Ultra-small B2O3 nanocrystals grown in situ on highly porous carbon microtubes for lithium–iodine and lithium–sulfur batteries

B₂O₃-modified carbon microtubes, which possess a highly porous structure and well-dispersed ultra-small B₂O₃ nanocrystals (ca. <5 nm) in the tube wall, are successfully fabricated for lithium–iodine and lithium–sulfur batteries.