Electrospinning Machine | Multifunctional integrated separator based on electrospinning structure engineering for high-stability lithium sulfur batteries

Lithium-sulfur batteries (Li-S) with elemental sulfur as the cathode and lithium as the anode have attracted widespread attention due to their high theoretical specific capacity (1675 mA·h g-1) and energy density (2600 Wh kg-1). However, the "shuttle effect" caused by the dissolution and migration of lithium polysulfides (LiPSs), intermediate products of the Li-S battery electrochemical reaction, the battery's self-discharge, and the irregular growth of lithium anode dendrites, all exacerbate the corrosion and passivation of the Li anode, the loss of active sulfur, and the decline in battery capacity. As an indispensable component of lithium-sulfur batteries, the separator is a hub for material transport between the sulfur cathode and the lithium anode, playing a core role in ion transport while avoiding short circuits. Functionalized separators are considered an efficient and convenient strategy to synergistically regulate sulfur electrochemical reaction activity and lithium anode deposition/stripping electrochemical behavior. 

Polymer nanofiber membranes prepared by electrospinning have outstanding electrolyte affinity, rich porosity, good chemical stability, structural flexibility, and lithium-ion transport properties, and are considered one of the promising candidate materials for battery separators. Therefore, developing novel functional separators with rationally ordered spatial structure and lithium-sulfur affinity through electrospinning structural engineering can better inhibit the "shuttle effect," reduce self-discharge, and alleviate the problem of lithium dendrite growth. Simultaneously, it can accelerate the redox kinetics of sulfur/lithium polysulfides and regulate the electrochemical behavior of lithium deposition/stripping.

Recently, Professor Li Wenhu from Shaanxi University of Technology, in collaboration with Professor Yang Rong from Xi’an University of Technology, Assistant Professor Jia Kai from Xi’an Jiaotong University, and Professor Bao Xichang from the Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, published their latest research titled "Multifunctional integrated separator based on electrospinning structure engineering for high-stability lithium sulfur batteries" in the journal Chemical Engineering Journal. This work innovatively introduced ZIF-67-derived metal oxide-modified nitrogen-doped carbon (NC-Co3O4) polyhedrons and graphene oxide (GO) into amino-functionalized polyacrylonitrile (PAN) electrospun nanofibers through electrospinning structural engineering, designing a bilayer-structured multifunctional integrated separator (APAN/Co-G). In the APAN/Co-G separator, APAN and NC-Co3O4 effectively enhanced redox reaction kinetics through adsorption or catalysis of LiPSs; the electrostatic interaction between oxygen-containing functional groups in GO and polysulfides improved the battery’s anti-self-discharge capability. Simultaneously, APAN significantly increased the separator’s electrolyte affinity, promoting uniform deposition/stripping of lithium ions (Li+). Due to its unique structural design, the APAN/Co-G separator exhibited high porosity (78.53%), electrolyte uptake (359.57%), ionic conductivity (1.73 mS cm-1), and Li+ transference number (0.72). The Li-S battery based on the APAN/Co-G separator demonstrated exceptional performance: only 2% self-discharge after 24 hours of rest, an average capacity decay rate as low as 0.016% per cycle over 2000 cycles at 0.5 C, an ultra-high areal capacity of 6.85 mA·h cm-2 at a high sulfur loading of 6.37 mg cm-2, and stable cycling for 100 hours even at a high current density of 5.0 mA cm-2 (5 mA·h cm-2). This study provides a novel approach through structurally engineered multifunctional integrated separators to address key bottlenecks in Li-S batteries, laying a crucial foundation for the practical application of next-generation high-energy-density energy storage devices.

Figure 1: Schematic diagram of the preparation and construction of the APAN/Co-G separator and its micro-morphology characterization.

The amination grafting reaction resulted from the reaction between -NH2 in branched polyethyleneimine (PEI) and -C≡N or methacrylate groups (C(=O)-OCH3) in polyacrylonitrile (PAN), forming N-C=N and C(=O)-NH groups. Figure 1 micro-morphology characterization results show: the separator thickness is about 155 μm; NC-Co3O4 polyhedrons and GO are loaded on the fibers and evenly distributed; the amination grafting process significantly improved the density and uniformity of the fiber network while maintaining the fiber morphology.

Figure 2: Phase and physical property characterization of the APAN/Co-G separator.

XRD/FTIR confirmed the successful loading of NC-Co3O4 and GO and amination grafting of the APAN/Co-G separator. The APAN/Co-G separator has high porosity (78.53%), high electrolyte uptake (359.57%), good thermal stability and mechanical stability, and efficient ion transport performance (ionic conductivity, 1.73 mS cm-1; lithium ion transference number, 0.72).

Figure 3: Electrochemical performance of the APAN/Co-G separator.

The APAN/Co-G separator used in the battery showed high specific capacities from 1180.94 to 586.61 mA·h g-1 at rates from 0.1 to 1 C (sulfur utilization reached 70%), a capacity decay rate of only 0.016% over 2000 cycles at 0.5 C, an areal capacity of 6.85 mA·h cm-2 at a high sulfur loading of 6.37 mg cm-2, and maintained a 77.68% capacity retention rate with low electrolyte usage (E/S=4 μL mg-1).

Figure 4: Analysis of redox kinetics and self-discharge performance.

The APAN/Co-G separator enhances Li-S battery performance through synergistic effects: the NC-Co3O4 component effectively adsorbs polysulfides and catalyzes their conversion, greatly increasing the lithium ion diffusion coefficient (8.49×10-8 cm2 s-1), reducing the redox reaction energy barrier (e.g., the activation energy for S8→Li2S4 is reduced by 52.11 kJ mol-1), and improving reaction reversibility; simultaneously, the APAN/Co-G separator strongly inhibits self-discharge (only 2% capacity loss after standing for 24 hours, far better than the 37% for the PP separator) and maintains a stable open-circuit voltage; additionally, the nitrogen-containing groups in APAN promote the formation of a stable SEI layer rich in high Li-ion conductive phases Li3N/Li2NxOy on the lithium anode, optimizing ion transport and homogenizing lithium deposition, while the oxygen-containing groups in GO selectively promote Li+ transport and electrostatically repel polysulfide anions. The synergistic effects of adsorption, catalysis, ion transport regulation, and anode protection mechanisms collectively inhibit the polysulfide shuttle, accelerate reaction kinetics, reduce self-discharge, and protect the anode, thereby achieving overall enhancement of electrochemical performance.

 Figure 5: Characterization of lithium ion electrochemical behavior based on the APAN/Co-G separator.

The APAN/Co-G separator significantly optimizes lithium deposition/stripping behavior and cycle stability: Li//Li symmetric cell tests show the cell stably cycles for over 600 hours at 1 mA cm-2 (overpotential only 15 mV), far better than the PP separator (short circuit at 30 hours); in Li//Cu half-cells, the APAN/Co-G cell maintains a high Coulombic efficiency of 99.03% over 100 cycles, with a lower nucleation overpotential. SEM analysis confirmed that the APAN/Co-G separator promotes more uniform and dense lithium ion deposition. These advantages stem from the enhanced electrolyte wettability and lithium ion transport kinetics due to the polar functional groups in the separator, which induce the formation of a stable SEI layer that uniformizes the interface current distribution and reduces the nucleation barrier, thereby effectively inhibiting "dead lithium" formation and improving cycle stability.

Figure 6: Schematic diagram of the mechanism of the APAN/Co-G separator for lithium-sulfur batteries.

Through systematic research, the multifunctional mechanism of the APAN/Co-G separator for Li-S batteries is as follows: (1) The fiber network structure, abundant functional groups, and NC-Co3O4 of the APAN/Co-G separator synergistically adsorb or catalyze LiPSs, inhibiting the "shuttle effect" and accelerating redox reaction kinetics; (2) The oxygen-containing electronegative groups in GO serve as Li+ hopping sites, reducing the migration of negatively charged species (Sn2-) through electrostatic interactions, thereby improving the anti-self-discharge capability of the Li-S battery; (3) The polar amino groups enhance the separator's affinity for the electrolyte and reduce the nucleation overpotential, which not only promotes uniform Li+ deposition but also helps form an Li3N-rich SEI layer, thereby improving cycle stability.

Paper link: https://doi.org/10.1016/j.cej.2025.166813