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

The serious voltage hysteresis phenomenon in Na$_3$MnTi(PO$_4$)$_3$ has received extensive research interests, which is determined by the intrinsic-antisite-defects (IASDs) of Mn$^{2+}$ 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 Na$_3$MnTi(PO$_4$)$_3$ system. Herein, we find that generation of Mn/Na$_□$ IASDs in Na$_3$MnTi(PO$_4$)$_3$ system is mainly related to Na vacancies and weaker Mn─O bonds in structure. The more Na vacancies, the more probability for Mn$^{2+}$ occupation on Na sites. Meanwhile, the weaker Mn─O bond, the more probability for Mn$^{2+}$ 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. Ti$^{4+}$), lower electronegativity (vs. Ti$^{4+}$), and good solid solubility in Na$_3$MnTi(PO$_4$)$_3$ system. Based on the guiding rule, we have selected several doping cations (including Cr$^{3+}$, Ti$^{3+}$, Fe$^{3+}$, and V$^{3+}$) to construct a Na-rich environment and enhance Mn─O strength. Among various dopants, the substitution of V$^{3+}$ for Ti$^{4+}$ leads to the strongest Mn–O interaction, thus demonstrating the most effective suppression of Mn/Na$_□$ IASDs. ... mehrWith these discoveries, we further developed a series of V-doped Mn-richer phosphate cathodes, Na$_{3.3+y}$Mn$_{1.15}$V$_y$Ti$_{0.85-y}$(PO$_4$)$_3$ (0.1 ≤ y ≤ 0.25) as the promising candidates for Na-ion batteries.