Phosphorus-encapsulated carbon nanotubes with nanopores on the sidewalls (P@DMWCNTs) were investigated as an improved anode material with high capacity for lithium ion batteries in order to achieve reversible insertion/extraction reactions of lithium ions through the nanoporous sidewalls with the effective pathway of lithium ions. A P@DMWCNT anode demonstrated excellent reversible capacities of approximately 850 mAh/g with high Coulombic efficiencies of 99.9% at the fiftieth charge-discharge cycle. However, lithium metal and/or lithium oxides were deposited on the surface of the anode, resulting in capacity fading compared with the capacity of < 1900 mAh/g at the first cycle. On the other hand, transmission electron microscopic (TEM)/energy dispersive X-ray spectroscopic (EDX) images and X-ray photoelectron spectroscopic (XPS) spectra indicated that phosphorus was stably encapsulated into the carbon nanotubes after the charge-discharge cycles. They suggest that a P@DMWCNT anode shows partially-reversible reactions assuming xLi + P <-> LixP during charge-discharge cycles. (C) 2018 The Electrochemical Society.