Suppressing Intrinsic Anti-Site Defects via Targeted Li-Occupation Unlocks Ultrahigh-Rate Capability in Vanadium-Free NASICON Cathodes.

Abstract

Vanadium-free Na superionic conductor (NASICON)-type Na₃MnTi(PO₄)₃ (NMTP) has been recognized as a prospective cathode material for sodium-ion batteries owing to the abundance of raw materials, and its eco-friendly composition. Yet the intrinsic anti-site defects (IASDs) derived from the occupation of Mn²⁺ on the Na-vacancy site (Mn/M2_v) in NMTP severely deteriorates the Mn redox kinetics, causing abnormal voltage hysteresis and unsatisfactory rate performance. This study rationally designed a targeted Li-occupation to effectively restrain the Mn/M2_v IASDs formation. Theoretical calculations identify that the tiny Li⁺ ions preferentially occupy the alkali-metal vacancies instead of Mn²⁺ and selectively occupy M2 (18e)-sites rather than M1 (6b)-sites, thereby effectively inhibiting anti-site occupation of Mn²⁺. This is experimentally validated in the prepared Na_2.95Li_0.05MnTi(PO₄)₃ (NMTP-Li0.05), where minimal Li doping leads to significant suppression of Mn/M2_v IASDs. During charge-discharge, the introduced Li⁺ exhibits high mobility between M1/M2 sites and low electrostatic repulsion with Na⁺. Therefore, the NMTP-Li0.05 achieves promoted kinetics with eliminated voltage hysteresis and an ultrahigh-rate capability of 70.2 mA h g^-1 at 100 C, which is exceptional among V-free NASICON cathodes. This work establishes an effective strategy for IASDs suppression in polyanionic cathodes and paves the way for developing high-performance and sustainable sodium-ion batteries.