Snarskis Gustautas, Pilipavičius Jurgis, Gryaznov Denis, Mikoliu Naitė Lina, Vilčiauskas Linas
Center for Physical Sciences and Technology (FTMC), Saulėtekio al. 3, LT-10257 Vilnius, Lithuania.
Institute of Solid State Physics, University of Latvia, Kengaraga 8, LV-1063 Riga, Latvia.
Chem Mater. 2021 Nov 9;33(21):8394-8403. doi: 10.1021/acs.chemmater.1c02775. Epub 2021 Oct 21.
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 Mn Ti (PO) (0.0 ≤ ≤ 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 NaMnTi(PO) ( = 1.0) and NaTi(PO) for < 1.0 or NaMnPO for > 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-NaMnTi(PO) (space group 2) 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.
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正引起广泛关注。锰基电极材料因其卓越的稳定性、资源非关键性和高电极电位而备受青睐。尽管大量研究致力于锂和钠离子电池的锰基氧化物，但对富锰聚阴离子中的热力学和相形成的理解仍然普遍不足。在本研究中，我们研究了基于NASICON结构的NaMnTi(PO)（0.0≤≤1.5）的双功能钠离子电池电极系统。为了分析热力学和相形成特性，我们使用密度泛函理论、团簇展开和半巨正则蒙特卡罗方法通过计算采样构建了成分-温度相图。结果表明该系统中可能的锰浓度存在有限的热力学极限，这主要由相分离为化学计量比的NaMnTi(PO)（=1.0）和<1.0时的NaTi(PO)或>1.0时的NaMnPO决定。通过对固态和溶胶-凝胶制备的样品使用X射线衍射和拉曼光谱获得的实验结果证实了理论预测。结果证实该系统不表现出固溶体型行为，而是相分离为热力学上更稳定的钠有序单斜α-NaMnTi(PO)（空间群2）和其他相。除了钠有序化，与Ti-O相比，Mn-O键的反键特性被认为是控制锰基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关于NASICON骨架热力学性质的一些重要问题，而且将有助于更全面地理解聚阴离子系统。
