Synergistic effect of sidewall holes and encapsulated phosphorus to improve lithium storage in single-walled carbon nanotubes

Abstract

The paper presents experimental and theoretical evidence that filling holey single-walled carbon nanotubes (hSWCNTs) with phosphorus creates an anode material with increased lithium storage capacity. Tuball SWCNTs were purified of the catalyst, opened at the ends, and treated with hot mineral acids, followed by annealing in argon to reduce the sizes of the bundle and create holes in the walls. According to transmission electron microscopy, Raman scattering and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy studies, sidewall defects affect the phosphorus vapor condensation into short and disordered chains inside the nanotubes. The interaction of thermally evaporated lithium with empty and phosphorus-filled SWCNTs was studied using model X-ray photoelectron and NEXAFS spectroscopy experiments at a synchrotron radiation source. In the case of phosphorus-filled holey SWCNTs (P@hSWCNTs), a high dispersion of lithium along the nanotube walls and the formation of lithium phosphide inside were detected. Density functional theory calculations showed that the presence of encapsulated phosphorus reduces the energy barrier for lithium to penetrate through the hole into the nanotube. Electrochemical testing confirmed the synergistic effect of sidewall holes and encapsulated fibrous phosphorus in improving charge storage capacity in the lithium-ion battery. The P@hSWCNTs electrode delivered 946 mA h g⁻¹ at a current density of 0.1 A g⁻¹, which is 2 times higher than that of the analogue with intact walls, and demonstrated stable operation at 2 A g⁻¹ for 100 discharge/charge cycles, achieving ∼100% Coulombic efficiency.