(Invited) Complex Phase Transitions in Layered Sodium Transition Metal Oxides

As an alternative to Li-based technologies to develop stationary energy storage systems, Na-ion batteries have attracted substantial interest. Unlike the Li-ion, layered sodium transition metal oxide (NaTMO 2 ) cathodes can diversify their redox centers from expensive Co to cheap Mn and Fe to achieve high energy density, offering a wide variety of chemistry selection to design low-cost, high-performance batteries. 1 These cathode materials often undergo discontinuous, yet reversible, phase transitions upon Na intercalation due to complex interatomic interactions, which result in multiple plateau-like features in their voltage profiles. However, large desodiation of Fe-containing Na-ion cathodes often leads to an unclarified, irreversible phase transition at high voltage, hindering the full use of Fe redox. 2 In this talk, we will discuss how transition metal selection affects the thermodynamic stability of desodiated structures of NaTMO 2 . First, we demonstrate this insight by comparing two prototypical compositions, NaNi 0.5 Co 0.5 O 2 and NaNi 0.5 Fe 0.5 O 2 , with detailed experimental and computational investigation. 3 The Na intercalation processes of the two compounds are carefully investigated by electrochemical methods and in situ X-ray diffraction. We confirm that although the two materials share the same Ni 3+ /Ni 4+ redox, their structural changes are very different from each other. Na x Ni 0.5 Co 0.5 O 2 goes through multiple phase transitions including three different O3-type and three different P3-type structures. In contrast, Na x Ni 0.5 Fe 0.5 O 2 only undergoes a generic O3-P3 transition with no sign of monoclinicity, leading to a smoothly increasing voltage profile. Second, our recent findings on a quaternary transition metal system, NaTi 0.25 Fe 0.25 Co 0.25 Ni 0.25 O 2 , 4 will be underlined. We will present structural evidence and characterization of the highly-desodiated, high-voltage phase that exhibits peculiar oxygen stacking. This phase consists of alternating octahedral and prismatic Na layers, namely OP2 stacking, in which formation is afforded by distortion-tolerant Ti and Jahn-Teller-active Fe. Moreover, we will demonstrate that this new phase participates in redox reaction reversibly, fundamentally distinct from inactive high-voltage phases in many Na-ion cathodes. Finally, a strategy to stabilize Fe-containing NaTMO 2 materials to create layered cathodes with high reversible Na capacity will be briefly discussed. Our results demonstrate that we can manipulate the redox reaction and associated structure change by transition metal mixing. References Yabuuchi, K. Kubota, M. Dahbi, and S. Komaba, Chem. Rev. , 2014, 114, 11636−11682 Jeong, H. Lee, J. Yoon, and W.-S. Yoon, J. Power Sources , 2019, 439, 227064 Vassilaras, D.-H. Kwon, S. T. Dacek, T. Shi, D. -H. Seo, G. Ceder, and J. C. Kim, J. Mater. Chem. A , 2017, 5, 4596–4606 Vassilaras, S. T. Dacek, H. Kim, T. T. Fister, S. Kim, G. Ceder, and J. C. Kim, J. Electrochem. Soc. , 164, 2017, A3484–A3486