With unique advantages over inorganic aerogels including higher strengths and compressive moduli, greater toughness, and the ability to be fabricated as a flexible thin film, polymer aerogels have the potential to supplant inorganic aerogels in numerous applications. Among polymer aerogels, polyimide aerogels possess a high degree of high thermal stability as well as outstanding mechanical properties. However, while the onset of thermal decomposition for these materials is typically very high (greater than 500 °C), the polyimide aerogels undergo dramatic thermally induced shrinkage at temperatures well below their glass transition (T) or decomposition temperature, which limits their use. In this study, we show that shrinkage is reduced when a bulky moiety is incorporated in the polymer backbone. Twenty different formulations of polyimide aerogels were synthesized from 3,3,'4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 4,4'-oxidianiline (ODA) or a combination of ODA and 9,9'-bis(4-aminophenyl)fluorene (BAPF) and cross-linked with 1,3,5-benzenetricarbonyl trichloride (BTC) in a statistically designed study. The polymer concentration, n-value, and molar concentration of ODA and BAPF were varied to demonstrate the effect of these variables on certain properties. Samples containing BAPF showed a reduction in shrinkage by as much as 50% after aging at elevated temperatures for 500 h compared to those made with ODA alone.

High areal performance from high cathode mass loading is an essential requirement to bring battery chemistries beyond the lithium (Li) ion, such as lithium–sulfur (Li–S) or lithium–selenium (Li–Se), toward practical applications. These conversion chemistry cathodes have been typically prepared by using conventional slurry-based techniques widely used for Li ion battery electrodes, requiring the use of solvent and binder and multiple steps such as mixing, casting, drying, and collecting and proper disposing of organic solvents. To increase active material mass loading, the processing steps become even more time-consuming when multiple casting-drying cycles are needed. Here we report an extremely facile procedure to prepare ultrahigh mass loading (>15 mg/cm2) with high active material content (>70%) conversion chemistry cathodes in a single step directly from neat active material, such as sulfur (S), selenium (Se), or selenium sulfide (SeS2), without the need of solvent or binder. This is achieved by the use of holey graphene (hG), a unique lightweight material that can be dry pressed by itself or as a host into neat or composite electrode forms. In the electrode preparation, hG, the neat active material, and hG are sequentially added to the pressing die, resulting in a sandwich architecture containing a neat active material layer with conveniently tunable ultrahigh mass loading. The sandwich electrodes exhibit excellent overall electrochemical performance with great active material utilization. Mechanistically, when Se is used as the example active material, the neat Se layer becomes electrochemically redistributed throughout the entire cathode thickness after the first cycle. The sandwich cathode not only does not crack or fail but also spontaneously densifies for stable and prolonged cycling. The sandwich electrode architecture is also compatible with the use of a fluorinated electrolyte solvent to significantly reduce polyselenide solubility and shuttling for improved cycling performance. Such sandwich electrodes from the hG-enabled, one-step, dry-press method offer an attractive fast fabrication option in bulk production of ultrahigh mass loading cathodes for practical applications.

Aerogels are promising materials for many aerospace applications, including high-performance antennae and flexible insulation, because of their inherent low density and high surface areas. Polymer aerogels, especially polyimide aerogels, provide excellent mechanical properties beyond traditional silica aerogels while maintaining the required thermal stability. Polyimide aerogel surface area, porosity, and pore volume are important properties; however, these measurements are traditionally conducted on the aerogel after removal of the solvent. Because of this, the impact of synthetic control and solvent presence on the nanoscale to mesoscale structure of polyimide aerogels in functional applications is unclear. In this report, we use small-angle neutron scattering to determine the dry and solvated skeletal strut size and composition of polyimide aerogels to deduce the impact of solvation on the structure of complex aerogel struts. Our results show that the aerogel contains a hierarchical assembly of pores, with pores present both within and between the supporting struts. This translates to a material with solvent in the larger pores, as well as absorbed in the supporting polyimide skeleton. The amount of solvent uptake in the struts varies with the solvent and polyimide properties. The insight from these results provides pathways to determine the correlations between aerogel nano- and mesoscale structural characteristics, fabrication processes, and their performance in functional applications such as polymeric battery separators. These results also broaden the characterization tools of polymeric aerogels that differentiate between dry and solvated nano- and mesoscale structures that exist in common operating conditions.

Solvents and binders are typical requirements in conventional lithium ion battery electrode fabrication to enable intimate material mixing, mechanical robustness, and reproducibility. However, for high energy density conversion chemistry cathodes such as sulfur (S) and selenium (Se), the time-consuming solvent-based methods are proven unreliable to achieve high mass loading cathodes with sufficient quality. Here, we report a facile solvent-free and binder-free method to prepare high mass loading composite Se cathodes that is enabled by the use of holey graphene (hG) as a lightweight conductive scaffold. Holey graphene is a derivative of graphene and can be dry-pressed into robust discs by itself. It can also serve as a matrix to host materials such as Se for composite disc preparation in a mix-and-press process free of solvent and binder. The method allows the preparation of ultrahigh Se content cathodes (up to 90 wt% Se) and ultrahigh Se mass loading (up to 15.6 mg cm−2 in this work). These cathodes exhibit excellent Se utilization, high areal capacity (up to 9 mAh cm−2), and good rate performance. The dry-press approach also allows for the preparation of a layered composite cathode architecture, where a thin hG layer is inserted between the composite and the current collector to improve the electrical contact. A solvent-free approach is also used to prepare hG-based hybrids with metal sulfides to be incorporated into a composite cathode to help entrap soluble polyselenide intermediates. The hybrid material is compatible with the solvent-free mix-and-press electrode fabrication approach and shows promise in improving the Se retention. While further improvements are still required, this work demonstrates the outstanding potential of using this facile, solvent-free approach enabled by hG for fabrication of high-performance, high mass loading conversion chemistry cathodes.

Conversion of lignin into foam-like aerogels using an environmentally benign freeze-drying process is described in this study. Interest in lignin as a bioresource has been gaining popularity in recent years, as it is currently viewed by most industries as a waste product that in most cases is simply burned as a fuel source. The use of lignin in a polymer/clay aerogel offers the potential for a high value-added foam-like material potentially usurping the use of traditional petroleum-derived foams in some applications. The present study demonstrates that lignin/clay and lignin/alginate aerogel samples can possess compressive moduli as high as 36·0 MPa.