Synthesis of highly substitutional nitrogen doped TiO2 via oxygen vacancy mediated strategy for ultrafast-charging lithium ion storage

Abstract

Substitutional nitrogen (SN) doping is an effective approach to simultaneously enhance the ionic diffusion kinetics and increase the electronic conductivity of TiO2 being as host for lithium-ion storage. Nevertheless, achieving SN doping with high concentration in well-crystallized TiO2 nano-particles is still a great challenge. Herein, guided by density functional theory (DFT) calculations, an oxygen vacancy-mediated SN doping strategy is purposed to modify the surface structure of TiO2 nano-particles. A reduced TiO2-x amorphous layer with rich oxygen vacancy on the surface of TiO2 nano-particles brings about high doping concentration of SN (∼13.8 atom%) and thus a TiO2@TiO2-x-SN [email protected] heterojunction is constructed. The high doping concentration of SN in TiO2-x-SN shell of TiO2@TiO2-x-SN heterojunction favors the diffusion kinetics of Li+ due to the formation of local electric field at its heterointerface region and reduced diffusion barrier of Li+. Moreover, TiO2@TiO2-x-SN heterojunction shows a decreased band gap of 2.37 eV after increasing the doping concentration of SN, facilitating the transport of electrons. Thus, the TiO2@TiO2-x-SN electrode displays high reversible specific capacity of 362.7 mAh g−1 at 0.1C and ultra-long cycling structural stability (with a capacity retention of 76.1% after 5000 cycles at 20C). A Li+ hybrid capacitor, assembled by activate carbon cathode and pre-lithitated TiO2@TiO2-x-SN anode, shows high energy/power densities (114.8 Wh kg−1/305.1 W kg−1 and 90.2 Wh kg−1/5659.8 W kg−1). This work paves a way for developing high concentration of heteroatom doping metallic oxides and offers comprehensive understandings for the relationship between doping concentration of heteroatoms and energy storage mechanism.