Robust fluorinated polyimide nanofibers membrane for high-performance lithium-ion batteries

Graphene Nanofibers by Integrated Manufacturing of Electrospinning and Laser Graphitization for Miniaturized Energy Storage Devices

Carbon nanofibers (CNFs) are emerging as promising materials for miniaturized energy storage devices (MESDs) due to their high specific surface area, excellent electrochemical performance, low internal resistance, and durability. Their versatility and tunability make them ideal candidates for various applications, making CNFs a key player in advancing compact and efficient energy storage solutions. Nonetheless, CNFs necessitate an extra step involving either physical or chemical treatments to regulate their morphology, augment surface area, and create micropatterns suitable for MESDs. Here, innovations in material fabrication using an integrated manufacturing process are reported that combines electrospinning and laser-induced graphitization to create graphene nanofibers (GNFs) from fluorinated polyimide nanofibers (fPI NFs). Initially, electrospinning yields uniformly sized and shaped fluorinated poly(amic) acid nanofibers, which are subsequently thermally imidized to form fPI NFs. Laser photothermal treatment of fPI NFs generates hierarchical meso- and nanopores in GNFs, enhancing specific surface area and electrochemical properties, including specific capacitance, cyclic stability, rate capability, areal capacitance, power density, and energy density. This integrated approach synergistically fabricates GNFs for MESD applications, particularly GNF-based micro-supercapacitors (MSCs), demonstrating a remarkable areal capacitance and an aerial energy density two orders of magnitude higher than MSCs based on laser-induced graphene derived from conventional polyimide film.

A NiCo oxide/NiCo sulfate hollow nanowire-coated separator: a versatile strategy for polysulfide trapping and catalytic conversion in high-performance lithium-sulfur batteries

Lithium-sulfur batteries (LSBs) are highly anticipated due to their remarkable theoretical specific energy and energy density. Nevertheless, the polysulfide shuttle effect severely curtails their cycle life, posing a significant obstacle to commercialization. Herein, we introduce nickel-cobalt oxide/nickel-cobalt sulfate hollow nanowires (NCO/NCSO-HNWs) as a separator modification material. The ingeniously designed hollow nanostructure of NCO/NCSO-HNWs endows it with a profusion of adsorption and catalytic active sites. This unique feature enables it to not only physically adsorb lithium polysulfides (LiPSs) but also catalytically convert them, thereby remarkably enhancing the anchoring and conversion efficiency of LiPSs. The LSBs equipped with NCO/NCSO-HNWs-modified separators exhibit an outstanding initial capacity of 1260 mA h g-1 at 0.2C. Even after 100 cycles, a high capacity of 956 mA h g-1 is retained, corresponding to an impressive retention rate of 75.9%. Notably, at 1C, after enduring 500 cycles, the discharge capacity still stabilizes at 695 mA h g-1. The utilization of such hollow nanowire-based separator modification materials represents a novel and effective strategy for elevating the performance of LSBs, holding substantial promise for surmounting the challenges associated with the shuttle effect and expediting the commercialization journey of LSBs.

Highly Stretchable Polyurethane Porous Membranes with Adjustable Morphology for Advanced Lithium Metal Batteries

A highly flexible, tunable morphology membrane with excellent thermal stability and ionic conductivity can endow lithium metal batteries with high power density and reduced dendrite growth. Herein, a porous Polyurethane (PU) membrane with an adjustable morphology was prepared by a simple nonsolvent-induced phase separation technique. The precise control of the final morphology of PU membranes can be achieved through appropriate selection of a nonsolvent, resulting a range of pore structures that vary from finger-like voids to sponge-like pores. The implementation of combinatorial DFT and experimental analysis has revealed that spongy PU porous membranes, especially PU-EtOH, show superior electrolyte wettability (472%), high porosity (75%), good mechanical flexibility, robust thermal dimensional stability (above 170 °C), and elevated ionic conductivity (1.38 mS cm-1) in comparison to the polypropylene (PP) separator. The use of PU-EtOH in Li//Li symmetric cell results in a prolonged lifespan of 800 h, surpasing the longevity of PU or PP cells. Moreover, when subjected to a high rate of 5 C, the LiFePO4/Li half-cell with a PU-EtOH porous membrane displayed better cycling performance (115.4 mAh g-1) compared to the PP separator (104.4 mAh g-1). Finally, the prepared PU porous membrane exhibits significant potential for improving the efficiency and safety of LMBs.

Direct Fabrication of PET-Based Thermotolerant Separators for Lithium-Ion Batteries with Ion Irradiation Technology

Lithium-ion batteries (LIBs) play a pivotal role as essential components in various applications, including mobile devices, energy storage power supplies, and electric vehicles. The widespread utilization of LIBs underscores their significance in the field of energy storage. High-performance LIBs should exhibit two key characteristics that have been persistently sought: high energy density and safety. The separator, a critical part of LIBs, is of paramount importance in ensuring battery safety, thus requiring its high thermal stability and uniform nanochannels. Here, the novel ion-track etched polyethylene terephthalate (ITE PET) separator is controllably fabricated with ion irradiation technology. Unlike conventional polypropylene (PP) separators, the ITE PET separator demonstrated vertically aligned nanochannels with uniform channel size and distribution. The remarkable characteristics of the ITE PET separator include not only high electrolyte wettability but also exceptional thermal stability, capable of withstanding temperatures as high as 180 °C. Furthermore, the ITE PET separator exhibits a higher lithium-ion transfer number (0.59), which is advantageous in enhancing battery performance. The structural and inherent advantages of ITE PET separators contribute to enhance the C-rate capacity, electrochemical, and long-term cycling (300 cycles) stability observed in the corresponding batteries. The newly developed method for fabricating ITE PET separators, which possess high thermal stability and a uniform channel structure, fulfills the demand for high-temperature-resistant separators without requiring any modification procedures. Moreover, this method can be easily scaled up using simple processes, making it a competitive strategy for producing thermotolerant separators.

In Situ Thermal Safety Aspect of the Electrospun Polyimide-Al2O3 Separator Reveals Less Exothermic Heat Energies Than Polypropylene at the Thermal Runaway Event of Lithium-Ion Batteries

Polyimide-Al₂O₃ membranes are developed as a direct alternative to current polyolefin separators by the electrospinning technique and their chemical structures confirm the carbonyl group with the presence of asymmetric and symmetric stretching and bending vibrations at 1778, 1720, and 720 cm^-1 and stretching vibration at 1373 cm^-1 for the imide group. Porous nanofiber architecture morphology is realized with a nanofiber thickness of ∼200 nm and shows an ultrasmooth surface and >1 μm pore size in the architecture, built with the chemical constituents of carbon, nitrogen, aluminum, and oxygen elements. The galvanostatic cycling study of the Li/PI-Al₂O₃/LiFePO₄ lithium cell delivers stable charge-discharge capacities of 144/143 mAh g^-1 at 0.2 C and 110/100 mAh g^-1 at 1 C for 1-100 cycles. The fabricated MCMB/PI-Al₂O₃/LiFePO₄ lithium-ion full-cell reveals less charge transfer resistance of R_ct ∼ 25 Ω and yields stable charge-discharge capacities of 125/119 mAh g^-1. The thermogravimetric curve for the PI-Al₂O₃ separator discloses thermal stability up to 525 °C, and the differential scanning calorimetric curve shows a straight line until 300 °C and depicts high thermal stability than the PP separator. In situ multimode calorimetry analysis of the MCMB/PP/LiFePO₄ full-cell showed a pronounced exothermic peak at 225 °C with a higher released heat energy of 211 J g^-1 at the thermal runaway event, while the MCMB/PI-Al₂O₃/LiFePO₄ full-cell revealed an almost 8-fold less exothermic released heat energy of 25 J g^-1 than the Celgard polypropylene separator, which was because the MCMB anode and LiFePO₄ cathode can be mechanically isolated without any additional separator's melting and burning reactions, as a fire-suppressant separator for lithium-ion batteries.

Polyimide-Based Materials for Lithium-Ion Battery Separator Applications: A Bibliometric Study

Polyimide (PI) has excellent thermal stability, high porosity, and better high-temperature resistance. It has the potential to become a more high-end separator material, which has attracted the attention of the majority of researchers. This review is aimed at identifying the research progress and development trends of the PI-based material for separator application. We searched the published papers (2012–2021) from the WOS core collection database for analysis and analyzed their research progress and development trend based on CiteSpace text mining and visualization software. The analysis shows that the PI-based composite separator material is a research hotspot in the future and the combination of nanofiber and cellulose materials with PI is also an important research direction in the future.

Preparation of porous fluorinated polyimide separator for lithium-ion batteries by non-solvent induced phase separation process

A series of novel porous fluorinated polyimide (FPI) separators containing trifluoromethyl group (–CF3) were prepared by the non-solvent induced phase separation (NIPS) strategy. The prepared FPI separator with 60% molar content (fluorinated dianhydride: non-fluorinated dianhydride: diamine = 60: 40: 100) of fluorinated groups (FPI-60%) could stably exist in the electrolyte as a LIBs separator. The resultant FPI-60% separator possesses high thermal stability with the Tg of 289.4°C and exhibits no shrinkage even at 200°C. The morphologies of the FPI-60% separators were adjusted by introducing small molecular non-solvent additives-ethanol, and the FPI-60% separators present the spongy-like and interconnected structure with different porosity as the amount of ethanol changed from 1 wt% to 10 wt%. The FPI-60% separators display excellent electrolyte uptake with 170%–200% and the ionic conductive could reach 1.17 mS/cm which is four times approximately than that of the PP separator. The lithium-ion batteries (LIBs) using FPI-60% separators with 10 wt% ethanol added show better rate capacities (102.8 mAh/g, 70.8 mAh/g of PI-10 and PP separator at 2 C, respectively) and the capacity retention rate is 93.2% after 50 cycles. The results prove that the porous FPI separator is a promising candidate for high-performance LIBs.

Highly Stable Porous Polyimide Sponge as a Separator for Lithium-metal Secondary Batteries

To inhibit Li-dendrite growth on lithium (Li)-metal electrodes, which causes capacity deterioration and safety issues in Li-ion batteries, we prepared a porous polyimide (PI) sponge using a solution-processable high internal-phase emulsion technique with a water-soluble PI precursor solution; the process is not only simple but also environmentally friendly. The prepared PI sponge was processed into porous PI separators and used for Li-metal electrodes. The physical properties (e.g., thermal stability, liquid electrolyte uptake, and ionic conductivity) of the porous PI separators and their effect on the Li-metal anodes (e.g., self-discharge and open-circuit voltage properties after storage, cycle performance, rate capability, and morphological changes) were investigated. Owing to the thermally stable properties of the PI polymer, the porous PI separators demonstrated no dimensional changes up to 180 °C. In comparison with commercialized polyethylene (PE) separators, the porous PI separators exhibited improved wetting ability for liquid electrolytes; thus, the latter improved not only the physical properties (e.g., improved the electrolyte uptake and ionic conductivity) but also the electrochemical properties of Li-metal electrodes (e.g., maintained stable self-discharge capacity and open-circuit voltage features after storage and improved the cycle performance and rate capability) in comparison with PE separators.

Progress on the Fabrication and Application of Electrospun Nanofiber Composites

Nanofibers are one of the most attractive materials in various applications due to their unique properties and promising characteristics for the next generation of materials in the fields of energy, environment, and health. Among the many fabrication methods, electrospinning is one of the most efficient technologies which has brought about remarkable progress in the fabrication of nanofibers with high surface area, high aspect ratio, and porosity features. However, neat nanofibers generally have low mechanical strength, thermal instability, and limited functionalities. Therefore, composite and modified structures of electrospun nanofibers have been developed to improve the advantages of nanofibers and overcome their drawbacks. The combination of electrospinning technology and high-quality nanomaterials via materials science advances as well as new modification techniques have led to the fabrication of composite and modified nanofibers with desired properties for different applications. In this review, we present the recent progress on the fabrication and applications of electrospun nanofiber composites to sketch a progress line for advancements in various categories. Firstly, the different methods for fabrication of composite and modified nanofibers have been investigated. Then, the current innovations of composite nanofibers in environmental, healthcare, and energy fields have been described, and the improvements in each field are explained in detail. The continued growth of composite and modified nanofiber technology reveals its versatile properties that offer alternatives for many of current industrial and domestic issues and applications.

A Heat-Resistant Poly(oxyphenylene benzimidazole)/Ethyl Cellulose Blended Polymer Membrane for Highly Safe Lithium-Ion Batteries

A blended membrane based on poly(oxyphenylene benzimidazole) (PBI) and ethyl cellulose (EC) exhibits heat resistance and good electrochemical performance. The prepared blended polymer gel membranes show no visible dimensional change after being held at 350 °C for 30 min, whereas the polyethylene (PE) separator almost completely melts. In addition to excellent thermal stability, the self-supporting blended membranes also exhibit a uniform thermal distribution during the heating process from 60 to 200 °C. Additionally, the ionic conductivities of the PBI/EC blended membranes with different ratios are 1.24 mS cm^-1 (1:1), 2.58 mS cm^-1 (1:2), and 1.68 mS cm^-1 (1:3), which are much higher than those of the PE separator (0.39 mS cm^-1). Compared to that of the PE separator (113 mAh g^-1), the cell with a separator of PBI/EC = 1:2 retained a discharge capacity of 131 mAh g^-1 after 150 cycles at 0.5C. Meanwhile, the rate performance of the cell was also better than that of the PE separator, especially at high currents (5C). All of the results indicate that this blended polymer gel membrane with good thermal stability is expected to be applied to high-performance lithium-ion batteries.

Preparation of Highly Porous PAN-LATP Membranes as Separators for Lithium Ion Batteries

Separators are a vital component to ensure the safety of lithium-ion batteries. However, the commercial separators employed in lithium ion batteries are inefficient due to their low porosity. In the present study, a simple electrospinning technique is adopted to prepare highly porous polyacrylonitrile (PAN)-based membranes with a higher concentration of lithium aluminum titanium phosphate (LATP) ceramic particles, as a viable alternative to the commercialized separators used in lithium ion batteries. The effect of the LATP particles on the morphology of the porous membranes is demonstrated through Field emission scattering electron microscopy. X-ray diffraction and Fourier transform infrared spectra studies suitably demonstrate the mixing of PAN and LATP particles in the polymer matrix. PAN with 30 wt% LATP (P-L30) exhibits an enhanced porosity of 90% and is more thermally stable, with the highest electrolyte uptake among all the prepared membranes. Due to better electrolyte uptake, the P-L30 membrane demonstrates an improved ionic conductivity of 1.7 mS/cm. A coin cell prepared with a P-L30 membrane and a LiFePO4 cathode demonstrates the highest discharge capacity of 158 mAh/g at 0.5 C-rate. The coin cell with the P-L30 membrane also displays good cycling stability by retaining 97.5% of the initial discharge capacity after 200 cycles of charging and discharging at a 1C rate.

Recent Development in Separators for High-Temperature Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are promising energy storage devices for integrating renewable resources and high power applications, owing to their high energy density, light weight, high flexibility, slow self-discharge rate, high rate charging capability, and long battery life. LIBs work efficiently at ambient temperatures, however, at high-temperatures, they cause serious issues due to the thermal fluctuation inside batteries during operation. The separator is a key component of batteries and is crucial for the sustainability of LIBs at high-temperatures. The high thermal stability with minimum thermal shrinkage and robust mechanical strength are the prime requirements along with high porosity, ionic conductivity, and electrolyte uptake for highly efficient high-temperature LIBs. This Review deals with the recent studies and developments in separator technologies for high-temperature LIBs with respect to their structural layered formation. The recent progress in monolayer and multilayer separators along with the developed preparation methodologies is discussed in detail. Future challenges and directions toward the advancement in separator technology are also discussed for achieving remarkable performance of separators in a high-temperature environment.

Synthesis and properties of benzocyclobutene-terminated imide-containing cyano group

Benzocyclobutene (BCB) resins have aroused much interest because of their excellent physical and chemical properties. Unfortunately the temperature required to induce cross-linking in typical BCBs is higher than 250°C, which restricts their applications. In this study, a novel cyano-containing BCB-terminated imide monomer was synthesized through the reaction of 1-cyano-5-amino-benzocyclobutene with 4,4′-oxydiphthalic anhydride. This monomer allows a 50–100°C lower curing temperature in comparison with typical BCBs, and it is highly soluble in various solvents and can easily convert to cured film at 150–200°C. Because of the presence of rigid imide group and strong polar cyano group, the cured film exhibits excellent mechanical strength with tensile strength up to 87.8 MPa, high glass transition temperature up to 350°C, low coefficient of thermal expansion (36.9 ppm K−1), and outstanding planarity with the average surface roughness as low as 0.26 nm.

Robust polyimide nanofibrous membrane with porous-layer-coated morphology by in situ self-bonding and micro-crosslinking for lithium-ion battery separator

Herein, we demonstrate a strategy to improve the tensile strength, thermal safety issues, and electrochemical performance of an as-synthesized polyimide separator. By spraying the solution of a specific chemical constituent on both sides of a poly(amic acid) non-woven membrane followed by thermal treatment, a novel polyimide nanofibrous membrane with porous-layer-coated morphology was successfully fabricated by in situ self-bonding and micro-crosslinking technique. The self-bonding and micro-crosslinking techniques improve the tensile strength of the nanofiber membranes from 5 MPa to 28 MPa by forming a crosslinked network structure, thereby reducing the risk of nanofiber disassembly during long-term operation. The rigid structure and aromatic groups in the polyimide chain enable the separator to have outstanding thermal dimensional stability at temperatures as high as 300 °C and thermal stability (5% weight loss at about 528 °C). Additionally, the unique flame retarding capability of polyimide ensures high security of the battery as well. Notably, the lithium-ion battery using porous-layer-coated polyimide separator exhibits a much higher capability (129.9 mA h g^-1, 5C) than that using a Celgard-2400 separator (95.2 mA h g^-1, 5C) and could work steadily at 120 °C, thus implying promising application in next generation high-safety and high-performance lithium-ion batteries.

Ion-Selective Polyamide Acid Nanofiber Separators for High-Rate and Stable Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have attracted great attention because of their high energy density and high theoretical capacity. However, the "shuttle effect" caused by the dissolution of polysulfides in liquid electrolytes severely hinders their practical applications. Herein, we originally propose a carboxyl functional polyamide acid (PAA) nanofiber separator with dual functions for inhibiting polysulfide transfer and promoting Li⁺ migration via a one-step electrospinning synthesis method. Especially, the functional groups of -COOH in PAA separators provide an electronegative environment, which promotes the transport of Li⁺ but suppresses the migration of negative polysulfide anions. Therefore, the PAA nanofiber separator can act as an efficient electrostatic shield to restrict the polysulfide on the cathode side, while efficiently promoting Li⁺ transfer across the separator. As a result, an ultralow decay rate of only 0.12% per cycle is achieved for the PAA nanofiber separator after 200 cycles at 0.2 C, which is less than half that (0.26% per cycle) of the commercial Celgard separator.