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

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.