NAtrium SuperIonic CONductor (NASICON) structured phosphate framework compounds are attracting a great deal of interest as suitable electrode materials for “rocking chair” type batteries. Manganese-based electrode materials are among the most favored due to their superior stability, resource non-criticality, and high electrode potentials. Although a large share of research was devoted to Mn-based oxides for Li- and Na-ion batteries, the understanding of thermodynamics and phase formation in Mn-rich polyanions is still generally lacking. In this study, we investigate a bifunctional Na-ion battery electrode system based on NASICON-structured Na_1+2xMn_xTi_2–x(PO₄)₃ (0.0 ≤ x ≤ 1.5). In order to analyze the thermodynamic and phase formation properties, we construct a composition–temperature phase diagram using a computational sampling by density functional theory, cluster expansion, and semi-grand canonical Monte Carlo methods. The results indicate finite thermodynamic limits of possible Mn concentrations in this system, which are primarily determined by the phase separation into stoichiometric Na₃MnTi(PO₄)₃ (x = 1.0) and NaTi₂(PO₄)₃ for x < 1.0 or NaMnPO₄ for x > 1.0. The theoretical predictions are corroborated by experiments obtained using X-ray diffraction and Raman spectroscopy on solid-state and sol–gel prepared samples. The results confirm that this system does not show a solid solution type behavior but phase-separates into thermodynamically more stable sodium ordered monoclinic α-Na₃MnTi(PO₄)₃ (space group C2) and other phases. In addition to sodium ordering, the anti-bonding character of the Mn–O bond as compared to Ti–O is suggested as another important factor governing the stability of Mn-based NASICONs. We believe that these results will not only clarify some important questions regarding the thermodynamic properties of NASICON frameworks but will also be helpful for a more general understanding of polyanionic systems.

中文翻译：

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Mn 基钠超离子导体 (NaSICon) 系统中相形成的特性：以 Na1+2xMnxTi2–x(PO4)3 (0.0 ≤ x ≤ 1.5) 为例

钠超离子导体 (NASICON) 结构的磷酸盐骨架化合物作为“摇椅”型电池的合适电极材料引起了极大的兴趣。锰基电极材料由于其优异的稳定性、资源非关键性和高电极电位而成为最受青睐的材料。尽管大部分研究致力于锂离子和钠离子电池的锰基氧化物，但对富含锰的聚阴离子的热力学和相形成的理解仍然普遍缺乏。在这项研究中，我们研究了一种基于 NASICON 结构的 Na _{1+2 x} Mn _x Ti _{2– x} (PO ₄ ) ₃ (0.0 ≤ x )的双功能钠离子电池电极系统。≤ 1.5)。为了分析热力学和相形成特性，我们使用密度泛函理论、簇扩展和半正则蒙特卡罗方法的计算采样构建了组成-温度相图。结果表明该系统中可能的 Mn 浓度的有限热力学极限，这主要由相分离成化学计量的 Na ₃ MnTi(PO ₄ ) ₃ ( x = 1.0) 和 NaTi ₂ (PO ₄ ) ₃ ( x < 1.0 或NaMnPO ₄ for x> 1.0。通过使用 X 射线衍射和拉曼光谱对固态和溶胶-凝胶制备的样品进行的实验证实了理论预测。结果证实，该体系没有表现出固溶型行为，而是相分离成热力学上更稳定的钠有序单斜晶系α-Na ₃ MnTi(PO ₄ ) ₃ (空间群C2）和其他阶段。除了钠有序之外，与 Ti-O 相比，Mn-O 键的反键特性被认为是控制 Mn 基 NASICON 稳定性的另一个重要因素。我们相信，这些结果不仅将阐明有关 NASICON 框架的热力学性质的一些重要问题，而且还将有助于更全面地了解聚阴离子系统。