Utilizing Hidden Surfaces: End-Cap Removal of Carbon Nanotubes for Improved Lithium Storage

The end-cap removal of carbon nanotubes (CNTs) refers to the structural modification method that makes inner-core spaces and voids between walls accessible. Specifically, the accommodation of alkali metal cations in those hidden surfaces for electrochemical energy storage has been a challenging task. Here we present open-ended vertically aligned CNTs (VA-CNTs) as an ideal structure for Li+ accommodation, which were produced by chemical vapor deposition, followed by CF4 reactive ion etching. A model study suggests a link between Li+ capacity and the surface area, more specifically, allows us to estimate the amount of additional Li+ accommodation, which is 2.3 times increased after end-cap removal. The relatively high capacity (889 mAh/g) has confirmed that open-ended VA-CNTs are highly active for Li+ intercalation as well as exposing interior surfaces, which can be compared to the control (338 mAh/g). The microstructural change observation combined with spectroscopic studies reveals that poor Li+ reversibility stems from the solid-electrolyte interface (SEI) layer formation on the interior and exterior walls of the CNTs, which results in poor initial Coulombic efficiency (similar to 23.3%) and cyclic stability (48.6%) after SO cycles. The significant capacity fades after the first cycle due to the accelerated formation of the SEI layer in the presence of a heteroatom, which degrades Li+ and electron mobility. The exposed inner-core space provides significant increased surface area as expected, but confined inner-core space leads to poor reversibility with channel blockage.