Recent Development in Novel Lithium-Sulfur Nanofiber Separators: A Review of the Latest Fabrication and Performance Optimizations

Abstract

Lithium-Sulfur batteries (LSBs) are one of the most promising next-generation batteries to replace Li-ion batteries that power everything from small portable devices to large electric vehicles. LSBs boast a nearly five times higher theoretical capacity than Li-ion batteries due to sulfur's high theoretical capacity, and LSBs use abundant sulfur instead of rare metals as their cathodes. In order to make LSBs commercially viable, an LSB's separator must permit fast Li-ion diffusion while suppressing the migration of soluble lithium polysulfides (LiPSs). Polyolefin separators (commonly used in Li-ion batteries) fail to block LiPSs, have low thermal stability, poor mechanical strength, and weak electrolyte affinity. Novel nanofiber (NF) separators address the aforementioned shortcomings of polyolefin separators with intrinsically superior properties. Moreover, NF separators can easily be produced in large volumes, fine-tuned via facile electrospinning techniques, and modified with various additives. This review discusses the design principles and performance of LSBs with exemplary NF separators. The benefits of using various polymers and the effects of different polymer modifications are analyzed. We also discuss the conversion of polymer NFs into carbon NFs (CNFs) and their effects on rate capability and thermal stability. Finally, common and promising modifiers for NF separators, including carbon, metal oxide, and metal-organic framework (MOF), are examined. We highlight the underlying properties of the composite NF separators that enhance the capacity, cyclability, and resilience of LSBs.

Keywords: carbon nanofiber; lithium-sulfur batteries; metal organic framework; separator.

Figure 5

(a) Electrolyte wettability and meniscus behavior of common LSB electrolytes and water on cross-linked PAN NF separator and PP separator. Reprinted/adapted with permission from Ref. [55]. (Copyright 2021, American Chemical Society). (b) Chemical structures of F-PI and PI and their binding energies to Li₂S₄ and Li₂S₆. Reprinted/adapted with permission from Ref. [56]. (Copyright 2020, Royal Society of Chemistry). (c) Schematic of a Janus-type aramid NF separator (ANF-JS), with the advantages of the dense and microporous structures listed. Reprinted/adapted with permission from Ref. [58]. (Copyright 2022, Royal Society of Chemistry). (d) Comparison of electrolyte uptake and storage time between a standard PP separator and CNF separator. Reprinted/adapted with permission from Ref. [59]. (Copyright 2022, Elsevier).

Figure 1

Schematic summarizing the strategies for tuning and enhancing NFs for LSBs via polymer modification and nanocompositing. The benefits of modifying NFs for LSBs are also summarized.

Figure 2

Schematic diagram showing the conversion of S₈ to Li₂S with intermediate LiPSs during LSB discharge (Top). The insolubility/solubility of the LiPSs is noted by red/blue colors. The main LiPS rejection mechanisms employed by NF separators.

Figure 3

(a) A schematic diagram of a standard electrospinning process. Reprinted/adapted with permission from Ref. [35]. (Copyright 2019, Elsevier). SEM (b) top view and (c) side view of an ammoniated PAN NF separator. A digital photograph of the NF is provided in the insert in (b). Reprinted/adapted with permission from Ref. [41]. (Copyright 2020, Elsevier).

Figure 4

The advantages and disadvantages of promising polymer materials for NF separator fabrication and application in LSBs.

Figure 6

(a) SEMs of CNF (Top Left) and CNF/CeO₂ (Top Right) and TEMs at low magnification (Bottom Left) and high magnification (Bottom Right). Reprinted/adapted with permission from Ref. [45]. (Copyright 2020, Elsevier). (b) Schematic showing the gradual changes from the loose Co-CNF layer to the dense PHB layer and the consequences of the bilayer structure. (c) The flame retarding ability of PHB and PHB/Co-CNF (Left) and thermal stability at 160 °C for the PHB and PHB/Co-CNF separators. Reprinted/adapted with permission from Ref. [109]. (Copyright 2022, Royal Society of Chemistry).

Figure 7

Schematic showing the basic structure of carbon nanoparticles, transition metal oxides, MOFs, and inorganic nitrides. The advantages (+) and disadvantages (-) are summarized.

Figure 8

(a) Schematic diagram illustrating the fabrication of cross-linked PVA/PAA modified with ZIF-8 MOF. Reprinted/adapted with permission from Ref. [131]. (Copyright 2021, American Chemical Society). (b) The capacity retention of an LSB with a PP, PMIA NF, and ZIF-modified PMIA NF separators. Reprinted/adapted with permission from Ref. [73]. (Copyright 2021, Elsevier). (c) Li plating/stripping performance in symmetric Li cells with PP, PVDF-HFP/PMIA, and ZIF-8-modified PVDF-HFP/PMIA separators. Reprinted/adapted with permission from Ref. [21]. (Copyright 2022, Royal Society of Chemistry). (d) DFT skeletons of ZIF-67 surrounding a substrate from 3 orthogonal viewpoints and the binding energy of various LiPSs and S₈ to the MOF as a substrate in the MOF cavity. Reprinted/adapted with permission from Ref. [117]. (Copyright 2020, Elsevier).