Size-Dependent Confining Effect of Phosphorus inside Carbon Nanotubes for Highly Stable Lithium-Ion Storage

By changing the diameter of carbon nanotubes (CNTs), the phase of the internal phosphorus could be adjusted, which was testified by spherical-chromatic aberration-corrected transmission electron microscopy. Phosphorus inside a double-walled CNT (P@DWCNT) delivers a stable capacity of 837 mA h g^-1 after 700 cycles at 1.0 A g^-1. A stable capacity of 155 mA h g^-1 could be delivered by P@DWCNT when forming a full cell with a LiFePO₄ cathode. According to the in situ Raman spectrum, the strain (24 GPa) on DWCNTs caused by the volume expansion of phosphorus during lithiation could be totally relieved along the axis of DWCNTs at the end of the delithiation process, showing high reversibility. This is because the filling rate of P@DWCNT is at an appropriate level and the DWCNTs are not fully filled, thus preventing the phosphorus from escaping and further pulverization. Phosphorus inside a single-walled CNT (P@SWCNT) can only deliver limited capacity since its single carbon layer cannot sustain the integrality of the internal phosphorus during multiple insertion/extraction processes of Li⁺. Because too much phosphorus is deposited in the multiwalled CNT (P@MWCNT) sample and the external phosphorus tends to break off rapidly upon cycling, P@MWCNT shows fast capacity degradation. The anodes are thoroughly studied by galvanostatic intermittent titration and pseudocapacitance investigations to reveal the size-dependent confining effect.