Electrospinning Techniques for Composite Gel Polymer Electrolytes in Electrochemical Stable Lithium–Sulfur Batteries
Authors: Chan Tzu-Ching , Chung Sheng-Heng 
Published: December 2024

Lithium–sulfur batteries are considered to be one of the promising next-generation rechargeable batteries due to the low cost, eco-friend, high theoretical capacity, and high energy density of the cathode materials. As a conversion-type battery, the lithium–sulfur battery could completely convert its chemical energy to electrical energy and would not be limited by the structural constraint of the electrode materials, resulting in higher electrochemical utilization, a higher theoretical capacity, and a higher energy density than general lithium-ion batteries. The theoretical capacity and theoretical energy density of lithium–sulfur batteries reach 1675 mAh g-1 and 2600 Wh kg-1, respectively.

During the charging and discharging processes, lithium–sulfur batteries undergo a series of complex electrochemical reactions. During the discharging process, the solid-state sulfur (S8) of the cathode reacts with lithium ions and then transforms to liquid-state polysulfides, and solid-state lithium sulfide (Li2S2/Li2S) in sequence. While during the charging process, solid-state Li2S2/Li2S first transforms to liquid-state polysulfides and then transforms to solid-state S8 with the reversible return of lithium ions to the metallic lithium anode. Although lithium–sulfur batteries feature so many advantages, lithium–sulfur batteries could not reach the criteria for commercialization because of some unsolved problems, such as the low sulfur loading, the high electrolyte-to-sulfur ratio, the low conductivity of the cathode, and the irreversible polysulfide diffusion. Among them, the irreversible polysulfide diffusion causes the continuous capacity decay of lithium–sulfur batteries. Liquid-state polysulfides, which are the intermediate products during the cycling process, have a high solubility in the common liquid-state electrolyte of lithium–sulfur batteries. Thus, it is easy for polysulfides to diffuse to the anode with liquid electrolyte during cycling, which results in the deposition of non-conductive sulfides on the surfaces of electrodes. Moreover, it also leads to the irreversible loss of the active material, the rapid decay of capacity, and the reduction of cycle life.

In this study, electrospinning technology is utilized to fabricate three kinds of membranes which consist of polyacrylonitrile (PAN), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), and PAN/PVDF-HFP composite, acting as separator/gel electrolyte membranes in lithium–sulfur cells. PAN features high thermal stability, proper morphology for electrolyte absorption, and the mitigation of polysulfide diffusion due to the cyanide functional group (−C≡N) on its molecular chains. As a result, PAN membranes are utilized as separators in lithium–sulfur cells. PVDF-HFP, which forms as a gel in the common ether liquid electrolyte for lithium–sulfur batteries, could stabilize lithium-ions transfer and capture the liquid electrolyte. Thus, PVDF-HFP membranes act as gel electrolyte membranes in lithium–sulfur cells. With a high sulfur loading of 4.2 mg cm-2, the cell with PAN separator achieved a discharge capacity of 980 mAh g-1 and maintained a high capacity of 680 mAh g-1 at C/10 rate after 100 cycles, showing a great ability to inhibit polysulfide diffusion. The cell with PVDF-HFP gel electrolyte achieved a high discharge capacity of 1190 mAh g-1 and demonstrated a long cycle life of 900 hours at lithium//lithium symmetrical cell, showing that PVDF-HFP gel electrolyte has a great ability to stabilize electrode surfaces and smooth the lithium-ions conduction. Therefore, a triple-layered sandwiched structural PVDF-HFP/PAN/PVDF-HFP (PH/P/PH) gel electrolyte membrane which consists of double layers of PVDF-HFP and a single layer of PAN in the middle is fabricated to maintain both the mitigation of polysulfide diffusion of PAN and smooth lithium-ion conducting channels of PVDF-HFP. The cell with this PH/P/PH composited gel electrolyte demonstrated a high discharge capacity of 1370 mAh g-1, a high areal capacity of 5.75 mAh cm-
2, a high energy density of 12.08 mWh kg-1, and a long cycle life of over 2000 hours at lithium//lithium symmetrical cell. (Fig. 1a and 1c) This PH/P/PH cell also demonstrated a better rate performance of C/10–C/3 rates than the cell with PAN separator and PVDF-HFP gel electrolyte, and it also exhibited a high capacity retention of 81% at C/3 rate after 100 cycles. (Fig. 1b) In conclusion, this triple-layered sandwiched structural PH/P/PH gel electrolyte membrane showed improvements in electrochemical utilization, efficiency, rate performance, and capacity retention, which also exhibited potential for developing practical lithium–sulfur cells.

Reference:

T.-C. Chan and S.-H. Chung, ACS Sustain. Chem. Eng.
2024 (in revision).

L.-L. Chiu and S.-H, Chung, Polymer
2023, 15, 1460.

Figure 1