Nanofiber Separator Research Articles

Cocross-Linked Nanofibrous Separator with High Ion Transport Capacity for Lithium-Ion Batteries

Cocross-Linked Nanofibrous Separator with High Ion Transport Capacity for Lithium-Ion Batteries

Nanofibrous Covalent Organic Frameworks as the Cathode, Separator, and Anode for Batteries with High Energy Density and Ultrafast-Charging Performance.

To meet the demand for longer driving ranges and shorter charging times of power equipment in electric vehicles, engineering fast-charging batteries with exceptional capacity and extended lifespan is highly desired. In this work, we have developed a stable ultrafast-charging and high-energy-density all-nanofibrous covalent organic framework (COF) battery (ANCB) by designing a series of imine-based nanofibrous COFs for the cathode, separator, and anode by Schiff-base reactions. Hierarchical porous structures enabled by nanofibrous COFs were constructed for enhanced kinetics. Rational chemical structures have been designed for the cathode, separator, and anode materials, respectively. A nanofibrous COF (AA-COF) with bipolarization active sites and a wider layer spacing has been designed using a triphenylamine group for the cathode to achieve high voltage limits with fast mass transport. For the anode, a nanofibrous COF (TT-COF) with abundant polar groups, active sites, and homogenized Li+ flux based on imine, triazine, and benzene has been synthesized to ensure stable fast-charging performance. As for the separator, a COF-based electrospun polyacrylonitrile (PAN) composite nanofibrous separator (BB-COF/PAN) with hierarchical pores and high-temperature stability has been prepared to take up more electrolyte, promote mass transport, and enable as high-temperature operation as possible. The as-assembled ANCB delivers a high energy density of 517 Wh kg-1, a high power density of 9771 W kg-1 with only 56 s of ultrafast-charging time, and high-temperature operational potential, accompanied by a 0.56% capacity fading rate per cycle at 5 A g-1 and 100 °C. This ANCB features an ultralong lifespan and distinguished ultrafast-charging performance, making it a promising candidate for powering equipment in electric vehicles.

Zwitterion Modified Polyacrylonitrile Fiber Separator for Long‐Life Zinc‐Ion Batteries

AbstractThe major challenges of aqueous zinc‐ion batteries (ZIBs) are the dendrite formation and the passivation of zinc metal anode, which restrict their practical applications. Herein, polyacrylonitrile (PAN) and zwitterionic surfactant (Sulfobetaine methacrylate, SBMA) are combined as a precursor to design a modified PAN nanofiber separator by electrospinning. SBMA with sulfonate group [SO3−] can alter the physical property of the precursor solution and electrostatic field during the electrospinning process. Besides, it can regulate the zinc ions crossing and homogenize the electric field by minimizing the zinc ion concentration polarization on the zinc foil surface due to the polar group. As a result, Zn symmetric cells with PAN@SBMA separators show long‐term stability (up to 1700 h), which outdistances the cell with GF‐D (only up to 50 h) under current density at 1 mA cm−2. Meanwhile, the Zn//PAN@SBMA//NH4V4O10 full cells have high specific capacity (236 mAh g−1) and excellent long‐term durability with 84.2% capacity retention after 2000 cycles at 5 A g−1. This work illustrates an easy and effective way to design a high ion transference number and functional PAN‐based separator for regulating Zn2+ deposition for the development of high‐performance ZIBs.

Nanoporous Aramid Nanofiber Separators with High Modulus and Thermal Stability for Safe Lithium-Ion Batteries.

Developing high-safety separators is a promising strategy to prevent thermal runaway in lithium-ion batteries (LIBs), which stems from the low melting temperatures and inadequate modulus of commercial polyolefin separators. However, achieving high modulus and thermal stability, along with uniform nanopores in these separators, poses significant challenges. Herein, the study presents ultrathin nanoporous aramid nanofiber (ANF) separators with high modulus and excellent thermal stability, enhancing the safety of LIBs. These separators are produced using a microfluidic-based continuous printing strategy, where the flow thickness can be meticulously controlled at the micrometer scale. This method allows for the continuous fabrication of nanoporous ANF separators with thicknesses ranging from 1.6 ± 0.1µm to 2.7 ± 0.1µm. Thanks to the double-side solvent diffusion, the separators exhibit controllably uniform pore sizes with a narrow distribution, spanning from 40 ± 5nm to 105 ± 9nm, and a high modulus of 3.3 ± 0.5GPa. These nanoporous ANF separators effectively inhibit lithium dendrite formation, resulting in a high-capacity retention rate for the LIBs (80% after 240 cycles). Most notably, their robust structural and mechanical stability at elevated temperatures significantly enhances LIB safety under transient thermal abuse conditions, thus addressing critical safety concerns associated with LIBs.

Batch Preparation and Performance Study of Boehmite-Based Electrospun Nanofiber Separators for Lithium-Ion Batteries.

The design and preparation of high-performance separators for lithium-ion batteries (LIBs) have far-reaching practical significance in enhancing the overall performance of LIBs. Electrospun nanofiber separators (ENSs) have the characteristics of large specific surface area, high porosity, small pore size and good affinity with the electrolyte, making them become ideal candidates for LIB separators. In this work, polyacrylonitrile (PAN)/polyurethane (PU) (PAU) ENSs loaded with boehmite (BM) particles (BM/PAU ENSs) were mass-produced using spherical section free surface electrospinning (SSFSE), and used as LIB separators. Their morphology, structures and performances were tested and characterized. The results showed that all BM/PAU ENSs maintained excellent thermal dimensional stability in the range of 140-180 °C, and had good electrolyte wettability and high porosity. The composite BM/PAU-2 ENS with the best performance had a porosity of 52.5%, an electrolyte uptake rate of 822.1%, and an ionic conductivity of 1.97 mS/cm. Additionally, the battery assembled with BM/PAU-2 separator also demonstrated best electrochemical performance, cycling performance, and rate capability, with a capacity retention rate of 94.4% after 80 cycles at 0.5 C, making it a promising high-performance separator for LIBs.

Core-shell nanofiber separator incorporated with PDA-PEI co-modified Li0.33La0.57TiO3 nanowires prepared by co-axial electrospinning for dendrite-free lithium metal batteries

The core-shell nanofibers containing PDA-PEI co-modified Li0.33La0.57TiO3 (LLTO) perovskite nanowires in the shell layer were fabricated by co-axial electrospinning technique to apply as separator in dendrite-free lithium metal batteries. The PVDF-HFP with good mechanical properties and polar C–F bonds was used as the core material, and polyacrylonitrile (PAN) with high thermal stability was employed as the shell layer in nanofibers. The PDA-PEI co-modified Li0.33La0.57TiO3 nanowires, reacting with PF6− anions in LiPF6 salt can not only capture free anions but also provide a lithium-ion migration pathway. Moreover, the core-shell structured nanofibers exhibited high mechanical strength (9.2 N/mm2), robust thermal stability (>0% at 200 °C for 1 h), and favorable electrolyte affinity (electrolyte uptake of 732%). Gelation of PVDF-HFP in liquid electrolyte leads to high-ionic conductivity (3.52 mS/cm), Li+ transference number (tLi+ = 0.68), uniform Li+ flux, and smooth Li deposition. The Li plating/stripping test consisting of Li–Li cells (with CS/LLTO-PDA-PEI) carried out at 0.4 mA/cm2 was conducted for 1000 h without a short circuit. NCM-Li and NCM-Gr coin cells using the CS (core-shell)/LLTO-PDA-PEI separator retained long-term cycle stability and good Coulombic efficiency (CE).

A Five Micron Thick Aramid Nanofiber Separator Enables Highly Reversible Zn Anode for Energy‐Dense Aqueous Zinc‐Ion Batteries

AbstractThe rampant dendrites growth caused by uncontrolled deposition of Zn2+ ions at Zn metal anode poses a significant obstacle to the practical applications of aqueous zinc‐ion batteries (ZIBs). Herein, an ultrathin (5 µm) aramid nanofiber (ANF) separator is reported to enhance the Zn anode stability and the ZIB energy density. Through systematic experimental studies and DFT simulations, it is demonstrated that the ANF separator with unique surface polarity can modify the solvation configuration, facilitate desolvation, and regulate the deposition orientation of Zn2+ ions. Consequently, the Zn anode with the ANF separator demonstrates an 85‐fold increase in running time beyond 850 h compared with the conventional glass fiber separator at 5 mA cm−2/2.5 mAh cm−2. Even under the harsh depth of discharge conditions of 50% and 80%, the Zn anodes still sustain extended cycling periods of over 475 and 200 h, respectively. As pairing this ANF separator with thin Zn anode and high‐areal‐capacity Mn2.5V10O24∙5.9H2O cathode in a low negative capacity/positive capacity ratio (2.64) full cell, superior gravimetric/volumetric energy density (129.2 Wh kg−1/142.5 Wh L−1) is achieved, far surpassing majority of the ZIB counterparts reported in the literature. This work offers a promising ultrathin separator for promoting the utilization of energy‐dense aqueous ZIBs.

Regulating pore structure of aramid nanofiber (ANF) separators for lithium–sulfur (Li–S) batteries

Excellent ionic conductivity and mechanical robustness are significant for separators of Li–S batteries. Aramid nanofiber (ANF) has been widely used in separators due to their excellent mechanical properties and high-temperature resistance. However, pure ANF separator possesses a dense pore structure resulting from the closely intertwined nanofibrous network, leading to inferior ionic conductivity. Herein, we propose a strategy of inhibiting of hydrogen bonding (IHB) among nanofibers to regulate the pore structure of ANF separators via employing pore-forming agent, solvation, and differentiated drying methods. Notably, a graph theoretical methodology (structural GT) is introduced to analyze the percolating network of ANF separator, revealing that the higher average nodal connectivity, the more abundant and homogeneous porous structure and higher conductivity. Excitingly, the pore size and the ionic conductivity of ANF separator by supercritical carbon dioxide drying (S-ANFs) is 44 nm and 0.171 mS/cm, which is 5 times and 1.9 times higher than pure ANF separator, respectively. Moreover, the ANF separator is dimensionally stable under 200 °C, demonstrating its desirable security under extreme conditions. Finally, the half-cell equipped resultant S-ANFs exhibits outstanding cycling stability (566 mAh/g after 200 cycles at 0.5 C) and Coulombic efficiency (99.25%). This work provides an efficient strategy to regulate the pore structure of ANF separator.

Tri-monomer polyamide acid nanofiber separator for safe and high performance lithium-ion batteries

The separators play a crucial role in ensuring the safety of lithium batteries and greatly affect the electrochemical performance of the battery. However, the weak thermal stability and non-polar surface of commercial polyolefin separators severely restrict the development of the high-performance lithium batteries. In this work, we initially proposed polyamide acid (PAA) nanofiber separator with three monomers through a two-step synthesis method, which was prepared by electrospinning. By controlling different processing temperatures, PAA separators containing different amount of carboxyl groups were obtained. PAA separator had better wettability and thermal stability than PE separator. Meanwhile, the electrospinning method brings ultra-high porosity (95.3 %) to the membrane, resulting a much higher electrolyte absorption and retention rate of PAA (initially 3214 %, 240 % at 50 min) compared to PE separator. In particular, carboxyl groups serve as functional groups to provide an electronegative atmosphere, promoting the transport of lithium ion between the anode and cathode, and playing an important role in improving ionic conductivity. Consequently, batteries with PAA-SOH-50 separator showed the best rate charging/discharging performance, maintaining 87.7 % and 73.6 % of its initial capacity at 2.0 C. Moreover, cells with PAA separator exhibited much higher initial discharge capacity (164.3 mAh g−1) than that with PE separator (126.8 mAh g−1). After 100 cycles at 0.5 C, the PAA nanofiber separator achieved an ultralow attenuation rate of only 0.04 % per cycle, which was less than half of the commercial PE separator (0.10 % per cycle). This work provides a new vision for the development of separator materials, revealing the great influence of functional groups, such as carboxyl groups, on the performance of separators, which provides a strong basis for the further research of high-performance separators.

Halloysite nanotubes enhanced polyimide/oxidized-lignin nanofiber separators for long-cycling lithium metal batteries

The high energy density and robust cycle properties of lithium-ion batteries contribute to their extensive range of applications. Polyolefin separators are often used for the purpose of storing electrolytes, hence ensuring the efficient internal ion transport. Nevertheless, the electrochemical performance of lithium-ion batteries is constrained by its limited interaction with electrolytes and poor capacity for cation transport. This work presents the preparation of a new bio-based nanofiber separator by combining oxidized lignin (OL) and halloysite nanotubes (HNTs) with polyimide (PI) using an electrospinning technique. Analysis was conducted to examine and compare the structure, morphology, thermal characteristics, and EIS of the separator with those of commercially available polypropylene separator (PP). The results indicate that the PI@OL and PI-OL@ 10 % HNTs separators exhibit higher lithium ion transference number and ionic conductivity. Moreover, the use of HNTs successfully impeded the proliferation of lithium dendrites, hence exerting a beneficial impact on both the cycle performance and multiplier performance of the battery. Consequently, after undergoing 300 iterations, the battery capacity of LiFePO4|PI-OL@ 10 % HNTs|Li stays at 92.1 %, surpassing that of PP (86.8 %) and PI@OL (89.6 %). These findings indicate that this new bio-based battery separator (PI-OL@HNTs) has the great potential to serve as a substitute for the commonly used PP separator in lithium metal batteries.

Facile preparation of surface-modified polypropylene nanofiber separators with enhanced ionic transport and welding performance for lithium-ion batteries

Nanofiber separators are promising for high-performance lithium-ion batteries due to their high porosity and electrolyte uptake. However, most of them have been criticized for the low mechanical strength, particularly polyolefin nanofiber separators, due to the poor welding performance caused by weak interaction among the fibers. The lack of mass-production techniques is another problem for polyolefin nanofiber separators though polyolefin has the competitive cost and stability. In present study, polypropylene nanofiber separators (PPNFS) prepared via nanolayer coextrusion, a new appeared method suitable for large-scale preparation, are surface modified via a simple dip-coating method. The electrostatic self-assembly on the surface of polypropylene nanofibers is achieved. The incorporation of modified polymers significantly enhances the interaction at the nanofiber overlaps, facilitating in-situ welding of nanofibers and resulting in an 800 % enhancement in separator mechanical properties. Polybenzimidazole (PBI) is a strange polymer with both electron-donating and electron-accepting sites. Experimental characterization and simulation calculations confirm that PBI-modified PPNFS exhibit a distinctive enhanced ionic transport and promote the formation of a more homogeneous solid electrolyte interphase (SEI) layer. Furthermore, PBI coated PPNFS (PPNFS@PBI) separators demonstrate exceptional thermal stability, superior electrolyte uptake, and enhanced fire resistance. Finally, the effect of molar concentration and molecular weight of PBI on battery performance is evaluated. 15 % Molar concentration of PBI with 88,000 molecular weight has been shown to maximize battery performance.

Amphoteric Supramolecular Nanofiber Separator for High‐Performance Sodium‐Ion Batteries

The separator is an essential component of sodium‐ion batteries (SIBs) to determine their electrochemical performances. However, the separator with high mechanical strength, good electrolyte wettability and excellent electrochemical performance remains an open challenge. Herein, a new separator consisting of amphoteric nanofibers with abundant functional groups was fabricated through supramolecular assembly of natural polymers for SIB. The uniform nanoporous structure, remarkable mechanical properties and abundant functional groups (e.g. −COOH, −NH2 and −OH) endow the separator with lower dissolution activation energy and higher ion migration numbers. These metrics enable the separator to lower the barrier for desolvation of Na+, accelerate the migration of Na+, and generate more stable solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI). The battery assembled with the amphoteric nanofiber separator shows higher specific capacity and better stability than that assembled with glass fiber (GF) separator.

Silkworm cocoon-inspired and robust nanofibrous composite separator with gradient structure for lithium ion batteries

Considering electrospun nanofiber-based membranes that generally exhibit high porosity but poor mechanical property, there is a growing need to fabricate nanofibrous separators with comprehensive performance for lithium-ion batteries (LIBs). Silkworm cocoon, a source of design inspiration from nature, comprises a multilayer structure with composition change in the thickness direction, thereby endowing the cocoon with relatively high porosity and outstanding mechanical strength. Therefore, a silkworm cocoon-like structured nanofibrous separator (GPS) is fabricated in this work by initially preparing several polyacrylonitrile (PAN) nanofibrous membranes with varied content of carboxyl styrene butadiene latex (SBR) and then laminating them in a concentration gradient sequence. Compared with the pristine PAN membrane, the GPS exhibits comparable porosity, thermal stability and electrolyte wettability due to the existence of PAN nanofibers, while the tensile strength and puncture resistance of the GPS are greatly raised by 5.5 times and 1.5 times, respectively. On account of the structure design, GPS also shows better mechanical strength than PAN/SBR separators without concentration gradient, and the strengthening mechanism has been verified by the finite element analysis. Meanwhile, such gradient structure of GPS renders the battery with desirable ion transport ability and stable cycle properties. Thus, the silkworm cocoon-like structured separator could be a promising separator candidate for LIBs, and the bioinspired design in structure will play an increasingly important role in the development of high-performance battery in the future.

A dual-network structured poly (m-phenylene isophthalamide) nanofiber separator with Li6.4La3Zr1.4Ta0.6O12 for dendrite-free and high-safety lithium-metal batteries

Separator plays an important role in improving the long-term cycle stability and safety performance of high-energy–density lithium-metal batteries (LMBs). Herein, a robust thermotolerant separator is prepared by one-step electrospinning technique, combing fluorinated poly (m-phenylene isophthalamide) nanofibers with Li6.4La3Zr1.4Ta0.6O12 (LLZTO), a fast ionic conductor. The synergistically incorporated poly (vinylidene fluoride-co-hexafluoropropylene) and LLZTO trigger off the formation of a fancy dual-network structure with high porosity and well-distributed nanopores, in which ultrafine secondary nanonets are tightly bound with three-dimensional nanofiber scaffold. LLZTO can not only provide sufficient Li+ migration channels, but also immobilize free anions to strengthen ionic transmission. Moreover, this nanofiber separator possesses high mechanical strength, excellent thermal stability and desirable electrolyte affinity. It can gel liquid electrolyte well, leading to high tLi+, homogenized Li+ flux and uniform Li deposition. Consequently, LMBs with this dual-network structured nanofiber separator deliver satisfactory cycle stability and high C-rate capability. This contribution provides new insights into the design and development of high-quality nanofiber separators for dendrite-free and high-safety LMBs.

Advancing lithium-ion battery technology with electrospun PVDF-HFP/SiO2 nanocomposite electrolyte membrane and CuCo2O4 high-performance anode material

Electrospinning technology represents a rapid, cost-effective, and secure method for fabricating separators designed for lithium-ion batteries. Within this study, we employed electrospinning technology to craft a PVDF-HFP-SiO2 nanofiber membrane. Throughout various characterizations, this membrane exhibited commendable mechanical attributes, with a maximum tensile stress reaching approximately 11.5 MPa and a maximum tensile strain of 156.3 %. Additionally, it demonstrated exceptional thermal stability, as evidenced by a degradation temperature reaching 450 °C. Simultaneously, we developed a novel flower-like CuCo2O4 anode material directly grown on a 3D nickel foam substrate, eliminating the need for a binder. Electrochemical assessments conducted on cells assembled with nanofiber membrane and CuCo2O4 anode revealed heightened ionic conductivity, with an interfacial resistance of 40 Ω·cm2, alongside exceptional capacity and cycling performance which maintained a discharge capacity of 613 mA h g−1 after 50 cycles, with a Coulombic efficiency consistently above ∼98 %. The enhanced electrochemical performance can be attributed to several factors, including the distinctive flower-shaped electrode, which provides a more extensive network of ion and electron transport channels. Additionally, the porous nanofiber separator, complemented by the presence of doped ceramic particles, plays a vital role in augmenting performance.

Channel engineering strategy of precisely modified MOF/nanofiber composite separator for advanced aqueous zinc ion batteries

As the combination of metal-organic framework (MOF) materials with adjustable channels and flexible polymeric matrix, MOF/nanofiber composite separators enable the controllable ion transport behavior, which offers promising candidates for developing novel battery separators. In this study, series MOF functionalized electrospun polyacrylonitrile nanofiber separators were successfully developed via a precisely surface grafting strategy and then served as the separators for highly efficient aqueous zinc (Zn)-ion batteries (ZIBs). Moreover, channel engineering was developed by placing amino (MOF-N) and sulfonic acid groups (MOF-NS) on the channel surface of UIO-66, which then effectively promoted the ion transport progress. In particular, the resultant MOF-NS shows an excellent ionic conductivity (22.81 mS cm−1), improved Zn2+ transference number (0.78), and outstanding cyclic durability. These enhanced properties can be contributed by the effectively promoted dissociation of zinc salts and desolvation processes of hydrated Zn ions through strong ion-dipole interactions, as confirmed by the theoretical simulations. Our work demonstrates the great importance of channel engineering in modifying porous MOFs materials and the strong power of the precisely modified MOF/nanofiber separators in regulating ion transmission behavior and thus offers a promising separator candidate for high performance aqueous ZIBs.

Electrospun of polyvinyl alcohol composite hydrogel nanofibers prepared by in-situ polymerization: A novel approach to fabricate hydrogel nanofiber membrane for lithium-ion batteries

The increasing demand for high-performance lithium-ion batteries has propelled the exploration of advanced materials to overcome limitations associated with commercially available polyolefin-based separators. This study introduces a pioneering approach involving the synthesis of a crosslinked hydrogel nanofiber membrane (PDs) composed of polyvinyl alcohol (PVA) and N, N-dimethylacrylamide (DMAAm) through the combination of the photo-polymerization with electrospinning process. To address key drawbacks observed in existing separators—namely, low porosity, inadequate thermal stability, and insufficient electrolyte wettability—the porosity of the PDs hydrogel nanofibers was systematically controlled through both heating drying (PD12-HD) and freeze-drying (PD12-FD) methods. The separator’s properties and performance were thoroughly examined, focusing on its chemical, mechanical, thermal, swelling, morphological, conductivity, and electrochemical properties. Comprehensive characterization of the PD12-FD separator revealed remarkable attributes surpassing those of a commercial counterpart (Celgard). The PD12-FD separator demonstrated exceptional mechanical strength (16.3 MPa), robust thermal stability (no shrinkage or deformation at 170 °C), high porosity (84.66 %), substantial electrolyte uptake capacity (672.86 %), superior ionic conductivity (3.405 mS/cm), and high lithium-ion transfer number (tLi+ = 0.678). The improvement is attributed to the cross-linked structure formed between PVA and DMAAm. Importantly, coin cells assembled with the PD12-FD separator exhibited outstanding electrochemical performance. Even after 150 cycles at 1C, the PD12-FD separator maintained a capacity above 95 % (145.48 mAh/g), surpassing both PVA and Celgard separators. This study proposes a novel methodology for fabricating hydrogel nanofiber separators and introduces a customizable synthesis approach for producing separators with enhanced performance and controlled characteristics.

Batch Fabrication of Electrospun PAN/PU Composite Separators for Safe Lithium-Ion Batteries

As an important element of lithium-ion batteries (LIBs), the separator plays a critical role in the safety and comprehensive performance of the battery. Electrospun nanofiber separators have a high porosity and good electrolyte affinity, which are favorable to the transference of lithium ions. In this paper, the batch preparation of polyacrylonitrile (PAN)-based nanofiber separators are obtained via spherical section free surface electrospinning (SSFSE). Introducing an appropriate amount of polyester polyurethane (PU) can effectively enhance the mechanical property of PAN nanofiber separators and help the separators resist the external force extrusion. The results show that when PAN:PU = 8:2, the porosity and electrolyte uptake rate of the composite nanofiber separator (PAN-2) are 62.9% and 643.3%, respectively, exhibiting a high ionic conductivity (1.90 mS/cm). Additionally, the coin battery assembled with PAN-2 as a separator (LiFePO4/PAN-2/lithium metal) shows good cycling performance and good rate performance, with a capacity retention rate of 93.9% after 100 cycles at 0.5 C, indicating that the battery with PAN-2 has a good application potential in advanced energy storage.

Fluorinated polymer coated cyanoethyl-chitin nanofiber composite separators for high performance lithium ion batteries

Natural polymer nanofiber-based separators with excellent thermal stability, mechanical strength, and electrolyte wettability have shown great potentials for advanced lithium ion batteries. However, there are two crucial issues to block their practical applications, including high moisture absorption and interfacial instability toward electrodes. Here, a surface coating strategy to overcome these disadvantages and promote natural polymer nanofiber-based separator performance is demonstrated. A hydrophobic polymer, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), is adopted to coating on the surface of cyanoethyl-chitin nanofiber (CCN) separators. The as-fabricated CCN/PVDF-HFP composite separators not only exhibit much lower moisture absorption and higher interfacial instability but also retain high mechanical strength and ionic conductivity in comparison to CCN separators. The LiFePO4/Li cells using CCN/PVDF-HFP separators exhibit much better electrochemical cycle performance and enhanced capacity retention than that using CCN separators.

Coordinated self-assembly of ultraporous 3D aramid nanofibrous separators with a poly(vinyl alcohol) sheath for lithium-metal batteries

Li-metal batteries with a low negative/positive electrode capacity ratio require significant improvement to ensure their practical implementation as high-energy-density batteries. In the present study, ultraporous aramid nanofiber (ANF) separators with a poly(vinyl alcohol) (PVA) sheath were fabricated using a coordinated self-assembly process to produce an ultrahigh bulk (>95 %) and surface (>48.6 %) porosity and a high ion affinity. In this process, sol films consisting of an optimized mixture of aramid nanoseed sols and PVA were sequentially immersed in ethanol and water to induce the formation of a 3D aramid network and the phase separation of PVA by their rate differences, followed by swelling-induced pore formation. The resulting separators exhibited an excellent ion conductivity and Li+ ion transference number of up to 2.73 mS cm−1 and 0.78, respectively, with the effective suppression of Li dendritic growth and the formation of a stable fluorine-rich solid electrolyte interphase (SEI) layer on the Li-metal surface. Li/Li half cells exhibited stable operation for up to 1500 cycles with a high current density of 10 mA cm–2 and an areal capacity of 1 mAh cm–2, with Li/Cu half cells operating for over 244 cycles even with a lithiophobic Cu current collector. LFP/Li full cells with a low N/P ratio (∼2.03) exhibited stable operation for 172 cycles with an average Coulombic efficiency of 99.6 %. NCM/Li full cell with extremely low N/P ratio (0.96) and lean electrolyte condition (4 g Ah–1) stably operated for 55 cycles with projected energy density of 350.7 Wh kg–1.