This is the first study to investigate the electrochemical Li ion insertion/deinsertion property of C-60 encapsulated single-walled carbon nanotubes (SWCNTs) (C-60-peapods). it was found that the reversible Li ion storage capacity of the C-60-peapod per unit weight is about 1.2 times greater than that of the empty tubes. This suggests that one peapod tube can store almost 1.7 times more reversible Li ions compared to one empty SWCNT tube. (C) 2008 Elsevier Ltd. All rights reserved.