Origin of Bonding between the SWCNT and the Fe₃O₄(001) Surface and the Enhanced Electrical Conductivity

Recent experiments have demonstrated that adding single-wall carbon nanotubes (SWCNTs) to Fe₃O₄ nanoparticle electrodes leads to dramatically improved electrical conductivity and performance of Li ion batteries. Our density functional theory (DFT) calculations reveal that the interactions between both pristine and B- or N-doped SWCNTs and the Fe₃O₄(001) surface are very weak. Although C vacancies in SWCNTs can lead to stronger chemical bonding between SWCNTs and Fe₃O₄(001) surfaces, the binding and electrical conductivity in this case are not ideal. Interestingly, we show that transition-metal (Fe, Ni) atoms or clusters facilitate the formation of strong chemical bonding between SWCNTs and Fe₃O₄(001) surfaces, providing excellent channels for electrons flowing between SWCNTs and Fe₃O₄(001) surfaces, which is essential for improving electrical conductivity of the mixed electrodes. The calculated electron conductance of the transition-metal-decorated system is improved by more than 2 orders of magnitude, in agreement with experimental observations. Our results, therefore, suggest a viable way for functionalizing SWCNTs.