Understanding the electrochemistry of a Prussian Blue Analogue

Rechargeable batteries will play a key role in the transition towards renewable and sustainable energy sources. Currently, batteries are mostly based on the lithium-ion technology and employ Ni or Co, which are not eco-friendly elements and whose mining raises ethical issues. Further, there are serious concerns on the scarce availability of Li resources, given the high market demand. To overcome these limitations, a huge effort is being made to find suitable alternatives to lithium for the development of the next-generation batteries. Promising candidates are potassium-ion batteries (PIBs), which may use Prussian Blue Analogues (PBAs) as positive electrodes due to their excellent electrochemical properties vs. potassium insertion.

PBAs can be described by the formula unit A_xM₁[M₂(CN)₆]_y (x ≤ 2), where A is an alkali cation, M₁ and M₂ are Transition Metal (TMs). TMs located in M₁N₆ or M₂C₆ octahedra form a three-dimensional cage-like structure with open channels, which facilitates the insertion of various guest cations. Manganese and iron are commonly chosen because they are cheap, non-toxic and redox active. The sequence of the redox reactions in PBAs is still not understood and previous investigations report the occurrence of reversible phase transition and volume expansion/shrinkage phenomena during the K⁺ ions (de)insertion. Such structural evolution could be triggered by both the variation of oxidation states of the TMs and the simultaneous (de)insertion of the cations within the cage structure. Since the electronic and local structures have a strong correlation with the electrochemical properties of electrode materials, and then to battery performance in terms of reversibility and cyclability, the elucidation of these phenomena is essential for the optimization of the design and performance of the next-generation batteries.

Our study focuses on the K_1.67Mn_0.65Fe_0.35[Fe(CN)₆]_0.92 0.45 H₂O cathode material, which we investigated under working conditions. In order to monitor the cathode structural evolution during charge/discharge cycles, we employed operando X-ray Absorption Spectroscopy (XAS). The experiments were performed at the XAFS beamline of Elettra, probing both the Fe and Mn K-edges.

Figure 1: XAS data collected during the first charge (depotassiation) on Fe and Mg edges on both XANES (a) and EXAFS (b) region. c) Computed concentration profiles obtained from MCR-ALS data treatment of the operando XAS data at the Fe (top) and Mn (bottom).

The XANES (X-ray Absorption Near Edge Structure) spectra acquired during the first charging cycle (see Fig. 1a) show a clear edge shift towards higher energies for both elements, in line with the expected increases in the oxidation states of the two TMs. The corresponding radial distributions extracted from the EXAFS (Extended X-ray Absorption Fine Structure) region (see Fig. 1b) reflect the modification in the coordination spheres of both metallic centres. In order to correlate the different reaction steps with specific redox couples, the two data sets have been analysed by Principal Component Analysis followed by MCR-ALS. The results are reported in Fig. 1c and show the concentration evolution of the three and two components identified for Fe and Mn, respectively.

Figure 2: Scheme of the phase transition occurring during depotassiation, as revealed by combined operando XRD and XAS analysis

By combining XAS spectroscopy, ex situ ⁵⁷Fe Mössbauer and operando XRD measurements, we were able to correlate the electrochemical reactions with the structural and bonding evolution of the Mn and Fe centres, which are summarized in Fig. 2. In the initial condition, the anode material is characterized by a monoclinic structure. Here, the oxidation state of Mn and Fe and their relative weight ratio was determined by XAS and Mössbauer spectroscopy. Upon charging, the extraction of K⁺ ions going along with the oxidation of Mn and Fe is accompanied by the de-tilting of the framework and the reduction of the M-N bond length. K⁺ de-insertion determines a phase transformation from monoclinic to cubic, structure along with an increase of the lattice size. This phase transition slows down the insertion of K⁺, and the continuous bond breathing of Mn, together with its likely slow partial dissolution in the electrolyte, might lead to an unrecoverable structure and deactivated Mn sites. This observation reveals that the optimization of the chemical composition of the electrode is a fundamental aspect in the design of the battery. Indeed, a decrease of Mn content will reduce the impact of framework distortion and will minimize battery failure due to significant global and/or local structural modifications.

The results of our work represent a crucial step towards the preparation of reliable and stable PBA-based cathodes, in order to make PIBs viable and sustainable alternatives to lithium-ion technology.

This research was conducted by the following research team:

Phuong Nam Le Pham^1,2, Romain Wernert^1,3,4, Maëlle Cahu¹, Moulay Tahar Sougrati^1,2,4, Giuliana Aquilanti⁵, Patrik Johansson^2,6, Laure Monconduit^1,2,4 and Lorenzo Stievano^1,2,4

¹ ICGM, Univ. Montpellier, CNRS, Montpellier, France
² Alistore-ERI, CNRS, Amiens, France
³ Univ. Bordeaux, CNRS, Bordeaux INP, ICMCB, Pessac, France
⁴ RS2E, CNRS, Amiens, France
⁵ Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
⁶ Department of Physics, Chalmers University of Technology, Göteborg, Sweden

Contact person email:

Local contact person email: