Na2+XTi6O13 As Potential Negative Electrode Material for Na-Ion Batteries

Na-ion as a charge carrier is one of the most promising alternatives for Li-ion storage due to its high abundance and low cost. Here we show that the reversible capacity of the recently reported layered Na₂Ti₆O₁₃ negative electrode material can be enhanced from 49.5 mAh/g (Na₂₊₁Ti₆O₁₃) to 196 mAh/g (Na₂₊₄Ti₆O₁₃) when the cut-off voltage is lowered from 0.3 V to 0 V. However, the lower cutoff voltage leads to a decrease in reversible capacity, most likely due to electrolyte decomposition. In-situ X-ray diffraction experiments in combination with DFT calculations reveal the structural changes, including the Na-ion positions, in Na_2+xTi₆O₁₃. Up to Na₂₊₂Ti₆O₁₃ a solid solution mechanism is demonstrated with small structural changes. Both DFT and X-ray diffraction indicate that Na₂₊₂Ti₆O₁₃ is the upper limit of the Na-ion storage in the bulk of the material. The 196 mAh/g (Na₂₊₄Ti₆O₁₃) capacity achieved at 0 V is suggested to be the consequence of a surface storage reaction. The combination of the large capacity and a solid solution mechanism with small structural changes make this material of potential interest for the use as negative electrode in Na-ion batteries.