RESUMEN

Silica-based aerogels are a promising low-cost solution for improving the insulation efficiency of single-pane windows and reducing the energy consumption required for space heating and cooling. Two key material properties required are high porosity and small pore sizes, which lead to low thermal conductivity and high optical transparency, respectively. However, porosity and pore size are generally directly linked, where high porosity materials also have large pore sizes. This is unfavorable as large pores scatter light, resulting in reduced transmittance in the visible regime. In this work, we utilized preformed silica colloids to explore methods for reducing pore size while maintaining high porosity. The use of preformed colloids allows us to isolate the effect of solution conditions on porous gel network formation by eliminating simultaneous nanoparticle growth and aggregation found when using typical sol-gel molecular-based silica precursors. Specifically, we used in situ synchrotron-based small-angle x-ray scattering during gel formation to better understand how pH, concentration, and colloid size affect particle aggregation and pore structure. Ex situ characterization of dried gels demonstrates that peak pore widths can be reduced from 15 to 13 nm, accompanied by a narrowing of the overall pore size distribution, while maintaining porosities of 70%-80%. Optical transparency is found to increase with decreasing pore sizes while low thermal conductivities ranging from 95 +/- 13 mW/m K are maintained. Mechanical performance was found to depend primarily on effective density and did not show a significant dependence on solution conditions. Overall, our results provide insights into methods to preserve high porosity in nanoparticle-based aerogels while improving optical transparency.

Asunto(s)

Nanopartículas , Dióxido de Silicio , Dióxido de Silicio/química , Rayos X , Geles/química , Coloides , Porosidad

RESUMEN

This paper presents a novel sol-gel method to synthesize large and thick silica aerogel monoliths at near ambient conditions using a commercial aqueous solution of colloidal silica nanoparticles as building blocks. To achieve slabs with high visible transmittance and low thermal conductivity, the method combines the strategies of (i) synthesizing gels on an omniphobic perfluorocarbon liquid substrate, (ii) aging at temperatures above room temperature, and (iii) performing solvent exchange with a low-surface-tension organic solvent prior to ambient drying. The omniphobic liquid substrates were used to prevent cracking and ensure an optically-smooth surface, while nanoparticle building blocks were small (<10 nm) to limit volumetric light scattering. Gels were aged at temperatures between 25 and 80 °C for up to 21 days to make them stronger and stiffer and to reduce shrinkage and cracking during ambient drying. Ambient drying was achieved by first exchanging water in the gel pores for octane, followed by drying in an octane-rich atmosphere to decrease capillary forces. The synthesized nanoparticle-based silica aerogel monoliths had thicknesses up to 5 mm, diameters up to 10 cm, porosities exceeding 80%, and thermal conductivities as low as 0.08 W m-1 K-1. Notably, the slabs featured visible transmittance exceeding 75% even for slabs as thick as 5 mm. The as-synthesized aerogel monoliths were exposed to TMCS vapor to induce hydrophobic properties resulting in a water contact angle of 140° that prevented water infiltration into the pores and protected the aerogels from water damage. This simple synthesis route conducted at near ambient conditions produces hydrophobic aerogel monoliths with promising optically transparent and thermally insulating properties that can be adhered to glass panes for window insulation and solar-thermal energy conversion applications.

Asunto(s)

Nanopartículas , Dióxido de Silicio , Geles , Suspensiones , Conductividad Térmica

RESUMEN

Tin-based alloying anodes are exciting due to their high energy density. Unfortunately, these materials pulverize after repetitive cycling due to the large volume expansion during lithiation and delithiation; both nanostructuring and intermetallic formation can help alleviate this structural damage. Here, these ideas are combined in nanoporous antimony-tin (NP-SbSn) powders, synthesized by a simple and scalable selective-etching method. The NP-SbSn exhibits bimodal porosity that facilitates electrolyte diffusion; those void spaces, combined with the presence of two metals that alloy with lithium at different potentials, further provide a buffer against volume change. This stabilizes the structure to give NP-SbSn good cycle life (595 mAh/g after 100 cycles with 93% capacity retention). Operando transmission X-ray microscopy (TXM) showed that during cycling NP-SbSn expands by only 60% in area and then contracts back nearly to its original size with no physical disintegration. The pores shrink during lithiation as the pore walls expand into the pore space and then relax back to their initial size during delithiation with almost no degradation. Importantly, the pores remained open even in the fully lithiated state, and structures are in good physical condition after the 36th cycle. The results of this work should thus be useful for designing nanoscale structures in alloying anodes.

RESUMEN

Fast-charging cathodes with high operating voltages are critical to the development of high energy and power density lithium-ion batteries. One route to fast-charging battery materials is through the formation of nanoporous networks, but these methods are often limited by the high calcination temperatures required for synthesis. Here, we report the synthesis of carbon-coated nanoporous LiVPO4F with excellent rate capabilities that can be stably cycled up to 4.6 V in standard LiPF6 electrolytes. During charge and discharge at 30C, 110 mAh/g (70% of theoretical capacity) was obtained, and only 9% of capacity was lost after 2000 cycles at 20C. These materials also showed excellent stability, with little self-discharge, an open-circuit voltage of 4.2 V, and a discharge capacity of 139 mAh/g obtained after holding for 12 h. Rate capabilities were further demonstrated in a proof-of-concept full cell made with a nanostructured Nb2O5. These devices were able to deliver 200 mAh/g at 1C and 100 mAh/g at 30C. Finally, operando X-ray diffraction and electrochemical kinetics were further used to provide insight into the nature of fast charging in these materials.