Scalable Template-Free Synthesis of Na2Ti3O7/Na2Ti6O13 Nanorods with Composition Tunable for Synergistic Performance in Sodium-Ion Batteries

Solid-state reactions are a simple and scalable synthetic method, but they lack controlled nanostructures at present. Here, we report an energy-efficient solid-state synthesis, via the addition of carbon, to mass-produce uniform, single-crystalline, one-dimensional metal oxide nanorods with tunable composition according to performance demands. The carbon added in the solid-state reaction provides an alternative low-temperature route for the metal oxide formation due to the extra local heat generation and CO2/CO release from carbon oxidation. To demonstrate the methodology, a series of single-crystalline Na2Ti3O7/Na2Ti6O13 nanorods with tunable composition are synthesized and applied in sodium-ion batteries. The local heat generated from carbon allows formation of Na2Ti3O7 at 450 °C, a reaction temperature much lower than that of conventional solid-state methods (750–1000 °C), and Na2CO3 is regenerated to be recycled in the synthesis. The high theoretical capacity of Na2Ti3O7 and low volume expansion of Na2Ti6O13 upon charge–discharge are synergistically exploited to achieve high electrochemical performance and stability.