly (5). However, the practical application of Li−S batteries is still plagued with numerous challenges. For example, the insulating nature of sulfur and discharge products Li 2 S/Li 2 S 2 leads to low active material utilization. In addition, the easy dissolution of lithium polysulfides (LiPSs) into the electrolyte causes LiPSs shuttling between cathode and anode and uncontrollable deposition of sulfide species on the lithium metal anode, inducing fast capacity fading and low coulombic efficiency (2, 6).Tremendous efforts have been taken to circumvent these concerns, with the nanostructuring of electrodes as one of the most effective approaches to overcoming the issues facing highcapacity electrode materials (2, 7). For example, the integration of nanostructured carbon materials with sulfur is one of the primary strategies for improving the electrical conductivity of the composites and suppression of polysulfide shuttling through physical confinement (8-14). However, it was first recognized by Zheng et al. (11) that the weak interaction between nonpolar carbon-based materials and polar LiPSs/Li 2 S species leads to weak confinement and easy detachment of LiPSs from the carbon surface, with further diffusion into the electrolyte causing capacity decay and poor rate performance. Therefore, the introduction of heteroatoms into carbonaceous materials (such as nitrogen, oxygen, boron, phosphorous, sulfur, or ...

Catalytic Effects in Lithium–Sulfur Batteries: Promoted Sulfur Transformation and Reduced Shuttle Effect

“…Cui and co‐workers found that metal sulfides accelerate the oxidation of Li 2 S to sulfur ( Figure 6 ). 12 Electrochemical measurements and XPS studies show that VS 2 , CoS 2 , and TiS 2 have higher binding energies toward Li 2 S 6 . First‐principles calculations show that the strong interaction between LiPSs and metal sulfides lowers the overpotential for the Li 2 S decomposition.…”

Controlling the Wettability between Freestanding Electrode and Electrolyte for High Energy Density Lithium-Sulfur Batteries

“…In particular, a sudden capacity drop was observed after the 50 th cycle, which was mainly attributed to the low utilization of sulfur and subsequent continuous formation of a large amount of insulating S/Li 2 S products during prolonged cycling. 46,47 A comparison of the different rate capabilities arising from the use of different electrolyte is given in Figure 7d. For all current densities, the cell using the 0.5 M LiTFSI electrolyte delivered a higher discharge capacity than the cell using the 1 M LiTFSI electrolyte.…”

Hierarchically Designed CNF/S−Cu/CNF Nonwoven Electrode as Free‐Standing Cathode for Lithium−Sulfur Batteries

“…The contents of Cu and S obtained from XPS results are much lower than added. [27,[34][35][36] A high ratio of pyridinic-N (41.3 % at. The XPS characterization technology only reflects the surface element information within the depth of 3 nm.…”