Electrochemically Controllable Synthesis of Low-Valence Titanium Sulfides for Advanced Sodium Ion Batteries with Ultralong Cycle Life in a Wide Potential Window.

Abstract

Low-valence titanium sulfides (LVTS) have metal-like electrical conductivities and a strong polysulfide binding abilities, which are promising anodes for sodium ion batteries with high capacities and long cycle lifes. However, it is difficult for traditional synthesis methods to synthesize LVTS without impurities. The electric field regulation method possesses the advantages of flexibility and high efficiency, achieving accurate control of the metal reduction process by adjusting the electrolysis potential and reaction time. In this work, we synthesized a series of LVTS (TiS and Ti2S) using electric field control methods and investigated their electrochemical behaviors as sodium storage anodes for the first time. Compared with traditional TiS2, LVTS display remarkable Na storage properties under the condition of complete electrochemical conversion at 0.005-3 V. Especially for TiS, it demonstrates a high capacity of 409 mAh g-1 at 1 A g-1 and inspiring cyclic stability over 6000 cycles. The large number of vacancies in the crystal structure can chemically anchor polysulfides and alleviate their dissolution in the electrolyte, resulting in superior long-term cyclic stability. The high intrinsic conductivity of LVTS is in favor of rapid transfer of electrons and promotes the fast conversion of polysulfides to sodium sulfides, thus realizing high reversible capacities. Moreover, with its fast Na+ transport kinetics, the as-prepared TiS demonstrates an impressive rate performance of 321 mAh g-1 at 15 A g-1. Overall, the electric field regulation method is flexible and efficient, which provides a new route for the preparation of high-performance electrode materials. Moreover, nonstoichiometric metal compounds possess abundant active sites and rapid electron transport kinetics, which provide a new choice for promising sodium storage materials in large-scale energy storage applications.