Abstract:

In the past few decades, new energy industries have developed rapidly due to the threat of the depletion of non-renewable resources. Among them, the lithium-ion battery has attracted significant attention of various researchers. However, lithium-ion batteries are limited by the uneven distribution of lithium resources and high cost. Sodium, which is in the same periodic group as Lithium, can help alleviate the problems related to the limited development of lithium ion batteries owing to the shortage of lithium resources. Sodium ion batteries are cheap, with varying choice of electrolytes, and have relatively stable electrochemical performances. However, the radius of a sodium ion is larger than that of a lithium ion, leading to slow ion transportation as well as changes in the volume of the host material during the charging and discharging processes. Therefore, compared with existing lithium ion batteries, sodium ion battery anode materials are very limited. Moreover, most sodium ion battery electrode materials have low specific capacities and poor cycle retention rates. Among these, ternary metal oxides, which have two different cations and can reversibly react with sodium ions are promising high-capacity anode materials for sodium ion batteries. In this study, Ti₂Nb₂O₉ nanosheets are obtained by ion-exchange and chemical-delaminate methods. The carbon-coated Ti₂Nb₂O₉ nanosheets are obtained after hydrothermal coating with sucrose and calcination. From the thermogravimetric analysis (TG) curve, the carbon content in the composites is calculated to be approximately 8.0%. Owing to the rich reactive sites and a short ion transport pathway, the Ti₂Nb₂O₉/C electrode delivers a high reversible capacity of 265.2 mAh·g^-1 at a current density of 50 mA·g^-1. Even at a high current density of 500 mA·g^-1, the electrode exhibits an excellent electrochemical performance with a reversible capacity of 160.9 mAh·g^-1 after 200 cycles (capacity retention of 75.3%). Additionally, the Ti₂Nb₂O₉/C nanosheets exhibit high reversible capacities of 251.3, 224.6, 197.4, 176.3, and 156.5 mAh·g^-1 at the current densities of 100, 200, 500, 1000, and 2000 mA·g^-1, respectively. It is demonstrated through the use X-ray photoelectron spectroscopy (XPS) that the following process involving the transfer of four electrons occurs: Ti⁴⁺/Ti³⁺, Nb⁵⁺/Nb⁴⁺, during the charging and discharging process of the Ti₂Nb₂O₉ electrode in the voltage range of 0.01–3.0 V. The theoretical specific capacity of Ti₂Nb₂O₉ in this process is calculated to be 252 mAh·g^-1, corresponding to the electrochemical data. Overall, this study demonstrates that the Ti₂Nb₂O₉/C anode nanosheets have an excellent charge-discharge performance, cycle stability, and rate performance in sodium ion batteries, thereby providing a feasible choice for sodium ion battery anode materials.

Key words: Sodium ion battery, Delaminate, Ti₂Nb₂O₉ nanosheet, Carbon coating, Anode material