Structural evolution from layered Na2Ti3O7 to Na2Ti6O13 nanowires enabling a highly reversible anode for Mg-ion batteries.

ABSTRACT

The development of suitable host materials for the reversible storage of divalent ions such as Mg2+ is still a big challenge and its progress to date has been slow compared to that of monovalent Li+ or Na+. Herein, we present the study of layered sodium trititanate (Na2Ti3O7) and sodium hexatitanate (Na2Ti6O13) nanowires as anode materials for rechargeable Mg-ion batteries. It is found for the first time that the structural evolution from layered Na2Ti3O7 to Na2Ti6O13 with a more condensate three-dimensional microporous structure enables remarkably enhanced Mg-ion storage performance. The Na2Ti6O13 electrode can achieve a large initial discharge and charge capacity of 165.8 and 147.7 mA h g-1 at 10 mA g-1 with a record high initial coulombic efficiency up to 89.1%. Ex situ XRD, Raman measurements and EDX mapping were used to investigate the electrochemical reaction mechanism. It is suggested that the irreversible structure change and the formation of insoluble NaCl with high yield and large particles when Na+ is replaced by inserted Mg2+ for the Na2Ti3O7 electrode could be ascribed to the rapid decline in capacity. By contrast, the Na2Ti6O13 electrode exhibits good structure stability during the Mg-ion insertion/extraction process, leading to good rate performance and cycling stability.