Nanofiber Separator Research Articles

Functional Gel Poly-m-phenyleneisophthalamide Nanofiber Separator Modified by Starch to Suppress Lithium Polysulfides and Facilitate Transportation of Lithium Ions for High-Performance Lithium-Sulfur Battery

A functionally fluorinated emulsion and starch particles co-doped electrospun poly-m-phenyleneisophthalamide (PMIA) nanofiber membrane-based gel polymer electrolyte was produced to inhibit the “shuttle effect” of lithium polysulfides (LiPSs) and promote the transport of lithium ions for lithium-sulfur (Li-S) batteries. The high mechanical property, high thermal stability, and significant preserving liquid electrolyte of the prepared membrane are derived from the addition of functionally fluorinated emulsion and starch particles. Specially, with the chemical adsorption of –OH in starch particles and physical prevention of a gel state deriving from the prepared membrane, the lithium polysulfides are effectively controlled in the side of the cathode. In addition, the polar group and stable helix structure existing in starch particles are beneficial to the fast migration of Li+. Therefore, the battery with the Starch-F PMIA separator possesses a first-discharge capacity of 1117.8 mAh g−1 at 0.5 C. Moreover, an Li-S battery assembled with the prepared membrane also reflects preeminent rate performance, low interface impedance, and admirable cycling stability. In consequence, the Starch-F PMIA separator is a promising candidate for the progressive lithium sulfur batteries.

Electrospun nanofiber separator derived from nano-SiO2-modified polyimide with superior mechanical flexibility for high-performance lithium-ion battery

Electrospun nanofiber separator derived from nano-SiO2-modified polyimide with superior mechanical flexibility for high-performance lithium-ion battery

An integrated separator/anode assembly based on electrospinning technique for advanced lithium-ion batteries

In this work, polyacrylonitrile (PAN) nanofiber separator is fabricated directly on the surface of the graphite anode of lithium-ion batteries, resulting in an integrated separator/anode assembly (SAA). On one hand, the porous structure and polar surface of PAN nanofiber separator enable the SAA with superior electrolyte affinity and wettability. On the other hand, the interlocking structure at the interface between graphite anode and nanofiber separator enhances the mechanical and thermal stabilities of the separator as well as the interface compatibility. As a result, the SAA possesses an electrolyte contact angle of about 0 o and a thermal shrinkage of less than 2% at 150 °C for 0.5 h, resulting from the coupling effect between nanofiber separator and graphite anode. The above advantages endow the LiCoO2/SAA full cell with better C-rate (capacity retention 44.3% at 32.0 C compared with that at 0.5 C) and cycling performances (capacity retention 98.0% after 200 cycles at 0.2 C) as well as higher thermal resistance compared with those of the common LiCoO2/polyolefin separator/graphite battery. Consequently, this work provides an advanced separator/anode assembly and the corresponding fabrication method, which may be a new strategy for improving the charge-discharge performance and assembly efficiency of lithium-ion batteries.

Core-sheath 3D printing of highly conductive and MoS2-loaded electrode with pseudocapacitive behavior

Pseudocapacitance holds great promise for energy density improvement of supercapacitors. However, how to enable a pseudocapacitor with combined feature of high mass loading of pseudocapacitive material, efficient ion/electron transport, as well as facile and scalable manufacturing, still remains a significant challenge. Herein, we demonstrated an efficient and scalable extrusion-based core-sheath 3D printing strategy for directly and controllably constructing pseudocapacitive carbon nanotube (CNT)- MoS2 (CM)-CNT/graphene electrode, with high mass loading up to 55% and uniform distribution of MoS2, hierarchically porous configuration and highly interconnected conductive network framework built by coupling graphene scaffold located in shell layer and CNTs situated in core layer. The unique architecture featuring adequate channels and pathways can act as a “superhighway” for ultrafast ion diffusion and electron transport throughout the entire device, thereby enabling fast kinetics of Faradaic reactions and abundant availability of electrochemical active sites. A symmetric pseudocapacitor assembled with 3D-printed electrode and in situ electrospun poly(vinylidene fluoride) (PVDF) nanofiber separator delivered high specific capacitance (558 mF cm−2) and energy density (49 µWh cm−2), remarkable cycling stability (87% after 8000 cycles), and superior capacity even at large electrode thickness. This work has shed light on new strategies for designing and fabricating high-performance pseudocapacitors toward future uses.

Rational designing of tree-like polymer gel membrane based on PVDF/lamellar organic montmorillonite nanofiber with excellent flame retardancy and superior ion conductivity for high-performance lithium-ion capacitor

A tree-like structured gel nanofibrous membrane (TLNM) including lamellar organic montmorillonite (OMMT) is designed and prepared via one-step electrospinning method. And its application as promising separator for lithium-ion capacitor were systematically researched. Based on the special structure and the addition of lamellar OMMT, the TLNMs are endowed improved mechanical performance, excellent thermal stability and flame-retardant, and good electrochemical properties for Li ion capacitor (LIC). As a result, the nanofibrous membranes exhibits enhanced electrolyte uptake performance (~870%, with an electrolyte contact angel of 0°) and robust thermal dimensional stability (stable at 160°C for 1.5 h). Furthermore, the limiting oxygen index of this membrane is as high as ~30%, which can significatively improve the safety of the nanofiber separator. And the assembled Li ion capacitor with the membrane presented superior ion conductivity (3.95*10−3 S cm−1). Additionally, the LIC also shows enhanced cycling stability (~60% capacity retention after 10,000 cycles at 0.5C), and favorable rate capability. All of the results indicate that the novel polymer gel membrane including excellent flame retardancy and superior ion conductivity is expected to be used for high-performance lithium-ion capacitor.

In situ construction of High-safety and Non-flammable polyimide “Ceramic” Lithium-ion battery separator via SiO2 Nano-Encapsulation

For the purpose of the needs of high-safety energy storage devices, we report a new type of high-performance core–shell structure PI/SiO2 inorganic–organic composite separator. This polymer separator has good flame retardant, strong mechanical properties, high thermal stability and high ion transmission efficiency. Unlike traditional ceramic modified separators, this hybrid separator is achieved by in situ nano-encapsulation hydrolysis. As a precursor of SiO2, Tetrabutyl orthosilicate (TBOS) is selected to coat on PI nanofibers surface, which endows a high retention rate of hydrolyzed silica and a dense/uniform coating. Due to the formation of silica nanoshell, the PI/SiO2 nanofiber separator shows an ultra-high 73.69 MPa tensile strength, a contact angle of only 6.8° and great thermal stability at 378 °C. The NCM811 half-cell equipped with composite separator shows a great capacity of 146 mAh·g−1 at 5C, which is much better than the battery using original PI separator and Celgard-2400 separator. It is worth nothing that the battery with PI/SiO2 separator exhibits superior cycle stability with little energy weaken during 100 cycles. This research provides a new strategy for preparing high-performance non-flammable organic–inorganic composite separators.

Oriented PAN/PVDF/PAN laminated nanofiber separator for lithium-ion batteries

Electrospun nanofiber separators have excellent properties in terms of large surface area and high porosity, which benefits the rate capability, electrochemical stability, and safety performance of lithium-ion batteries. Herein, a 0°PAN/PVDF/90°PAN-oriented composite nanofiber separator is prepared, which is prepared layer by layer by electrospinning technology and drum orientation collector. The results show that when the rotating speed of the drum is 600 r min−1, the 0°PAN/PVDF/90°PAN-oriented composite nanofiber separator has high porosity (about 85%), improved transverse and longitudinal tensile properties (10.33 MPa and 11.03 MPa, respectively), good dimensional stability at 160°C, and good electrochemical performance (specific charge capacity of 165 mAh g−1 at 0.5C and 142.7 mAh g−1 at 1C, capacity retention of 90% after 100 cycles).

A high-safety and multifunctional MOFs modified aramid nanofiber separator for lithium-sulfur batteries

High-level safety is the essential precondition for the future large-scale applications of the high-energy storage systems. In terms of the lithium-sulfur (Li-S) batteries, the low flash point and poor mechanical strength of the conventional polyolefin separator are the potential safety hazards to cause the fire and explosion in extreme environment. Herein, a thermotolerant and nonflammable hierarchical functional separator (Z-PMIA separator) is designed based on the poly(m-phenylene isophthalamide) (PMIA) membrane and the in-situ formed Co-containing zeolitic imidazolate framework ZIF-L(Co) secondary nanostructures. Benefiting from the feature of “Aramid” fibers and the special secondary ZIF-L(Co) nanostructures, the Z-PMIA separator not only guarantees high mechanical strength (suffering from a tensile strength of 15 MPa and a puncture force of 0.95 N) and remarkable thermal stability (no dimensional shrinkage even at 200 °C) but also effectively suppresses the polysulfide shuttle and lithium dendrite growth. As a result, the Z-PMIA separator delivers a high initial discharge capacity of 1391.2 mAh g−1 and a discharge capacity of 961.1 mAh g−1 after 350 cycles at 0.2C, with a slow capacity attenuation of 0.033% per cycle. Even under the conditions of high sulfur loading (9.23 mg cm−2) and low electrolyte/sulfur (E/S) ratio (8 mL g−1) or when running at a high temperature (80 °C), the Z-PMIA separator still enables a high charging/discharging performance, highlighting its promising commercial prospect for future high-safety and high-performance energy storage systems.

A dual-functional poly(vinyl alcohol)/poly(lithium acrylate) composite nanofiber separator for ionic shielding of polysulfides enables high-rate and ultra-stable Li-S batteries

Despite the high theoretical specific capacity, the main challenges of rechargeable lithium-sulfur (Li-S) batteries, including the unceasing shuttle of soluble lithium polysulfides (LiPSs) and severe Li corrosion, seriously hinder their commercial and practical applications. Herein, a bifunctional polyvinyl alcohol/poly(lithium acrylate) (C-PVA/PAA-Li) composite nanofiber separator is developed to address the main challenges in Li-S batteries by simultaneously allowing rapid lithium ion transport and ionic shielding of polysulfides. The C-PVA/PAA-Li composite nanofiber membrane is prepared via the facile electrospinning strategy, followed by thermal crosslinking and in-situ lithiation processes. Differing from the conventional Celgard-based coating methods accompanied by impaired lithium ion transport efficiency, the C-PVA/PAA-Li composite nanofiber membrane possesses well-developed porous structures and high ionic conductivity, thus synergistically reducing the charge transfer resistance and inhibiting the growth of lithium dendrites. The resulting Li-S batteries exhibit an ultra-low fading rate of 0.08% per cycle after 400 cycles at 0.2 C, and a capacity of 633 mAhg−1 at a high current density of 3 C. This study presents an inspiring and promising strategy to fabricate emerging dual-functional separators, which paves the pathway for the practical implementation of ultra-stable and reliable Li-S battery systems.

Enhanced the mechanical strength of polyimide (PI) nanofiber separator via PAALi binder for lithium ion battery

Polyimide (PI) nanofiber membrane prepared by electrospinning technique exhibits poor mechanical strength for the slippage of fibers, which limits it applied into lithium ion battery (LIB). In this study, we are devoted to promoting the macro mechanical strength of PI nanofiber membrane by inhibiting the slippage among the fibers. Lithium polyacrylate (PAALi) is a kind of excellent binder, and it contributes to increasing the adhesion effect between the fibers. As a result, the PI nanofiber membrane treated by PAALi solution shows the mechanical strength of 16.1 MPa, higher than 5.0 MPa of pristine PI membrane. The introduction of this binder makes the originally loose and disordered PI nanofibers cross-link with each other and prevents from the slip between the fibers. Surprisingly, the PAALi binder has no effect on the thermal stability and electrochemical performance of PI nanofiber separator. Furthermore, these LiCoO2/Li cells using the modified PI nanofiber separators exhibit better cycling stability and superior rate performance compared with the cells containing pristine PI separators. It demonstrates that the use of PAALi binder is a promising method to improve the mechanical strength of nanofiber membrane, which gets it applied in the high-power LIB.

Fabrication of metal-organic framework-based nanofibrous separator via one-pot electrospinning strategy

Metal-organic framework (MOF)/polymer composites have attracted extensive attention in the recent years. However, it still remains challenging to efficiently and effectively fabricate these composite materials. In this study, we propose a facile one-pot electrospinning strategy for preparation of HKUST-1/polyacrylonitrile (PAN) nanofibrous membranes from a homogeneous stock solution containing HKUST-1 precursors and PAN. MOF crystallization and polymer solidification occur simultaneously during the electrospinning process, thus avoiding the issues of aggregation and troublesome multistep fabrication of the conventional approach. The obtained HKUST-1/PAN electrospun membranes show uniform MOF distribution throughout the nanofibers and yield good mechanical properties. The membranes are used as separators in Li-metal full batteries under harsh testing conditions, using an ultrathin Li-metal anode, a high mass loading cathode, and the HKUST-1/PAN nanofibrous separator. The results demonstrate significantly improved cycling performance (capacity retention of 83.1% after 200 cycles) under a low negative to positive capacity ratio (N/P ratio of 1.86). The improvement can be attributed to an enhanced wettability of the separator towards electrolyte stemmed from the nanofibrous structure, and a uniform lithium ion flux stabilized by the open metal sites of uniformly distributed HKUST-1 particles in the membrane during cycling.

Planar Fully Stretchable Lithium-Ion Batteries Based on a Lamellar Conductive Elastomer.

Stretchable lithium-ion batteries (LIBs) have attracted great attention as a promising power source in the emerging field of wearable electronics. Despite the recent advances in stretchable electrodes, separators, and sealing materials, building stretchable full batteries remains a big challenge. Herein, a simple strategy to prepare stretchable electrodes and separators at the full battery scale is reported. Then, electrostatic spraying is used to make the anode and cathode on an elastic current collector. Finally, a polyvinylidene fluoride/thermoplastic polyurethane nanofiber separator is hot-sandwiched between the cathode and anode. The fabricated battery shows stable electrochemical performance during repeatable release-stretch cycles. In particular, a stable capacity of 6 mA•h/cm2 at the current rate of 0.5 C can be achieved for the fully stretchable LIB. More importantly, over 70% of the initial capacity can be maintained after 100 cycles with ∼150% stretch.

Homogeneous titanium hydroxide yields hydroxyl decorated nanofiber separator with improved electrolyte uptake and device capacitance

Good wettability of separators in batteries and capacitors is crucial for performance. Titanium hydroxide (Ti(OH)x) has been shown to greatly improve the wettability of polyvinylidene fluoride (PVDF) nanofibers (NFs). In this work, PVDF/Ti(OH)x NFs were evaluated for their potential use as separators. Results show that PVDF NFs incorporated with Ti(OH)x have improved electrolyte uptake (EU) between 550 and 1200 EU% compared to untreated PVDF with less than 100 EU%. Significant improvements in capacitance are seen from capacitors utilizing PVDF/Ti(OH)x NF separators with the best performing PVDF/Ti(OH)x NF at 15 wt% Ti(OH)x with 446.8 mF/cm2 compared to untreated PVDF NF separator with 262.7 mF/cm2.

Preparation of Poly-1-butene Nanofiber Mat and Its Application as Shutdown Layer of Next Generation Lithium Ion Battery.

Nonwoven nanofiber webs from polyolefin show great potential in various fields such as nanofilters, high performance membranes and separators in lithium ion batteries (LiB). Although nonwoven microfiber webs can be obtained by the well-established melt-blown method, it is relatively difficult to produce nonwoven nanofiber web using polyolefin (polyethylene and polypropylene). There have been several reports on the preparation of polyolefin nanofibers by melt-electrospinning, although this approach presents several intrinsic disadvantages, i.e., high processing costs, the requirement of complex equipment, and poor control over pore size or fiber diameter. Solution-based electrospinning has the potential to overcome the drawbacks of melt-electrospinning, but the solubility of most polyolefin is poor. In this study, we found that poly-1-butene, a member of the poly(alpha-olefin) family, can be used in the electrospinning process. We set the concentration of the polymeric solution for electrospinning at 0.65–1.7 g/mL. Here, we report on the fabrication of nonwoven fiber webs composed of poly-1-butene and their copolymers. The diameter of the nonwoven fiber mat was 0.2–0.4 μm, which can be applicable for shutdown layer. As a representative application, we prepared a poly-1-butene nanofiber separator with an appropriate pore size by electrospinning for use as the shut-down layer of a next-generation LiB. The PB-based nanofiber mat provided shutdown ability at around 100 to 120 °C.

A core@sheath nanofiber separator with combined hardness and softness for lithium-metal batteries

Lithium metal shows great promise to be the anode material of next-generation electrical energy storage, but the uncontrollable growth of dendritic and mossy lithium greatly restricts the practical application of lithium-metal batteries. Herein, we demonstrate a sustainable strategy for creating a core-shell structured polyimide@fluorinated poly-m-phenyleneisophthalamide (PI@F-PMIA) nanofiber separator coupled hardness with softness via coaxial electrospinning technique. The PI core with good heat-resistance and breaking tenacity acts as a steady and powerful skeleton support to guarantee the structural stability of the PI@F-PMIA separator, even in the wet environment of the working battery. Meanwhile, the gel F-PMIA shell can endow the PI@F-PMIA membrane with a more intimate electrolyte contact to further enhance the electrolyte affinity so as to improve the ion transport capacity. Therefore, the as-prepared PI@F-PMIA nanofiber membrane delivers the synchronous characteristics of the favorable wetting mechanical flexibility, laudable thermal stability, excellent electrolyte uptake as well as distinct gelation phase, which has the capability to overcome the obstacle of lithium dendrites growth and enable a homogeneous stable electrolyte interface. As a result, the good ionic conductivity and interfacial compatibility can be obtained by using the functionalized PI@F-PMIA separator, and the voltage of the relevant lithium symmetrical battery delivers the relatively minor change during 350 h at 2.0 mA·cm−2. More importantly, the as-assembled PI@F-PMIA-based lithium-metal cells present superior cycling stability with the capacity retention of 83.1% and Coulombic efficiency of 99.7% after 200 cycles at 0.5 C rate, accompanied by an excellent rate recovery capability. It follows that the newfangled PI@F-PMIA nanofiber membrane with softness and strength can be perceived to be a highly qualified separator for high-safety and dendrite-proof lithium-metal batteries.

The effects of polybenzimidazole nanofiber separator on the safety and performance of lithium-ion batteries: Characterization and analysis from the perspective of mechanism

Polybenzimidazole membrane exhibits a promising application prospect in the high-performance and safe operation of lithium-ion batteries (LIBs). However, the relevant characterization and analysis of their safety mechanism is extremely lacking. Herein, a poly(aryl ether benzimidazole) (OPBI) non-woven fabric is prepared by electrospinning technology as a highly safe separator of LIBs. The characterization and analysis of safety mechanism in terms of thermostability, flame resistance, electrochemical stability, as well as dendrite growth, are investigated. The OPBI membrane shows excellent stability, such as no shrinkage at 200 °C, remaining weight above 54% at 800 °C, even no combustion on fire, electrochemical stability window exceeding 5.75 V vs. Li+/Li. Compared with Celgard 2400 separator, the Li dendrite growth is effectively suppressed by the OPBI membrane due to the excellent interface compatibility with Li anode. The Li/LiFePO4 cells using OPBI membrane deliver outstanding cycling stability (a specific capacity retention rate of 94% over 300 cycles) and excellent rate capacity. The results indicate that the OPBI nanofiber membrane is an intriguing candidate for high-performance and safe LIBs.

Anisotropic semi-aligned PAN@PVdF-HFP separator for Li-ion batteries

Compared with the common electrospun nanofibers, the alignment of the nanofibers exhibits interesting anisotropic mechanical properties and structural stability. In this paper, semi-aligned PAN@PVdF-HFP nanofiber separators were prepared by a modified electrospinning method. The composite separators exhibit anisotropic mechanical properties and enhanced electrochemical performance compared with electrospun PAN films. The PAN@PVdF-HFP nanofiber separator can deliver an ionic conductivity of 1.2 mSċcm−1 with electrochemical stability up to 5.0 V. The LiFePO4/Li cell with semi-aligned PAN@PVdF-HFP separator shows excellent cycling performance, good rate capability, as well as high discharge capacity.

Biomineralized zircon-coated PVDF nanofiber separator for enhancing thermo- and electro-chemical properties of lithium ion batteries

As one of the components of the lithium ion batteries (LIBs), the separator plays a vital part in the safety and electrochemical performance. In this work, a ZrO2-ceramic-coated polyvinylidene fluoride (PVDF) nanofibrous separator for LIBs was successfully prepared by electrospinning, subsequent dopamine hydrophilic modification and biomimetic mineralization process. These preparation processes were environmentally friendly and simple. The ZrO2-ceramic coating endows the separator with outstanding electrolyte wettability and thermal stability. To be specific, the separator exhibits higher ionic conductivity (2.261 mS cm−1), high porosity (85.1%) and favorable electrolyte wettability (352%), and lower interfacial impedance (220 Ω). Compared with commercial polyolefin separator and PVDF nanofibrous separator, the ZrO2-ceramic-coated PVDF nanofibrous separator exhibits excellent rate performance and well cyclic stability. Most importantly the ZrO2–PDA/PEI–PVDF separator can still maintain a complete structure even at a high temperature of 300 °C. Compared with commercial separators, it greatly improves the safety of lithium ion batteries at high temperature. Therefore, this separator has far-reaching prospects for improving lithium ion safety and cycle stability.

Wet-spun PVDF nanofiber separator for direct fabrication of coaxial fiber-shaped supercapacitors

Coaxial graphene fiber-based supercapacitors have many enticing advantages in terms of large electrode/electrolyte contact interface area, short ion-transport path and high stability, but the polymer electrolyte separators upon which these supercapacitors are based typically lead to high resistance and the risk of short circuit upon severe deformations. To address these challenges, we have developed an efficient and controllable wet-spinning strategy for making poly(vinylidene fluoride) (PVDF) nanofiber separator with modulated thickness to directly fabricate high-performance coaxial fiber-shaped supercapacitors. The porous PVDF nanofiber separator tightly sandwiched in between graphene fiber electrodes featured an interconnected, thin and highly wrinkled network architecture, thereby enabling sufficient chemical reduction of electrode without concomitant destruction of the separator structure, good contact interface, and rapid infiltration and transport of electrolyte ions to the electrode without risk of short circuit. The coaxial fiber supercapacitor made using the wet-spun PVDF nanofiber separator delivered a remarkable specific capacitance of 346.5 mF cm−2 and energy density of 30.8 µWh cm−2, while maintaining high structural toughness and long cycling life (94% retention after 10,000 cycles). This coaxial architectural design with PVDF nanofiber as separator represents a promising strategy to fabricate high-performance fiber supercapacitors for flexible energy storage.

Highly reliable quinone-based cathodes and cellulose nanofiber separators: toward eco-friendly organic lithium batteries

Recently, organic compounds are considered as promising candidates for application in next-generation energy storage systems to overcome the disadvantages of conventional inorganic cathode materials, including their low specific capacity and poor disposal systems. In particular, pillar[5]quinone (P5Q) is very effective as it provides active sites that favor Li uptake and promote a high theoretical capacity. Herein, we propose P5Q-derived cathodes, which are enveloped in multi-walled carbon nanotubes and cellulose nanofibers (CNFs), fabricated by a simple vacuum-filtering method. The designed cathode solves the issues associated with organic materials, including their high solubilities in aprotic electrolytes and low conductivities. Furthermore, CNFs are introduced as alternatives to conventional polyolefin separators. CNF separators can effectively suppress the dissolution of active materials in liquid electrolytes. In addition, CNFs improve ionic conductivity (0.88 mS cm−1), electrolyte wettability (electrolyte uptake: 333.41%, porosity: 70 ± 5%), and thermal shrinkage in contrast to conventional polyolefin separators. The Li-ion battery, assembled with the suggested P5Q cathode and CNF separator, exhibits highly stable capacity retention (76.5% after 50 cycles at a 0.2 C rate) and good rate capability, although an organic electrolyte is used.