Critical Role of the Chemical Environment of Interlayer Na Sites: An Effective Way to Improve the Na Ion Electrode Activity of Layered Titanate

The chemical environments of the interlayer Na sites of layered titanate are finely controlled by the intercalation of n-alkylamine with various alkyl chain lengths to explore an effective way to improve its electrode functionality for sodium-ion batteries (SIBs). The n-alkylamine intercalation via ion-exchange and exfoliation-restacking routes allows the modification of in-plane structures of layered titanate to be tuned. Among the present n-alkylamine-intercalates, the n-pentylamine-intercalated titanate shows the largest discharge capacity with the best rate characteristics, underscoring the critical role of optimized intracrystalline structure in improving the SIB electrode performance of layered titanate. The creation of turbostratic in-plane structure degrades the SIB electrode performance of layered titanate, indicating the detrimental effect of in-plane structural disorder on electrode activity. ²³Na magic-angle spinning nuclear magnetic resonance spectroscopy demonstrates that the n-alkylamine-intercalated titanates possess two different interlayer Na⁺ sites near ammonium head groups/titanate layers and near alkyl chains. The intercalation of long-chain molecules increases the population of the latter site and the overall mobility of Na⁺ ions, which is responsible for the improvement of electrode activity upon n-alkylamine intercalation. The present study highlights that the increased population of interlayer metal sites remote from the host layers is effective in improving the electrode functionality of layered metal oxide for SIBs and multivalent ion batteries.