Aramid nanofibers/polyphenylene sulfide nonwoven composite separator fabricated through a facile papermaking method for lithium ion battery

Ultrafast self-assembly of supramolecular hydrogels toward novel flame-retardant separator for safe lithium ion battery

Traditional polyolefin separators for lithium-ion batteries (LIBs) often experience limited thermal stability and intrinsic flammability, resulting in great safety risks during their usage. Therefore, it is highly important to develop novel flame-retardant separators for safe LIBs with high performance. In this work, we report a flame-retardant separator derived from boron nitride (BN) aerogel with a high BET surface area of 1127.3 m² g^-1. The aerogel was pyrolyzed from a melamine-boric acid (MBA) supramolecular hydrogel, which was self-assembled at an ultrafast speed. The in-situ evolution details of the nucleation-growth process of the supramolecules could be observed in real-time using a polarizing microscope under ambient conditions. The BN aerogel was further composited with bacterial cellulose (BC) to form a BN/BC composite aerogel with excellent flame-retardant performance, electrolyte-wetting ability and high mechanical property. By using the BN/BC composite aerogel as the separator, the developed LIBs exhibited high specific discharge capacity of 146.5 mAh g^-1 and excellent cyclic performance, maintaining 500 cycles with a capacity degradation of only 0.012% per cycle. The high-performance flame-retardant BN/BC composite aerogel represents a promising candidate for separators not only in LIBs but also in other flexible electronics.

A nitrocellulose/cotton fiber hybrid composite membrane for paper-based biosensor

Nitrocellulose (NC) membrane was fabricated and tested for its potential use in various paper-based biosensors for use in point-of-care testing. However, contemporary technologies are complex, expensive, non-scalable, limited by conditions, and beset with potentially adverse effects on the environment. Herein, we proposed a simple, cost-effective, scalable technology to prepare nitrocellulose/cotton fiber (NC/CF) composite membranes. The NC/CF composite membranes with a diameter of 20 cm were fabricated in 15 min using papermaking technology, which contributes to scalability in the large-scale production of these composites. Compared with existing commercial NC membranes, the NC/CF composite membrane is characterized by small pore size (3.59 ± 0.19 μm), low flow rate (156 ± 55 s/40 mm), high dry strength (up to 4.04 MPa), and wet strength (up to 0.13 MPa), adjustable hydrophilic-hydrophobic (contact angles ranged from 29 ± 4.6 to 82.8 ± 2.4°), the good adsorption capacity of protein (up to 91.92 ± 0.07 μg). After lateral flow assays (LFAs) detection, the limit of detection is 1 nM, which is similar to commercial NC membrane (Sartorius CN 140). We envision the NC/CF composite membrane as a promising material for paper-based biosensors of point-of-care testing applications.

Separator with Nitrogen-Phosphorus Flame-Retardant for LiNix Coy Mn1- x - y O2 Cathode-Based Lithium-Ion Batteries

With the pursuit of high-energy-density for lithium-ion batteries (LIBs), the hidden safety problems of batteries have gradually emerged. LiNi_x Co_y Mn_{1- x - y} O₂ (NCM) is considered as an ideal cathode material to meet the urgent needs of high-energy-density batteries. However, the oxygen precipitation reaction of NCM cathode at high temperature brings serious safety concerns. In order to promote high-safety lithium-ion batteries, herein, a new type of flame-retardant separator is prepared using flame-retardant (melamine pyrophosphate, MPP) and thermal stable Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). MPP takes the advantage of nitrogen-phosphorus synergistic effect upon the increased internal temperature of LIBs, including the dilution effect of noncombustible gas and the rapidly suppression of undesirable thermal runaway. The developed flame-retardant separators show negligible shrinkage over 200 °C and it takes only 0.54 s to extinguish the flame in the ignition test, which are much superior to commercial polyolefin separators. Moreover, pouch cells are assembled to demonstrate the application potential of PVDF-HFP/MPP separators and further verify the safety performance. It is anticipated that the separator with nitrogen-phosphorus flame-retardant can be extensively applied to various high-energy-density devices owing to simplicity and cost-effectiveness.

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.

Effect of Cross-Linking and Surface Treatment on the Functional Properties of Electrospun Polybenzimidazole Separators for Lithium Metal Batteries

In this work, electrospun PBI separators with a highly porous structure and nanofiber diameter of about 90-150 nm are prepared using a multi-nozzle under controlled conditions for lithium metal batteries. Cross-linking with α, α-dibromo-p-xylene and surface treatment using 4-(chloromethyl) benzoic acid successfully improve the electrochemical as well as mechanical properties of the separators. The resulting separator is endowed with high thermal stability and excellent wettability (1080 to 1150%) with commercial liquid electrolyte than PE and PP (Celgard 2400) separators. Besides, attractive cycling stability and rate capability in LiFePO₄/Li cells are attained with the modified separators. Prominently, CROSSLINK PBI exhibits a stable Coulombic efficiency of more than 99% over 100 charge-discharge cycles at 0.5 C, which is superior to the value of cells using commercial PE and PP (Celgard 2400) separators. The half cells assembled using the CROSSLINK PBI separator can deliver a discharge capacity of 150.3 mAh g^-1 at 0.2 C after 50 cycles corresponding to 88.4% of the theoretical value of LiFePO₄ (170 mAh g^-1). This work offers a worthwhile method to produce thermally stable separators with noteworthy electrochemical performances which opens new possibilities to improve the safe operation of batteries.

Mechanically Strong Electrically Insulated Nanopapers with High UV Resistance Derived from Aramid Nanofibers and Cellulose Nanofibrils

Aramid nanofibers (ANFs) have great potential for civil and military applications due to their remarkable mechanical modulus, excellent chemical reliability, and superior thermostability. Unfortunately, the weak combination of neighboring ANFs limits the mechanical properties of ANF-based materials owing to their inherent rigidity and chemical inertness. Herein, high-performance nanopapers are fabricated by introducing a tiny amount of cellulose nanofibrils (CNFs) to serve as reinforcing blocks via vacuum filtration. As a result of the formation of nanosized building blocks and hydrogen-bonding interaction of CNFs, the resultant ANF/CNF nanopaper yields a record-high tensile strength (406.43 ± 16.93 MPa) and toughness (86.13 ± 5.22 MJ m^-3), which are 1.8 and 4.3 times higher than those of the pure ANF nanopaper, respectively. When normalized by weight, the specific tensile strength of the nanopaper is as high as 307.90 MPa·g^-1·cm³, which is even significantly superior to that of titanium alloys (257 MPa·g^-1·cm³). The ANF/CNF nanopaper also possesses excellent dielectric strength (53.42 kV mm^-1), superior UV-shielding performance (≥99.999% absorption for ultraviolet radiation), and a favorable thermostability (T_onset = 530 °C). This study proposes a new design strategy for developing ultrathin ANF-based nanopapers combined with high reliability and thermostability for application in high-end electrical insulation fields, such as 5G communication, wearable electronics, and artificial intelligence.

Ultrafast formation of ANFs with kinetic advantage and new insight into the mechanism

Aramid nanofibers (ANFs) have important applications in many fields, including electrical insulation and battery separators. However, a few limitations seriously restrict the application of ANFs currently, such as low preparation efficiency and the unclear preparation mechanism. To overcome these limitations, the present work proposes a new view-point from the perspective of reaction kinetics. The preparation efficiency was proven to essentially rely on the effective c(OH^-). With a simple pre-treatment, a kinetic advantage was created and the preparation time of ANFs was reduced from multiple hours to 10 minutes, which was a considerable step towards practical applications. Moreover, the resultant ANF membranes still exhibited excellent properties in terms of mechanical strength (tensile strength > 160 MPa), thermal stability, light transmittance, and electrical insulation (above 90 kV mm^-1). This work not only presents an ultrafast method to produce ANFs but also provides new insights into the mechanism that will benefit the subsequent development of ANF-based materials.

Application of Aramid Nanofibers in Nanocomposites: A Brief Review

The diameter of fibers is a critical factor in determining their final applications. When the diameter of aramid fibers changes from microns to nanoscale, its range of applications will be greatly extended. In this short review, the preparation of aramid nanofibers (ANFs) with diameters from ten nanometers to more than one hundred nanometers is introduced. Due to their excellent mechanical properties and their chemical and thermal stability, ANFs have been widely used as novel nanomaterials and composited with other materials, mainly for use in reinforced composites, energy storage, filtration and adsorption, biomedicine and electromagnetic fields. In this short review, the application of ANFs and their composites during the last 10 years is concisely summarized and a brief perspective on ANFs and their composites is also presented.

Evaporation and in-situ gelation induced porous hybrid film without template enhancing the performance of lithium ion battery separator

The current commercialized polyethylene (PE) separator has poor wettability and thermal stability which will seriously restrict the electrochemical performance and affect the safety of lithium ion battery. Herein, a porous hybrid layer coated separator with high thermal stability, good electrochemical performance and improved wettability was prepared by a template-free method via the synergistic effect between tetraethoxysilane (TEOS) and aramid nano fibers (ANFs) during the evaporation of solvent and the in-situ gelation of TEOS. The results show that the porous hybrid coating layers can enhance the thermal stability, wettability and electrolyte uptake of the separators. Moreover, the lithium ion transference number is also increased. As a result, the battery assembled with the composite separator exhibits enhanced electrochemical performance in terms of cycle stability and rate performance. When coupled with LiCoO₂cathode, the capacity retention rate is as high as 96.0% after 100 cycles at 0.2C.

Enhancement of the electrochemical performance of lithium-ion batteries by SiO2@poly(2-acrylamido-2-methylpropanesulfonic acid) nanosphere addition into a polypropylene membrane

Employing electrostatic self-assembly and free radical polymerization, the surface of SiO₂ nanospheres was coated with poly(2-acrylamido-2-methylpropanesulfonic acid) (SiO₂@PAMPS) bearing strong electron withdrawing sulfonic and amide groups, enhancing the dissociation ability of the lithium salt of the liquid electrolyte and absorbing anions via hydrogen bonds. After SiO₂@PAMPS nanospheres were introduced into the polypropylene (PP) membrane (SiO₂@PAMPS/PP), the electrolyte affinity and electrolyte uptake of the composite separators were significantly improved. The ionic conductivity of SiO₂@PAMPS/PP-18% (where 18% represents the concentration of the solution used for coating) soaked in liquid electrolyte was even 0.728 mS cm^-1 at 30 °C, much higher than that of the pristine PP membrane. The LiFePO₄/Li half-cell with SiO₂@PAMPS/PP-18% had a discharge capacity of 148.10 mA h g^-1 and retained 98.67% of the original capacity even after 120 cycles at 0.5C. Even at a rate of 1.0C, the cell capacity could be maintained above 120 mA h g^-1. Therefore, a coating formula was developed that could considerably improve the cycling ability and high rate charge-discharge performance of lithium ion batteries.

Enhanced thermal stability and wettability of an electrospun fluorinated poly(aryl ether ketone) fibrous separator for lithium-ion batteries

To enhance the comprehensive performance of lithium-ion batteries (LIBs), a novel fluorinated poly(aryl ether ketone) (FPAEK) compound was synthesized and further fabricated as a nonwoven fibrous separator for LIBs via an electrospinning method.