Stabilizing Li–S Battery Through Multilayer Encapsulation of Sulfur

Abstract

with the state-of-the-art high voltage cathode batteries with theoretical specific energy of ≈600 Wh kg −1 . In addition, elemental sulfur is environmentally friendly, inexpensive, and abundant.The Li-S battery poses some challenges that need to be overcome. One of the major challenges of the Li-S battery is that it generally suffers from the shuttling of soluble polysulfide species (Li 2 S x , x = 4-8) during cycling, which results in a low Coulombic efficiency and reduces its cycle life, typically to less than 100 cycles. In addition, since sulfur [1] and lithium sulfide are electronic insulators, the electronic conductivity of the cathode typically has to be augmented by addition of a significant amount of conductive carbon. To overcome these issues, most efforts have been directed to prevent soluble polysulfides from shuttling, by encapsulation of sulfur with conducting membranes or porous structures, most recently through designing 3D cathode architechtures [30][31][32][33][34] or other sulfur hosts [35,36] which allow improved stability as well as fast kinetics. Among conductive polymers, polypyrrole has been widely investigated [20,26,28,37] owing to its high electronic conductivity, low cost of precursors, and ease of synthesis. However, studies of polypyrrole encapsulated sulfur composites have shown no major successes perhaps due to the presence of large pores (≈5 nm, Figure S1, Supporting Information) in the encapsulating polymer. In addition, encapsulated sulfur nanocomposites require dissolution of a certain amount of sulfur by organic solvents to provide a void space in order to accommodate the large volumetric expansion of sulfur upon lithiation. [26] Here, for the first time, we demonstrate a strategy to improve the stability of Li-S battery using a multilayer encapsulation of sulfur nanoparticles. To ensure an effective encapsulation of sulfur, the following coating layers are selected. We coat the sulfur nanoparticles with MnO 2 particles as the interior shell since it was demonstrated that soluble lithium polysulfide species can be easily oxidized to thiosulfate groups by MnO 2 particles. The thiosulfate groups formed on the surface of the MnO 2 are proposed to anchor long-chain polysulfides by catenating them to form polythionates and therefore catalyze their conversion to insoluble short-chain polysulfides which significantly minimizes the shuttle effect. [29] We use polypyrrole as the exterior shell since (a) its porous structure allows electrolyte uptake, (b) its flexible network supports and contains the inner MnO 2 shell during discharging where the volumetric expansion of sulfur occurs, and (c) its remarkable electronic conductivity Advancements in portable electronic devices and electric powered transportation has drawn more attention to high energy density batteries, especially lithium-sulfur batteries due to the low cost of sulfur and its high energy density. However, the lithium-sulfur battery is still quite far from commercialization mostly because of incompatibility between all ma...

2D Conductive Metal–Organic Frameworks: An Emerging Platform for Electrochemical Energy Storage

“…Li‐S batteries, as an alternative in next generation energy storage devices, have attracted great interest because of their ultra‐high theoretical capacity (1675 mA h g −1 ), high energy density (2600 Wh kg −1 ), abundance of sulfur, cost effectiveness, and eco‐friendliness [39] . However, their performance in practical application are greatly limited due to the following aspects: i) intermediate polysulfides (LiPSs Li 2 S n , 2 < n ≤8) are usually formed during the charging/discharging processes, which can dissolve in polar organic electrolyte, leading to the loss of active materials and fast capacity fading; [40] ii) the insulating nature of sulfur and LiPSs increases the internal resistance of the batteries, resulting in limited electronic contact, sluggish redox kinetics, and serious polarization; [41] iii) the notorious shuttle effect of soluble LiPSs would cause severe self‐discharge behavior, leading to potential failures, poor cycle stability, and low Coulombic efficiency; [42] iv) big volume change during the conversion process of S to Li 2 S, giving rise to significant structure degradation during the cycles [43] . 2D c ‐MOFs featured with high electrical conductivity, broad diversity of organic ligands and metal ions, and hierarchical pore structures, hold great prospect as sulfur host materials and are promising to address the above‐mentioned challenges for Li‐S batteries.…”

Revisiting the Role of Conductivity and Polarity of Host Materials for Long‐Life Lithium–Sulfur Battery

“…The main issues of LSBs are related to poor cycle and rate performance associated with the loss of active sulfur due to continuous diffusion of highly soluble lithium polysulfides (Li 2 S x , 4 ≤ x ≤ 8) into the electrolyte solution and their parasitic reaction with the Li anode, and an expansion in sulfur volume (≈80%) upon lithiation. [ 10–12 ] Another issue occurs due to sulfur's intrinsic low electric conductivity (5 × 10 −30 S cm −1 at 25 °C). [ 13,14 ] So far, the poor conductivity issue has been overcome by employing highly conductive carbon host materials, which usually have a large surface area and pore volume for physical confinement.…”

Strongly Anchoring Polysulfides by Hierarchical Fe3O4/C3N4 Nanostructures for Advanced Lithium–Sulfur Batteries

“…Rechargeable lithium-sulfur batteries (LSBs) are widely expected to be the next-generation high-density energy storage technology due to their high theoretical capacity and high energy density, as well as the natural abundance and environmental compatibility of sulfur [1,2]. However, the practical applications of LSBs are still hampered by intrinsic issues, such as the large volumetric expansion (80%) of sulfur upon lithiation, insulating properties of sulfur, and dissolution of intermediate lithium polysulfides (LPS) species during cycling [1][2][3]. Shuttle effects, which are a result of the elution of LPS, are considered to be the main issue, resulting in the low Coulombic efficiency, high selfdischarge, poor rate performance, and low charge-discharge cycles of LSBs [3,4].…”

“…However, the practical applications of LSBs are still hampered by intrinsic issues, such as the large volumetric expansion (80%) of sulfur upon lithiation, insulating properties of sulfur, and dissolution of intermediate lithium polysulfides (LPS) species during cycling [1][2][3]. Shuttle effects, which are a result of the elution of LPS, are considered to be the main issue, resulting in the low Coulombic efficiency, high selfdischarge, poor rate performance, and low charge-discharge cycles of LSBs [3,4]. To address this main issue, several strategies to regulate the LPS have been explored by various research groups.…”