Uniform and ultrathin carbon-layer coated layered Na2Ti3O7 and tunnel Na2Ti6O13 hybrid with enhanced electrochemical performance for anodes in sodium ion batteries

Ultrathin and uniform carbon layer-coated layered Na2Ti3O7 and tunnel Na2Ti6O13 hybrids were synthesized via a facile and fast method using supercritical methanol and subsequent carbon coating with liquid carbon dioxide as a coating solvent. The carbon content and relative composition of the layered Na2Ti3O7 and tunnel Na2Ti6O13 phases in the hybrids were controlled by adjusting the calcination temperature. The drawbacks of each material could be overcome in the hybrid by taking advantage of the low volume expansion and high electronic conductivity (Na2Ti6O13), the high capacity (Na2Ti3O7), and a carbon coating. A careful examination of the cyclic voltammetry profiles of the sodium titanate hybrids revealed the existence of two different Na+ ion diffusion pathways in the layered Na2Ti3O7 phase. Under high discharge-charge conditions, some of the Na+ ion uptake in the layered Na2Ti3O7 and the structural integrity of tunnel Na2Ti6O13 resulted in excellent high-rate performance and long-term cyclability in the hybrid.