<p>Single-walled carbon nanotubes (SWCNTs) are typically bundled and long, which limits their ability to be filled with various compounds. In this work, we report a simple ultrasound pretreatment strategy and compare pristine and treated SWCNTs as a matrix for red phosphorus vapor condensation. Initially purified and opened commercial Tuball SWCNTs were subjected to a tailored sonication procedure in water, which allowed for efficient bundle separation and shortening of the nanotubes. Phosphorus was loaded into both pristine and sonicated SWCNTs (sSWCNTs) by a&#xa0;vaporization–condensation method at 800&#xa0;°C, followed by the removal of excess external phosphorus with a NaOH solution. X-ray photoelectron spectroscopy and thermogravimetric analysis determined a twofold increase in phosphorus content: from 8 at% (18 wt%) in the pristine SWCNTs to 18 at% (34 wt%) in the sSWCNTs. Most of the phosphorus was in elemental form, forming fibrous chains within the cavities of the nanotubes, which was demonstrated by Raman spectroscopy. Empty and phosphorus-filled nanotubes were studied as anodes in lithium-ion batteries. Ultrasound-induced changes in the structure of sSWCNTs resulted in increased specific capacity of both phosphorus-free and phosphorus-containing anodes as compared to their untreated counterparts. Phosphorus-filled sSWCNTs provided a capacity of 760 mAh/g at a current density of 0.1 A/g and demonstrated outstanding rate capability (50% capacity retention at 5 A/g) and remarkable long-term stability, maintaining a high capacity over 400 cycles at 1 A/g.</p> Graphical abstract <p></p>