Identifying the intrinsic anti-site defect in manganese-rich NASICON-type cathodes

Manganese-rich NASICON-type materials have triggered widespread attention for developing advanced polyanionic cathodes, primarily driven by their abundant reserves and promising cycling performance with high operating voltages (~3.8 V for Mn2+/3+/4+, versus Na+/Na). However, the charge/discharge profiles exhibit significant voltage hysteresis, which leads to a limited reversible capacity, thereby preventing their application. Here, we demonstrate that the voltage hysteresis in manganese-rich NASICON-type cathodes (Na3MnTi(PO4)3) is closely related to the intrinsic anti-site defect (IASD), which forms during synthesis and is captured in our characterizations. Combining electrochemical analysis and spectroscopic techniques, we draw a comprehensive picture of sluggish Na+ diffusion behaviours in the IASD-affected structure during cycling, and rationalize the relationship of voltage hysteresis, phase separation and delayed charge compensation. Furthermore, a Mo-doping strategy is developed to decrease the defect concentration, which enhances the initial Coulombic efficiency from 76.2% to 85.9%. Overall, this work sheds light on the voltage hysteresis in NASICON-type cathodes and provides guidelines for designing high-performance polyanionic electrodes. Manganese-rich NASICON-type compounds are promising cathode materials for sodium-ion batteries, but they suffer from severe voltage hysteresis. Here the authors uncover the root cause of voltage hysteresis in Na3MnTi(PO4)3 and demonstrate a doping strategy to mitigate the issue.