Formation Mechanism of Nitrogen-Doped Titanium Monoxide Nanospheres and Their Application as Sulfur Hosts in Lithium Sulfur Batteries

Lithium–sulfur batteries are one of the most promising next generation electrochemical devices with low cost and high energy density. Up to now, various metal oxides with strong interaction with lithium polysulfides (LiPs) have been designed as sulfur hosts to inhibit the notorious shuttle effect. Nitrogen doping on metal oxides can not only improve their shortcomings of poor electrical conductivity, but also introduce additional oxygen vacancies to further enhance their interaction with LiPs. In this work, the nitrogen-doped titanium monoxide (N–TiO) nanospheres have been first synthesized through a one-step method based on the synergistic effect of carbothermal reduction and nitridation reaction. The X-ray diffraction results suggested the gradually changed diffraction peak position, corresponding to the nitridation process. Through the calculation of Gibbs free energies in the temperature range of 800–1500 K, it was confirmed that TiO2 can directly transform to TiO and then to TiN. Moreover, the structural characterization and X-ray photoelectron spectroscopy results suggested the synthesized N–TiO nanospheres contain mixed valences of Ti4+, Ti3+, and Ti2+, together with a significantly increased O vacancy concentration. By using N–TiO/S as the cathode, a high initial capacitance of 1438 mA h g–1 at 0.2 C can be achieved, which was higher than those of the undoped TiO/S and TiN/S electrodes. In addition, such a cathode also delivered an initial discharge capacity of 1105 mA h g–1 at 2 C, and retained 474 mA h g–1 after 800 cycles, corresponding to the capacity decay rate of 0.071% per cycle. The strong interaction between O vacancies in N–TiO and LiPs, which has been confirmed through visible adsorption experiment and cyclic voltammetry curves of the Li2S6 symmetric cells, is the main reason for its excellent electrochemical performance. This work can provide new ideas for the preparation and application of N-doped metal oxide materials in the field of lithium–sulfur batteries.