Relationship between cation site preference and charge–discharge characteristics in TiNb₂O₇

Abstract

This study investigates the influence of synthesis conditions on electrode performance by growing TiNb₂O₇ (TNO) single crystals, conducting single-crystal X-ray structural analysis, and comparing the resulting structures. The findings revealed that the site preferences of Ti⁴⁺ and Nb⁵⁺ within the five octahedral sites (M1–M5) remained unaffected by the synthesis atmosphere at low temperatures below 800 °C. However, an increase in synthesis temperature from 800 °C onwards enhanced the site preference, particularly at the M1 and M5 sites, displaying significant changes. Based on the anisotropy of ion conduction and the conduction mechanism of TNO, it is proposed that ion conduction is facilitated by the positioning of Ti⁴⁺ at the M1 site, which is characterized by many shared edges within the tunnel structure and plays a critical role in enabling Li-ion conduction. While the site selectivity of Ti⁴⁺ and Nb⁵⁺ remained constant across synthesis atmospheres, the observed increase in the lattice constant and electronic conductivity contributed to the charge–discharge characteristics. In contrast, the sample synthesized at 800 °C, which exhibited the highest charge–discharge performance among the temperature-varied samples, had the highest occupancy rate of Ti⁴⁺ at the M1 site. This occupancy is hypothesized to underpin its superior electrochemical performance.