Prussian blue analogues (PBAs)[1] with the general composition A₂M[Fe(CN)₆] (A: alkali metal; M: Fe, Mn, etc) are an attractive positive electrode for potassium-ion batteries, owing to their chemical composition based on widely abundant materials, ease of synthesis, high electrochemical reversibility and higher average potential compared to its sodium congener [1]. The combination of a PBA electrode with a graphite negative electrode in a full cell configuration showed great promise as post-Li battery system. However, the upper cut-off potentials of K₂Fe[Fe(CN)₆] and K₂Mn[Fe(CN)₆] pose serious stability issues with respect to irreversible electrolyte degradation reactions. In addition, the electrolyte components have to be compatible with the potassium intercalation reaction at the graphite electrode[2], thus limiting the number of suitable electrolyte constituents.

In this presentation, we discuss aspects of the material synthesis and electrolyte degradation processes at high potentials in liquid carbonate-based electrolytes. As we will be shown the choice of the precursors is paramount to arrive at suitable particle sizes and has a great impact on the electrochemical behavior of the material. Likewise, the density of Fe-vacancies strongly depends on the chosen synthesis and may lead to significant losses in the achievable discharge capacity.

The electrode-electrolyte interface in half and full cell configurations was studied by in-house and synchrotron-based photoelectron spectroscopy (PES) for a detailed characterization of the surface layer and the oxidation states of iron in K₂Fe[Fe(CN)₆] electrodes. This combined analysis of electrochemical and surface-sensitive analytical studies provides a general picture of the electrode degradation at high potentials and fosters the development of better electrolyte mixtures. Our results further show, how a deliberate choice of electrolyte components can help to reduce irreversible reactions and improve cycling stability and cycle life of potassium-ion batteries. For this we have recently expanded our activities also to solid polymer electrolytes, showing superior capacity retention to liquid electrolyte systems[3].

Figure 1. left: K₂Fe[Fe(CN)₆] obtained using different Fe-precursors; right: capacity retention of PBA-K cells cycled in either a liquid (black) or solid polymer (red/purple) electrolyte.

References:

[1] Kim et al., Trends Chem. 1 (2019) 682–692.
[2] Allgayer et al., ACS Appl. Energy Mater. 5 (2022) 1136–1148.

[3] Khudyshkina et al., ACS Appl. Polym. Mater. 4 (2022) 2734–2746.