Porter Hanna Z, Foley Emily E, Jin Wen, Chen Eric, Lawrence Erick A, Bassey Euan N, Clément Raphaële J
Materials Department, University of California Santa Barbara, California 93106, United States.
Materials Research Laboratory, University of California Santa Barbara, California 93106, United States.
ACS Mater Au. 2024 Oct 30;5(1):170-181. doi: 10.1021/acsmaterialsau.4c00090. eCollection 2025 Jan 8.
Of the few weberite-type Na-ion cathodes explored to date, NaFeF exhibits the best performance, with capacities up to 184 mAh/g and energy densities up to 550 Wh/kg reported for this material. However, the development of robust structure-property relationships for this material is complicated by its tendency to form as a mixture of metastable polymorphs, and transform to a lower-energy Na FeF perovskite compound during electrochemical cycling. Our first-principles-guided exploration of Fe-based weberite solid solutions with redox-inactive Mg and Al predicts an enhanced thermodynamic stability of NaMg Fe F as the Mg content is increased, and the = 0.125 composition is selected for further exploration. We demonstrate that the monoclinic polymorph (space group 2/c) of NaFeF (Mg0) and of a new Mg-substituted weberite composition, NaMgFeF (Mg0.125), can be isolated using an optimized synthesis protocol. The impact of Mg substitution on the stability of the weberite phase during electrochemical cycling, and on the extent and rate of Na (de)intercalation, is examined. Irrespective of the Mg content, we find that the weberite phase is retained when cycling over a narrow voltage window (2.8-4.0 V vs Na/Na). Over a wider voltage range (1.9-4.0 V), Mg0 shows steady capacity fade due to its transformation to the Na FeF perovskite phase, while Mg0.125 displays more reversible cycling and a reduced phase transformation. Yet, Mg incorporation also leads to kinetically limited Na extraction and a reduced overall capacity. These findings highlight the need for the continued compositional optimization of weberite cathodes to improve their structural stability while maximizing their energy density.
在迄今为止探索的少数几种韦氏体型钠离子阴极材料中，NaFeF表现出最佳性能，据报道该材料的容量高达184 mAh/g，能量密度高达550 Wh/kg。然而，由于该材料倾向于以亚稳多晶型物的混合物形式形成，并在电化学循环过程中转变为能量更低的NaFeF钙钛矿化合物，因此建立这种材料稳固的结构-性能关系变得复杂。我们通过第一性原理指导对具有氧化还原惰性的Mg和Al的铁基韦氏体固溶体进行探索，预测随着Mg含量的增加，NaMgFeF的热力学稳定性会增强，并选择x = 0.125的组成进行进一步探索。我们证明，使用优化的合成方案可以分离出NaFeF（Mg0）和新的Mg取代韦氏体组成NaMgFeF（Mg0.125）的单斜多晶型物（空间群C2/c）。研究了Mg取代对韦氏体相在电化学循环过程中的稳定性以及对Na嵌入（脱嵌）的程度和速率的影响。无论Mg含量如何，我们发现当在狭窄的电压窗口（相对于Na/Na为2.8 - 4.0 V）内循环时，韦氏体相得以保留。在更宽的电压范围（1.9 - 4.0 V）内，Mg0由于转变为NaFeF钙钛矿相而显示出稳定的容量衰减，而Mg0.125则表现出更多的可逆循环和减少的相变。然而，Mg的掺入也导致动力学上受限的Na提取和整体容量降低。这些发现凸显了持续对韦氏体阴极进行成分优化以提高其结构稳定性同时最大化其能量密度的必要性。
