Spray Flame Synthesis and Multiscale Characterization of Carbon Black-Silica Hetero-Aggregates

Abstract

The increasing demand for lithium-ion batteries requires constant improvements in the areas of production and recycling to reduce their environmental impact. In this context, this work presents a method for structuring carbon black aggregates by adding colloidal silica via a spray flame with the goal of opening up more choices for polymeric binders. The main focus of this research lies in the multiscale characterization of the aggregate properties via small-angle X-ray scattering, analytical disc centrifugation and electron microscopy. The results show successful formation of sinter-bridges between silica and carbon black leading to an increase in hydrodynamic aggregate diameter from 201 nm to up to 357 nm, with no significant changes in primary particle properties. However, segregation and coalescence of silica particles was identified for higher mass ratios of silica to carbon black, resulting in a reduction in the homogeneity of the hetero-aggregates. This effect was particularly evident for silica particles with larger diameters of 60 nm. Consequently, optimal conditions for hetero-aggregation were identified at mass ratios below 1 and particle sizes around 10 nm, at which homogenous distributions of silica within the carbon black structure were achieved. The results emphasise the general applicability of hetero-aggregation via spray flames with possible applications as battery materials.

Keywords: carbon black; hetero-aggregation; nanoparticle characterization; silica; small-angle X-ray scattering; spray flame.

Figure 1

(a) SAXS data of carbon black before and after the spray flame. The original scattering data has been fitted with a unified fit. The local Guinier fit of the primary particles is highlighted with a dashed line. (b) Size distributions calculated from the fit parameters of the unified fit. Additional TEM data analysis of 224 counted primary particles of the carbon black after the spray flame yields a comparative size distribution. (c) Normalized volume PSDs of the carbon black aggregates before and after the spray flame, obtained via ACD, yield a hydrodynamic diameter. (d) HAADF-STEM image of the carbon black before the spray flame and (e) after the spray flame.

Figure 2

(a) SAXS data of the investigated colloidal silica particles before and after the spray flame. The scattering curve of the particles was fitted with local power law and Guinier fits. The colours of the local fits correspond to the colours of the structures formed, which are marked in the HAADF-STEM image of the sample after the spray flame in (b). The sample before the flame was measured as a dilute suspension.

Figure 3

(a) HAADF-STEM image of a hetero-aggregate with visible hetero-contact (white circle) (b–d) Closeup HAADF-STEM image with EDXS of the hetero-contact, with carbon (symbol C) coloured in green and silicon (symbol Si) coloured in blue. Experimental conditions were a mass ratio of silica to carbon black of 3 and original colloidal silica primary particle size of 30 nm.

Figure 4

(a) SAXS data of hetero-aggregates produced at a mass ratio of silica to carbon black of 1 with varying sizes of the colloidal silica particles ranging from 10 nm to 60 nm. The evaluated power law fit in the USAXS area is highlighted in red. (b) Derived exponents of the local power law fit in the USAXS area for hetero-aggregates with different mass ratios of silica to carbon black ranging from 0.3 to 3. Two different primary particle size ranges of the colloidal silica were investigated: 30 nm and 40 nm. In order to evaluate the experimental spread, three individual experiments were evaluated for a mass ratio of 0.33. (c) Observed homo-aggregation and coalescence of silica particles for a mass ratio of SiO₂ to carbon black of 3 and 30 nm silica particles in HAADF-STEM images.

Figure 5

(a,b) Normalized volume PSDs of hetero-aggregates, as measured via ADC. For better visual clarity, only every fifth data point is shown. The investigated hetero-aggregates were produced with mass ratios of silica to carbon black of 3 (a) and 0.33 (b) for 10 nm, 30 nm and 60 nm colloidal silica particles, respectively.

Figure 6

(a,c) HAADF-STEM images with EDXS of a hetero-aggregate produced with (a) 10 nm silica particles at a mass ratio of 0.33 and (c) 60 nm silica particles at a mass ratio of 3. Carbon is coloured in green (symbol C), silicon in blue (symbol Si) and oxygen in red (symbol O). Therefore, the silica (SiO₂) particles in (a) show a pinkish colour. (b) First mode (highlighted in Figure 5a with a straight line) of the PSD of hetero-aggregates for mass ratios of silica to carbon black ranging from 0.2 to 5 for three silica particle sizes. The data points each represent two experiments with three respective ADC measurements. The highlighted data points (a,c) correspond to the TEM images depicted in (a,c).

Figure 7

Carbon layer on the surface of the silica particles (diameter of silica = 60 nm; mass ratio = 3). Carbon is coloured in green (symbol C) and silicon in blue (symbol Si).