Nanofiber Separator Research Articles

Crosslinked PVA/Citric Acid Nanofibrous Separators with Enhanced Mechanical and Thermal Properties for Lithium-Ion Batteries

Electrospinning polyvinyl alcohol (PVA) nanofibrous membranes have gained increased attention for their uses as separators for lithium-ion batteries (LIBs) due to their high porosity and excellent electrolyte wettability, but their poor mechanical and thermal properties have limited their further development. In this work, a crosslinked PVA composite separator (PVA/CA-H) was first prepared via the electrospinning of the PVA and citric acid (CA) mixed solution and then the heating of the nanofibrous membrane, and the effects of the amount of CA on the structure and performance of the PVA/CA-H separator were investigated. The hydroxyl group of PVA and the carboxyl group of CA were crosslinked under the heat treatment, resulting in a slight reduction in the porosity and pore size of the composite separator compared to pure PVA, and to compensate for this issue, the mechanical strengths, as well as the thermal dimensional stability of the PVA/CA-H separator, were significantly improved. Meanwhile, the PVA/CA-H separator exhibited good electrolyte uptake (158.1%) and high ionic conductivity (1.63 mS cm−1), and, thus, the battery assembled with the PVA/CA-H separator exhibited a capacity retention of 96.3% after 150 cycles at 1 C. These features mean that the crosslinked PVA composite separator can be considered as a prospective high-safety and high-performance separator for LIBs.

A ZIF-8 modified aramid nanofiber separator against the shuttle effect for high performance lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries are considered the most promising energy storage systems due to their high specific capacity and energy density. However, the practical application of lithium-sulfur batteries has been hampered by capacity loss due to the shuttle effect and serious safety issues due to the flammability and shrinkage of commercial diaphragms at high temperatures. Herein, we designed a separator by adding ZIF-8 nanoparticles to aramid nanofibers named ZIF-8/ANF. ANF has high flame retardant property and high mechanical strength to prevent separators failure at high temperature, preventing lithium dendrites from punctured separators and causing short circuit. ZIF-8 has the porous structure, while exposing abundant metal nodes acting as active sites, which can effectively adsorb polysulfides and inhibit the shuttle effect. As a result, the Li-S batteries with ZIF-8/ANF separators exhibit high specific discharge capacity and cycle performance. The Li-S batteries show a high initial specific capacity of 1178.3 mAh g−1 at 0.1 C and 525.9 mAh g−1 after 400 cycles. In addition, they also exhibit discharge capacity is 825.6 mAh g−1 at 0.2 C with the loading of 3.5 mg cm−2. This work offers a guidance for safe and high-performance Li-S batteries.

Chitosan nanofiber paper used as separator for high performance and sustainable lithium-ion batteries

Separators are indispensable components in lithium-ion batteries (LIBs), providing efficient pathways for lithium ions to travel and isolating the positive and negative electrodes to avoid short circuits. However, traditional polyolefin-based separators exhibit inferior electrolyte affinities, limited porosities, and low thermal stabilities. In this study, a novel method was developed to prepare chitosan micro/nanofiber membranes as LIB separators using natural materials. The pore sizes of the chitosan micro/nanofibers separators were modulated by changing the diameters of the chitosan fibers. The results demonstrated that the chitosan nanofiber separators (CSNFs) had superior electrolyte uptake (281 %), excellent thermal dimensional stability, and electrochemical performance in LiFePO4/Li half-cell, as indicated by the higher discharge capacity after 100 cycles, and higher rate capacity than commercial Celgard2325 separator. This study paves the way for the fabrication of eco-efficient and environment-friendly separators for high-performance LIBs.

Thermally stable 3D cross-linked fluorinated polyimide/PVDF-HFP hybrid separator for lithium battery applications

Commercial Celgard 2325 separators suffer from poor flexibility, poor electrolyte wettability, and thermal instability, which can cause internal short circuits and thermal runaway in lithium rechargeable batteries. A novel 3D cross-linked and thermotolerant fluorinated polyimide (PI) / PVDF-HFP (PV) hybrid nanofiber separator rich in polar functional groups was fabricated using an electrospinning technique. PI and PV polymers are joined by a hydrogen bond through their exposed amide (–NH) and fluorinated (-CF) functional groups to build a supramolecular polymer cross-linked system. The separator exhibited high porosity (75 %), outstanding electrolyte uptake ability (775 %), strong mechanical properties (15.7 MPa), excellent thermal stability, and flexibility. As a result, the separator showed excellent Li+-conductivity (3.3 mS cm−1) and high Li+-transference number (tLi+ = 0.7) and maintained a proper-contacted electrolyte/electrode interaction for stable Li plating/stripping. Furthermore, when the PI/PV hybrid separator was applied in anode-free Li-metal batteries (a protocol of Cu/NMC cell), or Li-ion (MCMB /NMC) batteries, the cells achieved a high capacity and long lifetime. Remarkably, the Cu/NMC cell fabricated with a PI/PV hybrid separator is more robust and continues to function even after being heated at a high temperature of 140 °C for 1 h.

A Light-Thin Chitosan Nanofiber Separator for High-Performance Lithium-Ion Batteries.

With the development of portable devices and wearable devices, there is a higher demand for high-energy density and light lithium-ion batteries (LIBs). The separator is a significant component directly affecting the performance of LIBs. In this paper, a thin and porous chitosan nanofiber separator was successfully fabricated using the simple ethanol displacement method. The thickness of the CME15 separator was about half that of mainstream commercial Celgard2325 separators. Owing to its inherent polarity and high porosity, the obtained CME15 separator achieved a small contact angle (18°) and excellent electrolyte wettability (324% uptake). The CME15 separator could maintain excellent thermal dimensional stability at 160 °C. Furthermore, the CME15 separator-based LIBs exhibited excellent cycling performance after 100 cycles (117 mAh g-1 at 1 C). The present work offers a perspective on applying a chitosan nanofiber separator in light and high-performance lithium-ion batteries (LIBs).

Great potentials of lignin-based separator for Li-ion battery with electrospinning in aqueous system

Abstract Separator plays a key role in the safety and performances of lithium-ion batteries with the needs of good thermal stability and wettability. Compared with the commercial separator mainly made by polyolefins in organic solvents, the electrospinning of lignin/polyvinyl alcohol in an aqueous solution has been investigated as a green and economically viable method to produce high-performance separators without any other chemical additives. Lignin/polyvinyl alcohol (mass ratio: 3:7) nanofiber separators outperform commercial polypropylene separators in terms of porosity (71 %), liquid absorption (373 %), and outstanding thermal stability at 200 °C. Additionally, Li-ion battery cells that were constructed using different separators were assessed to demonstrate favorable electrochemical performance. The results showed that fabricated Li-ion battery with Lignin/polyvinyl alcohol (mass ratio: 3:7) nanofiber separators possesses a superior capacitance of 154.1 mA h g−1 at 0.5 C, and maintained outstanding cycle stability after 50 times of charging and discharging process at 0.5 C with the capacitance retention rate up to 98.7 %. The facile and sustainable synthesis strategy of lignin-based separator materials for Li-ion batteries developed in this work provides new perspectives for related research, especially based on environmentally friendly aqueous systems.

Pure cellulose nanofiber separator with high ionic conductivity and cycling stability for lithium-ion batteries

Conventional polyolefin separators are constrained by poor electrolyte wettability, inferior thermal stability, and low ionic conductivity, which seriously restrict their application in high-performance lithium-ion batteries (LIBs). Herein, cellulose nanofiber (CNF) as the matrix and tert-butyl alcohol (TBA) as the dispersion medium were used to prepare the pure CNF separators for LIBs by a facile filtration method. The effects of the drying temperature on the pore structure, electrolyte wettability, mechanical properties, thermal stability, and ionic conductivity of the separators were comprehensively investigated. The results showed that the freeze-dried separator at −80 °C with TBA as the dispersion medium (TBA-FD) had the best overall performance, with the porosity and electrolyte uptake up to 70.8 % and 296 %, respectively, as well as the ionic conductivity up to 1.90 mS/cm. The CNF separators had no apparent thermal shrinkage at 160 °C, illustrating good thermal stability. Moreover, the LiFePO4/lithium metal battery assembled with the TBA-HD (tert-butyl alcohol as the dispersion medium for heat-drying at 80 °C) and TBA-FD separators displayed superior cycling stability (with a capacity retention rate up to 97.5 % and 96.4 %, respectively) and rate performance. The pure CNF separators with good performance prepared by the facile method are greatly promising for high-performance LIBs.

Electrospun heterocycle aramid nanofiber separator with MOF-supported porous structure enabled excellent cycling stability for lithium metal batteries with high LiFePO4 loading

Recently, para-aramid nanofiber separators have attracted more attention in lithium metal batteries (LMBs); however, the balance between safety and electrochemical performance remains challenging. Herein, a robust, porous, and thermotolerant separator (ZPBIA) is proposed based on electrospun heterocycle aramid nanofibers and in-situ grown metal-organic framework (MOF). First, introducing H2SO4 enables improved electrospinnability of rigid-chain poly(p-phenylene -benzimidazole-terephthalate) (PBIA) due to hierarchical hydrogen bonds crosslinking network formation. Next, zeolitic imidazolate framework-67 (ZIF-67) nanosheets anchored in situ on PBIA nanofibers confer high porosity (91%) and good electrolyte wettability to the ZPBIA separator. As a result, the assembled LFP/ZPBIA/Li cell with a high LiFePO4 (LFP) loading (12 mg cm−2) delivers excellent rate capacity at 5.0 C (76 mAh g−1) and impressive long-term cycling stability at 1.0 C (a minor capacity decay of 0.031% per cycle after 500 cycles). Benefiting from the intrinsic properties of PBIA, the ZPBIA separator possesses high mechanical strength to prevent Li dendrites puncture. Moreover, the open circuit voltage stabilizes at ∼3.4 V from 30 to 150 ℃ for LFP/ZPBIA/Li cell, which is probably applied in high-temperature conditions. This work provides a novel fabrication of functional separators to balance the safety and electrochemical property of LMBs.

Nanofiber membrane coated with lithiophilic polydopamine for lithium metal batteries

Along with the cathode, anode, and liquid electrolyte in lithium-based secondary batteries, the separator is a crucial element for guaranteeing battery safety. However, conventional polyolefin separators suffer from inherent drawbacks such as inadequate compatibility with electrolytes and limited thermal stability. These limitations can lead to issues like high-temperature shrinkage, melting, and even combustion. Moreover, the vulnerability of separators toward lithium dendrite penetration exacerbates safety concerns associated with lithium-ion batteries. Hence, the design of high safety separators is currently a focus and challenge. In this study, we develop a multifunctional polymer-coupled nanofiber membrane by an electrospinning method that addresses the above issue as a separator of lithium metal battery. The nanofiber coating contains carbonyl oxygen, pyrrole nitrogen, and cross-linked networks with tertiary amine groups. These components effectively neutralize acidic compounds generated during the liquid electrolyte side reaction. X-ray micro-computed tomography analysis verifies the exceptional structural stability of the new separator, maintaining its 3D skeleton even after 2000 h of cycling. The nanofiber separator in a full Li||NCM811 cell achieves a high specific capacity of 136.6 mA h g−1 at 0.9 A g−1 and displays outstanding long-cycle stability over 500 cycles with a capacity retention of 88.5%.

In situ aluminide armored polyimide nanofiber separators with ultrahigh strength and high wettability for advanced lithium-ion batteries

Polyimide (PI) has been widely used in aerospace, electronic information, novel energy, and other industries due to its excellent high- and low-temperature, radiation, and chemical corrosion resistance. However, the use of electrospun PI separators in lithium-ion batteries (LIBs) is limited due to their poor electrolyte wettability and insufficient mechanical strength. Inorganic materials such as aluminide have typically been used in the coating composition of PI separators. To investigate the effects of different aluminide components, in this work, PI materials were prepared by the complexation–hydrolysis method and used as separators for LIBs. The physicochemical and electrochemical properties of the different nanofiber membranes were studied by adjusting the calcination temperature. The results showed that the sample with the highest tensile strength was 107.6 MPa, which is close to the commercial polyolefin separator. When the calcination temperature increased to 300 °C and then exceeded 400 °C, the aluminide shell initially transformed into thin boehmite and then into γ-Al2O3 phases. The mechanical properties of the nanofiber membranes were enhanced, and the wettability and electrochemical characteristics also improved to some extent. Furthermore, the nanofiber structure was destroyed at 500 °C and could not meet the long-term battery cycles. Therefore, when the calcination temperature was controlled between 300 °C and 400 °C, the aluminide-armored PI nanofiber membranes exhibited better comprehensive performance and were suitable for use as separators in LIBs, which was of great significance for improving the safety and stability of LIBs, especially power batteries.

WO3-composited polyimide nanofibrous separator with superior mechanical properties and high capacity for lithium-ion batteries

WO3-composited polyimide nanofibrous separator with superior mechanical properties and high capacity for lithium-ion batteries

Nanodiamond-Enhanced Nanofiber Separators for High-Energy Lithium-Ion Batteries.

Current lithium-ion battery separators made from polyolefins such as polypropylene and polyethylene generally suffer from low porosity, low wettability, and slow ionic conductivity and tend to perform poorly against heat-triggering reactions that may cause potentially catastrophic issues, such as fire. To overcome these limitations, here we report that a porous composite membrane consisting of poly(vinylidene fluoride-co-hexafluoropropylene) nanofibers functionalized with nanodiamonds (NDs) can realize a thermally resistant, mechanically robust, and ionically conductive separator. We critically reveal the role of NDs in the polymer matrix of the membrane to improve the thermal, mechanical, crystalline, and electrochemical properties of the composites. Taking advantages of these characteristics, the ND-functionalized nanofiber separator enables high-capacity and stable cycling of lithium cells with LiNi0.8Mn0.1Co0.1O2 (NMC811) as the cathode, much superior to those using conventional polyolefin separators in otherwise identical cells.

Non-flammable Phosphate-Grafted Nanofiber Separator Enabling Stable-Cycling and High-Safety Lithium Metal Batteries

Lithium metal batteries (LMBs) are promising rechargeable energy storage devices with high energy densities and long lifetimes. Nevertheless, safety concerns constitute a severe impediment to the commercial demand for LMBs, including uncontrollable lithium dendrites, heat-intolerant separators, and flammable electrolytes. Herein, a novel separator (PSVHM) containing poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(methyl methacrylate) (PMMA) and cage silsesquioxanes, phosphorous-containing acrylate monomers is fabricated by electrospinning with in situ ultraviolet-irradiation grafting polymerization. Attributed to the presence of phosphate groups in the membrane with flame retardant properties, the PSVHM separators have outstanding self-extinguish capabilities. In addition, the 3D-crosslinked PSVHM separator possesses high ion conductivity, and excellent mechanical strength, thus suppressing the uneven lithium dendrites and constructing a stable solid electrolyte interface (SEI). Moreover, cell assembled with the PSVHM separator exhibits an ultra-capacity retention of 98% after 200 cycles at a high current density of 2 C. This work provides a guideline for the preparation of high-safety and high-performance lithium metal batteries.

A Fluorinated‐Polyimide‐Based Composite Nanofibrous Separator with Homogenized Pore Size for Wide‐Temperature Lithium Metal Batteries

Lithium (Li) is known for excellent theoretical specific capacity and most negative electrochemical potential, while still restricted by the irregular lithium dendrites and safety risks in practical applications of lithium metal batteries (LMBs) due to thermal runaway. Herein, a fluorinated‐polyimide (F‐PI)‐based composite nanofibrous separator containing poly(vinylidene fluoride) (PVDF) component, which is designed as PVDF/F‐PI, is developed via a facile electrospinning strategy for wide‐temperature LMBs. First, abundant polar trifluoromethyl (–CF3) groups in F‐PI create an electronegative environment to facilitate rapid Li‐ions (Li+) transport. Meanwhile, the PVDF component, acting as both the physical linker between F‐PI nanofibers and the regulator of homogenized pore size, simultaneously improves the mechanical properties and homogenizes the Li+ flux on the electrode surface. Therefore, a steady circulation of 2400 h is achieved for the symmetric cell using PVDF/F‐PI separator, which still displays a stable cycle life with a low voltage polarization of 15 mV in 1000 h even under 60 °C. Therefore, the fluorinated‐PI‐based composite nanofibrous separator with high ionic conductivity and uniform pore structure offers a practical method for design of functionalized separators in wide‐temperature LMB applications.

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

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.

High-stability core–shell structured PAN/PVDF nanofiber separator with excellent lithium-ion transport property for lithium-based battery

The ion transport channel constructed by the separator is crucial for the practical performance of Li-ion batteries, including cycling stability and high rate capability under high current. Traditional polyolefin separator is the storage of electrolyte, which guarantees the internal ion transport process. However, its weak interaction with electrolyte and low cationic transport capacity limit the application of lithium ion battery in large current. In this study, a kind of core–shell structured polyacrylonitrile (PAN)/polyvinylidene fluoride (PVDF) nanofiber separator composed of PAN core and PVDF shell was prepared by coaxial electrospinning technique. As a result, the mechanical strength of PAN/PVDF nanofiber separator is increased from 0.6 MPa of PVDF to 3.6 MPa for PAN core. Furthermore, PAN/PVDF nanofiber separator exhibits an improved lithium-ion transference number (0.66), which is resulted from F functional groups of PVDF shell. It is believed that the interactions between the lithium ion and F functional group could construct a fast ion transport channel. The LiCoO2/Li half-cells assembled with PAN/PVDF exhibited higher discharge capacity (5C) than those cells using pristine PVDF, PAN separators and polyethylene (PE) separator. It is worth mentioning that the cells with PAN/PVDF separator also have excellent cycle stability. This study provides a new idea about separator-design strategy for high-performance lithium-based battery.

C60 and ZIF-67 synergistically modified gelatin-based nanofibrous separators for Li-S batteries

The lithium-sulfur (Li-S) battery has been attracting much more attention in recent years due to its high theoretical capacity and low cost, although various issues, such as the “shuttle effect” and the low use ratio of active materials, have been hindering the development and application of Li-S batteries. The separator is an important part of Li-S batteries, and its modification is a simple and effective strategy to improve the electrochemical performance of Li-S batteries. In this work, we explore separators with different functions on their two sides that have been produced by a step-by-step electrospinning method. The multifunctional separator on one side is pure gelatin, and the other side is zeolitic imidazolate framework-67 (ZIF-67)-C60-gelatin. The ZIF-67-C60-gelatin layer on the cathode side is of great importance. The chemisorption sites on it are provided by ZIF-67, and the transformation sites of lithium polysulfide are provided by C60. Gelatin, which is on the anode side, as an admirable separator material, makes the lithium flux uniform and thus prevents the generation of lithium dendrites. This type of multifunctional nanofiber separator based on double gelatin layers plays an important role in the adsorption and conversion of polysulfides, and it improves the overall performance of the Li-S battery. As a result, the Li-S batteries assembled with the prepared separator can still maintain the capacity of 888 mAh g-1 after 100 cycles at 0.2 C, and the capacity retention rate of the Li-S batteries is 72.9% after 400 cycles at 2 C. This simple preparation method and high-performance bilayer membrane structure provide a new route for commercial application.

Orientation gradient architecture of nanofibrous separator towards mechanical enhancement and ion transport acceleration for lithium-ion batteries

Designing rational structure is one of strategies as effective as supplementing proper materials for separator performance enhancement. Focusing on the nanofibrous separators that generally have high porosity but poor mechanical strength, in this work, we develop an architecture of orientation gradient along the thickness direction for polyacrylonitrile (PAN) nanofibrous membrane (OGP), which is facilely fabricated by sequential electrospinning with different collector rotating speeds. The obtained OGP separators not only share the advantages of PAN nanofibrous separator, such as high porosity, superior wettability and thermal stability, but also have considerable improvements in tensile strength and puncture resistance to compensate for its mechanical weakness. Meanwhile, when compared to the most commonly used nanofibrous separator with randomly oriented nanofibers, the OGP separators present more desirable lithium ion transport ability, including a larger amount of ions transported as well as a faster ion transport rate, thereby endowing the cell with superior cycling stability with a capacity retention 97.2% after 100 cycles at 0.5 C. Therefore, taking safety and battery performance into account, the as-prepared OGP nanofibrous membrane could be a promising separator candidate for lithium ion batteries.

A water-gating and zinc-sieving lignocellulose nanofiber separator for dendrite-free rechargeable aqueous zinc ion battery

The sustainable development of rechargeable aqueous zinc-ion batteries (RZIBs) is severely limited by the uncontrolled zinc dendritic growth and side reactions on the anode. Herein, a lignocellulose nanofiber (LCNF) separator from waste palm with dual-function of water-gating and zinc-sieving is developed for the dendrite-free zinc anode. The obtained LCNF separator offer hydrophobic surface (96°), uniform nanopores (∼20 nm), excellent mechanical performance (47.8 MPa), and high ionic conductivity (18.1 mS cm−1). Therefore, the LCNF separator enables increased zinc deposition kinetics, and reduced desolvation barrier of hydrated zinc ions, effectively suppressing the Zn dendrite growth and promoting the uniform Zn plating/stripping on zinc anode. Such separator enables excellent performance: a low polarization of 60.1 mV and steady charge-discharge in a Zn//Zn symmetric cell for 3500 h at the current density of 0.5 mA cm−1; dendrite-free zinc plating/stripping at 99.0 % coulombic efficiency in a Cu//Zn asymmetric cell for over 250 cycles at 1 mA cm−2; 280.1 mAh g−1 in MgVO//Zn full battery with the retention of 89.7 % for over 2000 cycles at 5 A g−1. In addition, this LCNF separator can be completely degraded in the natural soil within two weeks, exhibiting a low environmental impact with their natural biodegradability. This design of green separator for dendrite-free RZIBs also provides a new insight for the sustainable development of other metal-ion rechargeable batteries.

Phase Separation–Controlled Assembly of Hierarchically Porous Aramid Nanofiber Films for High‐speed Lithium‐Metal Batteries (Small 52/2022)

Porous Aramid Nanofiber Films In article number 2205355, Jeong Gon Son, Bongjun Yeom, and co-workers report highly porous aramid nanofiber separators with dual porosity (>97%), prepared by a two-step solvent exchange process, for Lithium-metal batteries. The hierarchical porous structure with maximized porosity and selective affinity of the aramid nanofibers to anions shows effectively reduced Li dendrite formation and stable cycling performance under high-speed charge/discharge conditions.