New Separators in Lithium/Sulfur Cells with High-Capacity Cathodes

Abstract

Lithium-sulfur cells exhibit poor cycle life, due to the 'polysulfide shuttle' enabled by the dissolution of sulfur reduction products in organic electrolyte. Different strategies have been implemented to reduce the shuttle effects, including novel cathode designs to sequester polysulfides within the cathode, new electrolytes with reduced polysulfide solubility, and blocking layers to hinder polysulfide migration towards the anode. However, these are less successful in the case of high-energy Li/S cells with cathodes featuring substantial sulfur loadings, as proportionately greater polysulfide dissolution rapidly degrades performance. Additionally, the electrolyte/sulfur ratio is a limiting factor in high-energy cells, indicating that a minimum polysulfide solubility is required. Herein we describe the benefits of modified separators with the objective of blocking polysulfide migration. Specifically, we demonstrate improved discharge performance in laboratory Li/S cells with ceramic-coated separators, including single-sided and double-sided Al 2 O 3 -coated polyolefin material from commercial sources. We also developed a new AlF 3 coating on a polyolefin separator that showed even greater improvements: higher initial specific capacity (∼800 mAh/g S ), coulombic efficiency (>96%), and cycle life (>100 cycles). Detailed analytical study via SEM and EDS of separators harvested from cycled cells indicated an approximately uniform sulfur distribution across the ceramic-coated separator. Finally, concentrated electrolytes, which reportedly minimize polysulfide diffusion and lithium dendrite growth, were tested and shown to yield high coulombic efficiency although with low sulfur utilization.

A Comprehensive Understanding of Lithium–Sulfur Battery Technology

“…Different inorganic materials have been demonstrated to be effective in trapping polysulfides such as oxides (including Al 2 O 3 , V 2 O 5 , SiO 2 , MnO x , and Ti x O y etc. ), AlF 3 , Ti 3 C 2 , C 3 N 4 , MoS 2 and black phosphorus . Aluminum oxide (Al 2 O 3 ), for example, was coated on porous separators in order to act as a physical obstacle to block polysulfide migrations 170a.…”

Nature-Derived Cellulose-Based Composite Separator for Sodium-Ion Batteries

“…Furthermore, our previous research reported that AlF 3 layer formed by scavenging HF plays an important role to act as a passive layer at high voltage in Na cells (Konarov et al, 2019). Bugga et al also presented AlF 3 coated separator, demonstrating that the AlF 3 layer also protected separator surface, preventing short-circuit by dendrites and significant impact than Al 2 O 3 coating on the cycling stability (Bugga et al, 2018;Sun et al, 2018). For the above reasons, the cell using cellulose-PAN-Al 2 O 3 separator presented the good electrochemical performance.…”

Enhancement in Electrochemical Performance of Lithium‐Sulfur Cells through Sulfur Encapsulation in Hollow Carbon Nanospheres Coated with Ultra‐Thin Aluminum Fluoride Layer

“…To our knowledge, there are only two reports available on this coating so far. In one paper, the authors have successfully applied AlF 3 on commercially available separators and observed improvement in cycling stability of Li−S cells . The other research on this topic is about Al 2 O 3 ‐coated sulfur cathodes .…”

“…Additionally, it partially helps to prevent the deposition of the discharge products on the surface of the cathode, which translates to better cycling stability. This coating has been applied on the separator before, but the cycling data did not improve significantly. Accordingly, we can conclude that the coated separator cannot completely prevent the polysulfide shuttle.…”