Microstructural control of new intercalation layered titanoniobates with large and reversible d-spacing for easy Na ion uptake.

Key issues for Na-ion batteries are the development of promising electrode materials with favorable sites for Na ion intercalation/deintercalation and an understanding of the reaction mechanisms due to its high activation energy and poor electrochemical reversibility. We first report a layered HTiNbO as a new anode material. This anode material is engineered to have dominant (200) and (020) planes with both a sufficiently large d-spacing of 8.3 Å and two-dimensional ionic channels for easy Na ion uptake, which leads to a small volume expansion of ~0.6 Å along the direction upon Na insertion (discharging) and the lowest energy barrier of 0.19 eV in the [020] plane among titanium oxide-based materials ever reported. The material intercalates and deintercalates reversibly 1.7 Na ions (200 mAh g) without a capacity fading in a potential window of 0.01 to 3.0 V versus Na/Na. Na insertion/deinsertion takes place through a solid-solution reaction without a phase separation, which prevents coherent strain or stress in the microstructure during cycling and ensures promising sodium storage properties. These findings demonstrate a great potential of HTiNbO as the anode, and our strategy can be applied to other layered metal oxides for promising sodium storage properties.