Phase Separation-Controlled Assembly of Hierarchically Porous Aramid Nanofiber Films for High-speed Lithium-Metal Batteries

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 ± 0.1 µm to 2.7 ± 0.1 µm. Thanks to the double-side solvent diffusion, the separators exhibit controllably uniform pore sizes with a narrow distribution, spanning from 40 ± 5 nm to 105 ± 9 nm, and a high modulus of 3.3 ± 0.5 GPa. 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.

Recent Advancements in Fabrication, Separation, and Purification of Hierarchically Porous Polymer Membranes and Their Applications in Next-Generation Electrochemical Energy Storage Devices

In recent years, hierarchically porous polymer membranes (HPPMs) have emerged as promising materials for a wide range of applications, including filtration, separation, and energy storage. These membranes are distinguished by their multiscale porous structures, comprising macro-, meso-, and micropores. The multiscale structure enables optimizing the fluid dynamics and maximizing the surface areas, thereby improving the membrane performance. Advances in fabrication techniques such as electrospinning, phase separation, and templating have contributed to achieving precise control over pore size and distribution, enabling the creation of membranes with properties tailored to specific uses. In filtration systems, these membranes offer high selectivity and permeability, making them highly effective for the removal of contaminants in environmental and industrial processes. In electrochemical energy storage systems, the porous membrane architecture enhances ion transport and charge storage capabilities, leading to improved performance in batteries and supercapacitors. This review highlights the recent advances in the preparation methods for hierarchically porous structures and their progress in electrochemical energy storage applications. It offers valuable insights and references for future research in this field.

Poly(3,4-Ethylenedioxythiophene):Sulfamic Acid Modified Aramid Nanofibers: An Innovative Conductive Polymer With Enhanced Electromagnetic Interference Shielding and Thermoelectric Performance

The development of alternative conductive polymers for the well-known poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is of great significance for improving the stability in long-term using and high-temperature environments. Herein, an innovative PEDOT:S-ANF aqueous dispersion is successfully prepared by using sulfamic acid (SA) to modified aramid nanofibers (S-ANF) as an alternative dispersant for PSS and the subsequent in situ polymerization of PEDOT. Thanks to the excellent film forming ability and surface negative groups of S-ANF, the PEDOT:S-ANF films show comparable tensile strength and elongation to unmodified PEDOT:ANF. Meanwhile, PEDOT:S-ANF has a high conductivity of 27.87 S cm-1, which is more than 20 times higher than that of PEDOT:PSS. The film exhibits excellent electromagnetic interference (EMI) shielding and thermoelectric performance, with a shielding effectiveness (SE) of 31.14 dB and a power factor (PF) of 0.43 µW m-1K-2. As a substitute for PSS, S-ANF exhibits significant structural and physicochemical properties, resulting in excellent chemical and thermal stability. Even under harsh conditions such as immersing to 0.1 M HCl, 0.1 M NaOH, and 3.5% NaCl solution, or high temperature conditions, the PEDOT:S-ANF films still maintain exceptional EMI shielding performance. Therefore, this multifunctional conductive polymer exhibits enormous potential and even proves its reliability in extreme situations.

Scalable electrospinning using a desktop, high throughput, self-contained system

Electrospinning is a specialized processing technique for the formation of submicron diameter fibers of polymeric and ceramic materials using an electrostatic field. The process has multiple advantages over other nano- and micro- fiber synthesis methods; however, generally suffers from very low fabrication speeds, making it undesirable for scalability. This work assesses the performance of a needle-less, self-contained, high throughput electrospinning system. It further compares the fiber fabrication rates obtained versus two single needle setups with different collectors: (i) a conventional single needle and flat plate geometry, and (ii) a single needle with a rotating collector geometry. Polyvinylpyrrolidone (PVP) in ethanol was used as the model material. The fabrication rate of the high throughput system "HTES" was measured at about 2.6 g/h and was about 15 times that collected of the flat plate. Comparing it to other systems reported in the literature also proved it to be a viable option for high throughput, lab scale electrospinning.

Biomimetic Aramid Nanofiber/β-FeOOH Composite Coating for Polypropylene Separators in Lithium-Sulfur Batteries

Aramid nanofibers (ANFs), with attractive mechanical and thermal properties, have attracted much attention as key building units for the design of high-performance composite materials. Although great progress has been made, the potential of ANFs as fibrous protein mimetics for controlling the growth of inorganic materials has not been fully revealed, which is critical for avoiding phase separation associated with typical solution blending. In this work, we show that ANFs could template the oriented growth of β-FeOOH nanowhiskers, which enables the synthesis of ANFs/β-FeOOH hybrids as composite coatings for polypropylene (PP) separators in Li-S batteries. The modified PP separator exhibits enhanced mechanical properties, heightened thermal performance, optimized electrolyte wettability, and improved ion conductivity, leading to superior electrochemical properties, including high initial specific capacity, better rate capability, and long cycling stability, which are superior to those of the commercial PP separators. Importantly, the addition of β-FeOOH to ANFs could further contribute to the suppression of lithium polysulfide shuttling by chemical immobilization, inhibition of the growth of lithium dendrites because of the intrinsic high modulus and hardness, and promotion of reaction dynamics due to the catalytic effect. We believe that our work may provide a potent biomimetic pathway for the development of advanced battery separators based on ANFs.

Revealing Surface/Interface Chemistry of the Ordered Aramid Nanofiber/MXene Structure for Infrared Thermal Camouflage and Electromagnetic Interference Shielding

The past decade has witnessed the advances of infrared (IR) thermal camouflage materials, but challenges remain in breaking the trade-off nature between emissivity and mechanical properties. In response, we identify the key role of a moderate reprotonation rate in the aramid nanofiber (ANF)/MXene film toward a surface-to-bulk alignment. Theoretical simulation demonstrates that the ordered ANF/MXene surface eliminates the local high electric field by field confinement and localization, responsible for the low IR emissivity. By scrutinizing the surface/interface chemistry, the processing optimization is achieved to develop an ordered and densely stacked ANF/MXene film, which features a low emissivity of 16%, accounting for sound IR thermal camouflage performances including a wide camouflage temperature range of 50-200 °C, a large reduction in radiation temperature from 200.5 to 63.6 °C, and long-term stability. This design also enables good mechanical performance such as a tensile strength of 190.8 MPa, a toughness of 12.1 MJ m-3, and a modulus of 7.9 GPa, responsible for better thermal camouflage applications. The tailor-made ANF/MXene film further attains an electromagnetic interference (EMI) shielding effectiveness (40.4 dB) in the X-band, manifesting its promise for IR stealth compatible EMI shielding applications. This work will shed light on the dynamic topology reconstruction of camouflage materials for boosting thermal management technology.

Wafer-Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle-Assisted Chemical Etching and Pseudomorphic Thermal Oxidation

Many biological materials exhibit a multiscale porosity with small, mostly nanoscale pores as well as large, macroscopic capillaries to simultaneously achieve optimized mass transport capabilities and lightweight structures with large inner surfaces. Realizing such a hierarchical porosity in artificial materials necessitates often sophisticated and expensive top-down processing that limits scalability. Here, an approach that combines self-organized porosity based on metal-assisted chemical etching (MACE) with photolithographically induced macroporosity for the synthesis of single-crystalline silicon with a bimodal pore-size distribution is presented, i.e., hexagonally arranged cylindrical macropores with 1 µm diameter separated by walls that are traversed by pores 60 nm across. The MACE process is mainly guided by a metal-catalyzed reduction-oxidation reaction, where silver nanoparticles (AgNPs) serve as the catalyst. In this process, the AgNPs act as self-propelled particles that are constantly removing silicon along their trajectories. High-resolution X-ray imaging and electron tomography reveal a resulting large open porosity and inner surface for potential applications in high-performance energy storage, harvesting and conversion or for on-chip sensorics and actuorics. Finally, the hierarchically porous silicon membranes can be transformed structure-conserving by thermal oxidation into hierarchically porous amorphous silica, a material that could be of particular interest for opto-fluidic and (bio-)photonic applications due to its multiscale artificial vascularization.