Guiding Design of Mn-Rich Phosphate Cathodes with Less Intrinsic Antisite Defects

The serious voltage hysteresis phenomenon in Na₃MnTi(PO₄)₃ has received extensive research interests, which is determined by the intrinsic-antisite-defects (IASDs) of Mn²⁺ resided in Na vacancies (Mn/Na_□) in structure. However, a general guideline to decrease IASDs is still lacking for the design of a higher-performance Na₃MnTi(PO₄)₃ system. Herein, we find that generation of Mn/Na□ IASDs in Na₃MnTi(PO₄)₃ system is mainly related to Na vacancies and weaker Mn─O bonds in structure. The more Na vacancies, the more probability for Mn²⁺ occupation on Na sites. Meanwhile, the weaker Mn─O bond, the more probability for Mn²⁺ delocalization/migration to other sites, finally leading to the Mn/Na_□ IASDs. To decrease Mn/Na_□ IASDs, we propose to introduce dopants with lower valence (vs. Ti⁴⁺), lower electronegativity (vs. Ti⁴⁺), and good solid solubility in Na₃MnTi(PO₄)₃ system. Based on the guiding rule, we have selected several doping cations (including Cr³⁺, Ti³⁺, Fe³⁺, and V³⁺) to construct a Na-rich environment and enhance Mn─O strength. Among various dopants, the substitution of V³⁺ for Ti⁴⁺ leads to the strongest Mn-O interaction, thus demonstrating the most effective suppression of Mn/Na_□ IASDs. With these discoveries, we further developed a series of V-doped Mn-richer phosphate cathodes, Na_{3.3+ y}Mn_1.15V_yTi_{0.85- y}(PO₄)₃ (0.1 ≤ y ≤ 0.25) as the promising candidates for Na-ion batteries.