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 via a facile thermal-treated process from poplar catkin, and then investigated for the first time as a capture interlayer for lithium-iodine (Li-I₂) and lithium-sulfur (Li-S) batteries. After introduction of the B₂O₃/carbon microtube composite interlayer, the iodine cathode demonstrates a reversible capacity of 176.9 mA h g^-1 after 500 cycles at a rate of 20C. When the rate increases to 100C, a high capacity of 140.7 mA h g^-1 can still be obtained after 5000 cycles, which makes this the most stable iodine cathode reported for Li-I₂ batteries to date. For Li-S batteries, the sulfur cathodes present excellent cycling performance with a high discharge capacity of 594 mA h g^-1 at the rate of 4C. Such excellent cycle stability for the Li-S and Li-I₂ batteries could be mainly attributed to strong interactions between the ultra-small B₂O₃ nanocrystals and polysulfide or iodine ions, which has been proven using first principle calculations.