Non-Equilibrium Transformation Mechanisms in a Prussian Blue Analogue Electrode

Sodium- and potassium-ion batteries are emerging as a promising complementary technology to lithium-ion batteries due to their prospective low cost and utilisation of abundant resources.^[1]

In my talk, I will discuss the non-equilibrium transformation mechanisms in a Prussian blue analogue electrode. Prussian blue analogues (PBAs) are an increasingly prominent family of cathode materials for both Na- and K-ion batteries,^[2] yet one whose electrochemical transformation mechanisms are remarkably poorly understood. Structural distortions are a particular characteristic of low-vacancy PBAs – targeted for high theoretical capacities.^[3] In the discharged state distortions are dominated by the alkali-metal cation,^[4] whereas in the charged state distortions can arise due to framework strain.^[5] The structural distortions drive characteristic phase transitions on cycling.^[3]

By combining operando X-ray absorption spectroscopy and X-ray diffraction with state-of-the-art Metropolis matrix factorisation analysis, we show that the nature of the phase transitions limits the ion transport kinetics, and therefore the fast-charging capabilities of the materials. The mechanisms at play are contrasted with conventional Li-ion cathodes, which reveals the significance of the highly flexible PBA framework. In this context, the framework’s ability to accommodate strain is detrimental for high rate cyclability. Our fundamental diagnosis of the structure-functionality relationship leads us to forming a strategy for designing optimised PBA cathode materials.^[6]

[1] Y. Tian, G. Zeng, A. Rutt, T. Shi, H. Kim, J. Wang, J. Koettgen, Y. Sun, B. Ouyang, T. Chen, Z. Lun, Z. Rong, K. Persson & G. Ceder Chem. Rev. 121, 1623–1669 (2021).
[2] K. Hurlbutt, S. Wheeler, I. Capone & M. Pasta Joule 2, 1950–1960 (2018).
[3] J. Cattermull, M. Pasta & A. L. Goodwin Mater. Horiz. 8, 3178–3186 (2021).
[4] J. Cattermull, N. Roth, S. J. Cassidy M. Pasta & A. L. Goodwin J. Am. Chem. Soc. 145, 24249–24259 (2023).
[5] E. A. Harbourne, J. Cattermull, H. L. B. Boström, N. Roth, M. Pasta, D. A. Keen & A. L. Goodwin In Review doi: 10.48550/arXiv.2408.13169 (2024).
[6] J. Cattermull, B. Jagger, S. J. Cassidy, S. Dhir, P. K. Allan, M. Pasta & A. L. Goodwin In Preparation.