Theoretical and experimental investigations of the Li storage capacity in single-walled carbon nanotube bundles

Abstract

Single-walled carbon nanotube bundles are investigated as Li-ion battery anode materials using a theoretical and experimental approach. Density functional theory yields Li binding energies and the intercalation capacity of a fixed density bundle of identical tubes. Two nanotube diameters are tested to explore the size effect on Li storage capacity. An infinite bundle model represents low surface area and a bundle with open tubes describes high surface area tuned by edges terminated in low-coordinated sites or functionalized with hydrogen. Charge and discharge curves are measured during the first 5 cycles and the chemistry of intercalation and interfacial reactions are characterized via in situ Raman spectroscopy. A significantly high capacity at the first discharge featuring Li intercalation and products from interfacial reactions evident from Raman spectroscopy is dramatically reduced at the 2^nd cycle and still decreases during successive cycles. The experimentally observed irreversible capacity loss is explained by the density functional theory analyses that demonstrate a much lower capacity of the nanotube bundle compared to graphite, in agreement with the results obtained in the 2^nd cycle. The excess capacity measured in the first cycle is attributed to the presence of defects and unsaturated edges and to the formation of a solid-electrolyte interphase.