Ultra-small TiO2 nanoparticles embedded in carbon nanosheets for high-performance sodium storage

With the properties of high theoretical capacity and excellent structural stability, TiO2 has been widely studied as anode material for sodium-ion batteries (SIBs). In this work, a composite structure of ultra-small TiO2 nanoparticles embedded in carbon nanosheets (TCNS) is obtained by annealing polyvinylpyrrolidone coated Zn-Ti layered double hydroxide under inert atmosphere, with zinc species removed by hydrochloric acid. As sodium-ion battery anode, the resultant TCNS shows a high reversible capacity of 271.2 mAh g−1 at 50 mA g−1 and considerable cycling stability (maintaining 196.7 mAh g−1 after 2000 cycles at 2 A g−1). Ex-situ XRD and TEM investigations clearly illustrate the structure changes of anatase during the sodium storage process. Specifically, ultra-small TiO2 nanoparticles in TCNS show obvious crystal distortion as triggered by the initial insertion of sodium ions, with the reversible sodium storage happening at the (1 0 1) active plane of anatase. As experimentally and theoretically evidenced, the twisted crystal structure is maintained in the subsequent cycles, which can effectively promote the sodium diffusion rate in anatase, resulting in the excellent rate and cycle performances of TCNS anode. This study provides informative guidance to explore high-performance TiO2 anodes for SIBs, with novel insights into the sodium ion insertion/extraction mechanism comprehensively elucidated during reversible sodium storage process.